Study on the use of redox electrolytes in supercapacitors

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

Mechanisms and designs of asymmetrical electrochemical capacitors

Different charge storage mechanisms in electrochemical energy storage devices are reviewed, including non-Faradaic capacitive, Faradaic capacitive, Faradaic non-capacitive, and their combinations. Specifically, Faradaic capacitive (pseudocapacitive) storage and Faradaic non-capacitive (Nernstian) storage are attributed to the transfer of delocalised and localised valence electrons, respectively. Mathematical and graphical expressions of the respective storage performances are presented. The account is made especially for asymmetrical electrochemical capacitors (AECs), supercapattery and supercabattery. Both hypothetical and experimental examples are presented to demonstrate the merits of supercapattery that combines capacitive and Nernstian electrodes. Enhanced storage performance is shown by properly pairing and balancing the properties of the negatrode (negative electrode) and positrode (positive electrode) in the AEC or supercapattery. In addition, the design, laboratory manufacturing and performance of several stacks of bipolarly connected AEC cells are assessed in terms of commercial feasibility and promise

Redox electrolytes in supercapacitors

Most methods for improving supercapacitor performance are based on developments of electrode materials to optimally exploit their storage mechanisms, namely electrical double layer capacitance and pseudocapacitance. In such cases, the electrolyte is supposed to be electrochemically as inert as possible so that a wide potential window can be achieved. Interestingly, in recent years, there has been a growing interest in the investigation of supercapacitors with an electrolyte that can offer redox activity. Such redox electrolytes have been shown to offer increased charge storage capacity, and possibly other benefits. There are however some confusions, for example, on the nature of contributions of the redox electrolyte to the increased storage capacity in comparison with pseudocapacitance, or by expression of the overall increased charge storage capacity as capacitance. This report intends to provide a brief but critical review on the pros and cons of the application of such redox electrolytes in supercapacitors, and to advocate development of the relevant research into a new electrochemical energy storage device in parallel with, but not the same as that of supercapacitors

Fundamental Consideration for Electrochemical Engineering of Supercapattery

Supercapattery is the generic name for various electrochemical energy storage (EES) devices combining the merits of battery (high energy density) and supercapacitor (high power density and long cycling life). In this article, the principle and applications of EES devices are selectively reviewed as the background for supercapattery development. The focus is on the engineering aspects for fabrication of two types of supercapattery: (i) by coupling a battery electrode with a supercapacitor electrode, or (ii) from materials that possess both the Nernstian and capacitive charge storage capacities. Fundamental rationales are discussed in relation with the designs, such as why the device is always asymmetrical, and what materials are suitable for making supercapattery. Whilst the key is how to optimize device performance in terms of energy capacity, power capability and cycle life, cost is also discussed on resource rich materials such as nanostructured composites and redox electrolytes

Charge Storage Properties of Aqueous Halide Supercapatteries with Activated Carbon and Graphene Nanoplatelets as Active Electrode Materials

Device level performance of aqueous halide supercapatteries fabricated with equal electrode mass of activated carbon or graphene nanoplatelets has been characterized. It was revealed that the surface oxygen groups in the graphitic structures of the nanoplatelets contributed toward a more enhanced charge storage capacity in bromide containing redox electrolytes. Moreover, the rate performance of the devices could be linked to the effect of the pore size of the carbons on the dynamics of the inactive alkali metal counterion of the redox halide salt. Additionally, the charge storage performance of aqueous halide supercapatteries with graphene nanoplatelets as the electrode material may be attributed to the combined effect of the porous structure on the dynamics of the non-active cations and a possible interaction of the Br−/(Br2+Br3−) redox triple with the surface oxygen groups within the graphitic layer of the nanoplatelets. Generally, it has been shown that the surface groups and microstructure of electrode materials must be critically correlated with the redox electrolytes in the ongoing efforts to commercialize these devices

Morphological, optical and electrical properties of spray coated zinc ethyl xanthates for decomposition within a poly(3-hexylthiophene-2,5-diyl) matrix

