Publication: Síntesis y caracterización de materiales con estructura tipo perovskita y procesado de ánodos microtubulares para pilas de combustible de óxido sólido
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2017-12
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2017-12-20
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Abstract
La labor de investigación referida en la presente memoria se enmarca dentro del
ámbito de las Pilas de Combustible de Óxido Sólido (SOFC, Solid Oxide Fuel Cell) y ha sido
desarrollada en el grupo de Síntesis y Procesado de Materiales (SIPMAT) adscrito al
Departamento de Ciencia e Ingeniería de Materiales e Ingeniería Química de la
Universidad Carlos III de Madrid, bajo la tutela del Dr. Alejandro Várez y la Dra. Belén
Levenfeld. Gran parte del trabajo que se presenta en este documento ha sido igualmente
realizado en el Departamento de Ciencia e Ingeniería de Materiales de la Universidad de
Sheffield (Department of Materials Science and Engineering), bajo la supervisión del Dr.
Anthony. R. West y del Dr. Nahum Masó. El documento contiene también resultados de
varios experimentos realizados en el Centro Nacional de Microscopia Electrónica (CNME)
de la Universidad Complutense de Madrid con el apoyo de la Dra. Ester García, en el
Instituto Max von Laue-Paul Langevin (Ill, Institut LaueLangevin,
Grenoble), y en el
Laboratorio Europeo de Radiación Sincrotrón (ESRF, European Synchrotron Radiation
Facility, Grenoble). Se incluyen además algunos resultados obtenidos en colaboración con
el Instituto de Ciencia de Materiales de Aragón (ICMA); en particular, con el Dr. Miguel
A. Laguna y del Dr. Hernán Monzón .
El trabajo engloba dos líneas de investigación independientes orientadas al desarrollo
de materiales y tecnologías de fabricación para pilas de combustible de óxido sólido en un
rango de temperaturas intermedio (600-800 °C; IT-SOFC: Intermediate Temperature Solid
Oxide Fuel Cell). La operación en este rango de temperaturas, sensiblemente inferior al
habitual de estos dispositivos (800-1000 °C), permitiría extender el ámbito de aplicación de
las pilas SOFC hacia el campo de las aplicaciones móviles y portátiles. Además, la
operación en este rango de temperaturas moderadamente más bajas implica otras ventajas
adicionales; tales como el aumento de la durabilidad y fiabilidad de los dispositivos, o la
posibilidad de incorporar materiales económicamente más viables. La primera línea de
investigación plantea la síntesis y caracterización de materiales con estructura perovskita
ABO3; donde la posición A está ocupada por cationes Ba2+ y la posición B por cationes
Ce4+, Pr4+/3+, Y3+, Zr4+ o una combinación de ellos. La segunda línea de investigación
pretende la aplicación del proceso de moldeo por extrusión de polvos (PEM, Powder
Extrusion Moulding) a la producción de ánodos microtubulares de Ni-YSZ.
La síntesis de todos los materiales de estructura ABO3 se llevó a cabo por reacción en
estado sólido; un método sencillo y de bajo coste, cuya eficacia ha sido ampliamente
justificada en la síntesis de compuestos similares. Todos los materiales sintetizados fueron
caracterizados desde un punto de vista estructural, microestructural y eléctrico
empleando técnicas de difracción, de microscopia electrónica y de espectroscopia de
impedancias, respectivamente. A excepción del material de composición nominal
BaCe0,8Y0,2O3-δ, todos los sistemas presentan una simetría ortorrómbica a temperatura
ambiente (G.E. No. 62 Pnma). Se registró además una secuencia de polimorfos de alta
temperatura para la serie de BaCe1-xPrxO3-δ análoga a la reportada para perovskitas
similares (i.e. Pnma → Imma → R3c → Pmm). Las temperaturas de transición se estimaron
entre 530-650, 530-700 y 1215 K, respectivamente; constatándose asimismo un adelanto de
las dos primeras transiciones a medida que aumenta el contenido en Pr. Los análisis
composicionales por EDS revelaron una pérdida irreversible de Ba relacionada con el
propio proceso de síntesis. Para compensar esta pérdida catiónica de Ba se proponen
algunos mecanismos, principalmente: (i) la reducción intrínseca del Pr y la consiguiente
redistribución hacia posiciones A para aliviar tensiones estructurales; (ii) la incorporación
de especies trivalentes Y3+ en posiciones A; (iii) la estabilización de las vacantes de Ba. La
detección de algunos efectos eléctricos poco habituales en el ámbito de los materiales
dieléctricos y semiconductores (e.g. descenso de la capacidad con la temperatura) puso en
valor la importancia de ajustar los datos de espectroscopia de impedancias mediante
circuitos equivalentes; confirmándose de esta forma el origen extrínseco de algunos
comportamientos. Desde un punto de vista exclusivamente eléctrico, la serie de materiales
BaCe0,9-xPrxY0,1O3-δ; en la que se combinan las ventajas de la sustitución Pr/Ce (aumento de
la componente electrónica y de la sinterabilidad) con las de la sustitución Y/Ce
(introducción de una componente iónica relacionada con la creación de VO ), se plantea
como posible alternativa para actuar como electrolito (x <0,2) o electrodo (x >0,2) en pilas
IT-SOFC.
