Unraveling the Effect of Alkali Cation on Fe Single Atom Catalysts with High Coordination Number

Abstract

Fe single atom catalysts (SACs) with high coordination number have emerged as high-performance catalysts for the conversion of CO2 to CO. However, the influence of alkali cation at the catalyst-electrolyte interface has not yet been understood clearly. Here, we investigate the role of the various alkali metal cations (Na+, K+, Rb+) on 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 CO2RR and such effects are enhanced with increasing cation concentration. Nevertheless, 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 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 condition (1 M KOH), our flow cell test demonstrate a maximum Farady efficiency of CO (FECO) of approximately 100% at 100 mA cm-2. This research provides significant insights for future SACs and electrolyte design.