Rambutan-like cobalt nickel sulfide (CoNi2S4) hierarchitecture for high-performance symmetric aqueous supercapacitors

[Display omitted] •A novel hierarchical CoNi2S4 nanostructure resembling the rambutan fruit is synthesized by hydrothermal method.•The hierarchical rambutan structure delivers excellent electrochemical properties in both three and two electrode systems.•A high specific capacitance of 1102.22Fg−1 is obtained at a current density of 1Ag−1.•The symmetric supercapacitor exhibits high power and energy density, and long-term cycling stability. A novel hierarchical cobalt nickel sulfide (CoNi2S4) nanostructure resembling the rambutan fruit was obtained by a simple one-step hydrothermal method using ethylene glycol as the solvent. Scanning and transmission electron microscopic analyses revealed that the as-synthesized CoNi2S4 consists of numerous hairy nanorods grown radially on top of individual sphere-like core structures. The CoNi2S4 hierarchical material was applied as a potential electrode for supercapacitors in both three- and two-electrode systems. The hairy nanorods and spheres were highly interconnected to form hierarchical rambutan-like CoNi2S4 structure with increased active areas of the electrode, and this facilitate effective charge transport from the nanorods to the spherical core structure. When evaluated as an electrode material in a three-electrode system, CoNi2S4 with the hierarchical structure delivered a high specific capacitance of 1102.22Fg−1 at a current density of 1Ag−1 with excellent rate capability (68.55% capacitance retention as the current increases from 1 to 10Ag−1) and significant cycling stability (75% retention after 3000 cycles). The electrochemical properties of the hierarchical CoNi2S4 were also investigated in a symmetrical cell arrangement using 2M aqueous KOH as the electrolyte. The symmetric aqueous supercapacitor exhibited a specific capacitance of 482Fg−1 at 1Ag−1 with maximum energy density of 16.74Whkg−1 and maximum power density of 10.2kWkg−1. Furthermore, the symmetric supercapacitor exhibited excellent cycling stability, showing 92.85% capacitance retention even after 5000 cycles.