Dissertations / Theses on the topic 'Solid oxide electrolysi'

This study has focused on solid oxide electrolyser cells for high temperature steam electrolysis. Solid oxide electrolysis is the reverse operation of solid oxide fuel cells (SOFC), so many of the same component materials may be used. However, other electrode materials are of interest to improve performance and efficiency. In this work anode materials were investigated for use in solid oxide electrolysers. Perovskite materials of the form L₁₋ｘSrｘMO₃ , where M is Mn, Co, or Fe. LSM is a well understood electrode material for the SOFC. Under electrolysis operation LSM performed well and no interface reactions were observed between the anode and YSZ electrolyte. LSM has a relatively low conductivity and the electrode reaction is limited to the triple phase boundary regions. Mixed ionic-electronic conductors of LSCo and LSF were investigated, with these materials the anode reaction is not limited to triple phase boundaries. The LSCo anode had adherence problems in the electrolysis cells due to the thermal expansion coefficient mismatch with the YSZ electrolyte. The LSCo reacted with the YSZ at the anode/electrolyte interface forming insulating zirconate phases. Due to these issues the LSCo anode cells performed the poorest of the three. The performance of electrolysis cells with LSF anode exceeded both LSM and LSCo, particularly under steam operation, although an interface reaction between the LSF anode and YSZ electrolyte was observed. In addition to the anode material studies this work included the development of solid oxide electrolyser tubes from tape cast precursor materials. Tape casting is a cheap processing method, which allows for co-firing of all ceramic components. The design development resulted in a solid design, which can be fabricated reliably, and balances strength with performance. The design used LSM anode, YSZ electrolyte, and Ni-YSZ cathode materials but could easily be adapted for the use of other component materials. Proper sintering rates, cathode tape formulation, tube length, tape thickness, and electrolyte thickness were factors explored in this work to improve the electrolyser tubes.

Actualment, la transició energètica cap a un escenari baix en carboni està impulsant la instal·lació global de fonts d’energia renovables, el seu desplegament per sobre de l’40%, implicarà l’ús de sistemes eficients d’emmagatzematge d’energia per cobrir la demanda. Les rutes d’hidrogen verd i power to gas es presenten com la millor alternativa per a aquest emmagatzematge al connectar les xarxes elèctriques i de gas. En aquest marc, les cel·les d’electròlisi d’òxid sòlid (SOEC), que produeixen hidrogen i gas de síntesi (H2 + CO) a partir de l’electròlisi de l’aigua o la co-electròlisi de l’aigua i el diòxid de carboni, són els electrolitzadors més eficients per a l’emmagatzematge d’energia. Les SOEC posseeixen altes taxes de conversió d’energia (≈80%) atorgades pel rang de temperatura d’operació (600-900 °C). No obstant, un dels principals inconvenients de les SOEC està relacionat amb les tècniques de fabricació, que impliquen molts passos per produir dispositius complets. A més, les seves prestacions i durabilitat encara s’estan investigant per augmentar la maduresa de la tecnologia i penetrar en el mercat competint amb altres tecnologies d’electròlisi que mostren menors eficiències. La present tesi està dedicada a l’exploració de nous conceptes de SOEC. Per a això, es consideren tres aspectes, que són: i) utilització de tècniques de fabricació additiva per a la fabricació replicable, automàtica i customitzable de dispositius energètics; ii) síntesi de nanocompostos mesoporosos en l’elèctrode d’oxigen per millorar el rendiment general i la durabilitat del dispositiu SOEC; i finalment iii) la producció de gas de síntesi per co-electròlisi i oxidació parcial de metà (POM) amb els dispositius desenvolupats. Robocasting (RC) i InkJet printing (IJP) s’han utilitzat per a la fabricació de cel·les simètriques impreses per tecnologia híbrides d’impressió 3D, que van ser co-sinteritzades a altes temperatures i provades electroquímicament. S’ha demostrat la viabilitat d’aquestes dues tècniques combinades per a la fabricació de dispositius ceràmics. S’ha sintetitzat ceria dopada mesoporosa (CGO) utilitzada com a suport per a elèctrodes d’oxigen nanocompostos. Per a això es proposa una ruta optimitzada per millorar l’activitat catalítica dels elèctrodes de base mesoporosa i per reduir la temperatura de sinterització mantenint la seva nanoestructura, i l’estudi dels seus efectes sobre el material. La millora del rendiment dels dispositius SOEC aplicant les rutes de síntesi i fabricació desenvolupades es demostra pels excel·lents resultats aconseguits, sense precedents per a aquest tipus de SOEC. El rendiment de dispositius complets amb elèctrodes d’oxigen mesoporosos es va provar a altes temperatures. El suport nanoestructurat optimitzat ha estat provat en una cel·la de botó (diàmetre = 2 cm) mostrant excel·lents rendiments observats en condicions de co-electròlisi i pila de combustible. També es va dipositar CGO mesoporós en cel·les d’àrea gran (25 cm2) per demostrar l’escalabilitat del material, per a dispositius d’interès comercial. Com a resum, el document presentat tracta de l’optimització de dispositius electroquímics innovadors d’alta eficiència com les SOEC, donant un nou pas més enllà de l’estat de l’art en les tecnologies de producció d’hidrogen a causa de la combinació de rutes de fabricació innovadores com la fabricació additiva de materials ceràmics amb funcionalitats avançades com els mesoporosos.
Actualmente, la transición energética hacia un escenario bajo en carbono está impulsando la instalación global de fuentes de energía renovables, su despliegue por encima del 40%, implicará el uso de sistemas eficientes de almacenamiento de energía. Las rutas de hidrógeno verde y power to gas se presentan como la mejor alternativa para este almacenamiento. En este marco, las celdas de electrólisis de óxido sólido (SOEC), que producen hidrógeno y gas de síntesis (H2 + CO) a partir de la electrólisis del agua o la co-electrólisis del agua y el dióxido de carbono, son los electrolizadores más eficientes. Las SOEC poseen altas tasas de conversión de energía (≈80%) otorgadas por el rango de temperatura de operación (600-900 ° C). Sin embargo, uno de los principales inconvenientes de las SOEC está relacionado con las técnicas de fabricación, que implican muchos pasos para producir dispositivos completos. Además, sus prestaciones y durabilidad aún se están investigando para aumentar la madurez de la tecnología y penetrar en el mercado compitiendo con otras tecnologías de electrólisis que muestran menores eficiencias. La presente tesis está dedicada a la exploración de nuevos conceptos de SOEC. Para ello, se consideran tres aspectos, que son: i) utilización de técnicas de fabricación aditiva para la fabricación replicable, automática y sintonizable de dispositivos energéticos; ii) síntesis de nanocompuestos mesoporosos en el electrodo de oxígeno para mejorar el rendimiento general y la durabilidad del dispositivo SOEC; y finalmente iii) la producción de gas de síntesis por co-electrólisis y oxidación parcial de metano (POM) con los dispositivos desarrollados. Robocasting e Inkjet Printing se utilizaron para la fabricación de celdas simétricas impresas por tecnología híbridas de impresión 3D, co-sinterizadas a altas temperaturas y probadas electroquímicamente. Se ha demostrado la viabilidad de estas dos técnicas para la fabricación de dispositivos cerámicos. Se ha sintetizado ceria dopada mesoporosa (CGO) utilizada como soporte para electrodos de oxígeno nanocompuestos. Para ello se propone una ruta optimizada para mejorar la actividad catalítica de los electrodos de base mesoporosa y para reducir la temperatura de sinterización manteniendo su nanoestructura. La mejora del rendimiento de los dispositivos SOEC aplicando las rutas de síntesis y fabricación desarrolladas se demuestra por los excelentes resultados conseguidos, sin precedentes para este tipo de SOEC. El rendimiento de dispositivos completos con electrodos de oxígeno mesoporosos se probó a altas temperaturas. El soporte nanoestructurado optimizado ha sido probado en una celda botón (diámetro = 2 cm) mostrando excelentes rendimientos observados en condiciones de COSOEC y SOFC. También se depositó CGO mesoporoso en celdas de área grande (25 cm2) para demostrar la escalabilidad del material. Ambos dispositivos se sometieron a una prueba de durabilidad, que mostró tasas de degradación en línea con la literatura más avanzada. Finalmente, se muestra la prueba de conceptos sobre la oxidación parcial de metano (POM) asistida electroquímicamente. Se produjo y probó un SOEC con CGO infiltrado por catalizadores de Ni y Cu como dispositivo POM. Se usó metano en el electrodo Ni-Cu-CGO como combustible. El oxígeno producido por la reacción de electrólisis del agua en el electrodo Ni-YSZ se utilizó para producir gas de síntesis a partir de CH4 en un proceso catalítico asistido electroquímicamente. Los principios de funcionamiento del experimento se demostraron con éxito. Como resumen, el presente documento trata de la optimización de dispositivos electroquímicos innovadores de alta eficiencia como las SOEC, dando un nuevo paso más allá del estado del arte en las tecnologías de producción de hidrógeno debido a la combinación de rutas de fabricación innovadores, como la fabricación aditiva con materiales cerámicos de funcionalidades avanzadas como los mesoporosos.
Nowadays, the energy transition to a low carbon scenario is promoting the global installation of renewable energy sources, its deployment above 40% will need the use of efficient energy storage systems for covering the demand. Green hydrogen and power to gas routes has arisen as the best alternative for this storage while connecting the electric and gas grids. In this frame, Solid Oxide Electrolysis Cells (SOECs), which produce hydrogen and syngas (H2+CO) from the electrolysis of water or the co-electrolysis of water and carbon dioxide, are the most efficient electrolysers for energy storage. SOECs possess high energy conversion rates (≈80 %) granted by the operation temperature range (600-900 °C). However, one of SOECs’ main drawbacks is related to the manufacturing techniques, which involves many steps to produce complete devices. Furthermore, their performances and durability are still being investigated to increase the maturity of the technology and penetrate to the market competing with other electrolysis technologies that show lower efficiencies. The present thesis is dedicated to the exploration of new concepts of SOECs. For this, three aspects are considered, which are: i) utilization of additive manufacturing (AM) techniques for reliable, automatic and tuneable fabrication of energy devices; ii) synthesis of mesoporous nanocomposites at the oxygen electrode to improve the general performances and durability of SOEC device; an finally iii) the production of syngas by co-electrolysis and partial oxidation of methane (POM) with the developed devices. Robocasting (RC) and Inkjet Printing (IJP) were used for the fabrication of hybrid 3D printed symmetrical cells, which were co-sintered at high temperatures and electrochemically tested. The feasibility of these two combined techniques for the fabrication of ceramic devices was demonstrated. Mesoporous doped ceria (CGO) was synthesized and used as a scaffold for nanocomposite oxygen electrodes. An optimized route to improve the catalytic activity of the mesoporous based electrodes and to reduce the sintering temperature to maintain their nanostructure, is proposed after the study of their effects on the material. The improvement of the SOEC devices performance applying the developed synthesis and fabrication routes is demonstrated by the achievement of unprecedented results for this type of SOEC. The performance of complete devices with mesoporous oxygen electrodes was tested at high temperatures. The optimized scaffold tested on a button test cell (diameter =2 cm) promoted the commented outstanding performances in both co-electrolysis and fuel cell conditions. Mesoporous CGO was also deposited on large area cells (25 cm2) to demonstrate the scalability of the material, for devices of commercial interest. Both devices underwent a durability test, showing degradation rates in line with state-of-the-art literature. Finally, the proof of concepts about electrochemically assisted partial oxidation of methane (POM) is shown. A SOEC with CGO scaffold infiltrated by Ni and Cu catalysers was produced and tested as POM device. Methane was supplied at the Ni-Cu-CGO electrode as fuel. The oxygen produced by the water electrolysis reaction at the Ni-YSZ electrode was used to produce syngas from CH4 on an electrochemical assisted catalytic process. The working principles of the experiment were successfully demonstrated opening a new research line. As a summary the present document deals with the optimization of innovative high efficient electrochemical devices as SOEC, bringing a new step beyond the state of the art on the hydrogen production technologies due to the combination of innovative fabrication routes such as the additive manufacturing with advanced functional ceramic materials like mesoporous.

