Dans le but d’améliorer les performances des micro-accumulateurs au lithium, de nouvelles voies de dépôt, compatibles avec des géométries texturées, sont actuellement explorées. Au cours de ce travail de thèse, un nouvel électrolyte solide déposé par voie « humide » a été développé. Ce matériau, composé d’un liquide ionique et d’un sel de lithium confinés dans une matrice solide, a été synthétisé par polymérisation in-situ d’un oligomère diméthacrylate. Afin de définir leurs caractéristiques de conduction ionique, de nouvelles méthodes, comme le suivi de la photo-polymérisation par impédance in-situ ou encore la réalisation d’un nouveau design de cellules à base de peignes interdigités, ont été développées. De plus, le transfert du lithium a été mesuré par RMN diffusionnelle. Une diminution significative de la vitesse de diffusion des ions Li+ après la photo-polymérisation a ainsi été mise en évidence. La spectroscopie Raman a permis de démontrer que celle-ci est due à la complexation des ions par les chaines de poly(oxyde d’éthylène) de la matrice solide. En outre, grâce aux observations de différentes compositions, un mécanisme de diffusion mixte des ions Li+ par migration dans le liquide et par sauts dans le solide a été identifié. Par conséquent, ces résultats nous ont permis de définir une stratégie pour améliorer la diffusion des ions Li+ : l’ajout d’un copolymère monofonctionnel a permis de diminuer la densité de réticulation de la matrice solide et ainsi d’optimiser la mobilité des chaines polymères. En effet, les performances de cyclage dans des empilements de micro-accumulateurs complets ont été améliorées. A température ambiante, ces résultats se sont révélés très proches de ceux obtenus avec l’électrolyte solide standard LiPON. En conclusion, l’analyse établie a permis de comprendre les liens entre structure et performances électrochimiques, ce qui a permis de dégager les voies d’amélioration les plus prometteuses pour ce type d’électrolytes. / New deposition techniques compatible with making tridimensional geometries are currently being investigated with the aim of improving the performances of lithium microbatteries. This work focuses on the development of a new quasi-solid electrolyte deposited by a “wet process”. An ionic liquid-based membrane containing a lithium salt was prepared by the photo-induced polymerization of a dimethacrylate oligomer. New methods such as a new type of conductivity cell based on planar interdigitated electrodes to measure ionic conductivity as well as in-situ monitoring of photo-polymerization using impedance spectroscopy were used. Transport properties of lithium ion were measured by PGSE-NMR. Interestingly, a significant reduction of lithium ion mobility was observed after UV-curing while the total ionic conductivity only decreased slightly. This phenomenon is due to the formation of lithium ion complexes with ethylene oxide moieties of the solid matrix, evidenced by Raman spectroscopy measurements. Additionally, we have shown that the structures of the complexes depend on the salt concentration and a dual solid/liquid transport mechanism was suggested. Hence, in order to improve lithium ion diffusion, a co-polymer was added in an attempt to decrease the cross-linking density of the solid matrix thus improving its segmental motion. The cyclability of the all solid state micro batteries was indeed improved. Comparable performances with the standard solid electrolyte LiPON were obtained at room temperature. In summary, it was established that electrochemical performances of the solid state microbatteries depend to a certain extent on the structure of the polymer electrolyte. Therefore it is possible to find new ways in designing these types of electrolytes for further improvement.

