Effect of cation vacancies in nonstoichiometric (LaMn)1-O3 on oxygen reduction reaction catalysis

Perovskite oxides (ABO3) containing Mn have received considerable attention because of their relatively high catalytic activity for the oxygen reduction reaction (ORR) and low preparation cost. In the ORR, both the adsorption of O2 and the desorption of OH on the surface significantly affect the efficiency of catalysis, which can be changed by the MnO bond length. In this study, the effect of the MnO bond length on the ORR catalytic activity in nonstoichiometric (LaMn)1-xO3 was investigated by changing the number of cation vacancies via heat treatment in the air to further enhance the catalytic activity. A volcanic relationship was found between the intrinsic catalytic activity for the ORR and MnO bond length. A low cation vacancy (x) results in a low valence state of Mn, leading to a long bond length. A long bond length with a weak Mn-O interaction promotes OH desorption, whereas a short bond length with a strong Mn-O interaction enhances O2 adsorption. Therefore, an appropriate MnO bond length exists to optimize ORR catalytic activity. This finding provides an important basis for the design of Mn-based ORR catalysts, as the same argument can be made regardless of the structure.