Linking faradaic efficiency for oxygen evolution to water oxidation reaction order on hematite photoanodes

Water oxidation on metal-oxide (photo)anodes is typically assumed to yield molecular oxygen. Here, we combine operando mass-spectroscopy oxygen detection with spectroelectrochemical rate-law analyses to directly link oxygen evolution selectivity to surface holes density on hematite photoanodes. At low current and hole densities, where first-order kinetics dominate, only a small fraction of the extracted charge produces molecular oxygen, indicating that isolated single-hole water oxidation is unable to drive oxygen evolution. As the surface-hole population increases, the Faradaic efficiency for oxygen evolution rises sharply and approaches a plateau coinciding with the emergence of cooperative third-order kinetics. Quantitative data analysis indicates that oxygen evolution exclusively follows the multihole pathway. Equivalent kinetic and Faradaic efficiency behaviour is also observed for purely electrochemical water oxidation on the same hematite samples, demonstrating that cooperative multihole chemistry is the intrinsic mechanism governing the oxygen evolution reaction on hematite and likely represents a general feature of metal-oxide catalysts.