Remarkable influence of the local symmetry of substituted 3d metal ion on bifunctional electrocatalyst performance of α-MnO2 nanowire

Efficient bifunctional electrocatalysts of V- and Co-substituted α-MnO2 materials can be synthesized by one-pot hydrothermal reaction of ion-adducts V3+/Co2+–MnO4−, leading to the successful substitution of both V and Co ions in the α-MnO2 lattice. According to extended X-ray absorption fine structure analysis, the substituted V ion is stabilized in highly distorted local symmetry whereas the substituted Co ion has regular octahedral local environment. Both the V- and Co-substituted α-MnO2 materials exhibit excellent bifunctional electrocatalytic functionalities with lowered overpotentials and enhanced current densities for oxygen evolution (OER) and oxygen reduction reactions (ORR), underscoring the beneficial role of cation substitution in enhancing the bifunctional electrocatalytic activity of α-MnO2. The Co-substituted α-MnO2 shows higher bifunctional oxygen electrocatalyst functionality than does the V-substituted homologue, which is attributable to the greater improvement of charge transfer kinetics upon the Co substitution than the V one. The poorer charge transfer property of V-substituted α-MnO2 can be ascribed to highly distorted local symmetry of V ion, which is unfavorable for increasing band width and depressing bandgap energy. Taking into account many advantages of the oxides of V, Mn, and Co elements such as low price, rich abundance, and environmentally benignity, the present study underscores that the partial substitution of neighboring 3d metal ion having regular octahedral symmetry can provide an effective and environmental friendly way of optimizing the oxygen electrocatalytic activity of manganese oxide.