Observation of a Potential-Dependent Switch of Water Oxidation Mechanism on Co-Oxide-Based Catalysts

O-O bond formation is a key elementary step of the water oxidation reaction. However, it is still unclear how the mechanism of O-O coupling depends on the applied electrode potential. Herein, using water-in-salt electrolytes, we systematically altered the water activity, which enabled us to probe the O-O bond forming mechanism on heterogeneous Co-based catalysts as a function of applied potential. We discovered that the water oxidation mechanism is sensitive to the applied potential: At relatively low driving force, the reaction proceeds through an intramolecular oxygen coupling mechanism, whereas the water nucleophilic attack mechanism prevails at high driving force. The observed mechanistic switch has major implications for the understanding and control of the water oxidation reaction on heterogeneous catalysts.