Solid Oxide Fuel Cells are fuel cells that operate at high temperatures usually in the range of 600oC to 1000oC and employ solid ceramics as the electrolyte. In Solid Oxide Fuel Cells oxygen ions (O2- ) are the ionic charge carriers. Solid Oxide Fuel Cells are known for their higher electrical efficiency of about 50-60% [1] compared to other types of fuel cells and are considered very suitable in stationary power generation applications. It is very important to study the effects of different parameters on the performance of Solid Oxide Fuel Cells and for this purpose the experimental or numerical simulation method can be adopted as the research method of choice. Numerical simulation involves constructing a mathematical model of the Solid Oxide Fuel Cell and use of specifically designed software programs that allows the user to manipulate the model to evaluate the system performance under various configurations and in real time. A model is only usable when it is validated with experimental results. Once it is validated, numerical simulation can give accurate, consistent and efficient results. Modeling allows testing and development of new materials, fuels, geometries, operating conditions without disrupting the existing system configuration. In addition, it is possible to measure internal variables which are experimentally difficult or impossible to measure and study the effects of different operating parameters on power generated, efficiency, current density, maximum temperatures reached, stresses caused by temperature gradients and effects of thermal expansion for electrolytes, electrodes and interconnects. iv Since Solid Oxide Fuel Cell simulation involves a large number of parameters and complicated equations, mostly Partial Differential Equations, the situation calls for a sophisticated simulation technique and hence a Finite Element Method (FEM) multiphysics approach will be employed. This can provide three-dimensional localized information inside the fuel cell. For this thesis, COMSOL Multiphysics® version 4.2a will be used for simulation purposes because it has a Batteries & Fuel Cells module, the ability to incorporate custom Partial Differential Equations and the ability to integrate with and utilize the capabilities of other tools like MATLAB ® , Pro/Engineer® , SolidWorks® . Fuel Cells can be modeled at the system or stack or cell or the electrode level. This thesis will study Solid Oxide Fuel Cell modeling at the cell level. Once the model can be validated against experimental data for the cell level, then modeling at higher levels can be accomplished in the future. Here the research focus is on Solid Oxide Fuel Cells that use hydrogen as the fuel. The study focuses on solid oxide fuel cells that use 3-layered, 4-layered and 6-layered electrolytes using pure YSZ or pure SCSZ or a combination of layers of YSZ and SCSZ. A major part of this research will be to compare SOFC performance of the different configurations of these electrolytes. The cathode and anode material used are (La0.6Sr0.4)0.95-0.99Co0.2Fe0.8O3 and Ni-YSZ respectively

Energy storage is crucial for intermittent renewable energy sources, electric vehicles, and portable devices. The continuously increasing energy consumption in these industries necessitates the enhancement of commercial lithium-ion batteries (LIB), especially regarding their safety and energy density. Historically, aqueous electrolytes were the norm in the battery industry. Prior to the development of lithium batteries, most commercially significant batteries used water as the solvent. In the past decade, "highly concentrated" electrolytes resurrected the notion of an aqueous lithium-ion battery (ALIB). Significant efforts have been made since then to comprehend the interfacial stability of these high-concentration electrolytes, and make them suitable for use in batteries especially high voltage ones. Another candidate for future batteries is All-Solid-State Batteries (ASSB) as they have the potential to double, or even triple, the energy density figures we currently achieve in LIBs mainly due to their ability to utilize lithium metal anode which has the highest specific capacity among anodes (3860 mAh g⁻¹), lowest reduction potential (-3.04 V vs SHE), and low density (0.53 g cm⁻³). This thesis first proposes a phenomenological model to describe the microstructure of aqueous electrolyte and the relation between their phase diagrams with ionic conductivity; highlighting a common correlation between the eutectic composition and peak ionic conductivity in conductivity isotherms. we then propose an empirical model correlating ionic conductivity with both molar concentration and temperature. The aim of this portion of the thesis is to provide an in depth understanding of aqueous electrolytes' physical properties in a way that can help researchers optimize the energy density and the cost of ALIBs. Moving further, the thesis presents two novel composite solid electrolytes (CSE) that were developed and fully characterized. Both of which were composed of the following four components; polyethylene oxide (PEO), lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) salt, lithium lanthanum titanate (LLTO) perovskite inorganic ceramic and the polymer plasticizer succinonitrile (SN). The careful formulation of these CSEs was based on the trade-off between film forming ability and ionic conductivity. The optimized polymer rich CSE proved to have better characteristics when compared to its ceramic rich alternative. ASSBs employing both CSEs were successfully charged and discharged when coupled with lithium metal anode and in-lab prepared composite cathode. The developed thin and flexible CSEs could be utilized in small applications (Wh-KWh) such as in consumer electronics and flexible biomedical devices (e.g., pacemakers) or larger applications (kWh-MWh) such as in EVs and large format storage for the electrical grid.

