Effect of Strain\nEngineering on the Spin State of\nthe Ni–N<sub>4</sub>/C Single-Atom Catalyst and Its Consequence\nin Electrocatalysis

Strain engineering is an effective strategy to improve\nthe activity\nof catalysts, especially for flexible carbon-based materials. Nitrogen-coordinated\nsingle atomic metals on a carbon skeleton (M–N<sub><i>x</i></sub>/C) are of interest in catalytic electroreduction\nreactions due to their high activity and atomic utilization. However,\nthe effect of strain on the structure–activity relationship\nbetween the electrochemical activity and the electronic and geometric\nstructures of Ni–N<sub><i>x</i></sub>/C remains unclear.\nHere, we found that by applying tensile strain on the Ni–N<sub>4</sub>/C, the spin state of the single atom can be changed from\na low-spin to a high-spin state. Moreover, the energy gap between\nthe highest occupied <i>d</i> orbital of Ni and the lowest\nunoccupied molecular orbital of the adsorbed species narrowed. With\nan increasing strain rate, the catalytic activity of O<sub>2</sub> and CO<sub>2</sub> electroreduction can be improved. Especially\nfor the 2e<sup>–</sup> O<sub>2</sub> reduction, the implicit\nsolvent model, constant-potential method, and microkinetic model were\nused to verify the positive effect of suitable stretching on the catalytic\nactivity from thermodynamic and kinetic viewpoints. This work can\nreveal the relationship between strain, spin state, and the catalytic\nactivity of Ni–N<sub><i>x</i></sub>/C.