Browsing by Author "Rampai, Tokoloho"

Now showing 1 - 13 of 13
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    Antarctic sea ice phytoplankton growth rates and survival mechanisms
    (2025) Kumadiro, Lisa; Rampai, Tokoloho; Fawcett, Sarah; Fietz, Susanne
    Phytoplankton play an important role in the Southern Ocean food web being the primary producers of food, particularly in winter, and partaking in the uptake of CO2 from the atmosphere via photosynthesis. Despite being photosynthetic organisms, phytoplankton survive at the bottom of sea ice where there is very little irradiance for up to 6 months. Sea ice phytoplankton are understudied. This is mainly because in situ studies on sea ice are not only expensive but logistically difficult. Some researchers have elected to bring sea ice phytoplankton from the Southern Ocean to land-based facilities. This has seen some logistical difficulties as it meant either changing the habitat phytoplankton would have been for transportation, thus changing the species originally found in the Southern Ocean or transporting phytoplankton in ice cores and losing species due to brine drainage or osmotic stress from temperature changes in the core. The objectives of this study were to optimize a previously designed hybrid tank for the purpose of obtaining and preserving phytoplankton species from the Marginal Ice Zone of the Southern Ocean to land-based facilities. The study also included design of an environmental chamber to be used for housing phytoplankton obtained during experimentation. Responses to temperature and irradiance variation on phytoplankton from the Marginal Ice Zone of the Southern Ocean were then evaluated using the designed environmental chamber. The solid-liquid hybrid system known as the hybrid tank was successfully optimized by reducing the size of the tank, adding irradiation to the tank, and making improvements to the sampling protocol. The tank was used to obtain ice cores from the Southern Ocean to the University of Cape Town in winter 2022. Post the winter cruise one hybrid tank sample was melted, and microscopic analysis conducted on the sample. In comparison with transportation of phytoplankton in a solid core and in a liquid melt in the dark, the hybrid tank resulted in an increase in phytoplankton cell concentration. Furthermore, the optimized hybrid tank improved preservation of species transported when compared to the initial tank. A desktop environmental chamber made from Perspex and insulated with polystyrene was successfully designed. The environmental chamber offers temperature and irradiation control by making use of a cold plate attached to a chiller and an LED light. Experiments conducted on the diatom species revealed that all the sea ice species were shade adaptive being photo inhibited at irradiances beyond 42μmolm-2s-1 with the exception of Navicula spp, Cylindrotheca closterium and the unidentified pennates. The diatom species also preferred warmer environments i.e., 8°C to 5°C.
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    Design and construction of a motor-actuated sea ice compression testing machine
    (2025) Keche, Tamuka; Rampai, Tokoloho; Govender, Reuben Ashley
    This thesis investigates the mechanical properties of sea ice in the Southern Ocean's Marginal Ice Zone, with the main emphasis on developing a specialized sea ice compression testing machine. The machine, designed for harsh sub-zero and saline conditions, would facilitate the immediate testing of ice samples onboard the SA Agulhas II vessel. The research entailed designing and constructing a compression machine capable of exerting up to 50 kN of force and accommodating strain rates critical for studying sea ice compressive strength. Field tests were conducted using ice cores collected from the MIZ consolidated ice. Compression tests were performed at strain rates around 4 x 10-4/s, which falls within the ductile-to-brittle transition zone, where maximum ice compressive strength is expected. The tested samples exhibited ductile failure modes in ice cores, with maximum stress observed before a gradual stress relaxation. Notably, the maximum stress observed was 6.91 MPa, aligning with established literature values for the compressive strength of sea ice. Additionally, the thesis provided a detailed analysis of the elastic moduli of sea ice. Despite the machine's slight deformation under load, the results indicated elastic moduli within the literature-reported range for sea ice. This data is crucial for understanding sea ice's elastic properties under varying environmental conditions. In conclusion, the newly designed sea ice compression machine functioned effectively at - 10°C, withstanding up to 44 kN of force and testing across a range of strain rates. Despite minor elastic deformation, it accurately measured ice strength, aligning with established data and proving its robustness for future research. Recommendations for future research include enhancements to the machine's design for improved precision and adaptability in testing. The findings suggest potential areas for further investigation in sea ice mechanics, emphasizing the importance of continued exploration for broader climate science implications.
