Unexpected effect of second-shell defect in iron-nitrogen-carbon catalyst for electrochemical CO2 reduction reaction: A DFT study

Metal-nitrogen-carbon catalysts (M-N-C) with single-atom active site are highly efficient catalysts for electrochemical CO₂ reduction reactions (CO₂RR). Abundant M-N-C catalysts have been developed, and the coordinated adjacent nitrogen atoms as first-shell environment have been the focus of research of activity-tuning. However, the effect of second-shell carbon environment around the metal-nitrogen moiety is still unclear. Moreover, it is confusing for the discrepancy between the experimental onset potential of around –0.2 V (vs. reversible hydrogen electrode, RHE, unless otherwise noted) and theoretical predictions of –0.5 V or higher by the widely-used computational hydrogen electrode (CHE) model. Herein, using the explicit solvent model and constant potential method (CPM), the electrochemical interface on Fe-N-C is simulated for CO₂RR. It reveals that the *COOH formation is facilitated in water solvent environment, while the CO₂ adsorption is potential-dependent. The predicted onset potential of around –0.2 V on Fe-N-C is consistent with experimental results. The sp² non-hexatomic defects introduced into second-shell carbon environment are significantly influential for the CO₂RR. The double five-seven ring (5577) defect is the most active, compared to that with triple five-seven ring (55577) or five-eight ring (58) defects. The highly flexible structure and altered density of states of Fe site induced by 5775 defects are key to CO₂ adsorption. This study provides new insights into the role of second-shell carbon environment for effective CO₂RR, and underlines the importance of CPM and solvent environment in accurate simulation for electrochemical interface.