Le gain en popularité de l’électricité dans le domaine énergétique, observable depuis plusieurs décennies, accentue l’urgence de développer des équipements de stockage efficaces et performants. Les batteries au lithium-ion (Li-ion), commercialisées depuis le début des années 1990, ont presque atteint les limites théoriques imposées par leurs composantes. La recherche s’oriente donc aujourd’hui vers les batteries tout-solide constituées d’une électrode négative de lithium métal. Ces batteries seraient en mesure d’atteindre des densités énergétiques supérieures à celles attribuables aux batteries lithium-ion utilisées et commercialisées à ce jour. Cependant, il subsiste toujours une impasse qui doit être solutionnée afin d’en assurer la viabilité : la formation de dendrites ou de mousse de lithium à la surface de l’électrode négative de lithium métal occasionne le court-circuit des batteries et en réduit l’espérance de vie. Plusieurs pistes de solutions sont proposées afin de réduire ou d’éliminer les problèmes de croissance dendritique et de mousse de lithium. Toutefois, il y a un manque d’information dans la littérature en lien avec la corrélation entre l’état de surface des électrodes négatives (anodes) de lithium métal et les performances électrochimiques de ces dernières. Ce projet de recherche visera donc, entre autres, à étudier l’impact de l’état de surface de l’électrode négative de lithium sur ses performances électrochimiques, dont son temps de vie, sa polarisation et son impédance. Une caractérisation a été effectuée sur les feuilles de lithium étudiées et sur l’interface lithium métal/électrolyte polymère. Suite à l’étude des feuilles sous leur forme native, des dépôts protecteurs d’or, d’aluminium et de fluorure de lithium ont été appliqués par déposition en phase vapeur (PVD) sur le lithium industriel de basse rugosité, afin d’évaluer si ces derniers amélioraient la performance électrochimique des cellules. La caractérisation physique a été effectuée par microscopie de force atomique à effet tunnel (Peakforce-TUNA) et microscopie électronique à balayage (MEB). Ensuite, la caractérisation chimique de chaque feuille de lithium utilisée a été caractérisée principalement par spectroscopie photoélectronique par rayons X (XPS) et par spectrométrie de masse à plasma induit (ICP-MS), permettant respectivement de connaître la composition chimique surfacique et complète des feuilles de lithium. Finalement, l’impact de l’interface lithium métal/électrolyte polymère sur la viabilité des cellules complètes a été déterminé par des cyclages galvanostatiques. Ces batteries ont enfin été observées post mortem par MEB afin d’observer l’impact du cyclage sur l’état interne des cellules. Il a été déterminé que la morphologie des feuilles de lithium et de l’interface lithium métal/électrolyte polymère ont un impact sans équivoque sur la durée de vie et sur la polarisation des cellules étudiées. Une méthode de préparation de surface électrochimique a donc été conçue, en cyclant les électrodes de lithium à basse densité de courant (0,130 mA.cm-2), améliorant ainsi la durée de vie des cellules symétriques exploitant des électrodes de lithium métal. / The increased use of electricity witnessed during the past few decades emphasizes the urgency of developing efficient and performing energy storing devices. Present on the market since the beginning of the 1990s, Lithium-ion (Li-ion) batteries have reached the theoretical limit inherent to their components. Research efforts currently aim at developing all-solid batteries composed of a negative lithium electrode. This type of electrode uses only lithium in its pure metallic state and it has the capacity to attain higher energy densities than those attributable to the lithium-ion batteries. Despite the potential of this promising technology, there is an obstacle that must be overcome in order to ensure its viability: the formation of dendrites and mossy lithium on the surface of the lithium metal negative electrode causes the batteries to short-circuit and reduces their life expectancy. Several solutions have been proposed in the literature in order to either eliminate or mitigate the issues of dendritic growth and mossy lithium. However, published studies do not specifically address the correlation between the state of the surface of the lithium metal and its electrochemical performance when used as the negative electrode (anode). This research project therefore focused on evaluating the impact of the state of the surface the lithium metal negative electrode on its electrochemical performance, such as its lifetime, polarization, and impedance. The lithium sheets and the lithium metal/polymer electrolyte interface were characterized in order to better understand the problematic processes related to the use of the lithium metal in batteries. In addition to studying the sheets in their native form, a protective gold deposit was applied by physical vapor deposition (PVD) on the lithium sheets to determine whether the deposit improved the electrochemical performance of the cells. The physical characterization was performed by using tunnelling atomic force microscopy (Peakforce-TUNA) and scanning electron microscopy (SEM). Each lithium x sheet used was then characterized by X-ray photoelectron spectroscopy (XPS) and coupled plasma mass spectrometry (ICP-MS). These chemical characterizations allowed to determine the surface and bulk chemical compositions of the lithium sheets. Finally, in order to understand the impact of the lithium metal/polymer electrolyte interface on the viability of complete cells, galvanostatic cycling, similar to true operating conditions of a battery, was performed. Cross-sections of these batteries were assessed post-mortem by SEM in order to analyze the impact of the cycling density on the internal state of the cells. It has been determined that the morphology of the lithium foils and the lithium metal/polymer electrolyte interface impacted the lifespan and the polarization of the studied cells. An electrochemical surface preparation method was therefore designed by cycling the lithium electrodes at a low current density (0.130 mA.cm-2), thus improving the life of the symmetrical cells composed of lithium metal electrodes.

