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Thesis (M.Sc.)--Memorial University of Newfoundland, 2000. / Restricted until November 2001. Bibliography: leaves 100-105.

Thesis--M. Phil., University of Hong Kong, 1980.

The driving force behind research in alternative clean and renewable energy has been the desire to reduce emissions and dependence on fossil fuels. In the United States, ground vehicles account for 30% of total carbon emission, and significantly contribute to other harmful emissions. This issue causes environmental concerns and threat to human health. On the other hand, the demand on fossil fuel grows with the increasing energy consumption worldwide. Particularly in the United States of America, transportation absorbs 75% of this energy source. There is an urgent need to reduce the transportation dependence on fossil fuel for the purpose of national security. Polymer electrolyte membrane (PEM) fuel cells are strong potential candidates to replace the traditional combustion engines. Even though research effort has transferred the fuel cell technology into real‒world vehicle applications, there are still several challenges hindering the fuel cell technology commercialization, such as hydrogen supply infrastructure, cost of the fuel cell vehicles, on‒board hydrogen storage, public acceptance, and more importantly the performance, durability, and reliability of the PEM fuel cell vehicles themselves. One of the key factors that affect the fuel cell performance and life is the run‒time thermal and water management. The temperature directly affects the humidification of the fuel cell stack and plays a critical role in avoiding liquid water flooding as well as membrane dehydration which affect the performance and long term reliability. There are many models exists in the literature. However, there are still lacks of control‒oriented modeling techniques that describe the coupled heat and mass transfer dynamics, and experimental validation is rarely performed for these models. In order to establish an in‒depth understanding and enable control design to achieve optimal performance in real‒time, this research has explored modeling techniques to describe the coupled heat and mass transfer dynamics inside a PEM fuel cell. This dissertation is to report our findings on modeling the temperature dynamics of the gas and liquid flow in the porous media for the purpose of control development. The developed thermal model captures the temperature dynamics without using much computation power commonly found in CFD models. The model results agree very well with the experimental validation of a 1.5 kW fuel cell stack after calibrations. Relative gain array (RGA) was performed to investigate the coupling between inputs and outputs and to explore the possibility of using a single‒input single‒output (SISO) control scheme for this multi‒input multi‒output (MIMO) system. The RGA analyses showed that SISO control design would be effective for controlling the fuel cell stack alone. Adding auxiliary components to the fuel cell stack, such as compressor to supply the pressurized air, requires a MIMO control framework. The developed model of describing water transport in porous media improves the modeling accuracy by adding catalyst layers and utilizing an empirically derived capillary pressure model. Comparing with other control‒oriented models in the literature, the developed model improves accuracy and provides more insights of the liquid water transport during transient response. / text

Megasonic cleaning is routinely used in the semiconductor industry to remove particulate contaminants from wafer and mask surfaces. Cleaning is achieved through proper choice of chemical solutions, power density and frequency of acoustic field. Considerable work has been done to increase understanding of particle removal mechanisms in megasonic cleaning using different solution chemistries with varying ionic strengths. However, to date, the focus of all these studies of particle removal has been either cavitation or acoustic streaming.The propagation of sound waves through a colloidal dispersion containing ions is known to result in the generation of two types of oscillating electric potentials, namely, Ionic Vibration Potential (IVP) and Colloid Vibration Potential (CVP). These potentials and their associated electric fields can exert forces on charged particles adhered to a surface, resulting in their removal. In addition, the pressure amplitude of the sound wave is also altered in solutions of higher ionic strengths, which can affect the cavitation process and further aid in the removal of particles from surfaces. To test the two hypotheses, investigations have been conducted on the feasibility of removal of charged particles from silicon wafers in electrolyte solutions of different ionic strengths irradiated with a megasonic field of different power densities. Cleaning experiments have been performed using potassium chloride (KCl) as a model electrolyte and silica particles as model contaminant particles. The cleaning performance in KCl solution has been compared to that in other electrolytes solutions such as sodium chloride, cesium chloride and lithium chloride. In order to characterize the cavitation events in KCl solutions, acoustic pressure and sonoluminescence measurements have been performed using hydrophone and cavitation probe respectively. The results indicate that particle removal efficiency (PRE) increases with KCl concentration and transducer power density and much lower power densities are required at higher KCl concentration for a comparable level of cleaning. Further, cleaning performances in NaCl and CsCl were found to be superior to those in KCl and LiCl solutions. Theoretical computations show that the removal forces due to CVP are much larger in magnitude than those due to IVP and are comparable to van der Waals forces.

