Reaction Mechanism Investigation of Hematite Photoanodes for Photoelectrochemical Water Splitting: Progress of In Situ and Operando Spectroscopy

Hematite photoanodes, a promising candidate for photoelectrochemical (PEC) water splitting, are far from reaching their theoretical photocurrent limit. Further designing high-performance hematite-based photoanodes requires an in-depth understanding of the reaction mechanism of PEC water splitting. In situ and operando spectral characterizations facilitate the mechanism investigation, revealing the dynamic surface structures and intermediates of hematite photoanodes during the PEC water splitting process. Herein, in situ and operando spectroscopic techniques were applied for a mechanistic understanding of hematite-based photoanodes in the PEC water splitting system, such as UV–visible spectroscopy, Raman spectroscopy, infrared spectroscopy, transient absorption spectroscopy, transmission electron microscopy, X-ray absorption spectroscopy, atomic force microscope, X-ray photoelectron spectroscopy, and electron paramagnetic resonance spectroscopy. The recent progress on charge carrier dynamics and interfacial reaction intermediates of hematite photoanodes for oxygen evolution reaction (OER) is summarized. An outlook on challenges and prospects of in situ and operando spectroscopic techniques for PEC water splitting is given. This review will inspire future mechanism investigation of photoanodes for PEC water splitting to achieve higher performance.