a novel process for fabricating membrane-electrode

A NOVEL PROCESS FOR FABRICATING

MEMBRANE-ELECTRODE ASSEMBLIES

WITH LOW PLATINUM LOADING FOR USE

IN PROTON EXCHANGE MEMBRANE FUEL

CELLS

by

Shahram Karimi

A thesis submitted in conformity with the requirements for the

degree of Doctor of Philosophy

Graduate Department of Chemical Engineering and Applied

Chemistry

University of Toronto

Copyright by Shahram Karimi (2011)

ii

A NOVEL PROCESS FOR FABRICATING MEMBRANE-ELECTRODE

ASSEMBLIES WITH LOW PLATINUM LOADING FOR USE IN PROTON

EXCHANGE MEMBRANE FUEL CELLS

Shahram Karimi

Doctor of Philosophy

Department of Chemical Engineering & Applied Chemistry

University of Toronto (2011)

ABSTRACT

A novel method based on pulse current electrodeposition (PCE) employing four different

waveforms was developed and utilized for fabricating membrane-electrode assemblies

(MEAs) with low platinum loading for use in low-temperature proton exchange

membrane fuel cells. It was found that both peak deposition current density and duty

cycle control the nucleation rate and the growth of platinum crystallites. Based on the

combination of parameters used in this study, the optimum conditions for PCE were

found to be a peak deposition current density of 400 mA cm-2

, a duty cycle of 4%, and a

pulse generated and delivered in the microsecond range utilizing a ramp-down waveform.

MEAs prepared by PCE using the ramp-down waveform show performance comparable

with commercial MEAs that employ ten times the loading of platinum catalyst. The

thickness of the pulse electrodeposited catalyst layer is about 5-7 µm, which is ten times

thinner than that of commercial state-of-the-art electrodes.

MEAs prepared by PCE outperformed commercial MEAs when subjected to a series of

steady-state and transient lifetime tests. In steady-state lifetime tests, the average cell

voltage over a 3000-h period at a constant current density of 619 mA cm-2

for the in-

house and the state-of-the-art MEAs were 564 mV and 505 mV, respectively. In

addition, the influence of substrate and carbon powder type, hydrophobic polymer

content in the gas diffusion layer, microporous layer loading, and the through-plane gas

permeability of different gas diffusion layers on fuel cell performance were investigated

and optimized.

iii

Finally, two mathematical models based on the microhardness model developed by

Molina et al. [J. Molina, B. A. Hoyos, Electrochim. Acta, 54 (2009) 1784-1790] and

Milchev [A. Milchev, ―Electrocrystallization: Fundamentals of Nucleation And Growth‖

2002, Kluwer Academic Publishers, 189-215] were refined and further developed, one

based on pure diffusion control and another based on joint diffusion, ohmic and charge

transfer control developed by Milchev [A. Milchev, J. Electroanal. Chem., 312 (1991)

267-275 & A. Milchev, Electrochim. Acta, 37 (12) (1992) 2229-2232]. Experimental

results validated the above models and a strong correlation between the microhardness

and the particle size of the deposited layer was established.

iv

ACKNOWLEDGEMENTS

I would like to take this opportunity to express my most heartfelt gratitude and deepest

appreciation to my supervisor, Professor Frank Foulkes, first for the immeasurable

amount of support, encouragement, and guidance he has provided throughout this

research and second, and equally important, for all the invaluable life lessons and

sometimes short but often long discussions on anything and everything. Professor

Foulkes’ insights and, more importantly, patience throughout this research have been

enlivening. I would also like to extend my sincerest thanks to my committee members

Professor Charles Mims and Professor Donald Kirk for their continued support and

guidance and, at times, a much-needed push to put me back on the right track.

Last, but not least, I am greatly indebted to my wife, Shahin, for her unending love,

unlimited patience and support, and for giving me the opportunity to follow my dreams.

Her unselfish character, kind nature, and constant encouragement have made a world of

difference and words cannot express my deepest feelings and appreciations. This is hers

as much as it is mine.

v

TABLE OF CONTENTS PAGE NO.

ABSTRACT……………………………………………………………….... ii

ACKNOWLEDGMENTS………………………………………………... ….. iv

TABLE OF CONTENTS………………………………………………… ….. v

LIST OF FIGURES………………………………………………………. ….. x

LIST OF TABLES………………………………………………………... ….. xviii

NOMENCLATURE………………………………………………………. ….. xx

1 INTRODUCTION

1.1 Rationale…………………………………………………… ….. 1

1.2 Catalyst Layer……………………………………………… ….. 2

1.3 Pulse Electroplating………………………………………... ….. 3

1.4 Thesis Objectives…………………………………………... ….. 5

2 BACKGROUND

2.1 Hydrogen Fuel Cells……………………………………….. ….. 6

2.1.1 A Condensed History………………………………. ….. 6

2.1.2 How Fuel Cells Work…………………………........ ….. 7

2.1.3 Different Types of Fuel Cells……………………… ….. 8

2.2 Main Components of a Fuel Cell………………………… ….. 9

2.2.1 Polymer Electrolyte Membrane……………………. ….. 9

2.2.2 Catalyst Layer……………………………………… ….. 13

2.2.3 Gas Diffusion Layer………………………………. ….. 16

2.2.4 Bipolar Plates……………………………………… ….. 20

3 FUEL CELL THERMODYNAMICS

3.1 Introduction………………………………………………… ….. 38

3.2 Reversible Cell Voltage under Non-Standard Conditions…. ….. 38

3.2.1 Introduction………………………………………… ….. 38

3.2.2 Reversible Cell Voltage as a Function of

Temperature……………………………………… ….. 38


Pressure…………………………………………… ….. 40


Concentration……………………………………… ….. 42

3.3 Fuel Cell Efficiency……………………………………… ….. 43

vi

4 FUEL CELL ELECTROCHEMISTRY

4.1 Introduction………………………………………………… ….. 45

4.2 Electrode Kinetics………………………………………… ….. 45

4.3 The Butler-Volmer Equation……………………………… ….. 47

4.4 Overvoltage and Current Density………………………… ….. 49

4.5 Fuel Cell Losses…………………………………………… ….. 51

4.5.1 Introduction……………………………………..… ….. 51

4.5.2 Activation Overvoltage…………………………… ….. 52

4.5.3 Ohmic Overvoltage………………………………… ….. 54

4.5.4 Concentration Overvoltage ...……………………… ….. 55

4.5.5 Mixed Potential at Electrodes……………………… ….. 56

5 CATALYST LOADING 5.1 Introduction…………………………………………………... 57

5.2 Catalyst Loading Methods………………………………… …... 58

5.2.1 Application of Catalyst to Gas Diffusion Layers ..... …... 59

5.2.1.1 Application of Catalyst by Spreading ………. 60

5.2.1.2 Application of Catalyst by Spraying……….... 61

5.2.1.3 Application of Catalyst by Painting………….. 62

5.2.1.4 Application of Catalyst by Powder

Deposition ……………………………………. 63

5.2.1.5 Application of Catalyst by Ionomer

Impregnation …………………………………. 63

5.2.1.6 Application of Catalyst by Electrodeposition. 65

5.2.2 Application of Catalyst to Membrane………………… 65

5.2.2.1 Application of Catalyst by Painting …………. 66

5.2.2.2 Application of Catalyst by Dry Spraying……. 67

5.2.2.3 Application of Catalyst by Sputtering ……….. 68

5.2.2.4 Application of Catalyst by Impregnation

Reduction …………………………………….. 72

5.2.2.5 Application of Catalyst by Evaporative

Deposition ……………………………………. 73

6 ELECTRODEPOSITION 6.1 Introduction ………………………………………………….. 75

6.2 Electroless Deposition ……………………………………….. 75

6.2.1 Electroless Palladium Deposition ……………………. 79

6.2.2 Electroless Platinum Deposition ……………………… 80

6.3 Pulse and Direct Current Electrodeposition………………….. 81

6.3.1 Introduction …………………………………………… 81

6.3.2 Direct Current Electrodeposition …………………….. 82

6.3.3 Pulse Current Electrodeposition………………………. 86

6.3.3.1 Introduction …………………………………… 86

6.3.3.2 Factors Influencing PC Electrodeposition……. 88

6.3.3.3 Major Types of Pulse Waveforms …………… 92

6.3.4 Nucleation and Growth during Electrocatalyzation….. 94

vii

6.3.4.1 Nucleation Rate ……………………………… 97

6.3.5 Electrodeposition of Metals and Alloys…………… ….. 98

6.3.5.1 Introduction………………………………….. 98

6.3.5.2 Copper and its Alloys………………………….. 98

6.3.5.3 Nickel and its Alloys………………………. ….. 103

6.3.5.4 Platinum and its Alloys……………………. ….. 106

6.3.5.5 Other Metals and Alloys…………………….. 115

7 EXPERIMENTAL PROCEDURES 7.1 Hydrophobic Polymer Coating……………………………..... 118

7.2 Sintering of Treated Carbon Substrates…………………….... 119

7.3 Carbon Ink Preparation: Microporous Layer Application…. …… 119

7.4 Nafion Impregnation………………………………………..... 120

7.5 Catalyst Electrodeposition………………………………….... 121

7.5.1 Platinum Electrodeposition…………………………… 121

7.5.2 Copper Electrodeposition…………………………….. 124

7.5.3 Nickel Electrodeposition……………………………… 124

7.6 MEA Fabrication and Testing……………………………….... 125

7.6.1 MEA Preparation……………………………………..... 125

7.6.2 Electrochemical Measurements……………………….. 125

7.6.2.1 Single Fuel Cell Tests………………………….. 125

7.6.2.2 Life Test and Durability Assessment of MEAs:

Static Testing…………………………………. 127

7.6.2.3 Life Test and Durability Assessment of MEAs:

Dynamic Testing………………………………. 128

7.6.2.4 X-Ray Diffraction…………………………….. 130

7.6.2.5 Scanning Electron Microscopy……………….. 130

7.6.2.6 Transmission Electron Microscopy…………... 131

7.7 Porosity Measurements of Gas Diffusion Layer………….... ….... 131

7.8 Through-Plane Gas Permeability……………………………... 132

8 RESULTS AND DISCUSSION 8.1 Influence of Hydrophobic Polymer (PTFE) Content in GDL 134

8.1.1 Influence of PTFE Loading on Cell Performance…… 135

8.1.2 Influence of PTFE Loading in Microporous Layer on

Cell Performance……………..……………………….. 138

8.1.3 Influence of MPL Loading on Cell Performance……. 141

8.2 Effects of Carbon Powder Characteristics on Cell

Performance…………………………………………………... 144

8.3 Nafion Impregnation…………………………………………….. 155

8.3.1 Impregnation Time……………………………………… 155

8.3.2 Nafion®

Ion-Exchange Capacity………………………… 160

8.4 Effect of Substrate Type on Cell Performance………………….. 161

8.4.1 Influence of Substrate Thickness and Other Physical

Parameters……………………………………………….. 161

8.4.2. Porosity Measurements………………………………….. 170

viii

8.4.3 Through-Plane Gas Permeability………………………. 171

8.5 Catalyst Electrodeposition……………………………………… 181

8.5.1 Copper Electrodeposition………………………………. 181

8.5.2 Elemental Analysis using EDX………………………. 184

8.5.3 Platinum Electrodeposition…………………………... 187

8.5.3.1 Direct Current Electrodeposition…………..... 187

8.5.3.2 Pulse Current Electrodeposition……………... 190

8.5.3.2.1 Influence of Cathodic Peak Current

Density……………………………… 190

8.5.3.2.2 Influence of Duty Cycle…………… 194

8.5.3.2.2.1 Regular Duty Cycles:

10%-100%...................... 194

8.5.3.2.2.2 Low Duty Cycles:

2%-10%......................... 196

8.5.3.2.3 Influence of Pulse Duration……….. 199

8.5.3.3 Influence of Plating Bath Concentration on

MEA Performance………………………….... 202

8.5.3.4 Platinum Distribution in Carbon Substrates

Fabricated by Pulse Electrodeposition………. 206

8.5.4 Effect of Pulse Current Waveform on

Electrodeposited Catalyst Layer Properties……..…… 209

8.5.5 Effect of Plating Solution Flow Rate on MEA

Performance……………………………………………. 213

8.5.6 Anode Platinum Loading………………………………. 215

8.5.7 Lifetime Behaviour of MEAs Prepared by Pulse

Electrodeposition and Conventional Techniques:

Static Testing…………………………………………. 217

8.5.8 Lifetime Behaviour of MEAs Prepared by Pulse

Electrodeposition and Conventional Techniques:

Dynamic Testing……………………………………... 221

9 MATHEMATICAL MODEL

9.1 Introduction………………………………………………….. 227

9.2 Mathematical Model Development…………………………. 227

9.2.1 Concentration and Overvoltage Profiles of

Different Current Waveforms……………………….. 227

9.2.2 Electrochemical Nucleation and Critical Nucleus…... 233

9.2.3 Different Types of Crystal Growth…… …... 238

9.2.3.1 Electrochemical Crystal Growth under Pure

Charge Transfer Control……………………… 239

9.2.3.2 Electrochemical Crystal Growth under Combined

Charge Transfer and Diffusion Control………… 240

9.2.3.3 Electrochemical Crystal Growth under Pure

Diffusion Control……………………………….. 242

9.2.3.4 Electrochemical Crystal Growth under

Ohmic Control……………………………….. 242

ix

9.2.3.5 Electrochemical Crystal Growth under

Combined Charge Transfer and Ohmic Control 243

9.3 Model Validation…………………………………………….. 245

9.3.1. Nickel Electrodeposition…………………………….. 245

9.3.2 Model Predictions for Nickel Concentration

Overvoltage for Various Waveforms………………. ….. 251

9.3.3 Platinum Electrodeposition………………………….. 261

9.3.3.1 Modification of the Mathematical Model for

Platinum Electrodeposition………………….. 262

9.3.3.1.1 Effect of Supporting Electrolyte….. 263

9.3.3.2 Comparison of Experimental and Model

Platinum Microhardness Data……………….. 280

9.3.3.3 Platinum Concentration Variation in the

Cathode Diffusion Layer……………………. 283

9.3.3.4 Concentration Overvoltage and Pulse Current

Waveforms…………………………………… 285

9.3.3.5 Nucleation Rate and Pulse Current

Waveforms…………………………………… 286

9.3.3.6 Duty Cycle and Pulse Current Waveforms… ….. 288

9.3.3.7 Influence of Waveform on Critical Nucleus

Size………………………………………….... 293

9.3.3.8 Influence of Cathodic Peak Deposition

Current Density on Nucleation Rate of

Platinum for Various Waveforms………….... 295

9.3.3.9 Comparison of Commercial and In-House

MEAs…………………………………………. 302

10 CONCLUSIONS……….................................................................. …... 305

11 RECOMMENDATIONS AND FUTURE WORK……………….. 309

12 REFERENCES………………………………………………………. 310

13 APPENDICES……………………………………………………….. 334

Appendix A: Physical data for platinum group metals……… 334

Appendix B: The electrical double layer…………………….. 335

Appendix C: Pump calibration data………………………….. 341

Appendix D: Derivation of equations……………………….. 344

Appendix E: Microhardness test.…………………………….. 382

x

LIST OF FIGURES

Figure 1-1 Schematic diagram of a square pulse current waveform

Figure 2-1 A simple proton exchange membrane fuel cell

Figure 2-2 Nafion® perfluorinated ionomer

Figure 2-3 (a) Toray carbon paper and (b) Toray carbon cloth

Figure 2-4 Classification of materials for bipolar plates in PEMFCs

Figure 2-5 Different flow field configurations: (a) parallel, (b) serpentine, (c) parallel-

serpentine, (d) interdigitated, and (e) pin or grid type

Figure 3-1 Reversible cell potential as a function of pressure for PEMFCs at 25 °C

Figure 4-1 A typical performance curve for a hydrogen fuel cell operated at STP

Figure 5-1 A simple schematic of a five-layer membrane-electrode assembly

Figure 5-2 Catalyst loading methods

Figure 6-1 A simplified equivalent circuit for single-electrode reaction

Figure 6-2 Actual and Nernst diffusion layers during non-steady-state electrolysis

Figure 6-3 Free energy of formation of a cluster as a function of size N

Figure 7-1 Drying of substrates in an oven

Figure 7-2 Electrodeposition flow cell

Figure 7-3 Different types of waveform

Figure 7-4 A simple representation of MEA fabrication process

Figure 7-5 A simple schematic of the experimental setup for MEA characterization

Figure 7-6 Schematic representation of a 200-W PEMFC system used in a tricycle

Figure 7-7 Fuel cell bicycle (direct drive)

Figure 7-8 Laboratory apparatus for through-plane permeability measurement of GDLs

Figure 8-1 Surface Morphology of Carbon Paper Substrates before (a) and after (b)

PTFE application (60 wt%)

Figure8-2(a) Impact of PTFE loading on through-plane resistivity of PTFE-treated

Toray TGP-H-090 carbon papers at various pressures (sintering

temperature: 360 °C; surface area: 10 cm2)

Figure 8-2(b) Impact of PTFE loading on through-plane resistivity of PTFE-treated

Toray TGP-H-090 carbon papers at various pressures (sintering

temperature: 360 °C; surface area: 10 cm2)

Figure 8-3 Influence of sintering temperature on through-plane resistivity of PTFE-

treated carbon papers subjected to varying pressures (PTFE loading: 110%;

surface area: 10 cm2)

xi

Figure 8-4 Influence of PTFE content on fuel cell performance operated at a cell

temperature of 50 °C in H2/Air with a platinum loading of 0.3 mg cm-2

per

electrode

Figure 8-5 Fuel cell potential with varying PTFE content at different current densities

operated at a cell temperature of 50 °C in H2/Air with a platinum loading of

0.3 mg cm-2

per electrode

Figure 8-6 Influence of diffusion layer loading on cell performance operated at a

cell temperature of 50 °C in H2/Air with a platinum loading of 0.3 mg

cm-2

per electrode

Figure 8-7 Surface morphology of diffusion layers containing Vulcan XC-72 and (a)

no PTFE; (b) 10 wt% PTFE; (c) 30 wt% PTFE; (d) 50 wt% PTFE

Figure 8-8 Pore volume distribution of several GDLs prepared using five different

types of carbon and graphite

Figure 8-9 Influence of carbon type in the MPL on cell performance of a H2/Air fuel

cell operated at a cell temperature of 50 °C with a platinum loading of 0.3

mg cm-2

per electrode

Figure 8-10 Cell performance as a function of MPL macropore volume at four different

current densities (hydrogen-air fuel cell with a cell temperature of 50 °C)

Figure 8-11 SEM images of microporous layers of a number of MEAs prepared by (a)

SAB, (b) Vulcan Xc-72, (c) Asbury 850

Figure 8-12 The impact of impregnation time on Nafion® loading

Figure 8-13 Effect of number of Nafion® applications on total Nafion

® loading

Figure 8-14(a) Untreated carbon electrode, ×130; (b) one Nafion® application, ×130 ;

(c) five Nafion® applications, ×153; (d) 10 Nafion

® applications, ×130;

all micrographs show the top surface of the GDE; floating method used

to load Nafion®

Figure 8-15(a) Micrograph of the surface of a carbon electrode with 14 Nafion®

applications, ×130; (b) cross section, ×130

Figure 8-16 Polarization curves for different substrates in H2/O2 with a platinum

loading of 0.3 mg cm-2

per electrode and a cell temperature of 50 °C

Figure 8-17 Polarization curves for different Substrates in H2/Air with a platinum


per electrode and a cell temperature of 50 °C

Figure 8-18(a) Cell voltage as a function of original GDL thickness with a platinum


for a H2/Air fuel cell at a cell temperatutre of

50 °C

Figure 8-18(b) A simple representation of compressed and uncompressed GDL

Figure 8-19 Illustration of various random fiber orientation distribution in 1, 2 and 3

dimensions

xii

Figure 8-20 Comparison of experimental and theoretical variations in through-plane

permeability as a function of medium porosity for carbon papers under

investigation

Figure 8-21 Influence of PTFE on the porosity of Toray TGPH 090 carbon paper

Figure 8-22 Effect of PTFE content on porosity and average pore diameter of Toray

TGPH 090 carbon paper

Figure 8-23 Differential pore volume for Toray TGPH 090 carbon paper substrates with

different PTFE loadings

Figure 8-24 Cumulative pore volume of Toray TGPH 090 carbon paper substrates with

different amounts of hydrophobic polymer

Figure 8-25 Percent of substrate pore volume with pore diameters of at least 7 m as a

function of permeability coefficient

Figure 8-26 Influence of pulse period on current efficiency of copper electrodeposition

Figure 8-27 Influence of duty cycle on current efficiency of copper electrodeposition

Figure 8-28 Influence of the number of applications on carbon electrode coverage

Figure 8-29 EDX spectrum of a carbon electrode cross section impregnated with 14

coatings and electroplated with copper

Figure 8-30 An EDX spectrum analysis of a carbon electrode cross section

impregnated with Nafion® and electroplated with copper

Figure 8-31 Effect of electrodeposition current density on cell performance in DC

electrodeposition (H2/Air; 20 wt% PTFE; 0.30 mg Pt/cm2; cell

temperature = 50 °C)


electrodeposition (H2/Air; 20 wt% PTFE; 0.30 mg Pt/cm2; cell


Figure 8-33 SEM cross-sectional micrographs of electrodes prepared by pulse

electrodeposition at peak current densities of a) 50 mA cm-2

and b)70 mA

cm-2

Figure 8-34 Effect of electrodeposition peak current density in square pulse

electrodeposition on fuel cell performance (H2/Air; 20 wt% PTFE;

0.30 mg Pt/cm2; cell temperature = 50 °C; TGPH-090 carbon paper)

Figure 8-35 Effect of electrodeposition peak current density on fuel cell performance in

square pulse electrodeposition (H2/Air; 20 wt% PTFE; pulse period = 750

ms; 0.30 mg Pt/cm2; cell temperature = 50 °C; TGPH-090 carbon paper)

Figure 8-36 Effect of PC and DC electrodeposition on fuel cell performance (H2/Air;

20 wt% PTFE; 0.30 mg Pt/cm2; cell temperature = 50 °C)

Figure 8-37 Effect of PC duty cycle on fuel cell performance (H2/Air; 20 wt% PTFE;

0.30 mg Pt/cm2; cell temperature = 50 °C; TGPH-090 carbon paper)

xiii

Figure 8-38 The relationship between duty cycle and cell voltage for different fuel cell

output current densities in fuel cells utilizing PC-electrodeposited catalysts

(H2/Air; 20 wt% PTFE; 0.30 mg Pt/cm2; cell temperature = 50 °C; TGPH-

090 carbon paper)

Figure 8-39 Effect of electrodeposition peak current density with low duty cycles (ф) (4% and 20%) in square pulse electrodeposition on fuel cell performance

(H2/Air; 20 wt% 20 wt% PTFE; 0.30 mg Pt/cm2; cell temperature = 50 °C)

Figure 8-40 Effect of square pulse electrodeposition peak current density with 4% duty

cycle (ф) on fuel cell voltage for fuel cell output current densities of 200-

1000 mA cm-2

(H2/Air; 20 wt% PTFE; 0.30 mg Pt/cm2; cell temperature =

50 °C)

Figure 8-41 Effects of duty cycle and pulse current density on fuel cell performance

(square pulse; H2/Air; 20 wt% PTFE; 0.30 mg Pt/cm2 per electrode; cell


Figure 8-42 Effects of pulse frequency (on-time/off-time) on cell performance (square

pulse; H2/Air; 20 wt% PTFE; 25% duty cycle; 0.30 mg Pt/cm2; cell


Figure 8-43 The relationship between pulse frequency and cell voltage for fuel cell

output current densities of 200-1000 mA cm-2

using PC electrodeposition

(square pulse; H2/Air; PTFE = 20 wt%; 20% duty cycle; 0.30 mg Pt/cm2;

cell temperature = 50 °C)

Figure 8-44 Effects of plating bath Pt(NH3)4Cl2 concentration on MEA performance

(square pulse; H2/Air; 20 wt% PTFE; 20% duty cycle; 0.35 mg Pt/cm2 per

electrode; cell temperature = 50 °C)

Figure 8-45 Cross-sectional platinum line scans for MEAs prepared from plating baths

with different platinum concentrations: (a) 1.0 mM; (b) 50 mM; (c) 100

mM; (d) 500 mM and (e) 1000 mM

Figure 8-46 Carbon substrate impregnated with platinum

Figure 8-47 Platinum distribution on a carbon substrate as a function of distance from

the centre of the substrate

Figure 8-48 An electron micrograph of a composite fuel cell MEA

Figure 8-49 Cell performance as a function of electrodeposition waveform (H2/O2; 20

wt% PTFE; 4% duty cycle; 0.35 mg Pt/cm2; cell temperature = 50 °C)

Figure 8-50 TEM images of platinum catalyst electrodeposited employing different

pulse waveforms: (a) ramp-down (b) triangular and (c) rectangular (peak

deposition current density = 400 mA cm-2

; 4% duty cycle; 0.35 mg Pt

cm-2

)

xiv

Figure 8-51 Size distribution of platinum nanoparticles according to the type of

waveform: (a) ramp-down (b) triangular and (c) rectangular waveforms

(peak deposition current density = 400 mA cm-2

; 4% duty cycle; 0.35 mg

Pt cm-2

per electrode)

Figure 8-52 Influence of plating solution flow rate on MEA performance (square pulse;

H2/Air; 20 wt% PTFE; 20% duty cycle; 0.35 mg Pt/cm2 per electrode;


Figure 8-53 Influence of plating bath flow rate on fuel cell voltage at four different

current densities (H2/Air; 20 wt% PTFE; 20% duty cycle; 0.35 mg

Pt/cm2 per electrode; cell temperature = 50 °C)

Figure 8-54 Influence of anode platinum loading on fuel cell performance (square

pulse; H2/Air; 20 wt% PTFE; cathode = 0.35 mg Pt cm-2

; 20% duty cycle;


Figure 8-55 Influence of anode Pt loading on electrode performance at four different

current densities (square pulse; H2/Air; 20 wt% PTFE; cathode = 0.35 mg

Pt cm-2

; 20% duty cycle; cell temperature = 50 °C)

Figure 8-56 Durability of single commercial and in-house MEAs with apparent areas

of 5 cm2 and platinum loadings of 0.5 mg cm

-2 on both anode and cathode

operated at a cell temperature of 60 °C and ambient pressure. Hydrogen

and air are used as fuel and oxidant, respectively, entering the cell at 100%

RH. The operation time is 4100 h.

Figure 8-57 Durability of single commercial and in-house MEAs with apparent areas

of 5 cm2 and platinum loadings of 0.5 mg cm

-2 on both anode and cathode

operated at a cell temperature of 60 °C and ambient pressure. Hydrogen

and air are used as fuel and oxidant, respectively, entering the cell at 100%

RH. The operation time is 280 h.

Figure 8-58 SEM images of the cross section of the in-house MEA showing

delamination at different magnifications (top) 125 (bottom) 200

Figure 8-59 Durability of single in-house MEA with apparent area of 5 cm2 and a

platinum loading of 0.5 mg cm-2

on both anode and cathode operated at a

cell temperature of 60 °C and ambient pressure. Hydrogen and air are used

as fuel and oxidant, respectively, entering the cell at 100% RH. The

operation time is 3000 h.

Figure 8-60 Open circuit voltage (OCV) data obtained at different time intervals for an

in-house and a commercial MEA

Figure 8-61 Power output of 200-W PEM fuel cell stacks containing in-house and

commercial MEAs running on hydrogen and air at ambient temperature

and pressure

xv

Figure 8-62 Initial OCV of in-house and commercial (E-TEK) MEAs in PEM fuel cell

stack in a dynamic system running on H2/Air at ambient temperature and

pressure (0.35 mg Pt/cm2 for anode and cathode of both MEAs)

Figure 8-63 Final OCV of in-house and commercial (E-TEK) MEAs in PEM fuel cell

stack in a dynamic system running on H2/Air at ambient temperature and

pressure (0.35 mg Pt/cm2 for anode and cathode of both MEAs)

Figure 8-64 Maximum power output of 42-cell fuel cell stacks containing in-house and

commercial (E-TEK) MEAs tested in a dynamic system running on H2/Air

at ambient temperature and pressure (0.35 mg Pt/cm2 for anode and

cathode of both MEAs)

Figure 9-1 A liquid droplet formed on (a) a flat solid surface and (b) its cross section

Figure 9-2 Comparison between the experimental and various models for Ni coating

hardness using rectangular waveform


hardness using ramp-up waveform


hardness using ramp-down waveform


hardness using triangular waveform

Figure 9-6 Concentration overvoltage for various waveforms with a peak current

density of 400 mA cm-2

, on-time & off-time of 5 ms (50% duty cycle) and

100 Hz (showing the first full cycle)

Figure 9-7 Nickel concentration in the cathode diffusion layer for various waveforms

with a peak current density of 400 mA cm-2

, on-time and off-time of 5 ms

each (50% duty cycle) and 100 Hz (showing the first full cycle , on-time

and off-time)



, on time of 0.1 ms, 1000 pulse

cycles and duty cycles of 10% - 100%. The initial bulk nickel

concentration is 1.427 mol L-1

.

Figure 9-9 Nucleation rate for a single cycle (on-time only is shown) for all

waveforms with a peak deposition current density of 400 mA cm-2

and

50% duty cycle

Figure 9-10 Calculated critical nucleus size for a single cycle (on-time only is shown)

for all waveforms with a peak deposition current density of 400 mA cm-2

and 50% duty cycle

Figure 9-11 XRD patterns exhibiting the influence of pulse duty cycle on crystal

orientation of nickel deposits with a constant peak current density of 400

mA cm-2

xvi

Figure 9-12 Surface morphology of electrodeposited nickel at a constant deposition

current density of 400 mA cm-2

and various duty cycles: (a) 20% and (b)

80%

Figure 9-13 Comparison of experimental and model microhardness data for various

pulse waveforms: a) Ramp-down, b) Triangular, c) Ramp-up and d)

Rectangular, deposited at different peak current densities (4% duty cycle;

50 mM Pt concentration)

Figure 9-14 Influence of different waveforms on platinum microhardness deposited at

various peak deposition current densities

Figure 9-15 Platinum concentration in the cathode diffusion layer for various


, on-

time and off-time of 5.0 ms each (50% duty cycle) and 100 Hz (showing

the first full cycle)

Figure 9-16 Concentration overvoltage for various waveforms with a peak deposition


, on-time & off-time of 5 ms (50% duty

cycle) and 100 Hz (showing the first fuel cycle)

Figure 9-17 Nucleation rate for various waveforms with a peak deposition current


, on-time of 5 ms and 100 Hz (showing the first

half-cycle)

Figure 9-18 Platinum ion concentration in the cathode diffusion layer as a function of

duty cycle in the Millisecond Range for all four waveforms with a peak

deposition current density of 400 mA cm-2

, on-time of 0.002 s and 100

pulse cycles


duty cycle in the Microsecond Range for all four waveforms with a peak



pulse cycles




, on-time of 0.02 s and 50 pulse

cycles


duty cycle in the Microsecond Range for all four waveforms with a peak



pulse cycles

Figure 9-22 Critical nucleus size of platinum as a function of time for various


, on-

time of 5 ms and 100 Hz (showing the first half-cycle)

Figure 9-23 Nucleation rate for various waveforms with an on-time of 1.0 ms, off-time

of 49 ms, 2% duty cycle, and 100 pulse cycles at different peak current

densities

xvii

Figure 9-24 Nucleation rate for various waveforms with an on-time of 100 ms, off-

time of 400 ms, 20% duty cycle, and 100 pulse cycles at different peak

current densities

Figure 9-25 Concentration overvoltage as a function of peak deposition current density

for all waveforms with a pulse on-time of 1.0 ms, pulse off-time of 49 ms,

2% duty cycle, and 100 pulse cycles

Figure 9-26 Platinum concentration as a function of peak deposition current density for

all waveforms with a pulse on-time of 1.0 ms, pulse off-time of 49.0 ms,



all waveforms with a pulse on-time of 1.0 ms, pulse off-time of 4.0 ms,


Figure 9-28 Nucleation as a function of peak deposition current density for all

waveforms with a duty cycle of 2%, pulse on-time of 2 ms, pulse off-time

of 98 ms and 1000 pulse cycles: (a) 50 mA cm-2

(b) 100 mA cm-2

, (c) 200

mA cm-2

, (d) 300 mA cm-2

, (e) 400 mA cm-2

, (f) 500 mA cm-2

, and (g) 600

mA cm-2

Figure 9-29 Influence of peak deposition current density on microhardness and grain

diamter of a Pt electrodeposited layer (ramp-down waveform; 2% duty

cycle; microsecond pulses)

Figure 9-30 Influence of average grain diameter on microhardness of an

electrodeposited layer (ramp-down waveform; 2% duty cycle;

microsecond pulses)

Figure 9-31 Influence of catalyst deposition method and loading on fuel cell

performance (ramp-down waveform; H2/Air; 20 wt% PTFE; operating

pressure = 1.0 bar; cell temperature = 40 °C)

Figure B-1 A parallel-plate capacitor

Figure B-2 Helmholtz compact double-layer model

Figure B-3 Gouy-Chapman model of electrical double layer

Figure B-4 Stern model of electrical double layer

Figure B-5 Grahame’s triple-layer model

Figure C-1 Electroplating bath flow rate as a function of pump speed and temperature

Figure E-1 Vickers indenter

xviii

LIST OF TABLES

Table 2-1 Potential coating materials for metallic bipolar plates

Table 2-2 Chemical composition of several stainless steels

Table 2-3 Primary coating materials and methods for 316 and 316L SS bipolar plates

Table 2-4 Chemical and physical properties of graphite and 316L SS

Table 3-1 Influence of temperature on reversible cell voltage at 1.0 bar pressure

Table 3-2 Influence of pressure on reversible cell voltage at a fixed temperature of

25 °C

Table 5-1 Effective surface area of several commercial carbon black powders

Table 6-1 Main characteristics of electrochemical and chemical deposition methods

Table 6-2 Chemical properties of several reducing agents

Table 7-1 Bath composition and electroplating conditions for Ni plating

Table 8-1 Manufacturers’ data: characteristics of different carbon powders in GDLs

Table 8-2 Diffusion coefficient of oxygen in water at different temperatures

Table 8-3 Diffusion coefficient of oxygen (as a binary mixture) in air at atmospheric

pressure

Table 8-4 Influence of carbon / graphite loading in MPLs on fuel cell performance

Table 8-5 Effects of impregnation time on Nafion® loading utilizing the floating

Method

Table 8-6 Nafion® impregnation of GDEs utilizing floating and brushing techniques

Table 8-7 Nafion® impregnation with 1 to 14 applications

Table 8-8 Ion exchange capacity data for different samples

Table 8-9 Physical properties of a number of different gas diffusion layers

Table 8-10 Porosity data for untreated GDLs

Table 8-11 Through-plane permeability values of untreated substrates

Table 8-12 Archie’s law parameters used to calculate absolute permeability using the

model of Tomakakis et al [634]

Table 8-13 Comparison of experimental and theoretical absolute permeability values of

different carbon substrates

Table 8-14 Influence of pulse period on current efficiency of copper electrodeposition

Table 8-15 Influence of duty cycle on current efficiency of copper electrodeposition

Table 8-16 EDX analysis of carbon substrates

xix

Table 8-17 Effect of plating bath flow rate on MEA performance

Table 9-1 Experimental and model hardness data for nickel (rectangular waveform)

Table 9-2 Experimental and model hardness data for nickel (ramp-up waveform)

Table 9-3 Experimental and model hardness data for nickel (ramp-down waveform)

Table 9-4 Experimental and model hardness data for nickel (triangular waveform)

Table 9-5 Physical constants and variables used for determining nickel-coating

microhardness and other characteristics

Table 9-6 Physical constants and variables used for determining the platinum-coating


Table 9-7 Nucleation rate for all waveforms for the first pulse cycle with a pulse on-

time of 5.0 ms and a peak deposition current density of 400 mA cm-2

Table 9-8 Platinum ion concentration in the cathode diffusion layer as a function of



Table 9-9 Platinum ion concentration in the cathode diffusion layer as a function of duty

cycle in the Microsecond Range for all four waveforms with a peak


Table 9-10 Performance comparison of conventional and PC electrodeposited MEAs

Table A-1 Physical data for Platinum Group Metals

Table C-1 Pump calibration, Tygon L/S 16 Tubing, 25 °C




xx

NOMENCLATURE

Symbol Quantity [units]

aH2 activity of hydrogen [unitless]

aO2 activity of oxygen [unitless]

aH2O activity of water [unitless]

A area [m2]

A active area of the electrode [cm-2

] A nucleation rate constant atm standard atmospheric pressure [=101325 Pa]

B constant of proportionality

c0 initial bulk concentration [mol L-1

]

cs surface concentration [mol L-1

]

C Celsius [C]

Cb bulk concentration [mol L-1

]

Cdl capacitance of the double layer [F·cm-2

]

CGC Gouy-Chapman capacitance [F·cm-2

]

CH Helmholtz capacitance [F·cm-2]

Cj concentration of species j [mol m-3

]

Ddl thickness of the double layer [cm]

Dj diffusion coefficient or diffusivity of species j [m2 s

-1]

partial differential operator

Ecell cell voltage [V]

Erev reversible cell voltage [V]

Eeqm equilibrium cell voltage [V]

E change in total energy of a system [J]

ƒ frequency [s-1

]

F Faraday’s constant [C mol-1

]

(FD)per mole driving force for diffusion [N mol-1

]

F’D driving force acting on one liter of solution [mol L-1

]

)( 0F wetting angle function [unitless]

G reactant consumption rate [mol s-1

]

G Gibbs free energy [J]

G standard Gibbs free energy [J]

G activation Gibbs function [J]

GC activation Gibbs function, chemical component [J]

G(N) Gibbs free energy of formation of a cluster [J]

GT,P change in Gibbs free energy at constant temperature and pressure [J]

h Planck’s constant [J s]

H enthalpy [J]

H microhardness [kg mm-2

]

H hydrogen atom

H+ hydrogen ion (proton)

H2 hydrogen gas

xxi

H(x) Heaviside function

H change in enthalpy [J mol-1

]

ia average current density [mA cm-2

]

iC capacitive current [mA cm-2

]

iF Faradic current [mA cm-2

]

iP peak current density [mA cm-2

]

itot or iT total galvanostatic current density [mA cm-2

]

iN nucleation current [mA cm-2

]

iG growth current [mA cm-2

]

i exchange current density [mA cm-2

]

i,a anode exchange current density [mA cm-2

]

i,c cathode exchange current density [mA cm-2

]

I current [A]

I1 single cluster growth current [mA cm-2

]

J(u) nucleation rate at time t = u [nuclei s-1

]

j flux of reactants reaching the surface of the electrode [mol s-1

cm-2

]

jb backward reactant concentration flux [mol s-1

cm-2

]

jf forward reactant concentration flux [mol s-1

cm-2

]

kb backward rate constant [L s-1

]

kB Boltzmann’s constant [J K-1

]

kf forward rate constant [L s-1

]

kCK Carman-Kozeny constant

K1 penetrability of the moving dislocation boundary [MPa m1/2

]

K0 value determined by dislocation boundary [MPa]

L length [m] (also a unit for volume, litre)

m mass of the system [kg]

m mass flux of a fluid [kg s-1

m-2

]

M metal

MA metal salt

n number of moles of gas [mol]

n number of moles of electrons transferred per mole of reaction [unitless]

ni number of moles of species i [mol]

nj molar flow rate of ion or species j through the electrolyte [mol s-1

]

jn molar flux of the ion through the electrolyte [mol s-1

cm-2

]

N number of pulse cycles

NC or nc critical radius of the cluster [m]

N maximum possible number of sites of nuclei on the substrate surface per

unit area

O2 oxygen gas

Ox oxidized species in a half-cell electrochemical reaction

P perimeter [m]

P pressure [Pa]

Pi partial pressure of species i [Pa]

P standard pressure [1 bar]

PH2 partial pressure of hydrogen gas [Pa]

xxii

PO2 partial pressure of oxygen gas [Pa]

q heat per mole [J mol-1

]

qGC Gouy-Chapman excess charge [C]

qH Helmholtz excess charge [C]

qS total charge on the solution side [C]

Q heat [J]

Qrev reversible heat [J]

r area-specific resistance [k cm2]

r radial distance from the centre of the cluster [m]

rC critical radius of the surface nucleus [m]

R resistance []

R radius of a cluster [m]

R’ universal gas constant [J mol-1

K-1

]

R average radius of the formed grains [m]

Rg radius of a growing nucleus [nm]

Red reduced species in a half-cell electrochemical reaction

s area occupied by one atom on the surface of the nucleus [m2]

s molar entropy [J K-1

mol-1

]

S entropy [J K-1

]

Sh Heaviside function

Sk Heaviside function

Sw Heaviside function

S surface area of a nucleus [m2]

S change in entropy [J K-1

]

S standard change in entropy [J K-1

]

t time [s]

tC charge time [µs]

tD discharge time [µs]

toff time the pulse is off [ms]

ton time the pulse is on [ms]

T temperature [K]

TH high temperature for a heat engine [K]

TL low temperature for a heat engine [K]

v velocity of the system [m s-1

]

v superficial velocity [m s-1

]

vM molar volume [m3 mol

-1]

vP specific molar volume of gas products [m3 mol

-1]

vR specific molar volume of gas reactants [m3 mol

-1]

V volume [m3]

V1 initial volume [m3]

V2 final volume [m3]

Vp pore volume [m3]

Vb bulk volume [m3]

Vs solid volume [m3]

V change in volume of a system [m3]

W work [J]

xxiii

W small change in work [J]

W1 mass of the membrane (SPE) before drying [g]

W2 mass of the membrane (SPE) after drying [g]

Wnet mechanical work delivered by a heat engine [J]

Greek Letters

α transfer coefficient [unitless]

σ specific free surface energy [J m-2

]

Forchheimer or inertial coefficient [m-1

]

ratio of the bulk concentration of supporting electrolyte to the bulk

concentration of the depositing electrolyte

dielectric constant [unitless]

edge energy [J cm-1

]

porosity

el specific conductivity of the solution [-1

cm-1

]

double-layer thickness [cm]

infinitesimal incremental amount of a path function

distance between two plates in a capacitor [m]

(x) Dirac delta distribution

change in a property

z change in elevation [m]

duty cycle [unitless]

membrane water content [g]

µ chemical potential [J mol-1

]

µ* chemical potential in the reference state [J mol-1

]

kinematic viscosity [Pa s]

toruosity

pulse period [ms]

number of states available to a molecule

overvoltage [V]

ohmic overvoltage [V]

act activation overvoltage [V]

C concentration overvoltage [V]

s overvoltage at the surface of the cluster [V]

energy conversion efficiency of a fuel cell [unitless]

j effective diffusivity [cm2 s

-1]

(N) surface potential [V]

electrical potential difference [V]

(0) potential at the electrode surface [V]

(x) local potential at a distance x from the electrode surface [V]

xxiv

Subscripts

atm of the atmosphere

eqm equilibrium value

ext ―external‖ value of a property

f final state of a system or process

f process of formation

g gas

i initial state of a system or process

irrev an irreversible process

L liquid

max maximum value

min minimum value

P at constant pressure

rev reversible process

s solid

soln solution phase

STP standard temperature and pressure (0 C and 1.0 atm pressure)

surr the surroundings

T at constant temperature

univ universe (defined as system plus surroundings)

v vapour

V at constant volume

x in the x-direction

y in the y-direction

z in the z-direction

Superscripts

dot (e.g., m ) denotes a rate (e.g., m is the rate of mass flowing through the system)

macron (e.g., g ) denotes the specific value of a property (e.g., g is the specific Gibbs

free energy in J kg-1

)

degrees Celsius

denotes standard conditions

Physical Constants

Symbol Quantity Value units

eo elementary charge 1.6022 10-19

C

F Faraday’s constant 96 487 C mol-1

g acceleration due to gravity 9.806 m s-2

kB Boltzmann’s constant 1.381 10-23

J K-1

NA Avogadro’s number 6.022 1023

mol-1

R’ Universal gas constant 8.3145 J mol-1

K-1

xxv

Glossary of Abbreviations

AES Auger Electron Spectroscopy

ASR Area-Specific Resistance

AST Accelerated Stress Test

BMC Bulk-Molding Compound

CE Current Efficiency

CL Catalyst Layer

CVD Chemical Vapour Deposition

DC Direct Current

DMFC Direct Methanol Fuel Cell

EDS Energy Dispersive Spectroscopy

FEP Fluorinated Ethylene Propylene

GDE Gas Diffusion Electrode

GDL Gas Diffusion Layer

HOPG Highly Ordered Pyrolytic Graphite

IC Integrated Circuit

ICP-OES Inductively Coupled Plasma-Optical Emission Spectroscopy

IEC Ion Exchange Capacity

IEM Ion Exchange Membrane

MEA Membrane-Electrode Assembly

OCV Open Circuit Voltage

ORR Oxygen Reduction Reaction

PANI Polyaniline

PC Pulse Current

PCE Pulse Current Electrodeposition

PCB Printed Circuit Board

PEM Proton Exchange Membrane or Polymer Electrolyte Membrane

PEMFC Proton Exchange Membrane Fuel Cell

PFSA Perfluorosulfonic Acid

PGMs Platinum-Group Metals

PPS Polyphenylene Sulfide

PRC Pulse Reverse Current

PSSA Polystyrene Divinylbenzene Sulfonic Acid

PTFE Polytetrafluoroethylene

PVD Physical Vapour Deposition

RDE Rotating Disk Electrode

RDS Rate-Determining Step

SEM Scanning Electron Microscopy

SPE Solid Polymer Electrolyte

TEM Transmission Electron Microscopy

US DOE United States Department of Energy

XPS X-ray Photoelectron Spectroscopy

XRD X-Ray Diffraction

1

1.0 INTRODUCTION

1.1 Rationale

Today, the burning of fossil fuels is closely associated with climate change, acid rain and

global warming. Furthermore, the prospect of finding significant new oil and gas fields is

slim. Even if significant new reserves of fossil fuels are discovered, will we be willing to

live with the consequences of ―burning‖ them? Anthropogenic carbon dioxide poses a

serious threat to our environment, economy and, more importantly, to our social well-

being. Energy issues such as resource depletion, energy security, end-use efficiency, and

climate change require deep thinking and timely solutions. Fuel cells can play a vital role

in alleviating some of the aforementioned problems and challenges facing humanity [1].

Proton exchange membrane fuel cells (PEMFCs) are electrochemical devices that convert

the chemical energy of the reactants (a fuel and an oxidant) directly to electrical energy in

the form of low voltage direct current (DC) and heat, without combustion. They have

been receiving significant attention due to their high power density, energy efficiency and

environmentally friendly characteristics [2-11]. However, one of the major impediments

in making them a viable power source is the high manufacturing cost due to the low

activity of the catalyst and hence, high platinum content [12-29] and the prohibitively

high cost of solid polymer electrolytes such as Nafion® [30-36 ]. In order to increase

platinum utilization, various methods have been developed to increase the effective

surface area of platinum through a particle size reduction and/or increase of the interface

between the catalyst, electrolyte and reactants [9, 25, 37-42]. A significant number of

patents also have been filed and granted over the past few decades underlying the

importance of such methods and processes in the PEMFC field [43-104].

Platinum deposition methods generally can be categorized in two different groups:

powder type and non-powder type. In a powder type process a platinum salt is first

chemically reduced by a reducing agent that leaves the platinum in a colloidal form.

Platinum particles in solution then are adsorbed on high surface area carbon to make

carbon-supported platinum. This is a simple and cost-effective process in which the final

amount of platinum on carbon can easily be controlled through the initial concentration

of the platinum salt. However, it is very difficult to increase the platinum-carbon ratio in

2

carbon-supported platinum beyond 40% without increasing platinum particle size [17].

The oxidation of hydrogen at the anode and, more importantly, the reduction of oxygen at

the cathode are directly dependent on the effective surface area of the electrocatalyst in

the catalyst layer and, therefore on the platinum particle size. As a result, maintaining

small catalyst particle size is paramount in fuel cell design. In addition, since increasing

the platinum ratio in the carbon-supported platinum is limited, this would hinder the

efforts to decrease the catalyst layer thickness with the result that a better mass transport

of reactants and products cannot be achieved [105].

On the other hand, in non-powder type processes the platinum particles are directly

deposited onto the surface of the carbon electrode or membrane. Sputter deposition is a

well-known process and several researchers have shown that it can be utilized to prepare

MEAs with high platinum utilization [105, 106]. However, this method requires high

initial investment due to the prohibitively high cost of vacuum equipment. Maintenance

costs are also high and extra care must be exercised to minimize contamination [106].

Another method is based on a two-step process in which a membrane such as Nafion®

initially undergoes an ion exchange reaction with metal salt of the catalyst to be deposited

and then reduced to the metal catalyst by a reducing agent. Although this process ensures

a good contact between the membrane and the metal catalyst, the electronic pathway

through the carbon particles is missing. A new approach based on pulse electrodeposition

to fabricate MEAs has exhibited promising results in laboratories [9, 107, 108].

1.2 Catalyst Layer

The fuel cell half reactions taking place on the surface of uncatalyzed electrodes are too

slow for practical applications, the oxygen reaction being especially problematic.

Accordingly, a catalyst layer is provided to increase the reaction rates. The electrodes are

made porous to facilitate the diffusion of the gases—both fuel and oxidant. Each

electrode has a carbon backbone in which are bonded small particles of platinum, about 2

nanometers in diameter [108]. This greatly increases the available surface area for the

half reactions to occur, thereby increasing their reaction rates. In addition, both platinum

3

and carbon are good electronic conductors, which facilitate the movement of electrons

through the electrodes to the external circuit.

Platinum, on account of its unique properties, has become an integral part of the proton

exchange membrane (PEM) fuel cell. The original membrane-electrode assemblies

(MEAs) were made in the 1960s for the Gemini space program and utilized about 4

milligrams of platinum per square centimeter of membrane area [108]. Although the

amount of platinum used in today’s PEM fuel cells has decreased to about 0.1-1.0

mg/cm2, this is still too high to support significant market penetration. It is essential,

therefore, to further lower catalyst loading without significant loss in catalytic activity.

Recent efforts to increase the platinum utilization without loss in performance have

resulted in the development of new methods for fabricating gas diffusion electrodes

(GDEs) with low or modest loadings of expensive platinum that provide unusually highly

efficient utilization of platinum metal. One such method was developed by Reddy et al.

[107] and was patented in 1992.

1.3 Pulse Electroplating

Electroplating with pulse current (PC) is becoming increasingly popular because it offers

several advantages over direct current plating, such as mass transfer enhancement and the

availability of additional process parameters—on-time, off-time and pulse current

density—which can be varied independently to influence deposit properties [9].

In PC electrodeposition, the current density or the applied potential is alternated rapidly

between two different values. This is done with a series of pulses of equal amplitude,

duration and polarity, separated by periods of zero current [6]. Each pulse consists of two

periods: an ―on-time‖ period in which the current is applied and an ―off-time‖ period

during which no current is applied. It is during the latter period when metal ions from the

bulk solution diffuse into the layer next to the working electrode; i.e., carbon paper or

carbon cloth in our experiments. When the current is applied during the on-time, more

evenly distributed ions are available for electrodeposition. A pulsing scheme is shown in

Figure 1-1.

4

Figure 1-1 Schematic Diagram of a Square Pulse Current Wavefor

It is evident from the above discussion and Figure 1-1 that in order to characterize a

simple direct current, it is sufficient to know the current density; however, the

characterization of a square pulse current waveform requires three parameters to be

known. These are cathodic current density, the cathodic pulse length (on-time) and the

interval between the pulses (off-time). A very useful term frequently encountered is the

duty cycle,

, representing the portion of the time when the current is on. It may be

defined as follows:

offon

on

tt

t

(1-1)

The average current density, ia, can be defined by the following equation:

pa ii (1-2)

ton toff

Pulse

Period

iP

Figure 1-1 Schematic diagram of a square pulse current waveform

Average current, ia

Peak current, ip

Time

5

1.4 Thesis Objectives

The primary objective of this work is to develop a novel PEM fuel cell catalyzation

technique to selectively deposit catalyst nanoparticles on carbon-based substrates where

both electronic and ionic pathways exist. This will ensure not only the existence, but also

the expansion of the three-phase interface leading to lower precious metal loadings.

Conventional deposition techniques, including spraying, brushing and rolling have been

proven to be unsuitable for attaining high catalyst utilization. Consequently,

unconventional methods have been employed to attain better results. One such method is

pulse current electrodeposition, which has shown great promise to selectively deposit

nanocatalysts where they are needed. Although the superiority of pulse electrodeposition

has been established in other applications, such as nickel electroforming and

semiconductors, its use in platinum electrodeposition has been limited to square-pulse

waveforms for fuel cell use. The influence of other types of waveform on the quality of

the electrodeposits has not been reported in the literature. Therefore, another objective of

this thesis is to evaluate the suitability and influence of other waveforms, specifically

triangular, ramp up and ramp down, on the quality of a number of metal deposits,

including platinum.

One of the main advantages of such methods cited by many researchers is the number of

independent variables that can potentially be optimized to improve deposit properties and

qualities. This can readily be done by employing parametric studies where the number of

variables is small—one or two. However, when the number of variables increase, it

becomes very difficult to optimize the process in a timely fashion. As a result, utilization

of more sophisticated techniques and strategies become essential. Consequently, another

objective of this thesis is to develop a modeling tool and optimization technique to

provide an accurate prediction of the influence of all these variables—pulse on-time,

pulse off-time, pulse frequency, duty cycle, type of waveform, peak current density and

total charge—on catalyst layer properties and, ultimately, on fuel cell performance.

6

1.0 BACKGROUND

1.1 Hydrogen Fuel Cells

2.1.1 A Condensed History

At first, fuel cells might seem to be a marvel of the 20th

and 21st centuries; however, they

have been around since 1839, when William Grove, a professor of experimental

philosophy at the London Royal Institution and a friend of Michael Faraday, first

discovered the principle of the device [109]. Thirty-seven years earlier, in 1802, British

chemist Sir Humphrey Davy discovered that water can be decomposed into its

constituents—hydrogen and oxygen—when an electric current is passed through it. This

process was later called electrolysis. Grove successfully demonstrated that the process of

electrochemical decomposition can be reversed, and that hydrogen and oxygen may be

combined to form water and energy in the form of heat and electricity [109]. Grove

realized both the simplicity of the process and its applications; however, he soon

abandoned his work on the fuel cell because he was not able to generate useful amounts

of energy. A few years later, when carbon-based fuels were used in an attempt to

eliminate the need for pure hydrogen with no success, efforts were abandoned until 1889

when Ludwig Mond and Charles Langer repeated the work of Grove and managed to

produce 1.5 W with 50% efficiency while replacing oxygen with air, and pure hydrogen

with impure industrial gas obtained from coal [2, 110]. This was a great achievement, and

for the first time the device was called a hydrogen fuel cell. But, once again, the project

stalled owing to the high cost of the platinum catalyst that was being poisoned by traces

of carbon monoxide present in the gas. Attempts were made to improve the device

without much success until 1932, when Francis Bacon developed the first modern

hydrogen fuel cell in the United States. For the next 20 years, much effort was expended

to increase the power output and efficiency of such devices until finally, in 1952, a 5-kW

fuel cell system was successfully tested [109]. The next push came from an unfamiliar

source—the U.S. space program. The appeal of the hydrogen fuel cell was apparent to

many scientists and engineers working for NASA; it was less dangerous than any nuclear

device, and much simpler to deal with in terms of installation, maintenance, and repair on

board shuttles. In addition, hydrogen fuel cells were more compact and lighter than any

7

other energy-producing device and storage medium, including state-of-the-art batteries

[109].

The transfer of hydrogen fuel cell technology to extraterrestrial applications proved to be

an easy task for two reasons: first, a large amount of fuel, pure hydrogen, was not needed,

and second, economics played a minuscule role. On the other hand, such a technology

transfer to terrestrial applications has been proven to be difficult. One of the main reasons

is the need for significant amounts of pure hydrogen. Another obvious reason is the cost

of such systems.

2.1.2 How Proton Exchange Membrane Fuel Cells Work

A fuel cell is an electrochemical device that converts the energy associated with the

combination of a hydrogen-rich fuel with an oxidant such as oxygen directly to electrical

energy without the need for the intermediate combustion steps found in internal

combustion engines. A fuel cell, in its simplest form, consists of a cathode, an anode, a

membrane electrolyte capable of conducting ionic current but not electrons, and an

external circuit connecting the two electrodes together to provide a path for the flow of

electronic current.

As hydrogen flows into the fuel cell on the anode side, a platinum catalyst promotes the

dissociation of hydrogen gas into hydrogen ions and electrons. The hydrogen ions pass

through the membrane from the anode compartment and migrate toward the cathode,

while the electrons that are released flow through the external circuit to the cathode.

Oxygen, on the other hand, flows into the fuel cell on the cathode side, where it combines

with the incoming hydrogen ions and electrons with the help of a noble metal catalyst to

produce electricity, water and heat. The electrons flow through an electric load (such as

an electric motor) located in the external circuit and generate low-voltage electricity, as

shown in Figure 2-1. The following are the electrochemical reactions taking place at both

the anode and the cathode:

Anode: H2 (g) 2H+

+ 2e- (2-1)

Cathode: ½O2 (g) + 2H+ + 2e

- H2O (liq) (2-2)

Overall: H2 (g) + ½O2 (g) H2O(liq) (2-3)

8

Figure 2-1 A simple proton exchange membrane fuel cell

The practical operating voltage from a single cell is about 0.7 V. Desired voltages of any

value can be obtained by connecting a predetermined number of cells in series. Although

the underlying electrochemical principles are similar to those of a battery, unlike

batteries, fuel cells are, in theory, capable of producing electricity indefinitely as long as

fuel and oxidant are supplied to them.

2.1.3 Different Types of Fuel Cell

Fuel cell design varies according to several parameters, including the power demands of a

given system and the operating temperature that is best suited to that particular

application [2, 109]. At present, there are five main types of fuel cell that are either under

development or commercially available. Because the electrolyte defines the key

properties, including the operating temperature, fuel cells are often named by the

electrolyte they employ. However, one also can classify fuel cells based on their

operating temperature or the state of the fuel used. The five main fuel cell technologies

are proton exchange membrane, alkaline, phosphoric acid, molten carbonate, and solid

H2 (g) 2H+

+ 2e-

½O2 (g) + 2H+ + 2e

- H2O (liq)

Solid Polymer Electrolyte

Load

FUEL

OXIDANT

9

oxide. Proton exchange membrane fuel cells are the smallest and lightest of the designs,

making them the best candidate for both transportation and portable applications. They

exhibit an overall maximum energy conversion efficiency of 40% to 60% and operate at a

temperature of about 80 °C [109, 111]. The applications, properties, advantages, and

disadvantages of these and other types of fuel cells have been fully discussed elsewhere

[2, 112-115].

2.2 Main Components of a PEM Fuel Cell

2.2.1 Polymer Electrolyte Membrane

Fuel cell design has gone through major changes with the advent of solid polymer

electrolytes (SPEs). Proton exchange membrane fuel cells employ a membrane electrode

assembly (MEA), which comprises an ion-exchange membrane sandwiched between the

two electrodes. Solid polymer electrolytes, also known as ion exchange membranes

(IEMs), were originally used as separators in electrochemical cells containing two

different electrolytes. Meyer [117] employed an IEM to separate the electrolytes of a

concentration battery. Jude et al. [118] presented a method in which the electrolytes of a

Daniell cell were separated with the help of an IEM. Pitzer [119] and Morehouse [120]

utilized IEMs to depolarize cells by removing reaction products. IEMs also have been

used to supply a cell with required reactants. This was shown by Robinson [121], who

employed a permanganate anion exchange resin in the cathode of a Leclanché cell.

The first use of an IEM as an electrolyte in an electrochemical cell was successfully

demonstrated by Grubb [121] where the current was entirely conducted through and by

the IEM. He used two different types of commercial membrane, namely Amberplex C-1

(a sulfonated polystyrene resin) and Nepton CR-51 (a sulfonated phenol formaldehyde

polymer formed into a homogenous sheet), to conduct battery experiments [121].

Significant progress and exposure were achieved when General Electric used

hydrocarbon-type polymers such as cross-linked polystyrene divinylbenzene sulfonic

acid (PSSA) and sulfonated phenol-formaldehyde to launch the first hydrogen fuel cell

aboard a space shuttle in the 1960s [112]. This gave a much needed impetus to further the

R&D activities in the field of SPEs. However, soon it was realized that the useful life of

hydrocarbon-based polymers are short due to the presence of C-H bonds, which

10

contribute to the instability of the complete chain via C-H bond cleavage. Research

continued and promising results were obtained when all the hydrogen atoms in carbon-

hydrogen bonds were replaced with fluorine atoms through a perfluorination process.

Carbon-fluorine bonds exhibit stronger affinity for each other, resulting in a much

stronger bond than a C-H bond. This translates into a rigid and long lasting structure,

prolonging its useful life.

All these led to the development and synthesis of a family of fluorocarbon copolymers

such as Nafion®. Nafion

® is a sulfonated tetrafluoroethylene copolymer discovered in

1962 by Walther Grot while working at DuPont de Nemours [122]. It all began when one

of DuPont’s scientific groups, known as the Plastics Exploration Research Group, was

persistently working on fluorine technology to develop monomers for copolymerization

with tetrafluoroethylene (TFE) [123]. The goal of the study was to manufacture a

material with exceptional dielectric and antistick properties as well as having a low

coefficient of friction. Such material would mirror the properties of another important

class of materials, Teflon. However, the product of the reaction between TFE and sulfur

trioxide exhibited some interesting properties that were initially considered to be inferior

to Teflon. The striking difference between the two was the inactivity of Teflon with

regards to its environment, while the new material showed a great tendency to interact

with its local surrounding. It was not until 1964 that DuPont realized some of the

desirable properties of Nafion®. It became apparent that this new material could be used

as a membrane separator in several industries, including chloralkali cells for production

of chlorine and sodium hydroxide. However, the above industries were in no rush to

adopt the new technology, since energy was considered to be abundant and relatively

cheap and there were no stringent environmental regulations. The push came from

General Electric while working on PEM fuel cell development for the U.S. space

program, where their state-of-the-art fuel cells were failing prematurely due to the

instability of the polystyrene sulfonic acid polymer membranes employed in oxidative

environments [123]. This provided the impetus for PEM fuel cells to gain acceptance in

the scientific community and arose interest and curiosity to further develop such fuel

cells.

11

Nafion®

membrane consists of a tetrafluoroethylene (Teflon) backbone with side chains

terminating in sulfonic acid groups. The Teflon backbone is strongly hydrophobic,

alleviating flooding problems, while the sulfonic acid groups are highly hydrophilic,

helping to retain enough water to keep the membrane hydrated during operation. The

strong nature of the C-F bonds ensures thermal and chemical stability in an acidic

environment. The protons on the sulfonic acid (SO3H) groups become mobile when

hydrated and can be transported from one side to another, traversing the whole membrane

structure in the form of hydronium ions. In hydrogen fuel cells, hydronium ions move

through the hydrophilic membrane structure from anode to cathode while sulfonate

groups are fixed. The chemical structure of Nafion® is shown in Figure 2-2.

Figure 2-2 Nafion® perfluorinated ionomer

There has been a considerable interest in SPEs, including Nafion®, over the past several

decades. This is mainly due to their desirable characteristics and their wide range of

applications in many industrial processes, including electrolytic cells used to produce

chlorine and sodium hydroxide, batteries, and fuel cells [124-128].

Perfluorosulfonic acid (PFSA) membranes must exhibit several key characteristics to be

successfully incorporated into PEM fuel cells. These include, but are not limited to, good

protonic conductivity, no electronic conductivity, stability in acidic environments,

impermeable to gases (i.e., hydrogen and oxygen), and both thermal and mechanical

stability. Much effort has been expended on the development of PFSAs for use in PEM

fuel cells over the years. Early work examined the water content of PSFAs, since it was

known that this would have a profound effect on proton conductivity. This kind of

research is still on-going. Koptizke et al. [128] studied the relationship between the water

O-----H

+

12

uptake and proton conductivity of several different PSFAs, including Nafion® 117, as a

function of temperature. Cappadonia et al. [129] also investigated the conductance of

Nafion®

membranes as a function of temperature utilizing impedance spectroscopy to

establish a link. In an early paper, Uosaki et al. [130] using Nafion® 117 and, employing

impedance spectroscopy and differential scanning calorimetry between 180 K and room

temperature, showed that the conductivity of Nafion® can be linked to the structure of

water in the membrane. Zawodzinski and co-workers [131] at Los Alamos National

Laboratory compared the intrinsic water uptake and water transport properties of

different membranes including Nafion® 117 and Dow’s XUS13204.10 developmental

fuel cell membrane, under conditions similar to operating PEM fuel cells. They

concluded that the higher density of ionic groups combined with higher water content per

sulfonic acid group gives rise to increased protonic conductivity. Earlier, Zawodzinski et

al. [132] reported water uptake and transport properties of Nafion® 117 membrane when

immersed in liquid water and exposed to water vapour of varying activities at 30 °C.

As mentioned above, the conductivity, and hence the performance, of SPEs depends on

the amount of free water within the membrane. Accordingly, diffusion of water within

such structures is crucial. Motupally et al. [133] reported experimental and simulated data

for the diffusion of water across Nafion® 115 membranes as a function of the water

activity gradient. The activity gradient was varied by changing the nitrogen flow rate into

the cell or by varying the cell pressure. Others have also reported different values for the

diffusion coefficient of water through Nafion®. Fuller [134] determined the diffusion

coefficient of water across Nafion® by measuring the water flux across membranes

having water on one side and nitrogen gas on the other. Zawodzinski et al. [135]

accomplished the task by employing a nuclear magnetic resonance technique. Many other

groups also have studied the conductivity of Nafion® membranes using diverse methods

such as ac impedance spectroscopy [136-140] and dc techniques [141, 142]. Furthermore,

a wide variety of environments also has been employed and reported, including water,

water vapour, 1.0 molar sulfuric acid, and humidified gases at various temperatures [132,

135, 140, 141, 143]. It is not surprising that different values have been reported in the

literature depending on the methods and test conditions used.

13

Membrane water content () can be defined as the number of moles of water divided by

the number of moles of ion exchange site:

2

21

02.18

)(

W

WWM (2-4)

Where M is the equivalent weight of the membrane, and W1 and W2 are the weights of the

membrane before and after drying, respectively.

Another important characteristic of Nafion® membranes is their ion exchange capacity

(IEC). The ion exchange capacity of a material may be defined as ―a reversible exchange

of ions between a solid and liquid in which there is no substantial change in the structure

of the solid‖ [144]. The total ion exchange capacity of an ion exchange material is the

number of ionic sites per unit weight or volume of resin. The dry weight total capacity is

usually expressed in milliequivalents per gram of dry resin in the H+ form. Ion exchange

capacity of Nafion® and Nafion

® composites have been extensively reported [145-153].

2.2.2 Catalyst Layer

Proton exchange membrane fuel cells utilize a solid polymer electrolyte membrane

disposed between two porous electrodes, often made of treated carbon paper or cloth.

This structure constitutes the MEA. Each electrode contains a catalyst layer, and,

depending on the required power output and operating conditions, in addition, also may

have an admixture of selected catalysts, an ionomer similar to that used for the ion

exchange membrane, and a binder such as polytetrafluoroethylene (PTFE). Furthermore,

the catalyst can be a metal black, an alloy (Pt/Ni or Pt/Ru for the anode of PEMFCs) or

an unsupported or supported metal catalyst (platinum supported on carbon being a prime

example). Regardless of the catalyst layer composition, it is strategically located next to

the solid polymer electrolyte; e.g., Nafion®.

The catalyst layer in conjunction with the SPE plays a vital role in the operation of

PEMFCs. It is here where electrochemical reactions (i.e., hydrogen oxidation and oxygen

reduction) take place. In order to catalyze these reactions, catalyst particles must establish

both electronic and ionic contacts. The former are achieved through the continuous

14

contact between catalyst particles and the highly conductive carbon substrate, while the

latter are accomplished through intimate contact with the ion-conducting membrane. The

contact point between catalysts, reactant gases and membrane is referred to as the three-

phase interface. The effective area of active catalyst sites must be several times greater

than the geometric area of the electrode in order to achieve reasonable reaction rates. One

method to accomplish this is by making the electrodes reasonably thin to create a three-

dimensional structure, where enough sites are available for catalyst particles to be

deposited [154].

