Improved energy storage phenomena in supercapacitor electrodes with Mn-Fe co-doped NiCo₂O₄/rGO composites

This study reports the synthesis of pristine nickel cobaltite (NiCo₂O₄) and manganese-iron (Mn-Fe) co-doped derivatives of Ni_1-xMn_xCo_2-yFe_yO₄ via co-precipitation method, followed by composite formation with reduced graphene oxide (rGO). X-ray diffraction (XRD) confirms the formation of a predominant cubic spinel phases along with minor secondary phases at higher doping levels, while Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM) reveal the bonding features as well as the uniform surface morphology. Brunauer-Emmett-Teller (BET) confirms a well-developed mesoporous structure that promotes effective electrolyte ion diffusion, while X-ray photoelectron spectroscopy (XPS) demonstrates the mixed surface chemical states and oxygen vacancies-related species, suggesting increased surface reactivity and charge transfer kinetics. The electrochemical properties are evaluated in a three-electrode setup with 3 M potassium hydroxide (KOH) electrolyte. Among all samples, the Ni_0.7Mn_0.3Co_1.4Fe_0.6O₄/rGO composite delivers the highest specific capacitance of 1639 F/g at 5 mV/s. Galvanostatic charge-discharge (GCD) studies demonstrate excellent rate performance and remarkable durability, with 94% capacitance retention after 5000 cycles at 5 A/g. Electrochemical impedance spectroscopy (EIS) analysis reveals a reduced series resistance (R_s = 0.150 Ω) compared to pristine NiCo₂O₄, confirming superior charge transfer efficiency. The synergistic effect of Mn-Fe co-doping with rGO, enhanced conductivity, active surface area, and redox kinetics, establishes these materials as strong candidates for high-performance supercapacitors.