【作者】 黄宛真；

【导师】 张孝彬；

【作者基本信息】 浙江大学 ， 材料物理与化学， 2005， 博士

【摘要】 人类越来越意识到能源危机所带来的灾害以及环境保护的重要性,世界各国纷纷寻找高效、环保、安全的可再生能源,制定一系列计划并花大量人力、物力、财力在开发研究氢能系统、锂离子电池以及超级电容器等方面,重点集中在能源材料的开发与研究。而随着一维纳米碳材料制备技术的日趋成熟,纳米碳管及纳米碳纤维在储能方面展示出良好的应用前景。由于纳米碳管和纳米碳纤维是由石墨卷曲而成的一维结构,具有比表面积大、密度低、导电性好以及优异的力学性能等特点,是储氢系统、锂离子电池以及超级电容器的理想材料。而鱼骨状纳米碳纤维由于互相平行的石墨层面的端部终止于纤维的外壁,暴露于纤维表面约0.34nm的层间距可提供多种原子及离子插入石墨层的机会,在储能方面具有潜在应用价值。基于此,本论文对本实验室制备工艺较成熟的多壁纳米碳管及鱼骨状纳米碳纤维进行系列预处理,并研究预处理方式对其储能特性的影响。 采用商业泡沫镍为催化剂,CVD法制备纳米碳纤维。该纳米碳纤维的石墨层面与其轴线的夹角在0～90°之间,为鱼骨状结构;由于制备温度较低,其石墨化程度不佳、石墨面不平整。分析讨论生长温度、氢气还原时间、生长时间及气氛流量等实验参数对产物形貌、结构及产量的影响,结果表明制备鱼骨状纳米碳纤维的最佳生长条件为:生长温度550℃、氢气还原时间3h、生长时间2h;气氛流量分别为:氮气300sccm、氢气50sccm、乙炔100sccm。该条件下制备的鱼骨状纳米碳纤维具有产率高、纯度高以及重复性好等优点。同时针对催化剂形貌及与之相联系的纳米碳纤维的关系,建立—“两端同时生长模型”讨论鱼骨状纳米碳纤维的生长机制。 采用CoO为催化剂、CVD法制备多壁纳米碳管,对其进行纯化、球磨、退火及掺杂处理,并研究各种预处理方式对多壁纳米碳管在室温常压下气态储氢性能的影响。结果表明退火及掺杂处理明显改善纳米碳管的储氢性能。而氮气退火比氧气退火效果明显,因为在氧气气氛中退火时会在纳米碳管表面引入大量有机含氧官能团,恶化其储氢性能。对纳米碳管进行KNO₃液相掺杂时则通过引入钾离子扩大氢气进出纳米碳管的孔道,从而提高纳米碳管的储氢量;掺杂纳米碳管的储氢量随掺杂液浓度的提高而提高,1.0M时得到最大储氢量3.2wt%。室温下更多还原

【Abstract】 With the development of human civilization, many challenges are presented to human being, such as crisis of fresh water and air pollution etc., one of which being felt and met for several decades is energy supplying. Many attentions have been focused on hydrogen system, lithium-ion batteries and super-capacitors. However, the materials suitable for these energy systems have not been found yet. It is said carbon nanotubes and graphite nanofibers could solve this problem because of their unique structure and novel properties such as high surface area, good conductivity and mechanical property etc. On this purpose, base on the large-scale synthesis of carbon nanotubes and graphite nanofibers in our laboratory, the hydrogen storage capacity, lithium insertion/extraction property and super-capacitor property of carbon nanotubes and graphite nanofibers are investigated in this paper.Fish-bone graphite nanofibers, where the angle between graphite sheets and fiber axis is 0~90° was synthesized on nickel foams by CVD method. The effect of synthesis parameters, such as growth temperature, growth time, hydrogen treated-time and gas flow ratio, on the formation and the yield of graphite nanofibers were studied in details. It is found that high yield and high purity fish-bone graphite nanofibers can be experimentally fabricated repeatedly after 3h hydrogen pretreatment of the catalysts and with the nitrogen flow of 300sccm, acetylene flow of lOOsccm and hydrogen flow of 50sccm for 2h at 550℃. The growth mechanism of fish-bone graphite nanofibers has also been discussed.Multi-walled carbon nanotubes were synthesized by cobalt-catalytic decomposition of acetylene. A series of pretreatments including purification, annealing and doping were performed before carrying out hydrogen storage experiments at room temperature and modest pressure. The results suggest that both annealing and doping processes play an important role in the hydrogen storage capacity of carbon nanotubes. Under the same conditions conducted, MWNTsannealed in nitrogen adsorbed more hydrogen than those annealed in air. The hydrogen adsorption capacity increased not only after doped in KNO3 solution but also with the increase of solution concentration. An optimal result (3.2%) was obtained when carbon nanotubes were annealed in nitrogen at 500 °C and doped in 1.0 mol/L KNO3 solution. Hydrogen desorption experiments were also carried out at room temperature. It is found that after a typical adsorption/ desorption cycle under ambient temperature, the MWNTs samples still retained significant amount of stored hydrogen. This phenomenon may be attributed to the existence of two types of adsorbed hydrogen in the structure: physical and chemical adsorptions, the former can be released easily and the latter is strongly bound and could be released more difficultly.Acid-treatment, ball-milling and heat-treatment of fish-bone graphite nanofibers were carried out and their effects on hydrogen adsorption capacity at room temperature and modest pressure were studied. It is found that there were shorten graphite nanofibers and end-opened graphite nanofibers after acid-treatment and ball-milling. The result also showed that the graphitization of graphite nanofibers was increased after heat-treatment. However, hydrogen adsorption capacity of fish-bone graphite nanofibers was not obviously increased, where the best result of hydrogen adsorption capacity was less than 0.5wt%. It indicated that fish-bone graphite nanofibers were not suitable for hydrogen storage system.Multi-walled carbon nanotube bundles were synthesized on Mo/MgC>4 catalysts by CVD method. Series of pretreatments such as oxidation, ball-milling, alkali-treatment and acid treatment were employed in multi-walled carbon nanotube bundles, and their lithium insertion/extraction properties were studied. The results showed that these pretreatments introduced defects or open ends to MWNTs. However, the lithium insertion/extraction property of MWNTs was not obviously increased due to the oxygen function groups, SEI film and the capillarity of MWNTs. A model of lithium insertion/extraction on MWNTs was proposed and the result showed that lithium insertion/extraction capacity on MWNTs is only 1/3-1/2 of the theoretical capacity of idea graphite (372mAh/g), which is consistent to our experiment results.The influence of different pretreatments such as acid-treatment and heat-treatment on the lithium insertion/extraction properties of fish-bone graphite nanofibers was also investigated. It is found that both acid-treatment and heat-treatment have positive and negative effects on lithium insertion/extraction properties of fish-bone GNFs. However, the fish-bone GNFs both treated by acid and heat-treatment showed good lithium insertion/extraction properties, where the insertion/extraction efficiency in first cycle was increased and the cycling capacity and stability were improved.Fish-bone graphite nanofibers were used as the electrode of super-capacitor and their property had been studied. The results of CV (Cycle Volt-ample) showed that after acid-treatment, fish-bone GNFs electrode includes both double-layered capacity and Farad capacity. The constant current charge/discharge curves indicated that the capacitor property of fish-bone GNFs electrode in acid electrolyte was better than in alkali electrolyte due to the increased Farad capacity in acid electrolyte. The capacity of fish-bone GNFs electrode in acid electrolyte and alkali electrolyte was 96F/g and 80F/g respectively, and the electrolyte showed good cycling stability after 10 cycles.更多还原