Solid oxide electrolysis of a mixture of water and carbon dioxide has many applications in space exploration. It can be implemented in propellant production systems that use Martian resources or in closed-loop life support systems to cleanse the atmosphere of facilities in extraterrestrial bases and of cabin spacecrafts. This work endeavors to quantify the performance of combined water and carbon dioxide electrolysis, referred to as "combined electrolysis", and to understand how it works so that the technology can be best applied. First, to thoroughly motivate the research, system modeling is presented that demonstrates the competitiveness of the technology in terms of electrolysis power requirements and consequential system mass savings. Second, to demonstrate and quantify the performance of the technology, experimental results are presented. Electrolysis cells were constructed with 8% by mol yttria-stabilized zirconia electrolytes, 50/50 by weight platinum/yttria-stabilized zirconia electrodes and chromium-alloy or alumina manifolds and tubing. Performance and gas chromatograph data from electrolysis of many different gas mixtures, including water, carbon dioxide, and a combined mixture of both, are presented. Third, to explain observations made during experiments and theorize about the phenomena governing combined electrolysis, data analyses and thermodynamic modeling are applied. Conclusions are presented regarding the transient response of combined electrolysis, the relative performance of it to that of other mixtures, how its performance depends on the water to carbon dioxide ratio, its effect on cell health, and its preference to water versus carbon dioxide. Procedures are also derived for predicting the composition of combined electrolysis exhaust for a given oxygen production rate, humidity content, and inlet flow rate. The influence of the two cell materials proves to be significant. However, in both cases it is proven that combined electrolysis does not encourage carbon deposition and the makeup of its products is governed by the water gas shift reaction. It is shown that the chromium-alloy system achieves water gas shift reaction equilibrium whereas the alumina system does not. Experimental observations support the argument that chromium oxide inside the chromium alloy cell forces its water gas shift reaction to equilibrium during electrolysis, influencing combined electrolysis performance.

This study has been mainly focused on high temperature solid oxide electrolysis cells (HT-SOECs) for steam electrolysis. The compositions, microstructures and metal catalysts for SOEC cathodes based on (La₀.₇₅Sr₀.₂₅)₀.₉₅Mn₀.₅Cr₀.₅O₃ (LSCM) have been investigated. Hydrogen production amounts from SOECs with LSCM cathodes have been detected and current-to-hydrogen efficiencies have been calculated. The effect of humidity on electrochemical performances from SOECs with cathodes based on LSCM has also been studied. LSCM has been applied as the main composite in HT-SOEC cathodes in this study. Cells were measured at temperatures up to 920°C with 3%steam/Ar/4%H₂ or 3%steam/Ar supplied to the steam/hydrogen electrode. SOECs with LSCM cathodes presented better stability and electrochemical performances in both atmospheres compared to cells with traditional Ni cermet cathodes. By mixing materials with higher ionic conductivity such as YSZ(Y₂O₃-stabilized ZrO₂ ) and CGO(Ce₀.₉Gd₀.₁O₁.₉₅ ) into LSCM cathodes, the cell performances have been improved due to the enlarged triple phase boundary (TPB). Metal catalysts such as Pd, Fe, Rh, Ni have been impregnated to LSCM/CGO cathodes in order to improve cell performances. Cells were measured at 900°C using 3%steam/Ar/4%H₂ or 3%steam/Ar and AC impedance data and I-V curves were collected. The addition of metal catalysts has successfully improved electrochemical performances from cells with LSCM/CGO cathodes. Improving SOEC microstructures is an alternative to improve cell performances. Cells with thinner electrolytes and/or better electrode microstructures were fabricated using techniques such as cutting, polishing, tape casting, impregnation, co-pressing and screen printing. Thinner electrolytes gave reduced ohmic resistances, while better electrode microstructures were observed to facilitate electrode processes. Hydrogen production amounts under external potentials from SOECs with LSCM/CGO cathodes were detected by gas chromatograph and current-to-hydrogen efficiencies were calculated according to the law of conservation of charge. Current-to-hydrogen efficiencies from these cells at 900°C were up to 80% in 3%steam/Ar and were close to 100% in 3%steam/Ar/4%H₂. The effect of humidity on SOEC performances with LSCM/CGO cathodes has been studied by testing the cell in cathode atmospheres with different steam contents (3%, 10%, 20% and 50% steam). There was no large influence on cell performances when steam content was increased, indicating that steam diffusion to cathode was not the main limiting process.

Solid oxide cells are best known in the energy sector as novel power generation devices through solid oxide fuel cells (SOFCs), which enable the direct conversion of chemical energy to electrical energy and result in high efficiency power generation. However, solid oxide electrolysis cells (SOECs) are receiving increased attention as a hydrogen production technology through high temperature electrolysis applications. The development of higher fidelity methods for modeling transport phenomena within solid oxide cells is necessary for the advancement of these key technologies. The proposed thesis analyzes the increased transport path lengths caused by constriction resistance effects in prevalent solid oxide cell designs. Such effects are so named because they arise from reductions in active transport area. Constriction resistance effects of SOFC geometry on continuum level mass and electronic transport through SOFC anodes are simulated. These effects are explored via analytic solutions of the Laplace equation with model verification achieved by computational methods such as finite element analysis (FEA). Parametric studies of cell geometry and fuel stream composition are performed based upon the models developed. These studies reveal a competition of losses present between mass and electronic transport losses and demonstrate the benefits of smaller SOFC unit cell geometry. Furthermore, the models developed for SOFC transport phenomena are applied toward the analysis of SOECs. The resulting parametric studies demonstrate that geometric configurations that demonstrate enhanced performance within SOFC operation also demonstrate enhanced performance within SOEC operation. Secondarily, the electrochemical degradation of SOFCs is explored with respect to delamination cracking phenomena about and within the critical electrolyte-anode interface. For thin electrolytes, constriction resistance effects may lead to the loss of electro-active area at both anode-electrolyte and cathode-electrolyte interfaces. This effect (referred to as masking) results in regions of unutilized electrolyte cross-sectional area, which can be a critical performance hindrance. Again analytic and computational means are employed in analyzing such degradation issues.

