<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>

Microbial fuel cells (MFC) are an emerging renewable technology which converts complex organic matter to electrical power using microorganisms as the biocatalyst. A variety of biological relevant organic matters such as glucose, acetate and ethanol have been utilized for the successful operation of a MFC. In this regard, the investigation of a MFC inoculated with ethanol oxidizing bacteria is of particular interest for this research due to its ability to simultaneously produce electricity while reducing ethanol pollution (a type of volatile organic carbon (VOC) pollutant) with potential use in modified biological air pollution control technology such as biofiltration. In this research, ethanol-oxidizing microbial species isolated from soil and compost samples were identified, with Acinetobacter calcoaceticus being the dominant strain. In order to understand the metabolism of the anode microbial cells, which is considered to be the key dictating the performance of a MFC, a systematic analysis/optimization of the growth rate and biomass production for A. calcoaceticus were carried out. A maximum specific growth rate with a final biomass concentration of 1.68 g/l was derived when aerated at a rate of 0.68 vvm. It has been recognized that one of the principle constraints in increasing the current density of MFCs is the electron transfer from the bacteria to the anode. In this sense, the addition of a redox mediator, which facilitates the process of the electron transfer, is desired for the efficient operation of a MFC. Thionine, methylene blue (MB), resorufin and potassium ferricyanide that have been profusely utilized as effective mediator compounds in many MFC studies, however, specific information on the biomass sorption of these compounds was lacking and therefore were selected for this research. All mediators tested were reduced biologically in A. calcoaceticus inoculated samples as indicated by the color transition from the pigmented oxidized form to the colorless reduced form. Subsequent tests on mediator color removal revealed that physical adsorption by the biomass, aggregation as well as precipitation accounted for a significant portion of the color loss for thionine and MB. It was speculated that the fraction of the initial mediator concentration sequestered, aggregated and/or precipitated no longer contributed to the electron transfer process, resulting in a current production which was proportional to the measurable mediator concentration remained in anode solution. To verify this hypothesis, chronoamperometric measurements were conducted for various mediator systems at known initial and measurable concentrations. The data obtained on the current produced were in good agreement with the theoretical predictions calculated from the actual mediator concentration, suggesting that the current produced depended on the concentration of mediator remaining in solution. Finally, the microbial reduction kinetics and the cytotoxicity of potassium ferricyanide were analyzed. The reduction of potassium ferricyanide followed zero order kinetics with the specific reduction rate increased as the initial mediator concentration increased from 1 mM to 200 mM. Inhibitory effects on cell growth were observed at initial potassium ferricyanide concentration of 50 mM.

This study presents a detailed gas-phase and surface kinetic model for n-tetradecane autothermal reforming to deconvolute the complex reaction network that provides the mechanistic understanding of reforming chemistry in a packed-bed reactor. A thermodynamic analysis study for diesel reforming was performed to map the carbon formation boundary for various reforming processes. Through a Langmuir-Hinshelwood-Hougen-Watson (LHHW) type of kinetic model, which was derived using a simple mechanistic study, the need for a detailed kinetic study including both gas-phase reactions and surface reactions was identified. Pt-CGO (Pt on Gd doped CeO2) and Rh-pyrochlore catalysts were synthesized and characterized. In an accelerated test for reforming of commercial-diesel, Rh-pyrochlore catalyst showed stable performance for 24 hrs, whereas Pt-CGO catalyst deteriorated in 4 hrs. Minimum structural change in Rh-pyrochlore catalyst compared to Pt-CGO catalyst was observed using redox experiments. An experimental kinetic study with an inert silica bed provided clear evidence that the gas-phase reactions are important to the kinetics of hydrocarbon reforming. “Reaction Mechanism Generator” (RMG) software was employed to generate a detailed gas-phase kinetic model containing nine thousand three hundred and forty-seven elementary reactions and four hundred and fifty-nine species. The model was validated against n-tetradecane ignition delay data, and inert bed autothermal reforming data. The RMG model was also extended to capture the high pressure and low temperature pyrolysis chemistry to predict pyrolysis experimental data. The reactor simulation using the RMG model identified the detailed chemistry of the reactions in the pre-catalytic zone. Gas-phase oxidation/pyrolysis converts the heavier hydrocarbons and oxygen in the pre-catalytic zone to lower molecular weight products prior to reaching the catalyst surface. The steam reforming reactions that are dominant on the surface of the catalyst primarily involve lower molecular weight oxidation/pyrolysis products. A multi-component micro-kinetic model containing two hundred and seventy surface reactions and fifty-two adspecies was developed using a semi-empirical Unity Bond Index-Quadratic Exponential Potential (UBI-QEP) method. Transition State Theory estimates were used for elementary reactions up to C3 species, and simple fragmentation reactions were assumed for higher hydrocarbon species. Model simulations indicated on the catalyst surface that hydrogen is initially produced by the water-gas-shift reaction and subsequently by steam reforming reactions. A major reaction path for ethylene formation from 1,3 butadiene in the post-catalytic zone of the reactor was also identified. / Thesis (Ph.D, Chemical Engineering) -- Queen's University, 2013-04-24 13:23:31.163

