The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis

TLDR

Water oxidation is a key bottleneck in converting light to storable solar fuels, and its mechanisms resemble those of surface oxides, multinuclear molecular catalysts, and the photosynthetic manganese–calcium complex. This review aims to compare and unify perspectives on water oxidation across diverse catalysis disciplines. The review examines electrochemical water splitting by metal oxides and cobalt oxide catalysts, outlines homogeneous catalysis with mononuclear ruthenium complexes, and discusses the natural manganese–calcium complex in photosystem II, focusing on thermodynamic efficiency, potential‑determining steps, redox‑potential leveling, proton removal, and O–O bond formation. It concludes by highlighting common themes and unifying concepts among these approaches.

Abstract

Abstract Striving for new solar fuels, the water oxidation reaction currently is considered to be a bottleneck, hampering progress in the development of applicable technologies for the conversion of light into storable fuels. This review compares and unifies viewpoints on water oxidation from various fields of catalysis research. The first part deals with the thermodynamic efficiency and mechanisms of electrochemical water splitting by metal oxides on electrode surfaces, explaining the recent concept of the potential‐determining step. Subsequently, novel cobalt oxide‐based catalysts for heterogeneous (electro)catalysis are discussed. These may share structural and functional properties with surface oxides, multinuclear molecular catalysts and the catalytic manganese–calcium complex of photosynthetic water oxidation. Recent developments in homogeneous water‐oxidation catalysis are outlined with a focus on the discovery of mononuclear ruthenium (and non‐ruthenium) complexes that efficiently mediate O 2 evolution from water. Water oxidation in photosynthesis is the subject of a concise presentation of structure and function of the natural paragon—the manganese–calcium complex in photosystem II—for which ideas concerning redox‐potential leveling, proton removal, and OO bond formation mechanisms are discussed. The last part highlights common themes and unifying concepts.