Research On Porous Carbon Electrode Materials For Electrochemical Supercapacitors

Electrochemical supercapacitor is a new type of energy storage device between traditional capacitors and rechargeable batteries. Because of its high specific power and long life and other outstanding advantages, in recent years, supercapacitors have attracted great interest in energy storage applications. However, its energy density is much lower than that of secondary batteries, such as lithium-ion battery, so many researchers are committed to improve the energy density of supercapacitor system. According to the energy equation of supercapacitors E=1/2 CV2, two effective apporaches can be used to improve the energy density of supercapacitors:one is increasing the specific capacitance of electrode material (C), the other one is to promote the output voltage (V). Based on this analysis, in order to improve the energy density of the porous carbon electrode, firstly, the researches of this thesis focused on the optimization of pore structure, by adjusting the pore size, pore connectivity and building hierarchical porous structure to improve the utilization of surface area, so improve the specific energy of electrode materials. Secondly, we creatively synthesize mesoporous carbon nanotubes/nanowires with a variety of novel structure and mesoporous carbon nanofiber arrays by MnO2 and natural carb shll templates, and the porous structures increase the surface area of he carbon materials which is benefit to improving the specific capacitance. Meanwhile, the nanotube and nanofiber structures will reduce the internal resistance of the electrode material, so increase the capacitor working voltage.1. Synthsis of three-dimensional ordered mesoporous carbon sphere arrays and its electrochemical performance in supercapacitorCurrently, activated carbon materials with a high surface area are widely used in capacitors, but because there are a lot of closed pores and disordered pore connectivity, the utilization of the surface area is low, thus it contributes little to specific capacitance. Ordered mesoporous carbon has attracted much attention due to its highly ordered pore structure and narrow pore size distribution in mesopore range. A novel three-dimensional ordered mesoporous carbon sphere arrays (MCSAs) with surface area of 601 m2/g, a large pore size of 10.4 nm and a window size of 60 nm was successfully synthesized through a two-step template method by the combination of the surfactant-templating organic resol self-assembly, colloidal crystals of polystyrene and silica hard templating routes. MCSAs deliver a larger specific area capacitance of than commercial activated carbon (14?F/cm2,84 F/g). The good performances might be owing to its large ordered pore size and the continuous connected windows between the spheres which are favorable for the penetration of electrolyte and ion transportation at high scan rates, so it shows a higher surface utilization and high specific capacitance. Also the arrays of electrode materials and the graphite structure should have better electrical conductivity, so the rate capability and cycling performance are greatly improved.2. Synthsis of hierarchical ordered mesoporous/microporous carbon and its electrochemical performance in supercapacitorMesoporous carbon has excellent rate capability characteristics, but its specific energy has been subjected to its low surface area. While there is an anomalous increase in carbon capacitance at pore size smaller than 2 nm. We develop a facile approach to prepare hierarchical ordered mesoporous/microporous carbon (OMMC) by synthesizing ordered mesoporous nanocrystalline titanium-carbide/carbon composites, followed by in situ chlorination of carbides. By adjusting the content of Ti in TiC, carbonization temperature, temperature of chlorine treatment, we regulate the micropore size, the mesopore size and the microporous/mesoporous ratio. The obtained hierarchical porous carbon materials consist of a narrow mesopore with size of 3.0 nm, and micropores on the walls with size of 0.69 and 1.25 nm. The ordered mesoporous channels ensure retention and immersion of the electrolyte, serving as a favorable ion-path for electrolyte penetration and allowing for fast ionic transport into the bulk of the OMMC particles, good electronic conductivity, thus show an excellent capacitance retention at high discharge rates for pulse power applications. At the same time, the micropores drilled on the mesopore walls allow the produced OMMC to exhibit a specific surface area up to 1917 m2/g and a specific capacitance up to 146 F/g for an outstanding energy density. The unique hierarchical porous structure contributes to attractive capabilities as a promising material in energy storage with both high energy density and high power density.3. Preparation of hierarchical mesoporous/microporous carbon from natural organism precursor and its electrochemical performance in supercapacitorUsing the template method and chlorination treatment method can prepare hierarchical porous carbon materials, but it is time-consuming, operation-complicated, and with a bad impact on the environment. However, novel carbon sources, seaweeds, are naturally occurring, simple and environmentally benign. Hierarchical mesoporous/microporous carbons are produced by freeze-drying and carbonization processes using various seaweeds as carbon