Trade-Off between the Coordination Environment and Active-Site Density on Fe–NxCy–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 (ΔGmax) step depended on the coordination environment. When Fe was fourfold-coordinated, the hydrogenation of CO2 to *COOH was the ΔGmax step, involving proton-coupled electron transfer (PCET; ΔGmax,PCET). When Fe was threefold-coordinated, the *CO desorption process was the ΔGmax step, which was without PCET (ΔGmax,nPCET). Notably, ΔGmax,PCET was negatively correlated with ΔGmax,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 3dz2 and 3dxz (3dyz) orbitals of Fe atom and the intermediates *COOH-π1* or *CO-2π*, 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 (φ) that can evaluate the binding stabilities and reactivities of different intermediates during CO2RR was proposed. Using φ, 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 φ-assisted screening: Fe–N4–C(IV), Fe–N3C–C(II), Fe–N2C2–C(IV), Fe–N3VC–C(I), Fe–N3P–C, Fe–N4–P/C, FeNi–N6–C, and FeCu–N6–C. Meanwhile, Fe–N3P–C, FeNi–N6–C, and FeCu–N6–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.