Transition metal-based heterojunctions for alkaline electrocatalytic water splitting

Electrocatalytic water splitting is an attractive strategy for generating hydrogen energy, involving the hydrogen evolution reaction (HER) at cathode and the oxygen evolution reaction (OER) at anode. Given the sluggish reaction kinetics often observed in alkaline electrolytes, there is a critical need for catalysts that can lower the energy barrier and accelerate the reaction rates. Transition metal-based catalysts hold significant potential to replace conventional noble metal catalysts due to their earth abundance, cost-effectiveness, and adjustable activity. To achieve electrocatalytic activity similar to, or even surpassing, that of precious metals, constructing heterogeneous structures proves to be an effective strategy. Within a heterojunction, the two components work synergistically to promote the electrocatalytic process. The lattice strain and electron transfer in a heterojunction can modulate the electronic structure of active sites, optimizing the adsorption of reaction intermediates and accelerating reaction kinetics. In this review, we offer an overview of the characteristics, types, characterization and synthesis methods of heterojunctions. Additionally, we delve into the application of heterojunctions in HER, OER, and overall water splitting. Finally, we present an overview of the existing challenges in developing heterojunctions and offer a perspective on the prospective utilization of heterojunctions for hydrogen generation.