L’enjeu majeur du développement des supercondensateurs reste focalisé sur l’augmentation de la densité d’énergie de ces systèmes tout en adoptant une démarche la plus respectueuse possible de l’environnement. Afin de satisfaire cet objectif, deux stratégies d’optimisation de supercondensateurs carbone/carbone ont été envisagées en fonction du milieu électrolytique utilisé: i) dans le cas du milieu aqueux, des solutions de sulfates alcalins neutres ont été considérées afin d’étendre la tension de fonctionnement du système; ii) dans le cas du milieu organique, une méthode douce d'activation a été mise en oeuvre afin d’obtenir des carbones microporeux avec une taille moyenne de pores adaptée à la taille des ions de l’électrolyte. L’utilisation d’électrolytes aqueux à base de sulfates alcalins dans des supercondensateurs carbone/carbone symétriques a permis d’étendre la fenêtre de tension jusqu’à 1,9 V ; cette dernière a même pu être étendue à 2,0 V par ajustement des masses d’électrodes. Enfin, des électrodes commerciales enduites ont été utilisées dans des cellules type « coffee-bag » offrant une excellente stabilité pendant 10,000 cycles à 2,1 V. En milieu organique, des carbones nanoporeux denses avec des pores adaptés à la taille des ions de l'électrolyte organique Et4NBF4/acetonitrile ont été obtenus par oxydation à haute pression et basse température (environ 200°C) d’un carbone non poreux. Une étape suivante de traitement thermique a ensuite permis d’éliminer les groupements fonctionnels de surface et ainsi d’améliorer l’accessibilité de la porosité. En raison de la faible oxydation, la densité des électrodes est remarquablement élevée permettant d’atteindre des valeurs élevées de capacité volumique. / The objective of this work is to improve the energy density of carbon/carbon supercapacitors. For achieving this objective, two different strategies were followed depending on the electrolyte used: i) in aqueous electrolytes, our efforts were focused on extending the operating cell voltage by using neutral alkali sulfate solutions; ii) in organic electrolyte, the target was to improve the volumetric capacitance by setting a mild activation method able to produce a porous carbon with average pore size matching the ion size, while not enlarging the pores upon porosity development. A practical cell voltage of 1.8 V has been demonstrated by implementing aqueous alkali sulfates in symmetric carbon/carbon capacitors. It has been shown that the voltage is limited by a partial destructive electro-oxidation of the positive electrode. Such irreversible electro-oxidation could be mitigated by mild chemical oxidation of the active carbon material with hydrogen peroxide; consequently, the voltage could be further expanded up to 1.9 V. Even 2.0 V could be attained after mass balancing the electrodes in order to allow them to operate in their stability window. Finally, pouch-cells with carbon coating on stainless steel current collector were realized by using 2 mol L-1 Li2SO4 as electrolyte. An exceptional cycling stability at cell voltages up to 2.1 V was obtained during 10,000 cycles. Hence, the use of alkali sulfate electrolytes is a cost-effective alternative to organic electrolytes for producing environment friendly and safe carbon/carbon supercapacitors. Dense nanoporous carbons with pores fitting the dimension of ions of the Et4NBF4/acetonitrile organic electrolyte were obtained by high pressure oxidation of non-porous carbon at low temperature, followed by a thermal desorption to remove the surface groups and unblock pore entrances. The activation mechanism consisted in drilling the narrow pores existing initially in the char. Due to the low burn-off, the density of the electrodes was remarkably high allowing high volumetric capacitance values to be reached. This novel production method associates the advantages of environment friendly, cost-effective, high yield and low energy consumption characteristics.

This dissertation reports on the results of investigations performed on the electrochemical energy ‎storage properties of carbon nanotubes. Specifically, the effect of heat treatment on the ‎electrochemical energy storage properties of carbon nanotubes is reported. We will focus on ‎single-walled carbon nanotubes (SWCNTs) produced by using the direct decomposition of ‎metallocene (for example ferrocene). For understanding the basic properties related to ‎electrochemical storage, electrochemical double-layer capacitors (EDLCs) were fabricated using ‎CNTs as active electrode materials. Electrochemical characterization such as Cyclic Voltammetry ‎‎(CV), Gavanostic Charge-Discharge (GCD) measurements as well as Electrochemical Impedance ‎Microscopy (EIS) were performed. Other measurements to understand the physical nature of the ‎nanotubes used were also performed. These include, Scanning Electron Microscopy (SEM), and ‎Volumetric Adsorption measurements. Our investigations indicate that heat treatment of as-‎produced SWCNTs at 350oC for 1 hour can improve the specific capacitance of these materials ‎several times > 300% when used with potassium hydroxide (KOH) as the electrolyte. We have ‎also performed EDLC measurements using room temperature ionic liquid-based polymer ‎electrolyte. This was performed to increase the operational voltages of the devices. Enhanced ‎capacitances for heat-treated samples as compared to as-produced samples were also obtained in ‎the case of this electrolyte. ‎ We have also shown that it is possible to fabricate flexible supercapacitors using aligned ‎multi-walled carbon nanotubes (MWCNTs) grown directly on a metal (Inconel 600) substrate. ‎The flexibility and robust electrode materials are the crucial characteristics to maintain through ‎the solid-state electrolytes (gel polymer: PVA/H2SO4). Applying this type of gel on MWCNTs ‎growth which is in the right alignment on the flexible metal sheet (Inconel) gives robust CNTs ‎supercapacitors. To go further in understanding the charging mechanism in these supercapacitors, ‎circuit modeling was applied to determine the role of pseudocapacitance in these devices. ‎ Even though the findings reported in this dissertation are performed on specific materials, ‎fundamental understandings can be easily extrapolated to other energy storage devices. ‎Understanding the role of electrochemical charge mechanisms to store the charges in various ‎assembling of energy storage devices is a valuable insight for future investigations. ‎

