Chemical Stability of CuWO4 for Photoelectrochemical Water Oxidation

Abstract

Pure-phase CuWO 4 photoanodes with 200 nm thickness were produced by spin-casting sol–gel precursors to evaluate their performance as photoelectrodes for water oxidation. The stability of CuWO 4 in potassium phosphate (KP i ) and potassium borate (KB i ) buffers was evaluated as a function of pH and irradiance. CuWO 4 photoanodes demonstrate higher stability at pH 3 and 5 in a 0.1 M KP i buffer and are significantly more stable over a 12 h period of illumination in a 0.1 M KB i buffer at pH 7 (∼75 μA/cm 2 photocurrent at 1.23 V vs RHE (reversible hydrogen electrode) and 1 sun illumination) than in a 0.1 M KP i buffer at pH 7. The onset of photoelectrochemical water oxidation and electrochemical O 2 reduction is dictated by Cu(3d x 2 – y 2 ) states that reside at 0.4 V vs RHE, determined by linear sweep voltammetry. The onset for water oxidation is hindered by a large charge-transfer resistance, as high as 4.6 kΩ at 1 V vs RHE. Nevertheless, CuWO 4 photoanodes show nearly quantitative faradic efficiency for water oxidation, even in the presence of chloride, an improvement over the binary oxide WO 3 .