La0.8Sr0.2Ga0.8Mg0.2O3-δ (LSGM) is an oxygen ion conducting electrolyte material widely used in solid oxide fuel cells (SOFC). La2NiO4+δ (LNO) is a mixed ionic-electronic conducting layered perovskite with K2NiF4 type structure which conducts oxygen ions via oxygen interstitials. LNO has shown promising results as an SOFC electrode in the literature. In this work the compatibility and performance of LNO electrodes on the LSGM electrolyte material for solid oxide electrolysis cell (SOEC) is investigated. The materials were characterised as SOEC/SOFC cells by symmetrical and three electrode electrochemical measurements using Electrochemical Impedance Spectroscopy (EIS). Conductivity and ASR values were obtained in the temperature range 300-800°C with varying atmospheres of pH2O and pO2. The cells were also subjected to varied potential bias, mimicking fuel cell or electrolysis use. Enhancement of LNO performance was observed with the application of potential bias in both anodic and cathodic mode of operation in all atmospheres with the exception of cathodic bias in pO2 = 6.5x10-3 atm. In ambient air at 800°C LNO ASRs were 2.82Ω.cm2, 1.83Ω.cm2 and 1.37Ω.cm2 in OCV, +1000mV bias and -1000mV bias respectively. In low pO2 at 800°C LNO ASRs were 9.17Ω.cm2, 1.74Ω.cm2 and 456.9Ω.cm2 in OCV, +1000mV bias and -1000mV bias respectively. The increase in ASR with negative potential bias in low pO2 is believed to be caused by an increase in mass transport and charge transfer impedance responses. Material stability was confirmed using X-Ray Diffraction (XRD), in-situ high temperature pH2O and pO2 XRD. In-situ XRD displayed single phase materials with no observable reactivity in the conditions tested. Scanning Electron Microscopy images of cells tested by EIS in all atmospheres displayed no microstructure degradation except for those cells tested in a humid atmosphere which display a regular pattern of degradation on the LNO surface attributed to reaction with the Pt mesh current collector.

The need of substituting the current energetic model by a system based on clean Renewable Energy Sources (RES) have gained more importance in the last decades due to the environmental issues related to the use of fossil fuels. These energy sources are site-specific and intermittent, what makes essential the development of Energy Storage Systems (ESS) that allows the storage of the electricity generated by renewable energies. Among the technologies under development for the storage of electrical energy, Solid Oxide Electrolysis Cells (SOECs) have been proposed in the last decades as a promising technology. Achieving efficiencies higher than 85%, SOEC technology is able to convert electrical energy into chemical energy through the reduction of H2O, CO2 or the combination of both; generating H2, CO or syngas (H2 +CO). The implementation of this technology based on renewable electrical energy, combined with fuel cells would allow closing the carbon cycle. The work presented in this thesis has been devoted to enhance the performance of SOEC. The approach that is presented for that propose is based on the implementation of high surface area and thermally stable mesoporous metal oxide materials on the fabrication of SOEC electrodes. High performance and stability of the electrodes was expected during its characterization. Structural and electrochemical characterization techniques have been applied during the development of this thesis for this purpose. The thesis is organized in eight chapters briefly described in the following: Chapter 1 briefly analyses the current energy scenario presenting electrolysers as a promising technology for the storage of electrical energy. Besides, basic principles of SOECs operation and the state-of-the-art materials of SOECs are reviewed. Chapter 2 describes all the experimental methods and techniques employed in this thesis for the synthesis and characterization of synthesised materials and fabricated cells. Chapter 3 presents the results obtained from the structural characterization of the mesoporous materials and fabricated electrodes, revealing the successful implantation of the hard-template method for obtaining Sm0.2Ce0.8O1.9 (SDC), Ce0.8Gd0.2O1.9 (CGO) and NiO mesoporous powders, and the fabrication of SDC-SSC (Sm0.5Sr0.5CoO3-δ), CGO- LSCF (La0.6Sr0.4Co0.2Fe0.8O3) and NiO-SDC electrodes based on mesoporous materials. The attachment of the mesoporous scaffold for the fabrication of oxygen electrodes has been optimized at 900 °C. Chapter 4 compares electrolyte- and fuel electrode-supported cell configurations based on the same oxygen electrode. The electrochemical performance and the microstructural characterization of these cells are considered for that purpose. Showing a maximum current density of -0.83 and -0.81 A/cm2 on electrolysis and co- electrolysis modes respectively, fuel electrode-supported cells are considered more suitable for SOEC fabrication. Chapter 5 presents a study focused on analysing the influence of the oxygen electrode interface on the SOEC performance. The electrochemical and microstructural characterization of barrier layers and oxygen electrodes fabricated applying different methods are discussed in this chapter. The combination of a barrier layer fabricated by Pulsed Laser Deposition (PLD) with an oxygen electrode based on mesoporous materials resulted on the injection of up to -1 A/cm2, what allows concluding that this interface microstructure is directed related with the best performing SOECs in this thesis. Chapter 6 shows the performance of SOEC cells on co-electrolysis mode containing the optimized oxygen electrode, fabricated by infiltration of mesoporous scaffolds. The long-term stability of infiltrated mesoporous composites have been demonstrated during 1400 h, registering degradation rates of 2%/kh and <1%/kh when current densities of -0.5 A/cm2 and -0.75 A/cm2 are injected, respectively. Chapter 7 shows results of the scale-up of the mesoporous-based electrodes for the fabrication of large area cells. Their electrochemical performance shows high fuel flexibility, injecting -0.82 A/cm2 on co-electrolysis mode; and long-term stability injecting -0.5 A/cm2 for 600 h. The conclusions of this thesis are presented in Chapter 8.
Una de las principales desventajas de las fuentes de energías renovables es que producen energía eléctrica de forma discontinua. Los electrolizadores de alta temperatura basados en óxidos sólidos (SOEC) se presentan como una tecnología prometedora para el almacenamiento de energía eléctrica. Alcanzando eficiencias mayores de un 85%, los electrolizadores SOEC permite convertir energía eléctrica en energía química mediante la reducción de las moléculas de agua (H2O), dióxido de carbono (CO2), o la combinación de ambas; generándose hidrógeno (H2), monóxido de carbono (CO) o gas de síntesis (H2 +CO) como producto. El trabajo que se presenta en esta tesis tiene como objetico mejorar el rendimiento de los electrolizadores SOEC mediante la utilización de óxidos metálicos mesoporosos, caracterizados por poseer alta área superficial y ser estables a altas temperaturas. Esta tesis está organizada en ocho capítulos. Los capítulos 3, 4, 5, 6 y 7 presentan los resultados alcanzados: El capítulo 3 presenta la caracterización estructural de los materiales mesoporosos y de los electrodos fabricados. Además, la temperatura de adhesión del material mesoporoso ha sido optimizada y se ha fijado a 900 °C. El capítulo 4 compara electrolizadores fabricados soportados por el electrodo de combustible y por el electrolito. Los resultados muestran que las densidades de corriente más altas fueron inyectadas en los electrolizadores soportados por el electrodo de combustible, considerándose esta configuración la más apropiada. El capítulo 5 presenta la influencia de la microstructura de la intercara del electrodo de oxígeno en el rendimiento de los electrolizadores SOEC. La caracterización electroquímica, apoyada por la caracterización microestructural, ha demostrado que la máxima densidad de corriente ha sido inyectada por el electrolizador cuya barrera de difusión ha sido depositado por láser pulsado (PLD) y la capa funcional del electrodo de oxígeno mediante infiltración de materiales mesoporosos. El capítulo 6 estudia el electrodo de oxígeno optimizado. Durante 1400 h de operación continua y caracterización microstructural, se ha demostrado la estabilidad de este electrodo. Por último, el capítulo 7 muestra los resultados obtenidos del escalado de los electrodos mesoporosos en celdas de mayor área (25 cm2). La caracterización electroquímica muestra alta flexibilidad ante las composiciones de gases utilizadas, y estabilidad de los electrodos mesoporosos propuestos.

Two general systems, brownmillerite-type Ba₂In₂O₅ and apatite-type silicates have been investigated for potential solid oxide fuel cell electrolyte applications. The combination of powder diffraction, NMR, TGA, Raman and AC impedance spectroscopy indicated the successful incorporation of phosphate, sulphate and silicate into the Ba₂In₂O₅ structure leading to a transition from an ordered brownmillerite-type structure to a disordered perovskite-type, which led to the conductivity enhancement below 800 °C, along with a significant protonic contribution in wet atmospheres. The CO₂ stability was also shown to be improved on doping. This oxyanion doping strategy has been extended to the analogous system, Ba₂Sc₂O₅, which resulted in samples with high conductivity and good stability towards CO₂. Neutron diffraction studies on La₉.₆Si₆O₂₆.₄ indicated that the interstitial oxide ion is located near the channel centre. Further interstitial anions could be accommodated through hydration, which led to displacement of the interstitial site away from the channel centre, with an accompanying swelling of the channel. Although long term annealing of these apatite silicates showed no apparent significant structural change, a reduction in the bulk conductivity was observed, while the grain boundary conductivity was improved, thus resulting in a small enhancement in the total conductivity below 400 °C.