No description available.

D. Tech. (Department of Mechanical Engineering, Faculty of Engineering and Technology), Vaal University of Technology. / On account of their lightweight, low-cost, corrosion resistance, and good formability, conductive polymer composites (CPCs) are promising for the production bipolar plate (BP) for polymer electrolyte membrane fuel cell (PEMFC). However, a high conductive filler loading is needed to impart the required level of electrical conductivity to the insulating polymer matrix and as a consequence, the toughness of the plate deteriorates considerably. By using immiscible blend of polymers that have complementary hardness and ductility as matrix, with conducting multi-fillers of different morphologies, it is possible to optimize the matrix strength characteristics and favour the formation of conducting network to produce CPC meeting BP performance standards. Of course, a lot will depend on the formulation of the most favourable composition and production variables. In this regard, polypropylene-epoxy and polyethylene-epoxy blends, filled with zero- and two-dimensional carbon forms – graphite, carbon black (CB), and graphene (Gr) – were investigated over an extensive range of compositions and compression moulding pressures, in this study. Several compounding runs (using melt mixing), at different stages, followed by compression molding, were done. The goal is to obtain combination of composite formulation and processing conditions that will produce the most promising combination of properties for BP application. In the first stage of the investigations, by using thermogravimetric analysis, two-stage decomposition behavior of PP-epoxy and PE-epoxy blends was revealed, which confirms the immiscibility of PP and PE with epoxy resin. Scanning electron microscope (SEM) micrographs of the PP-epoxy and PE-epoxy blends revealed a co-continuous structure, which can be attributed to the close-to-symmetric composition of the blend and compatibilizers added. Preferential localization of synthetic graphite (SG), CB, and Gr in the polymer blends was also revealed by the SEM micrographs. This confirms the fact that CPCs based on PP-epoxy and PE-epoxy blends can be explored further. PP-epoxy and PE-epoxy blends filled with only SG, 30 – 80 wt %, were produced and characterized for their electrical conductivity and flexural properties. In-plane electrical conductivity ranged from 12.09 to 68.03 Scm-1 for PP-epoxy/SG and 11.68 to 72.96 Scm-1 for PE-epoxy/SG composites produced. These are higher than values reported for several single matrix polymer composites at similar filler loadings. With reference to the United States Department of Energy performance targets for BPs, PE-epoxy/SG composites performed better in terms of electrical conductivity, while PP-epoxy/SG composites exhibited better flexural properties. Thereafter, using SG and CB double filler, PE-epoxy/SG/CB composites performed better than PP-epoxy/SG/CB composites in terms of electrical conductivity, while PP-epoxy/SG/CB composites exhibited superior flexural properties than the PE-epoxy/SG/CB composites at similar filler loadings. However, with respect to the DOE targets, composites based on PP-epoxy blend exhibited a more promising combination of electrical conductivity and flexural properties than PE-epoxy blend matrix. PP-epoxy filled with SG/CB was studied further, by using graphene (Gr) as second minor filler. In-plane and through plane electrical conductivities as well as thermal conductivity and thermal diffusivity of the PP-epoxy/SG/CB/Gr composites increased as total filler content was increased from 65 to 85 wt%. It implies that more conductive networks between filler particles were formed. Also, flexural strength, flexural modulus, and impact strength decreased as the total filler content increased from 65 to 85 wt%. The reduced flexural properties could be due to increased agglomeration of CB and Gr, and poor filler wetting at higher filler loadings and low matrix material, which leads to the formation of microvoids and a reduction of the load bearing capacity of composites. With respect to the DOE targets, PP-EP/SG/CB/Gr composite with 80 wt% (i.e., PP/EP/73G/6.2CB/0.8Gr) filler has the best combination of properties. Further improvement in properties of the PP-EP/SG/CB/Gr composite with 80 wt% filler was achieved by molding at higher pressures. As molding pressure was increased from 4.35 to 13.05 MPa, in-plane electrical conductivity increased from 116.31 to 144.99 Scm-1, while flexural strength increased from 29.62 to 42.57 MPa, satisfying the performance requirement targets for bipolar plates.