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    Design of a Small-Scale System for the Growth of Artificial Sea Ice
    (2021) Hall, Benjamin Andrew; Rampai, Tokoloho
    Sea ice plays a significant role in global climate systems, reflecting a significant portion of solar energy back into the atmosphere and maintaining ocean circulation currents. The effect of climate change on sea ice extent and seasonal changes is as yet unquantified. This is especially true for the initial growth processes and properties within the Antarctic Marginal Ice Zone (MIZ) front during the winter growth season. The Polar Engineering Research Group (PERG) at the University of Cape Town has conducted several research expeditions to the Antarctic MIZ along the 0° line of longitude, collecting samples of first year sea ice. Artificial sea ice has been used as a supplementary area of study because of the advanced control it provides over variables such as cooling rate or initial solution salinity. This allows for the effect of individual variables to be analysed through repeated experiments while adjusting only the variable of interest. Due to the complex nature and conditions of formation for Antarctic sea ice, this study focusses on the key properties of sea ice formed in predominantly calm conditions. These are observed as vertically elongated ice crystals with a c-axis located randomly within the horizontal plane. The profile of ice thickness over time displays a √ x shape. Brine inclusions are located in vertically orientated, interconnected channels, contained within the intracrystalline planes. The crystal planes have spacings of about 1 mm. Lastly, the salinity profile of the ice displays a characteristic c-shaped curve with depth, with higher values of salinity found at the top and bottom of the ice. Ice fitting this description is referred to as columnar S2 ice. The overall aim of this project is to design and test a small-scale system for the growth of artificial sea ice. This system will still enable method development of testing protocols for the testing of the Antarctic sea ice. Once this system has proven to reliably produce saline ice that can be termed as artificial sea ice with a columnar S2 structure, additional design implementations can then be undertaken to accomplish the growth of sea ice that more closely resembles the ice found in the Antarctic MIZ. The system is required to be large enough to produce samples of appropriate size and number to fit the testing protocols for mechanical testing set out by Schwarz et al. (1981), while being statistically sound. Secondary design objectives were to ensure the system is costeffective, portable and simple. A proof of design concept experiment, consisting of a 28 g kg−1 saline solution cooled at at a temperature of - 20 °C, was carried out in order to test the system design. The hypothesis is that the system design will be able to produce saline ice with properties similar to natural sea ice. Temperature profiles and ice growth within the tank were recorded, and ice samples were taken at the end of the run to determine in-ice salinity and crystal morphology. With some refinement of the system to identify the cause of the extended granular and transition layer, the system can be used to provide the necessary test samples for method development for the mechanical testing of sea ice samples collected from the Antarctic MIZ. Following on from this initial design, additional design implementations can be undertaken to accomplish the growth of sea ice that more closely resembles the ice found in the Antarctic MIZ. This will aid in the determination of sea ice properties and studies of the underlying growth processes that cause them.
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    DFT Study of MAX Phase Surfaces for Electrocatalyst Support Materials in Hydrogen Fuel Cells
    (2020-12-25) Gertzen, Jonathan; Levecque, Pieter; Rampai, Tokoloho; van Heerden, Tracey
    In moving towards a greener global energy supply, hydrogen fuel cells are expected to play an increasingly significant role. New catalyst support materials are being sought with increased durability. MAX phases show promise as support materials due to their unique properties. The layered structure gives rise to various potential (001) surfaces. DFT is used to determine the most stable (001) surface terminations of Ti2AlC, Ti3AlC2 and Ti3SiC2. The electrical resistivities calculated using BoltzTraP2 show good agreement with the experimental values, with resistivities of 0.460 µΩ m for Ti2AlC, 0.370 µΩ m for Ti3AlC2 and 0.268 µΩ m for Ti3SiC2. Surfaces with Al or Si at the surface and the corresponding Ti surface show the lowest cleavage energy of the different (001) surfaces. MAX phases could therefore be used as electrocatalyst support materials, with Ti3SiC2 showing the greatest potential.