Electrocatalyst can be deposited at the membrane-electrode interface in two different

ways to achieve the goal of catalyzing hydrogen oxidation and oxygen reduction. It can

either be applied as a thin layer on the solid polymer electrolyte or on the carbon

substrate (carbon cloth or carbon paper). In the former case, a thin layer of the

electrocatalyst is directly applied onto the solid membrane using various methods. In the

latter case, a catalyst ink is applied onto the electrode substrate by brushing, spraying,

rolling and other methods. Direct application on the solid membrane creates a thin

electrocatalyst layer resulting in a low catalyst loading and good mass transport

properties. In contrast, the application of the catalyst ink on the substrate may require a

higher catalyst loading since the catalyst ink tends to penetrate into the substrate,

resulting in inactive sites where no ionic contact is established. This issue, however, has

been resolved by impregnating a perfluorosulfonate ionomer into the substrate. Ticianelli

et al. [155] successfully demonstrated this method and lowered the platinum loading

without adverse impacts on fuel cell performance. Several years later, Wilson et al. [41,

156] proposed a method in which carbon-supported platinum particles were ultrasonically

mixed with Nafion® to make a catalyst ink and then applied onto a substrate. It should be

noted that the previous catalyst ink also can be applied directly onto the solid polymer

and then bonded, on both sides, to appropriate gas diffusion layers; i.e., carbon substrates,

to fabricate MEAs.

Recent efforts to decrease platinum loading through increased platinum utilization have

resulted in the development of a number of novel methods. These can be broadly

categorized into two groups: one is to use new materials to construct the catalyst layer

15

and the other is to optimize the structure of the catalyst layer to increase utility and

minimize irreversible losses. The former can be accomplished by modification of the

carbon-supported platinum. The latter is achieved by optimizing the structure of the

catalyst layer using different fabrication methods and materials. Uchida et al. [157]

examined the effects of the microstructure of the carbon support in conjunction with

PFSA distribution in the catalyst layer on the overall performance of a PEM fuel cell.

Their method was based on the process of PFSA colloid formation [17] and it was

concluded that fuel cell performance was directly affected by the PFSA and carbon

support content. Shin et al. [158] reported on the effects of the preparation method of the

catalyst ink on electrode structure and, ultimately, on fuel cell performance. It was

reported that the electrodes prepared by a colloidal method exhibited better results than

those prepared by a method in which the catalyst ink was a solution. Organic solvents

also have been used in the fabrication of thin catalyst layers. In a study published by

Yang et al. [159], five different organic solvents—butyl acetate, iso-amyl alcohol, diethyl

oxalate, ethylene glycol and ethylene glycol dimethyl ether—were used to prepare MEAs

using a transfer method. The MEAs prepared using ethylene glycol showed the best

results. It was, however, observed that the micropores in the MEAs showed partial

blockage by remaining solvent even after heat treatment. Increased porosity in the

catalyst layer also has been exploited by several researchers. Fisher et al. [160]

investigated the influence of pore-forming additives on overall cell performance.

Additional coarse porosity was obtained by adding pore formers to the catalyst ink.

The structure, properties and factors influencing the performance of catalyst layers in

PEM fuel cells have been extensively studied and reported in the literature. The cathode

catalyst layer has attracted much of the attention due to the greatest irreversible losses in

cell voltage occurring in this layer, compared with loses in the anode catalyst layer [161-

163]. Yoon et al. [164] examined the influence of the pore structure of the cathode

catalyst layer on overall cell performance. They utilized a spray-drying method to prepare

MEAs with varying degrees of porosity and pore structure and reported that the addition

of thermoplastic agents enhances structural stability and, consequently, overall cell

performance, concluding that pore-forming agents facilitate the transport of oxygen gas

through the catalyst layer. Yang et al. [165] studied the effects of ethylene glycol addition

16

to the catalyst slurry and observed an improved performance. They hypothesized that

propylene glycol acts as a pore-forming agent facilitating the transport of gas through the

catalyst layer. Chisaka et al. [166] investigated the effect of glycerol on the catalyst layer

when added to the catalyst ink. A decal method was employed to fabricate catalyst layers

with various mass ratios of glycerol to carbon in the catalyst ink, ranging from 0.0 to

20.0. Jia et al. [167] conducted research on the effect of surface oxidation of the carbon

support with nitric acid before platinum deposition, and reported an increase in cell

performance due to Pt particle size reduction. Mukerjee et al. [168] reported the result of

an in situ X-ray absorption study on several well-defined carbon-supported platinum

electrocatalysts with particle sizes in the neighbourhood of 25 to 90 Å.

2.2.3 Gas Diffusion Layer

In almost all PEM fuel cells, a reinforced material, often made of carbon paper or cloth

(Figure 2-3), is inserted between the bipolar plate and the catalyst layer. It is commonly

referred to as the gas diffusion layer (GDL). GDLs are porous in structure to allow the

passage of reactants to the catalyst layers on both sides of the cell and to ease the removal

of excess water from the cathode catalyst layer to avoid ―flooding‖. Flooding becomes a

major limiting factor in PEMFC performance when both cathode catalyst layer and GDL

become saturated with water. This fills the micropores inside the cathode GDL reducing

the paths for oxygen transport, contributing to the overall irreversible losses [112, 169].

Figure 2-3 (a) Toray Carbon Paper [116] (b) Toray Carbon Cloth [116]

17

In addition, GDLs must collect the current generated inside the fuel cell at the catalyst

layer and direct it to the external circuit while minimizing losses during transport. This is

accomplished by incorporating materials that are highly conductive into the structure;

carbon powder is a good example. Furthermore, the GDL needs to remove heat from

inside the cell and provide the MEA with stability and integrity. Some of the desired

characteristics are in contrast to each other; for example, water and heat removal from

inside the MEA is enhanced by increasing the porosity of the GDL, but this inherently

reduces mechanical strength as well as ionic and electrical conductivities of GDLs.

Hydrophobic agents such as PTFE are used to enhance water repellent and removal

characteristics of GDLs [17, 170-173]. However, it is well established now that as the

amount of hydrophobic polymer in the GDL increases, the conductivity and porosity will

decrease [174]. From the above discussion, it becomes clear that a well-balanced

approach is needed to successfully design an effective GDL.

Until recently, less attention had been devoted to the study, design, and fabrication of

GDLs. Instead, much time and effort has been spent on examining and optimizing the

membrane and the catalyst layer. The main reason for this has been economic, since both

the membrane and the catalyst layer are by far the most expensive components of an

MEA. However, it has been realized that an effective and reliable path between the

bipolar plates and the MEA of a PEM fuel cell is critical to overall cell performance. As a

result, systematic studies on various aspects of GDLs now have been carried out and

reported in the literature. De Sena et al. [175] studied the limiting structural effects of

porous GDLs on the oxygen reduction reaction (ORR) in 0.5 M sulfuric acid solution.

They reported that for an MEA with 10 wt% Pt/C, a PTFE content of 35 wt% results in

superior cell performance. Lee et al. [176] presented changes in the performance of a

PEM fuel cell as a function of the compression pressure from the bolts that clamp the fuel

cell. They utilized three different types of GDLs—Toray™, ELAT®, and CARBEL

®

combined with Toray™—to study the impact on the overall cell performance. The

performance changes are related to changes in porosity, the electrical contact resistance,

and the water management in the GDL. Carbon black and similar materials are routinely

used in GDLs to increase their electrical conductivities and pore-forming capabilities.

Passalacqua and coworkers [177] investigated the influence of several carbon blacks and

18

graphite on cell performance. Shawinigan Acetylene Black (SAB) was found to produce

the best results when compared with Vulcan XC-72, Mogul L, and Asbury 850 graphite.

This was attributed to improvements in gas diffusion characteristics, including better

water management properties in such GDLs. Antolini et al. [178] reported the effects of

two different carbon powders—Shawinigan and Vulcan XC-72—as materials for both

carbon cloth and carbon paper-based cathode GDLs on the performance of PEM fuel

cells. Cathode electrodes having Shawinigan carbon incorporated into their GDLs

exhibited better performance than electrodes prepared using Vulcan XC-72 carbon.

However, at high operating pressures, the best results were obtained when Vulcan carbon

was used in the catalyst side and Shawinigan in the gas side of cathode GDLs. It is worth

noting that the surface area of Vulcan XC-72 (around 250 m2 g

-1) is much greater than

Shawinigan carbon (about 70 m2 g

-1). Similar results also have been reported by other

workers [179-181].

It is generally agreed that the cathode of a PEMFC is its most important component,

controlling the overall cell performance. Much of the irreversible losses in a PEMFC

during operation are attributed to the losses due to an ineffective cathode electrode. The

oxygen reduction reaction is often the limiting kinetic step determining the overall cell

performance, and its rate is directly proportional to the rate of oxygen transport from the

flow field through the GDL and into the catalyst layer. There are several factors

hindering this diffusion process with accumulation of water inside the pores of both the

catalyst and gas diffusion layers considered to be the most serious. It is, therefore,

imperative to fabricate MEAs with adequate water management capabilities. To avoid

flooding the catalyst layer and enhance cell performance, a hydrophobic polymer such as

PTFE is often mixed with carbon powder and other materials and pasted onto a substrate

to prepare a GDL. Such a layer exhibits superior water repellent properties if proper care

is exercised during preparation and application. It is worth noting that PTFE (and all

hydrophobic polymers) cannot conduct electrons, therefore, its proportion to carbon

powder must be carefully determined to enhance its water impedance characteristics

while minimizing ohmic losses.

19

Numerous reports have been published on the appropriate use of such polymers in fuel

cells. Bevers et al. [182] examined the influence of various PTFE loadings and sintering

times on carbon papers. They highlighted the tradeoff between PTFE content (i.e.,

hydrophobicity) and conductivity of GDLs. The same authors [183] also presented their

findings on the influence of PTFE coating and sintering time on different paper

properties. Giorgi et al. [184] investigated the influence of the diffusion layer porosity as

well as the PTFE content and structure of the GDL on the performance of low platinum

electrodes in PEMFCs. Best performance was reported at low PTFE contents (less than

20 wt%) for the cathode electrode. Paganin et al. [185] studied the influence of PTFE

content for a three-layer structure with 20 wt% Pt/C and 0.4 mg Pt/cm2 and found that a

PTFE loading of 15 wt% produces the best result. Moreira and co-workers [186]

determined the optimum PTFE loading for both the anode and the cathode of a PEFMC.

They proposed a 10% and a 30% PTFE content (by mass) for the cathode and anode,

respectively. Lim and Wang [187] reported the impact of fluorinated ethylene propylene

(FEP) content in GDLs, ranging from 10% to 40% by weight, on the performance of

PEMFCs. A GDL impregnated with 10% FEP was recommended.

Several models also have been proposed to describe the microstructure of various

electrodes, and PTFE in GDLs has been reported to be present in different structural

forms [188-192]. Watanabe et al. [189] presented a model in which the PTFE present in a

GDL has an irregular shape, and, while binding the various components together,

including carbon black, it may prevent the latter from mixing with the electrolyte. Later,

Passalacqua et al. [193] introduced a set of parameters to characterize GDLs, using the

model presented by Watanabe et al. [189]. As mentioned earlier, PTFE is known to be

present in various forms, including agglomerate, films, and fibrils, when bonded to other

components of an MEA. Pebler [194] proposed a network of thin films of fibers during

various stages of electrode fabrication and Holze and Mass [195] reported both

agglomerate and fiber forms of Teflon in electrodes. Other workers [190, 196] have also

validated the models presented by Pebler and Holze by utilizing scanning electron

microscopy (SEM) to show a hydrophobic polymer layer as segregated regions in

different electrodes.

20

It has been reported that the addition of a microporous layer (MPL) between the GDL

and the catalyst layer of an MEA can improve the cell performance [178, 185, 200]. The

effects of various substrates, carbon powder, composition, and thickness of such a layer

have extensively been studied and reported [178, 184, 185, 197-200]. A series of

mathematical models also has been developed to explain the functions of such layers.

Martys [201] developed a numerical method to study the effect of water saturation in a

porous medium with spherical incursions. Nam et al. [202] employed a network model

for anisotropic solid structure and water to show the dependence of the effective

diffusivity of fibrous diffusion media on the porosity and saturation of such media. The

above model also has been used to show the increase in cell performance when a two-

layer diffusion medium is utilized.

2.2.4 Bipolar Plates

The practical operating voltage from a single cell is about 0.7 V. Desired voltages are

obtained by connecting a predetermined number of cells in series; this is accomplished by

inserting a highly conductive material—known as a bipolar plate—between two parallel

MEAs. Such plates are by volume, weight and cost the most critical component of a fuel

cell stack [203, 204]. They account for more than 40% of the total stack cost and about

80% of the total weight [205-211]. As a result, there have been significant R&D activities

in the past few years to lower their cost and reduce their size.

Bipolar plates perform a number of critical functions simultaneously in a fuel cell stack to

ensure acceptable levels of power output and a long stack lifetime. They act as a current

conductor between adjacent MEAs, provide pathways for reactant gases (hydrogen and

oxygen or air), facilitate water and heat management throughout the stack, and provide

structural support for the whole stack. Accordingly, they must exhibit excellent electrical

and thermal conductivity, corrosion resistance, mechanical and chemical stability, and

low gas permeability. Furthermore, raw materials must be widely available at reasonable

cost and be amenable to rapid and cost-effective fabrication methods and processes [203].

Due to the multifaceted characteristics of bipolar plates and a wide combination of

physical and chemical properties that are often contradictory, a large number of

21

candidates has been proposed and investigated over the years. As a result, a set of targets

and requirements has been established to develop suitable material for the fabrication of

bipolar plates. A summary of such requirements and targets is presented below [203,

204]:

Bulk electrical conductivity: > 100 S cm-1

Hydrogen permeability: < 2 × 10-6

cm3 cm

-2 s

-1

Corrosion rate: < 16 µA cm-2

Tensile strength: > 41 MPa

Flexural strength: > 59 MPa

Thermal conductivity: > 10 W m-1

K-1

Thermal stability: up to 120 °C

Chemical and electrochemical stability in acidic environments

Low thermal expansion

Acceptable hydrophobicity (or hydrophilicity)

Bipolar plates are commonly made of graphite for its corrosion resistance characteristics

as well as its low surface contact resistance. However, graphite is brittle, permeable to

gases, and exhibits poor mechanical properties. Furthermore, it is not suitable for mass

production, since the fabrication of channels in the plate surfaces requires machining, a

very time-consuming and costly process [204, 211-213]. Additionally, post processing

such as resin impregnation is often required to ensure the impermeability of graphite

plates to reactant gases. Other materials have been considered as replacements for

graphite plates; a break down of such materials is given in Figure 2-4 [210].

Polymer composites—both polymer-carbon and polymer-metal—offer alternative paths

for fabricating bipolar plates. Bin et al. [211] examined the conductivity and flexural

strength of polyvinylidene fluoride (PVDF)/titanium silicon carbide (Ti3SiC2) composite

bipolar plates prepared by compression molding. The effects of Ti3SiC2 content, particle

size, mould pressure and mould pressing time were investigated. Adequate electrical

conductivity and flexural strength were achieved by optimizing the above parameters.

22

Figure 2-4 Classification of materials for bipolar plates in PEMFCs [210]

Composites fabricated with 80 wt% Ti3SiC2, mould pressure of 10 MPa and a mould

pressing time of 10 minutes exhibited the best results. Kirchain et al. [214] reported that

significant cost reductions can be achieved when graphite-based material is replaced with

composites or metal alloys. Based on their cost model approximation, with composites,

the cost of bipolar plates is reduced to 15-29% of the stack cost. Polymer composites

exhibit all the desired characteristics of graphite plates, but also are amenable to easy and

cost effective high-volume processing methodologies. They can be molded into various

shapes and sizes in a cost-effective manner. It is, however, difficult to meet the required

thickness and resistance targets, because such polymers are inherently insulating. One

solution is to fill them with corrosion-resistant conductive particles such as graphite or

carbon black. In order to meet the resistance targets, however, high loadings of

conductive particles must be applied (greater than 50 v/o), which exceeds the percolation

METALS NON-METALS

s COMPOSITES

Coated Non-Coated

Bases:

-Al

-Ti

-Ni

-SS

Stainless steel

-Austenitic

-Ferric

Non-porous graphite Metal Based Carbon Based

Resin:

-Thermoplastics

-PVF

-Polypropylene

-Polyethylene

-Thermosets

-Epoxy resin

-Phenoic resin

-Furan resin

-Vinyl ester

-Graphite

-Polycarbonate

-SS

Filler:

-Graphite

-Carbon black

-Coke-graphite

Fibre:

-Graphite

-Cellulose

-Cotton flock

23

threshold concentration of 5-20 v/o and approaches, and in some cases surpasses, the

critical pigment volume concentration of 50-70 v/o [215]. At such concentrations,

conductive particles would form a long and continuous path for electrons to travel

through the whole thickness of the plate with ease, lowering the resistance by several

orders of magnitude. But, this insulator-conductor transition takes place at the expense of

the mechanical properties of the plate, since there will not be enough polymer binder to

hold everything together. This also will lead to the formation of more ―holes‖ or ―gaps‖

between various particles in the matrix, making the plate more brittle. Blunk et al. [215]

studied high-graphite-filled composites to find out if such plates can meet plate

resistance, thickness, and permeation rate targets simultaneously. In such plates, the

insulating polymer is mixed with high loadings of corrosion-resistant graphite particles to

meet the resistance target. However, at high graphite loadings (>50 v/o) not enough

polymer resin is present to fill the gaps between graphite particles, hence more pathways

for electron conduction are formed, increasing the conductivity of such plates. This,

however, is achieved at the expense of a porous and weaker plate. It is reported that this

also will increase H2 permeation rates when the plates are made thin enough to meet the

required high stack volumetric and gravimetric power densities.

Kuan et al. [216] investigated the impact of graphite content on the physical properties of

composite bipolar plates composed of vinyl ester resin prepared by a bulk-moulding

compound (BMC) process. They reported an increase in the porosity of the composite

bipolar plates from 0.06 to 2.64% as the graphite content increased from 60 to 80 wt%,

while the electrical resistance and flexural strength of the bipolar plates decreased from

20,000 to 5.8 mΩ and 38.47 MPa (60 wt% graphite) to 27.3 MPa (80 wt% graphite),

respectively. It is apparent that great care must be exercised when designing such plates.

Huang et al. [217] developed a method to fabricate bipolar plates using thermoplastic

composite materials consisting of graphite, thermoplastic fibers and glass or carbon

fibers. A wet-lay (paper-making) process was utilized to fabricate highly formable sheets

that were then stacked and compression molded to make bipolar plates with gas flow

channels. Graphite content was varied between 50 – 70 wt% and in-plane and through-

plane conductivity ranges of 140-310 S cm-1

and 15-50 S m-1

, respectively, were

24

reported. Yin et al. [218] studied the effects of resin content, molding temperature and

time on conductivity and bending strength of the composite materials made from phenol

formaldehyde resin powder and graphite using hot-pressure molding. Some interesting

results are reported: the conductivity decreases and bending strength increases with the

increase of resin content, while conductivity varies non-linearly (wave-like) with an

increase in temperature. Other workers have reported similar findings on resin-graphite

composites for bipolar plates [219-224].

Middleman et al. [225] proposed a process for large-scale production of composite

bipolar plates in which excellent physical and chemical properties are observed. Cho et

al. [226] demonstrated the viability of composite bipolar plates by developing plates with

performances comparable to that of graphite plates. Kuan et al. [227] utilized a bulk-

molding compound process to fabricate vinyl ester-graphite composite bipolar plates.

Such plates exhibited properties similar to those of graphite plates. Chun-hui et al. [228]

examined the flexural strength, pore size distribution, resistance to acid corrosion and

thermal properties of sodium silicate/graphite composite bipolar plates. A flexural

strength of 15 MPa was reported when the graphite content was about 40 wt% and it was

claimed that the plates withstood the force of machining flow fields. However, significant

amounts of pores have been reported and attributed to the formation of silica gel due to

solidification of sodium silicate. Furthermore, a corrosion current of 32 µA cm-2

has been

reported for bipolar plates with a 60 wt% graphite content in 1.0 M H2SO4 solution at

room temperature, showing their acid-resistant properties (the US Department of Energy

requires a corrosion rate of 10-5

A cm-2

for bipolar plates). Finally, it was claimed that the

water content of the above plates can reach 9 wt%, and that this water may be used to

humidify the incoming gases during operation. Xia et al. [229] determined the effects of

resin content, molding temperature and holding time on the conductivity and bending

strength of polyphenylene sulfide (PPS) resin/graphite bipolar plates fabricated using a

simple hot pressing technique. They reported a decrease in the electrical conductivity of

the composite with increase in the resin content, while the bending strength exhibited a

cyclic trend with an increase at very low and very high resin content and a gradual

decrease between the two extremes. An optimum resin content of 20 wt% was suggested.

25

An electrical conductivity of 118.9 S cm-1

and bending strength of 52.4 MPa are reported

for composite bipolar plates with 20% PPS resin content when molded at 380 °C for 30

min. Radhakrishnan et al. [230] also used PPS resin in conjunction with graphite to

fabricate bipolar plates by high-pressure compaction. Resistance values of 0.1 Ω were

achieved and reported. Huang et al. [231] utilized a wet-lay process to fabricate bipolar

plates from a mixture of PPS resin, graphite and glass or carbon fibers. In-plane

conductivities of 200-300 S cm-1

, tensile strength of 57 MPa, flexural strength of 96 MPa,

and impact strength of 81 J m-1

were achieved in their studies.

Bipolar plate manufacturers and suppliers rely heavily on the high concentration of

conductive graphite or carbon particles in composite bipolar plates to meet conductivity

targets set by various organizations such as the US DOE. Although such high loadings

yield composite plates that meet or surpass such targets, they become brittle, leading to

high scrap rates during plate manufacturing, adhesive bonding and stack assembly [232].

This becomes critical when thin plates are required to achieve high stack volumetric

power densities. It is expected that PEM fuel cells used for transportation applications

must possess a stack volumetric power density of at least 2 kW L-1

. This translates into

bipolar plates with thicknesses of 1.5 mm or less. As a result, extensive research has been

conducted with the aim of reducing graphite or carbon content while maintaining an

acceptable level of conductivity. Malliaris and Turner [233] studied the influence of

several parameters including binder type, filler particle size, and filler distribution.

Tchoudakow et al. [234] reported their findings on various polymer blends with lower

graphite content; and Sichel [235] investigated the impact of degree of mixing of low-

concentration polymer blends. Grunlan et al. [236] determined the impact of particulate

polymer microstructure, while Zhang et al. [237] examined the influence of polymer

crystallinity on the percolation threshold of conductive material (graphite or carbon

black).

Metals, on the other hand, offer better mechanical properties, higher electrical

conductivity, gas impermeability, and manufacturability; but suffer from corrosion and

are denser than graphite-based materials, thereby adding to the weight of the stack.

26

Considerable efforts have been expended to use noble metals, stainless steel, aluminum

and titanium as the material of choice for fabricating bipolar plates.

Metals such as titanium and stainless steel exhibit excellent mechanical properties and

have very low gas permeation rates. They are also suitable for mass production with low

scrap rates and are stable in a PEM fuel cell environment where low pH values are

common. The good corrosion characteristics of stainless steels and titanium are attributed

to the passivation of such metals in the presence of oxygen, where a protective oxide

metal film is formed on their surface. Such an advantage comes at the expense of metal

conductivity, which is hindered by the presence of such insulating protective layers. In

order to make these metallic plates conductive, it becomes necessary to remove such

oxide films or reduce their thickness to an acceptable level and apply a conductive and

corrosion-resistant film to retard the further formation of oxide films and decrease their

interfacial contact resistance. There are several concerns with such conductive layer

coatings. First, there is always the possibility of imperfections such as pinholes and micro

and macro cracks, which can lead to localized corrosion. This undermines the integrity of

the plates and, most importantly, poisons the membrane when dissolved ions diffuse into

the membrane and occupy the exchange sites, thereby lowering the ionic conductivity

and the overall performance of the stack [238]. Second, the cost and durability of such

conductive layers become an issue, since both are the main impediments to the

commercialization of fuel cells. Several fuel cell manufacturers have developed both

organic and inorganic protective and conductive coatings for metallic bipolar plates

[232]. A number of coating materials for metallic bipolar plates is presented in Table 2-1

[210]. Coating materials also have been classified as carbon- and metal-based. The

former includes conductive polymers, graphite, diamond-like carbon and organic self-

assembled monopolymers, while the latter utilizes noble metals, metal carbides and metal

nitrides. Extensive research on both carbon- and metal-based coatings has been

conducted and reported in the literature. It is generally agreed that such coatings must be

conductive and adhere to the substrate; i.e., metallic bipolar plates [238, 239, 246].

Proper adherence to the substrate is achieved by careful selection of coating materials

with similar thermal expansion coefficient as the substrate to minimize micro and macro-

27

Table 2-1 Potential coating materials for metallic bipolar plates [210]

Coatings Base Plate Materials

Aluminum Stainless Steel Titanium Nickel

Conductive Polymer [239] ×

Diamond-like Carbon [239] ×

Gold [240, 241] ×

Graphite Foil [242] × × ×

Graphite Topcoat [242] × × × ×

Indium Tin Oxide [243] ×

Lead Oxide [243] ×

Organic Monopolymer [239] ×

Silicon Carbide [243] ×

Titanium-Aluminum [243] ×

Titanium Nitride [243] ×

Oxides [244] ×

Chromium Nitride [245] Ni/Cr

crack formation. Woodman et al. [240] investigated the roles of thermal expansion

coefficient and corrosion resistance of a number of different bipolar plates on stack

performance. Li et al. [247] examined the corrosion behavior and conductivity of 316 SS

coated with TiN. It is reported that both corrosion resistance and electrical conductivity

of such plates are improved in simulated conditions similar to those of a real PEMFC.

Long-term studies on the stability of such plates have yet to be performed.

Cho et al. [208] conducted long-term studies on the corrosion behavior of TiN-coated 316

SS and reported significant improvement when compared with uncoated 316 SS. The

authors also have reported their findings in terms of surface energy, water contact angle

and surface wetability. Water contact angles of 90° and 60° are reported for TiN-coated

316 SS and 316 SS with no coatings. It is interesting to note that the water contact angle

of TiN-coated 316 SS is similar to that of graphite. This has been confirmed by Taniguchi

and Yasuda [248] who observed an increase in power output of a PEM fuel cell stack

when gas flow channels showed low water wetability.

A wide variety of coatings (see Table 2-1) and processes have been developed to address

the concerns associated with metallic bipolar plates. Joseph et al. [249] investigated the

viability of conductive polymer coatings such as polyaniline (PANI) and polypyrrole

28

(PPY) on 304 SS. They reported superior corrosion behavior and acceptable interfacial

contact resistance. Once again, long-term, economical viability and durability studies are

absent from this report.

Brady et al. [250] developed a preferential thermal nitridation process in which a nickel-

chromium alloy was coated with a thin layer of CrN/Ce2N. Based on their observations,

defect- and pinhole-free can be achieved with excellent corrosion resistance and

negligible interfacial contact resistance. Another substrate, 446 SS, also has been tested

with positive outcomes in terms of corrosion and contact resistance. But, admittedly such

coatings and processes are not cost effective. Physical vapor deposition (PVD) was

employed by Lee et al. [244] on two base metals, namely 316L SS and 5052 aluminum

alloy, with a YZU001-like diamond film. Tafel extrapolations from polarization curves

were used to compare the corrosion rates of the above metal plates with that of graphite.

Surprisingly, metallic plates outperformed graphite plates when placed in a single cell to

measure interfacial contact resistance and performance. According to the authors, 316L

SS plates exhibited better corrosion resistance with the above coating when compared

with aluminum plates. However, aluminum plates proved to be superior compared with

SS when interfacial contact resistance was concerned, although they had a shorter life

when tested in a single cell. There has been a great interest in carbon composite materials

for bipolar plates.

Due to their low cost, high strength and ease of machining, metallic bipolar plates have

attracted the attention of the research community. Stainless steel, by far, has been the

focus of on-going research over the past several years. The selection criteria for SS

bipolar plates is primarily the Cr, N, Mo content in accordance with the pitting resistance

equivalent [251, 252]. Stainless steels come in different compositions and, accordingly,

behave differently in various environments. Chemical compositions of some of the most

widely used SS are shown in Table 2-2 [251]. Stainless steel bipolar plates with different

compositions have been used extensively by different workers [253-258]. Davies et al.

[256] conducted a series of experiments on three types of stainless steel bipolar plates,

namely 310L, 316L and 904L. They reported that 904L performed the best while 316L

exhibited the worst result in an environment similar to that of a PEM fuel cell.

29

Table 2-2 Chemical composition of several stainless steels [252]

Material 316L 317L 904L 349TM

C ≤ 0.028 ≤ 0.028 0.011 0.05

Cr 16.20-16.80 18.10-18.60 20.48 23

Ni 10.10-10.30 12.45-12.75 24.59 14.5

Mn 1.70-1.95 1.60-1.90 1.53 1.5

Mo 2.03-2.25 3.05-3.35 4.5

Si 0.45-0.65 0.25-0.55 0.46 1.4

N 0.020-0.040 0.045-0.070 0.13

Cu ≤ 0.50 ≤ 0.50 1.4

Cb 0.4

Co ≤ 0.50

Fe Balance Balance Balance Balance

In another published paper, the same authors [257] utilized Auger Electron Spectroscopy

(AES) to determine the thickness of the passive film on the surface of various SS bipolar

plates. It was reported that the thickness of the passive layer decreases with the alloying

element content, and often varies between 3 to 5 nm. It was concluded that higher

alloying content promotes lower interfacial contact resistance. There have been some

disagreements amongst various groups on the relationship between alloying content and

corrosion and contact resistance behaviour of various stainless steel bipolar plates [253,

254, 257]. Such disagreements are, however, due to the pretreatment and the surface

conditions of such bipolar plates.

316 stainless steel has been receiving increasing attention as a replacement for non-

porous graphite in bipolar plates. Wang et al. [205] investigated the influence of oxygen

and hydrogen-containing environments on the corrosion behaviour of such bipolar plates.

Intergranular and pitting corrosion in both the oxygen and hydrogen-containing

environments were reported with a greater corrosion resistance shown by 316L SS

bipolar plates in simulated cathodes. This has been attributed to the cathodic protection

ability of the oxygen-containing environments. In addition, metal ion concentrations of

about 25 and 42 ppm at the anode and cathode, respectively, after 5000 h of operation are

reported utilizing potentiostatic tests (using ICP-OES). It was concluded that 316L SS

bipolar plates must be coated, since such levels of ion concentrations can adversely affect

30

the membrane. The above finding also was confirmed by another group, Ma et al. [259]

studied the corrosion behaviour of 316L SS bipolar plates and concluded that 316L SS

must be coated since it can be corroded in both anode and cathode environments. It is

common practice to coat 316L and 316 stainless steel bipolar plates for use in PEM fuel

cells [260-263]. Table 2-3 summarizes several coating methods and materials used on

316L and 316 SS for use in bipolar plates [205].

Wang et al. [264] developed a method to electro-polymerize polypyrrole films on 316L

stainless steel using both galvanostatic and cyclic voltammometric methods. The

aforementioned electrochemical methods yielded two different films with marked

differences in morphology. Optical microscopy revealed less intergranular corrosion for

plates covered with a dense coat of polypyrrole when compared with untreated 316L SS

plates. Feng et al. [265] investigated the impact of a nickel-rich layer with a thickness of

about 100 µm deposited on 316L SS by ion implantation. All tests showed higher

chemical stability of such plates in accelerated cathode environments containing 0.5 M

H2SO4 with 2 ppm HF at 80 °C. The superiority of Ni-coated plates is attributed to the

reduction in passive layer thickness caused by the Ni implantation. In recent years,

chromium-nitrogen films as a coating material for stainless steel plates have received

some attention. According to Fu et al. [266] 316L SS plates coated with Cr0.49 N0.51

delivered the best result in terms of interfacial conductivity, corrosion resistance, and

high surface energy. Cho et al. [267] applied a dense layer of chromium on 316L

austenitic stainless steel by pack cementation at 1100 °C for several time periods, ranging

from 2.5 to 10 hours. At short time periods; i.e., less than 5 hours, improvements in

Table 2-3 Primary coating materials and methods for 316 and 316L SS bipolar plates

[205]

Coating Material Coating Method

TiN Hollow Cathode Discharge Ion Plating

TiN PVD

PPY and PANI Electrochemical Method

Nitride Plasma Nitriding

No Coating Material Electrochemical Surface Process to Form Passive Film

31

corrosion resistance was reported. However, for longer running times; i.e., greater than 5

hours, no significant improvements were found.

Composite films also have been developed and applied on stainless steel bipolar plates to

improve their corrosion resistance and interfacial contact resistance. Fu et al. [268]

electrodeposited an Ag-polytetrafluoroethylene composite layer on 316L SS bipolar

plates and reported that the above plates exhibit lower interfacial contact resistance,

higher corrosion resistance and better hydrophobic characteristics. Novel composite

bipolar plates based on 316L SS also have been investigated. Kuo et al. [269] fabricated

bipolar plates using 316L SS and Nylon-6 polymer thermoplastic matrix using an

injection molding process. It was claimed that such bipolar plates are relatively light

weight, easy to process, inexpensive and exhibit favorable gas tightness, hardness and

corrosion resistance. However, they failed to match graphite bipolar plates as far as the

stack performance was concerned. Table 2-4 compares graphite and 316L SS in terms of

their chemical and physical properties [269].

Other types of stainless steel such as 304 SS also have been widely used as bipolar plates.

Chung et al. [270] and Fukutsuka et al. [271] both utilized carbon-coated 304 SS in

single-cell and three-cell PEM fuel cells, respectively. The first group employed thermal

chemical vapor deposition (CVD) to deposit a carbon film on Ni-coated 304 SS plates.

Table 2-4 Chemical and physical properties of graphite and 316L SS [269]

Property Graphite 316L SS

Cost (US $ kg -1

) 75 15

Density (g cm-3

) 2.25 8.02

Thickness of bipolar plate (mm) 2.5-4 1-2

Modulus of elasticity (Gpa) 10 193

Tesnile strength (Mpa) 15.85 515

Corrosion current (mA cm-2

) <0.1 <0.1

Electrical resistivity (Ω cm х 10-6

) 6000 73

Thermal conductivity (W m-1

K-1

) 23.9 16.3

Permeability (cm s-1

) 10-2

- 10-6

<10-12

32

Chemical stabilities comparable to PocoTM graphite were reported. The second group used

plasma-assisted CVD to deposit a carbon layer on 304 SS. It was reported that carbon-

coated 304 SS exhibited higher electrical conductivity compared with uncoated 304 SS,

while maintaining an acceptable level of corrosion resistance. As both studies were

carried out for relatively short times, long-term experiments are required to justify the use

of such materials as bipolar plates in PEM fuel cells. Noble metals such as gold also

have been used to coat stainless steel to improve its chemical and other properties in

harsh environments similar to that of PEM fuel cells [238, 241]. However, such metals

are prohibitively expensive and often thicker layers are needed to provide adequate

corrosion resistance of the base metal in acidic environments [271]. But, the greatest

level of success that has been reported on metal bipolar plates has been achieved with

noble metal coatings. Wang et al. [272] examined gold-plated titanium bipolar plates in a

single cell of 25 cm2 active area. A proprietary method was used to deposit gold coatings

of 2.5 μm thickness onto titanium plates. Polarization curves for graphite, pure titanium

and titanium coated with gold were presented at several different cell temperatures,

ranging from 40 to 80 °C. Gold-plated titanium plates performed poorly when the cell

temperature increased from 40 to 60 °C. However, a significant improvement was

reported at higher cell temperatures of 80 – 90 °C. In a paper published earlier by the

same group [273], three different types of metallic bipolar plates were examined: pure

titanium, titanium sintered with iridium oxide and titanium coated with platinum.

Titanium plates coated with platinum delivered the best result when tested in a single

PEM fuel cell operated at cell temperatures of 50 - 65 °C.

Copper alloys also have been studied as a potential candidate for bipolar plates in fuel

cells. Good corrosion resistance of copper and its alloys in weakly corroding

environments has been reported in the literature [274-276]. Nikam et al. [277] examined

the corrosion behaviour of copper-beryllium alloy C-17200 using a Tafel extrapolation

technique. It was noted that true corrosion representation can not be achieved from such

plots. As a result, chroamperometry was utilized to gain a better understanding by

analyzing the corrosion products by employing SEM, EDS and XPS. In an earlier work

[274], the authors presented their findings on the resistivity of the corrosion layer of the

above alloy using a four point probe apparatus in 0.5 M H2SO4 solution.

33

A large number of coating processes and techniques has been employed both in the

research community and in industry to deposit a wide array of coating materials on

different base materials. Pulse current electrodeposition has been successfully used to

apply a thin layer of gold over aluminum bipolar plates [278, 279]. Painting and pressing

have been employed to apply a graphite layer onto aluminum, titanium and nickel bipolar

plates [242], while electron beam evaporation can place a thin layer of indium-doped tin

oxide on a number of base metals, including stainless steel [243]. Physical vapor

deposition has been employed to deposit a number of coating materials such as chromium

or nickel-phosphorus alloy onto aluminum, stainless steel and titanium [280]. Vapor

deposition and sputtering has been reported to deliver excellent results when lead oxide

was deposited onto stainless steel bipolar plates [281].

As mentioned earlier, one of the primary functions of bipolar plates is to provide a path

for reactant gases to be homogeneously distributed over the surface of the catalyzed

electrodes. Accordingly, a number of different geometries are used and referred to as

flow fields. Figure 2-5 shows some of the most widely used flow-field configurations.

The proper design of such flow fields is not only critical in fuel and oxidant delivery, but

also in efficient heat and water management inside the stack. An extensive body of

research has been generated over the past few decades dealing with this special issue

[282-289].

Carrette et al. [282], Hertwig et al. [283] and Costamagna and Srinivasan [284] presented

detailed reviews of most of the flow-field designs and configurations and listed their

advantages and disadvantages. In a US patent filed and granted in 1992, Watkins et al.

[286] claimed as much as a 50% increase in overall cell performance by optimizing the

distribution of reactant gases via proper flow-field design. In another US patent, Reiser

and Sawyer [287] presented a flow-field network of many cubical or circular pins

arranged in a consistent pattern. In an interesting design, Pollegri and Spaziante [288]

showed a straight flow-field pattern in which an array of independent parallel flow

channels is connected to gas inlet and outlet. It was noted that the stack did not perform

very well when air was used as the oxidant after extended periods of operation. This drop

34

(a) (b) (c)

(d) (e)

Figure 2-5 Different flow field configurations: (a) parallel, (b) serpentine, (c) parallel-

serpentine, (d) interdigitated, and (e) pine or grid type

in performance was attributed to the inability of the bipolar plates to effectively transport

water out of the cell.

Two particular flow-field designs have been widely used in research and the fuel cell

industry: parallel and serpentine. The former configuration is often used because of its

simplicity, lower pressure drop between inlet and outlet of the gas channels and uniform

distribution of the reactant gases over the surface of the both electrodes. However, the

channels in a parallel flow field can easily be blocked by liquid water. This becomes

critical at the cathode where water is the byproduct and can easily lead to flooding and a

lowering of fuel cell performance. In addition, once a channel is clogged, the active area

of the electrode that is in direct contact with the bipolar plate becomes inactive as long as

the pathway is restricted to the flow of the reactant gas. As a result, the use of parallel

flow fields is restrictive in PEM fuel cells. On the other hand, the serpentine flow fields

35

can readily push the liquid water out of the channel due to high gas flow rate. The flow

field channels are often rectangular in cross section; however, other geometries such as

triangular, semi-circular and trapezoidal also have been reported in the literature [206,

290, 291]. The three critical variables in flow field design are channel depth, width and

land (rib) with average values of 1.5, 1.5, and 0.5 mm, respectively [206]. These

dimensions are always optimized to create a uniform gas distribution inside the cell or

stack with a reasonable pressure drop as the gases traverse the whole length of the

electrode, as well as providing a high contact area between the bipolar plate and the

electrodes for effective current collection. Watkins et al. [292] optimized the dimensions

of a bipolar plate for the cathode side of a PEM fuel cell. They reported that the optimal

ranges for the channel width, length and the land (rib) are 1.14-1.4 mm, 0.89-1.4 mm, and

1.02-2.04 mm, respectively.

Over the past few decades, in an attempt to improve fuel cell performance many studies

have been carried out to optimize all types of flow patterns. Mathematical models and

numerical simulations also have played a key role in gaining a better understanding of

how different flow-field configurations function in both a single cell and in stacks [293-

301]. Springer et al. [293] developed a one-dimensional steady state model that was

based on experimentally-derived parameters for PEM fuel cells. Another one-

dimensional model was presented by Gurau et al. [294] in which a cathode gas channel, a

gas diffusion layer and a membrane were all considered. A number of equations for

oxygen mass transport inside the cell and proton migration inside the membrane were

used and analytical solutions were derived. Two dimensional models were proposed by

Fuller and Newman [295] and Nguyen and White [296]. Numerical simulations were

presented to examine both water and heat management in PEM fuel cells. Ge and Yi

[297] also presented a two-dimensional model that highlighted the dependence of water

movement inside the cell on operating conditions and membrane thickness. Wang et al.

[298] studied the two-phased flow transport of air inside the cathode of a PEM fuel cell.

Their two-dimensional model was successful in predicting the transition between single

and two-phase regimes.

36

Yan et al. [299] developed a three-dimensional model to analyze the effects of the

contraction ratios of height and length on cell performance of PEM fuel cells. Their

model revealed that the reductions of the outlet channel flow areas increase the reactant

gas velocities in these regions and reactant transport and utilization as well as liquid

water removal. Recently, Cho et al. [300] proposed a three-dimensional, non-isothermal,

and steady state model to underline the heat transport characteristics of flow fields in

PEM fuel cells. According to this model, the maximum temperature occurs at the

cathode. This is primarily attributed to the formation of water at the cathode. Sinha et al.

[301] also developed a three-dimensional, non-isothermal PEM fuel cell model based on

serpentine and parallel flow-field designs operated at 95 °C and operating under different

inlet humidity conditions. It was noted that parallel flow fields outperformed the other

configuration at elevated cell temperatures and low inlet relative humidity.

The importance of bipolar plates as an integral component of PEM fuel cells becomes

clear when one looks at the large number of patents that have been filed and granted in

recent years [302-324]. Rock [302] proposed a design where serpentine flow channels are

arranged in a mirror-image fashion. According to this design, the inlet legs of each

channel border the inlet legs of the next adjacent channels in the same flow field. It is

claimed that in such arrangements the serpentine flow channels can be made longer and

may contain more medial legs than conventional serpentine flow channels. Low pressure

drops between adjacent legs also are claimed. In another patent [303] Rock presented a

serpentine flow field with a number of serially-linked serpentine segments extending

between inlet and outlet openings. Griffith [304] developed a serpentine flow design for

which each channel has an inlet, outlet, and at least one medial leg with hairpin curves

connecting the legs to each other. All legs extend in the same direction from the inlet to

the outlet with varying length. It was reported that this achieved better water

management. Rock [305] devised bipolar plates in which serpentine flow fields are

formed on one side and interdigitated flow fields on the opposite side. Furthermore, a

staggering seal arrangement was employed to direct gaseous reactant flow through the

stack in such a way that the seal thickness was maximized while the distance between

adjacent cells was minimized. A layered design was reported by Carlstrom [306] by

mating two interlocking layers that form an internal fluid channel between these two

37

layers, where a cooling fluid was circulated throughout the stack. One advantage of this

design is claimed to be its ability to be fabricated from a wide variety of materials,

including carbon, metals and composites. Boff and Turpin [307] believe that in most

cases, GDLs do not perform very well because the incoming gas appears to be unable to

access the whole area between the channels, but reaches only the area above the channel

and a small margin surrounding the channels. This is supported by the observation that

interdigitated channels achieve higher electrical efficiencies since the gas is forced into

the areas above the lands. It has been realized that by forming sufficiently fine channels

on the face of the flow fields (less than 0.2 mm in width) the aforementioned problem can

be curtailed without the use of separate GDLs. It also has been noted that narrow tracks

result in a reduction in resistive electrical losses.

In an interesting design, Trabold and Owejan [308] proposed a flow-field design in which

the channels include a number of side walls formed in different orientations. In addition,

an electrically-conductive member is incorporated at the surface of the plate to serve as

gas diffusion media. It is claimed that a better water transport is achieved. Rock et al.

[309] presented a serpentine flow design, where efficient thermal management is

obtained by creating coolant pathway between the two exposed faces of a single plate.

Mercuri [310] developed a fabrication method to manufacture graphite bipolar plates

from flexible graphite and preferably from uncured resin-impregnated graphite. Two

separate components; i.e., faces, one with a protrusion and another with a recession are

separately fabricated. The two pieces are then pressed together in such a way that the

protrusion of the first component is received in the recess of the second component and

finally heated to cure the resin and bond the components together.

There also have been significant activities in fabricating bipolar plates from a different

number of candidate materials with no winner to date. Metallic plates have been

fabricated and tested for their corrosion resistance and interfacial contact resistance

properties [311-316]. Composite materials also have been widely used in both fuel cells

and electrolyzers [317-324].

38

2.0 FUEL CELL THERMODYNAMICS

3.1 Introduction

Classical thermodynamics deals with transformation of energy from one form to another.

Since fuel cells convert the chemical energy of a fuel into electrical energy, it would be

beneficial to understand how such conversions take place. Thermodynamics can predict if

a particular reaction is energetically spontaneous. In addition, it will place an upper limit

on the quantity of the electrical energy that may be generated through the half-cell

reactions inside a fuel cell. This is referred to as ―ideal‖ work output and it should be

realized that real fuel cells operate below such limits due to a number of losses associated

with various cell components. These losses will be discussed in more detail in subsequent

chapters.

3.2 Reversible Cell Voltage Under Non-Standard Conditions

3.2.1 Introduction

It is often necessary to predict the reversible voltage of a fuel cell under non-standard

conditions. Fuel cells are usually run at temperatures higher than room temperature; even

PEM fuel cells are generally operated at 60 °C or higher to increase their efficiency.

Furthermore, it is often a requirement to pressurize the incoming gases (fuel and oxidant)

to increase the efficiency of the fuel cell. Lastly, the concentration (activity) of reactant

species can vary. It is important to be able to predict the voltage of a fuel cell at any

arbitrary temperature, pressure and fuel and oxidant concentrations. In the following

section, the influences of the above variables on the cell voltage will be explored.

3.2.2 Reversible Cell Voltage as a Function of Temperature

The cell voltage can be expressed in terms of Gibbs free energy formation at constant

temperature and pressure, ΔGT,P:

nF

GE

PT , (3-1)

The variation in cell voltage as a function of temperature at constant pressure can be

expressed as:

39

nF

S

dT

dE

P

(3-2)

where ΔS is the entropy change for the reaction, and

)()( TnF

SETE

(3-3)

For the reaction: H2 (g) + ½O2 (g) H2O (liq) (3-4)

The entropy change at 25 °C and one bar pressure is given by

)(2)(2)(2 21

ggliq OSHSOHSS (3-5)

= 69.91 – 130.684 – ½ [205.138]

= -163.343 J K-1

mol-1

This yields: 14104645.8)96487)(2(

343.163

KV

dT

dE

P

(3-6)

Over the narrow temperature range of 0 °C to 100 °C, ΔS will not vary significantly;

therefore the reversible voltage for the reaction will decrease with increasing temperature

by about 8.46 mV K-1

. When the reaction proceeds to water in the vapour state,

11

)(2 825.188 molKJOHS v

and .10303.2 14

KV

dT

dE

P

Table 3-1 shows values of E° at one bar pressure and various temperatures.

Table 3-1 Influence of temperature on reversible cell voltage at 1.0 bar pressure

Temperature (°C) Reversible Cell Voltage (V) Reversible Cell Voltage (V)

Liquid Water Water Vapour

25 1.229 1.184

40 1.216 1.181

50 1.208 1.179

80 1.182 1.172

100 1.165 1.167

40

Since S for a fuel cells is usually negative, the reversible voltage of a hydrogen fuel cell

will decrease as the cell temperature increases. Thus, for a reversible hydrogen-oxygen

fuel cell with water vapour as the by-product, there will be an approximate 23 mV

decrease in cell voltage for every 100 degree increase in cell temperature [113]. This

may not have a significant impact on PEM fuel cells, but will certainly manifest itself in

high-temperature fuel cells. This, however, does not mean that fuel cells must be operated

at low temperatures to increase their efficiency. Fuel cell kinetics dictates higher

temperature, since kinetic losses usually decrease with increasing temperature.

3.2.3 Reversible Cell Voltage as a Function of Pressure

Reactant pressure and concentration also can influence the reversible voltage of a fuel

cell. The pressure effects on reversible cell voltage are discussed here and concentration

impacts will be discussed in a later section.

Consider reaction (3-4), from thermodynamics

2222 2

1

2

1OHOHliq

T

vvvvv

VdP

G

(3-7)

Assuming ideal gas behaviour and :22 OH PP

P

TR

P

TR

P

TR

dP

G

OHT

5.1

2

1

22 (3-8)

but: EFnG PT , (3-9)

therefore: P

TRV

P

EFn

P

G

5.1 (3-10)

and: PFn

TR

P

E

5.1 (3-11)

41

Integrating equation (3-11) from standard pressure (P° = 1.0 bar) to an arbitrary pressure,

P, while keeping temperature constant yields,

P

P

Fn

TREET ln

5.1 (3-12)

The influence of pressure, like temperature, on reversible cell voltage is minimal. For a

PEM fuel cell operating at 25 °C, the reversible cell voltage as a function of pressure, for

both liquid water and water vapour as products, is shown in Table 3-2 and Figure 3-1.

Table 3-2 Influence of pressure on reversible cell voltage at a fixed temperature of 25 °C

Pressure (bar) Reversible Cell Potential (V) Reversible Cell Potential (V)

Liquid Water Water Vapour

1.0 1.229 1.185

2.0 1.242 1.189

3.0 1.250 1.192

4.0 1.256 1.194

5.0 1.260 1.195

6.0 1.264 1.196

7.0 1.266 1.197

8.0 1.269 1.198

9.0 1.271 1.199

10.0 1.273 1.200

When liquid water is the product of the above PEM fuel cell, increasing the pressure of

the incoming gases from 1.0 bar to 5.0 bar and then to 10.0 bar increases the reversible

cell potential by only 2.5% and 3.6%, respectively. The influence of the pressure is

clearly minimal. In practice, high gas pressures may not be desirable owing to

mechanical issues. However, the pressure effects on real fuel cells can be significantly

higher than those predicted by thermodynamics, owing to enhanced kinetics and mass

transport [112].

42

1.18

1.19

1.20

1.21

1.22

1.23

1.24

1.25

1.26

1.27

1.28

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Gas Pressure (atm)

Revers

ible

Cell

Po

ten

tial

(V)

Liquid Water

Water Vapour

Figure 3-1 Reversible cell potential as a function of pressure for PEMFCs at 25 °C

3.2.4 Reversible Cell Voltage as a Function of Concentration

Pure hydrogen and oxygen are seldom used in practice. Hydrogen fuel is usually

extracted from hydrocarbon fuels via reformation or partial oxidation processes.

Accordingly, the fuel stream consists of a mixture of hydrogen (about 50% - 70%), inerts

(water vapour, carbon dioxide, etc.), and active species (carbon monoxide).

The oxidant is generally taken from air, hence contains a large number of both inert and

active species. Even pure hydrogen and oxygen need to be humidified before entering the

fuel cell to ensure that the solid electrolyte membrane is adequately hydrated. Chemically

inert species such as water vapour influence the reversible cell voltage of PEMFCs by

lowering the concentrations of both fuel and oxidant, which, in turn, increase mass

transport resistance to the active sites on both the anode and cathode. Chemically active

species such as carbon monoxide have a more marked impact on the reversible cell

potential, the most important of which is poisoning of the noble metal catalyst.

43

Applying the Nernst equation to the PEM fuel cell reaction gives:

2

21

2

2ln2 OH

OH

aa

a

F

TREE

(3-13)

At the relatively low pressures found in PEM fuel cells the gas activities can be replaced

with their partial pressures. Furthermore, if the fuel cell is operated below 100 °C so that

liquid water is generated, the activity of water may be assumed to be close to unity.

Thus 2/1

22ln

2OH PP

F

TREE

(3-14)

The above expression confirms that pressurizing the incoming gases will result in a slight

increase in the reversible cell voltage. Compared with pure gases at 1.0 bar pressure, with

fuel and oxidant streams of 0.5 and 0.21 (mole fraction), respectively, the reversible

voltage will only decrease by 0.0189 V. This reveals that losses due to other

irreversibilities are much greater. Such irreversibilities include slow oxygen reduction at

the cathode and slow rates of mass and charge transfer. These irreversibilities are

discussed in the next section.

3.3 Fuel Cell Efficiency

The maximum thermal efficiency ε of a heat engine is defined as the maximum amount

of work that it can deliver with the thermal energy supplied to the engine. For a

combustion engine, this is simply the fraction of the enthalpy of combustion of the fuel

that can be delivered as work:

HI

LO

combust

net

in

net

T

T

H

W

Q

W

1max (3-15)

where THI is the temperature at which the combustion takes place and TLO is the

temperature of the heat sink (usually that of the surroundings).

Fuel cells, on the other hand, operate at constant temperature and both reactants and

products can enter and exit the system at similar temperatures. The following expression

is based on the fact that fuel cells are not subject to the above Carnot cycle limitation:

44

Thus combust

PT

H

G

,

fuel theof combustion ofenthalpy

producedenergy electrical (3-16)

Equation (3-16) is often referred to as the thermodynamic efficiency. It is also helpful to

express the efficiency of a fuel cell in terms of its output voltage. The voltage efficiency,

εv, of a fuel cell is largely determined by the cell voltage, Vc, divided by the reversible

OCV:

%100E

VC

v (3-17)

However, not all the fuel fed to a fuel cell participates in the electrochemical reaction

taking place at the anode. Some fuel will simply pass through the membrane and react

with the oxygen on the other side or leaves the cell unreacted. This lowers the overall

efficiency of the cell, as a result, a coefficient, known as the fuel utilization coefficient, εf,

is often introduced to account for such losses. This is just the fraction of the fuel that

reacts to generate electrical output. Thus the overall energy conversion efficiency of the

fuel cell is given by:

fv

combust

PT

H

G

, (3-18)

Although the ideal (maximum) efficiency of a fuel cell depends on thermodynamics, the

real (actual) efficiency depends on electrode kinetics. Electrode kinetics and fuel cell

electrochemistry are briefly discussed in the next section to present a simple overview of

electrode kinetics and further elucidate the problem of efficiency as related to

electrochemical conversion devices such as fuel cells.

45

3.0 FUEL CELL ELECTROCHEMISTRY

3.1 Introduction

A brief overview of fuel cell thermodynamics was presented in the preceding section.

Thermodynamic analysis provides us with necessary information to predict the

performance of a particular fuel cell based on a set of variables and state functions. The

main goal of thermodynamic calculations is to determine whether a particular process

proceeds to completion without help from external means; i.e., predicts the spontaneity of

a particular process. However, it cannot predict the rate of the electrochemical reactions

taking place on the surface of the electrodes or the mechanism of the reactions occurring

inside the cell. Furthermore, thermodynamic analysis is unable to tell us how much

energy is lost under real conditions; i.e., irreversible losses. To deal with these aspects we

must examine the electrochemical kinetics of the process.

4.2 Electrode Kinetics

Electrochemical reactions are defined as chemical reactions in which electrons are

transferred between an electrode—a metal or a semiconductor—and an electrolyte—a

solid polymer electrolyte, a molten salt or an aqueous electrolyte solution [331]. It is

apparent that both a transfer of electrical charge and a change in Gibbs free energy

accompany all electrochemical reactions. The primary function of electrode kinetics is to

investigate the sequence of partial reactions taking place on the surface of electrodes and

explain the mechanism of such reactions to determine the overall electrode reaction. This

information is then utilized to determine the rate-determining steps for the overall

electrode reaction. The overall rate of the reaction is evaluated by the rate of the slowest

step—the rate-determining step (RDS) [331].

The general half-cell reactions can be expressed as follows:

RedneOx (4-1)

The forward reaction presents a reduction reaction, where reactant ―Ox‖ undergoes a

reduction process by gaining ―n‖ electrons to form ―Red‖. For the opposite direction,

reactant ―Red‖ participates in an oxidation reaction and loses ―n‖ electrons to from ―Ox‖.

At equilibrium both processes take place at an equal rate resulting in no net electrode

46

current. In other words, for an electrode-electrolyte system at equilibrium, the rate of

electron generation equals the rate of electron consumption. When only one direction of

equation (4-1) is considered, the rate of electron consumption or generation—the current

that is produced—can be expressed as follows [330]:

jFAnI (4-2)

where I ≡ current in amperes

n ≡ number of electrons transferred

A ≡ the active area of the electrode in cm2

F ≡ Faraday’s constant (96487 C mol-1

e-)

j ≡ flux of reactants reaching the surface of the electrode in mol s-1

cm-2

Equation (4-2) can be expressed in terms of current density, i (A cm-2

), which makes

comparison of different electrodes easier and more manageable by eliminating the need

to account for the surface area of each electrode.

jFni (4-3)

The current that is produced as a result of the electrode processes can be determined by

the rate of the reactant conversion at the surface of the electrode, which, in turn, is a

function of surface concentration of the reactant(s). Considering equation (4-1), for the

reduction process (forward reaction), the flux is given by [330]:

sff Oxkj ][ (4-4)

Similarly, for the oxidation half reaction (backward reaction) the flux is:

sbb Redkj ][ (4-5)

where jf ≡ forward reaction flux in mol s-1

cm-2

jb ≡ backward reaction flux in mol s-1

cm-2

kf ≡ forward rate coefficient in L s-1

cm-2

kb ≡ backward rate coefficient in L s-1

cm-2

[—]s ≡ reactant concentration at surface in mol L-1

The net current density is the difference between the forward current density and the

backward current density:

47

)][Re][( sbsf dkFOxkFni (4-6)

As mentioned above, at equilibrium, the rate of forward reaction equals the rate of

backward reaction; accordingly, the net current is zero since the reaction will proceed at

the same rate in both directions. This rate, expressed as a current density, is known as

exchange current density, i₀.

4.3 The Butler-Volmer Equation

For an electrochemical process to proceed there must be a charge transfer across the

electrode/electrolyte interface. In moving from the electrolyte to the electrode or vice

versa, the charge must overcome an activation energy barrier [154, 330], the magnitude

of which is directly proportional to the change in Gibbs free energy of the products and

the reactants. The rate constant for each charge transfer process (in s-1

) can be written as:

TR

G

h

Tkk B exp (4-7)

where ΔG# = the activation energy barrier in J mol

-1

kB = Boltzmann’s constant in J K-1

h = Planck’s constant in J s

T = Temperature in K

Since electrochemical reactions involve the transfer of charge and are accompanied by a

change in the energy state of the system, the Gibbs activation energy may be thought of

as containing both electrical and chemical components. For the cathodic direction:

FnGG C (4-8)

For the anodic direction:

FnGG C )1( (4-9)

where

CG chemical component (subscript ―c‖ indicates the chemical

component)

potential difference component

charge transfer coefficient

48

Thus the heterogeneous rate constant for the forward direction, kf, can be expressed by

substituting equation (4-8) into (4-7) to give:

TR

Fn

TR

G

h

Tkk

fcBf

expexp

, (4-10)

The overvoltage or overpotential (also known as polarization) for an electrochemical

reaction is defined as:

revEE (4-11)

where E is the electrode potential and Erev is the reversible or equilibrium voltage for the

reaction. It is also customary to define the overvoltage as:

rev (4-12)

For a fuel cell, by convention, the overvoltage of the anode is positive, while that for the

cathode is negative. Equation (4-10) can be expressed for each direction in terms of the

overvoltage to give:

TR

Fn

TR

Fn

TR

G

h

Tkk revfcB

f

expexpexp

, (4-13)

TR

Fn

TR

Fn

TR

G

h

Tkk revbcB

b

]1[exp

]1[expexp

,(4-14)

All the terms in equations (4-13) and (4-14) with the exception of the last term, can be

gathered into a constant term k₀:

TR

Fnkk ff

exp,0 (4-15)

TR

Fnkk bb

]1[exp,0 (4-16)

Substitution of (4-15) and (4-16) into (4-6) gives the net electrode current density as a

function of the electrode overvoltage:

49

TR

FnkdFn

TR

FnkOxFni bsfs

]1[exp][Reexp][ ,0,0 (4-17)

The exchange current density, i0, was defined in section 4.2; a more comprehensive form

of exchange current density can be derived from equation (4-17) by noting that both the

overvoltage and external current are zero when the electrode is in equilibrium [330]:

0,00,00 ][Re][ ikdFnkOxFn bf (4-18)

An expression that relates overvoltage to current density can be derived by substituting

equation (4-18) into equation (4-17):

TR

Fn

TR

Fnii

]1[expexp0 (4-19)

The above equation is known as the Butler-Volmer equation and is a general expression

for an electrochemical reaction incorporating both reduction (left expression) and

oxidation (right expression) components. In an operating fuel cell, if the overvoltage of

an electrode is significantly positive, then the oxidation term (right side) becomes bigger,

while the reduction term (left side) becomes smaller resulting in a net current density that

is negative. This corresponds to an oxidation reaction, where electrons leave the

electrode, similar to the oxidation of hydrogen at the anode in PEM fuel cells. If the

overvoltage is, however, significantly negative the reduction term in equation (4-19)

becomes dominant, as is the case with the reduction of oxygen at the cathode in a PEM

fuel cell [330].

4.4 Overvoltage and Current Density

An important relationship between overvoltage and current density can be established by

examining the limiting form of the Butler-Volmer equation when the overvoltage is very

small. Consider equation (4-19), when the overvoltage is very small; i.e., 1TR

F, and

generally around 0.01 V or smaller, the expression can be expanded and written as [332]:

50

TR

FiTRFTRFii

0

0 ])/(1[/)1(1 (4-20)

This is based on the expansion of the exponential using xe x 1

Equation (4-20) shows that at low overvoltage the current density is directly proportional

to the overvoltage. Under these conditions, the interface between the electrode and the

electrolyte behaves similar to an ohmic conductor. If the overvoltage is slightly positive,

an anodic (positive) current will be generated, and when the overvoltage is small and

negative, a cathodic (negative) current will be generated. That is:

0i

iF

TR

(4-21)

Another limiting form of the Butler-Volmer equation arises when the overvoltage is

large—larger than 0.1 V. When the overvoltage is positive and greater than 0.1 V, the

first term in the Butler-Volmer equation becomes negligible so that:

TR

Fnii

)1(exp0 (4-22)

Similarly when the overvoltage is more negative than 0.1 V, the second term of equation

(4-19) becomes negligible:

TR

Fnii

exp0 (4-23)

And iFn

TRi

Fn

TRlnln

(4-24)

51

4.5 Fuel Cell Losses

4.5.1 Introduction

The ideal cell voltage for a PEM fuel cell can readily be evaluated. Under standard

conditions with water as the byproduct, at 25 °C the ideal cell voltage for a PEM fuel cell

is 1.229 V. In practice, however, the real cell voltage is usually considerably less than

this, even at open-circuit voltage, when no net current flows. This is attributed to a

number of irreversibilities associated with all types of fuel cells. The typical performance

of a hydrogen fuel cell—a polarization curve—operated under standard conditions is

shown in Figure 4-1, where three distinct regions can be observed.

It should be noted that even at the open-circuit voltage the actual cell voltage is less than

the reversible value. In addition to the three distinctive regions where voltage losses are

manifested, fuel crossover also can lower the actual cell voltage. The causes of voltage

losses for each of these regions are discussed and ways to minimize them are presented

below.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Current Density (mA cm-2

)

Ce

ll V

olt

ag

e (

V)

Ideal Voltage = 1.229 V at STD

Ohmic Polarization Concentration

Polarization

Activation

Polarization

Figure 4-1 A typical performance curve for a hydrogen fuel cell operated at 1.0 bar

pressure and 25 °C

Reversible Cell Voltage = 1.229 V at STP

52

4.5.2 Activation Overvoltage

The first part of a polarization curve for hydrogen fuel cells operating at temperatures

lower than 100 °C shows a sharp drop in cell voltage as it begins to move away from the

open circuit voltage. This drop is highly non-linear and is attributed to the slowness of

the charge transfer reactions occurring on the surface of both anode and cathode. It is

understood that for electrons to be transferred to or from an electrode in electrochemical

systems, a driving force must exist to attract or repel electrons through the external

circuit. This motive force is provided by a portion of the voltage that is initially generated

within the cell as it moves away from equilibrium. As a result, this portion of the voltage

will not be available to do useful work. In this region the difference between the open

circuit voltage and the actual voltage is called activation overvoltage or activation

polarization, act [328]:

0

logi

iaact (4-25)

Upon a closer examination, it becomes apparent that this is just another form of the Tafel

equation. It is also common practice to present the above equation in terms of natural

logarithms:

0

lni

iAact (4-26)

It is noted that the activation overvoltage can be lowered either by decreasing the

constant, A, or by increasing the exchange current density, i0. Accordingly, the exchange

current density is higher for faster reactions manifesting a more active electrode surface

while a smaller exchange current density translates into a slower reaction and a less

active electrode surface. It is, therefore, desirable to increase i0 at both the anode and the

cathode. This is especially critical for the cathode of a fuel cell where oxygen reduction

takes place and the exchange current density is much smaller compared with that of the

anode where hydrogen oxidation occurs. The difference in exchange current density can

sometimes be as high as 5 orders of magnitude. Consequently, activation overvoltage at

the anode usually can be ignored. For instance, for hydrogen fuel cells operated at low

temperatures, typical values for the exchange current density at the anode and the cathode

53

are 200 mA cm-2

and 0.1 mA cm-2

, respectively. A complete expression for the activation

overvoltage of a hydrogen fuel cell can be derived by taking into account both the anode

and the cathode overvoltages:

c

c

a

aacti

iA

i

iA

,0,0

lnln (4-27)

This can be rearranged and expressed as [328]:

b

iAact ln (4-28)

where A = Aa + Ac and )()( ,0,0A

A

cA

A

a

ca

iib

The constant A is found to be a function of temperature and a constant known as the

charge transfer coefficient, . Charge transfer coefficient values range from 0.0 to 1.0

with a value of about 0.5 for the anode and 0.1 to 0.5 for the cathode of a hydrogen fuel

cell. The constant A is known to be a function of temperature and electrode material. A

simple relationship between constant A, temperature and charge transfer coefficient has

been reported in the literature [328]:

F

TRA

2

(4-29)

It should be noted that the Tafel equation can be expressed in terms of current density

instead of overvoltage.

Aii act

exp0 (4-30)

Substituting equation (4-29) into equation (4-30) yields:

TR

Fii act2

exp0 (4-31)

The above equation is another way of expressing the Bulter-Volmer equation.

For a PEM fuel cell with no losses other than activation polarization at both anode and

cathode, the real cell voltage is given by:

54

c

c

a

ai

iA

i

iAEE

,0,0

lnln (4-32)

or simply,

actEE (4-33)

As mentioned previously, there are two ways of lowering the activation overvoltage for a

hydrogen fuel cell, and thereby increasing the cell voltage. First approach involves

lowering the constant A, while the second approach requires increasing the exchange

current density. The latter can be attained by increasing the cell temperature, utilizing

better catalysts, increasing reactant pressure, maximizing the true surface area of the

electrodes, and increasing reactant concentration.

4.5.3 Ohmic Overvoltage

The second region of a polarization curve for a low-temperature hydrogen fuel cell shows

a linear drop in cell voltage as more current is drawn. This is the simplest cause of

potential loss in hydrogen fuel cells and arises primarily because of the ohmic resistance

to the flow of electrons and ions. Electronic resistances appear in the catalyst and gas

diffusion layers, bipolar plates, and cell interconnects, while ionic resistance manifests

itself primarily in the electrolyte; i.e., Nafion®

. Ohm’s law can be used to approximate

this relationship; however, in order to compare the results with other losses, the equation

must be expressed in terms of current density. Starting with Ohm’s law:

RIV (4-34)

In equation (4-34), the current, I, in amperes, is readily converted to current density, i, in

mA cm-2

, but the resistance, R, has to be expressed in terms of resistance per unit cell

surface area; i.e., per cm2. This quantity is known as the area-specific resistance (ASR)

and is represented by the symbol r given in kΩ cm2. Equation (4-34) can be expressed in

terms of this new quantity:

riohmic (4-35)

The most effective way to reduce ohmic overvoltage is to make the membrane

electrolyte, the electrodes, and the MEAs as thin as possible. Making the electrolyte too

55

thin, however, may permit fuel crossover to become an issue. Furthermore, the integrity

of the whole MEA can become compromised if it is not thick enough to prevent short-

circuiting with the adjacent electrode. Equally important is the incorporation of a good

design and use of appropriate materials for the fabrication of the bipolar plates and other

cell interconnects.

4.5.4 Concentration Overvoltage

At the anode it is evident that the voltage depends on the partial pressure of the hydrogen

gas in the fuel mixture, and, as the hydrogen partial pressure decreases, the cell potential

also will decrease. Even using pure hydrogen as fuel, a pressure drop will exist between

the consumption site at the three-phase interface and the supply container, the magnitude

of which depends on the current being drawn from the cell and the design of the gas

delivery system [328].

