Effect of Strain Engineering on the Spin State of the Ni–N4/C Single-Atom Catalyst and Its Consequence in Electrocatalysis

Strain engineering is an effective strategy to improve the activity of catalysts, especially for flexible carbon-based materials. Nitrogen-coordinated single atomic metals on a carbon skeleton (M-N_x/C) are of interest in catalytic electroreduction reactions due to their high activity and atomic utilization. However, the effect of strain on the structure-activity relationship between the electrochemical activity and the electronic and geometric structures of Ni-N_x/C remains unclear. Here, we found that by applying tensile strain on the Ni-N₄/C, the spin state of the single atom can be changed from a low-spin to a high-spin state. Moreover, the energy gap between the highest occupied d orbital of Ni and the lowest unoccupied molecular orbital of the adsorbed species narrowed. With an increasing strain rate, the catalytic activity of O₂ and CO₂ electroreduction can be improved. Especially for the 2e^- O₂ reduction, the implicit solvent model, constant-potential method, and microkinetic model were used to verify the positive effect of suitable stretching on the catalytic activity from thermodynamic and kinetic viewpoints. This work can reveal the relationship between strain, spin state, and the catalytic activity of Ni-N_x/C.