In this thesis testing of solid oxide cells in fuel cell and electrolysis operation have been performed. Attempts to fabricate fuel cells are described, equipment for testing solid oxide electrolysis cells has been constructed and the development process for this described. Cells of a number of different types have been tested in which initial work was performed using microtubular cells. Work on the fabrication of planar solid oxide cells is described, anode supports were prepared by pellet pressing however the application of a suitably dense electrolyte was unsuccessful which resulted in a poor cell OCV. The initial degradation of commercial solid oxide cells has been investigated. During cyclic testing at low current density the cells were found to degrade at twice the rate in electrolysis operation compared to fuel cell operation. This leads to the conclusion that the degradation observed in electrolysis is reversible and that there is a disconnect between the electrolysis and fuel cell degradation processes. During testing at different current densities the cells were found to undergo severe degradation when operated with very low water content supplied to the cells. The degradation was 512 mV kh\(^{−1}\) at 2.5 vol% H2O and reduced to 45mV kh\(^{−1}\) at 50 vol% H2O. Over the timescales investigated in this work and due to the reversible nature of the electrolysis degradation identifying a degradation mechanism was very difficult.

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2004.
Vita.
Includes bibliographical references (leaf 35).
The Kroll process for refining titanium is an expensive batch process which produces a final product that still requires intensive post processing to create usable titanium. A new process, Solid Oxide Membrane Electrolysis with Rotating Cathode (SOMERC) process is being explored. The SOMERC process is a continuous process that could produce large quantities of high quality titanium at a fraction of the cost of the Kroll process. This paper examines the fluid flow around the ingot in the SOMERC Process. A large shear between the ingot and surrounding fluid will create a fully-dense ingot instead of dendrites, because dendrites are undesirable. Using a camera, a plane of light and titanium dioxide particles, videos and pictures of the water were taken and analyzed to find how to create a large amount of shear between the ingot and the fluid. Out of the speeds tested, a rotation rate of 900Ê»/s for the ingot proved to create the most shear, and therefore the shear between the ingot and fluid increases with increasing rotation rate, making it more likely to suppress the formation of dendrites.
by Chris Kinney.
S.B.

Research already carried out on the use of the recently patented electroless nickel ceramic codeposition technique as a method of manufacturing solid oxide fuel cell (SOFC) electrodes has thus far indicated that, while functional electrodes can be manufactured by the technique, for optimum performance of the cell, amplification of the ceramic content of the coatings is still required. By mainly employing external agents such as surface active agents (surfactants) and magnetic fields (in a bid to aid ceramic particle stability), this research focused on the prospect of increasing the ceramic content of cermets co-deposited for use as SOFC electrodes. A total of 137 co-deposited samples were produced from different bath compositions. As a prelude to the study, the interactions between the ceramic powders used (yttria stabilised zirconia (YSZ) / lanthanum strontium manganate (LSM)) and the medium for the deposition process – the electroless nickel solution, were investigated by zeta potentiometry and ultraviolet-visible spectroscopy techniques. The results obtained from the studies led to a variation of a series of fundamental plating factors such as the ceramic bath loading and particle size of the powders. While the former was found to yield the highest ceramic content in the coating at a bath loading of 50 g/l, variation of latter notably produced mixed results. With the introduction of surfactants, it was noted that above the surfactant's (sodium dodecyl sulphate) critical micelle concentration, the incorporation of ceramic particles (YSZ) into the nickel matrix steadily increased to as much as 60 volume %. An inverse relationship was though found to exist between the coating thickness and the surfactant's bath concentration. Uniform coatings were found to be associated with low magnetic field strengths while although increased magnetic field strengths positively resulted in the amplification of particle incorporation into the coating, a lack of cohesion between the coating and the substrate – as indicated by coating flake-off, was observed at such strengths. It is suggested that because the magnetic flux was more dominant than the normally ionic plating mechanism, the particles co-deposited under the influence of a high magnetic field were relatively unstable after the coating process. Since LSM is alkaline in nature this work confirms that future research on the application of electroless nickel ceramic co-deposition as a method of manufacturing SOFC cathodes, be focused on the use of alkaline electroless nickel baths rather than the acidic solutions, which better suite YSZ particles.

Steam electrolysis using a solid oxide electrolysis cell at elevated temperatures might offer a solution to high electrical energy consumption associated with conventional water electrolysers through a combination of favourable thermodynamics and kinetics. Although the solid oxide electrolysis cell has not. received significant attention over the past several decades and is yet to be commercialised, there has been an increased interest towards such a technology in recent years, aimed at reducing the cost of electrolytic hydrogen. Here, a one-dimensional dynamic model of a planar cathode-supported intermediate temperature solid oxide electrolysis cell stack has' been developed to investigate the potential for hydrogen production using such an electrolyser. Steady state simulations have indicated that the electrical energy consumption of the modelled stack is significantly lower than those of water electrolysers commercially available today. However, the dependence of stack temperature on the operating point has suggested that there is a need for temperature control. Analysis of a possible temperature control strategy by variation of the air flow rate through the stack has shown that the resulting changes in the convective heat transfer between the air flow and stack can alter the stack temperature. Furthermore, simulated transient responses indicated that manipulation of such an air flow rate can reduce stack temperature excursions during dynamic operation, suggesting that the p,oposed control strategy. has a good potential to prevent issues related to the stack temperature fluctuations.

This study mainly explores the development of alternative cathode materials for the electrochemical reduction of CO₂ by high temperature solid oxide electrolysis cells (HTSOECs), which operate in the reverse manner of solid oxide fuel cells (SOFCs). The conventional Ni-yttria stabilized zirconia (YSZ) cermets cathode suffered from coke formation, whereas the perovskite-type (La, Sr)(Cr, Mn)O₃ (LSCM) oxide material displayed excellent carbon resistance. Initial CO₂ electrolysis performance tests from different cathode materials prepared by screen-printing showed that LSCM based cathode performed poorer than Ni-YSZ cermets, due to non-optimized microstructure. Efforts were made on microstructure modification of LSCM based cathodes by means of various fabrication methods. Among the LSCM/YSZ graded cathode, extra catalyst (including Pd, Ni, CeO₂, and Pt) aided LSCM/GDC (Gd₀.₁Ce₀.₉O₁.₉₅) cathode, LSCM impregnated YSZ cathode, and GDC impregnated LSCM cathode, the GDC impregnated LSCM cathode, with porous LSCM as backbone for finely dispersed GDC nanoparticles, was found to possess the desired microstructure for CO₂ splitting reaction via SOEC. Incorporating of 0.5wt% Pd into GDC impregnated LSCM cathode gave rise to an Rp of 0.24 Ω cm² at open circuit voltage (OCV) at 900°C in CO₂-CO 70-30 mixture, comparable with the Ni/YSZ cermet cathode operated in the identical conditions. Meanwhile, the cathode kinetics and possible mechanisms of the electrochemical reduction of CO₂ were studied, and factors including CO₂/CO composition, operation temperature and potential were taken into account. The current-to-chemical efficiency of CO₂ electrolysis was evaluated with gas chromatography (GC). The high performance Pd and GDC co-impregnated LSCM cathode was also applied for CO₂ electrolysis without protective CO gas in feed. This cathode also displayed superb performance towards CO₂ electrochemical reduction under SOEC operation condition in CO₂/N₂ mixtures, though it had OCV as low as 0.12V at 900°C. The LSCM/GDC set of SOEC cathode materials were investigated in the application of steam electrolysis and H₂O-CO₂ co-electrolysis as well. For the former, adequate supply of steam was essential to avoid the appearance of S-shaped I-V curves and limited steam transport. The 0.5wt% Pd and GDC co-infiltrated LSCM material has been found to be a versatile cathode with high performance and good durability in SOEC operations.

The atmosphere of Mars is a potential source of the gases essential for human exploration missions. Many international space agencies and scientists have shown great interest in developing chemical plants to make propellants and life-support consumables utilizing the red planet's atmosphere and Earth-sourced H₂. Electrolyzing carbon dioxide to produce oxygen by a solid oxide electrolysis cell has been proven to be a potential candidate. A solid oxide electrolysis cell, which consists of 8mol% yttria-stabilized zirconia sandwiched between two electrodes, is designed, manufactured and tested. The electrode/electrolyte interfacial polarization characteristics are investigated with the aid of the current interruption method using a four-electrode set-up. Activation overpotentials, which are free of ohmic losses, are measured in the temperature range from 1023 to 1123K for the carbon dioxide electrode and the oxygen electrode. Both the electrode activation overpotentials show the Tafel behavior. In order to increase the active electrochemical reaction sites, platinum yttria-stabilized zirconia cermet electrode is investigated. A solid oxide electrolysis cell with cermet electrodes shows high performance and significantly reduces anode activation overpotentials, and ohmic resistance as well. A 1-D steady state solid oxide electrolysis cell model is set up to take into account different kinetics: (1) surface exchange kinetics at the gas/electrode interface involving adsorption/desorption; (2) mass transfer of the reactants and products involving the bulk diffusion and surface diffusion; and (3) electrochemical kinetics involving charge transfer at the triple phase boundary. The solid oxide electrolysis cell's performance and voltage are predicted at any given current based on this model. The effects of surface diffusion coefficients, adsorption/desorption rate constants, and anodic/cathodic reaction rate constants on carbon dioxide electrolysis are studied. A comparison of solid oxide electrolysis cells between the numerical results and the experimental results is made.

