ET Seminar

Cathode Materials forIntermediate OperatingTemperature SOFCs.

Srikanth GopalanDepartment of Manufacturing Engineering

Boston UniversityMay 30, 2003

ET Seminar

State of the Art- SOFCOperating Conditionsn Temperature: 900-1000°C

n Fuel: Reformed fossil fueln Fuel Utilization: 80-90%n Power Density: 0.2-2

W/cm2

Cost Targetn $3000-$1000/KW

Material Systemn Electrolyte: Dense Stabilized

Zirconia (5-50mm); YSZ

n Anode: Ni-ZrO2 Cermet (>30volume % Ni, 20-40%porous and 100mm-2mm)

n Cathode: Sr doped LaMnO3(20-40% Porous and 100mm-2mm); LSM

n Interconnect: Dense A/B sitedoped LaCrO3 (30mm-2mm)or Fe-Cr-Ni-Co alloys

ET Seminar

Limitations of the state of theart SOFC

High operating temperature and high current densityn Lack of chemical stability of individual components and

interdiffusion between cell componentsn Densification of porous electrodesn Delamination of cell components due to thermal expansion

mis-match and thermal cyclingn Expensive interconnect and manifolding materialHeat-up timen Several hoursCostn Order of magnitude more than the existing stationary power

generation systems

ET Seminar

Materials requirements for SOFCcomponents

n Cathode – High electrical conductivity in oxidizing atmosphere;stability in oxidizing atmosphere; electrocatalyst for oxygenreduction charge-transfer reaction

n Anode – High electrical conductivity in reducing atmosphere;stability in reducing atmosphere; electrocatalyst for fueloxidation charge-transfer reaction

n Electrolyte – High oxygen ion conductivity in oxidizing andreducing atmosphere; stability in oxidizing and reducingatmospheres (over up to 21 decades in pO2)

n Interconnect – High electrical conductivity in oxidizing andreducing atmosphere; stability in over 21 decades of pO2

ET Seminar

Microstructural requirements

n Cathode and anode –graded porosity

n Electrolyte andinterconnection -dense

ET Seminar

Advantages of lower operatingtemperaturen Greater lifetime of cell (decreases materials

interactions between electrode andelectrolyte)

n Cheaper manifolding materials

n Less time for startup of SOFC generatorgenerator

n Target : 600-750oC operating temperature

ET Seminar

Analyzing the performancecharacteristics of a single cell

n The overall cellvoltage can bedescribed by

aciREV hh ---= 0

˜˜¯

ˆÁÁË

Ê=

)(

)(0

2

2ln4 cO

aO

p

p

F

RTE Nernst potential

iR Ohmic resistance

ac hh , Cathode and anode polarization (concentration polarizationand charge-transfer polarization)

ET Seminar

Decreasing operatingtemperature: challengesn Ionic and electronic conduction processes in the

electrolyte and cathode are thermally activated

n Decreasing temperature leads to an exponentialdecrease in ionic conductivity of electrolyte andelectronic conductivity of cathode and an increase intotal cell resistance

n The most important source of performance loss inthe cell at lower temperatures is charge-transferpolarization

ET Seminar

What is charge-transferpolarization?

n Voltage loss associated with the kineticbarrier to reactions such as:

1/2O2 (g) + 2e- (Cathode) = O2- (Electrolyte)

H2 (g) + O2- (Electrolyte) = H2O (g) + 2e- (Anode)

ET Seminar

Research strategies for lowertemperature SOFCs

n Minimize ohmic resistance of cell, i.e. makecomponents thinner

n Higher conductivity cathode and electrolytematerial

n Engineer electrode microstructure to havefine grain size and porosity to increasenumber of sites for charge-transfer reactions

ET Seminar

SOFC materials research atBoston University

Outline

ß Introduction and research goalsß Description of experimental techniqueß Results and discussionß Summary and future work

ET Seminar

SOFC materials research atBU: Some preliminaries

Funding source: University coal researchprogram (UCR), Siemens WestinghousePower Corporation

Overall research goal: To develop SOFCsthat operate in the temperature regimeof 600-750oC

ET Seminar

Materials system for lower operatingtemperature SOFCs: Electrolyte

n Electrolyte: LSGM (La1-

xSrxGa1-yMgyO3-_). LSGMis a perovskite withexcellent oxygen ionicconductivity at lowertemperatures.

