Graphene Based Nanostructured Materials for Electrochemical Energy Storage

Abstract

The objective of this work is to gain fundamental insights into the reduction of graphene oxide (GO), chemical reduction induced self-assembly of graphene and assembly of novel three-dimensional graphene morphologies. It is motivated by the prospect of achieving better electrochemical performance by circumventing a common issue of “restacking” and design novel-electrode assemblies. The dissertation first provides insights into the photo-reduction of GO to reduced graphene oxide (RGO), which gives way to the photolytic release of hydroxyl radicals. This has several implications in the stability of GO and unveils potential new applications that take advantage of the OH● photolysis products. Next, relationships between processing conditions, material properties, and electrochemical response for self-assembled RGO electrode assemblies were studied. Electrode assemblies were prepared from GO dispersions, which are well suited to scaling up and economical production methods. L-ascorbic acid-assisted reduction of GO to RGO was utilized to self-assemble RGO sheets into 3D graphene frameworks, which were formed into electrode assemblies and fabricated into electrochemical double-layer capacitors. The aspect ratio of GO sheets and the solution processing conditions were varied to understand their impacts on the electrochemical response of the RGO capacitors. Ultra-large GO (UL-GO) sheets were synthesized using 50 mesh (297 µm) graphite flakes to fabricate graphene frameworks (GF). Their electrochemical performance was tested against GFs fabricated from small graphene oxide sheets synthesized using 325 mesh (44 µm) graphite flakes. GFs fabricated from UL-GO demonstrated a 25% higher specific capacitance, six times faster ion-transport, and six times lower charge transfer resistance. These results are indicative of higher energy density, efficient ion transport and improved electrical percolation in the 3D matrix of GFs formed by UL-GO. Next, the effect of concentration and pH for small GO and UL-GO was studied. 3D graphene frameworks were prepared using small GO with the concentrations from 2.0 mg/mL to 12 mg/mL and UL-GO with concentration from 0.5 mg/mL to 4 mg/mL. The superior electrochemical results such as high capacitance and low charge/discharge times were observed for optimum concentrations of 3.5 mg/mL and 1.5 mg/mL with a pH of 10 for GFs formed by small GO and UL-GO, respectively. Modifying GO morphology can result in interesting properties of 3D graphene frameworks. Therefore, a novel solution-based technique was developed to produce holey graphene. It was demonstrated that hydroxyl radicals are formed by microwave irradiation of H2O2, which can facilitate the bulk synthesis of holey graphene. Hydroxyl radicals produced from H2O2 attack both defective sites and sp2 hybridized carbon within RGO. Oxidative chemical etching removes RGO-carbons to locally produce holes. The achievement of superior electrochemical properties in 3D graphene frameworks by varying the sheet size and processing condition suggests that there are many opportunities where a fundamental understanding of assembling nanomaterials as basic building blocks can lead to desirable properties of bulk 3D materials. The work presented here could enable further improvements based on the availability of high specific surface area and more diffusion pathways in the graphene-based electrode assemblies.