Forming O–O bonds

The ability of transition metal oxides/oxyhydroxides to perform electrocatalytic water splitting is of tremendous interest, particularly for their proclivity to cycle multiple formal oxidation states. The cycling of five oxidation states is central to the oxygen evolution half-reaction (OER) in the absence of peroxide-based mechanisms. The first-row transition metals are of great interest due to their abundance and low toxicity. Many investigators have proposed attack on a metal-oxo motif by water or hydroxide. An alternative mechanism is MO+MO coupling. Here, a thermodynamic model is presented, which explains the trends in first-row transition metal OER electrocatalysts that can form O–O bonds. Based on these cycles, which are supported by experimental thermodynamic data rather than computations, one can predict that vanadium and chromium alone are poor catalysts; titanium may operate by an oxyl-coupling mechanism under irradiation; low-valent manganese, cobalt, iron, and nickel are not likely candidates for attack-based mechanisms; metallic copper is a good candidate; and high-valent iron species supported by nickel could support oxo-oxo coupling chemistry. This model can quantitatively describe experimental measurements on mixed-metal oxides and provide predictions of competent catalysts moving forward.