Design, Fabrication And Electrochemical Performance Study Of The 3D Graphene-Based Supercapacitors

Currently, eco-friendly and renewable clean energy such as solar energy, wind energy, and heat energy had been developed rapidly. At the same time, research and development of new type of energy conversion and storage devices with eco-friendly protection, high energy density and high power density became a key factor for promoting the development of new energy industry. Supercapacitor focused on by physical mechanism to store energy is a novel energy storage devices that bridges the gap between conventional capacitors and batteries, and possesses many advantages such as large capacity, high power density, fast charge and discharge, good cycle stability, free maintenance, and environmental pollution-free. However, the biggest problem of supercapacitor is that its energy density is too low. The synthesis of high performance electrode materials is the key factor for research and development of supercapacitor with high energy density and high power density that can compare with the battery.Graphene composed of sp2 hybridization of carbon atoms is a one-atom-thick two-dimensional carbon nano-material with honeycomb structure.Due to high specic surface area, high electrical conductivity, and chemical stability, which are favorable to the formation of electric double layer capacitors, graphene is regarded as the ideal electrode materials of supercapacitor. In this paper, three dimensional porous graphene and nitrogen-doped graphene had been synthesized by the hard template method, and studied as supercapacitor electrode materials.The main research contents and results are as follows:1. The three-dimensional porous graphene/nickel foam composite electrode was synthesized by the application of coating solution and reduction method with eco-friendly ascorbic acid. In the reduction process, graphene oxide (GO) is transformed into the high conductivity of graphene network and three-dimensional continuous and interconnected microporous structure with dependent on the porous foam nickel. The microporous structure is favorable to shorten the ion diffusion path, and facilitates the efficient diffusion of electrolyte ions to the surface of graphene, thereby effectively promote the charge transfer efficiency, and improve the utilization of active material. The results show that the specific capacitance is as high as 152 F/g, exhibited high rate capability performance, which specific capacitance still amounted to 107 F/g at 90.9 A/g of the test current, and good cycle stability that the attenuation rate of specific capacitance was only 11% after 2000 cycles test. The method is simple and requirements for equipment is low, but as a result of the GO directly deposited on the surface porous skeleton nickel foam, as-prepared graphene was formed a good contact with nickle foam,which can effectively reduce the contact resistance that provides the good condition of high rate capability performance. This experiment technology is easy to be master, and is likely to be applied to industrial levels on a large scale realize large-scale production in the industry. On the basis of this work, the graphene/nickel foam composite electrode was further etched by the hydrochloric acid, ably in the implementation of lightweight nickel preparation of conductive network and set in the three dimensional porous graphene formed an organic whole.The strategy preparation of composite electrode has the characteristics of lightweight flexible, and maintain a good specific capacity and rate performance.In addition, we adopt a simple assembly method to assemble the light thin flexible capacitor devices, in order to realize its practical application.2. On the basis of the first work, we further design and synthesize lightweight nickel network supported 3D flexible graphene composite foam electrode with using commercial 3D porous foam nickel as a template by taking advantage of shrinkage characteristic of HI acid-reduced graphene film thickness and anticorrosive effect of graphene for metal. With the support of the lightweight nickel conductive network, conductivity, mechanical strength and flexibility of graphene foam composite electrode with the three-dimensional structure have been improved significantly.3D lightweight graphene/nickel network compound foam completely "copy" the three-dimensional network structure of nickel foam, and nickel and the mass of its the conductive network can be as small as 1.1 mg/cm2. The results of the electrochemical research show that the specific capacitance of graphene active material is about 100 F/g, but on the based of calculation by the whole electrode mass, the specific capacitance can reach up to 40.9 F/g. A supercapacitor device based on the 3D graphene/nickel network composite foam exhibited high rate capability, good cycle stability, and excellent flexible properties. Our work provides a universal route for the rational design of high rate capability performance of supercapacitors with 3D lightweight flexible characteristics.3. In order to further improve the specific capacitance of graphene, we applied a photochemical reduction combined with ammonia reduction method to achieve the fast reduction and nitrogen doping of graphene oxide deposited in the three-dimensional porous foam. As compared to the chemical reductions and thermal treatments, photoreduction have been considered as a method with cheap but efficient, green and environmental protection, cleanliness, and tuneable reduction degree via irradiation. The nitrogen doped content of graphene reaches a high of 5.99 at% with 15 min of irradiation by high-pressure Hg lamp, and nitrogen-doped graphene composite electrode with high performance was obtained by irradiating for 30 min. with the support of the van der Waals force and nickel foam, nitrogen-doped graphene films was closely attached on the surface of nickel skeleton, which is favorable to overcome the fragile nature of graphene oxide film in the photoreduction process, and thereby significantly reduces contact resistance between the N-rGO sheets and the metal current collector. The combination of Nickel-based continuous and interconnected microporous structure and nitrogen doping of graphene greatly improve the electrochemical performance of graphene. Using KOH solution as the electrolyte, the specific capacitance of nitrogen doped graphene is as high as 253.3 F/g at 0.29 A/g of the test current; even at 92.3 A/g, specific capacitance still rearch up to 190 F/g, indicateing its high rate capability performance. By using Li2SO4 electrolyte and Li2SO4/KOH mixed electrolyte, the voltage window can be increased to 2 V and 1.5 V, respectively; the specific capacitance is as high as 235 F/g and 272.7 F/g, respectively; The energy density is as high as 32.6 Wh/kg and 21.2 Wh/kg, respectively. As compared to the pure Li2SO4 or KOH electrolyte, the Li2SO4/KOH mixed electrolyte more efficiently balances the energy density and power density.