<p> The progressive advancements in communication and transportation has changed human daily life to a great extent. While important advancements in battery technology has come since its first demonstration, the high energy demands needed to electrify the automotive industry have not yet been met with the current technology. One considerable bottleneck is the cathode energy density, the Li-rich layered oxide compounds xLi₂MnO₃.(1-x)LiMO₂ (M= Ni, Mn, Co) (0.5= Co) (0.5=discharge capacities greater than 280 mAh g^-1 (almost twice the practical capacity of LiCoO₂).</p><p> In this work, neutron diffraction under <i>operando</i> battery cycling is developed to study the lithium and oxygen dynamics of Li-rich compounds that exhibits oxygen activation at high voltage. The measured lattice parameter changes and oxygen position show movement of oxygen and lattice contractions during the high voltage plateau until the end of charge. Lithium migration kinetics for the Li-rich material is observed under operando conditions for the first time to reveal the rate of lithium extraction from the lithium layer and transition metal layer are related to the different charge and discharge characteristics.</p><p> In the second part, a combination of multi-modality surface sensitive tools was applied in an attempt to obtain a complete picture to understand the role of NH4F and Al₂O₃ surface co-modification on Li-rich. The enhanced discharge capacity of the modified material can be primary assigned to three aspects: decreased irreversible oxygen loss, the activation of cathode material was facilitated with pre-activated Mn³⁺ on the surface, and stabilization of the Ni redox pair. These insights will provide guidance for the surface modification in high voltage cathode battery materials of the future.</p><p> In the last part, the idea of Li-rich has transferred to the Na-ion battery cathode. A new O3 - Na_0.78Li_0.18Ni_0.25Mn_0.583O_w is prepared as the cathode material for Na-ion batteries, delivering exceptionally high energy density and superior rate performance. The single-slope voltage profile and ex situ synchrotron X-ray diffraction data demonstrate that no phase transformation happens through a wide range of sodium concentrations (0.8 Na removed). Further optimization could be realized by tuning the combination and ratio of transition metals.</p>

A thesis submitted to the Faculty of Science, in fulfilment of the requirements for the degree of Doctor of Philosophy. University of the Witwatersrand, Johannesburg May 23, 2016 / Amongst the major technological challenges of the twenty rst century is the harvesting of renewable energy sources. We studied the solar cell performance of the ternary compounds AgAlX2 (X = S, Se and Te) and AgInS2 as promising materials for meeting this challenge. Structural, electronic and optical properties of the compounds were investigated by means of the density functional theory and many body perturbation theory. Using cohesive energy and enthalpy, we found that among six potential phases of AgAlX2 and AgInS2, the chalcopyrite and the orthorhombic structures were very competitive as zero pressure phases. We predicted a low pressure-induced phase transition from the chalcopyrite phase to a rhombohedral phase. For the chalcopyrite phase, we found that the tetragonal distortion and anion displacement were the cause of the crystal eld splitting. The bandgaps from the general gradient approximation PBEsol were underestimated when compared to experiment and accurate bandgaps were obtained from the hybrid functioanl HSE06, the meta-general gradient approximation MBJ and GW approximation. Optical absorption from the Bethe-Selpeter equation indicated the presence of bound exciton in AgAlX2. We estimated the solar cell performance of the compounds using the Shockley and Queisser model and the spectroscopy limited maximum e ciency approach. We found that apart from AgAlS2, the estimated theoretical e ciency of the other compounds was greater that 13 %.

