Facile Aqueous Solution‐Gel Route toward Thin Film CuBi₂O₄ Photocathodes for Solar Hydrogen Production

Abstract A new benign aqueous route toward bismuth‐containing photoelectrodes is proposed to eliminate the need for harmful organic solvents and/or acids. A CuBi₂O₄ photocathode is prepared by stabilizing the metal ions through complexation in pH neutral aqueous solutions. Merits of the proposed approach are elemental homogeneity (with unique doping possibilities) and ease of fabrication (e.g., high scalability). The prepared aqueous CuBi₂O₄ precursor forms a nearly phase‐pure kusachiite crystalline phase free of organics residuals and capable of water reduction due to its sufficiently negatively positioned conduction band at −0.4 V versus RHE. Deposition on fluorine doped tin oxide coated glass (FTO/glass) substrates and thermal treatment leads to uniform but granular films of CuBi₂O₄ with excellent control over stoichiometry and thickness, owing to the facile and non‐destructive synthesis conditions. Ultimately, the optimized CuBi₂O₄ photocathodes produce AM1.5G photocurrent densities of up to −1.02 mA cm⁻² at 0.4 V versus RHE with H₂O₂ as an electron scavenger, competing with bare CuBi₂O₄ prepared through less benign non‐aqueous organic synthesis routes.