Photoelectrochemical oxidation of anions by WO₃ in aqueous and nonaqueous electrolytes

The behavior of WO₃ photoanodes has been investigated in contact with a combination of four anions (Cl^-, CH₃SO ₃^-, HSO₄^-, and ClO₄ ^-) and three solvents (water, acetonitrile, and propylene carbonate), to elucidate the role of the semiconductor surface, the electrolyte, and redox kinetics on the current density vs. potential properties of n-type WO ₃. In 1.0 M aqueous strong acids, although the flat-band potential (E_fb) of WO₃ was dominated by electrochemical intercalation of protons into WO₃, the nature of the electrolyte influenced the onset potential (E_on) of the anodic photocurrent. In aprotic solvents, the electrolyte anion shifted both E_fb and E _on, but did not significantly alter the overall profile of the voltammetric data. For 0.50 M tetra(n-butyl)ammonium perchlorate in propylene carbonate, the internal quantum yield exceeded unity at excitation wavelengths of 300-390 nm, indicative of current doubling. A regenerative photoelectrochemical cell based on the reversible redox couple B ₁₀Br₁₀^-/2- in acetonitrile, with a solution potential of ∼1.7 V vs. the normal hydrogen electrode, exhibited an open-circuit photovoltage of 1.32 V under 100 mW cm^-2 of simulated Air Mass 1.5 global illumination.