Engineering

Publication Search Results

Now showing 1 - 10 of 252
  • (2011) Hanaor, Dorian; Michelazzi, Marco; Chenu, Jeremy; Leonelli, Cristina; Sorrell, Charles
    Journal Article
    Thick anatase films were fabricated on graphite substrates using a method of anodic aqueous electrophoretic-deposition using oxalic acid as a dispersant. Thick films were subsequently fired in air and in nitrogen at a range of temperatures. The morphology and phase composition were assessed and the photocatalytic performance was examined by the inactivation of Escherichia coli in water. It was found that the transformation of anatase to rutile is enhanced by the presence of a graphite substrate through reduction effects. The use of a nitrogen atmosphere allows higher firing temperatures, results in less cracking of the films and yields superior bactericidal performance in comparison with firing in air. The beneficial effects of a nitrogen firing atmosphere on the photocatalytic performance of the material are likely to be a result of the diffusion of nitrogen and carbon into the TiO2 lattice and the consequent creation of new valence band states.

  • (2018) Hanumanth Rao, Narasinga Rao
    Thesis
    The novel PosiDAF process that uses cationic-polymer modified bubbles has been suggested as an alternative to conventional dissolved air flotation for the separation of algae. However, a cationic PosiDAF effluent compared to an anionic influent as detected by charge measurements indicated that effluent contained high polymer residuals and was undesirable. To prevent this, prior research investigated stronger polymer-bubble adhesion by developing hydrophobically modified polymers (HMPs) of poly(dimethylaminoethyl methacrylate) (PDMAEMA). However, while bench scale tests using the HMPs were successful, commercially available poly(diallyldimethylammonium chloride) (PDADMAC) outperformed the HMPs in pilot scale, suggesting that PDADMAC has a more suitable polymer backbone. Moreover, algal organic matter (AOM) released by cells, particularly biopolymers, was observed to influence cell separation. Further research is required to investigate alternate polymers and to determine more precisely the underlying mechanisms governing polymer-bubble-AOM interactions in PosiDAF. In this study, PDADMAC was modified with various aromatic and aliphatic pendant groups to generate several HMPs. Select HMPs of PDADMAC and previously investigated PDMAEMA were compared to evaluate polymer-bubble attachment and PosiDAF performance to separate algae and cyanobacteria. The composition of AOM, particularly biopolymers from each strain tested was characterised and their influence examined by conducting experiments with various AOM, protein and carbohydrate concentrations. The results showed that HMP coated bubbles had lower surface tensions and consequently, anionic effluents and strong polymer-bubble adhesion. Concurrently, cell separation was either comparable, or slightly better between HMPs. However, separation effectiveness varied for several algae, indicating that AOM impacted separation. Moreover, cell separation of the strains increased to > 95% when exudates from the best separated strain were added. On bulk and molecular characterisation of the cultures, the best separated strain was found to be biopolymer rich in comparison to the other strains. Hence, proteins and carbohydrates were dosed to study their influence and were observed to either depress or enhance the flotation depending on their character. It was concluded that the interplay of biopolymers with polymer-bubble-cell was responsible for the variations in cell removal observed across several strains. Overall, with a well-defined AOM character and low polymer residual in effluent, PosiDAF has been demonstrated as a robust and sustainable process.

