>Magister Scientiae - MSc / In the wide search for advanced materials for next generation lithium-ion batteries, lithium manganese orthosilicate, Li₂MnSiO₄ is increasingly gaining attention as a potential cathode material by virtue of its ability to facilitate the extraction of two lithium ions per formula unit, resulting in a two-electron redox process involving Mn²⁺/Mn³⁺ and Mn³⁺/Mn⁴⁺ redox couples. This property confers on it, a higher theoretical specific capacity of 333 mAhg⁻¹ which is superior to the conventional layered LiCoO₂ at 274 mAhg⁻¹ and the commercially available olivine LiFePO₄ at 170 mAhg⁻¹. Its iron analogue, Li₂FeSiO₄ has only 166 mAhg⁻¹ capacity as the Fe⁴⁺ oxidation state is difficult to access. However, the capacity of Li₂MnSiO₄ is not fully exploited in practical galvanostatic charge-discharge tests due to the instability of the delithiated material which causes excessive polarization during cycling and its low intrinsic electronic conductivity. By reducing the particle size, the electrochemical performance of this material can be enhanced since it increases the surface contact between the electrode and electrolyte and further reduces the diffusion pathway of lithium ions. In this study, a versatile hydrothermal synthetic pathway was employed to produce nanoparticles of Li₂MnSiO₄, by carefully tuning the reaction temperature and the concentration of the metal precursors. The nanostructured cathode material was further coated with a thin film of aluminium oxide in order to modify its structural and electronic properties. The synthesized materials were characterized by microscopic (HRSEM and HRTEM), spectroscopic (FTIR, XRD, SS-NMR, XPS) and electrochemical techniques (CV, SWV and EIS). Microscopic techniques revealed spherical morphologies with particle sizes in the range of 21-90 nm. Elemental distribution maps obtained from HRSEM for the novel cathode material showed an even distribution of elements which will facilitate the removal/insertion of Li-ions and electrons out/into the cathode material. Spectroscopic results (FTIR) revealed the vibration of the Si-Mn-O linkage, ascertaining the complete insertion of Mn ions into the SiO₄⁴⁻ tetrahedra. XRD and ⁷Li MAS NMR studies confirmed a Pmn21 orthorhombic crystal pattern for the pristine Li₂MnSiO₄ and novel Li₂MnSiO₄/Al₂O₃ which is reported to provide the simplest migratory pathway for Li-ions due to the high symmetrical equivalence of all Li sites in the unit cell, thus leading to high electrochemical reversibility and an enhancement in the overall performance of the cathode materials. The divalent state of manganese present in Li₂Mn²⁺SiO₄ was confirmed by XPS surface analysis. Scan rate studies performed on the novel cathode material showed a quasi-reversible electron transfer process. The novel cathode material demonstrated superior electrochemical performance over the pristine material. Charge/discharge capacity values calculated from the cyclic voltammograms of the novel and pristine cathode materials showed a higher charge and discharge capacity of 209 mAh/g and 107 mAh/g for the novel cathode material compared to 159 mAh/g and 68 mAh/g for the pristine material. The diffusion coefficient was one order of magnitude higher for the novel cathode material (3.06 x10⁻⁶ cm2s⁻¹) than that of the pristine material (6.79 x 10⁻⁷ cm2s⁻¹), with a charge transfer resistance of 1389 Ω and time constant (τ) of 1414.4 s rad⁻¹ for the novel cathode material compared to 1549 Ω and 1584.4 s rad-1 for the pristine material. The higher electrochemical performance of the novel Li₂MnSiO₄/All₂O₃ cathode material over the pristine Li₂MnSiO₄ material can be attributed to the alumina nanoparticle surface coating which considerably reduced the structural instability intrinsic to the pristine Li₂MnSiO₄ cathode material and improved the charge transfer kinetics.

