Nanostructured cobalt sulfide-on-fiber with tunable morphology as electrodes for asymmetric hybrid supercapacitors

Porous cobalt sulfide (Co₉S₈) nanostructures with tunable morphology, but identical crystal phase and composition, have been directly nucleated over carbon fiber and evaluated as electrodes for asymmetric hybrid supercapacitors. As the morphology is changed from two-dimensional (2D) nanoflakes to 3D octahedra, dramatic changes in supercapacitor performance are observed. In three-electrode configuration, the binder-free Co₉S₈2D nanoflake electrodes show a high specific capacitance of 1056 F g^-1at 5 mV s^-1vs. 88 F g^-1for the 3D electrodes. As sulfides are known to have low operating potential, for the first time, asymmetric hybrid supercapacitors are constructed from Co₉S₈nanostructures and activated carbon (AC), providing an operation potential from 0 to 1.6 V. At a constant current density of 1 A g^-1, the 2D Co₉S₈, nanoflake//AC asymmetric hybrid supercapacitor exhibits a gravimetric cell capacitance of 82.9 F g^-1, which is much higher than that of an AC//AC symmetric capacitor (44.8 F g^-1). Moreover, the asymmetric hybrid supercapacitor shows an excellent energy density of 31.4 W h kg^-1at a power density of 200 W Kg^-1and an excellent cycling stability with a capacitance retention of ∼90% after 5000 cycles.