Carbon-based magnetic nanohybrid materials for polymer composites and electrochemical energy storage and conversion

Graphene, the parent of all graphitic forms including fullerene, carbon nanotubes, and graphite, is a two dimensional building material for carbon materials of all other dimensionalities. Among them, graphene and carbon nanotubes have been extensively studied in such fields as polymer composites and electrochemical energy storage and conversion due to their high specific surface area, excellent conductivity, good stability, high electron mobility, and good mechanical performance. Thus, the goal of this study is to achieve improved physical properties of polymer composites and enhanced electrochemical performance of composite electrodes for lithium ion batteries by utilizing and modifying carbon nanotubes and graphene, respectively.;When carbon nanotubes are well-dispersed in polymer matrices, it exhibits improved mechanical, electrical, and thermal properties. Furthermore, when they are both dispersed and aligned, it could also show much enhanced anisotropic physical properties. There are various methods to obtain the alignment of carbon nanotubes in polymer matrices, which include electric, magnetic, electro-spinning, spin casting, or shear flow. Among them, the magnetic field is one of the efficient methods for aligning carbon nanotubes. However, the magnetic susceptibility of carbon nanotubes is very low; thus, strong magnetic field strength is required. One way to obtain facile alignment of carbon nanotubes by applying an external magnetic field is to engineer magnetic hybrid nanostructures, which illustrate that carbon nanotubes are tethered with magnetic nanoparticles, such as maghemite (γ-Fe2O3) or magnetite (Fe 3O4). For obtaining the hybrid nanostructures, a novel and easy synthesis method, modified sol-gel process, was developed, from which individual and separate magnetic nanoparticles could be obtained on the surface of carbon nanotubes. As-prepared magnetic carbon nanotubes (m-CNTs) showed strong magnetic response to a low magnetic field (< 0.3 T) and were aligned parallel to the direction of magnetic field in polymer matrices. If a sufficiently large homogeneous magnetic field is applied, the magnetic moments of the nanoparticles align in parallel, and the resulting dipolar interactions are sufficiently large to overcome thermal motion and to reorient the m-CNTs favoring the formation of chains of aligned carbon nanotubes. These chainlike structures are formed by connecting the m-CNTs in line, touching each other in a head-to-tail fashion, i.e., north to south pole. Since the north and south poles of the dipolar nanomagnets attract each other, while particles coming close to each other side by side with the magnetization direction parallel will repel each other, the m-CNTs could be dispersed as well as aligned. Moreover, due to this end-to-top connectivity, which generates higher aspect ratio, improved anisotropic electrical and mechanical properties could be realized.;Li-ion batteries (LIBs) have been widely used as power sources for portable electronic devices and electronic vehicles, and attracted a variety of attention due to their high energy density and power density. In this system, electrode materials are very significant for the enhanced electrochemical performance. Graphene has been widely used as anode materials for LIBs due to its outstanding properties. However, it has characteristics such as large irreversible capacity loss, low initial columbic efficiency, and rapid capacity fading. One efficient way to improve the capacity and cycle performance of LIBs is to introduce graphene-based hybrid nanostructured electrodes with another component being popular LIB materials such as oxide, metal, and carbon. Based on this perspective, graphene/iron oxide hybrid nanostructures were developed, where modified sol-gel process for the synthesis was adopted again, and its morphology represented that mono-disperse and individual magnetic nanoparticles were tethered on the graphene sheets as observed in carbon nanotube system. The as-prepared graphene/iron oxide composite, graphene/γ-Fe2O3, were used as an anode material for LIBs. This hybrid nanomaterial, due to their specific morphology and characteristics, exhibited improved large reversible capacity, long cycle life, and good rate performance.