A similar problem exists at the cathode. Although it is a well-established fact that

reduction in fuel and oxidant pressures will lead to lower cell voltages, it is not easy to

model such changes with certainty. A simple equation commonly used to express the

relationship between concentration polarization and reactant pressure is:

1

2lnP

P

Fn

TRconc (4-36)

where n is the number of electrons per mole of the reactant for the half-cell reaction

under investigation (2 for hydrogen oxidation and 4 for oxygen reduction in PEM fuel

cells), P1 is the pressure at zero current density, and P2 is the pressure at any current

density. Unlike the previous expressions for activation and ohmic polarizations, equation

(4-36) must be used with caution, since it is only approximate. This has been confirmed

by comparison with experimental values [328]. An empirical expression that is often

used in the fuel cell literature is:

ni

conc em (4-37)

m and n are fitted empirical constants. In practice, m and n are often taken as 3 × 10-5

V

and 8 × 10-3

cm2 m A

-1, respectively [328].

56

4.5.5 Mixed Potential at Electrodes

This type of voltage loss arises predominantly by the occurrence of undesired reactions

both at the cathode and the anode resulting in a lowering of the equilibrium potential. The

primary source of such losses is the crossover of the fuel through the electrolyte from the

anode to the cathode resulting in direct mixing with the oxidant and subsequent parasitic

consumption of reactants.

Electrolytes in general and solid polymer electrolytes in particular, are selected based on

their ionic conductivities, their inability to transport electrons, and their impermeability to

hydrogen fuel and oxidants. Selecting SPEs of higher thickness alleviates the fuel

crossover problem at the expense of ionic conductivity. Fortunately such flows of

electrons and fuels are often insignificant and can be ignored in PEM fuel cells. Such

losses are not easy to quantify, but can be measured indirectly by determining the rate of

fuel use at the anode or of oxidant at the cathode. The relationship between hydrogen use,

G in mol s-1

, and the generated current as a result of parasitic consumption is:

FGI 2 (4-38)

The small current, ir that would result from this parasitic reaction can be added to

equation (4-26) to take into account the above losses [328]:

0

lni

iiA r

act (4-39)

57

4.0 CATALYST LOADING

5.1 Introduction

The MEA of a hydrogen fuel cell is considered to be its most critical component. It

consists of two pretreated carbon substrates—either paper or cloth—known as gas

diffusion layers (GDLs), a membrane to separate the anode and the cathode, and two

catalyst layers, each strategically placed between the last two layers (see Figure 5-1). The

roles of both solid polymer electrolyte membranes and GDLs in PEM fuel cells were

discussed earlier (see sections 2.2.1 and 2.2.3). The catalyst layer is, by far, the most-

studied component of an MEA. It is the locus of all electrochemical reactions taking

place inside the cell and its primary function is to facilitate these reactions both at the

cathode and the anode. This is of special importance for the cathodic oxygen reduction

reaction (ORR), the rate of which can be lower by several orders of magnitude than that

of the anodic hydrogen oxidation reaction. One of the major requirements for the

cathodic reduction of oxygen in hydrogen fuel cells is that the reaction proceeds at low

overvoltages. Platinum and platinum-group metals (PGMs) satisfy this requirement, and,

in addition, are stable in the acidic environment of the fuel cell. However, they are

prohibitively expensive, and significant reduction in their loading without compromising

cell performance is required to ensure their feasibility in low-temperature fuel cells.

Figure 5-1 A simple schematic of a five-layer membrane-electrode assembly

Carbon

Substrate

Carbon

Substrate

Solid

Polymer

Electrolyte

Anode

Catalyst

Layer

Cathode

Catalyst

Layer

58

Electrocatalysts were originally made from pure platinum black crystallites with average

particle diameters in the range of 10 – 20 nm. The effective surface area of such

electrocatalysts is relatively small, owing to their large diameters, resulting in high

loadings to deliver an acceptable cell performance. By the late 1980s and early 1990s, a

ten-fold reduction in noble catalyst loading had been achieved, and the performance of

such cells was comparable, and in some cases, even better than, PEMFCs with higher

catalyst loadings [335]. Two innovations played critical roles in achieving this; first was

the replacement of platinum black with platinum supported on high surface-area carbon.

This not only created platinum catalysts that were smaller in size—2-5 nm diameter—but

also created a viable and extensive pathway for electrons to move away from the catalyst

layer, where they are generated, towards the external circuit, where they perform useful

work. Raistrick et al. [340] replaced platinum black with 10% carbon-supported platinum

particles in the range of 2-3 nm in diameter by preparing a catalyst ink containing carbon-

supported platinum, polytetrafluoroethylene, and solubilized Nafion®. Other workers,

including Ticianelli et al. [341] and Gottesfeld and Zawodzinski [342], further improved

cell performance by increasing the amount of Pt/C from 10% to 20% and optimizing the

amount of solubilized Nafion® in the catalyst ink. Second, it was realized that in order to

extend the three-phase interface, SPE must be able to penetrate inside the substrate, even

into the micropores, to come into full contact with the electrocatalyst and gas phases

during operation. Accordingly, a solubilized form of a SPE (e.g., 5% Nafion® in an

alcoholic solution) was impregnated into the catalyst side of a GDL. This had a positive

impact on cell performance, primarily by extending the three phase interface. However,

not all the methods commonly used today use ionomer impregnation.

5.2 Catalyst Layer Application Methods

Catalysts can be applied directly onto either two GDLs (e.g., carbon substrates) followed

by hot pressing of a SPE in the middle, or they can be loaded onto both sides of a solid

polymer electrolyte and then sandwiched between two GDLs. Regardless of the addition

sequence, there are several methods that are used for catalyst loading. These are grouped

and presented in Figure 5-2.

59

5.2.1 Application of Catalyst to Gas Diffusion Layers

There are many methods for catalyst application, but seven different techniques are

widely used to apply a thin layer of catalyst on carbon paper or cloth GDLs. Most of

these methods employ a two-step process to deposit noble metals onto the surface of a

GDL, consisting of a catalyst ink preparation step followed by an application phase.

However, a single sputtering process also can be used to perform this task with

comparable results. These methods are briefly discussed in the following subsections.

Figure 5-2 Catalyst loading methods

Catalyst Loading

Application to SPE Application to GDL

Spreading

Spraying

Painting

Sputtering

Powder Deposition

Ionomer Impregnation

Electrodeposition

DC Electrodeposition

Pulse Electrodeposition

Painting

Dry Spraying

Sputtering

Decaling

Evaporative Deposition

Impregnation-Reduction

60

5.2.1.1 Application of Catalyst by Spreading

Catalyst deposition by spreading is one of the earliest methods utilized to provide a

relatively consistent layer on various GDLs. The first step involves the preparation of a

catalyst ink that generally consists of carbon powder, PTFE and catalyst. The above ink is

mixed using mechanical agitators, and often ultrasonic mixers, to ensure a well-mixed

dough. The above catalyst ink then is applied onto a pre-treated carbon substrate using a

heavy metallic cylindrical roller on a flat surface.

Srinivasan et al. [343] utilized this method and reported a thin and uniform catalyst layer

on wet-proofed carbon cloth. With this technique the catalyst loading is directly

proportional to the thickness of the catalyst layer. Consequently, the amount of platinum

per unit area can readily be controlled during deposition by monitoring the catalyst layer.

Higher loadings can be easily obtained by repeating the second step (application phase).

This simple control comes at the expense of catalyst particle size (bigger particles result),

which can adversely influence cell performance, since increasing catalyst size decreases

catalyst activity. This becomes noticeable when the catalyst-to-carbon ratio increases to

40 wt% or greater. Catalyst layer cracking is another shortfall associated with this

method; however, several workers [344-346] have reported catalyst layers having cracks

whose area is less than 10% of the total area of the catalyst layer, and even crack-free

catalyst layers with acceptable performance. Ueyama [344] examined the impact of

several factors—including the thickness of the catalyst layer, the type of carbon support,

and the drying rate of the solvent used for making the catalyst ink—on the properties of

the catalyst layer in general, and catalyst consistency and freedom from cracks in

particular. In another patent, Sompalli et al. [345] presented an invention in which

significant reductions in ―mud-cracking‖ of catalyst layers is obtained either by pre-

treating the substrate with a wetting solvent prior to MEA fabrication or by utilizing a

post-treatment approach, in which the catalyst ink contains a solvent that is wetting to the

substrate. But, upon drying of the catalyst layer, it then is coated with a solution of an

ionomer and a solvent that is non-wetting to the substrate. It is claimed that both methods

control the drying rate to form a more uniform and robust electrode by preventing

electrode shrinkage and subsequent cracking of the catalyst layer. Iwasaki et al. [346]

61

proposed a method in which it is claimed that catalyst layers with less than 10% cracking

(compared with the whole catalyst layer area) can be formed. According to their method

a substrate—carbon paper or cloth—is coated with a slurry containing only carbon

powder and PTFE. This is followed by drying the previous coat under pressure to form a

hydrophobic layer. Carbon powder, PTFE and a suitable catalyst then are mixed and the

electrode is coated with this new paste. The amount of catalyst loading is simply

controlled by carefully monitoring the catalyst layer thickness. The electrode is once

again pressurized to dry the last coating. They suggested heating the MEA to remove any

remaining solvent in the hydrophobic layer before using it in a fuel cell. Uchida et al.

[347] developed a method for fabricating MEAs by first creating a paste containing

carbon-supported platinum, Nafion® solution, and a solvent. This paste was then spread

on a pre-treated carbon electrode. A significant improvement in cell performance was

reported and attributed to an increase in contact between the catalyst particles and the

electrolyte in the MEA.

The spreading method is popular because of its simplicity and cost effectiveness.

However, it suffers from lack of adequate control in terms of catalyst particle size when

higher catalyst loadings are desired. For higher catalyst loadings it is often necessary to

apply several coatings, and, to avoid re-dissolving of catalyst ink components in the

previous layer, the electrode generally is sintered between each application.

5.2.1.2 Application of Catalyst by Spraying

In this method, similar to the previous one, catalyst ink is prepared by mixing carbon

powder, PTFE, water, alcohol and a catalyst. The main difference between this catalyst

ink and that presented in the previous method is the respective viscosity. In the spreading

method, a rather thick and viscous ink is desirable, while in the spraying method, due to

the nature of the application process, the catalyst ink must be thin with a low viscosity to

ensure an adequate flow.

The ink then is simply sprayed onto a wet-proofed substrate. The amount of catalyst

loading is optimized by controlling the thickness of the catalyst layer. Higher catalyst

loadings are attained by repeatedly spraying the ink onto the electrode. The electrode is

62

always sintered between each application to prevent various components of the previous

layer from re-dissolving in the next layer [335]. It is a common practice to roll the

electrode to produce a catalyst layer that is thin, homogenous and of uniform thickness.

In addition, rolling has been found to produce layers with low porosity. Srinivasan et al.

[343] proposed a method in which the electrolyte is suspended in a mixture containing

alcohol, water and PTFE. They reported comparable performance when compared with

MEAs fabricated via the spreading method. Tanaka et al. [348] developed a method for

fabricating multi-layered gas diffusion electrodes in which both layers—a water repellent

layer and an electrocatalyst layer—are formed via spraying. A wide range of catalyst

particle size (2 to 100 nm diameter) was reported.

The spraying method has gained popularity amongst some workers owing to its ease of

use, scalability, and minimal equipment requirements. However, similar to the spreading

method, at higher catalyst loadings the catalyst particle size increases, consequently

decreasing its activity and lowering cell performance.

5.2.1.3 Application of Catalyst by Painting

This is one of the oldest and most cost-effective methods to deposit a noble metal catalyst

onto an electrode. Similar to the spray method, the catalyst ink must be thin with low

viscosity to ensure that a uniform layer is applied to the electrode with each application.

An artist’s paintbrush generally is used to transfer the catalyst from the mixing container

onto the electrode. Electrodes with carbon substrates are always wet-proofed to control

the depth of the catalyst ink penetration inside the electrode. This method is not intended

for mass production of GDLs or substrates with large surface areas; however, although it

can produce MEAs with acceptable results in a lab setting, it does require extra care and

effort to ensure reproducibility. Catalyst loading is optimized simply by controlling the

catalyst layer thickness. And, as is the case with the first two methods, the electrode is

sintered between each application to avoid dissolution of the components of the previous

layer into the next layer. This method suffers from the same shortcomings of the first two

methods.

63

5.2.1.4 Application of Catalyst by Powder Deposition

In catalyst powder deposition, a mixture of carbon powder, carbon-supported platinum

and PTFE is vigorously mixed in a fast running knife mill under forced cooling [335].

The mixture then is applied onto a wet-proofed carbon paper, carbon cloth or other

suitable substrate. Bevers et al. [349] reported significant improvements in both gas and

water transport properties of MEAs fabricated using this method, which differs from the

previous two in terms of catalyst ink preparation. While both spreading and spraying rely

on mechanical and ultrasonic mixers, catalyst powder deposition takes advantage of a

mill to produce a well-mixed and homogeneous mixture. Although the need for special

equipment is somewhat greater than for the previously-discussed methods, the catalyst

application step has many similarities with both spraying and spreading. As a result, most

catalyst layers produced via this method exhibit similar properties to those of catalyst

layers prepared by spraying or spreading techniques.

5.2.1.5 Application of Catalyst by Ionomer Impregnation

In most methods discussed so far, a layer of solubilized electrolyte (e.g., Nafion®) is

applied to the catalyst side of the electrode after catalyzation. This is believed to extend

the three-phase interface by maximizing the contact points between the electrolyte and

the catalyst particles (and of course, the reactants when the cell is in operation). Although

this will create more contact points between the electrolyte and the catalyst, the extent of

these contact points depends upon the depth of penetration of the electrolyte into the

catalyst layer. Some of the micropores in the catalyst layer can be as small as 5-20 nm in

diameter and, as a result, ionomer molecules cannot reach them. Gottesfeld and

Zawodzinski [342] reported a significant improvement in cell performance when a

solubilized electrolyte is added to the catalyst ink prior to application rather than after

catalyzation. This will further increase the protonic access to the majority of the catalyst

sites not in intimate contact with the solid electrolyte after hot pressing. Ionomer

impregnation is particularly effective in liquid-fed fuel cells such as direct methanol fuel

cells (DMFCs).

64

Zhang et al. [350] reported a technique in which the catalyst side of each electrode of a

DMFC was impregnated with a 1% solution of Nafion® before and after catalyzation.

They claim that the initial ionomer impregnation covers a significant surface area of the

substrate with a thin layer of the ionomer, resulting in a reduction of pore sizes inside the

substrate. This can lower the tendency of electrocatalyst to penetrate too deeply into the

substrate, and consequently become isolated and inactive. If this holds true, then catalyst

particles will remain very close to the substrate-membrane interface, and the likelihood of

them becoming active increases significantly. The second ionomer impregnation—after

catalyzation—further brings the deposited catalyst particles into contact with the

electrolyte, and hence increases cell performance. However, one must carefully optimize

the amount of solubilized electrolyte loading since it tends to alter the wetability

characteristics of the substrate, as pointed out by the authors. It is also worth investigating

the influence of an additional ionomer layer between the GDL and the catalyst layer on

the transport of electrons from the three-phase interface to the outside circuit. It is well-

known that one of the selection criteria for electrolyte membranes in fuel cells is their

inability to conduct electrons. An additional layer of Nafion®, for instance, between the

GDL and the catalyst layer will hinder the transport of electrons, resulting in inferior

performance if not designed and applied properly.

In another patent [351] by the same inventors (reference 350), the anode of a fuel cell

containing a solid polymer electrolyte is first oxidized in a simple electrochemical cell

comprising the electrode (working electrode) and a counter electrode both immersed in

an appropriate, preferably acidic, aqueous solution. A d.c. power supply is connected

across the electrodes (positive terminal connected to the working electrode) and electric

current allowed to pass through the solution. It is reported that by passing 20-80 C cm-2

of charge for about 5 to 10 minutes at a cell voltage of 4.0 V, the working electrode can

effectively be oxidized. The treated substrate is then impregnated with about 0.3 mg cm-2

of 1% Nafion® solution in isopropanol. This is followed by catalyzation and MEA

fabrication. It is reported that the above MEA performed better than other MEAs—both

oxidatively treated and non-treated MEAs with no ionomer impregnation. However, no

improvement in cell performance is reported for oxidatively-treated MEAs compared

with conventional MEAs.

65

In yet another patent, Wilson [352], argues that it is difficult to obtain high ionomer

loadings to maximize the contact between the catalyst particles and the ionomer if an

ionomer impregnation method is utilized. He points out that the differential swelling

between the SPE and the catalyst layers—arising from differing hydration

characteristics—leads to delamination between these two layers. This can not only

interrupt the ionic path and consequently lower cell efficiency, but also compromise the

integrity of the cell or the whole stack. This subject area is covered in detail elsewhere

[353-355].

5.2.1.6 Application of Catalyst by Electrodeposition

Considerable attention has been given to electrodeposition of metals from both acidic and

non-acidic solutions, primarily because of ease of use and relatively low-cost

requirements. In its most basic form, a porous carbon electrode—either carbon paper or

cloth—is first impregnated by a liquid ionomer followed by electrodeposition of platinum

from a suitable platinum complex. It is claimed that this technique deposits platinum only

at sites where both electronic and ionic pathways are present [38, 105, 335], thereby

ensuring higher-than-normal catalyst utilization rates.

Reddy et al. [107] developed an electrochemical catalyzation technique in which

platinum was electrodeposited from a commercial platinum complex solution onto an

uncatalyzed carbon electrode previously impregnated with a Nafion® solution. Due to the

affinity of Nafion® for cations, platinum ions readily diffuse through the Nafion

® layer

and are electrodeposited only in places where both ionic and electronic pathways exist. A

detailed discussion on electrodeposition of metals in general, and noble metals in

particular, is given in section 5.3.

5.2.2 Application of Catalyst to Membrane

Catalysts also can be deposited onto solid polymer electrolytes and then bonded to two

GDLs—one on each side—prior to hot pressing. There are several methods that can be

used to accomplish this task, which are briefly discussed in the following sections.

66

5.2.2.1 Application of Catalyst by Painting

This is a two-step technique in which, similar to many other methods, a catalyst ink is

prepared, followed by applying the ink onto both sides of the SPE using an artist’s brush.

The platinum ink is prepared by carefully mixing predetermined amounts of carbon-

supported platinum and solubilized Nafion®. The ink is thoroughly mixed using both

mechanical and ultrasonic mixers. Gottesfeld and Wilson [36, 156] examined this

technique by painting a layer of this ink directly onto a dry membrane in the Na+ form.

The ink consisted of 20 wt% carbon-supported platinum, 5 wt% Nafion® solution, water

and glycerol. A three-to-one ratio for the carbon-supported platinum and Nafion® was

reported, and NaOH was added after the catalyst, Nafion® and water were well mixed to

avoid gelling of the solution. A thin layer of this ink then was painted onto both sides of a

dry solid Nafion®

membrane and baked at about 160-190 C. A certain amount of

distortion was reported due to the swelling effect of the solvents; consequently, it was

recommended—especially for thinner membranes or heavy-ink applications—that the

painted electrode be dried on a special heated vacuum table. The authors suggested

baking the painted electrodes at temperatures relatively lower than the final curing

temperature, followed by rapid heating to the desired temperature. It was claimed that

this minimized the amount of distortion and cracking that is often associated with high-

temperature baking.

The last step involved the conversion of the membrane from the Na+ form to the H

+ form.

This was accomplished by immersing the membrane in lightly boiling 0.1 M aqueous

sulfuric acid for about 2 hours, followed by successive rinsing in lightly boiling deionized

water. Finally, the sample was air-dried for several hours and two GDLs hot pressed onto

each side to form an MEA. This is a simple and cost-effective technique for deposition of

platinum onto membranes, but it has several drawbacks. Although the catalyst layer

exhibits a uniform catalyst concentration profile, it is difficult to control the catalyst

particle size at high catalyst-to-carbon ratios; i.e.; platinum-to-carbon ratios of more than

40 wt%. Furthermore, this method is not suited for mass production of MEAs, where

robustness and reproducibility are of great importance.

67

5.2.2.2 Application of Catalyst by Dry Spraying

This is another two-step technique in which a catalyst ink is first prepared and then

sprayed onto a dry solid electrolyte. The catalyst ink is prepared by careful mixing of

carbon-supported platinum, electrolyte membrane in powder form, PTFE and filler

materials in a knife mill. The mixture is then atomized and sprayed through a slit nozzle

directly onto a dried solid membrane [335, 356]. The catalyst layer is air-dried and hot-

pressed or hot-rolled to ensure adequate electronic and ionic contact.

This technique can produce catalyst layers as thin as 5 m, if the degree of atomization is

optimized [335]. The platinum-to-carbon ratio can readily be controlled by monitoring

the initial concentration of the platinum salt; and consequently, the level of platinum

loading can be optimized. As is the case with many powder type deposition techniques, a

uniform catalyst distribution inside the catalyst layer is easily achieved; however, it is

very difficult to keep the catalyst size under 5 nm in diameter when the initial platinum-

to-carbon ratio exceeds 40 wt%.

Benitez et al. [357] developed a two-step method based on an electrospray technique. The

first phase involved the preparation of a catalyst ink by thoroughly mixing carbon-

supported platinum (20 wt%), Nafion® solution (5 wt%) and different solvents, including

ethanol, glycerol and n-butyl acetate. Three different methods for catalytic ink dispersion

onto a carbon cloth substrate were used and compared: normal spray, impregnation and

electrospray. For deposition based on the electrospray technique, a high electric field was

applied between the catalyst ink and the substrate. The catalyst ink was forced to flow

inside a capillary tube in which a 3300 to 4000 V was applied between this tube and the

carbon substrate. A resulting mist of highly charged droplets emerged from the apparatus

and deposited onto the surface of the carbon paper. All the MEAs were characterized by

means of scanning electron microscopy with EDAX-system, X-ray photoelectron

spectroscopy (XPS), and X-ray diffraction (XRD). Morphological and structural

information were obtained for all MEAs and it was claimed that MEAs fabricated by this

technique exhibited three times higher power density than those fabricated by the

impregnation method and eight times higher than those prepared using a conventional

68

spray technique. It also was claimed that this method is suitable for large-scale operations

using low-cost processes.

5.2.2.3 Application of Catalyst by Sputtering

Most of the research conducted to date on reducing the amount of noble metal catalyst in

fuel cells has focused either on lowering the size of the catalyst particles [106, 155, 341,

358-362] or localizing the catalyst in the close proximity of the electrolyte, where both

ionic and electronic pathways exist [115, 363]. The latter can be achieved by sputter

depositing a thin layer of catalyst close to or on the membrane. Conventional electrodes

have platinum loadings of about 0.4 mg cm-2

, while a normal sputtered platinum film of

about 5 nm in thickness has a Pt loading of around 0.014 mg cm-2

[362]. To achieve good

cell performance, an MEA with sputter-deposited platinum must satisfy several

requirements. First, it needs to maximize the three-phase interface to enhance the half-

cell reactions taking place on both anode and cathode. Second, the layer must be as thin

as possible to minimize ohmic losses, maximize gas transport within the layer, and

optimize water removal capabilities of the cell. Last, the sputtered layer must strongly

adhere to the membrane to function properly and effectively and to prolong its useful life

[362].

Hirano et al. [106] studied the performance of several MEAs prepared by a sputter

deposition technique. The sputter deposition was carried out on uncatalyzed and wet-

proofed (50 wt%) E-TEK electrodes in an argon atmosphere and a low pressure of

210-2

Torr. Furthermore, an accelerating potential of 470 V, a plate-current of 500 mA,

and a sputter-deposition rate of 0.039 mg cm-2

were utilized. The electrocatalyst layers of

both anode (conventional E-TEK electrode) and cathode (sputter-deposited catalyst layer)

were impregnated with Nafion®

solution. It was claimed that MEAs with a sputtered

platinum layer of only 0.1 mg cm-2

performed as well as state-of-the-art MEAs, with the

exception of high current density regions, where they were outperformed by conventional

E-TEK electrodes.

69

O’Hayre et al. [362] demonstrated the viability of the direct sputter deposition technique

by depositing platinum on Nafion®

117. It was claimed that MEAs prepared by this

method and having a platinum thickness of only 5-10 µm had a power output that was

several orders of magnitude greater than MEAs fabricated using conventional methods of

platinum deposition. Membrane-electrode assemblies with platinum loadings of 0.04 mg

cm-2

fabricated by the above method were compared with conventional MEAs having

platinum loadings of around 0.4 mg cm-2

. The power output of the sputter-deposited

MEAs was reported to be about 60% of that obtained from conventional state-of-the-art

MEAs. The authors emphasized the importance of carrying out the experiments at high

temperature and humidity to ensure the superiority of sputtered-platinum layers, since

almost all lab trials are performed under ideal lab conditions.

Gruber et al. [364] examined the influence of sputter-deposited catalyst layers on two

different types of GDL—SIGRACET GDL-HM (SGL Technologies, SGL Carbon

Group) and uncatalyzed ELAT (E-TEK Div., De Nora). Platinum catalyst layers were

deposited from a Pt diode target at 50 W rf (radio frequency) power and a pressure of 2.0

Pa at room temperature. The sputtered catalyst layers were characterized by SEM, EDX

and XRD techniques, and their performance was evaluated in a single cell (2.25 cm2

electrode area) and compared with state-of-the-art E-TEK electrodes (ELAT electrodes

with 1.0 mg cm-2

Pt loading). It was reported that although the performance of electrodes

with sputter-deposited platinum was lower than that of catalyzed ELAT electrodes, the

loadings of the former were lower than the latter by about one order of magnitude. E-

TEK electrodes with a loading of only 0.005 mg cm-2

of sputtered platinum delivered 124

mW cm-2

, compared with 203 mW cm-2

for ELAT’s state-of-the-art electrodes with as

much as 0.107 mg cm-2

of platinum. It also was stated that the inclusion of a small

amount of chromium can improve cell performance, as can the addition of two ―ultra-thin

Cr layers into a 25 nm Pt catalyst layer‖ [362] with platinum loading of 0.054 mg cm-2

. A

maximum power density of 200 mW cm-2

was reported, but the amount of chromium that

was added to the catalyst layer was not disclosed.

In a somewhat different approach, Nakakubo et al. [365] proposed a method in which a

thin platinum layer was sputter deposited onto a 50-m-thick PTFE sheet in air at low

70

pressure. This layer then was transferred to a solid polymer electrolyte membrane

(Nafion® 112) by hot pressing. A thin layer of gold also was sputter deposited to function

as a conductive layer. The sputtering process was conducted under a vacuum pressure of

6 Pa, a discharge current of 15 mA, a working distance of 30 mm, and an operating

temperature of 15 C. The sputtering process was performed under a nitrogen or air

atmosphere with a sputter-deposition rate of 0.0077 mg cm-2

min-1

. An additional layer of

ionomer (0.0 to 2.5 wt% Nafion®) was applied to the surface of the catalyst layer to

increase the three-phase interface. The influence of sputter-deposition time on single cell

performance was reported to be the highest for samples with the longest deposition time

of 90 minutes (deposition times of 15, 30, 60 and 90 minutes were reported). Cyclic

voltammograms (CV) in addition to polarization curves were provided to substantiate the

above claim. The high performance of the above MEAs was attributed to the high

catalyst activity and sufficient mass transport of such MEAs due to their ultra thin porous

and continuous catalyst layers.

Cha et al. [366] developed a multiple plasma-sputtering technique in which several

ultrathin layers of the catalyst were applied as opposed to a single layer. In their study,

Nafion® 115 in Na

+ form was used as the substrate and the sputtering system utilized a

radio frequency (rf) or direct current power source to form the plasma flame, with a basic

vacuum pressure of 1.33 mPa or less. The delivered power for the rf power source was

reported to be 50 W, while that for the d.c. source was 30 W. After each sputtering, air

was used to dry the catalyst layer. To further improve catalyst utilization, two additional

steps were implemented. First, a 5 wt% solution of Nafion® was brushed on the sputtered

catalyst layer to enhance the contact points between catalyst particles and the ionomer.

Second, a mixture of Nafion® solution (5 wt%), carbon powder and isopropyl alcohol

was simply brushed onto the catalyst layer. The substrate/catalyst layer structure was air

dried for an hour before being converted to the H+ form. Toray carbon papers with an

average thickness of 0.17 mm were placed on each side of the membrane/catalyst

assembly without hot pressing. The authors reported catalyst utilization efficiencies ten

times higher than those obtained from electrodes fabricated by conventional methods. It

71

is worth mentioning that these assemblies are not true MEAs, the latter usually being

prepared by hot pressing.

As stated earlier and reported by several workers [115, 363], at high current densities, a

large fraction of the current is generated near the front surface of the electrode, where the

catalyst layer is in close contact with the SPE. To ensure high efficiency and greater cell

output at higher current densities, both mass transport and ohmic limitations must be

minimized. Hence, depositing the catalyst particles near the front surface of the electrode,

where intimate contact between catalyst particles and the ionomer is maximized, is highly

desired. One of the earliest studies was conducted and reported by Ticianelli et al. [363]

in which three different methods were employed to fabricate MEAs: use of a higher wt%

Pt/C in the carbon-supported catalyst, use of a thin sputtered catalyst layer on Prototech

electrodes, and a combination of the two. Significant improvements in power output for

all three methods were reported. For the first method, in which electrodes were prepared

with 20 and 40 wt% Pt/C rather than with the standard 10 wt% Pt/C, the increase in

performance was attributed to the reduction in the thickness of the active layer from

about 100 m in the conventional electrodes to around 50 m and 25 m for electrodes

with 20 and 40 wt% Pt/C, respectively. This reduction in thickness provides for a better

supply of reactant gases to the catalyst sites and lowers the ohmic overvoltage since the

path traveled by reactant gases is considerably shortened. The authors claim that a

combination of higher Pt/C loading (20 wt%) and an ultrathin sputtered film of platinum

on the surface of the electrode delivered the best results in terms of cell voltage. This was

explained in terms of a reduction in the catalyst layer thickness and a higher

concentration of electrocatalyst particles near the front surface of the electrode, where

they come in direct contact with the solid polymer electrolyte, ensuring an ionic pathway.

Electrodes with 40 wt% Pt/C plus a sputtered film of platinum with an approximate

thickness of 50 nm were reported to be inferior to similar electrodes with 20 wt% Pt/C in

the catalyst layer. This was attributed to the larger catalyst particles, and hence lower

effective platinum surface area, in the layer with 40 wt% Pt/C.

Huang et al. [367] examined the impact of input power and sputtering-gas pressure on the

performance of PEMFC electrodes. A radio frequency magnetron sputter deposition

72

process was employed to prepare electrodes for MEA fabrication with three input power

levels of 50, 100, and 150 W. It was reported that at a Pt loading of 0.1 mg cm-2

and a

sputtering-gas pressure of 0.001 Torr, the electrodes prepared at 100 W delivered the best

performance when compared with electrodes fabricated at 50 and 150 W. The authors

also report a marked increase in the Pt sputter deposition rate with increasing rf power at

0.001 Torr. Average Pt sputter deposition rates of 22, 39, and 50 nm min-1

(corresponding

to 0.047, 0.084, and 0.107 mg cm-2

min-1

) for the rf powers of 50, 100, and 150 W,

respectively, were reported. These findings have been confirmed by other researchers

[368, 369]. Kawamura et al. [368] reported a similar trend in which the Pt deposition rate

on a glass substrate was observed to increase from 25 to 41 nm min-1

when the rf power

was increased from 25 to 40 W. Chapman et al. [369] attributed this increase in Pt

deposition rate to an increase in the plasma density at elevated RF values, which arises

from an increase in the plasma if Ar+.

Despite producing MEAs with excellent performance in terms of cell output, this method

has not gained any more ground than other methods discussed earlier. The main reason

for this is the need for expensive equipment, as well as the inability of this method to

effectively deposit catalyst particles on the surface of electrodes with unconventional

shapes.

5.2.2.4 Application of Catalyst by Impregnation Reduction

In this method a solid polymer electrolyte membrane in Na+ form is exposed to a

platinum salt solution such as (NH3)4PtCl2. This is generally followed by converting the

PFSA from its original Na+ form to the more useful H

+ form. The last step involves the

reduction of catalyst particles by exposing the membrane to an aqueous solution

containing a reducing agent such as NaBH4 [335]. Foster et al. [370] and Fedkiw and Her

[371] have independently reported significant improvements in the cell output obtained

from MEAs fabricated via this method with metal loadings in the order of 2 – 6 mg Pt

cm-2

.

In a similar fashion, surface ion-exchange can be employed to deposit noble metal

catalysts onto carbon black without using reducing agents or precursor salts. Yasuda et

73

al. [372], investigated the deposition of ultrafine platinum particles on carbon black by a

surface ion exchange method using several different carbon black powders, including

Vulcan XC-72R, Black Perals 2000, Denka Black and two trial samples from Denka:

ONB-250 and FX-35. The effective surface areas per unit weight of the first three

commercially available carbon black powders are tabulated in Table 5-1.

Table 5-1 Effective surface area of several commercial carbon black powders [372]

Carbon Black Powder Vulcan XC-72R Black Pearls Denka Black ONB-250 FX-35

Effective Surface Area (m2 g

-1) 257 1475 61 N/A N/A

5.2.2.5 Application of Catalyst by Evaporative Deposition

This technique involves a two-step process in which a catalyst salt is first deposited onto

a dry solid polymer membrane through evaporation followed by catalyst reduction. Foster

et al. [370] reported the evaporative deposition of a platinum salt—(NH3)4PtCl2—onto

dry Nafion® followed by the reduction of the platinum by exposing the treated Nafion®

to a solution of NaBH4. It has been claimed that platinum loadings of 0.1 mg Pt cm-2

or

less are achievable [335].

5.2.2.5 Application of Catalyst by Catalyst Decaling

This is another example of a multi-step process to deposit a highly dispersed catalyst

layer onto a dry solid polymer electrolyte membrane. Similar to many of the previously-

discussed techniques, the first step involves the preparation of catalyst ink. Carbon-

supported platinum, solubilized Nafion®, and a carefully selected solvent are thoroughly

mixed using both mechanical agitation and ultrasonic mixing. This ink is then applied,

often using a paintbrush or a spray gun, onto a Teflon blank and heated until dry.

Additional layers of the catalyst ink are added until the desired catalyst loading is

achieved. It is crucial, however, to dry each layer before the application of the next to

avoid the dissolution of the top layer into the previous layer. Catalyst loading and, more

importantly, catalyst layer thickness, also can be controlled by the initial concentration of

the Pt catalyst in the carbon-supported platinum. As is the case with many powder type

74

deposition techniques, a uniform catalyst distribution inside the catalyst layer is easily

achieved; however, it is very difficult to keep the catalyst size under 5 nm in diameter

when the initial platinum-to-carbon ratio exceeds 40 wt%. The next step involves hot

pressing the coated Teflon blanks to both sides of a dry solid Nafion® membrane. When

the PTFE layers—Teflon—are peeled away, a thin layer of catalyst is left on each side of

the Nafion® membrane. It is common practice to use Nafion

® membranes in the Na

+ form

and then convert them to the more useful H+

form after the addition of catalyst layers.

Gottesfeld and Wilson [41, 156] presented a decaling method where the protonated form

of the solubilized Nafion® is first converted to the TBA

+ (tetrabutylammonium) form by

mixing it with an alcoholic solution of TBAOH. Glycerol also is added to enhance the

paintability and the stability of the catalyst ink [335].

75

5.0 ELECTRODEPOSITION

6.1 Introduction

Electrochemical deposition of precious metals and their alloys from a number of media,

including aqueous and fused-salt electrolytes, involves the reduction of the desired metal

ions from the solution to the metallic state onto the surface of a substrate. The platinum

group metals (PGM)—Ru, Rh, Pd, Os, Ir, and Pt—are inert by nature and are placed at

the bottom of the emf (electromotive force) series. Physical properties of precious metals

are shown in Appendix A, Table 1. These metals in solution (ionic form) have a great

affinity for electrons and can readily be reduced and remain in the metallic state for as

long as is needed.

The reduction of a metal ion, Mz+

, from an aqueous solution can simply be represented

by:

Mz+

(solution) + ze- M(lattice) (6-1)

The above cathodic (reduction) reaction can proceed via two different processes:

electrodeposition or electroless deposition. Although both processes create nano- or

micro-metallic particles on the substrate, they follow two different paths to acquire the

electrons needed to reduce the metal ions. In the electrochemical method, the required

electrons are supplied via an external source such as a d.c. power supply. In addition, two

separate electrodes are needed for both cathodic and anodic reactions to proceed. In the

chemical or electroless deposition method—also known as the autocatalytic technique—

the need for an external electron source is eliminated since electrons are provided by a

reducing agent in the solution. A number of different characteristics and parameters of

electrochemical and chemical deposition methods are summarized in Table 6-1 [373].

6.2 Electroless Deposition

Interest and activity in electroless deposition has been on the rise since its disclosure by

Brenner and Riddell [374, 375] in 1946. In an electroless deposition, the basic

components include an electrolyte containing the desired cation, a substrate and a

reducing agent to act the electron source (electron donor).

76

Table 6-1 Main characteristics of electrochemical and chemical deposition methods [337]

Property Electrochemical Deposition Chemical Deposition

Driving Force External Power Supply Reducing Agent (RA)

Cathodic Reaction Mz+ + ze- → M Mz+ + RA + ze- → M

Anodic Reaction M → Mz+ + ze- RA → [RA]ox + ze-

Overall Reaction Manode → Mcathode Mz+ + RA → M + [RA]ox

Anodic Site Anode itself Work piece

Cathodic Site Work piece Work piece

The overall cell reaction for an electroless deposition can be expressed as follows:

Metal Ion (solution) + Reducing Agent (solution) M (lattice) + Oxidation Product (solution) (6-2)

It is noted that both electron transfer reactions take place at the same electrode having the

same electrolyte-electrode interface. It is evident that the electrode surface is divided into

two catalytic sites, namely anodic and cathodic sites, to initiate and promote both the

oxidation and reduction processes. A flow of electrons will take place between such sites

on the same substrate since both reactions are occurring on the same electrode [333].

Several electrochemical models have been proposed for electroless deposition processes.

One particular theory—the mixed-potential theory of corrosion processes—has been used

by several workers to describe the various processes taking place during electroless

plating [376, 377]. This theory, originally developed and presented by Wagner and Traud

[378] in 1938, treats the overall cell reaction for an electroless process as two partial

reactions: a reduction and an oxidation reaction as presented in equations (6-3) and (6-4).

Reduction: Mz+


Oxidation: Red (solution) Ox(solution) + ze- (6-4)

This theory explains the overall reaction in terms of three current-potential (i-v) curves:

two i-v curves for the partial reactions and another for the overall reaction. A thorough

discussion of this theory and its use in electroless deposition is given elsewhere [333,

379].

77

Electroless deposition is often selected over other plating and deposition processes for its

simplicity in terms of equipment selection and setup and/or for the production of deposits

with unique chemical, mechanical and magnetic properties. In addition, electroless

deposition generates deposits that are uniform, less porous, and can be formed directly on

non-conductive substrates. Since the mid 20th

century, a significant number of metals and

alloys have been employed as candidates for electroless deposition on a wide variety of

substrates. Copper and nickel deposits have been extensively studied using this method

with other metals such as cobalt, gold and silver being deposited and examined during the

past decades. More recently, platinum group metals, especially palladium, have been

deposited using electroless deposition techniques in the area of corrosion protection and

jewelry making. However, for fuel cell applications, there have been no significant

activities in employing this method for depositing catalysts on GDLs or PFSAs. This is

primarily attributed to the lack of control that such a technique provides in terms of the

catalyst layer deposition. One of the main reasons that this method has been gaining

acceptance both in the industry and in academia since its inception about 60 years ago is

its ability to produce ―uniform‖ deposits. This, however, may not be desirable when thin

catalyst layers with high catalytic activities are required.

Regardless of the above issues, PGM—especially platinum and palladium—have been

plated and examined using a number of different solution baths containing different

reducing agents. Hypophosphite, borohydride, alkylamine boranres, and hydrazine have

been extensively used as reducing agents in electroless plating of PGMs. Some of the

properties of the above chemicals are summarized in Table 6-2 [373, 380]. Borohydrides

such as sodium borohydride are mainly used in highly alkaline media, while for slightly

alkaline, neutral, and slightly acidic solutions, boro-amines are widely employed.

Substrates that do not exhibit catalytic activities in some deposition solution baths—such

as silver or copper in hypophosphite-containing baths—become catalytically active in

other plating baths—silver or copper in dimethyl amine borane-containing solution baths

[373].

78

Table 6-2 Chemical properties of several reducing agents [373]

Reducing Agent No. of Available

Electrons

Redox Potential (vs SHE) (V)

at 25 °C

Sodium hypophosphite

(NaPO2H2)

2 -1.40

Hydrazine

(N2H4)

4 -1.16

Dimethylamine borane

(CH3)2NH:BH3

6 -1.20

Diethylamine borane

(CH3CH2)2NH:BH3

6 -1.10

Sodium borohydride

(NaBH4)

8 -1.20

The catalytic activity of several metals, including platinum and palladium in a number of

plating baths with different compositions was investigated by Ohno et al. [381]. Several

reducing agents including formaldehyde, borohydride, hypophosphite, dimethyl amine

borane, and hydrazine were used to prepare the various baths. The authors reported

marked differences in the properties of the deposited layers depending on the reducing

agents used in the plating baths. Boron and phosphorus-based reducing agents created

amorphous deposits with addition of elemental boron and phosphorus to the layer. In

applications where a pure metallic layer such as platinum or palladium is desired, and

impurities cannot be tolerated, the use of hydrazine as the reducing agent is

recommended. In such cases purity levels of 97% or greater are reported with the balance

consisting of nitrogen and/or oxygen and small traces of other elements [382]. Hydrazine

has become a popular reducing agent in electroless plating, not only for its ability to

produce relatively pure deposit layers, but also for its wide range of applications in both

acidic and basic plating environments. Furthermore, hydrazine can reduce multi-valent

metals either to a lower valent state or to the metallic form (zero valent state). This is

simply accomplished by close monitoring and control of the plating environments [383].

Hydrazine, which is a stronger reducing agent in alkaline media than in acidic media,

functions according to the following reactions [373, 383, 384].

79

)66(7.8845 :Acid

)56(7.447444 :Alkaline

1298252

12982242

molkJGeHNHN

molkJGeOHNOHHN

In applications where the acidity or the alkalinity of the plating bath is critical, extra care

must be exercised when hydrazine is used as a reducing agent since some of it can be

oxidized to ammonia, changing the pH of the plating bath as deposition progresses. The

oxidation of hydrazine to ammonia occurs according to [373]:

)76(2222 3242 OHNHeOHHN

Hypophosphite also is used as the electron donor in some applications when utilization

efficiency is not the determining factor. As with most reducing agents, side reactions are

a common concern with hypophosphite complexes. Elemental phosphorus and hydrogen

are the main side products and can have significant impacts on the properties of the

deposited layers. In applications, such as medical applications, where impurities in the

deposited layer cannot be tolerated, other reducing agents must be considered. The

deposited layer also can adversely be affected if the rate of hydrogen evolution is above a

critical limit. The above reactions are shown below [373]:

)106(222

)96(

)86(224222

22

222

3222

OHHeOH

OHOHPPOHH

eHHHPOOHPOH

ads

ads

Borohydride and dimethyl amine borane complexes generate elemental boron as an

undesired by-product according to the following reactions [373]:

)136(653

)126(43)(

)116(84)(8

3322232

4

244

eHBOHNHROHBHNHR

OHBeOHB

eOHOHBOHBH

6.2.1 Electroless Palladium Deposition

Palladium is the most widely used noble metal that has been plated onto a number of

different substrates via electroless deposition. This can be attributed to the many

applications that palladium has found over the last several decades, especially in the

electronics industry, where it has been extensively used as a barrier layer or conductive

80

film on a number of electronics components to enhance their performance. In addition,

inorganic membranes covered with a layer of palladium metal have been utilized to

promote hydrogenation and dehydrogenation reactions [373]. Since the advent of fuel

cells, and especially after the oil crisis of 1973, palladium also has been used as a catalyst

along with platinum and ruthenium.

A number of different solution baths have been used for electroless palladium deposition,

with hydrazine and hypophosphite in alkaline media being the most widely utilized

reducing agents. In fact, hypophosphite was the first reducing agent used to deposit

palladium via an electroless process in 1969 [373, 385]. The main source of palladium

metal is palladium tetrammine chloride (II)—Pd(NH3)4Cl2—with the corresponding

reaction in an electroless deposition bath with hydrazine as the electron donor being

[373]:

2Pd(NH3)4

2 N2H4 4OH 2Pd0 8NH3 N2 4H2O (614)

It has been reported that the rate of electroless palladium deposition increases with

increasing operating temperature, palladium concentration and reducing agent

concentration in the plating bath [373]. It also has been reported that the deposition rates

decrease as deposition progresses in plating baths where hydrazine is used as a reducing

agent [373, 386]. This is primarily attributed to the breakdown of hydrazine to ammonia

by the freshly deposited palladium layer according to equation (6-7). Several solutions

have been proposed to rectify this problem, including the addition of a fresh supply of

hydrazine to the plating bath during the plating process [386, 387]. Palladium complexes

also have been reported to retard the breakdown of hydrazine in a number of plating

baths when coupled with a stabilizer such as ammonium chloride [388].

6.2.2 Electroless Platinum Deposition

Platinum, on account of its unique properties, also has found many applications,

including the medical field as well as catalysis. The first cases of electroless platinum

deposition on catalytic surfaces were reported in 1969 by Oster et al. [389] and Rhoda

and Vines [390]. The former used a combination of platinum sulfate and borohydride,

while the latter employed sodium hexahydroxy platinate in conjunction with hydrazine.

81

Rhoda and Vines [390] reported a deposition rate of 12 m h-1

at room temperature while

continuously supplying hydrazine to the plating bath. Both these processes were known

to be inefficient. Consequently, Leeman et al. [391] proposed a plating bath containing

hexachloroplatinic salts and hydrazine as the source of platinum and reducing agent.

Different hexachloroplatinic salts were suggested for alkaline and acidic plating baths.

Hexachloroplatinic acid (H2PtCl6) and hydrochloric acid in conjunction with a reducing

agent such as hydrazine is often used in acidic media with the following reduction

process:

H2PtCl6 2HClN2H4 8HClPtN2 (615)

In alkaline plating baths, another platinum salt, (NH4)2PtCl6, is used with hydrazine as the

reducing agent, often in the presence of ammonium hydroxide. The reduction of platinum

takes place according to:

(NH4 )PtCl6 4e 2NH4Cl Pt 4Cl (616)

One of the main uses of electroless platinum deposition is in the medical field, where

biologically inert materials are required for coating implantable electrodes. Platinum and

PGM electroless deposition methods are rarely used in manufacturing fuel cell

components, including the catalyst layers in MEAs. This is mainly because of the lack of

control provided by this method during the deposition process.

6.3 Pulse and Direct Current Electrodeposition

6.3.1 Introduction

Today’s state-of-the-art catalyst deposition techniques can be divided broadly into two

groups: powder and non-powder. In powder type techniques, the catalyst is often

deposited first on high-surface-area carbon (known as carbon-supported platinum, if

platinum is used as the catalyst). A slurry or ink then is prepared by ultrasonically mixing

the above carbon-supported catalyst with a hydrophobic polymer and often a lower

aliphatic alcohol and pre-determined amounts of solubilized Nafion® (or other liquid

SPEs). The last step involves the addition of this ―ink‖ onto the solid polymer electrolyte

membrane or onto the GDL by one of the methods discussed earlier in section 5. MEAs

then are fabricated by the addition of GDLs, if the ink was applied onto a SPE, or by the

82

addition of SPE, if the ink was applied to a GDL. MEAs fabricated according to such

techniques exhibit a uniform catalyst concentration profile, owing to the high degree of

catalyst dispersion and mixing with the binder. In addition, the thickness of the catalyst

layer can effectively be reduced by increasing the catalyst content in the Pt/C powder

without impacting the catalyst loading per unit area of electrode [38]. However, at high

catalyst-to-carbon ratios; i.e., greater than 40%, the catalyst particle size will inevitably

increase, resulting in a decrease in its effective surface area which, in turn, lowers the

oxygen reduction activity of the catalyst.

A number of non-powder type techniques have been developed and experimented with

over the past two decades [370, 371]. Such methods have attracted attention due to their

reproducibility, relative ease of use, and the ability to create very small catalyst particles

(less than 5 nm in diameter) and generate a high Pt/C ratio at the membrane-electrode

interface [371]. As previously mentioned, one of the most promising catalyst deposition

methods is electrodeposition, in which a direct current is passed through a solution

containing the desired electrocatalyst metal ions that is in contact with the substrate.

Metal ions first reach the deposition sites via surface diffusion on the substrate, where

they subsequently are reduced to metals by gaining electrons from an external power

supply or from an electron-donor compound in the solution.

6.3.2 Direct Current Electrodeposition

Direct current (DC) electrodeposition has been widely used in the plating industry, where

a number of metals are plated onto different substrates for various reasons, including

corrosion and wear protection, hardness improvement, and aesthetic enhancement. DC

electrodeposition can be carried out utilizing both batch and continuous flow processes;

however, the former are more prevalent. According to this method, the substrate is

immersed into a plating solution, where it comes into direct contact with the dissolved

metal ions (or alloys) of interest that will be deposited on its surface. A counter electrode

also is placed in the plating cell in close proximity to the substrate (the working

electrode) to complete the cell. A reference electrode often is placed near the working

electrode to take measurements during the electroplating process for future optimization.

83

An interface between the working electrode and the plating solution will be established as

soon as they come into contact. The thickness of this interface varies from several m to

several mm depending on the nature of the solvent, solute, the substrate, and the degree

of mechanical agitation. The interface on the solution side contains the metal ions of

interest, which will begin to migrate towards the electrode when the working electrode is

made more negative. The ions are first adsorbed on the surface of the electrode and then

reduced to metal adatoms in a single or multi-step process, depending on the cation type.

This involves one or several charge transfer steps in which the cation receives one or

more electrons to be reduced and finally incorporated into the substrate matrix. In theory,

this process will continue as long as sufficient metal ions are present in the interface close

to the substrate and electrons are supplied to the working electrode by an external power

supply. In reality, as the deposition process progresses, the metal ions in the interface

close to the working electrode become exhausted and the ion concentration near the

surface of the cathode approaches zero. As the ion concentration in this region

diminishes, new catalyst nuclei cease to be formed on the surface of the cathode; instead

the existing crystals begin to grow and dendrites are formed. The ―limiting current

density‖ is reached when the ion concentration near the surface of the cathode reaches

zero and dendritic crystals start to form via growth of the existing crystals.

The properties of the metal deposits can seldom be controlled by the current density,

which is the usual variable in DC electrodeposition. In a plating system, because the

concentration of ions near the surface of the substrate will gradually decrease as the

plating progresses, the ions must be replenished from the bulk solution for the process to

continue. Short of adding more salts to the bath, this can be accomplished by convection,

ionic migration and diffusion. Since ions carry charge, one-dimensional ion transport in

an electrolyte can be expressed as a current density, from:

)176( Fzni jj

where, jn is the molar flux of the ion through the electrolyte in mol s-1

cm-2

; zj is the

charge number on the ion in eq mol-1

; and F is Faraday’s constant in C eq-1

. Mass

diffusion occurs as a result of an ion concentration gradient in the electrolyte, and is

described mathematically by Fick’s law as:

84

x

cDn

j

jj

(6-18)

where, Dj is the diffusion coefficient or diffusivity of species j in cm2 s

-1, cj is the

concentration of the same species in mol cm-3

, and x is the position in cm.

Convection, on the other hand, is affected by net motion of the electrolyte:

xjj vcn (6-19)

where vx is the velocity of species j in cm s-1

and other symbols have their usual

meanings.

Lastly, ionic migration is driven by an electrical potential difference between two points.

In an electrolyte containing a conducting ion the molar flow rate of the ion can be

calculated according to the following expression,

x

cDTR

Fzn jj

j

j

(6-20)

where, is the electrical potential difference in V and other symbols have their usual

meanings. The total ion transport flux can be approximated by combining equations (6-

18) to (6-20) if the usual assumption is made that the three fluxes are mutually

independent:

x

cDTR

Fzvc

x

cDn jj

j

xj

j

jj

(6-21)

Equations (6-19) to (6-21) represent one-dimensional ion transport in an electrolyte. For

three dimensional transport the corresponding expressions based on the Nernst-Plank

equation are:

x

cDTR

Fzvc

x

cDn jxj

j

xj

j

xjxj

,,,

(6-22)

y

cDTR

Fzvc

y

cDn jyj

j

yj

j

yjyj

,,,

(6-23)

z

cDTR

Fzvc

z

cDn jzj

j

zj

j

zjzj

,,,

(6-24)

85

The net current density for one-dimensional ion transport, i, now can be represented by

substituting equation (6-21) into equation (6-17) to give:

]][[ Fzx

cDTR

Fzvc

x

cDi jjj

j

xj

j

j

(6-25)

The mobility of an ion, uj, is given by:

TR

DFzu

jj

j

2

(6-26)

where, uj is the mobility of the ion in m s-1

(V m-1

)-1

and other symbols have their usual

meanings. The mobility is a measure of how fast an ion moves under the influence of a

unit potential difference, and is a function of the ionic charge, operating temperature and

pressure, ionic concentration, and ionic size.

The relationship between ionic conductivity, mobility, and concentration can be

expressed as

jjjj cuzF (6-27)

It can be seen that as the charge of the ion, zj, is increased, the total current carried per ion

is increased proportionally, increasing, in turn, the effective conductivity, (in S m2

mol-1

). In addition, an increase in the mobility of the charge carriers also will increase the

ionic conductivity. The same holds true for the ionic concentration. Equation (6-27) also

can be written as

jjjj cDzTR

F 22

(6-28)

However, such processes cannot normally keep up with the electrodeposition rate and the

metal ion concentration drops as electrodeposition continues. This is even more

detrimental when the current density increases. As the deposition current density

increases, the metal ions close to the cathode will be consumed at a faster rate, which is

directly proportional to the magnitude of the applied current density (set and controlled

by the external power supply). As the metal ion concentration drops and the existing

crystals begin to grow, the catalytic activity of the deposited catalyst layer is adversely

affected. This problem can readily be rectified by the addition of relaxation times during

86

the electrodeposition; that is, by the inclusion of ―no current‖ times, when the external

power supply is turned off. This results in an ion replenishment in which metal ions are

transported from the bulk to the interface by diffusion and natural or forced convection.

This is the basis for pulsed current (PC) electrodeposition.

6.3.3 Pulsed Current Electrodeposition

6.3.3.1 Introduction

In DC and PC electrodeposition processes, metal ions are transported from the bulk to the

interface near the working electrode (the cathode of the electrochemical-plating cell).

One or all three transport modes discussed in the previous section contribute to this

transport. This is, however, accomplished more effectively in PC electrodeposition owing

to the use of relaxation times, which allow fresh ions to enter the interphase at the

electrode surface. In PC electrodeposition, either the current density or the applied

potential is alternated rapidly between two different values. This is done with a series of

pulses of equal amplitude, duration and polarity, separated by periods of zero current [6].

Each pulse consists of two periods: an ―on-time‖ in which the current is applied and an

―off-time‖ during which no current is applied. It is during the latter period that metal

ions from the bulk solution diffuse into the layer next to the working electrode; i.e.,

carbon paper or carbon cloth in our experiments. When the current is applied during the

on-time, more evenly distributed ions are available for electrodeposition.

It is well known that, compared with conventional DC electrodeposition, PC

electrodeposition exhibits many advantages in terms of deposited particle size, stronger

adhesion, and better hardness [9]. In addition, PC electrodeposition can positively affect

several key properties of the deposited metals, including their size, morphology, degree

of porosity, and even crystal orientation, by enhancing mass transfer. Thus it is possible

to effectively control a number of physical, chemical, and electrochemical properties of

the deposited layer by changing and controlling the PC electroplating parameters,

including pulse amplitude, width, and frequency. PC electrodeposition generally favours

finer and smaller grain sizes and increases the number of nuclei per unit area of substrate.

87

An external DC power supply provides the cathode with the flow of electrons needed for

the reduction of the metal ion. As a result, an electrical double layer is formed at the

electrode-electrolyte interface on the solution side (see Appendix B). The thickness of

this layer is dependant on several parameters, including the type of electrolyte, the degree

of agitation present, and plating parameters such as the applied current density, duty

cycle, etc. In DC electrodeposition, at the onset of deposition this layer begins to appear

and grows as plating continues until it reaches a defined thickness. After this critical

thickness has been reached, it remains relatively constant as long as no significant

changes are introduced to the plating cell. Due to the existence of opposite charges on

both sides of such a layer, it is inevitably charged and remains in this state as long as two

conditions are met: a steady flow of electrons continues to reach the cathode and there is

a sufficient amount of metal ions in the electrolyte. The former condition is easily met as

long as the power supply is operational and the resistance to the flow of electronic current

from the external source to the cathode is minimal. However, as the plating continues,

ions in the bulk solution will experience a resistance as they try to diffuse into this layer

and be reduced on the surface of the cathode. The concentration of metal ions in this

layer, known as the diffuse double layer, will decrease until it reaches a critical value at

which there will not be enough metal ions in the vicinity of the cathode to initiate the

formation of new nuclei. As a result, existing crystals will begin to grow and dendrites

will start to form. At this point, the system is in a mass-transport-limited mode and,

unless fresh metal ions are allowed to enter the diffuse layer, the formation of new nuclei

ceases and the deposited layer will lose some of its ductility and electrical conductivity,

amongst other important and desired characteristics.

As previously mentioned, PC electrodeposition can readily rectify the above problem by

significantly raising the limiting current density by replenishing the metal ions in the

diffuse layer during the time when the current (or potential) is interrupted; i.e., during

off-time [391]. It is important to recall that DC electrodeposition has only one parameter,

namely applied current density (or applied voltage). These can be varied, but there are

limitations, the most important of which is the limiting current density. The applied

current density in DC electrodeposition must be closely regulated and always be kept at

values lower than the limiting current density to avoid dendrite formation and, in more

88

severe cases, the loss of the deposition layer. But, in PC electrodeposition, there are three

parameters that can be independently varied: on-time (ton), off-time (toff), and peak

current density (iP). The first two variables—on-time and off-time—can be related

through another variable known as the duty cycle, γ, which is the percentage of the cycle

time during which the current (or potential) is on. This was previously defined in section

1.3 as

%100

offon

on

tt

t (1-1)

The frequency of a waveform is defined as the reciprocal of the cycle time (t):

ttt

foffon

11

(6-29)

Thus the duty cycle also can be defined in terms of cycle frequency:

)()( fton (6-30)

The average current density, iA, is defined as

)()( PA ii (6-31)

In practice, PC electrodeposition often involves a duty cycle of 5%-50% and on-times

and off-times from s to ms, depending on the application and desired characteristics of

the deposited layer [391].

6.3.3.2 Factors Influencing PC Electrodeposition

Electrical double layer – the nature and the thickness of this layer is critical in the

outcome of the deposited layer in terms of its morphology, hardness, and grain size. This

layer has been described in Appendix B. A constant current must be supplied to the above

layer to increase its potential to a level that is sufficient for metal deposition. When a

constant current is applied to a plating system, it is used to perform two varying tasks:

first, it must charge the double layer and, second, it needs to sustain the desired electrode

reaction that reduces metal from solution and leads to the incorporation of adatoms into

the substrate. Accordingly, the total galvanostatic current density supplied to the

electrode, itot, consists of two parts, a capacitive current, iC, that charges the double layer

89

and a Faradic current, iF, that corresponds to the rate of metal deposition. This can be

written as:

FCtot iii (6-32)

The first step after a galvanostatic current is supplied to the plating cell involves charging

the double-layer capacitance, Cdl, from the reversible potential, E, to a higher potential,

Ei, where the electrode reaction (reduction of the metal) begins to take place at a

measurable rate [333, 391]. Paunovic & Schlesinger have presented a simplified

equivalent circuit for a single-electrode reaction as shown in Figure 6-1, where RCT is the

charge-transfer resistance of the electrode reaction [333].

Figure 6-1 A simplified equivalent circuit for single-electrode reaction [333]

The time, t, needed to charge capacitor C in a resistance-capacitance circuit to 99% of the

imposed voltage is given by [333]:

RCCt VV 6.4)(99.0 (6-33)

where R is the resistance in Ω and C is the capacitance in µF. The time required to charge

the electrical double layer is referred to as the charge time, tC, defined as the time

required for the Faradic current, iF, to reach 99% of the peak current density, iP. The

discharge time, tD, is the time needed to decrease the Faradic current, iF, to 1% of the

peak current density, iP. Two simple formulae for selection of charge and discharge times

have been given by Chandrasekar and Pushpavanam [391] and Puippe and Leaman [392]:

P

Ci

t17

(6-34)

Cdl

itot

iC

iF

RCT

90

P

Di

t120

(6-35)

where tD (and tC) is in µs and ip is in A cm-2

.

Mass Transport – for the simple reduction of a metal ion, Mz+

, in an aqueous solution,

Mz+


the electrode reaction rate (or the current) is governed by the rates of several processes

[393]:

1. Mass transfer of reactant to the electrode from the bulk solution.

2. Electron transfer at the electrode surface.

3. Chemical reactions before and after the electron transfer step. These can be either

homogenous or heterogeneous processes taking place on the surface of the

electrode.

4. Physical processes, mostly in the form of surface reactions such as adsorption,

desorption, and crystallization.

The simplest reactions involve only mass transfer of the reactants from the bulk solution

to the surface of the electrode. However, this plays a critical role in limiting or enhancing

the deposition rate and, consequently, influences some of the key properties of the

deposited layer, including its grain size, hardness, and morphology. One of the most

important parameters for mass transport in both DC and PC electrodeposition is the

limiting current density, iL.

The concept of limiting current density can be better understood by considering the

Nernst Diffusion-Layer Model. According to this model, the concentration of the metal

ion in the bulk solution up to a distance from the surface of the cathode is cb and falls

linearly to cx as it approaches the working-electrode surface. The model assumes that the

contribution of the double-layer effect is negligible and that this diffuse layer—known as

the Nernst diffusion layer—is for all purposes stagnant. This stipulates that the existence

of any stirring has practically no impact on the diffuse layer, but will be effective beyond

this layer, at distances x > , as shown in Figure 6-2.

91

Figure 6-2 Actual and Nernst diffusion layers during non-steady-state electrolysis [333]

Based on this model the concentration gradient of the metal ion at the cathode is given

by:

0

0

xb

x

cc

x

c (6-36)

Furthermore, the rate of reaction based on the current density can be calculated as

follows:

0xb

j

ccDFni (6-37)

where Dj is the diffusion coefficient of the plating species j. For constant current

polarization, the diffusion-layer thickness increases with the square root of time

according to [333]:

tD j2 (6-38)

x = 0

Nernst Diffusion Layer - Model

Actual Concentration

Distance from Electrode, x

Concentration, C(x)

cb

Nernst Diffusion Layer Thickness

92

From Eq. (6-37) it is apparent that the maximum current density is obtained when the

term cx=0 is zero, giving rise to the maximum concentration gradient. At this current

density, known as the limiting current density, iL, the metal ions are reduced as soon as

they reach the electrode surface. This implies that the concentration of metal ion at the

electrode surface is practically zero and that the rate-limiting step is the transfer of metal

ions to the surface of the electrode.

In PC electrodeposition, the magnitude of the limiting current density is strongly

dependent on the pulse parameters, particularly, on the pulse on-time, ton. A primary

objective in all plating systems is to increase the magnitude of iL so higher peak current

densities (iP) and, consequently, greater average current densities (iA) can be applied. As

ton increases, the corresponding iL will decrease. Consequently, ton must be as short as

possible—low duty cycle—to stay below the corresponding limiting current density, but

sufficiently long to fully charge the electrical double layer [391]. In DC electroplating,

the properties of the deposited layers will be adversely affected if the applied current

density is close to the limiting current density. In practice, the allowed applied current

density is often 10%-20% of the corresponding limiting current density [391].

Conversely, in PC electrodeposition, smooth, crack-free, and uniform deposit layers are

obtained even when the applied current density approaches the limiting current density,

provided that the diffusion layer is relatively thin.

6.3.3.3 Major Types of Pulse Waveforms

A wide variety of waveforms has been used in a number of industries utilizing PC

electrodeposition, including printed circuit board (PCB) and integrated circuit (IC)

manufacturing. This has been accelerated over the past two decades owing to significant

improvements in modern electronics and microprocessor capabilities. Simple computer

programs and codes can easily be written to generate complex waveforms that previously

were either very tedious or almost impossible to achieve. Applied current waveforms are

broadly divided into two groups: (1) unipolar, where all the pulses have similar polarities

(unidirectional pulses) and (2) bipolar, where both anodic and cathodic pulses are

applied.

93

A large number of different waveforms has been created and experimented with for a

wide array of applications. Some of the most important waveforms, as well DC

electrodeposition, are briefly discussed below with reference to noble metal catalyst

deposition.

Direct Current Electrodeposition—conventional DC electrodeposition has been

extensively used and reported in the literature [9, 12, 394-399]. Lin-Cai and Pletcher

[394], who examined the activity per surface metal atom for hydrogen evolution on

electrodeposited platinum, observed that clusters less than 20 nm in diameter were

inactive and that a minimum of twenty platinum monolayers was required to achieve

normal activity. Itaya et al. [395] reported good catalytic activity of a platinum catalyst

layer electrodeposited through a PFSA membrane onto a glassy carbon electrode. A study

reported by Jannakoudakis et al. [396] outlined the electrodeposition of platinum from a

number of different plating baths, including Pt(NH3)64+

complex onto polyacrylonitrile

fiber supports. Srinivasan et al. [397] carried out a series of experiments to deposit

platinum onto Prototech GDLs using a chloroplatinic acid electrolyte. They reported that

the performance of MEAs fabricated by electrodeposition were comparable to MEAs

made using other methods. Shimazu et al. [398] and Kanevskii et al. [399] studied the

electrodeposition of platinum from chloride anion complexes onto more easily

characterized surfaces such as glassy carbon.

Cathodic Pulse followed by a Period of Zero Current (or Potential)—according to this

technique the current (or potential) is alternated between a maximum and a minimum

value with relaxation periods in between. A main feature of such waveforms is their

identical amplitude during the whole plating process. They may include anodic currents,

in which the current is intentionally reversed to create deposits with unique properties;

this is known as pulse reverse current (PRC), and has been widely used in the electronics

industry for the deposition of copper onto a number of substrates. There are many types

of waveforms that can be generated using a function generator and/or a galvanostat.

These include square pulse, ramp-up, ramp-down, and triangular, to name but a few.

94

Duplex Pulses—this type of waveform contains a series of pulses at one level followed

by another series of pulses at another level, all in one direction. The sudden variation in

the current or potential level of the pulses results in changes in the dynamics of the

plating bath, particularly in the vicinity of the working electrode, where metal ions must

be present at all times and, preferably, uniformly along the deposition surface.

Pulse-on-Pulse Waveform—these waveforms consist of pulses at specific amplitudes

followed by another set at higher or lower amplitudes. Similar to duplex pulses, pulse-on-

pulse waveforms offer complex waveforms that can effectively change the dynamics of

the plating bath to create deposits with specific properties. Such waveforms, however, are

not easy to generate and require more expensive equipment.

Superimposing Periodic Reverse on High Frequency Pulse—this type is characterized by

a series of pulses with controllable pulse parameters, including amplitude, frequency and

duration. The waveform is often sinusoidal with fast turn-ton and a slow turn-toff [391].

One of the main advantages of complicated waveforms over both DC and simple

waveforms is the increased number of variables at the researcher’s disposal, which

provide workers with tools to create deposits with unique properties. However, as the

number of variables increases, it becomes more difficult to fully understand the influence

of each variable on deposit properties [391].

6.3.4 Nucleation and Growth during Electrocatalyzation1

A good understanding of the processes involved in electrodeposition, from the

transportation of ions from the bulk solution to the reduction and incorporation of

adatoms into the crystal matrix, is invaluable in creating deposits with unique properties.

This is particularly important in the early stages of phase transition comprising nucleation

and crystal growth [400]. In the case of noble metals, a good understanding of the early

stages of electrocrystallization is critical in creating highly dispersed metal phases with

good catalytic activity. It is generally agreed that the primary processes leading to

1 Many of the formulas presented in this section are derived in Ref. 333.

95

nucleation and growth during the initial stages of electrocrystallization on a substrate are

now well understood [401-403].

The first atomic model of electrochemical crystal growth was developed by Erdey-Gruz

and Volmer [404, 405] in the early 1930s, and treated the substrate as a perfect crystal

surface. One of the main implications of perfect crystals with no imperfections is that

there is no site for crystal growth and the first step of the process must be nucleation. By

the mid 1950s, it was realized that real substrates contain imperfections, and

consequently, have many growth sites. This realization created a different way of

thinking with respect to crystallization and introduced several new models. These new

models were strengthened and validated by the results of in situ surface analytical

methods, including scanning tunnel microscopy (STM) and atomic force microscopy

(AFM).