Solid oxide fuel cells represent a potentially important application for ceramic materials. There are, however, some significant issues which can affect the reliability and durability of the cell. Mechanical failure owing to stress is one of the critical factors which can affect the stability and working life of the fuel cell stacks. These stresses generate in Solid Oxide Fuel Cells (SOFCs) owing to mechanical forces and change in temperature during fabrication, assembly and operating conditions. There can be chances of cell delamination and micro-cracks in cell electrodes if these stresses are too high. The elastic properties and thermal expansion coefficient play a vital role to improve cell stability and performance. These properties depend on the types of materials and geometries of the composites. In this research, a numerical framework to predict the effective elastic properties and the effective thermal expansion coefficient for porous Yttria-Stabilized Zirconia (YSZ) electrode microstructures in a Solid Oxide Fuel Cell is presented. The electrodes of Solid Oxide Fuel Cells are discretized as porous microstructures that are formed by randomly distributed and overlapping spheres with particle size distributions that match those of actual ceramic powder. Three-dimensional (3D) microstructures of YSZ-pore are formed with a porosity ranging from 25% to 40%. The technique involves the construction of the YSZ-pores microstructures based on measurable starting parameters and subsequent numerical prediction of effective elastic properties and effective thermal expansion coefficient. Three domain sizes are considered for the generation of YSZ-pore microstructures. The method of prediction of effective Young’s modulus (Eeff), effective Poisson’s ratio , effective bulk modulus effective shear modulus , and effective thermal expansion coefficients for various porosities (P) of Yttria-Stabilized Zirconia (YSZ) electrode material in Solid Oxide Fuel Cells is based on the Finite Volume analysis which in turn is based on the solution of the linear elastic stress analysis problem. The predicted results are compared with some theoretical correlations of two-phase composites for effective elastic properties and effective thermal expansion coefficient. It has been found that predicted results are falling inside of the upper and lower bounds. / Thesis (Master, Mechanical and Materials Engineering) -- Queen's University, 2013-10-01 17:01:05.068

The Liquid Salt Cooled Reactor (LSCR) provides several potential benefits compared to pressurized water-cooled reactor systems. These include low operating pressure of the liquid salt coolant, the high burnup tolerance of the fuel, and the high operating temperatures which leads to increases in efficiency. However, due to inherently low heavy metal loading, the fuel cycle design presents specific challenges. In order to study options for optimizing the fuel design and fuel cycle, SCALE6.1 was used to create simplified models of the reactor and look at various parameters. The primary parameters of interest included packing factor and fuel enrichment. An economic analysis was performed on these results by developing a simple fuel cycle cost (FCC) model that could be used to compare the different options from an economic standpoint. The lithium enrichment of the FLiBe coolant was also investigated. The main focus was to understand the practical limitations associated with the Li-7 enrichment and whether it could be used for beneficial purposes. The main idea was to determine whether a lower-than-equilibrium enrichment could be used at reactor start up so that the Li-6 isotope acts as a burnable absorber. The results for the lithium enrichment study showed that the enrichment converges over time, but the amount of time required to reach steady state is much too long and the FLiBe coolant could not be utilized for reactivity control as a burnable absorber. The results found through this research provide reasonable guidelines for expected costs and narrow down the types of configurations that should be considered as fuel design options for the LSCR. Additionally, knowledge was gained on methods for modeling the system not only accurately but also efficiently to reduce the required computing power and time.