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    Exploring the effects of single and dual phase culturing on the concentrations of Southern Ocean sea-ice algae and transporting living sea-ice algae from the Southern Ocean to land-based research facilities
    (2022) Hambrock, Mark; Rampai, Tokoloho; Walker, David
    Sea ice is a complex material with a significant impact on the global climate. Understanding the development of sea-ice properties based on the change in growth conditions is vital for the development of predictive models, which are key to providing forecasts of the influence global warming will have on sea ice. Algae found in sea ice form an important part of the oceanic food network, providing secondary producers with a source of food, particularly during winter, and are suspected of seeding algae blooms during spring and summer. Researching sea ice and sea-ice algae in situ is an expensive and logistically difficult undertaking, especially in the Southern Ocean. Consequently, many researchers elect to perform research on artificial sea ice, where conditions are more controlled while logistical and financial constraints are reduced. Despite the extent to which sea-ice research has been performed on artificial sea ice, relatively little research on sea-ice algae in artificial sea ice has been done. Sea-ice algae are strongly affected by the temperature, salinity, nutrient availability and intensity of photosynthetically active radiation in their environment. This makes the transportation of living sea-ice algae difficult. Little documentation of transportation of living sea-ice algae exists, with most of the laboratory research of sea-ice algae being performed on single-species liquid cultures. Such research is important but fails to address the complexity of real sea-ice algae communities. This dissertation investigates the effects of three sea-ice algae transportation methods on the concentration development of sea-ice algae, as well as the potential for experimentation with the algae transported with these methods. Two methods were adapted from literature: transportation in solid (1) and liquid (2) environments. In addition to these methods, a third method was explored: Transportation of living sea-ice algae in a hybrid solid-liquid system. The aim of transporting in the hybrid system was to minimise the changes from the natural to the artificial environment. Solid sea-ice storage was evaluated by means of an artificial sea-ice study: Artificial sea ice was grown, extracted and stored at -20 °C for 7 different durations, between 0 minutes and 35 weeks. Samples were segmented into 20 mm thick slices, melted, analysed for salinity and brine profile development was assessed. It was found that storage significantly impacted brine profiles, causing an average bulk desalination of 19% between samples stored for 1 day and 35 weeks, as well as a change in the shape of the salinity profile from a W to a C shape. Solid sea-ice transportation was thus eliminated as a transportation method for this work due to the high change algae communities would likely undergo during due to desalination and unfavourable environmental conditions associated with low temperatures. A solid-liquid hybrid system for the transportation of sea-ice algae was designed and constructed, consisting of a 30 litre Perspex tank, insulation, and a heating system. Two sea-ice cores were obtained from the Southern Ocean in winter of 2019 and transported in the hybrid system to land facilities. Issues with the system were identified, dedicated lights added, and an additional hybrid tank constructed. Six sea-ice cores were obtained from the Southern Ocean in spring of 2019 and parts of them transported in the hybrid tanks to land facilities, where they were melted and cultured in a liquid environment for 56 days. Sections of the cores were melted, and the algae preserved before transportation. Algae concentrations were determined via microscopy of preserved samples taken before transport, after transport and after liquid culturing. Taxonomic distributions of algae varied greatly between initial samples and concentrations ranged from 46 000 to 1 200 000 cells per ml. The hybrid transportation method caused the lowest degree of change in the community compositions and increased the overall concentrations of algae, whilst liquid incubation mostly decreased algae concentrations.
