Carnegie Mellon Salvador Research Group, h2p://neon.materials.cmu.edu/salvador/ Oxygen Exchange in Thin Layers of SOFC Cathode 1 Paul Salvador Department of Materials Science and Engineering Carnegie Mellon University Pi:sburgh, PA 15206 Lu Yan Philip Tsang K. R. Balasubramaniam Hui Du Shanling Wang Robin Chao Lam Helmick Sarthak Havelia Oleg Maksimov Joanna Meador s 200 nm Funded by DOE SECA, Thanks to L. Wilson, W. Surdoval, B. White, P. Burke, Shailesh Vora Wednesday, July 27, 11
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Carnegie MellonSalvador Research Group, h2p://neon.materials.cmu.edu/salvador/
Oxygen Exchange in Thin Layers of SOFC Cathode
1
Paul Salvador
Department of Materials Science and EngineeringCarnegie Mellon UniversityPi:sburgh, PA 15206
Lu Yan Philip Tsang K. R. Balasubramaniam Hui DuShanling Wang Robin Chao Lam Helmick
Sarthak Havelia Oleg Maksimov Joanna Meador
EISA vs SBA-15 HT
Hard Template EISA
Sintered particles vs.
Open pores
1um 1um
200 nm 200 nm
Funded by DOE -‐ SECA, Thanks to L. Wilson, W. Surdoval, B. White, P. Burke, Shailesh Vora
• Microstructure and oxygen exchange- Orientation of relaxed layers- Dislocations and strain- Extended boundaries in polycrystalline layers- Free surfaces
Carnegie MellonSalvador Research Group, h2p://neon.materials.cmu.edu/salvador/ 3
Infiltra>on: Surface Ac>ve nanopar>cles for SOFCs
What are the opQmal materials to use for infiltraQon?
What are the surface proper.es of cathode materials?
What are the mechanisms of enhanced performance / degradaQon?
What are the proper.es of nanosized par.cles?
What are the effects of the support on the proper.es of cathodes?
REVIE
W
www.advmat.de
cathode impedance decreased from greater than 3V ! cm2 to lessthan 0.5V ! cm2 upon application of a cathodic potential. Theresults with LCF–YSZ electrodes also rule out the possibility thatSr migration to the electrolyte interface might be responsible fordeactivation,[104] since no Sr was present in this electrode.
If the deactivation model we have presented is correct, thereare important implications. It is usually assumed that deactiva-tion is associated with solid-state reactions. Approaches used toavoid deactivation therefore generally involve inserting a barrierlayer to prevent contact between the perovskite and the electrolytelayers. Deactivation by solid-state reaction with the YSZelectrolyte undoubtedly occurs for some materials (e.g., LSCoand YSZ) and the use of barrier layers does help stabilize theperformance. However, the strategy needs to be quite different forstabilizing electrode performance when deactivation is due tosintering of the perovskite. One could attempt to prevent theformation of a dense layer (barrier layers may help achieve this ifthe interfacial energies between the perovskite and the barriermaterial prevent film formation) or to increase ionic conductivitythrough the perovskite layer. In either case, the approach tocathode development would be quite different from thosedesigned to avoid solid-state reactions.
3.3. LSCF–YSZ Electrodes
Sr-doped LaFe0.8Co0.2O3 (LSCF) can be considered a subset ofLSF. LSCF has better electronic conductivity than LSF and asimilar ionic conductivity.[104–106] It exhibits very good initialperformance as an SOFC cathode when a doped-ceria layer isused to separate it from the YSZ.[107,108] It is also used in thecurrent collector of some fuel-cell designs.[109] Very recently, workhas appeared in which an LSCF cathode was formed byinfiltration into porous YSZ.[110] Chen et al. have reported thatan infiltrated LSCF–YSZ electrode had an impedance as low as0.047V ! cm2 at 800 8C, comparable to the performance ofinfiltrated LSCo–YSZ and LSF–YSZ electrodes.[27,45] Unfortu-nately, the stability of infiltrated LSCF–YSZ electrodes was notreported.
Given the instability of LSF–YSZ and LSCo–YSZ electrodes, itseems likely that there will be stability issues with LSCF as well.The major issue is whether or not the Co in the LSCF will remainin the perovskite phase, so that La2Zr2O7 formation will beprevented. A study of La0.8Sr0.2Mn0.8Co0.2O3 (LSCM) infiltrationinto porous YSZ showed reasonable initial performance;however, the LSCM–YSZ electrodes were found to exhibitstability similar to that of LSCo–YSZ electrodes. Performancedeclined dramatically, even at 700 8C.[97] Therefore, we suggest itis likely that the LSCF electrodes will also undergo solid-statereactions with YSZ. Clearly, the stability of the infiltrated LSCFelectrodes needs to be checked given the intriguing performancethat was achieved.
