Efficient Photoelectrochemical Water Oxidation on Hematite with Fluorine-Doped FeOOH and FeNiOOH as Dual Cocatalysts.

An effective cocatalyst is usually required to improve the performance of photoelectrochemical (PEC) water splitting catalysts. A fluorine-doped FeOOH (F:FeOOH) cocatalyst on a hematite photoanode was used to lower the onset potential by 140 mV and significantly improve the PEC performance. Moreover, a more effective dual cocatalytic system was prepared by subsequent loading of a FeNiOOH cocatalyst, which resulted in a further decrease of the onset potential by 270 mV. The final onset potential of the Fe2 O3 /F:FeOOH/FeNiOOH photoanode was lowered to 0.45 V versus the reversible hydrogen electrode (RHE), which is one of the lowest onset potential values ever reported for hematite photoanodes. The photocurrent also dramatically increased by a factor of approximately 3 to 0.9 mA cm-2 at 1.0 V versus RHE. Based on the structural, chemical, and electrochemical impedance spectroscopy characterization, the enhanced performance was attributed to the F:FeOOH overlayer, which reduced the surface recombination and accelerated the oxygen evolution reaction activity, and the FeNiOOH cocatalyst, which further enhanced the reaction kinetics. The facile preparation of the F:FeOOH cocatalyst and the design of the dual cocatalytic system will allow the development of high-performance hematite photoanodes.