Preparation of Activated Carbon-Supported Iron Oxide Composite as a Supercapacitor Electrode Material

The growing demand for sustainable and high-performance energy storage systems has stimulated the development of biomass-derived electrode materials for supercapacitors. In this study, activated carbon (Cm-AC) was synthesized from Crataegus monogyna seeds and combined with magnetic iron oxide (Fe3O4) nanoparticles to fabricate a hybrid Cm-AC@Fe3O4 electrode. The incorporation of iron oxide into the activated carbon matrix altered the charge-storage mechanism from predominantly electrical double-layer capacitance to a hybrid process with a significant faradaic (diffusion-controlled) contribution. Electrochemical measurements in 1 M Na2SO4 showed that the specific capacitance increased from 145.92 to 270.68 F g−1 at a scan rate of 5 mV s−1. Dunn kinetic analysis revealed that diffusion-controlled faradaic processes accounted for up to 87.82% of the total capacitance at low scan rates, consistent with a dominant contribution from reversible Fe2+/Fe3+ redox reactions. Galvanostatic charge–discharge analysis yielded a specific capacitance of 351.51 F g−1 at 0.5 A g−1, while electrochemical impedance spectroscopy demonstrated reduced internal and charge-transfer resistances after Fe3O4 incorporation. A symmetric Cm-AC@Fe3O4 supercapacitor retained 87.5% of its initial capacitance after 10,000 charge–discharge cycles and delivered a maximum energy density of 10.35 Wh kg−1 at a power density of 396 W kg−1. These findings demonstrate that biomass-derived Cm-AC@Fe3O4 is a promising and sustainable electrode material for high-performance supercapacitor applications.