Earth-abundant photoelectrodes for water splitting and alternate oxidation reactions: recent advances and future perspectives

Solar water splitting by means of photoelectrochemical (PEC) cells offers the promise to produce cost-effective renewable and clean fuel from abundant sunlight and water. Lately, the realization of promise of concurrent hydrogen (H₂) production along with alternate oxidation reaction (which is less energetically demanding than the water oxidation reaction) has also become a subject of intense global research interests. At present, developing inexpensive, non-toxic, and earth-abundant semiconductor-based photoelectrodes (i.e. photocathode and photoanode) with a high stability is of great importance in achieving economically viable H₂ production and value-added chemicals. This review summarizes recent advances in these photoelectrodes along with contemporary understanding of key factors responsible for high solar-to-hydrogen efficiency, device stability, and highlights a promising new research trend of alternate oxidation reactions at photoanodes. First, we outline recent developments of novel photoelectrode materials using high-throughput computational screening integrated with ab-initio calculations. We proceed to discuss the merits and major challenges of these novel and existing photoelectrodes and links the strategies used to overcome these challenges to achieve economically viable solar H₂ generation. Several important studies on the emerging new trend of alternate oxidations reactions at photoanodes toward value-added chemicals are then detailed with particular emphasis is placed on dependency of photoanode design on type of organic feedstocks and desired products from the oxidation reaction. We also emphasize the development of tandem devices for overall water splitting using these photoelectrodes with high onset potentials. Finally, we provide not only promising future directions for each material system, but also a critical assessment and outlook on how these earth-abundant photoelectrodes could lead to a potential large-scale implementation of water splitting devices.