High-performance asymmetric supercapacitor based on Co9S8/3D graphene composite and graphene hydrogel

•The composite electrode of Co9S8 and 3DG is synthesized using CVD and hydrothermal methods.•The capacitive performance of composite electrode is enhanced by the consecutive CV scanning in a KOH electrolyte.•The synergistic effect of Co9S8 and conductive 3DG imparts high capacitive performance and great stability.•The ASC fabricated with the cathode of Co9S8/3DG and the anode of rGO hydrogel achieves 31.6 Wh kg−1 at 910 W kg−1.

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.

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