The objective of this thesis was to investigate the suitability of some different Li4Ti5O12 materials as a negative electrode in hybrid supercapacitors. A hybrid supercapacitor is a combination of a battery and an electric double-layer capacitor that uses both a battery material and a capacitor material in the same device. The target for these combination devices is to bridge the performance gap between batteries and capacitors and enable both high energy and power density. To achieve this, materials with high capacity as well as high rate capability are needed. To improve the rate of the commonly slow battery materials nanosizing has been found to be an effective solution. This study shows that Li4Ti5O12 has a significantly higher experimental capacity than the most common capacitor material, activated carbon. The capacity remained high even at high discharge rates due to a successful nanostructuring that increased the accessibility of the material and shortened the diffusion distance for the ions, leading to a much improved power performance compared with the bulk material. The use of a nanostructured Li4Ti5O12 material in a hybrid device together with activated carbon was estimated to double the energy density compared to an electric double-layer capacitor and maintain the same good power performance. To further increase the energy density also improved materials for the positive electrode should be investigated.

Electrochemical double layer capacitors (EDLCs, also called supercapacitors or ultracapacitors) are high power density energy storage devices that operate through the separation of charge at the electrochemical interface between an electrode and a supporting electrolyte. Numerous types of carbon materials with high surface area and internal porosity, such as activated carbon, carbon fabrics, nanotubes, and reduced graphene oxide have been studied as electrode materials. Electrolytes such as aqueous alkaline and acid solutions usually give high capacitance, while organic and ionic liquids provide a wider operation voltage. Graphene, due to its high theoretical surface area of 2630 m2/g, good electrical conductivity, and relatively low density, is being studied as an electrode material in EDLCs. The objective of this dissertation is thus to study effective methods for synthesis of graphene-based materials, and to investigate their behavior in EDLCs. This work explored microwave assisted synthesis of graphite oxide (‘MEGO’, prepared in less than one minute by irradiation of graphite oxide by microwave). This material was further chemically activated to obtain a unique carbon material, activated microwave exfoliated graphite oxide (‘a-MEGO’) with specific surface areas up to 3100 m2/g. Gas adsorption measurements were used to study the specific surface area and porosity of a set of a-MEGO samples, which were also studied by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) for their structure, and by combustion analysis (i.e., elemental analysis) and X-ray photoelectron spectroscopy (XPS) to understand their elemental composition. Cyclic voltammetry (CV), galvanostatic charge/discharge, and frequency response, tests were done in order to study the performance of these new carbon materials as electrodes in both aqueous and organic electrolytes in a two electrode cell set up. / text

This work focused on the study of self-discharge mechanisms of carbon electrochemical capacitor electrodes in 1.0 M H2SO4 electrolyte. Electrochemical capacitors have an increasingly important role in the future of energy storage for specific applications due to their high cycle lives, high power capabilities and the ability to use environmentally friendly materials. Remediation of the occurrence of self-discharge – the loss of charge over time when left in open-circuit configuration – must take place before electrochemical capacitors can be used more widely as this diminished potential results in a reduction of stored energy. By examining the now poorly understood causes and mechanisms of self-discharge, beneficial modifications to the electrochemical capacitors systems can be made, improving device performance. Three-electrode electrochemical set-ups were used to separate self-discharge mechanisms on the negative and positive electrodes. Various electrode and electrolyte reactions were investigated in relation to self-discharge, including Fe-contamination reaction, electrolyte decomposition, oxygen-reduction, carbon oxidation, and carbon surface group development. All experiments were conducted on porous carbon electrodes. It was determined that Fe-contamination increased self-discharge on both carbon electrodes at concentrations >10-3 M, and that previously developed planar kinetic models applied to these porous systems. Electrolyte decomposition did not result in increased self-discharge on either electrode. Electrolyte oxygen content must be minimized as oxygen is believed to undergo reduction to hydrogen peroxide on the negative-electrode, resulting in an increase in self-discharge. The carbon electrodes used in this work must be cycled prior to energy storage as the capacitance varies greatly with continued cycling, and the lack of cycling results in increased self-discharge. Additionally, interest in the carbon electrode’s surface functionalities resulted in the standardization of the Boehm titration.

