Identifying the nature of active centers and structure–performance correlations is of fundamental importance for the successful design of more energy-efficient and/or selective catalysts. Recently, the studies of metal-free catalysts containing sp2- or sp3-hybridized B sites have displayed an attractive prospect for N2 reduction but have obscured the nature of the optimal active B species. Herein, with the aid of first-principles calculations, we explicitly disclose that sp2-hybridized B is the optimal species for providing high activity for N2 reduction and particularly outstanding capability to suppress the competing hydrogen evolution reaction. Specifically, the system with B substituting an edge N atom in the cavity of C2N is proposed to be highly promising for N2 reduction under mild conditions. The developed comprehensive insight is of clear significance for the rational design of advanced catalysts for NH3 synthesis under mild conditions.