Intermediates-induced CO2 Reduction Reaction Activity at Single-Atom M-N2 (M=Fe, Co, Ni) Sites

Single-atom M-N-2 (M=Fe, Co, Ni) catalysts exhibit high activity for CO2 reduction reaction (CO2RR). However, the CO2RR mechanism and the origin of activity at the single-atom sites remain unclear, which hinders the development of single-atom M-N-2 catalysts. Here, using density functional theory calculations, we reveal intermediates-induced CO2RR activity at the single-atom M-N-2 sites. At the M-N-2 sites, the asymmetric *O*CO configuration tends to split into *CO and *OH intermediates. Intermediates become part of the active moiety to form M-(CO)N-2 or M-(OH)N-2 sites, which optimizes the adsorption of intermediates on the M sites. The maximum free energy differences along the optimal CO2RR pathway are 0.30, 0.54, and 0.28 eV for Fe-(OH)N-2, Co-(CO)N-2, and Ni-(OH)N-2 sites respectively, which is lower than those of Fe-N-2 (1.03 eV), Co-N-2 (1.24 eV) and Ni-N-2 (0.73 eV) sites. The intermediate modification can shift the d-band center of the spin-up (minority) state downward by regulating the charge distribution at the M sites, leading to less charge being accepted by the intermediates from the M sites. This work provides new insights into the understanding of the activity of single-atom M-N-2 sites.