3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Synthesis and application of boron-nitrogen doped carbon nano-onions in supercapacitors(2020) Shaku, BokomeThis work reports on the synthesis of carbon nano-onions (CNOs) via the chemical vapor deposition (CVD) and flame pyrolysis methods for application in supercapacitors (SCs).The synthesis of CNOs using CVD was achieved by reducing a Fe-Co catalyst to its metallic state, followed by decomposition of acetylene (C2H2) as a carbon source. The metal catalyst was supported on calcium carbonate (CaCO3). The synthesis of CNOs was carried out at different reaction temperatures (i.e. 450 °C, 550 °C and 650 °C). In addition to the final product encapsulating metal catalyst particles, the synthesized CNOs contained a number of by-products such as carbon nano-fibres and carbon nano-tubes. To overcome the problem associated with the undesired formation of carbon materials, a flame pyrolysis approach was used for the synthesis of CNOs with high purity, using grapeseed oil as a carbon source. The pristine material was annealed at different temperatures and doped with boron (B) and nitrogen (N). Physical properties of the materials were investigated and characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), a gas adsorption technique, Brunauer-Emmet-Teller (BET) analysis, Raman spectroscopy and thermogravimetric analysis (TGA). The results from the CVD method demonstrated a change as the temperature and flow rate increased. Microscopy analysis revealed successful dispersion of Fe and Co particles onto CaCO3 which produced a mixture of materials (CNOs, CNFs and CNTs). The XRD patterns confirmed the presence of highly active crystalline mixed oxide (CoFe2O4) as the dominant catalyst phase. TEM analysis of CNOs from the flame pyrolysis method revealed multi-layered concentric carbon layers with no by-products. The amount of substitutional nitrogen in the CNO samples was 1.67% which resulted in an increase in the specific surface area and ID/IGratio. The work further explored the use of CNOs, from flame pyrolysis, as electrode materials in a two electrode system. The capacitance and resistivity of the prepared carbon electrodes were evaluated using techniques such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). The electrochemical performance of N-doped CNOs was investigated and compared with those of the pristine, annealed and B-doped CNOs within an operating cell potential of 0.8 V in a 2 M KOH electrolyte. The specific capacitance increased upon nitrogen doping to 54 F/g as compared to pristine CNOs (7.5 F/g), annealed CNOs at 750 °C-2 h (22.5 F/g) and B-doped CNOs (28 F/g). The N-doped CNOs exhibited superior electrochemical performance compared to pristine, annealed and B-doped CNOs which was related to the high surface area and ID/IG ratio and indicated that the electrolyte ions could readily penetrate the pores of the electrode material. Our results demonstrated that N-doped CNOs are capable of delivering energy in a short timeframe in supercapacitorsItem Structure and thermoresponsive behaviour of porous and non-porous borophosphates(2016) Mogodi, Mashikoane WilsonIn this PhD thesis, the synthesis, crystal chemistry and thermoresponsive behaviour of non-porous borophosphates [ABPO5 (A= Ca, Sr, Ba) and BPO4] and porous borophosphates [NH4Fe(III)[BP2O8(OH)] and MIxMIIz(H2O)2[BP2O8].zH2O (MI = Na, NH4 and MII = Mn, Co)] phases were investigated. Understanding the crystal structure dynamics as a function of temperature of the selected porous and non-porous borophosphates revealed the thermal stability of the studied compounds, while serving as a predictive measure of the effects of temperature on other materials properties and subsequent applications. The non-porous borophosphates were synthesized using the solid state method, whereas the porous borophosphates were synthesized using the hydrothermal method. The powder X-ray diffraction (PXRD) technique, along with the application of the Rietveld refinement method, was the principle characterisation technique employed for the non-destructive and non-invasive thermoresponsive characterisation of the studied borophosphate phases. This thesis consists of seven chapters, four of which are independent papers corresponding to four chapters. Chapter 1 reviews the relevant scientific literature, while chapter 2 describes the methods of characterisation used in this thesis. Concerning the highly thermally stable non-porous borophosphates, chapter 3 presents the thermal expansion behaviour of ABPO5 compounds, which have been determined from the sequential application of the Rietveld refinement method of variable temperature powder X-ray diffraction (VT-PXRD) data. For trigonal ABPO5 compounds, a near linear expansion of the unit cell axes was found for all structures as a function of temperature. The variation of the crystal structure with temperature of ABPO5 compounds was also established. With the general understanding that borophosphates display intriguing crystal structure architectures, chapter 4 describes the synthesis and characterisation of four metal borophosphate hydrates: NaMII(H2O)2[BP2O8](H2O); MII = Co (I), Mn (II) and (NH4)0.5MII1.25(H2O)2[BP2O8](H2O)0.5; MII = Co (III), Mn (IV). The structures refined at room temperature from PXRD data revealed that isostructural phases I and II have an ordered arrangement of water molecules in the voids, whereas isostructural phases III and IV have fractional and disordered distribution of water molecules in the voids. Scanning electron microscope (SEM), fourier transform infrared spectroscopy (FT-IR), variable temperature powder X-ray diffraction (VT-PXRD) and thermogravimetric analysis (TGA) for all compounds are also presented. Chapter 5 was an investigation into the synthesis, crystal structure and thermal properties of the porous iron borophosphate NH4Fe(III)[BP2O8(OH)]. Variable temperature PXRD and thermogravimetric analysis were used to investigate the compounds thermal stability and expansion behaviour. Thermal investigations indicated that the compound is stable up to 470 °C. Of great importance to the accurate and/or precise measurement of the crystal structures and lattice parameters of the phases investigated using powder diffraction was the correct application of the Rietveld refinement method to the measured diffraction data. Therefore, chapter 6 is an investigation into a number of different Rietveld refinement approaches, which were aimed at modelling the changes in the atomic coordinates of BPO4 as a function of temperature. Parametric Rietveld refinements and rigid body Rietveld refinements were among the refinement strategies employed.