Decoding CO2RR selectivity and activity features on a series of well-defined iron single-atom catalysts

The electrocatalytic reduction reaction of CO2 (CO2RR) has been extensively studied, including on single-atom catalysts (SACs). However, a precise mechanistic understanding of the factors governing electron-transfer processes and Faradaic efficiency (FE) in SACs remains limited. Most established correlations address isolated aspects, such as activity or selectivity, without investigating their dependence on the critical parameter of current density. The mechanistic interpretation should elucidate all these aspects simultaneously within a well-defined series of electrocatalysts. In this series, only the electronic properties of the SAC are tuned, while all other factors should remain unchanged. We report here a well-defined series of FeN4-type SACs tailored with a fifth axial ligand, introduced via coordinatively assisted materials-surface engineering. By combining precise synthesis and extensive characterization, we have demonstrated that this series of electrocatalysts offers unique opportunities to understand these aspects. The electrocatalytic behavior of these SACFe electrocatalysts for CO2-to-CO conversion exhibits distinct trends in activity and FE with current density. In-depth computational modeling shows that these trends cannot be explained solely by the influence of the iron electronic structure and oxidation/spin state on the adsorption of key intermediates (*COOH, *CO, and *H), but rather by their effect on the ability of the metal center to accumulate and transfer charge to CO2 under operating conditions, with a clear dependence on charge localization (Bader charge). Controlled tuning of the iron electronic environment, therefore, provides a direct means to modulate charge redistribution and, consequently, catalytic performance. The structural control offered by this series is a distinctive advantage, enabling robust mechanistic insights.