William A. Rigdon

Diana Larrabee

Xinyu Huang, Ph.D.

Resilient Oxidation Catalysts for Electrochemical Hydrogen Pump

Final Presentation May 21, 2013

Electrochemical Hydrogen Pump

2

Pump serves to separate and compress hydrogen. Process is performed by applying power across the electrochemical cell. No moving parts in this design and this method provides the most efficient way to compress hydrogen.

Project Goals • Problem: Hydrogen oxidation electrocatalysts are used

in anode of hydrogen pump and fuel cell. They are subject to poisoning from impurities like carbon monoxide [CO] and durability concerns that arise from cleaning up CO.

• Challenge: Develop supports which can improve the activity and durability of electrocatalysts for H2 pump.

• Approach: Design a composite support structure which can aid in the improvement of both desired properties. Demonstrate performance improvements through working membrane electrode assemblies (MEA). Study the material behavior and elucidate the benefits.

3

Electrocatalyst Degradation

The corrosion mechanisms are all related, but it can be understood by four

simple schematics of the contribution to the detachment, dissolution, diffusion,

and re-deposition of Pt catalysts resulting in particle growth and loss of activity

4 Y. Shao-Horn, W. C. Sheng, S. Chen, P. J. Ferreira, E. F. Holby, D. Morgan. Instability of Supported Platinum Nanoparticles in Low-Temperature Fuel Cells. Topics in Catalysis. 46 (3-4), 285-305 (2007).

Project Approach

Prepare composite supports: CNT-Titania

Synthesize Pt electrocatalysts on supports

Characterize material structure/properties

Design and construct MEAs for testing

Test electrochemical performance

Observe carbon corrosion resistance

Report results and publish

5

Carbon Structure

6

F. Hasché, M. Oezaslan, P. Strasser. Activity, stability and degradation of MWCNT supported Pt fuel cell electrocatalysts. Physical Chemistry Chemical Physics. 12, 15251-15258, 2010.

Carbon chemistry and Pt support stability effects

-□- A carbon nanotube (CNT) demonstrates long range order and graphitic bonding with fewer defect sites on the surface

-o- High surface area amorphous carbon black supports have best activity, but have high defect density and poor stability

Titanium Dioxide Support Durability

7

S.-Y. Huang, P. Ganesan, S. Park, B. N. Popov. Development of a Titanium Dioxide-Supported Platinum Catalyst with Ultrahigh Stability for Polymer Electrolyte Membrane Fuel Cell Applications. Journal of the American Chemical Society. 131, 13898-13899, 2009.

A titanium dioxide platinum support was used to generate a performance similar to a commercial carbon black electrode with excellent durability, but required a very high platinum content.

Region of electrode operation

Passivation

Immunity

Corrosion

Metal and Oxide Stability

• Pourbaix Diagram

– Immunity

– Corrosion

– Passivation

8

E. Asselin , T. M. Ahmed , A. Alfantazi. Corrosion of niobium in sulphuric and hydrochloric acid solutions at 75 and 95 °C. Corrosion Science. 49, 694-700, 2007.

M. Pourbaix. Atlias of Electrochemical Equilibria in Aqueous Solutions. 1974.

Mechanistic Effect on Activity of CO Oxidation for Pt-TiOx

9

D. Jiang, S. H. Overbury, and S. Dai. Structures and Energetics of Pt Clusters on TiO2: Interplay between Metal-Metal Bonds and Metal-Oxygen Bonds. J. of Physical Chemistry. 116, 21880-21885, 2012.

S. Bonanni, K. Aït-Mansour, W. Harbich, H. Brune. Effect of the TiO2 Reduction State on the Catalytic CO Oxidation on Deposited Size-Selected Pt Clusters. J. of the American Chemical Society. 134, 3445-3450, 2012.

TiOx−OH + Pt−COad CO2 + Pt + TiO2 + H+ + e-

S. C. Ammal, A. Heyden. Nature of Ptn/TiO2(110) Interface under Water-Gas Shift Reaction Conditions: A Constrained ab Initio Thermodynamics Study. J. of Physical of Chemistry. 115, 19246–19259, 2011.

