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(2023) Merhebi, SalmaThesisLiquid metals are fast becoming a new class of materials and additives for composites synthesis. In particular, gallium (Ga) and Ga-based liquid metal and alloys exhibit fluidity and frictionless behaviours along with metallic conductivity properties. Liquid metals based on Ga also present low-toxicity and can be readily formed into micro and nanodroplets or utilised in the bulk as conductive liquid substrates. The resulting Ga-based composites present novel physio-chemical behaviours and multifunctional properties that remain to be explored for a range of applications. In this PhD thesis, the author investigates three liquid metal/polymer composite systems synthesised with low toxicity input materials for remote magnetic actuation, ionic sensing and separation, and cell electrostimulation capabilities. In the first project, the author aims to develop conductive and magnetic liquid metal polymeric gels. Electrically and magnetic conductive nanodroplets of Ga-based alloys are in-situ synthesised in a polyvinyl alcohol (PVA) solution using mild mechanical agitation methods. The resulting conductive and magnetic gels show additional self-healing properties and demonstrate great potential for the design of soft electronic systems and robotics. For the second project, Ga-based composites are investigated for the sensing and separation of alkali metal ions. Nanodroplets of Ga-based alloys embedded into a crosslinked PVA flat-sheet composite provide selectivity and sensing capability and stability in mixed ionic alkali metal solutions. The Ga-based flat-sheet composite has implications for the efficient and low-energy recovery of lithium ions from brines. In the third project, conductive liquid metal polymer composites are prepared for cell culture and electrostimulation. The composite substrates are composed of bulk Ga coated with polydopamine (PDA) to enhance cell adhesion capability. The Ga/PDA composites surfaces show high biocompatibility for cell culture. With added electrical stimulation protocols, the proliferation of mouse embryonic fibroblast cells is promoted. The conductive and biocompatible substrates lead to the use of liquid metals in regenerative medicine and tissue engineering. Collectively, the findings presented in this thesis provide deep insights and scientific findings for future research directions in the field of liquid metal-based composites for multifunctional materials in soft electronics, separation and sensing, and biomaterials.
(2023) Luo, XiaoxuanThesisComplex borohydrides have the potential to act as solid-state electrolytes for all-solid-state batteries. In this respect, sodium borohydride (NaBH4) is of high interest because it is thermally stable (up to 500 degrees celsius), and it has a high deformability and electrochemical stability against sodium anodes. However, its ionic conductivity at room temperature is extremely low ( ~ 10-10 S cm-1). Accordingly, this thesis aimed at investigating means to create defective NaBH4 structures with the intent to significantly enhance its ionic conductivity. To this aim, several strategies were investigated including the creation of intermediate interfaces, partial anionic substitution, the generation of defects and conducting interfaces through partial hydrolysis. By converting the surface of NaBH4 particles into Na2B12H12 of higher Na+ conductivity, to form NaBH4@Na2B12H12 core-shell structures, the resulting interface was found to lead to an ionic conductivity of 4 × 10-4 S cm-1at 115 degrees celsius, i.e., significantly higher to that of pristine Na2B12H12 (10-7 S cm-1). This demonstrates that it was possible to generate disordered interfaces trough anion mixing. The results suggested that the creation of defects may be more prone to lead to high ionic conductivity. Through partial substitution of BH4- anion by I- in NaBH4, defective NaBH4 structures with varied lattice constants could be created. This anion substitution strategy enhanced the ionic conductivity of NaBH4 doped with NaI to 1.6 × 10-3 S cm-1 at 65 degrees celsius. To further improve upon this, the idea of partial hydrolysis was also investigated with the idea to create both conductivity interfaces and defective NaBH4 structures by exposing NaBH4 to controlled amount of water. The disordered trapped interface located between alpha-NaBH4 and NaB(OH)4 showed fast Na+ dynamics, which led to a Na+ conductivity of 2.6 × 10-3 S cm-1 at 75 degrees celsius. Further addition of poly(ethylene oxide) (PEO) was found to help better control the levels of hydrolysis and the hydrolysed NaBH4-PEO composite electrolyte reached an ionic conductivity of 1.6 × 10-3 S cm-1 at 45 degrees celsius. These results indicate that the controlled formation of defects within NaBH4 is key to the conversion of such hydrides into superionic Na conductors.
