Solid Oxide Fuel Cells (SOFC) are electrochemical energy conversion devices which allow fuel gases, e.g. hydrogen or natural gas, to be converted to electricity and heat at much high efficiencies than combustion-based energy conversion technologies. SOFC are particularly suited to employment in stationary energy conversion applications, e.g. micro-combined heat and power (μ-CHP) and base load, which are certain to play a large role in worldwide decentralisation of power distribution and supply over the coming decades. Use of high-temperature SOFC technology within these systems is also a vital requirement in order to utilise fuel gases which are readily available in different areas of the world. Unfortunately, the limiting factor to the long-term commercialisation of SOFC systems is the redox instability, coking intolerance and sulphur poisoning of the state-of-the-art Ni-based cermet composite anode material. This research explores the ‘powder to power' development of alternative SOFC anode catalyst systems by impregnation of an A-site deficient La0.20Sr0.25Ca0.45TiO3 (LSCT[sub](A-)) anode ‘backbone' microstructure with coatings of ceria-based oxide ion conductors and metallic electrocatalyst particles, in order to create a SOFC anode which exhibits high redox stability, tolerance to sulphur poisoning and low voltage degradation rates under operating conditions. A 75 weight percent (wt. %) solids loading LSCT[sub](A-) ink, exhibiting ideal properties for screen printing of thick-film SOFC anode layers, was screen printed with 325 and 230 mesh counts (per inch) screens onto electrolyte supports. Sintering of anode layers between 1250 °C and 1350 °C for 1 to 2 hours indicated that microstructures printed with the 230 mesh screen provided a higher porosity and improved grain connectivity than those printed with the 325 mesh screen. Sintering anode layers at 1350 °C for 2 hours provided an anode microstructure with an advantageous combination of lateral grain connectivity and porosity, giving rise to an ‘effective' electrical conductivity of 17.5 S cm−1 at 850 °C. Impregnation of this optimised LSCT[sub](A-) anode scaffold with 13-16 wt. % (of the anode mass) Ce0.80Gd0.20O1.90 (CGO) and either Ni (5 wt. %), Pd, Pt, Rh or Ru (2-3 wt. %) and integration into SOFC resulted in achievement of Area Specific Resistances (ASR) of as low as 0.39 Ω cm−2, using thick (160 μm) 6ScSZ electrolytes. Durability testing of SOFC with Ni/CGO, Ni/CeO2, Pt/CGO and Rh/CGO impregnated LSCT[sub](A-) anodes was subsequently carried out in industrial button cell test rigs at HEXIS AG, Winterthur, Switzerland. Both Ni/CGO and Pt/CGO cells showed unacceptable levels of degradation (14.9% and 13.4%, respectively) during a ~960 hour period of operation, including redox/thermo/thermoredox cycling treatments. Significantly, by exchanging the CGO component for the CeO2 component in the SOFC containing Ni, the degradation over the same time period was almost halved. Most importantly, galvanostatic operation of the SOFC with a Rh/CGO impregnated anode for >3000 hours (without cycling treatments) resulted in an average voltage degradation rate of < 1.9% kh−1 which, to the author's knowledge, has not previously been reported for an alternative, SrTiO3-based anode material. Finally, transfer of the Rh/CGO impregnated LSCT[sub](A-) anode to industrial short stack (5 cells) scale at HEXIS AG revealed that operation in relevant conditions, with low gas flow rates, resulted in accelerated degradation of the Rh/CGO anode. During a 1451 hour period of galvanostatic operation, with redox cycles and overload treatments, a voltage degradation of 19.2% was observed. Redox cycling was noted to briefly recover performance of the stack before rapidly degrading back to the pre-redox cycling performance, though redox cycling does not affect this anode detrimentally. Instead, a more severe, underlying degradation mechanism, most likely caused by instability and agglomeration of Rh nanoparticles under operating conditions, is responsible for this observed degradation. Furthermore, exposure of the SOFC to fuel utilisations of >100% (overloading) had little effect on the Rh/CGO co-impregnated LSCT[sub](A-) anodes, giving a direct advantage over the standard HEXIS SOFC. Finally, elevated ohmic resistances caused by imperfect contacting with the Ni-based current collector materials highlighted that a new method of current collection must be developed for use with these anode materials.

