Photoinduced absorption spectroscopy (PIAS) study of water and chloride oxidation by a WO₃ photoanode in acidic solution

The mechanisms of water and chloride oxidation by a WO₃ photoanode are probed by photoinduced absorption spectroscopy (PIAS) coupled with transient photocurrent (TC) measurements. Linear sweep voltammograms (LSVs) and incident photon to current efficiencies (IPCEs) are obtained, in the water oxidation electrolyte (1 M HClO₄) and chloride oxidation electrolyte (3.5 M NaCl in 1 M HClO₄). Other work shows that the faradaic efficiency of water oxidation to O₂ in 1 M HClO₄ is ca. 1.0, and that for chloride oxidation to Cl₂ in 3.5 M NaCl plus 1 M HClO₄ is ca. 0.62. The PIAS/TC data reveals a 0.4 order dependency of the rate of water oxidation on the steady state concentration of photogenerated surface holes, [h_s⁺]_ss, and an approximately first order dependency of the rate of chloride oxidation on [h_s⁺]_ss. Associated mechanisms and rate determining steps for water and chloride oxidation at the photoanode surface that account for these reaction orders are proposed.