Spin-state-dependent oxygen evolution activity of strained Ni-N₄-C single-atom catalysts

Strain engineering offers an effective way to regulate the spin state and oxygen evolution reaction (OER) activity of single-atom catalysts (SACs). Here, spin-polarized density functional theory (DFT) calculations show that external biaxial tensile strain drives a spin-state transition of the Ni center in Ni-N₄-C. The theoretical OER overpotential decreases from 1.23 V without strain to a minimum of 0.49 V at 5% strain. Spin-state-controlled calculations further show that the HS configuration exhibits substantially lower overpotentials than the low-spin state at the same strain, whereas strain alone does not improve the activity within a fixed spin-state branch. These results demonstrate that tensile strain promotes OER activity mainly by stabilizing a more active spin configuration rather than through geometric deformation alone. This work clarifies the coupling among strain, spin state, orbital occupation, Ni-O bonding, and OER thermodynamics, providing theoretical guidance for the design of spin-regulated single-atom electrocatalysts.