The maximum theoretical energy efficiency of fuel cells is much larger than those of the steam-power-turbine cycles that are currently used for generating electrical power. Similarly, direct hydrocarbon fuel cells, DHFCs, can theoretically be much more efficient than hydrogen fuel cells. Unfortunately the current densities (overall reaction rates) of DHFCs are substantially smaller than those of hydrogen fuel cells. The problem is that the exchange current density (catalytic reaction rate) is orders of magnitude smaller for DHFCs. Other work at the University of Ottawa has been directed toward the development of polymer electrolytes for DHFCs that operate above the boiling point of water, making corrosion rates much slower so that precious metal catalysts are not required. Propane (liquefied petroleum gas, LPG) was the hydrocarbon chosen for this research partly because infrastructure for its transportation and storage in rural areas already exists. In this work nickel based catalysts, an inexpensive replacement for the platinum based catalysts used in conventional fuel cells, were examined using density functional theory, DFT. The heats of propane adsorption for 3d metals, when plotted as a function of the number of 3d electrons in the metal atom, had the shape of a volcano plot, with the value for nickel being the peak value of the volcano plot. Also the C-H bond of the central carbon atom was longer for propane adsorbed on nickel than when adsorbed on any of the other metals, suggesting that the species adsorbed on nickel was less likely to desorb than those on other metals. The selectivity of the propyl radical reaction was examined. It was found that propyl radicals

Understanding how the water produced in an operating polymer electrolyte membrane fuel cell (PEMFC) is transported in cathode catalyst layer (CCL) is crucial to improving performance and efficiency. In this thesis, a multiple-relaxation-time (MRT) lattice Boltzmann method (LBM) is employed to simulate the high density ratio, multiphase water transport in in the CCL. The three-dimensional structure of the catalyst layer is reconstructed based on experimental data acquired with a dual beam scanning electron microscope/focused ion beam system and a stochastic method using lower order statistical functions (e.g. porosity and two point correlation functions). Simulations of the water transport dynamics are performed to examine the effect of a range of physical parameters: wettability, viscosity ratio, pressure gradient, and surface tension. The water penetration patterns in the catalyst layers reveal a complex fingering process and transition of the water transport pattern from a capillary fingering regime to a stable displacement regime is observed when the wettability potential of the catalyst layer changes. The second part of the analysis focuses on quantifying the impact of liquid water distribution and accumulation in the catalyst layer on effective transport properties by coupling two numerical methods: the two-phase LBM is used to determine equilibrium liquid water distribution, and then a finite volume-based pore-scale model (FV-PSM) is used to compute transport of reactant and charged species in the CL accounting for the impact of liquid water saturation .The simulated results elucidate and quantify the significant impact of liquid water on the effective oxygen and water vapor diffusivity, and thermal conductivity in CLs. / Graduate

Polymer Electrolyte Membrane Fuel Cell (PEMFC) systems are promising technology for contributing to meet the deficiency of world`s clean and sustainable energy requirements in the near future. Metallic bipolar plate (BPP) as one of the most significant components of PEMFC device accounts for the largest part of the fuel cell`s stack. Corrosion for metallic bipolar plates is a critical issue, which influences the performance and durability of PEMFC. Corrosion causes adverse impacts on the PEMFC`s performance jeopardizing commercialization. This research is aimed at determining the corrosion resistance of metallic BPPs, particularly stainless steels, used in PEMFC from different aspects. Material selection, coating selection, manufacturing process development and cost considerations need to be addressed in terms of the corrosion behavior to justify the use of stainless steels as a BPP material in PEMFC and to make them commercially feasible in industrial applications. In this study, Ti, Ni, SS304, SS316L, and SS 430 blanks, and BPPs comprised of SS304 and SS316L were examined in terms of the corrosion behavior. SS316L plates were coated to investigate the effect of coatings on the corrosion resistance performance. Stamping and hydroforming as manufacturing processes, and three different coatings (TiN, CrN, ZrN) applied via the Physical Vapor Deposition (PVD) method in three different thicknesses were selected to observe the effects of manufacturing processes, coating types and coating thicknesses on the corrosion resistance of BPP, respectively. Uncoated-coated blank and formed BPP were subjected to two different corrosion tests: potentiostatic and potentiodynamic. Some of the substantial results: 1- Manufacturing processes have an adverse impact on the corrosion resistance. 2- Hydroformed plates have slightly higher corrosion resistance than stamped samples. 3- BPPs with higher channel size showed better corrosion resistance. 4- Since none of the uncoated samples meet the 2015 target of the U.S. Department of Energy, surface coating is required. 5- ZrN and CrN coated BPPs exhibited higher corrosion resistance meeting DOE target while TiN coated samples had the lowest corrosion resistance. Higher coating thicknesses improved the corrosion resistance of the BPPs. 6- Process sequence between coating and manufacturing is not significant for hydroforming case (ZrN and CrN) and stamping case (CrN) in terms of the corrosion resistance. In other words, coating the BPP`s substrate material before manufacturing process does not always decrease the corrosion resistance of the BPPs.

