Sulfur Doping versus Hierarchical Pore Structure: The Dominating Effect on the Fe-N-C Site Density, Activity, and Selectivity in Oxygen Reduction Reaction Electrocatalysis.

Nitrogen doping has been always regarded as one of the major factors responsible for the increased catalytic activity of Fe-N-C catalysts in the oxygen reduction reaction, and recently, sulfur has emerged as a co-doping element capable of increasing the catalytic activity even more because of electronic effects, which modify the d-band center of the Fe-N-C catalysts or because of its capability to increase the Fe-N site density (SD). Herein, we investigate in detail the effect of sulfur doping of carbon support on the Fe-N site formation and on the textural properties (micro- and mesopore surface area and volume) in the resulting Fe-N-C catalysts. The Fe-N-C catalysts were prepared from mesoporous carbon with tunable sulfur doping (0-16 wt %), which was achieved by the modulation of the relative amount of sucrose/dibenzothiophene precursors.