Photoelectrochemical Properties and Photostabilities of High Surface Area CuBi₂O₄ and Ag-Doped CuBi₂O₄ Photocathodes

Here, electrochemical synthesis methods were developed to produce CuBi₂O₄, a promising p-type oxide for use in solar water splitting, as high surface area electrodes with uniform coverage. These methods involved electrodepositing nanoporous Cu/Bi films with a Cu:Bi ratio of 1:2 from dimethyl sulfoxide or ethylene glycol solutions, and thermally oxidizing them to CuBi₂O₄ at 450°C in air. Ag-doped CuBi₂O₄ electrodes were also prepared by adding a trace amount of Ag+ in the plating medium and codepositing Ag with the Cu/Bi films. In the Ag-doped CuBi₂O₄, Ag+ ions substitutionally replaced Bi3+ ions and increased the hole concentration in CuBi₂O₄. As a result, photocurrent enhancements for both O₂ reduction and water reduction were achieved. Furthermore, while undoped CuBi₂O₄ electrodes suffered from anodic photocorrosion during O₂ reduction due to poor hole transport, Ag-doped CuBiO₄ effectively suppressed anodic photocorrosion. The flat-band potentials of CuBi₂O₄ and Ag-doped CuBi₂O₄ electrodes prepared in this study were found to be more positive than 1.3 V vs RHE in a 0.1 M NaOH solution (pH 12.8), which make these photocathodes highly attractive for use in solar hydrogen production. The optimized CuBi₂O₄/Ag-doped CuBi₂O₄ photocathode showed a photocurrent onset for water reduction at 1.1 V vs RHE, achieving a photovoltage higher than 1 V for water reduction. The thermodynamic feasibility of photoexcited electrons in the conduction band of CuBi₂O₄ to reduce water was also confirmed by detection of H₂ during photocurrent generation. This study provides new understanding for constructing improved CuBi₂O₄ photocathodes by systematically investigating photocorrosion as well as photoelectrochemical properties of high-quality CuBi₂O₄ and Ag-doped CuBi₂O₄ photoelectrodes for photoreduction of both O₂ and water.