The demand for energy storage technologies is rapidly increasing in portable electronics, transportation, and renewable energy systems. Thus, the objective of this research is to develop and enhance the performance of Ni-based electrochemical supercapacitors by optimizing synthesis conditions and design of the electrode materials. Conventional and on-chip supercapacitors were developed with notable performance enhancement. For conventional supercapacitors, a uniform and conformal coating process was developed to deposit Ni(OH)2 nanoflakes on carbon microfibers in-situ by a simple chemical bath deposition at room temperature. The microfibers conformally-coated with Ni(OH)2 make direct physical contacts with essentially every single nanoflakes, leading to more efficient electron transport. Using this strategy, we have achieved devices that exhibit five times higher specific capacitance compared to planar (non-conformal) Ni(OH)2 nanoflakes electrodes prepared by drop casting of Ni(OH)2 on the carbon microfibers (1416 F/g vs. 275 F/g). For on-chip storage applications, microfabricated supercapacitors were developed using a combination of top-down photolithography and bottom-up CBD. The resulting Ni(OH)2 micro-supercapacitors show high-rate redox activity up to 500 V/s and an areal cell capacitance of 16 mF/cm2 corresponding to a volumetric stack capacitance of 325 F/cm3. This volumetric capacitance is 2-fold higher than carbon and metal oxide based micro-supercapacitors. Furthermore, these micro-supercapacitors show a maximum energy density of 21 mWh/cm3, which is superior to the Li-based thin film batteries. To enhance cycling stability, Ni-Cu-OH and Ni-Co-OH ternary electrodes have been prepared with different Ni:Cu and Ni:Co ratios by CBD at room temperature on carbon microfibers. It is observed that the electrodes with Ni:Cu and Ni:Co composition ratio of 100:10 results in an optimum capacitance and cycling stability. For the optimum composition, Ni-Co-OH with graphene and carbon nanofibers electrode was tested, with resultant improvement in electrode potential window, equivalent series resistance, and cyclic stability. To further increase energy density, Ni(OH)2//Graphene asymmetric supercapacitor were fabricated with areal capacitance of 253 mF/cm2 at 5 mA/cm2 which is higher than NiO//rGO prepared by hydrothermal method. Ni-Co-OH/G-CNF//Graphene asymmetric supercapacitor results in a maximum power of 23 mW within an operating voltage of 2.2 V which are higher than of Ni(OH)2//Graphene (15.94 mW within 1.8 V). Our asymmetric supercapacitors have flexible-electrodes, low-cost fabrication process and environmentally friendly materials.

Thesis (M.Sc. (Chemistry)) -- University of Limpopo, 2021 / Recently, the use of electrochemical supercapacitors as energy storage devices has drawn great attention due to their high charge/discharge rate, long life span, high power and energy densities. However, the choice of electrode materials used is vital for the performance of supercapacitors. This study focused on the development of a low cost hybrid electrode based on reduced graphene oxide/metal organic framework composite (rGO/MOF) and a novel palladium (Pd) nanoparticles loaded on rGO/MOF termed Pd-rGO/MOF nanocomposite. The prepared nanocomposites were used for high performance electrochemical double layer capacitor-(EDLC) and battery-type supercapacitors known as supercabattery. The rGO material reported in this work was chemically derived through the oxidation reduction method using a hydrazine as a reducing agent. Furthermore, palladium nanoparticles were loaded on the rGO using the electroless plating method. The rGO/MOF and novel Pd-rGO/MOF nanocomposites were prepared using an impregnation method in dimethylformamide. The physical and morphological properties of the synthesised materials were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), energy dispersive X-ray spectroscopy (EDX), differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). The XRD and FTIR analyses showed crystalline phases and vibrational bands for both parent materials, respectively. The TGA/DSC results showed enhancement of the thermal stability of the composite as compared to MOF material. The SEM/EDS and TEM/EDX confirmed the presence of octahedral structure of MOF in the rGO sheet like structure and elemental composition of the synthesised composite. The resultant of Pd-rGO/MOF nanocomposite showed a morphology in which a thin layer of rGO coating existed over MOF with unique bright spots indicating the presence of Pd nanoparticles. This observation agreed well with the structural properties revealed by both XRD and FTIR with the reduction of MOF intensities upon Pd-rGO loading as well as enhancement of thermal stability of the nanocomposites. The electrochemical properties of the prepared electrodes were determined using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). To evaluate the electrochemical performance of the prepared electrode materials, both two and three electrode cells were assembled. From the CV and GCD results, the nanocomposites demonstrated a battery-type behaviour and therefore asymmetric supercabattery cells were assembled using the composites as positive electrodes, and activated carbon as a negative electrode. The specific capacity of rGO/MOF in three electrode cell was found to be 459.0 C/g at a current density of 1.5 A/g in 3M potassium hydroxide. Furthermore, the asymmetric supercapacitor based on the rGO/MOF nanocomposite and activated carbon (AC) as a negative electrode exhibited a maximum energy density of 11.0 Wh/kg and the maximum power density of 640.45 W/kg. The loading of palladium nanoparticles on the nanocomposite was to improve the electrochemical active sites and the performance of the supercapacitor electrode. After incorporation of Pd nanoparticles, the specific capacitance in three electrode cell improved to 712 C/g at a higher current density of 2 A/g with the same electrolyte. The assembled supercabattery has shown improved maximum energy and energy density of 26.44 Wh/kg and 1599.99 W/kg, respectively. Based on these findings, the synthesised rGO/MOF and Pd-rGO/MOF nanocomposites are promising electrode materials for future supercabattery applications. / NRF (National Research Foundation) and SASOL foundation

