Electrocatalytic water oxidation with manganese phosphates.

As inspired by the MnCaO 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 KMnPO and KMnPO•HO 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 KMnPO•HO, the Mn coordination structure did not change significantly during water oxidation. In KMnPO, the Mn coordination structure changed from 4-coordinated [MnO] to 5-coordinated [MnO] motif, which displays a triangular biconical configuration. The structure flexibility of [MnO] 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.