Electronic Theses and Dissertations (Masters)
Permanent URI for this collectionhttps://hdl.handle.net/10539/38003
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Item High entropy spinel oxides and iron-cobalt based electrocatalysts for rechargeable zinc-air batteries(University of the Witwatersrand, Johannesburg, 2024-08) Mongwe, Agnes Monosi; Ozoemena, Kenneth Ikechukwu; Haruna, Aderemi B.The development of effective and stable rechargeable zinc-air batteries (RZABs) using noble-metal free bifunctional electrocatalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) has been a key challenge to its practical applications. This MSc research work strategically investigated some synthetic methods aimed at tuning the physicochemistry and electrochemical properties of two electrocatalysts (i.e., noble-metal free high entropy spinel oxide (HESOx) ((CoCuFeMnNi)3O4) and spinel Fe2CoO4 for rechargeable zinc-air batteries). For HESOx, a simple and reproducible Pechini method was used to synthesize a homogeneous nanosized electrocatalyst HESOx-550. The HESOx-550 was thereafter supported on onion-like carbon (OLC) in (1) an acidic environment to produce HESOx-550/OLCAT (where AT stands for acid-treated) and (2) a nonacidic environment to produce HESOx-550/OLC. The effects of the different synthesis environments on these three samples were thoroughly investigated using different analytical techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, thermogravimetric analysis (TGA) and Nitrogen Gas Adsorption analysis. The Nitrogen Gas Adsorption analysis results show that the HESOx-550/OLCAT has the largest surface area and more volume. The electron paramagnetic resonance (EPR) and O1s XPS data consistently proved that HESOx-550/OLCAT has improved oxygen vacancies which are essential in improving conductivity and offering abundant reaction sites. The HESOx-550/OLCAT shows the best bifunctional ORR and OER electrocatalytic performance with a bifunctionality index (ΔE) of 0.70 V in 1 M KOH. In addition, the RZAB air electrode with HESOx 550/OLCAT exhibits high areal capacity (60 mAh cm-2) and areal energy density (73.2 mWh cm-2) with a long-term cycle stability over 112 h in 6.0 M KOH and 0.2 M zinc acetate. The HESOx-550/OLCAT RZAB shows better electrochemical performance than 10wt.% Pt/C- IrO2 when cycled over 315 h under 27% depth of discharge condition. For Fe2CoO4, iron cobalt-based electrocatalysts on Vulcan carbon support were synthesized using a simple reduction method to produce two composites (FeCo-Fe2CoO4/CAnnealed), and (FeCo Fe2CoO4/CMicrowave). The physicochemical analytical methods such as XRD, XPS, Raman, TGA and Nitrogen Gas Adsorption analysis were used to investigate the samples. The electrochemical analysis showed that the FeCo-Fe2CoO4/CAnn had a very low “bifunctionality index” (ΔE) of 0.76 V and the FeCo-Fe2CoO4/CAnn air cathode RZAB demonstrated good stability for over 50 h under harsh DOD conditions (35.2%). The assembled RZABs have areal energy densities of 48.4 mWhcm-2 and 60.5 mWhcm-2 which are higher than the minimum recommended areal energy density of 35 mWhcm-2 (and better than most electrocatalysts reported in the literature). This study has significant contributions to the progress of practical applications of RZABs.Item Synthesis of platinum-based electrocatalysts using nitrogen doped onion-like carbon and WS2 composites as the support for electrooxidation of ethanol in direct alcohol fuel cells(University of the Witwatersrand, Johannesburg, 2024-10) Bila, Laercia Rose; Gqoba, Siziwe; Maubane-Nkadimeng, Manoko S.The study reports on the synthesis of onion-like carbons (OLCs)/tungsten disulfide (WS2) composites as catalyst support for direct alcohol fuel cells (DAFC). OLCs were synthesized using waste engine oil over a flame pyrolysis (FP) method. The pristine OLCs (p-OLCs) were functionalized and purified using nitric acid (HNO3). The functionalized OLCs (F-OLCs) were further doped with nitrogen using melamine to increase the electronic properties of the OLCs. WS2 was synthesized using the colloidal method and oleylamine was used as the capping agent. Pt/p-OLCs, Pt/F-OLCs, and Pt/N-OLCs were synthesized using a reflux method where ethylene glycol was the reducing agent. Finally, WS2/N-OLCs were synthesized using the colloidal method and then Pt was dispersed on WS2/N-OLCs to form Pt/WS2/N-OLCs. High-resolution transmission electron microscopy showed the presence of onion-like rings in the OLCs and the quasi-spherical morphology, while a flower-like morphology was observed for WS2. Powder X-ray diffraction revealed that the synthesized WS2 had traces of WO3 due to the oxidation of WS2 which introduces WO3 impurities. Energy Dispersive