Three-phase porous composites containing electrolyte (ionic conductor), electronic conductor, and porosity phases are frequently used for solid oxide fuel cell (SOFC) electrodes. Performance of such electrodes is microstructure sensitive. Topological connectivity of the microstructural phases and total length of triple phase boundaries are the key microstructural parameters that affect the electrode performance. These microstructural attributes in turn depend on numerous process parameters including relative proportion, mean sizes, size distributions, and morphologies of the electrolyte and electronic conductor particles in the powder mix used for fabrication of the composites. Therefore, improvement of the performance of SOFC composite electrodes via microstructural engineering is a complex multivariate problem that requires considerable input from microstructure modeling and simulations. This dissertation presents a new approach for geometric modeling and simulation of three-dimensional (3D) microstructure of three-phase porous composites for SOFC electrodes and provides electrode performance optimization guidelines based on the parametric studies on the effects of processing parameters on the total length and topological connectivity of the triple phase boundaries. The model yields an equation for total triple phase boundary length per unit volume (LTPB) that explicitly captures the dependence of LTPB on relative proportion of electrolyte and electronic conductor phases; volume fraction of porosity; and mean size, coefficient of variation, and skewness of electrolyte and electronic conductor particle populations in the initial powder mix. The equation is applicable to electrolyte and electronic conductor particles of any convex shapes and size distributions. The model is validated using experimental measurements performed in this research as well as the measurements performed by other researchers. Computer simulations of 3D composite electrode microstructures have been performed to further validate the microstructure model and to study topological connectivity of the triple phase boundaries in 3D microstructural space. A detailed parametric analysis reveals that (1) non-equiaxed plate-like, flake-like, and needle-like electrolyte and electronic conductor particle shapes can yield substantially higher LTPB; (2) mono-sized electrolyte and electronic conductor powders lead to higher LTPB as compared to the powders having size distributions with large coefficients of variation; (3) LTPB is inversely proportional to the mean sizes of electrolyte and electronic conductor particles; (4) a high value of LTPB is obtained at the lowest porosity volume fraction that permits sufficient connectivity of the pores for gas permeability; and (5) LTPB is not sensitive to the relative proportion of electrolyte and electronic conductor phases in the composition regime of interest in composite electrode applications.

Solid oxide fuel cells (SOFC) in a flat-plate configuration require a hermetic seal between the fuel and air sides of the electrodes, and this seal must withstand a variety of thermally-induced stresses over the lifetime of the cell. In this study, quantitative microstructure-property relationships are developed to predict optimum seal structures for mechanical properties and thermal expansion coefficient criteria. These relationships are used to create an inverse approach to characterizing the processing method from the desired microstructure. The main focus of the work concentrates on providing tools to enable macroscopic property predictions from the constituent properties using homogenization techniques based on the individual phase properties and microstructure morphology. The microstructure is represented by two-point correlation functions. Statistical continuum mechanics models were then employed and developed to predict the mechanical and thermal properties of the material. The models enable the prediction of elastic modulus and coefficient of thermal expansion of the multi-phase material. The inelastic mechanical behavior was also studied, indicating microstructure dependence. These models will aid in predicting the a proper seal microstructure (with desired elastic stiffness, coefficient of thermal expansion, and viscoelastic behaviors) based on a desired level of crystallization glass-ceramic materials.

