Tuning transition metal atoms embedded in N6 cavity structure toward efficient electrocatalytic ammonia synthesis

Electrochemical nitrogen reduction reaction (NRR) has become a promising route for ammonia synthesis. Designing a providential catalyst is a key strategy to further improve electrocatalytic activity. In this study, we systematically explored the NRR catalytic performance of transition metal (TM) atoms supported in graphitic C3N with six-fold N cavity (TM/TM2@C3N) through density functional theory (DFT) calculations. Our results show that, compared to single atom catalysts (SACs), homonuclear double atom catalysts (DACs) can produce a strong d-p orbital coupling to facilitate the activation and reduction of N2. Among these catalysts, the low limiting potentials are required for Mn2@C3N and Fe2@C3N (-0.33 V/-0.47 V) along distal path, which can also suppress the competitive hydrogen evolution reaction. Our results unveil that the Mn_d orbitals accepted electrons while the dzx/dyz orbitals donate them back to *N2, resulting in the bonding states and antibonding states. Meanwhile, the higher d-band center of Mn2 results in fewer electrons in the equivalent antibonding states, increasing the binding strength of Mn-N2. Fe2@C3N is the opposite in Mn2@C3N. Importantly, the limiting potential of NRR on TM2@C3N illustrates a volcanic relationship with the adsorption free energy of the reaction intermediate *NH2 (Delta G*NH2). And the Delta G*NH2 can be used as descriptor to make the activity trend clear. Our work provides a new perspective for measuring and screening high-efficiency DACs for NRR.