Synthesis Of Carbon Microtube Composites And Their Application To Supercapacxtors

In recent years, supercapacitors with high power density and long-term cycling stability havebeen extensively explored and become an urgently demanded energy technologies to versatileapplications in high power electronic devices, electric vehicles or hybrid electric vehicles. As wellknown, electrode materials are the core and of vital importance for the further development ofsupercapacitors. Nowadays, carbon materials have attracted intense interests as electrode materialsfor electrochemical capacitors, because of their high surface area, good electrical conductivity, highchemical stability and low cost. Among currently rearched carbon materials, carbon microtubes（CMTs） are believed to be a type of ideal electrode materials because their tubular structure can befilled with guest molecules and serve as microchannels for ion or molecular transport. Manyartificial methods have been developed to prepare CMTs but their large-scale production is still abig challenge. Recently, we have put forward a route to prepare CMTs from plant biomass such aswillow and poplar catkins by simple carbonization, which provides a novel method to realize themass production of CMTs and the resource utilization of these pollutants simultaneously. In thisthesis, we are focusing on the further improvement of energy density and capacitance of CMTs bymodulating their structure—producing pores by activation, synthesizing carbon nanotube（CNT）-CMT and metal oxide-CMT composites. The main contents of this thesis are summarized asfollows:1. Activated CMTs have been prepared from carbon microtubes by the chemical activation of ZnCl2firstly, then we investigated the effects of different activation temperature on the structure andproperties of materials. The as-synthesized porous CMTs have large Brunauer-Emmett-Tellersurface areas of up to1400m²/g and a complex porous structure with the predominantmicropores （>80%） and small part of mesopores （<20%）. Two-electrode supercapacitor cellsconstructed with the activated CMTs yield higher values of gravimetric capacitance and energydensity with aqueous electrolyte as well as comparable stability in comparison with thecommercial activated carbon （AC）. The activated CMTs yield a specific capacitance of206F/gand an energy density of7.5Wh/kg with aqueous electrolyte, much higher than those of aactivated carbon （160F/g,5.5Wh/kg）.2. Taking the advantage of their large microtubular structure, we have grown CNTs on the inner andouter walls of CMTs by chemical vapor deposition using Ni nanoparticles as catalysts. This composite structure intergrates the merits of the high conductivity and larger surface area ownedby CNTs carbon nanotubes, reducing the internal resistance of the composite electrodes. Theprepared three-dimensional CMT/CNT composites were demonstrated to present good capacitiveproperties when used as electrode materials in two-electrode supercapacitor cells.3. Carbon materials alone usually can not satisfy the requirements of high-energy storage due totheir low specific capacitance and low energy density. An efficient method to overcome theseobstacles is to incorporate additional contributions from metal oxides. Herein, we preparedCMT/MnO₂composites using CMTs, KMnO4and PEG as raw materials at70℃. The PEG usedin this case can improve the adhesion of MnO₂and carbon microtubes. The supercapacitorproperties of CMT/MnO₂composites were tested under two-electrode system. The highestspecific capacitance of CMT/MnO₂composites could reach451F/g at a current density of1.5A/g.