▎ 摘　　要

Electrocatalytic nitrogen fixation using single-atom catalysts (SACs) offers a promising strategy for the sustainable production of NH3. On the basis of density functional theory, we systematically explored the potential for N-2 electroreduction of single-atom catalysts (SACs) covering V, Nb, and Ta transition metal (TM) centers supported by graphene and g-C3N4 substrates. The single Nb-atom embedded on g-C(3)N4 nanosheet possesses outstanding nitrogen reduction reaction (NRR) catalytic activity and exhibits better performance than graphene with a considerably smaller maximum Delta G value (0.05 eV). The single Nb atom on g-C3N4 with more negative valence provides structural advantages for hosting empty d-orbitals for strong N-2 and N2H adsorption, as well as more single d-electrons to further promote back-donation to activate the N N triple bond. This work may be helpful in developing more effective TM-based SACs for N-2 reduction through varying substrate effect toward the same single-atom catalysts.