Year

2013

Degree Name

Doctor of Philosophy

Department

School of Chemistry

Abstract

Supercapacitors are promising energy storage and power output technologies due to their improved energy density, rapid charge-discharge cycle, high cycle efficiency and long cycle life. Free standing poly(3,4-ethylenedioxythiophene) poly(styrene sulfonate) / single walled nanotube films have been characterised by scanning electron microscopy, Raman spectroscopy and thermo-gravimetric analysis to understand the physical properties of the films. Films with varying compositions of poly(3,4-ethylenedioxythiophene) / poly(styrene sulfonate) and single walled nanotubes were compared by electrochemical impedance spectroscopy, cyclic voltammetry and galvanostatic charge / discharge to understand their electrochemical properties. A comparison of the results shows that having single walled nanotubes dispersed throughout the polymer matrix increases the capacitance by 65 % and the energy density by a factor of 3 whilst achieving good capacity retention over 1000 cycles.

Graphene is sp2 hybridised carbon atoms in a honeycomb crystal lattice, which has attracted a lot of interest in materials science and condensed matter physics research due to its favourable electronic properties, abundance and low cost. Exfoliated graphene oxide which has also been partially reduced has been achieved using microwave irradiation through the use of a conventional microwave that has led to rapid expansion and a marked volume increase of the graphene oxide. This has enabled a porous material to be developed that can serve as a conductive scaffold and support for composite electrode materials.

Graphene based materials coupled with transition metal oxides are promising electrode materials in asymmetric supercapacitors owing to their unique properties which include high surface area, good chemical stability, electrical conductivity, abundance, and lower cost profile over time. A composite material consisting of graphene oxide exfoliated with microwave radiation (mw rGO), and manganosite (MnO) is synthesised in order to explore their potential as an electrode material. The composite material was characterised by scanning electron microscopy (SEM), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) was used to explore the process occurring at the electrode / electrolyte interface. Long term cyclability and stability was investigated using galvanostatic charge / discharge testing. From the resulting analysis, an asymmetric supercapacitor was constructed with the best composite containing 90% MnO- 10% mw rGO (w/w). The device exhibited a capacitance of 0.11 F/cm2 (51.5 F/g by mass) and excellent capacity retention of 82% after 15 000 cycles at a current density of 0.5 A/g.

Composites containing CNTs and graphene are materials of particular interest in the energy storage and conversion area due to their favourable properties which can result in unique optical, electrical, magnetic and chemical properties which are substantially different than that of the individual components. It has been shown that the combination of both CNTs and graphene allows an expressway of electron transport from the electrode material to the current collector. The ability of the CNTs to ‘sandwich’ in between the graphene sheets helps to alleviate restacking by acting as a spacer (thus maintaining surface area) while increasing electrical conductivity and mechanical stability.

We describe a facile method to develop electrodes of SWNT and exfoliated graphene oxide (mw rGO) with varying weight ratios via sonication, centrifugation and vacuum filtration. These composites are then optimised with the best performing weight ratio to be used in the fabrication of a supercapacitor. Extensive electrochemical testing revealed that the incorporation of SWNTs with mw rGO yielded an electrode material with large specific capacitances. The specific capacitances in all cases exceeded 125 F/g with the optimised / ideal weight ratio of 90% SWNT – 10% mw rGO exceeding 300 F/g. The thickness of the 90% SWNT – 10% mw rGO was optimised with the maximum attainable current per unit area arising at a thickness of 17 microns. Lastly, a device was fabricated that showed excellent reversibility upon current switching from 0.05 A/g up to 4.0 A/g, with a specific capacitance of 80 mF/cm2 and 128 F/g respectively at 0.05 A/g. Long term testing showed excellent stability over 10 000 cycles, with the maximum attainable energy and power density being 5.8 W.h/kg and 1.9 MW/kg.

Share

COinS