No description available.

M. Tech. (Department of Chemical Engineering, Faculty of Engineering and Technology), Vaal University of Technology. / Fuel cell technology is a promising clean energy source compared to internal combustion engines and electricity generating plants which are associated with high emissions of greenhouse gases. The aim of this study was to modify chitosan into polymer electrolyte membranes suitable for use in PEMFC and DMFC fuel cells. Chitosan modification was done with 2-aminoethanesulfonic acid (2-AESA), dimethylformamide (DMF) and silica nanoparticles. The effect of the modification on the properties of the developed chitosan membranes was studied using FTIR, XRD, SEM-EDS and TGA. The performance of the membrane electrode assemblies was investigated. The formation of electrostatic interactions in the developed sulfonated chitosan membranes was confirmed via the Fourier transform infrared (FTIR) analysis, indicating a shift in the wavenumber of the N-H bonds from 1581 cm-1 on the chitosan spectrum to a lower wavenumber of 1532 cm-1 in the FTIR spectra of the membranes and by the new peak at the wavenumber of ~1260 cm-1 attributed to the asymmetric O=S=O stretching vibrations of the sulphate groups and sulfonic acid groups from the cross-linking sulphuric acid solution and 2-aminoethanesulfonic acid incorporated on the chitosan polymer chain during the modification. Notably, the FTIR spectra of the developed sulfonated chitosan membranes lacked the peak at the wavenumber of ~1153 cm-1 attributed to the stretching of C-O-C bonds of the polysaccharide ring of chitosan. A reaction mechanism was proposed in this study illustrating the possible conversion of the polysaccharide rings of chitosan into a poly (cyclohexene-oxide) thermoplastic rings in the developed membranes. The TGA/DTGA results of the developed sulfonated chitosan membranes showed three degradation stages. The initial weight loss occurred at temperatures ˂100 °C due to the evaporation of volatile components and water molecules inside the membranes. The second degradation phase of the membranes occurred at 208 ℃ with a loss in weight of >30% resulting from the decomposition of cross-linking networks. The third degradation stage was associated with the decomposition of the main polymer backbone of the membranes and occurred at 263°C for the chitosan membranes modified with 2-aminoethanesulfonic acid and at 266 °C for the chitosan membrane modified with silica nanofiller. The TGA/DTGA curves of Nafion 117 showed a small loss in weight of ~ 5% before a sharp decomposition that occurred between 346–505 °C. The XRD diffractograms of the developed sulfonated chitosan membranes showed amorphous phases, the crystal peaks of chitosan at 2theta of 10° and 20° were flattened on the membranes. The SEM images showed a homogenous surface morphology for the sulfonated chitosan membrane with a higher weight percentage of 2-aminoethanesulfonic acid (13,6 wt.%). The SEM images performed on the surface of the sulfonated chitosan membrane modified silica nanoparticles showed a slight agglomeration associated with the migration of the unbonded silica to the surface. The methanol permeability coefficient of the developed sulfonated chitosan membrane modified with 2-aminoethanesulfonic acid was calculated to be 2.29x10-6 cm2/s. This value was close to the methanol permeability coefficient of 2.33x10-6 cm2/s associated with unfavourable depolarisation at the cathode in direct methanol fuel cells when using Nafion 117. The proton diffusion coefficient of Nafion 117 was calculated to be 1.64x10-5 cm2/s and that of the developed sulfonated chitosan membrane modified with 2-aminoethanesulfonic acid was found to be 6.56x10-6 cm2/s, respectively. The fuel cell performance of the developed sulfonated chitosan membrane modified with 2AESA was investigated in a hydrogen fuel cell (PEMFC) supplied with H2 and O2 directly from the electrolyser. The sulfonated chitosan membrane modified with 2-aminoethanesulfonic acid (13.6 wt.