Tuning the local coordination environment of Fe–N–C catalysts via sulfur doping for enhanced oxygen reduction

Abstract

Single iron atoms coordinated with nitrogen and dispersed within carbon-based materials (Fe–N–C) are recognized as the most active platinum group metal (PGM)-free cathode catalysts for the oxygen reduction reaction (ORR). To boost their performance, increasing the density of Fe–N–C active sites by incorporating more iron and/or modifying the local coordination structure to fine-tune the electronic properties of the sites have been key strategies. In this work, we present a highly active Fe–N–C electrocatalyst developed on a nitrogen and sulfur co-doped porous carbon substrate, derived from a polypyrrole (PPy) hydrogel. The precise selection of the complexing agent (SCN⁻) for Fe not only promotes sulfur doping and enhanced iron incorporation (up to 5.1 wt%) but alters the chemical environment of the Fe–N₄ sites, yielding an Fe single-atom catalyst (Fe SAC) with excellent ORR activity in both alkaline and acidic environments. Besides demonstrating outstanding 4e⁻ selectivity and tolerance to fuel cross-over the catalyst shows remarkable durability in accelerated stress tests and prolonged current–time response tests, making it a promising PGM-free option for sustainable energy technologies.