Supercapacitor electrodes were fabricated at a high active mass loading and exhibited enhanced electrochemical capacitance. A conceptually new salting-out extraction processing technique for the synthesis of dispersed Mn3O4-carbon nanotube (CNT) nanocomposites was developed, alleviating the need for hydrophobic solvents. The choice of isopropyl alcohol and NaCl for the extraction process offer advantages of an easy upscaling of this process. The salting out technique was shown to work with Octanohydroxyamic acid (OHA) and Lauryl Gallate (LG) as extractors and dispersants, critical to the success of the extraction. Mechanisms for surface adsorption on Mn3O4 and CNT for both OHA and LG are discussed. A secondary project was also undertaken, to investigate the use of chlorogenic acid and 3,4,5 – trihydroxybenzamide, as co-dispersing agents for MnO2 and CNTs. These molecules are used due to their unique structural properties, which are discussed. The electrodes fabricated using these co-dispersants showed significant increases in their specific capacitances and SEM imaging indicated improved mixing, compared to samples prepared without dispersants. A specific capacitance of 6.5 F g-1 was achieve at low electrical resistance, attributed to the microstructure of electrodes prepared with the co-dispersant molecules. / Thesis / Master of Applied Science (MASc) / The ever-growing realization that our energy consumption as a civilization is not sustainable, has fueled people around the globe to imagine and design new methods of energy storage, in attempts to mitigate this issue. From the foundational works of scientists, it has become clear to see that there is not “one right answer”. Instead, the unique benefits and drawbacks of energy storage technologies should be balanced and applied in situations where their properties permit a high efficacy. The intention of this work is to assist in the development of new materials to be used for energy storage devices called electrochemical supercapacitors. Novel colloidal processing techniques were developed, leading to the fabrication of high-performance electrodes, and providing further insight to the structure-properties relationship of organic extractors and co-dispersing agents for the design of nanocomposites.

<p> Electrochemical supercapacitors (ES) are urgently needed as components in many advanced power systems. The development of advanced ES is expected to enable radical innovation in the area of hybrid vehicles and electronic devices. Nanostructured manganese oxides in amorphous or various crystalline forms have been found to be promising electrode materials for ES. The use of composite electrodes of manganese oxide with carbon nanotubes is being proposed to improve the overall electrochemical performance of the ES.</p> <p> Electrodeposition methods have been developed for the fabrication of manganese oxide films with/without carbon nanotubes for applications in ES. Electrolytic deposition of manganese oxides was found to be possible using Mn2+ and Mn7+ species, co-deposition of multi wall carbon nanotubes (MWNT) and manganese oxide using cathodic electrosynthesis was successfully achieved.</p> <p> Novel chemical process has been developed for the synthesis of nano-size manganese oxide particles. Electrophoretic deposition of the nano-size manganese oxide particles was able to be performed in both aqueous and non-aqueous solutions. Electrophoretic co-deposition of the nano-size manganese oxide particles with carbon nanotubes was successfully achieved.</p> <p> The mechanisms and kinetics of all the deposition methods are discussed. Charge storage properties of the films prepared by different deposition methods are investigated and compared.</p> / Thesis / Master of Applied Science (MASc)

