New insights into the key bifunctional role of sulfur in Fe-N-C single-atom catalysts for ORR/OER.

Sulfur-doping of non-noble metal Fe-N-C single-atom catalysts (SACs) shows a key bifunctional role in promoting ORR and OER activity. The controversial claims about the enhanced ORR activity and the ambiguity of the OER activity brought about by S-doping demand in-depth investigation. Here, systematic theoretical investigation was carried out. Unlike previously believed, coordinative S-doping gives rise to a precisely regulated OOH* stabilization effect, which is revealed to be the origin of the bifunctional ORR/OER activity. The fine regulation is reflected in two aspects: (1) Compared with other intermediates, the regulation of OOH* adsorption is more obvious. (2) More sulfur-doping leads to excessive strong or weak stabilization, which is not conducive to ORR/OER. The single S doping elevates the charge density and opens the metallic spin channels of Fe-N 3 |S, moves the d-band center towards the Fermi level, all contributing to moderate OOH* stabilization. It is hoped that these results will promote the development of heteroatom-doped bifunctional SACs.