O manganito de neodímio dopado com estrôncio (NSM) é um dos materiais catódicos alternativos e que estão sendo estudados e pesquisados para aplicação em células a combustível de óxido sólido de temperatura intermediária (ITSOFCs). O estrôncio (Sr) auxilia consideravelmente na condutividade elétrica e na proximidade do coeficiente de expansão térmica do NSM com os eletrólitos de céria gadolínia (GDC) e céria samária (SDC), e que tornam o material adequado ao uso em temperaturas entre 500 e 800 ºC. Seguindo este contexto, o presente trabalho é uma contribuição ao estudo da síntese de NSM com diferentes concentrações molares de Sr (10, 30 e 50 %), através da técnica de reação no estado sólido. Os materiais foram obtidos e caracterizados visando avaliá-los quanto às características adequadas para emprego na ITSOFC. Após a síntese dos pós e processamento do material sinterizado, avaliou-se principalmente o teor do dopante Sr para a identificação das composições químicas obtidas, estrutura cristalina formada, morfologia dos pós e cerâmicas, além da expansão térmica e condutividade elétrica do material sinterizado. Verificou-se que os valores das concentrações molares dos elementos químicos constituintes para a formação do NSM estão próximos dos valores calculados estequiometricamente antes da etapa de calcinação. A porosidade se mostrou mais adequada para as amostras sinterizadas a 1100 e a 1200 ºC. Comprovou-se que, o teor de dopante não altera significativamente a área de superfície específica e o valor das densidades. Os coeficientes de expansão térmica encontrados estão bastante próximos aos eletrólitos comerciais e verificou-se que com o aumento das concentrações molares de estrôncio, ocorre o acréscimo nos valores de coeficientes de expansão térmica. A condutividade elétrica está adequada para aplicação como material catódico. Os resultados mostram que a síntese por mistura de sólidos apesar de ter as suas desvantagens, quando realizada com cuidados, proporciona pós de NSM, com boas características físicas, químicas e microestruturais. Conclui-se que as características do material com composição de 30 % em mol de Sr é a mais adequada para a preparação de suspensões cerâmicas para posterior deposição no eletrólito sólido de GDC e/ou SDC, embora sejam necessários outros estudos das características deste material como dispositivo eletroquímico para aplicação em ITSOFCs. / The strontium-doped neodymium manganite (NSM) is one of the alternative cathodic materials and they have been studied and searched for application in intermediate temperature solid oxide fuel cells (ITSOFCs). The strontium (Sr) assists considerably in the electric conductivity and in the proximity of the thermal expansion of the NSM with electrolytes of ceria doped with gadolinium (GDC) or samarium (SDC), allowing them to become the adequate material for the use in temperatures between 500 and 800 ºC. Following this context, the present work is a contribution to the study of the synthesis of NSM with different molar concentrations of Sr (10, 30 and 50 %), through the technique of solid state reaction. The materials were obtained and characterized to be adjusted to the requested characteristics for operating in the ITSOFC. After the synthesis of the powders and processing the sintered material, it was evaluated the concentration of Sr for the identification of chemical compositions, crystalline structure, powders morphology and ceramics, besides the thermal expansion and electric conductivity of the sintered material. It was verified that the values of the molar concentrations of the constituent chemical elements of the NSM are close to the values theoretically calculated before the stage of calcination. The porosity showed to be more adequate for the samples sintered at 1100 and 1200 ºC. One proved that, the increase of the Sr, relatively little, diminishes the specific surface area and the value of the densities and the coefficients of thermal expansion of the sintered samples. The found coefficients of thermal expansion are sufficiently close to the ones of commercial electrolytes and the electric conductivity is adequate for a cathodic application as material. The results show that the synthesis by solid state reaction, although having its disadvantages, provides powders of NSM with good physical, chemical and microstructural characteristics when carried with certain cares. It is concluded that the composition of 30 % in mol of Sr and sintered at 1200 ºC is the best for the preparation of ceramic suspensions for posterior deposition on the solid electrolyte of GDC and/or SDC, even so further studies are necessary to completely adjust this material to be used in a electrochemical device for application in ITSOFCs.