(2022) Zheng, ZhaozhiThesisUrban stormwater runoff possesses the properties of intermittent occurrence, unexpectable volume and variable pollution which lead to different environmental issues, including flooding and waterlogging, pollution transportation, damage to downstream and contamination of the receiving waters. On the other hand, the low-level contamination (relative to sewerage) and large volume supply of stormwater makes it suitable as an alternative water resource to relieve the water shortage in the urban areas. Stormwater harvesting is under the concept of Water Sensitive Urban Design (WSUD) trying to treat stormwater properly for the different end-uses (like irrigation, toilet flushing and even for the uses close to human contact). Several treatment technologies (e.g., biofilters, constructed wetlands) have already been implemented to purify the stormwater with effective performance prior to reuse. However, the refractory organic micropollutants (especially herbicides) presented resistance to these nature-based solutions by showing variable treatment outcomes. In order to provide harvested stormwater for end-uses with high quality requirement (e.g., close to human contact recreational waters), a reliable treatment technology for organic micropollutants is desired as a post-treatment method in the stormwater harvesting system. This thesis aims to develop advanced oxidation processes (AOPs), in particular, photoelectrochemical oxidation (PECO), as the post-WSUD treatment approach for stormwater using its oxidation capacity towards the refractory organic micropollutants. Following the technology development procedure, three steps have been conducted: (1) testing the feasibility of AOPs for stormwater herbicides treatment; (2) investigating the intrinsic mechanism in the stormwater herbicides degradation process; and (3) assessing the operation conditions impact towards PECO stormwater treatment system. Boron-doped diamond (BDD) anode was used in the preliminary lab-scale tests for the feasibility study of AOPs towards stormwater organic micropollutants (two representative herbicides, diuron and atrazine - selected as the target pollutants in the study). The results showed that the effective herbicides degradation could be achieved by PECO process under 5 V operation (which was regarded as the optimal voltage in the system). The positive impact coming from voltage increase has been found in the study. BDD showed a remarkably durable property with stable removal performance under challenging voltage application (9 V) without observed deterioration. The catalysts loading showed negligible effect in removal performance. While the thermal effect was observed as a supporting factor for the process (higher temperature supported the oxidation process). Since BDD is not a perfect choice for scaled-up implementation due to its high manufacture cost, carbon fiber anode was chosen in the following studies and operated under low voltage (2 V) to avoid the possible anode deterioration under high voltage application. In the mechanism investigation, the superoxide radicals were found to be the major reactive species in PECO process. Meanwhile, hydroxyl radicals and free chlorine also demonstrated supporting impact for the oxidation process. With the identified intermediate products, degradation pathways of diuron and atrazine were proposed for the first time for three AOPs (PECO, electrochemical oxidation (ECO) and photocatalytic oxidation (PCO)) in stormwater herbicides degradation process. PECO was certified to be the preferrable stormwater treatment technology with the ability for further oxidation reactions towards herbicides degradation compared with ECO and PCO. In the third study, a flow reactor was designed and used to test the impacts of operational conditions (flow rate, light intensity, and initial pollutant concentration) for PECO process. An obvious improvement was observed for flow rate towards removal performance, while the light intensity was found to influence atrazine removal only. The initial pollutant concentration study demonstrated the robust performance of PECO flow reactor towards herbicides removal under challenging (240 μg L-1) pollutant concentration condition. The real stormwater experiments suggested the possible impacts coming from the stormwater chemistry towards PECO process. Further based on the energy consumption analysis, high flow rate (610 mL min-1) and normal light intensity (100 mW cm-2) were regarded as the optimal operational conditions for flow reactor system. Also, the effective PECO degradation performance of herbicides under the real stormwater environment has been verified by using the stormwater collected from field as supporting electrolyte in the experiments. Overall, this thesis confirms PECO as a promising stormwater herbicides treatment technology (potentially for all organic micropollutants) to provide further purification for stormwater high-quality targets. It also discusses the implications for the practical implementation and points out the future research directions for the system optimization.