Energy storage systems such as batteries and supercapacitors store electrical energy in the form of chemical energy and release it when required. Among the various electrode materials, manganese oxides (MnOx) are promising electrode materials for these devices. Despite its outstanding theoretical capacitance, Mn-based oxide electrodes have several limitations that impede their electrochemical performance. Understanding how the charges are efficiently stored in the electrodes or across the electrode/electrolyte interface is crucial for developing advanced electrode material in the field of energy storage applications. The goal of my thesis is to develop and apply synchrotron-based scanning transmission X-ray microscopy (STXM) to investigate changes in the oxidation state of Mn and their spatial distributions in MnOx electrodes in the context of energy storage and release. To achieve high- precision qualitative and quantitative STXM identification and mapping of different MnOx species, calibrated and high-quality reference Mn 2p and O 1s NEXAFS (near edge X-ray absorption fine structure) spectra were measured. In collaboration with Wenjuan Yang and her PhD supervisor, Prof. Igor Zhitomirsky, I performed ex-situ STXM studies on Mn3O4-based supercapacitor electrode materials to investigate the influence of different synthesis methods and activation protocols on the charging behavior and capacitance performance. In collaboration with Pablo Ingino and his supervisor, Prof. Martin Obst (Bayreuth University), and my colleague, Dr. Chunyang Zhang, I helped develop a three-electrode, microfluidic-based flow electrochemical device for in-situ STXM. This device was used to electrodeposit MnO2 on the working electrode (WE) and track the oxidation state and morphological changes by STXM while scanning the potential of the cell in different electrolyte pH. The in-situ STXM studies showed a spontaneous reduction of the initially deposited MnO2 resulting from the local pH change at the WE. Additionally, a significant change from a quasi-uniform MnO2 film to a dendritic MnO2 structure was observed at oxidative potential. This dendritic growth resulted from dissolution/redeposition of MnO2 during charging/discharging processes, indicating a partial reversibility of dissoluble Mn species. The ex-situ and in-situ STXM studies I performed provide mechanistic insights that will help further improve Mn oxides-based electrodes and their applications as energy storage devices. / Thesis / Doctor of Philosophy (PhD)

Disertacijoje nagrinėjama tematika yra susijusi su superkondensatorių formavimu ir jų tyrimais naudojant plazmines technologijas. Superkondensatorių elektrodams formuoti panaudota plazminio purškimo technologija. Anglis ant nerūdijančio plieno paviršiaus užnešta panaudojant atmosferinio slėgio argono-acetileno plazmą. Nikelio oksidui nusodinti ant anglies elektrodų paviršiaus panaudotas magnetroninio garinimo metodas. Įvertinta acetileno kiekio įtaka superkondensatorių elektrinėms charakteristikoms ir elektrodų struktūrai. Tai pat įvertinta nikelio oksido kiekio įtaka superkondensatorių anglies elektrodų elektriniams parametrams, struktūrai bei mikroreljefui. Atliktas anglies elektrodų paviršiaus ėsdinimas deguonies plazma ir įvertintas jo poveikis kondensatorių elektriniams parametrams ir anglies elektrodų struktūrai. Matematiniu modeliavimu teoriškai įvertinta deguonies plazmos įtaka superkondensatorių anglies elektrodų paviršiaus mikroreljefui. Teoriškai pagrįstas anglies dangos reljefo anizotropinis augimas ir izotropinės paviršiaus erozijos mechanizmas. Kokybiškai paaiškintas acetileno kiekio ir plazmotrono galios įtakos mechanizmas anglies elektrodų struktūrai ir elektriniams parametrams. Pasiūlytas mechanizmas, aiškinantis NiO kiekio, esančio ant anglies elektrodų, įtaką superkondensatorių elektriniams parametrams ir elektrodų struktūrai. / The dissertation examines topics related to the formation of supercapacitors using plasma technology and their analysis. Plasma spray technology was used to form supercapacitors electrodes. Carbon was deposited on stainless steel surface using the atmospheric pressure argon-acetylene plasma. The deposition of nickel oxide on the surface of carbon electrodes was made using magnetron sputtering method. The influence of acetylene amount to the supercapacitors electrodes and the electrical characteristics of the structure were estimated. The nickel oxide influence to the electrical parameters of supercapacitor carbon electrodes, structure and microrelief was assessed too. The etching of carbon electrodes surface with oxygen plasma was performed and its impact on the capacitors electrical parameters and carbon electrode structure was evaluated. Mathematical modeling was used to theoretically estimate the influence of oxygen plasma to the supercapacitor carbon electrode surface microrelief. The anisotropic growth of carbon surface relief and isotropic surface erosion mechanism was theoretically explained. The impact mechanism of the acetylene content and plasmatron power to the carbon electrode structure and electrical parameters was qualitatively explained. The mechanism for understanding the influence of NiO content on the carbon electrodes to the supercapacitor electrical parameters and electrode structure was proposed.

