Unraveling the effect of alkali cations on Fe single atom catalysts with high coordination numbers†

Fe single atom catalysts (SACs) with high coordination numbers have emerged as high-performance catalysts for the conversion of CO₂ to CO. However, the influence of alkali cations at the catalyst–electrolyte interface has not yet been understood clearly. Here, we investigate the role of various alkali metal cations (Na⁺, K⁺, Rb⁺) in catalytic CO₂ reduction reaction (CO₂RR) behavior on high coordination number Fe SACs (FeN₅ and FeN₆) obtained from a facile hard template method. We find that larger cations can greatly promote the CO₂RR and such effects are enhanced with increasing cation concentration. Nevertheless, the hydrogen evolution side reaction (HER) on co-existing N heteroatom sites will be worsened. This trade-off highlights the importance of manipulating the reactive sites for SACs. From theoretical simulation and in situ spectroscopy results, we confirm that the functioning mechanism of cations on Fe SACs lies in the enhancement of the adsorption of key intermediates through direct coordination and indirect hydrogen bonding effects. With the rationally designed Fe SACs (FeN₅) and the electrolyte conditions (1 M KOH), our flow cell test demonstrates a maximum Faraday efficiency of CO (FE_CO) of approximately 100% at 100 mA cm⁻². This research provides significant insights for future SACs and electrolyte design.