sources. Different hierarchical porous carbon materials come from different type seaweed after carbonization, and the application in supercapacitor show a good electrochemical performance with a capacity of 150 F/g in 6 M KOH solution. Meanwhile, treatment with ZnCl2 as pore-enlarge activation was successfully achieved by a simple pyrolysis process. It improves the surface area, making it sufficient for electrolyte infiltration and storage, and providing a more efficient ion transmission channel, so it shows a lager specific capacitance of 194 F/g. Mesoporous materials can improve the power density, and the micropores play a crucial role in improving the energy density. We can also adjust microporous/mesoporous ratio through activation, in order to optimize the electrochemical performance.4. Direct synthesis of different morphology mesoporous carbon nanotubes using MnO2 as template and their electrochemical performance in supercapacitorsBecause of the stability of surface properties and high conductivity, carbon nanotubes are an ideal electrode material for use in high voltage capacitor. But the specific surface area of carbon nanotubes is small, so the capacitance contribution is very limited. Therefore, we use MnO2 as hard template, which is environmental friendly and morphology controllable, combined with the soft template self-assembly process, to prepare mesoporous carbon nanotubes, mesoporous carbon nano-thorn microspheres and mesoporous carbon nanowires in nanotubes. Mesoporous carbon nanotube with a high surface area of 1079 m2/g and a large pore size of 9.6 nm show a capacitance of 124 F/g. MnO2 can be easily removed by HCl, so greatly reduce the costs, improve the safety of experimental operation. Electrochemical test results show that the nanotube structure can reduce the internal resistance of carbon materials, thus improves the actual working voltage; meanwhile, a large number of mesoporous on the carbon nanotubes also contribute to the surface area, so that the material has a higher specific capacitance. In addition, the large mesopores and large inner tubes provide more channels for the electrolyte rapid penetration and ion transmission, thereby reducing the internal resistance of nanotubes, and improve the rate capability and cycling performance. At the same time, this synthesis method can be easily extended to preparation of other nano-sized mesoporous materials.5. Synthesis of highly ordered mesoporous carbon nanofiber arrays from a crab shell biological template and its electrochemical performance in supercapacitorsThe low surface area of carbon nanotubes/wires restricts the capacitance in supercapacitor. However, arranged them in an orderly array will not only increase the surface area, but also improve its conductivity. We develope a natural biological template approach for the successful synthesis of novel highly-ordered mesoporous carbon nanofiber arrays by combining surfactant-templating self-assembly of organic resols with natural crab shell hard-templating process. The obtained materials consist of a mesoporous carbon nanofiber (70 nm in mean diameter and 11 nm in mesopore), an interspacing void of 70 nm between nanofibers, and 1 micrometer of pores between nanofiber arrays. The structure of arrays and the large mesopores in the nanofiber greatly increase the surface area of materials to 1270 m2/g, and effectively provides transport channel for electrolyte ion, so it has a higher specific capacitance of 152 F/g. The micro-sized array structure also has higher electronic conductivity than activated carbon and the accumulation of single carbon nanotube/line, so it reduces the internal resistance, and ensures a higher actual working voltage, better rate capability and cycling performance. In addition, the biological templates are abundant, sustainable, low cost, environmental friendly, structural complex and ordered. Furthermore, the biomimetic strategy may present a new possibility for the synthesis of various nano-structured mesoporous materials with special morphologies.6. High-temperature treatment of mesoporous carbon nanowire arrays and its high voltage application in supercapacitorsIn the process of the preparation and purification of carbon nanutubes, the impurities of organic functional groups will be introduced on the surface, which may irreversibly reacted with the electrolyte at high voltage, thus affecting the charge voltage and specific capacitance of the capacitors. By further treatment of mesoporous carbon nanowires in nitrogen at high temperature, we remove the surface organic functional groups off. Tests show that the C=O, CO and OH groups on the carbon surface have a reduction with certain degree;and as the temperature further increased, the graphitization degree of the carbon materials increased accordingly. The above factors help to improve electrical conductivity, so it shows an improving specific capacitance at higher voltage of 4 V. Higher treatment temperatures effectively reduce the organic impurities functional groups on the surface of carbon nanowires, to avoid irreversible reaction at high voltage. So it improves the rated voltage of capacitors and the coulomb efficiency. The treatment of surface also increases the infiltration ability of electrolyte and reduces the internal resistance of the electrode material, thereby enhancing the rate capability and cycling performance of supercapacitor.