This work investigates the deposition of hybrid layers, for next generation in energy conversion, via spray coating. Understanding the effect that this deposition procedure has on these layers could lead to the rapid development of these technologies, for both laboratory applications and commercialisation. Synthesised zinc ethyl xanthate and poly(3-hexylthiophene-2,5-diyl) was spray-coated on substrates and heated to a temperature of 160 oC, to the hybrid film. Optical, morphological and conductive properties of these films were investigated and linked to the spray coating duration. It was revealed that shorter-duration spray times led to relatively low conductivity and smoother films, moreover, an increase in spraying duration also led to an increase in conductivity, but with increased roughness, from 6.178 nm to 8.317 nm. As the spray time was further increased factors, including film layering effects, led to a gradual decrease in conductivity accompanied by a decrease in the roughness. Smoother films were shown to result in higher light absorption, characterised by wider band gaps, which could be due to the crystal structure of the inorganic phase. The controllability of this rapid, facile, and inexpensive spray deposition process was then demonstrated in fabrication of prototype photovoltaic devices

Optimal Utilization of Combined Double Layer and Nernstian Charging of Activated Carbon Electrodes in Aqueous Halide Supercapattery through Capacitance Unequalization

Charge storage through electric double layer (EDL) charging of activated carbon (AC) and redox reactions of iodide and bromide ions in aqueous electrolytes and at the AC | electrolyte interface has been investigated by cyclic voltammetry and galvanostatic charging and discharging. Electrochemical experiments were carried out in both the three-electrode and two-electrode cells with the latter resembling the so-called supercapacitor-battery hybrid or simply supercapattery. By comparing the electrochemical behavior of bromide and iodide ions used as dissolved redox species (DRS), some observed features of the supercapattery are described and analyzed from the standpoint of the EDL charging of the AC electrodes, the thermodynamics and kinetics of the electrode reactions of the DRS, and the adsorption and transport of the charging reaction products. Furthermore, the effect of capacitance unequalization was explored for the adequate utilization of the charge storage from both the DRS and EDL contributions. It is also shown that counter-electrode oversizing has to be critically appraised for the design of optimal devices

Mechano‐fenton–piranha oxidation of carbon nanotubes for energy application

Emission of nitrogen oxides (NOx) from chemical processing of materials is a serious environmental concern, frustrating the development of many innovative technologies. For example, sulfonitric oxidation is the most widely used method for processing carbon nanotubes (CNTs), producing a large amount of NOx. As a result, large scale applications of CNTs for downstream purposes remain challenging. Herein, a NOx-free oxidation method is proposed for CNTs processing. It starts with mechanically grinding, and then oxidizing the CNTs by hydroxyl radicals in sealed reactors. Such processed CNTs are shorter, possess balanced surface oxygen containing groups without compromising the original CNT integrity, and can disperse readily in water. These are desirable for making various CNT composites, including those with conducting polymers for supercapacitors. The reactors in the process are industrially adoptable, promising a great technological and commercial future

Conversion of high moisture biomass to hierarchical porous carbon via molten base carbonisation and activation for electrochemical double layer capacitor

Biomass-derived carbon for supercapacitors faces the challenge of achieving hierarchical porous carbon with graphitic structure and specific heteroatoms through a single-stage thermal process that minimises resource input. Herein, molten base carbonisation and activation is proposed. The process utilises the inherent moisture of Moso bamboo shoots, coupled with a low amount of KOH, to form potassium organic salts before drying. The resultant potassium salts promote in-situ activation during single-stage heating process, yielding hierarchical porous, large specific surface area, and partially graphitised carbon with heteroatoms (N, O). As an electrode material, this carbon exhibits a specific capacitance of 327F g−1 in 6 M KOH and 182F g−1 in 1 M TEABF4/AN, demonstrating excellent cycling stability over 10,000 cycles at 2 A/g. Overall, this study presents a straightforward process that avoids pre-drying of biomass, minimises base consumption, and employs single-stage heating to fabricate electrode carbon suitable for supercapacitors

Study on the use of redox electrolytes in supercapacitors

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