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    Factors Influencing the Morphology of Sea Ice
    (2023) Matlakala, Boitumelo; Rampai, Tokoloho
    Due to the continuous melting of sea ice and rising sea levels, more studies are conducted on sea ice morphology and factors influencing the growth of ice. Sea ice is an integral part of the global climate system. It plays a vital role in the polar ecosystem, providing a habitat for organisms. Sea ice growth and behaviour has been observed to be largely affected by climate change and global warming. However, the consequences thereof, on sea ice extent and seasonal changes are still being studied. Artificial sea ice experiments have been used as they offer an advantage of control and help isolate variables during sea ice growth. Additionally, in-situ experiments are expensive, and can present logistical difficulty for measuring these variables long term. Factors such as growth dynamics, crystal texture and brine inclusions were investigated by variation of ambient temperature, starting artificial ocean salinity, reactor volume and the presence of microorganism' secretions: extracellular polymeric substances (EPS). Salinities of 10 psu, 20 psu, and 30 psu, were used at the following temperatures: -20°C, -10°C and -5°C. The temperature and salinity data showed that the growth rate of ice, increases with the decreasing ambient temperature, decreasing starting artificial sea ice salinity, decreasing reactor volume, as well as in the presence of microorganisms' secretions. The cross-polarisation results revealed a decreasing percentage of granular texture with increasing starting artificial ocean salinity and increasing ambient temperature. Similarly, the same trend was observed for increasing reactor volume. In the presence of microorganisms, however, a blotchy granular and a disordered columnar texture were observed. An increase in artificial sea ice porosity due to brine inclusions was revealed by the micro-ct scanning data for an increasing starting artificial ocean salinity. Furthermore, overall porosity increased with decreasing ambient temperature and in the presence of microorganism secretion.
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    Frazil Ice in the Antarctic Marginal Ice Zone
    (2021-06-10) Paul, Felix; Mielke, Tommy; Schwarz, Carina; Schröder, Jörg; Rampai, Tokoloho; Skatulla, Sebastian; Audh, Riesna R; Hepworth, Ehlke; Vichi, Marcello; Lupascu, Doru C
    Frazil ice, consisting of loose disc-shaped ice crystals, is the first ice that forms in the annual cycle in the marginal ice zone (MIZ) of the Antarctic. A sufficient number of frazil ice crystals form the surface “grease ice” layer, playing a fundamental role in the freezing processes in the MIZ. As soon as the ocean waves are sufficiently damped by a frazil ice cover, a closed ice cover can form. In this article, we investigate the rheological properties of frazil ice, which has a crucial influence on the growth of sea ice in the MIZ. An in situ test setup for measuring temperature and rheological properties was developed. Frazil ice shows shear thinning flow behavior. The presented measurements enable real-data-founded modelling of the annual ice cycle in the MIZ.
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    Investigating the air and liquid porosity of sea ice
    (2025) Swait, Hayley; Rampai, Tokoloho
    Micro-scale sea ice properties have cascading effects onto larger scale climate and ocean systems. Brine and air inclusions are influenced by the growth conditions, which are highly dynamic in the Antarctic Marginal Ice Zone (MIZ) compared to the calmer Arctic. There are varying depths within an Antarctic pancake floe due to these dynamic conditions, yet the spatial variability of brine and air porosity profiles has not been previously investigated. Additionally, the sea ice brine phase is greatly influenced by temperature, yet similarly there are limited studies on the impact of temperature storage and heating methods on brine inclusions. Understanding the impacts of environmental and storage conditions is essential to ensure accurate and representative analysis of sea ice's thermally responsive properties. In this study, non-destructive X-ray computer tomography (CT) analysis was used to investigate the brine and air inclusions within pancake ice from the Antarctic MIZ during winter 2019 and 2022 expeditions. Sampling protocols from the 2019 expedition were optimised during the 2022 expedition to store samples at the CT analysis temperature (-10°C) to minimise the impact of low temperature (-18°C) storage on these inclusions. Investigating the impacts of low temperature storage and heating methods on these inclusions was then conducted through uni- (UD) and multidirectional (MD) heating samples from -18°C to -10°C. This study showed on average reductions of 41% in the brine porosity of sea ice after being subjected to low temperature storage, unlike air porosity that showed no significant changes. However, there was no significant difference between the UD and MD heating methods. The number of brine inclusions increased after low temperature storage. The brine inclusions showed a decrease in sphericity in samples subjected to low temperature storage conditions. Air showed slight increases in the number of air inclusions in the sections in the upper region of the ice while the lower region showed a decrease in air number densities, potentially attributed to the gas saturation factor within the sea ice. In addition to the storage conditions influencing the inclusions, the spatial variability of coring locations within a pancake floe has also shown to have slight variation in the porosity profiles within sea ice samples. This study disqualifies the general assumption that sea ice porosity is not affected by low temperature storage conditions and therefore the storage conditions are of paramount importance in studying the microstructural properties of sea ice. Maintaining sea ice samples at higher analysis temperatures that is closer to in situ temperatures will minimize the effects of low temperature storage on the sea ice inclusions.
