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.