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[Display omitted] • Highly exposed isolated FeN 5 sites are embedded in N-doped carbon frameworks. • FeN 5 exhibits a maximum FE CO of 98% and over 95% from −0.4 to −1.0 V vs. RHE. • The axial coordinated N modulates the d -band center and facilitates CO desorption. • FeN 4 and axial coordinated N act synergistically to boost CO 2 -to-CO conversion. Electrochemical CO 2 conversion offers a sustainable approach to alleviate the energy crisis but remains a long-standing challenge. Herein, atomically dispersed FeN 5 sites anchored on N-doped carbon matrix templated by zeolitic imidazole framework-8 (ZIF-8) are facilely synthesized for efficient catalyzing CO 2 reduction. The FeN 5 single-atom catalyst (Fe-SA/ZIF) presents the Faradaic efficiency of 98% at −0.7 V vs. RHE and over 95% in a wide potential range from −0.4 to −1.0 V vs. RHE, surpassing most of the reported single-atom catalysts for CO 2 RR. Further density functional theory (DFT) calculations reveal that the outstanding activity of FeN 5 sites mainly originates from the lowered d -band center modulated by the out-of-plane coordinated pyridinic N, which reduces the CO adsorption energy from −1.71 to −1.49 eV compared to FeN 4 moieties. Thus, the synergistic effect between FeN 4 sites and the coordinated pyridine offers new insight in designing outstanding electrocatalysts for multiple applications. [ABSTRACT FROM AUTHOR]

[Display omitted] • A series of Co SACs were synthesized with tunable Co-N 5- x C x (x = 1, 2, 3) sites. • Engineering the hybrid N and C coordination greatly affect the CO 2 RR performance. • FE CO boosted from 54 to 76 and 92% at −0.8 V with increased ratio of Co-C. • Co-C promotes the electron donation and strengthen binding interaction with *COOH. Single-atom catalysts (SACs) with well-defined active sites provide an efficient route for catalyzing CO 2 reduction reaction (CO 2 RR). Although enormous attention has been focused on metal-N x moieties, understanding the effect of metal-C coordination and engineering of the hybrid metal-N/C sites have rarely been reported. Herein, we fabricated Co SACs with tunable isolated Co-N 5-x C x (x = 1, 2, 3) sites supported on porous carbon frameworks. Benefiting from the difference in electronegativity of the coordinated N and C atoms, the CO Faradaic efficiency (FE CO) enhanced considerably from 54% to 76% and 92% at −0.8 V vs. RHE for Co-N 4 C 1 , Co-N 3 C 2 , and Co-N 2 C 3 , respectively. Further density functional theory (DFT) calculations uncover that the increased ratio of Co-C coordination induced electron enrichment of Co atoms and upper-shift the d -band center to near Fermi level, and thus favorably promotes the electron-donating ability of Co centers and strengthens the adsorption of *COOH. [ABSTRACT FROM AUTHOR]

Table of contents for mechanism model of Cu-N 3 and Cu-N 4 moieties. • Highly exposed isolated Cu-N x sites were embedded in microporous carbon frameworks. • CO 2 -to-CO conversion was boosted with tailored Cu–N coordination environment. • Cu-N 4 exhibited a maximum FE CO of 98% and over 90% from −0.6 to −1.1 V vs. RHE. • Edge-hosted Cu-N 4 was identified as the accurate configuration of active site by DFT calculations. Although considerable progress has been achieved by Cu nanoparticles for catalyzing CO 2 reduction reaction (CO2RR), Cu single atom catalysts (Cu SACs) are generally suffered from inferior performance to that of widely investigated Fe, Co, Ni SACs. This phenomenon mainly ascribes to the lack of effective geometry and electronic engineering of copper active center from an atomic level. Herein, highly exposed atomically dispersed Cu-N x (x denotes Cu–N coordination number) sites anchored on 3D porous carbon matrix are successfully synthesized through facile one step thermal activation, and Cu-N 4 sites exhibit boosted activity and selectivity compared to its nearly inert Cu-N 3 counterparts. Aided by density functional theory (DFT) calculations, the edge-hosted Cu-N 4 moieties are revealed as key active sites for efficient CO generation via optimized local coordination environment and electronic properties, which strongly interact with *COOH intermediate and facilitate the desorption of *CO. As a result, Cu-N 4 catalyst achieves high CO Faradaic efficiency (FE CO) of over 90% from −0.6 to −1.1 V vs. RHE with a maximum value of 98%, surpassing the previously reported Cu SACs for CO 2 -to-CO conversion. This work provides new insight into proper Cu SACs design and fundamental mechanism understanding to boost CO2RR. [ABSTRACT FROM AUTHOR]