Earth-abundant Cu-based metal oxide photocathodes for photoelectrochemical water splitting

Photoelectrochemical (PEC) solar-fuel conversion is a promising approach to converting energy from sunlight into storable chemical fuels. The development of low cost, highly efficient, and stable semiconductor-based photoelectrodes is a key step in realizing economically viable PEC energy conversion on a global scale. The p-type Cu-based metal oxides possess a wide range of bandgap values and favorable band edges relative to the water splitting redox couples, thus providing promising candidates for PEC solar conversion applications. However, the improvement of the PEC performance for the binary and ternary copper-based metal oxides is severely hindered by the chemical instability and/or unsatisfactory optoelectronic properties. Thus, a fundamental understanding of the key limitations, improvement strategies, and progress of these materials is critical to design high performance and stable photocathodes. Here, we outline the development of p-type binary and ternary Cu-based metal oxide photocathodes, discuss the merits and major challenges of these p-type materials, and present the latest research effort in modifying the materials towards high-performance photocathodes. The critical strategies that have been successfully employed for Cu2O-based solar cells and photocathodes are emphasized to offer guidelines to advance emerging Cu-based photocathodes. Emphasis is placed on the determination of the faradaic efficiency and onset potential of hydrogen generation for the modified photocathodes to properly evaluate the performance and design tandem devices that achieve bias-free solar water splitting. Furthermore, perspectives regarding emerging issues yet to be addressed for the development of Cu-based metal oxide photocathodes with high photocurrent and photovoltage are also presented.