Facile microwave-hydrothermal synthesis of NiS nanostructures for supercapacitor applications

In the present investigation, nickel sulphide nanostructures (M, H and MH) have been synthesized through three different methods viz., microwave, hydrothermal and a combination of microwave and hydrothermal. The effect of preparation method on nickel sulphide nanostructures has been investigated through structural, morphological and electrochemical studies. XRD patterns reveal orthorhombic phase of Ni9S8 in the case of M while H and MH exhibit hexagonal NiS structure. SEM micrographs of M, H and MH indicate the nanoflake, spherical and layered structure, respectively. The electrochemical studies have been carried out via cyclic voltammetry, charge-discharge studies and electrochemical impedance analysis. MH provides the largest specific capacitance of 964 F g−1 (from galvanostatic charge-discharge studies at a specific current of 1 A g−1 in 2 M KOH electrolyte) combined with a charge transfer resistance of 0.6 Ω. The MH electrode could provide undiminished capacity retention after 2000 cycles. A symmetric supercapacitor device provides maximum specific capacitance of 119 F g−1 at 1 A g−1 with energy and power densities of 16.5 W h kg−1 and 250 W kg−1. These results imply that MH nanostructure is well suited as a supercapacitor electrode material.