Study on the use of redox electrolytes in supercapacitors

Abstract

Electric double layer (EDL) capacitors (a.k.a. supercapacitors) which are generally characterised by high power and long cycle lives, exploits the electrostatic interaction between electrons and ions within the porous matrix of carbonaceous materials with high specific surface area ranging from 500.00 to over 2,000.00 cm2/g. Majority of the research on supercapacitors focuses on developments of the electrodes in order to improve device performance metrics such as: capacitance, power, and cycle stability. The prevailing view on the electrolyte is that it should be chemically inert during the charge-discharge of a supercapacitor. Interestingly, in recent years, the adoption of electrolytes that display redox activity has garnered a lot of attention. This is mainly because redox electrolytes could enhance charge storage capacity. This research is focused on the use of redox electrolytes to improve the performance of supercapacitors. We have proposed that the charging of supercapacitors due to dissolved redox species (DRS) incorporates both Nernstian (battery) and EDL capacitance mechanisms, in line with the features of a supercapacitor-battery hybrid i.e. supercapattery. Accordingly, widespread confusion in the literatures regarding the charge storage mechanisms of these devices were critiqued and remedied through basic electrochemical considerations. Electrochemical analysis of supercapatteries with KI or KBr as redox electrolytes have been studied. Herein, the device characteristics were rigorously analysed from the standpoint of the polarisation of the activated carbon electrodes, the thermodynamics of the DRS, and the adsorption and transport of the charging reaction products. Therefore, the origin of charge capacity increase at high cell voltages have been fundamentally described. These findings are important to the design of high energy supercapatteries which was exemplified by devices with KBr as redox electrolyte. Additionally, fundamental electrochemical analysis have been used to meticulously assess the relationship between capacitive and non-capacitive storage mechanisms to understand the engineering design of supercapacitors with DRS. Correspondingly, through the variation of the mass ratio between the activated carbon materials on the positive electrode (positrode) and negative electrode (negatrode), supercapatteries with 1.00 mol/L KBr as redox electrolyte have been operated at energy range of 17.30 to 33.20 Wh/kg with a current load of ±0.25 A/g at 1.60 V. Optimal electrode mass ratio also resulted in high performing devices with specific energy and power of 21.31 Wh/kg and 703.82 W/kg respectively, at a current load of ±1.00 A/g. The characteristics of three different commercially available carbons which are broadly representative of the porous and surface physico-chemical properties of EDL capacitor electrodes have been rigorously compared and analysed. In this regard, the role of the pore size and surface physico-chemistry of the electrodes have been comprehensively correlated with the redox electrolytes based on the bromide anion, and with various cations such as Li+, Na+, K+ and Mg2+. Based on these investigations, bromides have been used as DRS in supercapatteries with either a micoroporous or mesoporous and highly graphitised carbon. Herein, it was shown that the cells are operable at 1.8 V and can retain ca. 80.00% of the initial specific energy after 10,000 galvanostatic charge-discharge cycles. Thus, fundamentally relevant and practically important properties of a mesoporous and highly graphitised carbon with bromides as DRS were reported for the first time. Hence, these findings could serve as useful benchmarks in the design of commercial supercapatteries with DRS. A redox organic molecule methyl hydroquinone (MH2Q) has been verified for the first time to be a viable DRS. Through electroanalytical, and spectroscopic comparisons with the more common hydroquinone (H2Q), it was observed that MH2Q adsorbs more strongly on the activated carbon electrode compared to H2Q. Furthermore, the structural basis to the charge storage mechanism of a highly graphitised carbon with MH2Q have also been revealed. These studies demonstrated that when organic redox molecules are to be selected as DRS, it is important to critically evaluate not just the enhancement of charge capacity, but also the relationship between the structure of the molecules and their interaction within the lattice of the carbon. Bi-electrolyte cells which are assembled in a manner that allows the positrode and negatrode to operate in two different electrolytes were demonstrated as a versatile means of designing high-performance supercapatteries. Accordingly, a novel bi-electrolyte cell with immiscible electrolytes wherein the positrode is operated in a redox electrolyte was shown to be characterised by slow rate of self-discharge. Also, these cells were operable at 2.00 V, displaying a specific energy of 35.83 Wh/Kg at an applied current load of ±0.50 A/g. This effectively introduces a new class of device engineering strategy with huge promise.

Date of Award	8 Jul 2019
Original language	English
Awarding Institution	Univerisity of Nottingham
Supervisor	George Zheng Chen (Supervisor), Chuang Peng (Supervisor) & Christian Klumpner (Supervisor)