Study On Microstructure And Thermal Conductivity And Electrochemical Properties Of Carbon Nanocomposites

Owing to their rich allotropes, excellent electrical conductivity, mechanical strength, chemical and thermal stabilities, carbon materials have been widely used as enhancement fillers. Graphene owns excellent thermal conductivity and can be used as the nanofiller to enhance the thermal conductivity of polymer. However, it is easy to aggregate due to its strong van der Walls force. So the dispersion of rGO, the interfacial interaction between rGO and PVA matrix, and the effects on the microstructure will be helpful to understand the thermal conductivity of composites. In this work, we investigated the micro-enhancement mechanism of thermal condctivity for dopping rGO nanosheets into rGO/PVA composites. On the other hand, a facial method has been developed for fabricating cross-linked carbon nanofibers by directly carbonizing electrospun polyacrylonitrile (PAN) nanofiber. Then cross-linked carbon nanofiber was used as scaffold to support polyaniline (PANi) for supercapacitor electrode materials, which show excellent electrochemical characteristics. The main results are as follows:1. Correlation between the free volume and thermal conductivity of porous reduced graphene oxide/poly(vinyl alcohol) composites studied by positron spectroscopyLight-weight porous reduced graphene oxide (rGO)/poly(vinyl alcohol) (PVA) composites were prepared by freeze-drying method. The effects of doping rGO nanosheets on the free volume and the thermal conductivity for rGO/PVA composites were systematically investigated by PALS, SEM, TEM, Fourier transform infrared spectrocopy and thermal conductivity measurement.a. The rGO nanosheets are most exfoliated in 0.5 wt%rGO/PVA composite. The agglometation of rGO nanosheets enhanced with the content of rGO.b. In rGO/PVA composites, there exists the interfacial interaction between rGO and PVA, such as hydrogen bond. According to the variation of the second lifetime intensity, h, the interaction parameter was caculated which demnostrated that the sample PVA/rGO-0.5 owns the strongest interfacial interaction.c. Interaction between rGO and PVA can restrain the movement of polymer chains resulting in the decreases of o-Ps lifetime and the free volume cavity. PVA/rGO-0.5 presents mostly exfoliated state of rGO nanosheets in which a bigger specific surface area of the excellent dispersion state is benefit to the formation of hydrogen bonding and limiting the movement of polymer chains and resulting in the smallest free volume.d. The thermal conductivity increased nearly linearly with rGO content. In order to understand the mechanism of thermal conductivity enhancement, the thermal conductivity was calculated using different kinds of models, including parallel model, series model and Maxwell-Garnett effective medium approximation (EMA), which were then compared with experimental result. The difference value between the experimentally measured thermal conductivity and the EMA theoretically calculated result was closely associated with interfacial interaction, revealing that the better the dispersion of rGO, the stronger the interfacial interaction, and the larger the thermal resistance.e. A direct relationshiD between the fractional free volume and the thermal conductivity has been obtained as ??e/0.18, suggesting that the interfacial free volume plays an important role in determining phonon scattering and the thermal conductivity of composites.2. Facile fabrication of cross-linked carbon nanofiber via directly carbonizing electrospun polyacrylonitrile nanofiber as high performance scaffold for supercapacitorsCross linked carbon nanofibers and non-cross-linked carbon nanofiber were prepared, and then were used as scaffold to support polyaniline (PANi) nanorod for supercapacitor electrode material.a. Cross-linked carbon nanofiber (CLCNF) was successfully prepared by directly carbonizing electrospun PAN nanofiber. Comparing to non-cross-linked carbon nanofiber (NCLCNF) obtained via carbonizing of pre-oxidation PAN nanofiber, CLCNF shows better conductivity owing to its cross-linked structure.b. CNF was used as scaffold to support PANi nanorod for supercapacitor electrode material. The PANi was synthesized through a chemical oxidation polymerization method. The hierarchical CLCNF/PANi composite shows a specific capacitance of 127 F/g at the current density of 800 A/g, with an excellent rate capability of 49% of its initial capacitance at 0.5 A/g, which is much higher than that of NCLCNF/PANi composite (17%). Moreover, supercapacitor device based on CLCNF/PANi achieves 75.3% capacitance retention after 10000 charge-discharge cycles at 10 A/g, suggesting excellent cycle stability. All these experimental results indicate that this method for fabricating CLCNF is a substantial advancement towards the practical applications of carbon nanofiber in energy conversion and storage field.