▎ 摘　　要

Electrochemical N-2 reduction reaction (NRR) is a very attractive route for ammonia production under ambient conditions. It is a challenging issue to develop efficient electrocatalysts in NRR process. Herein, we designed a series of NRR single-atom catalysts (SACs), namely, transition metal single-atom anchored on nitrogen-doped porous graphene with six-fold N6 cavity (TM@N6-G). Based on spin-polarized density functional theory (DFT) calculations, the stability and catalytic performance of TM@N6-G SACs were systemically investigated. Six promising catalysts (Nb, Mo, Tc, Ta, W, and Re@N6-G) are selected from 14 candidates by screening the free energy changes of the first and last electric chemical steps. Detailed NRR mechanism studies show that the six SACs have superior electrochemical NRR performance, especially W@N6-G, reaching low limiting potentials of-0.23 V. Furthermore, we extend the present study to another kind of graphitic carbon nitrides, g-C2N, which has a similar N6 cavity. Our calculations show that five TM@g-C2N SACs have good stability and high activity and selectivity for NRR. In addition, delta G(*NNH), d(N-N), and ICOHP can be used as descriptors to understand the origin of NRR catalytic activity of these two kinds of catalysts. The present work may provide valuable clues for exploring promising and efficient NRR electrochemical catalysts.