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    MAX phases as an electrocatalyst support material: a DFT study
    (2019) Gertzen, Jonathan; Rampai, Tokoloho; Van Heerden, Tracey; Levecque, Pieter
    The insatiable global demand for energy cannot be sustained by fossil fuels without irreparable damage to the environment. Various alternative energy sources are being investigated to provide renewable clean energy. One promising technology is the hydrogen fuel cell, which uses hydrogen and oxygen to produce electricity. However, the currently used catalyst support material, carbon black, corrodes in the low pH and oxidative environment. Therefore, new catalyst support materials are being sought. A new class of material, called MAX phases, shows potential because some possess a combination of properties of metals and ceramics. Three of them, Ti2AlC, Ti3AlC2, and Ti3SiC2, show good electrical conductivity and oxidation resistance. These MAX phases have been investigated using density functional theory (DFT) in this thesis to determine their properties. The density of states show that they are electrically conductive, with a continuous band over the Fermi level primarily from the Ti d orbital. Calculating the Boltzmann transport properties, yielded electrical resistivity values of 0.460 µΩ m for Ti2AlC, 0.370 µΩ m for Ti3AlC2, and 0.268 µΩ m for Ti3SiC2 at 300 K. Therefore, Ti3SiC2 should be the most electrically conductive of the three. The vacancy formation energy of an A group atom was investigated using a 2 x 2 x 2 supercell. The vacancy formation energies were calculated to be 2.882 eV for Ti2AlC, 2.812 eV for Ti3AlC2, and 2.167 eV for Ti3SiC2. The formation of a vacancy increases the electrical resistivity of the bulk MAX phases. As a catalyst support material, a MAX phase particle will have surfaces present. Due to the layered structure of the MAX phases, multiple terminations of (0 0 0 1) surfaces could be possible, which were investigated. It was shown that terminations where the Ti-C cage structure remained intact produced the lowest cleavage energies. For Ti2AlC, the two low cleavage energy surfaces are Al(Ti) and Ti(C), for Ti3AlC2, Al(Ti2) and Ti2(C), and for Ti3SiC2, Si(Ti2) and Ti2(C). The surfaces with the lowest cleavage energy should be more stable than other surfaces and would therefore be expected to be present on a MAX phase particle. Vacancies were also formed in the surface systems. The surfaces with the vacancy in the surface layer had the lowest vacancy formation energy, with that of Si(Ti2) being positive. The surface slabs generally showed a higher electrical resistivity than the bulk systems, while the formation of a vacancy generally increased the resistivity, in agreement with the bulk vacancy trend. These MAX phases are electrically conductive, however a quantifiable oxidation resistance was not able to be calculated. They do however show signs of being good electrocatalyst support materials.