Applications that require electrical energy storage are becoming increasingly diverse. This development is caused by a number of factors, such as an increasing global energy demand, the advent of electric vehicles, the utilization of intermittent renewable energy sources, and advances in disposable and organic electronics. These applications will set different demands on their electrical energy storage and, thus, there will be no single technology used for all applications. For some applications the choice of energy storage materials will be extremely important. Conventional batteries and supercapacitors rely on the use of nonrenewable inorganic materials mined from depleting ores, hence, requiring large amounts of energy for their processing. Such materials also add a significant cost to the final product, making them less attractive for large scale applications. Conducting polymers, on the other hand, constitute a class of materials that can be used for organic matter based energy storage devices. The aim of this thesis was to investigate the use of a composite consisting of the conducting polymer polypyrrole (PPy) and cellulose derived from Cladophora sp. algae for electrical energy storage. The polymer was coated onto the cellulose fibers by chemical polymerization resulting in a flexible material with high surface area. By using this composite as electrodes, electrochemical cells consisting of disposable and non-toxic materials can be assembled and used as energy storage devices. The resistances of these prototype cells were found to be dominated by the resistance of the current collectors and to scale with the thickness of the separator, and can hence be reduced by cell design. By addition of nanostructured PPy, the weight ratio of PPy in the composite could be increased, and the cell voltages could be enhanced by using a carbonized negative electrode. Composites of cellulose and poly(3,4-ethylenedioxythiophene) could also be synthesized and used as electrode materials. The porosities of the electrodes were controlled by mechanical compression of the composite or by coating of surface modified cellulose fibers with additional PPy. Finally, the self-discharge was studied extensively. It was found that oxygen was responsible for the oxidation of the negative electrode, while the rate of self-discharge of the positive electrode increases with increasing potential. Through measurements of the charge prior to and after self-discharge, as well as with an electrochemical quartz crystal microbalance, it was found that the self-discharge of the positive electrode was linked to an exchange of the counter ions by hydroxide ions. It is also demonstrated that the self-discharge rate of a symmetric PPy based device can be decreased dramatically by proper balancing of the electrode capacities and by reducing the oxygen concentration. The results of this work are expected to contribute towards future industrial implementation of electric energy storage devices based on organic materials.

A Dissertation submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science. 28 October 2016. / Energy storage has become an important issue for society, there is a need for affordable and efficient devices that can store energy optimally. Supercapacitors are energy storage devices that can solve society’s energy storage problem. They can store the energy generated by renewable energy systems. In this work approaches will be studied that may be used to estimate capacitance of materials that can be used as the electrode of these devices. These materials must have high energy density, which will address one of the limitations of supercapacitors. To estimate the capacitance of the double layer, the double layer theory and ab initio numerical tools based on density functional theory (DFT) are used. The ab initio tools work with periodic systems, when charging the system one violates the periodicity of the system. This is overcome by using the effective screening medium method, which prevents energy divergent of the system. In this work different configurations of the water molecules are used to average the different orientations of water molecules in the electrolyte. The Pt(111) electrode is used, and electrolyte of sodium ion and water. In different configurations the sodium ion in the electrolyte is located at different positions. The capacitances calculated using two different approaches that we developed in this work are comparable to previously estimated capacitance. This is achieved by using minimal computational efforts. We obtained capacitance within that range. Double layer capacitance can be estimated to a good accuracy with the methods developed in this work. Though there are improvements that can be made on the methods that have been developed in this work to better estimate the double layer capacitance. And also more research has to be done in this field to come up with a theory that will accurately estimate capacitance. At the moment calculating the double layer capacitance is not trivial due to the lack of theory that describe the processes taking place at the surface of the electrode where the capacitance is calculated. / LG2017

