Fe-embedded graphitic carbon nanofibers for efficient urea electrooxidation and stable oxygen reduction reaction in acidic media

Abstract

Fe nanoparticle-incorporated carbon nanofibers (Fe–CNFs) were successfully synthesized via electrospinning of a ferrous acetate/PVA solution followed by vacuum drying and calcination at 850 °C. SEM and TEM analyses confirmed the formation of uniform nanofibers containing well-dispersed metallic Fe nanoparticles encapsulated within partially graphitized carbon layers. XRD revealed the presence of crystalline α-Fe together with minor Fe₂O₃ and graphite reflections. Magnetic measurements demonstrated ferromagnetic behavior at both 5 K and 300 K with a clear divergence between ZFC and FC curves, confirming the presence of multi-domain Fe nanoparticles embedded in the carbon matrix. Electrochemically, the Fe–CNFs showed pronounced activation behavior in 1.0 M KOH, where strong Fe(II)/Fe(III) redox peaks in the first cycle diminished upon cycling due to surface reconstruction and the formation of Fe–oxyhydroxide species. Activated fibers exhibited excellent catalytic activity toward urea oxidation, achieving current densities above 120 mA cm⁻² in 2.0 M urea at 50 mV s⁻¹ and demonstrating a negative shift in onset potential with increasing urea concentration. In acidic medium (0.5 M H₂SO₄), the fibers displayed stable capacitive behavior with suppressed Fe redox activity. Remarkably, the Fe–CNFs exhibited strong oxygen reduction reaction activity in acid along with exceptional durability. Additionally, electrospinning directly onto a silicon wafer produced a compact Fe–CNF film (≈7.5 µm thick) strongly fused to the substrate, forming a conductive and mechanically stable coating suitable for integrated electrochemical devices Overall, the results demonstrate that the introduced Fe–CNFs combine robust structural stability, magnetic functionality, and versatile electrocatalytic performance, highlighting their potential as low-cost catalysts for different applications.