Dealing with degradation in solid oxide electrochemical cells: novel materials and spectroscopic probes

In this PhD thesis, we have focused on two of the main issues regarding solid oxide fuel cells and electrolysers. On the one hand, the high temperatures at which they work (800-1000ºC) is detrimental for their long-term performance, and novel combinations of electrolyte and oxygen electrode materials have been tested in order to establish their suitability to work in intermediate temperature (600-800ºC) solid oxide fuel cells. On the other hand, degradation issues affect these devices greatly when working in the electrolyser mode, often assigned to the development of high oxygen partial pressures within the electrolyte. Regarding this topic, we have developed an analytical procedure to monitor the oxygen activity inside a YSZ electrolyte using redox dopants as spectroscopic probes and used it in cells tested in different conditions in the electrolysis mode.First, an aluminium-doped lanthanum silicate compound (LSAO) with the apatite structure was chosen as electrolyte, and eight different strontium and cobalt-free compounds with a perovskite structure with the general formula LaMxN1-xO3 (where M: Fe, Mn, Cr; and N: Ni, Cu) were selected to be tested with the apatite electrolyte. The solid-state synthesis of the apatite and perovskite-type compounds was optimised, achieving perovskite compounds free from secondary phases or decomposition, both in powder form and also after sintering them in pellet form, as it was proven by X-ray diffraction and Raman spectroscopy. The chemical compatibility of the electrolyte and electrode materials was tested by mixing and heating the powders at temperatures above the operational and sintering ones, and no reaction between the compounds took place, as proven again by X-ray diffraction. The thermomechanical compatibility between sintered materials was tested by dilatometry, and no big differences could be found regarding the thermal expansion behaviour of the apatite and perovskites in a wide range of temperatures.Then, the electrochemical performance of the compounds was tested by electrochemical impedance spectroscopy. The LSAO ionic conductivity at intermediate temperatures (9.4·10-3 S·cm-1 at 800ºC) was close to the one shown by a conventional YSZ electrolyte, and the electrical conductivity of the oxygen electrode materials ranged from 10 to 100 S·cm-1 at 800ºC, with activation energies in the high temperature range between 0.1 and 0.3 eV. Among the perovskite materials, the ones containing manganese and copper showed the highest electrical conductivities and the lowest activation energies. The iron-containing compounds (LFN and LFC) exhibited a different activation behaviour with temperature than the rest of the compounds.The following step consisted on manufacturing symmetrical cells with the apatite electrolyte and perovskite electrodes. For that purpose, slurries of the electrode materials were prepared and the electrolyte pellet was coated with them by dip-coating and sintered. The microstructure of the cells was checked in terms of electrode thickness, porosity, particle size, and adherence of the electrodes. Among the compositions tested, LFC showed the lowest ASR with just 4.3 Ω·cm2 at 700ºC, value comparable to the state-of-the-art oxygen electrodes. When applying a small DC bias, the activation energies of the electrodes decreased, as well as their polarization resistances. Promising results were found in this thesis about novel electrolyte/electrode combinations for IT-SOFC, with room for improvement regarding electrode microstructure and the fabrication of composite electrodes with the LFC material.In order to examine the degradation issues concerning SOEC devices, the research began with finding a suitable spectroscopic probe that allowed us to track the oxygen activity in the cells. First, the optical signals in a YSZ electrolyte doped with redox ions were investigated. The objective was to select the ones suitable to track the oxygen activity, allow for a detection in the backscattering configuration and be operative at high temperatures. The samples tested in this part of the thesis were either commercial or solidified on purpose YSZ single-crystals doped with Ce, Mn, Mn-Nd, V or Tb; or polycrystalline ceramics of Tb or Pr-doped YSZ.Among the commercial samples, it was found that YSZ-Ce showed strong change in its optical signal upon redox treatment, and the Ce3+ backscattering signal could be used to monitor the oxygen activity, although the signal disappeared at temperatures above 300ºC. In the case of YSZ-Mn and