Enhancing d Electrons’ Delocalization of the Single-Atom Ni–N4 Site to Boost Electrochemical CO2 Reduction to CO by Axial d–d Orbital Coupling

Atomically dispersed nitrogen-coordinated transition metal (TM) anchored on graphene (TM–N x –C) provides a promising potential for an electrochemical CO2 reduction reaction (CO2RR). However, it is still a challenge to precisely control the electronic structures of TM single-atom catalysts (SACs) for optimizing the catalytic performance. Using first-principles calculations, we propose a novel strategy to regulate the electronic structure of the Ni–N4–C site by vertically coupling the 3-fold N atom-coordinated TM atom on graphene (TM–N3–C) for promoting CO2 reduction to CO. In contrast to the traditional TM–N4–C substrate that is weakly coupled with the N–N4–C site, the raised TM atoms on the TM–N3–C substrate relative to the basal plane of graphene shorten the distance from TM to Ni atoms and strengthen d orbital hybridization between them, thus leading to more delocalized charge distribution of the Ni active site. As a result, the improved axial d–d orbital coupling largely enhances the adsorption of the key *COOH intermediate on Ni SACs and, more importantly, maintains the facile desorption of adsorbed *CO. In particular, these Ni–N4–C SACs with axial coupling of Tc– and Ru–N3–C substrates not only exhibit high catalytic activity toward CO production, with low limiting potentials of −0.68 and −0.61 V, respectively, but also effectively suppress the competing hydrogen evolution (HER).