Effect of Substrate and Pyrolysis Atmosphere of FeN x Materials on Electrocatalysis of the Oxygen Reduction Reaction

In this work, several PGM-free materials supported on three types of carbon blacks (Vulcan XC-72, Monarch-1000, and Ketjenblack EC-600JD) and pyrolysed on N 2 and NH 3 atmosphere are investigated as catalysts for the oxygen reduction reaction (ORR) in acid and alkaline electrolytes. The synthesized materials are characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), energy-dispersive X-ray analysis (EDX), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The electrochemical characterization shows that for all N 2 - and NH 3 -treated catalysts, the ORR activity is higher in alkaline than acidic conditions, though in both cases, the main reaction product is water, thus evidencing a predominant occurrence of a 4-electron oxygen reduction mechanism. Further, a reasonable relationship is established between Fe-N x coordination (pyridinic-N) and pyrrolic-N contents and the ORR activity of catalysts in acid media (FeN x supported on Monarch-1000). In alkaline conditions, the iron coordination with pyridinic-N, coupled to a high carbon surface area (Ketjenblack), enhances the ORR activity regardless of the pyrolysis atmosphere. Finally, the ORR performance in alkaline medium of the FeN x /Ketjenblack catalyst pyrolysed under N 2 atmosphere is essentially the same as that achieved for the benchmark catalyst (Pt/C), while the durability of the best catalysts evidence to be quite promising in both acidic and alkaline media, as shown by a standard accelerated stability test. Graphical abstract