• La0.85Sr0.15Ga0.8Mg0.2O2.825_ La0.8Sr0.2Ga0.83Mg0.17O2.815

1- Bi0.75Y0.25O1.5 2 - Ce0.8Gd0.2O1.9 3 - Zr0.91Y0.09O1.955

M. Feng, J.B.Goodenough, K.Huang and C.Milliken, “Fuel Cells with Doped LanthanumGallate Electrolyte”, J.Power.Sources, vol. 63, 47-51 (1996)

ET Seminar

Materials system for lower operatingtemperature SOFCs: Cathoden State-of-the-art cathode

material: LSM (La1-xSrxMnO3)

n LSM is a perovskite p-typesemiconductor that exhibitssmall polaron hoppingconduction in the temperaturerange of 600-1200oC

n At high pO2’s (~10-4 – 1 atm)the Sr dopant is chargecompensated by holes, i.e. inKroger-Vink notation

n Hopping of holes from one Mnsite to another is themechanism for hole conduction

XOMnLa

LaMnO OMnSrSrMnO 333 ++æææ Ææ •'

ET Seminar

Materials system for lower operatingtemperature SOFCs: Cathode

n LSM is an excellent cathode material forhigher operating temperature SOFCs (800-1000oC)

n However it is a poor oxygen ion conductor;thus the cathodic charge transfer reaction isrestricted to the TPBs at the geometricalcathode-electrolyte interface

ET Seminar

n Site of charge-transfer reaction at thecathode electrolyte interface

Materials system for lower operatingtemperature SOFCs: Cathode

1/2O2(g) + 2e- (LSM) = O2- (YSZ)

Cathode: LSM

Electrolyte: YSZ

ET Seminar

Materials system for lower operatingtemperature SOFCs: Electrical measurementswith LSGM electrolyte and Pt cathode

n With decreasingtemperature the charge-transfer resistanceincreases more rapidlythan the electrolyteresistance

n Activation energy forRsystem is ~ 85 kJ/mol andfor Rct ~ 160 kJ/mol

n Activation energy forionic conduction in LSGMis ~ 82 kJ/mol (Huangand Goodenough)

1000/T (K-1)

.90 .95 1.00 1.05 1.10 1.15 1.20

Ln(T/R) (K/ohm.cm

2 )

2

3

4

5

6

7Rsystem (DC measurements)RSystem (AC measurements)RCharge transfer (AC measurements)

ET Seminar

Oxygen surface exchange coefficients forselected oxides (Kilner and Steele)

ET Seminar

Strategies for decreasing charge-transferresistance of cathode-electrolyte interface

n Find cathode materials with intrinsicallyhigh charge-transfer kinetics

n Design cathode microstructures withabundance of sites for charge-transferreactions

ET Seminar

Cathode materials systemsunder study

n LSM (La0.9Sr0.1MnO3)n LSM-LSGM compositen LSCF (La0.6Sr0.4Co0.8Fe0.2O3 )n LSCF – LSGM compositen Pt

ET Seminar

Electrochemicalcharacterization technique

n AC complex impedance spectroscopyon symmetrical cells , e.g.

1) air,LSM/LSGM/LSM,air2) air, Pt/LSGM/Pt,air

ET Seminar

AC complex impedancespectroscopyn Developed by J.E. Bauerle in the 60’s at

Westinghouse

n A small amplitude (10-50 mV) AC voltage is imposedon the sample and the response (current) ismeasured as a function of frequency

n A plot of the real part of the measured impedanceversus the imaginary part reveals details of the ohmicand the polarization resistances.

ET Seminar

Measurement Setup of Symmetrical Cell

Air

Air

alumina tube

current collector (platinum mesh)

electrode

electrolyte (LSGM)

electrode

Potentiostat/Galvanostat

&

Frequency Response Analyzer

ET Seminar

AC complex impedancespectroscopy

Rs Rp

Cd

ßRs – total area specific ohmic resistance of the cell (Rel + Rc +Ra)

ßRp – polarization resistance

ßCd – double layer capacitance; related to adsorption of gaseous speciesduring charge-transfer processes

