<p>Although numerous electrode materials exhibit high specific capacitance and strong initial electrochemical performance, achieving long-term stability under prolonged charge-discharge cycling remains a significant challenge. The gradual decline in electrochemical properties over time hinders their practical applicability in commercial energy storage systems. In this study, Cu–Bi–S nanocomposites capped with cetyltrimethylammonium bromide are synthesized via a microwave-assisted route, offering enhanced electrochemical stability as a negative electrode for supercapacitor applications. The nanocomposites, comprising CuS, Bi₂S₃, and metallic Bi, were prepared using varying molar concentrations of CTAB as a surfactant. The effect of CTAB concentration on the evolution of surface morphology and electrochemical characteristics has been comprehensively examined. The optimised electrode material exhibited a high specific capacity of 828 ± 8.4&#xa0;C g^− 1 at 10 mV s^− 1 and 586 ± 6.6&#xa0;C g^− 1 at 4&#xa0;A g^− 1 in a three-electrode configuration using 3&#xa0;M KOH electrolyte. Notably, it showed a meagre 6% degradation after 5000 cycles. An asymmetric device assembled with Cu–Bi–S as the negative electrode and activated carbon as the positive electrode delivered a high energy density of 77 ± 0.8 Wh kg^− 1 and a power density of 5100 ± 4&#xa0;W kg^− 1, while retaining 94% of its capacity after 8000 cycles.</p> Graphical Abstract <p></p>