Trade-Off between the Coordination Environment and Active-Site Density on Fe-N_x C_y -C Catalysts for Enhanced Electrochemical CO₂ Reduction to CO

Coordination environment and active site density are the two factors that affect the performance of single-atom catalysts (SACs). Herein, we performed systemic density functional theory calculations on the CO2 reduction reaction (CO2RR) catalyzed by a series of Fe-NxCy-C (x = 2-4, y = 0-2) SACs to shed light on this issue. It is found that the maximum free-energy change (.Gmax) step depended on the coordination environment. When Fe was fourfoldcoordinated, the hydrogenation of CO2 to *COOH was the.Gmax step, involving proton-coupled electron transfer (PCET;.Gmax,PCET). When Fe was threefold-coordinated, the *CO desorption process was the.Gmax step, which was without PCET (.Gmax,nPCET). Notably,.Gmax, PCET was negatively correlated with.Gmax,nPCET. Moreover, the Fe site density affected the catalytic activity, which required a balance between the coordination environment and density. The orbital hybridization between the 3dz 2 and 3dxz (3dyz) orbitals of Fe atom and the intermediates *COOH-p1* or *CO-2p*, which is strongly related to the spin characteristics of Fe sites, can promote this process. Accordingly, based on the magnetic moment, electronegativity, and catalytic site density of catalytic systems, a comprehensive descriptor (f) that can evaluate the binding stabilities and reactivities of different intermediates during CO2RR was proposed. Using f, we validated three strategies for screening catalysts: nearest and subnearest coordination shell regulation of the central atom and bimetallic structure construction. Eight outstanding electrocatalysts were obtained using f-assisted screening: Fe-N-4-C(IV), Fe-N3C-C(II), Fe-N2C2-C(IV), FeN3VC-C(I), Fe-N3P-C, Fe-N-4-P/C, FeNi-N-6-C, and FeCu-N-6-C. Meanwhile, Fe-N3P-C, FeNi-N-6-C, and FeCu-N-6-C yielded a high-throughput CO over a wide range of external potentials. We believe that the proposed coordination environment and active site density effects are crucial to understanding the intrinsic structure-property relationship under reaction conditions, thus offering potential design strategies for related catalysts.