T* based La₀.₉Ln₀.₉Sr₀.₂CuO₄ (Ln = Sm & Gd) has been investigated as cathode material for intermediate temperature solid oxide fuel cell using Ce₀.₉Gd₀.₁O₁.₉₅ (GDC) and La₀.₉Sr₀.₁Ga₀.₈Mg₀.₂O₃-δ (LSGM-9182) as the electrolyte material. Both oxides crystallize in tetragonal P4/nmm symmetry. The structural and phase stability has been confirmed up to 800 °C by High temperature XRD studies. The coefficient of thermal expansion (CTE) and oxygen content decrease with decreasing size of the Ln³+ ions from Ln = Sm to Gd. While the decrease in CTE is due to the increasing co-valence of the Ln–O bond, the decrease in electrical conductivity at high temperature is due to the increasing oxide ion vacancies and a bending of the O–Cu–O bonds. The highest value of DC conductivity has been observed for the LSSCu, which showed a metal like temperature dependence. LGSCu showed a semiconductor to metallic temperature dependence of conductivity with a maximum of 25 Scm-¹. From the microstructural characterization and the polarisation resistance measurement of the symmetric cells at temperature ranges from 700 - 800 °C, 900 °C has been chosen as the most suitable sintering temperature and LGSCu has shown the minimum polarization resistance of 0.35 Ωcm² and 0.09 Ωcm² at 800 °C using GDC and LSGM-9182 electrolytes respectively under OCV condition. To improve the ASR of LGSCu, the composite of LGSCu and GDC with varying wt. % of GDC has been optimised and it shows the ASR of 0.12 Ωcm² using GDC as the electrolyte because it enhance the triple phase boundary region. The maximum power density of single-cell SOFCs fabricated with the La₀.₉Ln₀.₉Sr₀.₂CuO₄ (Ln= Sm & Gd) cathodes, La₀.₉Sr₀.₁Ga₀.₈Mg₀.₂O₃-δ (LSGM-9182) electrolyte, and Ni–Ce₀.₉Gd₀.₁O₁.₉₅ cermet anode exhibit 720 and 824 mWcm-² at 800 °C respectively. The phase pure T* Nd₁.₃₂Ce₀.27Sr₀.₄₁CuO₄-δ (NCSCu) has been synthesized by combustion method and its crystal chemistry, thermal and electrochemical properties, and catalytic activity in SOFC were evaluated using LSGM-9182 as the electrolyte. It shows promising performance and can be used as potential cathode materials for IT-SOFC. The effect of B-site Ni and Co substitution for Cu on the structural and electrochemical properties of the T* La₀.₉Gd₀.₉Sr₀.₂CuO₄ has been investigated as cathode materials for intermediate temperature solid oxide fuel cells using LSGM-9182 as the electrolyte. At a given temperature, the electrical conductivity gradually increases with increasing Ni content and the CTE gradually decreases. Ni doping has also improved the electrochemical performance. Sr doped A /A //B₂O₅+δ (A / = Rare Earth, A // = Ba or Sr and B = Transition Metals) layered perovskites improves the electrochemical performance due to the increase in electrical conductivity and smaller size difference between Ln+³ and Sr+². However these layered perovskites suffer from high thermal expansion coefficient (20-23 x 10-6 K-1) which does not match with the state of the art electrolyte materials. B-site transition metal doped layered perovskites of compositions SmBa₀.₅Sr₀.₅Co₂-ₓO₅+δ (M = Cu, Ni, Fe) have been investigated as cathode material for intermediate temperature solid oxide fuel cell using LSGM-9182 as the electrolyte material. Phase purity has been confirmed by XRD technique. The crystal cell parameters have been found out using Rietveld refinement by FULLPROF software. The substitution of Cu, Ni and Fe for Co lowers the CTE of Co-based materials by suppression of the spin state transition of Co³+ which will be highly advantageous for long term SOFC application. The introduction of transition metals exhibit inferior electrochemical performance to pristine cathode using LSGM-9182 as the electrolyte but still shows reasonable power density with advantage of lower CTE value thereby can be explored as promising cathode material for IT-SOFCs.

