Photoelectrochemical (PEC) water oxidation has attracted heightened interest in solar fuel production. It is well accepted that water oxidation on hematite is mediated by surface trapped holes, characterized to be the high valent -Fe═O species. However, the mechanism of the subsequent rate-limiting O-O bond formation step is still a missing piece. Herein we investigate the reaction order of interfacial hole transfer by rate law analysis based on electrochemical impedance spectroscopy (EIS) measurement and probe the reaction intermediates by operando Fourier-transform infrared (FT-IR) spectroscopy. Distinct reaction orders of ∼1 and ∼2 were observed in near-neutral and highly alkaline environments, respectively. The unity rate law in near-neutral pH regions suggests a mechanism of water nucleophilic attack (WNA) to -Fe═O to form the O-O bond. Operando observation of a surface superoxide species that hydrogen bonded to the adjacent hydroxyl group by FT-IR further confirmed this pathway. In highly alkaline regions, coupling of adjacent surface trapped holes (I2M) becomes the dominant mechanism. While both are operable at intermediate pHs, mechanism switch from I2M to WNA induced by local pH decrease was observed at high photocurrent level. Our results highlight the significant impact of surface protonation on O-O bond formation pathways and oxygen evolution kinetics on hematite surfaces.

Hematite is a promising material for solar water splitting; however, high efficiency remains elusive because of the kinetic limitations of interfacial charge transfer. Here, we demonstrate the pivotal role of proton transfer in water oxidation on hematite photoanodes using photoelectrochemical (PEC) characterization, the H/D kinetic isotope effect (KIE), and electrochemical impedance spectroscopy (EIS). We observed a concerted proton-electron transfer (CPET) characteristic for the rate-determining interfacial hole transfer, where electron transfer (ET) from molecular water to a surface-trapped hole was accompanied by proton transfer (PT) to a solvent water molecule, demonstrating a substantial KIE (∼3.5). The temperature dependency of KIE revealed a highly flexible proton transfer channel along the hydrogen bond at the hematite/electrolyte interface. A mechanistic transition in the rate-determining step from CPET to ET occurred after OH(-) became the dominant hole acceptor. We further modified the proton-electron transfer sequence with appropriate proton acceptors (buffer bases) and achieved a greater than 4-fold increase in the PEC water oxidation efficiency on a hematite photoanode.