Electrocrystallization involves the incorporation of adatoms (or adions) into the substrate

crystal lattice by two different, but interdependent processes: (1) nucleation, and (2)

crystal growth. The first process—nucleation—becomes dominant with high adatom

population, high overvoltage, and low surface diffusion. By reversing the above factors,

the second process—crystal growth—becomes the dominant process. In PC

electrodeposition, a high population of adatoms and high overvoltage can readily be

achieved through high peak current density [391].

From a molecular point of view, a nucleus—a cluster of atoms—is only stable when it

reaches and then exceeds a critical size. Two processes play important roles in the

formation and growth of these clusters: (1) the arrival and adsorption of ions at the

surface of the substrate and (2) the movement of adsorbed ions across the surface. The

instability of these adsorbed ions stems from the fact that their binding energy to the

crystal is relatively low; as a result, they will stay on the surface as an adion only for a

very short time. However, their stability is significantly increased through formation of

clusters with other incoming and adsorbed adions. The free energy of formation of a

cluster containing N ions is given by:

)()( NezNNG (6-39)

96

where the first term refers to the energy that is required to bring N ions from the solution

to the surface of the substrate and the second term signifies the increase in the surface

energy as the cluster grows. It is apparent that both terms are functions of N, the first term

increases linearly with N, while the second term increases as N2/3

. The Gibbs free energy

of formation of a 2-dimensional cluster as a function of size is shown in Figure 6-3,

which shows that the free energy of formation of a cluster initially increases with N until

it reaches a critical value and then begins to drop sharply as N further increases [333].

Figure 6-3 Free energy of formation of a cluster as a function of size N [333]

The critical cluster size, NC, can be calculated from the following expression:

2

2

ez

sbNC (6-40)

where b is defined as

P 2

4S (P and S are the perimeter and the surface area of the nucleus,

respectively) and s is the area occupied by one atom on the surface of the nucleus,

is

the edge energy, e is the charge of a single electron, and

is the corresponding

overvoltage. Another important parameter, the critical radius of the surface nucleus, rc,

can be evaluated from:

ze

src (6-41)

It is seen that both NC and rc are strong functions of overvoltage.

G

N

NC

GC

97

6.3.4.1 Nucleation Rate

The rate of nucleation for conversion of a site on a real substrate into nuclei is given by:

ANNdt

dN)( (6-42)

where A is the nucleation rate constant and N₀ is the maximum possible number of

nucleus sites on the substrate surface per unit area. N₀ often depends on the applied

potential, but is always smaller than the atomic density. A wide range of values from 104

cm-2

to 1010

cm-2

has been reported for N₀ [401, 406]. Equation (6-42) can be rearranged

and integrated to obtain equation (6-43), which is easier to work with:

AteNN

1 (6-43)

There are two limiting cases for equation (6-43) at the start of the nucleation process; i.e.,

when time (t) is small:

Limiting case1: large nucleation constant, A:

NN (6-44)

This indicates that almost all the sites on the substrate surface are converted to nuclei

immediately after a potential change in the plating system is detected. This is known as

instantaneous nucleation and refers to the case in which the maximum number of nuclei

is formed. However, it is well known and widely reported in the literature that there is

actually a delay time between the first potential perturbation and the time the first nucleus

is formed. This has been mainly attributed to the build-up of the critical nuclei that must

be present at the beginning of the process before nucleation can be initiated [401, 407].

For all practical purposes, such a time delay can be ignored without severe implications

on experimental outcomes [401, 408-412].

Limiting case 2: small nucleation constant, A:

When 1At equation (6-43) reduces to:

tANN (6-45)

98

In this case the number of nuclei increase as plating progresses; i.e., N is a function of

time. This is referred to as ―progressive nucleation‖.

The nucleation rate, J, for 2-dimensional nucleation is given by:

Tkez

bskJ

2

1 exp (6-46)

where k1 is the rate constant, S

Pb4

2

is a geometric factor relating the perimeter of the

nucleus to its surface area (equal to for a spherical nucleus), is the edge energy in J

cm-1

, z is the charge of the ion, e is the electronic charge, k is Boltzmann’s constant, T is

the system temperature, and is the overvoltage.

From the above equation it can be seen that as the overvoltage increases, the nucleation

rate also will increase. The system temperature, T, will have the same effect, but its

impact will not be as significant as the change in overvoltage (for instance, it will be

easier to increase the overvoltage by a factor of 2 or 3 than to increase the system

temperature by the same magnitude; recall that the temperature is in kelvin).

6.3.5 Electrodeposition of Metals and Alloys

6.3.5.1 Introduction

The recent upsurge of interest in the electrodeposition of individual metals and their

alloys on various substrates has its roots in:

The electronics and microprocessor industries for the manufacture of PCBs and

ICs,

Metal deposition for magnetic devices such as disks and,

Deposition of single and multilayer structures for a number of chemical and

electrochemical processes, including the deposition of catalyst layers—mostly

noble metals—on GDLs for use in PEM fuel cells and electrolyzers.

6.3.5.2 Copper and its Alloys

The electrochemical deposition of copper has been studied extensively for two reasons:

(1) its technological use in the manufacture of ICs for the production of interconnection

99

lines, and (2) its scientific use as a base system to study the nucleation and growth of

metals under different conditions. Copper, by far, has been the most studied deposited

metal under different experimental conditions. Initial and advanced stages of copper

deposition in a large number of plating baths with different compositions have been

extensively examined and reported in the literature [413-424]. One of the unique

characteristics of any copper system is its relatively low exchange current density [413].

This results in charge transfer across the electrical double layer being the dominant force

in copper deposition, particularly when the deposition times are relatively short [413-415,

417]. It is customary to neglect the contribution of the charge transfer resistance when

dealing with dilute copper solutions. It is, however, beneficial to consider the

contributions of both charge transfer and diffusion limitations during the nucleation and

growth of copper crystals in all plating solutions. One of the few studies considering both

contributions has been reported by Melchev and Sapryanova [413], who examined the

nucleation and growth of copper crystals on an easily characterizeable substrate (glassy

carbon) under potentiostatic conditions. Current transients under different overvoltages

were employed to generate data to support the theory of progressive nucleation and

growth under combined charge transfer and diffusion limitations. It was concluded that

the progressive nucleation of copper on a glassy carbon electrode is dependent upon both

charge transfer and the diffusion mechanism of the copper ions.

One of the first investigations on the PC electrodeposition of copper was reported in 1979

by Cheh et al. [425]. This study was particularly concerned with the current efficiency

(CE) of copper electrodeposition under pulsed current conditions. It was reported that the

current efficiency of copper plating from an acidic plating bath was lower than a similar

system under DC conditions, and continued to drop below 100% as the off-time was

increased or the pulse duration was shortened. A mathematical model was formulated to

simulate a Cu/CuSO4 system under pulsed current conditions using a rotating disk

electrode (RDE) to explain the above observations. The effects of electrical double-layer

charging were ignored. Another study on the mechanism of copper electrodeposition by

PC electrodeposition and its impact on the current efficiency of the plating process was

made by Tsai et al. [426], who examined the effects of pulse period and duty cycle on the

CE of copper plating from an acidic bath under a wide range of pulse periods from 200 to

100

0.02 ms. A mathematical model based on an equivalent circuit was developed to simulate

the potential responses. It was reported that the current efficiency decreased with

shortening pulse period in the millisecond range, while it increased in the microsecond

range. These marked differences were explained in terms of dominant rate-determining

steps in both ranges.

More recently, Zhang et al. [427] reported the generation of 10 m copper films prepared

by pulse electrodeposition with different peak current densities and pulse on-times. These

authors reported that higher pulses lead to a stronger self-annealing effect on grain

recrystallization and growth. Similar findings also have been reported by Shen et al.

[428] and Ma et al. [429]. Furthermore, films electroplated at higher current densities

have been reported to take less time to complete the self-annealing process compared

with those electroplated at lower current densities [430-434].

Numerous studies have been carried out and reported on copper nucleation mechanisms

via electrodeposition on a number of substrates, including vitreous carbon [435-438],

sputtered TiN [439], and copper [440]. Grujicic and Pesic [435] studied the reaction and

nucleation mechanism of copper electrodeposition from plating solutions containing

ammonia. The effects of applied potential, copper concentration in the plating bath, and

substrate surface morphology were investigated. It was concluded that the copper

concentration is not an effective parameter to consider, and closer attention must be paid

to the pH of the plating bath during electrodeposition.

Alloy deposition techniques are as old as the electrodeposition methods employed for

individual metals; for instance, the deposition of brass was first carried out in 1840. The

electrodeposition of alloys is subject to the same principles as single metals, but when

deposited, may not exhibit the same properties as the individual metals present in the

alloy. In most cases, the deposits show superior and unique properties that are highly

desirable. Nickel-copper alloys have been extensively studied and used in the industry in

microsystems due to their superior corrosion resistance and magnetic and thermophysical

properties [441]. Other properties of great interest are wear resistance and magnetic and

optical characteristics.

101

Electrodeposition of nickel-copper alloys has been carried out employing galvanostatic

[442], potentiostatic [443], pulsed current [444] and pulse-reverse current [445, 446]

techniques from a number of different plating baths, including sulfamate [447], cyanide

[448], citrate [449], sulfate-oxalate [450], and pyrophosphate [451]. The Ni-Cu alloy

plating process is a codeposition process in which the more noble metal, copper, is

preferentially deposited first followed by nickel. As with many alloys, the standard

reduction potentials of copper and nickel, +0.337 V and -0.25 V, respectively, vs. SHE at

25 °C, are quite far apart in the standard emf series. The difference can be narrowed or

eliminated by including a significant change in ionic concentration through complex ion

formation, such as by the addition of a complexing agent like citrate. Roos et al. [452]

studied the electrodeposition of Ni-Cu alloys from plating baths with varying

compositions and citrate as a complexing agent on RDEs and Hull cell electrodes. The

former substrate was replaced with rotating cylinders, since the current distribution on

RDEs was observed to be non-uniform. Quang et al. [453] also utilized a number of

different citrate baths and reported crack-free deposits with thicknesses in excess of 100

microns. Later, in 1997, Bonhote et al. [444] showed that the microstructure of a Ni-Cu

deposited multilayer could effectively be controlled by controlling the applied current

density. It also was reported that when the applied current density used for copper

deposition was lower than its limiting current density, the multilayer structure was

oriented along the (110) direction.

In addition to citrate, other complexing agents such as sulfamate also have been utilized

in Ni-Cu electrodeposition. Tench et al. [454] examined the influence of Ni-Cu plating

baths on the quality of multilayers containing sulfamates. Plating baths with low

concentrations of copper were recommended. Furthermore, Ni-Cu multilayers

electrodeposited by PC electrodeposition techniques were found to be superior in terms

of tensile strength, compared with multilayers obtained by DC plating with identical

plating bath compositions. Bradley et al. [447] and Menezes et al. [455] studied the pulse

plating of Ni-Cu alloys from plating baths containing sulfamates. They both suggested

the use of citrate baths for thick Ni-Cu multilayers. Lin et al. [456] investigated the

influence of ammonium ions in sulfamate electrolytes on deposited nickel layer. It was

102

reported that ammonium ions increased the internal stress by greatly refining the grain

structure and increasing the defect density. However, an increase in hardness also was

observed. The influence of ammonium on Ni-Cu systems was reported by Chassaing et

al. [457, 458]. The influence of other compounds on the dynamics of copper, nickel, and

copper-nickel systems has been periodically reported in the literature; these include

tartrate and ammonium chloride [459], chloride and nitrate ions [460] and ammonia-

based media [461].

The influence of other important plating parameters such as plating bath pH also has been

widely studied. However, the majority of the plating baths have their pH in the acidic

region, where sulfuric acid is often used to maintain a constant pH during electroplating.

Green et al. [462] experimented with citrate electrolytes with two different pH values: 4.1

and 6.0. They found that the plating bath with pH 6.0 was more stable than that with pH

4.1. Arvinda et al. [463] investigated the deposition of copper from two different alkaline

solutions—hydroxide and triethanolamine—with citrate as the complexing agent.

Mathematical models have been developed by many researchers to describe and predict

the codeposition of nickel and copper from various plating baths. One of the first

mathematical models was formulated by Guglelmi [464] in the early 1970s. This model is

based on a two-step adsorption, one weak (physical in nature) and the other rather strong

(electrochemical in nature). According to this model, particle concentration in the deposit

increases with increasing electrolyte concentration and decreases with increasing current

density. Mass transport and convective flows are ignored since the model predictions are

based on the assumption that adequate mixing is provided throughout the deposition

process. Roos et al. [465] verified the validity of the above model by examining the

codeposition of alumina and copper from a sulfate electrolyte. Later in the year, Celis et

al. [466] carried out similar experiments on the codeposition of alumina and copper from

a copper sulfate electrolyte. Their results were in good agreement with Guglelmi’s model.

Several years later, in 1987, Celis et al. [467] developed a new model based on

Guglelmi’s original model, but proposed a five-step adsorption model as opposed to

Guglelmi’s original two-step model. The validity of this new model was tested and

verified by a series of experiments on the codeposition of alumina with copper from a

103

copper sulfate plating bath. Another model by Valdes et al. [468] attempted to explain the

codeposition process by considering the electrochemical and hydrodynamic forces acting

on particles in the plating bath. This model was based on very fast kinetics leading to the

capture of all particles approaching within a certain critical distance of the substrate. A

new concept—―perfect sink‖—was introduced that would predict an increase in the

deposition rate with increasing current density.

The development of a significant number of mathematical models since the early 1980s

has paved the way to a better understanding of the electrodeposition process with its

many variables. Fransaer et al. [469] proposed a trajectory model for large particles (> 1

micron) and later developed another model to describe the influence of various particles

in the plating bath on the applied current [470]. Wang et al. [471] introduced a model

based on adsorption strength, postulating that particles adsorbed on the surface of the

electrode can detach themselves depending on the deposition conditions. Vereecken et al.

[472] formulated a model by considering the convective diffusion of particles to the

electrode surface. They were successful in describing the kinetics of nanoparticles during

electrodeposition. A model based on primary current distribution and ohmic resistance in

the plating bath was introduced by West et al. [473]. A one-dimensional diffusion model

was postulated by Bade et al. [474], who investigated the deposition of Ni in microholes

using micro RDEs. Griffith et al. [475] proposed both one- and two-dimensional models,

where the 2-D model was limited to the deposition of a single species. Another one-

dimensional model was proposed by West et al. [476] for electrodeposition of copper

using pulse and pulse-reverse plating methods. Andricacos et al. [477] proposed another

model for the deposition of copper for chip interconnects. The validity of the above

model was later verified through experimental work carried out by the authors [478].

Other models by West et al. [479], Gill et al. [480], and Panda [441] have shed more light

on these interesting but not fully-understood phenomena.

6.3.5.3 Nickel and its Alloys

Nanocrystalline materials have found many uses due to their unique mechanical,

chemical, and electrochemical properties [481-483], and nickel coatings are amongst the

104

earliest commercially-available electrodeposited thin films [484-486]. One of the

simplest baths, developed by Watts in 1916, is still in use today owing to its low cost,

ease of preparation, and simple control [484-487]. As with other metals, nickel can be

deposited on many different substrates using various techniques, including DC and Pulse

electrodeposition.

The synthesis of nanocrystalline nickel by electrodeposition has been widely reported in

the literature. Direct current electrodeposition has been successfully used to produce

nanocrystalline materials, including nickel and its alloys. Ebrahimi et al. [488] have

produced nanocrystalline nickel layers from a nickel sulphamate bath. They reported thin

layers of electrodeposited Ni with grain sizes ranging from 35 to 97 nm. Bakonyi et al.

[489] synthesized nanocrystalline nickel by DC electrodeposition using a plating bath

containing Na2SO4, NiSO4, and HCOOH.

For reasons previously discussed, PC electrodeposition has become the method of choice

for creating nanocrystalline materials. It is well known and well documented in the

literature that the properties of metals and alloys can readily be altered and improved by

modifying their microstructures by controlling the pulse parameters, including applied

peak current density, on-time, and off-time [490, 491]. Erb et al. [492], Klement et al.

[493], and Brooks et al. [494] studied the impact of various electroplating parameters on

the structure and properties of pulse electroplated nickel. They all reported the synthesis

of nickel nanoparticles with grain sizes in the range of 6-100 nm, utilizing a Watts bath

with small amounts of saccharin. Pulse electrodeposition was carried out with on-time

and off-time of 2.5 ms and 45 ms, respectively. Natter et al. [495-497] in a series of

papers reported the creation of nanocrystalline nickel with grain sizes of less than 50 nm

using pulse electrodeposition with on-time and off-time ranging from 1-5 ms and 50-100

ms, respectively. The plating bath was reported to contain NiSO4, H3PO4, NH4Cl, and

Na-K tartarate. Jeong et al. [498] investigated the influence of pulse parameters on the

grain size reduction of nanocrystalline nickel deposits and correlated and reported

superior wear resistance compared with similar pure nickel layers deposited by

conventional methods. In a similar study, Chen et al. [499] examined the influence of

pulse frequency on the microstructure of Ni-Al2O3 composite coating. They also studied

105

the impact of pulse frequency on the hardness and wear resistance of the above

composite, and found that the pulse frequency significantly influenced the preferred

orientation of the composite coatings; as the pulse frequency increased, the texture of the

coatings changed from a preferred and strong (111) to a more random orientation. Yang

et al. [500] examined the influence of pulse parameters on the corrosion resistance and

porosity of electrodeposited nickel coatings. Direct current electrodeposition was also

employed and it was observed that the porosity of nickel deposits created by PC

electrodeposition significantly decreased, resulting in better corrosion-resistant properties

compared with DC electrodeposition. Qu et al. [501] experimented with ultra narrow

pulse width and high peak current density to produce nanocrystalline nickel coatings with

low porosity, small grain size, and minimal internal stress. It was claimed that grain sizes

ranging from 50 to 200 nm were obtained when the current density changed from 300 to

60 A dm-2

. The hardness of the nickel deposits also was reported to increase with

increasing current density in the range of 20 to 150 A dm-2

; however, it decreased at

current densities greater that 150 A dm-2

.

Much of the published work on pulse electrodeposition has focused on a rectangular

waveform. Accordingly, most of the comparative studies between pulse electrodeposition

and other methods are based on this waveform. Several workers, however, have shifted

their focus to different types of waveforms. Wong et al. [502] presented their

experimental and theoretical works on the surface finish of nickel electroforms prepared

by using five different waveforms: rectangular, triangular, ramp-up, and ramp-down, all

with relaxation times (off-times) and a ramp sawtooth waveform without any relaxation

period. They claimed that at constant electrodeposition parameters—peak current density,

pulse period, electrodeposition thickness—ramp waveforms produced the best results in

terms of surface roughness, followed by triangular and rectangular waveforms. It was

noted that surface roughness could readily be reduced by as much as two to three times

when ramp (both up and down) and triangular waveforms were utilized. In a similar

study, Chan et al. [503] investigated the influence of rectangular, triangular, and

sinusoidal waveforms on crystallographic texture, microstructure, and the surface

finishing of electroformed nickel, and reported that smaller grain size and a better surface

finish were obtained by employing a triangular waveform. In another study conducted

106

and reported by Wong et al. [504], the highest hardness and the smallest grain size of

nickel electroforms were obtained when a ramp-down waveform with relaxation times

was used. Analytical equations for overvoltage and nucleation rates at a constant current

density and deposition thickness were also derived.

6.3.5.4 Platinum and its Alloys

In recent years, there has been a growing interest in the development of highly dispersed

catalyst layers containing Pt and PGMs with high catalytic activity for use in low-

temperature hydrogen fuel cells [505-508].

A number of different methods have been devised to deposit a predetermined amount of

catalyst on GDLs or SPEs. Chemical reduction of commercially available platinum salts

such as H2PtCl6 and Pt(NH3)4Cl2 by a reducing agent such as NaBH4 is one of the most

commonly used methods [509]. The impregnation-reduction method also is widely used

to deposit Pt onto different types of SPEs [510-512]. A combination of reduction and

decal transfer processes also has been explored, in which the reduced platinum alloy

particles are first precipitated out of solution and made into an electrode by the decal

transfer technique [41, 509]. However, the utilization of catalyst particles prepared using

the above methods normally is less than 20%, primarily due to their relatively large size

or lack of ionic and/or electronic contacts. Another common technique to prepare carbon-

supported platinum is to employ a ―sulphito route‖, where chloride is first removed from

H2PtCl6 by converting it into Na6Pt(SO3)4 and then precipitating platinum oxide colloids

onto suspended carbon-black particles, and finally producing Pt/C by a simple chemical

reduction process [509, 513]. An average platinum particle diameter of 1.5-1.8 nm has

been reported using this technique [509, 513]; however, due to the nature of this method,

more than half the deposited electrocatalyst may not be available to participate in the

redox reactions inside the fuel cell [513, 514].

As a result, there has been a significant level of activity in the field of electrocatalyst

deposition for use in sensors, medical devices, and fuel cells, particularly proton

exchange membrane and direct methanol fuel cells. To ensure high utilization of

deposited catalyst particles, two conditions must be met: small particle size—often less

107

than 5 nm in diameter—and the presence of the catalyst in the three-phase interface,

where both the reduction and oxidation reactions take place. The former can be

accomplished by many of the methods explained in this section as well as in section 5.2.

The latter is more difficult to achieve, since most of the conventional deposition

techniques cannot ensure contact between the polymer electrolyte membrane and all—or

even most—of the deposited catalyst particles. Of the various deposition techniques

available today, pulse electrodeposition has received much attention for a number of

reasons [12, 107]. It provides the worker with more control over the deposition process,

where, at least in theory, catalyst particles can selectively be deposited in places where

both ionic—via the polymer electrolyte—and electronic—through the carbon network—

pathways are readily available. The creation of nanoparticles in the range of 1.5-3.0 nm is

ensured by controlling and optimizing the electrodeposition parameters, such as the

applied cathodic current density, pulse on-time, and pulse off-time. However,

maintaining small particle size during electrodeposition is challenging, since after the

initial phase of nucleation, when small catalyst crystals are formed, these crystals have a

tendency to grow by incorporating future incoming platinum particles. Accordingly,

major studies have been conducted to gain a better understanding of the electrodeposition

process from a number of different plating bath solutions and under varying experimental

conditions. A brief review of some of the published work is given here.

In order to understand the influence of various plating parameters, such as applied current

density and pulse period, on the properties of the electrodeposited catalyst layer, it is

imperative to be able to characterize this layer with ease and certainty. A number of

studies have been conducted on the electrodeposition of platinum from different solutions

onto glassy carbon substrates [13, 394, 398, 509, 515-518]. Such substrates are easy to

prepare and work with and, more importantly, they can easily be characterized.

In 1986, Itaya et al. [517] reported the electrodeposition of platinum into Nafion® on

glassy carbon substrates. A solution of potassium hexachloroplatinate (K2PtCl6) in 1.0 M

H2SO4 was used as the electrolyte, while a 3 wt% Nafion®

solution in ethanol was

utilized to prepare the electrode coatings. Cyclic voltammetry (CV) was employed to

deposit platinum into the Nafion® film. The average size of the electrodeposited platinum

108

crystals was reported to be in the range of 10-20 nm. Ye et al. [518] also electrodeposited

Pt within a Nafion® film coated on glassy carbon electrodes. Two potential-control

methods—cyclic and constant potential—were employed to form an electrodeposited Pt

layer on the working electrode in a three-electrode cell with a Pt foil and a mercurous

sulfate electrode as the counter and reference electrodes, respectively. It was claimed

that the Pt particles prepared by the cyclic-potential method were smaller than those

prepared by the fixed-potential technique. Thompson et al. [509] investigated the

electroreduction of platinum from a solution of H3Pt(SO3)2OH onto glassy carbon as a

base material and then onto carbon black (CB) based electrodes. A three-cell electrode

containing a platinum wire as the counter electrode, a saturated calomel electrode as the

reference electrode, and glassy carbon and CB-based substrates as the working electrode

was used. All the experiments were carried out at room temperature. The electroreduction

of H3Pt(SO3)2OH to form platinum nanoparticles is considered to take place according to

the following reaction:

2

3

02

23 22)( SOPteSOPt (6-47)

It is noted that the production of sulphite ions (SO32-

) or other sulphur-containing

compounds can potentially lead to the poisoning of the platinum electrocatalysts by

sulphide ions (S2-

). A possible pathway for the formation of sulphide ions from sulphite

ions is

2

2

2

3 366 SOHHeSO (6-48)

The authors argued that the poisoning effect of sulphide ions during the electroreduction

may actually be beneficial in inhibiting the growth of platinum crystals, leading to very

small catalyst particles on the substrate. No concrete evidence was provided, but the use

of surface techniques such as X-ray photoelectron spectroscopy (XPS) was suggested to

verify the above claim.

In the early 1990s, Taylor et al. [107, 519] described and patented an electrochemical

catalyzation technique in which it was claimed platinum particles in the range of 25 to 35

Å can selectively be deposited in places where both ionic and electronic pathways exist.

Electrodes fabricated via the above process exhibited higher catalytic activities compared

109

with other state-of-the-art electrodes prepared utilizing powder-type techniques. This was

attributed to the enhancement of the three-phase interface by extending it beyond what

was possible with conventional methods. This was in agreement with McBreen’s [520]

findings that platinum particles not in contact with the solid polymer electrolyte will not

be utilized in either the hydrogen oxidation or oxygen reduction reactions in PEMFCs.

Zubimendi et al. [521] examined the early stages of platinum electrodeposition on highly

ordered pyrolytic graphite (HOPG) from a H2PtCl6 solution, and reported that the initial

stages of platinum electrodeposition involve nucleation and growth at surface defects on

the above substrate. Low cathodic potentials and low charge densities produced rounded

platinum clusters of about 2 nm in diameter, while compact platinum crystals with flat

terraces were formed as the applied cathodic potential and charge density were increased.

Hogarth et al. [522] presented a simple technique to electrodeposit platinum on pre-

treated carbon cloth for use in electrooxidation of methanol in sulphuric acid electrolytes.

A three-electrode cell fitted with the pre-treated carbon cloth as the working electrode, a

platinum gauze as the counter electrode, and a Hg/Hg2SO4 reference electrode were used

to carry out the electrodeposition. A 0.02 M solution of chloroplatinic acid was used as

the electrolyte and the platinum was electrodeposited onto the working electrode under

potentiostatic conditions (-0.30 V vs. Hg/Hg2SO4). The performance of the above

electrode was compared with that of conventional electrodes (chemically-deposited Pt)

for the oxidation of methanol in 2.5 M H2SO4 at 60 ºC. Similar performances were

reported, although the platinum crystals obtained by electrodeposition were twice the size

of those deposited using chemical methods. This was attributed to the higher catalytic

activity of platinum particles deposited by the electrodeposit technique.

Mechanistic investigations of electrochemical nucleation and growth of platinum clusters

on various substrates and under different experimental conditions are of great importance

in understanding the influence of various plating parameters on the deposit properties,

including crystal size, geometry, effective surface area, and the thickness of the deposited

layer. Kelaidopolou et al. [523] studied the kinetics of nucleation and growth of platinum

clusters on a tungsten substrate from an aqueous solution of 0.002 M K2PtCl6 and 0.1 M

HClO4. Milchev et al. [400] performed a series of experiments under the same

experimental conditions and electrolyte solution as in the previous study by

110

Kelaidopolou, but replaced the working electrode with titanium. It was reported that, as

was the case with tungsten, the nucleation and growth of platinum clusters on an oxidized

titanium surface is controlled by kinetic factors. The cathodic current transients under

potentiostatic conditions were recorded and analyzed with respect to the theory of

progressive nucleation with overlap of diffusion zones and limited number of active sites.

As previously discussed, the electrodeposition of platinum on HOPG can provide useful

information about the early stages of the nucleation process; however, the deposition

process must be halted at a very early stage of nuclei growth to avoid overlap of the

diffusion zone with the crystal growth [521, 524-526]. In addition, the electrodes

fabricated by this method contain very small amounts of catalyst, which falls short of the

critical minimum needed for use in PEM fuel cells. It is believed that not only the small

size of platinum particles, but also the distance between adjacent Pt clusters can impact

their catalytic activity in fuel cells [524, 527-530]. The latter is known to be related to the

platinum loading that, in turn, depends on the deposition time. As the deposition time is

lengthened, the amount of the deposited catalyst is increased; however, this also

encourages crystal growth and will adversely affect cell performance by lowering the

effective surface area of catalyst nanoparticles. Chen et al. [524] investigated the

nucleation and growth mechanism of Pt on microelectrodes less than 10 nm in diameter.

The extremely small surface area of such substrates allows for single nuclei to form,

making the investigation easier and more manageable. Single nucleation and growth can

be achieved at either low or high overvoltages, nucleation being controlled by the rate of

charge transfer, while growth is predominantly under diffusion control. Larger electrodes

(up to 100 nm in size) can effectively be used for single-nuclei deposition when the

overvoltage is maximized; i.e., diffusion-controlled deposition. The diffusion coefficient

of the PtCl62-

anion and the exchange current density for the reduction reaction of the

above anion to metallic Pt were reported to be (1.2 ± 0.1) 10-5

cm2 s

-1 and (8 ± 1) 10

-6

A cm-2

, respectively.

The electrodeposition of platinum often proceeds via the reduction of Pt(II) or Pt(IV)

complexes with a number of complexing ligands, including Cl, NO2, and NH3 [531]. The

first attempt to electrodeposit platinum from an aqueous solution dates back to the

111

beginning of the 19th

century in which an acidic solution based on [PtCl6]-2

was employed

[532]. Platinum has been electroreduced from a number of different aqueous solutions

such as H2PtCl6 [9, 513, 524, 533-541], K2PtCl6 [400, 509, 510, 542], K2PtCl4 [543],

K2Pt(NO2)4 [544], H3Pt(SO3)2OH [509], Pt(NH4)2Cl6 [531], Pt(NH3)4HPO4 [545], and

Pt(NH3)4Cl2 [12, 107, 519, 533]. The resulting platinum solutions are classified as acidic

or basic. In acidic solutions, in the presence of chloride ions and Pt(0), the reaction

proceeds according to:

ClPtPtClPtCl 22

2

6

2

4 (6-49)

In acidic media (HCl), the formation of (PtCl4)2-

is dominant, since the equilibrium is

shifted to the left [531]. The current efficiency of such plating baths is relatively high, but

the films deposited from such solutions contain small grain sizes and large internal

stresses, which result in cracking of films thicker than about 1m [531]. However, this

does not pose a problem in fabricating MEAs, since the goal is not to deposit thick layers

on substrates, but to electrodeposit well dispersed nano-particles at the three-phase

interface. Nevertheless, acidic solutions are not compatible with most of the existing

SPEs, almost none of which transport anions, because in electrodeposition, the catalyst

compounds must traverse the SPE layer to reach the carbon network on the other side to

be reduced. As a result, most researchers have tried to perform the catalyst

electrodeposition prior to SPE addition [9, 105, 536]. Another solution sought by several

workers has been the inclusion of a supporting salt in the aqueous electrolyte to enhance

the diffusion of anions across the membrane [535, 546].

Alkaline plating baths, on the other hand, contain the cationic form of platinum

compounds and do not suffer from the mobility restrictions inside the SPE layer like

acidic deposition solutions. Alkaline baths consist mainly of solutions of [Pt(NH3)4]2+

and

Pt(NH3)2(NO2)2, but Pt deposition from other plating baths such as [Pt(OH)6]2-

also has

been reported [531, 532]. Although platinum-containing cations can readily diffuse

through the solid polymer electrolyte, the current efficiency of such solutions is very low

at room temperature. Accordingly, the temperature of the plating bath must be increased

to enhance the current efficiency. The current efficiency of Pt(NH3)2(NO2)2 is a strong

function of temperature and increases from less than 10% at room temperature to over

112

50% at 60 C. Tetrammineplatinum (II)—[Pt(NH3)4]2+

—behaves in a similar manner, but

in order to achieve current efficiencies in excess of 50%, the plating bath temperature

must be raised to 90 C [531]. It is also common practice to maintain the pH of the

plating bath in the narrow range of 10.0-10.5 using a buffer solution such as a dilute

solution of sodium phosphate [544, 547-549]. However, deposition solutions at other pH

values also have been reported. Whalen et al. [531] investigated the deposition of

platinum film from a plating solution containing 17 mM (NH4)2PtCl6 and 250 mM

Na2HPO4 at pH 7.8.

Pulse electrodeposition of metals on conductive surfaces can be performed either in

galvanostatic or potentiostatic modes. The former can simply be carried out in a two-

electrode cell, a current source, and an arbitrary waveform generator. The potentiostatic

technique, on the other hand, requires a three-electrode cell and is not convenient for

electrodes having a geometric surface area greater than 10 cm2 [542]. Gloaguean et al.

[550] and Thomson et al. [551] evaluated the performance of PEM fuel cell MEAs

fabricated using potentiostatic pulse electrodeposition, while Choi et al. [9], Kim et al.

[105], and Coutanceau et al. [542] evaluated the performance of MEAs prepared by

galvanostatic pulse electrodeposition.

Several researchers have examined the influence of various pulse parameters on the

properties of the deposited catalyst layers for use in hydrogen fuel cells. Choi et al. [9]

investigated the effects of pulse duty cycle and applied cathodic current density on the

activity of the deposited layer and compared it with electrodes fabricated by DC

electrodeposition. With DC electrodeposition, the best electrodes are reported to be

prepared at a current density of 25 mA cm-2

. It is claimed that at higher current densities,

electrode performance drops sharply, possibly due to the growth of dendritic crystals and

the loss of the deposition layer, as a result of hydrogen evolution. However, in PC

electrodeposition, the peak current density can be significantly higher than the DC

electrodeposition and the best performance is reported with electrodes prepared at a


. The optimum pulse conditions for the above

electrodes are reported to be an on-time of 100 ms and an off-time of 300 ms for a duty

cycle of 25%. The conditions of PC electrodeposition in terms of off- and on-times and

113

the peak current density were given as 10 to 100 ms and 10 to 50 mA cm-2

, respectively.

The total amount of charge for fabrication of a single electrode through both DC and PC

electrodeposition techniques is reported to be 4 C cm-2

. The effect of electrode roughness

on the electrodeposition also was briefly discussed. The preparation of carbon electrodes

(application of the carbon ink prior to Pt electrodeposition) by brushing was suggested,

since smaller platinum particles in the neighborhood of 150 nm in diameter were

observed after the completion of the pulse electrodeposition. In contrast, the size of the

platinum deposits on electrodes prepared by the rolling method was reported to be around

250 nm in diameter.

Kim et al. [536] described a pulse electrodeposition technique in which platinum particles

smaller than 5 nm in diameter were claimed to be deposited directly on the surface of

carbon electrodes. Using an acidic plating bath containing H2PtCl6, SPE impregnation

was performed after catalyzation. It was claimed that the Pt-to-carbon ratio at a distance

of 1m from the membrane was about 75 wt% and was lowered to almost zero at a

distance of about 7 m from the membrane; therefore, it was possible to deposit a very

thin (about 5 m) layer of platinum close to the membrane, where both ionic and

electronic contacts could be secured. The authors reported that MEAs fabricated using

this technique exhibited a current density of 0.33 A cm-2

at 0.8 V with a platinum loading

of 0.25 mg cm-2

. The electrodeposition parameters for electrodes with the best fuel cell

performance were an applied peak current density of 200 mA cm-2

, a duty cycle of 4.6%,

and a total charge density of 11 C cm-2

. In a later study [105] the same authors reported a

different set of PC electrodeposition conditions under which the best electrodes were

prepared using a peak current density of 400 mA cm-2

, a duty cycle of 2.9%, and a total

charge density of 8 C cm-2

. These electrodes exhibited a high catalyst performance of 380

mA cm-2

at 0.8 V. Any increase in total charge density above 8 C cm-2

was claimed only

to increase the catalyst loading and its thickness without enhancing catalytic activity. A

range of 6 to 16 C m-2

was reported with 8 C cm-2

being the optimum charge density

under the above conditions (peak current density of 200 mA cm-2

and duty cycle 4.6%).

Ye et al. [541] studied the influence of shape control agents such as polyethylene glycol

(PEG-10000) and lead (II) acetate on the performance of electrodes prepared by pulse

114

electrodeposition. The morphology and microstructure of the platinum deposits were

reported to change from spherical clusters to clump-like crystal aggregations and finally

to elongated leaf-like flake clusters for no additives, PEG-10000 and lead (II) acetate,

respectively. It was claimed that the addition of PEG-10000 to the catalyst ink can

significantly improve the catalytic activity of the electrode towards oxygen reduction in

fuel cells. The pulse electrodeposition parameters under which the electrodes were

prepared were not clearly stated. Chen et al. [552] examined the influence of plating bath

viscosity on the performance of pulse electrodeposited platinum nanoclusters on carbon

nanotubes. Pulse electrodeposition of Pt nanocrystals from plating baths of increased

viscosity containing H2PtCl6 and glycerol was claimed to improve their activity towards

oxygen reduction. This higher catalytic activity was attributed to the ability to control the

size of the deposited Pt particles by optimizing the viscosity of the plating bath. It was

claimed that the addition of glycerol can significantly lower the diffusional mass

transport of the metal ions and, consequently, inhibit the growth of the deposited metal

crystals. Verbrugge [12] further researched the Electrocatalyzation technique first

proposed by Taylor et al. [107, 519] using two different plating baths, one containing

copper sulfate and the other tetrammine platinum (II) chloride. A Pt(NH3)4Cl2 threshold

concentration of 10 mM was recommended to ensure an acceptable current efficiency for

platinum deposition and inhibit hydrogen evolution that becomes the dominant reaction

at the cathode. Furthermore, a potential range of 0.0 V to -0.8 V (vs. Ag/AgCl) was

suggested for platinum deposition from a 10 mM Pt(NH3)4Cl2 plating bath.

Electrodeposition of bimetallic Pt alloy systems under both potentiostatic [553, 554-558]

and galvanostatic [542, 559, 560] control also has been reported. Coutanceau et al. [542]

investigated the influence of the relaxation time on the electrocatalyst particle size and

performance of anodes fabricated by galvanostatic pulse electrodeposition of Pt-Ru on

carbon electrodes for use in DMFCs. It was claimed that shorter off-times produced

smaller particle size with higher performance. This is in sharp contrast with other

workers’ findings using Pt alone [9, 107, 519]. It was reported that at loadings of 2 mg

cm-2

of Pt-Ru alloy, most catalyst particles range from 5 to 8 nm in size, with the best

performance delivered by MEAs containing a Pt-Ru atomic ratio of 80:20 in the

operating temperature range 50-110 C. Fujita et al. [555] also studied the influence of

115

pulse off-time on the properties of Co-Pt thick film magnets, including current efficiency,

internal stress, and film composition. The pulse on-time and current density were

constant at 2.0 ms and 50 mA cm-2

, respectively, while the pulse off-time was varied

between 0.0 and 58.0 ms. The current efficiency was observed to increase from 5% to

50% as the pulse off-time was increased from 0.0 to 30.0 ms, and then stayed constant at

pulse off-times greater than 30 ms. The observed improvement in the current efficiency

was attributed to the retardation of hydrogen evolution.

Wei et al. [553] pulse electrodeposited a thin layer of Pt-Ru catalyst on a glassy carbon

rotating disk electrode on which a layer of Nafion® had previously been applied. All

electrodepositions were performed under potentiostatic conditions and it was claimed that

the performance of electrodes fabricated via the above method with an estimated loading

of 77 g Pt-Ru/cm2 was superior to that of conventional electrodes with loadings of 100

g Pt-Ru/cm2. Nishimura et al. [558] developed a method for the electrodeposition of Pt-

based nanoparticles in conjunction with Ni by means of double potential step electrolysis.

Platinum-nickel alloy was deposited at the first step, followed by the dissolution of Ni

during the second step. It was reported that Pt-Ni nanoparticles in the order of 5.4 ± 1.5

nm could be formed. It was further claimed that the oxygen reduction electrocatalytic

activity of electrodes fabricated by the above technique was twice that of electrodes

containing pure platinum. Leistner et al. [560] developed a pulse electrodeposition

method for preparing Fe/Pt multilayers with low oxygen content for use in micromagnets

and high density magnetic data storage systems. The pulse electrodeposition was carried

out under potential control using a single bath technique. Pulse durations ranging from 25

s to 20 min were studied; however, other pulse parameters were not stated. It was claimed

that Fe/Pt bilayers as thin as 40 nm with low oxygen content can be achieved using the

above method.

6.3.5.5 Other Metals and Alloys

The electrodeposition of metallic alloys has been extensively studied for decades [561,

562]. These alloys have been found to be very useful in a number of industries, including

microelectronics for high density magnetic recording devices, high-end tool

116

manufacturers for corrosion and wear protection, and biomedical applications in which

there is an ever-growing demand for precision devices such as implants.

Zinc alloys have been investigated by several workers because of their superior protective

properties [563-565] and their observed catalytic activity [566-569]. Zn-Ni alloy has been

the alloy of choice for many reasons, the most important of which is its superior

corrosion resistance. It is widely reported that pulse electrodeposited Zn-Ni layers exhibit

better corrosion resistance than similar layers deposited using conventional methods. This

has been attributed primarily to the existence of a -phase and a more compact and

relatively homogeneous layer [391, 570-573]. The use of Zn-Co alloys also has been

investigated by several workers for their anti-corrosion characteristics [574] as well as for

their promising catalytic activity [575, 576]. Fei et al. [577] studied the influence of

pulse-reverse parameters on the properties of electrodeposited Zn-Co layers of varying

cobalt content prepared under different plating conditions. Most studies have reported a

maximum Co content of 6-7 wt% due to limitations arising from anomalous co-

deposition [577, 578-580]. Fei et al. [577] showed that the cobalt content in Zn-Co alloy

deposits is a strong function of both the average current density and the fraction of the

pulse reverse in square pulse reverse electrodeposition. The frequency of the pulse was

reported to have little impact on the outcomes, but does influence the microstructure of

the resulting Zn-Co layer. High cobalt content (greater than 90 wt%) was achieved at

average current densities of 10 mA cm-2

or lower, with a sharp decrease when the current

density was increased from 10 to 20 mA cm-2

, followed by a gradual increase beyond 20

mA cm-2

. Zn-Mn alloys have been studied mainly for their exceptional protective

properties [564, 581], with alloys having a Mn content of 30-40% exhibiting the highest

corrosion resistance [565, 582]. Müller et al. [565] examined the influence of pulse,

pulse reverse, and superimposed current modulations on the properties of Zn-Mn

deposits. The alloys were characterized by their composition, structure, thickness, and

morphology. It was reported that all the pulse forms employed having the same average

current density created deposits with higher manganese content compared with deposits

prepared using direct current electrodeposition, but that the current efficiencies were

lower. Deposits created by pulse reverse electrodeposition are reported to exhibit the best

117

results in terms of thickness and composition. Pulse electrodeposition of pure zinc as

protective layers also has been studied. Youssef et al. [583] conducted a series of

corrosion experiments on electrodeposited zinc and electrogalvanized steel in de-aerated

0.50 M NaOH solution. It was claimed that the estimated corrosion rate (90 A cm-2

) of

the electrodeposited zinc specimen was about 60% lower than that of the

electrogalvanized steel (229 A cm-2

).

In recent years, the electrodeposition of metal oxide films has attracted the attention of

many researchers in a number of different fields, including electronic, catalytic, and

biomedical. Of special interest is the electrodeposition of manganese dioxides for use in

lithium batteries [584], sensors [585], and supercapacitors [586]. Moses et al. [587]

investigated the microstructure and properties of manganese dioxide films prepared by

galvanostatic, pulse, and pulse reverse electrodeposition techniques using KMnO4

solutions of various concentrations, ranging from 0.01 to 0.10 M. Oxide films prepared

by pulse-reverse electrodeposition exhibited higher specific capacitance than those

generated by other techniques, including galvanostatic methods. The plating conditions in

pulse reverse were a constant cathodic current density of 1-2 mA cm-2

for a period of 2-5

min followed by a reverse current density (i.e., opposite polarity) of 0.5-1 mA cm-2

for

0.5-1 min. Wu et al. [588], who examined the influence of PC electrodeposition on the

properties of deposited Mn-Co films for in SOFCs as interconnects, found that the Mn

content decreased with increasing off-time and that the surface morphologies changed

from flake-like particles to crystalline structures.

Other metals and alloys that have benefited from pulse electrodeposition—both under

potentiostatic and galvanostatic control—are gold [589, 590], Au-Co [591], Au-Ni [592],

chromium [593, 595], Cr-Ni [596], silver [597], Ag-Sn [392], and Ag-Pd [598]. In fact,

the benefits of PC electrodeposition became apparent in the early 1950s when it was

originally employed to deposit gold from a number of acidic electrolyte solutions onto

different substrates, where it had been tried unsuccessfully in the past using DC

electrodeposition [599, 600].

118

6.0 EXPERIMENTAL PROCEDURE

7.1 Hydrophobic Polymer Coating

Commercial carbon papers (Toray TGPH-030, 060, 090, 120; E-TEK) and cloths (ELAT,

E-TEK) were made hydrophobic by treating with PTFE (60 wt% solid, Alfa Aesar)

according to the following procedure. Substrates were first cut into desired shapes

(circles of 3.5 cm in diameter, in most cases) and immersed in acetone (ACS reagent

grade) for 1 h to remove any dust and foreign matter from the carbon fibers. The

substrates were rinsed for 5 minutes with fresh acetone and then dried in an oven at 80 C

for several hours. The carbon papers/cloths were slowly lowered into a PTFE suspension

and left for 5 minutes before they were removed and placed in an oven to dry. To ensure

a uniform drying and to avoid the migration of PTFE to only small areas on the

substrates, they were placed on six long needles (with pointed ends up) inside an oven

and dried for several hours at 80-90 C as shown in Figure 7-1. PTFE loadings were

determined by gravimetric analysis and the above procedure was repeated until the

desired loading was achieved. A brushing method also was employed, in which PTFE

was applied using an artist’s paint brush. Extra care was exercised to ensure uniform

PTFE distribution by applying constant paintbrush strokes. Samples were finally placed

in an oven and sintered at 360 C for 20 min. A number of MEAs were fabricated using

pre-treated carbon substrates (treated with FEP or PTFE by the manufacturer); such

samples are clearly identified in this study.

Figure 7-1 Drying of substrates in an oven

119

7.2 Sintering of Treated Carbon Substrates

The impact of sintering temperature and duration on the through-plane electrical

resistance of PTFE-treated carbon substrates was determined by evaluating through-plane

electrical resistance as a function of PTFE content and applied contact pressure, ranging

from 100 to 500 bars. After PTFE application, each sample was placed in an oven and

rapidly heated up to a pre-set temperature, ranging from 100 C to 430 C. For various

sintering temperatures, the sintering time was kept constant at 30 minutes and was

recorded from the time the oven reached the desired temperature. The oven then was

quickly turned off and the samples removed several minutes after being cooled down; the

oven temperature was in the neighbourhood of 50-60 C at the time of sample removal.

The samples were then transferred into a moisture-free environment for future analysis.

The influence of sintering duration on through-plane electrical resistance of treated-

carbon substrates was determined by keeping the sintering temperature constant at 360

C for all samples, while varying the sintering time from 10 to 50 minutes. The oven

temperature was 360 C at the time of sample placement and maintained at this

temperature for the duration of the experiment. The PTFE-to-substrate ratio (based on

mass) was around 1.0-1.1 for all samples.

The through-plane electrical resistance of each sample was measured by placing it

between two polished copper plates each with a contact surface area of 25 cm2 and

connected by wires to a milliohm meter (Chroma 16502, four terminal test cable with

temperature compensation card, at 1 kHz).

7.3 Carbon Ink Preparation: Microporous Layer Application

Membrane-electrode assemblies were first fabricated by preparing a homogeneous

suspension referred to as ―carbon ink‖. The first step involved the treatment of carbon

black (Vulcan XC-72 or another carbon/graphite powder) at 600 C for 3 hours to remove

any trace organic matter. Pretreated carbon black, PTFE, and isopropyl alcohol (HPLC

grade) then were mixed in an ultrasonic bath and/or ultrasonic homogenizer. Electrodes

with various diffusion layer loadings (0.5-2.0 mg cm-2

) were prepared by brushing the

120

prepared paste onto one side of a wet-proofed carbon cloth or paper and then dried in an

oven at 200 C for 6 hours. This layer is known as the micro-porous layer (MPL). This

diffusion layer or MPL is strategically placed between the catalyst and gas diffusion

layers to remove water from inside the catalyst layer via capillary action, to enhance the

electronic conductivity of the MEA, and to prevent the movement of the catalyst layer

further into the GDL. The side with the carbon ink is referred to as the ―carbon side‖,

while the other side with only PTFE is known as the ―gas side‖. Each electrode was

weighed before and after the application of the carbon ink. The resulting carbon substrate

has a strong hydrophobic nature. Several carbon substrates also were treated with a

hydrophilic paste to add a hydrophilic layer on top of the hydrophobic layer. Glycerol

was added to the carbon ink and then homogenized in an ultrasonic bath for at least one

hour before brushing onto the hydrophobic layer. The hydrophilic layer loading was 0.5

or 1.0 mg/cm2 in all cases. Both the anode and cathode were prepared in this manner.

7.4 Nafion® Impregnation

Nafion® impregnation was achieved utilizing two widely used methods: floating and

brushing. In the floating method, a partially fabricated gas diffusion electrode (GDE),

from the previous step, was floated, carbon-face down, on the surface of an alcoholic

solution of 5% Nafion® (by weight) in a shallow beaker. The solid polymer electrolyte

solution was allowed to penetrate part way into the gas permeable face. An impregnation

time of 30 seconds was used, which provided Nafion® loadings of about 1.0 mg cm

-2.

Excess solution was allowed to drain off the electrode surface, which then was dried in

air for 12 hours. In the brushing method, an artist’s brush was used to gently brush

Nafion® solution onto the carbon face of the electrode. Extra care was exercised to

maintain an identical process throughout the experiments by keeping the number of

strokes and soaking time (soaking the brush in the Nafion® solution) constant. The extent

of the impregnation was determined by using scanning electron microscopy.

The ion-exchange capacity (IEC) of a number of samples was determined by placing

them in 250 mL of 2.0 M NaCl solution while purging nitrogen gas through the solution

for one hour to convert the membrane from the H+

form to the Na+ form. The resulting

121

solution then was titrated with standardized 0.05 M NaOH to an end point. The volume

of NaOH consumed was noted and used to calculate the moles of H+ in solution.

Assuming complete conversion of the membrane to the Na+ form, the ion-exchange

capacity was calculated using:

IEC (VNaOH ) (CNaOH ) (1/mass of the sample) (1eq mol1) (1000 meq eq1) (71)

where, VNaOH = volume of NaOH (L)

CNaOH = concentration of NaOH (mol L-1

)

IEC = ion-exchange capacity (meq g-1

)

7.5 Catalyst Electrodeposition

7.5.1 Platinum Electrodeposition

Electrodeposition was primarily carried out at 20 C with several test runs at 50 C in a

flow cell (see Figure 7-2) using a platinum solution containing 0.05 M Pt(NH3)4Cl2. All

solutions were prepared from analytic grade chemicals and deionized water (18 MΩ·cm).

The uncatalyzed carbon substrate was mounted inside the flow cell on a sample holder

coupled with a platinum disk and a platinum wire as current collectors. The substrate size

was kept constant at 3.5 cm diameter throughout the experiments. A platinum disk of 2.5

cm in diameter was used as the anode. A Masterflex peristaltic pump was used to

circulate the electrolyte through the flow cell during electrodeposition. The electrolyte

container was kept in a water bath to ensure a constant temperature during the

electrodeposition process. Two different sizes of tubing—Tygon L/S 16 and L/S 25—

were used with the pump to achieve various flow rates. Also, calibrations were performed

using the copper plating bath solution at two different temperatures, 25 °C and 50 °C.

The solution was run through the pump and into a graduated cylinder where it was

carefully measured. The results are tabulated in Tables C-1 through C-4 and shown

graphically in Figure C-1 in Appendix C.

122

Figure 7-2 Electrodeposition flow cell

Plating Solution

OUT

Plating Solution

IN

Platinum

Disk

(Anode)

Carbon

Substrate

(Cathode)

123

An EG&G (model 371, PARC or model 273, PARC) potentiostat coupled with an EG&G

universal programmer (model 175, PARC) was used to control both the pulse wave and

the deposition current density. An oscilloscope (HP model 54504A) was employed to

monitor the waveform and other parameters such as duty cycle and peak current density

throughout each experiment. The peak deposition current densities, pulse on-time, pulse

off-time and the duty cycles were varied to optimize the deposition process and

consequently, the dispersion and size of the catalyst particles. Catalyst loadings in the

range of 0.05-0.50 mg cm-2

for both anode and cathode were prepared by controlling the

electrodeposition peak current density and total charge density. At the completion of

electrodeposition, each electrode was washed with deionized water (18 MΩ·cm) for at

least 15 minutes to remove any free platinum from the substrate pores. The electrodes

then were heated in air at 300 C for 1 h to remove the solvent in the hydrophobic layer

of the electrode. Four different waveforms—rectangular, triangular, ramp-up and ramp-

down—were employed to perform electrodeposition (Figure 7-3).

Figure 7-3 Different types of waveform

ip

ip

t t

i

i

Rectangular Triangular

Ramp up Ramp down

a b

a a

124

7.5.2 Copper Electrodeposition

Copper electrodeposition on carbon substrates was initially carried out to gain experience

with the proposed technique and to lower cost because of the prohibitive cost of

platinum. Electrodeposition was performed at 20 C using an acidic bath containing 0.05

M CuSO4.5H2O and 0.5 M H2SO4 (all analytic grade chemicals) under both direct current

(DC) and pulse current (PC) electrodeposition. Similar to platinum electrodeposition, pre-

treated carbon papers and cloths were used as the substrates of choice. The equipment

setup and experimental procedure were similar to the method utilized for platinum

electrodeposition—section 7.5.1.

7.5.3 Nickel Electrodeposition

Nickel electrodeposition also was experimented with for the same reasons as copper. A

modified Watts nickel bath and electroplating conditions are presented in Table 7-1. All

solutions were prepared from analytic grade chemicals and deionized water. The

electrolyte bath was agitated by a mechanical stirrer at 500 rpm and the temperature

maintained at 50 °C throughout the plating process. The initial pH of the electrolyte was

4.2, a value commonly used for Ni electroplating. AISI 431 stainless steel (3.5 cm in

diameter) was used as a substrate.

Table 7-1 Bath composition and electroplating conditions for Ni plating

Nickel Sulphamate 330 g L-1

Nickel Chloride 15 g L-1

Boric Acid 30 g L-1

Sodium Dodecyl Sulphate 0.2 g L-1

pH 4.2

Temperature 50 ± 1 °C

Peak Current Density 10 – 1000 mA cm-2

Duty Cycle 2 – 100%

Period of One Cycle µs - ms

125

Prior to placement inside the electroplating cell, the substrate was first ground to a finish

on grade 180 emery paper, rinsed with deionized water, and then scrubbed with alcohol

and acetone and finally rinsed with deionized water. After electrodeposition, the surface

morphology of each specimen was examined and characterized using SEM. The grain

sizes were measured according to ASTM E112-95, while microhardness tests were

performed in accordance with ASTM E384. A brief description of the latter method is

provided in Appendix E.

7.6 MEA Fabrication and Testing

7.6.1 MEA Preparation

The Pt-containing electrodes (both cathode and anode) fabricated according to the

method presented in section 7.5.1 in conjunction with a solid membrane (Nafion® 112,

115 or 117) were bonded to form an MEA by hot pressing at 130 C and 1500 kPa for 2

min for carbon cloth and at 125 C and 1400 kPa for 3 min for carbon paper substrates

(see Figure 7-4). A single 5-cm2 fuel cell with serpentine flow pattern (EFC05-01SP,

Electrochem Inc.) in conjunction with a gas supply and measuring devices was used to

evaluate the performance of all MEAs.

7.6.2 Electrochemical Measurements

7.6.2.1 Single Fuel Cell Tests

Two different experimental set-ups were used to characterize the MEAs in a 5-cm2 fuel

cell. The first set-up included two mass flow meters to accurately and independently

control and monitor the flow rates of reactant gases, two humidifiers to control the

humidity levels of entering gases, several digital multimeters to monitor and record the

cell outputs (Flukes 187 and 189 multimeters, Fluke Corp.), a resistance decade box

(model 1434-N, General Radio CO), and a single 5-cm2 PEM fuel cell with internal

heaters (EFC05-01SP, Electrochem Inc.). A simple schematic of this set-up is shown in

Figure 7-5. A fully automated multi-range fuel cell test station (Series 850e, Scribner

Associates Inc.) and a manual fuel cell test system (Electrochem Inc.) also were

employed to perform some of the tests in the later stages of the research.

126

Figure 7-4 A simple representation of the MEA fabrication process

(1) (2) (3) (4)

(5) (6)

Carbon

Substrate

(untreated)

Microporous

Layer

(PTFE + C)

Nafion®

Layer

(deposited)

Platinum

Nanoparticles

(electrodeposited)

+ +

Anode SPE Cathode

Hot-bonded and

pressed

Complete MEA

127

Figure 7-5 A simple schematic of the experimental setup for MEA characterization

7.6.2.2 Life Test and Durability Assessment of MEAs: Static Testing2

The durability of different MEAs was evaluated using a 5-cm2 single cell with serpentine

flow fields at a constant cell temperature of 60 °C with both reactants being fully

humidified at 65 °C before entering the cell. Hydrogen and air (or pure oxygen with a

stoichiometry of 1.5) were used as fuel and oxidant with stoichiometries of 1.2 and 2.5,

respectively. Polarization and potential-time curves were obtained using a fully

automated fuel cell test station (Series 850e, Scribner Associates Inc.) in a galvanostatic

polarization mode. Potential-time curves were obtained at a constant load; i.e., 619 mA

cm-2

, for a maximum of 4000 h, at specified time intervals. Each MEA was conditioned

with fully-humidified fuel and oxidant at 65 °C and a constant load of 50 mA cm-2

for 24

h prior to testing to ensure the high ionic conductivity of the Nafion® layers in the

MEA—the SPE separating the anode and the cathode and the solubilized Nafion® that

2 These tests were carried out at Lambton College under the supervision of the author

Anode

Cathode

SPE

H2

O2

or

Air

N2

Humidifiers Single-Cell

Fuel Cell

Mass

Flow Meters

Back-Pressure

Regulators

128

was applied onto the GDE before catalyzation. MEAs also were analyzed after the

completion of lifetime tests using SEM to observe any change in their morphology.

7.6.2.3 Life Test and Durability Determination of MEAs: Dynamic Testing3

Two series of experiments were carried out to determine the durability and reliability of

MEAs prepared according to the pulse-electrodeposition technique proposed in this

thesis. In the first series, the behaviour of the MEAs in a real application under constant

load was determined by installing them into a 200-W PEM fuel cell stack (Horizon Fuel

Cell Technologies Pte. Ltd.) running on hydrogen and air without external

humidification. Hydrogen was supplied by a 900-L metal hydride canister (Ovonics,

USA). The fuel cell stack then was used to charge a battery bank containing three 12

VDC lead-acid batteries each with 7.2 A-h capacity. The battery bank, in turn, was used

to power a tricycle with a 350-W electric motor (BionX, Canada). The power output of

the stack was monitored and recorded over a period of twenty-six weeks for a total of 60

individual charges. A schematic representation of the tricycle fuel cell system is shown in

Figure 7-6.

In the second series, the true dynamic behaviour of in-house and commercial MEAs was

assessed in two separate 200-W PEM fuel cell stacks (Horizon Fuel Cell Technologies

Pte. Ltd.), one containing in-house MEAs and another using commercial MEAs. In

contrast to the tricycle fuel cell system, this setup did not have a battery bank. Instead, the

fuel cell stack was used to directly power a bicycle. Real-time data were collected at three

different stages: before the operation of the bicycle, during the operation (when the

bicycle was on the road relying on fuel cell stack power) and after the completion of road

trials. Polarization and open-circuit voltage (OCV) curves at different operating times

were compared. In addition, maximum power outputs from single 200-W PEM fuel cell

stacks at full and partial loads while the bicycle was in operation were recorded. This

system is shown in Figure 7-7. The bicycle was powered by only one of the stacks at any

time during the experimental phase and each stack was operated for three hours per day

for 112 days.

3 These tests were carried out at Lambton College under the supervision of the author

129

Figure 7-6 Schematic representation of a 200-W PEMFC system used in a tricycle

200-W

FUEL CELL

STACK

130

Figure 7-7 Fuel cell bicycle (direct drive)

7.6.2.4 X-Ray Diffraction

Catalyzed substrates were characterized using an X-ray diffractometer (Rigaku MSC,

Woodlands, TX, USA) employing a graphite crystal counter monochromator filtering Cu

Kβ radiation. The operating parameters of the X-ray source were 40 kV and 40 mA with

the patterns recorded in a 2θ, ranging from 20-90° and a step scanning rate of 1° per

minute.

7.6.2.5 Scanning Electron Microscopy

Gas diffusion electrodes and membrane-electrode assemblies were examined by scanning

electron microscopy using a high resolution SEM (S-4700, Hitachi Ltd.) coupled with an

EDX. Incident electron beam energies from 3 – 30 keV were utilized. SEM was used to

study the surface morphology of GDLs before and after the application of PTFE, MPLs

and catalyst layers, as well as to determine the thicknesses of the various MEA

components.

131

7.6.2.6 Transmission Electron Microscopy

Catalysts were characterized by transmission electron microscopy (TEM) using a high-

resolution TEM (JEOLEM-2010EX, Japan) with a spatial resolution of 1.0 nm. Particle

size analysis was performed by digital image processing of bright field transmission

electron micrographs of platinum particles with background correction. The latter was

carried out by digitally reducing the background levels and increasing the global intensity

differences between platinum nanoparticles and the support material.

7.7 Porosity Measurements of Gas Diffusion Layer

The bulk porosity of each untreated GDL was determined using a mercury porosimeter

(Micromeritics Autopore IV 9500, Micromeritics, USA). Mercury Porosimetry requires

the specimen to be completely dry; accordingly, all samples were dried in a vacuum oven

at 200 °C for 12 hours prior to testing. After placing each sample inside the sample

holder, the mercury pressure was gradually increased from 0.003 to 200 MPa and the

corresponding intruded volume measured and recorded. The total pore volume of each

sample was taken as the total cumulative volume of intruded mercury. Pore size

distributions also were estimated by measuring and recording the incremental volume of

mercury at each pressure. A contact angle of 130° and a mercury/vapour surface tension

of 0.485 N m-1

were assumed for calculation purposes.

The bulk porosity of a number of GDLs also was determined by immersing the samples

in decane—a wetting fluid—and utilizing gravimetric analysis. ―Dry‖ samples were first

weighed using a semi-micro balance and then were completely immersed in a shallow

beaker filled with decane and left undisturbed for 20 minutes, after which they were

removed from the beaker and excess solution allowed to drain. Samples were

immediately dried using felt-free papers and their masses measured and recorded. This

method was found to be simple and yielded results in good agreement with those

obtained from porosimetry.

132

7.8 Through-Plane Gas Permeability Measurement

Gas permeability through the plane (along the z direction) of a number of untreated GDLs

was measured using the in-house-fabricated apparatus, similar to the device used by

Gostick et al. [620], shown in Figure 7-8. The size of each sample was 3.3 cm in diameter

and, as can be seen in Figure 7-8, each GDL was compressed and secured between two

plates and a tight gas seal obtained by tightening the eight bolts. Air was fed to the GDL

via the circular channel at the centre of the top plate and the flow rate of the gas (air in

this study) at the outlet was measured using a digital flow meter (Omega FVL-1616A;

with a maximum flow rate of 20 sccm). The pressure change across the GDL was also

measured with a differential pressure transducer (Omega PX653) in conjunction with a

data acquisition system (National Instrument, NI USB-6009).

The pressure drop of each sample for a constant flow rate was measured six times and an

average value was calculated. The thickness of each substrate was measured using a

caliper. The gas permeability of the samples was evaluated by calculating the

corresponding gas permeability coefficients, k, based on the following rearranged form of

Darcy’s law.

)( vP

Lk

(7-2)

where, k is the substrate permeability coefficient, m2; L is the substrate thickness, m;

P is the pressure drop across the substrate, Pa; v is the superficial velocity, calculated

from the air flow rate divided by the substrate area, m s-1

; and

is the fluid viscosity (1.8

10-5

Pas, for air at 25 °C).

133

Figure 7-8 Laboratory apparatus for through-plane permeability measurement of GDLs

ΔP

Air In

Air Out

Data

Logger

Test Sample

134

7.0 RESULTS AND DISCUSSION

8.1 Influence of Hydrophobic Polymer (PTFE) Content in GDL

The original two-layer electrodes of the 1980s have evolved into the more effective three-

layer electrodes of today. Early electrodes comprised a wet-proofed support layer and a

catalyst layer consisting of carbon-supported platinum, solubilized SPE (e.g., Nafion®)

and PTFE. Today’s electrodes have incorporated a microporous layer (MPL) sandwiched

between the catalyst and the gas diffusion layers. The performance of three-layer

electrodes is generally superior to that of dual-layer electrodes. However, the

performance of fuel cell electrodes is influenced by many factors, including (1) type of

support material; i.e., carbon paper or cloth and whether it is woven or non-woven, (2)

the thickness and porosity of the support material, (3) the amount of hydrophobic

polymer, (4) type of electrocatalyst, including size, morphology, loading, type of carbon

support, and presence of other metals (bimetallic electrocatalysts), (5) SPE loading and

type (thickness and equivalent weight), (6) thickness of the GDL, and (7) fabrication

method and thermal treatment [10, 328].

The primary function of a GDL is twofold: to ensure a homogeneous distribution of the

reactant gases towards the catalyst layer and to remove the water generated inside the

catalyst layer by effectively removing it from the interior of the MEA and transporting it

towards the bipolar plates, where it leaves the cell or stack. The latter is achieved by

employing a hydrophobic polymer such as PTFE, which is first applied on both sides of

untreated substrates (carbon paper or cloth) to make them hydrophobic and then, in an

additional step, mixed with carbon powder and applied onto the gas diffusion electrode

(GDE) prior to Nafion®

impregnation and catalyst deposition to form a microporous

layer. Since PTFE is not electrically conductive, its proportion with respect to carbon

powder must be carefully selected and optimized to enhance its water blocking properties

while at the same time minimizing ohmic losses. It is, therefore, critical to determine the

optimum weight ratio between PTFE and the substrate to provide an adequate network of

macropores for fluid transport during fuel cell/stack operation while maintaining high

electronic conductivity. It is also imperative to optimize the ratio of PTFE to carbon

powder in the microporous layer of the GDL to enhance fuel cell performance. In

addition to the above functions, the GDL provides a support for the MEA membrane and

135

catalyst layers, and, more importantly, creates a pathway for the effective transport of

electrons from inside the MEA to the current collectors, and ultimately, to the external

circuit.

8.1.1 Influence of PTFE Loading on Cell Performance

A series of experiments was performed to determine the effect of PTFE loading, ranging

from 20% to 150% (based on the untreated weight of the substrate) on the substrate

through-plane conductivity. The extent of the surface coverage of the carbon fibers by the

hydrophobic polymer can clearly be seen in the SEM images of Figure 8-1, where

untreated (a) and wet-proofed (b) carbon papers are shown. The through-plane

conductivity of all carbon substrates decreased as the level of hydrophobic polymer

increased. This is expected since higher PTFE loadings result in more coverage of the

substrate fibers, which conduct electrons from one side of the GDL to the other.

(a) (b)

Figure 8-1 Surface morphology of carbon paper substrates before (a) and after (b) PTFE

application (60 wt%)

The through-plane resistance of Toray TGPH-090 carbon papers treated with varying

amounts of PTFE and subjected to applied pressures ranging from 50 to 480 bars is

shown in Figures 8-2(a) and (b). Similar results were obtained for other types of carbon

papers and cloths.

136

According to Figure 8-2(a), at all pressures the resistance increases almost linearly with

increased PTFE loading, but drops with increased pressure, especially if the PTFE

loading is high. This can be attributed primarily to the changes in PTFE distribution on

the surface, and ultimately, inside the micro- and macro-pores of the substrate as the

applied pressure is increased. Initially, when the pressure is negligible, most of the PTFE

resides on the surface of the substrate, creating an insulating barrier to the transport of

electrons across the carbon substrate, resulting inevitably in high through-plane electrical

resistance. Also, the applied pressure forces the substrate fibers to come into closer

contact, creating an additional series of pathways for electronic conduction, further

lowering the resistance.