The surface of a dense Ni-YSZ anode was modified with a thin-film coating of niobium oxide (Nb2O5) in order to understand the mechanism of sulfur tolerance and the behavior of carbon deposition. Results suggest that the niobium oxide was reduced to NbO2 under operating conditions, which has high electrical conductivity. The NbOx coated dense Ni-YSZ showed sulfur tolerance when exposed to 50 ppm H2S at 700°C over 12 h. Raman spectroscopy and XRD analysis suggest that different phases of NbSx formed on the surface. Further, the DOS (density of state) analysis of NbO2, NbS, and NbS2 indicates that niobium sulfides can be considered as active surface phases in the H2S containing fuels. It was demonstrated that carbon formation was also suppressed with niobium oxide coating on dense Ni-YSZ in humidified CH4 (3% H2O) at 850ºC. In particular, under active operating conditions, there was no observable surface carbon as revealed using Raman spectroscopy due probably to electrochemical oxidation of carbon. Stable performances of functional cells consisting of Pt/YSZ/Nb2O5 coated dense Ni-YSZ in the fuel were achieved; there was no observable degradation in performance due to carbon formation. The results suggest that a niobium oxide coating has prevented carbon from formation on the surface probably by electrochemically oxidation of carbon on niobium oxide coated Ni-YSZ. On the other hand, computational results suggest that, among the metals studied, Mo seems to be a good candidate for Ni surface modification. Ni-based anodes were modified with Mo using wet-impregnation techniques, and tested in 50 ppm H2S-contaminated fuels. It was found that the Ni-Mo/CeO2 anodes have better sulfur tolerance than Ni, showing a current transient with slow recovery rather than slow degradation in 50 ppm H2S balanced with H2 at 700°C.

In this work, a broad range of analytical methods was applied to the study of the following three materials systems: yttria-stabilised zirconia (YSZ), samarium-doped ceria (SDC) and SDC-supported metal catalysts. YSZ and SDC were studied in the light of their application as solid electrolytes in Solid Oxide Fuel Cells. The SDC-supported metal catalysts were evaluated for application in the reforming of methanol. The conductive properties of YSZ pellets derived from powders of different Y contents and particle size ranges were investigated using Impedance Spectroscopy (IS). Comparative studies of the crystallography (by X-ray Powder Diffraction (XRD)), morphology (by Scanning and Transmission Electron Microscopy (SEM, TEM)), chemical composition (by Energy Dispersive X-ray Spectroscopy (EDX) and Inductively Coupled Plasma Mass Spectroscopy (ICP-MS)) and sintering behaviour (dilatometry) were employed in the overall assessment of the conductivity results collected. Detailed studies of three SDC compositions were performed on nanopowders prepared by a low temperature method developed in the Baker group. Modifications led to a simple and reliable method for producing high quality materials with crystallites of ~10 nm diameter. The products were confirmed by XRD and TEM to be single-phase materials. Thermogravimetric analysis, dilatometry, specific surface area determination, elemental analysis and IS were carried out on these SDC powders. The relationships between particle size, chemical composition, sintering conditions and conductivity were studied in detail allowing optimum sintering conditions to be identified and ionic migration and defect association enthalpies to be calculated. Finally, the interesting results obtained for the SDC nanopowders were a driving force for the preparation of SDC-supported metal catalysts. These were prepared by three different methods and characterised in terms of crystallographic phase, specific surface area and bulk and surface chemical composition. Isothermal catalytic tests showed that all catalysts had some activity for the reforming of methanol and that some compositions showed both very high conversions and high selectivities to hydrogen. These catalysts are of interest for further study and possibly for commercial application.

Cerias, appropriately doped with trivalent rare earth ions in particular, can have high oxide ion conductivity and are attractive as both SOFC (solid oxide fuel cell) electrolytes and anodes. Here, four groups of candidate electrolyte materials were synthesised using a low temperature method in order to determine the effect of multiple doping on their microstructure and ionic conductivity. In an initial study, seven compositions of Ce0.8SmxGd[sub]yNd[sub]zO1.9 (where x, y and z = 0.2, 0.1, 0.0667 or 0 and x + y + z = 0.2) were synthesised and the properties of multiply-doped materials were compared with the corresponding singly-doped parent materials. The effect of co-doping with Gd and Sm was investigated in more detail by preparing and studying five compositions of Ce1−2xSmxGdxO2−x (where x = 0.125, 0.1, 0.0875, 0.075 or 0.05) and seven compositions of Ce0.825SmxGd0.175−xO1.9125 (where x = 0.175, 0.14, 0.105, 0.0875, 0.07, 0.035 or 0). The effect of additional doping with a divalent ion- Ca2+- was studied in six compositions of Ce[sub](0.825+y)Sm[sub](0.0875-y)Gd[sub](0.0875-y)Ca[sub]yO1.9125 (where y = 0, 0.00875, 0.0175, 0.02625, 0.035 or 0.04375). The materials were characterised using scanning and transmission electron microscopy, inductively coupled plasma mass spectrometry and X-ray diffraction. Crystallite sizes were determined in the powders and relative densities and grain size distributions were obtained in sintered pellets. Total, bulk and grain boundary conductivities were obtained using impedance spectroscopy and corresponding activation energies and enthalpies of ion migration and defect association were calculated. The most promising material for SOFCs operating at intermediate temperatures was found to be Ce0.825Sm0.0875Gd0.0875O1.9125 which had a total conductivity at 600 °C of 2.23 S m−1. Lastly, doped ceria materials, primarily Ce0.8Sm0.2O1.9, were employed as catalytic supports for Pd and PdO nanoparticles and these were investigated as SOFC anode materials.

