Electrochemical charge-storage behavior of olive-stone-derived rGO anchored with Co-Ni ferrites

Biomass-derived carbon materials provide sustainable platforms for electrochemical energy-storage electrodes. In this study, graphene oxide was synthesized from olive stones and combined with mixed cobalt-nickel ferrites, Co _x Ni_1-x Fe₂O₄ (x = 0.25, 0.50, and 0.75), through an in situ co-precipitation route followed by thermal treatment. The treatment partially reduced GO to rGO and enabled the anchoring of spinel ferrite nanoparticles on the carbon framework. XRD, FTIR, BET, SEM, EDX, TEM, and XPS analyses confirmed the formation of rGO-supported Co-Ni ferrites with controlled Co/Ni ratios, mesoporous texture, and nanoscale ferrite domains. Electrochemical measurements in 3.0 M KOH showed that the Ni-rich Co₀.₂₅Ni₀.₇₅Fe₂O₄/rGO electrode delivered a specific capacitance of 697 F g^-1 at 1.0 A g^-1 and retained 93.5% of its initial capacitance after 10 000 cycles. EIS analysis showed lower charge-transfer resistance for this composition, indicating improved electron-transfer kinetics and electrode-electrolyte interfacial behavior. The improved charge-storage response is associated with the combined contribution of Ni/Co redox activity, reduced crystallite size, mesoporosity, and conductive rGO pathways. A symmetric two-electrode device assembled from Co₀.₂₅Ni₀.₇₅Fe₂O₄/rGO operated within 0-1.2 V and delivered 264 F g^-1. These results show how Co/Ni compositional tuning and biomass-derived rGO support affect pseudocapacitive charge storage in alkaline electrolyte.