<p>The development of electrode materials with high specific capacitance and long-term cycling stability remains a key challenge for advanced supercapacitor applications. In this work, NiCo₂O₄/g-C₃N₄ nanocomposite was synthesised via a hydrothermal–calcination method to promote strong interfacial interaction between NiCo₂O₄ and g-C₃N₄. The electrochemical performance of the composite electrode was evaluated using cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy techniques. In a three-electrode system, the NiCo₂O₄/g-C₃N₄ electrode delivered a high specific capacitance of 411.5&#xa0;F g⁻¹ at a current density of 1 A g⁻¹ and retained 99% of its initial capacitance after 2000 cycles. Furthermore, a symmetric supercapacitor device assembled using identical electrodes achieved an energy density of 36.54 Wh kg⁻¹ at a power density of 500&#xa0;W kg⁻¹ and exhibited 86.36% capacitance retention after 12,000 charge–discharge cycles. These results demonstrate that interfacial engineering of NiCo₂O₄ with g-C₃N₄ is an effective strategy for improving the electrochemical performance of supercapacitor electrodes.</p>