Enhancing d Electrons’ Delocalization of the Single-Atom Ni–N<sub>4</sub> Site to Boost Electrochemical CO<sub>2</sub> Reduction to CO by Axial d–d Orbital Coupling
Xiaohang Wang, Zongchang Mao, Guanping Wei, Lingli Liu, Tiantian Hao, Baolei Li, Ling Zhu, Simin Xu, Shaobin Tang
Abstract
Atomically dispersed nitrogen-coordinated transition metal (TM) anchored on graphene (TM–N x –C) provides a promising potential for an electrochemical CO 2 reduction reaction (CO 2 RR). 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–N 4 –C site by vertically coupling the 3-fold N atom-coordinated TM atom on graphene (TM–N 3 –C) for promoting CO 2 reduction to CO. In contrast to the traditional TM–N 4 –C substrate that is weakly coupled with the N–N 4 –C site, the raised TM atoms on the TM–N 3 –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–N 4 –C SACs with axial coupling of Tc– and Ru–N 3 –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).