Structural Design And Property Studies Of Hollow Carbon Nanomaterials

The development of advanced energy-storage materials and adsorbents has been extensively explored in order to address the energy and environmental challenges. Due to their advantageous features including the merits of carbon and nanomaterials, the new carbon nanomaterials have seen practical use in many fields, including supercapacitors, water treatment, lithium ion battery, photochemical cell, catalyst, etc. Generally, hollow carbon nanomaterials can be classified into two categories:tubular carbon nanomaterials and hollow carbon nanospheres. Carbon nanotubes (CNTs) as a significant class of tubular carbon nanomaterials have been widely explored as the electrode materials. Although CNTs have great electrical conductivity due to their unique carbon framework structure, their main drawback is their relatively small specific surface area, which limits their practical application in supercapacitors. Hollow carbon nanospheres are widely adopted carbon-based adsorbents. However, it has been well-established that the adsorption performance of the carbon materials is highly dependent on their pore size distribution. Hollow carbon nanospheres with micropores are effective treatment of heavy metal and/or organic matters with low molecular weight. In this dissertation, a research has been made on the synthesis of the new hollow carbon nanomaterials and their applications in supercapacitors and the removal of humic acid. The main conclusions are depicted as follow.(1) The synthesis involves the preparation of rod-like templates of NHC in a reverse micellar system, sequential coatings of RF resin and silica onto the templates, thermal treatment at high temperatures to carbonize the resorcinol-formaldehyde (RF) resin, and finally removal of the silica shell and remaining nickel to release tubular carbon nanorods. A Tecnai T12transmission electron microscope (TEM), X-ray diffraction (XRD), the nitrogen adsorption-desorption isotherms, and FT-IR spectroscopy have been used to study the physicochemical properties of the as-prepared carbon nanotubes. We have demonstrated that the aspect ratio of the tubular carbon nanorods can be tuned in the range5-80by controlling the hydrazine/nickel ratio; the thickness of RF resin shell could also be controlled in the range5-8nm by adjusting the amount of RF solution.(2) The effects of carbonization temperature on zeta potentials, surface areas, crystallinity and electrochemical performances of tubular carbon nanorods were studied. In order to further investigate the electrochemical characteristics of tubular carbon nanomaterials, a series of electrochemical tests and comparisons were performed on tubular carbon nanorods carbonized at1000℃. The specific capacitance of tubular carbon nanorods is132F/g at a current density of0.9A/g in the potential range of0-1V. Specifically, the energy density is7.8Wh/kg at power density of22.50kW/kg which far exceeds the power requirement of "Partnership for a New Generation of Vehicles"(PNGV,15.00kW/kg).(3) We report the preparations of carbon nanoshells, porous carbon nanoshells and the porous wrinkled carbon shells (WCS) through a template-mediated process, and the synthesis of carbon nanospheres by sol-gel method. Carbon nanospheres and carbon nanoshells possess mircopores which result from the pyrolysis of the RF resin shell, while porous carbon nanoshells possess mircopores and mesopores which result from the templates. Furthermore, the pores of WCS are radial.(4) A adsorption studies of carbon nanoshell, porous carbon nanoshell, WCS and activated carbon were conducted, and it was found that the hollow structure and radial pores are beneficial to the removal of humic acid. We have demonstrated that the thickness of WCS can be tuned by controlling the RF precursor. The effects of the thickness of carbon shell on zeta potentials, pore sizes, pore volumes, surface areas, and surface areas of WCS were studied. The WCS possess an enhanced adsorption performance as demonstrated by a series of analyses, such as the adsorption capacities and adsorption kinetics. Furthermore, equilibrium study of optimized WCS were examined and the Langmuir equation represents a better fit of the experimental data.