Electrochemically Identified Ultrathin Water-Oxidation Catalyst in Neutral pH Solution Containing Ni²⁺ and Its Combination with Photoelectrode

Water oxidation electrocatalyzed by Ni²⁺ under neutral conditions was investigated using various electrochemical analyses. The addition of Ni²⁺ in a phosphate-buffered solution catalyzed the oxidation of water, as confirmed by the detection of oxygen generation via scanning electrochemical microscopy. A combination of cyclic voltammetry, coulometric titration, and electrochemical quartz microbalance measurements identified the catalysis as heterogeneous and the catalyst as a Ni-based ultrathin (<4 nm) layer (“Ni-Pi”). Analysis of the potential- and pH-dependency of the titrated amount of charge revealed that the catalyst was deposited only under anodic polarization conditions and was removed under unpolarized conditions; the catalyst may be Ni­(III) oxide, and its formation and oxidation appeared to be chemically irreversible. The diffusion-limited nature of water oxidation catalyzed by Ni²⁺ was closely related to the phosphate ions involved in the catalyst formation and the accompanying catalysis. Although the catalytic performance of Ni²⁺ alone was not remarkable, it exhibited a synergetic effect with BiVO₄ for photoelectrochemical water oxidation, which can compete with Co-Pi-decorated BiVO<sub>4