Electrochemical reduction of CO2 is a sustainable solution for both CO2 remediation and fuels generation. However, the sluggish rate of CO2 reduction reaction (CO2RR) and the inevitable competition over hydrogen evolution reaction (HER) hinders the application of CO2 electrolysis. Transition metal (M) and nitrogen (N) co-doped carbon (M-N-C) materials have emerged as promising non-precious metal catalysts for CO2RR. However, the chemical nature of nitrogen-coordinated metal sites active for CO2RR has remained elusive, which hinders further catalyst development with enhanced performance. Herein, we report the exploration of understanding the reactivity and active sites of M−N4 (M = Fe and Co) for CO2RR through a combination of experimental studies and theoretical density functional theory (DFT) computations. Single atomic Fe and Co based M-N-C with well-defined M−N4 coordination without metallic phase were prepared through thermal activation of chemically Fe or Co-doped zeolitic imidazolate frameworks (ZIF), respectively, which were studied as ideal model catalysts. Electrochemical CO2RR results indicated that Fe is intrinsically more active than Co in M−N4 for the reduction of CO2 to CO, demonstrating a higher CO Faradaic efficiency (93% vs. 45%) and a more positive onset potential (−0.29 V vs. −0.38 V). By means of DFT predictions on limiting potentials in CO2RR and the competitive hydrogen evolution reaction (HER) as well as reaction energy in the dissociation of *COOH intermediate, we identified that the edge-hosted M−N2+2-C8 bridging two adjacent armchair-like graphitic layers is the active site for the CO2RR, which is much more efficient than traditionally proposed bulk-hosted M−N4-C10 embedded compactly in a graphitic layer. The DFT results also validated the higher capability of the Fe-N-C catalyst in suppressing HER than the Co-N-C. The discovery of edge-hosed M−N2+2-C8 clusters containing adjacent carbon atoms with dangling bonds provides new insight in designing highly efficient M-N-C catalysts for selective CO2 reduction to CO. In addition, this unique design may be extended to other atomic metal cen-ters beyond Fe and Co in the search of most efficient CO2RR catalysts.

Unveiling active sites of CO2 reduction on nitrogen coordinated single atomic iron and cobalt catalysts

2019 Spring National Meeting

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