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

This article investigates the role and mechanism of cobalt oxide (CoOx) as a co-catalyst loaded on bismuth vanadate (BiVO4) photoanodes to enhance the oxygen evolution reaction (OER) kinetics in photocatalytic water splitting for green hydrogen production. Using electrochemical impedance spectroscopy and simultaneous transient measurements of photoinduced absorption and photocurrent, the study reveals that CoOx actively participates in OER by stabilizing reaction intermediates—likely superoxide species—in a surface state formed due to oxygen vacancies created by the reduction of V5+ in BiVO4. The OER proceeds via a three-electron oxidation step with a rate-determining water adsorption–oxygen release process, and at higher light intensities, a new reaction pathway involving dual active sites in an amorphous CoOx(OH)y layer emerges, improving charge transfer and suppressing recombination. These findings demonstrate that CoOx functions as a genuine catalyst rather than merely passivating the surface, providing insights for designing more efficient photocatalytic systems for water splitting.