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    Synthesis of Ti₂AlC, Ti₃AlC₂ and Ti₃SiC₂ MAX phase ceramics; and their composites with c-BN
    (2011) Rampai, Tokoloho; Lang, Candace
    MAX phase ceramics are ternary ceramics with both metallic and ceramic properties. The existing backing materials in grinding wheels can be made of ceramics or metals. In these applications, ceramics have the disadvantage of low toughness, and most metals have the disadvantages of relatively high density and intolerance to some very high temperatures. The MAX phases have a combination of the main advantages of both metals and ceramics: they are soft and machinable yet also heat-tolerant, strong and lightweight. Cubic boron nitride (c-BN) is a widely used abrasive in grinding wheels, which is exceeded in hardness only by diamond. Composites of c-BN and selected MAX phases may result in materials of some interesting and useful properties for application in industry. Firstly MAX phases, Ti₃SiC₂; Ti₃AlC₂ and Ti₂AlC were synthesised, then reaction couples of MAX-cBN are made in order to investigate the best conditions for composite synthesis, and to analyse the interfacial phases which occur. Finally, the MAX-cBN composites were synthesised from the reaction couple studies. The following results were obtained: 1. Samples synthesised to obtain Ti₃AlC₂ were largely composed of the Ti₂AlC, and thus synthesis of the Ti₃AlC₂ MAX phase was deemed unsuccessful. 2. Nearly pure samples of Ti₂AlC and Ti₃SiC₂ were successfully synthesised with high densities, 99.16% and 98.21%, respectively, of the theoretical density. 3. Reaction couple studies revealed that the Ti₃SiC₂ /c-BN couple was successfully made at 1400°C, 10MPa pressure for 30 minutes, and Ti₂AlC/c-BN couple was successfully made at 1500°C, 10MPa pressure for 30 minutes. The interfacial phases characterised by XRD and SEM found here were TiN, TiC, TiB₂ and AlN for the latter and TiN, TiS₂ and TiB₂ for the former. 4. These conditions were used to successfully synthesise MAX/c-BN composites where both could react and still remain intact. The interfacial phases characterised by XRD and SEM found here were TiAl, TiC, TiB₂ and AlN for Ti₂AlC/c-BN and TiN, TiC, TiS₂ and TiB₂ for Ti₃SiC₂ /c-BN. From these results the following conclusion was drawn: Ti₂AlC and Ti₃SiC₂ are fully compatible with c-BN in order to synthesise a composite with notable properties such as the fracture toughness, suggested by the observed fracture mechanism seen from the fracture surface of these composites.
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    The hydrated lime dissolution kinetics in acid mine drainage neutralization
    (2021) Mgabhi, Senzo Mntukhona; Petersen, Joachim; Lewis, Alison; Rampai, Tokoloho
    Hydrated lime, Ca(OH)₂, has been rediscovered as an environmentally sustainable product, which could be of help in the remediation of acid mine drainage (AMD), especially in the AMD neutralization process. This is due to its ease of acquisition, affordable price and unique versatile properties such as reactivity and neutralization efficiency. AMD is an acidic wastewater containing high concentrations of sulphates and dissolved heavy metals mainly ferrous iron. The dissolution of Ca(OH)₂ in aqueous solution is complex, which make its kinetics in AMD neutralization difficult to understand. The aim of this study was therefore to understand the Ca(OH)₂ kinetics in simplified solutions such as de-ionized water and CH₃COOH. The neutralization process is an acid-base reaction; therefore, pH was used as a critical parameter in determining Ca(OH)₂ dissolution rate. The determination of the dissolution rate was attempted in two ways – measurement of dissolved calcium and determining change of particle size distribution. There were two methods of determining calcium assays investigated, that is EDTA-EBT titration method and OCPC spectrophotometric method. Both methods worked successfully for a Ca(OH)₂-H₂O system. The EDTA-EBT titration method worked better even at higher concentrations of calcium (up to 100 ppm) while the complexometric spectrophotometric method was consistent with Beer-Lambert Law for a narrow calcium concentration range of 1 to 2 ppm, when a small amount of magnesium was introduced. However, both methods failed in the presence of appreciable quantities of magnesium, sulphates and ferric ion. The investigation for particle characterization found that image analysis of SEM images was a better particle-size characterization option than laser diffraction measurement, which tended to cause blinding of the instrument window, but still yielded only qualitative results. There were four reactor configurations used, that is batch reactor for determining the effect of the