Strategies to improve cobalt-based electrocatalysts for electrochemical water splitting

Water electrolysis involving the hydrogen evolution reaction (HER) on the cathode and oxygen evolution reaction (OER) on the anode plays a significant role in eco-friendly energy conversion of water to hydrogen. The common catalysts for HER and OER are composed of precious metals as well as their alloys and compounds, but the high cost and natural scarcity are impeding wider application. In this respect, cobalt-based complexes have been investigated as homogeneous molecular catalysts for HER and OER in order to satisfy the low overpotential demand. Owing to the earth abundance and simple chemistry to form compounds with different valence states, cobalt-based compounds are promising alternatives. However, wide application of cobalt-based complexes has been hampered by synthetic issues and at the same time, the catalytic properties of common cobalt-based compounds are still not satisfactory because of the intrinsic electronic structure and limited active sites on the surface. In this review, recent strategies to enhance the catalytic properties of cobalt-based compounds for electrochemical water splitting are discussed comprehensively and the associated HER and OER mechanisms are described. Different methods to maximize the extrinsic activity and enhance the intrinsic activity are summarized and finally, the challenges and perspectives of cobalt-based electrocatalysts are discussed and suggested from the perspective of water splitting.