Co-optimization of CuBi₂O₄ photocathode by heterojunction and hole-selective layer for efficient photoelectrochemical water splitting

CuBi₂O₄ is identified as a promising photocathode in photoelectrochemical (PEC) water splitting systems. However, the PEC performance of CuBi₂O₄ is far from expected due to the limited separation and transport efficiency of photogenerated carriers. To address the above issues, a cost-effective ternary Cu:NiO_X/CuBi₂O₄/CuO composite photocathode was designed. Firstly, a thin Cu:NiO_X film was inserted between CuBi₂O₄ and FTO conducting substrate as a hole-selective layer, which promotes the transmission of photogenerated holes to the FTO substrate effectively. Furthermore, the modification of CuO film on the CuBi₂O₄ electrode not only increases the absorption of sunlight and generates more photogenerated carriers, but also constitutes a heterojunction with CuBi₂O₄, creating a built-in electric field, which facilitates the separation of electrons and holes, and accelerates the electrons transfer to electrode–electrolyte interface. The fabricated Cu:NiO_X/CuBi₂O₄/CuO composite photocathode exhibits a surprisingly high photocurrent density of − 1.51 mA·cm⁻² at 0.4 V versus RHE, which is 2.6 times that of the pristine CuBi₂O₄ photocathode. The improved PEC performance is attributed to the synergy effect of the Cu:NiO_X hole-selective layer and the CuBi₂O₄/CuO heterojunction. Moreover, the combination with the BiVO₄/CoS, an unbiased overall water splitting was achieved, which has a photocurrent of 0.193 mA·cm⁻².