2D Materials for Energy Storage

Abstract

The work presented in this thesis investigates the incorporation of graphene within supercapacitors. The methods for the production of graphene yield materials with different morphologies and physical properties leading to differences in their electrochemical performance. This is highlighted in the literature with different capacitances and energy storage mechanisms being reported for graphene. Therefore, the effect of the graphene production method on their electrochemical performance in aqueous electrolytes was investigated. This work highlights the importance of the oxygen content, defect density and flake structure on the displayed properties of the supercapacitor influencing the possible applications of the device, i.e. high power or energy devices. Furthermore, the majority of research on graphene utilises aqueous electrolytes meaning direct comparison with commercial devices, utilising organic electrolytes with rated voltages of up to 2.7 V, is not possible. The design of the cell becomes vital in enabling the higher cell voltage to be reached, due to the asymmetry of the positive and negative electrode potential limits. Therefore, a technique to determine these limits and optimise the cell design was developed. The validity of this technique was verified by aging measurements on optimised and non-optimised cells: the operating voltage, from 2.7 to 3 V, and the potentiostatic cycle lifetime, 500 % increase, of the optimised devices was significantly improved. Finally, most literature on graphene supercapacitors focuses on small scale devices with low electrode mass loadings and thicknesses inflating their performance. This means their larger scale device analogues will show reduced performance. Therefore, the use of graphene as a conductive additive in activated carbon supercapacitors was investigated in large scale devices (pouch cells). It was found that the increase in capacitance due to the inclusion of graphene was minimal (1 – 2 %) compared to carbon black measured up to a voltage of 2.7 V. However, when the graphene containing pouch cells were measured at higher voltages (3 V) they showed superior capacitance retention with potentiostatic (5 % increase) and galvanostatic (20 % increase) cycling compared to carbon black.

Date of Award	1 Aug 2019
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Robert Dryfe (Supervisor) & Andrew Forsyth (Supervisor)