Copper tungstate photoanodes with enhanced solar water splitting performance

Abstract

Copper tungstate (CuWO₄) has recently emerged as a potential high-performing photoanode for photoelectrochemical water splitting due to its suitable optical properties and high chemical resistance. Here, we present a simple deposition method for the fabrication of CuWO₄ photoanodes with record performance, and discuss many outstanding challenges with this material. First, we show how the presence of water within the reaction mixture is vital for achieving high performance, due to the more effective complexation of both copper and tungsten species, subsequently preventing the precipitation of separate WO₃ species. We also demonstrate undoubtedly that the Cu in CuWO₄ displays an oxidation state higher than +2, which is stabilized by an effective oxidation state for tungsten lower than the expected +6. After optimizing deposition conditions including annealing temperature, annealing atmosphere, and thickness, we were able to fabricate photoanodes reaching a photocurrent of 0.55 mA cm⁻² at 1.23 V (further increasing to 0.7 mA cm⁻² after the deposition of a surface co-catalyst) which is a record for undoped copper tungstate. Our work provides not just clear guidelines for the deposition of high performance CuWO₄ photoanodes, which could be adopted for the synthesis of doped or heterostructured materials, but also critical insights and deeper understanding of some of the key fundamental properties of this promising photoactive material.