A quantitative assessment of the competition between water and anion oxidation at WO3 photoanodes in acidic aqueous electrolytes

The faradaic efficiency for O 2 (g) evolution at thin-film WO 3 photoanodes has been evaluated in a series of acidic aqueous electrolytes. In 1.0 M H 2 SO 4 , persulfate was the predominant photoelectrochemical oxidation product, and no O 2 was detected unless catalytic quantities of Ag + (aq) were added to the electrolyte. In contact with 1.0 M HClO 4 , dissolved O 2 was observed with nearly unity faradaic efficiency, but addition of a hole scavenger, 4-cyanopyridine N -oxide, completely suppressed O 2 formation. In 1.0 M HCl, Cl 2 (g) was the primary oxidation product. These results indicate that at WO 3 photoanodes, water oxidation is dominated by oxidation of the acid anions in 1.0 M HCl, H 2 SO 4 , and HClO 4 , respectively. In 1.0 M HCl, H 2 SO 4 , and HClO 4 , oxidation of the respective acid anions dominates oxidation of water at illuminated WO 3 photoanodes.