Thesis (M.S.)--Montana State University--Bozeman, 2005. / Typescript. Chairperson, Graduate Committee: Vic A. Cundy. Includes bibliographical references (leaf 80).

Thesis (Ph. D.)--University of Texas at Austin, 2007. / Vita. Includes bibliographical references.

Thesis (M.S.)--West Virginia University, 2007. / Title from document title page. Document formatted into pages; contains vii, 92 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 89-92).

The MgO:TiO2 :ZrO2 ternary system was investigated as a possible novel anode material in a solid oxide fuel cell. Titanium-substituted yttria-stabilised zirconias have the necessary electrical conductivity properties for a ZrO2 -based fluorite electrode but problems have been encountered such as a decrease in unit-cell size upon reduction leading to mechanical failure. By incorporating magnesium into the titanium-stabilised zirconia structure, it was thought that the cubic-fluorite structure might be stabilised. A phase diagram study was made of the MgO:TiO2 : ZrO2 ternary system at 1500°C. Upon researching the literature phase diagram of the MgO:TiO2 :ZrO2 system, it was found that the authors had not studied the single-phase region in the ZrO2 -rich area extensively and did not use a consistent temperature for their analysis of samples. This has meant that the phase diagram has had to be reinvestigated. The results obtained at 1500°C are in disagreement with the previously published phase diagram. A large area bounded by single-phase cubic-fluorite was detected; however the central region of this domain contained both tetragonal and cubic-fluorite domains. This implies that for the central region of this phase area that the cubic- fluorite phase is not stable at 1500°C. Selected stabilised cubic-fluorite samples with ~ 10 atom% Mg were annealed at l000°C after preparation at 1500°C and it was found that due to the presence of other phases present at 1000°C, that the cubic-fluorite phase is thermodynamically unstable at lower temperatures. DTA analysis revealed that as the titanium content in the cubic-fluorite solid-solution increased, the phase transition from tetragonal phase (+ MgO) to cubic-fluorite phase decreased. These results were used to provide a basis for a temperature phase diagram showing the likely phase transitions that occur at a particular temperature range. The activation energy for conduction increased and ionic conductivity decreased with increasing titanium content in the solid solution, due to the effects of local distortions created by the smaller ionic radius of titanium when compared to zirconium.

Solid oxide fuel cells (SOFCs) represent a much cleaner and more efficient method for harnessing fossil fuel energy than conventional combustion; however, the challenge with making SOFCs mainstream lies in reducing operating costs and staving off their rapid degradation. High cathode polarization remains a bottleneck for lowering operation temperature. On the anode side, supplying SOFCs with hydrocarbon-based fuels poses many problems for systems using state-of-the-art material specifications such as composites of Ni and yttria-stabilized zirconia (YSZ). Various novel materials and surface modifications have been found to mitigate these problems, but more information towards a more profound understanding the role of these materials is desired. In this work, advanced Raman spectroscopic techniques were applied toward this end. Raman spectroscopy was used for the tracking of the evolution of water, carbon, sulfur, and oxygen species as well as new phases at SOFC electrode surfaces following or during exposure to various temperatures, atmospheres, and electrochemical stimuli. This information, coupled with performance data and other characterizations, would help to clarify the mechanisms of anode contamination reactions and oxygen reduction reactions. Knowledge gained from this work would also help to connect electrode modifications with performance enhancement and poisoning tolerance, offering insights vital to design of better electrodes. In addition, lack of adequate Raman signal from certain species, which is one of Raman spectroscopy’s limitations, was addressed. Surface enhanced Raman scattering (SERS) techniques were used in both in situ and ex situ analyses to increase signal yield from gas species and phases that are found only in trace amounts on electrode surfaces. Finally, a more practical thrust of this work was the application of this study methodology and the knowledge gained from it to cells with NASA's bielectrode supported cell (BSC) architecture. These types of cells also offer great prospects for superior specific power density due to their low weight. Ultimately, the goal of this thrust was progress towards achieving optimum performance of SOFCs operating under hydrocarbon fuels.

