Development of Electro-active Graphene Nanoplatelets and Composites for Application as Electrodes within Supercapacitors

Abstract

The mounting concern for renewable energies from ecologically conscious alternatives is growing in parallel with the demand for portable energy storage devices, fuelling research in the fields of electrochemical energy storage technologies. The supercapacitor, also known as electrochemical capacitor, is an energy storage device possessing a near infinite life-cycle and high power density recognized to store energy in an electrostatic double-layer, or through a pseudocapacitance mechanism as a result of an applied potential. The power density of supercapacitors far exceeds that of batteries with an ability to charge and discharge stored energy within seconds. Supercapacitors compliment this characteristic very well with a cycle life in excess of 106 cycles of deep discharge within a wide operational temperature range, and generally require no further maintenance upon integration. Conscientious of environmental standards, these devices are also recyclable. Electrochemical capacitors are currently a promising candidate to assist in addressing energy storage concerns, particularly in hybridized energy storage systems where batteries and supercapacitors compliment each other’s strengths; however specific challenges must be addressed to realize their potential. In order to further build upon the range of supercapacitors for future market applications, advancements made in nanomaterial research and design are expected to continue the materials development trend with a goal to improve the energy density through the development of a cost-efficient and correspondingly plentiful material. However, it is important to note that the characteristic power performance and exceptional life-cycle should be preserved alongside these efforts to maintain their niche as a power device, and not simply develop an alternative to the average battery. It is with this clear objective that this thesis presents research on an emerging carbon material derived from an abundant precursor, where the investigations focus on its potential to achieve high energy and power density, stability and integration with other electroactive materials. Activated carbons have been the dominant carbon material used in electric double-layer capacitors since their inception in the early 1970s. Despite a wide range of carbon precursors and activation methods available for the generation of high surface area carbons, difficulties remain in controlling the pore size distribution, pore shape and an interconnected pore structure to achieve a high energy density. These factors have restricted the market growth for supercapacitors in terms of the price per unit of energy storage. Activation procedures and subsequent processes for these materials can also be energy intensive (i.e. high temperatures) or environmentally unfriendly, thus the challenge remains in fabricating an inexpensive high surface-area electroactive material with favourable physical properties from a source available in abundance. Double-layer capacitive materials researched to replace active carbons generally require properties that include: high, accessible surface-area; good electrical conductivity; a pore size distribution that includes mesopore and micropore; structural stability; and possibly functional groups that lend to energy storage through pseudocapacitive mechanisms. Templated, fibrous and aerogel carbons offer an alternative to activated carbons; however the drawbacks to these materials can include difficult preparation procedures or deficient physical properties with respect to those listed above. In recent years nanostructured carbon materials possessing favourable properties have also contributed to the field. Graphene nanoplatelet (GNP) and carbon nanotube (CNT) are nanostructured materials that are being progressively explored for suitable development as supercapacitor electrodes. As carbon lattice structured materials either in the form of a 2-dimensional sheet or rolled into a cylinder both of these materials possess unique properties desirable in for electrode development. In the proceeding report, GNPs are investigated as a primary material for the synthesis of electrodes in both a pure and composite form. Three projects are presented herein that emphasize the suitability of GNP as a singular carbon electrode material as well as a structural substrate for additional electroactive materials. Investigation in these projects focuses on the electrochemical activity of the materials for supercapacitor devices, and elucidation of the physical factors which contribute towards the observed capacitance. An initial study of the GNPs investigates their distinct capacitive ability as an electric double-layer material for thin-film applications. The high electrically conductivity and sheet-like structure of GNPs supported the fabrication of flexible and transparent films with a thickness ranging from 25 to 100 nm. The thinnest film fabricated (25 nm) yielded a high specific capacitance from preliminary evaluation with a notable high energy and power density. Furthermore, fast charging capabilities were observed from the GNP thin film electrodes. The second study examines the use of CNT entanglements dispersed between GNP to increase the active surface area and reduce contact resistances with thin-film electrodes. Through the use MWNT/GNP and SWNT/GNP composites it was determined that tube aspect ratio influences the resulting capacitive performance, with the formation of micropores in SWNT/GNP yielding favourable results as a composite EDLC. The third study utilizes electrically conducting polypyrrole (PPy) deposited onto a GNP film through pulse electrodeposition for use as a supercapacitor electrode. Total pulse deposition times were evaluated in terms of their corresponding improvements to the specific capacitance, where an optimal deposition time was discovered. A significant increase to the total specific capacitance was observed through the integration PPy, with the majority charge storage being developed via psuedocapacitive redox mechanisms. A summary of the studies presented here centers on the development of GNP electrodes for application in high power supercapacitor devices. The potential use for GNP in both pure and composite electrode films is explored for electrochemical activity and capacitive capabilities, with corresponding physical characterization techniques performed to examine influential factors which contribute to the final results. The work emphasizes the suitability of GNP material for future investigations into their application as carbon or carbon composite electrodes in supercapacitor devices.