El procesado por PEM se optimizó con éxito utilizando un sistema ligante
termoplástico compuesto por polipropileno (50 %vol.), cera parafina (46 %vol.) y ácido
esteárico (4 %vol.). Además, con objeto de satisfacer los requerimientos de materiales
anódicos (50/50 %vol. Ni/YSZ y 50 %vol. de porosidad), se empleó una ratio NiO/YSZ de
63/37 %vol. y maicena como agente formador de poros. Asimismo, para determinar la formulación óptima se procesaron feedstocks con cargas en volumen de polvo variables
(45-65 %vol.). Igualmente, a fin de evaluar la influencia del tamaño y de la distribución de
tamaños de partícula del NiO sobre la procesabilidad de los feedstocks y las propiedades
finales de los microtubos, se emplearon dos polvos de NiO con un tamaño medio de
partícula entre 6-10 y 1-2 μm. Todos los feedstock procesados presentaron un
comportamiento de tipo pseudoplástico entre 10-1000 s1
y una viscosidad inferior al
máximo recomendado para este tipo de procesos (1000 Pa∙s). Desde un punto de vista
reológico, los feedstocks que contienen polvo de NiO con un tamaño medio de partícula
entre 1-2 μm resultaron más adecuados (i.e. menor viscosidad, menor carácter
pseudoplástico y mayor energía de activación). La etapa de extrusión se optimizó para
obtener “piezas en verde” libres de defectos de aproximadamente 15 mm de longitud,
4 mm de diámetro nominal y 0,5-1 mm de espesor de pared. La eliminación del sistema
ligante se programó de forma gradual en dos etapas. La primera de ellas consistió en una
extracción con disolventes, mientras que la segunda implicó una degradación térmica
para eliminar por completo la parte orgánica. Las “piezas en marrón” se sinterizaron en
aire (1400-1600 °C, 2 h) y, posteriormente, se redujeron en H2 (800-850 °C, 2 h). La
microestructura, la porosidad y la conductividad de las piezas finales resultaron estar
íntimamente ligadas al tamaño de partícula del NiO. Los mejores resultados se registraron
para perfiles procesados con NiO de 1-2 μm y una carga de polvo del 65 %vol.,
obteniéndose microestructuras con una elevada porosidad (43 %vol.) y una red fina de
partículas Ni0 interconectadas. La conductividad eléctrica medida en d.c. (9,3·103 S∙cm1)
fue además cercana al máximo teórico para esta composición (12,5·103 S∙cm1).
Las celdas
fabricadas utilizando el perfil procesado por PEM (Ni-YSZ|YZS|LSM-YSZ) exhibieron
densidades de potencia comparables a las reportadas para celdas similares (0,51 W∙cm2,
a
850 °C y 0,7 V). Estos resultados ponen de manifiesto el potencial de esta tecnología de
procesado; fácilmente escalable a nivel industrial y de bajo coste, para la producción de
ánodos microtubulares porosos de Ni-YSZ aptos para operar en pilas IT-SOFC.
