Sulfur-Doped Fe–N–C Nanomaterials as Catalysts for the Oxygen Reduction Reaction in Acidic Medium

Iron atoms coordinated with nitrogen (FeNx) sites in Fe–N–C catalysts are widely accepted as the main active sites for the oxygen reduction reaction (ORR). Introducing sulfur (S)-functionalities in the carbon to tailor FeNx active centers can further promote their ORR performance in acidic media. Here, we report an approach to synthesize the FeN4 active sites supported on N/S-codoped graphitic carbon (FeN4/NSC) by the in situ addition of thiourea molecules into Fe-doped zeolitic imidazole framework (ZIF-8) precursors and subsequent pyrolysis. The thiourea molecules positively provoke changes in the catalyst structure, including S-doping into the graphitic carbon, improved density of FeN4 active sites, and an increase in the Brunauer–Emmett–Teller surface area (up to 760 m2 g–1). FeN4/NSC exhibits an enhanced ORR activity with a higher half-wave potential of 0.83 V (vs reversible hydrogen electrode (RHE)) compared to that of pristine FeN4/NC (0.80 V), in addition to superior durability with a small activity decay that occurs after 40 000 cycles. The improved catalytic performance is mainly due to the high site density of FeN4 sites and electron-withdrawing S-doping.