Incorporation of Fe₃C and Pyridinic N Active Sites with a Moderate N/C Ratio in Fe–N Mesoporous Carbon Materials for Enhanced Oxygen Reduction Reaction Activity

A facile and effective route was reported to synthesize iron–nitrogen codoped mesoporous carbon nanomaterials (Fe–N–C) as a high-performance oxygen reduction reaction (ORR) catalyst by pyrolyzing glycine, polyvinylpyrrolidone (PVP), citric acid, and FeCl3 through a heat-treatment (700–900 °C) process accompanied by an acid leaching technique. Among the as-prepared catalysts, the Fe–N codoped carbon-800 after treatment with acid (Fe–N–C-800–acid) and the Fe–N–C-900 catalyst possessed excellent activity with onset potentials of 0.93 and 0.92 V, respectively, and basically identical half-wave potentials of 0.75 V as well as a higher current density of 4.3 and 2.7 mA cm–2 at −0.56 V (vs RHE) through selective four-electron transfer kinetics (n = 3.5 and 3.7, respectively). Moreover, they exhibit comparable Tafel slope values of 100 and 108 mV dec–1, respectively, compared to 20% Pt–C (99 mV dec–1) as well as excellent durability and resistance to methanol. The relevant abundance of Fe3C and pyridine N in the Fe–N–C-800–acid and Fe–N–C-900 has been proposed as the active sites in facilitating the desired ORR reaction. Interestingly, the physical and chemical characterization of all as-prepared catalysts indicated that the high Fe content in these catalysts does not play a key role in affecting the ORR performance but that a moderate N/C ratio, i.e., 1% in Fe–N–C-800–acid catalyst, is pivotal in achieving the high activity toward the ORR.