Figure 8-2 (a) Impact of PTFE loading on through-plane resistivity of PTFE-treated

Toray TGP-H-090 carbon papers at various pressures (sintering temperature: 360 °C;


137

Figure 8-2 (b) Impact of PTFE loading on through-plane resistivity of PTFE-treated

Toray TGP-H-090 carbon papers at various pressures (sintering temperature: 360 °C;


As the pressure begins to increase and passes a critical point (around 150 bar for most

samples), the through-plane electrical resistance starts to fall less rapidly with further

increases in pressure. At applied pressures greater than about 350 bar, the change in

through-plane electrical resistance is negligible for most PTFE loadings. At this point, the

PTFE is well distributed both on the substrate surface and inside the pores. Regardless of

the applied pressure, no new contact points are created between the carbon fibers, hence

no more clear pathways for the conduction of electrons, and no further increase in

conductivity.

The influence of sintering temperature on the through-plane electrical resistance of a

number of carbon substrates also was investigated. The result for one of the carbon-paper

substrates—Toray TGPH-090—is presented in Figure 8-3. The first noticeable trend is

the marked increase in the through-plane electrical resistance of substrates sintered at

higher temperatures. This is expected since higher sintering temperatures result in a more

uniform distribution of the hydrophobic polymer both on the substrate surface and within

the micropores, consequently lowering the electronic conductivity by effectively covering

138

Figure 8-3 Influence of sintering temperature on through-plane resistance of PTFE-

treated Toray TGPH-090 carbon papers subjected to varying sressures (PTFE loading:

110%; surface area: 25 cm2)

the carbon substrate fibers. As before, the decrease in through-plane electrical resistance

of all samples with increasing applied pressure is mainly due to the formation of new

carbon networks inside the GDL.

8.1.2 Influence of PTFE Loading in Microporous Layer on Cell Performance

A series of experiments was carried out to examine the influence on fuel cell performance

of hydrophobic polymer content in the MPL of GDEs and to determine the optimal

amount for carbon paper substrates. Both anodes and cathodes were prepared in the same

way. Figure 8-4 shows the polarization curves for MEAs made with Toray TGHP-090

carbon paper and different PTFE loadings, ranging from 10 wt% to 60 wt% (with respect

to the carbon powder in the MPL). Each data point represents an average of at least five

steady-state runs. The amount of PTFE was kept constant in both anode and cathode in

this series of experiments. As can be seen from Figure 8-4, the best result was obtained

with GDLs containing 20 wt% PTFE. This is in agreement with observations reported by

other researchers [426, 601].

139

Figure 8-4 Influence of PTFE content on fuel cell performance operated at a cell


per electrode;

fully humidified fuel and oxidant with stoichiometries of 1.2 and 2.5, respectively

The curve for 30 wt% PTFE shows nearly equal performance, especially at higher current

densities. The reason that the 30 wt% PTFE becomes increasingly as effective as the 20

wt% PTFE as the current density increases may be attributed to the better water removal

ability of the GDL with a higher PTFE content. At these high water-producing levels, 30

wt% PTFE is more effective in preventing flooding of the GDL by effectively removing

the excess water away from the catalyst and gas diffusion layers. Overall, however, the

20 wt% PTFE was found to be the best for all practical situations, considering the fact

that PEMFCs are not normally operated at high current densities. At higher PTFE levels

(40 wt% and greater), however, ohmic losses become dominant and the overall cell

performance decreases. In addition, high PTFE levels can influence the pore-forming

capability of GDLs; high PTFE levels in GDLs are known to decrease the porosity of

diffusion layers, which adversely affects the GDL by lowering its ability to remove water

from inside the MEA and transport gases to and from the catalyst layers.

140

Figure 8-5 shows the correlation between PTFE content and the voltage obtained at

several different current densities ranging from 200 to 1000 mA cm-2

. As previously

mentioned, the best result is obtained for electrodes with 20 wt% PTFE. Hydrophobic

polymer contents of greater than 20 wt% (with respect to carbon) would contribute to

ohmic losses caused by the non-conducting PTFE.

On the other hand, PTFE contents less than 20 wt% would allow better electric

conductivity, but may limit the access of reactant gases to the reaction sites, resulting in a

lower performance due to greater diffusion and activation losses. Furthermore,

inadequate PTFE content would allow the retention of water inside the MEA, creating a

barrier, which limits the transport of gases to and products away from the reaction sites

and retarding half-cell reaction rates. In addition, liquid water can cover the surface of

active catalyst sites required for half-cell reactions and hinder hydrogen oxidation at the

anode, and more importantly, oxygen reduction at the cathode.

Figure 8-5 Fuel cell potential with varying PTFE content at different current densities

operated at a cell temperature of 50 °C in H2/Air with a platinum loading of 0.3 mg cm-2

per electrode; fully humidified fuel and oxidant with stoichiometries of 1.2 and 2.5,

respectively

141

As mentioned previously, the slight increase in the voltage at high current densities when

the PTFE content is increased from 20 to 30 wt% occurs in the mass-transport limited

region (i > 0.9 A cm-2

), and is due to better GDL water management at higher PTFE

content. Most fuel cells, however, are operated in the ohmic region, where the operating

voltage for a single cell is about 0.6-0.8 V. Fig. 8-5 clearly shows that for all practical

operating conditions, 20 wt% PTFE content exhibits the best performance.

8.1.3 Influence of MPL Loading on Cell Performance

Extensive experimental work has been performed and reported by many researchers on

the composition of the diffusion layer and the type of carbon powder utilized [9-11].

However, little attention has been directed at the effect of microporous layer (MPL)

loading or thickness on the electrode flooding level and, ultimately, on fuel cell

performance. It is apparent from Fig. 8-6 that thin MPLs (i.e., lower loadings) are very

sensitive to liquid water accumulation. The pore volume that is required to conduct fluid

transport inside the MEA—fuel and oxidant gases to the catalyst layers and liquid water

away from the catalyst layers—is smaller for thinner MPLs. Therefore, when liquid water

enters the GDL at a given rate, a thinner MPL is expected to have a higher liquid water

saturation level than a thicker one, resulting in less available pore volume for fluid

transport. Conversely, a thicker MPL is capable of retaining more liquid water and still

have enough free pores to transport fluids to and from the catalyst layers. However, as

discussed above, excessive PTFE loading, which results in thicker MPLs, reduces the

electronic pathways, making it more difficult for electrons to leave the GDL, thereby

lowering overall cell performance.

Figure 8-6 indicates that the best results are obtained for electrodes with 1.5 mg PTFE-C

cm-2

. Microporous layers with less than 1.0 mg PTFE-C cm-2

exhibit the worst cell

performance due to lack of sufficient hydrophobic polymer in the diffusion layer. This

contributes to diffusion losses since most pores in this layer are filled with liquid water,

thereby impeding gas transport to and from the active sites in the catalyst layer. There

also is the possibility of the catalyst sites themselves being covered with liquid water

since the cell now operates under flooded conditions.

142

Figure 8-6 Influence of diffusion layer loading on cell performance operated at a cell


per electrode;


Accordingly, even if the reactants can diffuse through the GDL and reach the catalyst

layer, they will have to dissolve in a water film and diffuse to the catalyst layer; this will

slow down the reaction rates. On the other hand, a thicker MPL may pose other

limitations, such as creating a longer pathway for diffusion of gases to and from the

catalyst layer as well as a higher electrical resistance due to the longer pathways for

electrons to reach the external circuit. It is clear from this series of experiments that the

gas permeability of the MPL is a major factor influencing electrode performance.

Figure 8-7 presents SEM micrographs of the diffusion layers. Examination of these

images, obtained at the same magnification, shows that Vulcan XC-72 particles are

covered by a PTFE film and that some of the macropores are blocked by this film,

especially at higher PTFE content. The extent of this coverage is directly proportional to

the amount of PTFE present in the diffusion layer. At low PTFE content—around 10

wt%—although the diffusion layer shows high electronic conductivity, it is not

sufficiently hydrophobic, and cell performance drops because of mass transport

limitations due to water accumulation inside the MEA. On the other hand, when the

concentration of PTFE in the diffusion layer is increased to 40 wt% or greater, a large

143

percentage of the carbon fibers are covered with a non-conducting material that is

resistant to the flow of electrons, leading to a marked drop in cell performance. Aside

from the apparent surface coverage of the carbon fibers by the hydrophobic polymer,

internal coverage also will be higher due to the migration of PTFE inside the diffusion

layer as the PTFE content increases. Accordingly, a delicate balance must be maintained

to maximize performance.

(a) (b)

(c) (d)

Figure 8-7 Surface morphology of MPLs containing Vulcan XC-72 and (a) No PTFE;

(b) 10 wt% PTFE; (c) 30 wt% PTFE; (d) 50 wt% PFE

144

8.2 Effects of Carbon Powder Characteristics on Cell Performance

It has been reported that the physical properties of the carbon powder in the MEAs of

low-temperature fuel cells can significantly alter the chemical and physical characteristics

of both the gas diffusion and catalyst layers [602]. In the case of the catalyst layers,

carbon has been established as the best support candidate on account of its high effective

surface area, good electrical conductivity, adequate surface hydrophobicity, good

mechanical and chemical stability in harsh environments and relatively low cost [603]. In

addition, the carbon pore structure can be modified to create a wide array of pore size

distributions needed for various applications [604]. Granular, powder-activated carbons

and carbon blacks have been extensively used as catalyst supports; however, there has

been a growing interest in the use of other types of carbon materials such as activated

carbon fibers, nanofibers and nanotubes. A comprehensive review of such materials for

use as catalysts or catalyst supports has recently been published [605].

In this part of the study, a large number of GDEs were prepared using six different

carbon/graphite materials: Vulcan XC-72, Shawinigan Acetylene Black (SAB), Ketjen

Black DJ-600, Black Pearls 2000, Asbury 850, and Mogul L. Important physical

characteristics of the above materials are listed in Table 8-1. It is important to note that

the list of carbon materials used for fabrication of carbon-supported catalyst and MPLs is

long and, in addition to carbon blacks and graphite, includes active carbons and active

carbon fibers, glassy carbon, pyrolytic carbons, fullerenes, and nanotubes [603].

A large number of MEAs were fabricated according to the method explained in section 7.

Both anodes and cathodes of all MEAs were prepared in the same manner with the type

of carbon/graphite used in the diffusion layer being the primary variable. Several MEAs

were fabricated with different carbon loadings to evaluate the impact on fuel cell

performance. All MEAs were tested in a 5-cm2 fuel cell with hydrogen and air supplied

to the cell as fuel and oxidant, respectively.

Porosimetry experiments also were carried out on several electrodes before catalyst

deposition to characterize the diffusion layers based on the type of carbon or graphite

used in the MPL. The pore distribution patterns of several MPLs fabricated using five

145

Table 8-1 Manufacturers’ data: characteristics of different carbon powders in GDLs

Property SAB Vulcan

XC-72

Ketjen Black

DJ-600

Black Pearls

2000

Asbury

850

Mogul L

Source

Acetylene

Oil-furnace

Specific Surface

Area (m2 g

-1)

64

252

1300

1500

13

140

Area of Mesopores

(m2 g

-1)

64

177

1230

1020

Total Pore Volume

(cm3 g

-1)

0.2

0.63

2.68

2.56

Micropore Volume

(cm3 g

-1)

0.0

0.037

0.029

0.208

Avg Pore Diameter

(nm)

14.4

15.9

9.45

20.6

> 30

>45

Primary Particle

Diameter (nm)

42

20 - 30

35 - 40

10 - 15

Projection Area of

Aggregates (m2)

0.52

0.14

Crystallite Size

(nm)

4.1

2.0

1.4

1.1

different carbon and graphite materials are shown in Figure 8-8. Pore distribution curves

are arbitrarily divided into three regions: less than 0.05 m, 0.05 m to 1.00 m and

greater than 1.00 m. It is interesting to note that the pore size distributions in the first

region are similar for almost all types of carbon and graphite. The differences begin to

appear when the pore radii increase and reach the second region—0.05 m to 1.00 m—

and significant differences become apparent when the pore radii enter the third region—

greater than 1.00 m.

146

Figure 8-8 Pore volume distribution of several GDLs prepared using five different types

of carbon and graphite

Figures 8-9 shows the cell performance of a number of MEAs prepared with different

carbon/graphite in the microporous layer with a constant carbon loading of 1.5 mg cm-2

.

The cell temperature was maintained at 50 C, while the temperatures of the incoming

hydrogen gas and air were kept at 5 C higher than the cell temperature. At low current

densities—less than 100 mA cm-2

—the effect of carbon type on fuel cell performance is

negligible. As the current density passes 200 mA cm-2

, marked differences become

noticeable. At current densities higher than 500 mA cm-2

, MEAs with SAB and Vulcan

XC-72 show better performance than the other four, and at current densities higher than

600 mA cm-2

(diffusion-limited region), MEAs with SAB outperform those with Vulcan

XC-72.

One of the primary differences between these carbon and graphite materials is their

specific surface area, ranging from 13 m2 g

-1 for Asbury 850 to 1500 m

2 g

-1 for Black

Pearls 2000. Upon a closer investigation, however, no correspondence between specific

surface area and cell performance is observed. A better correlation, however, is observed

between total macropore volume and cell performance.

147

Figure 8-9 Influence of carbon type in the MPL on cell performance of a H2/Air fuel cell

operated at a cell temperature of 50 °C with a platinum loading of 0.3 mg cm-2

per

electrode; fully humidified fuel and oxidant with stoichiometries of 1.2 and 2.5,

respectively

By examining Table 8-1 and Fig 8-9, a noticeable trend is revealed: at high current

densities the cell voltage increases with increasing macropore (> 1 µm diameter) volume.

In other words, cell performance increases as macropore volume in the MPL increases.

Shawinigan acetylene black has the highest macropore volume of all types of

carbon/graphite investigated in this study and delivered the best performance when tested

in a fuel cell, while Mogul L has the smallest volume of macropores, and, accordingly

exhibited the worst cell performance when tested in the same fuel cell under identical test

conditions. This difference in cell performance is most pronounced at higher current

densities, as can be seen from Figure 8-10. At high current densities (the diffusion-

controlled region) the removal of water from the cell becomes critical, and failure to do

so will result in a sharp decrease in cell performance, primarily due to formation and

stagnation of water inside the pores of the MPL as well as the GDL. This will, in turn,

hinder the transport of gases to and from the catalyst layer, lowering cell performance.

The aforementioned explanation is in agreement with the pore volume distribution of the

various carbon/graphite shown in Figure 8-8.

148

Figure 8-10 Cell performance as a function of MPL macropore volume at four

different current densities (H2/Air fuel cell with a cell temperature of 50 °C; fully

humidified fuel and oxidant with stoichiometries of 1.2 and 2.5, respectively)

As previously mentioned, pore distribution curves are conventionally divided into three

regions: less than 0.05 m, 0.05 m to 1.00 m and greater than 1.00 m, identified as

primary, secondary and tertiary (macro) pores, respectively. SAB has the smallest pore

volume in the primary range (micropores), while possessing the highest pore volume in

the tertiary range (macropores). On the other hand, a significant portion of the pores in

Ketjen Black DJ-600 is in the micro range—primary pores. Such pores are more prone to

flooding since liquid water can easily fill them, blocking the passage of reactants and

products to and from the catalyst layer. This is amplified in the cathode of an MEA at

high current densities, where the rate of water generation is high.

The results also might be explained in terms of capillary condensation theory. It has long

been known that if a capillary is immersed in a liquid capable of wetting its walls, the

liquid rises in the capillary, forming a meniscus that is concave toward the vapour phase

[606]. The vapour pressure over this meniscus is often lower than the vapour pressure of

the liquid by an amount equal to pressure exerted by the liquid in the capillary [606]. A

mathematical expression, known as the Young-Laplace equation, is often used to

149

calculate the vapour pressure over a capillary. The difference in pressure between the two

sides of the (spherical) meniscus is:

r

P2

(8-1)

Where, ΔP is the pressure difference between the two sides of a meniscus in Pa , γ is

surface tension of the liquid in N m-1

, and r is the radius of the capillary in m. Cylindrical

capillaries have been assumed in deriving the above expression. It can readily be seen

that as the radius r of the capillary decreases, the difference between normal and

equilibrium pressure increases, causing the liquid to condense at pressures far below the

normal vapour pressure.

A simple explanation of the capillary condensation theory is necessary to fully

understand the influence of total porosity and micropores in the various carbon and

graphite materials employed to prepare MPLs. In its simplest form, where a flat solid

surface adsorbent is in contact with a gas, monomolecular adsorption is the dominant

process occurring as the pressure of the gas increases. This will continue until the surface

of the adsorbent is covered by a single layer of gas adsorbate. The formation of additional

layers requires an increase in pressure. However, if the adsorbent contains pores that are

at least several times greater in width than the diameter of the adsorbate molecules, even

more layers of adsorbate molecules will be adsorbed on the walls of the adsorbent. This

happens because in such pores under pressure, in addition to multimolecular layer

formation, condensation of the gaseous molecules also will occur. As the gas pressure is

further increased, the thickness of the multimolecular layer also is increased, causing

adjacent multimolecular layers to join and form a meniscus of condensed adsorbate. This

will initially take place inside the pores with the smallest cross-section and then will

move along to other pores forming additional menisci [607]. If the adsorbate molecules

are capable of wetting the adsorbent walls, the resulting meniscus is concave, and

molecules of adsorbate are readily condensed on the meniscus at pressures lower than the

saturation vapor pressure [606, 607]. The molecules in a concave surface are more tightly

held together due to a larger number of neighboring molecules than on a flat surface at

the same temperature. This lowers the equilibrium vapour pressure of a gas over a

150

concave surface, compared with a flat surface, and speeds up the condensation process

[607].

According to this theory, in PEMFCs the condensation of water inside the pores of the

MPLs and GDLs occurs below the actual saturation vapour pressure, and will occur

sooner in smaller pores. On this basis, the superior performance of SAB in MPLs of low-

temperature fuel cells can readily be explained in terms of its pore distribution. SAB

possesses the highest percentage of macropores among all types of carbon and graphite

investigated in this study, while having the least amount of micropores. Equation (8-1)

shows that the difference between the normal and equilibrium vapour pressures is

inversely proportional to the radius of the capillary; consequently, small pores will

experience more adsorbate condensation than large pores. Conversely, higher capillary

radii ensure the availability of free and clear passages inside the MPL for transport of

gases to and from the catalyst layer. Thus carbon materials containing a significant

amount of micro- and meso-pores, such as Ketjen Black and Black Pearls 2000, will

undergo condensation of water sooner, restricting the flow of gases and leading to

inferior cell performance. However, even the most widely used carbon material in

PEMFCs—Vulcan XC-72—also contains a significant number of micropores, which will

adversely influence cell performance at high current densities.

The importance of gas transport inside both the MPL as well as the GDL can be

explained using the standard equation for the diffusion-limited current density:

xx cDAFni lim (8-2)

where n is the number of electrons released by the half-cell reaction, F is Faraday’s

constant, A is the effective area of the microporous or gas diffusion layer, Dx is the

diffusion coefficient of species x inside the GDL or MPL, cx is the concentration of

species x before entering the GDL/MPL and

is the thickness of the GDL/MPL layer.

Thus, for the cathode side of an MEA operating under flooding conditions equation (8-2)

shows that ilim decreases owing to the lower diffusion coefficient of oxygen in water

compared with air. The diffusion coefficient of oxygen in water can be estimated using

151

the following correlation developed by Wilke and Chang [608], which is based on

Stokes-Einstein equation:

))((

104.7

22

22

22

2/1

8

OOH

OHOH

OHOV

MTD

(8-3)

where OHOD22

is the diffusion coefficient of oxygen in water (cm2 s

-1)

T is the absolute temperature of the system (K)

H2O is a parameter used to describe the solvent (= 2.26 for water (ref.

609))

M H2O is the molar mass of water (g mol-1

)

H2O is the viscosity of water (cp)

VO2 is the molar volume of oxygen, which is 25.6 cm

3 mol

-1 at STP (ref.

610)

The diffusion coefficient of oxygen in air can be estimated using the following

expression [609]:

j

i

iTiPjTjP

TD

TD

i

j

j

i

BABAT

T

P

PDD

2

3

)()( ,, (8-4)

where D(A-B) is the diffusion coefficient of the binary pair (cm2 s

-1)

D /T is the collision integral for molecular diffusion

P (Pa) and T (K) are the pressure and temperature of the components,

respectively

i and j are reference and modeled conditions, respectively

It should be noted that

D /T is a function of

T AB , where

is the Boltzmann’s

constant and

AB is the energy of molecular interaction. For binary mixtures of oxygen

with carbon dioxide, nitrogen and water vapour,

T AB varies between 1.3 to 3.5 over

the temperature range of 0 C to 80 C [611].

Based on equations (8-3) and (8-4), the diffusion coefficients of oxygen in water and air

at several temperatures are presented in Tables 8-2 and 8-3 [611, 612]. Tables 8-2 and

8-3 show that the effect of temperature on the diffusion coefficient of oxygen in water is

152

more significant than in air. Increasing the temperature from 20 to 40 C increases the

diffusion coefficient of the former by more than 60% and the latter by only about 25%.

This will have important consequences for PEMFCs containing carbon with significant

amounts of micropores in their GDLs and/or MPLs. At high current densities, where the

rate of water generation at the cathode reaches its peak, the micropores in both GDL and

MPL tend to clog quickly, restricting the flow of gases to and from the catalyst layer. The

problem is amplified if the cell is operated at low temperatures, because of the decrease

in

DO2.

Table 8-2 Diffusion coefficient of oxygen in water at different temperatures [611, 612]

Temperature

(C)

Viscosity of Water

(cp)

DO2

(10-5

cm2 s

-1)

20 1.002 1.97

40 0.653 3.24

50 0.547 3.99

60 0.467 4.82

Table 8-3 Diffusion coefficient of oxygen (as a binary mixture) in air at atmospheric

pressure [613]

Binary Mixture Temperature

(C)

A

DO2

(cm2 s

-1)

O2 – CO2

20

40

146 0.153

0.193

O2 – H2O(v) 20

40

201 0.240

0.339

O2 – N2 20

40

102 0.219

0.274

153

To sum up, equation (8-2) indicates that the limiting current density is directly

proportional to2OD . Furthermore, the oxygen diffusion coefficient in air is three to four

orders of magnitude greater than in water. Consequently, MPLs with a small fraction of

micropores and a large fraction of macropores are best suited for applications requiring

high current densities and lower temperature operation.

A series of experiments was carried out to further determine the influence of carbon

loading in the MPL on cell performance. Different amounts and types of carbon or

graphite were applied to both the anodes and cathodes of a number of MEAs. The

experimental results are presented in Table 8-4, which shows that membrane-electrode

assemblies employing SAB, Vulcan XC-72, Ketjen Black and Black Pearl delivered the

best performance with a carbon loading of 1.5 mg cm-2

in both H2-Air cells as well as

H2-O2 cells. The cell performance fell sharply as the carbon in the microporous layer

further increased to 3.0 mg cm-2

. As previously discussed, increasing the carbon loading

beyond 1.5 mg cm-2

causes cell performance to decrease, primarily because of the

creation of a longer pathway for reactant gases to travel from the GDL to the catalyst

layer via the MPL, and similarly, for products, especially liquid water, to exit the MEA.

However, a somewhat different trend was observed with Asbury 850 and Mogul L, for

which performance slightly improved as the loading was increased from 1.0 to 3.0 mg

cm-2

, but leveled off at loadings greater than 3.0 mg cm-2

. The slight improvement in cell

performance for MEAs with higher-than-normal loadings of Asbury 850 is attributed to a

better coverage of the electrode surface with increased loading. At lower loadings, such

as 1.5 mg cm-2

, SAB shows a homogenous surface with randomly distributed micro

channels. In contrast, large flaky agglomerates and non-homogenous surfaces are

observed from micrographs of MPLs prepared with Asbury 850 and Mogul L at the same

loadings.

154

Table 8-4 – Influence of carbon / graphite loading in MPLs on cell performance

Sample Carbon Type Carbon Loading Oxidant Max. Power

(mg cm-2

) (mW cm-2

)

A01 SAB 1.0 Air 273

A02 SAB 1.0 Oxygen 771

A03 SAB 1.5 Air 286

A04 SAB 1.5 Oxygen 806

A05 SAB 3.0 Air 229

A06 Vulcan XC-72 1.0 Air 251

A07 Vulcan XC-72 1.0 Oxygen 744


A09 Vulcan XC-72 1.5 Oxygen 782


A11 Ketjen Black 1.0 Air 213


A13 Ketjen Black 1.5 Oxygen 698



A16 Black Pearl 1.0 Air 200


A18 Black Pearl 1.5 Oxygen 692



A21 Asbury 850 1.0 Air 162

A22 Asbury 850 1.5 Air 169

A23 Asbury 850 2.0 Air 177

A24 Asbury 850 3.0 Air 196

A25 Asbury 850 3.0 Oxygen 554

A26 Asbury 850 5.0 Air 193

A27 Mogul L 1.0 Air 149




A31 Mogul L 3.0 Oxygen 521


It was noted that an increase in MPL carbon loading with Mogul L beyond 3.0 mg cm-2

did not result in better cell performance. The existence of such large flakes is indicative

of incomplete coverage of the carbon fibers of the GDL, which leads to the creation of

non-homogenous surfaces such as those seen in the micrographs of Figure 8-11. This will

155

(a) (b) (c)

Figure 8-11 SEM images of microporous layers of a number of MEAs prepared by (a)

SAB, х 500 (b) Vulcan XC-72, х500 (c) Asbury 850, х500.

adversely influence the movement of reactant gases through the MPL and, equally

important, the transport of product water away from the MEA interior via the MPL.

Increasing of carbon (or graphite) content in the MPL results in a better coverage of the

GDL and, consequently, a better cell performance.

8.3 Nafion® Impregnation

8.3.1 Impregnation Time

Nafion®

impregnation was achieved by either floating or brushing, as described in section

7.3. The Nafion® film should be gas tight and must be non-electron conducting.

Consequently, sufficient Nafion®

must be applied on the carbon electrode surface. First,

a series of experiments was performed to determine the optimum impregnation time

based on the floating method. Table 8-5 shows the difference between 30 s, 60 s, 1 h,

and 24 h Nafion® impregnation times using the above method. The results indicate that

beyond about one minute, longer contact with Nafion®

solution does not significantly

affect the Nafion® loading. The results are also presented graphically in Figure 8-12. To

prepare GDEs with non-electron conducting properties on one side using the brushing

technique, several layers of Nafion® solution, ranging from one to fourteen applications,

were applied to one side of the GDE. The results of these experiments are presented in

Tables 8-6 and 8-7, which show that somewhat higher Nafion® loadings are obtained by

employing the brushing method.

156

Table 8-5 Effects of impregnation time on Nafion® loading utilizing the floating method

Impregnation Mass of the Electrode Nafion Average Literature

Time Before After Difference Loading Nafion Value

Impregnation Impregnation Loading

(g) (g) (g) (mg/cm2) (mg/cm

2) (mg/cm

2)

30 s 0.12695 0.14202 0.01507 1.57

30 s 0.13046 0.14581 0.01535 1.60

30 s 0.12523 0.13976 0.01453 1.51 1.57 1.51*

30s 0.11856 0.13416 0.0156 1.62

60 s 0.12520 0.14426 0.01906 1.98

60 s 0.12464 0.14373 0.01909 1.98

60 s 0.12073 0.13918 0.01845 1.92 1.97

60 s 0.13147 0.15050 0.01903 1.98

1 h 0.1152 0.13641 0.02121 2.20

1 h 0.11957 0.14132 0.02175 2.26

1 h 0.12564 0.14658 0.02094 2.18 2.22

1 h 0.13044 0.15206 0.02162 2.25

24 h 0.12651 0.14973 0.02322 2.41

24 h 0.13147 0.15533 0.02386 2.48

24 h 0.11783 0.14081 0.02298 2.39 2.43 2.5*

24 h 0.12460 0.14815 0.02355 2.45

* N.R.K. Vilambi, E.B. Anderson, and E.J. Taylor, U.S. Patent 508144 (Jan. 28, 1992)

Figure 8-12 The impact of impregnation time on Nafion® loading

157

Table 8-6 Nafion® impregnation of GDEs utilizing floating and brushing techniques

Method Mass (g)

Original 1st

Nafion 2nd

Nafion 3rd

Nafion 4th

Nafion 5th

Nafion

Applic. deposited Applic. deposited Applic. deposited Applic. deposited Applic. deposited

Floating 0.12520 0.14466 0.01946 0.16331 0.01865 0.18010 0.01679 0.19740 0.01730 0.21390 0.01650

Floating 0.12464 0.14373 0.01909 0.16148 0.01775 0.18170 0.02022 0.19505 0.01335 0.20880 0.01375

Floating 0.11856 0.13463 0.01607 0.15371 0.01908 0.17120 0.01749 0.18797 0.01677 0.20030 0.01233

Floating 0.13147 0.15050 0.01903 0.17247 0.02197 0.18634 0.01387 0.20314 0.01680 0.21899 0.01585

Floating 0.12073 0.13918 0.01845 0.15918 0.02000 0.17469 0.01551 0.18992 0.01523 0.20481 0.01489

Floating 0.12033 0.13820 0.01787 0.15389 0.01569 0.17259 0.01870 0.18990 0.01731 0.20002 0.01012

Floating 0.12619 0.15020 0.02401 0.17400 0.02380 0.19429 0.02029 0.21293 0.01864 0.23661 0.02368

Floating 0.11855 0.13920 0.02065 0.15926 0.02006 0.18214 0.02288 0.21010 0.02796 0.23190 0.02180

Brushing 0.11758 0.14349 0.02591 0.17214 0.02865 0.18626 0.01412 0.20136 0.01510 0.21606 0.01470

Brushing 0.12023 0.14349 0.02326 0.16722 0.02373 0.18186 0.01464 0.19708 0.01522 0.21147 0.01439

Brushing 0.11946 0.14225 0.02279 0.16360 0.02135 0.17898 0.01538 0.19474 0.01576 0.21018 0.01544

Brushing 0.12333 0.14494 0.02161 0.16177 0.01683 0.17748 0.01571 0.19445 0.01697 0.21042 0.01597

Table 8-7 Nafion® impregnation with 1 to 14 applications

No of Nafion Cumulative Amount of Nafion Deposited (mg/cm2)

Applications Floating Brushing

1 2.01 (0.06)* 2.43 (0.04)

2 4.05 (0.09) 4.78 (0.07)

3 6.09 (0.08) 7.05 (0.14)

4 7.95 (0.11) 9.17 (0.12)

5 9.63 (0.12) 11.22 (0.11)

6 11.31 (0.19) 13.19 (0.18)

7 12.87 (0.26) 14.89 (0.22)

8 14.22 (0.23) 16.46 (0.24)

9 15.41 (0.25) 17.79 (0.29)

10 16.64 (0.31) 19.05 (0.27)

11 17.71 (0.42) 20.23 (0.38)

12 18.66 (0.39) 21.36 (0.44)

13 19.51 (0.54) 22.45 (0.52)

14 20.28 (0.60) 23.46 (0.63)

* Standard deviations are shown in brackets

As shown in Figure 8-13, for the first 5-6 applications, the amount of deposited Nafion®

increases linearly with the number of applications, after which the slope decreases to a

158

0.00

5.00

10.00

15.00

20.00

25.00

0 5 10 15

Number of Nafion Coatings

Mas

s of

Naf

ion

(mg/

cm2 )

Floating Method

Brushing Method

Figure 8-13 Effect of number of Nafion® applications on total Nafion

® loading

somewhat lower value. This is probably due to the fact that during the first 5-6

applications, the Nafion® solution rapidly penetrates into the region of the carbon

substrate near the surface, effectively sealing it off to further penetration. After the sixth

application, the Nafion® layer begins to build up on the surface and its thickness starts to

grow.

Scanning electron microscopy was used to examine the microstructure of a number of

carbon electrodes. Figure 8-14(a) shows the surface microstructure of untreated carbon

paper, while Figures 8-14(b)-(d) show the surface microstructure of carbon electrodes

treated with 1, 5, and 10 Nafion® applications (using floating method), respectively, in

which the carbon electrodes were air cured for 12 h prior to the next application. From

these micrographs, it is evident that initially Nafion® covers the surface, but there still

remain big channels on the surface even after 5 applications. However, after 10

applications, these channels begin to disappear, and by the 14th

application they are

hardly visible, as seen from Figure 8-15(a), confirming that after the sixth application

Nafion®

starts to build on the surface of the electrode after sealing off most of the gaps.

As the number of applications increases, more Nafion® covers the surface, until finally a

uniform Nafion® film is formed on the carbon electrode. This is confirmed by Fig. 8-

15(b), which is a scanning electron micrograph of the cross-section of a carbon electrode

159

with 14 Nafion®

applications, and indicates that the thickness of the Nafion® film is about

50-60 µm.

Figure 8-14 (a) Untreated carbon electrode, ×130; (b) one Nafion®

application, ×130 ;

(c) five Nafion® applications, ×153; (d) 10 Nafion

® applications, ×130; all micrographs

show the top surface of the GDE; floating method used to load Nafion®

160

Figure 8-15 (a) Micrograph of the surface of a carbon electrode with 14 Nafion®

applications, ×130; (b) cross section, ×130

8.3.2 Nafion® Ion-Exchange Capacity

The total capacity of an ion exchange material is the number of ionic sites per unit weight

or volume of resin. The dry weight total capacity is usually expressed in milliequivalents

per gram of dry resin in the H+ form. Ion-exchange capacities (IECs) of several Nafion

®-

impregnated samples were determined following the procedure outlined in section 7.3. In

addition, the IEC of pure Nafion® film also was determined for comparison purposes.

The IEC of the samples impregnated with Nafion® was found to be lower than both

commercial Nafion® membrane and the Nafion

® solution used for impregnation. This

may partly be attributed to the incomplete conversion of H+

to Na+ form inside the

substrate prior to titration or perhaps to some elements in the carbon binding with some

of the exchange sites. The results are tabulated in Table 8-8.

Carbon Deposited

Nafion®

161

Table 8-8 Ion exchange capacity data for different samples

Sample Dry Mass Titrant Vol. IEC Avg. IEC

(g) 0.05 M NaOH (mL) (meq/g)* (meq/g)

Nafion Membrane (117) 0.37520 6.69 0.910

Nafion Membrane (117) 0.35893 6.43 0.914 0.908

Nafion Membrane (117) 0.36841 6.51 0.902

Air-Dried Nafion Solution 0.19651 3.44 0.893

Air-Dried Nafion Solution 0.21048 3.72 0.902 0.896

Air-Dried Nafion Solution 0.20954 3.67 0.894

Nafion-impregnated Sample 0.12161 1.89 0.793



Nafion-impregnated Sample 0.14050 1.76 0.639 0.689




* meq per gram of dry H+ form

8.4 Effects of Different Types of Substrates on Cell Performance

8.4.1 Influence of Substrate Thickness and other Physical Parameters

A number of MEAs were fabricated using seven carbon substrates with different physical

properties. All other parameters, including diffusion layer loading, PTFE content and

catalyst and Nafion® loadings were kept constant. In addition, both the anode and cathode

of each MEA were made from the same type of carbon substrate. The physical

characteristics of these substrates are reported in Table 8-9.

Figs. 8-16 and 8-17 show that MEAs prepared with ETEK-Elat from BASF and TGP-H-

090 from Toray Industries, Inc. exhibited the best results when fed with either pure

oxygen or air as oxidant and pure hydrogen as fuel. All experiments were performed in a

single 5-cm2 fuel cell running at atmospheric pressure and a cell temperature of 50 C

without external humidification. MEAs fabricated from Toray’s TGP-H-030 performed

the worst. .

162

Table 8-9 Physical properties of a number of different gas diffusion layers

Property Unit ETEK-Elat Toray Toray Toray Toray Stackpole Ballard Avcarb

LT 1200-W TGP-H-030 TGP-H-060 TGP-H-090 TGP-H-120 PC-206 1071 HCB

Thickness µm 275 110 190 280 370 330 380

Bulk Density* g cm-3

0.727 0.4 0.44 0.44 0.45 0.42 0.31

Porosity*

% 78 80 78 78 78 > 70 > 50

Porosity (experimental, Porosimetry) Surface Roughness*

%

µm

79.9

--

81.6 8

80.4 8

79.5 8

77.1 8

--

--

86.0

--

Gas Permeability* ml min-1

> 900 2500 1900 1700 1500 > 1000 > 700

Electrical Resistivity*

through plane mΩ.cm 410 80 80 80 80 -- 110

in-plane mΩ.cm -- -- 5.8 5.6 4.7 -- 9

Thermal Conductivity*

through plane (room temp) W m-1

K-1

-- -- 1.7 1.7 1.7 -- --

in-plane (room temp) W m-1

K-1

-- -- 21 21 21 -- --

in-plane (100 °C) W m-1

K-1

-- -- 23 23 23 -- --

* Indicates Specification from Manufacturer

163

Figure 8-16 Polarization curves for different substrates in H2/O2 with a platinum loading

of 0.3 mg cm-2

per electrode, Nafion® 112, cell temperature of 50 °C and fully humidified

fuel and oxidant with stoichiometries of 1.2 and 1.5, respectively

Figure 8-17 Polarization curves for different substrates in H2/Air with a platinum loading

of 0.3 mg cm-2

per electrode, Nafion® 112, cell temperature of 50 °C and fully humidified

fuel and oxidant with stoichiometries of 1.2 and 2.5, respectively

164

Figure 8-16 compares the cell performance of MEAs made from different substrates in a

hydrogen-oxygen fuel cell. At low current densities (less than 300 mA cm-2

) all MEAs

perform equally well; however, at current densities higher than 300 mA cm-2

, the thickest

and the thinest GDLs from Toray Industries Inc.—TGP-H-120 (370 µm) and TGP-H-030

(110 µm)—start to lag behind the others. The differences in cell performance become

very apparent when the current density reaches 1500 mA cm-2

. At this current density, the

MEAs containing the above two substrates are operating under flooded conditions. On

the other hand, MEAs fabricated from ETEK-Elat carbon cloth and TGP-H-090 and PC-

206 carbon papers exhibit satisfactory performance even at high current densities. The

MEA fabricated from the carbon paper from Ballard performed equally well until about

1000 mA cm-2

, after which the cell voltage gradually declined followed by a sharp

decrease beyond 1800 mA cm-2

. A similar trend also is observed in Figure 8-17, where

air is employed as the oxidant. However, when air was used as oxidant, the thickest

substrate—TGP-H-120—exhibited a better performance compared with both TGP-H-030

and TGP-H-060. This was surprising since the lower partial pressure of oxygen in air

requires shorter pathways from the bipolar plate to the catalyst layer to minimize pressure

drop within the channels of GDL. Also, a thinner GDL exhibits better electronic

conduction owing to the shorter paths that electrons must traverse from the catalyst layer

to the bipolar plates. On the other hand, a thicker GDL, provided that all other parameters

are unchanged, provides more macropores for diffusion of reactant gases and

transportation of water to and from the catalyst layer. This is undoubtedly important

when the cell is operated at high current densities, where the rates of oxygen

consumption and water generation are high. As expected, the cell voltage for all MEAs

dropped when oxygen was replaced with air, on account of the lower partial pressure of

oxygen in air. The differences in cell performance with respect to the various substrate

materials can be understood by referring to the physical characteristics of each carbon

cloth or paper given in Table 8-9.

It is important to understand the primary functions of the GDL prior to a full discusion of

why and how the differing physical properties of this layer influence the performance of

an MEA. A GDL is a layer of porous, electronically conductive and mechanially and

165

chemically stable material that is strategically placed between the catalyst layer and the

flow field both on the anode and cathode sides of a PEM fuel cell. Although most

attention has been paid to the optimization of the catalyst and electrolyte layers, GDLs

also provide a number of important functions, and their optimization can significantly

improve cell performance.

Thus, they facilitate the transport of reactant gases to the catalyst layers; act as an

electronic conductor between the catalyst layers and biopolar/end plates; serve as a

thermal conductor by effectively removing heat from the inside of MEAs; aid in water

management of the MEA by transporting excess water from the catalyst layers to outside

the cell; provide mechanical supports for electrolyte and catalyst layers; and minimize

electronic contact resistance by ensuring good electrical and physical contact between the

GDL and the catalyst layer on one side, and the GDL and the flow-field plate on the other

side. In some instances, GDLs also act as substrates for the deposition of the

electrocatalyst layer before MEA fabrication.

In view of the above functions, it becomes apparent that a number of the material

properties required are inherently diametrically opposite in nature. For example, the

porosity of the GDL needs to be high enough to ensure effective transport of reactant

gases to the catalyst layers; however, the porosity cannot be so high that the through-

plane electronic conductivity of the material is compromised. Furthermore, GDLs must

be able to withstand compressive forces while maintaining mechanical integrity. This is

especially important in fuel cell stacks where the compression is relatively high to ensure

stack integrity, low electrical contact resistance and adequate gas tightness. However,

porosity levels tend to decrease as compression is increased. Another important function

of the GDL is the effective removal of excess water from inside the MEA; this is

particulary critical at the cathode at high current densities, where the rate of water

generation is high and, if not effectively removed, causes the cell to operate under

flooded conditions, which restrict the flow of reactant gases to the catalyst layers. GDLs

often are treated with a hydrophobic polymer to alleviate this problem; however, all

hydrophobic polymers are non-electronic conductors. Accordingly, their content must be

166

optimized to facilitate the movement of water within the MEA, while maintaining good

electronic conductivity. Additionally, porosity decreases with increasing hydrophobic

content.

The thickness of a typical GDL ranges from 100 to 400 m, with the optimal thickness

being determined by several parameters, including the effective transport of reactants and

products to and from the catalyst layer, through-plane electronic conductivity, and pore

stability under compression. Typical porosity vlaues for GDLs vary from 70% to 90%

(by volume), depending on operational conditions. Pore stability and resilience increase

with GDL thickness, permitting higher compression to be applied; however, the transport

properties of GDLs are often improved when the thickness is decreased, due to shorter

pathways for reactants and products. All these variables must be closely examined and

controlled when optimizing GDLs in fuel cells.

When examining the polarizaton curves shown in Figures 8-16 and 8-17 in conjunction

with the data presented in Table 8-9 (no PTFE or MPL), no apparent correspondence

appears between cell performance and GDL thickness with the exception that GDLs with

medium thickness tend to perform better than very thin or very thick GDLs.

Nevertheless, the use of different carbon substrates with varying thickness reveals a

noticeable influence on cell performance in both H2/O2 and H2/air fuel cells. As

previously discussed, the optimal GDL thickness is a compromise between the number of

pores available for effective gas and water transport within the MEA and resilience of the

substrate to pore destruction under compression. In this study, the thickness of untreated

GDLs varied from 110 to 380 m and the best two performances were observed with

MEAs fabricated from ETEK-Elat and TGP-H-090 with corresponding thicknesses of

275 m and 280 m (untreated state under no compression), respectively. For

comparison purposes, all GDLs in this study were divided into three groups based on

their original thickness: low (less than 200 m), medium (between 200-300 m) and high

(greater than 300 m). As shown in Figure 8-18(a), cell performance is not significantly

influenced by GDL thickness at low current densities (less than 400 mA cm-2

). However,

it improves as GDL thickness increases and reaches a maximum at around 280 m at

167

Figure 8-18(a) Cell voltage as a function of original GDL thickness with a platinum


for a H2/air fuel cell, Nafion® 112, cell temperatutre of 50 °C and


current densities higher than 400 mA cm-2

and then begins to decrease as the thickness is

further increased beyond 300 m. The effect of thickness on cell performance is most

pronounced when the cell is operating at high current densities, which corresponds to the

diffusion-controlled region, where mass transport losses become dominant and reactant

delivery to the catalyst sites and water removal from within the MEA become crucial.

As mentioned above, although thin GDLs reduce mass transport losses and enhance

electronic conduction, they are more prone to flooding than thicker ones, leading to

significant losses when operated at high current densities.

The gas permeability of the GDL plays an important role in gas and water transport and

can significanlty influence cell performance. The porosity, , of a fibrous material,

including carbon-based electrodes, is defined as the ratio of the pore volume to the total

volume of the sample [618]:

sample

pore

V

V (8-5)

168

The porosity of a carbon-based GDL is calculated from its weigth per unit area (W/A),

solid phase density ( real ) and compressed thickness (d):

))()((

1Ad

W

real (8-6)

The solid phase density of carbon-based materials varies from 1.6 to 1.95 g cm-3

[619].

The porosity of a compressed GDL (see Fig 8-18(b)) also can be expressed in terms of its

compressed pore volume (Vp,c) and uncompressed bulk pore volume (Vb,u) [620]:

)( ,,,, cbubupcp VVVV (8-7)

)( ,,, upubcb VVV (8-8a)

)( ,,, ubuubcb VVV (8-8b)

)1(,, uubcb VV (8-9)

Where

u is the bulk porosity of the uncompressed fibrous material. The above equations

assume that the solid fibres are incompressible and that deformation takes place only in

the direction of the compressive force, perpendicular to the fibre orientation in the x-y

plane. The effective diffusion coefficients (Deff) of the hydrogen and oxygen are

influenced by the GDL bulk porosity,

u , according to [621]:

22 NO

effD

D

(8-10)

where

is the tortousity and 22 NOD is the binary diffusion coefficient of oxygen in

nitrogen. The tortousity is estimated using the Bruggeman equation [622]:

1

(8-11)

The relationship between permeability and porosity is often described by the Carman-

Kozeny equation, which relates the absolute permeability Kabs to the porosity

u and grain

size

. In fuel cell studies, the grain size is replaced with the fiber diameter df, and the

generalized form of this equation becomes [623]:

32 fabs dK (8-12)

169

Figure 8-18(b) A simple representation of compressed and uncompressed GDL

More specifically, a fitting parameter known as the Carman-Kozeny parameter, kKC, is

used to determine the permeability of GDLs, yielding [620]:

2

32

)1(16

KC

f

absk

dK (8-13)

Porosity of several untreated carbon substrates were determined using mercury intrusion

porosimetry. The experimental data, shown in Table 8-9, are in good agreement with the

manufacturers’ data.

Both the in-plane and through-plane permeability of different GDLs have been

investigated and reported in the literature. Williams et. al [614] presented a correlation

Pore

Fibre

Uncompressed Fibrous

Material

Compressed Fibrous Material

Uncompressed pore volume = Vp,u = εu Vb,u

170

between the through-plane permeability and the limiting current density. It is well-

established that diffusion is the primary driving force behind the through-plane transport

of fluids within GDLs. However, more recently, a large number of researchers have come

to the conclusion that in-plane permeability is more critical than through-plane

permeability in PEMFCs [615-617]. The primary driving force for in-plane permeability

is convection—forced convection in most cases. It is worth noting that both natural and

forced convection can enhance mass transport within the flow-field channels.

8.4.2 Porosity Measurements

The bulk porosity of each untreated GDL was determined according to the methods

described in section 7.6 and reported in Table 8-10 supplied by the manufacturers. The

greatest significance of the bulk porosity of the GDL is its ability to influence the

effective diffusion coefficient of the porous medium according to equation (8-10). The

relationship between the permeability and porosity of a material, as reported in equation

(8-12), is further developed and discussed in the following section. It suffices to state that

MEAs fabricated from Toray TGP-H-090 and E-TEK LT 1200-W delivered the best

results when used in a single cell. Both have porosities in the neighbourhood of 80%,

which lies in the middle of the porosity spectrum (77% - 86%).

Table 8-10 Porosity data for untreated GDLs

Material Porosity Porosity Porosity

Porosimetry Gravimetric Difference Manufacturer

(%) (%) (%) (%)

Toray TGPH 030 81.6 82.4 0.8 82

Toray TGPH 060 80.4 82.3 1.9 80

Toray TGPH 090 79.5 78.1 -1.4 79

Toray TGPH 120 77.1 78.6 1.5 77

E-TEK LT 1200-W 79.9 79.3 -0.6 79

Ballard Avcarb 1071 HCB 86.0 88.7 2.7 82

171

To sum up, the greatest difference beween the highest and lowest porosity is about 8.9%.

This is a strong indication that other important parameters are involved and that the bulk

porosity of untreated GDLs, although important, does not impart a significant impact on

cell performance.

8.4.3 Through-Plane Gas Permeability

The through-plane permeability of a number of the substrates used in section 8.4.1 to

fabricate MEAs was evaluated using the method described in section 7.7. The

permeability of these GDLs was determined using equations (7-2) and (8-15), based on

the assumption that the Forchheimer effect (see below) does not apply to this series of

substrates.

This assumption is valid since in the creeping flow regime, inertial forces are negligible

in comparison with pressure and viscous forces, of which the latter is the dominant

source of pressure loss. It is only at higher velocities where the former becomes the

dominant source of pressure loss due to acceleration and deceleration of the fluid inside

the porous medium. This is known as the Forchheimer effect, and must be taken into

account when determining the absolute permeability of a porous material in which the

fluid velocity cannot be considered low. This is accomplished by adding the above

contribution to equation (7-2):

vvk

vP

(8-14)

where

is the density of the fluid and

is the inertial coefficient, known also as the

Forchheimer or non-Darcy coefficient.

For a compressible fluid flowing inside a porous medium at a relatively low velocity, the

solution of Darcy’s law has been shown to be [620, 627]:

))(()(2

22

mk

MTRL

PPfluid

outin

(8-15)

where Pin and Pout are the inlet and outlet pressures of the fluid, L is the substrate length,

R’ is the universal gas constant, T the temperature, Mfluid the molecular weight of the

fluid (air in this study), µ is the viscosity of air (1.85 × 10-5

Pa·s) and m is the mass flux

172

through the substrate. It should be noted that equation (8-15) is valid only for one-

dimensional flows.

At higher fluid velocities, the contribution from inertial forces becomes significant and

can no longer be ignored; accordingly, Darcy’s law must be modified to account for such

losses. For a compressible fluid the Forchheimer term must be added to the right-hand

side of equation (8-15) transforming it to the following equation:

2

22

))(())(()(2

mmk

MTRL

PPfluid

outin

(8-16)

Experimental values for the through-plane permeability of a number of untreated

substrates are reported in Table 8-11. These values are the average of at least five

samples taken from the same sheet of material. The through-plane permeability of

Stackpole PC-206 substrate was not determined because of a lack of adequate material.

Comparison of these results with those reported by other researchers shows good

agreement. Mathias et al. [615] reported a range of values for the through-plane

permeability of Toray TGP-H-060, ranging from 5.0 х 10-12

to 10 х 10-12

m2. Gostick et

al. [620] evaluated the through-plane permeability of Toray TGP-H-090 and reported a

value of 8.99 х 10-12

m2. Williams et al. [614] tested the the through-plane permeability of

Toray TGP-H-120 and found it to be 8.69 х 10-12

m2.

Table 8-11 Through-plane permeability values of untreated substrates

Material Permeability Coefficient,

kz × 1012

Number of Replicates

Average Deviation

(m2) (%)

Toray TGPH 030 10.2 5 3.13

Toray TGPH 060 9.41 5 2.71

Toray TGPH 090 8.94 6 1.84

Toray TGPH 120 8.63 5 3.27

E-TEK LT 1200-W 63.9 5 3.46

Ballard Avcarb 1071 HCB

9.88 6 5.07

173

The viscous permeability of fibrous materials has been studied by many workers.

Johnson et al. [628] proposed a transport parameter based on electrical conduction

phenomena that would allow permeability approximations from readily available or

easily measured properties such as porosity, specific surface area and formation factor.

Jackson and James [629] introduced a set of dimensionless viscous permeabilities derived

from earlier experimental measurements to help with theoretical predictions. Fluid

permeability also has been linked to diffusion parameters, including effective diffusivity

and mean survival time [630-633]. More recently, Tomadakis and Robertson [634]

presented a comprehensive model to predict the viscous permeability of different types of

fibers with random structures. They considered structures formed by cylindrical fibers

with random distribution in 1, 2 and 3 directions, as depicted in Figure 8-19. Comparison

with a large number of experimental data has shown good agreement. The strength of this

model is based on several premises: first, the fibers in all dimensions are allowed to

overlap, freely mimicking the fibers of real structures. Second, the model requires only

readily measureable parameters such as fiber diameter and porosity, without relying on

hard-to-obtain fitting parameters. A detalied description of the model is provided

elsewhere [634-637] and the absolute permeability, k, is estimated using the

mathematical expression:

2

2

2

2 ])1[()1(

)(

)(ln8rk

pp

p

(8-17)

where

is the material porosity, r is the average radius of the fibers, and

and

p are

constants that depend on the direction of the flow with respect to the planes of the fibers

as well as the fiber arrangement in 1-, 2-, and 3-dimensions.

The two constants

and p are known as Archie’s law parameters for the bulk diffusion

tortuosity and conduction-based permeability of randomly overlapping fiber structures

[634], and are given in Table 8-12 for the above model.

174

1-D 2-D 3-D

Figure 8-19 Illustration of various random fiber orientation distribution in 1, 2 and 3

dimensions

Table 8-12 Archie’s law parameters used to calculate absolute permeability using the

model of Tomakakis et al [634]

Structure Flow Direction εp α

1-D Parallel 0 0

Perpendicular 0.33 0.707

2-D Parallel 0.11 0.521

Perpendicular 0.11 0.785

3-D All directions 0.037 0.661

The data obtained from the above model are compared with the experimental results of

the present study in Table 8-13. The absolute permeabilities, kz, are calculated for one,

two and three dimensional random fiber structures. The above model predicted the

absolute permeability to flow perpendicular to the planes of the fibers quite well with the

exception of the substrate from Ballard, for which the deviation is attributed primarily to

the presence of considerable quantities of filler inside the matrix.

Comparison of predicted through-plane permeabilities with the corresponding

experimental data show an excellent agreement with 2-D structures with the exception of

carbon substrates from BASF and Ballard (see Table 8-13). A comparison between the

above model in 2-D and the experimental values is indicated in Table 8-13 and presented

graphically in Figure 8-20. An excellent agreement between the experimental through-

175

plane permeability data and the corresponding values predicted by the above model for 2-

D structures is observed. For instance, the through-permeability of Toray TGP-H-060 is

estimated to be 9.65 × 10-12

m2, which is in good agreement with the experimental value

of 9.41× 10-12

m2. It is worth mentioning that Tomadakis and Robertson’s model also

takes into consideration the influence of the anisotrophy of the material. This is important

for materials with highly-aligned fibers, since they exhibit the highest anisotrophy, which

can have a direct impact on the through-plane permeability of a substrate. The

permeability of substrates with high anisotrophy can differ from others by as much as a

factor of 2 [620]. Gas diffusion layers are often treated with a hydrophobic polymer to

ensure sufficient water removal capability, especially when operated at high current

densities. However, the addition of a hydrophobic polymer alters the total porosity and,

consequently, the diffusibility.

Table 8-13 Comparison of experimental and theoretical absolute permeability values of

different carbon substrates

Material Porosity Fiber

Diameter Absolute Permeability, K % Difference

(%) (µm) (×10-12

m2) for 2-D Values

Experimental Mathematical Modeling

1-D 2-D 3-D

Toray TGPH 030 81.6 8.8 10.2 7.96 10.9 14.5 0.7

Toray TGPH 060 80.4 9.0 9.41 12.8 9.65 12.8 0.24

Toray TGPH 090 79.5 9.4 8.94 6.6 9.3 12.4 0.36

Toray TGPH 120 77.1 10.8 8.63 6.18 8.96 12.1 0.33

E-TEK LT 1200-W 79.9 14.9 63.9 17.6 24.7 32.9 -39.2

Ballard Avcarb 86.0 7.1 9.88 10.9 14.4 18.8 4.52

176

Figure 8-20 Comparison of experimental and theoretical variations in through-plane 2-D

permeability as a function of medium porosity for carbon papers under investigation

A number of GDLs were fabricated from Toray TGPH 090 carbon paper pre-treated with

varying amounts of PTFE, ranging from 10 to 50 wt%. As expected, the substrate

porosity decreased with increasing PTFE content in an almost linear fashion, as can be

seen from Figure 8-21. The average pore diameter of the substrate also decreased with

increasing PTFE content; however, the trend was rather non-linear, as shown in

Figure 8-22. To fully understand the effect of PTFE content on pore volume and,

ultimately, on gas diffuseability and cell performance, the pore diameter as a function of

differential pore volume for substrates containing 10-30 wt% PTFE was determined.

177

Figure 8-21 Influence of PTFE on the porosity of Toray TGPH 090 carbon paper

Figure 8-22 Effect of PTFE content on porosity and average pore diameter of Toray

TGPH 090 carbon paper

178

The results (Figure 8-23) show that GDLs containing 10 wt% PTFE exhibit a wider pore

volume distribution than those treated with higher PTFE loadings. This is true for both

small pores (less than 5 µm) and large pores (greater than 100 µm), indicating that the

amount of applied PTFE is not sufficient to cover most of the pores. However, at PTFE

loadings greater than 20 wt%, a rather narrow pore size distribution is observed,

indicating that most pores at both ends of the spectrum are covered by PTFE. It also can

be seen that the total pore volume decreases with increasing PTFE content, in turn,

restricting the flow of gases inside the GDL. These findings are in agreement with those

reported by other researchers [200, 640, 641].

Cumulative pore volumes of substrates treated with varying amounts of PTFE also were

evaluated and are presented in Figure 8-24, which shows that the cumulative pore volume

of all substrates decreases as PTFE content is increased from 10 to 50 wt%. In addition,

when each curve is individually analyzed, it can be seen that regardless of the PTFE

content, pores with diameters of about 4 µm or smaller are not affected by PTFE

treatment, a constant cumulative pore volume being observed for all the examined

substrates in this region. However, a sharp decrease can be seen for pores with diameters

Figure 8-23 Differential pore volume for Toray TGPH 090 carbon paper substrates with

different PTFE loadings

179

Figure 8-24 Cumulative pore volume of Toray TGPH 090 carbon paper substrates with

different amounts of hydrophobic polymer

greater than 4 µm and is amplified for those in the neighborhood of 50 – 300 µm in

diameter. This implies that most of the PTFE infiltrates into pores with diameters of 4 µm

or greater. One of the implications of this is that small pores (less than 4 µm in diameter)

will not be sufficiently hydrophobic, significantly increasing the probability of water

saturation and flooding leading to performance losses. Furthermore, there exist two

different primary types of diffusion in porous media: Knudsen and Maxwellian (bulk

diffusion). The former arises from the continuous collision of gas molecules with the pore

walls and often takes place in long pores with narrow diameters (2 – 50 nm). In other

words, this form of diffusion is dominant when the mean-free path is significantly larger

than the diameter of the pore. It has been reported that Knudsen diffusion is the dominant

mode when the pore diameter is less than 70 nm, which is one-tenth of the mean-free

path of air [614]. Bulk diffusion, on the other hand, dominates when the pore diameter is

at least 100 times greater than the mean-free path of the flowing gas molecules. Williams

et al. [614] also have noted that the effective diffusion coefficient is one order of

magnitude higher for conventional carbon substrates when bulk diffusion is the dominant

mode of diffusion.

180

Figure 8-23 shows that when the PTFE content is greater than 20 wt%, some of the pores

are unnecessarily narrowed or even blocked. This will lower the permeability of the GDL

and, ultimately, lower cell performance. Evidently, there is an optimal PTFE content that

will achieve the two critical objectives of ensuring adequate hydrophobicity to prevent

flooding without restricting the flow paths for mass transport to and from the catalyst

layer.

As stated above, when the pore diameter is greater than the mean-free path of air by a

factor of two, bulk diffusion becomes the dominant form of mass transfer. To illustrate

this, a plot of the pore volume contributed by pores having a minimum diameter of 7.0

m (100 times the mean-free path of air) as a function of the oxygen permeability

coefficient is presented in Figure 8-25, which shows as expected that the permeability

increases with the percentage of large pores. This can be attributed to the enhanced

convection of the oxidant inside the GDL. The ability of the substrate to effectively

remove water also will benefit from larger pores.

Figure 8-25 Percent of substrate pore volume with pore diameters of at least 7 m as a

function of permeability coefficient

181

8.5 Catalyst Electrodeposition

8.5.1 Copper Electrodeposition

For preliminary experiments, platinum was replaced with copper on account of the

prohibitive cost of the former. Copper electrodeposition was carried out following the

procedure outlined in section 7.4.2. The effects of different pulse current

electrodeposition parameters—pulse on-time, pulse off-time and duty cycle—on the

current efficiency of copper electrodeposition from an acidic bath were determined.

Since the properties of an electrodeposit can be influenced by its thickness, this parameter

was held constant at 0.05 mm throughout for each experiment. The parallel plate flow

cell was used with a 3.0-cm-diameter platinum screen as a counter electrode to deposit

copper onto carbon substrates at pulse current densities between 10-50 mA cm-2

. The

current efficiency was calculated by gravimetric analysis. Current efficiencies of copper

electrodeposition with different pulse periods, ranging from 4 to 400 ms, and duty cycles

of 25% and 50% are presented numerically in Tables 8-14 and 8-15 and graphically in

Figures 8-26 and 8-27, respectively.

Figure 8-26 Influence of pulse period on current efficiency of copper electrodeposition

182

Figure 8-27 Influence of duty cycle on current efficiency of copper electrodeposition

As shown in Figure 8-26, the current efficiency decreases with decreasing pulse period in

the millisecond region from about 92% to 76% for pulse periods between 400 ms to 4 ms,

respectively. This can be explained in terms of the dissolution of copper adatoms with

shortening pulses in the millisecond range. The reduction mechanism of copper has been

extensively studied [425, 426, 435, 436, 623]. It is believed that cupric ions are initially

adsorbed onto the surface of the substrate and then reduced to copper in two steps. The

copper adatoms then diffuse to the kink sites and steps, where they are incorporated into

the substrate matrix. However, if the pulse period is too short, the copper adatoms will

not have sufficient time to get into the kink sites and will, during the off period, degrade

as copper atoms suspended in the bulk solution and will re-dissolve into bulk solution.

Steps involved in the copper deposition process are shown below [601]:

Cu+2

(aq) Cu+2

(ad) adsorption (8-18)

Cu+2

(ad) + e- Cu

+1(ad) first charge transfer (8-19)

Cu+1

(ad) + e- Cu(ad) second charge transfer (8-20)

Cu(ad) Cu(cry) surface diffusion (8-21)

Cu(ad) Cu metal degradation (8-22)

2Cu+1

(ad) Cu+2

(aq) + Cu(s) disproportionation (8-23)

183

The decrease in current efficiency corresponding to the shortening of the pulse periods is

gradual from 400 ms to 40 ms; however, a sharp decrease is observed when the pulse

period is further shortened to 4 ms, as seen in Figure 8-26. If the on-time becomes very

short, as is the case for pulse periods less than 40 ms, there will not be sufficient time for

many copper adatoms to reach the kink sites and they may diffuse back into the solution

as suspended solids during the off-time, thereby lowering the current efficiency.

Figure 8-27 shows the influence of the duty cycle on the current efficiency of copper

deposition. The current efficiency decreases from about 99%, for DC electroplating at

100% duty cycle, to about 82% for a duty cycle of 10%. This is reasonable since a 10%

duty cycle corresponds to a longer off-time and, consequently, less time for the copper

adatoms to be incorporated into the substrate matrix, leading naturally to a lowering of

the current efficiency.

Table 8-14 Influence of pulse period on current efficiency of copper electrodeposition

Method Pulse Period Average Current Current Efficiency Average

Density Current Efficiency

(ms) (mA/cm2) (%) (%)

DC 10 98.7

DC 10 99.4 99.1

DC 10 99.1

PC* 400 10 91.6

PC* 400 10 91.0 91.6

PC* 400 10 92.3

PC* 100 10 88.5

PC* 100 10 90.0 88.9

PC* 100 10 88.3

PC* 40 10 87.0

PC* 40 10 86.1 86.3

PC* 40 10 85.7

PC* 4 10 76.2

PC* 4 10 74.5 75.9

PC* 4 10 76.9

* For a duty cycle of 25%

184

Table 8-15 Influence of duty cycle on current efficiency of copper electrodeposition

Method Duty Cycle Average Current Current Efficiency Average

Density Current Efficiency

(%) (mA/cm2) (%) (%)

PC 10 4 83.4

PC 10 4 80.6 81.8

PC 10 4 81.5

PC 25 10 91.6

PC 25 10 91.0 91.6

PC 25 10 92.3

PC 50 20 94.3

PC 50 20 95.1 94.8

PC 50 20 94.9

PC 75 30 96.7

PC 75 30 97.5 97.2

PC 75 30 97.4

DC 100 10 98.7

DC 100 10 99.4 99.1

DC 100 10 99.1

DC 100 20 98.2 98.5

DC 100 20 98.8

8.5.2 Elemental Analysis using EDX

Scanning electron microscopy was used in combination with EDX to determine the

elemental composition of the deposits. The extent of Nafion® coverage was evaluated by

comparing the amount of carbon present on the surface of a carbon electrode after each

Nafion® application. The results are presented numerically and graphically in Table 8-16

and Figure 8-28, respectively. Figure 8-28 shows that the amount of carbon present on

the surface of a carbon electrode decreases sharply from about 100% to only 25% with

the first three Nafion®

applications, and then follows a more gradual decline until the

electrode surface is completely covered with Nafion® after 7 applications. After the 7

th

application, Nafion® begins to build up only on the surface, thickening the Nafion

® coat.

Fig. 8-29 shows the EDX spectrum of a carbon electrode that first had been coated with

185

Nafion®, followed by depositing copper. The spectrum analyzed the cross-sectional

region at the carbon/Nafion® interface and confirms that the copper was deposited at the

correct location (see Figure 8-30).

Table 8-16 EDX analysis of carbon substrates

Number of Nafion % Carbon on

Applications the substrate

0 99.4

1 52.0

2 36.4

3 25.5

4 21.8

5 11.3

6 8.4

7 1.9

8 0

9 0

10 0

Figure 8-28 Influence of the number of applications on carbon electrode coverage

186

Figure 8-29 EDX spectrum of the interfacial cross-section of a carbon electrode

impregnated with 14 coatings of Nafion® and electroplated with copper

Figure 8-30 An EDX spectrum analysis of a carbon electrode cross section

impregnated with Nafion® and electroplated with copper

Carbon

Nafion

®

Cross-sectional

area analyzed

Sample: <Untitled>

Comments:

Acquired: 13-Aug-2004, 17:38

Processed: 13-Aug-2004, 17:41

Standardized: 16-Apr-2002, 10:54

Detector window: Beryllium

Accelerating voltage: 20

Tilt: 15

Elevation: 10

Azimuth: 13

Correction method automatically selected: PAP

Element wt% ZAF factor 3 sigma

C 24.04 0.368 191.9888 !

S 26.70 0.726 11.425 !

O 39.97 0.863 63.542 !

Cu 9.26 2.181 57.1153 !

Total 99.97

187

It is important to note that a correct sulfur-to-oxygen mass ratio of 0.6680 can be

calculated from data in Fig. 8-30, proving the presence of sulfonic groups (-SO3-) on the

substrate surface.


8.5.3.1 Direct Current Electrodeposition

A series of experiments was carried out to study the effects of various electrodeposition

parameters in both Direct Current (DC) and Pulse Current (PC) electrodeposition. In this

section the results for DC electrodeposition are presented. The findings for PC

electrodeposition are discussed in the following section.

Since DC electrodeposition has only one variable, namely, the current density, iDC, only

minimal control is possible in DC systems. As the deposition current density increases,

the metal ion concentration is depleted near the surface of the cathode and dendrites

begin to form. Eventually, the concentration near the surface of the cathode approaches

zero and the ―limiting current density‖, iL, is reached, and crystal growth becomes the

dominant process as opposed to nuclei formation. Mass transport limitations can clearly

be seen in Figure 8-31, where the performance of electrodes fabricated by DC

electrodeposition of Pt at various current densities, ranging from 10 to 50 mA cm-2

, is

shown. The best performance was observed at a plating current density of about 15 mA

cm-2

.

Figure 8-32 presents cross-plots of the same results as a function of the cell voltage vs.

the applied Pt deposition current density at different cell current densities. Again, it is

evident that the best performance was obtained using a deposition current density in the

neighbourhood of 15-20 mA cm-2

. This behaviour can be explained in terms of Pt crystal

growth and nuclei formation. At very low current densities, the rate at which electrons are

supplied to the surface is low compared with the diffusion rate of platinum metal ions.

Consequently, platinum ions will crystallize at stable places on the electrode surface and

the growth of existing crystals will dominate. This increases the particle size of the

deposited catalyst and, at the same time, decreases the available surface area for reaction,

and more importantly, for the reduction of oxygen at the cathode. However, as the

188

Figure 8-31 Effect of Pt electrodeposition current density on cell performance in DC

electrodeposition (H2/Air, 20 wt% PTFE, 0.30 mg Pt cm-2

per electrode, Nafion® 112,

cell temperature of 50 °C, fully humidified fuel and oxidant with stoichiometries of 1.2

and 2.5, respectively)


electrodeposition (H2/Air, 20 wt% PTFE, 0.30 mg Pt cm-2

per electrode, Nafion® 112,

cell temperature of 50 °C, fully humidified fuel and oxidant with stoichiometries of 1.2

and 2.5, respectively)

189

deposition current density increases, the rate at which electrons are supplied to the

surface is no longer slower than the metal ion diffusion rate. This increases the rate of

nuclei formation and consequently, decreases the deposited platinum particle size.

