Sulfur-Mediated Microenvironment Modulation of High-Density Fe-N₄ Sites for High-Efficiency Oxygen Reduction and...

Single-atom catalysts (SACs) featuring Fe-N₄ active sites hold significant potential for the oxygen reduction reaction (ORR). However, achieving high-density Fe-N₄ active sites while precisely modulating their microenvironment to enhance ORR activity remains a formidable challenge. Here, an S-mediated strategy is presented for the preparation of Fe single-atom-loaded S,N-doped carbon (FeNSC). This strategy leverages the interactions between S and N during pyrolysis to significantly suppress N loss, thereby achieving a high density of Fe-N₄ sites. Concurrently, the precise doping of S into the second coordination shell of Fe-N₄ centers modulates their electronic structure, leading to a significant weakening of ^*O and ^*OH intermediates adsorption during the ORR. Consequently, the FeNSC catalyst exhibits excellent pH-universal ORR performance with half-wave potentials of 0.928 V (0.1 M KOH), 0.806 V (0.1 M HClO₄), and 0.755 V (0.1 M phosphate buffer solution). A FeNSC-based quasi-solid-state zinc-air battery (QSS-ZAB) achieves smooth operation over a broad temperature range of -40 to 60 °C. Notably, it sustains continuous operation for over 940 h at -40 °C, showcasing unprecedented cryotolerance. This work provides novel insights into the electronic microenvironment engineering of Fe-N₄ sites in SACs for high-efficiency ORR and cryotolerant QSS-ZABs.