3.4. LSM–YSZ Electrodes
Some of the key properties of LSM have already been discussedearlier in this review and have been reviewed extensivelyelsewhere.[33] Under typical cathode conditions, LSM has an
electronic conductivity greater than 200 S cm"1 but has negligibleionic conductivity. It undergoes a solid-state reaction with YSZ toform La2Zr2O7 above 1 250 8C,[66] but LSM–YSZ mixtures arestable at lower temperatures. Because LSM–YSZ composites arethe standard material for cells with YSZ electrolytes, there is avery large body of work characterizing the performance of theseelectrodes.
Much of the effort in improving LSM–YSZ electrodes hascentered on developing an optimal composite structure. Forexample, Virkar and coworkers have developed mathematicalmodels to determine what that ideal structure should be.[55] Asdiscussed earlier and shown in Figure 3, it is common practice forfuel-cell developers to engineer LSM–YSZ electrodes so that theregion near the electrolyte, the functional layer, has a differentmicrostructure from that of the region farther from theelectrolyte, the current-collection layer. Micrographs of thefunctional and conduction layers show that the functional layertends to have smaller pores and less overall porosity. This reflectsthe need to enhance the concentration of TPB sites. Highporosities are not required for high gas-phase transport rates inthin layers. Infiltration procedures are ideal for preparing thefunctional layer, but are not really needed for the current-collection layer.
The performance of electrodes formed by infiltration of LSM isvery good and comparable to that of conventional LSM–YSZcomposites. For example, Armstrong and Virkar preparedinfiltrated LSM electrodes using nitrate-salt solutions andachieved power densities as high as 1.2W cm"2 for a celloperating in hydrogen at 800 8C.[78]
As with conventional LSM–YSZ composites, structure appearsto be critical. Sholklapper et al. reported a power density of0.3W cm"2 at a temperature of only 650 8C for a cell prepared byinfiltrating a porous YSZ scaffold with LSM nanoparticles.[81] Thestructure of these electrodes is shown in the SEMmicrographs inFigure 10 and demonstrates that the YSZ scaffold is coated withsmall LSM particles. The picture in Figure 10 is similar to that of
Figure 10. Cross-sectional SEM image showing the microstructure of aLSM–YSZ cathode that was fabricated by infiltrating a porous YSZ scaffoldwith LSM nanoparticles. Reproduced with permission from reference [81].Copyright 2006 The Electrochemical Society.
implies a change in the surface area of the LSF phase. If electrodeperformance is limited by the rate of surface oxidation and thesurface area decreases, the performance would be expected todecline proportionally. A smaller active surface area could alsoexplain the decrease in impedance with increasing currentdensity. In the Butler–Volmer picture of electrode reactions, ratesare expected to accelerate as one moves farther from equilibrium.However, a plot of the cathode impedance did not show theexpected logarithmic dependence between current and over-potential that would be expected for Butler–Volmer kinetics.[85]
Furthermore, while we cannot totally rule out this model, it seemsunlikely that the slope of the V–i relationship would remainconstant in going from anodic to cathodic polarization if thesurface kinetics were limiting.
An alternative picture for understanding deactivation of theLSF–YSZ electrodes is shown schematically in Figure 8. Based onthe SEM results, the LSF deposits calcined at 850 8C exist as smallparticles on the YSZ scaffold, separated by gaps that allow gas-
phase oxygen to diffuse to the YSZ interface. This is depicted inFigure 8a, which is similar to the diagram in Figure 2b. Whenthese particles sinter, either over time at operating conditions orfollowing high-temperature calcination, the LSF forms a dense,polycrystalline layer over the YSZ scaffold, as depicted inFigure 8b. Because the ionic conductivity of LSF is much lowerthan that of YSZ (8! 10"4 S cm"1 for LSF and 1.89! 10"2
S cm"1 for YSZ at 700 8C[20,101]), the transport of oxygen ionsthrough the LSF could be limiting. The coupling of oxygen-iontransport up the YSZ ‘‘fingers’’ and through the LSF film wouldthen be responsible for the current-dependent impedances.
Data for cathode performance with infiltrated Ca- and Ba-doped LaFeO3 (LCF and LBF)[101] provide additional support forthe picture in Figure 8. LCF and LBF have nearly the sameelectronic conductivity as that of LSF, but their ionic conductiv-ities are significantly lower. LCF, in particular, has an ionicconductivity that is 50 times lower than that of LSF at 700 8C.Following calcination at 850 8C, the initial performance of LCF–YSZ and LBF–YSZ electrodes was indistinguishable from thatobserved with LSF–YSZ electrodes. Assuming the morphology ofthe electrodes is similar to that shown in Figure 8a, it isreasonable that the ionic conductivity would not be critical in thiscase.
