Oxygen Transport Membranes (OTMs) can drastically reduce the energy and cost demands of processes that require pure oxygen, as they offer the possibility to combine a separation unit with a chemical reactor. One of the most commercially viable applications of OTMs is the partial oxidation of hydrocarbons for syngas production. A typical OTM configuration is a sequential arrangement of layers, i.e. an inactive support, a fuel oxidation layer, a dense layer and an oxygen reduction layer. However, one of the limitations of the OTM system is the low catalytic activity and stability of the materials currently used for the fuel oxidation layer. Moreover, the traditional deposition techniques that are used for the catalysts preparation are difficult to perform, as the fuel oxidation layer is buried deeply in the structure of the OTM. To simplify the OTM fabrication and improve the catalysts activity and stability, this thesis explores the exsolution of Ni nanoparticles from two different host lattice compositions, as potential materials for the fuel oxidation layer of OTMs. The (La₀.₇₅Sr₀.₂₅)(Cr₀.₅Mn₀.₄₅Ni₀.₅)O₃ (LSCMNi5) perovskite was selected, as the first candidate material for the OTMs. During reduction, the exsolution of Ni nanoparticles from the perovskite lattice took place and enhanced significantly the catalytic activity of the material regarding methane conversion. However, these nanoparticles were oxidised during the first hours of the testing and slowly reincorporated into the perovskite structure, leading to drop in the performance. Thereafter, the (La₀.₇₅Sr₀.₂₅)(Cr₀.₅Mn₀.₄₅Ni₀.₅)O₃ (LSCMNi5) perovskite was selected as an alternative composition. When the oxide lattice was sufficiently reduced, the exsolution of Fe-Ni alloy nanoparticles occurred. The catalytic testing suggested that the Fe-Ni alloy nanoparticles on LSCFNi5 presented lower activity for methane conversion comparing to the Ni nanoparticles on LSCMNi5, but higher stability in oxidising conditions. By increasing the Ni doping on the B-site of LSCF to 15 mol%, the catalytic activity of the material regarding methane conversion was increased and exceeded that of LSCMNi5. A CH₄ conversion of 70% was achieved, which was 20 times higher than that of the initial LSCF perovskite. Therefore, by tailoring the perovskite composition and the exsolution of the Fe-Ni alloy nanoparticles, it was possible to synthesize a material for the fuel oxidation layer of OTMs, which combined the high catalytic activity of Ni and the good redox stability of Fe.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第17560号 / 工博第3719号 / 新制||工||1566(附属図書館) / 30326 / 京都大学大学院工学研究科航空宇宙工学専攻 / (主査)教授 吉田 英生, 教授 中部 主敬, 准教授 松本 充弘 / 学位規則第4条第1項該当

As the demand for clean and renewable energy sources continues to grow, much attention has been given to solid oxide fuel cells (SOFCs) due to their efficiency and low operating temperature. However, the components of SOFCs must still be improved before commercialization can be reached. Of particular interest is the solid electrolyte, which conducts oxygen ions from the cathode to the anode. Samarium-doped ceria (SDC) is the electrolyte of choice in most SOFCs today, due mostly to its high ionic conductivity at low temperatures. However, the underlying principles that contribute to high ionic conductivity in doped ceria remain unknown, and so it is difficult to improve upon the design of SOFCs. This thesis focuses on identifying the atomistic interactions in SDC which contribute to its favourable performance in the fuel cell. Unfortunately, information as basic as the structure of SDC has not yet been found due to the difficulty in experimentally characterizing and computationally modelling the system. For instance, to evaluate 10.3% SDC, which is close to the 11.1% concentration used in fuel cells, one must investigate 194 trillion configurations, due to the numerous ways of arranging the Sm ions and oxygen vacancies in the simulation cell. As an exhaustive search method is clearly unfeasible, we develop a genetic algorithm (GA) to search the vast potential energy surface for the low-energy configurations, which will be most prevalent in the real material. With the GA, we investigate the structure of SDC for the first time at the DFT+U level of theory. Importantly, we find key differences in our results from prior calculations of this system which used less accurate methods, which demonstrate the importance of accurately modelling the system. Overall, our simulation results of the structure of SDCagree with experimental measurements. We identify the structural significance of defects in the doped ceria lattice which contribute to oxygen ion conductivity. Thus, the structure of SDC found in this work provides a basis for developing better solid electrolytes, which is of significant scientific and technological interest. Following the structure search, we perform an investigation of the electronic properties of SDC, to understand more about the material. Notably, we compare our calculated density of states plot to XPS measurements of pure and reduced SDC. This allows us to parameterize the Hubbard (U) term for Sm, which had not yet been done. Importantly, the DFT+U treatment of the Sm ions also allowed us to observe in our simulations the magnetization of SDC, which was found by experiment. Finally, we also study the SDC surface, with an emphasis on its structural similarities to the bulk. Knowledge of the surface structure is important to be able to understand how fuel oxidation occurs in the fuel cell, as many reaction mechanisms occur on the surface of this porous material. The groundwork for such mechanistic studies is provided in this thesis.

