Purification, processing, and separation of carbon nanotubes and application in lithium ion batteries

Single wall carbon nanotubes\u27 (SWCNTs) unique structural and electronic properties have made them an ideal candidate for use in electrochemical devices like lithium ion batteries, PEM fuel cells, and supercapacitors. SWCNTs were investigated as free-standing anodes, current collector supports, and conductive additives in lithium ion batteries. Before incorporation into batteries, the effect of organic solvents on SWCNT material properties and separation of SWCNTs by electronic type was studied to enable characterization and control of SWCNT properties. Variations in purity, solvent processing, and electronic type of SWCNTs are shown to have a significant effect on SWCNT characterization, optoelectronic properties, and performance in lithium ion batteries as described further below. Organic solvents from the alkyl amide and halogenated aromatic classes have been analyzed as dispersion agents for high purity single wall carbon nanotubes (SWCNTs). The resulting dispersions from two novel SWCNT solvents, N,N,N\u27,N\u27-Tetramethylmalonamide (TMMA) and 1-Chloronaphthalene (1-CLN), have been compared to well-established solvents (i.e., N,N Dimethylacetamide (DMA) and 1,2 Dichlorobenzene (DCB)). The spectroscopic results for the halogenated aromatic solvents are consistent with a sonopolymerization that results in a polymer wrapping of the SWCNTs. In comparison, the alkyl amide solvents (DMA and TMMA) show similar dispersion limits with no significant change in absorbance as a function of ultrasonication. These solvents also have the additional benefit of being able to be removed without damaging the SWCNT structure. Density-gradient ultracentrifugation (DGU) has enabled separation of SWCNTs by diameter, electronic type, and chirality. The DGU method uses surfactant coated SWCNT material that rises or falls to the point in the gradient matching its density with ultracentrifugation. SWCNTs produced through laser vaporization were separated by electronic type and materials were characterized through optical absorption. The effect of purity, organic solvent processing, and DGU electronic type separation on lithium ion capacity of free-standing anodes was studied which showed improved performance for SWCNT material with high purity and uniformity. The use of a SWCNT paper as a current collector for a traditional anode coating was found to improve energy density by reducing electrode mass while retaining high capacity. As a conductive additive, SWCNTs formed an effective percolation network at extremely low mass loadings in traditional cathode and anode coatings. The SWCNT additives were shown to increase rate capability and usable capacity of the electrodes compared to higher mass loadings of typical conductive carbon additives

Synthesis of nanocomposites from polyaniline, polypyrrole and carbon nanotubes, and unzipping of multi-walled carbon nanotubes for the obtention of new graphitic nanomaterials

