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    Tesis

    Propiedades de nanocerámicos basados en CeO2 para celdas de combustible de óxido sólido de temperatura intermedia

    2019



    TesistaMarina Soledad BELLORA
    Ingeniera Química, Universidad Nacional del Comahue, Argentina
    Doctora en Ciencia y Tecnología, Mención Materiales - Instituto Sabato UNSAM/CNEA - Argentina
    DirectorDr. Diego German Lamas. UNSAM, CONICET - Argentina
    CodirectorDr. Joaquín Sacanell. CNEA – Argentina
    Lugar de realizaciónGerencia Investigación y Aplicaciones (CAC - CNEA) - Escuela de Ciencia y Tecnología (UNSAM – CONICET)
    Fecha Defensa26/12/2019
    JuradoDra. Graciela Carmen Abuin. INTI - Argentina
    Dr. Federico Andrés Viva. CNEA - Argentina
    Dr. Pablo Martín Botta. UNMdP - Argentina
    CódigoITS/TD 134/19

    Título completo

    Propiedades de nanocerámicos basados en CeO2 para celdas de combustible de óxido sólido de temperatura intermedia

    Resumen

    Las celdas de combustible de óxido sólido (SOFCs) se presentan como una alternativa para la generación de energía más eficiente y con menor impacto ambiental. El desafío actual es el diseño de nuevos materiales de alta performance que permitan reducir las temperaturas de trabajo para minimizar los problemas de degradación y los costos.

    Para el ánodo, se han propuesto ánodos basados en ceria (CeO2) por su conductividad iónica y electrónica debido a la reducción parcial de Ce4+ a Ce3+. Se usan en materiales compuestos con metales. El Ni es un excelente conductor electrónico y electrocatalizador para la oxidación de H2 o hidrocarburos.

    Este trabajo se centra en el estudio de la influencia del tamaño de cristalita de materiales basados en CeO2 en la performance catalítica para su aplicación como ánodo de SOFCs de temperatura intermedia. Se trabajó con materiales basados en CeO2 dopados con Gd2O3, Sm2O3 e Y2O3, con los que se sintetizaron materiales compuestos con NiO, y se varió el tamaño de cristalita a través de tratamientos térmicos.

    En esta tesis se presenta el estudio del comportamiento redox de los materiales basados en CeO2, utilizando las técnicas de reducción a temperatura programada y la espectroscopía in-situ de absorción de rayos X con radiación sincrotrón. Se observó que Ni promueve la reducción del Ce y que la temperatura de inicio de la reducción depende del tamaño medio de cristalita, comenzando a temperaturas menores en muestras de menor tamaño. Este efecto resulta de gran interés para la aplicación del material como ánodo de una IT-SOFCs.

    Se realizó la evaluación del comportamiento catalítico de los materiales dopados con Sm2O3 para la oxidación de metano. Se ensayan dos atmósferas y se analiza el efecto de la presencia de Ni y del tamaño de cristalita del material. Los ensayos de oxidación parcial y total de metano mostraron una clara interacción entre el comportamiento redox del cerio y las partículas de Ni de la fase activa.

    Se presenta un estudio preliminar del comportamiento electroquímico del NiO/SDC utilizando espectroscopía de impedancia electroquímica a fin de comprender los procesos limitantes involucrados y el mecanismo de oxidación del H2.

     

    Palabras claves: Generación de energía, Energías limpias, Celdas de Combustible, SOFCs, Nanomateriales basados en CO2.

    Complete Title

    CeO2-based nanomaterials propierties for Intermediate Temperature Solid Oxide Fuel Cell

    Abstract

    In view of the need to find alternatives for more efficient energy generation and with less environmental impact, solid oxide fuel cells are presented as a viable alternative. These devices have significant advantages over conventional technologies but their high operating temperatures present a lock for their use since it imposes restrictions on the choice of materials used and significantly increases costs. The current challenge of researchers in this field is the design of new high-performance materials that reduce working temperatures in order to achieve more economical devices and minimize degradation problems.