YSZ-V, even though a change in the optical signal upon oxidation/reduction could be found, there were no backscattering signals that could be used for tracking the oxygen activity. The luminescence of minority rare earth dopants in these samples (Pr3+, Er3+ or Nd3+) was measured and a change in the backscattering signal could be observed upon redox treatment. Nevertheless, the quantification of these signals would have been complicated due to a possible interaction with the major dopants, and these commercial samples were not used to track the oxygen activity within the electrolyte.In the case of praseodymium-doped YSZ, a change in optical signal was observed by diffuse reflectance, and bands due to Pr3+ and Pr4+ could be found. In the case of backscattering signal, Pr3+ ions exhibited an intense luminescence band which decreases upon oxidation to Pr4+, and this signal held up to 700ºC. This could be useful to make in-situ or in-operando measurements of the oxygen activity inside the electrolyte. Nevertheless, ion-ion interaction and concentration quenching of the luminescence band prevented an easy quantification of the oxygen activity, and this probe was also discarded.Terbium-doped YSZ was found suitable in order to track the oxygen activity within YSZ. Changes in optical and luminescence signals could be observed and attributed to different oxidation states of terbium (Tb3+ and Tb4+) upon redox treatments. Tb3+ was not affected by concentration quenching and a quantitative analysis could be carried out. It was found that Tb4+ absorbance was proportional to PO2^(1⁄4), as expected for the electron trapping model. A relation between the Tb3+ luminescence intensity and the oxygen partial pressure could be found, and it proved to be useful in the high PO2 range (10-4-100 bar). Terbium was therefore the selected probe in order to carry out the electrochemical experiments to detect degradation mechanisms in electrolysers.3%Tb-doped 8YSZ shows appropriate oxide ion conductivity to be used as the electrolyte in solid oxide cells. Then, an electrolyte-supported solid oxide cell was prepared using a LSM/YSZ composite for the oxygen electrode and a NiO/YSZ composite for the fuel electrode, and its electrochemical properties were tested in a bicameral cell at 800ºC. Using EIS and changing the atmosphere in the oxygen side, the electrochemical properties of the system were described, and the polarization resistance of each electrode was assigned. After those measurements, several experiments were carried out polarising different cells using a range of biases in the electrolyser mode. When a steady-state was reached around 48 hours after applying a constant voltage, cells were quenched to freeze the high temperature polarization state.The post-mortem cells were analysed in terms of the Tb3+ luminescence across the electrolyte thickness. The luminescence values were transformed into oxygen partial pressures using the relation mentioned above and profiles of the oxygen activity within the electrolyte could be obtained. These measurements were noisy and a couple of corrections were made in order to obtain a suitable signal. An edge-correction due to the loss of signal near the electrodes and a saturation correction due to microstructural aspects of the cell were applied. The method presented here has potential to visualize PO2 profiles in SOEC. Further experiments should be done in order to achieve higher accuracies.Finally, numerical solutions to the transport equations for describing the oxygen activity within the electrolyte were found and compared with the experimental results. It was found that the simulations assuming polarization resistances as derived from EIS spectra at the beginning of the CA experiments did not agree with the oxygen activity profiles obtained from the luminescence experiments. By analysing the SEM micrographs of the post-mortem cells, we could find that the most degradation had occurred near the fuel electrode. The nickel particles tended to agglomerate, especially for high polarization biases, and the porosity of the electrode decreased with applied bias. Besides, cracks within the electrolyte were found near this electrode and even a complete delamination of the fuel electrode was observed for the highest polarization experiment. These observations allowed us to assign a higher polarization resistance to the fuel electrode and then the numerical model results were closer to the results of the luminescence measurements. In order to get better insights of the degradation conditions of the cell while working on electrolyser mode, more experiments should be done.<br /