Solid oxide fuel cell platforms consisting of alternating cellular layers of yttria-stabilized zirconia electrolyte and Fe-Ni metallic interconnects (Fe45Ni, Fe47.5Ni, Fe50Ni) were produced through the co-extrusion of two particulate pastes. Subsequent thermal treatment in a hydrogen atmosphere was used to reduce iron and nickel oxides and co-sinter the entire structure. Issues surrounding this process include the constrained sintering of the layers and the evolution of residual stress between the dense, fired layers. Sintering curves for individual components of the layers were measured by dilatometry to ascertain each materials impact on overall sintering mismatch. X-ray diffraction, scanning electron microscopy and weight loss were utilized to examine phase evolution within the Fe-Ni alloys during reduction. YSZ powders densified above ~1050C and shrinkage was rapid above the sintering temperature. Shrinkage of the interconnect occurred in two stages: reduction and the initial stages of sintering concluded around ~600C, plateauing shortly and continuing at ~900C as pore removal and grain growth ensued simultaneously. Constrained sintering resulted in the formation of remnant porosity within the interconnect layers. Interconnect compositions were chosen in efforts to minimize disparities in thermal expansion with the electrolyte. Residual strains on the surfaces of the layers were measured by x-ray diffraction. Corresponding stresses were calculated using the sin2y method. Grain growth within the interconnect prohibited random planes to be measured so stress measurements were confined to the ceramic layers. Various material properties such as thermal expansion were collected and employed in a modified finite element model to estimate residual stresses in the platform. A method for determining a crucial parameter, the zero stress temperature was outlined and incorporated. Modeled values were found to agree well with XRD values, providing indirect confirmation of the zero stress temperature calculations. Discrepancies were attributed to microcracks found within the layer that arose due to residual stress values surpassing the tensile strength of the zirconia.

Thesis (Ph.D.) - University of St Andrews, March 2010.

This study has focused on the implementation of a stack system for a novel design of solid oxide fuel cell (SOFCRoll). The issues affecting the commercialization of SOFCs are mainly based on durability and cost. The new design offers a number of advantages over the existing designs; it seeks to retain the specific advantages of both the tubular (high unit strength, no sealing problems) and planar arrangements (high power density). This design also aims to achieve low manufacturing cost by utilizing a cheap, easily scalable production technique: tape casting, together with co-firing all components, in one single step. In this study aspects of the design and operation of SOFCRoll stacks were studied particularly those affecting the single cell test reproducibility such as pre test quality control and scale up issues such as bundle and stack gas distribution. Initially the performance of single cells was characterized and the variation of their power output with temperature was observed. The maximum power, 0.7W at 800°C was achieved with a high silver content. The OCV and total resistance of this cell were 0.93V, 0.30Ω respectively. A standard pre-test quality control and current collection technique was introduced. At 800°C reproducible performance of 0.5W power obtained, average OCV was 0.935V and average series and polarization resistances of 0.18Ω and 0.19Ω was achieved respectively. Once single cell reproducibility was achieved, the design and operation of a 5 cell SOFCRoll bundle was investigated. A FLUENT CFD model was used to optimize the gas distribution in the five cell manifold design. The value of the model as a design tool was demonstrated by the comparison of 3 different gas manifold designs. The final manifold design M3 achieved 2.5W which is consistent with the 0.5W per a cell target. This manifold was then used as the basis for the development of a 25 cell stack which was built and tested. The 25 cell stack testing results were down to 0.35W per a cell. The performance drop highlighted the problem of fuel cell manufacturing reproducibility and also the importance of introducing reproducible manufacturing tequniques. That been the case for single cell manufacturing reproducibility issue, the fundamental concern for performance drop remains a design issue. To optimize the SOFCRoll design and to assist with the development program a single-cell CFD model was developed using FLUENT. The model was validated by comparison with data from experimental measurements for the single cell. The model work was used to predict the geometrical effect of the SOFCRoll tubular and the spiral gas channel configuration and current collector configuration. Results indicate the outlet gas flow velocity is higher around the spiral, near the gas inlet (the gas interring the cell preferentially flows around the spiral) therefore, velocity decrease as the gas moves along the cell. The lowest outlet velocity is registered opposite to the gas inlet, thus creating non-uniform gas distribution. The current density distribution is not uniform and is affected primarily by reactant flow distributions along the cell and possible current collection issues particularly around the spiral part of the cell.