X-ray Spectroscopy revealed that the OLCs derived from waste engine oil present some impurities that were attributed to the motor wear as well as the fuel. When Pt was loaded onto the WS2/N OLCs composite, the WS2 lost its original nanoflower morphology, which was attributed to the presence ethylene glycol used as a reducing agent. X-ray photon spectroscopy confirmed the successful synthesis of the Pt electrocatalysts. Cyclic voltammetry was used to determine the oxidation of ethanol and the current density of the synthesized electrocatalysts. Interestingly, the Pt/p-OLCs electrocatalyst had a higher current density compared to Pt/F-OLCs and Pt/N-OLCs. This was attributed to metal impurities found in p-OLCs, which were reduced during the purification process. The Pt/WS2/N-OLCs electrocatalyst showed higher current density compared to Pt/WS2 but this was low compared to Pt/N-OLCs. The data reveals that the addition WS2 shows a co-catalyst behaviour, rather than a support.Item Diastereoselective conjugate addition reactions using diverse nucleophiles on a variety of Morita-Baylis-Hillman (MBH) adducts(University of the Witwatersrand, Johannesburg, 2023-09) Bhom, Nafisa; Bode, Moira L.The Morita-Baylis-Hillman (MBH) reaction involves the formation of a new carbon-carbon bond, generating an MBH adduct. These MBH adducts are multi-functional molecules, which can be used as synthons for the generation of complex and diverse compounds. The first part of the work described here involved the synthesis of a series of diverse ester and nitrile MBH adducts obtained as racemic mixtures. The MBH adducts were protected using different protecting groups, which could potentially control the diastereoselectivity and the formation of alternative products in the subsequent conjugate addition reaction. Conjugate addition reactions were performed on the protected MBH adducts using different nucleophiles to obtain the product as diastereomers. These reactions were monitored to detect whether diastereomers were obtained or not. The diastereomeric ratios obtained using different substrates, protecting groups and nucleophiles were determined. The best diastereomeric ratio was 3:1, obtained for the piperidine and benzylamine addition on the TBDMS protected nitrile adducts 192a/b and 196a/b. The addition of sulfur nucleophiles gave the conjugate addition product only and the addition of nitrogen nucleophiles gave both conjugate addition and allylic substitution products. It was found that the protecting groups did not have an effect on the diastereomeric ratio obtained, nor on the formation of alternative products. The last step performed in the sequence was the deprotection of the conjugate addition products. The configuration of the major and the minor diastereomers were determined, the major product was assigned as the syn diastereomer. The major:minor diastereomeric ratio for compound 208a/b was 3:1 and for compound 209a/b, a ratio of 2:1 was obtained. The next part of the work involved the synthesis of MBH adducts with amide as the electron withdrawing group. The originally proposed route involved the synthesis of MBH esters and their conversion into amides. The conjugate addition reactions were attempted on these amide adducts, but were unsuccessful. A number of alternative routes were attempted for the synthesis of amide adducts and conjugate addition products resulting from these adducts. From all the alternative routes, the best route was the originally proposed route.Item Manganese-Rich Nickel-Manganese-Cobalt Oxides as Hybrid Supercapacitor Electrode Materials(University of the Witwatersrand, Johannesburg, 2023-09) Tshivhase, Funanani; Ozoemena, Kenneth IkechukwuFossil fuels used as the conventional energy source play a substantial negative role in climatic changes and global warming. Their reservoirs on earth keep getting constrained, thus limiting their reliability. These issues make renewable energy sources an excellent alternative due to their abundance, environmental safety, affordability, and renewability. However, renewable energy is subjected to geographic limitations, and some sources are intermittent, which can be solved by applying energy storage devices. Asymmetric hybrid supercapacitors are an excellent choice due to the safety of aqueous electrolytes, exploitation of abundant metals in the metal oxides used, improvement of power and energy density and simple assembly and application. In this work, manganese-rich nickel-manganese-cobalt (MR-NMC) was studied and applied in asymmetric hybrid supercapacitors as a cathode material, and reduced graphene oxide (rGO) was used as an anode. Synthesis was done using co-precipitation-(Conv), laminar Taylor vortex flow