Unraveling Mechanistic Reaction Pathways of the Electrochemical CO2 Reduction on Fe-N-C Single-Site Catalysts

We report a joint experimental-computational mechanistic study of electrochemical reduction of CO2 to CH4, catalyzed by solid-state Fe-N-C catalysts, which feature atomically dispersed, catalytically active Fe-N-x sites and represent one of the very rare examples of solid, non-Cu-based electrocatalysts that yield hydrocarbon products. Work reported here focuses on the identification of plausible mechanistic pathways from CO2 to various C-1 products including methane. It is found that Fe-N-x sites convert only CO, CO2 and CH2O into methane, whereas CH3OH appears to be an end product. Distinctly different pH dependence of the catalytic CH4 evolution from CH2O in comparison with that of CO2 and CO reduction indicates differences in the proton participation of rate-determining steps. By comparing the experimental observations with density functional theory derived free energy diagrams of reactive intermediates along the CO2 reduction reaction coordinates, we unravel the dominant mechanistic pathways and roles of CO and CH2O during the catalytic CO2-to-CH4 cascades and their rate-determining steps. We close with a comprehensive reaction network of CO2RR on single-site Fe-N-C catalysts, which may prove useful in developing efficient, non-Cubased catalysts for hydrocarbon production.