▎ 摘　　要

As one of the most important subjects in chemistry, nitrogen activation and reduction to yield ammonia is still a big challenge. The lack of deep understanding of the nitrogen reduction reaction (NRR) impedes the development of high-performance catalysts. In the present study, we introduce a second transition metal (M = Mn, Fe, Co, Ni, Cu, Zn, and Mo) into the active site of a single-atom Fe-N-C catalyst to tune the electronic structure and study the activity of the as-designed neighboring bimetal Fe/M-N-C catalyst in the electrochemical NRR under acidic conditions, by performing first-principles calculations. By checking the stability of the catalysts, the adsorption ability for N-2, the Gibbs free energy change for the potential-determining step in the NRR, and the hydrogen evolution reaction (HER) activity, only the Fe/Mn-N-C catalyst is predicted to be a promising candidate for the NRR as it shows significantly improved catalytic activity and strong selectivity against the HER. A mechanistic study reveals the synergistic effects of the bimetal active sites, and the introduced Mn atom generates a strong multi-reference effect on the electronic configuration to create more tunnels to transfer the d-orbital electrons to activate the inert N & xe002;N triple bond, inducing the "acceptance-donation" process to facilitate the activation and reduction of N-2. The current results provide an effective strategy to design stable, active, and selective catalysts for the electrochemical NRR.