A numerical model of the steady state and alternating current behaviour of a solid-oxide fuel cell is presented to explore the possibilities to diagnose and identify degradation mechanisms in a minimally invasive way using impedance spectroscopy. This is the first report of an SOFC impedance model to incorporate degradation, as well as the first one to include the ribbed interconnect geometry, using a 2-D approximation. Simulated degradation modes include: electrode/electrolyte delamination, interconnect oxidation, interconnect/electrode interface detachment, and anode sulfur poisoning. Detailed electrode-level simulation replaces the traditional equivalent circuit approach, allowing the simulation of degradation mechanisms that alter the shape of the current path. The SOFC impedance results from calculating the cell response to a small oscillatory perturbation in potential. Starting from the general equations for mass and charge transport, and assuming isothermal and isobaric conditions, the system variables are decomposed into a steady-state component and a small perturbation around the operating point. On account of the small size of the imposed perturbation, the time dependence is eliminated, and the original equations are converted to a new linear, time independent, complex-valued system, which is very convenient from a numerical viewpoint. Geometrical and physical modifications of the model simulate the aforementioned degradation modes, causing variations in the impedance. The possibility to detect unique impedance signatures is discussed, along with a study of the impact of input parameter inaccuracies and parameter interaction on the presented results. Finally, a study of pairs of concurrent degradation modes reveals the method’s strengths and limitations in terms of its diagnosis capabilities. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

This work focused on the study of three independent projects involving perovskite oxide materials and their applications as solid oxide fuel cell (SOFC) anodes and electrochemical oxygen sensors. The underlying theme is the versatility and tune-ability of the perovskite structure. Reactivity and conductivity (ionic as well as electronic) are modified to optimize performance in a specific application. The effect of Ce doping on the structure and the conductivity of BaFeO3 perovskite materials is investigated and the resulting materials are applied as oxygen sensors. The new perovskite family, Ba1-xCexFeO3-δ (x=0, 0.01, 0.03, and 0.05), was prepared via a sol-gel method. Powder XRD indicates a hexagonal structure for BaFeO3 with a change to a cubic perovskite upon Cerium doping at the A site. The solubility limit of Ce at the A site was experimentally determined to be between 5-7 mol %. Bulk, electronic and ionic conductivities of BaFeO3-δ and Ba0.95Ce0.05FeO3-δ were measured in air at temperatures up to 1000˚C. Cerium doping increases the conductivity throughout the entire temperature range with a more pronounced effect at higher temperatures. At 800˚C the conductivity of Ba0.95Ce.05FeO3-δ reaches 3.3 S/cm. Pellets of Ba0.95Ce.05FeO3-δ were tested as gas sensors at 500 and 700˚C and show a linear, reproducible response to O2. Promising perovskite anodes have been tested in high sulfur fuel feeds. A series of perovskite solid oxide fuel cell (SOFC) anode materials: Sm0.95Ce0.05FeO3-δ, Sm0.95Ce0.05Fe0.97Ni0.03O3-δ and Sm0.95Ce0.05Fe0.97Co0.03O3-δ have been tested for sulfur tolerance at 500°C. The introduction of the extreme 5% H2S enhances the performance of these anodes, verified by EIS and CA experiments. Post mortem analyses indicate that the performance XII enhancement arises from the partial sulfidation of the anode, leading to the formation of FeS2, Sm3S4 and S on the perovskite surface. Testing in lower concentrations of sulfur, more common in sour fuels, 0.5% H2S, also enhances the performance of these materials. The SCF-Co anode shows promising stability and an increase in exchange current density, io, from 13.72 to 127.02 mA/cm2 when switching from H2 to 0.5% H2S/99.5% H2 fuel composition. Recovery tests performed on the SCF-Co anode conclude that the open cell voltage (OCV) and power density of these cells recover within 4 hours of H2S removal. We conclude that the formation of metal sulfide species is only partially reversible, yielding an anode material with an overall lower Rct upon switching back to pure H2. Combining their performance in sulfur containing fuels with their previously reported coke tolerance makes these perovskites especially attractive as low temperature SOFC anodes in sour fuels. A new perovskite family Ba1-xYxMoO3 (x=0-0.05) has been investigated in regards to electrical conductivity and performance as IT-SOFC anode materials for the oxidation of H2. Refinement of p-XRD spectra as well as SEM imaging conclude that the solubility limit of Y doping at the A site is 5 mol%, beyond which Y2O3 segregation occurs. The undoped BaMoO3 sample has a colossal room temperature conductivity of 2500 S/cm in dry H2. All materials maintain metallic conductivity in the temperature range of 25-1000°C with resistance increasing with Y doping. The Ba1-xYxMoO3 (x=0, 0.05) materials exhibit good performance as SOFC anode materials between 500-800°C, with Rct values at 500°C in dry H2 of 3.15 and 6.33 ohm*cm2 respectively. The catalytic performance of these perovskite anodes is directly related to electronic conductivity, as concluded from composite anode performance.

