In this study it was proposed to investigate the use of 3D metal oxides (specifically titanium oxides) as potential electrode materials for lithium ion batteries. Three different approaches were taken: mesoporous materials to increase the surface area and improve the capacity; nanocrystalline materials to increase the surface area and to investigate any changes that may occur using nanocrystals; and microporous materials that are more open, allowing rapid diffusion of lithium and higher capacities. Of the three categories of materials studies, mesoporous TiO2 was the least promising with low reversible capacities (20 mAh??g-1) due to densification resulting in a loss of surface area. In nanocrystalline rutile an irreversible phase change occurred upon initial intercalation, however after this intercalation occurred reversibly in a single phase mechanism giving capacities of 100 mAh??g-1. A trend in intercalation potential was observed with crystallite size that was related to the ability of the structure to relax and accept lithium. Doping of rutile yielded no real improvement. Brookite gave only low capacities from a single phase intercalation mechanism. TiO2 films produced by a novel electrochemical technique showed that while amorphous films give greater capacities, more crystalline (anatase) films give greater reversibility. Overall, microporous titanosilicates showed the most promise with sitinakite giving a reversible capacity of 80 mAh??g-1 after twenty cycles or double this when dried. The intercalation was found to occur by two steps that generate large changes in crystallite size explaining the capacity fade witnessed. While doping did not improve the performance, cation exchange has proven beneficial. The remaining titanosilicates did not perform as well as sitinakite, however a trend was observed in the intercalation potentials with the wavenumber of the Ti-O Raman stretch. This was due to the covalent nature of the bonding. Upon reduction an electron is added to the bond meaning the energy of the bond determines intercalation potential. Overall, most promise was shown by the microporous titanosilicates. The capacities of sitinakite after drying, are comparable to those of the "zero strain" material Li4Ti5O12. Investigation of the titanosilicates and their ion-exchanged derivatives is a promising path for new lithium-ion battery electrode materials.

In this study it was proposed to investigate the use of 3D metal oxides (specifically titanium oxides) as potential electrode materials for lithium ion batteries. Three different approaches were taken: mesoporous materials to increase the surface area and improve the capacity; nanocrystalline materials to increase the surface area and to investigate any changes that may occur using nanocrystals; and microporous materials that are more open, allowing rapid diffusion of lithium and higher capacities. Of the three categories of materials studies, mesoporous TiO2 was the least promising with low reversible capacities (20 mAh??g-1) due to densification resulting in a loss of surface area. In nanocrystalline rutile an irreversible phase change occurred upon initial intercalation, however after this intercalation occurred reversibly in a single phase mechanism giving capacities of 100 mAh??g-1. A trend in intercalation potential was observed with crystallite size that was related to the ability of the structure to relax and accept lithium. Doping of rutile yielded no real improvement. Brookite gave only low capacities from a single phase intercalation mechanism. TiO2 films produced by a novel electrochemical technique showed that while amorphous films give greater capacities, more crystalline (anatase) films give greater reversibility. Overall, microporous titanosilicates showed the most promise with sitinakite giving a reversible capacity of 80 mAh??g-1 after twenty cycles or double this when dried. The intercalation was found to occur by two steps that generate large changes in crystallite size explaining the capacity fade witnessed. While doping did not improve the performance, cation exchange has proven beneficial. The remaining titanosilicates did not perform as well as sitinakite, however a trend was observed in the intercalation potentials with the wavenumber of the Ti-O Raman stretch. This was due to the covalent nature of the bonding. Upon reduction an electron is added to the bond meaning the energy of the bond determines intercalation potential. Overall, most promise was shown by the microporous titanosilicates. The capacities of sitinakite after drying, are comparable to those of the "zero strain" material Li4Ti5O12. Investigation of the titanosilicates and their ion-exchanged derivatives is a promising path for new lithium-ion battery electrode materials.

