Revisiting the Role of Sulfur Functionality in Regulating the Electron Distribution of Single-Atomic Fe Sites Toward Enhanced Oxygen Reduction

Introducing sulfur functionalities is reported as an efficacious route to regulate the electron distribution of single-atomic Fe sites for improving the oxygen reduction reaction (ORR) activity, however, it remains controversial about the role of type of sulfur functionalities in enhancing the ORR activity. Herein, this study revisits the role of sulfur functionalities in regulating the electron distribution of single-atomic Fe sites by the construction of single-atomic Fe-N-4 sites with sulfur functionalities (FeSNC) using sulfur-containing molecules with different molecular structures. X-ray absorption spectroscopy and theoretical calculations confirm that thiophene S and oxidized S both possess the electron-donating properties for regulating the electronic distribution of Fe-N-4 sites, which is beneficial to weaken the adsorption of the ORR intermediates. As expected, the optimal FeSNC has attractive ORR activity with half-potential of 0.76 V in 0.5 m H2SO4 and 0.91 V in 0.1 m KOH along with remarkable Zn-air battery performance. Moreover, the developed synthetic method has also been extended to construct B and P regulated FeNC catalysts.