Kinetics and Faradaic Efficiency of Oxygen Evolution on Reduced H_xWO₃ 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 H_xWO₃ 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 H_xWO₃ during photoelectrocatalysis.