<p>Despite extensive research on ferrite-based electrode materials, the development of high-surface-area NiFe₂O₄ with enhanced electrochemical performance and long-term cycling stability remains a challenge. In this study, NiFe₂O₄ spinel ferrite nanoparticles were successfully synthesized via the polyol method and systematically investigated for supercapacitor applications. Rietveld refinement confirmed the formation of a crystalline single-phase cubic spinel structure. Fourier transform infrared spectroscopy (FTIR), Raman, and X-ray photoelectron spectroscopy (XPS) analyses verified the characteristic bonding and chemical states of Ni and Fe within the ferrite lattice. Field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectroscopy (EDAX) results revealed agglomerated spherical nanoparticles with uniform elemental distribution and a Ni/Fe ratio close to the theoretical composition. Brunauer–Emmett–Teller <b>(</b>BET) surface area of 147.857 m²/g and a pore volume of 0.555 cm³/g, confirming its mesoporous architecture. Electrochemical studies demonstrated a high specific capacitance of 676 F/g (0.676 F/cm²) at 1&#xa0;mA/cm² in 1&#xa0;M KOH, along with excellent cycling stability, retaining 80.87% of its initial capacitance after 5000 cycles. Furthermore, the electrochemical impedance spectroscopy (EIS) analysis indicated low charge transfer resistance and efficient ion diffusion. A symmetric liquid-state supercapacitor device assembled using NiFe₂O₄ electrodes delivered a maximum energy density of 26.04 Wh/kg and a power density of 892.85 W/kg, while maintaining 17.85 Wh/kg at 9&#xa0;mA/cm². These findings demonstrate that NiFe₂O₄ is a promising electrode material for advanced energy storage applications.</p>