Thesis (Ph.D.)--Boston University PLEASE NOTE: Boston University Libraries did not receive an Authorization To Manage form for this thesis or dissertation. It is therefore not openly accessible, though it may be available by request. If you are the author or principal advisor of this work and would like to request open access for it, please contact us at open-help@bu.edu. Thank you.
Direct electrolysis of magnesmm from its oxide is less expensive and more environmentally friendly than current methods of magnesium production. The solid oxide membrane (SOM) process is a viable method for production of magnesium via direct electrolysis. In the SOM process magnesium oxide is dissolved in a molten flux, which acts as a supporting electrolyte. A yttria stabilized zirconia (YSZ) membrane is immersed in the flux and separates the anode from the cathode. When an electrical potential is applied between electrodes, magnesium cations travel through the flux to a steel cathode where they are reduced. Simultaneously, oxygen anions travel through the YSZ to a liquid metal anode where they are oxidized. However, in order for the SOM process to be commercially successful it must run for thousands of hours at high current efficiencies. It is believed the degradation of the YSZ membrane determines the lifetime and operating costs of the SOM process. This study investigates the mechanisms of YSZ membrane degradation. There are two main pathways of YSZ degradation: 1) yttria (yttrium oxide) diffusion out of the membrane, and 2) electronic conductivity in the flux providing a pathway for the applied potential to reduce the YSZ membrane. It is shown through diffusion experiments that the loss of yttria from the membrane into the oxy-fluoride flux can be prevented by adding yttrium fluoride to the flux, so that the activity of yttria in the flux is equal to the activity of yttria in the membrane. The electronic conductivity then becomes the primary source of membrane degradation in the SOM process. Electronic conductivity lowers the current efficiency of the SOM process. It is shown through measurements that the electronic conductivity is reduced by lowering the magnesium solubility in the flux. This is accomplished by performing SOM electrolysis at a reduced pressure (<0.1 atm). When SOM electrolysis of magnesium oxide is carried out at reduced pressure, the membrane is not degraded and the current efficiency is high (>70%). Thus tlus process provides a basis for a successful commercial operation for the direct electrolysis of magnesium oxide.

Solid oxide fuel cells (SOFCs) have attracted much attention because of their potential of providing an efficient, environmentally benign, and fuel-flexible power generation system for both small power units and for large scale power plants. However, conventional SOFCs with yttria-stabilized zirconia (YSZ) electrolyte require high operation temperature (800-1000°C), which presents material degradation problems, as well as other technological complications and economic obstacles. Therefore, numerous efforts have been made to lower the operating temperature of SOFCs. The discovery of new electrolytes for low-temperature SOFCs (LTSOFCs) is a grand challenge for the SOFC community.

 Nanostructured materials have attracted great interest for many different applications, due to their unusual or enhanced properties compared with bulk materials. As an example of enhanced property of nanomaterials, the enhancement of ionic conductivity in the nanostructured solid conductors, known as “nanoionics”, recently become one of the hottest fields of research related to nanomaterials, since they can be used in advanced energy conversion and storage applications, such as SOFC. So in this thesis, we are aiming at developing a novel nanocomposite approach to design and fabricate ceria-based composite electrolytes for LTSOFC. We studied two ceria-based nanocomposite systems with different SDC morphologies.

 In the first part of the thesis, novel core-shell SDC/amorphous Na₂CO₃ nanocomposite was fabricated for the first time. The core-shell nanocomposite particles are smaller than 100 nm with amorphous Na₂CO₃ shell of 4~6 nm in thickness. The nanocomposite electrolyte shows superionic conductivity above 300 °C, where the conductivity reaches over 0.1 S cm-1. The thermal stability of such nanocomposite has also been studied based on careful XRD, BET, SEM and TGA characterization after annealing samples at various temperatures, which indicated that the SDC/Na₂CO₃ nanocomposite possesses better thermal stability on nanostructure than pure SDC. Such nanocomposite was applied in LTSOFCs with an excellent performance of 0.8 W cm-2 at 550 °C. The high performances together with notable thermal stability make the SDC/Na₂CO₃ nanocomposite as a potential electrolyte material for long-term SOFCs that operate at 500-600 °C.

In the second part of the thesis, we report a novel chemical synthetic route for the synthesis of samarium doped ceria (SDC) nanowires by homogeneous precipitation of lanthanide citrate complex in aqueous solutions as precursor followed by calcination. The method is template-, surfactant-free and can produce large quantities at low costs. To stabilize these SDC nanowires at high operation temperature, we employed the concept of “nanocomposite” by adding a secondary phase of Na₂CO₃, as inclusion which effectively hindered the grain growth of nanostructures. The SDC nanowires/Na2CO3 composite was compacted and sintered together with electrode materials, and was then tested for SOFCs performance. It is demonstrated that SOFCs using such SDC nanowires/Na₂CO₃ composite as electrolyte exhibited better performance compared with state-of-the-art SOFCs using conventional bulk ceria-based materials as electrolytes.

Solid oxide fuel cells (SOFCs) are considered as one of the most promising powergeneration technologies due to their high energy conversion efficiency, fuel flexibilityand reduced pollution. The current SOFCs with yttria-stabilized zirconia (YSZ)electrolyte require high operation temperature (800-1000 °C), which not only hinderstheir broad commercialization due to associated high cost and technologicalcomplications. Therefore, there is a broad interest in reducing the operating temperatureof SOFCs. The key to development of low-temperature SOFCs (LTSOFCs) is to explorenew electrolyte materials with high ionic conductivity at such low temperature (300-600 °C).Recently, ceria-based composite electrolyte, consisting of doped cerium oxide mixedwith a second phase (e.g. Na2CO3), has been investigated as a promising electrolyte forLTSOFCs. The ceria-based composite electrolyte has shown a high ionic conductivityand improved fuel cell performance below 600 °C. However, at present the developmentof composite electrolyte materials and their application in LTSOFCs are still at an initialstage. This thesis aims at exploring new composite systems for LTSOFCs with superiorproperties, and investigates conductivity behavior of the electrolyte. Two compositesystems for SOFCs have been studied in the thesis.In the first system, a novel concept of non-ceria-salt-composites electrolyte, LiAlO2-carbonate (Li2CO3-Na2CO3) composite electrolyte, was investigated for SOFCs. TheLiAlO2-carbonate electrolyte exhibited good conductivity and excellent fuel cellperformances below 650 oC. The ion transport mechanism of the LiAlO2-carbonatecomposite electrolyte was studied. The results indicated that the high ionic conductivityrelates to the interface effect between oxide and carbonate.In the second system, we reported a novel core-shell samarium-doped ceria(SDC)/Na2CO3 nanocomposite which is proposed for the first time, since the interface isdominant in the nanostructured composite materials. The core-shell nanocompositeparticles are smaller than 100 nm with amorphous Na2CO3 shell. The nanocompositeelectrolyte was applied in LTSOFCs and showed excellent performance. Theconductivity behavior and charge carriers have been studied. The results indicated that H+conductivity in SDC/Na₂CO₃ nanocomposite is predominant over O2- conductivity with1-2 orders of magnitude in the temperature range of 200-600 °C. It is suggested that theinterface in composite electrolyte supplies high conductive path for proton, while oxygenions are most probably transported by the SDC nano grain interiors. Finally, a tentativemodel “swing mechanism” was proposed for explanation of superior proton conduction.

QC 20100930

Solid Oxide Fuel Cells are fuel cells that operate at high temperatures usually in the range of 600oC to 1000oC and employ solid ceramics as the electrolyte. In Solid Oxide Fuel Cells oxygen ions (O2-) are the ionic charge carriers. Solid Oxide Fuel Cells are known for their higher electrical efficiency of about 50-60% [1] compared to other types of fuel cells and are considered very suitable in stationary power generation applications. It is very important to study the effects of different parameters on the performance of Solid Oxide Fuel Cells and for this purpose the experimental or numerical simulation method can be adopted as the research method of choice. Numerical simulation involves constructing a mathematical model of the Solid Oxide Fuel Cell and use of specifically designed software programs that allows the user to manipulate the model to evaluate the system performance under various configurations and in real time. A model is only usable when it is validated with experimental results. Once it is validated, numerical simulation can give accurate, consistent and efficient results. Modeling allows testing and development of new materials, fuels, geometries, operating conditions without disrupting the existing system configuration. In addition, it is possible to measure internal variables which are experimentally difficult or impossible to measure and study the effects of different operating parameters on power generated, efficiency, current density, maximum temperatures reached, stresses caused by temperature gradients and effects of thermal expansion for electrolytes, electrodes and interconnects. Since Solid Oxide Fuel Cell simulation involves a large number of parameters and complicated equations, mostly Partial Differential Equations, the situation calls for a sophisticated simulation technique and hence a Finite Element Method (FEM) multiphysics approach will be employed. This can provide three-dimensional localized information inside the fuel cell. For this thesis, COMSOL Multiphysics version 4.2a will be used for simulation purposes because it has a Batteries & Fuel Cells module, the ability to incorporate custom Partial Differential Equations and the ability to integrate with and utilize the capabilities of other tools like MATLAB, Pro/Engineer, SolidWorks. Fuel Cells can be modeled at the system or stack or cell or the electrode level. This thesis will study Solid Oxide Fuel Cell modeling at the cell level. Once the model can be validated against experimental data for the cell level, then modeling at higher levels can be accomplished in the future. Here the research focus is on Solid Oxide Fuel Cells that use hydrogen as the fuel. The study focuses on solid oxide fuel cells that use 3-layered, 4-layered and 6-layered electrolytes using pure YSZ or pure SCSZ or a combination of layers of YSZ and SCSZ. A major part of this research will be to compare SOFC performance of the different configurations of these electrolytes. The cathode and anode material used are (La0.6Sr0.4)0.95-0.99Co0.2Fe0.8O3 and Ni-YSZ respectively.
ID: 031001387; System requirements: World Wide Web browser and PDF reader.; Mode of access: World Wide Web.; Adviser: .; Title from PDF title page (viewed May 22, 2013).; Thesis (M.S.M.E.)--University of Central Florida, 2012.; Includes bibliographical references (p. 101-107).
M.S.M.E.
Masters
Mechanical and Aerospace Engineering
Engineering and Computer Science
Mechanical Engineering; Thermofluids

