3-Dimentional Metal (Metal=Fe, Co, Ni) Sulfide/graphene nanocomposite for high performance energy storage applications

In this thesis, cobalt sulfide based electrode materials for supercapacitors have been studied systematically. Cobalt sulfide nanoparticles were successfully synthesized by electrochemical deposition methods. Both constant current density and cyclic voltammetry (CV) methods were applied. Comparing the specific capacitance values of the products from different methods, the sample under cyclic voltammetry had a higher specific capacitance of 2982.25 F/g compare to 320.16 F/g by using the constant current method at 1 A/g charging density. However, the sample electrodeposited by CV had extremely low stability during the electrochemical tests, in which the retention dropped to 16.9 % of initial capacitance after 3000 cycles. Carbon-based materials then were introduced as additives for cobalt sulfide electrode. 3-dimentional (3D) carbon material vertical graphene (VG) was proven to enhance the electrochemical performance of this kind of supercapacitors. Vertical graphene has unique 3D porous morphology and large surface area with good conductivity. The enhanced migration of the ions improved the specific capacitance after the cycling process. In detail, the ions migration between cobalt sulfide nanoparticles and graphene was owing to highly conductive graphene materials. Therefore, electrochemical performance with graphene was better than that of cobalt sulfide directly deposited on nickel foam during the charging/discharging processes. In this way, an excellent specific capacitance of 2649.25 F/g at 1 A/g charge density and 150.7 % retention after 3000 cycles was reached with electrodeposited on vertical graphene. In order to improve the retention performance, anchoring metal sulfide to carbon-based vertical graphene was proved to increase the retention after 3000 cycles from 16.9 % to 150.7 %, which have great potential to achieve both high capacitance and cycling stability for electrodes