Publication: Estudio de los sistemas BaCe0,9-xPrxY0,1O3-ð/ BaCe0,8-xPrxY0,2O3-ð : búsqueda de conductores protónicos para su utilización como electrolitos en pilas de combustible de óxido sólido (SOFCs)
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2012-10
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2012-10-22
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Abstract
Existen evidencias sobre la posible utilización de óxidos sólidos de estructura tipo perovskita; fundamentalmente basados en BaCeO3, BaZrO3 ó SrCeO3, como electrolitos en pilas de combustible de óxido sólido, Solid Oxide Fuel Cells (SOFC’s). En general, su interés radica en la creación de vacantes de oxígeno; principales responsables de la reportada conducción protónica de estos materiales a altas temperaturas, al sustituir parcialmente en posiciones B elementos de valencia 3+. En este proyecto, se aborda la síntesis, caracterización estructural y eléctrica de materiales, de estructura tipo perovskita (ABO3), basados en el ceriato de bario, BaCeO3, en los que se sustituye parcialmente el cerio por itrio y/o praseodimio. Concretamente, se sintetizaron materiales de fórmula general: BaCe0,9‐xPrxY0,1O3‐δ con x = 0 – 0,2 – 0,45 – 0,7 – 0,9 y BaCe0,8‐ xPrxY0,2O3‐δ con x = 0 – 0,2 – 0,4 – 0,6 – 0,8 . Se estudió la evolución de la reacción desde 1000 hasta 1600°C, así como la estructura, simetría cristalina y microestructura de estos materiales mediante difracción de rayos X (DRX) y microscopía electrónica de barrido (SEM). Se confirmó la presencia de una estructura tipo perovskita (ABO3) y de una fase única a partir de 1200⁰C en todas las muestras y un progresivo aumento de cristalinidad y mejora de la homogeneidad con la temperatura. El refinamiento de Rietveld de los difractogramas evidenció una disminución de los parámetros de red y de las distancias interatómicas B‐O con la sustitución de praseodimio. Todos los perfiles se ajustaron a una estructura ortorrómbica con un grupo espacial Pbnm (62) (√2ac x √2ac x 2ac), excepto la muestra BaCe0,8Y0,2O3‐δ que presentaba a una estructura monoclínica con un grupo espacial I2/m (12) (√2ac x √2ac x 2ac). En dicha muestra, a su vez, se encontraron trazas de Y2O3. Por otro lado, se detectó una disminución del volumen de la celda con el aumento de la cantidad de itrio. En general, la densidad de las muestras aumenta con el contenido en Pr. Igualmente, el tamaño de grano aumenta a medida que aumenta el contenido en Pr. Las propiedades eléctricas de materiales calcinados a 1500°C se midieron en condiciones oxidantes mediante espectroscopia de impedancia compleja (EIC). Estas medidas se realizaron en un rango de temperatura desde 10 hasta 700K aproximadamente y realizando un barrido en frecuencia desde 5 hasta 10MHz. Todas las muestras presentaron un comportamiento típico de materiales semiconductores; aumentando hasta en varios órdenes de magnitud la conductividad con la temperatura. Sin embargo, energía de activación disminuye al aumentar el contenido de praseodimio. En general, aumentos del contenido en itrio del 10 al 20% no mejoraron la conductividad eléctrica. Se encontraron indicios de una posible conductividad protónica a altas temperaturas en algunos casos, probablemente combinada con conductividad electrónica. Por ejemplo, La muestra BaCe0,7Pr0,2Y0,1O3‐δ mostró una conductividad de “bulk” de σ=3∙10‐3S∙cm‐1 a 600⁰C y de 0,01S∙cm‐1 para la muestra BaCe0,9Y0,1O3‐δ. Para este mismo material, se han encontrado estudios con valores próximos al medido. ____________________________________________________________________________________________________________
There is evidence of the possible use of solid perovskite structure, mainly based BaCeO3 or BaZrO3 or SrCeO3, as electrolytes in Solid Oxide Fuel Cells (SOFC’s). Usually, their interest lies in the creation of oxygen vancancies. These oxygen vacancies which come from the substitution in the B sites a trivalent dopant cation are reported as the mains responsible for protonic conduction at high temperatures. This proyect discussed the synthesis, structural and electrical characterization of materials with perovskite structure based on Y/Pr doped barium cerate BaCeO3. In particular, the synthesized materials have as general formulation: BaCe0,9‐xPrxY0,1O3‐δ with x = 0 – 0,2 – 0,45 – 0,7 – 0,9 and BaCe0,8‐xPrxY0,2O3‐δ with x = 0 – 0,2 – 0,4 – 0,6 – 0,8 . Firstly, evolution the perovskite