Electrochemical measurements. Electrochemical ORR measurements were conducted in the three-electrode system connected to CH instruments electrochemical workstation. All data were collected in a 0.1 M KOH electrolyte. A Ag/AgCl electrode saturated with KCl and a graphite rod were used as reference and counter electrodes, respectively. Catalyst inks were prepared by adding 5 mg of the catalyst to 1.0 mL of solution containing 576 µL IPA, 384 µL water, and 40 μL of a 5% Nafion solution, followed by ultrasonication for several hours to form a homogeneous ink. The electrocatalysts studied in this work were Fe/C-1, FeSN-1/C-2, FeSN-3/C-3, FeSN-2/C-3, SN/C-3, FeS-1/C-3 and a commercial 20 wt.% Pt/C catalyst. The electrocatalyst ink was dripped onto the surface of a pretreated RDE surface (diameter 0.196 cm 2 ) and the resulting modified electrodes allowed to dry in air. Catalyst loadings were 0.38 mg/cm 2 for all electrocatalysts.