Influence of the Purification Strategy on the Activity and Stability of FeNC Catalysts for the Oxygen Reduction Reaction

FeNC materials are a promising alternative to substitute platinum catalysts in proton exchange membrane fuel cells (PEMFCs) for electrocatalysis of the oxygen reduction reaction (ORR). However, it remains an active research challenge to improve their stability, based on the interconnection of the multiple degradation mechanisms such as carbon corrosion, active site (FeN₄) demetalation, decrease of hydrophobicity, etc. In this study, we compare the impact of different purification strategies involving ball milling and forming gas (N₂/H₂) heat treatment followed by acid leaching designed to remove efficiently the remaining Fe-inorganic species after pyrolysis, which may promote the degradation of the catalyst and membrane during FC tests. Through a comprehensive structural characterization of the final composition by X-ray diffraction, X-ray absorption, and X-ray emission spectroscopies and carbon morphology (transmission electron microscopy and Raman spectroscopy) of the FeNCs, we achieved an efficient removal of Fe₃C species, accompanied by distinct alterations to the carbon morphology, which prove to be crucial for the material’s stability. Our results highlight the enhanced stability of the catalyst treated under N₂/H_2, which retained 82% of its initial current density while held at 0.5 V for 24 h in H₂/Air during FC testing. With this study, we prove the importance of FeNC catalyst’s purification and its benefit in fuel cell activity and stability.