Electro-organic reactions are often complicated by the need to add supporting electrolytes and co-solvents. In many cases these additives take part in side reactions causing low yields and hinder the purification stages. The solid polymer electrolyte (SPE) reactor uses an ion exchange membrane to transfer charged species between the electrodes and so eliminates the need for any additives. In this way improvements in electrochemical processing can be achieved. The SPE reactor has only been studied for model organic and aqueous based electrochemical reactions. The aims of this project were to develop the reactor for use as a suitable means of synthesising alcohols and acids based on substituted toluenes. This involved selection of suitable electrode material, polymer electrolyte pre-treatment and reactor modelling. According to published reports the direct electro-oxidation of toluene takes place with maximum yields of 19% with an acetic acid co-solvent and a nitric acid supporting electrolyte. Higher yields are possible with inorganic mediators such as Mn³⁺ and Cr⁶⁺. 30% yields of methoxylated products are possible from electrolysis in methanol although many non volatile by-products are formed. Initial research was spent investigating the oxidation of toluene in sulphuric acid at a lead dioxide rotating disk electrode. It was found that the reaction is mass transfer limited in the potential region below gas evolution. The order of reaction with respect to toluene was 0.5. Electrolysis of toluene on platinum mesh in nitric acid with and without acetic acid was found to produce benzyl alcohol and benzaldehyde with low current efficiencies. Without co-solvent the maximum current efficiency was 10% at 2S0Alm². An SPE reactor fabricated from glass with an active electrode area of Scm2 was used to perform electrode tests. Highest yields of benzaldehyde were obtained using nickel foam, graphite felt and palladium coated mesh electrodes. The current efficiencies were 52.4%, 20.3% and 10.7% respectively. This work highlighted the need for a good membrane-electrode contact. The oxidation of benzyl alcohol in the same reactor using nickel foam Abstract was accomplished with a current efficiency of 85.4% showing that the difficult step in the oxidation of toluene was the first one to benzyl alcohol. Pre-treatment of the membrane by swelling in solvents was considered to be an important factor in the performance of the SPE reactor. Several ion exchange membranes were pre-treated in a variety of aqueous and organic solvents including methanol, toluene, DMF, water and sulphuric acid. Nafion® 117 was found to increase in size more than the other tested membranes in all solvents except water and sulphuric acid. Many of the pre-treated membranes were tested in an SPE reactor made from steel with an active electrode area of 2lcm2 for the oxidation of toluene in methanol. The anode-membrane potential was measured as a function of time and current density with Nafion® 117 having the lowest values of potential. Selection of the pre-treatment method for future use was determined by assessing the performance in the reactor, contamination of products and chemical hazards. Swelling in aqueous solvents was the chosen procedure. The steel SPE reactor was operated in continuous mode with recycle for the oxidation of toluene in methanol. Galvanostatic electrolysis took place at several current densities, temperatures and feed concentrations. Two products were identified as ⍺-methoxytoluene and ⍺,⍺-dimethoxytoluene and these were formed at low current efficiencies between 1.4% and 9%. The main product was thought to be an oligomer of toluene. The gas generated was found to be mainly hydrogen with a small amount of oxygen thought to come from residual water in the pre-treated membrane. A computer simulation of the SPE reactor for toluene oxidation in methanol was based on two series and one parallel reaction. These were first order in reactant species and followed Tafel type kinetics. Mass transfer of dilute reactants was based on Fickian diffusion. Parameters not available in the literature such as membrane potential and electro-osmotic flow were correlated to applied variables using experimental data and multiple linear regression. The importance of electro-osmotic flow in the SPE reactor was demonstrated by considering its effect on product distribution. The model showed that the oligomerisation of toluene was the dominant reaction making the SPE reactor unsuitable for the oxidation of toluene.

Magister Scientiae - MSc / 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. / South Africa

