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 Co9S8 2D nanoflake electrodes show a high specific capacitance of 1056 F g(-1) at 5 mV s(-1) vs. 88 F g(-1) for the 3D electrodes. As sulfides are known to have low operating potential, for the first time, asymmetric hybrid supercapacitors are constructed from Co9S8 nanostructures 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(-1) at a power density of 200 W Kg(-1) and an excellent cycling stability with a capacitance retention of similar to 90% after 5000 cycles.