Verification and applicability of symmetric cell configuration for mechanistic study of oxygen electrode reactions of solid oxide cells

Abstract

Symmetric cell configuration is commonly used for the study of reaction mechanism and kinetics of oxygen reduction reaction (ORR) and/or oxygen evolution reaction (OER) of solid oxide cells (SOCs) to avoid the issues associated with the possible inaccuracy and errors of the reference electrode in three-electrode cell configuration. The electrode impedance behavior and reaction parameters such as electrode polarization resistance are taken as half of the measured overall value of the cell, based on the assumption that the oxygen reaction measured on the symmetric electrodes is completely reversible. However, the applicability of symmetric cell configuration for such mechanistic studies of oxygen reaction has never been verified with respect to the electrode materials. In this paper, the reversibility of symmetric cell configuration is investigated on model electrodes of platinum (Pt), La0.8Sr0.2MnO3 (LSM) and La0.6Sr0.4Co0.2Fe0.8O3 (LSCF) using a three-electrode configuration. The results indicate the reversibility of ORR at working electrode and OER at counter electrode depends strongly on the reversibility of the electrode/electrolyte interface for the oxygen incorporation/exsolution process, which in turn is closely related to the oxygen ionic conductivity of the electrode and in less extent to the ionic conductivity of the electrolyte. Therefore, LSCF electrode shows a much high degree of reversibility as compared to LSM, while for the reaction on Pt electrode, the oxygen reaction is essentially not reversible. This study clearly indicates that the applicability of the symmetric cell configuration for the mechanistic study of electrode reactions in SOCs is closely related to the reversibility in the molecular level (the individual reaction steps) as well as the interface level (the regions where the individual reaction steps occur).