The dissertation examines topics related to the formation of supercapacitors using plasma technology and their analysis. Plasma spray technology was used to form supercapacitors electrodes. Carbon was deposited on stainless steel surface using the atmospheric pressure argon-acetylene plasma. The deposition of nickel oxide on the surface of carbon electrodes was made using magnetron sputtering method. The influence of acetylene amount to the supercapacitors electrodes and the electrical characteristics of the structure were estimated. The nickel oxide influence to the electrical parameters of supercapacitor carbon electrodes, structure and microrelief was assessed too. The etching of carbon electrodes surface with oxygen plasma was performed and its impact on the capacitors electrical parameters and carbon electrode structure was evaluated. Mathematical modeling was used to theoretically estimate the influence of oxygen plasma to the supercapacitor carbon electrode surface microrelief. The anisotropic growth of carbon surface relief and isotropic surface erosion mechanism was theoretically explained. The impact mechanism of the acetylene content and plasmatron power to the carbon electrode structure and electrical parameters was qualitatively explained. The mechanism for understanding the influence of NiO content on the carbon electrodes to the supercapacitor electrical parameters and electrode structure was proposed. / Disertacijoje nagrinėjama tematika yra susijusi su superkondensatorių formavimu ir jų tyrimais naudojant plazmines technologijas. Superkondensatorių elektrodams formuoti panaudota plazminio purškimo technologija. Anglis ant nerūdijančio plieno paviršiaus užnešta panaudojant atmosferinio slėgio argono-acetileno plazmą. Nikelio oksidui nusodinti ant anglies elektrodų paviršiaus panaudotas magnetroninio garinimo metodas. Įvertinta acetileno kiekio įtaka superkondensatorių elektrinėms charakteristikoms ir elektrodų struktūrai. Tai pat įvertinta nikelio oksido kiekio įtaka superkondensatorių anglies elektrodų elektriniams parametrams, struktūrai bei mikroreljefui. Atliktas anglies elektrodų paviršiaus ėsdinimas deguonies plazma ir įvertintas jo poveikis kondensatorių elektriniams parametrams ir anglies elektrodų struktūrai. Matematiniu modeliavimu teoriškai įvertinta deguonies plazmos įtaka superkondensatorių anglies elektrodų paviršiaus mikroreljefui. Teoriškai pagrįstas anglies dangos reljefo anizotropinis augimas ir izotropinės paviršiaus erozijos mechanizmas. Kokybiškai paaiškintas acetileno kiekio ir plazmotrono galios įtakos mechanizmas anglies elektrodų struktūrai ir elektriniams parametrams. Pasiūlytas mechanizmas, aiškinantis NiO kiekio, esančio ant anglies elektrodų, įtaką superkondensatorių elektriniams parametrams ir elektrodų struktūrai.

Recent advances in innovative energy storage devices such as supercapacitors have made battery-supercapacitor hybrid energy storage systems technically attractive. However the field of hybrid energy storage system control is relatively new, involving the major challenge of developing control techniques optimised for improved battery-life or other performance metrics. This thesis presents the design and analysis of an actively controlled hybrid energy storage system. Detailed information is given regarding the system implementation and dynamic controls developed as a part of the research. Novel use of the sliding-mode or hysteretic current-controlled DC/DC converter is shown to provide a versatile and robust power electronic building block for the power-control hardware implementation. Current state of the art in the field has converged around a frequency-domain approach to the overall power sharing strategy within hybrid energy storage systems employing batteries and high-power, low-energy density storage such as supercapacitors, with benefits in terms of reduced battery current maxima and an (un-quantified) increase in battery life having been reported. This research extends previous studies by considering the frequency-domain approach in further detail and providing quantitative simulation results confirming how an estimated increase in battery cycle-life of ~18% can be achieved. A systematic simulation framework used for the development and assessment of novel hybrid energy storage system control strategies is described and demonstrated in the context of a remote wind power application. The hardware design of all systems considered is described in detail and demonstrated by experiment.

