Nickel sulfide-based electrocatalysts for overall water splitting

Hydrogen is a green energy with sustainability and high energy density. Electrochemical water splitting (EWS) is a promising green strategy for hydrogen production. Noble metal electrocatalysts exhibit excellent electrocatalytic activity in EWS. However, the applications of noble metals in EWS are limited because of their scarcity and high price. Therefore, the research on non-noble metal electrocatalysts has attracted much attention. Among them, nickel sulfide electrocatalysts, with a unique 3D structure, pretty conductivity, and adjustable electronic structure, show significant electrocatalytic activity in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). In this review, the mechanism of the electrocatalytic reaction, electrochemical parameters, and preparation methods of nickel sulfide are introduced first. Then, the five methods including atomic doping (including cations, anions and diatoms), morphological control, hybridization, integration with nanocarbon, and high-index facets exposure to regulate the electronic structure and active sites of nickel sulfide were illustrated, so as to improve the electrocatalytic activity of nickel sulfide. The electrocatalytic properties of these nickel sulfides were reviewed. However, there are some problems in the research of electrocatalysis, such as how to further improve the conductivity of the electrocatalyst, and the calculation method of current density is not unified. Therefore, our future development direction is to prepare a stable nickel sulfide electrocatalyst, study relevant strategies to simultaneously increase active sites and improve conductivity, and effectively make nickel sulfide into an EWS catalyst with higher performance.