<p>Solar photovoltaics (PV), the technology that converts sunlight into electricity, has immense potential to become a significant electricity source. Nevertheless, the laws of economics dictate that to grow from the current 2% of U.S. electricity generation and to achieve large scale adoption of solar PV, the cost needs to be reduced to the point where it achieves grid parity. For silicon solar cells, which form 90% of the PV market, a significant and slowly declining component of the cost is due to the high-temperature (> 900 &deg;C) processing required to form p-n junctions. In this thesis, the replacement of the high-temperature p-n junction with a low-temperature amorphous titanium dioxide (TiO₂)/silicon heterojunction is investigated. The TiO₂/Si heterojunction forms an electron-selective, hole-blocking contact. A chemical vapor deposition method using only one precursor is utilized, leading to a maximum deposition condition of 100 &deg;C. High-quality passivation of the TiO₂/Si interface is achieved, with a minimum surface recombination velocity of 28 cm/s. This passivated TiO₂ is used in a double-sided PEDOT/n-Si/TiO₂ solar cell, demonstrating an open-circuit voltage increase of 45 mV. Further, a heterojunction bipolar transistor (HBT) method is developed to investigate the current mechanisms across the TiO₂/p-Si heterojunction, leading to the determination that 4nm of TiO₂ provides the optimal thickness. And finally, an analytical model is developed to explain the current mechanisms observed across the TiO₂/Si interface. From this model, it is determined that once &#916;E_V (TiO₂/Si) is large enough (400 meV), the two key parameters are the Schottky barrier height (resulting in band-bending in silicon) and the recombination velocity at the TiO₂/Si interface. Data corroborates this, indicating the hole-blocking mechanism is due to band-bending induced by the unpinning of the Al/Si interface and TiO₂ charge, as opposed to due to the TiO₂ valence band edge.

<p> When mixed with lithium salts, polymers that contain more than one chemical group, such as block copolymers and endgroup-functionalized polymers, are promising electrolyte materials for next-generation lithium batteries. One chemical group can provide good ion solvation and transport properties, while the other chemical group can provide secondary properties that improve the performance characteristics of the battery. Secondary properties of interest include non-flammability for safer lithium ion batteries and high mechanical modulus for dendrite resistance in high energy density lithium metal batteries. Block copolymers and other materials with multiple chemical groups tend to exhibit nanoscale heterogeneity and can undergo microphase separation, which impacts the ion transport properties. In block copolymers that microphase separate, ordered self-assembled structures occur on longer length scales. Understanding the interplay between structure at different length scales, salt concentration, and ion transport is important for improving the performance of multifunctional polymer electrolytes.</p><p> In this dissertation, two electrolyte materials are characterized: mixtures of endgroup-functionalized, short chain perfluoropolyethers (PFPEs) and lithium <i> bis</i>(trifluoromethanesulfonyl) imide (LiTFSI) salt, and mixtures of polystyrene-<i>block</i>-poly(ethylene oxide) (PS-<i> b</i>-PEO; SEO) and LiTFSI. The PFPE/LiTFSI electrolytes are liquids in which the PFPE backbone provides non-flammability, and the endgroups resemble small molecules that solvate ions. In these electrolytes, the ion transport properties and nanoscale heterogeneity (length scale ~1 nm) are characterized as a function of endgroup using electrochemical techniques, nuclear magnetic resonance spectroscopy, and wide angle X-ray scattering. Endgroups, especially those containing PEO segments, have a large impact on ionic conductivity, in part because the salt distribution is not homogenous; we find that salt partitions preferentially into the endgroup-rich regions. On the other hand, the SEO/LiTFSI electrolytes are fully microphase-separated, solid, lamellar materials in which the PS block provides mechanical rigidity and the PEO block solvates the ions. In these electrolytes longer length scale structure (&sim;10 nm &ndash; 1 &mu;m) influences ion transport. We study the relationships between the lamellar grain size, salt concentration, and ionic conductivity using ac impedance spectroscopy, small angle X-ray scattering, electron microscopy, and finite element simulations. In experiments, decreasing grain size is found to correlate with increasing salt concentration and increasing ionic conductivity. Studies on both of these polymer electrolytes illustrate that structure and ion transport are closely linked.</p>