%) achieved an open-circuit voltage of ~0.9 V and a maximum power output of 64.7 mW/cm2 at a maximum current of 70 mA. The current produced by the developed chitosan membrane was applied into the load and was able to turn (power) the electric fan. The sulfonated chitosan membrane modified with silica nanoparticles (2 wt.%) yielded an open-circuit voltage of ~0.9 V and attained a maximum power output of 58 mW/cm2 at a maximum current output of 60 mA/cm2. The current generated by the membrane was also able to turn the electric fan. The Nafion 117 membrane was also investigated under similar conditions and obtained an open-circuit voltage of 0.6 V and a maximum power output of 130 mW/cm2 at the maximum current output of 308 mA. The current produced by Nafion 117 was supplied into the load and was able to turn the electric fan.

No description available.

No description available.

Polymer electrolyte membrane (PEM) fuel cell (FC) stacks require elastomeric gaskets for each cell to keep the reactant gases within their respective regions[1]. If any gasket degrades or fails, the reactant gases can leak or mix with each other directly during operation or standby, affecting the overall operation and performance of the FC. The elastomeric gaskets used as FC seals are exposed to a range of environmental conditions, and concurrently, subjected to mechanical compression between the bipolar plates forming the cell. The combination of mechanical stress and environmental exposure may result in degradation of the seal material[2] over a period of time. In order to address the durability and make reliability predictions, the long-term stability of the gaskets in FC assemblies is critical. The aim of this study is to investigate the performance of elastomeric seals in a simulated FC environment in the presence of mechanical stresses. The overall scope of the study includes mechanical and viscoelastic properties characterization, and lifetime durability predictions based on an accelerated characterization approach. With the help of finite element analysis software, ABAQUS, a fixture was designed to perform strain-based accelerated characterization of seal material in air, deionized (DI) water, 50v/50v ethylene glycol/water solution, and 0.1M sulfuric acid solution. Dogbone samples were strained to different levels in the custom fixture and submerged in liquid solutions at 90°C and in air at 90°C and 120°C. It was observed that mechanical properties such as tensile strength, strain to break, 100% modulus, crosslink density, and tensile set degrade due to aging and the extent of change (increase or decrease) depends significantly on the strain level on the specimen. Trouser tear tests were conducted on reinforced specimens in air and deionized water (DI) to evaluate the tear resistance of an elastomeric seal material intended for proton exchange membrane fuel cells. Plots relating the crack growth rate with tearing energy were obtained at various temperatures and provided significant insight into the rate and temperature dependence of the tearing strength of the seal material. Stick-slip crack propagation was observed at all temperatures and loading rates, although the behavior was suppressed significantly at low loading rates and high temperatures. Crack growth rate versus tearing energy data at different temperatures was shifted to construct a master curve and an estimate on the threshold value of tear energy was obtained which may be helpful in designing components where material tear is of concern. Strain energy release rate (SERR) value, calculated using the J-integral approach for a pre-existing crack in ABAQUS, was used to estimate the crack growth rate in a given seal cross-section to predict lifetime. In order to assess the viscoelastic behavior and to investigate the long term stress relaxation behavior of the seal material, compression stress relaxation (CSR) tests were performed on molded seals, called as SMORS, over a range of environmental conditions using a custom-designed fixture. The effect of temperature and environment was evident on material property changes and presented in terms of momentary properties and stress relaxation behavior. Various mechanisms involved in material degradation, chain