School of Chemistry (ETDs)

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A systematic study on the use of the sol-gel synthetic method for lithium manganese oxide-based cathode materials
(University of the Witwatersrand, Johannesburg, 2024-09) Muntswu, Zwivhuya; Billing, Caren; Ferg, Ernst E.; Billing, David G.
This dissertation investigated the synthesis of two lithium manganese oxide-based cathode materials (Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4) using the sol-gel method and probing the phase transitions during the synthesis. The sol-gel synthetic method involved dissolving stoichiometric amounts of lithium nitrate, manganese nitrate hydrate, and citric acid in distilled water forming an aqueous solution. The starting precursor materials were dried at 140 °C which formed a crystalline phase of -Aqua-S-citrato (2-)-manganese(II) with an orthorhombic crystal system and P222 space group. The thermal behaviour of the precursor was explored to understand the effects of calcination/annealing temperatures. Thermal analysis of precursors prepared using nitrate salts with a 1:1 total metal ion to citric acid ratio displayed thermal stability to temperatures higher than 380 °C with the formation of a final metal oxide after 70% mass loss due to the decomposition of the organic and nitrate materials. However, when increasing the concentration of the complexing agent, an increase in material decomposition due to an increase in organic material is seen. The precursor materials prepared with a lower complexing agent concentration result in materials that have thermal instability when exposed to high temperatures. Thermal analysis of Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 prepared using acetate salts as starting materials shows material decomposition at high temperature of ~600 °C Calcining both undoped and Al-doped nitrate precursors at moderate temperatures (380 °C to 500 °C) resulted in the formation of Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 with a pure cubic spinel structure and an Fd-3m space group, however, increasing the calcining temperature to 800 °C for the undoped nitrate-based precursor revealed an impurity phase formation relating to dilithium manganese oxide with a monoclinic crystal system. On the other hand, calcining acetate-based precursors at moderate temperatures (380 °C to 500°C) results in metal oxides with low crystallinity compared to metal oxides prepared with nitrate-based precursors. Calcining acetate-based precursors at 800 °C was more favourable since they form the desired metal oxides without any impurities which might imply structural phase stability at high temperatures. The local and average crystallographic structures (via PDF and XRD respectively) of various nitrate-based metal oxides were investigated, where a good agreement between collected data and a calculated structural model revealed the formation of a cubic spinel structure of space group Fd-3m. Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 metal oxides were achieved from calcining precursors at moderate temperatures of 380 °C and 450 °C. The PDF high r-value signal displays a good fit which confirms to the average structure data information where the r-value signal which correspond to the local structure refinements have a minor discrepancy when fitted with a cubic spinel of space group Fd-3m.
Investigation of rhombohedral 𝑩𝒊𝟐𝑶𝟑 as an oxide conducting electrolyte for solid oxide fuel cell applications
(University of the Witwatersrand, Johannesburg, 2023-09) Kerspuy, Tanner Royele Rowan; Billing, Caren; Erasmus, Rudolph M.; Billing, Dave Gordon
The synthesis of a bismuth system co-doped with neodymium (Nd3+) and yttrium (Y3+) was at the core of this project. The focus was placed on the synthesis of the rhombohedral phase of bismuth oxide, which has not been observed in pure bismuth oxide. Neodymium was selected as the main dopant (the one used in highest dopant concentration), due to its Shannon ionic radii. Upon doping with Nd3+ as a single dopant, it is observed that a mixture of the rhombohedral and monoclinic phases is obtained, thus noting that the single dopant system using Nd3+ does not stabilise the rhombohedral phase. When using a co-doped system of 15 mol % Nd3+ and 5 mol % Y3+ (15Nd5YSB), it is observed that we are able to obtain a stable phase pure rhombohedral phase, with a total dopant concentration of 20 mol%. The total dopant concentration % ranges selected ranged between 8.5-10 mol %, 20 mol % and 22.5 mol %. The Rietveld refinement of the X-ray diffraction data obtained for both the laboratory and synchrotron-based techniques indicate sample phase purity and phase stability for the samples under investigation. The refinements obtained for the samples indicated that not only one structure model was used to fit the experimental data. The structural models which fit the Rietveld refinements of the experimental data resulted in the observation of pure phase and mixed phase rhombohedral samples being observed. The Nd0.15Y0.05-Bi2O3 (15Nd5YSB)sample resulted in a phase pure rhombohedral structural model. Hereafter all samples will be referred to with the shorthand notation. The thermal analysis techniques are used to indicate the thermal dependence of the samples, this analysis also indicated phase stability across the temperature range of investigation as no phase transitions occurred throughout the heating and cooling cycles, and minimal weight loss is observed. The samples of importance in this study were the 12.5Nd10YSB sample which obtained a conductivity of 2.4511×10-5 S.cm-1 at 500 ℃, and the 15Nd5Y2.5TbSB sample which obtained a conductivity of 2.1725×10-5 S.cm-1 at 500 ℃. The Arrhenius plots obtained indicated stability 3 of these samples across the 200-500 ℃ temperature range with no discontinuities, which suggests no phase transitions, or order-to-disorder transitions. Variable temperature Raman spectroscopy indicated that the behaviour for all the samples analysed using Raman spectroscopy is consistent, however, a deviation was observed for the 15Nd5Y2.5ScSB sample which has a distinctive spot which exhibits different Raman shift behaviour as compared to all other samples. The VT-Raman spectroscopy spectra indicate a distinctive signature Raman peak at ~250 cm-1, which can be concluded to be the Raman peak which is indicative of the rhombohedral 𝐵𝑖2𝑂3, this peak also appears in the low cubic phase % sample after cooling back to room temperature. This assignment of the Raman spectral peak is confirmed through this peak being evident throughout all the spectra obtained and it being consistent throughout all the spectra observed.
