Boosting Nitrogen Activation with Asymmetric Coordinated Ni–N1–N4 Site through p–d Hybridization

Abstract

Single-atom catalysts (SACs) hold great promise for the electrocatalytic N₂ reduction reaction (NRR), yet a comprehensive understanding of coordination microenvironment modulation remains elusive. Herein, Ni SACs with diverse Ni–N_x–C configurations (square-planar: Ni–N₄, triangle-cone: Ni–N₁–N₃, pentagon-planar: Ni–N₅, and square-cone: Ni–N₁–N₄) were theoretically studied. Ni SACs with the Ni–N₁–N₄ configuration demonstrated the highest stability and suitability for N₂ adsorption, hydrogenation (*NN → *NNH), and inhibition of *H adsorption for H₂ evolution. Notably, the additionally coordinated N disrupted the axisymmetric distribution of electrons in Ni–N₁–N₄, particularly in the z-direction of Ni d_z² orbitals, inducing the end-adsorbed N₂ to adopt an inclined position. The variation in electronic states facilitated the simultaneous σ and π interaction between Ni d_z² and N₂, forming a strong Ni–N bond (1.89 Å) and promoting N₂ activation via p–d hybridization. In light of the calculation results, Ni SACs with the Ni–N₁–N₄ configuration (Ni₂₀-FAN) were fabricated and displayed a superior NRR activity and Faradaic efficiency of 35.74% at −0.4 V vs RHE. In situ spectroscopic techniques together with density functional theory calculations further unraveled a facile distal pathway for ammonia evolution over Ni–N₁–N₄ SACs. This work offers an innovative perspective for theory-guided modulation of the SAC coordination microenvironment, introducing asymmetric coordination as an effective strategy for the activation of symmetric inert molecules.