Photoelectrochemical cells (PEC) and microbial fuel cells (MFC) are two promising environmental technologies with the purposes of energy production and pollutant degradation. In this study, p-type Cu2O thin film electrodes were synthesized by electrodeposition on the ITO glass. The influences of various electrodeposition conditions, including the deposition potential, temperature, electrolyte pH, substrates and deposition duration on the morphology and the photoelectrochemical properties of the Cu2O films were investigated. The so-called p-type micro-crystal Cu2O thin film photocathodes were synthesized at -0.4 V, 70 °C and pH 10. An innovative composite Cu2O/TiO2 photoelectrode was developed by dip-coating TiO2 on the surface of the Cu2O film. The outer TiO2 layer would help reduce the electron-hole recombination and hence improve the catalyst stability. The photocatalyst was shown to be capable of photocatalytic degradation of model pollutants. Under simulated solar irradiation, methylene blue, acridine orange, and bromocresso brilliant blue G were effectively degraded in the Cu2O-based PEC. The composite Cu2O/TiO2 photoelectrode could further enhance the photodegradation of the dyes. For the study on MFC with the saline wastewater-inoculated MFCs, an electricity output of 581 mW/m2 could be achieved at a NaCl concentration of 200 mM. Based on the characterization of the bioande using the electrochemical impedance spectroscopy (EIS) technique, the R(QR)(QR) model, instead of the conventional R(QR) model, was found to fit well with the EIS data of the carbon cloth bioanode. The results support the two-interface-based physical model for the description of the bioanode, including an interface on the flat electrode and the other for the porous biofilm matrix. The new model was employed to monitor the biofilm formation and development on the carbon clothe anode during the MFC start-up. In addition, photocatalytic MFC was developed by using the Cu2O film as the photocathode for the MFC. With the simulated solar light illumination, the PMFC open circuit voltage could be increased by 200 mV comparing to the MFC test. Moreover, the cathode material (Cu2O) is much less expensive than Pt used by common MFCs. / published_or_final_version / Civil Engineering / Master / Master of Philosophy