As the deposition current density increases beyond 15 mA cm-2

, concentration

polarization becomes increasingly significant and, metal once more moves to the tips of

the existing crystals, which begin to grow. Again, the crystals become dendritic and this

diminishes the surface area of the catalyst necessary for both hydrogen oxidation and

oxygen reduction and decreases electrode performance. As can be seen from Figure 8-31,

at high deposition current densities (40 mA cm-2

and greater) the performance of the

MEA drops sharply. This is primarily attributed to the generation of hydrogen at such

current densities, which adversely affects the deposited catalyst layer. This is confirmed

by examining the cross section SEM images (Fig. 8-33) of two electrodes; one fabricated

at a peak current density of 50 mA cm-2

and the other at a higher peak current density of

70 mA cm-2

. While a strong platinum peak is observed for the electrode prepared at the

lower peak current density (micrograph (a)), no such peak is observed for the higher

current density. At the higher peak current density, the deposited catalyst layer is

removed due to the generation of hydrogen.

Carbon Carbon

Figure 8-33 Enhanced SEM cross-sectional micrographs of electrodes prepared by pulse

electrodeposition at peak current densities of (a) 50 mA cm-2

and (b) 70 mA cm-2

(a) (b)

Nafion®

Nafion®

15 µm 15 µm

190

8.5.3.2 Pulse Current Electrodeposition

8.5.3.2.1 Influence of Cathodic Peak Current Density

It is known that PC electrodeposition has several advantages over DC electrodeposition

in terms of controlled particle size, better adhesion to the substrate and uniform

distribution of deposited metals within the catalyst layer [9]. It has also three independent

variables as opposed to only one with DC electrodeposition. These variables—peak

deposition current density, on-time and off-time—can be manipulated to optimize the

performance of the catalyzed electrode. The effect of peak deposition current density on

fuel cell performance is shown in Figures 8-34 and 8-35.

In this experiment, the peak current density was varied while keeping the duty cycle,

pulse period and charge density constant. As can be seen from Figures 8-34 and 8-35, the

electrodes prepared at a peak current density of 50 mA cm-2

exhibit better performance

than those prepared at either lower or higher peak current densities. This increase in

performance can be attributed to an increase in the active surface area of the deposited

platinum and selective platinum loading in the catalyst layer close to the Nafion®

membrane, leading to the extension of the three-phase interface. In an electroplating

process, metal ions are transferred to the cathode, where adatoms are formed by the

electron transfer process and, consequently, incorporated into the crystal lattice. As

discussed earlier, two competing processes are involved: nucleation and crystal growth.

At low current densities, because the rate of diffusion from solution is higher than the rate

of charge transfer, metal ions have sufficient time to find stable places on existing

crystals to attach themselves and be incorporated into the crystal lattice and the crystal

grows, leading to lower catalyst surface area. As the pulse deposition current density

increases, there is no longer sufficient time for adatoms to diffuse across the surface to be

incorporated into a growing crystal. Instead, as each new adatom is deposited, it becomes

a single nucleus or part of a very small number of nuclei. The result is an increase in the

number, but a decrease in the size, of the Pt crystallites, resulting in a catalyst layer with

superior properties in terms of hydrogen oxidation and oxygen reduction.

191

Figure 8-34 Effect of electrodeposition peak current density in square pulse

electrodeposition on cell performance (H2/Air, 20 wt% PTFE; 0.30 mg Pt cm-2

per

electrode, cell temperature of 50 °C, Nafion® 112, TGPH-090 carbon paper, fully


Figure 8-35 Effect of electrodeposition peak current density on fuel cell performance in

square pulse electrodeposition (H2/Air, 20 wt% PTFE, 0.30 mg Pt cm-2

per electrode, cell

temperature of 50 °C, Nafion® 112, TGPH-090 carbon paper, fully humidified fuel and

oxidant with stoichiometries of 1.2 and 2.5, respectively)

192

As the peak deposition current density increases beyond 50 mA cm-2

, surface diffusion

becomes the rate-determining step, the system approaches its ―limiting current density‖,

and dendrites begin to form. Crystal growth then becomes the dominant process and the

size of the catalyst particles starts to increase, resulting in a lower catalyst effective

surface area and, consequently lower performance. At very high peak current densities—

greater than 70 mA cm-2

—hydrogen may be evolved contributing to spalling of the

catalyst layer and a sharp decline in performance.

The rate of nuclei formation can be expressed by [333]:

Tkez

bskJ

2

1 exp (8-24)

where k1 is a rate constant; b is a geometric factor; s is the area occupied by one atom on

the surface of the cluster;

is specific edge energy; and k is Boltzmann’s constant. T, z, e

and

have their usual meanings.

As can be seen from the above equation, the rate of nuclei formation increases as the

reaction overvoltage,

, increases4. Furthermore, under activation control the overvoltage

is given by the Tafel equation:

)(log i (8-25)

Equations (8-24) and (8-25) show that as the applied current density, i, increases the

overvoltage, η, also increases, increasing the nucleation rate and promoting a catalyst

layer with smaller platinum particle size, and hence better performance.

One of the main advantages of pulse electrodeposition is its ability to impede the onset of

crystal growth by increasing the limiting current density. This is achieved by

replenishment of ions in the vicinity of the cathode during pulse off-time, allowing a

higher cathodic current density to be applied at the electrode surface compared with DC

electrodeposition because of the higher concentration of platinum ions near the electrode

surface. On the other hand, the application of a continuous current in DC

4 In this treatment, as explained later (section 9.2.3.1 ), cathodic overvoltages are treated as positive

quantities.

193

electrodeposition leads to a steady decline in the concentration of the electroactive

species in the diffusion layer, eventually to a point where it becomes zero. At this point,

dendrites will start to form and existing crystals will grow, diminishing the effective

surface area of the deposited layer. Figure 8-36 shows the polarization curves of the

MEAs prepared by DC and PC electrodeposition. The pulse-electrodeposited electrode

was prepared under the conditions of 50 mA cm-2

of peak deposition current density,

100 ms on-time and 300 ms off-time. A continuous current density of 15 mA cm-2

was

applied for DC electrodeposition. Total charge density was kept identical in both cases.

The results clearly show the superiority of PC electrodeposition, and are attributed to the

smaller Pt particle size of the electrodes prepared by PC electrodeposition.

Figure 8-36 Effect of PC and DC electrodeposition on fuel cell performance (H2/Air, 20

wt% PTFE, 0.30 mg Pt cm-2

per electrode, cell temperature of 50 °C, Nafion® 112, fully


194

8.5.3.2.2 Influence of Duty Cycle

8.5.3.2.2.1 Regular Duty Cycles : 10%-100%

Figure 8-37 shows the polarization curves for a number of MEAs prepared by square

pulse current electrodeposition with a varying duty cycle. The duty cycle was varied by

changing the off-time while the peak current density, on-time and charge density were

fixed at 50 mA cm-2

, 150 ms and 6 C cm-2

, respectively. The results indicate that the

duration of off-time plays an important role in the deposition of platinum since it is

during this period that fresh platinum ions are replenished close to the surface of the

cathode from the bulk solution. It is also during the off-time that platinum ions diffuse to

the surface of the electrode, making it possible to perform electrodeposition at a higher

pulse current density, raising the overvoltage and increasing the rate of nuclei formation.

This trend in Figure 8-38, can be explained in terms of the limiting current density and its

effect on the quality of the deposited platinum. When the pulse off-time is too long with

respect to pulse on-time—low duty cycle—the limiting current density becomes higher

than the applied current density requiring a longer deposition time to achieve the same

platinum loading.

Figure 8-37 Effect of PC duty cycle on fuel cell performance (H2/Air, 20 wt% PTFE,

0.30 mg Pt cm-2

per electrode, cell temperature of 50 °C, Nafion® 112, TGPH-090 carbon

paper, fully humidified fuel and oxidant with stoichiometries of 1.2 and 2.5, respectively)

195

Figure 8-38 The relationship between duty cycle and cell voltage for different fuel cell

output current densities in fuel cells utilizing PC-electrodeposited catalysts (H2/Air, 20


per electroede, cell temperature of 50 °C, TGPH-090 carbon

paper, fully humidified fuel and oxidant with stoichiometries of 1.2 and 2.5, respectively)

This is analogous to electrodeposition with a low pulse current density. As mentioned

before, low pulse current densities lower the formation of new nuclei resulting in poor

electrode performance. One approach to rectify this is to raise the peak current density so

charge transfer will no longer be the rate-determining step. This is discussed in the

following section, where low duty cycles and high peak current densities are utilized to

electrodeposit fine catalyst layers. On the other hand, if the off-time is too short—high

duty cycle—there is not enough time for fresh platinum ions to diffuse into the diffusion

layer from the bulk solution and reach the surface of the cathode. Furthermore, the

limiting current density of an electroplating system is inversely proportional to the

applied duty cycle. In other words, as the duty cycle increases, the limiting current

density of the system will inevitably decrease. This will, in turn, impose a limitation on

the size of the applied peak current density that can be used. Generally, a higher peak

current density must be applied to promote nuclei formation and avoid crystal growth.

For a particular electroplating system, if the peak current density is greater than its

corresponding limiting current density, then crystal growth becomes the dominant

process, increasing the size of the deposited catalysts and, consequently, decreasing the

196

effective surface area of the deposited platinum. It is important to note that in this series

of experiments the limit for the applied peak current density is set at 25 mA cm-2

. Higher

peak current densities were found to surpass the limiting current density of the system,

leading to crystal growth and dendrite formation. Electrodes fabricated at higher peak

current densities; i.e., greater than 25 mA cm-2

, exhibit poor single cell performance, as

can be seen in Figure 8-37, where a peak current density of 50 mA cm-2

was employed.

A duty cycle of 10% is not adequate to effectively promote nucleation. In this case,

charge transfer is the rate-limiting step and adatoms will have enough time to reach stable

places—existing crystals—on the surface of the cathode.

Therefore, it is crucial to optimize the duty cycle in order to decrease the platinum

particle size and to increase the fuel cell performance. A duty cycle of 20% was found to

provide the best results under the specified experimental conditions.

8.5.3.2.2.2 Low Duty Cycles: 2%-10%

Membrane-electrode assemblies prepared by square pulse electrodeposition with low

duty cycles, ranging from 2% to 10%, have been reported to perform better than MEAs

fabricated utilizing higher duty cycles [105, 536].

Figures 8-39 and 8-40 show the effect of a very low duty cycle of 4% on electrode

performance in a fuel cell. A slight improvement in the performance of electrodes

fabricated at a low duty cycle (4%) is observed compared with electrodes prepared at

higher duty cycles (greater than 10% with 20% being the best at 50 mA cm-2

). As before,

this can be explained in terms of the nucleation rate and crystal growth of deposited

platinum catalyst. As the duty cycle decreases, the average current density decreases as

well. As a result, higher cathodic peak current densities can be applied, leading to higher

cathodic overvoltages. This promotes the formation of new nuclei, resulting in finer and

smaller platinum particles on the carbon substrate and, ultimately, in better fuel cell

performance. This can clearly be seen in Figure 41 where the electrode prepared with a

duty cycle of 4% and a peak current density of 400 mA cm-2

exhibits better performance

than the electrode fabricated at a duty cycle of 20% and a corresponding cathodic peak

197

Figure 8-39 Effect of electrodeposition peak current density with low duty cycles (ф) (4% and 20%) in square pulse electrodeposition on fuel cell performance (H2/Air, 20


per electrode, cell temperature of 50 °C, Nafion® 112, fully


Figure 8-40 Effect of square pulse electrodeposition peak current density with 4% duty

cycle (ф) on fuel cell voltage for fuel cell output current densities of 200-1000 mA cm-2

(H2/Air, 20 wt% PTFE, 0.30 mg Pt cm-2

per electrode, cell temperature of 50 °C, Nafion®

112, fully humidified fuel and oxidant with stoichiometries of 1.2 and 2.5, respectively)

198


. It must be noted that the average current density in both

cases is below the critical limiting current density of 15 mA cm-2

for DC

electrodeposition. This can be further substantiated by considering the electrodes

prepared using a pulse current density of 600 mA cm-2

. In this case, the average current

density is greater than the limiting current density and, consequently, an inferior

performance is observed, probably as a result of dendrite formation and crystal growth.

On the other hand, at a duty cycle of 4% and low cathodic peak current density (50 mA

cm-2

) the average current density is not high enough to support the formation of new

nuclei, and results in the growth of platinum crystals. This lowers the effective surface

area of the platinum for hydrogen oxidation, and more importantly, for oxygen reduction,

leading to a lower-than-expected fuel cell performance.

This can partially be explained by determining ―where‖ electrocatalysts are likely to be

deposited under such electroplating conditions. This requires a good understanding of the

current density distribution throughout the substrate and all the parameters influencing it.

It is known that the current density distribution throughout electronically conductive

carbon substrates is impacted by mass transport limitations, kinetics of the catalyst

deposition process and ohmic drop throughout the substrate [12, 624-626]. Mass

transport limitations are generally reduced by increasing the concentration of the catalyst

ions in the solution. This will result in the deposition of catalyst particles away from the

active layer since platinum ions can now penetrate further into the substrate and then be

reduced, if the cathodic peak current density is relatively low. Although in this study the

concentration of platinum is low to ensure its deposition in regions where both ionic and

electronic pathways exist, the applied cathodic peak current density must be high enough

to inhibit its penetration into the substrate, where it will become inactive. On the other

hand, if the cathodic peak current density is too high, hydrogen evolution becomes the

predominant reaction, destroying the deposited platinum layer. In addition, according to

Equation (8-24) high cathodic peak current densities increase the nucleation rate by

increasing the overvoltage. This will, in turn, improve fuel cell performance.

199

Figure 8-41 compares the performance of electrodes fabricated at various duty cycles,

ranging from 2% to 20% and corresponding average current densities of 6 – 16 mA cm-2

.

Similar trends to the case of 4% duty cycle were observed, and the same argument can be

used to explain these findings.

Figure 8-41 Effects of duty cycle and pulse current density on fuel cell performance

(square pulse, H2/Air, 20 wt% PTFE, 0.30 mg Pt cm-2

per electrode, cell temperature of

50 °C, fully humidified fuel and oxidant with stoichiometries of 1.2 and 2.5, respectively)

8.5.3.2.3 Influence of Pulse Duration

Figure 8-42 presents the performance plots of MEAs fabricated by square-pulse

electrodeposition with varying pulse periods, ranging from 40 to 4000 ms, at a deposition

peak current density of 50 mA cm-2

and a duty cycle of 25%. The results indicate that

pulse frequency plays an important role in the deposition of active catalysts. When the

pulse period is too short (for instance on-time/off-time of 10/30 ms), there is not enough

time for fresh platinum ions in the bulk solution to reach the surface of the electrode

during off-time since the on-time is relatively long. Although the duty cycle is set at

200

25%, a substantial amount of platinum ions will move into the substrate from the

diffusion layer and will be deposited both on the surface and inside of the substrate due to

the relatively long on-time. This will lower the concentration of the platinum ions inside

the diffusion layer, where the onset of limiting current density is inevitable. On the other

hand, if the pulse period is too long, the corresponding on-time will be long as well (1000

ms in the worst case) contributing to the exhaustion of platinum ions near the surface of

the cathode, resulting in the growth of the platinum crystals and a lowering of the

nucleation rate. This contributes to an inferior MEA performance by decreasing the

effective surface area of the platinum for both oxidation and reduction.

An on-time of 150 ms and off-time of 450 ms was found to deliver the best results under

the specified experimental conditions. Figure 8-43 shows the relationship between pulse

frequency and cell performance at five different fuel cell output current densities, ranging

from 200 – 1000 mA cm-2

. It can be seen that electrodes prepared using long pulse

periods; i.e., greater than 1500 ms, deliver the worst results when compared with

electrodes fabricated employing very low pulse frequencies—less than 50 ms. This can

be explained in terms of the available platinum ions close to the cathode during the on-

time of the pulse period. With low pulse frequencies, the on-time is relatively short and

the average current density is often below the limiting current density. This ensures the

presence of an ample supply of platinum ions near the surface of the cathode during the

electrodeposition period. On the other hand, with high pulse frequencies, the on-time is

relatively long and the number of platinum ions in the diffusion layer will diminish

during this period. At this point mass transport will become the rate-determining step and

crystal growth will become the dominant process, leading to a lower catalyst surface area

and, ultimately, lower fuel cell performance. According to Figure 8-43, a pulse frequency

with an on-time of 150 ms and an off-time of 450 ms (for a pulse period of 600 ms)

delivers the best performance at all current densities investigated in this series of

experiments. MEAs fabricated with a pulse frequency of 400 ms performed equally well;

however, a sharp decrease in performance was observed when the pulse frequency was

further decreased to 200 ms.

201

Figure 8-42 Effects of pulse frequency (on-time/off-time) on cell performance (square

pulse, H2/Air, 20 wt% PTFE, 20% duty cycle, 0.30 mg Pt cm-2

per electrode, cell

temperature of 50 °C, Nafion® 112, fully humidified fuel and oxidant with

stoichiometries of 1.2 and 2.5, respectively)

Figure 8-43 The relationship between pulse frequency and cell voltage for fuel cell

output current densities of 200-1000 mA cm-2

using PC electrodeposition (square pulse,

H2/Air, 20 wt% PTFE, 20% duty cycle, 0.30 mg Pt cm-2

per electrode, cell temperature of

50 °C, fully humidified fuel and oxidant with stoichiometries of 1.2 and 2.5, respectively)

202

8.5.3.3 Influence of Plating Bath Concentration on MEA Performance

A series of experiments was conducted to determine the optimal concentration of the

cation in the plating solution. Five plating solutions with Pt(NH3)4Cl2 concentrations,

ranging from 1.00 to 1000 mM were prepared and used to deposit platinum onto identical

carbon (TGPH-090, Toray) substrates using square pulse electrodeposition. All other

parameters, including peak current density, off-time, on-time, duty cycle and platinum

loading were kept constant. Figure 8-44 shows that a plating solution concentration of

50 mM delivered the best results. Platinum line scans of the cross sections of the above

MEAs were obtained to estimate the thickness of the catalyst layers as well as their

distribution inside the GDL using EPMA. Figures 8-45 (a)-(e) compare the Pt content at

the electrode surfaces adjacent to the Nafion® layer as well as the extent of platinum

penetration into the GDL. According to these line scans, the Pt content; i.e., its intensity,

is at its maximum at the interface between the GDL and the Nafion® layer for all cases.

However, the extent of Pt penetration inside the GDL shows the highest level for MEAs

prepared with concentrated plating solutions; i.e., 500 mM and 1000 mM.

Figure 8-44 Effects of plating bath Pt(NH3)4Cl2 concentration on MEA performance

(square pulse, H2/Air, 20 wt% PTFE, 20% duty cycle, 0.35 mg Pt cm-2

per electrode, cell



203

Figure 8-45 Cross-sectional platinum line scans for MEAs prepared from plating baths

with different platinum concentrations: (a) 1.0 mM; (b) 50 mM; (c) 100 mM; (d) 500 mM

and (e) 1000 mM

Nafion® GDL

204

On the other hand, MEAs prepared with plating solutions containing 50 mM and 100 mM

of cations show a high Pt intensity at the interface and a fast decline into the GDL and

away from the Nafion®

-GDL interface. Reference to Figure 8-44 shows that the catalyst

deposited from the 50 mM and 100 mM baths delivered the best fuel cell performance.

The MEA made from the least-concentrated bath—1.0 mM—exhibits the worst cell

performance, although the extent of its penetration into the GDL is relatively low.

However, the Pt intensity at the GDL-Nafion®

interface is the lowest amongst all the

examined MEAs.

The primary objective of employing a pulse current electrodeposition technique is to

selectively deposit platinum particles within the active catalyst layers, where both

electronic and ionic pathways exist. To achieve this selective deposition, a number of

parameters must be carefully selected and optimized, including the concentration of the

Pt(NH3)42+

cation in the plating bath.

The cation concentration in the bath must be relatively low for two reasons. First, for the

cation to reach the active layer and then be deposited, it first must pass through the

cation-conducting selective Nafion® membrane adjacent to the active layer. To ensure the

conduction of metal cations through the Nation®

, the concentration of the metal cation

must be lower than the fixed charge of the membrane. The fixed charge concentration of

most SPEs is around 1.0 mol L-1

(based on total membrane volume) [12, 141].

Accordingly, the concentration of the metal cation in the plating bath must be lower than

1.0 mol L-1

. Second, the current density distribution throughout the carbon substrate

dictates the localization of the catalyst particles and, as discussed below, the metal cation

concentration can play a major role.

Two scenarios can be considered: very low and high cation concentrations. For the

former, the current-density distribution throughout the carbon substrate is controlled

primarily by the extent of the cation transport inside the substrate. Although the low

concentration limits the movement of metal ions and ensures their reduction as soon as

they reach the active layer, they can become inactive due to simultaneous side reactions

in the same region of the electrode, such as the reduction of water. Since the Pt ion is

205

dissolved in an aqueous solution, it is possible for the solvent to be reduced before the

platinum. This can take place according to the following reaction:

2H2O + 2e- H2 (g) + 2OH

- (aq) (8-26)

This, of course, depends on the kinetic resistance associated with the reduction of

platinum being minimal [12]. Reaction (8-26) not only retards the reduction of the metal

cations, but also destroys the existing catalyst layer due to evolution of hydrogen. In

addition, owing to the presence of a very small number of metal cations, the deposition of

significant amounts of the catalyst may not be possible.

On the other hand, if the concentration of the cation in the plating solution is too high,

then the current-density distribution throughout the substrate is no longer governed by

ionic mass transport, but rather by deposition kinetics and, more importantly, by the

ohmic drop throughout the substrate. In this case, a large portion of the catalyst ions can

move away from the active layer and be deposited inside the carbon substrate, and,

ultimately, become inactive. In other words, some of the metal ions will have enough

time to move away from the region where both ionic and electronic connections are

available (GDL-Nafion®

interface), and will be reduced in places where only electronic

pathways are present. This can be overcome by providing the metal ions with sufficient

electrons to facilitate the reduction process by increasing the current density. However,

this will inevitably lead to hydrogen generation and the loss of the catalyst layer, as

previously explained.

The reduction of the platinum-complex cation into metallic platinum can be represented

by the following reaction:

Pt(NH3)42+

+ 2e- Pt + 4NH3 (8-27)

Since ammonia is a weak base, the solution in the diffusion layer adjacent to the electrode

surface can become basic as the electroplating progresses. Furthermore, NH3 can react

with Brønsted acids (proton donors) to form ammonium, a relatively strong conjugated

acid, according to the following chemical reaction:

NH3 + H2O ↔ NH4+ + OH

- (8-28)

206

Selective electrodeposition of platinum inside the active layer is a sensitive process and

the inhibition of competing processes is critical. Therefore, it is imperative to maintain an

ammonium concentration that is well below that of the platinum [12]. The extent of

ammonium production in an aqueous solution is a function of pH. If the pH is low, the

equilibrium shifts to the right (according to equation (8-28)) and more ammonium is

produced. On the other hand, if the pH is high, the equilibrium shifts to the left.

Furthermore, high average current densities facilitate the production of ammonium in

aqueous solutions. This was not deemed to be a problem in this series of experiments,

since the applied average current densities were low enough to inhibit the onset of

ammonium production.

8.5.3.4 Platinum Distribution in Carbon Substrates Fabricated by Pulse

Electrodeposition

A series of experiments was performed to determine the platinum distribution in 3.5 cm

diameter carbon substrates (TGPH-090, Toray). Each carbon substrate was cut into 10

equal area annuli (Figure 8-46) and the amount of platinum determined using gravimetric

analysis. Five identical samples were analyzed to ensure the accuracy and reproducibility

of the reported data. The results, presented in Figure 8-47, indicate an almost uniform

platinum distribution in the area used as active electrode (a 2.24 cm 2.24 cm electrode

is cut to make a 5-cm2

MEA). It should be noted that no platinum was found near the

edges of the circular electrodes. This was expected since there was no contact between

this part of the electrode and the plating solution during electrocatalyzation, since the

substrate was sitting inside the sample holder on an o-ring.

Figure 8-48 shows an electron micrograph of a composite fuel cell MEA consisting of a

cathode prepared using our PC electrodeposition technique bonded to a commercial

Nafion®

112 membrane. The Nafion®

membrane in turn is bonded to an anode, which

consists of a commercial E-TEK electrode, prepared using a commercial rolling

technique. All five layers of the MEA are clearly visible and their thicknesses can readily

be determined. The bright portions on either side of the Nafion® membrane indicate the

presence of a heavy metal such as platinum. The thickness of the Nafion® 112 membrane

can readily be confirmed as 50 µm.

207

Figure 8-46 Carbon substrate impregnated with platinum

It is important to note the thickness of the pulse-electrodeposited platinum layer on the

cathode side of the MEA and compare it with the E-TEK electrode on the anode side.

The thickness of the former is about 5 m while the thickness of the latter is almost ten

times higher at about 50 m. The thickness of the catalyst layer can play a vital role in

the distribution of gases and water inside the MEA and hence, can directly influence the

performance of the fuel cell. It is known that thick catalyst layers (greater than 10 m)

lead to lower cell performance due to the longer paths for reactants to reach the three-

phase interface deep inside the MEA, where the redox reactions take place. In addition,

ohmic losses also increase since electrons are forced to take a longer path to reach the

GDL, and, ultimately, the external circuit.

1.75 cm

2.24 cm

Active area

used in fuel

cell

208

Figure 8-47 Platinum distribution on a carbon substrate as a function of distance from

the centre of the substrate

Figure 8-48 An electron micrograph of a composite fuel cell MEA

Cathode

GDL Bonded

Nafion®

Membrane

~ 50 m

E-TEK Pt/C

Layer

~ 40 m

0.3 mgPt/cm2

Anode

GDL

Pt Layer

Deposited

by Pulse Electrod-

eposition

~ 5-7 m

0.3 mgPt/cm

2

E-TEK Electrode

20 m

--------

Pt zone

Pt zone

17.5 13.5 8.8 4.4 0.0 4.4 8.8 13.5 17.5

Distance from the centre of the substrate (mm)

mg Pt cm-2

209

8.5.4 Effect of Pulse Current Waveform on Properties of Electrodeposited

Catalyst Layer

Most of the research carried out globally on electroforming and electrodeposition is

focused on utilizing square-wave pulse current to improve deposit quality. There has

been a limited number of published articles on the use of other types of waveform to

enhance surface finishing and hardness of some deposited metals such as nickel [503]

and nickel alloys [504], but no reports on using different types of waveform to

electrodeposit platinum and platinum-group metals for use as electrocatalysts.

Therefore, a series of experiments was conducted to determine the influence of different

pulse current waveforms on deposit quality, particularly the catalytic activity of platinum

towards hydrogen oxidation and oxygen reduction in a PEM fuel cell. Figure 8-49

compares the performance of electrodes prepared by employing different waveforms—

rectangular, ramp up, triangular and ramp down—with a duty cycle of 4% and a peak


. The best performance was exhibited by the MEA

prepared with a ramp-down waveform. The MEA fabricated using a triangular waveform

performed equally well, followed by that created using a ramp-up waveform. The MEA

made by employing a rectangular waveform delivered the worst performance. It is

important to note that at low current densities (less than 1000 mA cm-2

), the performance

curves are very similar; and only a slight improvement is achieved; however, at higher

current densities (greater than 1700 mA cm-2

), an almost 20% improvement in fuel cell

current density output is observed, when using waveforms other than the conventional

rectangular one.

The above results can be explained in terms of the size and distribution of the deposited

platinum. When a rectangular waveform is used, the current density is instantaneously

increased to the peak current density and then kept constant for the duration of on-time.

The continuous high cathodic current encourages crystal growth and hence fewer nuclei

are formed, resulting in a decrease in the number and an increase in the size of the Pt

crystallites and, consequently, a decrease in the effective surface area of the deposited

platinum. On the other hand, if the nucleation rate can be increased at the beginning of

the process, the overall crystal growth is retarded as the electrodeposition continues, and

210

Figure 8-49 Cell performance as a function of electrodeposition waveform (H2/O2, 20

wt% PTFE, 4% duty cycle, 0.35 ± 0.02 mg Pt cm-2

per electrode, cell temperature of 50

°C, fully humidified fuel and oxidant with stoichiometries of 1.2 and 1.5, respectively)

well-dispersed and relatively small catalyst particles can be obtained. This takes place

when a ramp-down waveform is employed. To elaborate, a sharp increase in the applied

peak current density at the start of the pulse (similar to a square-pulsed waveform)

establishes a high concentration overvoltage leading to an increase in nucleation rate in

accordance with equation (6-47). If continued; however, the number of electroactive

species in the diffusion layer close to the surface of the cathode diminish faster than they

can reach the substrate surface from the bulk solution. At this point, the nucleation rate

decreases; but, since the cathodic current is still at its maximum, crystal growth becomes

the dominant mode of electrodeposition, leading to inevitable growth of existing crystals.

In other words, when a high peak current density is applied and maintained for an

extended period of time—as is the case for a rectangular pulse waveform—the system is

no longer limited by charge transfer, but by diffusion. In this case, adatoms will have

enough time to find stable places on the surface of the substrate to be reduced; i.e., on the

surface of existing crystals, rather than forming new nuclei. This diminishes the effective

surface area of the deposited catalyst, leading to inferior fuel cell performance. The

aforementioned problem is rectified by limiting the duration of the peak current density

to inhibit crystal growth. A ramp-down waveform is a prime example of such a strategy,

211

in which the peak current density is sharply increased to promote nuclei formation,

followed by a gradual decrease to impede crystal growth (by preventing the onset of a

diffusion-controlled process) while new nuclei can still be formed.

A triangular waveform also is designed to promote nuclei formation, while inhibiting

crystal growth when the pulse current is applied. Similar to a ramp-down waveform, a

rapid increase at the beginning of the pulse ensures the creation of high concentration

overvoltage at the electrolyte-electrode interface, transforming the electrodeposition

process to be dominated by nucleation. As the applied current density increases, the

concentration of the electroactive species at the interface decreases, shifting the system

into one that is dominated by crystal growth. However, as soon as the peak current

density is reached, the pulse current changes direction and starts to decrease as

electrodeposition continues. As a result, the concentration of the electroactive species in

the diffusion layer starts to increase once again, before the limiting current density is

reached. The end result is a well-dispersed catalyst layer with high active surface areas.

A ramp-up waveform is similar to a triangular one, at least at the beginning. A gradual

increase is observed at the start of the pulse promoting nucleation rather than crystal

growth. However, compared with a triangular waveform, it takes twice as long to reach

the designated peak current density. This additional time allows the adatoms to find

stable places on the surface of the electrode to be reduced (i.e., the existing crystals) and

the nucleation rate will decrease as a result. However, a sharp drop in the applied current

density at the conclusion of the on-time (ton) ceases crystal growth, ensuring smaller

particle size compared with a square-pulse waveform, for which the peak current density

is applied for the complete duration of pulse on-time.

Figure 8-50 shows TEM images of platinum particles for electrodes fabricated using (a)

ramp-down, (b) triangular and (c) rectangular waveforms with a peak deposition current


, a duty cycle of 4%, and 0.35 ± 0.02 mg Pt cm-2

. Electrocatalysts

prepared by the ramp-down and triangular waveforms exhibited a more uniform

distribution and smaller particle size than obtained by the conventional rectangular

waveform. These micrographs further validate the superiority of ramp-down and

212

Figure 50 TEM images of platinum catalyst electrodeposited employing different pulse

waveforms: (a) ramp-down (b) triangular and (c) rectangular (peak deposition current

density = 400 mA cm-2

, 4% duty cycle, 0.35 ± 0.02 mg Pt cm-2

per electrode)

Figure 8-51 Size distribution of platinum nanoparticles according to the type of

waveform: (a) ramp-down (b) triangular and (c) rectangular waveforms (peak deposition

current density = 400 mA cm-2

, 4% duty cycle, 0.35 ± 0.02 mg Pt cm-2

per electrode)

(a) (b) (c)

50 nm

50 nm

50 nm

213

triangular waveforms over the rectangular waveform in depositing a more uniform layer

with smaller nanoparticles. Figure 8-51 shows the histograms of the platinum particle

size for all three waveforms, where a significant proportion of the platinum nanoparticles

are in the neighbourhood of 1-3 nm (about 90% for the ramp-down and around 85% for

the triangular waveforms), while for the rectangular waveform, the majority of the

platinum nanoparticles are in the range of 3-5 nm (more than 80%). These clearly prove

the superiority of the ramp-down and triangular waveforms over the conventional

rectangular waveform and validate the experimental fuel cell performance results. They

also are in good agreement with the mathematical modeling predictions presented in

section 9.3.

8.5.5 Effect of Plating Solution Flow Rate on MEA Performance

Figure 8-52 presents the polarization curves for electrodes fabricated with different

electrolyte (platinum plating solution) flow rates in the plating cell. The amount of

platinum loading in the cathode and anode, duty cycle and peak current density were kept

constant at 0.35 ± 0.02 mg cm-2

, 20% and 50 mA cm-2

, respectively. The influence of

flow rate during electrodeposition is minimal; however, marginal improvements in

performance were observed for electrodes fabricated using higher flow rates. An

electrolyte flow rate of 454 mL min-1

was found to provide the best results (see Table 8-

17 and Figure 8-53). This can be attributed to a better mixing during deposition,

especially near the surface of the cathode, where the solution velocity is much lower than

that in the bulk solution. A shift from laminar to turbulent flow at higher flow rates also

may contribute to higher electrode performance, since more platinum ions will be

available near the surface of the cathode, resulting in a more uniform platinum

distribution.

214

Figure 8-52 Influence of plating solution flow rate on MEA performance (square pulse,

H2/Air, 20 wt% PTFE, 20% duty cycle, 0.35 ± 0.02 mg Pt cm-2

per electrode, cell



Table 8-17 Effect of plating bath flow rate on MEA performance

Electrolyte Flow Rate

(mL min-1

)

Voltage (V) at

200 mA cm-2

800 mA cm-2

92 0.689 0.314

235 0.690 0.320

454 0.706 0.331

581 0.685 0.327

215

Figure 8-53 Influence of plating bath flow rate on fuel cell voltage at four different

current densities (H2/Air, 20 wt% PTFE, 20% duty cycle, 0.35 ± 0.02 mg Pt cm-2

per

electrode, cell temperature of 50 °C, Nafion® 112, fully humidified fuel and oxidant with


8.5.6 Anode Platinum Loading

It is well established that the cathode of a PEM fuel cell is the most influential component

controlling cell performance. In addition, more than half the total voltage loss in a PEM

fuel cell can be attributed to the poor cathode performance. The oxygen reduction

reaction is the rate-limiting reaction and many researchers have tried to optimize its

performance. As a result, the amount, distribution and size of the platinum electrocatalyst

play an important role in fuel cell design and operation. Consequently, the anodic

hydrogen oxidation reaction is fast and may not need as much platinum catalyst as the

cathode.

A series of experiments was performed to examine the effects of anode platinum loading

on fuel cell performance. Figure 8-54 presents the polarization curves of different

electrodes with varying amounts of platinum on the anode. The amount of platinum

loading at the cathode was kept constant at 0.35 mg cm-2

, along with duty cycle (20%)

216

and peak current density (50 mA cm-2

). The results indicate that the lowering of the

platinum loading at the anode from 0.35 to 0.15 ± 0.02 mg cm-2

has only a very minor

effect on fuel cell performance. However, anode loadings less than 0.15 ± 0.02 mg cm-2

result in a sharp decline in performance.

The above finding also is presented in Figure 8-55 in terms of platinum loading and cell

voltage for four different fuel cell output current densities. It is clear that reducing the

platinum loading at the anode from 0.35 to 0.15 ± 0.02 mg cm-2

has a very small impact

on cell performance. However, loadings less than 0.15 ± 0.02 mg cm-2

adversely affect

the performance of the anode, most notably, at high current densities, where a marked

decline is observed.

Figure 8-54 Influence of anode platinum loading on fuel cell performance (square pulse,

H2/Air, 20 wt% PTFE, cathode = 0.35 ± 0.02 mg Pt cm-2

, 20% duty cycle, cell

temperature of 50 °C, Nafion® 112, and fully humidified fuel and oxidant with


217

Figure 8-55 Influence of anode Pt loading on electrode performance at four different

current densities (square pulse, H2/Air, 20 wt% PTFE, cathode = 0.35 ± 0.02 mg Pt cm-2

;

20% duty cycle, cell temperature of 50 °C, Nafion® 112, and fully humidified fuel and

oxidant with stoichiometries of 1.2 and 2.5, respectively)

8.5.7 Lifetime Behaviour of MEAs Prepared by Pulse Electrodeposition and

Conventional Techniques: Static Testing

Steady-state lifetime tests were performed on both commercial and in-house MEAs. The

former were obtained from E-TEK (E-TEK Div. of De Nora N.A., Inc., USA) and the

latter were prepared using the catalyzation technique described in this report. The type of

catalyst, gas diffusion layers and SPEs were identical for both MEAs. Although most

researchers prefer to employ accelerated stress tests (AST) to evaluate the durability of

different MEAs in PEM fuel cells, a steady-state lifetime test was utilized here to gain a

better understanding of various failure modes. The emphasis was placed on catalyst

degradation, since this was the primary difference between the two specimens under

investigation.

Catalyst degradation in PEM fuel cells operated for extended periods of time has been

associated with delamination of the catalyst layer, catalyst migration, catalyst ripening,

catalyst washout and carbon corrosion [427]. All these undesirable but inevitable

processes can contribute to apparent activity loss in the catalyst layer, and generally result

from changes in the microstructure of this layer and/or the loss of electronic and ionic

contact with the GDL and/or SPE.

218

A durability test on two different MEAs—E-TEK MEA and in-house MEA—was

performed in accordance with the experimental procedure outlined in section 7.6.2.2.

Figure 8-56 compares the potential-time curves for these MEAs, both of which were

fabricated with similar components, including identical Nafion® layers and catalyst type

on both anode and cathode. The in-house MEA generated a higher cell voltage for the

first 240 hours of operation compared with the commercial MEA from E-TEK, as shown

in Figure 8-57. However, the in-house MEA experienced a sharp decline in cell voltage at

about the 240-h mark, its performance dropping by approximately 10%, from 555 mV to

around 500 mV, after which it maintained a relatively constant output of 495 mV until

the 2800-h mark, when the cell voltage again began to fall. This time, however, it did not

recover and the cell voltage reached zero after 200 hours of operation after the decline at

the 2800-h mark. On the other hand, the commercial MEA performed well until it was

removed from the test cell after 4100 hours of continuous operation.

The sharp decrease in cell voltage after the second decline, indicated that a major failure

inside the MEA had occurred. Subsequent scanning electron microscopy revealed a

significant separation between the SPE and the catalyst layer on the cathode side of the

in-house MEA, as shown in Figure 8-58. Minor separation on the anode side also was

evident. These problems were attributed to an ineffective MEA fabrication process in this

case. Accordingly, another MEA identical to the first in-house MEA was prepared to

prove the above hypothesis. This time extra care was exercised during the fabrication and

testing phases to minimize human and mechanical errors. The durability of this second

MEA was determined according to the experimental procedure given in section 7.6.2.2.

After an operation of nearly 3000 hours, the MEA’s performance was comparable to that

of the commercial MEA without a noticeable decay in cell voltage during the complete

operation cycle. Life test results for this second MEA are presented in Figure 8-59. The

decrease in cell voltage for the in-house and the commercial MEAs for the initial 3000 h

of operation were found to be 2.1% and 2.8%, respectively.

219

Figure 8-56 Durability of single commercial (0.50 mg Pt cm-2

per electrode) and in-house

(0.35 mg Pt cm-2

per electrode) MEAs with apparent areas of 5 cm2 operated at a cell

temperature of 60 °C. Hydrogen and air are used as fuel and oxidant entering the cell at

100% RH with stoichiometries of 1.2 and 2.5, respectively. The operation time is 4100 h.


per electrode) and in-house

(0.35 mg Pt cm-2

per electrode) MEAs with apparent areas of 5 cm2 operated at a cell

temperature of 60 °C. Hydrogen and air are used as fuel and oxidant entering the cell at

100% RH with stoichiometries of 1.2 and 2.5, respectively. The operation time is 280 h.

220

Figure 8-58 SEM image of the cross section of the in-house MEA showing delamination

on one side of the Nafion® membrane


per electrode) and in-

house (0.35 mg Pt cm-2

per electrode) MEAs with apparent areas of 5 cm2 operated at a

cell temperature of 60 °C and ambient pressure. Hydrogen and air are used as fuel and

oxidant entering the cell at 100% RH with stoichiometries of 1.2 and 2.5, respectively.

The operation time is 3000 h.

04589 20 kV 50m

Apparent

delamination on

one side of the SPE

Nafion® 112

Cathode catalyst layer

Anode catalyst layer

221

Figure 8-60 shows the open circuit voltage data for the commercial and in-house MEAs

obtained at different time intervals. It can be seen that OCVs initially increase slightly

with time, reach a maximum value at about 2000 h (for both MEAs) and then decrease.

Furthermore, the OCV of the in-house MEA was higher than that of the commercial

MEA for the first 250 hours and between 2000 and 2500 hours after which the OCV of

the commercial MEA was higher for the remainder of the test. The initial rise in OCV of

both MEAs is attributed to the continuous hydration of Nafion®, where ionic conductivity

is improved resulting in a slight increase in cell performance.

Figure 8-60 Open circuit voltage (OCV) data obtained at different time intervals for

an in-house and a commercial MEA

8.5.8 Lifetime Behaviour of MEAs Prepared by Pulse Electrodeposition and

Conventional Techniques: Dynamic Testing

Steady-state lifetime tests reveal important information about the durability of MEAs;

however, such information is only pertinent to systems under a constant load. Real

systems, on the other hand, operate under transient conditions, where the load is

constantly changing in response to real-time variations in operational demands. To gain a

better understanding of the influence of changes in operational conditions on MEAs, a

222

number of dynamic life tests were carried out according to the experimental procedure

outlined in section 7.6.2.3.

Figure 8-61 shows the power output of two 200-W PEM fuel cell stacks, one containing

in-house and the other commercial MEAs. Both stacks were used to charge a battery

bank comprising three 12-V lead acid batteries sixty-three times over a 60 day period.

The initial performance of the stack containing the in-house MEAs was superior to that of

the commercial MEAs. This can clearly be seen in Figure 8-61 for the first 30 charges,

where the average power outputs are 218 W and 214 W for in-house and commercial

MEAs, respectively. The degree of variation in stack output also was lower for the in-

house MEAs for the first 30 charges. The drop in performance, however, was more

pronounced for the in-house MEAs after the 30th

charge. This is most likely due to settled

changes in the morphology and distribution of catalyst particles in the MEAs.

Figure 8-61 Power output of 200-W PEM fuel cell stacks containing in-house and

commercial MEAs running on hydrogen and air at ambient temperature and pressure

223

In another series of experiments, two 200-W PEM fuel cell stacks, each containing 42

cells, were used to directly power an electric bicycle (see Figure 7-6) to assess the

durability and performance of different MEAs operated with changing loads according to

the experimental procedure outlined in section 7.6.2.3. Figure 8-62 shows the OCVs for

both MEAs at the start of each trial run. The initial OCV of the stack containing in-house

MEAs was higher for every run compared with that of the stack utilizing commercial

MEAs. In addition, the variation in OCV throughout the experiment was greater for the

commercial MEAs. At 1.007 V the average initial OCV for the fuel cell stack containing

in-house MEAs was about 6% higher than its commercial counterpart. Since the primary

difference between the above MEAs was the method employed to deposit catalysts, it can

be concluded that the noticeable improvement of in-house MEAs is due to higher catalyst

utilization.

The OCVs of both fuel cell stacks also were recorded at the end of each trial run; the

results are presented graphically in Figure 8-63. Initially, the OCVs of both stacks

increased and then leveled off for the remainder of the experiment. Similar to the

previous series of initial OCVs, the final OCVs of the stack containing in-house MEAs

were higher than those of the stack containing commercial MEAs. The average final

OCV of the fuel cell stack utilizing in-house MEAs was 1.022 V, while that of the

commercial stack was 1.012 V; this is an increase of 10 mV (about 1%). This small

improvement may result from a more effective catalyst layer based on the deposition

method described in this thesis. As can be seen from Figure 8-48, the catalyst layer

deposited by the pulse electrodeposition technique is about 5 m in thickness compared

with the commercial layer that is approximately 10 times thicker at 50 m. The benefits

of a thin catalyst layer are twofold: better water management and lower ohmic losses,

both of which contribute to higher cell/stack performance. Due to shorter pathways from

the catalyst/membrane interface to the GDL, water is more effectively removed from the

interior of the MEA avoiding flooding conditions, where stack performance is

compromised. In addition, elevated hydration levels of the catalyst and electrolyte layers

are possible, which results in higher ionic conductivity of the electrolyte.

224

Figure 8-62 Initial OCV of in-house and commercial (E-TEK) MEAs in PEM fuel cell

stack in a dynamic system running on H2/Air at ambient temperature and pressure (0.50 ±

0.02 mg Pt cm-2

for anode and cathode of the commercial MEAs and 0.35 ± 0.02 mg Pt

cm-2

for the in-house MEAs)

Figure 8-63 Final OCV of in-house and commercial (E-TEK) MEAs in PEM fuel cell

stack in a dynamic system running on H2/Air at ambient temperature and pressure (0.50 ±

0.02 mg Pt cm-2

for anode and cathode of the commercial MEAs and 0.35 ± 0.02 mg Pt

cm-2


225

Furthermore, thin catalyst layers lead to a superior cell performance since reactants (and

products) are no longer required to travel through long pathways to reach the three-phase

interface deep inside the MEA, where the electrochemical reactions are taking place.

Lastly, ohmic losses decrease in the presence of a thin catalyst layer, since electrons now

are only required to traverse a shorter distance between the place they are generated (the

catalyst/SPE interface) and the current collectors.

The next series of experiments on MEA durability involved recording and analyzing the

maximum power points for both fuel cell stacks—one containing in-house MEAs and

another employing commercial MEAs—during the operation of the direct-drive bicycle

under real-life conditions in accordance with the method presented in section 7.6.2.3.

Figure 8-63 shows the maximum power points obtained from both fuel cell stacks while

the bicycle was running powered directly by a single stack at any one time. The reported

maximum power points in Figure 8-64 are the single highest recorded power output

during a given run. A similar trend for both fuel cell stacks was observed: a sharp

increase in fuel cell power output for the first 10 runs followed by a more gradual

increase in stack output. Initially, the performance of the fuel cell stack containing the

commercial MEAs was slightly higher than those employing in-house MEAs; however,

this trend was reversed quickly and, after about 15 runs, the latter stack exhibited a

slightly better performance for the remainder of the trial runs. The maximum power

points of in-house and commercial MEAs were 231 W and 222 W, respectively. More

importantly, the average peak power points (the highest recorded power output during a

single run—about 90 minutes) for the stacks utilizing in-house and commercial MEAs for

the complete duration of this series of experiment were 225 W and 217 W, respectively.

The observed 4% increase in the average cell power output is ascribed to a more effective

catalyst layer present in the in-house MEAs compared with commercial MEAs.

226

Figure 8-64 Maximum power output of 42-cell fuel cell stacks containing in-house and

commercial (E-TEK) MEAs tested in a dynamic system running on H2/Air at ambient

temperature and pressure (0.50 ± 0.02 mg Pt cm-2

for anode and cathode of the

commercial MEAs and 0.35 ± 0.02 mg Pt cm-2


227

8.0 Mathematical Model

9.1 Introduction

A mathematical model based on the works of Molina et al. [642] and Milchev [643, 652,

653] was further developed and refined to describe the dynamics of the experimental

metal electrodeposition process onto a solid surface. The model predicts and explains a

number of key concepts and variations in such systems, including species concentration

in the vicinity of the working electrode with time, grain size distribution of the

electrodeposited species, the magnitude of the overvoltage during the process, the

nucleation rate, the grain growth and the hardness of the resulting layer using four

different waveforms: rectangular, triangular, ramp down and ramp up, all with relaxation

times. Nickel was initially used to carry out the pulse electrodeposition to show the

generality of the model and to curtail cost.

9.2 Mathematical Model Development

9.2.1 Concentration and Overvoltage Profiles of Different Current Waveforms

It is imperative to understand the mass transfer mechanism during metal deposition.

There are three primary modes of mass transfer: diffusion, convection, and ionic

migration. Diffusion is usually described by Fick’s first law, where the diffusion flux of

a solute in a one-dimensional binary system is given by:

x

CDJ

(9-1)

where J is the diffusion flux of the species (mol cm-2

s-1

), D is the diffusion coefficient or

diffusivity (cm2 s

-1), C is the concentration (mol cm

-3) and x is the direction of the flux

(cm).

The driving force FD for diffusion of any given species is the gradient in its chemical

potential:

1

molper

molNdx

dFD

(9-2)

where (FD)per mol is the driving force per mole.

228

The driving force F'D acting on one liter of solution of concentration c in mol L-1

is

L

N

mol

N

L

mol

DD cFF (9-3)

dx

dc

dx

dcFD

(9-4)

For realistic concentration gradients the diffusion flux JD is directly proportional to the

driving force:

dx

dcB

dx

dcBFBJ DD

(9-5)

where B is a constant of proportionality.

The chemical potential µ of any given species in solution is given by

)(ln cyTR (9-6)

where µ*, a function of temperature only, is the chemical potential in the reference state

(an ideal interaction-free one molar solution) and y is the activity coefficient of the

species. Differentiating (9-6):

dx

dy

ydx

dc

cTR

dx

d 11 (9-7)

If the concentration gradient is not too great, it can be assumed that the activity

coefficient is approximately constant over the gradient, so that dy/dx ≈ 0, and (9-7)

becomes

dx

dc

c

TR

dx

dc

cTR

dx

d 1 (9-8)

dx

d

TR

c

dx

dc (9-9)

Experimentally it is observed that the diffusion flux JD is proportional to the

concentration gradient (Fick’s Diffusion Law):

229

dx

dcDJ D (9-10)

where D is the diffusion coefficient for the species.

Substituting (9-9) into (9-10) gives

dx

d

TR

cD

dx

d

RT

cD

dx

dcDJ D

(9-11)

Fick’s first law assumes that the concentration of the species under investigation is

independent of time. In real systems, however, as electrodeposition progresses, the

concentration of the species in the vicinity of the working electrode will inevitably

change. Fick’s second law takes this into account and predicts how concentration varies

with time as a result of diffusion. Fick’s second law is generally derived from Fick’s first

law:

x

cD

xJ

xt

C

(9-12)

Two cases can be considered:

The diffusion coefficient, D, is independent of the coordinate and/or species

concentration:

2

2

x

cDc

xxD

x

cD

xt

c

(9-13)

The diffusion coefficient, D, is not independent of the coordinate and/or the species

concentration:

cDt

c

(9-14)

Metal deposition or electrodeposition can be described by Fick’s second law of diffusion

with a constant diffusion coefficient, namely

2

2

x

cD

t

c

(9-15)

This equation can readily be solved by assigning an initial condition (9-16) and two

boundary conditions (9-17 and 9-18) [642]:

ocxc ),0( (9-16)

230

octc ),( (9-17)

zFD

ti

x

c

x

)(

0

(9-18)

The current density function, i(t), in expression (9-18) is unique for each type of current

waveform and can be expressed by a unit step function that is discontinuous with a

―zero‖ value for negative argument and a value of ―one‖ for a positive argument. This is

known as a Heaviside function and is routinely used in signal processing to define signals

that switch on and off at specified time intervals. Generally speaking, the Heaviside

function H(x) is the cumulative distribution function of a random variable and can be

expressed as the integral of the Dirac delta distribution,

:

x

dxxxH )()( (9-19)

where the Dirac delta distribution,

(x), is expressed as:

0,0

0,)(

x

xx (9-20)

Based on the above definitions, the Heaviside function is commonly defined as:

0,0

0,1)(

x

xxH (9-21)

Based on the Heaviside function, Molina et al. [642] have defined the mathematical

expressions for four different current waveforms—rectangular, triangular, ramp up and

ramp down—as follows:

Rectangular: )()()( tStSiti kwp (9-22)

Triangular:

)()()()()( tStS

b

thtStS

a

wtiti hkkwp (9-23)

Ramp up: )()()( tStSwta

iti kw

p (9-24)

Ramp down: )()()( tStStka

iti kw

p (9-25)

231

where a and b are defined in Fig. 7-3 and the Heaviside function is defined in terms of

Sk(t):

ktif

ktiftSk

0

1)( (9-26)

In equations (9-22) to (9-25):

Nw (9-27)

aNk (9-28)

baNh (9-29)

where N is the number of pulse cycles (N = 0, 1, 2, ….., n) and

is the pulse period.

To obtain analytical solutions, the partial differential equation (9-15) is solved using

Laplace transforms. The full solution for the rectangular waveform is presented below

and a detailed solution for the ramp-down waveform is provided in Appendix D.

The mass transfer in most electrochemical systems, including electrodeposition processes

employing different waveforms, is customarily described by Fick’s second law of

diffusion—equation (9-15). Taking the Laplace transform of this equation yields:

02

2

ccsdx

cdD (9-30)

According to the initial condition—equation (9-16)—the concentration at the start of the

first pulse (t = 0) is equal to the bulk concentration; the Laplace transform of (9-16)

results in:

s

cxc 0),0( (9-31)

Similarly, the Laplace transform of the boundary condition (9-17) gives:

s

csc 0),( (9-32)

Applying the Laplace transform to the mathematical expression representing the

rectangular waveform, expression (9-22), yields:

232

22)(

s

e

s

eiti

ksws

p (9-33)

Substituting the transformed Heaviside function—(9-33)—into boundary condition

(9-18) results in:

22

0s

e

s

e

DFz

i

x

c kswsp

x

(9-34)

The ordinary differential equation (ODE) that was derived from Fick’s second law of

diffusion (presented in equation (9-30)) can readily be solved using expressions (9-22)

and (9-32) to give

s

ceAc

xD

s

0

(9-35)

The constant A in the above expression can be evaluated by taking its derivative and

putting it into equation (9-34):

s

c

s

D

DFz

i

s

c

e

dx

cdA

p

D

s

001

(9-36)

Simplifying the above equation:

)379(

1

2/32/3

22

s

e

s

e

DzF

i

s

e

s

e

sDzF

iA

kswsp

kswsp

Putting equation (9-37) into (9-35):

s

ce

s

e

s

e

DFz

ixsc

xD

sksws

p 0

2/32/3),(

(9-38)

The surface concentration can be expressed as follow:

233

s

ce

s

e

s

e

DFz

ixsc

D

sksws

p 0)0(

2/32/3),(

(9-39)

2/32/3

0),(s

e

s

e

DFz

i

s

cxsc

kswsp

(9-40)

The solution to the original diffusion equation is obtained by applying the inverse

Laplace transform to equation (9-40):

)419()(

)2/3()(

2/3)0,(

2/12/1

0

tS

kttS

wt

DzF

ictc kw

p

The concentration overvoltage for a diffusion-controlled process is represented by:

0

)0,(ln

c

tc

Fz

TR (9-42)

Substituting equation (9-41) into (9-42) yields the overvoltage equation for a rectangular

waveform:

0

2/12/1

0 )()2/3(

)(2/3

lnc

tSkt

tSwt

DzF

ic

zF

RTkw

p

(9-43)

)(

)2/3()(

2/31ln

2/12/1

0

tSkt

tSwt

DFcz

i

Fz

TRkw

p (9-44)

Equations (9-41) and (9-44) are used to determine the concentration and concentration

overvoltage profiles of electrochemical systems employing square-pulse

electrodeposition.

9.2.2 Electrochemical Nucleation and the Critical Nucleus

Electrochemical nucleation occurs between two different phases: the electrolyte

containing the metal ions and the electron-conducting electrode. Under special

conditions, ions in the electrolyte that are in close proximity to the working electrode

begin to exchange electrons, resulting in an interfacial charge transfer and, ultimately,

234

electrochemical nucleation. For the nucleation and growth of any metal nanocrystallite,

including platinum on a carbon substrate, two limiting cases must be examined:

instantaneous and progressive nucleation. In the case of the former, the number of nuclei

present on the surface of the substrate at any time after the plating current is applied will

remain constant. For the latter; however, the number of nuclei is a function of time; i.e., it

continuously increases as plating progresses.

Earlier theories of electrochemical crystal growth at the atomic level considered the

substrates to be perfect surfaces without growth sites. As a result, nucleation was

perceived as the first step in deposition [333]. Later, researchers realized that ―real‖

surfaces have imperfections and, consequently, an array of growth sites. For progressive

nucleation, the total applied current,

iT (t) , is divided into two parts: a nucleation current,

iN (t) , and a growth current,

iG(t), such that

)()()( tititi GNT (9-45)

The nucleation rate J is related to the nucleation current,

iN (t) [642, 643]:

c

N

nez

tiJ

)(

(9-46)

where nc represents the size of the critical nucleus, which, according to Milchev [643] is

the ―cluster of the new phase that is in unstable equilibrium with the supersaturated

parent phase.‖ Here the ―unstable equilibrium‖ refers to the state of the system, since,

adding more atoms to the cluster from the parent phase will convert the critical nucleus

into a stable cluster leading to its irreversible growth. On the other hand, removal of

atoms from the cluster will lead to its irreversible decay [643].

According to Milchev [643] & Molina et al. [642], the critical nucleus size is defined as:

)(3

3203

23

F

vn M

c

(9-47)

where, is the specific free surface energy (J m-2

), vM is the molecular volume of the

metallic deposit (m3 mol

-1), and

is the electrochemical potential of the building units

or components (J), given by

235

ez (9-48)

and

F(0) is the wetting angle function, defined as:

)(cos4

1)cos(

4

3

2

1)( 0

3

0

0

0

V

VF (9-49)

In the above equation, V is the volume of a spherical segment with radius R formed by a

liquid droplet on a flat solid surface with a cap-shaped form as shown in Figure 9-1(a),

and V0 is the volume of a normal sphere. V and V0 are defined in equations (9-50) and (9-

51), respectively [643]:

0

3

0

3

0 coscos323

1),(

RRV (9-50)

3

03

4RV

(9-51)

(a) (b)

Figure 9-1 A liquid droplet formed on (a) a flat solid surface and (b) its cross section

[643]

In Figure 9-1 (b) and equations (9-49) and (9-50),

0 is the wetting angle (Rad). The

required energy for an accumulation of atoms to form a stable cluster—the critical

nucleus—is given in terms of changes in the electrochemical Gibbs free energy of the

system [642]:

cc nnG

2

1)( (9-52)

Substituting equation (9-47) into (9-52) yields:

236

)(3

16)( 02

23

F

vnG M

c

(9-53)

For the case of progressive and instantaneous nucleation without overlap, the initial

creation of nuclei on the substrate does not influence future nuclei formation or

interaction amongst growing clusters since the actual surface fraction covered by the

growing nuclei is negligible compared with the total surface area of the substrate. The

growth current, iG(t), for an electrochemical system with progressive nucleation is given

by [642, 643]:

t

G duutIuJti0

1 )()()( (9-54)

where, J(u) is the nucleation rate at time t = u and I1(t – u) is the growth current of a

single cluster formed at t = u.

The single cluster growth current, I1, is an important parameter in defining the

crystallization process and is given by the following expression [643]:

)(exp1 2/1

2/3

1 tTR

FzAI

(9-55)

where 2/3

0

2/1

0

2/5 ][][)(][2 cDvFzFA M (9-56)

The radius Rc of a homogeneously formed hemisphere on the surface of a solid substrate

can be calculated using the Gibbs-Thomson equation:

M

c

vR

2 (9-57)

Consider a single nucleus formed on the surface of a substrate at t = u. As

electrodeposition continues, this nucleus will grow and it is necessary to determine its

radius at any point in time during electrodeposition. The radius Rg of a growing nucleus

at time t since its formation on a foreign solid surface is calculated from:

)(

)(

)(4

3)(

0

3

tJ

ti

FzF

vtR

gM

g

(9-58)

237

If a cap-shaped grain is assumed (refer to Figure 9-1, ref. [643]), the final size Rf

distribution can be evaluated knowing Rc, Rg and the wetting angle,

0:

33

0

3 )(sin gcf RRR (9-59)

The average radius of the formed grains can readily be obtained by combining equations

(9-46) and (9-59):

3/1

0

0

3

)(

)()(

2

t

t

f

dttJ

dttJtRd

R (9-60a)

Lastly, the grain average radius R (or diameter d ) obtained from equation (9-60) can be

used to obtain the hardness (H) of the deposited layer using the Hall-Petch equation

[642]:

g

kH

y3 (9-61)

In the above equation, g is the acceleration due to gravity and ky is defined as [642]:

d

kkk l

oy (9-62)

where k0 is a materials constant for the starting stress for dislocation movement; this also

can be thought of as the resistance of the lattice to dislocation motion, and kl is the

strengthening coefficient, which is a constant unique to each material.

Substituting (9-62) into (9-61) and rearranging, gives the average grain diameter d as a

function of the hardness H:

2

3

3

o

l

kgH

kd (9-60b)

In this study, the average particle size of an electrodeposited platinum layer is correlated

with its hardness both experimentally and theoretically. The latter is accomplished

through mathematical modeling of such a layer using the Hall-Petch relationship, based

on the work of Molina et al. [642]. It is well established that material hardness increases

with decreasing particle size as long as the critical grain size has been established. The

238

hardness decreases with decreasing grain size below this critical grain size. It is also

proven that the catalytic activity of any catalyst, including platinum, increases with

increasing effective surface area. In other words, smaller catalyst particles will exhibit a

higher catalytic activity.

9.2.3 Different Types of Crystal Growth5

The catalytic activity of a catalyst layer correlates strongly with its particle size and, in

turn, with its average grain diameter. Hence, it is necessary to calculate the nucleation

rate, J(t), of an electrodeposition process in order to evaluate its average grain diameter

using equation (9-60b). The evaluation of nucleation rate, however, requires prior

knowledge of both nucleation and growth currents in the case of progressive nucleation.

Most published works [502, 540, 642] have assumed that the contribution of one of these

currents—usually the growth current, iG(t)—is negligible compared with the other.

In this study, contributions from both nucleation and growth currents for all waveforms

are considered. Hence the nucleation rate in equation (9-46) is evaluated from equation

(9-45) by considering a number of unique electrochemical growth conditions. The

growth current, iG(t), for any electrodeposition process involving progressive nucleation

can be readily evaluated by solving the convolution integral shown in equation (9-54). A

closer examination of this expression reveals that I1(t)—the current of a hemispherical

cluster growing at a constant electrode potential—must be determined. Based on the

works of Milchev [643], two mathematical expressions for the radius R(t) and the current

I1(t) of a single growing hemispherical cluster will be derived for five unique cases of

electrochemical growth under pure charge transfer control (ionic), pure diffusion control,

pure ohmic control, combined charge transfer and diffusion control and combined charge

transfer and ohmic control.

It is worth noting that the growth of any cluster in this fashion is similar to the growth of

a hemispherical liquid drop on a solid surface. This dictates that the faces of a 3-

dimensional crystal must contain a large number of growth sites [643], and is a frequently

5 These are based on the works of Milchev [643] and full derivations of equations indicated with an asterisk

(*) are given in the appendix.

239

used approximation that is satisfied in this study, owing to the existence of a considerable

number of growth sites on carbon substrates.

9.2.3.1 Electrochemical Crystal Growth under Pure Charge Transfer Control

Consider a single-ion electrochemical system where pulse current electrodeposition is

about to be carried out to deposit a nanocrystallite metallic layer with a cluster of radius R

and a surface area, SR, on a foreign solid substrate. During the very first transient stage of

this process, the double layer is charged, and the overvoltage at time t, t, is given by

[643]:

dlel

tC

texp1 (9-63)

where η is the steady state overvoltage, and el and Cdl are the ohmic resistance of the

electrolyte and the double layer capacitance, respectively. It is evident that after some

time t ≈ 5(el)(Cdl), the double layer is fully charged and the overvoltage becomes .

Beyond this time the kinetics are governed by charge transfer kinetics, with

= Erev – E > 0. From this time onwards, the formation of a new phase—nuclei—on the

electrode surface becomes the dominant process. Initially, the growth of the cluster on the

electrode surface is dominated by the rate of charge transfer across the electrical double

layer at the electrode-electrolyte interface. The growth current, I1(t), is approximated by

the Butler-Volmer equation as [643]:

TR

Fz

TR

FzitStI R

)1(expexp)()( 01 (9-64)

where i0 is the exchange current density (A cm-2

) and SR = 2 R2(t). Substituting for SR

yields

TR

Fz

TR

FzitRtI

)1(expexp)(2)( 0

2

1 (9-65)

It should be noted that in this treatment both cathodic currents and cathodic overvoltages

are treated as positive quantities. This simplifies the logarithmic treatment of negative

numbers.

240

The mass balance during the growth of a single spherical nucleus controlled by ion mass

transfer also can be described by Faraday’s law according to [643]:

dt

dRR

v

FztI

M

2

1

2)(

(9-66)*

It should be noted that this equation describes the mass balance irrespective of the growth

mechanism. Explicit formulae for I1(t) and R(t) can be obtained by combining equations

(9-65) and (9-66) and solving the resulting differential equation with the boundary

condition R(t) = 0 at t = 0. As a result:

tTR

Fz

TR

Fz

Fz

vitR M

)1(expexp)( 0 (9-67)*

2

3

2

3

0

2

1

)1(expexp

)(

2)( t

TR

Fz

TR

Fz

Fz

ivtI M

(9-68)*

Equation (9-68) is used to determine the growth current in the convolution integral

presented in equation (9-54) for cases where crystal growth is controlled by charge

transfer across the electrical double layer.

9.2.3.2 Electrochemical Crystal Growth under Combined Charge Transfer and

Diffusion Control

The previous case is applicable only to systems where the electrodeposition is carried out

in a relatively short time period, the primary assumption being that the concentration of

the electroactive species at the surface of the electrode, cs, is virtually identical to c₀, that

in the bulk solution. However, for longer times when the surface concentration of the

electroactive species is lower than the bulk concentration, in addition to charge transfer,

diffusion also must be considered. For the simple case of steady-state diffusion, the

concentration of the electroactive species around the growing hemispherical cluster is

given by [643]:

c

c

r

Rcc s11 (9-69)*

241

where R is the radius of each cluster and r is the radial distance from the centre of the

cluster. If an instantaneous steady state is assumed, the concentration gradient at the

cluster-electrolyte interface is given by

R

cc

dr

dc s

Rr

)(

(9-70)*

Combining Faraday’s law (equation (9-66)) with equations (9-65) and (9-70) results in:

TR

FziRcDFz

TR

Fz

TR

FzcDiv

dt

dRM

exp

)1(expexp

0

0

(9-71)*

Imposing the initial condition, R(t) =0 at t = 0, gives [643]:

121)(2/1 ntmtR (9-72)*

where Pi

cDFzm

0

(9-73)

TR

FzP

exp (9-74)

cDFz

Qivn M

2

2

0

)(

(9-75)

TR

Fz

TR

FzQ

)12(exp

2exp (9-76)

Substituting equations (9-72) – (9-76) into Faraday’s law (9-65) and solving for the

growth current yields:

1

)21(

1)(

2/11nt

ntptI (9-77)*

where

3

0

24

Pi

QcDFzp

(9-77a)

242

Equations (9-72) and (9-77) are used to calculate R(t) and the growth current, I1(t), in the

convolution integral of equation (9-54) for cases where crystal growth is controlled by a

combination of charge transfer and diffusion.

