Browsing by Author "Williams, Mario"

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    Characterization of platinum-group metal nanophase electrocatalysts employed in the direct methanol fuel cell and solid-polymer electrolyte electrolyser
    (University of the Western Cape, 2005) Williams, Mario; Linkov, V.; Khotseng, L.; Dept. of Chemistry; Faculty of Science
    Characterization of nanophase electrocatalysts, which are an essential part in the direct methanol fuel cell (DMFC) and solid-polymer electrolyte (SPE) electrolyser, have been studied in this work. Their nanoparticulate size raises significant challenges in the analytical techniques used in their structural and chemical characterization. Hence, the applicability of analytical protocols for the qualitative and quantitative characterization of structural and chemical properties of nanophase platinum and platinum-ruthenium electrocatalysts was investigated. Also, fabricated carbon-supported platinum, platinum-ruthenium, iridium oxide, and mesoporous silica-templated platinum electrocatalysts were screened on the basis of their electrocatalytic activity. A set of structural and chemical parameters influencing the performance of nanophase electrocatalysts was identified. Parameters included crystallinity, particle size, particle size distribution, agglomeration, aggregation, surface area, thermal stability, chemical speciation, electrocatalytic activity, and electrochemically-active surface area. A large range of analytical tools were employed in characterizing the electrocatalysts of interest. High accuracy and precision in the quantitative and qualitative structural characterization of nanophase electrocatalysts, collected by x-ray diffractometry and transmission electron microscopy, was demonstrated. Selected-area electron diffraction was limited to a rapid qualitative evaluation of electrocatalyst polycrystallinity and crystal symmetry. Scanning electron microscopy was limited to the qualitative evaluation of the agglomeration state of supported electrocatalysts. High-performance particle sizing was unable to resolve the particle size of the electrocatalyst from that of the support and was therefore employed in the quantitative investigation of aggregate size and size distribution in supported electrocatalysts. The technique produced high precision data illustrating the reproducibility of the aggregate size data. N2-physisorption produced surface area and pore size distribution data of high quality, but was unable to determine surface areas specific to the metal phase in supported electrocatalysts. The technique was deemed inconsistent in the accurate determination of average pore size. The resolution of scanning electrochemical microscopy and proton-induced x-ray emission spectroscopy (SECM) did not allow for an investigation of characteristics at the nanoscale. Quantitative chemical information was difficult to extract from SECM maps and the technique was limited to the qualitative characterization of surface topography. Thermogravimetry was suitable for the qualitative investigation of the thermal stability of the nanophase electrocatalysts of interest. In this study, temperature-programmed reduction was able to qualitatively speciate the surface chemical state and investigate the strength of the metal-support interaction in supported nanophase electrocatalysts. Cyclic voltammetry and linear-sweep voltammetry were employed in the electrochemical characterization of nanophase electrocatalysts and both qualitative and quantitative information were obtained. The techniques were able to discriminate between various commercial and fabricated electrocatalysts and identify new highly-active materials. Preparation variables could be critically evaluated for the fabrication of cost-effective highly-active nanophase electrocatalysts. Certain techniques were deemed to be highly applicable in discriminating between high and low activity nanophase electrocatalysts based on their structural and chemical properties. The electrocatalyst characterization strategy and methodology was developed and will be implemented for future characterization of nanophase electrocatalysts.
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    Palladium surface-modified rare earth metal-based ab5 type hydride-forming materials
    (University of the Western Cape, 2008) Williams, Mario; Linkov, V.M.; Nechaev, A.N.
    Driven by mounting standards of living and a growing population, South African energy consumption is expected to increase dramatically within the next decade. The increased demand for more energy will require enormous growth in the capacity for energy generation, more secure and diversified energy sources, and a successful strategy to reduce greenhouse gas emissions. The wellbeing of the South African economy depends on reliable and affordable supplies of energy; whilst environmental wellbeing, from improving urban air quality to abating the risk of global warming, requires energy resources that emit less greenhouse gases compared to petrochemicals. Amongst the various alternative energy strategies, building an energy infrastructure that utilises hydrogen as the primary energy carrier may enable a non-polluting energy security in the future, when it is produce using renewable energy sources (e.g. water electrolysis). Hydrogen has been acknowledged as a key element in the future generation of energy and will be essential in increasing and maintaining economic growth. The significance of hydrogen as a future energy source is due to its large abundance and an energy density that is three times greater than that of an average hydrocarbon fuel. Roughly 80% of hydrogen is produced by natural gas reforming, partial oxidation of light alcohols, and autothermal reforming. In addition, a number of alternative technologies exist in which hydrogen can be generated from starting materials such as coal; biomass; and water, including electrolysis, fossil fuel processing, and coal gasification. However, most of these technologies produce a hydrogen product which is of poor purity. Purification is achievable considering equipment costs are extremely high and the process is therefore mostly economically unfeasible.