This thesis details research performed in modeling, simulation, and rational design of porous SOFC cathodes via development, extension, and use of the key tools to aid in the fundamental understanding and engineering design of cathode materials. Phenomenological modeling of triple phase boundary (TPB) reactions and surface transport on La₁₋ₓSrₓMnO₃ (LSM) was conducted, providing insight into the role of the bulk versus surface oxygen reduction pathway and the role of sheet resistance in thin-film patterned electrode measurements. In response to observation of sheet resistance deactivation, a modeling study was conducted to design thin-film patterned electrodes with respect to sheet resistance. Additionally, this thesis outlines the application of phenomenological chemical kinetics to describe and explain the performance and stability enhancements resulting from surface modification of La₁₋ₓSrₓCo₁₋yFeyO₃₋delta (LSCF) with a conformal LSM coating. The analysis was performed in close coordination with electrochemical experiments and transmission electron microscopy. Finally, the thesis describes conformal modeling of porous cathode microstructures using chemical kinetics and transport models. A novel application of conservative point defect ensembles was developed to allow simulations with complicated chemical surface kinetics to be efficiently coupled with bulk transport within the porous structure. The finite element method was employed to simulate electrochemical response conformal to sintered porous ceramic structures using actual 3D microstructural reconstructions obtained using x-ray microtomography. Mesh refinement, linear, and nonlinear reaction rate kinetics were employed to study the bulk versus surface oxygen reduction pathways and the effect of near-TPB nanostructure.

Solid oxide fuel cells (SOFCs) are solid-state ceramic cells, typically operating between 1073 K and 1273 K. Because of high operating temperature, SOFCs are mostly applicable in stationary power generation. Among various configurations in which SOFCs exist, the planar configuration of solid oxide fuel cell (SOFC) has the potential to offer high power density due to shorter current path. Moreover, the planar configuration of SOFC is simple to stack and closely resemble the stacking arrangement of polymer electrolyte membrane (PEM) fuel cells. However, due to high operating temperature, there are problems associated with the development and commercialization of planar SOFCs, such as requirement of high temperature gas seals, internal stresses in cell components, and high material and manufacturing costs. Mathematical modeling is an essential tool for the advancement of SOFC technology. Mathematical models can help in gaining insights on the processes occurring inside the fuel cell, and can also aid in the design and optimization of fuel cells by examining the effect of various operating and design conditions on performance. A multi-component and multi-dimensional mathematical model of SOFCs has been developed in this thesis research. One of the novelties of the present model is its treatment of electrodes. An electrode in the present model is treated as two distinct layers referred to as the backing layer and the reaction zone layer. Reaction zone layers are thin layers in the vicinity of the electrolyte layer where electrochemical reactions occur to produce oxide ions, electrons and water vapor. The other important feature of the present model is its flexibility in fuel choice, which implies not only pure hydrogen but also any reformate composition can be used as a fuel. The modified Stefan-Maxwell equations incorporating Knudsen diffusion are used to model multi-component diffusion in the porous backing and reaction zone layers. The coupled governing equations of species, charge and energy along with the constitutive equations in different layers of the cell are solved for numerical solution using the finite volume method and developed code written in the computer language of C++. In addition, the developed numerical model is validated with various experimental data sets published in the open literature. Moreover, it is verified that the electrode in an SOFC can be treated as two distinct layers referred to as the backing layer and the reaction zone layer. The numerical model not only predicts SOFC performance at different operating and design conditions but also provides insight on the phenomena occurring within the fuel cell. In an anode-supported SOFC, the ohmic overpotential is the single largest contributor to the cell potential loss. Also, the cathode and electrolyte overpotentials are not negligible even though their thicknesses are negligible relative to the anode thickness. Moreover, methane reforming and water-gas shift reactions aid in significantly reducing the anode concentration overpotential in the thick anode of an anode-supported SOFC. A worthwhile comparison of performance between anode-supported and self-supported SOFCs reveals that anode-supported design of SOFCs is the potential design for operating at reduced temperatures. A parametric study has also been carried out to investigate the effect of various key operating and design parameters on the performance of an anode-supported SOFC. Reducing the operating temperature below 1073 K results in a significant drop in the performance of an anode-supported SOFC; hence ionic conductivity of the ion-conducting particles in the reaction zone layers and electrolyte needs to be enhanced to operate anode-supported SOFCs below 1073 K. Further, increasing the anode reaction zone layer beyond certain thickness has no significant effect on the performance of an anode-supported SOFC. Moreover, there is a spatial limitation to the transport of oxide ions in the reaction zone layer, thereby reflecting the influence of reaction zone thickness on cell performance.