The advantages of alkaline anion exchange membrane fuel cells (AAEMFCs) over proton exchange membrane fuel cells is the motivation for this dissertation. The objectives of this dissertation were to develop durable membranes with high anion conductivity and an understanding of the ion conductivity relationship with morphology. The research results presented in this dissertation focuses on developing different architectures of ionic copolymers including diblock copolymers and random copolymers for AAEMFCs. A novel, and stable cobaltocenium cation, was incorporated into polymer for stable AAEM. Because of its 18 electron closed valence-shell configuration, the cobaltocenium cation is promising for use in AAEMFC. Two block copolymers, polystyrene-b-poly(vinyl benzyl trimethyl ammonium hydroxide) (PS-b-[PVBTMA][OH]) and poly(vinyl benzyl trimethyl ammonium bromide)-b-poly(methylbutylene) ([PVBTMA][Br]-b-PMB), were studied in chapters 2 and 3 respectively. The major difference between these two chapters was the type of hydrophobic block employed. The membranes fabricated from PS-b-[PVBTMA][OH] were too brittle to be mechanically durable nor flexible enough for use as membranes due to the high Tg of polystyrene. The flexible, and robust, [PVBTMA][Br]-b-PMB membranes were successfully fabricated because of the low Tg and fully saturated backbone of poly(methylbutylene). The morphological structures were characterized by environmental controlled scattering experiments. The morphology relationship with ion conductivity was investigated in terms of the type of structure, various degree of ion content and degree of orientation of structure. In chapter 2, block copolymers of PS-b-[PVBTMA][OH] were synthesized by sequential monomer addition by ATRP and then post polymerization anion exchange from tetrafluoroborate to the hydroxide counter anion. The morphology of the membranes of PS-b-[PVBTMA][BF4] and PS-b-[PVBTMA][OH] block copolymers were determined by small angle X-ray scattering (SAXS) at different humidity and temperature conditions. The effects of the morphologies on the ionic conductivity, measured by impedance spectroscopy, were investigated in terms of type of structure, size of d-spacing and presence of grain boundaries. The block copolymers of [PVBTMA][Br]-b-PMB membranes were successfully fabricated in chapter 3. The membranes cast from different solvents exhibited different degree of structural ordering and values of ionic conductivity. The conductivity dependence on humidity, temperature and casting solvents were fully studied to understand the relationship between conductivities and morphologies. The membranes cast from THF showed highest bromide conductivity (0.02 S/cm) at 90 oC and 95% RH. High bromide conductivity (∼0.04 S/cm) and a low percolation point were achieved because of the formation of well-connected ion conducting channel. Effects of ion clusters on conductivities were studied by SANS and SAXS. Increasing the degree of functionality in the ionic domain is another avenue to eliminate ion cluster and achieve high ion conductivity in block copolymers. Investigations of cross-linked polyisoprene-ran-poly(vinyl benzyl trimethyl ammonium chloride) (PI-ran-[PVBTMA][Cl]) in chapter 4 was explored to fabricate robust AAEMs and to offer comparison of block copolymers to random copolymers in terms of ion conductivity, water uptake and morphology. The random copolymers were solvent processable, and were cross-linked by thermal treatment. High chloride ion conductivity (0.061 S/cm at 90oC and 95% RH) could be achieved. The ion conductivities were influenced by water uptake and ion exchange capacity of the membranes. The ion cluster effects on the conductivities were studied by SAXS as well. Finally, the comparison of ionic block copolymers and random copolymers membranes indicated that the ionic block copolymers membranes showed lower percolation point, lower water uptake and higher ion conductivity with the similar ion content relative to random copolymers membranes. Therefore, using ionic block copolymers as AAEM is promising for achieving higher performance. In chapter 5, a novel monomer, styrene cobaltocenium hexafluorophosphate (StCo+PF6-), was synthesized by a one-pot reaction without the need for purification by column chromatography. It showed excellent alkaline stability (negligible degradation after 7 days at 80oC in 2 M KOH solution) because of its 18 electron closed valence-shell configuration and the steric hindrance of the phenyl group. The excellent alkaline stabilities of phenyl cobaltocenium confirmed that membranes containing cobaltocenium are promising for use in AAEMFC. The dissertation concludes with a summary chapter 6 where the major results from the previous chapters are discussed. Suggestions are also offered for future investigations.

