Photoinduced Absorption Spectroscopy of CoPi on BiVO4: The Function of CoPi during Water Oxidation

This paper employs photoinduced absorption and electrochemical techniques to analyze the charge carrier dynamics that drive photoelectrochemical water oxidation on bismuth vanadate (BiVO₄), both with and without cobalt phosphate (CoPi) co‐catalyst. These results are correlated with spectroelectrochemical measurements of Co^II oxidation to Co^III in a CoPi/FTO (fluorine doped tin oxide) electrode during dark electrocatalytic water oxidation. Electrocatalytic water oxidation exhibits a non‐linear dependence on Co^III density, with a sharp onset at 1 × 10¹⁷ Co^III cm^?2. These results are compared quantitatively with the degree of CoPi oxidation observed under conditions of photoinduced water oxidation on CoPi–BiVO₄ photoanodes. For the CoPi–BiVO₄ photoanodes studied herein, ≤5% of water oxidation proceeds from CoPi sites, making the BiVO₄ surface the predominant water oxidation site. This study highlights two key factors that limit the ability of CoPi to improve the catalytic performance of BiVO₄: 1) the kinetics of hole transfer from the BiVO₄ to the CoPi layer are too slow to effectively compete with direct water oxidation from BiVO₄; 2) the slow water oxidation kinetics of CoPi result in a large accumulation of Co^III states, causing an increase in recombination. Addressing these factors will be essential for improving the performance of CoPi on photoanodes for solar‐driven water oxidation.