Ultra-efficient and durable photoelectrochemical water oxidation using elaborately designed hematite nanorod arrays

Ultrahigh-efficiency photoelectrochemical water oxidation using modified hematite (α-Fe₂O₃) nanorod arrays is reported. The hematite nanorod arrays are synthesized using chemical bath deposition and further modified by hydrogen treatment, loading of a ~3.5-nm-thick TiO₂ overlayer, and deposition of a cobalt phosphate (CoPi) catalyst. Although each modification method is well known, an elaborate optimization of the combined modification methods achieves a stable photocurrent density of ~6 mA cm⁻² at 1.23 V vs. RHE over 100 h under AM 1.5G irradiation (100 mW cm⁻²) with the stoichiometric O₂ and H₂ evolutions at ~95% of Faradaic efficiency. To the best of our knowledge, this is the highest photocurrent density obtained using a hematite-based photoanode, and such long-term durability coupled with this level of efficiency has been rarely reported. The modified-hematite photoanodes are thoroughly characterized using various spectroscopic and electrochemical techniques. While the hydrogen treatment enhances the electrical conductivity, the ultrathin TiO₂ overlayer reduces the surface charge recombination and effectively preserved the integrity of the hydrogen-treated hematite electrode.