Following calcination at 1 100 8C, SEM showed that LSF, LCF,and LBF each tended to form a dense film over the YSZ, similar tothat pictured schematically in Figure 8b. Cathode performancefor each of the three composite cathodes also decreaseddramatically. Figure 9 shows impedance data at open circuitand at 100mA cm"2 for three cells made with identical anodesand electrolytes, but with infiltrated cathodes based on LSF, LBF,or LCF.[101] The impedance at open circuit was much larger forthe electrodes based on LCFand LBF, as would be expected for themodel in Figure 8, given the significantly lower ionicconductivities of the LCF and LBF. Similar to the findings withLSF, the impedance of LCF–YSZ and LBF–YSZ cathodesexhibited a strong current dependence. With LCF–YSZ, the
Figure 7. SEM images of a) the porous YSZ scaffold, b) LSF film on theYSZ scaffold after calcination at 850 8C, c) LSF film after testing for 1000 has 700 8C, and d) LSF film after calcination at 1100 8C. Reproduced withpermission from reference [85]. Copyright 2007 The ElectrochemicalSociety.
Figure 8. Schematic diagram of infiltrated LSF–YSZ cathode a) aftercalcining in air at 850 8C and b) after calcining at higher temperatures(>1000 8C) or long term aging.
0.0
1.0
2.0
3.0
4.0
-1.0 0.0 1.0 2.0 3.0 4.0Z Re / cm-2
-Z Im
/ c
m-2
Figure 9. Impedance spectra obtained from fuel cells with infiltratedelectrodes as a function of the active component, LSF (&), LBF (#),and LCF (~), used in the cathode. The composite cathodes in each ofthese cells were calcined at 1100 8C. The filled symbols show datameasured at open circuit while the open symbols were obtained at acurrent density of 100mA cm"2. Reproduced with permission fromreference [101]. Copyright 2008 The Electrochemical Society.
Carnegie MellonSalvador Research Group, h2p://neon.materials.cmu.edu/salvador/ 4
Probe the nature of atomic scale surface chemistry or interface crystallographyrather than the device scale micro-structural perturbations in SOFC conditions:
T = 500 - 900 °C, PO2 ≈ 10-5 - 1 atm, Overpotential ≈ 0 - 0.4 V
Conceptual Thin Film Sample: Proxy to Crystals
Need High Quality Samples with Controlled Microstructural Features:
Single Crystals or Thin Films
Mixed Conductor
Surface
Substrate
Mass Uptake / Release
Transient Conductivity
Surface Electronic StatesSurface Structure
Surface Stoichiometry
Theory / Modeling /Simulation
Reaction Pathways
Surface Chemical Species
• thickness < D / k
• surface sensitive : k
• characterization of film quality / microstructural states
Carnegie MellonSalvador Research Group, h2p://neon.materials.cmu.edu/salvador/
Temperature dependence of two mechanisms
17
-15
-14
-13
-12
-11
-10
-9
-8
0.8 0.9 1 1.1 1.2
ln K
ch
em
/ c
ms-1
1000/T (K-1)
lnKred-2
lnKoxi-2
lnKred-1
lnKoxi-1
Ea,1=1.57 eV
Ea,2=0.87 eV
ln k
chem
/ cm
s-1
lnkoxi,1
lnkoxi,2
lnkred,1
lnkred,2
I
(a) kgrain kgb kgb
(b)
1. la O’ et al, J. Electrochem sec. (2009). 2. De Souza et al, Mater. lett. (2000). 3. Yan et al, Solid state ionics. (2011). 4. Kan et al, Solid state ionics. (2010). 5. Yasuda et al, J Solid State Chem. (1996).
Our data agree with literature:
Kchem is on the order of 10-5cm/s.1-5
Ea1: 1.48 eV for microelectrode1, 1.32 eV dense pellet2,
Ea2: 0.8 eV (100) and (111) on STO3, 0.07-0.8 eV powder4.
Carnegie MellonSalvador Research Group, h2p://neon.materials.cmu.edu/salvador/
Summary
21
Two apparent processes occuring on the surface (Ea) for Kchem .
These were interpreted as belonging to: (1) the native surface response of individual grains/variants and (2) the variants boundaries / grain boundaries of the textured films.
The first (native surface) process : EA,1 ≈ 1.5 eV, the second (extended defect) process EA,2 ≈ 0.75 eV. The Kchem,2 values are almost 3 orders of magnitude higher than the Kchem,1 values at low temperatures (< 700°C)
Depends on the density of the defects.
At higher temperatures, the data can be fit with one Kchem Intermediate value of EA indicate that both processes contribute to overall exchangeThe native surface, higher activation energy process is competitive with
The native surface processes are Strain dependent Orientation Dependent Substrate dependent