A dissertation submitted to the Faculty of Engineering, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Master of Science in Engineering Johannesburg, 1994 / A quantitative description of galvanic interactions between sulphide minerals based on thermodynamic and kinetic parameters has been developed. The basis for quantitative description involves conducting a voltage balance over the galvanic couple. The contributions to the voltage balance include the galvanic couple cell emf, kinetic descriptions of the anodic and cathodic half reactions, the voltage characteristics 'of mineral-mineral contacts and solution voltage losses. The rates of the anodic and cathodic half' reactions were modelled by the Butler-Volmer equation and ti1ediffusion equation. A potentiostat was used to vary the voltages losses across mineral-mineral contacts. TIle galvanic couples were constructed. as rotating ring disc electrodes and hence electrolyte voltage losses were negligible. Three galvanic couples, copper-platinum, copper-pyrite and galena-pyrite, were electrochemically characterised under different conditions of ferric concentration, electrode rotation rate and temperature. The effect of illumination on the anodic dissolution of galena was investigated. The electrochemical model is in good agreement with experimentally measured galvanic currents. Galvanic interaction is a dynamic function and various models are developed which account for dynamic behaviour in galvanic cells. / MT2017

Graduation date: 1999

Thesis (Ph. D.)--Columbia University, 1926. / Page 53 wrongly numbered 3. Vita.

Thesis (M.S.)--Ohio University, March, 2004. / Includes bibliographical references (leaves 69-71).

Thesis (Ph. D.)--Columbia University, 1926. / Page 53 wrongly numbered 3. Vita.

Thesis (M.Sc.(Chemistry))--University of Pretoria, 2001. / Summaries in Afrikaans and English.

Thesis (M.S.)--Ohio University, March, 2004. / Title from PDF t.p. Includes bibliographical references (leaves 69-71)

Conducting diamond electrodes provide unique advantages for electrochemistry such as a wide potential window, low baseline current, chemical inertness and resistance to fouling. De Beers boron-doped diamond electrodes, manufactured by chemical vapour deposition and containing varying amounts of boron, were therefore investigated in order to determine their suitability for future electrochemical applications. These electrodes were initially characterised using techniques such as SEM, LA-ICP-MS, Raman spectroscopy and XPS. The electrochemical behaviour of these electrodes was investigated in two redox systems (potassium iron (III) cyanide and cerium (III) sulphate) and two biological systems (dopamine and ascorbic acid). These results were compared against that of the conventional glassy carbon electrode. Porous boron-doped diamond, a novel electrode material, was used for the electrochemical detection of thyroid hormones (L-T3 and L-T4). These hormones have never previously been investigated using a boron-doped diamond electrode. The De Beers boron-doped diamond electrode was found to outperform the conventional glassy carbon electrode, which fouled very easily, in the detection of dopamine. Peak separation between dopamine and the interfering ascorbic acid was attained at a pretreated boron-doped diamond electrode. The feasibility of detecting thyroid hormones using a porous boron-doped diamond electrode was demonstrated, and the electrode material was patented. / Dissertation (MSc (Chemistry))--University of Pretoria, 2006. / Chemistry / unrestricted

This investigation was conducted to attempt to find an electrolyte that would be suitable for the electrodeposition of boron and to duplicate work performed by previous investigators. Methods for the electrochemical production of boron previously investigated have either yield a product too impure for practical use, or have involved such serious operating difficulties as to be entirely impractical as commercial processes. In the present investigation, the systems 4.5KCl-KBF₄-6B₂O₃ and 8.5KCl-KBF₄ were investigated at 800-850°C in an attempt to duplicate the methods reported in the literature as suitable for use as commercial processes. When the electrolyte 4.5KCl-KBF₄-6B₂O₃ was used, boron, 91.2 per cent pure, was electrodeposited at a cathode current density of 0.83 amperes per square centimeter. When the electrolyte 8.5KCl-KBF₄ was used, boron, 96 per cent pure, was electrodeposited at a cathode current density of 1.41 amperes per square centimeter. The system 5Na₂O-1-LA₂O-6B₂O₃ was investigated at 650 ± 5°C. The deposit obtained proved to contain 68 per cent carbon and only a trace of boron when a cathode current density of 1.04 amperes per square centimeter was employed. The melt attacked the graphite crucible's binder and in no way showed any promise as an electrolyte for commercial use in the production of elemental boron. The systems 3K₂O-KBF₄-B₂O₃ and 2K₂O-2KBF₄-B₂O₃ were also investigated. The melts alkaline in nature and attacked the graphite crucible's binder. The product obtained when the 3K₂O-KBF₄-B₂O₃ system was investigated at 850 ± 5°C proved to contain 78 per cent boron and 13 per cent carbon. The system 2K₂O-2KBF₄-B₂O₃ was studied to determine the effect of decreasing the alkalinity of the bath. The product obtained when this mixture was studied at 850 ± 5°C proved to contain 85 per cent boron and 6 per cent carbon. / Master of Science