Lowering the operating temperature of the high-temperature, solid oxide fuel cell (SOFC) improves both the thermodynamic efficiency and the lifetime of this energy efficient technology. Unfortunately the rate of oxygen-ion transport through the solid electrolyte is temperature dependent, and materials previously employed as electrolytes in the high-temperature SOFC perform poorly at intermediate temperatures. Therefore new oxygen-ion conductors with enhanced ionic conductivity at intermediate temperatures are required. The bulk of the existing literature on high-temperature SOFCs has focussed on zirconia-based binary systems as electrolytes, due to their high ionic conductivity and negligible electronic conductivity. Only select compositions within the zirconia-scandia system have demonstrated acceptable ionic conductivity levels at intermediate temperatures; however unstable phase assemblage and the high economic cost of scandia are clear disadvantages. Ceria-based binary systems have demonstrated improved oxygen-ion conductivity at intermediate temperature compared to many zirconia systems, however significant levels of n-type electronic conductivity are observed at low oxygen partial pressures. Consequently it was thought unlikely that significant increases in ionic conductivity would be found in existing zirconia- and ceria-based binary systems, therefore another approach was required in an attempt to improve the performance of these established fluorite systems. The fluorite systems Zr0.75Ce0.08M0.17O1.92 (M = Nd, Sm, Gd, Dy, Ho, Y, Er, Yb, Sc) were prepared and investigated as possible, intermediate-temperature SOFC electrolytes in an attempt to combine the higher conductivity found in the ceria systems with the low electronic conductivity observed in the zirconia systems. Also it was anticipated that systems containing dopants not previously observed to confer high ionic conductivity in either zirconia- and ceria-based binary systems, might exhibit enhanced ionic conductivity with expansion of the zirconia lattice resulting from the addition of ceria. All the as-fired Zr0.75Ce0.08M0.17O1.92 compositions possessed the face-centred cubic structure and lattice parameter measurements revealed the anticipated unit cell enlargement as the size of the dopant cation increased. No unusual microstructural parameters were identified that could be expected to interfere with the ionic transport properties in the as-fired compositions. The electrical conductivity was found to be influenced by the dopant-ion radius, the presence of ceria, low oxygen partial pressures and, in some compositions, the formation of poorly conducting, ordered-pyrochlore microdomains dispersed amongst the cubic defect-fluorite matrix. In a second approach to the formulation of new oxygen-ion conductors suitable for the intermediate-temperature SOFC, compounds possessing structures other than the fluorite structure were considered. An examination of the literature for oxides having the pyrochlore, scheelite and perovskite structures showed that the Sr+2- and Mg+2-doped LaGaO3 perovskites (LSGM) possessed ionic conductivity equal to the highest conducting, zirconia and ceria binary compounds. Therefore the perovskite systems La0.9Sr0.1Ga(0.8-x)InxMg0.2O2.85 (X = 0, 0.05, 0.1, 0.2, 0.4, 0.6, 0.8) (I-LSGM) were prepared and examined, the objective being to favourably influence structural parameters believed responsible for optimal ionic conductivity, namely the unit cell symmetry and volume. It was found that In+3 systematically substituted for Ga+3 on to the B-site of LSGM at least up to the X = 0.4 composition. While In+3 was found to replace the Ga+3 as expected, Mg+2, which occupies the same crystallographic site, was also replaced by In+3. Up to the X = 0.2 composition, at least two trace level secondary phases were observed to form along with the bulk I-LSGM phase. For I-LSGM compositions with X > 0.2, significantly larger concentrations of the secondary phases were identified. Evidence of a strontium-rich, high-temperature liquid phase was observed also near the grain boundaries on as-sintered and thermally etched surfaces in LSGM and I-LSGM compositions. It is believed that the observed, high sintered density in the complex, doped-LaGaO3 systems is due to the formation of this high-temperature liquid phase. Increasing levels of diffuse scatter and superstructure formation were observed in electron diffraction patterns in the I-LSGM bulk phase (up to X = 0.2), indicating a possible decrease in vacancy concentration and reduced, localised unit cell symmetry. The electrical conductivity in the I-LSGM compositions was believed to be influenced by the distortion of the oxygen-ion conduction path, a reduction in vacancy concentration, formation of stronger dopant-vacancy associates at low temperature and the presence of ordered structures. In addition, phase instability, in the form of subtle ordering in specific crystalline planes, was observed to influence the electrical conductivity as a function of time at intermediate temperatures.

Thesis (M.S.)--University of Missouri--Rolla, 2007. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed April 17, 2007) Includes bibliographical references (p. 77-80).