Dissertation (PhD)--Stellenbosch University, 2004. / ENGLISH ABSTRACT: This study investigates the electrochemical oxidation of phenol. Phenol is a major toxin and water pollutant. In addition, during water treatment it reacts with chlorine to produce carcinogenic chlorophenols. lts treatment down to trace levels is therefore of increasing concern. For this purpose, dynamically stable anodes for the breakdown of phenols to carbon dioxide or other less harmful substances were developed and characterized. The anodes were prepared from mixed oxides of tin (Sn) and the precious metals ruthenium (Ru), tantalum (Ta) and iridium (Ir), which in tum were prepared using sol-gel techniques. This involved dip-coating the aqueous salts of the respective metals onto titanium substrates and heating to temperatures of several hundreds of degree Celsius. The properties of these mixed oxide thin films were investigated and characterized using thermal gravimetric analysis (TGA), scanning electron microscopy (SEM), atomic force microscopy (AFM), elemental dispersive energy X-ray analysis (EDX), X-ray diffraction (XRD), Rutherford backscattering spectrometry (RBS), particle induced X-ray emission (PIXE) and electrochemical measurements. A variety of different electrode materials including Til Sn02-Ru02-Ir02, Ti/Ta20s-Ir02 and Ti/RhOx-Ir02 were developed and tested for their potential as oxidation catalysts for organic pollutants in wastewaters. Depending on the anode type, phenol was found to be electrochemically degraded, to different extents, on these surfaces during electrolysis. It was however found that the oxidation rate not only depended on the chemical composition but also on the oxide morphology revealed, resulting from the preparation procedure. The Ti/SnOz-Ru02-Ir02 film was found to be the most efficient surface for the electrolytic breakdown of phenol. This film oxidized phenol at a potential of 200 mV vs Ag/AgC!. The activity of the catalytic systems was evaluated both on the basis of phenol removal efficiency as well as the kinetics of these reactions. Phenol removal efficiency was more than 90% for all the film surfaces prepared and the rate of the reaction followed first order kinetics. A pathway for the electrochemical degradation of phenol was derived using techniques such as HPLC to identify the breakdown products. These pathway products included the formation of benzoquinone and the further oxidation of benzoquinone to the carboxylic acids malic, malonic and oxalic. / AFRIKAANSE OPSOMMING: Die onderwerp van hierdie studie is die elektrochemiese oksidasie van fenol deur nuwe gemengde-oksied elektrodes. Fenol is 'n belangrike gifstof en besoedelingsmiddel in water. Daarbenewens kan fenolook met chloor reageer tydens waterbehandeling om sodoende karsinogeniese chlorofenole te vorm. Dit is dus belangrik dat metodes ondersoek word wat die konsentrasie van fenol in water verminder. Hierdie studie behels die bereiding en karakterisering van nuwe dinamiese stabiele anodes (DSA) vir die afbreek van fenol tot koolstofdioksied en ander minder gevaarlike verbindings. Hierdie nuwe anodes is berei vanaf die gemengde-okside van die edelmetale tin (Sn), ruthenium (Ru), tantalum (Ta) en iridium (Ir), met behulp van sol-gel tegnieke. Die finale stap in die bereiding behels kalsinering van die oksides by temperature van "n paar honderd grade Celsius. Hierdie nuwe elektrodes is later gebruik om die oksidasie van fenol te evalueer. Die gemengde-oksied dunlae/anodes IS d.m.v. die volgende analitiesetegnieke gekarakteriseer: termiese-gravimetriese analise (TGA), skandeerelektronmikroskopie (SEM), atoomkragmikroskopie (AFM), elementverstrooiingsenergie- X-straalanalise (EDX), X-straaldiffraksie (XRD), Rutherford terug-verstrooiingspektroskopie (RBS), partikel-geinduseerde X-straal emissie (PIXE), en elektrochemiese metings. 'n Verskeidenheid elektrodes van verskillende materiale is berei en hul potensiaal as oksidasie-kataliste vir organiese besoedelingsmiddels in afloopwater bepaal. Hierdie elektrodes het die volgende ingesluit: Ti/Sn02-Ru02-Ir02, Ti/Ta20s-Ir02 en Ti/RhOx-Ir02. Gedurende elektrolise is fenol elektrochemies afgebreek tot verskillende vlakke, afhangende van die tipe elektrode. Die oksidasietempo het egter nie alleen van die chemiese samestelling van die elektrode afgehang nie, maar ook van die morfologie van die okside, wat op hulle beurt van die voorbereidingsprosedure afgehang het. Daar is bevind dat die Ti/Sn02-Ru02-Ir02 elektrode die mees effektiewe oppervlakke vir die afbreek van fenol is. Hier het die oksidasie van fenol by 'n potensiaal van 200 mV plaasgevind. Die aktiwiteite van die katalitiese sisteme IS bepaal op grond van hulle fenolverwyderingsdoeltreffendheid. Die kinetika van die reaksies is ook bepaal. Al die elektrodes het >90% fenolverwyderingsdoeltreffendheid getoon en die reaksietempos was van die eerste-orde. Deur van analitiese tegnieke soos hoëdrukvloeistofchromatografie (HPLC) gebruik te maak is die afbreekprodukte van fenol geïdentifiseer en 'n skema vir die elektrochemiese afbreek van fenol uitgewerk. Daar is bevind dat bensokinoon gevorm het, wat later oksidasie ondergaan het om karboksielsure te vorm.

Thesis (PhD)--Stellenbosch University, 2004. / ENGLISH ABSTRACT: Please see fulltext for abstract / AFRIKAANSE OPSOMMING: Sien asb volteks vir opsomming

Thesis (Ph. D.)--University of Washington, 2006. / Vita. Includes bibliographical references (leaves 204-217).

We have developed a thermionic cathode capable of high emission currents. The structure of this cathode is oxide coated carbon nanotubes (CNTs) on a tungsten (W) substrate. This cathode was superior in emission due to the combination of the field enhancement effect from the CNTs and the lowered work function from the semiconducting oxide surface. Such oxide coated CNTs were excellent electron emitters. Conventional electron emission theories, such as Richardson's and Fowler-Nordheim's, did not accurately describe the field enhanced thermionic emission from such emitters. A unified electron emission theory was adopted and numerical simulations were performed to explain the deviation of electron emission from conventional field and thermionic emission theories. Also, the thermionic measurement system and measurement methods were improved in order to measure and characterize the strong electron emission from this new cathode. Large electron emission current from such structures also made a new thermionic cooling device a possibility. Cooling due to the electron emission was measured in terms of temperature drop, and a large temperature drop was observed from this cathode structure. Finally, applications of this cathode in plasma discharge devices were explored. This new cathode was tested in a plasma environment and initial results were obtained. / Department of Physics and Astronomy