Previous research indicates compositional variation of pentlandite [(Fe,Ni)9S8] and the effect this variation may have on the electrochemical behaviour of pentlandite is poorly understood. Pentlandite is the primary source of nickel and an important base metal sulfide (BMS) in the platinum industry. It hosts significant amounts of PGEs especially palladium and rhodium when compared to chalcopyrite and pyrrhotite. The aim of the project was to investigate the possible compositional variations of natural pentlandite and the effect of these variations on the electrochemical behavior thereof. To study possible compositional variations, single pentlandite particles - in the order of 100μm in size from flotation concentrates (PGM deposits) and massive samples (massive ore bodies) - from various sources were employed. Electron microprobe analysis indicated a compositional variation of the pentlandite particles hand-picked from the flotation concentrate samples. Variation was observed in the cobalt, iron and nickel content and this was independent of the deposit. A slight compositional variation was observed from the massive pentlandite samples. The effect the compositional variation may have on the electrochemical reactivity of pentlandite was investigated using electrochemical techniques, i.e. measurement of the polarisation resistance and mixed potential as well as performing linear anodic voltammetry, current density–transients and electrochemical impedance spectroscopy (i.e. capacitance). Poor electrochemical response of the pentlandite microelectrodes was observed. Pre–existing pores, deep pores, cracks and the brittle nature of pentlandite microelectrodes may have contributed to the poor electrochemical response of natural pentlandite particles hand-picked from the flotation concentrate. Slight compositional variations of the massive pentlandite sample influenced the electrochemical behaviour. In aerated solutions, iron enriched pentlandites were less reactive after progressive oxidation. The lower reactivity of the electrodes was a result of thick oxide films formed. This was illustrated by polarisation resistance and capacitance measurements. The lower reactivity of the electrodes was also related to the mechanism of the reduction of oxygen at oxidised passive electrode surfaces. It is however difficult to distinguish if the differences in the reactivity was a result of the Fe/Ni ratio or the influence of cobalt. Current density transients confirmed that the reactivity of a pentlandite electrode to be time dependent. The reactivity of the electrode decreased during oxidation. A variation in the electronic properties of the formed oxide film was observed. Slight compositional variation of pentlandite did not have a significant effect on the rest potential values as do changes in the type of sulfides (e.g. pyrite vs. pentlandite). This was confirmed by similar rest potential values of various pentlandite electrodes. The oxidation of synthetic pentlandite may be influenced by the chemical composition. In de-aerated solutions, anodic oxidation (as indicated by the linear anodic voltammogram) of synthetic pentlandite started at a potential lower than of the natural electrodes. In aerated solutions, the synthetic pentlandite was less reactive and formed thicker oxide films. Copyright / Dissertation (MEng)--University of Pretoria, 2010. / Materials Science and Metallurgical Engineering / unrestricted

This PhD project aimed to investigate the possibility of using Odorant Binding Proteins (OBPs) as sensing layers of chemical sensors, for the detection of organic compounds in both vapour and liquid phases. OBPs are small soluble proteins present in high concentrations in the olfactory system of vertebrates and insects. OBPs are attractive in the biosensor field since they can bind odorants and pheromones in a reversible way. They are resistant to high temperatures and protease activity and they can be easily expressed in large amounts. OBPs belonging to different species of mammals and insects were utilised for developing biosensors relied on different transduction mechanisms. Recombinant OBPs were grafted on the gold electrode of transducers by using Self-assembled monolayers (SAMs) of alkanethiols. The efficiency of the immobilisation method was proved by using electrochemical techniques. Quartz crystal microbalances (QCMs), screen-printed electrodes (SPEs) and interdigitated electrodes (IDEs) were employed for developing three types of OBP-based biosensors. I. QCMs functionalised with OBPs were tested against pheromones (i.e. bombykol and bombykal) and volatile compounds found in foodstuffs (i.e. pyrazine derivatives and geosmin) in vapour phase. The QCM based biosensors showed a good degree of selectivity and a detection limit of the order of parts per billion, in air. II. In liquid phase, impedimetric biosensors based on SPEs also showed a good selectivity and sensitivity being able to detect analyte concentrations of the order of 10-9 M. III. OBPs immobilised on the gold electrodes of IDEs were instead tested against S-(+) carvone vapour, proving that the binding activity of the proteins was preserved in vapour phase and can be quantified as variation of capacitance. The developed OBP biosensors showed good selectivity, sensitivity and stability over time in both liquid and vapour phase. The responses of the sensors were reversible, allowing to the device to be used several times. Moreover, the biosensors were label-free, hence the interaction between OBPs and ligand was directly detected without using auxiliary probes/species. With these findings, we envisage the use of our biosensors in several applications, including monitoring of the quality of food along the transportation and storage, controlling of pests and useful insects in agriculture, or as analytical devices for studying the dynamics in binding processes.

The diploma thesis deals with evaluation of electrochemical corrosion characteristics of Mg-Zn systems prepared by powder metallurgy in SBF solution. The main aim of the thesis is to analyze the influence of chemical composition, achieved structure and parameters of the process of production of Mg-Zn systems on their electrochemical corrosion characteristics. The basic electrochemical properties of the prepared materials will be evaluated by electrochemical impedance spectroscopy. In order to clarify the mechanism of corrosion of materials, the immersion tests will be used, accompanied by metallographic observations.