R. E. Fuentes, B. L. GarcÍa, and J. W. Weidner. Effect of Titanium Dioxide Supports on the Activity of Pt-Ru toward Electrochemical Oxidation of Methanol. Journal of the Electrochemical Society. 158 (5), B461-B466, 2011.

Support Effect on Methanol Electrocatalytic Oxidation

10

Metal Oxides & Defect Chemistry

11

By metal oxide doping of Ti site with Nb,

𝑁𝑏2𝑂5

2 𝑇𝑖𝑂2 2 𝑁𝑏𝑇𝑖

· + 4 𝑂𝑂𝑋 +

1

2 𝑂2 + 2 𝑒−

The equilibrium reaction for oxygen at low pressures is:

𝑂𝑂𝑋 ⇌ 𝑉𝑂

·· + 2 𝑒− +1

2𝑂2

The mass action law follows this expression for the equilibrium constant K for electrons

𝑉𝑂

·· ∗[𝑛]2

[𝑂2]1/2 = 𝐾𝑛 where [O2] = Partial pressure of O2 or P(O2)

At low P(O2), where e- compensates for the oxygen vacancies [n] ≈ 2 𝑉𝑂

··

1

2𝑛 ∗ 𝑛 2 = 𝐾𝑛 ∗ 𝑃(𝑂2)−

1

2 therefore,

𝑛 = (2𝐾𝑛)1

3 ∗ 𝑃(𝑂2)−1

6

TiOx-CNT Support Synthesis

12 N. G. Akalework , C.-J. Pan , W.-N. Su , J. Rick , M.-C. Tsai , J.-F. Lee , J.-M. Lin , L.-D. Tsai and B.-J. Hwang. Journal Materials Chemistry. 22, p. 20977-20985, 2012.

MEA Manufacturing • Novel in our approach for application of electrocatalysts for benefit to

CO oxidation in working electrochemical cells

• Prepared electrocatalyst powders and mixed into inks

• Ultrasonic spray deposition to prepare MEAs

• MEA is greater design challenge than half cell study

• Compared 3 symmetric 10 cm2 electrode designs with 0.3 mgPt/cm2

1. Pt-CNT

2. Pt-TiOx-CNT

3. Pt-TiNbOx-CNT (10 atomic % Nb substituted for Ti)

13

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cu

rre

nt

(A)

Potential (V) -0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cu

rre

nt

(A)

Potential (V)

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

1.0

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cu

rre

nt

(A)

Potential (V)

a) b)

c)

14

Pt-CNT Pt-TiOx-CNT

Pt-TiNbOx-CNT

32.1 m2/gPt

0.683 V max

36.5 m2/gPt

0.646 V max

38.7 m2/gPt

0.631 V max 0.601 V peak 1

Figure 1. Electrodes are first exposed to 100 ppm CO for 60 minutes and then purged with N2 gas. Cyclic voltammetry is performed and 1st scan is compared to 3rd. The onset for CO oxidation is left-shifted more than 50 mV for 10% Nb doped titania supported Pt electrocatalysts.

N. Wagner, E. Gülzow. Change of electrochemical impedance spectra (EIS) with time during CO-poisoning of the Pt-anode in a membrane fuel cell. Journal of Power Sources. 127, 341-347, 2004.

15

Electrochemical Impedance Spectroscopy Shows CO Deactivation of Electrode

16

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

0.000

0.014

0.028

0.042

0.056

0.070

C22

C18

C14

C10

C6

C2

Z real (ohms)

-Z im

ag

ina

ry (

oh

ms)

Tim

e (5

min

ute

inte

rval

s)

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

0.000

0.014

0.028

0.042

0.056

0.070

E2

C18

C14

C10

D6

D2

Z real (ohms)

-Z im

ag

ina

ry (

oh

ms)

Tim

e (5

min

ute

inte

rval

s)

0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16

0.000

0.014

0.028

0.042

0.056

0.070

C22

C18

C14

C10

C6

C2

Z real (ohms)

-Z im

ag

ina

ry (

oh

ms)

Tim

e (5

min

ute

inte

rval

s)

Figure 2. Anodes under open

circuit condition after exposure to

100 ppm CO in H2 gas stream at 50

mL/min at 70 °C measured every 5

minutes up to 1 hour show the

magnitude of catalyst deactivation

(CO poisoning). The Pt-TiNbOx-

CNT shows best tolerance to CO at

these conditions (least deactivation).

