Preparation, Characterization And Supercapacitive Properties Of Graphene-Based Conducting Polymer Composites

Supercapacitors play an increasingly important role in modern power source application due to their high capacitance, high power density, long cycle life and good operational safety. Conducting polymers with high Faradaic pseudo-capacitor are important electrode materials for supercapacitor applications. However, the poor electrochemical utilization, low electronic conductivity in dedoping state and rapid capacity decay limit their further advancements in promising applications. The combination of conducting polymers and graphene (GN) with high specific surface area, good conducting property and exceptional mechanical strength has been proposed as perfect electrode materials with good electrochemical properties for supercapacitors due to the synergistic effect of GN and conducting polymers. Consequently, the thesis is aimed at the preparation and characterization of the GN-based conducting polymer composites and their application in supercapacitors in order for the synchronous realization of large specific energy density and good cycle stability.1. A simple and effective ionic liquid (IL)-assisted mechanochemical route is used to synthesize GN/PANI and GN/PPy composites. Functionalized IL1-(3-sulfonic acid) propyl-3-methylimidazolium hydrogen sulfate ([MIMPS][HSO4]) acts as the dispersant of GN and the dopant of PANI to prepare GN/PANI composite. Functionalized IL1-butyl-3-methylimidazolium tetrachloroferrate (Bmim[FeCl4]) acts as not only the dispersant of GN, but also the catalyst and dopant in the synthesis of GN/PPy composite. GN serve as a support material for depositing PANI or PPy during polymerization process, while in-situ produced PANI or PPy deposited onto GN can be used as spacer to effectively avoid the restacking of GN. The strong mechanical energy makes the laminated composites random stack and reconstructs hierarchical architecture, which is convenient for diffusion of the electrolyte ions into the inner region of electrodes to take place redox reaction. Electorchemical tests indicate that the electroactive and cycling stability of PANI and PPy have obvious improving. The GN/PANI shows a specific capacitance of616F g-1at0.2A g-1and a capacity loss of7%after500continuous cycles. For GN/PPy composite, a specific capacitance of375F g-1at0.2A g-1and a capacity degradation of13%after1000continuous cycles can be obtained.2. A series of GN/PANI/carbon nanotube (CNT) and GN/PPy/CNT ternary composites have been fabricated via in situ polymerization method using poly(sodium4-styrene sulfonate)(PSS) for dispersing GN and CNT. The electrostatic interaction between negatively charged PSS and pyrrole or aniline monomer facilitates the generation of homogeneous polymer layer. The conducting polymers preferentially deposit on the surface of GN due to the high chemical activity and high theoretical surface area of GN. The introduction of one-dimensional CNT effectively inhibits the stacking of nanosheet-like GN/PANI or GN/PPy to form three-dimensional hierarchical architecture, favoring the contact between the electrolyte ions and electroactive materials. The high conducting of CNT and GN can construct a3-D conductive architecture for electron transfer and fast ions transport. Owing to the synergistic effect between two-dimensional GN and one-dimensional CNT, electrochemical results demonstrate that the electrochemical properties of ternary composites are better than pure conducting polemers and binary composites of conducting polemers with GN or CNT. A specific capacitance of909F g-1at0.2A g-1and a capacity loss of7%at4A g-1after2000continuous cycles can be obtainedand for GN/PANI/CNT composite with GN:CNT=5:1. The GN/PPy/CNT composite with GN:CNT=8:1has a maximum specific capacitance of372F g-1at0.2A g-1and a capacity degradation of4%after2000continuous cycles at4A g-1.3. Freestanding "sandwich-like" films with PANI nanofibres or PANI/CNT nanocables uniformly distributed between GN sheets have been fabricated by reducing a graphite oxide (GO)/PANI or GO/PANI/CNT precursors prepared by flow-directed assembly from a complex dispersion of GO and PANI or PANI/CNT, followed by reoxidation and redoping of the reduced PANI in the composite to restore the conducting PANI structure. In the composite film, the GN sheets can act as the current collector to improve the electronic and ionic transportation during the redox process of PANI, and elastic buffering to accommodate the volumetric change of the PANI chains. The PANI or PANI/CNT provides high faradaic capacitance and increases the basal spacing between GN sheets to enhance the accessibility to the GN surfaces. Especially for ternary GN/PANI/CNT composite film, rigid CNT core can not only effectively enhance the electroactive of PANI, but also further improve the mechanical stability of PANI. The GN/PANI/CNT film shows that the mass and volume specific capacitances of are569F g-1and188F cm-3at a current density of0.2A g-1, and a capacity loss of4%after5000continuous charge/discharge cycles.4. Unique flexible film with PPy/CNT composite homogeneously distributed between GN sheets is successfully prepared by flow-assembly of the mixture dispersion of GN and PPy/CNT. In such layered structure, the coaxial PPy/CNT nanocables can not only enlarge the space between GN sheets but also provide pseudo-capacitance to enhance the total capacitance of electrodes. According to the galvanostatic charge/discharge analysis, the mass and volume specific capacitances of GN/PPy/CNT are211F g-1and122F cm-3at a current density of0.2A g-1, higher than those of the GN film (73F g-1and79F cm-3) and PPy/CNT (164F g-1and67F cm-3). Significantly, the GN/PPy/CNT electrode shows excellent cycling stability (5%capacity loss after5000cycles) due to the flexible GN layer and the rigid CNT core synergistical releasing the intrinsic differential strain of PPy chains during long-term charge/discharge cycles.