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

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 1102.22 F g⁻¹ at a current density of 1 A g⁻¹ with excellent rate capability (68.55% capacitance retention as the current increases from 1 to 10 A g⁻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 2 M aqueous KOH as the electrolyte. The symmetric aqueous supercapacitor exhibited a specific capacitance of 482 F g⁻¹ at 1 A g⁻¹ with maximum energy density of 16.74 Wh kg⁻¹ and maximum power density of 10.2 kW kg⁻¹. Furthermore, the symmetric supercapacitor exhibited excellent cycling stability, showing 92.85% capacitance retention even after 5000 cycles.