In this PhD thesis, new materials for solid oxide fuel cells have been researched. It focuses on both the cathode and electrolyte components. Two general systems, the perovskite-type ABO3 and apatite-type M10-x(XO4)6O2+y structures, have been investigated. The structural characteristics, conductivity and stability have been examined. The perovskite work for the cathode uses doping strategies to introduce disorder into the system and change the conduction characteristics through a structure change to cubic. It has been shown that only small amounts of dopants are required to cause this structural change and effect the conductivity. In addition, thermal and chemical compatibility tests, along with ASR tests with known fluorite and apatite electrolytes, have been investigated. Their stability in a CO2 containing environment was tested and a full-scale production of a fuel cell was attempted (Chapters 3 and 4). The electrolyte investigations focussed on doping the Ba2Sc2O5 sample to form a perovskite structure that possesses both oxide ion and protonic conductivity. The doping has decreased the amount of scandium present with cheaper elements such as rare earth Yb3+, or transition metals Fe4+ and Ti4+, all in an attempt to form the cubic structure that results in high oxide ion/proton conductivity and increased stability in CO2 environments (Chapter 5). The final chapter focuses on phosphate and rare earth doping of BaPrO3, to form the cubic perovskite structure. These samples were seen to have increased water incorporation and stability in CO2. However, this was at the expense of the ionic conductivity due to vacancy trapping.

Solid oxide fuel cells (SOFCs) are considered as one of the most promising power generation technologies due to their high energy conversion efficiency, fuel flexibility and reduced pollution. There is a broad interest in reducing the operating temperature of SOFCs. The key issue to develop low-temperature (300~600 °C) SOFCs (LTSOFCs) is to explore new electrolyte materials. Recently, ceria-based composite electrolytes have been developed as capable alternative electrolyte for LTSOFCs. The ceria-based composite electrolyte has displayed high ionic conductivity and excellent fuel cell performance below 600 °C, which has opened up a new horizon in the LTSOFCs field. In this thesis, we are aiming at exploring nanostructured composite materials for LTSOFCs with superior properties, investigating the detailed conduction mechanism for their enhanced ionic conductivity, and extending more suitable composite system and nanostructure materials.In the first part, core-shell samarium doped ceria-carbonate nanocomposite (SDC/Na2CO3) was synthesized for the first time. The core-shell nanocomposite was composed of SDC particles smaller than 100 nm coated with amorphous Na2CO3 shell. The nanocomposite has been applied in LTSOFCs with excellent performance. A freeze dry method was used to prepare the SDC/Na2CO3 nanocomposites, aiming to further enhance its phase homogeneity. The ionic conduction behavior of the SDC/Na2CO3 nanocomposite has been studied. The results indicated that H+ conductivity in the nanocomposite is predominant over O2- conductivity with 1-2 orders of magnitude in the temperature range of 200-600 °C, indicating the proton conduction in the nanocomposite mainly accounts for the enhanced total ionic conductivity. The influence of Na2CO3 content to the proton and oxygen ion conductivity in the nanocomposite was studied as well.In the second part, both the proton and oxygen ion conduction mechanisms have been studied. It is suggested that the interface in the nanocomposite electrolyte supplies high conductive path for the proton, while oxygen ions are probably transported by the SDC grain interiors. An empirical “Swing Model” has been proposed as a possible mechanism of superior proton conduction, while oxygen ion conduction is attributed to oxygen vacancies through SDC grain in nanocomposite electrolyte.In the final part, a novel concept of non-ceria-salt-composites electrolyte, LiAlO2-carbonate composite electrolyte, has been investigated for LTSOFCs. The LiAlO2-carbonate electrolyte exhibits good conductivity and excellent fuel cell performances below 650 °C. The work not only developed a more stable composite material, but also strongly demonstrated that the high ionic conductivity is mainly related to interface effect between oxide and carbonate. As a potential candidate for nanocomposite, uniform quasi-octahedral CeO2 mesocrystals was synthesized in this thesis work as well. The CeO2 mesocrystals shows excellent thermal stability, and display potential for fuel cell applications.

QC 20120529

Thesis (Ph.D.)--Boston University
Magnesium is the least dense engineering metal, with an excellent stiffness-to-weight ratio. Magnesium recycling is important for both economic and environmental reasons. This project demonstrates feasibility of a new environmentally friendly process for recycling partially oxidized magnesium scrap to produce very pure magnesium at low cost. It combines refining and solid oxide membrane (SOM) based oxide electrolysis in the same reactor. Magnesium and its oxide are dissolved in a molten flux. This is followed by argon-assisted evaporation of dissolved magnesium, which is subsequently condensed in a separate condenser. The molten flux acts as a selective medium for magnesium dissolution, but not aluminum or iron, and therefore the magnesium collected has high purity. Potentiodynamic scans are performed to monitor the magnesium content change in the scrap as well as in solution in the flux. The SOM electrolysis is employed in the refining system to enable electrolysis of the magnesium oxide dissolved in the flux from the partially oxidized scrap. During the SOM electrolysis, oxygen anions are transported out of the flux through a yttria stabilized zirconia membrane to a liquid silver anode where they are oxidized. Simultaneously, magnesium cations are transported through the flux to a steel cathode where they are reduced. The combination of refining and SOM electrolysis yields close to 100% removal of magnesium metal from partially oxidized magnesium scrap. The magnesium recovered has a purity of 99.6w%. To produce pure oxygen it is critical to develop an inert anode current collector for use with the non-consumable liquid silver anode. In this work, an innovative inert anode current collector is successfully developed and used in SOM electrolysis experiments. The current collector employs a sintered strontium-doped lanthanum manganite (La0.8Sr0.2Mn03-δ or LSM) bar, an Inconel alloy 601 rod, and a liquid silver contact in between. SOM electrolysis experiments with the new LSM-Inconel current collector are carried out and performance comparable to the state-of-the-art SOM electrolysis for Mg production employing the non-inert anode has been demonstrated. In both refining and SOM electrolysis, magnesium solubility in the flux plays an important role. High magnesium solubility in the flux facilitates refining. On the other hand, lower magnesium solubility benefits the SOM electrolysis. The dissolution of magnesium imparts electronic conductivity to the flux. The effects of the electronic conductivity of the flux on the SOM electrolysis performance are examined in detail through experiments and modeling. Methods for mitigating the negative attributes of the electronic conductivity during SOM electrolysis are presented.

One of the major trends of development of solid oxide fuel cells is to reduce the operating temperature from the high temperature range (>950°C) and intermediate temperature range (750-850°C) to the low temperature range (450-650°C). Development of low temperature oxygen ion conducting electrolytes is focused on single-phase materials including Bi2O3 and CeO2-based oxides. These materials have high ion conductivity at the low temperature range, but they are unstable in reducing environments and they are also electronic conductors. In the present research, three types of multiphase materials, Ce0.887Y0.113O1.9435 (CYO)-ZrO2, CYO- yttria-stabilized zirconia (YSZ), and CuO-CYO were investigated. We found that the conductivity of multiphase electrolyte CuO-CYO with a mass ratio of 1:3 is at least 4 times greater than that of CYO and 10 times greater than that of YSZ, the most commonly used material, obtained in the present experiments at 600°C. The enhancement of conductivity in multiphase materials correlates with the level of mismatch between the two phases. Large mismatches in terms of valance and structure result in high vacancy density and hence high oxygen ion conductivity at grain boundaries. This study demonstrates that synthesis of multiphase ceramic materials is a feasible new avenue for development of oxygen ion electrolyte material for low temperature SOFCs.

Low-temperature routes to solid electrolytes are important for construction of solid-state batteries, electrochromic devices, electrolyte-gated transistors, high-energy capacitors and sensors. Here we report an environmentally friendly aqueous solution route to amorphous thin films of solid lithium based electrolytes and related multi-layered structures. This route allows production of high quality films at very low temperatures, up to 600 °C lower than traditional melt quenching routes. Pinhole free films of thicknesses ranging from 13-150 nm produced by this route are extremely smooth and fully dense, with temperature dependent conductivities similar to those reported for samples made by energy intensive techniques. Processing conditions were examined by TGA-DSC; film evolution was monitored by FTIR; and, resulting films were characterized using FTIR, XPS, SEM, and XRD. These techniques indicate that water and nitrate removal is complete at low temperatures, and the films remain amorphous to 400 °C. Electrical analysis suggests the presence of ionic double layer capacitor behaviour as observed in similar metal oxide systems. Large magnitudes of ε'app are reported for two separate systems herein, surpassing values reported in the literature for similar materials produced by other synthetic methods. A two-fold increase in the breakdown strength of nanolaminate structures over their single-phase counterparts is also reported. The approach developed demonstrates a simple, inexpensive and environmentally benign deposition route for the fabrication of inorganic solid electrolyte thin-films and related nanolaminates, using LiAlPO, LiAlO, and TiO2:LiAlO as model systems.