  • (2019) Tian, Yuheng
    Thesis
    Ever-increasing worldwide energy requirements and concerns on global warming have stimulated the development of renewable energy resources. However, intermittency of renewables is the biggest challenge for their widespread application. Advanced large-scale energy storage technologies are thus urgently demanded to effectively utilise these renewable energy resources and increase the stability and reliability of power generation. Batteries are considered as a promising energy storage system, which has been under intensive investigation for overcoming the current limitation of renewables. In the PhD research, due to the safety (using the non-combustible aqueous electrolytes) and cost-effectiveness (low-cost separators and aqueous electrolyte salts), aqueous batteries are focused in terms of electrocatalyst design for optimising the aqueous redox reaction, development of novel aqueous battery systems by exploring the existing electroactive materials in the new systems and by exploring new redox-active materials for achieving the high-performance aqueous batteries. Specifically, in the first project, hydrophilic tannic acid modified WS2 nanosheets are developed as polysulfide conversion electrocatalysts in alkaline aqueous solutions, which overcome the sluggish reaction kinetics of polysulfide and improve the electrocatalytic activity for polysulfide reactions in aqueous solution. This work opens the way to the preparation of optimal electrocatalysis of polysulfide redox reactivity and provides an alternative option to improve the aqueous polysulfide-based batteries. In the second project, a novel alkaline redox flow battery is studied by using methyl viologen (MV) as anolyte and potassium ferrocyanide as catholyte. MV is for the first time explored in the alkaline condition, which shows enhanced electrochemical kinetics compared to in the neutral system. This work demonstrates the potential of MV as the anode material candidate in the alkaline battery. In the third project, a non-persistent radical precursor, N-hydroxyphthalimide (NHPI), is reported for the first time as a low-cost, high-potential organic cathode with rapid kinetics in a semi-aqueous redox battery. A novel combined strategy is proposed in stabilising non-persistent radicals, which includes the binary electrolyte system, cluster-anchoring to polymer chains and cold quenching. This work offers a measure to access the high-potential and kinetic radical chemistry for high-voltage aqueous redox batteries.

  • (2019) Liang, Jiaxing
    Thesis
    Due to the severe environment pollution and critical energy issues around the world, fossil fuel is being replaced by new energy sources, e.g. solar power, wind power, etc. To enhance the efficiency of those new energies, electrochemical energy systems (EESs), including lithium-ion battery (LIB), supercapacitor (SC) and lithium-ion capacitor (LIC) are widely used in our daily life. Combining the advantages of LIB and SC, LIC seems to be the most competitive candidate for the next generation EESs. The similarities and differences between three of them are discussed in chapter 1. Also, evaluation methods, device design principles, together with configurations are summarized. By reviewing the development status of LIC, it’s worth to pointing out that pre-lithiation step is necessary in almost all the device design. Moreover, to further enhance the energy density of the device, there is also the call for new cathode material with wide operational voltage and large capacity. Lastly, symmetric LIC design is not popular because the lack of suitable active materials. Aiming to build up a high performance symmetric LIC, 2-dimensional tungstate acid-link polyaniline (TALP), which has a wide working potential range from 0.01V to 4.5V versus Li/ Li+, is used as active material in this thesis. Solvent exchange effect in TALP is observed and confirmed. Water, NMP and electrolyte could diffuse in the interlayer to form the nanoconfined fluid and replace the former existed solution. Plus, as spotted by XPS, not just Li+, but also PF6- is able to intercalate/ de-intercalate into/ from TALP interlayer, thanks to the large interlayer spacing, resulting in a high areal capacity of 39 mAh cm-3 under a large current of 2000 mA g-1. As anode, getting use of the solvent exchange effect and layered structure, interlayer SEI is formed for pre-lithiaiton. The modified sample (5FEC-SEI-TALP) exhibits 4 times higher initial columbic efficiency than the fresh sample. Later, symmetric LIC is produced with TALP as cathode and 5FEC-SEI TALP as anode, which demonstrates a high energy density of 102.63 Wh kg-1 under a power density of 224.5 W kg-1, which is superior to the AC-based symmetric LIC.

  • (2019) Sun, Ju
    Thesis
    The increasing demand for energy and limited availability of fossil fuels trigger the development of renewable resources. However, large scale-up application of renewable energy is inhibited by the inability of storage. High efficient electrical energy storage (EES) becomes one of the tools for utilizing electricity produced from intermittent renewable sources. Despite Lithium ion batteries (LIBs) being the most mature technology in current portable devices, metal-sulfur batteries (MSB) show great promise as the future energy storage device due to the low cost ($0.065/kg) and high theoretical capacity (1675 mAh/g) of sulphur. This thesis starts with the well-renowned Li-S batteries by exploring safer Li metal replacement. Pairing Al-Li alloy with S is of great promise to suppressing the dendrite growth and enhancing ambient stability, thus achieving high-energy rechargeable batteries with improved safety. A parallel interface engineering (PIE) strategy is proposed in the full cell design, and the improvement is attributable to the more efficient and uniform lithium sulphides deposition on the chemically uniform surfaces of the carbon cathode, as well as the suppressed growth of dendritic species on the Li-Al alloy anode with an implantable solid-electrolyte interphase. The second stage of the research focuses on the studies of Na-S batteries. A unique intertwined sponge is also prepared to facilitate ion diffusion/mass transfer of Na-S batteries, and to provide reservoir to adsorb polysulfides. The last section presents Mg-S batteries with a dual-doped (Co and N) Lychee-like sulfur host (Co@NC) to effectively mitigate magnesium polysulfides shuttling. The mesopores of Co@NC precursor can physically confine the sulfur species, while Co and N elements doping play significant role in strongly binding polysulfides. This strategy provides a promising example for the development of novel post Li-ion batteries in terms of the rational design of carbon materials.

