Synthesis Of Hierarchical Porous Carbon/Graphene Composite Materials And Study On Their Electrochemical Performance

Supercapacitor is considered to be a new type of energy storage devices with characters of high power density, long cycle life, and environment friendly. It becomes more and more mature and has broad application prospects in the field of new energy vehicles, information and communication, aeronautics and astronautics. Its critical component is the electrode material that is also the main factor of influencing the properties of supercapacitor. Graphene, as a new form of nano-carbon, with speciality of two-dimensional plane single atomic structure, high specific surface area(2630 m~2/g),unique electrical conductivity(107 S/m),etc. All of which make it become a kind of good functional materials, nanometer additive that has a huge potential in the field of supercapacitor. In this paper, the author discusses the preparation of grapheme materials first, and then deals with the hierarchical porous carbon materials. The composition promotes its structures and properties, which make the hierarchical porous carbon/graphene material exhibit prior electrochemical performance in supercapacitors.1. Thermally reduced graphene prepared by vacuum promoted thermal expansion of graphite oxide. The microstructure of graphene is without any significant change after annealing at various temperatures 300 oC and 900 oC, respectively. However, the number of oxygen-containing functional groups from graphite oxide falls from 27.81 wt.% to 9.04 wt.%. And oxygen-containing functional groups are almost removed under the high temperature heat treatment(900 oC). The capacitance of G300 can reach up to 149 F/g, and the value decreases to 43 F/g when the annealing temperature is 900 oC. The oxygen containing functional groups can provide the abundant active site between electrolyte and electrode materials, which enhances the capacitance performance of graphene. Thus it can be seen that low temperature reduction is more advantageous to the graphene as supercapacitor electrode materials.2. Nitrogen-doped hierarchical porous carbon was successfully synthesized using block copolymers as precursor by a self-template method and subsequent KOH chemical activation. The self-template agent, polystyrene, promotes the formation of mesopores, which is favorable for the rapid transport of both electrons and electrolyte ions. The micropores of nitrogen-doped hierarchical carbons were then extensively developed by activation to provide large electrochemical double-layer capacitance. As the activation temperature increased from 600 oC to 900 oC, the specific surface area of the materials increased dramatically. Thus, the as-obtained nitrogen-doped hierarchical carbon(NHPC-800) possesses a high surface area of 2104.5 m~2/g with nitrogen content of 5.79%. It exhibits a maximum specific capacitance of 257 F/g at a current density of 0.5 A/g in 6 M KOH aqueous electrolyte as measured in a three-electrode system. Furthermore, the nitrogen-doped hierarchical carbon electrode exhibits excellent rate capability with 128 F/g remaining at 20 A/g and good cycling stability of 90.38% retention over 10,000 cycles in a two-electrode system. The tunable porous structure and effective nitrogen doping via block polymer precursor shed a light for designing advanced electrode materials for a high-performance electrochemical energy system.3. Based on the previous research results, hierarchical porous carbon/graphene composite materials were prepared from block polymer which is synthesized by the polymerization of polyacrylonitrile(PAN) and polystyrene(PS) with a few percents of G300 by a simple and effective process carbonization and activation with KOH. The composite materials have exhibited specific surface area of 2683 m~2/g and good conductivity. These outstanding properties are attributed to the following reasons: Firstly,the insertion of graphene can prevent the agglomeration of pure block polymer during the carbonization reaction. And at the meantime,hierarchical porous structure is formed due to the cross-linking, which is helpful to form more macro/meso/micropores in the later activation process.. Secondly, graphene has a good conductivity, which can form a good conductive network in the composites and thus improve the conductiviy of the composites. By exploring the amount of grapheme, the author finds that it is best when the mass fraction of grapheme is 4%. Supercapacitor devices using the composites as electrode active materials exhibits excellent electrochemical performance in organic electrolyte and aqueous systems, with the highest specific capacitance up of 270 F/g in aqueous systems and energy density of 33 Wh/kg in organic electrolyte, as well as good cycle stability.