Οι σημαντικές περιβαλλοντικές επιπτώσεις που απορρέουν από τις ανθρώπινες δραστηριότητες έχουν οδηγήσει την επιστημονική κοινότητα σε αναζήτηση πιο αποδοτικών και φιλικών προς το περιβάλλον τεχνολογιών παραγωγής ενέργειας. Σε αυτά τα πλαίσια η τεχνολογία των κελιών καυσίμων έχει προσελκύσει σημαντικό ενδιαφέρον. Τα κελιά καυσίμου μετατρέπουν τη χημική ενέργεια που είναι αποθηκευμένη σε ένα καύσιμο απ’ ευθείας σε ηλεκτρική, χωρίς να υπόκεινται στους περιορισμούς του κύκλου Carnot. Συγκριτικά με τα υπόλοιπα κελιά καυσίμου, το κελί καυσίμου στερεού οξειδίου (SOFC), ξεχωρίζει κυρίως λόγω της υψηλής απόδοσής του. Στην παρούσα εργασία γίνεται εκτίμηση των παραμέτρων μαθηματικών προτύπων κελιών καυσίμου στερεού οξειδίου, με σκοπό να αναλυθούν οι ηλεκτροχημικές διεργασίες που πραγματοποιούνται κατά τη λειτουργία τους. Αρχικά περιγράφεται η λειτουργία του SOFC και αναπτύσσονται οι εξισώσεις που συνιστούν το μαθηματικό πρότυπο SOFC. Επίσης, παρουσιάζονται επιλεγμένα μαθηματικά πρότυπα που αναφέρονται στη βιβλιογραφία για την προσομοίωση των διεργασιών που λαμβάνουν χώρα στο SOFC. Στην συνέχεια, περιγράφεται η μέθοδος βελτιστοποίησης που χρησιμοποιείται για την εκτίμηση των παραμέτρων των μαθηματικών προτύπων. Τέλος, παρατίθενται τα αποτελέσματα και τα συμπεράσματα που απορρέουν από την προσομοίωση και την εκτίμηση παραμέτρων καθώς και προτάσεις για μελλοντική έρευνα. / Human impact on the environment, has led the scientific community in research of more efficient and environmentally friendly energy production technologies. In this frame, fuel cell technology has attracted considerable attention. Fuel cells convert the chemical energy stored in a fuel into electrical without being subject to the Carnot cycle limitations. Compared to other types of fuel cells, solid oxide fuel cell (SOFC) stands out mainly due to its high performance. In the present work, parameter estimation of solid oxide fuel cell mathematical prototypes is implemented, in order to analyze the electrochemical processes taking place during SOFC operation. Initially, SOFC performance is described and the SOFC prototype equations are developed. Furthermore, selected mathematical prototypes reported in literature for the simulation of the processes taking place in the SOFC are presented. Then, the simulation method used for the parameter estimation of the mathematical prototypes is described. Finally, simulation results and conclusions derived from parameter estimation as well as suggestions for future work are given.