Solid oxide fuel cells (SOFCs) are solid-state ceramic cells, typically operating between 1073 K and 1273 K. Because of high operating temperature, SOFCs are mostly applicable in stationary power generation. Among various configurations in which SOFCs exist, the planar configuration of solid oxide fuel cell (SOFC) has the potential to offer high power density due to shorter current path. Moreover, the planar configuration of SOFC is simple to stack and closely resemble the stacking arrangement of polymer electrolyte membrane (PEM) fuel cells. However, due to high operating temperature, there are problems associated with the development and commercialization of planar SOFCs, such as requirement of high temperature gas seals, internal stresses in cell components, and high material and manufacturing costs. Mathematical modeling is an essential tool for the advancement of SOFC technology. Mathematical models can help in gaining insights on the processes occurring inside the fuel cell, and can also aid in the design and optimization of fuel cells by examining the effect of various operating and design conditions on performance. A multi-component and multi-dimensional mathematical model of SOFCs has been developed in this thesis research. One of the novelties of the present model is its treatment of electrodes. An electrode in the present model is treated as two distinct layers referred to as the backing layer and the reaction zone layer. Reaction zone layers are thin layers in the vicinity of the electrolyte layer where electrochemical reactions occur to produce oxide ions, electrons and water vapor. The other important feature of the present model is its flexibility in fuel choice, which implies not only pure hydrogen but also any reformate composition can be used as a fuel. The modified Stefan-Maxwell equations incorporating Knudsen diffusion are used to model multi-component diffusion in the porous backing and reaction zone layers. The coupled governing equations of species, charge and energy along with the constitutive equations in different layers of the cell are solved for numerical solution using the finite volume method and developed code written in the computer language of C++. In addition, the developed numerical model is validated with various experimental data sets published in the open literature. Moreover, it is verified that the electrode in an SOFC can be treated as two distinct layers referred to as the backing layer and the reaction zone layer. The numerical model not only predicts SOFC performance at different operating and design conditions but also provides insight on the phenomena occurring within the fuel cell. In an anode-supported SOFC, the ohmic overpotential is the single largest contributor to the cell potential loss. Also, the cathode and electrolyte overpotentials are not negligible even though their thicknesses are negligible relative to the anode thickness. Moreover, methane reforming and water-gas shift reactions aid in significantly reducing the anode concentration overpotential in the thick anode of an anode-supported SOFC. A worthwhile comparison of performance between anode-supported and self-supported SOFCs reveals that anode-supported design of SOFCs is the potential design for operating at reduced temperatures. A parametric study has also been carried out to investigate the effect of various key operating and design parameters on the performance of an anode-supported SOFC. Reducing the operating temperature below 1073 K results in a significant drop in the performance of an anode-supported SOFC; hence ionic conductivity of the ion-conducting particles in the reaction zone layers and electrolyte needs to be enhanced to operate anode-supported SOFCs below 1073 K. Further, increasing the anode reaction zone layer beyond certain thickness has no significant effect on the performance of an anode-supported SOFC. Moreover, there is a spatial limitation to the transport of oxide ions in the reaction zone layer, thereby reflecting the influence of reaction zone thickness on cell performance.

Among all existing fuel cell technologies, the planar solid oxide fuel cell (SOFC) is the most promising one for high power density applications. A planar SOFC consists of two porous ceramic layers (the anode and cathode) through which flows the fuel and oxidant. These ceramic layers are bonded to a solid electrolyte layer to form a tri-layer structure called PEN (positive-electrolyte-negative) across which the electrochemical reactions take place to generate electricity. Because SOFCs operate at high temperatures, the cell components (e.g., PEN and seals) are subjected to harsh environments and severe thermomechanical residual stresses. It has been reported repeatedly that, under combined thermomechanical, electrical and chemical driving forces, catastrophic failure often occurs suddenly due to material fracture or loss of adhesion at the material interfaces. Unfortunately, there have been very few thermomechanical modeling techniques that can be used for assessing the reliability and durability of SOFCs. Therefore, modeling techniques and simulation tools applicable to SOFC will need to be developed. Such techniques and tools enable us to analyze new cell designs, evaluate the performance of new materials, virtually simulate new stack configurations, as well as to assess the reliability and durability of stacks in operation. This research focuses on developing computational techniques for modeling fracture failure in SOFCs. The objectives are to investigate the failure modes and failure mechanisms due to fracture, and to develop a finite element based computational method to analyze and simulate fracture and crack growth in SOFCs. By using the commercial finite element software, ANSYS, as the basic computational tool, a MatLab based program has been developed. This MatLab program takes the displacement solutions from ANSYS as input to compute fracture parameters. The individual stress intensity factors are obtained by using the volume integrals in conjunction with the interaction integral technique. The software code developed here is the first of its kind capable of calculating stress intensity factors for three-dimensional cracks of curved front experiencing both mechanical and non-uniform temperature loading conditions. These results provide new scientific and engineering knowledge on SOFC failure, and enable us to analyze the performance, operations, and life characteristics of SOFCs.