Caracterización, integración y comportamiento de nuevos materiales tipo oxiapatita en una pila de combustible de óxido sólido microtubular

Una SOFC (Solid Oxide Fuel Cell) o pila de combustible de óxido sólido, es el tipo de pila de hidrógeno más eficiente actualmente. Normalmente, su temperatura de trabajo se encuentra entre los 600 ºC y 1000 ºC, lo cual conlleva un problema de estabilidad termoquímica de los componentes y sus interfases, que acaba derivando en una disminución de la efectividad y duración de la celda. Una posible solución que se está estudiando en el ICMA, en el grupo de Procesado y Caracterización de Cerámicas Estructurales y Funcionales (grupo de investigación dentro del cual se va a realizar este proyecto), es sustituir el material del electrolito que se utiliza actualmente, la YSZ (circona estabilizada con itria), por nuevos materiales de tipo oxiapatita, los cuales presentan mejores valores de conducción de los iones de oxígeno a temperaturas intermedias, lo que permitiría bajar el punto de operación del sistema al rango de los 700 - 800ºC. Sin embargo, estos materiales presentan importantes limitaciones en su procesado, debido fundamentalmente a su difícil densificación, lo que hace necesario un estudio minucioso de su proceso de sinterización. En este proyecto se estudiarán dos oxiapatitas en concreto: La9.67Si6O26.5 y La9.67Si3Ge3O26.5 El objetivo principal de este proyecto es fabricar una pila microtubular soportada sobre ánodo con este nuevo tipo de material. Para ello habrá que: 1. Sintetizar las oxiapatitas, realizando las mezclas, moliendas y tratamientos térmicos adecuados, comprobando después su cristalinidad mediante difracción de rayos X y espectroscopia Raman así como sus propiedades de conducción iónica mediante espescroscopia de impedancias (EIS). 2. Caracterizar y optimizar la morfología del polvo de los compuestos obtenidos para su posterior utilización en la fabricación de las pilas. Para ello, se estudiará la distribución del tamaño de partícula, se realizarán ensayos de dilatometría para determinar las temperaturas de sinterización del electrolito y por último, se realizarán ensayos de reología de las suspensiones para optimizar el proceso de dipcoating por el que se depositarán el electrolito y el cátodo sobre el ánodo. 3. Una vez optimizados los parámetros, se fabricarán los ánodos de NiO y oxiapatita, mediante CIP (prensado isostático en frío). Posteriormente,se depositará el electrolito sobre el ánodo mediante la técnica de dipcoating y por último, se depositará el cátodo (mezcla de Pr2NiO4 y oxiapatita) sobre el electrolito. La integración entre las diferentes capas que la componen , así como los espesores de cada una de las fases, se estudiará mediante técnicas de microscopia óptica y de barrido (SEM). 4. Finalmente, si la integración de las multicapas resulta ser adecuada, se realizará la caracterización electroquímica mediante el control del voltaje a circuito abierto (OCV), medidas de las curvas intensidad-voltage (I-V). El objetivo global de este proyecto es que el estudiante adquiera los conocimientos necesarios para trabajar en un laboratorio de fabricación y caracterización de materiales cerámicos

Optimización de cátodos de pilas de combustible SOFC mediante infiltración de catalizadores

El presente proyecto tiene como objetivo el estudio del efecto de la adición de nanopartículas metálicas u óxidos metálicos sobre el comportamiento del electrodo de aire (cátodo) en una pila de combustible de óxido sólido (SOFC). Para llevar a cabo este propósito se procederá a la fabricación de una pila plana simétrica con dos cátodos (en vez de ánodo y cátodo como en las pilas convencionales) depositados mediante la técnica de screen-printing y de dip-coating, y a los cuales se les incorporarán posteriormente nanopartículas metálicas a partir de sus sales por técnicas de infiltración. Los procedimientos que han sido llevados a cabo para la elaboración de una celda simétrica han sido los siguientes: Partiendo de sustratos comerciales de YSZ (circona estabilizada con itria), se cortarán con láser electrolitos planos circulares a los cuales se les aplicarán las suspensiones de electrodo basadas en LSM (manganita de lantano y estroncio) e YSZ mediante las técnicas de dip-coating y de screen-printing. Esta última técnica de fabricación favorece la reproducibilidad y automatización del proceso. A continuación se procede a la caracterización microestructural de los electrodos y de su interfase con el electrolito de YSZ, para comprobar la homogeneidad de la deposición, tamaño y distribución de las fases, y adherencia con la YSZ. Para ello se observarán las muestras en el microscopio electrónico de barrido (SEM) que nos permitirán establecer cuáles son las mejores condiciones de trabajo (proporción LSM/YSZ, espesor de electrodo, temperatura de sinterizado y porosidad). Como parte central del proyecto, se procede a la incorporación mediante técnicas de infiltración a vacío de sales metálicas que favorecerán las reacciones electroquímicas en el electrodo, reduciendo así su resistencia de polarización. En este paso se parte de disoluciones de nitrato de cobalto, cerio, manganeso y praseodimio que tras un tratamiento térmico formen nanopartículas metálicas o de óxido metálico. Posteriormente se procede a caracterizar y optimizar las infiltraciones mediante el uso de la espectroscopia de impedancias. Los resultados obtenidos muestran que las disoluciones con una carga de sales metálicas de 2,5 μmoles por electrodo reducen en mayor medida la resistencia eléctrica de las pilas de combustible tipo SOFC. Finalmente, tras analizar todas las sales metálicas infiltradas se comprueba que la infiltración de sales metálicas de cerio son las que mejores resultados presentan. Las mejorías obtenidas son del orden del 30 % cuando nos encontramos a “bajas temperaturas” (700 ºC). En el caso de altas temperaturas (900 ºC) las mejorías siguen siendo notables, un 18 % respecto a una celda sin infiltrar. Se concluye por tanto que la infiltración de catalizadores en los cátodos de las pilas de combustible de tipo SOFC mejoran su comportamiento debido a un incremento de la catálisis de la reducción del oxígeno

