Study On The Preparation Mechanism And Electrochemical Performance Of Potato Starch-Based Carbon Spheres

In view of environmental pollution and increasingly exhausted of fossil resource, renewable biomass resource as alternate resources attracts more and more attention. Starch is natural polymer and can be obtained from various renewable plant resources. Starch as carbon precursor is developed because its unique characteristics, e.g., low price, biodegradable, and containing abundant carbon element. Besides, starch granules have special shape. Therefore, starch as carbon precursor not only is able to reduce production cost, but also can prepare carbon spheres with different particle size according to the application requirement. In this research, potato starch was selected as the raw material, and carbon spheres with the original shape of potato starch were prepared by two different methods. The preparation mechanism and the main effect factor in the preparation were investigated. Furthermore, the effect of the pore structure of activated carbon spheres on the electrochemical performance was studied. The primary study of preparation mechanism of ZnCl₂ was carried out simultaneously.In this paper, two different methods were used to prepare potato starch-based carbon spheres retaining the original shape of potato starch. The stabilization-method included stabilization at 220°C and carbonization at 600°C. Stabilization is the key step in this process. The water adsorbed in the starch and containing in the chain-water crystal is lost at the first four hours. With the extension of the stabilization time, the intermolecular cross-linking and rearrangement results in the destruction of chain-chain crystallite structure, which prevents the phase transition of starch granules during the later carbonization. The impregnation-method included impregnation by NH₄Cl solution and carbonization at 600°C. NH₄Cl is adsorbed physically by potato starch during impregnation. The HCl from the decomposition of NH₄Cl promotes the dehydration of starch during carbonization, which leads to the destruction of the chain-chain crystallite structure and benefits the preparation of carbon spheres. It can be concluded that the destruction of chain-chain crystallite structure in potato starch is necessary during the preparation.Potato starch-based carbon spheres from impregnation-method show high graphitization degree after graphitization which is beneficial to the intercalation and deintercalation of lithium ions. Moreover, perfect integrity of carbon spheres prevents the exfoliation of lamellar of graphite during the charge-discharge cycle. Therefore, it exhibits a reversible capacitance of 323mAh/g as anode materials of Li-ion batteries, along with an initial coulombic efficiency of 88%. The coulombic efficiency maintains 98% after the first cycle, and the reversible capacitance can still reach 316mAh/g after 20 cycles, which indicates good electrochemical performance.Potato starch-based activated carbon spheres (PSAC) were prepared by CO₂ and KOH activation. The effect of activation conditions on the pore structure was studied, and the effect of pore structure on the electric double-layer capacitance was investigated. The results indicate there is a cavity in the center of the PSAC with high specific surface area. PSAC prepared by KOH activation shows good power and cycle performance. Sample with surface area of 2520m²/g has the highest specific capacitance of 335F/g.The activation mechanism of ZnCl₂ activation was also investigated in this thesis. The results indicate simonkolleite appears on the surface of potato starch after impregnation by ZnCl₂ solution. The simonkolleite is decomposed into ZnO, HCl and H₂O during activation. The catalytic dehydration of HCl makes the activated carbon spheres keep the original shape of potato starch. ZnO crystal as one of the decomposition product of simonkolleite is removed by HCl washing after activation, which is contributed to the surface area of activated carbon spheres.