| Supercapacitor is a new energy storage device between traditional electrostaticcapacitors and second battery. In recent years, supercapacitors have drawn considerableattention because of its high power rating, long cycle life and broad using temperaturerange. However, its energy density is lower than that of secondary battery. How toimprove the energy density of supercapacitors is the core subject in this field and theelectrode material is a key factor influencing the performance of supercapacitor.Graphene, two-dimensional single-atom-thick carbon material, is considered as idealelectrode material for supercapacitors due to its excellent physical and chemical properties,such as tremendous specific surface area, high conductivity, thermal stability. However,the practical capacitance measured is no more than half of theoretical capacitance ofgraphene. It is necessary to improve the capacitance of graphene through activation,modification and combination with other materials. Three effective approaches areconducted in this thesis in order to improve the electrochemical performance ofsupercapacitors. First is nitrogen doping modification of graphene surface, the second ispreparing porous graphene and the third is preparation of graphene-based composites withunique structure. The details of research are as follows:1. A pressure-promoted low temperature approach was investigated to preparenitrogen doped graphene (NG). The doping can be successfully realized at the temperatureas low as150℃by heating GO and NH4HCO3in a sealed autoclave. The influences ofthe weight ratio of GO/NH4HCO3, the heating temperature and the heating time forpreparation of NG were studied. In addition, the influence of N-configurations at grapheneplane on the electrochemical performance of NG was investigated. The results show that the nitrogen content of NG increases with the increment of mass ratio and reaction time.The study of electrochemical performance demonstrates that pyridinic-N is very importantto improve the capacitance of NG. Furthermore, the NG exhibits a high specificcapacitance of170F/g at0.5A/g, and a high retention rate of96.4%of its initialcapacitance after10000cycles at a current density of10A/g.2. Few-layer graphene with few defects and high conductivity was prepared fromnatural flake graphite using a modified solvent-exfoliated method. Graphene microspheres(GMS) with3D porous structure were fabricated from two-dimensional graphenenanosheets through spray-drying process. And a serious of PANI/GMS composites weresynthesized from solution with different aniline concentration (CAN). Electrochemicalmeasurements confirm that when CANis0.05M, the performance of PANI/GMScomposite is prime, with a high specific capacitance of350F/g at a current density of20mV/s, as well as the retention ratio of90.5%after10000cycles at the scan rate of100mV/s.3. A simple method of preparation of porous graphene was proposed. The mixture ofmetal powder and graphene oxide was treated at high temperature under the protection ofinert gas atmosphere. The porous graphene was obtained after remove the metal with acid.The influences of processing temperature, holding time and type of metal for preparingporous graphene were studied and the electrochemical performance of porous graphenewas also investigated. The results demonstrate that when the temperature reaches up to700℃and above, nickel can react with graphene forming porous. Iron can also be used toprepare porous graphene, however other metals, such as copper, zinc, titanium andaluminum, are difficult to react with GO. Electrochemical measurements confirm that atthe scan rate of20mV/s, the specific capacitance of porous graphene is107F/g, which ismore than twice that of reduced graphene oxide. |
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