Synthesis, Preparation and Assembly of Carbon-Nanotube-Based Electrode Materials

Abstract

Carbon nanotubes are a promising material for supercapacitor electrodes. There are issues, relating to synthesis, assembly and the effect of certain properties on electrode performance, which need to be addressed before they can be incorporated into commercial supercapacitors. I combine the supergrowth and the catalyst re-generation techniques and apply them to large-scale CNT manufacture. Also, I investigate a procedure for synthesising CNT directly onto Aluminium foam and investigate the effect that the ratio of metallic to semi-conducting SWCNT has on electrode performance. In the ‘supergrowth’ technique, an oxidiser is introduced into the synthesis reactor with the carbon source to gasify amorphous carbon deposits that may impede CNT growth. This technique has been demonstrated to increase CNT yields and is applied here to a catalyst support material better suited to large-scale production, with mixed results. There was no significant increase in CNT yield; however, an increase in the IG/ID ratio was observed, this suggests that the CO2 gas oxidized some of the amorphous carbon. Commercial samples of ‘metallic enriched’ and ‘unsorted’ SWCNT were characterised by Raman spectroscopy at 488 nm and 633 nm to determine their m/s-SWCNT ratio. Unfortunately, due to the nature of the sample, Raman spectroscopy alone could not be used to quantify their m/s-SWCNT ratio. The two samples were then compared as electrode material on Al foil current collectors in aqueous solution. Unexpectedly, the ‘unsorted’ sample displayed a higher peak specific power density. Direct CNT growth on Al foam was investigated. Fe was deposited on the foam using wet impregnation. The foam was immersed in an ethanol / Fe(NO3)3.9H2O solution. A low synthesis temperature is required to prevent the foam from melting, this results in reduced graphitisation of the carbon in the foam. The Fe coating was non-uniform and the consequent carbon product consisted mostly of carbon fibre. The floating catalyst method may be more suitable for this substrate. A fluidised-bed reaction system was constructed to test the scalability of a process described in the literature. The process combined a floating-catalyst technique, fluidisation and gas-cyclone separation of carbon nanotubes to provide a simple and effective technique for large-scale production of high-purity CNT. The first step undertaken in this study was to test the effect of catalyst preparation technique (floating catalyst and wet impregnation) in a small-scale fixed bed on the carbon product produced. The floating catalyst produced a more uniform carbon product. The fluidised bed reactor system was then built and the technique attempted on a large-scale. At first, the reaction temperature on the first run was too low for CNT synthesis, and on-going issues with the furnace prevented repetition of the experiment at higher temperatures. However, SEM micrographs of the product found on the wall further up from the distributor plate (which was held at a higher temperature during the reaction) showed evidence of CNT growth.