The research work referred in this report is defined within the field of Solid Oxide Fuel Cells (SOFCs) and has been developed in the University Carlos III of Madrid, Department of Science and Engineering of Materials and Chemical Engineering, Group of Synthesis and Processing of Materials (SIPMAT) under the supervision of Dr. Alejandro Várez and Dr. Belén Levenfeld. Likewise, a great part of the research work has been done in the Department of Materials Science and Engineering of the University of Sheffield under the supervision of Dr. Anthony. R. West and Dr. Nahum Masó. The present document also contains results from experiments carried out at the National Center for Electron Microscopy of the Complutense University of Madrid with the support of Dr. Ester García, at the Institute Laue-Langevin in Grenoble (ILL), and at the European Synchrotron Radiation Facility (ESRF). Some results were equally obtained in collaboration with the Institute of Materials Science of Aragon (ICMA); in particular, with Dr. Miguel A. Laguna and Dr. Hernán Monzón. The work encompasses two independent research lines oriented to the development of novel materials for Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC), which typically work within the interval 600-800 ° C. This lower operation temperature range than usual SOFC (800-1000 °C), account for extending their applications into the field of mobile and portable devices. In addition, the operation in this range of moderate temperatures involves further advantages such as increasing the durability and reliability of the fuel cells, and the possibility of incorporating more cost-effective materials. The first research line addresses the synthesis and characterization of high temperature proton conductors (HTPC) with potential application to work as electrolyte (and electrodes) in IT-SOFC devices. The investigated materials are ceramics showing perovskite type structure ABO3; where the A position is occupied by Ba2+ cations and the B position by Ce4+, Pr4+/3+, Y3+, Zr4+ cations or a combination of them. The syntheses of all the compositions were carried out by solid-state reaction, a simple and low-cost method whose efficiency has been widely justified in the synthesis of similar compounds. In order to synthesize highly homogeneous materials with a high degree of crystallinity, moderately high temperatures (1500 °C) and relatively long calcination times (>12 h) were required. All the synthesized ABO3 materials were characterized from a structural, microstructural and electrical point of view, using diffraction, electron microscopy and impedance spectroscopy techniques. The results obtained from the corresponding Rietveld refinements found that, except for the material with nominal composition BaCe0.8Y0.2O3-δ, all the prepared systems presented an orthorhombic symmetry at room temperature (S.G. No. 62 Pnma). A sequence of high temperature polymorphs was recorded for the BaCe1-xPrxO3-δ series, which is in accordance to that reported for similar perovskites: Pnma → Imma → R3c → Pmm. The transition temperatures were estimated between 650-530, 530-700 and 1215 K, respectively. Equally, a decrease in the temperature of the first two transitions with the increase of the Pr content was also found. Electron microscopic images and compositional analyses by EDS revealed an irreversible loss of Ba during the synthesis, which is compensated by: (i) autoionization and redistribution of Pr in positions A (Pr3+) and B (Pr4+) and/or (ii) the substitution of trivalent Y3+ species in antisite, thus conditioning the electrical response of the materials. The detection of some unusual electrical effects in the context of dielectric and semiconductor materials emphasized the importance of adjusting the impedance spectroscopy data through equivalent circuits; confirming in this way the extrinsic origin of some behaviors. Within the context of proton IT-SOFCs, and from an electric point of view, the series of materials BaCe0,9-xPrxY0,1O3-δ; in which the advantages of the Pr4+/Ce4+ substitution (increase of the electronic component of the conductivity) are combined with those of the Y3+/ Ce4+ substitution (introduction of an ion/proton component related to the creation of "VO "), is considered as a solid alternative to act as electrolyte (x <0.2) or electrode (x> 0.2). The second research line assesses the application of Powder Extrusion Moulding (PEM) process to the production of Ni-YSZ microtubular profiles for anode-supported MT-IT-SOFC. The processing by this method was successfully optimized using a thermoplastic binder system composed of polypropylene (50 vol%), paraffin wax (46 vol%) and stearic acid (4 vol%). Furthermore, in order to satisfy the requirements of anodic materials (50/50 vol% Ni/YSZ and 50 vol% porosity), a 63/37 vol% Ni/YSZ ratio and cornstarch as a pore forming agent were used. Likewise, aiming an optimum formulation, different feedstocks were processed with variable powder loadings (45-65 vol%). Similarly, in order to evaluate the influence of size and particle size distribution of the NiO on the processability of the feedstocks and the final properties of the microtubes, two NiO powders showing average particle size of 6-10 and 1-2 were used. All the processed feedstocks presented a pseudoplastic behavior between 10-1000 s1 and viscosity lower than the maximum recommended for these processes (1000 Pa ∙ s). From a rheological point of view, the feedstocks containing NiO powder with the smaller particle size presented enhanced properties for the extrusion of microtubular profiles (i.e. lower viscosity, lower pseudoplastic character and higher activation energy). The extrusion step was optimized to achieve "green parts" free of defects and showing dimensions around; 15 mm in length, 4 mm nominal diameter and 0.5-1 mm wall thickness. The removal of the binder was programmed in two steps; a solvent extraction debinding followed by a thermal degradation, in order to ensure the complete elimination of the organic compounds. The "brown parts" were sintered in air (1400-1600 °C, 2 h) and, subsequently, reduced under H2 (800-850 °C, 2 h). The microstructure, porosity and conductivity of the final parts were directly linked to the NiO particle size. Thus, the best results were obtained for the profiles processed with NiO of 1-2 μm particle size and a powder content of 65 vol%, which showed a microstructure with a high porosity (43 vol%) and a fine network of interconnected Ni0 particles. Likewise, the measured electrical conductivity of this part in d.c. (9.3·103 S∙cm1) was very close to the theoretical maximum for this composition (12.5·103 S∙cm1). Finally, a MT-IT-SOFC (Ni-YSZ|YZS|LSM-YSZ) supported on this profile processed by PEM exhibited power densities comparable to those reported for similar fuel cells (0.51 W ∙ cm2, at 850 °C and 0.7 V). These results highlight the potential of thermoplastic powder extrusion moulding technology (cost-effective and easily scalable to an industrial level) as an alternative for the production of porous microtubular Ni-YSZ profiles suitable to operate in MT- IT-SOFC anode-supported devices.
The research work referred in this report is defined within the field of Solid Oxide Fuel Cells (SOFCs) and has been developed in the University Carlos III of Madrid, Department of Science and Engineering of Materials and Chemical Engineering, Group of Synthesis and Processing of Materials (SIPMAT) under the supervision of Dr. Alejandro Várez and Dr. Belén Levenfeld. Likewise, a great part of the research work has been done in the Department of Materials Science and Engineering of the University of Sheffield under the supervision of Dr. Anthony. R. West and Dr. Nahum Masó. The present document also contains results from experiments carried out at the National Center for Electron Microscopy of the Complutense University of Madrid with the support of Dr. Ester García, at the Institute Laue-Langevin in Grenoble (ILL), and at the European Synchrotron Radiation Facility (ESRF). Some results were equally obtained in collaboration with the Institute of Materials Science of Aragon (ICMA); in particular, with Dr. Miguel A. Laguna and Dr. Hernán Monzón. The work encompasses two independent research lines oriented to the development of novel materials for Intermediate Temperature Solid Oxide Fuel Cell (IT-SOFC), which typically work within the interval 600-800 ° C. This lower operation temperature range than usual SOFC (800-1000 °C), account for extending their applications into the field of mobile and portable devices. In addition, the operation in this range of moderate temperatures involves further advantages such as increasing the durability and reliability of the fuel cells, and the possibility of incorporating more cost-effective materials. The first research line addresses the synthesis and characterization of high temperature proton conductors (HTPC) with potential application to work as electrolyte (and electrodes) in IT-SOFC devices. The investigated materials are ceramics showing perovskite type structure ABO3; where the A position is occupied by Ba2+ cations and the B position by Ce4+, Pr4+/3+, Y3+, Zr4+ cations or a combination of them. The syntheses of all the compositions were carried out by solid-state reaction, a simple and low-cost method whose efficiency has been widely justified in the synthesis of similar compounds. In order to synthesize highly homogeneous materials with a high degree of crystallinity, moderately high temperatures (1500 °C) and relatively long calcination times (>12 h) were required. All the synthesized ABO3 materials were