Nanostructured cobalt oxide and cobalt sulfide for flexible, high performance and durable supercapacitors

Transition metal oxides and sulfides have great potential for energy storage devices due to their large theoretical energy storage capacities. A facile technique was used for the synthesis of nanostructured and phase pure cobalt oxide (Co3O4) and subsequently converting it to cobalt sulfide (Co9S8). The effect of sulfurization on energy storage capacity of the cobalt oxide was explored. Microstructural characterizations using X-ray diffraction and scanning electron microscopic reveal formation of phase pure and nanostructured Co3O4 and Co9S8. It was observed that the areal capacitance of Co3O4 (983 mF/cm2) improved significantly after converting to Co9S8 (7358 mF/cm2). The CV curves of the Co9S8 electrode on bending showed outstanding stability with no change in energy storage properties. New insights into the better performance of Co9S8 over Co3O4 based on electrochemical investigations are presented. The performance of the Co9S8 as an electrode material for energy storage applications was further investigated by fabricating a supercapacitor device. The supercapacitor device showed outstanding stability up to 5000 cycles of charge-discharge study. The performance of the supercapacitor was observed to be improving with temperature. The supercapacitor displayed ~100% enhancement in energy storage property on increasing temperature from 10 to 70 °C. Our results suggest that hydrothermally grown Co9S8 on nickel foam can be utilized for high capacity, flexible and binder free electrode for energy storage applications.