Solid oxide fuel cells (SOFCs) have attracted much interest as the most efficient electrochemical device to directly convert chemical energy to usable electrical energy. The porous Ni-YSZ anode known as the state-of-the-art cermet anode material is found to show serious degradation when using hydrocarbon as fuel due to carbon deposition, sulphur poisoning, and nickel sintering. In order to overcome these problems, doped strontium titanate has been investigated as a potential anode material due to its high electronic conductivity and stability in reducing atmosphere. In this work, A-site deficient strontium titanate co-doped with lanthanum and calcium, La₀.₂Sr₀.₂₅Ca₀.₄₅TiO₃ (LSCT[sub](A-)), was examined. Flat multilayer ceramics have been produced using the aqueous tape casting technique by controlling the sintering behaviour of LSCT[sub](A-), resulting in a 450µm thick porous LSCT[sub](A-) scaffold with a well adhered 40µm dense YSZ electrolyte. Impregnation of CeO₂ and Ni results in a maximum power density of 0.96Wcm⁻² at 800°C, higher than those of without impregnation (0.124Wcm⁻²) and with impregnation of Ni alone (0.37Wcm⁻²). The addition of catalysts into LSCT[sub](A-) anode significantly reduces the polarization resistance of the cells, suggesting an insufficient electrocatalytic activity of the LSCT[sub](A-) backbone for hydrogen oxidation, but LSCT[sub](A-) can provide the electronic conductivity required for anode. Later, the cells with the configuration of LSCT[sub](A-)/YSZ/LSCF-YSZ were prepared by the organic tape casting and impregnation techniques with only 300-m thick anode as support. The effects of metallic catalysts in the anode supports on the initial performance and stability in humidified hydrogen were discussed. The nickel and iron impregnated LSCT[sub](A-) cell exhibits a maximum powder density of 272mW/cm² at 700°C, much larger than 43mW/cm² for the cell without impregnation and 112mW/cm² for the cell with nickel impregnation. Simultaneously, the bimetal Ni-Fe impregnates have significantly reduced the degradation rates in humidified hydrogen (3% H₂O) at 700°C. The enhancement from impregnation of the bi-metal can possibly be the result of the presence of ionic conducting Wustite Fe₁₋ₓO that resides underneath the Ni-Fe metallic particles and better microstructure. Third, in order to improve the ionic conductivity of the anode support and increase the effective TPBs, ionic conducting ceria was impregnated into the LSCT[sub](A-) anode, along with the metallic catalysts. The CeO₂-LSCT[sub](A-) cell shows a poor performance upon operation in hydrogen atmosphere containing 3% H₂O; and with addition of metallic catalysts, the cell performance increases drastically by almost three-fold. However, the infiltrated Ni particles on the top of ceria layer cause the deposition of carbon filament leading to cell cracking when exposure to humidified methane (3% H₂O). No such behaviour was observed on the CeO₂-NiFe impregnated anode. The microstructure images of the impregnated anodes at different times during stability testing demonstrate that the grain growth of catalysts, the interaction between the anode backbone and infiltrates, and the spalling of the agglomerated catalysts are the main reasons for the performance degradation. Fourth, the YSZ-LSCT[sub](A-) composites including the YSZ contents of 5-80wt.% were investigated to determine the percolation threshold concentration of YSZ to achieve electronic and ionic conducting pathways when using the composite as SOFC anode backbone. The microstructure and dilatometric curves show that when the YSZ content is below 30%, the milled sample has a lower shrinkage than the unmilled one due to the blocking effect from the well distributed YSZ grains within LSCT[sub](A-) bulk. However, at the YSZ above 30% where two phases start to form the individual and interconnected bulk, the composites without ball milling process show a lower densification. The impact of YSZ concentration and ball milling process on the electrical properties of the composites reveals that the percolation threshold concentration is not only dependant on the actual concentration, but also related to the local arrangement of two phases. In Napier University, the electroless nickel-ceramic co-depositon process was investigated as a manufacturing technique for the anodes of planar SOFCs, which entails reduced costs and reduced high-temperature induced defects, compared with conventional fabrication techniques. The Ni-YSZ anodes prepared by the electroless co-deposition technique without the addition of surfactant adhere well to the YSZ electrolyte before and after testing at 800°C in humidified hydrogen. Ni-YSZ anodes co-deposited with pore-forming starch showed twice the maximum power density compared with those without the starch. It has therefore been demonstrated that a porous Ni-YSZ cermet structure was successfully manufactured by means of an electroless plating technique incorporating pore formers followed by firing at 450°C in air. Although the use of surfactant (CTAB) increases the plating thickness, it induces the formation of a Ni-rich layer on the electrolyte/anode interface, leading to the delamination of anode most likely due to the mismatched TECs with the adjacent YSZ electrolyte.