Preparation Of Carbon Nanotubes By Electrolysis In Molten Salts And Its Electrocatalytic Properties

Carbon nanotubes, discovered in 1991 by lijima, are a new form of carbon structures. They are like two-dimensional graphite sheets wrapped into seamless hollow tubes consisting of cyclindrical network of hexagonal rings. Because of their extraordinary electrical, mechanical properties and unimaginable applications, carbon nanotubes have attracted much attention and has been an avalanche of research in the fields of physics, chemistry and materials etc. Based on the review of the research and development of carbon nanotubes, the preparation of carbon nanotubes by electrolysis in molten salts and its electrocatalytic properties were selected as objects in this thesis. By means of XRD, TEM/EDS, SEM and electrochemical measurements, electrolytic production of carbon nanotubes and its growth mechanism were studied. The electrochemical properties such as storage of hydrogen, oxygen gas diffusion electrode and electrocatalytic oxidation of methanol using carbon nanotubes as a novel nano-electrode material were investigated as well.At first, the effects of the composition of the melts and electrolytic parameters on the microstructure, morphology and phase composition of the as-prepared materials were evaluated in this paper. It was found that carbon nanotubes and the related nanoparticles were formed in situ during the electrolysis of graphite cathode in molten alkali chlorides e.g. LiCl, NaCl and KC1. The extent of carbon nanotubes formed in LiCl was highest indicating that LiCl electrolyte was favourable for carbon nanotubes' growth. Howerver, samples taken from the LiOH electrolyte contained hardly any carbon nanotubes. Addition of CaCl2,CuCl2, CoCl2, ZnCl2, NiCl2 (1-30% by weight) to the LiCl resulted in the formation of carbonaceous nanoparticles, graphite pieces and spherical metal particles reduced by the corresponding salts. However, in these experiments, no carbon nanotubes or metal-containing were nanotubes observed. Notably, the tin-containing nanotubes (metal nanowires) were produced by the electrolysis of LiCl/SnCl2 mixtures (SnCl2<10% by weight) in addition to the carbon nanotubes, and reduction of SnCl2 content would lead to the increase of nanotube yields. The p-Sn nanowires can be oxidized into SnO2 within carbon nanotubes, identified as tetragonal structure ranging from 20-50 nm in diameters. Microscopic investigations have shown that the majority of electrolytically formed carbon nanotubes were curved with outer diameters of 75~100nm and a length of up to several micrometers, similar to those prepared by catalytic decomposition and electric arcs. It was implied that the occurrence of LiC6 produced from cathodic reaction had significant influence on the formation of carbon nanostructure during electrolysis.The electrochemical hydrogen storage of carbon nanotubes and ABs-type multicomponent alloys with carbon nanotubes additives were investigated systematically. The results showed that hydrogen storage capacityvof carbon nanotubes ranged from 16.7 to 24.6 mAh/g. The polarization of ABs type multicomponent alloy electrode decreased with the addition of carbon nanotubes and improved the rate of change transfer reaction on the sunface accordingly. It was also found that addition of carbon nanotube and nickel powder improved the activation performance and high-rate dischargeability of hydrogen storage alloy electrode. For example, this alloy electrode can be completely activated after 11 charging-discharging cycles with maxium capacity of 270.5 mAh/g and also showed a high-rate dischargeability of 70.6% at a discharge current of 300 mAh/g, however activation cycles of another electrode containing nickel powder was 24 and the high-rate dischargeability was 44.5% correspondingly. From their kinetic analysis, carbon nanotubes added to the hydrogen storage electrode increased greatly the values of exchange current density (i0) and diffusion coefficient (Da). The electrochemical impedance characteristics changing with the depth of discharge was associated with the surface adsorption resistance of hydrogen diffusion in hy...