Regulating the Local Microenvironment of an Fe−N₄ Single-Atom Catalyst for Enhanced Oxygen Reduction Reaction

Tailoring the local microenvironment of a single-atom active site is an effective pathway to promote electrocatalytic performance. Herein, we introduced a sulfur site to address the undesirable electron selectivity of a single-atom Fe−N4 catalyst for oxygen reduction reaction (ORR). Comprehensive X-ray spectroscopic results, coupled with theoretical calculations, reveal that the introduction of a second-shell S site optimizes the local electron distribution of the Fe−N4 moiety, enhancing its ORR activity. Furthermore, in situ synchrotron spectroscopy demonstrates that the presence of S in the Fe−N4 moiety facilitates the breakage of the O−O bonding by reinforcing the adsorption of *OOH intermediates, accounting for enhanced ORR activity and four-electron selectivity. The well-designed catalyst exhibits satisfactory alkaline ORR activity, featuring a half-wave potential of 0.93 V versus reversible hydrogen electrode. Importantly, it effectively suppresses the yield of hydrogen peroxide to 4%, achieving a maximum power density of 241.1 mW cm−2 in a Zn–air battery.