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

Abstract

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 NiO_X layer was attached to the copper vacancy-induced CuBi₂O₄ (CBO-O₂) 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-O₂ and Fe:NiO_X. The CBO-O₂ photocathode exhibits a higher hole-concentration with superior charge-separation, and the Fe:NiO_X layer acts as an efficient hole-transport layer (HTL) at the back interface. The PEC performance characteristic of the Fe:NiO_X/CBO-O₂ photocathode increased four fold than that of a conventional-heating CBO photoelectrode (CBO-Air). Additionally, the Fe:NiO_X/CBO-O₂ 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.