Boosting faradaic efficiency of CO2 electroreduction to CO for Fe-N-C single-site catalysts by stabilizing Fe3+ sites via F-doping

The atomically dispersed Fe3+ sites of Fe-N-C single-site catalysts (SSCs) are demonstrated as the active sites for CO2 electroreduction (CO2RR) to CO but suffer from the reduction to Fe2+ at similar to -0.5 V, accompanied by the drop of CO faradaic efficiency (FECO) and deterioration of partial current (J(CO)). Herein, we report the construction of F-doped Fe-N-C SSCs and the electron-withdrawing character of fluorine could stabilize Fe3+ sites, which promotes the FECO from the volcano-like highest value (88.2%@-0.40 V) to the high plateau (> 88.5%@-0.40--0.60 V), with a much-increased J(CO) (from 3.24 to 11.23 mA.cm(-2)). The enhancement is ascribed to the thermodynamically facilitated CO2RR and suppressed competing hydrogen evolution reaction, as well as the kinetically increased electroactive surface area and improved charge transfer, due to the stabilized Fe3+ sites and enriched defects by fluorine doping. This finding provides an efficient strategy to enhance the CO2RR performance of Fe-N-C SSCs by stabilizing Fe3+.