Uncovering the coordination effect on the Ni single-atom catalysts for CO2 reduction including vacancy defect and non-vacancy defect structures

The Ni single atom catalysts (SACs) are suggested to be efficient and popular for electrochemical CO2 reduction reaction (CO2RR) with high Faradaic efficiency (FE). Although lots of studies have been performed to explore the structure–property relationship with the aim to develop the robust Ni SACs, there are conflict and totally inverse viewpoints about the influence of the Ni coordination environment on its catalytic performance. Moreover, the full systematical investigations for all possible structures are still lack, especially for the influence of vacancy defect and the comparison for the structures with and without vacancy defect. Seventeen Ni SACs on graphene have been built including the full coordinated structures and vacancy-defect structures as much as possible in this work. The free energies related with three steps to generate CO are all calculated by the first principle to explore the effect of coordination environment on the catalytic activity of Ni SACs. Moreover, the hydrogen evolution reaction (HER) is also considered to judge their selectivity. Inclusion of the vacancy-defect or decreasing the coordinated number of N atoms are favorable to promoting the formation of *COOH, however, the desorption of *CO step becomes more difficult. On the basis of above criteria, the structure with three coordinated N atoms and one vacancy-defect (Ni-N3-V) is a potential active catalyst standing out from other coordinated situations. According to the partial density of states (PDOSs) result, the activity of Ni-N3-V is affected by both the Ni atom and the C atoms around the Ni atom. The effect of support materials should be carefully considered to build the Ni SACs.