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Abstract
Carbon Nanotubes (CNTs) have been at the forefront of nanotechnology research over the past decade. The development of synthesis techniques such as chemical vapor deposition (CVD) and floating catalyst CVD (FCCVD) have transformed CNTs from being property enhancing fillers in composites to structural materials of the future. CNT sheets and yarns are being produced at industrial scale and are seeing applications in fields like aerospace and energy storage.
In this work, we fabricate CNT sheets by dry-spinning technique. This approach helps produce light-weight, strong and conductive CNTs sheets free from catalyst impurities. Plasma functionalization is a fast, clean and efficient technique for modification of CNTs. Here, we demonstrated for the first-time atmospheric pressure plasma functionalization of CNT sheets carried out in-situ during their manufacturing. Helium/oxygen plasma was shown to create oxygen-based functional groups on the CNTs in a matter of seconds, which enables the interaction between nanotubes and other chemical compounds including polymers. This technique was successfully employed for fabrication of improved CNT/epoxy composites. Plasma functionalization of CNTs enables the formation of covalent bonding between CNTs and uniform infiltration of the epoxy resin. This resulted in formation of a crosslinked CNT/epoxy composite which showed an improvement of strength and modulus over pristine CNT sheets. High strength CNT/Epoxy composites with high wt. % CNT content lead to a role reversal with epoxy resin which serves to reinforce a CNT sheet matrix, where the tubes dominate in quantity.
CNT sheets demonstrate high electrical conductivity and thus find application in electronics. This work also explores the application of CNT sheets as free-standing electrodes for energy storage. For the first time, carbon nanotube synthesis was carried out directly on CNT sheet as substrate by plasma enhanced chemical vapor deposition (PECVD). This experiment allowed creation of a 3-D dimensional structure made entirely of carbon nanotubes with vertically aligned CNT array growing on horizontally aligned CNT sheet substrate (NCNT). The PECVD process conducted in an ammonia plasma atmosphere enabled incorporation of nitrogen-based functional groups into the nanotube sidewalls which further enhance its electrochemical activity. NCNT sheets used as freestanding, metal-free were employed as scaffold for polyaniline (PANI), a conductive polymer and pseudo-capacitive material. PANI coating duration was optimized and a core-shell morphology was obtained comprising of PANI shell supported by CNT core. PANI/NCNT composite electrode delivered a high specific capacitance and retained a high capacitance value when the current density was increased 30-fold. A symmetric supercapacitor (SC) device was made with PANI/NCNT electrodes and PVA/H2SO4 gel electrolyte. The resulting SC was a mechanically flexible device which delivered a high specific capacitance and retained more than 92% of its initial capacitance after 10000 cycles of charge-discharge at high current density of 24.7 A/g (based on mass of PANI). This kind of performance is a result of the synergy between the core-shell PANI-CNT structure and the direct growth of CNTs on CNT sheet, which allows rapid charge transport. The obtained results prove the efficacy of CNT sheets applied as free-standing electrodes for next-generation light-weight energy storage devices.