| As novel energy storage devices, flexible supercapacitors can satisfy the needs of ultrathin/lightweight electronic products and wearable units, due to their high specific capacitance, long cycle life, short charging time, high power density, wide temperature range and environmentally friendly operation. Light and safe power sources are driving the fast development of the electronics industries and the key factor is flexible electrode materials. Carbon materials, the common conductive substrates, can be incorporated with conducting polymers to fabricate composite electrodes, aiming to increase the electrochemical properties of flexible supercapacitors. Among the carbon materials, graphene (RGO) is widely used as a scaffold for hybrid electrodes due to its intrinsically superior electrical conductivity, high surface area, excellent mechanical flexibility and good chemical stability; carbon fibers with high conductivity are also used as flexible electrode supports; Conducting polymers (such as polyaniline, polypyrrole) exhibit excellent electrochemical performances as pseudocapacitance electrodes, but their poor cycle stabilities limit the practical utility in supercapacitors. Therefore, we are striving to develop synergistic flexible composites through combining conducting polymers (high pseudocapacitance) and graphene and/or carbon fibers (double layer capacitance) with the purpose to obtain novel materials with high capacitance, high power density and good cycle life.In this thesis, the main subject is graphene/polyaniline (PANI) composites, and we chose the microwave-exfoliated graphene, three-dimensional porous graphene foam and plasma etching nitrogen-doped carbon fiber cloth as the flexible substrate for depositing PANI to design and fabricate several hybrid composites. We systematic studied the relationship between their microstructures, morphologies and electrochemical and explored the effects of conductive substrates and assemble methods on the electrochemical performances of composites, thus simplify the preparation of electrodes and assemble procedure of storage device. The main contents are follows:1. We used tetrabutylammonium hydroxide (TBAOH) as surfactant to stabilize microwave-exfoliated graphene sheets (TMEG), and then deposited PANI nanowires on the surface of TMEG by in situ polymerization to obtain high performance PANI/TMEG. The morphologies of composites were controlled by changing the loadings of TMEG:aggregation of PANI particles was obtained with a low contents of TMEG due to the unavoidable self-nucleation of PANI; when the percentage of TMEG increase to10wt%, well-ordered PANI nanowires (50nm in diameter and100nm in length) were coated on TMEG nanosheets. A series of PANI/TMEG films were produced by filtration and their electrochemical performances were tested by two electrode system. Symmetric flexible supercapacitors based on PANI/TMEG film yielded a specific capacitance of693F g-1, obtained a capacitance of390F g-1even at the current density of20A g-1and retained82%capacitance after5000cycles. PANI/TMEG film can be directly used as electrode to assemble flexible devices, avoiding the traditional preparation of electrodes. However, the obtained electrochemical performances is not as good as the one fabricated by traditional method, owing to low porous structure caused by the aggregation and restack of graphene sheets in the composite film.2. With the aim to improve the behavior of PANI/TMEG film, we reported a facile and cheap method to prepare freestanding and lightweight flexible RGO foam (RGO-F). We used nickel foam as template, and then vertically deposited PANI nanowire arrays on the3D macroporous RGO networks (RGO-F/PANI) as supercapacitor electrode. Nickel foams (NF) were immersed in the GO suspension, then RGO-F foam with good electrical conductivity (1600S m-1) was obtained by removal of NF and hydroiodic acid (HI) reduced processes. The3D flexible lightweight RGO-F as skeletons to construct RGO-F/PANI nanostructured hybrid electrodes is the key to our fabrication process. The RGO-F/PANI5composite electrodes (PANI loading is40.2wt%) are binder-free electrodes, symmetric flexible supercapacitors were fabricated that obtained a specific capacitance of725F g-1(volumetric:188.5F cm-3), maximum energy density and power density of15.9Wh kg-1.and45kW kg-1and a cycling performance of83%of initial capacitance over5000cycles.3. With the aim to modify the porous structure of R.GO-F and improve the utilization of the electrode materials, we demonstrate that new hierarchically porous functional graphene foam and PANI composites were fabricated (fRGO-F/PANI). The preparation process is similar to that of GO foam with polystyrene particles (PS). After calcinations in N? atmosphere, an interconnected hierarchically porous (100-300μm and0.5-2μm) graphene (fRGO-F) framework was got by the removal of template. PANI nanowires with50nm average diameters and approximately150nm in length were deposited on the structure of porous fRGO (fRGO/PANI) by in situ polymerization. The as-prepared fRGO-F/PANT composites maintained the three dimensional hierarchically porous structure as fRGO-F and PANI nanowires had a homogeneous dispersion throughout the macroporous architectures. Directly used as electrodes to assemble supercapacitor, fRGO-F/PANI7(PANI loading is52.4wt%) exhibited a specific capacitance of939F g-1with a cycle life of88%after5000cycles, which was beyond the RGO/PANI electrode. The maximum energy density and power density is up to20.9Wh kg-1at a power density of7.2kW kg-1and25.9kW kg-1at a energy density of17.8Wh kg-1, respectively.4. With the aim to improve the flexibility of electrode materials, a combination of vertical PANI nanowire arrays and nitrogen plasma etched carbon fiber cloths (eCFC) was fabricated to create3D nanostructured PANI/eCFC composites. By the plasma etching process in N2/O2atmosphere, the nitrogen-doped eCFC with functional groups such as carboxylic acids and pyrrole N on the surface can better adsorb the aniline monomer unit by electrostatic interactions. The morphologies of PANI nanowires deposited on eCFCs can be controlled by varying the concentrations of the aniline monomer. Lower concentrations of aniline result in sparse and short nanowires, in contrast, self-nucleation of PANI nanowires is unavoidable at higher concentrations of aniline, resulting in both random connected PANI nanowires and aligned PANI nanowires. The optimized concentration of aniline is0.05M because of the uniform PANI nanowire arrays. The small size of highly ordered PANI nanowires can greatly reduce the scale of diffusion length, allowing for the improved utilization of electrode materials. A two-electrode flexible supercapacitor based on PANI/eCFC5demonstrates a high specific capacitance (1035F g-1at a current density of1A g-1), good rate capability (88%capacity retention at8A g-1), and long-term cycle life (10%capacity loss after5000cycles).5. In order to improve the cycle stability and specific capacitance of composites, we reported the synthesis of reduced graphene oxide (RGO) sheets wrapped PANI/eCFC to form RGO/PANI/eCFC flexible composites. The RGO coating layer is important to accommodate volume change and mechanical deformation of the coated PANI nanowires arrays during the long-term charge/discharge processes. The optimal thickness of RGO layer is7.4nm, which is about7-8nanolayer graphene. The resulting hierarchical symmetric supercapacitor based on RGO/PANI/eCFC composites shows an enhanced capacitive behaviour with a maximum energy density of25.4Wh kg-1, a maximum power density of92.2kW kg-1and a specific capacitance of1145F g-1which is higher than that of PANI/eCFC (1050F g-1) and GO/PANI/eCFC (940F g-1). Moreover, the assembled supercapacitor exhibits excellent charge/discharge rates and a good cycling stability, retaining over94%of its initial capacitance after5000cycles. The results indicated that the high conductive RGO layer with optimal thickness wrapped PANI/eCFC can enhance the cycle stability of flexible electrodes. |
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