High-Density CuBi₂O₄ Photocathodes Using Well-Textured Buffer Layers and Their Unassisted Solar Hydrogen Production Performances

Solar hydrogen production using photoelectrochemical (PEC) cells requires the selection of cost-effective materials with high photoactivity and durability. CuBi₂O₄ photocathodes possess an appropriate bandgap for efficient hydrogen production. However, their performance is limited by poor charge transport and interface voids formed due to the porous structure during annealing, which complicates the deposition of passivation overlayers. To address this, effective suppression of the porous structure in CuBi₂O₄ is essential. Here, the study proposes the strategic use of an Sb-Cu₂O buffer layer with a uniform (111) crystal orientation prior to the electrodeposition of Cu-Bi-O. This buffer layer facilitates 2D film growth during electrodeposition, enhancing Cu supply via out-diffusion from the buffer during annealing. Moreover, the uniform orientation of the buffer layer promotes the crystallization of CuBi₂O₄, significantly improving charge transport efficiency. By incorporating an Al-ZnO/TiO₂ overlayer, the study achieves a photocurrent of 2.56 mA cm⁻² at 0 V_RHE and an onset potential of 1.04 V_RHE, with excellent stability exceeding 60 hours. In a glycerol oxidation reaction coupled with hydrogen production, an unassisted PEC cell with a BiVO₄ photoanode demonstrates the highest H₂ production (750.5 µmol cm⁻²) among Cu-based ternary oxides, with 97% Faradaic efficiency over 20 hours while producing DHA, and formic acid.