Understanding the Mechanism of Fe–N–C Catalyst Oxygen Reduction Reaction Performance Enhancement: The Impact of Iron Valence State and Nitrogen Content

Optimizing non-noble catalysts for the oxygen reduction reaction (ORR) is crucial for advancing energy storage and conversion technologies. This study investigates the influence of iron valence states and nitrogen precursor contents on the physicochemical properties and electrochemical performance of lignin-based Fe-N-C catalysts. Catalysts are synthesized using different valence iron sources and varying pyrrole contents. The results reveal that iron(II) doping increases microporosity and specific surface area, while iron(III) doping enhances mesoporous and iron contents of the carbon materials. XPS and Raman spectroscopy confirm the successful incorporation of Fe-N_<i>x</i> active sites in the as-prepared LFe_IIIP_<i>x</i> catalysts. Electrochemical tests demonstrate that LFe_IIIP₁₀ exhibits the highest ORR activity, surpassing commercial Pt/C in half-wave potential (<i>E</i>_1/2) and limiting current density (<i>J</i>_L). The direct formate fuel cell (DFFC) with an LFe_IIIP₁₀ air cathode achieves a maximum power density of 24.7 mW cm^-2, 32% higher than that with a Pt/C cathode. This study highlights the critical role of tailoring iron valence states and nitrogen levels to develop high ORR performance, cost-effective Fe-N-C catalysts, providing valuable insights for the future design of non-noble metal catalysts in energy applications.