As the global energy crisis and environmental pollution problem continues, there is an increasing demand for clean and sustainable energy storage and conversion technologies, such as water-splitting electrolysis. Water electrolysis is a process of running an electrical current through water in separating the hydrogen and oxygen. Oxygen evolution reaction (OER) is a key reaction in this electrochemical process, and the electrochemical performance of these systems is usually hindered by the slow OER reaction kinetics. In order to achieve high energy conversion efficiency, the development of efficient OER catalysts is the key. To achieve that, abundant research is done by using noble metal oxides as catalyst, such as IrO2 and RuO2. However, considering their high cost, a cheap earth-abundant material with a high OER catalytic activity is required. Accordingly, this study has been focused on the synthesis of three dimensional porous structured Ni-based OER catalysts. First, a 3D porous Ni meso-foam was developed through a facile high-temperature one-pot synthesis method, and its catalytic activity towards OER was explored. Specifically, the as-synthesized Ni meso-foam material, referred to as raw NMF, has a wire-linked structure and high surface area. A reduction procedure was introduced to obtain reduced Ni meso-foam materials, referred to as NMF-H2. It was also oxidized in air at 600 ℃ to form a semi-hollow NiO crosslinking phase and subsequently reduced in H2 at 300℃, forming a regenerated porous Ni foam material, referred to as NMF-O2/H2. The composition and morphology of all materials were investigated by XRD and SEM, respectively. The SEM image reveals that, in the porous NMF-O2/H2, the cross-linked meso-wire structure was maintained, and the average pore size is between 0.5-5 μm. Electrochemical analysis show that the OER activity of the Ni foam catalysts follows NMF-O2/H2 > NMF-H2 > raw NMF. In addition to the NMF-based materials, a Ni/Ni(OH)2 layer-structured electrocatalyst, referred to as NiDHBT, was also developed using a dynamic hydrogen bubble templating (DHBT) method. First, the 3D-porous micro Ni/Zn nanoplatelets were constructed in a two-step DHBT deposition method. The Ni/Zn foil was used as a scaffold, featured with the open porous structure and high surface area, for the subsequent electrodeposition of Ni(OH)2. Then, the Zn was etched from the as-prepared Ni/Zn/Ni(OH)2 nanocomposite to obtain the NiDHBT. The catalytic performance of the NiDHBT toward OER reaction was evaluated, and the optimal catalysts developed from different electro deposition potentials were determined. On the recognition of the high catalytic activity of NMF-O2/H2 and NiDHBT, porous structured FeOx-Nickel meso-foam, referred to as Fe@NMF-O2/H2, and FeOx- Ni/Ni(OH)2 layered-structure materials, referred to as Fe@NiDHBT, was further developed to explore the benefits of FeOx deposition for its OER catalytic performance. The deposition of FeOx is achieved by physical mixing FeOx colloid with NMF-O2/H2 and NiDHBT, and the electrochemical performance of these materials was examined in 1 M KOH. Among the developed materials, the best performing catalyst is Fe@NiDHBT synthesized by loading FeOx colloid onto the NiDHBT support. The overpotential for Fe@NiDHBT to reach 10 mA·cm-2 is 247mV, and the corresponding Tafel slope is 48.10mV·dec-1. Therefore, it was concluded that the FeOx¬¬ loading modification is an effective strategy to improve the OER activity of Ni foam-based catalysts.

As atmospheric carbon dioxide (CO2) levels continue to rise due to anthropogenic fossil fuel utilization, the need to develop and employ alternative energy carriers becomes more and more critical. In recent years, interest in hydrogen (H2) has significantly increased as it is a clean and sustainable, alternative fuel, which can be both produced and utilized without greenhouse gas emissions; H2 can be produced via water electrolysis powered by renewable energy sources (RES), such as wind and solar energy, then, H2 can be utilized as a fuel in a hydrogen fuel cell, emitting only water as a by-product. Not only is H2 a clean alternative fuel, but it also provides an economically feasible way of storing renewable energy so that RES supply can be better regulated according to demand. Of the existing water electrolysis technologies, not many offer the ability to produce hydrogen both efficiently and at low cost. The alkaline environment of the more commonly employed traditional alkaline electrolyser allows for the use of non-noble metal electrocatalysts, as well as inexpensive cell materials. This process however suffers from an inefficient cell design. Conversely, the proton exchange membrane water electrolyser (PEMWE) utilizes a solid polymer electrolyte membrane, which allows for a compact, low resistance cell design. However, the harsh acidic environment of this device requires expensive platinum group metal (PGM) catalysts and expensive cell components. Anion exchange membrane water electrolysis (AEMWE) is a promising technology for low-cost, efficient H2 production as it combines the compact cell design of the PEMWE with the favourable alkaline environment of the traditional alkaline electrolyser. The electrochemical water splitting process is limited by the kinetically unfavourable oxygen evolution half-cell reaction (OER), which requires expensive rare catalysts such as iridium, to efficiently carry out the reaction. Nickel (Ni) is a promising inexpensive and abundant catalyst for the OER in alkaline media, due to its high activity and corrosion resistance. A significant increase in OER activity can be achieved by iron (Fe) incorporation into Ni catalysts. The addition of ceria (CeO2), a mixed ionic-electronic conductor with favourable oxygen storage and release properties, can also have a positive effect on catalytic performance. While developing electrocatalysts for improved OER performance is important, evaluating the studied materials as anodes in practical AEMWE devices is imperative as it accounts for the efficiency of the catalysts in electrode layers formed using an anion exchange ionomer (AEI). An AEI is a solid polymer electrolyte that serves as a binder for the particles as well as a hydroxide ion conductor in a catalytic layer of an AEMWE. The main objectives of this thesis are to (i) develop highly active NiFe-based nanoparticle (NP) catalysts with and without CeO2 for the promotion of the OER in AEMWE devices, and (ii) study the effects of commercial AEI type and amount on the efficiency of the produced NiFe-based particles in AEMWE anodes. These objectives will help further understand the behaviour of Ni-based catalysts in AEMWE systems, as well as the effects that catalyst-ionomer interactions can have on anode efficiency in carrying out the OER. The nanoparticles developed in this work were synthesized by an easily scalable chemical reduction method in ethanol using sodium borohydride. Results show that Ni NPs, which are around 4-6 nm in size, with 10 and 20 at% Fe, provide the highest OER performance. Incorporating small amounts of CeO2 into the NiFe materials results in better charge and mass transfer of the catalysts, however it introduces an additional ohmic resistance, which prevails over any OER-promoting interactions between NiFe and CeO2. The best NiFe-based catalysts with and without CeO2 were evaluated as anodes in a single cell AEMWE in combination with the commercial Fumatech Fumion® ionomer as well as the commercial Ionomr Innovations AemionTM ionomer. The single-cell AEMWE analysis of the various catalytic layers shows that Ni90Fe10 with 15 wt% Fumion® shows the best catalytic performance of 1.72 V at 0.8 A cm-2 in 1 M potassium hydroxide (KOH) at 50°C. Ni90Fe10 is also the most stable under operating conditions in comparison to the other tested Ni-based materials. While it was found that using 7 wt% AemionTM provided similar catalytic activity to 15 wt% Fumion®, results show that the AemionTM ionomer interacts with NiFe to inhibit the formation of NiOOH, the OER active phase. The results of this work highlight the complex interactions between Ni-based nanoparticles and anion exchange ionomers towards the OER and provide possible directions for future research and development in high performing Ni-based anodes for AEMWE.