characterized from a structural, microstructural and electrical point of view, using diffraction, electron microscopy and impedance spectroscopy techniques. The results obtained from the corresponding Rietveld refinements found that, except for the material with nominal composition BaCe0.8Y0.2O3-δ, all the prepared systems presented an orthorhombic symmetry at room temperature (S.G. No. 62 Pnma). A sequence of high temperature polymorphs was recorded for the BaCe1-xPrxO3-δ series, which is in accordance to that reported for similar perovskites: Pnma → Imma → R3c → Pmm. The transition temperatures were estimated between 650-530, 530-700 and 1215 K, respectively. Equally, a decrease in the temperature of the first two transitions with the increase of the Pr content was also found. Electron microscopic images and compositional analyses by EDS revealed an irreversible loss of Ba during the synthesis, which is compensated by: (i) autoionization and redistribution of Pr in positions A (Pr3+) and B (Pr4+) and/or (ii) the substitution of trivalent Y3+ species in antisite, thus conditioning the electrical response of the materials. The detection of some unusual electrical effects in the context of dielectric and semiconductor materials emphasized the importance of adjusting the impedance spectroscopy data through equivalent circuits; confirming in this way the extrinsic origin of some behaviors. Within the context of proton IT-SOFCs, and from an electric point of view, the series of materials BaCe0,9-xPrxY0,1O3-δ; in which the advantages of the Pr4+/Ce4+ substitution (increase of the electronic component of the conductivity) are combined with those of the Y3+/ Ce4+ substitution (introduction of an ion/proton component related to the creation of "VO "), is considered as a solid alternative to act as electrolyte (x <0.2) or electrode (x> 0.2). The second research line assesses the application of Powder Extrusion Moulding (PEM) process to the production of Ni-YSZ microtubular profiles for anode-supported MT-IT-SOFC. The processing by this method was successfully optimized using a thermoplastic binder system composed of polypropylene (50 vol%), paraffin wax (46 vol%) and stearic acid (4 vol%). Furthermore, in order to satisfy the requirements of anodic materials (50/50 vol% Ni/YSZ and 50 vol% porosity), a 63/37 vol% Ni/YSZ ratio and cornstarch as a pore forming agent were used. Likewise, aiming an optimum formulation, different feedstocks were processed with variable powder loadings (45-65 vol%). Similarly, in order to evaluate the influence of size and particle size distribution of the NiO on the processability of the feedstocks and the final properties of the microtubes, two NiO powders showing average particle size of 6-10 and 1-2 were used. All the processed feedstocks presented a pseudoplastic behavior between 10-1000 s1 and viscosity lower than the maximum recommended for these processes (1000 Pa ∙ s). From a rheological point of view, the feedstocks containing NiO powder with the smaller particle size presented enhanced properties for the extrusion of microtubular profiles (i.e. lower viscosity, lower pseudoplastic character and higher activation energy). The extrusion step was optimized to achieve "green parts" free of defects and showing dimensions around; 15 mm in length, 4 mm nominal diameter and 0.5-1 mm wall thickness. The removal of the binder was programmed in two steps; a solvent extraction debinding followed by a thermal degradation, in order to ensure the complete elimination of the organic compounds. The "brown parts" were sintered in air (1400-1600 °C, 2 h) and, subsequently, reduced under H2 (800-850 °C, 2 h). The microstructure, porosity and conductivity of the final parts were directly linked to the NiO particle size. Thus, the best results were obtained for the profiles processed with NiO of 1-2 μm particle size and a powder content of 65 vol%, which showed a microstructure with a high porosity (43 vol%) and a fine network of interconnected Ni0 particles. Likewise, the measured electrical conductivity of this part in d.c. (9.3·103 S∙cm1) was very close to the theoretical maximum for this composition (12.5·103 S∙cm1). Finally, a MT-IT-SOFC (Ni-YSZ|YZS|LSM-YSZ) supported on this profile processed by PEM exhibited power densities comparable to those reported for similar fuel cells (0.51 W ∙ cm2, at 850 °C and 0.7 V). These results highlight the potential of thermoplastic powder extrusion moulding technology (cost-effective and easily scalable to an industrial level) as an alternative for the production of porous microtubular Ni-YSZ profiles suitable to operate in MT- IT-SOFC anode-supported devices.
Description
Keywords
Extrusión, Ánodos microtubulares, Solid Oxide Fuel Cell (SOFC), Pilas de combustible de óxido sólido, Estructura perovskita