Browsing by Author "Ndungu, Patrick"

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    Carbonation of brine impacted fractionated coal fly ash: Implications for CO2 sequestration
    (Elsevier, 2011) Muriithi, Grace; Gitari, Wilson; Petrik, Leslie; Ndungu, Patrick
    Coal combustion by-products such as fly ash (FA), brine and CO2 from coal fired power plants have the potential to impact negatively on the environment. FA and brine can contaminate the soil, surface and ground water through leaching of toxic elements present in their matrices while CO2 has been identified as a green house gas that contributes significantly towards the global warming effect. Reaction of CO2 with FA/brine slurry can potentially provide a viable route for CO2 sequestration via formation of mineral carbonates. Fractionated FA has varying amounts of CaO which not only increases the brine pH but can also be converted into an environmentally benign calcite. Carbonation efficiency of fractionated and brine impacted FA was investigated in this study. Controlled carbonation reactions were carried out in a reactor set-up to evaluate the effect of fractionation on the carbonation efficiency of FA. Chemical and mineralogical characteristics of fresh and carbonated ash were evaluated using XRF, SEM, and XRD. Brine effluents were characterized using ICP-MS and IC. A factorial experimental approach was employed in testing the variables. The 20–150 μm size fraction was observed to have the highest CO2 sequestration potential of 71.84 kg of CO2 per ton of FA while the >150 μm particles had the lowest potential of 36.47 kg of CO2 per ton of FA. Carbonation using brine resulted in higher degree of calcite formation compared to the ultra-pure water carbonated residues.
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    Consolidated nanomaterials synthesized using nickel micro-wires and carbon nanotubes
    (University of the Western Cape, 2007) Davids, Wafeeq; Linkov, Vladimir M.; Nechaev, Alexander; Ndungu, Patrick; Dept. of Chemistry; Faculty of Science
    Nano-devices are the next step in the application of nanomaterials in modern technology. One area of research that is receiving an increased amount of attention globally is the fabrication of new nano-devices for applications in hydrogen energy technologies. The current work focuses on the synthesis and characterization of nano-devices with potential application in alkaline electrolysis and secondary polymer lithium ion batteries. Previous work with Nickel micro-wires demonstrated the potential to use these nanomaterials as electrodes in alkaline electrolysis. Carbon nanotubes have been shown to posse excellent electrochemical properties. A new direction in research is explored by combining nickel micro-wires with CNT, a new consolidated composite carbon nanocomposite can be realized and the characterization of such a novel composite was the focus of this thesis. Novel composite carbon nanomaterials were synthesized using an electrochemical template technique and a hydrocarbon pyrolysis step. The first step involved the deposition of nickel within the pores of ion track etched Polyethylene terephthalate (PET) membrane; with pore diameters of 1μ, 0.4μ and 0.2 μ. Electrochemical deposition of nickel was carried out galvanostatically in a nickel hard bath between 35-40°C, and using a deposition current density of 75 mAcm2. Carbon nanotubes were then deposited directly onto the surface of the nickel micro-wires via a chemical vapour deposition (CVD) technique using liquid petroleum gas (LPG) as the carbon source. CVD was done at a temperature of 800°C and the deposition time was 5 minutes. The morphology and structural studies of these novel composite nanomaterials were studied by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). Electrochemical investigations were done using Cyclic Voltammetry (CV), Chronoamperometry (CA) and Electrochemical Impedance Spectroscopy (EIS). After removal of the template, before CNT CVD growth, SEM images revealed free standing arrays of nickel micro-wires, and after CNT growth via CVD the SEM micrographs showed that the morphology of the Ni micro-wires was moderately altered by the CVD process. From the XRD results it was shown that the crystallinity of the Nimicro-wires was persevered after the CVD process. The XRD of the nickel micro-wires with CNT grown directly on the surface revealed the characteristic CNT peak at 2θ =24.60. Cyclic Voltammetry (CV) was performed on the consolidated composite nanomaterial in an alkaline solution. The CV revealed that the novel composite carbon nanomaterial was the most active for hydrogen evolution when compared to unmodified Ni micro-wires and a flat nickel electrode. This was attributed to the increase in electrochemical accessible surface area. Electrochemical impedance spectroscopy (EIS) showed that the novel composite carbon nanomaterial had a much higher capacitance than the nickel micro-wires, a flat nickel electrode, a flat nickel substrate modified with CNT, and a graphite electrode. When a similar comparison was done using a commercially available anode for lithium ion battery applications, the novel consolidated composite carbon nanomaterial had double the capacitance of the commercial anode. The consolidated composite carbon nanomaterial was modified by depositing Pt on to the surface of the CNT via electroless deposition. The presence of Pt was determined by Energy dispersive spectrometry and the electrocatalytic activity of the Pt modified consolidated composite carbon nanomaterial was significantly improved. The work presented in this thesis provides a new and unique direction in the synthesis and application of novel consolidated carbon nanomaterials through true synergistic effect between nickel micro-wires and CNT. The exploration of the characteristics of the system and the ability to functionalize the CNT with different moieties allows for a wide range of application in energy conversion devices.
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    Hall measurements on carbon nanotube paper modified with electroless deposited platinum
    (Springer, 2010) Petrik, Leslie; Ndungu, Patrick; Iwuoha, Emmanuel
    Carbon nanotube paper, sometimes referred to as bucky paper, is a random arrangement of carbon nanotubes meshed into a single robust structure, which can be manipulated with relative ease. Multi-walled carbon nanotubes were used to make the nanotube paper, and were subsequently modified with platinum using an electroless deposition method based on substrate enhanced electroless deposition. This involves the use of a sacrificial metal substrate that undergoes electro-dissolution while the platinum metal deposits out of solution onto the nanotube paper via a galvanic displacement reaction. The samples were characterized using SEM/EDS, and Hall-effect measurements. The SEM/EDS analysis clearly revealed deposits of platinum (Pt) distributed over the nanotube paper surface, and the qualitative elemental analysis revealed co-deposition of other elements from the metal substrates used.