Conducting polymer hydrogels (CPHs) is a class of unique materials that synergize the advantages of conducting polymers (CPs) and polymer hydrogels together. It has been employed in many high-performance electrochemical devices for years, such as energy storage and biosensors. However, large limitations of applying CPHs into the abovementioned areas have been facing the researcher for a long time, mainly due to the difficulties from complicated materials synthesis and untenable nanostructures for potential applications. The drawbacks of previously reported CPHs have put numerous disadvantages onto their applications, partially because they have, for example, high prices, untunable microscale or nanoscale architectures, environmentally hazardous properties, and unscalable and time-consuming synthesis processes. In this thesis, we proposed a novel route for carrying out CPHs by one-step organics synthesis at ambient conditions. The CPHs have hierarchically porous nanostructures crosslinked in a three-dimensional (3D) way, which enable its stable mechanical, unique chemical and physical properties, and outstanding electrochemical properties for potential applicability in long-term energy storage devices and highly sensitive biosensors. With highly controllable nanostructures of the CPHs, our novel concept and material system could possibly be utilized in a broad range of electrochemical applications, including but not limited to lithium-ion batteries (LIBs) electrodes, electrochemical capacitors (ECs), biofuel cells, medical electrodes, printable electronic devices, and biosensors. / text

Le présent travail a porté sur l’élaboration d’électrodes à base de polymère conducteur et d’un tapis de nanotubes de carbone alignés pour les applications de supercondensateur.Dans la première partie de ce travail, nous avons porté notre attention sur le choix de nos matériaux et nous avons pu déterminer leur comportement électrochimique en milieu liquide ionique. Les deux polymères choisis : le poly(3-méthylthiophène) et le poly(3,4-diméthylthiophène) possèdent des propriétés très différentes en terme de capacitance, de potentiel d’oxydation, de résistance ou de cyclabilité. Afin d’allier ces propriétés entre elles, nous avons réalisé la synthèse de copolymères obtenus par voie électrochimique en milieu liquide ionique aprotique. Cette électro-co-polymérisation nous a permis d’obtenir une large gamme de matériaux aux propriétés variables selon la composition du polymère.Dans la deuxième partie de ce travail, nous avons optimisé la synthèse électrochimique d’un nanocomposite poly(3-méthylthiophène)/nanotubes de carbone alignés en milieu liquide ionique. Les matériaux obtenus offrent la propriété d’être « auto-supportés » et permettent ainsi de s’affranchir de collecteur de courant. La nanostructuration grâce aux nanotubes et l’absence de collecteur de courant nous ont permis d’améliorer d’un facteur 3 la capacitance spécifique de nos électrodes. Une optimisation des conditions d’élaboration des nanocomposites permettent d’atteindre une capacitance de 180F.g-1 dans EMITFSI à 30°C. / The present work concerned the elaboration of electrodes containing electronic conducting polymer and a carpet of aligned carbon nanotubes for supercapacitor applications. In the first part of this work, we put our attention on the choice of our materials and we were able to determine their electrochemical behavior in ionic liquid. Both chosen polymers : poly(3-méthylthiopène) and poly(3.4-diméthylthiophène) possess very diffrent properties in term of capacitance, oxidation potential, resistance or cyclability. In the second part of this work, we optimized the electrochemical synthesis of a nanocomposite poly(3-méthyltiophène)/aligned carbon nanotubes in ionic liquid. The obtained materials offer the property to be "self-supported" and allow using themselves without current collector. And optimization of the conditions of nanocomposites elaboration allows reaching a capacitance of 180 F.g-1 in EMITFSI in 30°C.

