Synthesis and characterization of high surface area CuWO4 and Bi2WO6 electrodes for use as photoanodes for solar water oxidation

Abstract

Two n-type W-containing ternary oxides, CuWO₄ and Bi₂WO₆, were prepared as high surface area electrodes and characterized for use as photoanodes in a water-splitting photoelectrochemical cell. The synthesis involved electrochemical preparation of porous WO₃ electrodes and annealing them with Cu²⁺- or Bi³⁺-containing solutions on their surfaces to form the respective electrodes. The resulting CuWO₄ electrode had a bandgap of 2.3 eV, and showed excellent photostability and photocurrent-to-O₂ conversion efficiency (ca. 100%) in 0.1 M borate buffer solution (pH 9). Bi₂WO₆ had a bandgap of 2.8 eV but, regardless of its higher bandgap energy, Bi₂WO₆ showed an earlier photocurrent onset and much higher photocurrent than CuWO₄ due to its more favorable CB edge and flatband potential position for water splitting. Bi₂WO₆ also showed chemical stability over a wide pH range (−0.26 ≤ pH ≤ 9.0). The photocurrent-to-O₂ conversion efficiency of Bi₂WO₆ was in the range of 50–75% and its photocurrent decayed over time, indicating photocorrosion. However, stable photocurrent was obtained when H₂O₂, which has faster oxidation kinetics than water, was introduced into the electrolyte as a hole scavenger. This suggests that the photocorrosion of Bi₂WO₆ can be suppressed when an oxygen evolution catalyst is placed on its surface to improve interfacial hole transfer kinetics. With proper oxygen evolution catalysts and improved charge transport properties, both CuWO₄ and Bi₂WO₆ have the possibility of achieving better photoelectrochemical performances than WO₃.