Equivalent Circuit Model Analysis of Microstructure-Controlled LSM/ScSZ Composite Cathodes by Powder Slurry Impregnation Method

Abstract

Improved electrochemical performance of strontium-doped lanthanum manganite (LSM)/scandia-stabilized zirconia (ScSZ) composite cathodes for solid oxide fuel cells was accomplished by using the powder slurry impregnation (PSI) method where a prefabricated thin porous ScSZ electrolyte layer is impregnated with LSM/ScSZ powder mixture in slurry. The PSI method enabled control of the electrolyte surface roughness, resulting in a decreased electrode polarization resistance despite a tendency of the ohmic resistance to increase due to the added porous ScSZ electrolyte layer. Resistance separation by electrochemical impedance spectroscopy with an equivalent circuit model (ECM) revealed that the proposed composite cathodes exhibited a significant reduction in the oxygen reduction reaction (ORR) resistance at the triple-phase boundary (TPB). Optimization of the electrolyte surface structure is therefore critical so as not to offset the improvement in the ORR resistance. Furthermore, besides the ECM used to separate the resistances, two ECMs that represent the microstructure of actual composite electrodes were evaluated to clarify the electrode structural factors determining the measured impedance spectra. The analysis revealed that impedance response is influenced by the area ratio of the double-phase boundary to the TPB, although cathode performance is mainly determined by the TPB area and the ionic conduction resistance within composite electrodes.