formation, the cristal structure and symmetries and the microstructure of these materials was analyzed by X‐ray diffraction (XRD) and scanning electron microscopy (SEM) from 1000 to 1600°C. In general, all the samples wer single phase with a perovskite structure (ABO3) above 1200°C. Crystallinity and homogeneity increase with the temperature. Increasing praseodymium content lattice parameters and interatomic distances B‐O decrease lightly. On the other hand, a reduction in the volume of the cell was unregistered with amount of yttrium. All profiles were matched with an orthorhombic group Pbnm (62) (√2ac x √2ac x 2ac) except the composition which was indexed in a monoclinic group I2/m (12) (√2ac x √2ac x 2ac). In samples with BaCe0,8Y0,2O3‐δ composition some amount of Y2O3 were found by scanning microscopy. Regarding to microstructural characterization, it was found that samples density increase with Pr content. The same behavior was concluded for grain size increases. Electrical properties were measured by impedance spectroscopy (IS). These experiments were performed at 1500°C, in air and in a frequency range from 5 to 10MHz. All samples showed a semiconductor typical behavior of were conductivity increase with temperature. However, activation energy decreases with praseodymium content. Overall, increases in yttrium content from 10 to 20% did not improve electrical conductivity. It was found indications of possible protonic conductivity at elevated temperature in some cases, probably combined with electronic conductivity. For example, the sample BaCe0,7Pr0,2Y0,1O3‐δ showed a conductivity of σ=3∙10‐3S∙cm‐1 at 600⁰C and 0,01S∙cm‐1 for BaCe0,9Y0,1O3‐δ. For this material, some studies had reported conductivities close to the measured values.
There is evidence of the possible use of solid perovskite structure, mainly based BaCeO3 or BaZrO3 or SrCeO3, as electrolytes in Solid Oxide Fuel Cells (SOFC’s). Usually, their interest lies in the creation of oxygen vancancies. These oxygen vacancies which come from the substitution in the B sites a trivalent dopant cation are reported as the mains responsible for protonic conduction at high temperatures. This proyect discussed the synthesis, structural and electrical characterization of materials with perovskite structure based on Y/Pr doped barium cerate BaCeO3. In particular, the synthesized materials have as general formulation: BaCe0,9‐xPrxY0,1O3‐δ with x = 0 – 0,2 – 0,45 – 0,7 – 0,9 and BaCe0,8‐xPrxY0,2O3‐δ with x = 0 – 0,2 – 0,4 – 0,6 – 0,8 . Firstly, evolution the perovskite formation, the cristal structure and symmetries and the microstructure of these materials was analyzed by X‐ray diffraction (XRD) and scanning electron microscopy (SEM) from 1000 to 1600°C. In general, all the samples wer single phase with a perovskite structure (ABO3) above 1200°C. Crystallinity and homogeneity increase with the temperature. Increasing praseodymium content lattice parameters and interatomic distances B‐O decrease lightly. On the other hand, a reduction in the volume of the cell was unregistered with amount of yttrium. All profiles were matched with an orthorhombic group Pbnm (62) (√2ac x √2ac x 2ac) except the composition which was indexed in a monoclinic group I2/m (12) (√2ac x √2ac x 2ac). In samples with BaCe0,8Y0,2O3‐δ composition some amount of Y2O3 were found by scanning microscopy. Regarding to microstructural characterization, it was found that samples density increase with Pr content. The same behavior was concluded for grain size increases. Electrical properties were measured by impedance spectroscopy (IS). These experiments were performed at 1500°C, in air and in a frequency range from 5 to 10MHz. All samples showed a semiconductor typical behavior of were conductivity increase with temperature. However, activation energy decreases with praseodymium content. Overall, increases in yttrium content from 10 to 20% did not improve electrical conductivity. It was found indications of possible protonic conductivity at elevated temperature in some cases, probably combined with electronic conductivity. For example, the sample BaCe0,7Pr0,2Y0,1O3‐δ showed a conductivity of σ=3∙10‐3S∙cm‐1 at 600⁰C and 0,01S∙cm‐1 for BaCe0,9Y0,1O3‐δ. For this material, some studies had reported conductivities close to the measured values.
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Keywords
Pilas de combustible, Óxidos sólidos, Conducción protónica