Modeling Impedance Response of SOFC Cathodes Prepared by Infiltration

A 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 whereperovskite-coated, YSZ fins extend from the electrolyte. Two rate-limiting cases are considered: oxygen ion diffusion through theperovskite film or reactive adsorption of O2 at the perovskite surface. Adsorption is treated as a reaction between gas-phase O2 andoxygen vacancies, using equilibrium data. With the exception of the sticking probability, all parameters in the model are experimentallydetermined. Resistances and capacitances are calculated for LSF-YSZ and there is good agreement with experimentalvalues at 973 K, assuming adsorption is rate limiting, with a sticking probability between 10-3 and 10-4 on vacancy sites. Accordingto the model, perovskite ionic conductivity does not limit performance so long as it is above ~10-7 S/cm. However, the structureof the YSZ scaffold, the ionic conductivity of the scaffold, and the slope of the perovskite redox isotherm significantly impactelectrode impedance. Finally, it is shown that characteristic frequencies of the electrode cannot be used to distinguish when diffusionor adsorption is rate-limiting.