Pt- CNT Pt- TiOx-CNT

Pt- TiNbOx-CNT

0.0

2.5

5.0

7.5

10.0

12.5

15.0

0.00 0.05 0.10 0.15 0.20

Cu

rren

t (A

)

Potential (V)

Pt-CNT 0

5

10

15

0.0

2.5

5.0

7.5

10.0

12.5

15.0

0.00 0.05 0.10 0.15 0.20

Cu

rren

t (A

)

Potential (V)

Pt-TiOx-CNT 0

5

10

15

Electrochemical Output from Pump

17

Figure 3. Hydrogen pump polarization at 5 minute intervals under 100 ppm CO in H2 at 50 mL/min, 70 °C, 95% RH. The Pt-TiNbOx-CNT electrocatalyst show the greatest tolerance. An earlier onset for oxidation can be seen at 15 minute scan above 150 mV. 0.0

2.5

5.0

7.5

10.0

12.5

15.0

0.00 0.05 0.10 0.15 0.20

Cu

rren

t (A

)

Potential (V)

Pt-TiNbOx-CNT 0

5

10

15

XRD Spectra of Composite Support and effect of [C:Ti] atomic ratio

0.E+00

1.E-04

2.E-04

3.E-04

4.E-04

5.E-04

6.E-04

7.E-04

8.E-04

9.E-04

0 100 200 300 400

Tita

niu

m M

ole

s A

dd

ed

[Ti:C] Atomic Ratio

Effect of Titanium Isopropoxide added to fixed 0.1 g mass of CNT

Ti moles

Power (Ti moles)

XRD scans show the presence of small anatase crystallites on the carbon nanotube support. A higher titanium loading of 10:1 had a greater resistance and also lacked sufficient electronic contact to function as electrocatalyst as evidenced by the minimal ECSA and lack of i-V performance. A lowered ration of C:Ti [80:1] (5% mass ratio of Ti) was used successfully.

[10:1]

[80:1]

0

10000

20000

30000

40000

50000

60000

70000

80000

10 30 50 70 90

Inte

nsi

ty (

cou

nts

)

2Ѳ

XRD Spectra of TiOx-CNT Catalyst Supports

[80:1]

[10:1]

18

Raman Spectra of Composite Support

0 500 1000 1500 20000

2000

4000

6000

8000

10000

12000

14000

16000

18000

Inte

ns

ity

(a

. u

.)

Raman Shift (cm-1)

Titania-CNT

Oxidized-CNT

W. F. Zhang, Y. L. He, M. S. Zhang, Z Yin, Q. Chen. Raman scattering study on anatase TiO2 nanocrystals. J. Phys. D: Appl. Phys. 33, 912–916 (2000).

Raman data from red laser also shows the confirmation of dual phase support with presence of anatase. The concentration of titania on the surface may have an effect on the material’s band gap, Eg. Later, dopant Nb atoms wer added to effectively reduce the titanium oxidation state and increase its electronic conductivity.

19

0

5000

10000

15000

20000

25000

0 500 1000 1500 2000In

ten

sity

(a.

u.)

Raman Shift (cm-1)

Raman Spectra of CNT:Titania

[80:1]

[10:1]

TiNbOx

20

0

500

1000

1500

2000

2500

3000

0 500 1000 1500 2000

Inte

nsi

ty (

a.u

.)

Raman Shift (cm-1)

O-CNT

TiNbOx-CNTEmergence of peak at 160 cm-1 in 10% Nb doped composite titania supports

Carbon Corrosion Resistance

A method to quickly screen electrocatalyst durability achieved by scanning cell potential and monitoring the evolution of carbon dioxide [CO2

+] ion current by mass spectrometer from sample capillary attached to the exhaust line. Real time concentrations can be correlated with potential dynamic.

L. M. Roen, C. H. Paik, and T. D. Jarvi. Electrocatalytic Corrosion of Carbon Support in PEMFC Cathodes. Electrochemical and Solid-State Letters. 7 (1), A-19-A22, 2004.

