No description available.

Magister Scientiae - MSc / 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. / South Africa

Magister Scientiae - MSc / The main objective of this work it to produce membrane electrode assemblies (MEAs) that have improved performance over MEAs produced by the conventional manner, by producing highly efficient, electroactive, uniform catalyst layers with lower quantities of platinum electrocatalyst. The catalyst coated membrane (CCM) method was used to prepare the MEAs for the PEM fuel cell as it has been reported that this method of MEA fabrication can improve the performance of PEM fuel cells. The MEAs performances were evaluated using polarisation studies on a single cell. A comparison of polarisation curves between CCM MEAs and MEAs produced in the conventional manner illustrated that CCM MEAs have improved performance at high current densities (>800 mA/cm2). / South Africa

Oxytetracycline (OTC) is a broad spectrum antibiotic used extensively in the agricultural and human-health sector, and is effective against various gram positive and –negative bacteria as well as large viruses and certain pathogenic Rickettsiae. This study addresses the lack of voltammetric knowledge regarding the electroanalytical characterisation of OTC and its analysis in complex matrices. Cyclic voltammetry (CV) revealed several irreversible anodic peaks for OTC at a bare glassy carbon electrode (GCE). These current responses were improved through the selection of a diluent for OTC stock preparation, electrolyte solution and electrolyte pH, stir time and applied preconditioning potential. Under enhanced adsorptive conditions and using square wave voltammetry (SWV), a detection limit of 24.3 nM was achieved. The electrode surface could be renewed in vitro for 10 successive scans. OTC oxidation was characterised as a one electron:one proton ECiE mechanisms. Next, investigating the viability of voltammetry in various complex microbiological growth media revealed that selected growth media contained interfering redox active components, which, while simultaneously coating the electrode surface, effectively reduced GCE performance and lowered the active electrode surface area, as ascertained through CV and electrochemical impedance spectroscopy (EIS) studies. This interference lowered OTC current response in the presence of growth media which was partially recovered by appropriate growth media selection and sample dilution. In testing the use of acid functionalised multi-walled carbon nanotubes (MWCNTs) to improve anodic OTC response, charge-based attraction was observed between the MWCNT dispersal agent Nafion® and OTC, while increased surface area associated with prolonged acid functionalisation time aided in improving OTC current response.

A glass electrode is described which is capable of detecting photosynthetic CO₂ uptake by monitoring the concentration of CO₂ dissolved in a small drop of algal suspension. Consideration of current theory suggests that transients should occur in the rate of CO₂ uptake which are similar to but distinct from those observed in O₂ evolution rate. These transients should be most readily observed when cells are illuminated alternately with wavelengths of 650 nm and 705 mn. The preliminary results show that C0₂ uptake by photosynthesis can be monitored by this apparatus. Although transients were not observed, this may be due to the rather low sensitivity. There is a discussion of shortcomings of the apparatus, and of improvements necessary before transients can be observed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

No description available.

No description available.

Electrochemiluminescence (ECL) is a phenomenon whereby electrochemical reactions generate a product that is capable of emitting light. ECL’s high sensitivity, selectivity, extremely low background, and relatively simple instrumentation make it particularly well-suited for chemical sensing and biosensing strategies. Here we report a simple ECL imaging system based on a camera interfaced with a zoom lens to compare pencil graphite electrode (PGE) and screen-printed carbon electrode (SPCE) arrays as ECL platforms. With this system, ECL signals generated from tris(2,2′- bipyridine)ruthenium(II) chloride hexahydrate ([Ru(bpy)3]2+) using co-reactant tri-n-propylamine (TPA) were linear with respect to [Ru(bpy)3]2+ concentrations from 9 to 450 μM. Detection limits for [Ru(bpy)3]2+ were found to be 1.8 μM with PGEs and 0.9 μM with SPCEs. Immobilization of a thin polyvinylpyridine (PVP) film ECL reporter [Ru(bpy)2(PVP)10]2+ on SPCEs was also investigated. Overall, the combination of PGEs or SPCEs with the simple ECL imaging system offers a cost-effective approach to ECL-based sensing and biosensing.

Doctor of Philosophy / Department of Chemistry / Takashi Ito / This research was focused on understanding how external stimuli affect the permeability of the chemically modified electrodes, and how the materials used in modifying the working electrodes respond to the changes in the surface charge. We adopted a voltammetric type electrochemical sensor to investigate the permeability effects induced by pH and organic solvents. The working electrodes used in this research were chemically modified with thioctic acid self assembled monolayer (TA SAM), track etched polycarbonate membranes (TEPCM) and PS-b-PMMA nanoporous films (polystyrene-block-polymethylmethacrylate). We studied the permeability behavior of each of the material upon application of external stimuli. In chapter 3, the permeability changes induced by change in surface charge of thioctic acid SAM was investigated. The surface charge of the monolayer was tuned by changing pH of the medium, which resulted in decrease of redox current of a negatively charged marker due to deprotonation of the surface –COOH groups of TA SAM. Decrease in redox current reflected a decrease in the reaction rate, and by using closed form equations the effective rate constants at several pKa values were extracted. In chapter 4, permeability changes induced by pH in TEPCM were investigated. We assessed the surface charge of these membranes via cyclic voltammetry generated for neutral and charged redox molecules. Limiting current of charged markers were affected by the surface charge induced by pH, where as the redox current for the neutral marker was not affected. Experimental redox currents were larger than the theoretical current, indicating that redox molecules preferentially distributed in a surface layer on the nanopore. Organic solvent induced permeability changes of PS-b-PMMA nanoporous films were investigated via electrochemical impedance spectroscopy and AFM. Higher response of pore resistance in the presence of organic solvents indicated either swelling of the nanoporous film or partitioning of organic solvents in the pores. However AFM data revealed that the permeability changes are due to partitioning of the solvents rather than swelling of the porous film, since there was no appreciable change if the pore diameter in the presence of solvents.

Typescript (photocopy). / Digitized by Kansas Correctional Industries