Síntesis y caracterización de electrolitos híbridos polímero-cerámica para baterías en estado sólido.

El uso de litio metal como ánodo en las baterías de los coches eléctricos es una de las opciones más prometedoras debido a que posee el potencial de reducción más negativo, es el tercer elemento más ligero y uno de los cationes más pequeños. Un protocolo para la síntesis y caracterización de un electrolito-sólido-compuesto basado en óxido de polietileno / Li6.4La3Zr1.4Ta0.6O12 conteniendo LiTFSI como la sal de litio es desarrollado, posibilitando el uso de baterías recargables de Li metal en estado sólido. El electrolito-solido-compuesto (SE-compuesto) se ha comparado con un electrolito polimérico de referencia. SE-compuesto alcanza una conductividad para el Li+ de of 1.33·10-5 S cm-1, un número de transporte de 0.59 y energía de activación de 0.93 eV, cuyos valores son equivalentes a los encontrados en bibliografía, probando así el protocolo. Se han hecho ensayos en baterías de litio de estado sólido con un cátodo basado en LiFePO4. Para ello se han establecido rutas optimizadas tanto para la composición como para la preparación del cátodo, y para el ensamblaje de las baterías, y se han caracterizado con un electrolito líquido, y posteriormente con ambos SEs. La ciclabilidad del SE-compuesto mejora, y la polarización se reduce con respecto al de referencia. Se han alcanzado capacidades de hasta 100 mAh g-1.<br /

Effects of synthetic conditions on the structural, stability and ion conducting properties of Li0.30(La0.50Ln0.50)0.567TiO3 (Ln= La, Pr, Nd) solid electrolytes for rechargeable lithium batteries

The structure, thermal stability, morphology and ion conductivity of titanium perovskites with the general formula Li3xLn2/3−xTiO3 (Ln = rare earth element; 3x= 0.30) are studied in the context of their possible use as solid electrolyte materials for lithium ion batteries. Materials are prepared by a glycine-nitrate method using different sintering treatments, with a cation-disorder-induced structural transition from tetragonal to cubic symmetry, detected as quenching temperature increases. SEM images show that the average grain size increases with increasing sintering temperature and time. Slightly higher bulk conductivity values have been observed for quenched samples sintered at high temperature. Bulk conductivity decreases with the lanthanide ion size. A slight conductivity enhancement, always limited by grain boundaries, is observed for longer sintering times. TDX measurements of the electrolyte/cathode mixtures also show a good stability of the electrolytes in the temperature range of 30-1100ºC.This research has been funded by the Consejería de Industria, Innovación, Comercio y Turismo (SAIOTEK 2012 programmes), by Dpto. Educación, Política Lingüística y Cultura of the Basque Goverment (Grupos de Investigación del Sistema Universitario Vasco 2013 – 2018; IT-630-13), by Ministerio de Ciencia e Innovación (MAT2010-15375 and MAT2012-30763) and by Ministerio de Economía y Fondos Feder (MAT2010-19837-C06-06). The authors thank SGIker technical support (UPV/EHU, MEC, GV/EJ and European Social Fund). L.O.S.M. acknowledges the Departamento Ciencias, PUCP, for the allocation of research time