Kinetics and Faradaic Efficiency of Oxygen Evolution on Reduced H_<i>x</i>WO₃ Photoelectrodes

Tungsten oxide (WO3) electrodes were synthesized by spin-coating an ammonium metatungstate sol. Instability in photocurrent during water oxidation applications has previously been attributed to formation of destructive peroxide intermediates. Under constant illumination, repeated cycles of poising WO3 electrodes at 0.98 V vs Ag/AgCl in pH 1 sulfate solution followed by measuring the open-circuit potential for several hours show reversibility in the photocurrent decay. This behavior is attributed to photochromic HxWO3 generated at low concentration within the electrode, which serves to increase the donor density. The Mott–Schottky analysis of electrochemical impedance spectroscopy measurements on WO3 electrodes before and after performing the oxygen-evolution reaction (OER) exhibits a decrease in donor density from 2.8 × 1022 to 6.0 × 1021 cm–3 with a corresponding 110 mV positive shift in the flat-band potential, indicative of tungsten oxidation during the OER. Tungsten oxidation is corroborated by a decrease in W5+ signal in the X-ray photoelectron spectroscopy data. Measuring the OER rate by gas chromatography during water oxidation shows concurrent recovery of catalytic activity after resting at open circuit under illumination, illustrating the key role of HxWO3 during photoelectrocatalysis