<p>A fabrication and performance study of an asymmetric supercapacitor which employed a two-step hydrothermally synthesized cobalt sulfide (Co₃S₄) via regulated sulfidation as the positive electrode has been documented. Novel findings on the effect of hydrothermal synthesis duration on the structural, morphological and electrochemical properties of Co₃S₄ are reported. The efficacy of the as-synthesized Co₃S₄ has been demonstrated through several characterizations including X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and nitrogen (N₂) adsorption-desorption. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) were conducted to examine the electrochemical behavior of the Co₃S₄. Among four variations of synthesis duration, the 20&#xa0;h is the most suitable duration to synthesize a highly effective Co₃S₄ active material to be utilized as a positive electrode for a supercapacitor since the as-prepared electrode exhibited a specific capacitance (<i>C</i>_<i>sp</i>) value of 480.40&#xa0;F g^− 1 at 1&#xa0;A g^− 1, which is 1.9 times higher than the lowest synthesis duration, Co₃S₄-12&#xa0;h, and 2.1 times higher than the highest synthesis duration, Co₃S₄-24&#xa0;h. The fabricated Co₃S₄-20&#xa0;h/KOH/GO asymmetric supercapacitor reveals an energy density of 9.22 Wh kg^− 1, a power density of 4000&#xa0;W kg^− 1 and 85.42% capacitance retention after 5000 charge-discharge cycles, measured at 1&#xa0;A g^− 1.</p>