Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting

A Boost for Bismuth Vanadate In theory, given its light-absorption spectrum, bismuth vanadate should be an effective photoanode for solar water-splitting. However, in prior studies, few of the “holes” generated upon photoexcitation have persisted long enough to strip electrons from water. Kim and Choi (p. 990, published online 13 February) now show that the use of a hydrophobic vanadium source in the semiconductor's synthesis results in a high-surface-area morphology with substantially enhanced hole lifetimes. Deposition of two successive catalyst layers enhanced the proportion of holes that reacted with water at the surface, thereby raising the efficiency of the oxygen evolution reaction. A high-surface-area morphology of bismuth vanadate enhances the proportion of photogenerated holes that can oxidize water. Bismuth vanadate (BiVO4) has a band structure that is well-suited for potential use as a photoanode in solar water splitting, but it suffers from poor electron-hole separation. Here, we demonstrate that a nanoporous morphology (specific surface area of 31.8 square meters per gram) effectively suppresses bulk carrier recombination without additional doping, manifesting an electron-hole separation yield of 0.90 at 1.23 volts (V) versus the reversible hydrogen electrode (RHE). We enhanced the propensity for surface-reaching holes to instigate water-splitting chemistry by serially applying two different oxygen evolution catalyst (OEC) layers, FeOOH and NiOOH, which reduces interface recombination at the BiVO4/OEC junction while creating a more favorable Helmholtz layer potential drop at the OEC/electrolyte junction. The resulting BiVO4/FeOOH/NiOOH photoanode achieves a photocurrent density of 2.73 milliamps per square centimenter at a potential as low as 0.6 V versus RHE.