▎ 摘　　要

The electrocatalytic nitrogen reduction reaction (eNRR) is widely regarded as a viable route to artificial N-2 fixation towards NH3 production under ambient conditions. Herein, using density functional theory and the computational hydrogen electrode method, we systematically explored the eNRR on M-n@C2N (M = Fe, Co, Ni, Cr, Mo, and W; n = 1, 2, and 3), representing single-, double-, and triple-atom catalysts on graphene-like C2N. Our results demonstrate that *NHx intermediates on M-n@C2N are highly stable for n = 3 and unstable for n = 1, rendering M-2@C2N as the optimal candidate for driving the eNRR due to its moderate binding with NHx (x = 0, 1, 2, 3). With the ensemble size of M-n increasing from n = 1 to 3, the N-affinity of active sites can be enhanced to a certain extent, constrained by the oxidation state of M-n(delta+). The limiting potential (U-L) of the eNRR yields a well-defined trend on either the M-1 (i.e., MN2) or M-2 (i.e., N3MMN3) active site and is critically dependent on the N-affinity of M-n(delta+), contrasting to that (U-L) on the M-3 site which is both metal- and ensemble-size-dependent. Our study provides theoretical guidance for rational design of atomic active sites driving efficiently the eNRR.