The Co₉S₈ nanoparticles are homogeneously deposited on the conductive backbone of 3D graphene (3DG) by using a glucose-assisted hydrothermal method. The activation process for the composite of Co₉S₈ and 3DG involving the consecutive cyclic voltammetry scanning in a 1 M KOH solution increases surface roughness of the composite electrode. As a result, the increase in the active surface area of the activated composite electrode leads to significant enhancement of electrode performance. Because the combination of the Co₉S₈ nanoparticles and conductive 3DG generates a profound effect on the electrode, the activated composite electrode shows a high specific capacitance of 1721 F g⁻¹ and great cycling stability at a relatively high current density of 16 A g⁻¹. Furthermore an asymmetric supercapacitor device assembled from the composite of Co₉S₈ and 3DG and reduced graphene oxide hydrogel is tested to evaluate the capacitive performance of the composite electrode in a full-cell configuration. The fabricated device is capable of functioning with an output voltage of 1.8 V and delivering a maximum energy density of 31.6 Wh kg⁻¹ at a power density of 910 W kg⁻¹. More importantly, the device exhibits great long-term stability with 86% capacitance retention after 6000 charge/discharge cycles.