Designing binder-free, flexible electrodes for high-performance supercapacitors based on pristine carbon nano-onions and their composite with CuO nanoparticles

The increasing demand for energy has triggered tremendous research efforts for the development of lightweight and durable energy storage devices. This requires exploring simple and economical methods to prepare the active materials and to design lightweight, flexible, free-standing supercapacitor electrodes in an inexpensive binder-free process. Herein, we try to address both these critical issues using CNOs and their composite with CuO as the active material. Active materials were supported on cotton wipes by a simple sonication and drying process to obtain light-weight, flexible and free-standing binder-free electrodes. In a symmetrical two-electrode cell, a pristine CNO electrode delivers a specific capacitance of 102.16 F g(-1) (20 mV s(-1)), an energy density of 14.18 W h kg(-1) and a power density of 2448 W kg(-1), which are the highest values reported so far for CNO-based materials. CNO-CuO nanocomposites demonstrate a very significant specific capacitance of 420 F g-1 (10 mV s(-1)) with deliverable energy and power density at 58.33 W h kg(-1) and 4228 W kg(-1), respectively. Electrodes of both the active materials show an excellent cyclic performance and stability, retaining up to 90-95% of their initial capacitance after 5000 charge-discharge cycles at a current density of 5 A g(-1). A simple cost estimation indicates that our device can deliver an energy density of 58.33 W h kg(-1) at an estimated cost of less than 1 $.