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This thesis presents a compendium of work related to performance analyses of a proton electrolyte membrane (PEM) fuel cell with two novel design configurations. The finite element based numerical analysis has been carried out to solve the numerical transport models involved in a PEM fuel cell coupled with the flow in a porous medium, charge balance, electrochemical kinetics and membrane water content. The scope of this research work focuses on improving the performance of the PEM fuel cell by optimizing the thermo-fluid properties of the reactant species instead of analysing the complex electro-chemical interactions. Two new design configurations have been numerically analyzed; in the first design approach, a perforated-type gas flow distributor is used instead of a conventional gas flow distributor such as a serpentine, straight or spiral shape; the second design approach examines the effect of reactant flow pulsation on the PEM fuel cell performance. Results obtained from the numerical analyses were also compared with the experimental data and a good agreement was found. Performance of the PEM fuel cell with a perforated-type gas distributor was analyzed at different operating and geometric conditions to explore the merits of this new design configuration. Two-dimensional numerical analyses were carried out to analyze the effect of varying the different operating parameters; threedimensional numerical analyses were carried out to study the variation of different geometric parameters on overall performance of the new design configuration of the PEM fuel cell. The effects of the reactant flow pulsation on the performance of PEM fuel cell were analyzed using a two-dimensional numerical approach where both active and passive design configurations were numerically simulated to generate the pulsations in the reactant flow. The results showed a considerable increase in overall performance of the PEM fuel cell by introducing pulsations in the flow.

It is known that during fluid restriction, obligatory water losses continue and hypohydration develops and that restricted energy intake leads to a concomitant restriction of all other dietary components, as well as hypohydration, but the specific effects of periods of fluid and/ or energy restriction on fluid balance, electrolyte balance and exercise performance have not been systematically described in the scientific literature. There were two main aims of this thesis. Firstly, to describe the effects of periods of severe fluid and/ or energy restriction on fluid and electrolyte balance; secondly, to determine the effect of electrolyte supplementation during and after energy restriction on fluid and electrolyte balance as well as energy exercise performance. The severe restriction of fluid and/ or energy intake over a 24 h period all resulted in body mass loss (BML) and hypohydration, but whilst serum osmolality increases during fluid restriction (hypertonic hypohydration), serum osmolality does not change during energy restriction (isotonic hypohydration), despite similar reductions in plasma volume (Chapter 3). These differences in the tonicity of the hypohydration developed are most likely explainable by differences in electrolyte balance, with fluid restriction resulting in no change in electrolyte balance over 24 h (Chapter 3) and energy restriction (with or without fluid restriction) producing significant reductions in electrolyte balance by 24 h (Chapter 3; Chapter 4; Chapter 5; Chapter 6; Chapter 7). Twenty four hour combined fluid and energy restriction results in large negative balances of both sodium and potassium, and whilst the addition of sodium chloride to a rehydration solution ingested after fluid and energy restriction increases drink retention, the addition of potassium chloride to a rehydration solution does not (Chapter 4). Supplementation of sodium chloride and potassium chloride during periods of severe energy restriction reduces the BML observed during energy restriction and maintains plasma volume at pre-energy restriction levels (Chapter 5; Chapter 6; Chapter 7). iv These responses to electrolyte supplementation during energy restriction appear to be related to better maintenance of serum osmolality and electrolyte concentrations and a consequential reduction in urine output (Chapter 5; Chapter 6; Chapter 7). Additionally, 48 h energy restriction resulted in a reduction in exercise capacity in a hot environment and an increase in heart rate and core temperature during exercise, compared to a control trial providing adequate energy intake. Whilst electrolyte supplementation during the same 48 h period of energy restriction prevented these increases in heart rate and core temperature and exercise capacity was not different from the control trial Chapter 8). In conclusion, 24-48 h energy restriction results in large losses of sodium, potassium and chloride in urine and a large reduction in body mass and plasma volume and supplementation of these electrolytes during energy restriction reduces urine output, attenuates the reduction in body mass and maintains plasma volume and exercise capacity.

