cathode materials for itsofc
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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
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