Microwave-assisted synthesis of palladium-based ferroalloy electrocatalysts for application in alkaline direct alcohol fuel cells
(University of the Witwatersrand, Johannesburg, 2024-11) Ramashala, Kanyane Nonhlanhla Eugenia; Billing, Caren; Modibedi, R. Mmalewane; Ozoemena, Kenneth Ikechukwu
This research work describes the study of Pd-based ferro-electrocatalysts for application towards direct ethanol fuel cells (DEFCs), direct ethylene glycol fuel cells (DEGFCs), direct glycerol fuel cells (DGFCs) and oxygen reduction reaction (ORR) operated in a basic environment. The initial part of the research was to explore the Pd-based monometallic and bimetallic (Pd/C and PdFe/C) by utilising varied methods such as the conventional sodium borohydride (NaBH4) and microwave-assisted technique (MW) towards the oxidation of glycerol (gly), intending to choose the best method viable for these catalysts. This study revealed that MW techniques tuned the physicochemical properties of Pd/C and PdFe/C by augmenting their crystallinity and defect. These led to improved electrocatalytic activities towards glycerol oxidation reaction (GOR) over NaBH4 technique. MW process as a powerful tool was further used in the entire study to synthesise bimetallic and trimetallic electrocatalysts in ethanol (EtOH), ethylene glycol (EG) and glycerol (Gly) oxidation reaction in an alkaline environment. The synthesised bimetallic catalysts studied in this research work were (PdFe/C, PdCo/C, and PdMn/C) at varied ratios of Pd: M (Pd2M/C (2:1) and PdM/C (1:1)). Amongst them all, Pd2Fe/C and PdFe/C were observed to be the most favourable catalysts towards all the alcohols, with the excellent specific activity of about, for EtOH (11.59 and 4.15 mA cm-2), EG (9.82 and 5.51 mA cm-2) and Gly (8.94 and 4.73 mA cm-2), respectively. The satisfactory performance exhibited by the PdFe/C electrocatalyst prompted the exploration of the second 3d transition metal (PdFeMn/C and PdFeCo/C), intending to investigate the synergistic behaviour between the non-noble metals and Pd. The XRD confirmed that these electrocatalysts are in a crystalline nature with a decrease in d spacing (from 0.2247 nm, PdFe/C to 0.2236 nm (PdFeMn/C)) after the insertion of Mn into PdFe/C. This was supported by the TEM images obtained for the PdFeMn/C catalyst with a particle size of sub 10 nm. The comparison studies towards EtOH, EG and Gly were investigated for all the electrocatalysts and there was a remarkable observation, which is dissimilar from the theoretical studies (DFT). Density Functional Theory (DFT) revealed that PdFeCo performed better in terms of Gibbs free energy, binding energy, and energy band gap than PdFeMn; however, the experimental studies favoring the performance of PdFeMn. The PdFeMn/C delivered the best electrochemical activities, including a superior electrochemical active surface area (ECSA), larger current densities and mass activity response, and less susceptibility to poisoning and high conductivity as compared to PdFe/C and PdFeCo/C electrocatalysts. Furthermore, the PdFeMn/C electrocatalyst exhibited remarkable electrochemical properties during the ORR (basic medium). Ultimately, the best two anode electrocatalysts (PdFe/C & PdFeMn/C) were explored and tested for the proof-of-concept in the two-electrode configuration with the micro-3D printed cell. The PdFeMn/C delivered improved µ-ethylene glycol fuel cell, µ-glycerol fuel cell, and µ-ethanol fuel cell activities with respective to high voltage and power density of 33.27 mW cm-2, 11.00 mW cm-2 and 45,80 mW cm-2 respectively, operated at 100 mV / s. These electrocatalysts have demonstrated promising results in advancing ADAFCs.
Structural Characterization of Bimetal-Phosphate Based Solid-State Electrolytes: A PXRD, PDF and XAS Study
(University of the Witwatersrand, Johannesburg, 2024) Nkala, Gugulethu Charmaine; Billing, David G.; Billing, Caren; Vila, Fernando D.; Forbes, Roy P.
In this work, NASICON-type lithium titanium phosphate (LiTi2(PO4)3, LTP) was synthesized following the conventional solid-state reaction methodology. Single and double-doped formulations of LTP were made, with the primary objective of improving the room-temperature ionic conductivity, for their application as potential solid-state electrolytes for all-solid-state Li ion batteries. The primary characterization technique applied was ambient-temperature powder X-ray diffraction (PXRD) at both laboratory and synchrotron experimental conditions. The Rietveld refinement approach was used to determine the qualitative and quantitative phase compositions of each sample, revealing the rhombohedral (R-3c, space group #167) main phase, with phosphate-based secondary phases. Total scattering data, through the pair distribution function (PDF) was applied, revealing lattice site preference during the substitution of Ti with Al, Sn and Dy at the 12c site. Further analysis through small-box modelling indicated the local structure deviation below 10 Å, from rhombohedral (R-3c) to monoclinic (P21/n, space group #14). The application of experimental X-ray absorption spectroscopy (XAS) revealed a stable 4+ oxidation state for Ti regardless of doping. However, the extended X-ray absorption fine structure (EXAFS) data showed that the replacement of Ti with Sn results in heavy disorder and subsequent changes in the PO4 tetrahedra, corroborating the findings from Raman spectroscopy. Theoretical XAS spectra were computed using FEFF, providing insights into the origins of experimentally observed XAS features from first-principles. Applying electrochemical impedance spectroscopy (EIS) to assess the ambient-temperature ionic conductivity, co-doped systems showed an improvement in the conductivity. The application of characterization techniques at various length scales has been demonstrated to provide insights into the mechanisms governing the performance of the solid-state electrolytes.