Electrocatalytic water oxidation with manganese phosphates

As inspired by the Mn₄CaO₅ oxygen evolution center in nature, Mn-based electrocatalysts have received overwhelming attention for water oxidation. However, the understanding of the detailed reaction mechanism has been a long-standing problem. Herein, homologous KMnPO₄ and KMnPO₄•H₂O with 4-coordinated and 6-coordinated Mn centers, respectively, are prepared. The two catalysts constitute an ideal platform to study the structure-performance correlation. The presence of Mn(III), Mn(IV), and Mn(V) intermediate species are identified during water oxidation. The Mn(V)=O species is demonstrated to be the substance for O-O bond formation. In KMnPO₄•H₂O, the Mn coordination structure did not change significantly during water oxidation. In KMnPO₄, the Mn coordination structure changed from 4-coordinated [MnO₄] to 5-coordinated [MnO₅] motif, which displays a triangular biconical configuration. The structure flexibility of [MnO₅] is thermodynamically favored in retaining Mn(III)-OH and generating Mn(V)=O. The Mn(V)=O species is at equilibrium with Mn(IV)=O, the concentration of which determines the intrinsic activity of water oxidation. This study provides a clear picture of water oxidation mechanism on Mn-based systems.