With the rapid development of portable electronics, hybrid-electric and electric cars, there is great interest in utilization of sulfur as cathodes for rechargeable lithium batteries. Lithium/sulfur batteries implement inexpensive, the earth-abundant elements at the cathode while offering up to a five-fold increase in energy density compared with the present Li-ion batteries. However, electrically insulating character of sulfur and solubility of intermediate polysulfides in organic liquid electrolytes, which causes rapid capacity loss upon repeated cycling, restrict the practical application of Li/S batteries. In this thesis, the gel polymer and solid polymer electrolytes were synthesized and applied in Li/S batteries. A gel polymer electrolyte (GPE) was formed by trapping 1 M lithium bistrifluoromethane-sulfonamide (LiTFSI) in tetraethylene glycol dimethyl ether (TEGDME) electrolyte in a poly(vinylidenefluoride-co-hexafluoropropylene) (PVDF-HFP) /poly(methylmethacrylate) (PMMA) polymer matrix. The electrochemical properties of the resulting GPE were investigated in lithium/sulfur battery. The gel polymer battery exhibited a high specific capacity of 753.8 mAh gˉ¹ at the initial cycle, stable reversible cycling and a capacity retention about 80% over 40 cycles along with a high Coulombic efficiency. Comparative studies conducted with the 1 M LiTFSI liquid electrolyte cell demonstrated that a cell with liquid electrolyte has remarkably low capacity retention and Coulombic efficiency compared with the GPE cell. In the further studies, a solid polymer electrolyte (SPE) based on poly- (ethylene-oxide)/nanoclay composite was prepared and used to assemble an all-solid-state lithium/sulfur battery. The ionic conductivity of the optimized electrolyte has achieved about 3.22×10ˉ¹ mS cmˉ¹ at 60 °C. The Li/S cell with this SPE delivered an initial discharge capacity of 998 mAh gˉ¹ when operated at 60 °C, and retained a reversible capacity of 634 mAh gˉ¹ after 100 cycles. These studies has revealed that the electrochemical performance of lithium/sulfur cells, including charge-discharge cyclability and Coulombic efficiency, can be significantly improved by replacing liquid electrolytes with solid polymer and gel polymer electrolytes, which reduce the polysulfide shuttle effect and could protect the lithium anode from the deposition of the electrochemical reaction, leading to higher sulfur utilization in the cell.

No description available.

Among the many and varied fields of present day chemistry, that of optical activity occupies a not unimportant position. The rotatory powers of the alkali and ammonium salts of d-sec-βoctyl hydrogen phthalate have been studied in alcoholic and aqueous solution. The rotations of the salts were found to be in the order H > Li > NH4 > Na > K > Rb. It has been concluded that the variation of the molecular rotation of the salts with concentration may be explained by assuming changes in the degree of dissociation of the electrolytes and a deforming influence of the cations on the optically active anions. The effect of the addition of inorganic salts of various valence types to 0.2 M aqueous solutions of d-sec-β-octyl sodium phthalate has been investigated. The series for the depressive influence of the alkali and ammonium chlorides on the rotatory power of the optically active electrolyte is Cs+> Rb+> K+> Na+> NH4+> Li+. The corresponding series for the divalent chlorides is Ba++> Sr++> Ca++> Mg++. It is concluded that the observed effects may be attributed mainly to a deforming influence of the positive ion on the optically active negative ion, the explanation involving a consideration of the ionic radii. It is worthy of comment that the above results have been obtained by use of a carboxylic acid containing no hydroxyl group.

