Overlooked Mechanism of the Oxygen Reduction Reaction on Non-coordinated Sulfur-Doped Fe–N–C Catalysts: A Dual Enhancement in Site Density and Activity

Abstract Sulfur doping has been widely recognized as an effective strategy to enhance the oxygen reduction reaction (ORR) performance of Fe–N–C single atom catalysts (SACs). Despite the observed variations in catalytic performance, the precise role and spatial distribution of S dopants, along with the underlying mechanism, remain highly debated. To address this challenge, we establish a defect-driven modeling framework for sulfur-doped FeN4 SACs, explicitly accounting for the diversity of sulfur-related defect configurations and their structural evolution. Our calculations reveal that sulfur doping into the second and more distant coordination shells of Fe centers can effectively enhance their ORR performances. More interestingly, the adjacent carbon atoms of dopants, upon being functionalized by hydrogen or hydroxyl from the reaction environments, are also activated as efficient active centers, substantially increasing the density of active sites without increasing metal loading and broadening the accessible operational potential window. Moreover, we uncover a cooperative interplay among sulfur-containing defects, H/OH functionalization, and the FeN4 moiety, leading to a multisite ORR mechanism involving both Fe and adjacent carbon sites. Our proposed framework offers a rational explanation for the widely scattered ORR activities and provides general theoretical guidance for the rational design of chalcogen-doped SACs for various electrochemical reactions.