<p>In polymer electrolyte fuel cells (PEFC) chemical energy, in for example hydrogen, is converted by an electrochemical process into electrical energy. The PEFC has a working temperature generally below 100 °C. Under these conditions water management and transport of oxygen to the cathode are the parameters limiting the performance of the PEFC.</p><p>The purpose of this thesis was to better understand the complex processes in different parts of the PEFC. The rate-limiting processes in the cathode were studied using pure oxygen while varying oxygen pressure and humidity. Mass-transport limitations in the gas diffusion layer using oxygen diluted in nitrogen or helium was also studied. A large capacitive loop was seen at 1-10 Hz with 5-20 % oxygen. When nitrogen was changed to helium, which has a higher binary diffusion coefficient, the loop decreased and shifted to a higher frequency.</p><p>Steady-state and electrochemical impedance spectroscopy (EIS) models have been developed that accounts for water transport in the membrane and the influence of water on the anode. Due to water drag, the membrane resistance changes with current density. This gives rise to a low frequency loop in the complex plane plot. The loop appeared at a frequency of around 0.1 Hz and varied with <em>D</em>/<em>L_m</em>², where <em>D</em> is the water diffusion coefficient and <em>L_m</em> is the membrane thickness. The EIS model for the hydrogen electrode gave three to four semicircles in the complex plane plot when taking the influence of water concentration on the anode conductivity and kinetics into account. The high-frequency semicircle is attributed to the Volmer reaction, the medium-frequency semicircle to the pseudocapacitance resulting from the adsorbed hydrogen, and the low-frequency semicircles to variations in electrode performance with water concentration. These low-frequency semicircles appear in a frequency range overlapping with the low-frequency semicircles from the water transport in the membrane. The effects of current density and membrane thickness were studied experimentally. An expected shift in frequency, when varying the membrane thickness was seen. This shift confirms the theory that the low-frequency loop is connected to the water transport in the membrane.</p>

<p>This thesis aims at formulating and validating models for electrochemical energy storage devices. More specifically, the devices under consideration are lithium ion batteries and polymer electrolyte fuel cells.</p><p>A model is formulated to describe an experimental cell setup consisting of a Li_xNi_0.8Co_0.15Al_0.05O₂ composite porous electrode with three porous separators and a reference electrode between a current collector and a pure Li planar electrode. The purpose of the study being the identification of possible degradation mechanisms in the cell, the model contains contact resistances between the electronic conductor and the intercalation particles of the porous electrode and between the current collector and the porous electrode. On the basis of this model formulation, an analytical solution is derived for the impedances between each pair of electrodes in the cell. The impedance formulation is used to analyse experimental data obtained for fresh and aged Li_xNi_0.8Co_0.15Al_0.05O₂ composite porous electrodes. Ageing scenarios are formulated based on experimental observations and related published electrochemical and material characterisation studies. A hybrid genetic optimisation technique is used to simultaneously fit the model to the impedance spectra of the fresh, and subsequently also to the aged, electrode at three states of charge. The parameter fitting results in good representations of the experimental impedance spectra by the fitted ones, with the fitted parameter values comparing well to literature values and supporting the assumed ageing scenario.</p><p>Furthermore, a steady state model for a polymer electrolyte fuel cell is studied under idealised conditions. The cell is assumed to be fed with reactant gases at sufficiently high stoichiometric rates to ensure uniform conditions everywhere in the flow fields such that only the physical phenomena in the porous backings, the porous electrodes and the polymer electrolyte membrane need to be considered. Emphasis is put on how spatially resolved porous electrodes and nonequilibrium water transport across the interface between the gas phase and the ionic conductor affect the model results for the performance of the cell. The future use of the model in higher dimensions and necessary steps towards its validation are briefly discussed.</p>

A test system for the performance analysis of a novel thermally regenerative fuel cell (TRFC) using propiophenone and hydrogen as the oxidant and fuel respectively was designed and built. The test system is capable of either hydrogen-air or hydrogen-propiophenone operation. Membrane electrode assemblies (MEAs) were made using commercial phosphoric acid-doped polybenzimidazole (PBI) membranes and commercial electrodes. Using Pt/carbon paper electrodes with a catalyst loading of 1mg/cm2 and a membrane with an acid doping level of 10.2 mol acid/mol of polymer repeat unit, a maximum performance of 212 mW/cm2 at a current density of 575 mA/cm2 was achieved for baseline hydrogen-air testing at 110°C. Problems were encountered, however, in achieving consistent, reproducible performance for in-house fabricated MEAs. Furthermore, ex-situ electrochemical impedance spectrometry (EIS) showed that the phosphoric acid-doped PBI was unstable in the propiophenone and that acid-leaching was occurring. In order to have MEAs with consistent characteristics for verifying the test system performance, commercial phosphoric acid-doped PBI membrane electrode assemblies were used. At a temperature of 160°C and atmospheric pressure with hydrogen and air flowrates of 150 mL/min and 900 mL/min respectively a maximum power density of 387 mW/cm2 at a current density of 1.1 A/cm2 was achieved. This performance was consistent with the manufacturer’s specifications and these MEAs were subsequently used to verify the performance of TRFC test system despite the EIS results that indicated that acid-leaching would probably occur. The Pt catalyzed commercial MEAs achieved very limited performance for the hydrogenation of the ketone. However, the performance was less than but comparable to similar results previously reported in the literature by Chaurasia et al. [1]. For pure Pt catalyst loading of 1 mg/cm2, using a commercial PBI MEA operating at 160°C and atmospheric pressure, the maximum power density was 40 µW/cm2 at a current density of 1.3 mA/cm2. A 16 hour test was conducted for these conditions with a constant 1 ohm load, successfully demonstrating the operation of the test system. The test system will be used in the development of better catalysts for ketone hydrogenation. / Thesis (Master, Chemical Engineering) -- Queen's University, 2012-10-12 10:00:58.854