reactor-(Lam), and microwave irradiation-(MW) approaches. Physical characterization was performed using XRD and TEM. Electrochemical studies were done using CV, GCD and EIS. Three full cells/two electrode systems were assembled and studied. Those cells were rGO//Conv MR-NMC, rGO//Lam MR-NMC and rGO//MW MR-NMC. The data obtained from electrochemistry tests was used for the calculations of specific capacitance, energy and power densities. rGO//MW MR-NMC cell had the highest specific capacitance response compared to rGO//Conv MR-NMC and rGO//Lam MR-NMC over the entire current density range used, where at the current density of 0.2 A g-1, rGO//MW MR-NMC had 44.77 F g-1, followed by rGO//Lam MR-NMC with 15,89 F g-1, then rGO//Conv MR-NMC with 13.68 F g-1. There was no significant difference in the specific capacitance responses of rGO//Conv MR-NMC and rGO//Lam MR-NMC. rGO//MW MR-NMC also exhibited higher energy density for the entire range of power density over rGO//Conv MR-NMC and rGO//Lam MR NMC. At the power density of 678,08 W kg-1, rGO//MW MR NMC had a specific energy density of 65 Wh kg-1, followed by rGO//Lam MR NMC with 23.45 Wh kg-1, then rGO//Conv MR-NMC with 19.82 Wh kg-1. Overall, the electrochemistry and the calculated perimeters thereafter showed that microwave irradiation is a reliable approach that can be used in the preparation of metal oxides used in energy storage devices for the improvement of electrochemical performance, which potentially enables the commercialization of these systems and management of energy crisis in South Africa, Africa and the world as a whole, hence the rGO//MW MR-NMC material performed better than the other two.Item Imputation of missing values and the application of transfer machine learning to predict water quality in acid mine drainage treatment plants(University of the Witwatersrand, Johannesburg, 2024) Hasrod, TaskeenAccess to clean water is one of the most difficult challenges of the 21st century. Natural unpolluted water bodies are becoming one of the most dramatically declining resources due to environmental pollution. In countries like South Africa which has a mining-centred economy, toxic pollution from mine tailing dumps and unused mines leach into the underground water table and contaminate it. This is known as Acid Mine Drainage (AMD) and poses a grave threat to humans, animals and the environment due to its toxic element and acidic content. It is, therefore, imperative that sustainable wastewater treatment procedures be put in place in order to decrease the toxicity of the AMD such that clean water may be recovered. An efficient circular economy is created in the process since original wastewater can be recycled to not only provide clean water, but also valuable byproducts such as sulphur (from the elevate sulphate content) and other important minerals. Traditional analytical chemistry methods used to measure sulphate are usually time-consuming, expensive and inefficient, thereby, leading to incomplete analytical results being reported. To address this, this study aimed at imputing missing values for sulphate concentrations in one AMD treatment plant dataset and then using that to conduct transfer learning to predict concentrations in two other AMD treatment plants datasets. The approach involved using historical water data and applying geochemical modelling as a thermodynamical tool to assess the water chemistry and conduct preliminary data cleaning. Based on this, Machine Learning (ML) was then used to predict the sulphate concentrations, thus, addressing limited data on this parameter in the datasets. With complete and accurate sulphate concentrations, it is possible to conduct further modelling and experimental work aimed at recovering important minerals such as octathiocane, S8 (a commercial form of sulphur), gypsum and metals. Historical data obtained from the three AMD treatment plants in Johannesburg, South Africa (viz., Central Rand, East Rand and West Rand) were obtained and the larger Central Rand dataset was split into smaller untreated AMD (Pump A and Pump B) subsets. Thermodynamic and solution equilibria aspects of the water were assessed using the PHREEQC geochemical modelling code. This served as a preliminary data cleanup step. Eight baseline as well as three ensemble machine learning regression models were trained on the Central Rand subsets and compared to each other to find the best performing model that was then used to conduct Transfer Learning (TL) onto the East Rand and West Rand datasets to predict their sulphate levels. The findings pointed to a high correlation of sulphate to temperature (°C), Total Dissolved Solids (mg/L) and most importantly, iron (mg/L). The linear correlation between