This thesis aimed to study some of the general principles underlying electrodeposition together with experimental facts regarding the effects of changing constituents of the plating solutions, variations in hydrogen-ion concentrations, and variation in current density used in the electrodeposition of lead.

No description available.

published_or_final_version / abstract / Mechanical Engineering / Doctoral / Doctor of Philosophy

<p>This study has shown that OH˙ radicals can be generated in an Fe/O2 cell from the electrode products via Fenton&rsquo / s reaction and used for water disinfection. The cell system in which the experiments were carried out was open and undivided and contained two electrodes with iron (Fe) as the anode and oxygen (O2) gas diffusion electrode. Typically, 100 ml of Na2SO4.10H2O (0.5M) solution was used as a background electrolyte. OH˙ radicals were produced in-situ in an acidic solution aqueous by oxidation of iron (II), formed by dissolving of the anode, with hydrogen peroxide (H2O2). The H2O2 was electrogenerated by reduction of oxygen using porous reticulated vitreous carbon (RVC) as a catalyst.</p>

<p>A comparative study of electrolysers using different anodic materials for the electrolysis of brine (sodium chloride) for the production of sodium hypochlorite as a source of available chlorine for disinfection of rural water supplies has been undertaken. The electrolyser design used was tubular in form, having two chambers i.e. anode inside and cathode outside, separated by a tubular inorganic ceramic membrane. The anode was made of titanium rod coated with a thin layer of platinum and a further coat of metal oxide. The cathode was made of stainless steel wire. An assessment of these electrolysers was undertaken by studying the effects of some variable parameters i.e.current, voltage and sodium chloride concentration. The cobalt electrolyser has been shown to be superior as compared to the ruthenium dioxide and manganese dioxide electrolysers in terms of hypochlorite generation. Analysis of hydroxyl radicals was undertaken since there were claims that these are produced during brine electrolysis. Hydroxyl radical analysis was not successful, since sodium hypochlorite and hypochlorous acid interfere using the analytical method described in this study.</p>

<p>Hydrogen has been called the fuel of the future, and as it’s non- renewable counterparts become scarce the economic viability of hydrogen gains traction. The potential of hydrogen is marked by its high mass specific energy density and wide applicability as a fuel in fuel cell vehicles and homes. However hydrogen’s volume must be reduced via pressurization or liquefaction in order to make it more transportable and volume efficient. Currently the vast majority of industrially produced hydrogen comes from steam reforming of natural gas. This practice yields low-pressure gas which must then be compressed at considerable cost and uses fossil fuels as a feedstock leaving behind harmful CO and CO2 gases as a by-product. The second method used by industry to produce hydrogen gas is low pressure electrolysis. In comparison the electrolysis of water at low pressure can produce pure hydrogen and oxygen gas with no harmful by-products using only water as a feedstock, but it will still need to be compressed before use. Multiple theoretical works agree that high pressure electrolysis could reduce the energy losses due to product gas compression. However these works openly admit that their projected gains are purely theoretical and ignore the practical limitations and resistances of a real life high pressure system. The goal of this work is to experimentally confirm the proposed thermodynamic gains of ultra-high pressure electrolysis in alkaline solution and characterize the behavior of a real life high pressure system.</p> / Dissertation