This thesis was devised as a fundamental study of the Li-S system by the use of 7Li Magic Angle Spinning (MAS) Nuclear Magnetic Resonance (NMR), X-ray Absorption Near- Edge Structure (XANES), and Non-Resonant Inelastic X-ray Scattering (NRIXS). The first part of this thesis reports the first evidence of a stable solid-phase intermediate between elemental sulfur (α-S8) and Li2S, Li2S6, which can be used to understand deeper Li-S battery. The second part of this thesis is based on operando XANES measurements made in the Argonne Photon Source (APS).Linear combination fit (LCF) analyses are performed to interpret the data; and, noticeably, the distinction between short-chain and long-chain polysulfides can be made due to the use of proper reference materials. The results reveal the first detailed observation of typical sulfur redox chemistry upon cycling, showing how sulfur fraction (under-utilization) and sulfide precipitation impact capacity. It also gives new insights into the differences between the charge and discharge mechanisms, resulting in the hysteresis of the cycling profile. Operando XANEs were also performed on het-treated material, which exhibits a particular electrochemical signature, which has never explained. After a preliminary electrochemical study by potentiodynamic cycling with galvanostatic acceleration (PCGA), operando XANES measurements at the sulfur K-edge are performed on heat-treated PCNS. Noticeably, the difference in the XANES signatures of the pristine and the recharged state shows the irreversible process that occurs during the first discharges. At last, electrolytes are investigated by the compilation of quantitative physico-chemical parameters – viscosity, ionic conductivity, and solubility of Li2S and Li2S6 – on novel class of solvents that are glymes with non-polar groups and acetonitrile (ACN) complexed with LiTFSI. 1,1,2,2-Tetrafluoroethyl 2,2,3,3-tetrafluoropropyl ether (HFE) is chosen to decrease their viscosities. (ACN)2:LiTFSI attracts particular attention because of the particularly low Li2Sn solubility and. Its good electrochemical performance when mixed with 50 vol% HFE. Operando XANES proves the formation of polysulfides in this electrolyte, although constrains imposed by this novel electrolyte to the XANES experiment complicate the data analysis. The low energy feature evolution shows a more progressive mechanism involved in this electrolyte, which could be linked to the particularly low Li2Sn solubility

This thesis is concerned with investigating the suitability of room temperature ionic liquids as solvents in which to perform voltammetry, and in characterising electrochemical processes within these media. After providing a general introduction and a background to the ionic liquid field, the results of six original studies are presented, dealing in turn with the following subjects: • The oxidation of N,N,N',N'-tetraalkyl-para-phenylenediamine (TAPD) in five ionic liquids each incorporating the bis(trifluoromethylsulfonyl)imide anion. • The reduction of oxygen in four ionic liquids based on quaternary alkyl -onium cations and heavily fluorinated anions in which the central ion is either nitrogen or phosphorous. The simulation of double potential step chronoamperometry at a disk electrode for the case of unequal diffusion coefficients and its experimental validation using a variety of aqueous, traditional nonaqueous and ionic liquid solutions. • The rate of diffusion of N,N,N',N'-tetramethyl-para-phenylenediamme (TMPD), its radical cation and dication as a function of temperature and ionic liquid viscosity and four such solvents. • The temperature dependence of the viscosity of five ionic liquids along with the translational and rotational diffusion coefficients of dissolved 2,2,6,6- tetramethylpiperidine-N-oxyl (TEMPO). • The kinetics of the reaction between N,N-dimethyl-para-toluidine (DMT) and its electrogenerated radical cation in an ionic liquid solvent. The experimental strategy common to each report involves the application of cyclic voltammetry and chronoamperometry at disk electrodes immersed in uL-samples of ionic liquid solution. The data so measured is then analysed via the appropriate theoretical equations or, as is commonly necessary, by comparison with simulated voltammetry. Combined, these chosen redox systems provide access to information on various aspects of electrochemistry within ionic liquids, specifically (a) mass transport (b) the nature of the electron transfer process and (c) the rate of follow-up homogeneous reactions. It is the overall finding herein that while both diffusion and heterogeneous electron transfer are significantly slowed relative to the same processes in a conventional organic solvent, the rate of subsequent homogeneous chemistry remains largely unchanged.