The long-term outdoor durability of dye-sensitized solar cells (DSSCs) is still a challenging issue for the large-scale commercial application of this promising photovoltaic technique. In order to study the degradation mechanism of DSSCs, ageing tests under selected accelerating conditions were carried out. The electrolyte is a crucial component of the device. The interactions between the electrolyte and other device components were unraveled during the ageing test, and this is the focus of this thesis. The dynamics and the underlying effects of these interactions on the DSSC performance were studied. Co(bpy)32+/3+-mediated solar cells sensitized by triphenylamine-based organic dyes are systems of main interest. The changes with respect to the configuration of both labile Co(bpy)32+ and apparently inert Co(bpy)33+ redox complexes under different ageing conditions have been characterized, emphasizing the ligand exchange problem due to the addition of Lewis-base-type electrolyte additives and the unavoidable presence of oxygen. Both beneficial and adverse effects on the DSSC performance have been separately discussed in the short-term and long-term ageing tests. The stability of dye molecules adsorbed on the TiO2 surface and dissolved in the electrolyte has been studied by monitoring the spectral change of the dye, revealing the crucial effect of cation-based additives and the cation-dependent stability of the device photovoltage. The dye/TiO2 interfacial electron transfer kinetics were compared for the bithiophene-linked dyes before and after ageing in the presence of Lewis base additives; the observed change being related to the light-promoted and Lewis-base-assisted performance enhancement. The effect of electrolyte co-additives on passivating the counter electrode was also observed. The final chapter shows the effect of electrolyte composition on the electrolyte diffusion limitation from the perspectives of cation additive options, cation concentration and solvent additives respectively. Based on a comprehensive analysis, suggestions have been made regarding lithium-ion-free and polymer-in-salt strategies, and also regarding cobalt complex degradation and the crucial role of Lewis base additives. The fundamental studies contribute to the understanding of DSSC chemistry and provide a guideline towards achieving efficient and stable DSSCs. / <p>QC 20160517</p>

There has been a remarkable progress in the development of organic electronic materials since the discovery of conducting polymers more than three decades ago. Many of these materials can be processed from solution, in the form as inks. This allows for using traditional high-volume printing techniques for manufacturing of organic electronic devices on various flexible surfaces at low cost. Many of the envisioned applications will use printed batteries, organic solar cells or electromagnetic coupling for powering. This requires that the included devices are power efficient and can operate at low voltages. This thesis is focused on organic thin-film transistors that employ electrolytes as gate insulators. The high capacitance of the electrolyte layers allows the transistors to operate at very low voltages, at only 1 V. Polyanion-gated p-channel transistors and polycation-gated n-channel transistors are demonstrated. The mobile ions in the respective polyelectrolyte are attracted towards the gate electrode during transistor operation, while the polymer ions create a stable interface with the charged semiconductor channel. This suppresses electrochemical doping of the semiconductor bulk, which enables the transistors to fully operate in the field-effect mode. As a result, the transistors display relatively fast switching (≤ 100 µs). Interestingly, the switching speed of the transistors saturates as the channel length is reduced. This deviation from the downscaling rule is explained by that the ionic relaxation in the electrolyte limits the channel formation rather than the electronic transport in the semiconductor. Moreover, both unipolar and complementary integrated circuits based on polyelectrolyte-gated transistors are demonstrated. The complementary circuits operate at supply voltages down to 0.2 V, have a static power consumption of less than 2.5 nW per gate and display signal propagation delays down to 0.26 ms per stage. Hence, polyelectrolyte-gated circuits hold great promise for printed electronics applications driven by low-voltage and low-capacity power sources.