Template-free synthesis of hierarchical hollow NiS_x microspheres for supercapacitor

Hierarchical nickel sulfide (NiS_x) hollow microspheres can be successfully synthesized with a template-free method using α-Ni(OH)₂ microspheres as a precursor by calcination and sulfidation. The intermediate Ni hollow spheres, formed at different calcination temperatures (300°C and 400°C) under H₂/N₂ atmosphere, can be easily transformed into NiS_x with similar morphology and mixed phases of Ni₃S₂ and NiS during the followed sulfidation process. The formation processes for the hollow structure of NiS_x are also discussed in this work. Particularly, the NiS_x prepared from Ni intermediate spheres at 300°C show a high specific capacity of 153.6mAhg^-1 at 0.5Ag^-1 at high mass loading due to its small crystal size, hierarchically porous structure and high electrical conductivity. A hybrid capacitor was assembled by using it as positive electrode and activated carbon as negative electrode to examine their practical applications in a full-cell configuration. The hybrid capacitor exhibited excellent comprehensive performances in 1.6V. The hybrid capacitor also showed good cycling stability, with 81.25% of the initial specific capacitance after 1000 cycles at 2Ag^-1. Above results indicate the great potential of the NiS_x hollow spheres as a promising electrode material for supercapacitor applications.