Napolitano, Federico R. (2014) Caracterización estructural y electrónica de titanatos de lantano estroncio dopados con cobalto para su uso como electrodo de celdas de combustible simétricas / Structural and electronic characterization of lanthanum strontium titanares doped with cobalt for symmetrical fuel cells electrodes. Tesis Doctoral en Física, Universidad Nacional de Cuyo, Instituto Balseiro.
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Resumen en español
La estructura básica ABX_3 de las perovskitas forma la familia base de un amplio rango de estructuras relacionadas por la combinación entre distorsiones topológicas, y sustituciones en los sitios A, B y X. Estos compuestos exhiben una gran diversidad de propiedades magnéticas, eléctricas, ópticas y catalíticas de potencial aplicación en física de estado solido, química y ciencia de materiales. Los óxidos tipo perovskitas ABO_3 son candidatos ideales para ser utilizados en aplicaciones de alta temperatura, en particular si presentan buenas propiedades de transporte eléctrico, iónico y actividad catalítica con respecto a ciertas reacciones químicas. Un ejemplo de aplicación se encuentra en las celdas de combustible de oxido solido (SOFC), cuyo estado del arte involucra la utilización de este tipo de compuestos. Las celdas de combustible, en general, son dispositivos electroquímicos capaces de convertir energía química en energía eléctrica con gran eficiencia. Debido a la simplicidad en el concepto de funcionamiento y a su versatilidad poseen un gran potencial para desempeñar un papel importante en una futura matriz de generación de energía eléctrica. Las SOFC, en particular, combinan los beneficios de ser una tecnología compatible con el cuidado del medio ambiente y la posibilidad de un amplio rango de generación de potencia y flexibilidad en el combustible utilizado (Ej.: hidrogeno con bajos requerimientos de pureza e hidrocarburos como gas natural). Una nueva propuesta de materiales a ser utilizados surgió recientemente como un nuevo concepto de SOFC, la celda de combustible de oxido solido simétrica (S-SOFC) que utilizan el mismo material para cátodo y ánodo simultáneamente. Este nuevo concepto podrá resolver dos de los principales problemas asociados al envejecimiento de las SOFC: el envenenamiento por azufre y el deposito de carbón, posibilitando la extensión de la vida útil del dispositivo. El objetivo general del presente trabajo es la síntesis, desarrollo y evaluación de nuevos materiales cerámicos con conductividad mixta para su potencial uso en dispositivos de alta temperatura, analizando su posible uso simultaneo como cátodo y ánodo de SOFC. Para ello se planteo la solución solida entre (La,Sr)CoO_3 y (La,Sr)TiO_3, compuestos que han sido ampliamente reportados para aplicaciones como cátodo y ánodo de SOFC, respectivamente. Especialmente se estudio la familia de compuestos La_0.4Sr_0.6Ti_1-yCo_yO_3±δ con 0 ≤ y ≤ 1 (LSTC). Al ser un compuesto novedoso, el primer paso consistió en identificar y optimizar la ruta de síntesis mas conveniente para obtener la estructura perovskita (La,Sr)(Ti,Co)O_3 con el objetivo de obtener compuestos nanoestructurados, estableciendo una ruta química de baja temperatura basada en una solución de etilenoglicol y acido cítrico como la mas conveniente en función de la aplicación. Dado que los parámetros estructurales ejercen fuerte influencia en las propiedades electroquímicas y ante la notable falta de información cristalográfica, se procedió a realizar una completa caracterización estructural de la sistemática sustitucional con datos de calidad obtenidos mediante técnicas de difracción de rayos X de polvos con luz sincrotrón y neutrones. A temperatura ambiente se encontró que la muestra sin cobalto adopta una estructura tipo perovskita cubica de grupo espacial Pm3m con deficiencia catiónica en el sitio A, en acuerdo con lo reportado para este compuesto al ser sintetizado por una ruta de baja temperatura en aire. Al realizar la sustitución de Ti por Co la simetría de la estructura disminuye a una romboédrica (grupo espacial R3c) debido a la rotación rígida de los octaedros de oxígeno en la dirección a¯a¯a¯ , distorsión que alcanza su máximo para la muestra con 30% de cobalto. Estos resultados fueron complementados con la caracterización del orden local del Ti y del Co a través de espectroscopia de absorción de rayos X extendida en energía (EXAFS). Se determino que, si bien los entornos de ambos metales de transición no son equivalentes, en general mantienen el modelo de entorno octaédrico regular (no distorsionado) del grupo R3c. La caracterización de la estructura electrónica de la serie LSTC se realizo a través de espectroscopía de absorción de rayos X cerca del borde de absorción (XANES). Del análisis de estas mediciones se obtuvo que el estado de oxidación del titanio permanece estable en +4 en toda la serie, mientras que el Co se encuentra mayoritariamente en estado +3. De esta forma, se obtuvo información de cuales son los mecanismos de compensación de carga en la serie LSTC. En las muestras con bajo contenido de Co la electroneutralidad de la estructura es lograda por medio de la expulsión de estroncio de la estructura (formando clústers amorfos de SrO). La sustitución de Ti"4+ por Co"3+ permite compensar estos defectos hasta llegar a la ocupación total del sitio A en la muestra con 30% de cobalto. Al aumentar el contenido de cobalto se encontraron