O desenho planar para as células a Combustível de Óxido Sólido (SOFC) é melhor do que o tubular devido a sua maior densidade de corrente e menor custo de fabricação. No entanto, o projeto de SOFC planar requer selantes para evitar o vazamento de combustível e a mistura de gases em altas temperaturas. Os vidros e os vitrocerâmicos têm demonstrado serem os mais adequados por apresentarem boa compatibilidade com outros componentes da célula nas temperaturas de trabalho das SOFCs (700-1000°C). No presente estudo, uma série de composições pertencentes ao sistema BaO-Al2O3-SiO2 (BAS) com a adição de B2O3 foram sintetizados tomando as proporções apropriadas de cada óxido constituinte. Propôs-se melhorar este sistema utilizando-se formadores e teores relevantes de modificadores estruturais, de forma a compatibilizar tanto o desempenho térmico por meio do coeficiente de expansão térmica (CET) como a compatibilidade química com os demais componentes da célula. A originalidade deste estudo está na busca destas características em regiões de composições ainda não exploradas, localizadas dentro do triangulo de compatibilidade BS-B2S-BAS2 na região rica em bário do sistema ternário. Entre estes vidros sintetizados quatro composições (BAS-4, BAS-5, BAS-6 e BAS-7) foram escolhidas porque são as mais adequadas às solicitações termomecânicas exigidas para um material vítreo atuar como selante em SOFC. / The design for planar cells Fuel Solid Oxide (SOFC) is better than the tubular due to its higher current density and lower manufacturing cost. However, the design of planar SOFC requires sealant to prevent leakage of fuel and the mixture of gases at high temperatures. Glasses and glass-ceramics have proven to be the most suitable because they have good compatibility with the other components of the cell at working temperature (700-1000°C). In the present study, a series of compositions belonging to the BaO-Al2O3-SiO2 (BAS) system with the addition of B2O3 were synthesized having the appropriate proportions of each component oxide. It was proposed to improve this system using relevant levels of formers and structural modifiers oxides, in order to match both the thermal performance of thermal expansion coefficient (TEC) and chemical compatibility with other components of the cell. The originality of this study is to search for these characteristics in regions of compositions not yet explored, located inside the compatibility triangle BS-B2S-BAS2 at the barium rich part of the ternary diagram. Among the synthesized glasses four batch compositions (BAS-4, BAS-5, 6-BAS, BAS-7) were chosen because best matched the thermo-mechanical required for a glassy material to act as SOFCs sealant.

Os filmes micrométricos contendo óxido misto de lantânio, estrôncio, cobalto e ferro (La0,60Sr0,40)(Co0,20Fe0,80)O3-δ - LSCF, misturado com (Ce0,90Gd0,10)O1,95 - CGO, são relevantes para a utilização como camada funcional para o cátodo da Célula a Combustível de Óxido Sólido de Temperaturas Intermediárias. Estes filmes foram depositados no um substrato cerâmico e denso de CGO ou CGO sobre (ZrO2)0.92(Y2O3)0.08 - YSZ. O estudo deste cátodo é fundamental, pois é nele que ocorre a reação de redução do gás oxigênio, e o seu desempenho eletroquímico depende da interface destes dois materiais. Neste sentido, o presente trabalho contribui para a síntese dos particulados de LSCF para o processamento de filmes, utilizando a técnica de deposição com uso de aerógrafo e para sua conformação em camadas contendo porosidade com espessuras entre 30 e 50 μm. Inicialmente, os particulados de LSCF foram sintetizados pela técnica do citratos e de LSCFCGO obtidos por mistura mecânica, sendo caracterizados por DRX para a confirmação da formação da estrutura cristalina ortorrômbica para o LSCF e cúbica para CGO. Em seguida, foram preparadas suspensões orgânicas de LSCF, LSCFCGO e CGO que foram alimentadas por gravidade em um aerógrafo manual para deposição sobre substrato do eletrólito. Para a conformação dos substratos de CGO ou YSZ, utilizou-se prensa uniaxial e isostática, sinterização e retificação. Verificaram-se, pelas micrografias, que os substratos CGO e YSZ apresentaram densidades (> 92%) suficientes para serem utilizados como eletrólitos. Os filmes de LSCF e LSCFCGO apresentaram-se com porosidades adequadas (> 30%) e espessura total de aproximadamente 40 μm, com boa aderência ao eletrólito. A presença do cátodo compósito contendo eletrólito de CGO sobre YSZ possibilitou aumento de 25% no desempenho eletroquímico (2,50 Ω.cm2 para 650ºC) em decorrência da melhora na reação de redução do oxigênio na interface cátodo/eletrólito. / The study of micrometrics films of (La0.60Sr0.40)(Co0.20Fe0.80)O3-δ - LSCF mixture with (Ce0.90Gd0.10)O1.95 - CGO is relevant for use as functional cathode of Intermediate Temperature Solid Oxide Fuel Cells (ITSOFC). These films were deposited on the CGO or CGO and YSZ dense ceramic substrate, used as electrolyte, structural component of the module. The study of this cathode is fundamental, because is there that occurs oxygen reduction reaction, and the electrochemical performance depends on the interface of these two materials. In this sense, this work contributes for the synthesis of LSCF particulates, for processing films using the wet powder spraying technique, adopted for the conformation of the ceramic films for allowing the attainment porous layers with thicknesses between 30 and 50 μm. Initially, the LSCF particulates were synthesized by the citrate technique and the LSCFCGO produced by solid mixture were characterized by XRD to confirm the formation of LSCF orthorhombic structure and CGO cubic structure. In the stage of formation were prepared organic suspensions of LSCF, LSCFCGO and CGO fed by gravity in a manual airbrush for electrolyte substrate deposition, sintering and grinding for thickness reduction. The micrographs showed that the CGO and YSZ substrates were dense (> 92%) enough to be used as solid electrolyte. The LSCF and LSCFCGO films presented with adequate porosity (> 30%) and total thickness of approximately 40 μm, with good adhesion to electrolyte. The presence of the composite cathode containing CGO or YSZ electrolyte allowed the increase of 25% in the electrochemical performance (2.50 Ω.cm2 to 650ºC) due to improvement in the oxygen reduction reaction at the interface cathode/electrolyte.