21

22

0.5

0.8

1.0

1.3

1.5

1.5E-11

2.5E-11

3.5E-11

4.5E-11

0 50 100 150 200

44

AM

U I

on

Cu

rre

nt

(Am

ps)

Time (Seconds)

Comparison of Carbon Dioxide Evolution from Support

Pt-CNT

Pt-TiOx-CNT

Pt-TiNbOx-CNT

Potential

Cell T = 80 C Humidifier T = 70 C Relative Humidity = 66% Helium flow on cathode @ 50 mL/min Cyclic Voltammetry from 0.5 to 1.5 V at 10 mV/sec

Po

ten

tial (V

olts)

Electron Microscopy

23

2-2.5 2.5-3 3-3.5 3.5-4 4-4.5 4.5-5 5-5.50.00

0.05

0.10

0.15

0.20

0.25

0.30

0.35

0.40

Dis

trib

uti

on

of

Pt

Cry

sta

llit

es

Pt Crtystallite Diameter (nm)

Frequency

Atomic ratio near 1:1 between Ti:Pt in this image from STEM and EDX

[Pt]

[Ti]

[O]

HRTEM of Pt particle distribution on support (above) TEM at USC shows area for improvement and also a single CNT/Pt electrocatalys (below; left and right)

Credit: Haijun Qian and JoAn Hudson at Clemson EMF for HRTEM and STEM images & EDX data

Industry Collaboration: Sustainable Innovations, LLC

24

Template design for MEA construction

Before After

Worked closely with industry partner to prepare a resilient hydrogen oxidation catalyst and delivered MEA for testing. Electrochemical hydrogen pump results will be presented at the 2013 Fuel Cell Seminar & Energy Exposition.

Conclusions Advantageous modification of both activity and

durability of electrocatalyst through design of a composite support structure for platinum

Experimental results measured in working cells show benefits to hydrogen oxidation reaction

Resilient effects in CO tolerance and carbon corrosion resistance can prolong the life of the cell which is critical to reducing material costs

Reduced upper potential required for CO removal

Decreased number of cycles required for cleaning

25

26

Backup Slides

27

Background and Introduction

• Application for H2 Pumps

• Cost of Materials, Platinum

• Cost of Fuel, Pure H2

• High Pressure Delivery, Mechanical v. EC

• Sources of CO and Impurities – Natural Gas, water-gas shift

– Biofuels

• Carbon Monoxide Effect on Pt Catalysis

• CO clean up leads to corrosion!

28

0

1

2

3

4

5

0 100 200 300 400 500 600 700 800 900

Cu

rre

nt

(A)

Time (seconds)

50 mV hold test + CO 100 ppm

Pt-C (TKK)

Pt-TiNbOx-CNT

Pt-TiOx-CNT

Pt-CNT

29

0.0

2.5

5.0

7.5

10.0

12.5

15.0

0.00 0.05 0.10 0.15 0.20

Cu

rren

t (A

)

Potential (V)

Pt-CNT 0

5

10

15

0.0

2.5

5.0

7.5

10.0

12.5

15.0

0.00 0.05 0.10 0.15 0.20

Cu

rren

t (A

)

Potential (V)

Pt-TiOx-CNT 0

5

10

15

Polar

30

0.0

2.5

5.0

7.5

10.0

12.5

15.0

0.00 0.05 0.10 0.15 0.20

Cu

rren

t (A

)

Potential (V)

Pt-TiNbOx-CNT 0

5

10

15

Figure 3. Hydrogen pump polarization at 5 minute intervals show the greater tolerance to 100 ppm CO in the fuel stream Hydrogen Pump Polarization under CO 100 ppm in H2 at 50 mL/min, 70 °C, 95% RH

What’s Remaining? Durability measurements by CO2 evolution

X-ray photoelectron spectroscopy

Electron Microscopy (TEM, STEM, FESEM)

Prepare MEA materials for stack tests by S. I.

Experimental data quantification + present

Submit abstracts to relevant conferences o Electrochemical Society

o Fuel Cell Seminar & Exposition

o American Chemical Society

31

Raman Spectroscopy

32

0

5000

10000

15000

20000

25000

0 200 400 600 800 1000 1200 1400 1600 1800 2000

Inte

nsi

ty (

a.u

.)