9.2.3.3 Electrochemical Crystal Growth under Pure Diffusion Control

The electrochemical crystal growth is assumed to be under pure diffusion control when

electrodeposition times are relatively long. The same holds true when the exchange

current density is high provided that nt > 50. In such cases I1(t) and R(t) can be

expressed as [643]:

2/1

2/1

2/1exp12)( t

TR

FzvcDtR M

(9-78)*

2/1

2/3

2/33/1

1 exp12)()( tTR

FzvcDFztI M

(9-79)*

Equations (9-78) and (9-79) can be further simplified, provided that the following

conditions are satisfied:

the surface concentration of the electroactive species approaches zero; i.e., cs 0,

the cathodic overvoltage is greater than FzTR /5 (69.6 mV at 50 °C)

Under these conditions,

2/12/12)( tvcDtR M (9-80)

2/12/33/1

1 2)( tvcDFztI M (9-81)

9.2.3.4 Electrochemical Crystal Growth under Complete Ohmic Control

The concentration of the electroactive species at the surface of the substrate, cs, stays

very close to the bulk concentration, c₀, during pulse electrodeposition if two conditions

are met:

the cluster is growing at a relatively low rate,

the electrolyte is continuously stirred during the electrodeposition process

In this case, the total overvoltage η consists of the overvoltage at the surface of the

cluster, ηs, plus the ohmic drop through the solution:

243

)()(1 RtIs (9-82)

where (R) is the ohmic resistance of the electrolyte around the cluster, calculated from

L

R

RR

el

12

1)(

(9-83)*

where, el (-1

cm-1

) is the specific conductivity of the solution and L is the distance

between the cluster and the counter electrode. Equation (9-83) can be simplified if the

distance between the cluster and the counter electrode is much greater than the radius of a

single cluster formed on the surface of the working electrode; i.e., L >> R. This is

actually the case in this study; hence the following equation has been employed to

calculate (R):

R

Rel2

1)( (9-84)

If the overvoltage at the surface of the cluster is entirely accounted for by the ohmic drop,

then, from equation (9-82), the total overvoltage η reduces to the ohmic drop through the

solution:

)()(1 RtI (9-85)

Substituting equation (9-84) into (9-85) gives:

R

tI

el

2

)(1 (9-86)

From which RtI el2)( (9-86a)

Thus, when the growth kinetics of the cluster is under complete ohmic control, Equations

(9-66) and (9-86a) give the radius of the cluster and its growth current as [643]:

2/1

2/1

2)( t

Fz

vtR Mel

(9-87)*

2/1

2/1

2/32/3

1 2)( tFz

vtI M

el

(9-88)*

244

9.2.3.5 Electrochemical Crystal Growth under Combined Charge Transfer and

Ohmic Control

This case is, by far, the most complicated of those discussed so far. In this case, the

overvoltage at the cluster surface is given by Equation (9-82) and (9-84) as [643]:

)()(1 RtIs

R

tI

el

2

)(1 (9-89)

And the Butler-Volmer equation takes the form:

R

tI

TR

Fz

R

tI

TR

FziRtI

elel

2

)()1(exp

2

)(exp2)( 11

0

2

1 (9-90)

Equating Eqn (9-66) with Eqn (9-90) gives [643]:

dt

dRRPP

dt

dRRPP

dt

dR4321 expexp (9-91)*

where

TR

Fz

Fz

iv oM exp1P (9-92)

MvTR

Fz

2

2P (9-93)

TR

Fz

Fz

iv oM )1(exp3P (9-94)

MvTR

Fz

2)1(

4P (9-95)

Approximate analytical expressions for the above equation are provided for low and high

ohmic drops by Melchev [643], since an exact solution to this equation is not possible.

245

9.3 Model Validation

The proposed electrodeposition technique should work for the deposition of any metal

cation onto a conductive substrate provided that the concentration of the metal in the

plating bath is low. In addition, the mathematical model developed in the previous

section is applicable to any electrodeposition system, irrespective of the type of metal. To

demonstrate the generality of this hypothesis, a series of experiments was carried out

using nickel and copper. The results for copper have been presented in previous sections;

here the findings for nickel are discussed, followed by the findings for platinum. The

validity of the mathematical model developed is verified by comparison with the

experimental results.

9.3.1 Nickel Electrodeposition

Nickel electrodeposition was carried out according to the method described in section

7.5.3. After electrodeposition, the surface morphology of each specimen was examined

and characterized using SEM. The grain sizes were measured according to ASTM E112-

95, while microhardness tests were performed in accordance with ASTM E384 (refer to

Appendix E for a brief description of the latter method). Experimental results—taken

from the present study as well as from published work [504]—then were compared with

hardness results generated by the model developed in this study and a similar model

reported by Molina et al. [642]. Both experimental and model data are presented in

tabulated and graphical formats in Tables 9-1 through 9-4 and Figures 9-2 to 9-5,

respectively.

Experimental and model results for a rectangular waveform are given in Table 9-1. Our

experimental data first are compared with a set reported by Wong et al. [504], who used

identical electrolyte bath composition and electrodeposition parameters. A comparison

between the two experimental sets shows a strong agreement. Mathematical model

hardness data were obtained by considering five different sets of conditions in terms of

nucleation and growth currents as described in sections 9.2.3.1 through 9.2.3.5. It should

be noted that the peak current density of the rectangular waveform must be half that of

the other waveforms to generate the same average current density.

246

Table 9-1 Experimental and model hardness data for nickel (rectangular waveform)

Hardness (kg mm-2)

Average current density (mA cm

-2)

Experimental Mathematical

Wong et al.

This study

Molina et al.

model

Pure diffusion

Pure charge

Transfer (ionic)

Pure ohmic

Combined charge

transfer & diffusion

Combined charge

transfer & ohmic

100 140 136 118 105 148 147 155 155

200 160 157 166 147 208 208 220 220

300 175 173 203 181 255 255 269 269

400 200 202 235 208 295 295 311 311

500 260 254 264 233 331 331 348 349

600 350 347 290 255 363 363 383 383

Table 9-2 Experimental and model hardness data for nickel (ramp-up waveform)

Hardness (kg mm-2)


-2)


Wong et al.

This study

Molina et al.

model

Pure diffusion

Pure charge

Transfer (ionic)

Pure ohmic

Combined charge


Combined charge

transfer & ohmic

100 153 155 129 125 125 125 104 104

200 175 173 182 177 177 177 147 147

300 195 199 223 216 216 216 179 179

400 225 229 259 249 249 249 207 207

500 290 286 290 279 279 279 231 231

600 390 388 319 305 305 305 253 253

247

Table 9-3 Experimental and model hardness data for nickel (ramp-down waveform)

Hardness (kg mm-2)


-2)


Wong et al.

This study

Molina et al.

model

Pure diffusion

Pure charge transfer (ionic)

Pure ohmic

Combined charge


Combined charge

transfer & ohmic

100 162 166 124 148 148 148 180 180

200 195 196 175 209 209 209 254 254

300 225 228 214 256 256 256 311 311

400 268 272 248 295 295 295 359 359

500 330 334 278 330 330 330 402 402

600 450 447 305 362 362 362 441 441

Table 9-4 Experimental and model hardness data for nickel (triangular waveform)

Hardness (kg mm-2)


-2)


Wong et al.

This study

Molina et al.

model

Pure diffusion

Pure charge transfer (ionic)

Pure ohmic

Combined charge


Combined charge

transfer & ohmic

100 158 159 129 140 140 140 141 141

200 180 182 182 198 198 198 199 199

300 210 212 223 242 242 242 243 243

400 245 251 258 280 280 280 281 281

500 310 303 289 313 313 313 314 314

600 410 412 318 343 343 343 344 344

248


hardness using rectangular waveform


hardness using ramp-up waveform

249


hardness using ramp-down waveform


hardness using triangular waveform

250

As can be seen from Table 9-1 and Figure 9-2, the model predictions for pure diffusion

control, pure ohmic control and pure charge transfer control give the best approximation

of hardness values, the only exception being when the system is operated at the lowest

average current density of 100 mA cm-2

. All other models overestimate the hardness

values for average current densities between 200 and 500 mA cm-2

. In general, as the

current density is increased, the microhardness of the deposit layer also increases.

Table 9-2 and Figure 9-3 compare the hardness results generated by a ramp up waveform

with the experimental data. Here the closest fit seems to be obtained for crystal growth

under pure diffusion, ohmic or ionic control. The model based on a combined ion and

diffusion control underestimates the hardness values and deviates significantly from the

experimental ones at higher average current densities—500 mA cm-2

and beyond. The

predictions from our model based on pure diffusion, for example, are very close to those

made by Molina et al. [642] with the former providing a better approximation for peak

current densities, ranging from 200 to 500 mA cm-2

, while the latter gives closer

approximation for low (100 mA cm-2

) and high (500 mA cm-2

) average current densities.

Similar to the rectangular and ramp up waveforms, for the ramp down waveform (Figure

9-4), the model based on pure diffusion control generates the best results for applied

average current densities below 500 mA cm-2

. For the triangular waveform (Figure 9-5),

all models performed equally well. Noticeable changes cannot be observed from Figure

9-5, where all experimental and model values are graphically presented; however, upon

inspection of the data in Table 9-4, a very slight improvement in predicted hardness

values in the neighbourhood of 200 – 500 mA cm-2

can be seen. The model under pure

diffusion control generated data that are in good agreement with the corresponding

experimental values.

In general, the results indicate that the mathematical models based on pure diffusion,

ohmic and charge transfer control generate data which are in reasonably good agreement

with experimental values reported in the literature by Wong et al. [540] and as well as

with those obtained in the present study. As mentioned previously, both model and

experimental data present a clear and consistent trend with respect to applied average

251

current density and nickel-coating microhardness: as the applied average current density

increases, the microhardness increases as well.

The physical constants, variables and electrodeposition parameters that were employed to

solve the mathematical expressions in the models are summarized in Table 9-5.

Table 9-5 Physical constants and variables used for determining nickel-coating


Property Symbol Value Unit Reference

Specific edge energy σ 0.255 J m-1 [644]

Diffusion coefficient D 1.329 × 10-9 m2 s-1 [645]

Molecular volume vm 1.1410 х 10-29 m3 mol-1

Dislocation blocking value K0 7 MPa [333, 642]

Penetrability of the moving Kl 0.18 MPa m-1/2 [333, 642]

dislocation boundary

Wetting angle γ0 π/2 Rad [642]

Temperature T 328.15 K

Initial Ni concentration C0 1.472 kmol m-3

Pulse duty cycle θ 0.5

Frequency f 100 Hz

Ionic charge z +2

Electronic charge e 1.602 × 10-19 C

Faraday's constant F 96487 C mol-1

Universal gas constant R 8.314 J mol-1 K-1

Acceleration due to gravity g 9.8 m s-2

9.3.2 Model Predictions for Nickel Concentration Overvoltage for various Waveforms

Figure 9-6 shows the concentration overvoltage predicted by the model developed in the

previous section that is based on the model presented by Molina et al. [642]. As can be

seen from this figure, the ramp-down waveform generates the highest concentration

overvoltage at the beginning of the pulse, and then subsides as electrodeposition

continues. A similar trend can be observed for the rectangular waveform, where the

concentration overvoltage has the second highest value initially. However, for the latter

waveform, the concentration overvoltage continues to rise as long as the current is on.

252

Figure 9-6 Concentration overvoltage for various waveforms with a peak current density

of 400 mA cm-2

, on-time & off-time of 5 ms (50% duty cycle) and 100 Hz (showing the

first full cycle)

These observations can be explained by examining the waveforms and how the current is

initially imposed on the system. In the case of a ramp-down waveform, the peak current

density rises quickly (much more like a step function) resulting in a sharp decrease in the

concentration of the electroactive species (Ni++

ion in this case) in the diffusion layer

leading to a sharp increase in concentration overvoltage. As soon as the maximum peak

current density is reached, however, the applied current density begins to recede, causing

the corresponding concentration overvoltage to gradually decline until the pulse on-time

is complete and the relaxation period (off-time) begins. Similarly, with the onset of a

rectangular waveform, the concentration overvoltage quickly rises; however, contrary to

the ramp-down waveform, the resulting concentration overvoltage continues to rise due

to the rapid depletion of nickel ions close to the cathode as a result of a high and

continuous peak current density. Accordingly, the concentration overvoltage steadily

increases until the pulse on-time period has elapsed and the relaxation period

commences.

According to Figure 9-6, both the ramp-up and triangular waveforms exhibit a slow rise

in concentration overvoltage at the beginning of the pulse on-time. A closer examination

253

of these waveforms shows that the applied current density in both cases does not reach its

highest value (i.e., the peak current density) until the middle or the last part of the pulse

on-time for the triangular and ramp-up waveforms, respectively. Consequently, the

decrease in the concentration of the nickel ions very close to the surface of the cathode is

more gradual than the previous rectangular and ramp-down waveforms. In addition, the

rise in the concentration overvoltage is faster for the triangular waveform than for the

ramp-up waveform. This is anticipated since the peak current density of the former

waveform is reached in half the time as the latter. This, in turn, causes a faster drop in the

concentration of the electroactive species in the cathode diffusion layer, leading to higher

concentration overvoltage in a shorter time period.

Similar to ramp-down waveform, the concentration overvoltage of the triangular

waveform reaches a maximum about 3.5 ms after the pulse on-time is activated (1.0 ms

after it reaches the maximum applied current density) and then begins to decline as the

applied current density starts to decrease. In contrast, the corresponding concentration

overvoltage of the ramp-up waveform continues to rise until the end of the pulse on-time.

Again, this is expected since the applied current density during pulse on-time of a ramp-

up waveform is constantly increasing and reaches its maximum value only at the

completion of the pulse on-time. It can be seen that the highest concentration overvoltage

occurs with the ramp-up waveform. However, this does not translate into a higher

nucleation rate, since the total concentration overvoltage for the entire pulse on-time of

the ramp-up waveform is smaller than the other three, as can be observed by comparing

the corresponding areas under each curve in Figure 9-6.

The above findings are in good agreement with the results reported by other workers

[504, 642]. However, most pulse electrodeposition models, including the two cited in this

study, use a 50% duty cycle with the assumption that the concentration of the

electroactive species in the diffusion layer will recover and reach its initial bulk value

after the completion of each pulse period. In other words, it is assumed that the

concentration of the metal ions in the diffusion layer is replenished during the pulse off-

time, when ions from the bulk solution diffuse into the diffusion layer. Despite this

relaxation time, the ion concentration in the vicinity of the cathode does not regain its

254

initial (bulk) concentration and will always decrease with increasing pulse cycles; this is

especially true for large duty cycles (50% and higher), when the pulse off-time is not long

enough (relative to the on-time) to allow ion recovery in the cathode diffusion layer. This

is predicted by the model developed in this study and presented in Figure 9-7. Figure 9-7

shows the calculated variation in nickel ion concentration in the cathode diffusion layer

during a single 10-ms pulse with a peak current density of 400 mA cm-2

and a duty cycle

of 50% for the four waveforms examined in this study. The concentration profiles are

distinctively characteristic for each waveform. The highest initial decline in nickel

concentration during pulse on-time is observed for rectangular and ramp-down

waveforms. This results from the sharp increase in applied current density at the start of

the pulse, leading to a higher deposition rate and a marked decline in nickel

concentration. For the ramp-down waveform the ion concentration in the diffusion layer

will recover and start to increase as the pulse on-time progresses, unlike the rectangular

waveform for which the concentration decreases throughout the complete pulse on-time.



, on-time and off-time of 5 ms each (50%

duty cycle) and 100 Hz (showing the first full cycle , on-time and off-time)

255

For the triangular and ramp-up waveforms, the initial decline in the concentration of

nickel ions in the cathode diffusion layer is more subtle compared with ramp-down and

rectangular waveforms. This is attributed to the gradual increase in the applied current

density during the pulse on-time in contrast to the sharp increase for the other two

waveforms. In addition, the decrease in nickel ion concentration in the diffusion layer

when a triangular waveform is employed is initially more pronounced than with a ramp-

up waveform, since the peak current density of the former is reached in half the time as

that of the latter. Furthermore, in the case of a triangular waveform, the concentration of

the electroactive species in the cathode diffusion layer begins to increase as the applied

current density reaches its maximum and then starts to decline. At this point, the rate of

nickel electrodeposition begins to fall and the diffusion rate from the bulk solution into

the diffusion layer becomes greater, increasing the concentration within the diffusion

layer. The concentration in the diffusion layer for a ramp-up waveform, however,

continues to decrease as the electrodeposition progresses owing to the continuous rise in

applied current density until the completion of the pulse on-time.

The vertical dashed line marks the end of the pulse on-time and the commencement of its

off-time. As expected, the nickel ion concentration decreases during pulse on-time when

the ions migrate from the diffusion layer to the surface of the substrate and are

subsequently reduced. Accordingly, a concentration gradient between the bulk solution

and the cathode diffusion layer is established and nickel ions begin to enter the latter

layer from the bulk solution. However, the rate of nickel electrodeposition is significantly

higher than the diffusion rate resulting in a reduction in nickel ion concentration in the

diffusion layer.

With time, the nickel ion concentration in the diffusion layer continues to decrease until it

approaches zero, when the limiting current density is reached and crystals begin to grow

at a faster rate. However, it is anticipated that nickel ions enter the cathode diffusion

layer when the current is interrupted. This ensures the presence of free metal ions in the

diffusion layer for the next pulse cycle. The extent of nickel ion replenishment in the

diffusion layer depends on several factors, including applied peak current density, pulse

duration, and duty cycle. One of the most important variables is the pulse off-time and if

256

its duration (with respect to on-time) is not long enough, the concentration of the metal

ion in the diffusion layer will not reach its initial value and the concentration in the

cathode diffusion layer will decrease with each pulse cycle. In most studies, a duty cycle

of 50% is employed with the assumption that the concentration of the electroactive

species will reach its original value at the completion of the pulse off-time. According to

the model developed in this study, the concentration of nickel ion in the cathode diffusion

layer does not reach its initial value at the conclusion of the pulse off-time for any of the

waveforms. The difference is most pronounced for the rectangular waveform, where the

difference between the initial and final nickel ion concentrations is twice that of the other

waveforms. It is thus evident that a 50% duty cycle is not sufficient to ensure the return

of the metal ion concentration to its initial level after the conclusion of the pulse off-time.

This is predicted by the model developed in this study and presented in Figure 9-8, where

concentrations at the end of the last pulse cycle (i.e.; 1000th

) for all waveforms at

different duty cycles are shown. The decrease in final concentration with increasing duty

cycle is more gradual for ramp-down and triangular waveforms compared with ramp-up

and rectangular waveforms, where the concentration declines more rapidly at higher duty

cycles.



, on-time of 0.1 ms, 1000 pulse cycles and

duty cycles of 10% - 100%. The initial bulk nickel concentration is 1.427 mol L-1

.

257

The nucleation rate and critical nucleus size of the deposited nickel for one pulse cycle

for all four waveforms are presented in Figures 9-9 and 9-10, respectively. According to

Figure 9-9, the ramp-down waveform exhibits the highest nucleation rate at the beginning

of pulse on-time, while the ramp-up waveform shows its highest nucleation rate at the

end of pulse on-time. This is expected since the nucleation rate is directly proportional to

the applied peak current density; as the cathodic peak current density increases, the

nucleation rate also increases. In case of the ramp-down waveform, the highest cathodic

peak current density is applied at the start of the pulse on-time, resulting in an increase in

the formation of new nuclei, which gradually will decrease as the applied current density

subsides with the progression of pulse on-time.

Figure 9-9 Nucleation rate for a single cycle (on-time only is shown) for all waveforms

with a peak deposition current density of 400 mA cm-2

and 50% duty cycle

258

Fig 9-10 Calculated critical nucleus size for a single cycle (on-time only is shown) for all


and 50% duty cycle

In contrast, for the ramp-up waveform, the cathodic current density is at its minimum at

the beginning of the pulse on-time and gradually increases until it reaches its maximum at

the end of the pulse on-time. Accordingly, the nucleation rate is very slow at the start of

the pulse on-time, but exhibits an exponential growth as the applied current density

increases. The rectangular waveform shows a gradual increase in nucleation rate as the

pulse on-time progresses and, similar to the ramp-up waveform, reaches its maximum at

the end of the pulse on-time. The triangular waveform, on the other hand, displays an

initial slow rise in nucleation rate followed by a rapid rise as the current density reaches

its maximum, and subsequently tapers off as the applied current density subsides.

According to Figure 9-9, the highest nucleation rate is attained by the ramp-up waveform;

however, this does not guarantee the generation of the smallest crystallites, as can be

observed from Figure 9-10. The evolution of the critical nucleus size for all waveforms

shown in Figure 9-10 indicates that the ramp-down waveform results in a smaller nucleus

size near the start of the pulse, while larger nuclei are obtained at the end of the

waveform. Interestingly, the nucleus size increases for both ramp-down and triangular

waveforms at the end of the pulse on-time. This is attributed to the lower current density

259

at the end of the pulse on-time, which, in the case of triangular and ramp-down

waveforms, favours the growth of existing nuclei. Overall, however, the smallest critical

nucleus size is attained with the ramp-down waveform.

To verify the model predictions, the influence of pulse duty cycle on deposit quality was

compared with X-ray Diffraction (XRD) spectra obtained according to the method

outlined in section 7.6.2.4. The XRD patterns shown in Figure 9-11 reveal the preferred

orientation of the nickel deposits obtained using the ramp-down waveform with a

constant peak current density of 400 mA cm-2

and varying duty cycles. As can be seen,

the diffraction intensity of the (200) orientation increases with decreasing duty cycle.

This is primarily attributed to the reduction in grain size of the deposited nickel and is

further substantiated by comparing the SEM images of the nickel deposits shown in

Figure 9-12. Figure 9-12 (a) shows the surface morphology of nickel deposits obtained at

a low pulse duty cycle of 20% in which small grains are dominant.

In contrast, when a duty cycle of 80% is employed, the surface morphology is

significantly altered and the nickel deposits contain a large number of crystallites that are

pyramidal in shape and significantly larger than the crystals shown in Figure 9-12 (a),

where a lower duty cycle has been utilized. The necessary condition for creating new

crystalline phase from the electrolyte solution is the presence of nickel ions in the cathode

diffusion layer during the pulse on-time to inhibit crystal growth and promote nucleation.

A lengthy pulse on-time—long duty cycle—results in fast diminution of electroactive

species in the cathode diffusion layer and, consequently, leads to crystal growth.

260

Figure 9-11 XRD patterns exhibiting the influence of pulse duty cycle on crystal

orientation of nickel deposits with a constant peak current density of 400 mA cm-2

Figure 9-12 Surface morphology of electrodeposited nickel at a constant deposition


and various duty cycles: (a) 20% and (b) 80%

(a) (b) 1 µm

___

1 µm

___

261


Platinum electrodeposition was carried out according to the method outlined in section

7.5.1 with two minor modifications. First, for microhardness tests, the substrate was

changed from carbon paper or cloth to AISI 431 stainless steel, since it is almost

impossible to perform a Vicker’s test on the actual MEA and it has been necessary to

assume that the grains deposited on the AISI 431 stainless steel plates are the same as

those deposited onto the carbon substrates. Second, the exposed surface area of all

stainless steel plates to the plating solution inside the electroplating flow cell was limited

to 1.0 cm2 and the plating bath temperature was maintained at 25 °C throughout the

plating process. Prior to placement inside the electroplating cell, the substrate was first

ground to a finish on grade 180 emery paper, rinsed with deionized water, and then

scrubbed with alcohol and acetone and finally rinsed with deionized water.

After electrodeposition, the surface morphology of each specimen was examined and

characterized using SEM and microhardness, the latter obtained in accordance with the

ASTM E384 method. Experimental findings were subsequently compared with the

hardness results generated by the model developed in this study. Physical constants and

variables used for determining the platinum-coating microhardness and other

characteristics are summarized in Table 9-6. The number of test samples was limited to a

few compared with nickel due to the prohibitive cost of platinum. Based on the findings

in the previous section for nickel electrodeposition modeling, the mathematical model for

platinum electrodeposition is based on crystal growth under pure diffusion control.

However, a number of modifications were made to ensure the validity of the model; these

are discussed in following section (9.3.3.1).

The ―size effect‖ of platinum particle on the electroreduction of oxygen is one of the

most important and widely-studied areas in electrochemical science. As discussed in

previous sections, platinum metals electrodeposited by a pulse current electrodeposition

technique tend to deposit platinum catalyst particles that are smaller in size, selectively

deposited on the carbon substrate, and in direct contact with both carbon support and the

membrane electrolyte, leading to higher fuel cell performance. In addition, certain pulse

waveforms have been shown to create smaller platinum particles, resulting in higher fuel

262

cell performance compared with other type of waveforms. The primary objective of this

part of the study is to use the mathematical model that was refined and developed (based

on the works of Molina et al. [642]) in the preceding section to predict the influence of

different pulse parameters such as peak cathodic current density, duty cycle, pulse

frequency, millisecond and microsecond pulses, and pulse off-time on the microhardness

of platinum deposits and, ultimately, on fuel cell performance. Similar to nickel

electrodeposition, model microhardness data are compared with the experimental values

to validate the mathematical model developed in this study and to gain an understanding

of the influence of electrodeposition parameters on platinum microhardness and,

subsequently, fuel cell performance.

Table 9-6 Physical constants and variables used for determining the platinum-coating

microhardness and other characteristics [646, 647]

Property Symbol Value Unit

Specific edge energy σ 0.240 J m-1

Diffusion coefficient D 1.20 × 10-9 m2 s-1

Molecular volume vm 1.5095 х 10-29 m3 mol-1

Dislocation blocking value K0 7 MPa

Penetrability of the moving Kl 0.168 MPa m-1/2

dislocation boundary

Wetting angle γ0 π/2 Rad

Temperature T 298.15 K

Initial Pt concentration C0 0.05 mol L-1

Pulse duty cycle θ 0.5

Frequency f 100 Hz

Ionic charge z +2

Electronic charge e 1.602 × 10-19 C

Faraday's constant F 96487 C mol-1

Universal gas constant R 8.314 J mol-1 K-1

Acceleration due to gravity g 9.8 m s-2

9.3.3.1 Modification of the Mathematical Model for Platinum Electrodeposition

One of the long standing issues in the theory of crystal growth (and nucleation) is the

prediction of the growth rates of spherical or hemispherical crystals on foreign substrates.

263

Most of the theoretical works have considered three rate-determining cases: (i) slow bulk

diffusion of the electroactive species from the bulk solution to the surface of the growing

crystal, (ii) slow interfacial charge transfer, and (iii) mixed kinetics or rate control by

both bulk diffusion and interfacial ion processes [648-651]. A number of studies also

have considered the more complicated case of joint ohmic, diffusion, and charge transfer

limitations, where the electrochemical nucleation and growth take place in the complete

absence of any supporting electrolyte [652-654]. This was the case in this study where

platinum was electroreduced on different substrates in the absence of a supporting

electrolyte. The model developed and tested in the preceding section (nickel deposition),

however, was based on a plating bath containing supporting electrolyte. In such cases, the

theoretical model only accounts for joint diffusion and charge transfer limitations.

Consequently, the model needs to be modified to account for ohmic limitations, where

platinum electrodeposition is carried out in the complete absence of supporting

electrolyte. In the following section we refine and further develop the model presented in

the preceding sections to include ohmic limitations and validate the model by comparing

it with experimental data obtained for platinum electrodeposition. A detailed treatment of

electroplating systems where a supporting electrolyte is present also is provided.

9.3.3.1.1 Effect of Supporting Electrolyte6

(a) In the Absence of Supporting Electrolyte

The standard form of the Butler-Volmer equation for the metal deposition process

Mzze

M (9-91)

is

TR

Fz

TR

Fzii actact

o

)1(expexp (9-92)

where act is the charge transfer overvoltage (activation overvoltage), is the transfer

coefficient for the charge transfer reaction, and io is the exchange current density. In the

above expression the cathodic contribution to the net current density (the first term in the

brackets) is taken as positive, the anodic contribution (the second term in the brackets) is

6 These are based on the works of Milchev et al. [643, 652, 653] and full derivations of equations indicated

with an asterisk (*) are given in the appendix.

264

taken as negative, and cathodic overvoltages are taken as positive. By denoting cathodic

overvoltages and currents as positive quantities in the following derivations, the

complexities of dealing with the logarithms of negative numbers are avoided. Also, to

avoid confusion between the radius R of the deposited metal cluster and the gas constant,

the gas constant is designated as R .

When the rate of (9-91) is completely controlled by the rate of the charge transfer process

(activation-controlled), the exchange current density io is related to the concentration co

of the metal ion in the bulk solution. However, when the concentration cS of the metal ion

adjacent to the metal surface is different from that in the bulk solution, as is the case

when there are solution mass transport limitations, the exchange current density io must

be replaced with io, S, which is the exchange current density referred to the concentration

cS . The relationship between io, S and io is given by

1

,

o

S

oSoc

cii (9-93)*

Furthermore, in the presence of a concentration gradient in the depositing species, in

addition to the charge transfer overvoltage act, the total cathodic overvoltage will include

a concentration overvoltage C given by

S

o

Cc

c

Fz

RTln (9-94)

such that the total cathodic overvoltage T will be

T = ∆E = act + C (9-95)

where ∆E is the external cathodic voltage applied between the reference electrode and the

metal surface. Rearranging (9-95):

act =

o

S

c

c

Fz

RTE ln (9-96)

Substituting (9-93) and (9-96) into (9-92) rearranges to give

265

i =

TR

EFz

TR

EFz

c

ci

o

S

o

)1(expexp

(9-97)

i.e.,

TR

EFz

TR

EFz

c

ciS

o

S

oR

)1(expexpI

=

TR

EFz

TR

EFz

c

ciR

o

S

o

)1(expexp2 2

(9-98)

where SR 2R2 is the surface area of the hemispherical cluster of radius R. When

cS co , C 0 and Eqn (9-98) reduces to Eqn (8-92). Eqn (9-98) must be used

whenever cS < co.

The external cathodic potential ∆E applied to the working electrode (where the metal

deposition takes place) is measured against a reference potential Eref . For convenience we

take this reference potential as the stable equilibrium potential set up between an

electrode made of the bulk metal M and its

ions Mz

at the concentration of the bulk

solution, where the metal M is the same

metal that is to be deposited on the surface

of the working electrode. We designate this

equilibrium potential as o and the potential

at the surface of the working electrode as

S. The potential o

is taken as the potential

in the solution at the reference point in the

solution that corresponds to the tip of the

Luggin capillary (or the reference

electrode). Thus any applied external potential ∆E applied to the working electrode can

be expressed as

∆E = o S (9-99)

If the cluster is growing at a fixed external applied voltage ∆E, reference to the diagram

shows that

c

x

o

S

solution metal

act

∆E

266

∆E = C + + act (9-100)

where act is the charge transfer overvoltage, is the ohmic overvoltage (the ohmic

drop present in the solution between the reference electrode and the working electrode),

and C is the concentration overvoltage between the reference electrode and the working

electrode. All the quantities in Eqn (9-100) are taken as positive.

In the absence of supporting electrolyte part of the ion transport occurs by ionic migration

of the charged ions under the influence of the applied voltage gradient.

The positive x–direction is taken as away from the electrode surface where the metal

deposition takes place. Taking cathodic currents as positive, the cationic diffusion flux

towards the electrode is given by

xd

dcDJ D

(9-101)

where c+ is the concentration of the cation in solution and D+ is its diffusion coefficient.

Using the selected sign convention, since 0dxdc , the cation diffusion flux JD

is

positive, which means that the cations diffuse from the bulk solution towards the

electrode. The conduction (or ionic migration) flux is given by

xd

dUcJ C

(9-102)

where U+ is the mobility of the cation and d dx is the voltage gradient in the solution.

The mobility, which is the ionic migration velocity per unit field strength, is always

defined as a positive quantity. Since d dx is positive (the voltage becomes less negative

as the distance from the cathode increases), it follows from (9-102) that the cation

conduction flux JC

, like the diffusion flux JD

, also is positive. Therefore both fluxes

move towards the electrode surface and, neglecting convection in the diffusion zone, the

total cationic current density iT

is given by

iT

=

CDCD FJzFJzii

= xx d

dUcFz

d

dcFDz

(9-103)

In dilute solution the Einstein relation links diffusion to conduction by

267

U i | zi | FDi

R T (9-104)

where R is the gas constant and T is the absolute temperature. Substituting (9-104) into

(9-103) gives

iT

=

xx d

d

TR

FDzcFz

d

dcFDz

=

xx d

d

TR

cDFz

d

dcFDz

2

(9-105)

Now consider the anions: For the anions, since the negatively-charged electrode exerts a

repulsive effect, the anionic conduction flux JC

is away from the electrode. This reduces

the anionic concentration S)(c adjacent to the electrode, establishing a positive anionic

concentration gradient that sets up an anionic diffusion flux JD

towards the electrode.

Since it is assumed that no anions are discharged at the electrode and that there is no

convective mass transfer taking place in the diffusion layer, it follows that there is no net

anionic flux JT

in the diffusion layer, and that the anionic diffusion flux towards the

electrode is exactly balanced by the anionic migration flux away from the electrode.

Thus, for the anions in the diffusion layer

iT

=

xx d

dUcFz

d

dcFDzii CD

||||

= xx d

d

TR

FDzcFz

d

dcDFz

|||||| (9-106)

The negative sign in front of the second term indicates that the anionic conduction flux is

in the opposite direction to the (positive) anionic diffusion flux.

But, since iT

= 0, xx d

d

TR

FDzcFz

d

dcFDz

|||||| = 0

Rearranging: d

dx =

xd

dc

TR

FDzcFz

FDz

||||

|| =

xd

dc

cFz

TR

|| (9-107)

But, from charge neutrality, czcz || (9-108)

268

Putting (9-108) into (9-107): xx d

dc

Fcz

TR

d

d

(9-109)

From (9-108),

cz

zc

||

Differentiating: xx d

dc

z

z

d

dc

||

(9-110)

Substituting (9-110) into (9-109): xx d

dc

z

z

Fcz

TR

d

d

||

(9-111)

Putting (9-111) into (9-105): iT

=

xx d

dc

z

z

Fcz

TR

TR

cDFz

d

dcFDz

||

2

= xx d

dc

z

zFDz

d

dcFDz

||

and therefore xd

dc

z

zFDziT

||1 (9-112)

Thus, from Eqn (9-112), when there is transport by both diffusion and ionic migration,

the total cationic current density at the cathode surface (where x = 0) is

0

1

xxd

dcaFDziT (9-113)

where a is defined as a z

| z | (9-114)

From Faraday’s Law the total current is

I 2zF

vM

R2 dR

dt (9-115)

For a hemispherical cluster of radius R the surface area is 12 4R

2 2R2; therefore,

from (9-113), the total current passing through the surface of the cluster is

R

d

dcaFDzRI

xx

12 2 (9-116)

The concentration gradient at the surface of the cluster is

269

o

SoSoSo

Rc

c

R

c

R

c

R

c

R

cc

d

dc1

xx

(9-117)

Substitution of (9-117) into (9-116) gives the total current passing through the surface of

the cluster as

o

S

oc

cCaFDRzI 112 (9-118)

The term (1 + a) accounts for the contribution of ionic migration to the total current.

When diffusion and ionic migration are both present, the charge transfer overvoltage act

that must be used in Eqn (9-92) is a function of the applied cathodic voltage and the

concentration and ohmic overvoltages. This relationship is given by Eqn (9-100) as

act = ∆E – C – (9-119)

The concentration overvoltage C is given by Eqn (9-94). The ohmic overvoltage is

just the voltage drop through the diffusion zone. This is determined as follows:

From Eqns (9-111) and (9-114), xx d

dc

cFz

TRa

d

d

1

from which

c

dc

Fz

TRad

Integrating

1

2

12 lnc

c

Fz

TRa (9-120)

At R = , 2 = o and c2 = co

At R = R, 1 = S

and c1 = cS

Putting these boundary conditions into Eqn (9-120) gives the ohmic overvoltage as

S

o

Soc

c

Fz

aTRln (9-121)

Putting (9-94) and (9-121) into (9-119):

270

act = ∆E –

S

o

c

c

Fz

TRln –

S

o

c

c

Fz

aTRln

= ∆E

S

o

c

ca

Fz

TRln1 (9-122)

As discussed above, when cS < co , the exchange current density io in Eqn (9-92) must be

replaced with iS,o given by Eqn (9-93) to yield

I = SR i

=

TR

Fz

TR

FziR actact

So

)1(expexp2 ,

2

=

TR

Fz

TR

Fz

c

ciR actact

o

S

o

)1(expexp2

1

2 (9-123)

Substituting (9-122) into (9-123):

I =

o

S

o

S

oc

ca

Fz

TR

TR

Fz

TR

EFz

c

ciR ln1exp2

1

2

–

o

S

c

ca

Fz

TR

TR

Fz

TR

EFzln1

)1()1(exp

=

o

S

o

S

oc

ca

TR

EFz

c

ciR ln1exp2

1

2

–

o

S

c

ca

TR

EFzln1)1(

)1(exp

=

)1(1

lnexpexp2 2

a

o

S

o

S

oc

c

TR

EFz

c

ciR

–

)1)(1(

lnexp)1(

exp

a

o

S

c

c

TR

EFz

271

=

a

o

S

o

S

oc

c

TR

EFz

c

ciR

exp2

1

2 –

aa

o

S

c

c

TR

EFz1

)1(exp

=

TR

EFz

c

ciR

a

o

S

o

exp2

1

2 –

TR

EFz

c

ca

o

S )1(exp

)1(

(9-124)

Eqn (9-124) gives the expression for the growth current I of a hemispherical cluster of

radius R in the presence of joint diffusion and ionic migration (ohmic) control in the

absence of any supporting electrolyte.

Equations (9-115) and (9-118) each give expressions for the total current I. Equating

these two equations gives:

o

S

o

M c

ccaFDRz

dt

dRR

v

Fz112

2 2

from which dt

dR

cDva

R

c

c

oMo

S

)1(1 (9-125)

Eqn (9-115) rearranges to

22 FRz

Iv

dt

dR M

(9-126)


I =

TR

EFz

dt

dR

cDva

RiR

a

oM

o

exp)1(

12

1

2

–

TR

EFz

dt

dR

cDva

Ra

oM

)1(exp

)1(1

)1(

(9-127)


dR

dt =

TR

EFz

c

c

TR

EFz

c

ciR

FRz

vaa

o

S

o

S

o

M )1(expexp2

2

)1(1

2

2

272

=

TR

EFz

dt

dR

cDva

RiR

FRz

va

oM

o

M

exp)1(

122

1

2

2

–

TR

EFz

dt

dR

cDva

Ra

oM

)1(exp

)1(1

)1(

=

TR

EFz

dt

dR

cDva

R

Fz

iva

oM

oM

exp)1(

1

1

–

TR

EFz

dt

dR

cDva

Ra

oM

)1(exp

)1(1

)1(

(9-128)

In Eqn (9-128) the term exp (1)zFE R T can be expanded to

exp(1 )zFE

R T

= exp

( 1)zFE

R T

=

TR

EFz

TR

EFzexpexp

(9-129)

Substituting (9-129) into (9-128) gives

dR

dt =

TR

EFz

dt

dRR

cDvaFz

iva

oM

oM

exp)1(

11

1

–

TR

EFz

TR

EFz

dt

dRR

cDva

a

oM

expexp)1(

11

)1(

(9-130)

Eqn (9-130) gives the rate of cluster growth as a function of time, and can be expressed

more concisely as [653]:

dR

dt A1F1 A2F2

(9-131)

where F1 = 44AA

3A

o

S

c

c

dt

dRR1 (9-132)

F2 = 55AA

3A

o

S

c

c

dt

dRR1 (9-133)

273

A1 =

TR

EFz

Fz

iv oM exp (9-134)

A2 =

TR

EFzexp1A (9-135)

A3 = TRatv

FzacDv

M

oM

1

1

2

1

(9-136)

A4 = a1 (9-137)

A5 = 1 a (9-138)

where t+ is the transport number of the cation and is the specific conductivity of the

solution.

Although Eqn (9-130) cannot be solved analytically, for short and long periods of time

the following two approximate solutions are valid [652].

Short times and low ohmic and concentration overvoltages

At short times and low current, oS cc 1 and dt)dRR(A3 0, so that the terms F1

and F2 in Eqn (9-131) can be expressed through the first two terms of binomial

expansions, which, for x2 < 1, state that

1 x n 1 nx

(n)(n1)x 2

2!

(n)(n 1)(n 2)x3

3! . . . ≈ 1– nx

and 1 x n 1 nx

(n)(n 1)x2

2!

(n)(n 1)(n 2)x3

3! . . . ≈ 1+nx

Under these conditions and, with the initial conditions that R = 0 at t = 0, Eqn (9-131)

can be solved to give

R R Tt 1 a

zFio B1

2vMio

2 AB t

R Tt 1 a

1/ 2

1

(9-139)

where A = expzFE

R T

exp

(1 )zFE

R T

(9-140)

B = 1 a expzFE

R T

1 a exp

(1 )zFE

R T

(9-141)

274

Using Eqn (9-139) with Eqn (9-124) for the current gives

1)1(21

)1(1142/12

2

2

2

aTtRtvi

aTtRtvi

Fz

aTtRI

Mo

Mo

AB

AB

B

A

oi (9-142)

Equations (9-139) and (9-142) give I and R as a function of time for short times when

I

R 0.1

2t(1 a) R T

zF

(9-143)

Long times and high ohmic and concentration overvoltages

For very high ohmic and concentration overvoltages the charge transfer overvoltage tends

towards zero and the concentration of the cation at the electrode surface will be at the

limiting value cS,m. Under these conditions Eqn (9-122) becomes

act = ∆E

S

o

c

ca

Fz

TRln1 = 0

from which TRa

EFz

c

c

mS

o

)1(ln

,

TRa

EFz

c

c

mS

o

)1(exp

,

(9-144)

Under these conditions the maximum concentration overvoltage C,m will be [653]:

C,m =

mS

o

c

c

Fz

TR

.

ln =

TRa

EFz

Fz

TR

)1(expln

= R T

zF

zFE

1 a R T =

E

1 a (9-145)

and, from Eqn (9-121) the maximum ohmic overvoltage ,m will be

, m = o S,m

=

mS

o

c

c

Fz

TRa

,

ln = a R T

zFln exp

zFE

(1 a) R T

= a R T

zFln exp

zFE

(1 a) R T

=

aE

1 a (9-146)

In this case the functions F1 and F2 in Eqn (9-126) can be approximated by the first two

275

terms of the Taylor expansion around cS,m co to give

R R Tt

iozFP1

2vMio

2(1 a)Qt

R Tt+

1/ 2

1

(9-147)

where P = expzFE

(1 a) R T

(9-148)

Q = exp2zFE

(1 a) R T

exp

(21)zFE

(1 a) R T

(9-149)

Combining Eqn (9-147) with Eqn (9-115) gives [653]:

I 4 R Tt

2(1 a)Q

io zF 2

P3

1 vMio2 (1 a)Q t R Tt

1 2vMio

2(1 a)Q t R Tt

1 / 2 1

(9-150)*

Equations (9-147) and (9-150) give I and R as a function of time for long times when

I

R

2t(1 a) R T

zF1 1

0.2

a

exp

zFE

(1 a) R T

(9-151) I

R

2t(1 a) R T

zF1 1

0.2

a

exp

zFE

(1 a) R T

. . . (61)

(b) In the Presence of Supporting Electrolyte

Milchev’s treatment presented above in part (a) deals with the absence of supporting

electrolyte. The same author also has derived similar equations that take into account the

presence of supporting electrolyte [653]. For this latter case, the rate of cluster growth

with time is given by

a

dt

dRR

acDvTR

EFz

Fz

iv

dt

dR

oM

oM

1

11

11exp

1

a

dt

dRR

acDvTR

EFz

TR

EFz

Fz

iv

oM

oM

)1(

11

11

)1(expexp

. . . (9-152)

where D+ and co are the diffusion coefficient and the bulk concentration of the depositing

cation Mz

, respectively.

276

Eqn (9-152), which gives the rate of cluster growth with time in the presence of

supporting electrolyte, is identical to Eqn (9-130), which gives the rate of cluster growth

with time in the absence of supporting electrolyte, with the exception of the factors

and . In the derivation of Eqn (9-152), Milchev has chosen as supporting

electrolytes salts with the same anion as that of the salt MA from which the desired

metal deposits. Also, the valences of the cations and anions of the supporting electrolytes

BA have been chosen to be the same as those of the depositing salt MA . Thus, if

the metal M is being deposited from its ions Mz

from the salt MA , the supporting

electrolyte will consist of salts of the type BA whose cation Bz

has the same valence

z as that of Mz

. Accordingly, the factor is defined as the ratio of the bulk

concentration of supporting electrolyte to the bulk concentration of the depositing

electrolyte:

oM

oB

o

z

o

z

c

c

M

B

)(

)(

][

][

(9-153)

Thus is a measure of the relative level of supporting electrolyte present. If there is no

supporting electrolyte, = 0 and Eqn (9-152) reduces to Eqn (9-130).

Eqn (9-152) can be expressed more concisely as [653]:

dR

dt A 1 F 1 A 2 F 2 (9-154)

where F 1 = 44AA

3A

oB

SB

c

c

dt

dRR

)(

)(1 (9-155)

F 2 = 55AA

3A

oB

SB

c

c

dt

dRR

)(

)(1 (9-156)

A 1 =

TR

EFz

TR

EFz

Fz

iv oM )1(expexp

(9-157)

A 2 = 111

oM cDv (9-158)

A 3 = 111

oM cDv (9-159)

A 4 = a1 (9-160)

277

A 5 = 1 a (9-161)

In Eqn (9-158) and all those that follow below, D+ and co are, respectively, the diffusion

coefficient and bulk concentration of the depositing cation, Mz

.

As in the case of Eqn (9-130), Eqn (9-152) cannot be solved analytically, although

approximate expressions for R and I as functions of time can be made, as follows:

Short times and low ohmic and concentration overvoltages.

At short times and low current, oBSB )(c)(c 1 and A 3R(dR dt) 0, so that the terms

F 1 and F 2 in Eqn (9-154) can be expressed through the first two terms of binomial

expansions, as shown earlier for the case without supporting electrolyte. When this is

done, Eqn (9-154) reduces to

01 2132413 AAdt

dRRAAAAA (9-162)

With the initial condition that at t = 0 R = 0, Eqn (9-162) can be solved to give R as a

function of time t as

11

21

1

exp

2/1

2

2

tFzc

vi

TREFzi

FzcDR

o

Mo

o

o

BA

B+D

(9-163)*

where A =

TR

EFz

TR

EFz )21(exp

2exp

(9-164)

B =

TR

EFz

a

a

a

aexp

1

1

1

1 (9-165)

Substitution of Eqn (9-163) into Eqn (9-115) gives the current I as the following function

of time:

1)121

)111

3exp

42/122

2222

(Dt)(

(Dt)(

+

+

Fzcvi

Fzcvi

TREFzi

FczDI

oMo

oMo

o

o

BA

BA

B

. . . (9-166)

278

Equations (9-163) and (9-166) satisfactorily describe the growth process if [653]:

ocDFza )1)(1(2.0 R

I

1.0

dt

dRR3A (9-167)

Long times and high ohmic and concentration overvoltages.

In this case the ion transport from the bulk solution to the electrode determines the rate of

the growth process, and the activation overvoltage required to discharge the ions onto the

metal surface becomes negligibly small compared with the magnitude of the

concentration and ohmic overvoltages. This approaches the limiting condition act 0 as

t for which the surface concentration of the anion B attains its limiting

value cB,m S. This permits a determination of the following minimum value of the ratio

oBSB )(c)(c [653]:

)1(1

)(

)( , aM

c

c

oB

SmB (9-168)

where M = exp zFE R T

1 (9-169)

From these relationships it is possible to obtain the maximum possible values of the

concentration overvoltage C,m and ohmic overvoltage ,m for any given values of ,

∆E, and T. The maximum concentration overvoltage is

C,m R T

zFln

Ma

(1 )M1a (9-170)

and the maximum ohmic overvoltage is

, m R T

zF 1 a lnM (9-171)

Under these conditions approximate solutions for R and I as functions of time can be

obtained by expanding the terms F 1 and F 2 as the first two terms of a Taylor’s series to

transform Eqn (9-154) into

279

M(1 A 5) (1a)

A 1 A 4M A 2 A 5 A 3R dR

dt

– A 1M A 4 A 4 1 M1(1a) – A 2 A 5 1 A 5 M1 (1a) = 0 (9-172)

Eqn (9-172) can be solved with the usual boundary conditions that R = 0 at t = 0 to give

1)1)(1(2

1

2/1

2

22

o

oM

o

o

cFzD

taiv

i

FczDR

PQ

Q

(9-173)

where P 1M1 (1a)

(9-174)

TR

EFzM

aa exp

)1(Q (9-175)

Substitution of Eqn (9-173) into Eqn (9-115) gives the current I as a function of time for

long times as

1)1)(1(21

)1)(1(11142/1222

2222

ooM

ooM

o

o

cFzDtaiv

cFzDtaiv

i

aFczDI

PQ

PQ

Q

P

. . . (9-176)

Eqns (9-173) and (9-176) are valid approximations for long times when [653]:

)1(12.011)1)(1(2

M

aaFczD o

R

I (9-177)

The transients evaluated when = 0, correspond to the cluster growth in the absence of

supporting electrolyte under joint ohmic, diffusion, and charge transfer limitations. The

values at, say, = 100, correspond to growth in the presence of a large excess of

supporting electrolyte, for which the cluster growth is controlled only by joint diffusion

and charge transfer limitations. The above theory has been verified for the

electrodeposition of Ag crystals from a solution consisting of 0.5 M AgNO3 in 1.0 M

HNO3 [654]. The results of this latter study show that the presence of even small amounts

of supporting electrolyte can significantly decrease the crystal growth rate.

280

9.3.3.2 Comparison of Experimental and Model Platinum Microhardness Data

Figure 9-13 compares the experimental microhardness results with those of the models

presented in the preceding sections, one based on pure diffusion (see section 9.2) and

another based on ohmic, ionic and diffusion control with no supporting electrolyte

(referred to as modified model; section 9.3.3.1). Figure 9-13(a) shows the experimental

result (dashed line) and compares it with both model predictions for the ramp-down

waveform at different peak deposition current densities. A comparison of the

experimental and model results reveal a strong correlation between the experimental and

the modified model for peak deposition current densities ranging from 200 to 400 mA

cm-2

, while a better agreement is obtained at the lowest and highest peak deposition

current densities when the model is based on pure diffusion control. A similar trend also

is observed for the triangular waveform (Fig. 9-13(b)).

In case of the ramp-up waveform, the modified model underestimates the microhardness

of the deposited platinum layer, while the model based on pure diffusion control

generates data that are in better agreement with the experimental findings. These trends

can be explained by examining the characteristics of all the waveforms examined in this

research. In case of the ramp-down and triangular waveforms, although the rise in

deposition current density is faster than the ramp-up waveform, the decline begins almost

instantaneously in the case of ramp-down waveform and half-way through the pulse on-

time for the triangular waveform. This ensures that diffusion of the electroactive species

from the bulk solution into the diffusion layer is no longer the rate-determining factor

after the deposition current density begins to decline, and other factors become more

dominant. For instance, charge transfer across the deposition layer and ohmic losses

within the electrolyte (and more specifically inside the diffusion layer) become more

important and must be taken into account. For the ramp-up waveform, however, the

deposition current density continues to rise as long as the current is on. This would

greatly diminish the concentration of the platinum ion in the diffusion layer, leading to a

higher concentration overvoltage and, consequently, creating an electroplating system

where diffusion of the electroactive species into the diffusion layer becomes the rate-

determining factor.

281

Figure 9-13 Comparison of experimental and model microhardness data for various

pulse waveforms: a) Ramp-down, b) Triangular, c) Ramp-up and d) Rectangular,

deposited at different peak current densities (4% duty cycle; 50 mM Pt concentration)

282

For the rectangular waveform, the deposition current density rises very quickly, similar to

that of the ramp-down waveform; however, it maintains its highest value (i.e., peak

current density) throughout the pulse on-time. Similar to the ramp-up waveform, the

platinum concentration in the diffusion layer begins to decline and never recovers until

the onset of pulse off-time. Similar to the ramp-up waveform, the diffusion of the

electroactive species from the bulk solution into the diffusion layer becomes the

dominant factor, influencing the plating process and inevitably the microhardness of the

resulting layer.

Figure 9-14 shows the microhardness of platinum layers predicted by the modified

mathematical model as well as the corresponding experimental values deposited by

various waveforms at different peak deposition current densities. Similar to nickel

electrodeposition, ramp-down waveform generates the highest hardness followed closely

by triangular waveform and then by ramp-up and rectangular waveforms. These trends

can be explained by considering the influence of each waveform on a number of

parameters during electrodeposition such as the variation of platinum concentration in the

cathode diffusion layer, changes in concentration overvoltage, critical nucleus size, and

platinum nucleation rate. These are discussed in the following sections.

Figure 9-14 Influence of different waveforms on platinum microhardness deposited at

various peak deposition current densities

283

9.3.3.3 Platinum Concentration Variation in the Cathode Diffusion Layer

The calculated variation of platinum ion concentration in the diffusion layer adjacent to

the cathode with a pulse on-time of 5.0 ms for the four different waveforms is shown in

Figure 9-15. As in the case of nickel electrodeposition, the concentration profiles are

markedly different and are characteristic of each waveform. The highest initial decrease

in platinum concentration during pulse on-time is observed for the rectangular and ramp-

down waveforms. This is attributed to the sharp increase in the applied current density at

the beginning of the pulse on-time, resulting in a higher deposition rate and a marked

decline in platinum concentration near the surface of the cathode for both waveforms.

The platinum ion concentration in the cathode diffusion layer will, however, recover and

start to increase as the pulse progresses for the ramp-down waveform, while, in case of

the rectangular waveform the concentration continues to decrease with the progression of

pulse on-time due to the application of a continuous high current density.

Figure 9-15 Platinum concentration in the cathode diffusion layer for various


, on-time and off-

time of 5.0 ms each (50% duty cycle) and 100 Hz (showing the first full cycle)

284

For the triangular and ramp-up waveforms, on the other hand, the initial decline in the

concentration of platinum ion in the diffusion layer is subtler than for the previous two

waveforms. This is due to the gradual increase in the applied current density during the

pulse on-time, compared with the sharp increase in current density for the ramp-down

and rectangular waveforms. Furthermore, the diminution of platinum ion concentration

for the triangular waveform is initially more pronounced than for the ramp-up waveform,

since the peak current density of the former is reached in half the time as the latter.

Additionally, for the triangular waveform, the platinum concentration in the diffusion

layer begins to increase as the applied current density approaches its maximum and then

starts to level off. At this point, the rate of platinum electrodeposition begins to fall and

the diffusion rate from the bulk solution into the diffusion layer becomes greater, leading

to an increase in the metal ion concentration within the diffusion layer. The platinum ion

concentration in the diffusion layer for the ramp-up waveform, however, continues to

decrease as the electrodeposition proceeds because of the continuous rise in the applied

cathodic current density until the completion of the pulse on-time and the onset of the

pulse off-time.

Figure 9-15 shows the concentration of the platinum ion in the cathode diffusion layer for

a complete pulse cycle for each waveform with a duty cycle of 50%. As can be seen, the

platinum concentration begins to recover before the conclusion of the pulse on-time in

the case of the ramp-down and triangular waveforms, while a recovery does not take

place until the onset of pulse off-time for the ramp-up and rectangular waveforms. For

the ramp-down and triangular waveforms, the applied current density rapidly increases,

reaching its maximum, and begins to decline before the pulse on-time is concluded. After

reaching the peak current density the rate of electrodeposition begins to fall, while the

platinum diffusion rate into the cathode diffusion layer starts to rise. This is clearly

demonstrated in Figure 9-15, where the platinum concentration in the diffusion layer

reaches its minimum half way through the pulse on-time in case of the ramp-down

waveform and around the 3.5-ms mark for the triangular waveform. On the other hand,

for both the rectangular and ramp-up waveforms, the platinum ion concentration close to

the cathode continues to decrease as electrodeposition advances until the end of pulse on-

time. Since, the minimum concentration is reached at the conclusion of the pulse on-time,

285

the ion replenishment in the cathode diffusion layer does not occur until the

commencement of the pulse off-time, resulting in a lower platinum concentration at the

end of the pulse off-time and an increase in the possibility of crystal growth due to

insufficient platinum ions in the diffusion layer when the current begins to flow again.

This is especially critical when high duty cycles are employed (e.g., 50% or greater) as is

the case with most pulse electrodeposition studies [502, 642].

9.3.3.4 Concentration Overvoltage and Pulse Current Waveforms

The concentration overvoltage has been linked directly to the size of the critical radius

and nucleation rate via equations (6-40) and (6-47), respectively. It can be seen that as the

concentration overvoltage increases, the nucleation rate also increases, while the size of

the critical radius diminishes. Accordingly, higher concentration overvoltages are

desirable in electroplating. Figure 9-16 shows the model predictions for the concentration

overvoltage of platinum electrodeposition using the four waveforms for a full pulse cycle.

As can be seen, the ramp-down waveform generates the highest overvoltage initially,

followed by the rectangular waveform. The explanations for these observations are

similar to those for the nickel electrodeposition and the reader is referred to section 9.2.3

for a more complete discussion.

Figure 9-16 Concentration overvoltage for various waveforms with a peak deposition


, on-time & off-time of 5 ms (50% duty cycle) and 100

Hz (showing the first fuel cycle)

286

When the rectangular waveform is applied, the concentration overvoltage initially

increases in the same manner as the ramp-down waveform; but, continues to rise for the

duration of pulse on-time because of the high and continuous peak current density. One

key observation is the marked difference between the magnitudes of the concentration

overvoltages of these two waveforms where that of the rectangular waveform is

significantly lower than that of the ramp-down waveform. This is anticipated since the

peak current density of the former is half that of the latter (and other waveforms as well)

in order to generate the same average current density.

According to Figure 9-16, the concentration overvoltages of the triangular and ramp-up

waveforms exhibit a more gradual rise than those of the ramp-down and rectangular

waveforms. The explanation provided in section 9.2.3 for nickel electrodeposition also is

valid in the case of platinum.

9.3.3.5 Nucleation Rate and Pulse Current Waveforms

Figure 9-17 shows the nucleation rate associated with the four waveforms. Initially, the

ramp-down and rectangular waveforms exhibit the highest nuclei formation per unit time,

with the former beginning to decline as electrodeposition progresses, while the latter

maintains a constant rise during pulse on-time. The explanation given in the previous

section for nickel electrodeposition also is true here and the reader is referred to the

discussion provided in section 9.3.2. From Figure 9-17, it is clear that the ramp-down and

rectangular waveforms generate the smallest and the largest number of nuclei during the

first pulse cycle, respectively. The number of nuclei formed during the first pulse cycle is

obtained by determining the area under each curve. The number of nuclei generated by

each waveform for different pulse cycles (i.e., 1st, 10

th, 30

th, 50

th, and the cumulative of

the first 100 cycles) with a peak current density of 400 mA cm-2

, pulse on-time of 5.0 ms,

and a duty cycle of 50% predicted by our model is shown in Table 9-7.

287

Figure 9-17 Nucleation rate for various waveforms with a peak deposition current


, on-time of 5 ms and 100 Hz (showing the first half-cycle)

Table 9-7 Nucleation rate for all waveforms for the first pulse cycle with a pulse on-time

of 5.0 ms and a peak deposition current density of 400 mA cm-2

Waveform

Number of Nuclei

1st C 1

st Cycle 10

th Cycle 30

th Cycle 50

th Cycle Cumulative for 100 Cycles

Rectangular

3.96 х1011

1.76х1014

0.00 0.00 1.44 х1017

Triangular 3.34 х1011

6.13 х1013

3.61 х1015

1.08 х1017

1.11 х1018

Ramp-up 3.31 х1011

7.07 х1013

4.79 х1015

0.00 1.18 х1018

Ramp-down 2.51 х1011

5.18 х1013

2.83 х1015

1.50 х1017

1.30 х1018

Contrary to experimental findings where electrodeposited layers produced with a ramp-

down waveform exhibit the best results both in terms of microhardness and fuel cell

performance, the model data, at least for the first pulse cycle, predict that the most

number of nuclei will be generated by employing a rectangular waveform, while a ramp-

down waveform generates the least number of nuclei. This is anticipated since at the

onset of pulse on-time, the concentration of the electroactive species in the cathode

diffusion layer is virtually identical to its bulk concentration, and the continuous high

288

peak current density of a rectangular pulse current ensures high concentration

overvoltages, leading to higher nucleation rates. However, as electrodeposition advances,

the concentration of platinum ion in the cathode diffusion layer begins to decrease faster

than for other waveforms, causing a sharp decline in nucleation rate, as can be seen from

Table 9-7. This is more pronounced at high duty cycles, where the off-time is not long

enough to ensure the replenishment of fresh platinum ions in the cathode diffusion layer.

9.3.3.6 Duty Cycle and Pulse Current Waveforms

The influence of low duty cycles (i.e., 2% – 10%) on platinum ion concentration in the

cathode diffusion layer both in millisecond and microsecond ranges is shown in Figures

9-18 and 9-19, respectively. The total pulse on-time, bulk platinum ion concentration, and

peak cathodic current density were kept constant at 200 ms, 0.05 M, and 400 mA cm-2

,

respectively, for all waveforms in both millisecond and microsecond ranges. Pulse duty

cycles for all waveforms were varied by keeping the pulse on-time constant (0.002 s and

0.0002 s for millisecond and microsecond pulses, respectively) and varying the pulse off-

time. For low duty cycles in the millisecond range, as can be seen from Figure 9-18, the

concentration of platinum ion in the cathode diffusion layer drops sharply as duty cycle

increases from 2% to 10% for a rectangular waveform and approaches zero when duty

cycle is greater than 8%. Even at a duty cycle of 5%, the platinum ion concentration is

one-fifth of its initial value. The decline in platinum ion concentration in the diffusion

layer for other pulse waveforms is more gradual under the same conditions; nevertheless,

they all tend to decrease as duty cycle increases.

The above predictions based on the model developed in this study are in good agreement

with the experimental results presented in section 8.5.4.2.2.2, where the best and worst

fuel cell performances were found to be for MEAs prepared by ramp-down and

rectangular pulse current waveforms, respectively. Additionally, the best fuel cell

performance was observed with MEAs fabricated with the lowest duty cycle of 2%, as

discussed in section 8.5.4.2.2.2. Furthermore, as the duty cycle decreases, the applied

peak cathodic current density can be increased, leading to higher nucleation rates and

smaller deposited particle size. It is evident that the assumption that a 50% duty cycle in

289


duty cycle in the Millisecond Range for all four waveforms with a peak deposition


, on-time of 0.002 s and 100 pulse cycles


duty cycle in the Microsecond Range for all four waveforms with a peak deposition



Millisecond

Range

Microsecond

Range

Rectangular

Rectangular

290

pulse current electrodeposition is adequate for replenishment of electroactive species in

the cathode diffusion layer is not valid, at least for the rectangular waveform in the

millisecond range.

Figure 9-19 shows the platinum ion concentration in the cathode diffusion layer when

both pulse on-time and off-time are in the microsecond range for all four waveforms. It

can be seen that the decline in platinum ion concentration is more gradual for all

waveforms than with the pulses in the millisecond range (Figure 9-18). This is to be

expected, since, in the microsecond pulses, the on-time is relatively short, resulting in a

faster platinum ion recovery during the off-time. Even in the case of the rectangular

waveform, the platinum ion concentration in the cathode diffusion layer stays close to the

bulk concentration even at higher duty cycles. The presence of a high concentration of

platinum ions close to the surface of the substrate during pulse on-time ensures a high

nucleation rate and avoids crystal growth and the formation of dendrites by increasing the

corresponding limiting current density. Predicted values of platinum ion concentration in

the cathode diffusion layer for both milli- and micro-second pulses are given in Tables 9-

8 and 9-9, respectively. It is evident that longer pulses (e.g., in the millisecond range)

result in a sharper decline of the electroactive species in the close vicinity of the cathode

during pulse on-time. Although the ion concentration in the above layer may recover

during the pulse off-time, the possibility of full recovery decreases significantly as the

duty cycle increases and/or the pulse on-time becomes relatively long (i.e.; going from

micro to milli or even second pulses) regardless of the pulse off-time.


cycle in the Millisecond Range for all four waveforms with a peak deposition current


# of pulse on-time off-time Duty Cycle Pt Concentration (mmol L-1

)

cycles (s) (s) (%) Rectangular Ramp up Triangular Ramp down

50.00 50.00 50.00 50.00

100 0.002 0.0980 2 25.52 37.24 37.85 37.86

100 0.002 0.0480 4 16.15 32.51 33.16 33.21

100 0.002 0.0314 6 8.38 28.58 29.28 29.38

100 0.002 0.0230 8 1.90 25.28 26.04 26.21

100 0.002 0.0180 10 0.00 20.84 21.68 21.85

291


cycle in the Microsecond Range for all four waveforms with a peak deposition current


# of pulse on-time off-time duty cycle Pt Concentration (mmol L-1

)

cycles (s) (s) (%) Rectangular Ramp up Triangular Ramp down

50.0000 50.0000 50.0000 50.0000

1000 0.0002 0.00980 2 42.2538 46.1169 46.1270 46.1369

1000 0.0002 0.00480 4 39.0518 44.3892 44.7562 44.9055

1000 0.0002 0.00314 6 36.8392 43.2266 43.4473 43.4808

1000 0.0002 0.00230 8 34.7900 42.1818 42.4232 42.4763

1000 0.0002 0.00180 10 32.8258 41.1742 41.4419 41.5198

The influence of high duty cycle (i.e., 10% – 100%) on the platinum ion concentration in

the cathode diffusion layer in both the millisecond and microsecond ranges is shown in

Figures 9-20 and 9-21, respectively. As with low duty cycles, the platinum ion

concentration decreases as the duty cycle increases from 10% to 100% (100% being

direct current with no interruption). As with low duty cycles discussed previously, the

application of a rectangular waveform results in the sharpest decline of the diffusion layer

platinum ion concentration during electrodeposition. This is anticipated since the high

and continuous application of peak current density during pulse on-time leads to the

consumption of almost all the available platinum ions in the cathode diffusion layer. At

duty cycles greater than 90%, the platinum ion concentration in the diffusion layer

reaches zero before the end of the pulse on-time, leading to the possible formation of

dendrites and the promotion of crystal growth. Similar trends can be observed for the

other waveforms; however, for these waveforms the extent of the concentration drop is

more gradual than for the rectangular waveform, and never reaches zero, with the ramp-

down waveform exhibiting the highest platinum concentration at the end of the pulse

cycle for all duty cycles. As expected, the variation in platinum ion concentration when

high duty cycles are employed is greater than that observed with low duty cycles. The

main reason for such marked changes is the longer pulse on-time with respect to pulse

off-time in the former compared with the latter.

292


duty cycle in the Millisecond Range for all four waveforms with a peak deposition




duty cycle in the Microsecond Range for all four waveforms with a peak deposition



293

However, unlike low duty cycles, the model predictions for the variations in platinum

concentration in the cathode diffusion layer for both milli- and micro-second pulses are

somehow similar. This is primarily attributed to the lower peak cathodic current density

(i.e., 50 mA cm-2

compared with 400 mA cm-2

for low duty cycles) when duty cycles

greater than 10% are used. Comparing milli- and micro-second pulses (Figures 9-20 and

9-21) for duty cycles ranging from 10%-100%, it is apparent that the decline in platinum

ion concentration in the cathode diffusion layer at the end of each pulse period is very

similar, leading to the conclusion that, regardless of the type of waveform, the duration of

the pulses in the milli- and micro-second ranges has a minimal influence on the

concentration of the electroactive species in the cathode diffusion layer during

electrodeposition.

9.3.3.7 Influence of Waveform on Critical Nucleus Size

In catalyst deposition, one of the main objectives is to produce catalyst particles with the

smallest size possible in order to increase their surface area and, ultimately, enhance their

catalytic activity. Figure 9-22 shows the critical nucleus size of platinum as a function of

time for a single 5-ms pulse on-time for the four waveforms examined in this study. High

nucleation rates yield small nucleus sizes and, as can be seen from Figure 9-22, for the

ramp-down waveform at the beginning of the pulse on-time when the applied current

density is at its highest value, the critical nucleus size is at a minimum, but increases as

the current density decreases, resulting in the greatest critical nucleus size amongst all the

waveforms at the end of the pulse on-time. A similar trend was observed for the

rectangular waveform at the start of the pulse on-time; however, unlike the ramp-down

waveform, the critical nucleus size continued to decrease as the pulse on-time progressed,

on account of the high and continuous peak current density.

As expected, the critical nucleus size of the platinum particles predicted by the model for

the triangular and ramp-up waveforms was greater than that predicted for the rectangular

and ramp-down waveforms at the beginning of the pulse on-time, primarily owing to the

gradual rise in the cathodic current density of the first two waveforms. The ramp-up

waveform exhibited the largest critical nucleus size initially, but the smallest towards the

294

end of the on-time. In the case of the ramp-up waveform, the cathodic current density

rises very slowly initially and does not reach its maximum until the end of the pulse on-

time, leading to low nucleation rates for much of the pulse on-time and, subsequently,

higher critical nucleus size. However, after the 4-ms mark of the pulse on-time, the ramp-

up waveform produces the smallest platinum particles, owing to its high cathodic peak

current density. As can be seen from Figure 9-22, the ramp-down and ramp-up

waveforms generate the smallest and largest critical nucleus sizes, respectively, for a 5-

ms pulse on-time with a peak cathodic current density of 400 mA cm-2

and 50% duty

cycle.