This master thesis deals with supercapacitors based on electrical double layer and proper carbon electrodes for this type of supercapacitors. In theoretical part of work is described theory of supercapacitors, energy storage principles and their properties. In the next part are described carbon materials, their properties and electrochemical methods of measurements that we used. In the experimental part is described preparation of electrodes, results and conclusion.

This work is focused on optimalisation of coposite materials for civil engineering. In the theoretical part of the project is introduction of the composite materials and materials which are used for their production. There are also concerned their properties and possible application areas. In conclusion of theoretical part this project is a summary of the possible use of composite materials. The practical part compares the characteristics of the different types of composites made in terms of conductivity and the rate of corrosion.

This work deals with the preparation of gold nanostructures for future usage in electrochemical sensors and biosensors, methods for their characterization and production. The emphasis is focused on the template-based electrodeposition method of gold and on study of the effect of manufacturing conditions on physical properties, mainly electrical and topological of nanostructures. Thesis is focused also on overall function and sensitivity of the gold nanostructured electrode.

The evaluation of concentration of ions and molecules with the help of biosensors have been regarded as an emerging technology. Bio and chemical sensors have a variety of applications in the field of medicine, military, environmental and food industries alike. With an estimated investment growth of over 4.31% in the development of pH sensors in the next five year, the objective of a developing a robust measurement system is all the more required. The scope of this research is to evaluate the ability of conducting polymer-based gel electrolytes for pH sensitivity, as a function of the transistor characteristics using an Extended Gate Field Effect Transistor or a conducting film in an electrochemical cell. Polymer gels were prepared by dissolving a suitable conducting polymer in an acidic media. The interaction of the gel with a buffer solution of known pH was collected as electric signals using a glassy carbon as an electrode. The electrochemical cell was further connected to the gate of a Metal-Oxide-Semiconductor Field Effect transistor (MOS-FET). The drain current was measured under two conditions; a) voltage across the gate (VGS) was kept constant, with varying voltage across the drain (VDS) and b) voltage across drain was fixed, while gate voltage changed. The drain current versus voltage of the transistor was plotted as a function of the ion interaction between the gel and the buffer. Different plots were recorded for different values of pH solutions. Final results were plotted to calculate the change in threshold voltage, for every change in pH of the observed solution. pH sensitivity of the gels was further tested through the Electrochemical Impedance Spectroscopy method, using a potentiostat and a three-electrode electrochemical cell. With a small excitation, the AC current flowing through the circuit at different frequencies were recorded and the plots discussed, to evaluate sensitivity to pH.

The goal of this study is to design and manufacture a microfluidic device capable of measuring changes in impedance valuesof microfluidic cell cultures. Tocharacterize this, an interdigitated array of electrodes was patterned over glass, where it was then bonded to a series of fluidic networks created in PDMS via soft lithography. The device measured ethanol impedance initially to show that values remain consistent over time. Impedance values of water and 1% wt. saltwater were compared to show that the device is able to detect changes in impedance, with up to a 60% reduction in electrical impedance in saltwater. Cells were introduced into the device, where changes in impedance were seen across multiple frequencies, indicating that the device is capable of detecting the presence of biologic elements within a system. Cell measurements were performed using NIH-3T3 fibroblasts.

Today there are numerous methods to slow down a corrosion process of metallic materials. However, due to environmental effects and health risk issues, several traditional corrosion inhibitors have to be phased out. Hence, it is of great importance to develop new corrosion inhibitors that are “green”, safe, smart and multifunctional. In this essay, the focus is on mussel adhesive protein (MAP) and its possibility to reduce the rate of the corrosion process. The protein exhibit great adhesive strength and protective properties, allowing it to adhere to a multitude of different surfaces and is therefore of great interest of corrosion science. The protein Mefp-1, derived from the blue mussel´s foot, had been pre-adsorbed on the carbon steel surface and provided good corrosion inhibition in a basic chloride solution for a short exposure time. The protection was further improved with the assist of iron and ceria ions by formation of protein/ions complexes within the surface films and thus enhanced the corrosion protection for longer exposure time. Ceria nanoparticles were used in order to create a multi-layer composite film with an even higher corrosion protection. The results suggest a denser film compared to previous samples and a more uniform surface.