Impact of iron precursors on the properties and activities of carbon-supported Fe-N oxygen reduction catalysts

Two iron precursors (iron(III) chloride and iron(II) ammonium sulfate) with a nitrogen-containing compound (ethylenediamine) were pyrolyzed on commercially available carbon blacks (Vulcan XC-72) under a nitrogen flow in this study. The properties of the resultant nonnoble electrocatalysts (Fe(Z)N/C-2 and Fe(Z)N/C-8) effected by using different iron precursors during the synthetic process were investigated by X-ray-based spectroscopies including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS). The electrocatalytic performance toward oxygen reduction reaction (ORR) of these electrocatalysts was also comparatively studied by rotating disk electrode and chronoamperometric techniques. The obtained results indicate that iron precursors play an essential role on the chemical microstructure, elemental states, and ORR performance of electrocatalysts. The electrocatalysts (Fe(III)N/C-8) prepared by using iron(III) chloride as starting precursors exhibit better electrochemical ORR activity and durability among all the synthesized catalysts. As evidenced by XPS and XAS studies, we conclude that this may be due to the formation of active FeN_x sites, more surface Fe/C and N/C atomic ratios, and the coexistence of pyridinic-N and graphitic-N species in the Fe(III)N/C-8 electrocatalyst.