Manganese pyrophosphate with multiple coordinated water molecules for electrocatalytic water oxidation

Abstract

The coordinated water in the Mn₄CaO₅ clusters in natural water oxidation center is believed to play an important role in promoting water oxidation. However, its specific role is unclear. In this work, based on a new manganese phosphate (Mn₂P₂O₇·3H₂O) with well-defined crystal surfaces (crystalline MnPi) and its amorphous counterpart (amorphous MnPi), the effects of coordinated water molecules on water oxidation have been systematically investigated. There are four coordinated water molecules on one Mn site, which is very rare and valuable to study relevant effects from water coordination. Unusually, the crystalline MnPi outperformed the amorphous MnPi in electrocatalysis. The exposed well-defined surface of the crystalline MnPi contains continuous Mn sites with multiple coordinated water molecules. The kinetics and thermodynamics of surface oxidation have been quantitatively studied based on the appealing catalyst platform. The interaction between adjacent Mn sites leads to a 3H⁺/2e dual site oxidation in crystalline MnPi, while this process is 2H⁺/1e single site conversion in amorphous MnPi. The higher level of charge neutralization of oxygen atoms from continuous H-bond network in crystalline MnPi is helpful for the Mn^II/III oxidation, which subsequently promotes water oxidation. This study provides valuable insight into the role of coordinated water molecules in initiating water oxidation in Mn-based catalytic systems.