    In the case of the anode, it is essential to find materials of high electrocatalytic activity for the oxidation of the fuel. For intermediate temperature devices, ceria-based anodes (CeO2) have been proposed since they have excellent properties due to their mixed, ionic and electronic conductivity due to the partial reduction of Ce4+ to Ce3+. They are used doped with divalent or trivalent metal oxides to promote ionic conductivity. They are generally used in composite materials with metals such as Ni, Co, Cu or noble metals such as Pt, Ru. Ni is an excellent electronic conductor and electrocatalyst for the oxidation of H2 or hydrocarbons, and has the advantage of having a low cost compared to noble metals. Normally part of the oxide, NiO, and metallic Ni is formed in the reducing atmosphere of the fuel.

    On the other hand, the optimization of the microstructure of the electrodes is key to obtain a better performance. It has been observed that electrodes, both anodes and cathodes, based on mixed nanostructured conductors have a better response than those with conventional micrometric materials. This may be due to the very significant increase in the number of active sites for the oxidation reaction of the fuel.

    This research work focuses on the study of the influence of the crystallite size of CeO2-based materials on the intermediate temperature catalytic performance for possible application as an intermediate temperature solid oxide fuel cell (IT-SOFC) anode. We worked with CeO2-based materials with three different dopants (Gd2O3, Sm2O3 and Y2O3: GDC, SDC and YDC, respectively), with which composite materials were synthesized with NiO, and the crystallite size was varied by subjecting them to different heat treatments temperatures.

    This thesis is organized in nine chapters. In Chapter 1 a general introduction to the subject of study is presented. Besides, the main motivations and objectives of this work are also presented.

    In Chapter 2 the experimental techniques used in the course of this work are described.

    In Chapter 3 the synthesis and structural and morphological characterization of the chosen materials are presentd. The crystalline phases present were identified and the average crystallite size of each was determined by X-ray powder diffraction (XPD), electron microscopy techniques such as TEM and SEM that also provided information on the morphology of each phase, the porous structure by isothermal fisisortion of N2 and dispersion of X-rays at low angles (SAXS). In addition, chemical analyzes, such as X-ray fluorescence (XRF) and energy dispersive microanalysis were performed to determine the composition of the synthesized materials.

    In Chapter 4 the redox behavior of CeO2-based materials doped with Gd2O3 (GDC) are discussed. The techniques of programmed temperature reduction (TPR) and in situ X-ray absorption spectroscopy near the absorption edge (XANES) with synchrotron radiation were used. The oxidation states of Ce and Ni were studied for a function of the temperature in the atmosphere of diluted hydrogen. Two methods of analyzing the data obtained by the XANES technique were compared. The results showed that the presence of Ni promotes the reduction of the Ce present in the support. In addition, it was observed that the reduction start temperature strongly depends on the average crystallite size of both phases, starting at lower temperatures in smaller samples.

    In Chapter 5 the redox behavior of materials doped with Sm2O3 and Y2O3 is studied, using again the techniques of TPR and XANES in situ with synchrotron radiation. The results confirm the observations made in the previous chapter regarding the effect of Ni on the reduction of Ce and the reduction of the reduction temperature in materials with smaller crystallite size. This effect is of great interest for the application of the material as an anode of an IT-SOFCs since it favors the decrease in the working temperature of said device.

    In Chapter 6 the results of the catalytic evaluation of materials doped with Sm2O3 for the oxidation of methane are presented. Two atmospheres are tested and the effect of the presence of Ni and the crystallite size of the material is analyzed. In situ XANES assays allowed monitoring of the oxidation states of Ce and Ni and the mass spectrometer coupled to the experiment allowed checking the occurrence of methane oxidation reactions. In situ partial and total methane oxidation tests showed a clear interaction between the redox behavior of cerium and the Ni particles of the active phase.

    A preliminary study of the electrochemical behavior of the NiO / SDC composite material using electrochemical impedance spectroscopy is presented in Chapter 7. The effects of temperature, the percentage of H2 present in the working atmosphere and the crystallite size on the electrochemical response of the material were studied in order to understand the limiting processes involved and the mechanism of oxidation of H2.

    Finally, in Chapter 8 the conclusions and perspectives of this thesis work are presented and in Chapter 9 the scientific articles published in international journals and the participation in national and international conferences based on the results of this research work are mentioned.


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