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

A new family of perovskite titanates with formulae La4+nSr8-nTi12-nMnnO38 and La4Sr8Ti12-nMnnO38-δ have been investigated as potential fuel electrode materials for SOFCs. The series La4+nSr8-nTi12-nMnnO38 present layered domains within their structure. As such layers appear to have a large negative effect over the electrochemical properties only a few compounds have been characterised. The series La4Sr8Ti12-nMnnO38-δ present a rhombohedral (R-3c) unit cell at room temperature which becomes cubic when increasing the temperature up to 900°C both in air and in reducing conditions. The primitive volume correlates with the oxygen content for the reduced samples. TGA and magnetic studies have revealed that the Mn present is mainly as Mn⁺³. Preliminary HRTEM investigations have revealed that some crystallographic shears distributed randomly within a perovskite matrix remain in the structure, which implies that the oxygen overstoichiometry is compatible with rhombohedral distortions in the oxygen sublattice. Mn substitution does not have a large impact on the bulk conductivity of the phases studied, which remains close to the values observed in other related titanates, although the grain boundary contributions are largely improved. Relatively low polarisation resistances were observed under both hydrogen and methane conditions for the lowest n compounds of the series. The anodic overpotential for n=1 was fairly low to those reported in the literature for other materials and especially for titanate-based anodes, i.e. a value of 55mV at 0.5A/cm2, at 950°C, under wet hydrogen was obtained. Additionally, a value 72mV was obtained in the same conditions under methane. These values indicate that the use of Mn as dopant for perovskite-related titanates enhanced electrochemical performance of these anodes, especially at high temperatures.

Stabilised zirconias have a remarkable variety of technological and commercial applications, e.g., thermal barrier coatings, gas sensors, solid oxide fuel cells, ceramic knives and even fashion jewelry. This amazing versatility seems to originate from the creation of atomic defects (oxide ion vacancies) in the zirconia crystal. Indeed, these vacancies, and their interactions with other vacancies or cations, dramatically affect the structural, thermal, mechanical and electrical properties of zirconia. This thesis is concerned with the study of the role of the vacancy interactions on the conducting properties of these materials. This study was performed by using realistic, first-principles based molecular dynamics simulations. The first system studied in this thesis is Zr0:5 0:5xY0:5+0:25xNb0:25xO7. This has a fixed number of vacancies across the series but its conductivity changes by almost two orders of magnitude as a function of x. For this reason, Zr0:5 0:5xY0:5+0:25xNb0:25xO7 represents an ideal test-bed for the role of the cation species on the defect interactions and therefore on the ionic conductivity of these materials. Realistic inter-atomic potentials for Zr0:5 0:5xY0:5+0:25xNb0:25xO7 were developed on a purely first-principles basis. The observed trends of decreasing conductivity and increasing disorder with increasing Nb5+ content were successfully reproduced. These trends were traced to the influences of the cation charges and relative sizes and their effect on vacancy ordering by carrying out additional calculations in which, for instance, the charges of the cations were equalised. The effects of cation ordering were considered as well and their influence on the conductivity understood. The second part of this thesis deals with Sc2O3–doped (ScSZ) and Y2O3–doped (YSZ) zirconias. These systems are of great academic and technological interest as they find use in solid oxide fuel cells. Inter-atomic potentials were parametrised and used to predict the structural and conducting properties of these materials, which were found to agree very well with the experimental evidence. The simulations were then used to study the role of the vacancy interactions on the conducting properties of these materials. Two factors were found to influence the ionic conductivity in these materials: cation-vacancy and vacancy-vacancy interactions. The former is responsible for the difference in conductivity observed between YSZ and ScSZ. Vacancies, in fact, prefer to bind to the smaller Zr4+ ions in YSZ whereas there is not a strong preference in the case of ScSZ, since the cations have similar sizes in this case. This effect is observed at temperatures as high as T = 1500 K. Finally, it was found that vacancies tend to order so that they can minimise their mutual interaction and that this ordering tendency is what ultimately is responsible for the observed anomalous decrease of the ionic conductivity with increasing dopant concentration. The consequences of such a behaviour are discussed.