This thesis explores some new applications in power electronics for supercapacitors. This involves the design and development of dc-dc converters to interface the supercapacitor banks with the rest of the power electronic system. Two applications for supercapacitors are proposed and analyzed. The first application is aimed at high power applications such as motor drives. The proposed approach compensates the effect of voltage sags in the dc link of typical adjustable speed drives, thus reducing speed fluctuations in the motor and eliminating the possibility of nuisance tripping on the drive control board. The second approach presented in this thesis explores the use of supercapacitors to extend run-time for mobile devices such as laptop computers and hand held devices. Three possible approaches are explored: a) Supercapacitors connected directly across the battery; b) Battery-inductor-supercapacitor connection; and c) Supercapacitor, and battery connected via a DC-DC converter. Analytical models, simulation and experimental results on a typical laptop computer are presented.

Almost every form of alternative energy and energy system being implemented today, e.g., wind, solar, hybrid electric and hydrogen fuel cell vehicles, depends on electrical energy storage (EES). At the DOE basic energy sciences workshop on Basic Research Needs for Electrical Energy Storage held April, 2007, the conclusion was reached that "revolutionary breakthroughs in EES are perhaps the most crucial need for this nation’s secure energy future." The workshop, chaired by John Goodenough, University of Texas-Austin, focused on the two primary methods of EES - batteries and electrochemical double-layer capacitors (also referred to as ‘ultracapacitors’ and ‘supercapacitors’). As stated in the report from this DOE workshop, “The performance of current EES technologies falls well short of requirements for using electrical energy efficiently in transportation, commercial, and residential applications.” In this dissertation, increasing the energy storage capacity of ultracapacitors through the use of graphene electrode materials is investigated. Chapter 1 is a basic overview of EES applications and ultracapacitor technology. In Chapter 2, best practice experimental procedures to accurately evaluate a material’s performance are described. Because current measurement methods for determining a material’s performance for use as an ultracapacitor electrode are not well standardized, the different techniques currently being employed lead to wide variations in reported results. Reliable methods that would accurately test a large number of samples involving minute quantities of material were required. In Chapter 3, the performance of graphene-derived materials is investigated. Chemically modified graphene materials gave values competitive with current activated carbons and an ultracapacitor based on activated graphene electrodes yielded the highest specific capacitance values reported to date. Chapter 4 describes a lithium ion hybrid supercapacitor using this novel material that gave energy densities greater than lead acid batteries. The exceptional performance of these graphene derived materials will likely result in their rapid adoption as well as an expanded range of applications utilizing ultracapacitors. As increasingly higher surface area graphene materials are developed, a fundamental understanding of the components that affect interfacial capacitance is critical for further capacity increases. In the last chapter, the first direct measurement of the interfacial capacitance for one and two sides of single layer graphene is presented. The results show that the quantum capacitance increasingly becomes a factor with the result being a reduced increase in capacitance, not the linear increase with surface area as would be expected for bulk conductive materials. These results indicate that the development of higher surface area graphene materials alone is not sufficient for additional increases in performance; the modification of the electronic properties will also be required. / text

This thesis explores some new applications in power electronics for supercapacitors. This involves the design and development of dc-dc converters to interface the supercapacitor banks with the rest of the power electronic system. Two applications for supercapacitors are proposed and analyzed. The first application is aimed at high power applications such as motor drives. The proposed approach compensates the effect of voltage sags in the dc link of typical adjustable speed drives, thus reducing speed fluctuations in the motor and eliminating the possibility of nuisance tripping on the drive control board. The second approach presented in this thesis explores the use of supercapacitors to extend run-time for mobile devices such as laptop computers and hand held devices. Three possible approaches are explored: a) Supercapacitors connected directly across the battery; b) Battery-inductor-supercapacitor connection; and c) Supercapacitor, and battery connected via a DC-DC converter. Analytical models, simulation and experimental results on a typical laptop computer are presented.

