Chloride Oxidation as an Alternative to the Oxygen-Evolution Reaction on H<i><sub>x</sub></i>WO<sub>3</sub> Photoelectrodes
Andrew G. Breuhaus‐Alvarez, Quintin Cheek, Joshua J. Cooper, Stephen Maldonado, Bart M. Bartlett
Abstract
A comparison of photoelectrochemical oxygen-evolution reaction (OER) and chloride oxidation is performed on semiconducting HxWO3 thin films. Over a 3 h controlled potential coulometry (CPC) experiment, the photocurrent density recorded during OER in a nitrate electrolyte decreases to half the starting photocurrent. However, if the same electrolysis experiment is performed with a chloride electrolyte, the photocurrent density is much more stable, degrading by only 5% over the same period. Linear sweep voltammetry (LSV) in the nitrate electrolyte exhibits a foot of the wave approximately 150 mV more positive than in the chloride electrolyte and the saturated photocurrent density is approximately 20% greater in the chloride electrolyte compared to the nitrate electrolyte. Also, the Faradaic efficiency (FE) for the OER is 87 ± 2% in the nitrate electrolyte compared to an FE of 100% for the oxidation of the chloride electrolyte to hypochlorous acid. These results suggest that the chloride ion rapidly injects electrons into the photogenerated holes in the HxWO3 valence before these holes destructively recombine with W5+ electron donors. The result is an increase in HxWO3 stability during photoelectrochemical chloride oxidation when compared to water oxidation. FeOOH electrocatalysts are known to efficiently remove holes from photoresponsive metal oxides, and FeOOH was deposited on HxWO3. The HxWO3|FeOOH material also exhibits a negligible loss of photocurrent during OER and chloride oxidation, supporting this hypothesis.