Two-dimensional C2N-based single-atom catalyst with complex microenvironment for enhanced electrochemical nitrogen reduction: A descriptor-based design

The catalytic descriptor with operational feasibility is highly desired towards rational design of high-performance catalyst especially the electrode/electrolyte solution interface working under mild conditions. Herein, we demonstrate that the descriptor Omega parameterized by readily accessible intrinsic properties of metal center and coordination is highly operational and efficient in rational design of single-atom catalyst (SAC) for driving electrochemical nitrogen reduction (NRR). Using two-dimensional metal (M)-BxPySzNm@C2N as prototype SAC models, we reveal that *N-2 + (H+ + e(-)) -> *N2H acts predominantly as the potential-limiting step (PLS) of NRR on M-B2P2S2@C2N and M-B1P1S1N3@C2N regardless of the distinction in coordination microenvironment. Among the 28 screened M active sites, with Omega values close to the optimal 4, M-B2P2S2@C2N (M = V (Omega = 3.53), Mo (Omega = 5.12), and W (Omega = 3.92)) and M-B1P1S1N3@C2N (M = V (Omega = 3.00), Mo (Omega = 4.34), and W (Omega = 3.32)) yield the lowered limiting potential (U-L) as -0.45, -0.54, -0.36, -0.58, -0.25, and -0.24 V, respectively, thus making them the promising NRR catalysts. More importantly, these SACs are located around the top of volcano-shape plot of U-L versus Omega, re-validating Omega as an effective descriptor for accurately predicting the high-activity NRR SACs even with complex coordination. Our study unravels the relationship between active-site structure and NRR performance via the descriptor Omega, which can be applied to other important sustainable electrocatalytic reactions involving activation of small molecules via sigma-donation and pi*-backdonation mechanism.