Linking Oxygen Evolution Selectivity to Water Oxidation Reaction Order on Hematite Photoanodes by Operando Photoelectrochemical Mass Spectrometry

Water oxidation on metal-oxide (photo)anodes is often assumed to yield molecular oxygen. Here, we combine operando photoelectrochemical mass-spectrometry oxygen detection with spectroelectrochemical rate law analyses to link oxygen selectivity to surface-hole density on hematite photoanodes. At low current and hole densities, where first-order kinetics dominate, a near-zero Faradaic efficiency for oxygen evolution is observed, revealing that isolated single-hole water oxidation is poorly coupled to molecular oxygen formation. Increasing surface-hole density sharply increases O2 Faradaic efficiency, showing that the kinetic crossover to cooperative third-order water oxidation is also a selectivity switch toward O2 evolution. Equivalent behavior under dark electrochemical operation shows that multihole chemistry is intrinsic to hematite and governs selective oxygen evolution, likely representing a general feature of semiconducting metaloxides.