A library of graphene-based hybrid materials was synthesized as novel hybrid electrochemical electrodes for electrochemical energy conversion and storage devices and electrocatalytical sensing namely enzymeless glucose sensing. The materials used were supercapacitive graphene-family nanomaterials (multilayer graphene-MLG; graphene oxide-GO, chemically reduced GO-rGO and electrochemical reduced GOErGO) and pseudocapacitive nanostructured transition metal oxides including cobalt oxide polymorphs (CoO and Co3O4) and cobalt nanoparticles (CoNP). These were combined through physisorption, electrodeposition, and hydrothermal syntheses approaches. This project was carried out to enhance electrochemical performance and to develop electrocatalytic platforms by tailoring structural properties and desired interfaces. Particularly, electrodeposition and hydrothermal synthesis facilitate chemically-bridged (covalently- and electrostatically- anchored) interfaces and molecular anchoring of the constituents with tunable properties, allowing faster ion transport and increased accessible surface area for ion adsorption. The surface morphology, structure, crystallinity, and lattice vibrations of the hybrid materials were assessed using electron microscopy (scanning and transmission) combined with energy dispersive spectroscopy and selected-area electron diffraction, X-ray diffraction, and micro-Raman Spectroscopy. The electrochemical properties of these electrodes were evaluated in terms of supercapacitor cathodes and enzymeless glucose sensing platforms in various operating modes. They include cyclic voltammetry (CV), ac electrochemical impedance spectroscopy, charging-discharging, and scanning electrochemical microscopy (SECM). These hybrid samples showed heterogeneous transport behavior determining diffusion coefficient (4⨯10-8 – 6⨯10-6 m2/s) following an increasing order of CoO/MLG < Co3O4/MLG < Co3O4/rGOHT < CoO/ErGO < CoNP/MLG and delivering the maximum specific capacitance 450 F/g for CoO/ErGO and Co3O4/ rGOHT. In agreement with CV properties, these electrodes showed the highest values of low-frequency capacitance and lowest charge-discharge response (0.38 s – 4 s), which were determined from impedance spectroscopy. Additionally, through circuit simulation of experimental impedance data, RC circuit elements were derived. SECM served to investigate electrode/electrolyte interfaces occurring at the solid/liquid interface operating in feedback probe approach and imaging modes while monitoring and mapping the redox probe (re)activity behavior. As expected, the hybrids showed an improved electroactivity as compared to the cobalt oxides by themselves, highlighting the importance of the graphene support. These improvements are facilitated through molecular/chemical bridges obtained by electrodeposition as compared with the physical deposition.

Le stockage par supercondensateurs de l'énergie photovoltaïque est modélisé afin de disposer d'un modèle suffisamment fin et accessible permettant de les intégrer dans des chaînes de conversion de l'énergie solaire. Les supercondensateurs sont modélisés par un circuit multibranche comportant des résistances et capacités variables suivant la tension, dont les valeurs sont déterminées par une expérience de caractérisation ayant une bonne précision. Par ailleurs, tous les éléments d'une chaîne typique de conversion de l'énergie photovoltaïque sont modélisés avec le logiciel Matlab/Simulink (gisement solaire, conversion photovoltaïque des panneaux, régulateur, batterie et charges). Le modèle de stockage de l'énergie photovoltaïque par supercondensateurs est ensuite validé par la bonne concordance des mesures relevées en conditions réelles avec les résultats donnés par les simulations. Enfin, deux exemples d'exploitation du modèle sont proposés et discutés : la détermination du temps de charge des supercondensateurs suivant l'éclairement solaire et la température ambiante, et l'intégration des supercondensateurs dans la chaîne d'alimentation électrique d'un moteur à courant continu permettant de réduire les sollicitations électriques sur la batterie lors de l'appel de puissance nécessaire au démarrage du moteur / The storage by ultracapacitors of photovoltaic energy is modeled in order to have an accurate and accessible model to integrate ultracapacitors into solar energy conversion systems. Ultracapacitors are modeled by a multibranch circuit representation composed of resistors and capacitors with variable voltage whose values are determined by an accurate characterization experiment. Moreover, all the elements of a typical photovoltaic energy conversion system are modeled by using the Matlab/Simulink software (solar radiation, photovoltaic panels, batteries and charges). The energy storage model by ultracapacitors is then validated by the good agreement of measured values taken in real conditions with the results provided by simulations. Finally, two examples are proposed and discussed: the determination of the storage duration of ultracapacitors versus solar irradiance and ambient temperature, and the integration of ultracapacitors in the electrical feeding system of a DC motor to reduce the electrical current peak of the battery at the start of the motor

