O–O bond formation mechanisms during the oxygen evolution reaction over synthetic molecular catalysts

Water oxidation is one of the most important reactions in natural and artificial energy conversion schemes. In nature, solar energy is converted to chemical energy via water oxidation at the oxygen-evolving center of photosystem II to generate dioxygen, protons, and electrons. In artificial energy schemes, water oxidation is one of the half reactions of water splitting, which is an appealing strategy for energy conversion via photocatalytic, electrocatalytic, or photoelectrocatalytic processes. Because it is thermodynamically unfavorable and kinetically slow, water oxidation is the bottleneck for achieving large-scale water splitting. Thus, developing highly efficient water oxidation catalysts has attracted the interests of researchers in the past decades. The formation of O–O bonds is typically the rate-determining step of the water oxidation catalytic cycle. Therefore, better understanding this key step is critical for the rational design of more efficient catalysts. This review focuses on elucidating the evolution of metal-oxygen species during transition metal-catalyzed water oxidation, and more importantly, on discussing the feasible O–O bond formation mechanisms during the oxygen evolution reaction over synthetic molecular catalysts.