The increased development of intermittent renewable energy supplies not only demands robust storage technology, but also alternative means to produce materials in ways to avoid fossil fuel consumption and make use of the increasing electricity supply. Power to gas (PtG) through solid oxide cell (SOC) co-electrolysis reactors provide an attractive manner to overcome both challenges. The performance of co-electrolysis reactors for sector coupling purposes was investigated through mathematical models at the stack and system level.The system level model involved the development of an ideal power to methane (PtM) system with no losses in the auxiliary units and ideal SOC operation. This model was used to determine the maximum achievable efficiencies independent of technology, for a co-electrolysis and steam electrolysis based PtM in two different schemes: atmospheric SOC with pressurised methanation reactor and equal pressure between the SOC and methanation reactor. The performance of the system was analysed through the exergy method for different operating temperatures and pressures. The system was designed to be completely coupled, where the heat generated by one process was usable for another. Functional exergy efficiency was one of the main performance criteria used for comparison. It was found that for an ideal system, co-electrolysis operation was marginally beneficial over steam electrolysis at the system level based on exergetic efficiency. This is further compounded when considering the product yield, where the co-electrolysis systems outperform the steam electrolysis systems significantly.The stack level model involved introducing a new modelling framework based on fundamental charge transfer interactions to modify a transient steam/𝐻𝐻2 based SOC reactor modelled with Modelica at the DLR. This also involved modifying the reversible potential model to account for co-electrolysis as well as novel implementation of the DGM for co-electrolysis. The model was validated against experimental results of steady state operation for 1.4bar, 4bar and 8bar and feed gas compositions of 60% steam, 30% 𝐶𝐶𝑂𝑂2 and 10% 𝐻𝐻2; and 45% steam, 45% 𝐶𝐶𝑂𝑂2 and 10% 𝐻𝐻2 by volume. The model results agree with the experimental results. Further analysis of the reactor under co-electrolysis operation was performed. The 𝐶𝐶𝑂𝑂2 consumption mechanism was investigated as well as various electrochemical and thermal phenomena, to understand the operating behaviour of co-electrolysis stacks and to obtain general trends in operation with different operating conditions. The SOC reactor model was also used to predict the reactor behaviour under elevated pressure operation outside of the validation scope. Elevated pressure operation reduced polarisation overpotentials and ohmic resistance due to higher methanation rate, this led to lower cell voltages at high operating current densities thereby reducing the power demand compared to the lower pressure operation. However, the higher methanation rate led to higher methane content in the reactor outlet.The trends with pressure and temperature in the stack model were used to determine the theoretical limits of the PtM system with a state-of-the-art reactor. Invariable efficiencies were applied to the auxiliary units as average efficiencies to consider a wide range of equipment efficiencies. The system performance was analysed for different operating temperatures, pressures, current densities, and stack active areas. System and stack performance increased with temperature, while pressure had marginal impact on system performance but reasonable impact on the stack performance especially for lower auxiliary unit efficiency. The system and stack performance decreased with current density while increasing the SOC area resulted in higher efficiencies to nearly ideal, for constant flow rates.The results of the models suggest that SOC based co-electrolysis reactors provide an attractive method for sector coupling purposes. The exergy method provided a broad method to analyse and compare different systems. More research is required, especially on the thermal aspects of SOC reactor and 𝐶𝐶𝑂𝑂2 consumption mechanisms in co-electrolysis reactors.
Den ökade utvecklingen av förnybara energikällor kräver inte bara pålitlig lagringsteknik utan också alternativa sätt att producera material på sätt att undvika fossila bränsleförbrukningar och använda sig av den ökande elförsörjningen. Kraft till gas (PtG) genom fasta oxidceller (SOC) samelektrolysreaktorer ger ett attraktivt sätt att övervinna båda utmaningarna. Prestanda hos samelektrolysreaktorer för sektorkopplingsändamål undersöktes genom matematiska modeller på komponent- och systemnivå.Systemnivåmodellen involverade utvecklingen av ett idealiskt kraft-till-metan-system (PtM) utan förluster i hjälpenheterna och idealisk SOC-drift. Denna modell användes för att bestämma de maximalt uppnåbara effektiviteterna oberoende av teknik, för en samelektrolys och ångelektrolysbaserad PtM i två olika scheman: atmosfärisk SOC med trycksatt metaneringsreaktor och lika tryck mellan SOC och metaneringsreaktorn. Systemets prestanda analyserades genom exergimetoden för olika driftstemperaturer och tryck. Systemet var utformat för att vara helt kopplat, där värmen som genereras av en process kunde används vidare. Funktionell energieffektivitet var ett av de viktigaste prestationskriterierna som användes för jämförelse. Det visade sig att för ett idealiskt system var samelektrolysoperation marginellt fördelaktig jämfört med ångelektrolys på systemnivå baserat på exergetisk effektivitet. Detta blandas ytterligare när man överväger produktutbytet, där samelektrolyssystemen överträffar ångelektrolyssystemen avsevärt.Stacknivåmodellen involverade införandet av ett nytt modelleringsramverk baserat på grundläggande laddningsöverföringsinteraktioner för att modifiera en övergående ånga/𝐻𝐻2-baserad SOC-reaktor modellerad med Modelica vid DLR. Detta involverade också modifiering av den reversibla potentiella modellen för att ta hänsyn till samelektrolys samt ny implementering av DGM för samelektrolys. Modellen validerades mot experimentella resultat vid stationärt förhållande för 1,4bar, 4bar och 8bar och matargaskompositioner av 60% ånga, 30% 𝐶𝐶𝑂𝑂2 och 10% 𝐻𝐻2; och 45% ånga, 45% 𝐶𝐶𝑂𝑂2 och 10% 𝐻𝐻2 i volym. Modellresultaten överensstämmer med de experimentella resultaten. Ytterligare analys av reaktorn under samelektrolysoperation utfördes. 𝐶𝐶𝑂𝑂2-förbrukningsmekanismen undersöktes liksom olika elektrokemiska och termiska fenomen, för att förstå driftsbeteendet hos samelektrolysstaplar och för att få generella trender i drift med olika driftsförhållanden. SOC-reaktormodellen användes också för att förutsäga reaktorns beteende under förhöjd tryck utanför valideringsområdet. Förhöjt tryckdrift minskade polariseringsöverpotentialen och ohmskt motstånd på grund av högre metaneringshastighet, vilket ledde till lägre cellspänningar vid höga driftsströmtätheter, vilket minskade effektbehovet jämfört med lägre tryckoperation. Den högre metaneringshastigheten ledde emellertid till högre metanhalt i reaktorutloppet.Trenderna med tryck och temperatur i stackmodellen användes för att bestämma de teoretiska gränserna för PtM-systemet med en toppmodern reaktor. Konstanta verkningsgrader applicerades på hjälpenheterna som genomsnittliga verkningsgrad för att överväga ett brett spektrum av utrustningsverkningsgrad. Systemets prestanda analyserades med avseende på olika driftstemperaturer, tryck, strömtäthet och stack-aktiva områden. Systemets och stackens prestanda ökade med temperaturen, medan trycket hade marginell inverkan på systemets prestanda men rimlig inverkan på stackens prestanda, särskilt för de lägre hjälpaggregatens verkningsgrad. Systemets och stackens prestanda minskade med strömtätheten medan en ökning i SOC yta-resulterade i högre effektivitet till nästan idealisk för konstanta flödeshastigheter.Resultaten av modellerna antyder att SOC-baserade samelektrolysreaktorer ger en attraktiv metod för sektorkoppling. Exergimetoden gav en bred metod för att analysera och jämföra olika system. Mer forskning krävs, särskilt om de termiska aspekterna av SOC-reaktorn och 𝐶𝐶𝑂𝑂2-förbrukningsmekanismerna i samelektrolysreaktorer.

The interactions between different components of solid oxide cells (SOCs) are critical issues for achieving the tens of thousands of hourâ s goal for long-term performance stability and lifetime. The interactions between the ceramic electrolyte, porous ceramic air electrode, and metallic interconnect materials â including solid state interfacial reactions and vaporization/deposition of some volatile elements â have been investigated in the simulated SOC operating environment. The interactions demonstrate the material degradation mechanisms of the cell components and the effects of different factors such as chemical composition and microstructure of the materials, as well as atmosphere and current load on the air electrode side. In the aspect of materials, this work contributes to the degradation mechanism on the air electrode side and provides practical material design criteria for long-term SOC operation. In this research, an yttria-stabilized zirconia electrolyte (YSZ)/strontium-doped lanthanum manganite electrode (LSM)/AISI 441 stainless steel interconnect tri-layer structure has been fabricated in order to simulate the air electrode working environment of a real cell. The tri-layer samples have been treated in dry/moist air atmospheres at 800°C for up to 500 h. The LSM air electrode shows slight grain growth, but the growth is less in moist atmospheres. The amount of Cr deposition on the LSM surface is slightly more for the samples thermally treated in the moist atmospheres. At the YSZ/LSM interface, La enrichment is significant while Mn depletion occurs. The Cr deposition at the YSZ/LSM interface is observed. The stoichiometry of the air electrode is an important factor for the interactions. The air electrode composition has been varied by changing the x value in (La0.8Sr0.2)xMnO3 from 0.95 to 1.05 (LSM95, LSM100, and LSM105). The enrichment of La at the YSZ/LSM interface inhibits the Cr deposition. The mechanisms of Cr poisoning and LSM elemental surface segregation are discussed. A 200 mA·cm-2 current load have been applied on the simulated cells. Mn is a key element for Cr deposition under polarization. Excessive Mn in the LSM lessens the formation of La-containing phases at the YSZ/LSM interface and accelerates Cr deposition. Deficient Mn in LSM leads to extensive interfacial reaction with YSZ forming more La-containing phase and inhibiting Cr deposition.
Ph. D.