Porous ceramic-metal composites, or cermets, such as nickel zirconia (Ni-YSZ), are widely used as the anode material in solid oxide fuel cells (SOFC). These materials need to enable electrochemical reactions and provide the mechanical support for the layered cell structure. Thus, for the anode supported planar cells, the thermomechanical behavior of the porous cermet directly affects the reliability of the cell. Porous cermets can be viewed as three-phase composites with a random heterogeneous microstructure. While random in nature, the effective properties and overall behavior of such composites can still be linked to specific stochastic functions that describe the microstructure. The main objective of this research was to develop the relationship between the thermomechanical behavior of porous cermets and their random microstructure. The research consists of three components. First, a stochastic reconstruction scheme was developed for the three-phase composite. From this multiple realizations with identical statistical descriptors were constructed for analysis. Secondly, a finite element model was implemented to obtain the effective properties of interest including thermal expansion coefficient, thermal conductivity, and elastic modulus. Lastly, nonlinear material behaviors were investigated, such as damage, plasticity, and creep behavior. It was shown that the computational model linked the statistical features of the microstructure to its overall properties and behavior. Such a predictive computational tool will enable the design of SOFCs with higher reliability and lower costs.

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.

Solid oxide fuel cells (SOFCs) offer the advantages of (i) employing less expensive catalysts compared to the expensive Pt catalyst used in proton exchange membrane fuel cells and (ii) directly using hydrocarbon fuels without requiring external fuel reforming due to the high operating temperature. However, the conventional high operating temperatures of 800 - 1000 °C lead to interfacial reactions and thermal expansion mismatch among the components and limitations in the choice of electrode and interconnect materials. These problems have prompted a lowering of the operating temperature to an intermediate range of 500 - 800 °C, but the poor oxygen reduction reaction kinetics of the conventional La[subscript 1-x]Sr[subscript x]MnO₃ perovskite cathode remains a major obstacle for the intermediate temperature SOFC. In this regard, cobalt-containing oxides with perovskite or perovskite-related structures have been widely investigated, but they suffer from large thermal expansion coefficient (TEC) mismatch with the electrolytes. With an aim to lower the TEC and maximize the electrochemical performance, this dissertation focuses on perovskite-related and trigonal RBaCo₄O₇-based oxide cathode materials. First, the effect of M = Fe and Cu in the perovskite-related layered LnBaCo₂₋xMxO₊[delta] (Ln = Nd and Gd) oxides has been investigated. The Fe and Cu substitutions lower the polarization resistance and offer fuel cell performance comparable to that of La[subscript 1-x]Sr[subscript x]CoO₃₋[delta] perovskite due to improved chemical stability with the electrolyte and a better matching of the TEC with those of standard electrolytes. Second, the perovskite-related intergrowth oxides Ln(Sr,Ca)₃Fe₁.₅Co₁.₅O₀ and La₁.₈₅Sr₁.₁₅Cu[subscript 2-x]Co[subscript x]O[subscript 6 +delta] and their composites with gadolinia-doped ceria (GDC) have been investigated. The electrical conductivity, TEC, and catalytic activity increase with increasing Co content. The composite cathodes exhibit enhanced electrochemical performance due to lower TEC and increased triple-phase boundary. Third, RBa(Co,Zn)₄O₇ (R = Y, Ca, and In) oxides with a trigonal structure and tetrahedral-site Con+ ions have been investigated. The chemical instability normally encountered with this class of oxides has been overcome by appropriate cationic substitutions as in (Y₀.₅Ca₀.₅)Ba(Co₂.₅Zn₁.₅)O₇ and (Y₀.₅In₀.₅)BaCo₃ZnO₇. With an ideal matching of TEC with those of standard electrolytes, the RBa(Co,Zn)₄O₇ (R = Y, Ca, and In) + GDC composite cathodes exhibit low polarization resistance and electrochemical performance comparable to that of perovskite oxides. / text