<p>Graphene, first isolated in 2004 and the subject of the 2010 Nobel Prize in physics, has generated a tremendous amount of research interest in recent years due to its incredible mechanical and electrical properties. However, difficulties in large-scale production and low as-prepared surface area have hindered commercial applications. In this dissertation, a new material is described incorporating the superior electrical properties of graphene edge planes into the high surface area framework of carbon nanotube forests using a scalable and reproducible technology.</p><p>The objectives of this research were to investigate the growth parameters and mechanisms of a graphene-carbon nanotube hybrid nanomaterial termed “graphenated carbon nanotubes” (g-CNTs), examine the applicability of g-CNT materials for applications in electrochemical capacitors (supercapacitors) and cold-cathode field emission sources, and determine materials characteristics responsible for the superior performance of g-CNTs in these applications. The growth kinetics of multi-walled carbon nanotubes (MWNTs), grown by plasma-enhanced chemical vapor deposition (PECVD), was studied in order to understand the fundamental mechanisms governing the PECVD reaction process. Activation energies and diffusivities were determined for key reaction steps and a growth model was developed in response to these findings. Differences in the reaction kinetics between CNTs grown on single-crystal silicon and polysilicon were studied to aid in the incorporation of CNTs into microelectromechanical systems (MEMS) devices. To understand processing-property relationships for g-CNT materials, a Design of Experiments (DOE) analysis was performed for the purpose of determining the importance of various input parameters on the growth of g-CNTs, finding that varying temperature alone allows the resultant material to transition from CNTs to g-CNTs and finally carbon nanosheets (CNSs): vertically oriented sheets of few-layered graphene. In addition, a phenomenological model was developed for g-CNTs. By studying variations of graphene-CNT hybrid nanomaterials by Raman spectroscopy, a linear trend was discovered between their mean crystallite size and electrochemical capacitance. Finally, a new method for the calculation of nanomaterial surface area, more accurate than the standard BET technique, was created based on atomic layer deposition (ALD) of titanium oxide (TiO2).</p> / Dissertation

Ce travail contribue à l’optimisation d’un système de stockage hybride couplant une batterie lithium-ion et des supercondensateurs pour les véhicules électriques. La complémentarité entre ces deux sources d’énergie permet l’amélioration des performances globales du système. Notre étude porte sur la mise en oeuvre de techniques avancées de contrôle et de gestion de l’énergie. Notre objectif est d’avoir une meilleure utilisation du système de stockage. Dans ce cadre, notre démarche est de développer une gestion d’énergie en temps réel qui tient compte des contraintes électriques et thermiques des systèmes de stockage. Une étude comparative sur les avantages et les inconvénients de différentes techniques de gestion d’énergie nous a permis d’effectuer le choix entre un partage de puissance à moindre coût et un partage performant de l’énergie entre les systèmes de stockage. Un banc expérimental a été mis en oeuvre afin de concrétiser la démarche théorique / This work contributes to the optimization of a hybrid storage system that combines lithium-ion batteries with supercapacitors used for electric vehicles. This hybridization structure was chosen due to the complementarity between both used storage devices. Our study focuses on the implementation of advanced energy control and management techniques. Using better the storage system represents the goal of this thesis. Our approach is to develop a real time algorithm of energy management taking into account battery electrical and thermal behaviors. A comparative study evaluates the benefits and the drawbacks of each proposed strategy in order to offer various choices between low cost power sharing solutions and control strategy with high performances. An experimental bench was implemented to apply the theoretical concept