Visible-light-absorbing semiconductor/molecular catalyst hybrid photoelectrodes for H2 or O2 evolution: recent advances and challenges

The research on the conversion of solar energy and its storage as an eco-friendly and momentarily available chemical fuel, such as H-2, by sunlight-driven water splitting is closely related to the sustainable development of the global economy and to the continuous improvement of the modern living standards of human beings. One of the most promising approaches to sunlight-driven water splitting is to construct a dual-illuminated photoelectrochemical (PEC) cell by integrating a photoanode with a photocathode in a tandem configuration. To this end, the important work is to individually develop highly efficient, durable, inexpensive, and readily scalable photoanodes and photocathodes for each half reaction of water splitting, either O-2 or H-2 evolution reaction (OER or HER). A promising approach emerging in recent years towards OER photoanodes and HER photocathodes is the immobilization of molecular catalysts (MC) onto the surface of visible-light-absorbing semiconductor (VLASC) electrodes. Very recently, some encouraging results have been achieved in the construction of MC-modified VLASC photoanodes and photocathodes. This review is focused on the recent advances in hybrid photoelectrodes for OER and HER, which were built by the integration of MCs with VLASC materials. After a brief introduction of three major units, viz. VLASC materials, MCs, and anchor groups, used to date for fabricating hybrid photoelectrodes for OER and HER, the construction strategy and the performance of the VLASC/MC photoanodes and photocathodes are described in two respective chapters. Finally, challenges and developments in future studies of VLASC/MC hybrid photoelectrodes are discussed.