Evaluation of Copper Vanadate (β-Cu₂V₂O₇) as a Photoanode Material for Photoelectrochemical Water Oxidation

Monoclinic copper vanadate (n-type Cu₂V₂O₇) thin film photoanodes were prepared for the first time by spray pyrolysis and evaluated for photoelectrochemical (PEC) water oxidation. The spray pyrolysis parameters were optimized to obtain phase-pure photoanodes of β-Cu₂V₂O₇ (ziesite)_. The bandgap energy of β-Cu₂V₂O₇ is ∼2.0 eV, which corresponds to a theoretical solar-to-hydrogen (STH) efficiency of 16% if it can be paired with an appropriate photocathode in a tandem device to drive overall water splitting. However, all Cu₂V₂O₇ photoanodes prepared so far have shown relatively low photoconversion efficiencies, and the properties that limit the efficiency have not yet been fully identified. In this work, many key physical and photoelectrochemical properties of β-Cu₂V₂O₇, such as optical band gap, doping type, flat-band potential, band positions, charge carrier dynamics, and chemical stability are reported. The photoelectrochemical performance of the β-Cu₂V₂O₇ photoanodes is found to be limited by a short carrier diffusion length and slow water oxidation kinetics. Time-resolved microwave conductivity (TRMC) measurements reveal that the short carrier diffusion length (∼28 nm) is mainly due to a relatively low carrier mobility (∼3.5 × 10^-3 cm² V^-1 s^-1). The slow water oxidation kinetics can be improved by using cobalt phosphate (CoP_i) as a water oxidation cocatalyst, resulting in a doubling of the photocurrent. © 2020 American Chemical Society.