Oxygen Reduction Activity and Interfacial Structures of La_0.8Sr_0.2CoO₃ at Initial Electrochemical Process in an Alkaline Solution

Abstract

Oxygen reduction and evolution reactions (ORR and OER, respectively) of perovskite-type La_0.8Sr_0.2CoO₃ were characterized using two-dimensional model electrodes with different reaction planes (001), (110), and (111). Synthesized by pulsed laser deposition, these thin (30 nm) and flat (roughness < 1 nm) electrodes can reveal the reaction plane dependence of the ORR activity. From steady-state polarization measurements in KOH (aq.), the ORR activity was the highest on the (001) film during the first ORR/OER cycle, and it decreased significantly during the second cycle. In-situ synchrotron X-ray diffraction clarified crystal structure changes in the bulk and surface regions of La_0.8Sr_0.2CoO₃, and these changes are associated with forming oxygen defects during the initial electrochemical process. Furthermore, the La_0.8Sr_0.2CoO₃ surface partially decomposed upon reacting with the aqueous solution, as clarified by hard X-ray photoemission spectroscopy. Therefore, the interfacial structures formed in the electrochemical reaction field is important for enhancing ORR and OER activities.