High levels of water stress and increased demand for potable water generated via desalination pose significant challenges for sustainable waste brine management in arid regions. Electrochemical techniques, like brine electrolysis, offer an approach for treating brine, preventing environmentally harmful disposal, and facilitating the recycling of valuable ions found in brine. As the large concentration of ions can precipitate and degrade conventional electrolyzer components, membraneless electrolyzers, which lack membranes, can be an alternative for direct brine electrolysis. The absence of membranes enables operation in the presence of impurities and a wide range of pH environments. However, membraneless electrolyzers suffer from a trade-off between current density and current utilization that stems from undesired back-reactions that arise from the crossover of gaseous and aqueous products between the anode and cathode. In this dissertation work, a combination of in situ high-speed video, colorimetric pH imaging, modeling, and electroanalytical methods were used to evaluate how the performance of a porous flow-through cathode is affected by operating current density, electrolyte flow rate, and choice of catalyst placement on a porous support. It was found that catalyst placement is a key knob to control the location of product generation and thereby minimize product crossover and maximize pH differential. Placing the catalyst on the outer surface of the cathode resulted in an average increase of 51% in current utilization, a metric for measuring crossover, compared to the opposite configuration. This finding is explained by the ability of the porous electrode support to serve as a barrier to suppress crossover for the outward-facing catalyst configuration. In addition, the outward-facing catalyst configuration leads to more stable operation while incurring minor increases (90-170 mV) in overpotentials. For both catalyst configurations, it was also shown that the Damköhler number (𝐷𝑎) is a practical descriptor for predicting operating conditions that maximize the concentration of OH⁻ in the cathode effluent stream. Furthermore, this dissertation evaluated the performance of a platinized cathode within a membraneless electrolyzer in the presence of Mg²⁺ impurities. In a 3-hour stability test at 50 mA cm⁻² during brine electrolysis, electrolytes with Mg²+ concentration below 5 mM showed a negligible influence on cathode performance. Electrolytes with Mg²⁺ concentration below 1.2 mM at similar operating conditions exhibited improved cathode performance compared to Mg-free brine. All learnings during this study were captured in a mathematical model that predicts the tolerance threshold at which the cathode would cease to operate due to accumulations of Mg(OH)₂ deposits at different current densities and superficial velocities. Overall, these studies demonstrated the potential of membraneless electrolyzers as an emerging technology for treating brine and converting it into high-value products. Finally, applying an oxide overlayer to planar electrodes has been demonstrated to improve their stability, activity, and/or selectivity. This is relevant for direct brine electrolysis, as brine contains many impurities that can compromise the integrity of electrodes and promote undesirable reactions, generating toxic products like chlorine gas. However, given that high-surface electrodes are required for industrial applications, it is necessary to develop a method to encapsulate high-surface-area electrodes. Applying nanoscopic oxide encapsulation layers to high-surface-area electrodes such as nanoparticle-supported porous electrodes is not an easy task. This dissertation work demonstrated that the recently developed condensed layer deposition (CLD) method can be used for depositing nanoscopic (sub-10 nm thick) titanium oxide (TiO₂) overlayers onto high surface area platinized carbon foam electrodes. Characterization of the overlayers by transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) showed they are amorphous, while X-ray photoelectron microscopy confirmed that they exhibit TiO₂ stoichiometry. Electrodes were also characterized by hydrogen underpotential deposition (Hupd) and carbon monoxide (CO) stripping, demonstrating that the Pt electrocatalysts remain electrochemically active after encapsulation. Furthermore, copper underpotential deposition (Cuupd) measurements for bare Pt and TiO₂-encapsulated Pt electrocatalysts revealed that the TiO₂ overlayer effectively prevented Cu₂+ from reaching the buried, allowing this method to determine the coverage of the TiOx coating. In summary, this portion of the dissertation demonstrated that CLD is a promising method for applying nanoscopic overlayers on high-surface electrodes.