ET Seminar

AC complex impedancespectroscopy

Rs Rp

Cd

st

pst

dp

pdt

dp

pst

dct

ctst

RZ

RRZ

CR

RCZ

CR

RRZ

jCRjR

RZ

=

+=

+=

++=

++=

•Æ

Æ

)][Re(lim

)][Re(lim

)Im(

)Re(

w

w

w

w

w

w

0

222

2

222

1

1

ET Seminar

Typical impedance plot (frequency: 65535Hz~0.001Hz; amplitude: 10mv)

-ImZ

RealZRs Rs+Rp

0

3.75 4.00 4.25 4.50

-0.25

0

0.25

0.50

Z'(ohm)

Z''(ohm)

Serialresistance

Totalresistance

Polarizationresistance(Rp)

Inductancetrail

Rs Rs+Rp

3.75 4.00 4.25 4.50

-0.25

0

0.25

0.50

Z'(ohm)

Z''(ohm)

3.75 4.00 4.25 4.50

-0.25

0

0.25

0.50

Z'(ohm)

Z''(ohm)

ET Seminar1000/T (K-1)

0.90 0.95 1.00 1.05 1.10 1.15 1.20

Ln(T/R

A) (K

/ohm.cm

2 )

-2

0

2

4

6

8

10

Arrhenius Plot of PolarizationResistance

LSCF -- 15µm

LSCF -- 10 µm

Pt --- 5 µm

LSM+LSGM -- 30 µm

LSM -- 30 µm

LSCF – 30 µm

At 750oC, Rp for Pt is 30.9 Ohm.cm2

and for 30 _m LSCF electrode is 0.13 Ohm.cm2

ET Seminar

Two different approaches toan effective cathode

n LSM-LSGM composite cathode

n LSCF mixed conducting electrode

LSMElectrocatalystLSGM

Electrolyte

Three-Phase Boundary (site of chargetransfer reactions)

Com

positeInterlayer

LSGMElectrolyte

LSCF mixed conductingelectrocatalyst

LSGMElectrolyte

ET Seminar

Electrode thickness effect

n Increasing thickness of LSCF electrodedecreases polarization resistance

n This is a geometrical effect which isattributed to increase in the number ofsites available for charge-transfer withincreasing electrode thickness

ET Seminar

Prior work on thickness effect incomposite cathode(Virkar et al. US Patent 5,343,239)

ET Seminar

Prior work on thickness effectin composite cathode(Virkar et al. US Patent 5,343,239)

ET Seminar

AC Impedance Spectra of LSCF/LSGM/LSCFSymmetrical Cell at 1073K: Effect of thickness

Z' (ohm.cm2)

Z''(o

hm

.cm2

)

-0.3

-0.2

-0.1

0.0

0.1

0.2

0.3

5.2 5.3 5.4 5.5 5.6 5.7 5.8 5.9

10µm thickness

15µmthickness

Higher thickness

ET Seminar

Polarization resistance versusthickness for LSCF/LSGM/LSCF cell at1073 K

0

0.1

0.2

0.3

0.4

0.5

0 10 20 30 40 50 60 70

Rp

(W.cm2 )

t (mm)

ET Seminar

Typical cathode microstructures

LSCF LSM

LSM/LSGMComposite

ET Seminar

Summary

n Of the cathode materials investigated,LSCF has the lowest effective charge-transfer resistance in the desiredoperating temperature range of 650-800oC

ET Seminar

Ongoing work

n Thickness optimization of LSCF cathode

n Study of LSCF + LSGM compositeelectrodes

ET Seminar

Other ongoing work funded byUCR grant: Anode development

n It is known that LSGM reacts with Ni to forminsulating lanthanum nickelate phase

n Our target anode is Ni-(Gd2O3-CeO2) with abuffer layer of (Gd2O3-CeO2) to preventlanthanum nickelate phase formation

ET Seminar

n Our target geometry is anode supported LSGMelectrolyte cell

Other ongoing work funded by UCRgrant: Process development

2 m

m20

¶Ãm

50¶Ãm

Ni- (Gd2O3-CeO2)cermet

(Gd2O3-CeO2)

LSGM

LSCF

ET Seminar

Ongoing work

n Anode developmentn Cell process development

ET Seminar

Acknowledgments

n DOE-NETL and Siemens Westinghouse forfinancial support

n Graduate students: Wenquan Gong and CuiHengdong

n Post-doc: Dr. Chris Manning

n Collaborator: Prof. Uday B. Pal