Electrocatalytic water oxidation with manganese phosphates.

Area of Science:

• Inorganic Chemistry
• Electrocatalysis
• Water Oxidation Mechanisms

Background:

• Manganese-based electrocatalysts, inspired by nature's Mn₄CaO₅ center, are crucial for water oxidation.
• A detailed understanding of the reaction mechanism for these catalysts remains a significant challenge.

Purpose of the Study:

• To investigate the structure-performance correlation in homologous KMnPO₄ and KMnPO₄•H₂O catalysts.
• To elucidate the water oxidation mechanism on manganese-based systems, identifying key intermediates and reaction pathways.

Main Methods:

• Preparation of homologous KMnPO₄ (4-coordinated Mn) and KMnPO₄•H₂O (6-coordinated Mn) catalysts.
• Identification of intermediate species (Mn(III), Mn(IV), Mn(V)) during water oxidation.
• Analysis of changes in Mn coordination structure during the catalytic process.

Main Results:

• The Mn(V)=O species was identified as the key intermediate for O-O bond formation.
• KMnPO₄•H₂O maintained its Mn coordination structure during water oxidation.
• KMnPO₄ exhibited a flexible coordination structure, changing from [MnO₄] to a triangular biconical [MnO₅] motif, favoring Mn(III)-OH retention and Mn(V)=O generation.
• The equilibrium between Mn(V)=O and Mn(IV)=O, and the concentration of Mn(V)=O, dictates the intrinsic water oxidation activity.

Conclusions:

• The study provides a clear mechanistic picture of water oxidation on manganese-based electrocatalysts.
• Structural flexibility in the [MnO₅] motif of KMnPO₄ is crucial for efficient water oxidation.
• The Mn(V)=O species is the active intermediate for O-O bond formation in these systems.