Functionalized graphene materials for high performance supercapacitors

Abstract

Graphene has been considered as a promising electrode material for supercapacitors due to its outstanding properties. However, graphene produced from reduction of graphene oxide (GO) always suffers from restacking and aggregation issue, which reduces the effective surface area and porosity for ion diffusion and storage. This dissertation aims to solve this problem through chemical functionalization of graphene and provide an in-depth understanding on the structure-property relationship of the corresponding electrodes. To realize controlled chemical functionalization, various “molecular spacers” were covalently grafted onto graphene surface to create 3D graphene-based covalent organic frameworks. Delicate selection of organic spacers can tune the surface chemistry, interlayer spacing and specific surface area of graphene at molecular level. The chemical reaction mechanism and evidence of crosslinking structures were elucidated by a series of chemical, thermal and physical analyses. The negligible structural change before and after GO reduction demonstrated the effectiveness and stability of the “molecular spacers”. Organic pseudocapacitive materials have also been incorporated into graphene networks to from larger conjugated networks with enhanced conductivity and pronounced redox charge transfer characteristics. Molecular dynamic (MD) and density function theory (DFT) calculations were performed to verify the experimental findings. In addition, the effect of polymer binders were studied to optimize the formulation of free-standing electrodes. The functionalized graphene materials have shown considerably improved electrochemical properties with high capacitance, large energy and power densities, and long-term cycling stability. Moving from material design to device fabrication, micro-supercapacitors (MSCs), as miniaturized energy storage units, have been developed using highly conductive and aqueous processable graphene nanocomposites as micro-electrodes. For MSC fabrication, interdigitated patterns were formed by a combination of thin film deposition, shadow masking, and plasma etching processes. The high quality materials and design merits resulted in an ultrahigh energy density and ultrafast frequency response. The planar MSC architecture can be further extended to make flexible or even foldable MSC arrays with controllable operation potential window and much higher areal capacitance for wearable electronic devices.