With the world’s increasing energy demand and the depletion of fossil fuels, there is a growing demand for the development of alternative and clean energy sources. Batteries and fuel cell technologies have been cited as next generation technologies to provide sustainable energy; however, these technologies are insufficient in supplying high power in short time periods that can be produced by oil as an energy source. In contrast, electrochemical capacitors possess fast charging/discharging capabilities with high power output. As a result, the use of electrochemical capacitors in commercial applications has generated strong interest. Examples of commercial applications include emergency back-up power, consumer electronics, and hybrid vehicles. Commercially available electrochemical capacitors are based on carbonaceous materials with high surface area, excellent electrical conductivity, and wettability which statically store the charges in pores. In contrast, pseudocapacitive materials, namely transition metals, utilize fast reversible faradaic reactions on the surface of the materials which allow for greater energy storage than carbonaceous materials. Currently, many research activities are being focused on pseudocapacitive materials in an effort to enhance their energy storage capabilities. This thesis presents research on a pseudocapacitive material: nickel hydroxide/oxide hybrid. Also, it identifies the hybrid material’s lack of conductivity which can negatively impact its capacitive performance. An addition of carbon supports is recommended to enhance the conductivity. There are two parts to this study. The first study addresses the synthesis of the nickel hybrid structures through solvothermal process and calcination. The materials are thoroughly analyzed through physical and electrochemical characterizations. The issue of using the hybrid material as pseudocapacitor electrodes are identified at this stage. The second part of the study addresses the effect of different carbon additives in the nickel hybrid material. Commonly known carbon additives are incorporated into the nickel hybrid material and analyzed through electrochemical characterization to distinguish the best carbon support for the nickel hydroxide/oxide.

Sustainable, environmentally friendly, and renewable energy sources are urgently needed as concerns about carbon emissions and the depletion of fossil fuels are becoming worrying. It is vital to explore cost-effective and environmentally sustainable energy sources to ensure adequate provision for the ever-increasing energy demand. Supercapacitor devices enable storage of energy and its delivery at high power over a short period. These devices have the advantage of being manufactured at low cost, being safe to use, and having a long-life cycle. This study investigated the effect of incorporating a carbon-based material (graphene foam) within a ternary transition-metals hydroxide (Nickel, Cobalt, and Manganese) to obtain its optimal electrochemical properties for supercapacitors applications. It involved a low-cost and environmentally sustainable synthesis method whereby a constant quantity of the ternary metal hydroxides (NiCoMnTH) was loaded onto various amounts of graphene foam (GF). Typical energy storage characterisation techniques were performed on the synthesised material. The physical characterisation provided results regarding the structural, morphological and surface particularities of the different nanostructured materials. The electrochemical characterisation (EC) allowed the evaluation of the materials' electrochemical behaviours and performances. The EC results also revealed the optimised composite, which demonstrated outstanding electrochemical performances. The integration of graphene foam within the pristine material enhanced its surface area improving its specific capacity to about 178,6 mAh g-1. This specific capacity was close to the triple of the initial value having a specific capacity value equivalent to 76,2 mAh g-1 when evaluated in the same configuration and under the same settings. The improved nanomaterial was then utilised as a positive electrode material for the design of a novel hybrid device. The hybrid device was assembled with the optimised material (NiCoMnTH/GF) on the positive end and activated carbon on the negative end. The device demonstrated a sustaining specific capacity of 23,4 mAh g-1at a specific current of 0,5 A g-1. The device also yielded sustaining specific energy and power densities of values of 22,32 Wh kg-1 and 439,7 W kg-1 respectively at the same specific current. The battery-supercapacitor materials combination developed a synergetic effect on the electrochemical properties, thereby enhancing the specific energy and power densities. After a 15000 cycles stability test, the device displayed an outstanding Coulombic efficiency of 99,9 % and capacity retention of 80 % within a potential range of 1,6 V at a specific current of 3 A g−1. These results have demonstrated the prodigious electrochemical potentials of the as-prepared novel nanomaterial and its capability to be utilised as a positive electrode for energy storage applications. / Dissertation (MSc (Chemical Technology))--University of Pretoria, 2020. / National Research Foundation / Chemical Engineering / MSc (Chemical Technology) / Unrestricted

In a world that relies heavily on electricity and portable energy, the development of high performing energy storage devices is crucial. The ongoing push for energy storage devices such as batteries and supercapacitors to store more energy and charge/discharge faster has become exponentially stronger over the past decade. In order to meet the high demands, new materials and processing techniques must be developed. A particle extraction through the liquid-liquid interface (PELLI) technique was used with a versatile extracting molecule, Octyl Gallate (OG). It was found that OG was able to extract a variety of materials including oxides, oxyhydroxides, and pure silver. The advantage of PELLI is that it circumvents the drying stage that occurs in many electrode synthesis techniques where metal oxides are synthesized in aqueous then dried and mixed with conductive additives dispersed in organic solvent. This drying stage causes a practically irreversible agglomeration which hinders mixing with conductive additives as well as reduces the surface area of the material, limiting its electrochemical performance. Using hydroxamates such as octanohydroxamic acid and bufexamac, a novel PELLI technique was developed based on the use of OHA as an extracting agent as well as a capping agent. In addition, a preliminary investigation was started on advanced negative electrode material for supercapacitors. FeOOH-based electrodes exhibit high capacitance but low cyclic stability. Zn2+ ions were introduced during synthesis forming a doped Zn/FeOOH electrode which showed a significant increase in cyclic stability. / Thesis / Master of Applied Science (MASc)