Activation, Doping And Supercapacitive Properties Of Graphene Nanomaterials

Portable supercapacitor has many advantages of high power density, small size, flexibility, wide potential window, no electrolyte leakage(safety) and ease of processing, etc. It could be used in portable, wearable and flexible electronics which have received extensive attention. Among these, thin and flexible electrode is the key for these supercapacitors. A two-dimensional plane material as an electrode material should be with good mechanical properties, can be folded arbitrarily, and without structural collapse. At the same time, it could avoide noticeable performance loss as supercapacitor. Graphene has a large theoretical specific surface area, good electronic conductivity, good mechanical strength and has been used in supercapacitor. However, van der Waals force between the graphene sheets makes the sheets stack seriously, blocking the diffusion of the electrolyte ions, causing the actual inter-graphene low specific surface area utilization with worse electrochemical properties of graphene. Therefore, in order to expand the value of graphene films, researchers active the graphene oxide. Surface activation could introduce large number of oxygen-containing functional groups on graphene oxide. These functional groups can not only weaken the Van der Waals force between the graphene sheets preventing stacked sheets blocking, providing unobstructed passage for the electrolyte ion transport, but also provide active sites for subsequent chemical modification, which changes its electrochemical properties effectively. In this thesis we use different activators to activate graphene oxide film and graphene oxide solution, then dop N and P element and study its electrochemical properties. This commerical prospects method is simple and fast. Main studies are as follows:1. Graphene oxide film is served as base material, and HF as an activator in the introduction of a large number of oxygen functional groups on graphene oxide surface. Then ammonia as the doping and reducing agent in the preparation of nitrogen-doped graphene film material. HF can introduce a large number of pore structure in the graphene oxide surface, increasing the electrolyte ion transport while maintain the film-like structure of graphene oxide film. On the one hand the introduced pores can provide active sites for doping nitrogen element. Doped nitrogen element can improve the wettability of the graphene film, increase electrochemical reaction efficiency, and provide pseudo-capacitance performance. The electrochemical test results show that: the specific capacitance can reach 282 F g-1 at the current density of 1 A g-1, which is higher than the specific capacitance of graphene film’s 245 F g-1 without activation treatment. At a current density of 10 A g-1, its specific capacitance can still be maintained at 177 F g-1. At a current density of 1 A g-1 the capacity retention ratio is 87 % after 10000 charge and discharge test.2. Strong oxidizing agent potassium permanganate as an activator to active graphene oxide solution. Then vacuum filtration has been used to obtain a porous graphene oxide film, after that porous graphene film is reduced and doped by ammonia solution through hydrothermal reaction. Since the same charged functional groups repel each other between graphene oxide sheets hinder sheet stacking, after that, subsequent hydrothermal incorporation of nitrogen, and intercalated water molecules increase the distance between the graphene sheets, which provides a three-dimensional pore structure for electrolyte ions transmission. The specific capacities can reach 313 F g-1 at the current density of 1 A g-1 which is much higher than the the specific capacities of graphene film’s 270 F g-1 without activation treatment. When the current density increase to 10 A g-1, the specific capacitance can still be maintaine at 183 F g-1, the capacitance retention rate is 58 %.3. The porous graphene oxide solution which activated by potassium permanganate serve as the substrate material and achieve nitrogen and phosphorus doping simultaneously by hydrothermal method. Nitrogen doping on the one hand can increase the electrode materials and electrolyte wettability, on the other hand can provide Pseudo-capacitance. Phosphorus doped can forming C-P bond to substitute irreversible C-O bond which can broaden the potential window. Electrochemical test results show that the potential window in aqueous electrolyte can be increased to 1.5 V, the energy density can up to 12.1 Wh k g-1. At the same time, the energy density of the material can up to 67.8 Wh kg-1 in an organic system. At a current density of 1 A g-1, the capacity retention ratio is 98 % after 5000 charge-discharge test.