Küngas, Rainer
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Publication Modeling Impedance Response of SOFC Cathodes Prepared by Infiltration(2011-03-21) Bidrawn, Fred; Küngas, Rainer; Vohs, John M; Gorte, Raymond JA mathematical model has been developed to understand the performance of electrodes prepared by infiltration of La0.8Sr0.2FeO3 (LSF) and La0.8Sr0.2MnO3 (LSM) into yttria-stabilized zirconia (YSZ). The model calculates the resistances for the case where perovskite-coated, YSZ fins extend from the electrolyte. Two rate-limiting cases are considered: oxygen ion diffusion through the perovskite film or reactive adsorption of O2 at the perovskite surface. Adsorption is treated as a reaction between gas-phase O2 and oxygen vacancies, using equilibrium data. With the exception of the sticking probability, all parameters in the model are experimentally determined. Resistances and capacitances are calculated for LSF-YSZ and there is good agreement with experimental values at 973 K, assuming adsorption is rate limiting, with a sticking probability between 10-3 and 10-4 on vacancy sites. According to the model, perovskite ionic conductivity does not limit performance so long as it is above ~10-7 S/cm. However, the structure of the YSZ scaffold, the ionic conductivity of the scaffold, and the slope of the perovskite redox isotherm significantly impact electrode impedance. Finally, it is shown that characteristic frequencies of the electrode cannot be used to distinguish when diffusion or adsorption is rate-limiting.Publication Doped-Ceria Diffusion Barriers Prepared by Infiltration for Solid Oxide Fuel Cells(2010-05-24) Küngas, Rainer; Bidrawn, Fred; Vohs, John M; Gorte, Raymond JTo stabilize solid oxide fuel cells cathodes prepared by infiltration of La0.8Sr0.2CoO3 (LSCo) into porous yttria-stabilized zirconia (YSZ), a coating of Sm-doped ceria (SDC) was first deposited onto the YSZ scaffold. The dense SDC coating was prepared by infiltration with aqueous solutions of SM(NO3)3 and Ce(NO3)3, followed by calcination to 1473 K. The SDC coating prevented ~ 20 mΩ cm2, at 973 K, with acceptable degradation after heating to 1373 K.Publication Modeling Impedance Response of SOFC Cathodes Prepared by Infiltration(2011-03-21) Bidrawn, Fred; Küngas, Rainer; Vohs, John M; Gorte, Raymond JA mathematical model has been developed to understand the performance of electrodes prepared by infiltration of La0.8Sr0.2FeO3 (LSF) and La0.8Sr0.2MnO3 (LSM) into yttria-stabilized zirconia (YSZ). The model calculates the resistances for the case where perovskite-coated, YSZ fins extend from the electrolyte. Two rate-limiting cases are considered: oxygen ion diffusion through the perovskite film or reactive adsorption of O2 at the perovskite surface. Adsorption is treated as a reaction between gas-phase O2 and oxygen vacancies, using equilibrium data. With the exception of the sticking probability, all parameters in the model are experimentally determined. Resistances and capacitances are calculated for LSF-YSZ and there is good agreement with experimental values at 973 K, assuming adsorption is rate limiting, with a sticking probability between 10-3 and 10-4 on vacancy sites. According to the model, perovskite ionic conductivity does not limit performance so long as it is above ~10-7 S/cm. However, the structure of the YSZ scaffold, the ionic conductivity of the scaffold, and the slope of the perovskite redox isotherm significantly impact electrode impedance. Finally, it is shown that characteristic frequencies of the electrode cannot be used to distinguish when diffusion or adsorption is rate-limiting.Publication Systematic Studies of the Cathode-Electrolyte Interface in SOFC Cathodes Prepared by Infiltration(2011-04-01) Küngas, Rainer; Vohs, John M; Gorte, Raymond JIn this study, the effect of the morphology and ionic conductivity of the electrolyte material in SOFC composite cathodes is systematically studied. The specific surface area of prous yttria-stabilized zirconia (YSZ) scaffolds was varied by almost two orders of magnitude using different pore formers and surface treatment with hydrofluoric acid (HF). The effect of ionic conductivity on the performance of SOFC cathodes was studied for electrodes prepared by infiltration of 35 wt % LSF into 65% porous scandia-stabilized zirconia (ScSZ), YSZ, or yttria-alumina co-stabilized zirconia (YAZ) scaffolds of identical microstructure cathodes.Publication Effect of the Ionic Conductivity of the Electrolyte in Composite SOFC Cathodes(2011-04-22) Küngas, Rainer; Vohs, John M; Gorte, Raymond JSolid oxide fuel cell (SOFC) cathodes were prepared by infiltration of 35 wt % La0.8Sr0.2FeO3 (LSF) into porous scaffolds of three, zirconia-based electrolytes in order to determine the effect of the ionic conductivity of the electrolyte material on cathode impedances. The electrolyte scaffolds were 10 mol % Sc2O3-stabilized zirconia (ScSZ), 8 mol % Y2O3-stabilized zirconia (YSZ), and 3 mol % Y2O3- 20 mol % Al2O3-doped zirconia (YAZ), prepared by tape casting with graphite pore formers. Each electrolyte scaffold was 65% porous, with identical pore structures as determined by scanning electron microscopy (SEM). Both symmetric cells and fuel cells were prepared and tested between 873 and 1073 K, using LSF composites that had been calcined to 1123 or 1373 K. Literature values for the electrolyte conductivities were confirmed using the ohmic losses from the impedance spectra. The electrode impedances decreased with increasing electrolyte conductivity, with the dependence being between to the power of 0.5 and 1.0, depending on the operating temperature and LSF calcination temperature.