Tesis (Doctorado en Nanociencias y Nanotecnología)"La síntesis de compositos a partir de nanotubos de carbono ha tenido como principal reto el lograr una buena dispersión de los primeros en la matriz polimérica y para ello se han utilizado diversos métodos de funcionalización. Así mismo, se busca obtener una buena interacción entre ambos componentes y que el composito resultante se beneficie de las propiedades presentes en los materiales de partida. En este contexto, en la presente tesis presentamos los principales resultados concernientes a la síntesis de compositos a partir de nanotubos de carbono multicapa (MWNTs), MWNTs dopados con nitrógeno (CNx) y de polímeros conductores electrónicos, específicamente de polianilina (PAni), polianilina sulfonada (SPAni) y polipirrol (PPy). Dicho estudio surge a partir de nuestro interés en combinar las propiedades únicas de cada componente y así obtener nuevos materiales con propiedades electrónicas y mecánicas mejoradas. Por primera vez en el ámbito científico, sintetizamos estos compositos mediante la polimerización in situ de los monómeros correspondientes mediante un método de alquilación reductiva en amoníaco líquido, al cual se denomina sales de nanotubos, y que ha sido ampliamente utilizado para la funcionalización de fulerenos, nanotubos de una (SWNTs) y de varias capas (MWNTs). En este método se disuelve litio metálico en amoníaco líquido, al que se agregan los nanotubos de carbono formando entonces una sal de nanotubos. Durante la síntesis de compositos encontramos que, ocasionalmente, tanto MWNTs como CNx se exfoliaban en los extremos o en segmentos. Decidimos entonces seguir esta línea de investigación, logrando exitosamente la obtención de nanocintas de carbono a partir de MWNTs. Encontramos que los MWNTs pueden ser abiertos longitudinalmente mediante la intercalación de litio y amoníaco, seguida por exfoliación. Los mejores resultados se obtuvieron mediante intercalación en tubos cortados y abiertos en los extremos y exfoliación con tratamiento ácido y calentamiento abrupto. El material resultante consistió en: (i) estructuras grafíticas de multicapa (nanocintas), (ii) MWNTs parcialmente abiertos y (iii) hojuelas de grafeno. A los nanotubos completamente abiertos les llamamos ex-MWNTs, los cuales se caracterizan por su gran cantidad de bordes, lo cual los hace candidatos muy atractivos para muchas aplicaciones, tales como: elaboración de nanocompositos, adsorción de gases, baterías recargables, capacitores, etc.""The synthesis of composites from carbon nanotubes has its most notable challenge in the good dispersion of carbon nanotubes within the polymer matrix. Moreover, a good interaction is also desired between both components and a synergic effect in the composite as well, resulting from the properties of each component. On this respect, in this thesis we present the main results concerning the synthesis of composites from multi-walled carbon nanotubes (MWNTs), nitrogen-doped MWNTs (CNx), and electronic conducting polymers, specifically from polyaniline (PAni), sulfonated polyaniline (SPAni), and polypyrrole (PPy). This study was motivated by our interest too combine the unique properties of each component and the obtention of composites with improved electronic and mechanical properties. For first time in science, we synthesized these composites by in situ polymerization of the corresponding monomers by means of a reductive alkylation method in liquid ammonia which is called nanotube salts. This method has been widely used for functionalization of fullerenes, single- (SWNTs), and multi-walled carbon nanotubes (MWNTs). In this method, metallic lithium is dissolved in liquid ammonia, to which carbon nanotubes are added, thus forming nanotube salts. During the synthesis of these composites we occasionally observed that both MWNTs ans CNx were opened at the tips or in segments. We thus decided to follow this research line, successfully obtaining carbon nanoribbons from MWNTs. We found that these MWNTs can be opened longitudinally by intercalation of lithium and ammonia followed by exfoliation. Intercalation of open-ended tubes and exfoliation eith acid treatment and abrupt heating provided tjhe best results. The resulting material consists of: (i) multilayered flat graphitic structures (nanoribbons), (ii) partially open MWNTs, and (iii) graphene flakes. We called the completely unwrapped nanotubes ex-MWNTs, which are characterized by a large number of edges that make them very attractive for many applications such as: composites, gas adsorption, rechargeable batteries, capacitors, etc. Characterization of their morphology, vibrational, and structure properties allowed us to propose an exfoliation mechanism for MWNTs.

