Graphene/Conducting Polymer Nanocomposites And Their Electrochemical Research

Graphene (GE) is one of the hottest interdisciplinary research spots in areas of material science, information science, bioscience, new energy and so on, it is showing great prospect in new energy, electronics, catalysis, sensing devices, biomedical, optical and many other fields due to its great surface area and excellent electrical properties. This thesis mainly concentrated on the controllable synthesis and application of GE based composites and graphene oxide (GEO) based composites in electrochemical capacitors. Electrochemical capacitors is a novle energy storage device between conventional capacitors and batteries with high power density, fast charge-discharge rate, long cycle life, high efficiency, wide temperature range, reliability and many other advantages. Electrode material is a key point which determines the performances of electrochemical capacitors. Compared with conventional carbon materials such as active carbon and carbon nanotube, GE is expected to achieve large-scale industrial applications because it has a higher electron transfer velocity, surface area and mechanical performance. However, there are few studies on the GE composite electrode materials and those studies are lack of the understanding of GE electrodes' energy storage characteristics and advantages.In order to solve these problems and meet the demand of electrochemical capacitors for high-performance electrode mateirials, this dissertation studied the preparation and energy storage characteristics of GE (GEO)/conducting polermer composite electrodes. The relationships between category, structure, and performance of these materials were studied systematically; the energy storage characteristics of these novel materials were illustrated. The main contents are as follows:(1) GEO contains a large number of oxygeneous functional groups, it can form stable suspension in water and is easy to adsorb monomer to fabricate composites. To make full use of the inherent advantages of GEO, we creately introduced GEO to the preparation process of polyaniline (PANI) to form GEO/PANI composite by in situ polymerization method. Two typical morphologies are acquired, one is assemblyed by PANI nanofiber in the surface and GEO in the backbone, the other is constituted with individual lamella GEO and nanofiber PANI. We suggested that there are four probable combining modes between GEO and PANI:??—?stacking,?electrostatic action,?hydrogen bond,?chemical doping. These combining modes can unite individual components firmly and make the composites more stable. We stablished the relevance between the specific capacitance and graphite raw material sizes (12500 and 500 mesh) as well as the content of GEO added. Also, we determined that the composite can reach the highest initial specific capacitance of 746 and 627 F/g at the mass ratio (GEO/polyainline) of 1:200 (12500 mesh) and 1:50 (500 mesh), respectively. These values are much higher than PANI electrode, showing excellent electrochemical properties.(2) Since the conductivity of GEO is not so good as GE and the enhancing space of its energy storage is limited, so it is necessary to converse GEO to GE. Based on this analysis, two routes are designed to fabricate GE/PANI composite. One is to prepare GE first and then introduce it into the synthesis process of PANI. The other is to prepare GEO/PANI first and then add NaOH solution to help deleting redundant oxygeous groups, PANI was dedoped at the same time, finally PANI was redoped by HCl solution. Experiment results demonstrate that the unifonnity of the morphology and electrochemical performances for the second route is better, because it can delete the oxygeous groups and dedope PANI at the same time. In this way, we successfully prepared highly dispersed GE nanosheets covered by uniform PANI nanoparticles in ethylene glycol medium, the composite has unusual flexibility, and can be bent without being splitting into pieces. When used as supercapacitor electrode, the composite shows a specific capacitance of 1126 F/g with the cycle life of 84% after 1000 cycles, which is beyond the single component PANI or GE electrode, exhibiting excellent electrochemical performance. At the same time, the power density and energy density of the composite materials are also increased significantly, up to 141.1 W/kg and 37.9 Wh/kg, corresponding to nearly 10 times for GE electrode.In addition, we tried to fabricate the composite adopting ethanol as solvent by the second route, and PANI conducting nano wires bridged GEO composite was observed. Different GEO sheets are connected by PANI nanowires, forming a larger conjugated system. This strategy is simple, effective and inexpensive, which is an ideal method to fabricate GE/PANI composite and can also be applied to other conducting polymers such as polypyrrole (PPY) and polythiophene composites.(3) The suspension of GE can be more stable in alkaline solution, however, acid moleculars must be added in the preparation of GE/PANI composite, which makes the suspension unstable. In order to enhance the dispersion degree of GE and its energy storage performance, we grafted sulfonated functional groups to GE sheets and introduce it into the synthesis process of PANI by in situ method and interfacial method to obtain sulfonated GE (SGE). Reasearch results indicate that PANI exhibits nanorod or nanofiber morphology in the composites by interfacial method, while it is nano particles or nanofiber by in situ method. The specific capacitance of composite is higher for interfacial method. For the interfacial method, the morphology of PANI can be adjusted by adding another kind of doping acid and amount of GE. When only SGE was adopted, PANI show nanorod structure in the composites, when another acid-hydrochloride was added, PANI exhibits nanoparticle and nanofiber structure. Moreover, SGE can be used as both doping acid and template, which could reduce the amount of smaller acid and enhance the dispersion degree of GE. Also, the morphology plays an important role in the capacitance and cycling life on the composite electrode.(4) PPY is yet another promising conducting polymer, we fabricated GEO/PPY composites and GE/PPY composites, studied the electrochemical properties of them and compared the electrochemical properties between them. By changing the type of doping acid we realized the controllable fabrication of PPY in the composites:By using H2SO4 and 1,5-NDA as doping agents, the sizes of prepared PPY on GEO are 10 and 20 nm and the specific capacity values are 532 and 475 F/g respectively, higher than PPY electrode. The pyrrole amount and acid species had great impact on the microstructure, morphology and electrochemical properties of the composite. The more amount of pyrrole added, the more loaded amount of PPY and the thicker the layers.Meanwhile, we introduced GE synthesized by chemical method into the synthesis process of PPY, PPY nanofiber and nanoparticles are formed on GE sheets. The GE/PPY composites exhibited higher specific capacitance than GEO/PPY under the same conditions, the specific capacitance enhancement reached 65%, showing good electrochemical performances.