Chloride Oxidation as an Alternative to the Oxygen-Evolution Reaction on H_xWO₃ Photoelectrodes

A comparison of photoelectrochemical oxygen-evolution reaction (OER) and chloride oxidation is performed on semiconducting H x WO 3 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 H x WO 3 valence before these holes destructively recombine with W 5+ electron donors. The result is an increase in H x WO 3 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 H x WO 3 . The H x WO 3 |FeOOH material also exhibits a negligible loss of photocurrent during OER and chloride oxidation, supporting this hypothesis.