indicios de que es factible un aumento en la no-estequiometria de oxígeno. Teniendo en cuenta la aplicación potencial en dispositivos de alta temperatura, se realizo la caracterización de la estructura cristalina y electrónica en función de la temperatura (20 - 750 C) y atmosfera oxidante y reductora. Se observo que en ambiente oxidante, la serie La_0.4Sr_0.6Ti_1-yCo-yO_3±δ es estable en todo el rango de temperatura ensayado, mientras que en atmosfera reductora los son para y 0:6. En todos los casos se observo que los LSTC tienen una transición de fase reversible desde la romboédrica R3c (a temperatura ambiente) a la cubica Pm3m (a alta temperatura), estando la temperatura de transición relacionada directamente con el grado de distorsión romboédrica de la estructura a temperatura ambiente. Esta transformación es de segundo orden y se debe a la rotación rígida del octaedro de oxígenos. La evolución del estado de oxidación de los metales de transición fue caracterizada a través de XANES in-situ y reducción programada en temperatura y correlacionada con cambios estructurales observados por difracción de rayos X in-situ. En atmosfera reductora, en la muestra sin dopar solo el 7% del titanio se reduce a Ti"3+ al alcanzar los 750 C, este mecanismo es activado a temperaturas superiores a los 500 C, mientras que en las muestras dopadas con 30 y 50% de cobalto se encontró que este elemento se reduce totalmente a Co"2+ entre los 200 y 450 C. Por el contrario, en atmosfera oxidante no se encontraron indicios de reducción en ambos metales de transición. Se estudiaron las propiedades de transporte eléctrico de los LSTC con y = 0.0; 0.3; 0.5 en función de la temperatura y de la presión parcial de oxígeno (pO_2). Se encontró un incremento en la conductividad total, en aire, de entre 7 y 9 ordenes de magnitud entre la muestra sin dopar (10"-7 S/cm) y las correspondientes a y = 0.3 (0.2 S/cm) e y = 0.5 (20 S/cm) a 600 C, respectivamente. Se determino que en la muestra sin dopar, los portadores son mayoritariamente electrones provistos por la reducción de una pequeña fracción de Ti. Por su parte, la adición de cobalto introduce portadores tipo hueco-electrón que incrementan la densidad de portadores disponibles y explica, en parte, el incremento de la conductividad de los LSTC al aumentar el contenido de cobalto. El estudio de las propiedades estructurales y electrónicas, y su relación con los mecanismos de compensación de carga en la serie La_0.4Sr_0.6Ti_1-yCo_yO_3±δ realizados en este trabajo permiten establecer una base para el estudio y comprensión de las propiedades que presentan esta familia de compuestos, sean magnéticas, eléctricas, catalíticas, etc. Como resultado de este trabajo, fueron reportadas al International Centre for Diraction Data 11 nuevas estructuras cristalinas correspondientes a la familia La_0.4Sr_0.6Ti_1-yCo_yO_3±δ y aceptadas para su inclusión en la base de datos de compuestos inorgánicos.
Resumen en inglés
The basic ABX3 perovskite structure turns out to be the prototype of a large range of related structures due to the combination of topological distortions and substitutions in the A, B or X sites. These compounds exhibit a broad variety of magnetic, electric, optic and catalytic properties, potentially useful in solid state physics, chemistry and materials science topics. The ABO_3 perovskite oxides are ideal candidates for high temperature applications, particularly if they present good electric or ionic transport properties and proper catalytic activities for certain chemical reactions. Solid Oxide Fuel Cells (SOFC) are one of the most interesting examples, and the use of perovskite compounds are considered the state of the art for SOFC development. Fuel cells are the most efficient electrochemical devices able to convert chemical to electrical energy. They have a great potential within the future electrical energy generation matrix due to their working principle simplicity and versatility. Within all the fuel cells types, SOFC combine the advantages of being environmental friendly and a wide range of power generation together with the fuel exibility (i.e. SOFC can use flow purity hydrogen or hydrocarbon fuels such as natural gas). A new materials search branch has emerged recently called the Symmetrical Solid Oxide Fuel Cells (S-SOFC) concept, where the same material is used as anode and cathode simultaneously. This new approach could solve two of the main aging SOFCs problems, sulfur poisoning and carbon deposits when the cell is using hydrocarbon as fuel, by simply inverting the gas flux, extending the useful life span. The general objective of this thesis is the synthesis, development, characterization and evaluation of new ceramic compounds with mixed conductivity for the simultaneous use as SOFC and/or IT-SOFC cathode and anode materials. For this end we proposed a solid solution between (La,Sr)CoO_3 and (La,Sr)TiO_3, which have been reported as good cathode and anode materials, respectively. The systematic substitution of Ti by Co, in order to obtain La_0.4Sr_0.6Ti_1-yCo_yO_3±δ (0 ≤ y≤ 1) series has been especially studied. Since this is a novel compound, the rst step was the optimization of the synthesis method to obtain (La,Sr)(Ti,Co)O3 nanostructured perovskite material which was achieved through a low temperature chemical route based in citric acid and ethylene glycol. A complete structural characterization of SOFC electrodes is required