The research presented in this thesis focuses on developing new functional polymeric materials that can conduct ions, H+, or OH - or Li+. The motivation behind this work was to understand the similarities and/or differences in the structure property relationships between polymer membranes and the conductivity of H+ and OH - ions, and between polymer membranes and the anhydrous conductivity of H+ and Li+ ions. This understanding is critical to developing durable polymer membranes with high H+, OH - and Li+ ion conductivity for proton exchange membrane fuel cells (PEMFCs), alkaline anion exchange membrane fuel cells (AAEMFCs) and lithium ion batteries respectively. Chapter 1 describes the basic functioning of PEMFCs, AAEMFCs and lithium ion batteries, the challenges associated with each research topic, and the fundamental mechanisms of ion transport. The proton conducting properties of poly(4-vinyl-1H-1,2,3-triazole) were investigated on a macroscopic scale by impedance spectroscopy and microscopic scale by solid state MAS NMR. It was found that proton conductivity is independent of molecular weight of the polymer, but influenced by orders of magnitude by the presence of residual dimethylformamide. To improve the mechanical properties of otherwise liquid-like 1H-1,2,3-triazole functionalized polysiloxane homopolymers, hybrid inorganic-organic proton exchange membranes (PEMs) containing 1H-1,2,3-triazole grafted alkoxy silanes were synthesized, using sol-gel chemistry. This method enabled self-supporting membranes having proton conductivity comparable to uncrosslinked homopolymers. One of the biggest challenges with AEMs for use in AAEMFCs is finding a cationic polyelectrolyte that is chemically stable at elevated temperatures in high pH environment. Novel triazolium ionic salts were developed, having greater chemical stability under alkaline conditions compared to existing imidazolium ionic salts. However, the chemical stability of triazolium cations was not sufficient for AAEMFC applications. Excellent chemical stability of (C5H5)2Co+ in 2 M NaOH at 80°C over 30 days was demonstrated and polymerizable vinyl functionalized cobaltocenium monomers were synthesized. This work paves the way for future development of AEMs containing cobaltocenium moieties to facilitate hydroxide ion transport. Polymers containing covalently attached cyclic carbonates were synthesized and doped with lithium triflate and their lithium ion conductivities were investigated. The findings highlight the importance of high charge carrier density and flexibility of the polymer matrix to achieve high lithium ion conductivity. These results are similar to the key factors influencing anhydrous proton transport.

Hydrogen has long been touted as an alternative fuel which could form the basis of a sustainable energy system: the hydrogen economy. This thesis advances the application of light metal amide materials in the realisation of this transformative potential. One of the most vexing technical challenges to the widespread adoption of hydrogen in transportation applications is its low volumetric energy density, which makes the storage of a sufficient amount of hydrogen in a vehicle very difficult. In their conventional application, light metal amides (<b>M(NH₂)_x</b>),where M is a Group I or II metal) have been promoted as a means of storing large quantities hydrogen in the solid state, significantly increasing this energy density. This thesis highlights the impressive characteristics of amide-based materials, primarily the facile nature of the reversibility of the hydrogen storage reaction, as a model for the development and optimisation of solid-state hydrogen stores. The study of the relationship between the crystal structures of the relevant materials and their hydrogen storage properties through in situ X-ray and neutron powder diffraction measurements is reported for the lithium amide - lithium hydride (Li-N-H) hydrogen store. These investigations provide strong evidence for ionic mobility as the basis of reversible hydrogen storage in the Li-N-H system. The hydrogen storage and release reactions are seen to progress through a continuum of non-stoichiometric states, a transformation which is facilitated by its topotactic nature. The structural and energetic properties of these non-stoichiometric phases are reported, showing that they are intrinsically disordered and thermodynamically unstable relative to their parent structures. The study of the behaviour of the Li-N-H system is extended to many tens of hydrogenation-dehydrogenation cycles to examine practical performance, confirming the mechanism of capacity loss through the formation of parasitic lithium hydride, and showing that the addition of nitrogen improves the cycling lifetime of the system. An unexplored aspect of light metal amide chemistry is also presented, where the hydrogen storage and release reactions of sodium amide are performed simultaneously. Together, these reactions effect the chemical decomposition of ammonia. Ammonia is a high energy density liquid hydrogen carrier which has been largely overlooked, partly due to the difficulty extracting its stored hydrogen. This work demonstrates a new method of ammonia decomposition which gives comparable performance to the expensive rare-metal catalysts which are currently used for the productions of high-purity hydrogen. A survey of the ammonia decomposition efficiency of a number of light metal amides and imides is presented, showing that it is not only amides which decompose into their constituent elements (such as sodium amide) which are active in ammonia decomposition, but also imide-forming amides. Indeed, imides and imide-forming amides are shown to be advantageous from the perspective of containing the catalyst material. Neutron diffraction and isotope exchange measurements provide some initial insights into the mechanism of reaction, identifying clear avenues for development of these systems, and inviting further discussion of the potential of ammonia as a sustainable energy vector.