Raman Shift (cm-1)

Raman Spectra of Carbon:Titanium Catalyst Supports

[80:1]

[10:1]

TiNbOx

33

0

10000

20000

30000

40000

50000

60000

70000

80000

30 35 40 45 50 55 60 65 70

Inte

nsi

ty (

a.u

.)

2Ѳ

XRD of Pt Composite Electrocatalysts

Pt-CNT

Pt-TiOx

Pt-TiNbOx

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020

Cu

rre

nt

De

ns

ity

(m

A/c

m2)

Cell Potential (V)

Initial

10000

30000

0 200 400 600 800 1000 12000.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Ce

ll P

ote

nti

al (V

)

Current Density (mA/cm2)

Initial

10000

30000

0.0 0.2 0.4 0.6 0.8 1.0 1.2

-0.020

-0.015

-0.010

-0.005

0.000

0.005

0.010

0.015

0.020

Cu

rre

nt

De

ns

ity

(m

A/c

m2)

Cell Potential (V)

Initial

12300

32000

0 200 400 600 800 1000 12000.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Cell P

ote

nti

al (V

)

Current Density (mA/cm2)

Initial

12300

32000

Carbon

34

F. Hasché, M. Oezaslan, P. Strasser. Activity, stability and degradation of MWCNT supported Pt fuel cell electrocatalysts. Physical Chemistry Chemical Physics. 12, 15251-15258, 2010.

Cyclic Voltammetry Polarization Air

Pt-CNT

Carbon chemistry and Pt support stability effects Pt-TiOx-CNT

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cu

rre

nt

(A)

Potential (V)

CV Composite Graph 100 mV/sec

Pt-TiOx-CNT

Pt-TiNbOx-CNT

Pt-CNT

35

Support m2/g Pt (UPD)

Pt-TiOx-CNT 15.84045873

Pt-TiNbOx 13.17344286

Pt-CNT 18.99396032

Pt-C(TKK) 47.47620635

J. Ma, A. Habrioux, N. Guignard, and N. Alonso-Vante. Functionalizing Effect of Increasingly Graphitic Carbon Supports on Carbon-Supported and TiO2−Carbon Composite-Supported Pt Nanoparticles. Journal of Physical Chemistry C. 116, 21788−21794, 2012.

CO Stripping Voltammetry

36

X-ray Photoelectro Spectroscoopy

37

L. R. Baker, A. Hervier, H. Seo, G. Kennedy, K. Komvopoulos, and G. A. Somorjai. Highly n-Type Titanium Oxide as an Electronically Active Support for Platinum in the Catalytic Oxidation of Carbon Monoxide. J. Physical Chemistry C. 115, 16006-16011, 2011.

B. Y. Xia, B. Wang, H. B. Wu, Z. Liu, X. Wang, X. Wen Lou. Sandwich-structured TiO2–Pt–graphene ternary hybrid electrocatalysts with high efficiency and stability. Journal of Materials Chemistry. 22, 16499-16505. 2012

38

-0.05

-0.03

-0.01

0.01

0.03

0.05

0.07

0.40 0.60 0.80 1.00 1.20 1.40 1.60

Cu

rre

nt

(A)

Potential (V)

CVs during Accelerated Testing coupled with Mass Spec

Pt-CNT

Pt-TiOx-CNT

Pt-TiNbOx-CNT

39

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.0 0.5 1.0

Cu

rre

nt

(A)

Potential (V)

Pt-TiNbOx-CNT Before & After

After

Before

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.0 0.2 0.4 0.6 0.8 1.0 1.2

Cu

rre

nt

(A)

Potential (V)

Pt-C (TKK) Before & After ADT

After

Before

-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.0 0.5 1.0

Cu

rre

nt

(A)

Potential (V)

Pt-CNT Before & After ADT

After

Before-0.4

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

0.0 0.5 1.0

Cu

rre

nt

(A)

Potential (V)

Pt-TiOx-CNT Before & After

After

Before

Industry Collaboration: Sustainable Innovations

40