for a comprehensive understanding of their electrochemical properties and there was a notable lack of crystallographic information about LSTC compounds at the beginning of this study. Therefore, a complete structural characterization of the systematic substitution of Ti by Co was performed using diraction techniques with high quality synchrotron radiation and neutron data. At room temperature the Co free sample adopts a cubic perovskite Pm3m structure with cationic deciency in the A-site, according to previously reported data for this compound. The as-synthesized LSTC powders show a rhombohedral R3c structure for all Co substitution studied range due to the rigid rotation of oxygen octahedral in the a¯a¯a¯ direction which angle reach a maximum value for the sample with 30% of cobalt content. It was also observed that the A-site vacancy concentration decreases with Co content increase, becoming negligible for samples with Co content larger than 30%. These results were complemented with the local structure characterization of the transition metals (Ti and Co) that occupy the perovskite B site. This characterization was performed using X-ray absorption techniques (EXAFS and XANES) which allow to detect possible local distortions and the electronic state of the transition metals. From EXAFS data analysis, the crystallographic model given by the R3c space group showing undistorted regular octahedron around each transition metal was conrmed, although the local environment around Ti and Co are not equivalent. From XANES data analysis, the Ti oxidation state was found to be stable at +4 in the whole LSTC series while Co was found to be mostly in +3 state. In this way, the information about which are the active charge compensation mechanisms and their dependency with the Co doping level was obtained. For the low doped samples, the electroneutrality is achieved by the generation of strontium vacancies (generating amorphous SrO clusters). As Ti"4+ is substituted by Co"3+ up to a 30%, this point defects are gradually compensated by the decrease of the average B-site valence. For higher Co doped samples, evidences of an increase of the sample oxygen non-stoichiometry was found. Due to the potential application of this compounds in high temperature electrochemical devices, the characterization of crystalline and electronic structure at high temperature (20 - 750 C) and dierent atmospheres were performed. While, the LSTC series was found to be stable in oxidative environmet in the whole studied temperature range, in reductive atmospheres a limit in the Co content of y ≤ 0.6 was found in order to retain the perovskite phase up to 750 C. A reversible second order phase transition was found in the whole LSTC series from rombohedral R3c (at room temperature) to cubic Pm3m (at high temperature) in both atmospheres, related with the rigid rotation of the oxygen octahedra. The phase transition temperature was found to be directly related to the room temperature rombohedral distortion level. The evolution of the transition metals oxidation state was characterized through XANES in-situ and programmed temperature reduction (TPR) and correlated with the structural changes observed by XRD in-situ. At reductive atmospheres, in the Co free sample only 7% of the titanium is reduced to Ti"3+ at 750 C, mechanism that is activated at T > 500 C, in the 30% and 50% Co doped samples it was found that Co is reduced entirely to Co"2+ between 200 and 450 C. Meanwhile, no evidences of Ti or Co reduction was found in oxidative atmosphere up to 750 C. The electric transport properties of the LSTC with y = 0.0; 0.3; 0.5 and its dependence with temperature and oxygen partial pressure (pO2) was studied. An increment of 7 and 9 orders of magnitude in the conductivity was observed between the undoped sample and y = 0.3 and y = 0.5 samples, at 600 C, respectively. Charge carriers at the undoped sample was found to be dominated by electrons due to the Ti reduction in a small proportion. Meanwhile, in the doped samples, the hugh increase in the conductivity is explained in part by the holes charge carriers introduced by the addition of cobalt into the sample. The study of the structural and electronic properties, and their relationship with the charge compensation mechanism in the La_0.4Sr_0.6Ti_1-yCo_yO_3 series performed in this thesis provides the base knowledge to understand the properties presented by this family of compounds such as their magnetic, electric or catalytic characteristics. As a consequence from this work, 11 of the new crystalline structures here presented were reported to the International Centre for Data Diraction and accepted for its inclusion to their inorganic compounds data base.
| Tipo de objeto: | Tesis (Tesis Doctoral en Física) |
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| Palabras Clave: | Electrodes; Electrodos; Crystal structure; Estructura cristalina; Electronic structure; Estructura electrónica; Synchrotrons; Sincrotones |
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| Materias: | Ingeniería > Ciencia de los materiales |
| Divisiones: | Gcia. de área de Aplicaciones de la tecnología nuclear > Gcia. de Investigación aplicada > Caracterización de materiales |
| Código ID: | 1090 |
| Depositado Por: | Tamara Cárcamo |
| Depositado En: | 21 Jul 2022 16:04 |
| Última Modificación: | 21 Jul 2022 16:04 |
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