Participation of the unstable lattice oxygen of cation-exchanged δ-MnO2 in the water oxidation reaction

Abstract

Among 3d transition metal oxides, Mn oxides function as electrocatalysts for the oxygen evolution reaction (OER) in alkaline media. At the OER potential, some Mn oxides are oxidized to a layered structure birnessite-type δ-MnO₂, so strategies to improve the OER activity of δ-MnO₂ and elucidate its reaction mechanism have been suggested. Here, we demonstrated for the first time that δ-MnO₂ can follow the lattice oxygen participation mechanism (LOM) rather than the conventional adsorbate evolution mechanism and the contribution of LOM can improve the OER activities of δ-MnO₂. Consistent with the electrochemical evidence for LOM, we found the isotopic shift of Mn–O and active oxygen in an ¹⁸O-labeling experiment by in situ Raman analysis. The oxygen vacancies introduced by the intercalated cation between MnO₆ layers thermodynamically activate the lattice oxygen of δ-MnO₂ and absorb OH⁻ ions in the alkaline electrolyte, enabling the unstable lattice oxygen to participate in the OER as the reactant.