Study On Flexible All-carbon Electrodes For Supercapacitors

Supercapacitors, also called electrochemical capacitors, have been the most promising energy storage devices, due to its high power density,rapid charge-discharge ability, long cycle life and excellent safety performance. With emerging of portable or wearable electronic devices and multifunctional integrated circuit, flexibility has become an urgent and mainstream demand for energy storage equipment. Flexible supercapacitor, comparing with other flexible energy storage devices, has the characteristics of simple structure, excellent safety, high power, long cycle life and light-weight. As is known, the electrode material determines the overall performance of the energy storage behavior. And in flexible supercapacitor electrodes, the carbon-based material, such as graphene and carbon nanotube, is usually used as a flexible framework to build a 3D self-supporting composite structure. In this research, graphene was leading as the 3D structure cell of the flexible architecture, and the function of the self-assembly of graphene nano sheets was used to make the integrated self-supporting flexible electrode.1. The aqueous solution of graphene oxide was prepared by modified Hummer's method. Then, through different preparation methods, three kinds of flexible graphene films were made, including filtration molding-heat reduction, chemical reduction (liquid phase)-filtration molding and filtration molding-chemical reduction (gas phase). On this basis, the three different kinds of graphene flexible film were applied to the supercapacitor as the flexible electrodes and electrochemical behaviors were compared and analyzed in detail. The results showed that as electrodes of electrical double-layer capacitor, electrochemical behaviors of the three kinds of electrodes were similar, and energy storage performances were different, but not significant (147-171 F g-1).Further studies found that the electrochemical performance of filtration molding-thermal reduction graphene films is optimal. Furthermore, this preparation method is simple, easy operation and the product exhibits a smooth surface, better mechanical properties, easier to prepare composite electrode.2. An integrated flexible composite electrode was made of activated carbon powder and graphene nano sheets by blending in solution and filtration-thermal reduction method. By adjusting the mass ratio of graphene oxide and activated carbon, a series of composite membranes were obtained and applied in inorganic electrolyte (6 mol.L-1) symmetric supercapacitor system. Due to high specific surface area the activated carbon possessed, with the increase in proportion of activated carbon composite flexible electrode (from 0% to 89%), the specific surface area of the electrode (from 70 to 2250 m2 g-1) and gravimetric capacitance of the whole electrode (from 171 to 302 F g-1) were significantly improved.Meantime, without any addition of conductive agent, the amount of active material was up to 89%, the graphene bond electrode still maintains the integrity of the "three-dimensional embedded structure" and good cycling stability. In this part, a novel type of flexible high-performance self-supporting carbon composite electrode was prepared and in which provided a simple and feasible method with composite proportion controllable.3. Choosing a hierarchical porous carbon with pore size widely distributed as active material and graphene nano-sheets as binder to prepare graphene-bonding electrode, at the meantime, a comprehensive comparison was carried out with conventional powder-paste-molding method electrode. The graphene-bonding electrode exhibits an outstanding advantage in electrode conductivity (0.177 S m-1-0.337 S m-1),specific surface area (from 2201 m2 g-1to 2561 m2 g-1), pore size distribution, electrode structure and inorganic/organic electrolyte electrochemical energy storage capability (capacity increased by 28%,321 F g-1 in 6 mol.L-1 KOH and 41%, 147 F g-1 in 1 mol.l-1Et4NBF4/AN).4. Furthermore, this novel method-graphene co-assemble with other carbon particle into an integrate composite architecture, can be applied as a universal method for preparation of flexible electrodes. It further extended to various porous carbon with different particle sizes, different morphologies and different pore size distributions (polymer-based porous activated carbon, activated porous carbon fiber and activated porous carbon sphere). Moreover, in this method, graphene was played a role of binder with high conductivity, high flexibility, porosity and self-assembly characteristic. It also overcomes adverse effects of reduced conductivity and pore-blocking from traditional polymer binder and the energy storage performance significantly improves from the conventional paste electrode(mass capacity increased by 27% -40%, volume capacity increased by 4%-24%). Besides, this graphene-bonding method successfully pioneered a brandnew flexible energy storage application from various porous carbon particles and inspired a new idea for further designing flexible composites.