Physicochemical analysis of infiltrated cathode symmetric cells for intermediate temperature SOFCs

This study investigated the cathode parameter changes before and after infiltration based on Jamnik and Maier's MIEC transmission line model. To consider additional factors such as surface adsorption capacitance, chemical capacitance Cchem, and general capacitance effect CHN were introduced in parallel. To obtain the appropriate temperature dependence, oxygen partial pressure dependence, and current density dependence trends of transmission line parameters, the trend of Cchem was fixed according to each measurement condition using the thermodynamic parameters of previous papers. Infiltrating Sm0.5Sr0.5CoO3-delta (SSC), a better catalyst material, into the widely used La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF6428) perovskite electrode resulted in a decrease in surface reaction resistance Rp by about 1 order of magnitude and a decrease in oxygen ion diffusion resistance Rs. The oxygen ion adsorption capacitance CHN0 on the surface is larger after infiltration due to the larger surface area of the infiltrated electrode, which can be explained by the two arcs clearly separated in the impedance spectra of the infiltrated cell. Additionally, after infiltration, the diffusion characteristics depending on oxygen partial pressure changed relatively from bulk diffusion to surface diffusion. The value of Rs is larger than Rp under almost all conditions, indicating that the overall rate-determining step (rds) is the diffusion reaction in the current direction. Furthermore, to observe the changes in surface reaction characteristics only, the characteristics of Rp before and after infiltration were analyzed based on the Butler-Volmer-type equation and oxygen partial pressure dependence of the exchange current density of surface reactions. As a result, the rds of surface reactions changed from the reduction reaction of the intermediate stage of electrode reactions after infiltration to the dissociative adsorption reaction of the initial reaction stage.