Rate-Limiting O–O Bond Formation Pathways for Water Oxidation on Hematite Photoanode

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 r...