AN ABSTRACT OF THE THESIS OF James Morrow, for the Master of Science degree in Mechanical Engineering, presented on November 3, 2017, at Southern Illinois University Carbondale. TITLE: INVESTIGATION OF THE MATERIAL PROPERTIES OF CERIUM OXIDE WITH DOPANTS FOR AN OXYGEN TRANSPORT MEMBRANE MAJOR PROFESSOR: Dr. Kanchan Mondal Many physical properties of cerium oxide both undoped and doped have been studied herein. These properties include electrical conductivity, hardness, sintered density, and microstructure. These will be used to help determine a cerium oxide compound to use as an oxygen transport membrane in a combustion system. These compounds have been readily studied beforehand with exception to compounds with multiple dopants. Along with single doped cerium oxide, dual doped was investigated as well. The samples to be tested were created using co-precipitation and the subsequent powders were sintered at 1500°C to generate solid pellets. Once the pellets were formed the physical properties were tested. It was found that hardness and sintered density had little to no effect on electrical conductivity and the microstructures of the samples were shown to be favorable. As far as single or dual dopants were concerned, it was found that by including a second dopant along with zirconium that the electrical conductivity was reduced. Except for in the case where iron was doped along with zirconium, where the conductivity was increased. It was suggested to use samarium as the second dopant along with zirconium for the membrane.

Hybrid lead halide perovskites, AMX 3 compounds in which A = CH 3 NH 3 (MA), CH(NH 2 ) 2(FA), Cs; M = Pb,Sn; X = I, Br, Cl, display remarkable performance in solution-processed optoelectronic devices, including > 22% efficient thin film photovoltaic cells. These compounds represent the first class of materials discovered to exhibit properties associated with high performance compound semiconductors, while being formed at or near room temperature using simple solution chemistry techniques. This thesis is focused on the synthesis, structural characterisation and phase behaviour of MAPbI 3 , FAPbI 3 , A-site solid solutions and novel organic metal halide framework materials. The complete atomic structure and phase behaviour of methylammonium lead iodide is elucidated for the first time, including hydrogen positions, using high flux, constant wave-length neutron powder diffraction. At 100 K an orthorhombic phase, space group Pnma, is observed, with the methylammonium cations ordered as the C–N bond direction alternates in adjacent inorganic cages. Above 165 K a first order phase transition to tetragonal, I4/mcm, occurs with the unlocking of cation rotation, which is disordered primarily in the ab plane. Above 327 K a cubic phase, space group Pm3m, is formed, with the cations isotropically disordered on the timescale of the crystallographic experiment. The high temperature phase of formamidinium lead iodide, α-FAPbI 3 is shown for the first time to be cubic, (Pm3m), at room temperature using time-of-flight, high resolution neutron powder diffraction. Polymorphism and the low temperature phase behaviour of FAPbI 3 have been further investigated using reactor and spallation neutron sources with high resolution in temperature. A tetragonal phase, P4/mbm, is confirmed in the temperature range 140-285 K.The composition, structural and optical parameters of ’A’ site solid solutions (MA/FA)PbI 3 have been investigated by single crystal X-ray diffraction, UV-vis spectroscopy and 1 H solution NMR. A composition-dependent transition in the crystal class from tetragonal to cubic(or pseudo-cubic) at room temperature is identified and correlated to trends in the optical absorption. Novel hybrid materials with inorganic frameworks of varying dimensionality from 0D to 2D, including imidazolium lead iodide and piperazinium lead iodide, have been synthesised using various templating organic cations and their atomic structures solved by single crystal X-ray diffraction.

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