Les piles à combustible à oxyde solide (SOFC) offrent une alternative réelle aux technologies classiques de génération d’électricité en étant à la fois propre, efficace et respectueuse de l’environnement. Toutefois, leur principale limitation réside en leur durée de vie et fiabilité limitées dues à leur haute température de fonctionnement. Des recherches intenses de matériaux pour SOFC sont actuellement poursuivies pour essayer d’abaisser la température de fonctionnement de ces dispositifs afin de dépasser ces limitations. Parmi les différents candidats qui ont émergé, le Silicate de Lanthane (LSO) et le Zirconate de Baryum dopé à l'Yttrium (BZY) ont été identifiés comme des alternatives potentielles à utiliser comme matériaux d’électrolyte pour SOFC à température intermédiaire.De manière surprenante, alors que de nombreuses études concernent l’optimisation microstructurale et électrochimiques des composants de la pile, très peu d’études concernant l’évaluation de leurs propriétés mécaniques et de leur influence sur la durée de vie du dispositif.La fiabilité et durée de ces dispositifs dépend non seulement de leur stabilité électrochimique, mais aussi de la capacité de leur structure à supporter les contraintes résiduels issus du procédé de fabrication et de contraintes mécaniques de fonctionnement. En raison du fait que les SOFC sont composés d'empilement de plusieurs cellules individuelles qui, à leur tour, sont constituées de couches fragiles individuelles en contact étroit, ces contraintes proviennent principalement de la différence entre le coefficient de dilatation thermique et les propriétés élastiques des couches adjacentes et la déformation du fluage. Des contraintes non coordonnées peuvent entraîner une défaillance mécanique d'une seule cellule et avoir des conséquences dramatiques sur l'ensemble de la pile. De ce fait, la connaissance des propriétés mécaniques des composants de la cellule est une étape importante pour préserver l’intégrité et le développement des SOFC. Le but de cette thèse est la fabrication et l’étude des propriétés structurale, microstructurales et mécaniques de matériaux de type LSO et BZY. / Solid oxide fuel cells (SOFCs) offer a real alternative to classical technologies for the generation of electricity by clean, efficient and environmental-friendly means. Nevertheless, the main limitation of SOFCs lies in their unsatisfactory durability and reliability due to the high operating temperatures and thermal cycling characteristic of these devices. An intense search is currently underway for materials for SOFCs with the objective of lowering the working temperature and then overcoming these limitations. Among the different candidates which have emerged, Lanthanum Silicate (LSO) and Yttrium-doped Barium Zirconate (BZY) were considered as potential alternatives to be used as electrolyte materials for SOFC at intermediate-temperature. While numerous studies have been devoted to characterizing and optimizing the microstructural and electro-chemical properties of SOFC components, as yet there is little research available on mechanical properties and the influence they have on SOFC lifespan.The reliability and durability of these devices depends not only on their electro-chemical stability, but also on the ability of their structure to withstand residual stresses arising from the cell manufacturing process and mechanical stresses from operation. Owing to the fact that SOFCs are composed by stacking of several single cells which in turn are made up of individual brittle layers in close contact, these stresses mainly originate from the difference between the coefficient of thermal expansion and elastic properties of adjacent layers and creep deformation. Mismatched stresses can result in the mechanical failure of a single cell and have dramatic consequences on the whole stack. Therefore, knowledge of mechanical properties of the cell components becomes an important issue for the mechanical integrity and development of SOFCs.The aim of this PhD thesis is the fabrication and structural, microstructural and mechanical characterization of LSO and BZY.

No description available.

No description available.

京都大学 / 新制・課程博士 / 博士(工学) / 甲第24227号 / 工博第5055号 / 新制||工||1789(附属図書館) / 京都大学大学院工学研究科機械理工学専攻 / (主査)教授 岩井 裕, 教授 中部 主敬, 教授 黒瀬 良一 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DGAM

No description available.