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 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.