Fabrication Of Functionalized Graphene-based Materials And Their Applications To Energy Storage

Graphene is one of the most promising miracle materials under investigation, due to itsatomically thick two-dimensional structure, fascinating physical and chemical properties. Up tonow, graphene has been widely explored for applications in energy-related areas, including solarcells, lithium ion batteries, supercapacitors and catalysis. In this regard, functionalization playsan important role in the modification of structural, electronic and chemical properties ofgraphene for further applications. Hence, both covalent and noncovalent strategies have beenexplored for graphene functionalization in recent years. In this dissertation, we develop simpleand effective methods to fabricate functionalized graphene-based materials and their applicationsto energy strorage are also investigated.1. Three-dimensional (3D) graphene-based materials exhibit large surface area, high porosityand remarkable electronic properties, offering a wide range of applications in catalysis, drugdelivery, tissue engineering, energy storage, sensors and actuators. Previous reports demonstratethat3D graphene oxide gel (GOG) can be achieved by hydrogen bonding or electrostaticinteraction, through the introduction of a hydrophilic polymer or metal ion as the cross-linker.However, the main challenge is to effectively assemble the2D graphene-based sheets into the3Dnetworking architecture, especially with controllable pore structure or interlayer spacing. Wehave developed an effective strategy to synthesis GOG by employing a hydrophobic interlayercross-linker via supramolecular π-π interaction at room temperature. This method also providesan effective way to control fairly well the interlayer distance and the porosity of GOG.2. Graphene oxide (GO) is usually synthesized by oxidization and exfoliation of graphite,together with carboxyl, hydroxyl and epoxy groups attached to the edge and plane of graphene.Nevertheless, the strong hydrophilic oxygen-containing groups also make GO incompatible withmost organic materials, especially as the filler for polymer nanocomposites. On the other hand,the GO sheets tend to either form irreversible agglomerates or restack during the solutionreduction process, leading to loss of the surface area and porosity for energy storage applications.In our work, we have developed an easy and versatile method for the covalent functionalizationof GO to give increased solubility in organic solvents, which involves one-step reaction inaqueous solvent. The obtained functionalized GO shows high solubility in most organic solventswithout aggregation or visible restacking. Later, we found the introduced functional groups can not only partially prevent the restacking of graphene sheets during the solution reduction, butalso improve the electrochemical performance for supercapacitor due to the contribution ofadditional redox capacitance.3. Conventional Li-ion batteries (LIBs) configuration consist of lithium transition metaloxides or phosphates as the positive electrode (cathode) and graphite-type materials as thenegative electrode (anode), thus charge storage capability is inherently limited to about300mAh/g, due to the low theoretical specific capacity of the cathode (150-200mAh/g) and anode(372mAh/g) materials. Thus, significant research efforts have been focused on searching foradvanced carbon-based anode materials with enhanced Li storage capacity for next-generationLIBs. Up-to-date, various carbon-based materials, such as carbon nanotube, carbon fiber, porouscarbon and their hybrids have been well investigated as a possible anode material for LIBs.Graphene and its derivatives have also been considered as a potential electrode material for LIBs,primarily due to their extraordinary electrical conductivity, large specific surface area and stablechemical property. However, the facile synthesis of graphene-based anode materials for highperformance LIBs, especially suitable for scalable continuous manufacturing remainschallenging. In our work, we have used a simple method to fabricate functionalizedgraphene-based materials and its application to Li-ion battery are also investigated.