  • (2019) Wu, Hao
    Thesis
    Nanostructured semiconductors, which includes zero-dimensional (0-D), one-dimensional (1-D), two-dimensional (2-D) and three dimensional (3-D) metal oxide nanostructures play determining roles in solar energy conversion. While extensive efforts have been paid in the generation of efficient photo(electro)catalyst with different compositions or alteration of structures, a true breakthrough is when efficiency enhancement is achieved not only at the low cost of raw materials but also at the efficiency of the synthesising method. Cuprous oxide (Cu2O) and zinc oxide (ZnO) have attracted considerable attention due to their abundant resources, low market prices, and excellent optical or electrical properties. However, the single component photo(electro)catalysts cannot meet the requirement of effective solar energy conversion process at the most time. This Feature Thesis concisely investigates the surface coating of nanostructured Cu2O and ZnO thin films with two models, including semiconductor/semiconductor heterojunctions and semiconductor@metal-organic framework, which are explored for effective improvement of photo(electro)catalytic performance via promotion of separation and transportation of the photoinduced charge carriers, enhancement of the photo(electro)catalytic stability and facilitation of surface gas molecule absorption and reaction. The work started with an investigation on the influence of pulse electrodeposited ZnOx ultrathin interlayer (with amorphous TiO2 outer layer) on the photostability of 2-D Cu2O with respect to the charge transfer efficiency of the multilayered material. 2-D Cu2O thin films were synthesised on the fluorine-doped tin oxide (FTO) substrate by a facile electrodeposition method. Amorphous TiO2 layers were subsequently dip-coated on bare Cu2O/FTO and ZnOx/Cu2O/FTO electrodes. The photoelectrochemical activity and stability of TiO2/ZnOx/Cu2O/FTO photoelectrode were found to be significantly improved compared to that of bare Cu2O/FTO and TiO2/Cu2O/FTO photoelectrodes. The as-designed multilayered structure was shown to facilitate the charge separation and electron transfer, which is crucial to the reduction activity and stability of Cu2O. Compare to the 2-D counterparts, 1-D Cu2O nanostructures with a higher surface-to-volume ratio, shorter lateral transport length, and lower light reflectivity were further studied for the synergetic effect of Cu2O and Cu-based metal-organic framework (Cu-MOF). The crystalline 1-D Cu2O nanowires were prepared by a simple anodization method followed by an annealing treatment in Ar. Cu-MOF was homogeneously coated on the surface of Cu2O via a partial topotactic conversion strategy. The integrated core-shell nanowires exhibited enhanced gaseous photocatalytic CO2 reduction to produce methane (CH4) selectively. It could be attributed to the enlarged surface area, high CO2 uptake, improved charge separation, and prolonged lifetime of separated electrons introduced by the surface-coated Cu-MOF. Following these findings, the proposed surface coating methods including pulsed electrodeposition and solvothermal method were examined to decorate 1-D ZnO nanorods, respectively. CdS nanocrystals were pulse electrodeposited on the 1-D ZnO nanorods which extended the light absorption to visible light. The as-prepared ZnO-CdS hybrid photoelectrode achieved an exceptional anodic photocurrent in a near-neutral pH solution under visible light illumination. Moreover, a dynamic photoelectrochemical sensing device using prepared ZnO-CdS as the working photoelectrode was constructed to detect the Cu2+ ions. It showed a remarkable sensitivity with high selectivity with respect to the previously reported photoelectrochemical sensors. The last section of the work adopted the partial topotactic conversion strategy to coat zeolitic imidazolate frameworks (ZIFs) on the outer surface of 1-D ZnO nanorods. The ZnO@ZIF-8 photocatalyst performed an enhanced photocatalytic activity and better reproducibility than the neat ZnO for acetaldehyde production under the full-spectrum illumination at 200 oC. The enhanced photocatalytic activity of ZnO@ZIF-8 composites could be attributed to the well-matched band position and the intimate contact interfaces between ZnO and ZIF-8, which resulted in an efficient separation and transfer of the photoinduced charge carriers. Moreover, the ZnO@ZIF-8 composites showed an enlarged surface area within the microporous structures introduced by the ZIF-8 which offers more surface active sites during the gaseous photocatalytic reactions.