iron and sulphate substantiated pyrite (FeS2) as their source following weathering. Water quality parameters were found to be dependent on factors such as weather and geography this was evident in the treated water that had quite different chemistry to that of the untreated AMD. Neutralisation agents used were based on those parameters, thus, further delineating the chemistry of the treated and untreated water. The best performing ML model was the Stacking Ensemble (SE) regressor trained on Pump B’s data and combined the best performing models namely, Linear Regressor (LR), Ridge Regressor (RD), K-Nearest Neighbours Regressor (KNNR), Decision Tree Regressor (DT), Extreme Gradient Boosting Regressor (XG), Random Forest Regressor (RF) and Multi-Layer Perceptron Artificial Neural Network Regressor (MLP) as the level 0 models and LR as the level 1 model. Level 0 consisted of training heterogenous base models to obtain the crucial features from the dataset. These individual predictions and features were then fed to a single meta-learner model in in the next layer (level 1) to generate a final prediction. The stacking ensemble model performed well and achieved Mean Squared Error (MSE) of 0.000011, Mean Absolute Error (MAE) of 0.002617 and R2 of 0.999737 in under 2 minutes. This model was selected to be used for TL to the East Rand and West Rand datasets. Ensemble methods (bagging, boosting and stacking) outperformed individual baseline models. However, when comparing stacking ensemble ML that combined all the baseline models with stacking ensemble ML that only combined the best performing models, it was found that there was no significant improvement in excluding bad models from the stack as long as the good models were included. In one case, it was actually beneficial to include the bad performing models. All models were trained in under 2 minutes which proved the benefit of using ML approaches compared to traditional approaches. The treated water data was highly uncorrelated such that model training was unsuccessful with the highest achievable R2 value being 0.14, thus, no treated water model was available for TL. TL was successfully conducted on the cleaned and modelled East Rand AMD dataset using the Central Rand (Pump B) stacking regressor and a high level of accuracy with respect to Mean Square Error (MSE), Mean Absolute Error (MAE) and R2 (MSE:0.00124, MAE:0.0290 and R2:0.963) between the predicted and true sulphate values was achieved. This was achieved despite a marked difference in the distributions between the Central Rand and East Rand datasets which further proved the power of utilizing ML for water data. TL was successful in imputing missing values in the West Rand dataset following prediction of sulphate levels in the cleaned and modelled West Rand AMD and treated water datasets. No true values for sulphate levels in the West Rand dataset were given, as such, accuracy comparisons could not be made. However, a general baseline idea of the amount of sulphate present in the West Rand treatment plant could now be understood. The sulphate levels in all three treatment plants (Central Rand, East Rand and West Rand) were found to greatly differ from each other with the Central Rand having the most normal distribution, the East Rand having the most precise distribution and the West Rand having the most variable distribution. Whilst the sulphate levels in the treated effluent waters could not be reliably predicted due to inherent issues (e.g., analytical inaccuracies and inconsistences) and poor correlations within the treated water datasets, sulphate levels in all three of the untreated AMD datasets were successfully predicted with a high degree of accuracy. This underpinned the observation made previously about the discrepancies between treated and untreated water. The study has shown that it is possible to impute missing values in one water dataset and use transfer learning to complete and consolidate another similar, but scarce, dataset(s). This approach has been lacking in the water industry, resulting in the reliance and use of traditional methods that are expensive and inadequate. This has caused water practitioners to abandon scarce datasets, thus, losing potentially valuable information that could be useful for water remediation and recovery of valuable resources from the water. As a spin off from the study, it has been indicated that automation of such data analysis is possible. This was achieved by developing a Graphical User Interface (GUI) for ease of use of the SE-ML model by those with little to no programming background nor ML knowledge e.g., the laboratory staff at the AMD treatment plants. This can also be used for teaching purposesin academia.