Overpotential-Induced Introduction of Oxygen Vacancy in La_0.67Sr_0.33MnO₃ Surface and Its Impact on Oxygen Reduction Reaction Catalytic Activity in Alkaline Solution

Oxygen reduction reaction (ORR) catalytic activity of La 0.67 Sr 0.33 MnO 3 epitaxial thin films was investigated in a KOH solution by using a rotating-disk electrode. We found that while the films exhibit ORR current, the current is not limited by oxygen transport resulting from the film electrode rotation and shows the large hysteresis against the potential sweep direction. This behavior is in stark contrast to the oxygen reduction reaction activity of an electrode ink made from LSMO bulk powder, whose ORR current is oxygen-transport limited. In situ synchrotron X-ray absorption spectroscopy also reveals that the valence state of Mn in the LSMO film surface is lowered under the reducing atmosphere caused by the overpotential. This indicates the overpotential-induced introduction of oxygen vacancies in the film surface. We also show that the ORR current of the LSMO films exposed to the reducing atmosphere is lowered than that of the original surface. These results indicate that the ORR catalytic activity of LSMO surfaces is strongly influenced by oxygen vacancies.