This thesis attempts to test the effects of barium non stoichiometry and varying calcination temperatures on the microstructure and electrical conductivity of BaxZr0.7Pr0.1Y0.2O3- δ (x = 0.9, 1.0, 1.1). BZPY powders were fabricated using a combustion method, with the quantity of barium carefully controlled to create powders with a 10% molar excess or deficiency of barium. Then, portions of the precursor were calcined at 900 ºC, 1000 ºC, 1100 ºC, 1200 ºC and 1300 ºC for 5 h. The resulting calcined powders were pressed into pellets and sintered at 1600 ºC for 10 h, in a powder bath of the same chemical composition. In all, three chemically different powders were synthesized, and each composition was subjected to five different calcination temperatures, resulting in fifteen different samples to characterise. The precursor from the combustion method was characterised by using an STA to perform both TG and DSC simultaneously. The chemical composition of the precursor and calcined samples was analysed using ICP-OES. XRD was used to characterise the phases of both the powders and the sintered pellets. Lattice parameter indexing using Topaz and Scherrer's equation were used to extract the lattice parameters and crystallite sizes respectively. The microstructure of the pellets was examined using an SEM, the grain size measured using a linear intercept method and pore size using ImageJ. Finally, EIS was used to measure the conductivity of the pellets in dry and wet Argon atmospheres, with silver electrodes. Unfortunately, neither changes to barium stoichiometry nor partial calcination could improve the performance of BZPY. Partially calcined samples did not give rise to dense pellets, barium deficient samples showed inferior conductivity and barium excess samples, while showing higher conductivity than the barium deficient pellets at high temperature, were fragile and had to be handled carefully. Ultimately, the attempt to improve the performance of BZPY did not succeed and alternate methods of improving the grain growth need to be sought.

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第17890号 / 工博第3799号 / 新制||工||1581(附属図書館) / 30710 / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 江口 浩一, 教授 安部 武志, 教授 阿部 竜 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第19729号 / 工博第4184号 / 新制||工||1645(附属図書館) / 32765 / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 江口 浩一, 教授 安部 武志, 教授 陰山 洋 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

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Master Thesis, Department of Mechanical Engineering / A comprehensive analytical model of an indirect internal reforming type tubular Solid Oxide Fuel Cell (IIR-T-SOFC) has been developed. Two-dimensional axisymmetric multicomponent gas flow fields and quasi-three-dimensional electric potential/current fields in the tubular cell are simultaneously treated in the model with consideration of the involved phenomena such as internal reforming, electrochemical reactions and radiative heat transfer. By using this model, the characteristics of the operating state of an IIR-T-SOFC were numerically examined. As a result, it was shown how the thermal field and power generation characteristics of the cell were affected by the gas inlet temperature, air flow rate, steam-methane ratio, reforming catalyst distribution and thickness of the electrodes. In particular, the optimized catalyst distribution greatly reduced both the maximum temperature and temperature gradients of the cell with little negative impact on the power generation performance of the cell. / 京都大学 / 0048 / 修士 / 修士(工学) / Kyoto University / TFtmp

京都大学 / 新制・課程博士 / 博士(工学) / 甲第23577号 / 工博第4932号 / 新制||工||1770(附属図書館) / 京都大学大学院工学研究科物質エネルギー化学専攻 / (主査)教授 江口 浩一, 教授 阿部 竜, 教授 岩井 裕 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

No description available.