3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Preparation of Cu2ZnSnS4/N-MWCNTs nano-hybrid systems for potential application in dye-sensitized solar cells as counter electrode(2020) Mxakaza, Lineo FlorenceSolar energy is deemed as one of the best alternative energy sources to fossil fuels because it is abundant and renewable. Dye-sensitized solar cells (DSSCs) are one of the photovoltaic devices that have attracted much of attention due to easy fabrication methods and relatively low-cost of production. DSSC architecture includes a photoanode sensitized with an organic dye, an electrolyte and a counter electrode (CE). Platinum is a common CE material because of it has high catalytic activity and conductivity. However, platinum is a noble metal and has a low surface area and alternative materials that result in comparable or even higher power conversion efficiencies are sort after. Cu2ZnSnS4 (CZTS) has been extensively studied as an absorber layer in p-n junction solar cells because it exhibits high absorption coefficient and low band gap. CZTS has recently been reported to have promising electro-catalytic activity properties that can be utilized in DSSC as CE material to facilitate the reduction of the electrolyte. However, poor results have been obtained thus far. Several articles have attributed the poor performance of CZTS as a CE material to its poor electron transport properties since it is a semiconductor. To counteract this shortcoming, CZTS can be coalesced with other high electron conductivity materials such as carbon nanotubes. Nitrogen doped multi-walled carbon nanotubes (N-MWCNTs) have been reported to exhibit high electron conductivity although they are poor electro-catalysts, due to defects caused by the nitrogen atoms and these defects can also be utilized in the formation of CZTS/N-MWCNTs hybrid systems. To counteract the poor catalytic behaviour of MWCNTs and poor conductivity of CZTS, a nanocomposite mixture of CZTS/N-MWCNT can be employed to provide good catalytic activity and electron transfer. Herein, we report on the colloidal synthesis and characterization of CZTS nanoparticles in oleylamine (OLA). The successful synthesis of CZTS was confirmed by X-ray diffraction(XRD) and Raman spectroscopy where the kesterite phase was obtained. The X-ray photoelectron spectroscopy (XPS) also confirmed the formation of CZTS. The transmission electron microscope (TEM) images showed polydispersed particles. To overcome the polydispersity, digestive ripening was undertaken. Ripening for 2hrs resulted in a more monodispersed sample. To ensure good anchoring of nanoparticles on the surface of the N-MWCNTs, ligand exchange was done where shorter and bidentate ligands were used namely pyridine (py), mercaptopropionic acid (MPA) and ethanedithiol (EDT). The py was used as it a traditionally use exchange ligand while MPA and EDT with two functional groups could bind to the nanoparticle surface with one group and have the other available for anchorage to the N-MWCNTs. The XRD and TEM images showed that the long-chained OLA capped particles were far better than the py, MPA and EDT capped particles. The N-MWCNTs were synthesized using the chemical-vapour deposition method. The nitrogen doping was confirmed by XPS and the bamboo morphology observed on the TEM images. Raman spectroscopy also showed that more defects were present after doping. The ex-situ synthesis method that involves sonicating the CZTS and N-MWCNTs in dimethyl formamide was employed to prepare the nanocomposites. Raman spectroscopy and the TEM images confirmed the interaction between these two materials hence the formation of the nano-hybrids/nanocomposites. Cyclic voltammetry using nanocomposite ink fabricated on glassy carbon electrode in the I−/I3−electrolyte was run to determine the maximum reduction peak current of the various weight percent ratios of CZTS and N-MWCNTs in the CZTS/N-MWCNTs nanocomposites. The CZTS/NMWCNTs (0.3ratio) was found to exhibit the highest reduction peak current, and relatively low potential and was therefore deemed as the best of all the ratios and a possible Pt CE material alternativeItem 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.Item Earthing electrodes power frequency and impulse behaviour(1995) Eytani, Dan MordechaiPower frequency and Impulse tests were performed on earth electrodes, w'iich are being used in MV networks. The electrodes are combinations of conductors buried in a trench and earth rods, which are driven vertically into the ground [Abbreviated Abstract. Open document to view full version]