Synthesis Of Fish-bone Graphite Nanofibers And Carbon Nanotubes And Their Application On Hydrogen Storage System, Lithium-ion Battery And Super-capacitor

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.