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  • (2021) Djuandhi, Lisa
    With a theoretical capacity of 1672 mA h g-1, more than five times higher than any commercially available lithium-ion (Li-ion) cell systems, the lithium-sulfur (Li-S) cell is an attractive candidate for next generation energy storage. Despite this high theoretical capacity, Li-S cells generally suffer from poor capacity retention and working lifetimes that prevent them from mass commercialisation. This is mainly due to current limitations in managing the inherent Li-S redox reactions which involve diffusion and migration of electrochemically active polysulfides. One approach to prevent polysulfide migration is by rational design of the sulfur electrode framework. The aim of this research is to investigate the electrochemical implications of using different frameworks for entrapment of redox active species, mainly designed for the Li-S cell system. The two types of frameworks investigated are: (1) mixed-morphology carbon feeds derived from waste sources wherein the intention is for the carbon to purely act as a structural framework to trap lithium polysulfides, and (2) sulfur-rich copolymers wherein redox active sulfur is covalently bound within the framework. More specifically, the goals involve determining: (1) whether carbon acts purely as a structural framework to trap redox active species during electrochemistry, and (2) whether sulfur-rich copolymers act purely as a sulfur feed. Achieving these goals requires a thorough understanding of what properties in each framework are ideal for the Li-S cell. The main conclusion drawn from this work is that neither of the materials studied behaved as pure structural or covalent frameworks partaking in various side processes. Using specialised techniques such as X-ray powder diffraction, solid-state NMR, and X-ray absorption near-edge structure spectroscopy, the beneficial and parasitic side processes involved in each framework are able to be determined. Overall, a significantly enhanced understanding of the Li S cell chemistry when using these materials is presented in this work.

  • (2021) Rathbone, Harry
    Photosynthesis has played a key role in the evolutionary trajectory of life on Earth. The ways in which organisms harvest sunlight have diversified over the billions of years since photosynthesis emerged in the quest for more efficient use of this energy source. The evolutionary origins of some organisms’ light harvesting apparatus, however, have remained elusive as have the causes for stark architectural changes between evolutionarily related organisms. In this thesis, I firstly provide a detailed exploration of published data describing photosynthetic efficiency through the lens of structural biology and quantum mechanics, examining observations from a range of antenna systems. After having built a framework for how an efficient photosynthetic antenna may be constructed, the rest of this thesis explores the evolutionary trajectory of the light harvesting antenna of the cryptophyte algae. Cryptophytes are a clade of secondary endosymbiotic algae which gained their photosynthetic chloroplasts from an engulfed red alga, but produced in an architecturally distinct antenna. Red algae have an antenna comprised of stacked protein rings that form an energetic funnel to the photosynthetic reaction centre which generates chemical energy from photon excitations. Cryptophytes took this energy funnel and dismantled it; complexing one of its component proteins with a peptide of unknown origin (‘cryptophyte alpha’) and packing them at high density within the chloroplast. By examining recently published cryo-electron microscopy maps of red algal antennas, I have discovered the evolutionary ancestor of the unique cryptophyte alpha subunit. Through this discovery, I reveal possible evolutionary events following secondary endosymbiosis leading to the origin of the cryptophyte light harvesting system. Finally, I examine the light harvesting antenna of a particular cryptophyte species, Hemiselmis andersenii, isolating multiple protein components and determining their crystal structures at high resolution. Through this, I discover a more complex antenna than previously thought with multiple protein components and a rich energetic structure. Some of these antenna proteins show previously unrecognised spectral properties and chromophore architecture. This structural data aids in understanding the architectural change between the red algal and cryptophyte light harvesting antennas and further diversification within the cryptophyte clade.

  • (2022) Paull, Oliver
    This thesis represents an effort to understand the structure of anisotropically strained Bismuth Ferrite (BiFeO3 BFO). This is executed by using anisotropic epitaxy and exploring the structure, magnetism, and electromechanical response in anisotropically strained BFO at various levels of average in-plane strain. This includes in the vicinity of the strain-induced morphotropic phase boundary where large enhancements to the electromechanical performance are identified. Bismuth ferrite (BFO) is a room-temperature magnetoelectric material that is able to easily adapt its crystal structure to accomodate any strain that is applied to it. By utilising high-index crystallographic substrates the effect anisotropic epitaxial strain has been explored using three different substrate materials (SrTiO3, (LaAlO3)0.3(Sr2TaAlO6)0.7 (LSAT), and LaAlO3) each with four orientations. The unit cell parameters of the BFO films behave linearly when weakly compressively strained on SrTiO3, and become more non-linear on LSAT. The strain-driven morphotropic phase boundary in BFO films grown on tilted LaAlO3(310) surfaces is able to stabilise a low-symmetry bridging phase between the well known M_A and M_C symmetries of BFO when deposited on SrTiO3 and LaAlO3 respectively. The anisotropic strain conditions of the substrate miscut force the BFO film to maintain strain along a high-symmetry in-plane direction whilst partially relaxing in the orthogonal low-symmetry in-plane direction. Interferometric displacement sensor (IDS) measurements indicate that the intrinsic piezoresponse of this new phase of BFO is double that of the R'-like version. Moreover we see spectroscopic indications through IDS and band-excitation frequency response measurements that there is a field-induced phase transition occurring under electric field wherein the low-symmetry phase is reversibly interconverted into the tetragonal-like phase creating a giant effective electromechanical response. These observations are fully supported by density functional theory and effective Hamiltonian calculations. When growing thicker films of this soft low-symmetry phase, a rich and detailed phase coexistence between the R', T', and bridging phase arise that is reminiscent of a highly tilted mixed-phase BFO. The topography of these samples also exhibit domain-like periodic stripes that evolve with the crystallography and are intimately linked together. \\ At the end of this thesis a number of neutron scattering experiments are presented on BFO films on YAlO3, LaAlO3, LSAT, and SrTiO3 substrates. Despite calculations and some experimental hints of a C-type antiferromagnetic phase in T'-BFO, there appears to be no evidence of this magnetic phase in BFO//YAO and BFO//LAO. Additionally, a cycloid model has been developed and implemented in order to fit ambiguous cycloidal peaks with a constrained model. This model is applied to two different systems of BFO with the results and interpretations discussed.