Transparent Nanoparticulate FeOOH Improves the Performance of a WO3 Photoanode in a Tandem Water-Splitting Device

Oxygen evolution catalysts (OEC) are often employed on the surface of photoactive, semiconducting photoanodes to boost their kinetics and stability during photoelectrochemical water oxidation. However, the necessity of using optically transparent OEC to avoid parasitic light absorption by the OEC under front-side illumination is often neglected. Here, we show that furnishing the surface of a WO 3 photoanode with suitable loading of FeOOH as a transparent OEC improved the photocurrent density by 300% at 1 V versus RHE and the initial photocurrent-to-O 2 Faradaic efficiency from ∼70 to ∼100%. The data from the photovoltammetry, electrochemical impedance, and gas evolution measurements show that these improvements were a combined result of reduced hole-transfer resistance for water oxidation, minimized surface recombination of charge carriers, and improved stability against photocorrosion of WO 3 . We demonstrate the utility of transparent FeOOH-coated WO 3 in a solar-powered, tandem water-splitting device by combining it with a double-junction Si solar cell and a Ni–Mo hydrogen evolution catalyst. This device performed at a solar-to-hydrogen conversion efficiency of 1.8% in near-neutral K 2 SO 4 electrolyte.