Augmented photoelectrochemical water reduction: influence of copper vacancies and hole-transport layer on CuBi2O4 photocathode

A ternary metal oxide CuBi2O4 has received immense attention in the research field of photoelectrochemical (PEC) water or CO2 reduction owing to its ideal optical bandgap and positive photocurrent onset potential. However, CuBi2O4 photocathodes have limitations regarding charge-carrier separation within and transport across the interface to an n-type FTO substrate. In this study, a novel thin Fe-doped NiOX layer was attached to the copper vacancy-induced CuBi2O4 (CBO-O-2) back-interface. X-ray photoelectron and energy dispersive spectroscopies provided evidence for copper vacancies within CBO; atomic force microscopy and electrochemical impedance spectroscopy revealed improved surface conductivity and interface charge transfer between CBO-O-2 and Fe:NiOX. The CBO-O-2 photocathode exhibits a higher hole-concentration with superior charge-separation, and the Fe:NiOX layer acts as an efficient hole-transport layer (HTL) at the back interface. The PEC performance characteristic of the Fe:NiOX/CBO-O-2 photocathode increased four fold than that of a conventional-heating CBO photoelectrode (CBO-Air). Additionally, the Fe:NiOX/CBO-O-2 photocathode with a protective layer revealed about 4 h stability and 96% faradaic efficiency. The improved photocurrent of the device is attributed to the increased acceptor density in CBO and effective hole transportation across the back interface with positive-band alignment. This work provides an efficient strategy for constructing high-performance ternary metal oxide-based photocathodes/photoanodes for sustainable and environmentally friendly energy applications.