High areal capacitance and long cycling stability in asymmetric supercapacitors using binder-free, hierarchical nanostructured Ni3S2/MnO2 hybrid electrodes

Supercapacitors have become inevitable energy storage devices for portable electronics and electric vehicles. Among the various material choices, transition metal sulfides with high pseudocapacitive nature commands attention as supercapacitor electrode materials. Nevertheless, transition metal sulfides suffer from poor elec-trochemical cycling stability, which hinders their use in practical applications. Incorporating transition metal oxides into these materials to form hybrids can effectively rectify these stability issues. Here, we report a two-step hydrothermal synthesis of nanocrystalline Ni3S2/MnO2 hybrids and use them as electrodes to fabricate super -capacitors. The incorporation of MnO2 improves the surface area of the electrode, and the synergistic effect of the Ni3S2/MnO2 nanostructured hybrid helps achieve the best electrochemical performance. The Ni3S2/MnO2 hybrid electrode exhibits a high gravimetric capacitance of 806.6 F g-1 (2892 mF cm-2) at a current density of 1 A g-1 in 1 M KOH aqueous electrolyte. Along with the enhanced pseudocapacitance, the hybrid electrode exhibits exceptional cycling stability of 96.8 % even after 25,000 charge/discharge cycles performed at a current density of 5 A g-1. An asymmetric supercapacitor fabricated using activated carbon (AC) (Ni3S2/MnO2//AC) exhibits a cell capacitance of 34.8 F g-1 (139.9 mF cm-2) at a current density of 1 A g-1.