Highly active epitaxial La_(1-x)Sr_xMnO₃ surfaces for the oxygen reduction reaction: Role of charge transfer

Most studies of oxide catalysts for the oxygen reduction reaction (ORR) use oxide powder, where the heterogeneity of exposed surfaces and the composite nature of electrodes limit fundamental understanding of the reaction mechanism. We present the ORR activity of epitaxially oriented La_(1-x)Sr_xMnO₃ surfaces and investigate, by varying Sr substitution, the relationship between the role of charge transfer and catalytic activity in an alkaline environment. The activity is greatest for La_(1-x)Sr_xMnO₃ with 33% Sr, containing mixed Mn^3+/4+, and the (110) and (111) orientations display comparable activities to that of the (001). Electrochemical measurements using the facile redox couple [Fe(CN)₆]^3-/4- illustrate that increasing ORR activity trends with faster charge-transfer kinetics, indicating the importance of facile charge transfer at the oxide/water interface and mixed Mn valence in promoting ORR kinetics.