  • (2019) Wei, Zengyi
    Thesis
    As emulsions are thermodynamically unstable, stabilizers are added to prevent creaming and coalescence, and can greatly improve the customer appeal and shelf life of formulated products. Investigations of the structures of surfactants and polymers are key to understanding how they meditate the properties of the oil/water interface. Studying molecules at the oil/water interface is challenging, and only a handful of techniques are suitable for detailed structural studies of molecules at the oil/water interface. In this thesis, an easy-to-use sample environment suitable for oil/water interfacial characterization is described. End attached polydimethylsiloxane (PDMS) films mimic silicone layers in a way that permits the amounts and molecular structures of materials at the interface to be measured. Molecular conformations at the oil/water interface are seen to differ from those at the air/water and solid/liquid interfaces. Penetration of surfactant tails into the oil phase was observed. Addition of small hydrocarbon surfactants and salts was seen to affect polymeric surfactants. Displacement of the polymeric surfactants from the PDMS surface, producing poorly stabilized interfaces, was observed; co-adsorption behavior was also measured. The efficiency of steric stabilization could also be tuned by the addition of salts. A reduction in polymer adsorption and a thinner steric barrier were seen when salts were added as co-solutes. The effect was salt-identity specific. Adsorbed polymeric structures were also probed using neutron reflection in a sample environment that creates a confined geometry at the oil/water interface. Multilayer structures were formed under confinement and the polymeric surfactants were seen to be repelled from the hydrophobic surface. These are the first experiments on confinement at liquid/liquid interfaces ever attempted. Interfacial structures were seen to be highly dependent on the phases that were used, with these data from the PDMS/water interface indicating that it is difficult to draw direct conclusions from measurements at the air/water or solid/liquid interface. The PDMS brush layer is seen to be an excellent model oil surface and enables detailed structural studies of polymers and surfactants at the PDMS/water interface.

  • (2019) Chung, Hoi Ying
    Thesis
    Sunlight-driven water splitting has emerged as a potential method in the production of clean energy carriers. The bottleneck of water splitting reaction lies on water oxidation because of the involvement of four-electron transfer mechanism. Bismuth tungstate (Bi2WO6) is a photoactive semiconductor capable of oxidising water into oxygen and with a band gap of 2.7-2.9 eV (visible-light active). In this thesis, Bi2WO6 was designed to achieve improved photocatalytic and photoelectrochemical (PEC) efficiency. Important factors and the underlying mechanisms involved were studied systematically. Firstly, the synthesis of hierarchical Bi2WO6 microspheres with controllable crystallinity and surface area was achieved through a combination of hydrothermal/calcination processes. Though both crystallinity and surface area are affecting the overall activities, it was found the degree of crystallinity dominates the reaction. As the bond length of tungsten (W) was shorten accompanying the improved crystallinity, improved charge transportation was observed. Following the investigation of crystallinity and surface area, by tuning the W precursor concentration in the synthesis, Bi2WO6 was made into platelike-structure with well-defined crystal facets, in which the role of each crystal facet was investigated. Bi2WO6 with a higher exposure extent of electron-dominated crystal facet reduces the charge transport resistance and decreases the charge trapping. Moreover, by introducing more W in the Bi2WO6, a self-doped phenomenon that increases the charge carrier concentration and contains more W5+ species were observed. These factors inhibit the charge recombination and beneficial to the photo-oxidation process. The performance of Bi2WO6 was further improved by the creation of heterojunction. Controlled transformation of tungsten oxide (WO3) to Bi2WO6 via electrodeposition and hydrothermal method successfully introduced heterojunction Bi2WO6/WO3. A higher interface between the Bi2WO6 and WO3 has lowered the amount of interfacial trapping of photoelectrons. It directly optimises the PEC water splitting to an ideal situation, facilitated by smaller applied bias. Another heterojunction was formed with the oxygen-deficient bismuth tungstate (Bi14WO24) and bismuth oxide, in which the photocorrosion was suppressed owing to the promoted interfacial charge transfer between the two components. The findings in this thesis could be applicable to other oxide-based photocatalysts.