Batteries are complex, multidisciplinary, electrochemical energy storage systems that are crucial for powering our society. During operation, all battery technologies suffer from voltage losses due to energetic penalties associated with the electrochemical processes (i.e., ohmic resistance, kinetic barriers, and mass transport limitations). A majority of the voltage losses can be attributed to processes occurring on/in the battery electrodes, which are responsible for facilitating the electrochemical reactions. A major challenge in the battery field is developing strategies to mitigate these losses. To accomplish this, researchers must i) identify the processes limiting the performance of the electrode, ii) characterize the main, performance-limiting processes to understand the underlying mechanisms responsible for the poor performance, and iii) mitigate the voltage losses by developing strategies which target these underlying mechanisms. In this thesis, three studies are presented which highlight the role of electrochemical engineers in alleviating the performance limiting processes in battery electrodes. Each study is focused on a different step of the research approach (i.e., identification, characterization, and mitigation) and analyzes an electrode from a different battery system. The first part of the thesis is focused on identifying the processes limiting the capacity in nanocomposite lithium-magnetite electrodes. To accomplish this, the mass transport processes and phase changes occurring within magnetite electrodes during discharge and voltage recovery are investigated using a combined experimental and modeling approach. First, voltage recovery data are analyzed through a comparison of the mass transport time-constants associated with different length-scales in the electrode. The long voltage recovery times are hypothesized to result from the relaxation of concentration profiles on the mesoscale, which consists of the agglomerate and crystallite length-scales. The hypothesis was tested through the development of a multi-scale mathematical model. Using the model, experimental discharge and voltage recovery data are compared to three sets of simulations, which incorporate crystal-only, agglomerate-only, or multi-scale transport effects. The results of the study indicate that, depending on the crystal size, the low utilization of the active material (i.e., low capacity) is caused by transport limitations on the agglomerate and/or crystal length-scales. For electrodes composed of small crystals (6 and 8 nm diameters), it is concluded that the transport limitations in the agglomerate are primarily responsible for the long voltage recovery times and low utilization of the active material. In the electrodes composed of large crystals (32 nm diameter), the slow voltage recovery is attributed to transport limitations on both the agglomerate and crystal length-scales. Next, the multi-scale model is further expanded to study the phase changes occurring in magnetite during lithiation and voltage recovery experiments. Phase changes are described using kinetic expressions based on the Avrami theory for nucleation and growth. Simulated results indicate that the slow, linear voltage change observed at long times during the voltage recovery experiments can be attributed to a slow phase change from α¬-LixFe3O4 to β¬-Li4Fe3O4. In addition, simulations for the lithiation of 6 and 32 nm Fe3O4 suggest the rate of conversion from α¬-LixFe3O4 to γ-(4 Li2O + 3 Fe) decreases with decreasing crystal size. The next part of the thesis presents a study aimed at characterizing the formation of PbSO4 films on Pb in H2SO4, which has been previously identified as a performance-limiting process in lead-acid batteries. Transmission X-ray microscopy (TXM) is utilized to monitor, in real time, the initial formation, the resulting passivation, and the subsequent reduction of the PbSO4 film. It is concluded with support from quartz-crystal-microbalance experiments that the initial formation of PbSO4 crystals occurs as a result of acidic corrosion. Additionally, the film is shown to coalesce during the early stages of galvanostatic oxidation and to passivate as a result of morphological changes in the existing film. Finally, it is observed that the passivation process results in the formation of large PbSO4 crystals with low area-to-volume ratios, which are difficult to reduce under both galvanostatic and potentiostatic conditions. In a further extension of this study, TXM and scanning electron microscopy are combined to investigate the effects of sodium lignosulfonate on the PbSO4 formation and the initial growth of PbSO4 crystals. Sodium lignosulfonate is shown to retard, on average, the growth of the PbSO4 crystals, yielding a film with smaller crystals and higher crystal densities. In addition, an analysis of the growth rates of individual, large crystals showed an initial rapid growth which declined as the PbSO4 surface coverage increased. It was concluded that the increase in PbSO4 provides additional sites for precipitation and reduces the precipitation rate on the existing crystals. Finally, the potential-time transient at the beginning of oxidation is suggested to result from the relaxation of a supersaturated solution and the development of a PbSO4 film with increasing resistance. The final part of the thesis presents a study aimed at mitigating the ohmic losses during pulse-power discharge of a battery by the adding a second electrochemically active material to the electrode. Porous electrode theory is used to conduct case studies for when the addition of a second active material can improve the pulse-power performance. Case studies are conducted for the positive electrode of a sodium metal-halide battery and the graphite negative electrode of a lithium-ion battery. The replacement of a fraction of the nickel chloride capacity with iron chloride in a sodium metal-halide electrode and the replacement of a fraction of the graphite capacity with carbon black in a lithium-ion negative electrode were both predicted to increase the maximum pulse power by up to 40%. In general, whether or not a second electrochemically active material increases the pulse power depends on the relative importance of ohmic-to-charge transfer resistances within the porous structure, the capacity fraction of the second electrochemically active material, and the kinetic and thermodynamic parameters of the two active materials.