Functional Properties of Mechanically Exfoliated Graphene

Abstract

Graphene is an exciting 2-dimensional material with a wide range of superior properties, such as excellent thermal and electrical conductivity, high transparency, surface area and mechanical strength. However, a wider scale application still depends on finding viable large-scale production methods of high quality graphene. This thesis demonstrates a feasible method for fabrication of graphene with scalability potential. High concentration and quality of mechanically exfoliated graphene (in short, pristine graphene), was achieved at short processing time using shear exfoliation. This mechanically exfoliated graphene was further used to fabricate functional nanopapers with a wide range of properties, and as electrode materials for supercapacitor application. The properties of pristine graphene were also compared to other types of graphene, namely graphene oxide (GO) and reduced graphene oxide (RGO), in nanocomposite films designed for functional applications formed as graphene-doped microfibrillated cellulose (MFC) nanopapers. The different surface chemistry of these grades of graphene led to the fabrication of composites with a wide range of properties due to their unique interaction with the MFC polymer matrix. For example, pristine graphene led to a high electrical conductivity and thermal stability, while GO and RGO led to enhanced mechanical properties of the nanopapers. The composites can be tailored for a wide range of potential applications, such as flexible electronics, sensors, electrodes etc. Following these foreseen advantages, the nanopapers were further developed for testing in application as electrode materials in supercapacitors. This was realized by converting into carbon hybrids by activation with KOH to introduce properties suitable for good electrochemical performance. MFC was used to stabilize and exfoliate the graphene and as a binder in the electrodes. The electrodes showed excellent electrochemical performance with potential for application in various devices. Furthermore, the performance of the carbon hybrids was compared with willow-derived activated carbon in supercapacitors. The willow-derived activated carbon showed even higher capacitance than the carbon hybrids. However, the carbon hybrids showed much better cycling stability, 99% capacitance retention after 5 000 cycles versus 94% for willow derived activated carbons. The work summarized in this thesis, in conclusion, contributes to the development of graphene for larger scale commercial application, identifying potential routes for fabrication of high performance functional carbon materials based on alternative renewable and sustainable materials to replace the currently potentially toxic, and hazardous chemical additives.