  • (2019) Chen, Ming
    Thesis
    Radio frequency electric field treatment (RFEF) is a non-thermal method of food preservation that utilizes high intensity oscillating electric field to inactivate bacteria. Past studies reported that RFEF operated at lower frequencies resulted in a higher inactivation rate of bacteria. However, the electrode may corrode at lower frequencies, especially at frequencies lower than 20 kHz, which could potentially degrade the quality of the processed liquid food. This research studied on the electrode corrosion occurred during the high intensity RFEF operated under 20 kHz, also called the audio frequency electric field treatment (AFEF), and it proposed a novel method to reduce the corrosion rate of electrodes. Inspired by the electrical double layer (EDL) and its equivalent electrical circuit, applying a high permittivity coating on the surface of electrodes was proposed to reduce the corrosion rate. Three coatings composed of titanium dioxide, barium titanate, and CCTO, having low, moderate, and high permittivities, respectively, were selected as the coatings applied on the surface of electrodes to explore how the permittivity of the coatings affect the corrosion rate. Mathematical modeling and experiments were conducted to test whether the three coatings could mitigate the electrode corrosion. In mathematical modeling, an equivalent electrical circuit of the coating-solution system was employed to calculate the phase angle and voltage drop across the three coatings and the solution (saline water). After that, the experiment of the electrode corrosion, with/without applying the three coatings on electrodes during the high intensity AFEF treatment, was conducted. Furthermore, the electrical impedance, atomic and molecular structures, surface morphology, and elemental analysis of the three coatings were conducted. Finally, the concentration of the metallic ions in the AFEF-processed saline water was measured. In conclusion, the corrosion rate generally increased with the increase of the electric field strength and the conductivity and the reduction in frequency, respectively. The CCTO coating performed the best on the reduction of the electrode corrosion among the three proactive coatings, while the titanium dioxide coating marginally reduced the electrode corrosion. However, calcium ions were detected in the processed saline water when the CCTO coating was applied, although the concentration was deficient. As a novel study in this area, this research contributes to the effective mitigation of electrode corrosion, during the electric field treatment, thereby increasing the Industrial applicability of the electric field treatments.

  • (2019) Jantarang, Salina
    Thesis
    The anthropogenic emission of carbon dioxide has led to detrimental environmental impacts. One method to mitigate emissions is by carbon dioxide hydrogenation to produce methane. As one of the fuels currently heavily relied on in society, methane production is a preferable process. In order to promote sustainability, photothermal conditions are practical as they solely require solar energy for light to heat conversion. The area of photothermal methanation has not been extensively studied and therefore there are limits in the understanding of the reaction and suitable catalyst design. This work probes the design of nickel catalysts for the photothermal methanation of carbon dioxide. Nickel was selected as the active metal due to its high conversion under conventional thermal conditions, as well as its economic value. Catalyst supports comprising varied metal oxides were studied to alter the catalyst characteristics and understand their impact on the methanation reaction. With ceria and titania, changing the support composition provided an understanding on how the ease of catalyst reduction governed the photothermal carbon dioxide methanation. As catalyst reduction occurs in situ, partial reduction lead to catalyst activation and therefore methane formation. The exothermic nature of methanation aided catalyst reduction and product formation. The findings led to subsequent probing of those characteristics important in the catalyst support, including a semiconductor with oxygen vacancies (ceria), a semiconductor (titania), and an insulator support (alumina). The different catalyst support properties highlighted the importance of oxygen vacancies. The role of ceria oxygen vacancies was further studied by the addition of silica to suppress their impact. While the oxygen vacancies were inhibited at a 9:1 ceria to silica ratio, the catalytic activity was similar to those of nickel/ceria, highlighting the importance of oxygen vacancy proximity to the nickel sites. As nickel manipulation has been identified to result in a catalytic performance comparable to a catalyst with oxygen vacancies, plasma treatment was utilised to induce defects on nickel/silica and nickel/titania catalysts for enhancing photothermal methanation activity.