hydrodynamics (stirring rate and powder addition) and three types of slurry CSTRs. The jacketed chemostat was found to be the optimal reactor configuration while the other two were used as base cases. The Ca(OH)₂ dissolution rate in de-ionized water decreased from 4.0×10⁻⁵ to 1.6×10⁻⁵ mol‧L⁻¹‧s⁻¹ when the temperature was increased from 26 °C to 42 °C. Correspondingly, the pH decreased with Ca(OH)₂ dissolution rate from 11.89 to 11.6. The dissolution rate expression was first order and consistent with the Nernst-Brunner Equation, with the dissolution rate constant of 2.34×10⁻³ s⁻¹ and the activation energy of 18.1 kJ mol ⁻¹ respectively. The overall Ca(OH)₂ dissolution rate in CH₃COOH solution decreased from 2.6×10⁻⁴ to 1.7×10⁻⁴ mol‧L⁻¹‧s⁻¹ when the temperature was increased from 25 °C to 44 °C. At constant ambient temperature (22°C), the Ca(OH)₂ dissolution rate increased with the decrease in pH from 12.1 to 4.38, then decreased with the decrease in pH from 4.38 to 3.5. Using pH to correlate dissolved calcium data and then to determine the rate of reaction, it was found that the dissolution rate is zeroth-order to hydrogen proton and first-order with respect to calcium concentration with the dissolution rate constant of 1.2×10⁻² s⁻¹ and the activation energy of 5.7kJ mol ⁻¹ respectively. These results confirmed that the dissolution of Ca(OH)₂ in DI water and the acetic acid solution is complex. The lower values of the activation energies (5.7 – 18.1 kJ mol ⁻¹), signifies that the kinetics of the Ca(OH)₂ dissolution are mass transfer controlled. Furthermore, these results were confirmed by the weak dependence of the dissolution rate to temperature. However, it was found that slurry CSTR is an efficient reactor system to study the effect of pH on the kinetics of hydrated lime at steady-state conditions.
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    Ti3C2Tx as an Advanced Support Material for Polymer Electrolyte Fuel Cell Catalysts to Facilitate the Oxygen Reduction Reaction
    (2022) America, Tyler Daniel; Rampai, Tokoloho; Levecque, Pieter
    Polymer electrolyte fuel cell's (PEFC's) have the potential to offer a leading energy conversion technology. These fuel cells make use of hydrogen and oxygen and by means of a chemical reaction, electricity, heat and liquid water are produced. In 2015 the Department of Energy (DoE) of the United States declared a 5000-hour lifetime target for transport applicable fuel cells. With current technological limitation, the achieved lifespan is, however, restricted to only 1700-hours. An assessment to find a more active but primarily more durable support for the oxygen reduction reaction (ORR) in a PEFC than the currently employed carbonaceous support was therefore undertaken. A MXene, Ti3AlC2 was selected for assessment based on its theoretically suitable electrical and thermal conductivities, as well as its possession of among the strongest resistance to oxidation of the many different MAX phases. The synthesis of high (>50 m2 g -1 ), specific surface area, delaminated Ti3C2Tx flakes was attempted first with mild in-situ HF conditions. While this method could both etch and delaminate flakes in a single stage, because the flake size remained large and unchanged, the specific surface area was not seen to increase to the outlined requirements. To synthesize Ti3C2Tx flakes with a high specific surface area, HF etching was therefore employed. In this report, 0.5 g of 400-mesh Ti3AlC2 flakes synthesized by hot pressing were etched in 10 ml of 48 wt % HF for 24 hours at 30 °C. After micronizing for 10 minutes and probe sonicating in solution for a further 40 minutes, high specific surface area (86 m2 g -1 ), delaminated Ti3C2Tx flakes were attained. Using metal organic chemical deposition, well dispersed 2- 5 nm platinum particles were successfully deposited onto the support material. Initial electrochemical performance evaluations indicated a lack of conductivity which restricted electron transport and therefore limited catalyst activity. This was determined to be the result of more defective flakes and by correlation, an increase in interfaces leading to increased resistance. With the incorporation of carbon to the catalyst material to synthesize a hybrid electrode, a positive result confirming ORR activity was attained. While the electrochemical surface area (ECAS) was less than half of that of Pt/C (80 vs 28 m2 g -1 ), it confirmed where the synthesis constraints lie. In review of the durability results, it was found that trapped intermediates between high specific surface area MXene sheets not only restricts access to catalytic sites but are further protonated and reduced to form hydrogen peroxide which causes irreversible damage the PEFC's catalyst membrane.