NANOSTRUCTURED ARRAYS FOR SENSING AND ENERGY STORAGE APPLICATIONS

Vertically aligned multi walled carbon nanotube (MWCNT) arrays fabricated by xylene pyrolysis in anodized aluminum oxide (AAO) templates without the use of a catalyst, were integrated into a resistive sensor design. The steady state sensitivities as high as 5% and 10% for 100 ppm of NH3 and NO2 respectively at a flow rate of 750 sccm were observed. A study was undertaken to elucidate (i) the dependence of sensitivity on the thickness of amorphous carbon layers, (ii) the effect of UV light on gas desorption characteristics and (iii) the dependence of room temperature sensitivity on different NH3 and NO2 flow rates. An equivalent circuit model was developed to understand the operation and propose design changes for increased sensitivity. Multi Walled Carbon NanoTubes (MWCNTs) – Polymer composite based hybrid sensors were fabricated and integrated into a resistive sensor design for gas sensing applications. Thin films of MWCNTs were grown onto Si/SiO2 substrates via xylene pyrolysis using chemical vapor deposition technique. Polymers like PEDOT:PSS and Polyaniline (PANI) mixed with various solvents like DMSO, DMF, 2-Propanol and Ethylene Glycol were used to synthesize the composite films. These sensors exhibited excellent response and selectivity at room temperature when exposed to low concentrations (100ppm) of gases like NH3 and NO2. Effect of various solvents on the sensor response imparting selectivity to CNT – Polymer nanocomposites was investigated extensively. Sensitivities as high as 28% was observed for a MWCNT – PEDOT:PSS composite sensor when exposed to 100ppm of NH3 and -29.8% sensitivity for a MWCNT-PANI composite sensor to 100ppm of NO2. A novel nanostructured electrode design for Li based batteries and electrochemical capacitor applications was developed and tested. High density and highly aligned metal oxide nanowire arrays were fabricated via template assisted electrochemical deposition. Nickel and Molybdenum nanowires fabricated via cathodic deposition process were converted into respective oxides via thermal treatments and were evaluated as electrodes for batteries and capacitor applications via Cyclic Voltammetery (CV). Several chemical baths were formulated for the deposition of pristine molybdenum nanowires. Superior electrochemical performance of metal (Ni and Mo) oxide nanowires was observed in comparison to the previously reported nano-particle based electrodes

N-DOPED MULTIWALLED CARBON NANOTUBES: FUNCTIONALIZATION, CHARACTERIZATION AND APPLICATION IN LI ION BATTERIES

The focus of this dissertation is to utilize chemical functionalization as a probe to investigate the reactivity of N-doped multiwalled carbon nanotubes (N-MWCNTs). The surface of N-MWCNTs, being a set of potentially reactive graphene edges, provides a large number of reactive sites for chemical modification, so considerable changes in chemical and physical properties can be envisaged. We observed that both reduction (dissolving metal reduction/alkylation) and oxidation (H2SO4/HNO3 and H2SO4/KMnO4 mixtures) of N-MWCNTs lead to formation of interesting spiral channels and spiraled carbon nanoribbons. A variety of techniques, including TGA, SEM, TEM, XRD and surface area measurements were used to analyze these new textural changes. We have developed methods to demonstrate that specific chemistry has occurred on these new structures. To this end, we introduced metal-binding ligands that could be used as probes in imaging and spectroscopic techniques including TEM, STEM, EDX, and EELS. A proposal for the underlying structure of N-MWCNTs responsible for the formation of the new textures is presented. We have investigated the performance of our materials as potential negative electrodes for rechargeable lithium ion batteries

Electrical charge distribution studies in SWNT, C60@SWNT, and DWNT structures.

Single Walled Carbon Nanotubes (SWNTs), Double Walled Carbon nanotubes (DWNTs) and C60@SWNT (peapods) are three of well-ordered all-carbon nanostructures. Peapods are SWNTs filled with C60 fullerene peas. At high temperatues, C60 molecules coalesce into a long internal SWNT forming a DWNT. Charge transfer properties on SWNTs (both donor and acceptor type) have been studied intensively by various groups. Few groups have conducted such studies on peapods and DWNTs. In a recent study on Br2 doping of DWNTs using Raman spectroscopy, it has been found that the net charge primarily resides on the outer tube revealing molecular Faraday cage effect even when the outer layer is semiconducting and inner layer is metallic. We are investigating charge distribution behavior in these structures by a systematic electrical transport study (both resistivity and Seebeck coefficient) by way of doping wtih potassium (donor)

Growth, properties, and applications of branched carbon nanostructures

Nanomaterials featuring branched carbon nanotubes (b-CNTs), nanofibers (b-CNFs), or other types of carbon nanostructures (CNSs) are of great interest due to their outstanding mechanical and electronic properties. They are promising components of nanodevices for a wide variety of advanced applications spanning from batteries and fuel cells to conductive-tissue regeneration in medicine. In this concise review, we describe the methods to produce branched CNSs, with particular emphasis on the most widely used b-CNTs, the experimental and theoretical studies on their properties, and the wide range of demonstrated and proposed applications, highlighting the branching structural features that ultimately allow for enhanced performance relative to traditional, unbranched CNSs