This thesis proposes a modified structure model for graphite oxide (GO), an important precursor in graphene chemistry, develops a new strategy to convert GO back to graphene-like structure, and demonstrates its possible applications in both water purification and supercapacitor technologies. GO, a nontraditional compound first obtained from graphite oxidation over 150 years ago, is now becoming an important player in the production of graphene-based materials, which has high technological relevance. GO structure and reduction have been vigorously investigated, but its precise chemical structure still remains obscure, and the complete restoration of the sp2 carbon lattice has not yet been achieved. In our work, solid state 13C NMR (MAS) analysis offered a piece of evidence for five or six-membered ring lactol structure existing in GO that had never been assigned before, leading to a modified Lerf-Klinowski model for GO. A three-step reduction strategy, involving sodium borohydride (NaBH4), sulfuric acid, and high temperature thermal annealing, described in the thesis, successfully reduced GO back to chemically converted graphene (CCG) with the lowest heteroatom abundance among all those previously reported. In addition to the chemical significance of graphene/CCG production, GO and its derivatives were used as novel adsorbents in water purification. GO-coated sand showed higher retention than ordinary sand for both Rhodamine B and mercuric ion (Hg2+) contaminants in water. Further functionalization of GO with thiophenol resulted in better adsorption capacity toward Hg2+ than that of activated carbon. In addition, free-standing films of GO were treated and reduced with a CO2 laser beam into different conductive reduced GO (RGO) patterns, and directly used as supercapacitor devices which showed good cyclic stability and energy storage capacities comparable to that of existing thin film ultracapacitors. GO turned out to be a solid electrolyte with anisotropic proton conductivity similar to Nafion, while the large amount of trapped water in GO played an important role.

>Magister Scientiae - MSc / Supercapacitors are one of the important subjects concerning energy storage which has proven to be a challenge in this country. Currently, the electrodes of most commercial supercapacitor are made of carbon which is known to be inexpensive and has high resistance to corrosion. These carbon based supercapacitors operate under EDLC. They offer fast charging/discharging rates and have the ability to sustain millions of cycles without degrading. With their high power densities, they bridge the gap between batteries which offer high energy densities but are slow in charging/discharging and conventional dielectric capacitors which are very fast but having very low energy densities. The objective of this work was to develop a high performance supercapacitor using polyanilino-graphene oxide intercalated with platinum group metal nanocomposites. Specific capacitance of each material was investigated with the objective of ascertaining the material that has the best capacitance. In this work, GO was functionalized with aniline and intercalated with Pt, Pd and Pd-Pt nanocomposites. The nanomaterials were characterized with FTIR, Ultravioletvisible (UV-visible) spectroscopy, high resolution scanning electron microscopy (HRSEM), high resolution transmission electron microscopy (HRTEM), energy dispersive x-ray microanalysis (EDS) and X-ray diffraction (XRD) analysis. The composites were tested for possible application as supercapacitor materials using potentiostatic-galvanostatic constant current charge/discharge. The synthesized materials had good electronic, mechanical, optical, physical etc. properties as proven by the various characterization techniques but they proved not to be ideal for application as supercapacitor materials. The materials tested negative when tested for both anodic and cathodic materials therefore we can conclude that the materials are not good supercapacitor materials and therefore cannot be used in application as novel as supercapacitors.

Nanostructured materials provide new and exciting approaches to the development of supercapacitor electrodes for high-performance electrochemical energy storage applications. One of the biggest challenges in materials science and engineering, however, is to prepare the nanomaterials with desirable characteristics and to engineer the structures in proper ways. This dissertation presents the successful preparation and application of very promising materials in the area of supercapacitor energy storage, including manganese dioxide and its composites, polyaniline and activated carbons. Attention has been paid to understanding their growth process and performance in supercapacitor devices. The morphological and electrochemical cycling effects, which contribute to the understanding of the energy storage mechanism of MnO2 based supercapacitors is thoroughly investigated. In addition, MnO2 based binary (MnO2-carbon nanocoils, MnO2-graphene) and ternary (MnO2-carbon nanotube-graphene) nanocomposites, as well as two novel electrodes (MnO2-carbon nanotube-textile and MnO2-carbon nanotube-sponge) have been studied as supercapacitor electrode materials, showing much improved electrochemical storage performance with good energy and power densities. Furthermore, a general chemical route was introduced to synthesize different conducting polymers and activated carbons by taking the MnO2 nanostructures as reactive templates. The electrochemical behaviors of the polyaniline and activated nanocarbon supercapacitors demonstrate the morphology-dependent enhancement of capacitance. Excellent energy and power densities were obtained from the template-derived polyaniline and activated carbon based supercapacitors, indicating the success of our proposed chemical route toward the preparation of high performance supercapacitor materials. The work discussed in this dissertation conclusively showed the significance of the preparation of desirable nanomaterials and the design of effective nanostructured electrodes for supercapacitor energy storage applications.