Understanding the reaction mechanism and kinetics of photocatalytic oxygen evolution on CoO_x-loaded bismuth vanadate

Photocatalytic water splitting for green hydrogen production is hindered by the sluggish kinetics of oxygen evolution reaction (OER). Loading a co-catalyst is essential for accelerating the kinetics, but the detailed reaction mechanism and role of the co-catalyst are still obscure. Here, we focus on cobalt oxide (CoO_x) loaded on bismuth vanadate (BiVO₄) to investigate the impact of CoO_x on the OER mechanism. We employ photoelectrochemical impedance spectroscopy and simultaneous measurements of photoinduced absorption and photocurrent. The reduction of V⁵⁺ in BiVO₄ promotes the formation of a surface state on CoO_x that plays a crucial role in the OER. The third-order reaction rate with respect to photohole charge density indicates that reaction intermediate species accumulate in the surface state through a three-electron oxidation process prior to the rate-determining step. Increasing the excitation light intensity onto the CoO_x-loaded anode improves the photoconversion efficiency significantly, suggesting that the OER reaction at dual sites in an amorphous CoO_x(OH)_y layer dominates over single sites. Therefore, CoO_x is directly involved in the OER by providing effective reaction sites, stabilizing reaction intermediates, and improving the charge transfer rate. These insights help advance our understanding of co-catalyst-assisted OER to achieve efficient water splitting.