Figure 9-22 Critical nucleus size of platinum as a function of time for various


, on-time of 5 ms and

100 Hz (showing the first half-cycle)

295

9.3.3.8 Influence of Cathodic Peak Deposition Current Density on Nucleation Rate

of Platinum for Various Waveforms

Figures 9-23 and 9-24 present the influence of the cathodic peak current density on the

nucleation of platinum at both low and high peak current densities, respectively. It is

evident that changes in the shape of the applied peak current density, while maintaining

the other parameters constant, results in significant variations in the platinum nucleation

rate. More importantly, the impact of the cathodic peak current density is correctly

predicted, which is in good agreement with the experimental results discussed in section

8.5.3.2.1. For a low duty cycle of 2%, a peak deposition current density of 400 mA cm-2

is predicted to yield the highest number of nuclei per unit time for all waveforms, with

the ramp-down waveform generating the largest number of nuclei per unit time (100 ms

in this case). For a high duty cycle of 20%, a peak deposition current density of 40 mA

cm-2

is predicted to produce the highest number of nuclei per unit time for all waveforms

with the ramp-down waveform yielding the largest number of nuclei per unit time (10 s

in this case).

Figure 9-23 Nucleation rate for various waveforms with an on-time of 1.0 ms, off-time

of 49 ms, 2% duty cycle, and 100 pulse cycles at different peak deposition current

densities

296

Figure 9-24 Nucleation rate for various waveforms with an on-time of 100 ms, off-time

of 400 ms, 20% duty cycle, and 100 pulse cycles at different peak current densities

According to Figure 9-23, for a low duty cycle of 2% with a 1.0 ms pulse on-time and

49.0 ms pulse off-time, the highest nucleation rate is achieved by employing a peak


regardless of the shape of the pulse current

waveform. This is supported by our experimental findings, where MEAs fabricated at

low duty cycles of 2% and 4% and a peak deposition current density of 400 mA cm-2

delivered the best fuel cell performance. It also was observed that MEAs prepared with a

peak deposition current density of 780 mA cm-2

and a low duty cycle of 2% perform

equally well (section 8.5.3.2.1). The model predictions as well as the experimental

observations can be explained by considering the electrodeposition mechanism, in which

there is a competition between nucleation and crystal growth. At low peak deposition

current densities, the rate of charge transfer needed for the reduction of platinum adatoms

is very slow compared with the diffusion rate of platinum ions from the bulk solution into

the diffusion layer. Consequently, platinum adatoms have sufficient time to reach the

existing crystals and crystal growth becomes the dominant process, resulting in a low

nucleation rate. As the peak deposition current density increases, however, the rate of

charge transfer also increases, and most platinum adatoms become single nuclei or part of

a small nucleus. This, of course, requires an adequate diffusion rate to ensure the

presence of fresh platinum ions in the diffusion layer during pulse on-time; however, as

297

the peak deposition current density further increases (beyond 400 mA cm-2

), the diffusion

rate becomes the rate-determining step and, once again, crystal growth becomes the

dominant process, lowering the nucleation rate as shown in Figure 9-23.

For a duty cycle of 20%, Figure 9-24 shows that a peak deposition current density of 40

mA cm-2

generates the highest number of platinum nuclei, regardless of the shape of the

waveform. Our experimental results indicated that MEAs prepared with a peak deposition


and a duty cycle of 20% delivered the best result when

tested in a single fuel cell (Figure 8-34) followed by MEAs fabricated with a peak


. The model prediction is slightly different from

the experimental one, nevertheless they are reasonably close.

High peak deposition current densities lead to higher concentration overvoltages, which,

in turn, enhance nucleation according to equation (6-47). However, as the concentration

overvoltage increases, the concentration of the electroactive species in the diffusion layer

decreases, increasing the possibility of dendrite formation. Therefore, the peak deposition

current density for an electroplating system must be carefully selected.

Figure 9-25 shows the model predictions for the concentration overvoltage of a platinum

electroplating system for four different waveforms at high peak deposition current

densities (i.e., 50 – 600 mA cm-2

). As can be seen, at low peak current densities, the

resulting overvoltages are virtually identical for all the waveforms; however, as the peak

current density increases beyond 200 mA cm-2

, the concentration overvoltage generated

by the rectangular waveform begins to rise faster than for the other waveforms. This is

primarily attributed to the high and continuous application of the peak deposition current

density during the pulse on-time, and may lead to the incorrect conclusion that the

rectangular waveform yields the highest nucleation rate. It is imperative to consider the

concentration of the electroactive species in the cathode diffusion layer during pulse on-

time. Figure 9-26 predicts the concentrations as a function of peak current density. It can

be seen that for the rectangular waveform, the concentration of platinum ion in the

diffusion layer drops at a faster rate as the peak deposition current density increases from

50 to 600 mA cm-2

compared with the other waveforms. This, of course, influences the

298

Figure 9-25 Concentration overvoltage (overpotential) as a function of peak deposition

current density for all waveforms with a pulse on-time of 1.0 ms, pulse off-time of 49 ms,

duty cycle of 2%, and 100 pulse cycles


all waveforms with a pulse on-time of 1.0 ms, pulse off-time of 49.0 ms, duty cycle of

2%, and 100 pulse cycles


all waveforms with a pulse on-time of 1.0 ms, pulse off-time of 4.0 ms, duty cycle of

20%, and 100 pulse cycles

299

nucleation rate and, ultimately, the surface area of the deposited platinum particles. A

similar trend also is predicted for low peak deposition current densities; i.e., 10 – 80 mA

cm-2

(Figure 9-27), concurring indirectly with the experimental results that MEAs

prepared using a rectangular waveform deliver poorer fuel cell performance than MEAs

prepared using the other three waveforms.

Figure 9-28 shows a series of graphs illustrating the influence of peak deposition current

density (50 – 600 mA cm-2

) on nucleation rate for the four waveforms as predicted by the

model. It can be seen that at high peak deposition current densities (≥ 400 mA cm-2

), the

ramp-down waveform yields the highest nucleation rates, again indirectly confirming the

experimental findings in which MEAs fabricated with a ramp-down waveform employing

high peak deposition current densities of 400 mA cm-2

delivered the best fuel cell

performance.

At low current densities, the rate of platinum ion diffusion from the bulk solution into the

diffusion layer and, consequently into the substrate macropores is higher than the rate of

charge transfer; as a result, metal ions have sufficient time to find stable places on

existing crystals and the crystal grows, leading to lower catalyst surface area. As the

pulse deposition current density increases, there is no longer adequate time for adatoms to

diffuse across the surface to be incorporated into a growing crystal. Instead, as each new

adatom is deposited, it becomes a single nucleus or part of a very small number of nuclei.

The result is an increase in the number, but a decrease in the size of the Pt crystallites,

resulting in a catalyst layer with superior catalytic properties. This trend can be seen for

all waveforms for peak current densities from 50 to 400 mA cm-2

. As the peak deposition

current density increases beyond 400 mA cm-2

, surface diffusion becomes the rate-

determining step, the system approaches its ―limiting current density‖, and no further

increase in nucleation rate is observed. At low current densities (50-100 mA cm-2

), the

rectangular waveform generates the highest number of nuclei per unit time, due to its

continuous high cathodic current density. Furthermore, the peak current density is not

high enough to diminish the platinum concentration in the diffusion layer during pulse

on-time, and the crystal growth is minimal. As the peak current density increases and

reaches 400 mA cm-2

and beyond, however, ramp-down waveform exhibits the highest

300

ip = 50 mA cm-2

Figure 9-28 Nucleation as a function of

peak deposition current density for all

waveforms with a duty cycle of 2%,

pulse on-time of 2 ms, pulse off-time of

98 ms and 1000 pulse cycles: (a) 50 mA

cm-2

(b) 100 mA cm-2

, (c) 200 mA cm-2

,

(d) 300 mA cm-2

, (e) 400 mA cm-2

, (f)

500 mA cm-2

, and (g) 600 mA cm-2

(a) (b)

(c) (d)

(e) (f)

(g)

301

nucleation rate amongst all the waveforms, followed by the triangular waveform. It is

interesting to note that the highest nucleation rate for ramp-up and triangular waveforms

takes place at a peak deposition current density of 400 mA cm-2

, while for the ramp-down

waveform, it almost stays the same from 400 - 600 mA cm-2

. Another important

observation is the fact that the onset of the highest nucleation rate for any of the peak

deposition current densities happens faster for the rectangular waveform relative to the

other three waveforms. This is again due to different inherent characteristics of the

rectangular waveform compared with others.

Furthermore, Figures 9-29 and 9-30 show the model predictions for the influence of peak

deposition current density on microhardness and grain diameter and the relationship

between the microhardness and the grain diameter of an electrodeposited platinum layer,

respectively. As expected, the microhardness of the Pt layer increases as the peak

deposition current density increases, while its average grain diameter decreases (Fig. 9-

29). According to Fig. 9-30, as microhardness increases, average grain diameter

decreases, leading to smaller nanoparticles.

Figure 29 Influence of peak deposition current density on microhardness and grain

diameter of a Pt electrodeposited layer (ramp-down waveform; 2% duty cycle;

microsecond pulses)

302

Figure 30 Influence of average grain diameter on microhardness of an electrodeposited Pt

layer (ramp-down waveform; 2% duty cycle; microsecond pulses)

9.3.3.9 Comparison of Commercial and In-House MEAs

Two MEAs with different platinum loadings, one containing 0.05 and another 0.35 mg Pt

cm-2

per electrode (anode and cathode), were prepared using the technique developed in

this research. Based on our experimental and theoretical findings and the optimization

process utilized to select the best electrodeposition parameters, both in-house MEAs were

fabricated using a ramp-down waveform with a duty cycle of 4%, peak deposition current


and pulses generated and delivered in the microsecond range.

The above MEAs were tested in a single 5-cm2 fuel cell and their performance was

compared with a commercial MEA (from E-TEK) with 0.50 mg Pt cm-2

per electrode. All

fabrication and operating parameters were kept constant with the exception of the catalyst

loading method and the amount of catalyst per electrode, as can be seen from Table 9-10.

Figure 9-31 shows the polarization curves of the above MEAs using pure hydrogen and

air as fuel and oxidant, respectively. The in-house MEA with 0.35 mg Pt cm-2

outperformed the commercial MEA with 0.50 mg Pt cm-2

. The maximum power output

from the in-house MEA (0.35 mg Pt cm-2

) was 379 mW cm-2

, compared with a maximum

power output from the commercial MEA (0.50 mg Pt cm-2

) of only 318 mW cm-2

. This

represents a 19% improvement with 30% less platinum catalyst. Furthermore, when the

Pt loading of the in-house MEA was lowered by a factor of 7 from 0.35 mg Pt cm-2

to

0.05 mg Pt/cm2, its power output still was almost identical (312 mW cm

-2) with that from

303

the commercial MEA containing 0.50 mg Pt cm-2

(318 mW cm-2

). These results clearly

confirm the superiority of pulse current (PC) electrodeposition over the conventional

methods currently employed for fabricating high performance MEAs.

Table 9-10 Performance comparison of conventional and PC electrodeposited MEAs

Commercial MEA In-House MEA In-House MEA

0.50 mg Pt/cm2 0.35 mg Pt/cm

2 0.05 mg Pt/cm

2

Substrate Carbon Cloth (E-TEK) Carbon Cloth (E-TEK) Carbon Cloth (E-TEK)

Solid Polymer Electrolyte Nafion® 112 Nafion® 112 Nafion® 112

Microporous Layer Loading (mg cm

-2)

Not Known 1.5 1.5

Carbon Powder Vulcan XC-72 Vulcan XC-72 Vulcan XC-72

Catalyst Type Platinum Platinum Platinum

PTFE Loading (wt%) 20 20 20

Catalyst Loading Method Conventional

(Rolling) PC Electrodeposition

(Ramp-down Waveform) PC Electrodeposition

(Ramp-down Waveform)

Fuel/Oxidant H2/Air (both dry) H2/Air (both dry) H2/Air (both dry)

Fuel Cell Operating Pressure (bar)

1.00 1.00 1.00

Fuel Cell Operating Temperature (°C)

40 40 40

Maximum Power (mW) 318 379 312

Figure 9-31 Influence of catalyst deposition method and loading on fuel cell

performance (ramp-down waveform, H2/Air, 20 wt% PTFE, cell temperature = 40 °C,

Hydrogen and air are used as fuel and oxidant entering the cell at 100% RH with


304

Membrane-electrode assemblies with low platinum loadings have been reported to

perform equally well compared with conventional MEAs with higher catalyst loadings.

Su et al. employed a novel catalyst-sprayed membrane technique to lower the platinum

loadings of the anodes and the cathodes of MEAs to 0.04 and 0.12 mg cm-2

, respectively,

without lowering MEA performance [656]. Platinum loadings of 0.10-0.20 mg cm-2

also

have been reported to deliver high performance by a number of researchers [657-658].

However, MEAs with ultra-low platinum loadings (i.e.; less than 0.1 mg cm-2

) have been

known to significantly lower its performance. Leimin et al. [659] claimed only a 5%

decrease in cell performance, when the platinum loading was decreased from 0.30 to 0.15

mg cm-2

, but a 35% decrease in cell performance when the catalyst loading was further

lowered to 0.06 mg cm-2

. Martin et al. [660] utilized scanning electron microscopy and

single-cell fuel cells to examine MEAs prepared by a novel electrospray method, where

platinum loadings as low as 0.0125 mg cm-2

were employed. MEAs with ultra-low

platinum loadings (less than 0.10 mg cm-2

) exhibited inferior performance compared with

MEAs containing 0.10 mg cm-2

. More recently, Gasteiger and Markovic [661] and

Secanell et al. [662] presented novel non-precious metal catalysts for oxygen reduction

and an MEA optimization method based on a two-dimensional isothermal, isobaric and

single phase MEA model, respectively.

305

10.0 CONCLUSIONS

The main conclusions drawn from the research are as follows:

1. Membrane-electrode assemblies (MEAs) prepared by pulse current (PC)

electrodeposition using a ramp-down waveform show performance comparable with

commercial MEAs, but with only one-tenth of the platinum of the latter. When the

platinum loading of in-house MEAs is increased to almost two-thirds that of

commercial E-TEK electrodes, the in-house MEAs exhibit better performance than

the commercial electrodes. MEAs fabricated using a triangular waveform performed

equally well, while those prepared using rectangular and ramp-up waveforms

performed slightly less well.

2. The thickness of the pulse electrodeposited Pt electrocatalyst layer is about 5-7 m,

which is about ten times thinner than that of commercial electrodes. In all likelihood,

this reduction in catalyst layer thickness has a significant impact on MEAs

performance and may arguably be the most important factor in improving fuel cell

performance.

3. MEAs prepared with pulse electrodeposition exhibit better fuel cell performance than

those fabricated by direct current electrodeposition. This is primarily attributed to the

deposition of smaller platinum crystallites and thinner catalyst layers using PC

electrodeposition.

4. For the plating cell employed, the optimal plating conditions were found to be: (i) a

peak deposition current density of 400 mA cm-2

, (ii) a duty cycle of 4%, (iii) pulses

generated and delivered in the microsecond range, (iv) a platinum bath concentration

of 0.05 M aqueous Pt2(NH3)4Cl2, (v) a plating solution flow rate of about 450 mL

min-1

, and (vi) a ramp-down waveform.

5. Based on the combination of parameters used in this study, a deposition current


produced the electrode with the best performance in DC

electrodeposition, while in PC electrodeposition a pulse current density of 50 mA

cm-2

and 400 mA cm-2

in high (10%-90%) and low (2%-10%) duty cycles,

respectively, give the best results.

306

6. MEAs fabricated using a 4% duty cycle (at a pulse deposition current density of 400

mA cm-2

) exhibited the best fuel cell performance, while at high duty cycles (10%-

90%), MEAs prepared with a duty cycle of 20% (at a pulse current density of 50 mA

cm-2

) delivered the best performance. However, MEAs fabricated using lower duty

cycles (less than 10%) generally outperformed those fabricated using higher duty

cycles. Other combination of parameters would have to be verified and optimized.

7. In a series of lifetime tests MEAs prepared by pulse electrodeposition performed

better than commercial MEAs. In static lifetime tests (constant load), the average cell

voltages over a 3000-h period at a constant current density of 619 mA cm-2

for the in-

house and the commercial MEAs were 564 mV and 505 mV, respectively. More

importantly, the decrease in cell voltage for the in-house MEA was only 2.1%, while

the commercial MEA experienced a drop of 2.8% by the end of the experiment.

Furthermore, there was significantly less variation in the output cell voltage of the in-

house MEA compared with the commercial MEA during the 3000-h operation.

8. In dynamic (varying load) lifetime experiments, an adult tricycle powered by a fuel

cell stack containing 42 in-house MEAs outperformed a similar fuel stack containing

commercial MEAs. Both stacks were used to charge a battery bank comprising three

12-V lead acid batteries sixty-three times over a 60 day period. The initial

performance of the stack containing the in-house MEAs was slightly superior to that

of the commercial MEAs, where, for the first 30 charges, the average stack power

outputs were, respectively, 218 W and 214 W. The variation in stack output also was

lower for the in-house MEAs for the first 30 charges. Furthermore, the average final

OCV of the fuel cell stack employing in-house MEAs was 1.022 V, while that of the

commercial stack was 1.012 V; this is an increase of 10 mV (about 1%). This small

improvement may result from a more effective catalyst layer based on the deposition

method described in this thesis.

9. A mathematical model, based on the works of Molina et al. [642] and Milchev [643,

652-654], for the electrodeposition based on joint diffusion, ohmic and charge

transfer control in the absence of any supporting electrolyte correctly predicted the

influence of peak deposition current density, duty cycle, and type of waveform on the

microhardness of the deposited layer. A strong correlation between microhardness

307

and particle size of the deposited layer was established. According to this model, the

highest nucleation rate of both platinum and nickel is obtained by employing the

ramp-down waveform, followed by the triangular waveform and then ramp-up, and,

finally, the rectangular waveform.

10. According to the model, as the peak deposition current density increases, the

microhardness rises, owing to a decrease in metal grain size, thereby improving the

catalytic activity of the catalyst layer in the fuel cell. The model predictions were

consistent with the experimental results.

11. At high peak deposition current densities (≥ 400 mA cm-2

), the ramp-down waveform

yielded the highest nucleation rates, indirectly confirming the experimental findings

in which MEAs fabricated with a ramp-down waveform employing a high peak


delivered the best fuel cell performance.

12. A hydrophobic polymer (PTFE) loading of 20 wt% on both cathode and anode on

carbon substrates gives the best results.

13. Under low fuel cell load conditions (< 200 mA cm-2

), MEAs prepared with different

carbon/graphite powders show similar performance. However, at current densities

greater than 300 mA cm-2

, the cell voltage increases with increasing macropore (> 1

µm) volume. Shawinigan Acetylene Black (SAB) had the highest macropore volume

of all the carbon/graphite materials investigated in this study and delivered the best

performance when tested in a single fuel cell, while Mogul L had the smallest volume

of macropores, and, consequently exhibited the poorest fuel cell performance under

identical test conditions. The improvement in gas diffusion layers prepared with SAB

is attributed to reduced mass transport limitations, most likely as a result of better

water management, especially at high current densities.

14. A diffusion layer loading (PTFE + carbon powder) of 1.5 mg cm-2

delivers the best

performance. Electrodes with less than 1.0 mg PTFE-C/cm2 exhibit poor cell

performance, while excessive loadings—greater than 2.0 mg PTFE-C/cm2—in most

cases, can pose other limitations, thereby lowering fuel cell performance. A slight

improvement in performance is observed when the carbon loading is increased from

308

1.0 to 3.0 mg cm-2

for MEAs prepared with Asbury 850 and Mogul L. This marginal

improvement is attributed to a better coverage of the electrode surface.

15. Carbon paper (TGP090, Toray Inc.) and carbon cloth (Elat, E-TEK Inc.) exhibit very

similar performance when the PC electrodeposition technique is employed. At low

current densities (less than 300 mA cm-2

) all MEAs performed equally well; however,

the performance at current densities higher than 300 mA cm-2

of the thickest and the

thinest GDLs from Toray Industries Inc.—TGP-H-120 (370 µm) and TGP-H-030

(110 µm)—starts to lag behind the others. These differences in fuel cell performance

become very apparent when the current density reaches 1500 mA cm-2

using pure

oxygen as the oxidant. This variation in performance is attributed to the differences in

the electronic conduction and the water management (related to the amount of

macropores) capability of each substrate.

16. Reducing the platinum loading at the anode from 0.35 to 0.15 mg cm-2

using the PC

electrodeposition technique has a negligible impact on the performance of a PEM fuel

cell.

17. Thirty-second impregnation time (floating method) for Nafion® loading produces

carbon substrates with a sufficient amount of Nafion®. Higher Nafion

® loadings can

be obtained using the brushing method.

309

11.0 RECOMMENDATIONS AND FUTURE WORK

1. Examine the impact of fuel cell temperature on MEAs preparation using the PC

electrodeposition technique.

2. Study the influence of external humidification of reactant gases on MEAs fabricated

using the PC electrodeposition technique.

3. Further characterize both diffusion and catalyst layers utilizing various techniques

such as cyclic voltammetry (CV), Inductively coupled plasma atomic emission

spectroscopy (ICP-AES) and electrochemical impedance spectroscopy (EIS).

4. Further develop the mathematical model based on nucleation theory.

310

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334

APPENDIX A

Table A-1 Physical data for Platinum Group Metals [655]

Property/Metal Ru Rh Pd Os Ir Pt

Atomic Number 44 45 46 76 77 78

Atomic Weight (g mol-1

) 101.07 102.9 106.4 190.2 192.2 195.09

Group, Period, Block 8, 5, d 9, 5, d 10, 5, d 8, 6, d 9, 6, d 10, 6, d

Ionic Radii (Å) / ox. State 0.82 (+3) 0.81 (+3) 0.78 (+2) 0.78 (+2) 0.82 (+3) 0.74 (+2)

Density at 20 °C (g cm-3

) 12.45 12.41 12.02 22.61 21.65 21.45

Liquid Density at m.p. (g cm-3

) 10.65 10.7 10.38 20 19 19.77

Crystal Structure hcp fcc fcc hcp fcc fcc

Melting Point (K) 2607 2237 1828.05 3306 2739 2041.4

Boiling Point (K) 4423 3968 3236 5285 4701 4098

Heat of Fusion (kJ mol-1

) 38.59 26.59 16.74 57.85 41.12 22.17

Heat of Vaporization (kJ mol-1

) 591.6 494 362 738 563 469

Specific Heat Capacity (J mol-1

K-1

) 24.06 24.98 25.98 24.7 25.1 25.86

Oxidation States 1,2,3,4,6,8 1, 2, 3, 4 2, 4 -1, -2, 1-8 2, 3, 4, 6 1,2,3,4,5,6

Electronegativity (Pauline Scale) 2.3 2.28 2.2 2.2 2.28

Ionization Energies (kJ mol-1

)

1st: 710.2 719.7 804.4 840 880 870

2nd: 1620 1740 1870 1600 1600 1791

3rd: 2747 2997 3177

Electrical Resistivity at 273 K (nΩ m) 71 43.3 105.4 (293K)81.2 47.1 105 (293 K)

Thermal Conductivity at 300 K (w m-1

K-1

) 117 150 71.8 87.6 147 71.6

emf (vs. SHE) (M/M2+

) +0.680 +0.758 +0.951 +0.850 +1.156 +1.188

Speed of Sound (thin rod) (m s-1

) 5970 4700 3070 4940 4825 2800

Young's Modulus (Gpa) 447 380 121 528 168

Shear Modulus (Gpa) 173 150 44 222 210 61

Bulk Modulus (Gpa) 220 275 180 320 230

Poisson Ratio 0.3 0.26 0.39 0.25 0.26 0.38

Hardness (kg mm-2

)

(i) Metal (annealed) 200-350 120-140 37-40 300-500 200-240 37-42

(ii) Electrodeposited 900-1000 800-900 200-400 n/a 900 200-400

Mohs Hardness 6.5 6 4.75 7 6.5 4.0-4.5

CAS Registry Number 7440-18-8 7440-16-6 7440-05-3 7440-04-2 7439-88-5 7440-06-4

335

APPENDIX B

The Electrical Double Layer

Introduction

Electrical double layer phenomena arise when two different phases come in contact. The

resulting interphase has a different composition than both bulk phases and its structure is

of great importance in electrochemistry. This is important in electrodeposition, where a

substrate (usually a metal) and an electrolyte solution (containing the metallic ions) come

in contact. A good understanding of the structure of this substrate-electrolyte interphase

is important since the electrodeposition process takes place in this very thin region, where

the electric field is in the neighbourhood of 106 to 10

7 V cm

-1 [333]. In an electrical

double layer between two different phases, one phase carries a positive charge and the

other a negative charge at the phase boundary, analogous to a simple parallel plate

capacitor (Figure B-1).

Figure B-1 A parallel-plate capacitor

The structural properties of the interphase play a vital role in the nature of the

electrodeposited layer. Water contains dissolved ions of interest that are hydrated to a

varying degree, while water molecules have the ability to form clusters. Such clusters

either can be structured, where they contain hydrogen-bonded water molecules, or

unstructured, where single and independent water molecules exist. According to this

- - -

- - -

+

+

+

+

+

+

336

model, the lifetime of such clusters is about 10-10

s [333, 334]. On the other hand, metals

contain a fixed lattice of positive ions surrounded by a number of negatively-charged

electrons. A distinction is made between the bulk structure of a metal and that of its

surface. The latter is often referred to as the top layer of atoms, but in electrochemistry

the metal surface is considered to contain the top two to three atomic layers. A thorough

discussion of the electrical double layer is provided by Paunovic and Schlesinger [333].

Electrical Double Layer Models

It is clear that the formation of any interface between an electrolyte and a metal will

inevitably impact the reactions taking place at the metal surface, and such reactions will

be markedly different from those occurring in the bulk solution. In electrodeposition, the

electrode will often be under potentiostatic or galvanostatic control via an external power

supply or current rectifier, hence not only adding to the complexity of the system, but

also imparting additional constraints on the metal by dictating the amount of charge that

can be held at the electrode. All these factors strongly influence the rates and the types of

interactions that can take place between ions and molecules in the electrolyte and the

electrode surface. With this interface being the locus of the electrodeposition, where all

desired reactions take place, it becomes important not only to gain an understanding of all

the factors influencing such reactions, but also be able to control them to achieve

desirable outcomes. The first step in attaining the above goals is to successfully explain

the behaviour of such interfaces under different conditions. Many models have been put

forward to explain the behaviour of the metal-electrolyte interface, including the

Helmholtz, Gouy-Chapman, Stern, Grahame, and Bockris, Devanathan & Muller models.

For reference purposes only, simple schematics of the above models are shown in Figures

B-2 to B-5.

337

Figure B-2 Helmholtz compact double-layer model

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

XH

Metal Solution

Hydrated

Anions

Distance from the

Metal Surface

Helmholtz

Fixed Plane

(volts)

Distance into Solution

Adsorbed

Water Dipoles

+

338

Figure B-3 Gouy-Chapman model of electrical double layer

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

Metal Solution

+

+

+


1

+

+

+

+

+

+

+

+

+

-

-

-

-

-

-

-

-

-

339

Figure B-4 Stern model of electrical double layer [333]

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

Metal Solution

+


+

+

Helmholtz Plane

Fixed Ions

Mobile Ions

Diffuse Layer

Compact

layer

CH CGC

340

Figure B-5 Grahame triple-layer model

+

+

+

+

+

+

+

+

+

+

+

+

+

+

+

XH,i

Metal Solution

Helmholtz Planes

(inner & outer)

+

- -

+

+

+

XH,O

Distance to Solution

Diffuse Layer

-

Partially-Hydrated Ion

Fully-Hydrated Ion

341

APPENDIX C


SPEED

Flow rate

(mL min-1

)

(%) Run 1 Run 2 Run 3 Run 4 Average

5 13 13 13 13 13

10 41 39 39 39 40

20 92 94 93 93 93

30 168 170 169 169 169

40 235 235 236 237 236

50 304 301 300 307 303

60 375 377 375 378 376

70 454 454 452 456 454

80 515 514 517 517 516

90 572 574 574 576 574

92 (Maximum) 581 582 582 585 583


Speed

Flow

rate ( (mL

min-1

)


5 13 14 14 13 14

10 42 40 39 39 40

20 94 95 94 93 94

30 170 170 171 169 170

40 237 236 236 237 237

50 304 303 302 306 304

60 378 377 379 378 378

70 454 457 456 456 456

80 516 517 516 517 517

90 578 577 575 576 577

92 (Maximum) 583 584 584 585 584

342


Speed

Flow rate

(mL min-1

)


5 27 27 28 27 27

10 84 85 85 85 85

20 193 194 193 192 193

30 350 348 350 351 350

40 490 490 492 490 491

50 633 632 632 630 632

60 781 782 781 784 782

70 945 944 947 944 945

80 1073 1076 1078 1075 1076

90 1190 1193 1191 1190 1191

92

(Maximum) 1210 1208 1212 1209 1210


Speed

Flow rate

(mL min-1

)


5 28 27 29 28 28

10 86 86 87 85 86

20 195 196 194 194 195

30 352 351 350 350 351

40 493 494 492 490 492

50 632 635 634 631 633

60 78 783 780 783 783

70 949 946 946 948 947

80 1075 1078 1079 1078 1078

90 1192 1193 1194 1194 1193

92

(Maximum) 1212 1211 1215 1216 1214

343

Figure C-1 Electroplating bath flow rate as a function of pump speed and temperature

344

APPENDIX D

Derivation of Equations

Obtaining an Analytical Solution for the Ramp-Down Waveform

The mass transfer in most electrochemical systems, including electrodeposition processes

employing different waveforms, is customarily described by Fick’s second law of

diffusion—equation (9-15). Taking the Laplace transform of this equation yields:

02

2

ccsdx

cdD (1)

According to the initial condition—equation (9-16)—the concentration at the start of the

first pulse (t = 0) is equal to the bulk concentration; the Laplace transform of (9-16)

results in:

s

cxc 0),0( (2)

Similarly, the Laplace transform of the boundary condition (9-17) gives:

s

csc 0),( (3)

Applying the Laplace transform to the mathematical expression representing the ramp-

down waveform, expression (9-25), yields:

22)(

s

e

s

e

s

ea

a

iti

kswswsp

(4)

Substituting the transformed Heaviside function—(4)—into boundary condition (9-18)

results in:

22

0s

e

s

e

s

ea

aDFz

i

x

c kswswsp

x

(5)

The ordinary differential equation (ODE) that was derived from Fick’s second law of

diffusion can readily be solved using expressions (2) and (3) to give

345

s

ceAc

xD

s

0

(6)

The constant A in the above expression can be evaluated by taking its derivative and

putting it into equation (5):

s

c

s

Da

DFz

i

s

c

edx

cdA

p

D

s

001

(7)

Simplifying the above equation:

)8(

1

2/52/52/3

22

s

e

s

e

s

ea

aDFz

i

s

e

s

e

s

ea

saDFz

iA

kswswsp

kswswsp

Putting equation (8) into (6):

s

ce

s

e

s

e

s

ea

aDFz

ixsc

xD

skswsws

p 0

2/52/52/3),(

(9)

The surface concentration can be expressed as follow:

s

ce

s

e

s

e

s

ea

aDFz

isc

D

skswsws

p 0)0(

2/52/52/3)0,(

(10)

2/52/52/3

0)0,(s

e

s

e

s

ea

aDFz

i

s

csc

kswswsp

(11)

The solution to the original diffusion equation is obtained by applying the inverse

Laplace transform to equation (11):

)12()(

)2/5()(

)2/5()(

2/3)0,(

2/32/32/1

0

tS

kttSw

wttS

awt

aDFz

ictc kw

p

346

The concentration overvoltage for a diffusion-controlled process is represented by:

0

)0,(ln

c

tc

Fz

TR (9-42)

Substituting equation (12) into (9-42) yields the overvoltage equation for a ramp-down

waveform:

0

2/32/32/1

0 )()2/5(

)()2/5(

)(2/3

lnc

tSkt

tSwwt

tSawt

aDFz

ic

zF

RTkw

p

)(

)2/5()(

)2/5()(

2/31ln

2/32/32/1

0

tSkt

tSwwt

tSawt

aDFcz

i

Fz

TRkw

p

347

Derivation of Eqn (9-66)

Faraday’s Law for Hemispherical Cluster

If n = number of moles, then, from Faraday’s law the total current I is given by

I zFdn

dt

eqmol

Ceq

mols C

s . . . (a)

The volume V of a hemisphere of radius R is

V 12

43 R

3 23 R

3

No. moles in the hemisphere: n V

vM

23R3

vM

2R3

3vM

. . . (b)

Rate of deposition mol

s

: dn

dt =

dn

dRdR

dt

= d

dR

2R3

3vM

dR

dt

= 2R2

vM

dR

dt . . . (c)

Putting (c) into (a): I zFdn

dt zF

2R 2

vM

dR

dt

Therefore I 2zF

vM

R

2 dR

dt (9-66)

348

Derivation of Eqns (9-67) and (9-68)

I1(t) 2R2io exp

zF

R T

exp

(1 )zF

R T

. . . (9-65)

where SR is the cluster surface area and io is the exchange current density. In order not to

confuse the gas constant with the cluster radius, R is used for the gas constant and R for

the radius of the hemispherical cluster.

I1(t) 2zF

vM

R2 dR

dt . . . (9-66)

from which dR

dt

vMI1(t)

2zFR2 . . . (a)

Substituting (9-65) into (a):

dR

dt =

vM

2zFR2 2R

2io exp

zF

R T

exp

(1)zF

R T

= vMio

zFexp

zF

R T

exp

(1)zF

R T

. . . (b)

dR vMio

zFexp

zF

R T

exp

(1)zF

R T

dt

Integrating, noting that at t = 0, R = 0, gives

R(t) vMio

zFexp

zF

R T

exp

(1)zF

R T

t . . . (9-67)


I1(t) =

2vMio

zFexp

zF

R T

exp

(1 )zF

R T

t

2

io expzF

R T

exp

(1 )zF

R T

i.e., I1(t) = 2vMio

3

(zF)2 exp

zF

R T

exp

(1)zF

R T

3

t2

. . . (9-68)

349


co = bulk concentration

c = concentration at r

cS = concentration at surface of cluster

At r = , c = co

At r = r, c = c

At r = R, c = cS

If the total cation flux is diffusion flux, then J Ddc

dr

i

zF

I

AzF (I = total current)

Therefore, I

AzF D

dc

dr

dc

dr

I

AzFD . . . (a)

Area of hemispherical surface is A = 12 4r

2 = 2πr2 . . . (b)

Putting (b) into (a): dc

dr

I

2r2zFD

Cross-multiplying: dc I

2zFD

dr

r2

dcc1

c 2

I

2zFD

dr

r2

r1

r2

Integrating: c2 – c1 = I

2zFD

1

r2

1

r1

k

1

r2

1

r1

. . . (c)

When c2 c, r2 r

c1 co, r1

c co k1

r

1

k

1

r

. . . (d)

When c2 co, r2

c1 cS, r1 R

co cS k1

1

R

k

1

R

. . . (e)

Dividing (d) by (e): c co

co cS

1 r

1 R

R

r

Cross-multiplying: c co R

rco cS

dr

r

R

350

c = co R

rco cS co

R

rco

co

co cS . . . (f)

= co 1R

r

co

co

cS

co

i.e., c co 1R

r1

cS

co

. . . (9-69)

From (f), c co R

rco cS

Differentiating: dc

dr

dco

drR co cS

d(1 r)

dr 0 R co cS

1

r2

Therefore, dc

dr

R co cS r

2

When r = R, dc

dr

rR

R co cS

R2

i.e., dc

dr

rR

co cS

R . . . (9-70)

351


We start with I1(t) 2zF

vM

R2 dR

dt . . . (9-66)

from which dR

dt

vMI1(t)

2zFR2 . . . (a)

Also, I1(t) 2R2io

cS

co

expzF

R T

exp

(1 )zF

R T

. . . (9-61d)

Substituting (9-61d) into (a):

dR

dt =

vM

2zFR2 2R

2io

cS

co

expzF

R T

exp

(1 )zF

R T

= iovM

zF

cS

co

expzF

R T

exp

(1 )zF

R T

. . . (b)

Also, I1(t) 2RzFDco 1cS

co

. . . (9-61b)

from which 1cS

co

I1(t)

2RzFDco

or cS

co

1I1(t)

2RzFDco

. . . (c)

Substituting (c) into (b):

dR

dt =

iovM

zF1

I1(t)

2zFRDco

exp

zF

R T

exp

(1 )zF

R T

= iovM

zFexp

zF

R T

I1(t)

2zFRDco

expzF

R T

exp

(1 )zF

R T

. . . (d)

Substituting (9-57) into (d):

dR

dt =

iovM

zFexp

zF

R T

2zF

vM

R2 dR

dt

2zFRDco

expzF

R T

exp

(1 )zF

R T

= iovM

zFexp

zF

R T

R

vMDco

expzF

R T

dR

dt exp

(1 )zF

R T

352

= iovM

zF exp

zF

R T

Rio

zFDco

expzF

R T

dR

dt

iovM

zF exp

(1 )zF

R T

dR

dt1

Rio

zFDco

expzF

R T

iovM

zF exp

zF

R T

iovM

zFexp

(1 )zF

R T

dR

dt =

iovM

zF exp

zF

R T

iovM

zFexp

(1 )zF

R T

1Rio

zFDco

expzF

R T

=

iovM

zFexp

zF

R T

exp

(1 )zF

R T

1Rio

zFDco

expzF

R T

. . . (e)

Multiplying both top and bottom of (e) by zFDco:

dR

dt =

zFDcoiovM

zFexp

zF

R T

exp

(1 )zF

R T

zFDco Rio expzF

R T

Finally, dR

dt

DcovMio expzF

R T

exp

(1 )zF

R T

zFDco Rio expzF

R T

. . . (9-71)

353


Start with: dR

dt

DcovMio expzF

R T

exp

(1 )zF

R T

zFDco Rio expzF

R T

dR zFDco Rio expzF

R T

Dco vMio exp

zF

R T

exp

(1 )zF

R T

dt

zFDco dR io expzF

R T

RdR DcovMio expzF

R T

exp

(1 )zF

R T

dt

zFDco dR0

R

io expzF

R T

RdR0

R

DcovMio expzF

R T

exp

(1)zF

R T

dt0

t

zFDco R io expzF

R T

R2

2 DcovMio exp

zF

R T

exp

(1 )zF

R T

t

io expzF

R T

R2

2 zFDco R DcovMio exp

zF

R T

exp

(1)zF

R T

t 0

This is a quadratic equation. Rearranging:

R2

zFDco

io

2 exp

zF

R T

R

DcovMio expzF

R T

exp

(1 )zF

R T

t

io

2 exp

zF

R T

0

R2

2zFDco

io expzF

R T

R

2DcovMio expzF

R T

exp

(1 )zF

R T

t

io expzF

R T

0

R2

2zFDco

io expzF

R T

R 2DcovM 1 expzF

R T

t 0

354

Solving the quadratic, taking the positive root since the radius R of the cluster must be >0:

R =

2zFDco

io expzF

R T

2zFDco

2

io

2 exp

2zF

R T

4 1 2DcovM 1 exp(1 )zF

R T

expzF

R T

t

2 1

=

2zFDco

io expzF

R T

2zFDco

2

io

2 exp

2zF

R T

4 1 2DcovM 1 expzF

R T

t

2 1

= zFDco

io expzF

R T

1

2

2zFDco 2

io

2 exp2zF

R T

4 1 2DcovM 1 expzF

R T

t

= zFDco

io expzF

R T

1

4

2zFDco 2

io

2 exp2zF

R T

2D covM 1 expzF

R T

t

= zFDco

io expzF

R T

zFDco

2

io

2 exp2zF

R T

2DcovM 1 expzF

R T

t

= zFDco

io expzF

R T

zFDco

2

io

2 exp2zF

R T

zFDco

2

zFDco 2 2Dco vM 1 exp

zF

R T

t

= zFDco

io expzF

R T

zFDco

1

io

2 exp2zF

R T

1

zFDco 2 2DcovM 1 exp

zF

R T

t

= zFDco

io expzF

R T

zFDco

exp2zF

R T

io

2

2

zF 2

Dco

vM 1 expzF

R T

t

355

= zFDco

io expzF

R T

zFDco

exp2zF

R T

io

2

io

2

io

2

2

zF 2

Dco

vM 1 expzF

R T

t

= zFDco

io expzF

R T

zFDco

io

exp2zF

R T

2io

2

zF 2

Dco

vM 1 expzF

R T

t

=

zFDco

io expzF

R T

zFDco

io

exp2zF

R T

exp2zF

R T

exp2zF

R T

2io

2

zF 2

Dco

vM 1 expzF

R T

t

=

zFDco

io expzF

R T

zFDco

io

exp2zF

R T

11

exp2zF

R T

2io

2

zF 2

Dco

vM 1 expzF

R T

t

=

zFDco

io expzF

R T

zFDco

io

expzF

R T

11

exp2zF

R T

2io

2

zF 2

Dco

vM 1 expzF

R T

t

=

zFDco

io expzF

R T

zFDco

io

expzF

R T

1 exp2zF

R T

2io

2

zF 2

Dco

vM 1 expzF

R T

t

=

zFDco

io expzF

R T

zFDco

io expzF

R T

1 2vMio

2

zF 2

Dco

exp2zF

R T

exp

2zF

R T

exp

zF

R T

t

356

=

zFDco

io expzF

R T

zFDco

io expzF

R T

1 2vMio

2

zF 2

Dco

exp2zF

R T

exp

(2 1)zF

R T

t

. . . (a)

Define the following:

P = expzF

R T

. . . (b)

m = zFDco

ioP . . . (c)

Q = exp2zF

R T

exp

(2 1)zF

R T

. . . (d)

n = vMio

2Q

zF 2

Dco

. . . (e)

Substituting (b), (c), (d), and (e) into (a) transforms (a) to

R(t) = zFDco

io P

zFDco

io P1 2

vMio

2

zF 2

Dco

Q t

= m m 1 2n t

= m 1 2n t 1

Therefore, R(t) = m 1 2nt 1 / 2

1 . . . (9-72)

357


Show that I p1 nt

1 2nt 1 / 2 1

We showed earlier that I 2zF

vM

R

2 dR

dt . . . (9-66)

and also that R m 1 2nt 1 / 2

1 . . . (9-72)

We have defined: p 4 zFDco

2Q

io P3 . . . (a)

n = vMio

2Q

zF 2

Dco

. . . (b)

Q = exp2zF

R T

exp

(2 1)zF

R T

. . . (c)

P = expzF

R T

. . . (d)

m = zFDco

ioP . . . (e)

Noting that p, n, Q, P, and m are not functions of t, we can differentiate (9-72) to give

dR

dt =

d

dtm 1 2nt

1 / 21 m

d

dt1 2nt

1/ 2

= m 12 1 2nt

1/ 22n

= mn

1 2nt 1/ 2 . . . (f)

Dividing (a) by (b): p

n=

4 zFDco 2

Q

io P3

vMio

2Q

zF 2Dco

= 4 zFDco

2

io P3

zF 2D co

vMio

2

= 4zF

vM

zFDco

io P

3

358

= 4zF

vM

m3

Rearranging: 2zF

vM

p

2m3n

. . . (g)

Substituting (g), (9-72), and (f) into (9-66):

I = p

2m3n m 1 2nt

1 / 21

2

mn

1 2nt 1/ 2

= p1

2mn1 2nt

1/ 21

2

mn

1 2nt 1 / 2

= p1

2 1 2nt 1/ 2 1 2nt 1/ 2

1 2

= p1 2nt 2 1 2nt 1/ 2

1

2 1 2nt 1 / 2

= p12 1 2nt 1

2 2 1 2nt 1 / 2 1

2

1 2nt 1/ 2

= p

12 nt 1 2nt 1/ 2

12

1 2nt 1 / 2

= p1 nt

1 2nt 1 / 2 1 2nt 1/ 2

1 2nt 1/ 2

= p1 nt

1 2nt 1 / 2 1

. . . (9-77)

359


Pure Diffusion Control

From Eqn (9-72), R(t) m 1 2nt 1/ 2

1 . . . (9-72)

When io is very large or t is very long, nt >> 50, and 12nt 1/21 2nt

1/2

Therefore, under these conditions, R(t) m 2nt 1/ 2

. . . (a)

Putting in values for m and n:

R(t) ≈zFDc

io expzF

R T

2vMio

2

zF 2

Dc

exp

2zF

R T

exp

(2 1)zF

R T

t

1/ 2

=

zFDc

io

2vMio

2

zF 2

Dc

1 / 2

expzF

R T

exp

2zF

R T

exp

(21)zF

R T

1/ 2

t1/ 2

=

zFDc

io

2vMio

2

zF 2

Dc

1 / 2

exp2zF

R T

1 / 2

exp2zF

R T

exp

(21)zF

R T

1 / 2

t1/ 2

=

2vMDc 1 / 2

exp2zF

R T

exp

2zF

R T

exp

2zF

R T

exp

(2 1)zF

R T

1/ 2

t1/ 2

= 2vMDc 1 / 2

exp 0 expzF

R T

1 / 2

t1/ 2

= 2DcvM 1 / 2

1 expzF

R T

1/ 2

t1 / 2

. . . (9-78)

where R is the radius of the cluster and R is the gas constant.

360


Pure Diffusion Control

From Eqn (9-77) I1(t) p1 nt

1 2nt 1/ 2 1

. . . (9-77)

where p 4 zFDc

2Q

ioP3 . . . (a)

When io is very large or t is very long, nt >> 50,

and 1+nt ≈ nt and 1+2nt ≈ 2nt

and (9-77) reduces to I1(t) pnt

2nt 1/ 2

p

nt 1/ 2

2

. . . (b)

Putting the expressions for p and n into (b):

I1(t) = 4 zFDc

2Q

2 ioP3

vMio

2Qt

(zF)2Dc

1/ 2

= 4 zFDc

2

2 io

vM

1 / 2io

zF(Dc)1/ 2

Q3 / 2

P3 t

1 / 2

= zF 2DcvM

1/ 3 3 / 2 Q3 / 2

(P2)

3/ 2 t1/ 2

. . . (c)

Q

P2

3 / 2

=

exp2zF

R T

exp

(2 1)zF

R T

expzF

R T

2

3 / 2

=

exp2zF

R T

exp

(2 1)zF

R T

exp2zF

R T

3 / 2

= 1 exp(zF

R T

3 / 2

. . . (d)

Putting (d) into (c):

361

I1(t) = zF 2DcvM

1/ 3 3 / 2

1 exp(zF

R T

3 / 2

t1 / 2

. . . (9-79)

362


The ohmic resistance of an electrolyte is given by

1

A

where is the length of the conduction path and A is the cross-sectional area

perpendicular to the flow of current.

Therefore d 1

d

A . . . (a)

Referring to the figure below:

The area of a hemisphere of radius r is A = 12 4r

2 = 2πr2 . . . (b)

and the differential path length is d = dr . . . (c)

Putting (b) and (c) into (a): d1

dr

2r2

Integrating: dr=R

r=L

1

2

dr

r2

R

L

R-L = 1

2

1

r

R

L

= 1

2

1

L

1

R

= 1

2

1

R

1

L

= 1

2

R

R

1

R

1

L

= 1

2R

R

R

1

L

i.e., R -L 1

2R1

1

L

. . . (9-83)

Location ofreference electrode

dr

r

L

R

363


From Eqn (9-86a), I = 2elR . . . (9-86a)

and from Eqn (9-66), I 2zF

vM

R

2 dR

dt . . . (9-66)

Equating the two expressions for I:

2 e lR2zF

vM

R2 dR

dt

dR

dt

2 e lRvM

2zFR2

e lvM

zFR . . . (a)

RdR e lvM

zFt

Integrating, with R = 0 at t = 0, R2

2 elvM

zF t . . . (b)

R 2 e lvM

zF

1 / 2

t1/ 2

. . . (9-87)

Substituting (9-87) and (a) into (9-66):

I = 2zF

vM

2e lvMt

zF

e lvM

zF (2e lvMt) (zF) 1/ 2

= 2zF

vM

2e lvM

zF

t

(zF)1/ 2 e lvM

zF(2e lvM)1 / 2

t1 / 2

= 23/ 2 e l

3/ 2 vM

zF

1/ 2

t1/ 2

. . . (9-88)

364


Eqn (9-66): I 2zF

vM

R

2 dR

dt . . . (9-66)

from which dR

dt

vMI

2zFR2 . . . (a)

Eqn (9-90):

I 2R2io exp

zF

R T

I

2R

exp

(1)zF

R T

I

2R

. . . (9-90)

Substituting (9-90) into (a):

dR

dt =

vM

2zFR2 2R

2io exp

zF

R T

I

2R

exp

(1)zF

R T

I

2R

= vMio

zFexp

zF

R T

I

2R

exp

(1)zF

R T

I

2R

. . . (b)

Using the expression for I given by Eqn (9-66),

I

2R

2zFR2 vM dR dt

2R

zFR

vM

dR

dt . . . (c)

Therefore,

zF

R T

I

2R

zF

R T

zFR

vM

dR

dt

zF

R T zF

2R

R TvM

dR

dt . . . (d)

and, similarly,

(1 )zF

R T

I

2R

= (1 )zF

R T

zFR

vM

dR

dt

= (1 )zF

R T

(1 ) zF 2R

R TvM

dR

dt . . . (e)

Therefore expzF

R T

I

2R

= exp

zF

R T zF

2R

R TvM

dR

dt

= expzF

R T

exp

zF 2R

R TvM

dR

dt

. . . (f)

365

Similarly,

exp(1 )zF

R T

I

2R

= exp

(1 )zF

R T

(1 ) zF 2R

R TvM

dR

dt

= exp(1)zF

R T

exp

(1) zF 2R

R TvM

dR

dt

. . . (g)

Substituting (f) and (g) into (b) gives

dR

dt

vMio

zFexp

zF

R T

exp

zF 2R

R TvM

dR

dt

vMio

zFexp

(1 )zF

R T

exp

(1) zF 2R

R TvM

dR

dt

By using the definitions

TR

Fz

Fz

iv oM exp1P ,

MvTR

Fz

2

2P

TR

Fz

Fz

iv oM )1(exp3P ,

MvTR

Fz

2)1(

4P

we can make Eqn (g) more compact, giving

dR

dt P1 exp P2R

dR

dt

P3 exp P4R

dR

dt

. . . (9-91)

366

Derivation of Eqn (9-93): io,S io cS co 1

[Appears on page 267 following Eqn (1) in J. Electrochim. Acta, 312 (1991) 267-275]

For the metal deposition reaction Mzze

M

the forward (cathodic) reaction partial current density is

if zFk f[Mz

]expzFa ct

R T

zFkf co exp

zFa ct

R T

. . . (a)

and the backward (anodic) reaction partial current density is

ib zFkb[M]exp(1 )zFact

R T

zFkb (1)exp

(1 )zFact

R T

. . . (b)

where kf and kb are the electrochemical rate constants and co is the bulk concentration of

the metal ion. Note that since the activity of the metallic phase is unity, there is no

concentration term for the backward (anodic) contribution to the current density. Again,

cathodic overvoltages and currents are taken as positive.

At electrochemical equilibrium, = 0 and if = ib so that

zFkfco zFkb io . . . (c)

putting (c) into Eqns (a) and (b) gives the net current density as

i = if – ib

i = zFk fco expzFact

R T

– zFk b exp(1 )zFa ct

R T

= io expzFact

R T

exp

(1 )zFact

R T

. . . (d)

Eqn (d) is the standard Butler-Volmer equation, which assumes that the concentration cS

of the metal ion at the electrode surface is the same as the concentration co in the bulk

solution. If cS is less than co, Eqn (a) for the forward reaction must be modified to

i f zFk f

cS

co

exp

zFa ct

R T

. . . (e)

and the net current density then becomes

i = zFk f

cS

co

exp

zF a ct

R T

– zFkb exp(1 )zF act

R T

. . . (f)

367

Owing to the presence of the concentration gradient between the bulk solution and the

metal surface, part of the total applied cathode overpotential ∆E will consist of a

concentration overpotential C as well as an activation overpotential act. It is the

activation overpotential act that must be used in Eqn (f). Exchange current densities

usually are expressed in terms of bulk concentrations; therefore it is useful to find an

expression that can relate the surface exchange current density io,S to the bulk solution

exchange current density io.

We let this relationship be io,S io f . . . (g)

such that i = io,S expzF act

R T

exp

(1)zF act

R T

. . . (h)

The parameter f is determined as follows:

The total applied overpotential is = ∆E = act C . . . (i)

from which act = EC

= E RT

zFln

co

cS

= E RT

zFln

cS

co

. . . (j)

Substituting (g) and (j) into (h):

i = io f expzF

R TE

RT

zFln

cS

co

exp

(1)zF

R TE

RT

zFln

cS

co

= io f expzFE

R T

exp

zF

R T

RT

zFln

cS

co

– exp(1 )zFE

R T

exp

(1 )zF

R T

RT

zFln

cS

co

= io f expzFE

R T

exp ln

cS

co

– exp(1 )zFE

R T

exp (1 )ln

cS

co

368

= io f expzFE

R T

exp ln

cS

co

exp(1 )zFE

R T

exp ln

cS

co

(1)

= io f expzFE

R T

cS

co

exp(1 )zFE

R T

cS

co

(1 )

. . . (k)

For reasons discussed above, for a metal deposition reaction, as indicated in Eqn (h),

there should be no pre-exponential term associated with the anodic partial current density

in Eqn (k). The unwanted term cS co ()

following the second exponential term in

Eqn (k) is removed by setting

f cS

co

1

. . . (l)

and, consequently, from Eqn (g), io,S iocS

co

1

. . . (m)

Substituting (l) into (k):

i = io

cS

co

1

expzFE

R T

cS

co

exp(1 )zFE

R T

cS

co

(1 )

= io

cS

co

expzFE

R T

exp(1 )zFE

R T

. . . (n)

Eqn (n) must hold for all values of cS , including the limiting case when cS co ; i.e.,

when there is no concentration overpotential and C 0. Under this limiting condition,

from Eqn (i),

∆E = act . . . (o)

and Eqn (n) becomes

i = io expzFact

R T

exp(1 )zFact

R T

. . . (p)

Eqn [n] is the form of the Butler-Volmer equation that is used for the electrodeposition of

369

a metal when the concentration of the metal ion at the electrode surface is less than the

concentration of the metal ion in the bulk solution. In the limiting case when cS co ,

Eqn (n) reduces to Eqn (p), which is just the standard Butler-Volmer equation given as

Eqn (d).

370

Derivation of Eqn ( 1 ) in Milchev Transport Paper Part I

[Milchev, A., Electrochim. Acta, 312 (1991) 267-275]

[Numerically-numbered equations are the numbers used by Milchev in his paper.]

Putting SR 2R2 into Eqn (1): gives

I 2R2io,S exp

zFact

R T

exp

(1 )zFact

R T

. . . (a)

Eqn (8): act E R T

zF1 a ln

co

cS

E

R T

zF1 a ln

cS

co

. . . (8)

Putting (8) into (a):

I =

2R2io,S exp

zF

R TE

R T(1 a)

zFln

cS

co

exp

(1)zF

R TE

R T(1 a)

zFln

cS

co

= 2R2io,S exp

zFE

R T

exp

zF

R T

R T(1 a)

zFln

cS

co

– exp(1)zFE

R T

exp

(1)zF

R T

R T(1 a)

zFln

cS

co

= 2R2io,S exp

zFE

R T

exp (1 a)ln

cS

co

–

exp(1 )zFE

R T

exp (1 )(1 a)ln

cS

co

= 2R2io,S exp

zFE

R T

exp ln

cS

co

(1a)

– exp(1 )zFE

R T

exp ln

cS

co

(1)(1a)

= 2R2io,S exp

zFE

R T

cS

co

(1a)

exp(1 )zFE

R T

cS

co

(1) (1a)

371

= 2R2

io,S

cS

co

(1a)

expzFE

R T

io,S

cS

co

(1) (1a)

exp(1 )zFE

R T

. . . (b)

The relationship between io, S, the exchange current density related to the concentration cS

of the metal ions at the cluster surface and io, the exchange current density related to the

bulk concentration of the metal ions can be shown to be [see separate derivation]

io,S iocS

co

1

. . . (c)

Therefore

io,S cS

co

(1a)

iocS

co

1

cS

co

(1a)

io

cS

co

1a

io

cS

co

1a

. . . (d)

Similarly,

io,S cS

co

(1) (1a)

= io

cS

co

1

cS

co

(1 )(1a)

io

cS

co

(1)(1aa)

= io

cS

co

11aa

io

cS

co

aa

iocS

co

(1)a

= o

cS

co

(1)a

. . . (e)

Substituting (d) and (e) into (b):

I = 2R2

io

cS

co

1a

expzFE

R T

io

cS

co

(1)a

exp(1 )zFE

R T

Therefore,

I = 2R2io

cS

co

1a

expzFE

R T

cS

co

(1)a

exp(1 )zFE

R T

. . . ( 1 )

372


Given dR

dt A1F1 A2F2

. . . (9-131)

where F1 1 A3RdR

dt

A4

. . . (9-132)

F2 1 A3RdR

dt

A5

. . . (9-133)

and, defining f zFE R T ,

A1 vMio

zF exp

zFE

R T

vMio

zF exp f . . . (9-134)

A2 A1 expzFE

R T

vMio

zF exp f exp f

vMio

zFexp (1 ) f

. . . (9-135)

A3 zF

2

vMt 1 a R T . . . (9-136)

A4 1a . . . (9-137)

A5 1 a . . . (9-138)

Using the binomial expansions for x2 < 1 that 1 x n

1 nx and 1 x n 1 nx

Eqn (9-132) becomes F1 1A3RdR

dt

A4

1 A4A3RdR

dt . . . (a)

373

and (9-133) becomes F2 1A3RdR

dt

A5

1A5A3RdR

dt . . . (b)

Putting (a) and (b) into (9-131):

dR

dt= A1 1 A 4A3R

dR

dt

A2 1A5A3R

dR

dt

= A1 A2 A3 A1A4 A2A5 RdR

dt

Rearranging: dR

dt1 A3 A1A4 A2A5 R A1 A2

from which dR A3 A1A4 A2A5 RdR A1 A2 dt . . . (c)

Integrating (c) with the boundary condition that R = 0 at t = 0,

R A3 A1A4 A2A5

2R

2 A1 A2 t . . . (d)

Eqn (d) rearranges into the quadratic equation

A3 A1A4 A2A5

2R

2R A2 A1 t 0

i.e., R2

2

A3 A1A4 A2A5

R

2 A2 A1 tA3 A1A4 A2A5

0 . . . (e)

Solving the quadratic:

374

R =

2

A3 A1A 4 A 2A 5

2

A3 A1A 4 A2A5

2

4 1 2 A 2 A1 t

A3 A1A4 A2A5

2

= 1

A3 A1A4 A 2A5

2

2 A1 A2 t

A3 A1A4 A2A5

–

1

A3 A1A 4 A 2A 5

= 1

A3 A1A4 A2A5 2

2 A1 A2 A3 A1A4 A2A5 t

A3 A1A4 A2A5 2

1

A3 A1A4 A2A5

= 1

A3 A1A4 A2A5 1 2 A1 A2 A3 A1A4 A2A5 t

1

A3 A1A4 A2A5

= 1

A3 A1A4 A2A5 1 2 A1 A2 A3 A1A4 A2A5 t 1 . . . (f)

Substituting Eqns (9-1344), (9-135), (9-136), (9-137) and (9-138) into (f) gives

R = 1

zF 2

vMt 1 a R T

vMio

zF exp f 1 a

vMio

zF exp (1 ) f (1 )a

1 2vMio

zF exp f

vMio

zF exp (1) f

zF 2

vMt 1 a R T

vMio

zF exp f 1a

vMio

zFexp (1) f

vMio

zF exp (1) f t

1

= R Tt 1 a

zF io 1a exp f 1 a exp (1) f

375

1 2vMio

zF

exp f exp (1) f

zF 2

vMt 1 a R T

vMio

zF

exp f 1a exp (1) f exp (1) f t

1/ 2

1

= R Tt 1 a


1 2vMio

zF

2

zF 21

vMt 1 a R T

exp f exp (1) f exp f 1a exp (1) f exp (1) f t

1/2

1

= R Tt 1 a


12vMio

2

t 1 a R T

exp f exp (1 ) f exp f 1 a exp (1 ) f exp (1 ) f t

1 / 2

1

= R Tt 1 a


12vMio

2exp f exp (1 ) f exp f 1 a exp (1 ) f exp (1 ) f t

t 1 a R T

1 / 2

1

. . . (g)

Defining A expf exp(1) f . . . (h)

376

and B 1a expf 1 aexp(1) f . . . (i)

and substituting these into (g) gives

R R Tt 1 a

zFio B1

2vMio

2 AB t

R Tt 1 a

1/ 2

1

. . . Eqn (9-139)

377

Derivation of Eqn (9-150) in Effect of Supporting Electrolyte

Eqn (9-66): I 2zF

vM

R2 dR

dt . . . (1)

Eqn (9-147): R R Tt

iozFP1

2vMio2(1 a)Qt

R Tt+

1/2

1

. . . (2)

Re-write (1) as I C1R2 dR

dt . . . (3)

where C1 2zF

vM

. . . (4)

Re-write (2) as R C2 1 2C3t 1 /21 . . . (5)

where C2 R Tt

iozFP . . . (6)

C3 vMio

2(1 a)Q

R Tt+ . . . (7)

Differentiating (5): dR

dt C2

12 1 2C3t 1/2

2C3 C2C3

1 2C3t 1/ 2 . . . (8)

Substituting (5) and (8) into (3):

I = C1 C2 1 2C3t 1/ 21

1 / 2

2C2C3

1 2C3t 1/ 2

= C1C23C3

1 2C3t 2 1 2C3t 1/ 2

1

1 2C3t 1/ 2

= C1C23C3

2 2C3t 2 1 2C3t 1/ 2

1 2C3t 1/ 2

= C1C23C3

2 1C3t 2 1 2C3t 1/ 2

1 2C3t 1/ 2

= 2C1C23C3

1C3t 1 2C3t 1/ 2

1 2C3t 1/ 2

378

= 2C1C23C3

1C3t 1 2C3t

1/2 1 2C3t

1/2

1 2C3t 1/2

= 2C1C23C3

1C3t 1 2C3t

1/2 1

. . . (9)

Finally, putting (4), (6), and (7) into (9):

I = 22zF

vM

R Tt

iozFP

3v

Mio2(1 a)Q

R Tt+

1v

Mio2 (1 a)Q t

R Tt+

12v

Mio2(1 a)Q t

R Tt+

1 /2 1

= 4 R Tt

2(1 a)Q

io zF 2P3

1 vMio2(1 a)Q t R Tt

1 2vMio2(1 a)Q t R Tt

1 / 2 1

. . . Eqn (9-150)

379


We start with Eqn (9-162): 1 A 3 A 1 A 4 A 2 A 3 R dR

dt A 1 A 2 0 . . . (9-162)

where A 1 vMio 1

zFexp f . . . (a)

in which f =zFE

R T

A 2 vMio

zFexp f exp (1 ) f . . . (b)

A 3 = vMDco 1 1 1

. . . (c)

A 4 = 1a . . . (d)

A 5 a 1 . . . (e)

which, upon integration yields the quadratic equation

i.e., R2

2

A 3 A 1 A 4 A 2 A 5

R

2 A 2 A 1 tA 3 A 1 A 4 A 2 A 5

0

the solution of which is

R = 1

A 3 A 1 A 4 A 2 A 5 1 2 A 1 A 2 A 3 A 1 A 4 A 2 A 5 t 1 . . . (f)

[For clarity, below we shall drop the ―primes‖ from the A terms.]

We evaluate the two main terms in (f) in two parts, starting with the term

1

A3 A1A 4 A 2A 5 . . . (g)

Substituting (a), (b), (c), (d), and (e) into (g):

1

A3 A1A 4 A 2A 5

=

1

vMDco 1 a 1 1 vMio 1

zF exp f 1 a

vMio

zFexp f exp (1 ) f a 1

380

= vMDco 1 a 1

vMio

zF1 exp f 1 a exp f exp (1 ) f a 1

= DcozF 1 a 1

io 1a 1 exp f a 1 exp f a 1 exp (1) f

= DcozF 1 a 1

1a 1 a 1 io exp f a 1 io exp (1) f . . . (h)

Expand the term in front of the first exponential:

1a 1 a 1 = 1aa aa

= 1a 1a . . . (i)

Substituting (i) into (h):

= DcozF 1 a 1

1 a 1a io exp f a 1 io exp (1) f

= DcozF 1 a 1

1 a 1a io exp f a 1 io exp f exp f

= DcozF 1 a 1

io exp f 1 a 1a a 1 exp f

= DcozF 1

io exp f 1 a 1 a a 1 exp f

1 a

= DcozF 1

io exp f 1a

1 a

a 1

1 a exp f

. . . (j)

Defining B 1a

1 a

a 1

1 a exp f . . . (k)

Eqn (j) becomes 1

A3 A1A4 A2A5

DcozF

io exp f 1

B . . . (l)

Next evaluate the square root term in (f) by substituting (a), (b), and (m) into this term:

381

12 A1 A2 A3 A1A4 A2A5 t

= 1 2 A1 A2 DcozF

io exp f 1

B

1

t

= 1 2 A1 A2 io exp f DcozF

B

1

t

= 1 2vMio 1

zF exp f

vMio

zFexp f exp (1 ) f

io exp f

DcozF B

1

t

= 12vMio

2

Dco zF 2 1 exp f exp f exp (1) f exp f

B

1

t

= 12vMio

2

Dco zF 2 1 exp 2f exp (1) f exp f

B

1

t

= 12vMio

2

Dco zF 2 exp 2f exp (1 2) f

B

1

t . . . (m)

If we define A exp 2f exp (12) f

then (m) becomes

12 A1 A2 A3 A1A4 A2A5 t = 12vMio

2A

Dco zF 2

B

1

t . . . (n)

Finally, substituting (l) and (n) into (f) gives

R = 1

A3 A1A4 A2A5 1 2 A1 A2 A3 A1A4 A2A5 t 1

= DcozF

io exp f 1

B1

2io2vM A

Dco zF 2

B

1t

1/2

1

. . . Eqn (163)

382

APPENDIX E

Vickers Microhardness Test

The description for the Vickers microhardness test presented below is taken from ASTM

E384-05a: Standard Test Method for Microindentation Hardness of Materials.7

The Vickers hardness test involves indenting the test material with a square-based

pyramidal-shaped diamond indenter with face angles of 136° subject to a force (i.e.; load)

of 1 to 1000 gf ( 9.8 х 10-3

to 9.8 N), as shown in Figure E1. The full load is usually

applied for a period of 10 to 15 seconds and the indentation diagonals are measured with

a light microscope after load removal. It is important to assume that the indentation does

not undergo elastic recovery after force removal. For optimum accuracy of measurement,

the test samples must be free of oil, grease and foriegn objects and the test must be

performed on a flat specimen. Accordingly, all the speciemen were rinsed with acetone

and deionized water and further cleaned in an ultrasonic bath filled with deionized water

for atleast 20 minutes prior to testing. The following procedure was employed to test all

the speciemen:

Examine the indenter and replace if it is worn, dulled, chipped or cracked. Clean

the indeter, if necessary,

Turn on the illumination system and power and select the appropriate indenter,

Place the test sample inside the stage clamps and make sure that the specimen

surface is perpendicular to the indenter axis,

Turn on the microscope and select a low power objective so that the specimen

surface can clearly be observed,

Adjust the light intensity and apertures for optimum resolution and contrast,

Select the appropriate area for the microhardness determination, and change the

microscope setting to the highest magnification available,

7 Manual Book of ASTM Standards (2006), Section Three: Metals Test Methods and Analytical

Procedures, Vol. 03.01, Metals-Mechanical Testing; Elevated and Low-Temperature Tests: Metallography,

pp. 444-476.

383

Figure E1 Vickers indenter

Select the desired force (micro force ranges: 10 g – 1.0 kg) and activate the tester

so that the indenter is automatically lowered, making contact with the specimen

for the selected time period,

Remove the load and change to measuring mode, selecting the appropriate

objective lens, light intensity, and apertures to attain optimum resolution and

contrast,

136 ° between

opposite faces

Force

d1

d2

384

Inspect the indentation to ensure its occurrence at the desired spot, if one half of

either diagonal is more than 5% longer than the other half of that diagonal, or if

the four corners of the indentation are not in sharp focus, the test must be

repeated. Consult manufacturer’s manual for proper alignment procedure,

Determine the lengths of both diagonals of the Vickers indentation to within 0.1

µm and then average the two diagonal length measurements,

Compute the Vickers hardness number using the following equations, noting that

test loads are in grams-force and indentation diagonals are in micrometers,

HV = 1.000 х 103 х P/As = 2.000 х 10

3 х P sin(α/2)/d

2

OR

HV = 1854.4 х P/d2

where: P = force, gf,

As = surface area of the indentation, µm2,

d = mean diagonal length of the indentation, µm, and,

α = face angle of the indenter, 136° 0'

The Vickers hardness reported with units of kgf mm-2

is determined as follows:

HV = 1.8544 х P1/d12

where: P1 = force, kgf,

d1 = length of long diagonal, mm.

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