Synthsis, Characterization And Applications Of Carbon Spheres Based On Hydrothermal Carbonization Process

The discovery of fullerene boosted the scientists’ interest and development of shaped nano carbon materials. The shaped carbon materials include carbon nanotubes, carbon nanofibers, carbon onions, carbon spheres, carbon nanorods, and carbon capsules, etc. Among others, spherical carbon materials (carbon spheres) attracted great concerns due to the high mechanical strength, excellent loading performance and low flow resistance. Thanks to these unique characteristics, carbon spheres have been extended to various applications such as catalyst supports, adsorption materials, printing inks, intensifiers for rubber and electrode materials.The commercialized synthetic strategy using coal tar pitch or petroleum heavy oil as raw materials and some laboratory methods (e.g., templating method, arc discharge method, or chemical vapor deposition method) always suffer from the drawbacks of complicated operating process, high production cost and wide size distribution of the prepared spheres. The shortcomings of these methods accelerate the steps of developing greener, more energy-saving, and more sustainable approaches for carbon spheres.Hydrothermal carbonization (HTC) of carbohydrates is an ideal costless and highly sustainable process to produce carbon spheres. Reports have demonstrated that both biomass (e.g., bagasse, rice) and their building blocks (e.g., glucose, fructose, cellulose, starch and cyclodextrin) could be transfrommed into carbon spheres with size between several nanometers to dozens of micron by hydrothermal treatment at a low temperature range of 150 ℃-300 ℃. Although holding many advantages, the wide size distribution, the crosslinking, the low surface area, and the poor repeatability of the spheres are still the common problems remain to be solved. To address these issues, we developed the polyelectrolyte-directing HTC process and the air-activation method for HTC carbon spheres.The main results are summarized as follows:1. Production of monodisperse, uniform carbon spheresThe crosslinking and wide size distribution are common features for the carbon spheres produced by HTC processes no matter what the raw materials are, which limits the applications of the carbon spheres to a great extend. In this thesis, we take glucose as the carbon source, PAANa as the structure-directing agent to synthesize uniform, monodisperse carbon spheres. We found that only 0.3 g/L of PAANa was enough for this transformation, much less than the concentration of glucose (160-400 g/L). The addition of PAANa didn’t affect the surface functionalization of the obtained carbon spheres. The production yield and repeatability of the HTC process were both improved.The as-prepared carbon spheres were completely monodisperse without crosslinking or aggregation. The sphere size was characterized by narrow size distribution. The sphere size could be tuned between 400-1000 nm by varing the reaction time, temperature and glucose concentration. The obtained soft carbon spheres could be directly carbonized to hard carbon spheres with better structure order at high temperatures or used as template for hollow oxides.2. Fabrication of carbon spheres with diameter under 100 nmIt is great challenge to synthesize carbon spheres with size under 100 nm because the high surface energy of small carbon spheres would drive them to grow to large ones with size of more than 300 nm. By adding PSS as the structure-directing agent, the HTC of carbohydrate could result in carbon spheres with a size range of 60-80 nm. The small nanospheres could construct a 3D porous structure by packing or slightly aggregation. The BET surface area of the material can reach up to 78 m2/g, much larger than that of traditional HTC material (<5 m2/g).3. Production of hierarchically porous carbon spheres by air activationUsing air as the activating agent, porous structure was introduced into the carbon spheres at high temperatures. After activation at 1000 ℃, porous carbon spheres with BET surface areas of 1704 m2/g and pore volume of 1.22 cm3/g with 51% mesopore ratio could be prepared. The carbon spheres activated at 900 ℃ exhibited high adsorption capability for dyes, especially for methylene blue. The adsorption capacity reached 1068 mg/g, much better than that of the traditional activated carbon (100-1000 mg/g). This method eliminated the disadvantage of complicated process, severe pollution problems and lacking of mesopores in the product for traditional activation methods using strong acids or bases.4. Design of hierarchically porous carbon material composed of carbon spheres3D macro-meso-microporous carbon mateirals were fabricated by air activation of carbon spheres produced by PSS-assisting HTC method. The porous structure was characterized with BET surface area as high as 1306 m/g, pore volume of 1.01 cm3/g and mesopore ratio of 63.9%. When used as the supercapacitor electrode material, the activated carbon spheres showed a specific capacitance of 170 F/g and low equivalent series resistance.To sum up, we developed electrolyte-directing HTC processes for the production of carbon spheres with tunable size and structure. An air activation method was also designed to introduce hierarchically porous structure into HTC carbon spheres. These developed carbon spheres were extended to some promising applications. We expect that our research would contribute to the further design and synthesis of HTC materials.