Structure Modification And Performance Of Graphene Based Electrode Materials For Supercapacitors

Supercapacitors, which combine the advantages of high power density, long cycle life, and high reliability, are a competitive solution to electrical energy storage in various applications, such as new energy, electronic devices, electric vehicles, etc. At present, the development of supercapacitors is focused on increasing the energy density while maintaining their high power nature. Based on the calculation equation for energy density E=1/2（CV2）, the energy density（E） could be enhanced by:（1） developing active materials with high specific capacitance（C）;（2） designing of capacitor systems with high cell voltage（V）. In this dissertation, aiming to develop active materials with high specific capacitance, we use graphene oxide as starting material, based on the unique properties of graphene oxide（reducible, modifiable, as well as the molecular and anionic behavior）, we have designed and prepared three types of electrode materials for supercapacitor. In addition, in order to achieve high cell voltage, an asymmetric supercapacitor based on MnO₂ and graphene as positive and negative electrode materials, respectively, has been designed and fabricated.By using the molecular behavior of graphene oxide, carbon spheres have been successfully converted from graphene oxide by adjusting the hydrothermal environment. With the oxyacids（sulfuric and phosphoric acids） or strong reductant（hydrazine） as additives, graphene oxide can be converted to carbon sphere via structure reorganization. Because of the thermo-unstable core, the graphene oxide converted carbon spheres exhibit a unique stimulus-responsive behavior（volume expansion and ballooning） when the electron beam is used as external trigger. However, because of the formation of carbon sphere is via extensive aggregation of graphene oxide, the inner area of the carbon sphere is not accessible to the electrolyte, thus the existence of GO derived carbon sphere in the hydrothermal reduced graphene shows negative effect on the electrochemical performance.The surface of graphene was decorated by Mn₃O₄ nanoparticles（⁵nm in size） through hydrolysis of manganese acetate in a reduced graphene oxide dispersion with a mixed solvent of N,N-dimethylformamid and water. Appropriate content of Mn₃O₄（58 wt.%） could not only effectively prevent the compact restack of graphene sheets during drying process, but also avoid the self aggregation of Mn₃O₄ nanoparticles, thus increase the electroactive surface of the composite material. The graphene/Mn₃O₄ composite has a specific capacitance of 343 F/g at a discharge current density of 2 m A/cm², and maintains at 215 F/g when the current density increases to 50 m A/cm².By using the anionic behavior of graphene oxide, a graphene oxide/polypyrrole（GO/PPy） composite film with thickness of 50μm was prepared by electrochemical co-deposition in a precursor aqueous solution of pyrrole, graphene oxide and lithium perchlorate. The GO/PPy composite film is assembled by three dimensionally interconnected PPy coated GO sheets, thus providing interconnected tunnels for fast diffusion of electrolyte ions through the film electrode. The morphologies of the composite film can be well controlled by adjusting the concentration of GO in the precursor solution. The GO/PPy composite electrode with small mass loading of 0.26 mg/cm² exhibits the highest specific capacitance of 481.1 F/g, while the electrode with a larger mass loading of 1.02 mg/cm² delivers the best area capacitance of 387.6 m F/cm², at a current density of 0.2 m A/cm². Moreover, the GO/PPy composite electrodes all exhibit good rate capability with capacitance retentions over 80% when the current density load increased by 50 times（from 0.2 to 10 m A/cm²）. Both of the aqueous and solid-state supercapacitors based on GO/PPy composite electrodes show excellent electrochemical capacitive properties with good cycling stability. The aqueous supercapacitor with a cell voltage of 0.8 V could deliver an energy density of 16.4 Wh/kg at a power density of 80 W/kg, and maintains 78%（12.9Wh/kg） of the energy density at high power density of 4000 W/kg.An asymmetric MnO₂//graphene supercapacitor with high cell voltage of 2.0 V was fabricated via matching the electrode potential window of positive MnO₂ and negative graphene electrodes with 1mol/L sodium sulfate aqueous solution as electrolyte. The MnO₂/graphene asymmetric supercapacitor shows an energy density of 25.2 Wh/kg at a power density of 100 W/kg, which is much better than those of symmetric MnO₂//MnO₂ supercapacitors（4.9 Wh/kg） and graphene/graphene supercapacitor（3.6 Wh/kg）. Moreover, the asymmetric system exhibits capacitance retention of 96% after 500 cycles.