A review of defect engineering in two-dimensional materials for electrocatalytic hydrogen evolution reaction

The exploration of efficient and earth-rich electrocatalysts for electrochemical reactions is critical to the implementation of large-scale green energy conversion and storage techniques. Two-dimensional (2D) materials with distinctive structural and electrochemical properties provide fertile soil for researchers to harvest basic science and emerging applications, which can be divided into metal-free materials (such as graphene, carbon nitride and black phosphorus) and transition metal-based materials (such as halogenides, phosphates, oxides, hydroxides, and MXenes). For faultless 2D materials, they usually exhibit poor electrochemical hydrogen evolution reaction (HER) activity because only edge sites can be available while the base surface is chemically inactive. Defect engineering is an effective strategy to generate active sites in 2D materials for improving electrocatalytic activity. This review presents feasible design strategies for constructing defect sites (including edge defects, vacancy defects and dopant derived defects) in 2D materials to improve their HER performance. The essential relationships between defect structures and electrocatalytic HER performance are discussed in detail, providing valuable guidance for rationally fabricating efficient HER electrocatalysts. The hydrogen adsorption/desorption energy can be optimized by constructing defect sites at different locations and by adjusting the local electronic structure to form unsaturated coordination states for efficient HER.