3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item A DFT based study of the gold-water interface(2019-10) Masike, Tshegofatso TitusA gold/water interface has been investigated with the DFT-based self-consistentcharge density-functional tight-binding (SCC-DFTB) method using a full cell model with periodic boundary conditions. Born-Oppenheimer molecular-dynamics simulations for water on the surfaces of two Au60 electrode slabs were computed. We have demonstrated the applicability of this method to study the structural and dynamical properties of the gold/water interface in a system with periodic-boundary conditions. The results of the simulation clearly show a dependency on the charged electrodes by means of the orientation of the water molecules and the ensuing polarisation of the cell. However, it was also shown that this polarisating e ect is con ned almost exclusively to rst few angstroms away from the electrode . This is indicative of the rather short-range screening behaviour of water. The present study is a feasibility study of models with periodic-boundary conditions and beckons further analysis of metal/electrolyte interfaces on a dependable atomic scale, using periodic boundary conditions.Item Colloidal synthesis of molybdenum diselenide nanomaterials for supercapacitor applications(2018) Ndala, Zakheleii Abstract Herein we report on the synthesis of MoSe2 nanomaterials using novel colloidal synthetic methods and their application as electrode materials in supercapacitors. Supercapacitors are energy storage devices with high power density and high cycle stability that can be used in applications that require rapid charge/discharge. The drawback in supercapacitors is their low energy density. Nanomaterials with high surface area are being explored as alternatives to activated carbon which has been a commonly used electrode material in supercapacitors. This is done to increase the energy density of supercapacitors. MoSe2 has been identified as an excellent candidate for use as an electrode material in supercapacitors because of its interesting properties. MoSe2 is a layered transition metal dichalcogenide (TMD) that is similar to graphene in structure and possesses interesting structural, optical and electronic properties. MoSe2 also has a high surface to volume ratio. A colloidal synthetic process was used for the synthesis of the MoSe2 nanomaterials, after which the effect of various reaction parameters was investigated. The reaction was run at 300 °C using oleylamine (OAm) as the solvent and surfactant. A time study on the synthesis of the nanomaterials revealed MoSe2 few-layer nanosheets grow from a flocculate formed in the initial stages of the reaction (30 min). At longer reaction times (e.g. 90 min) the flocculate is consumed to form wrinkled few-layer nanosheets. A variation of the metal precursor in the reaction results in changes to the morphology of the MoSe2 nanomaterials. The formation of the flocculate in the initial stages of the reaction when molybdenum hexacarbonyl is used as the metal precursor results in the formation of wrinkled few-layer nanosheets. The use of molybdic acid as the metal precursor results in the formation of nanoparticles with a central core which leads to the formation of MoSe2 nanoflowers. The effect of adding a co-surfactant to the reaction system was also investigated. The effect of adding oleic acid as a co-surfactant in the reaction along with oleylamine resulted in changes to the thickness of the nanosheets synthesized and slight changes in the morphology. The use of 1-octadecene as a co-surfactant resulted in the increased reactivity of the selenium precursor which increased the number of nanosheets growing per nanoflower. The electrochemical properties of the MoSe2 nanomaterials were investigated to determine the suitability of the nanomaterials for use as supercapacitor electrodes. The MoSe2 nanomaterials synthesized using colloidal synthesis exhibited electric double layer capacitance behaviour. The effect of the morphology on the electrochemical performance of the MoSe2 nanomaterials was investigated using MoSe2 nanoflowers and few-layer nanosheets. The MoSe2 nanoflowers were shown to have a higher specific capacitance at 81 Fg-1 than the few-layer nanosheets at 30 Fg-1. The nanoflowers also had better capacitance retention at higher current densities in the charge-discharge analysis compared to the few-layer nanosheets. The nanoflowers had higher capacitance retention of 68% compared to 20% for the few-layer nanosheets. The nanoflowers also had a lower equivalence series resistance (ESR) of 34.0 Ω compared to that of the few-layer nanosheets at 57.1 Ω.Item Effects of TiO2 and CS supports on hydrogen spillover in Co and Ru supported catalysts(2018) Dlamini, SthembileHydrogen spillover is the surface migration of activated hydrogen atoms from a metal catalyst particle, on which they are generated, onto the catalyst support. A lot of research work has been done on hydrogen spillover since its discovery in 1964, and its incidence on reducible supports such as titanium oxide is established, yet questions remain about the role of the support in hydrogen spillover in heterogeneous catalysis. The aim of this research was to investigate the role of a support in hydrogen spillover, using cobalt and ruthenium supported on TiO2 and CS catalysts. These two catalysts were prepared by deposition precipitation-urea, incipient wetness impregnation and polyol methods and characterized using TGA, TEM, BET, XRD and Raman spectroscopic analysis. The Fischer-Tropsch evaluation of the catalysts was done in a fixed bed reactor and the products were analyzed on offline gas chromatographs. The results show a significant shift to lower reduction temperatures for the CoO to Co peak for the physical mixture of Co/TiO2 and Ru/TiO2. However, when Co/TiO2 and Ru/TiO2 catalysts were packed in a bed system separated by different amounts of TiO2, no significant change was observed compared to the “hybrid” catalyst. The separation distance between the two catalysts had no effect in the reduction temperature. The improved reducibility of CoO to Co was attributed to the dissociation of H2 on the Ru, which made the Ru/TiO2 catalyst the donor phase causing the hydrogen to spillover to the acceptor phase which is the Co/TiO2. The same finding was observed for the Ru and Co catalysts supported on the carbon spheres where the CoO to Co peak was shifted to lower reduction temperature compared to the monometallic Co/CS catalyst. The reducibility was attributed to the presence of Ru. For the Co and Ru catalysts supported on carbon spheres the physically mixed Co/CS and Ru/CS catalysts resulted in the increase of C5+ selectivities compared to the monometallic catalysts of Co catalysts. Microwave irradiation had a positive effect on the dispersion and surface area on the catalyst prepared using the polyol and incipient wetness methods.