scission and crosslinking, were suggested and insights were gained into how cure state and level of antidegradants in a material dictate the material behavior during the first phase of environmental exposure leading to change in material properties. Ring samples made of silicone were also tested using the fixture to obtain insight additional into material degradation due to aging. Results presented from testing on SMORS showed a lot more variation in data as compared to neat silicone rings due to the complexity involved in making SMORS. For understanding the deformation behavior of an elastomeric seal and its sealing performance, finite element characterization of seal cross-section was carried out on O-ring and SMORS cross-section. The effect of a seal's layout on distribution and magnitude of contact stresses and contact width was investigated for the O-ring and the information obtained thereby helped to analyze a complex assembly such as SMORS, where several interfaces and boundary conditions are involved. Stress/strain profiles were generated to visualize their concentration and distribution in the seal cross-section. Frictionless and rough interfacial conditions between seal material and platens were assumed and it was found that its effect on contact width and peak contact pressure was insignificant. Results obtained from FEA on SMORS were validated through comparison with contact mechanics approach and experimental data and it was found that Lindley's equation correlates well with experimental data whereas ABAQUS overestimates the load values at a given compression. Lindley's approach may be used to develop contact pressure profiles that may help estimate peak contact pressure at a given time so leaking can be avoided. / Ph. D.

The transition from fossil fuels to renewable resources has created more demand for energy storage devices. Lithium-oxygen (Li-O2) batteries have attracted much attention due to their high theoretical energy densities. They, however, are still in their infancy and several fundamental challenges remain to be addressed. Advanced analytical techniques have revealed that all components of a Li-O2 battery undergo undesirable degradation during discharge/charge cycling, contributing to reduced cyclability. Despite many attempts to minimize the anode and cathode degradation, the electrolyte remains as the leading cause for rapid capacity fading and poor cyclability in Li-O2 batteries. In this dissertation, composite gel polymer electrolytes (cGPEs) consisting of a UV-curable polymer, tetragylme based electrolyte, and glass microfibers with a diameter of ~1 µm and an aspect ratio of >100 have been developed for their use in Li-O2 battery application. The Li-O2 batteries containing cGPEs showed superior charge/discharge cycling for 500 mAh.g-1 cycle capacity with as high as 400% increase in cycles for cGPE over gel polymer electrolytes (GPEs). Results using in-situ electrochemical impedance spectroscopy (EIS), Raman spectroscopy, and scanning electron microscopy revealed that the source of the improvement was the reduction of the rate of lithium carbonates formation on the surface of the cathode. This decrease in formation rate afforded by cGPE-containing batteries was possible due to the decrease of the rate of electrolyte decomposition. The increase in solvated to the paired Li+ ratio at the cathode, afforded by increased lithium transference number, helped lessen the probability of superoxide radicals reacting with the tetraglyme solvent. This stabilization during cycling helped prolong the cycling life of the batteries. The effect of ion complexes on the stability of liquid glyme based electrolytes with various lithium salt concentrations has also been investigated for Li-O2 batteries. Charge/discharge cycling with a cycle capacity of 500 mAh·g-1 showed an improvement as high as 300% for electrolytes containing higher lithium salt concentrations. Analysis of the Raman spectroscopy data of the electrolytes suggested that the increase in lithium salt concentration afforded the formation of cation-solvent complexes, which in turn, mitigated the tetragylme degradation.