Les piles à combustible à oxyde solide (SOFC) offrent une alternative réelle aux technologies classiques de génération d’électricité en étant à la fois propre, efficace et respectueuse de l’environnement. Toutefois, leur principale limitation réside en leur durée de vie et fiabilité limitées dues à leur haute température de fonctionnement. Des recherches intenses de matériaux pour SOFC sont actuellement poursuivies pour essayer d’abaisser la température de fonctionnement de ces dispositifs afin de dépasser ces limitations. Parmi les différents candidats qui ont émergé, le Silicate de Lanthane (LSO) et le Zirconate de Baryum dopé à l'Yttrium (BZY) ont été identifiés comme des alternatives potentielles à utiliser comme matériaux d’électrolyte pour SOFC à température intermédiaire.De manière surprenante, alors que de nombreuses études concernent l’optimisation microstructurale et électrochimiques des composants de la pile, très peu d’études concernant l’évaluation de leurs propriétés mécaniques et de leur influence sur la durée de vie du dispositif.La fiabilité et durée de ces dispositifs dépend non seulement de leur stabilité électrochimique, mais aussi de la capacité de leur structure à supporter les contraintes résiduels issus du procédé de fabrication et de contraintes mécaniques de fonctionnement. En raison du fait que les SOFC sont composés d'empilement de plusieurs cellules individuelles qui, à leur tour, sont constituées de couches fragiles individuelles en contact étroit, ces contraintes proviennent principalement de la différence entre le coefficient de dilatation thermique et les propriétés élastiques des couches adjacentes et la déformation du fluage. Des contraintes non coordonnées peuvent entraîner une défaillance mécanique d'une seule cellule et avoir des conséquences dramatiques sur l'ensemble de la pile. De ce fait, la connaissance des propriétés mécaniques des composants de la cellule est une étape importante pour préserver l’intégrité et le développement des SOFC. Le but de cette thèse est la fabrication et l’étude des propriétés structurale, microstructurales et mécaniques de matériaux de type LSO et BZY
Solid oxide fuel cells (SOFCs) offer a real alternative to classical technologies for the generation of electricity by clean, efficient and environmental-friendly means. Nevertheless, the main limitation of SOFCs lies in their unsatisfactory durability and reliability due to the high operating temperatures and thermal cycling characteristic of these devices. An intense search is currently underway for materials for SOFCs with the objective of lowering the working temperature and then overcoming these limitations. Among the different candidates which have emerged, Lanthanum Silicate (LSO) and Yttrium-doped Barium Zirconate (BZY) were considered as potential alternatives to be used as electrolyte materials for SOFC at intermediate-temperature. While numerous studies have been devoted to characterizing and optimizing the microstructural and electro-chemical properties of SOFC components, as yet there is little research available on mechanical properties and the influence they have on SOFC lifespan.The reliability and durability of these devices depends not only on their electro-chemical stability, but also on the ability of their structure to withstand residual stresses arising from the cell manufacturing process and mechanical stresses from operation. Owing to the fact that SOFCs are composed by stacking of several single cells which in turn are made up of individual brittle layers in close contact, these stresses mainly originate from the difference between the coefficient of thermal expansion and elastic properties of adjacent layers and creep deformation. Mismatched stresses can result in the mechanical failure of a single cell and have dramatic consequences on the whole stack. Therefore, knowledge of mechanical properties of the cell components becomes an important issue for the mechanical integrity and development of SOFCs.The aim of this PhD thesis is the fabrication and structural, microstructural and mechanical characterization of LSO and BZY

La technologie d’électrolyse à oxydes solides (SOE) pourrait permettre d’améliorer l’efficacité des installations de conversion d’électricité en gaz naturel de synthèse (SNG) et de réduire leur coût, grâce à une integration thermique performante, à l’industrialisation de la technologie et une flexibilisation des unités pour la pénétration de l’électricité renouvelable. Une analyse énergétique détaillée de trois concepts d’installations power-to-SNG innovants est d’abord réalisée avec une intégration thermique détailllée. Les installations intégrant des unités SOE et produisant du GNC ou du GNL présentent des rendements d’au moins 78,5% sur base PCS, bien plus élevés que pour les installations intégrant des unités d’électrolyse PEM qui produisent du GNC avec un rendement de 64,4%. La réponse thermique des unités SOE soumises à des variations de charge électrique est ensuite étudiée sur la base d’un modèle dynamique 1D à l’échelle d’une cellule (SOEC). Les cellules « électrolyte support » sont thermiquement plus stables que les « électrode support » et donc plus adaptées à des charges électriques variables. Le modèle est ensuite étendu à une unité entière de production et de stockage d’H2 et couplé à différents profils électriques. L’unité affiche une consommation énergétique de 3,4-3,8 kWh·Nm-3 H2 et un rendement élevé de l’électricité vers l’H2 (93-103%) par récupération de la vapeur de méthanation. Un dimensionnement du réservoir d’H2 et de l’unité de méthanation est réalisé avec un profil électrique éolien. Les charges électriques variables réduisent l’efficacité des installations power-to-SNG, en augmentent les coûts et en complexifient l’opération. Les installations multifuels semblent être l’option la plus prometteuse pour gérer l’intermittence de la production d’électricité. Etendre la gamme d’opération des SOECs aux modes exotherme et endotherme améliorerait les rendements de l’électricité vers l’H2 en comparaison au mode marche/arrêt. Pour une charge électrique constante, les SOECs doivent préférablement être opérées au thermoneutre ou en mode exotherme. Enfin, les coûts de production du SNG sont évalués, en commençant par une estimation ascendante des coûts d’investissement d’unités SOE. Les coûts de production du SNG des concepts étudiés vont de 82 à 89 €·MWh-1 CH4 (PCS) avec des unités SOE, valeurs plus faibles que pour des unités PEM, mais qui restent deux fois supérieures au prix moyen du gaz naturel en France
The solid oxide electrolysis technology (SOE) could improve the conversion efficiency of power-tosynthetic natural gas (SNG) plants and reduce their costs, provided that i) a performant thermal integration is implemented ii) the technology is implemented at industrial scale, and iii) plants can absorb the intermittency of renewable power sources. First, the energy analysis of three innovative power-to-SNG plant concepts is implemented. For each concept, a full explicit thermal integration is proposed. Plants with integrated SOE units show efficiencies higher than 78.5% (based on the HHV of the SNG) for the production of CNG and LNG, significantly higher than plants with PEM units with a 64.4% efficiency for CNG production. Second, the thermal response of SOE units to electrical power loads is investigated with a 1D dynamic model at the cell level (SOEC). Electrolyte support cells present a higher thermal stability than electrode support cells and should be preferred for fluctuating power applications. The model was then extended to a full H2 production and storage unit and coupled with different electrical power profiles. The units shows an energy consumption of 3.4-3.8 kWh·Nm-3 H2 and a high power-to-H2 conversion efficiency (93-103%) because of the steam recovery from the methanation unit. A first dimensioning of the H2 storage tank and the methanation unit is proposed, assuming a windmill power profile. Fluctuating power profiles reduce the efficiency of power-to-SNG plants, increase their costs and complexify their operation. Multifuel plants seem to be the most promising option to tackle the issue of intermittent power production. Extending the operation range of SOECs to exothermic and endothermic modes would improve power-to-H2 conversion efficiencies compared to on/off operation. In case of constant power load though, SOECs should preferably be operated at the thermoneutral point or in exothermic mode. Third, SNG production costs corresponding to the aforementioned plant concepts are evaluated, starting with a bottom-up cost evaluation of SOE units. The SNG production costs are in the range of 82-89 €·MWh-1 CH4 (HHV) with SOE units, which is lower than with PEM units, but remains two times higher than the average price of conventional natural gas for all sectors in France

A one dimensional mass transfer analysis was performed for convective transport as well as mass transport within a porous media. This analysis was based on the analogous average heat transfer within a duct. Equations were developed to calculate the concentration of gas species at the triple phase boundary sites present at the interface of a porous electrode and a nonporous electrolyte. The mass transport analyzed on the steam side electrode of a solid oxide electrolysis cell was performed for a ternary gas mixture. In this analysis two gas species were actively diffusing in the presence of a third inert carrier gas. Multicomponent diffusion coefficients were determined for each species in the steam side electrode mixture. The mass transport analysis performed on the air side electrode utilized a binary gas mixture, namely air. At less than one percent of the total mixture of air, the combined effects of Argon and Carbon Dioxide were assumed to be negligible. This assumption allowed us to consider air a binary mixture. A comprehensive model was developed to determine cell performance under various operating condition and multiple cell geometries. The output of this model was used to optimize various physical features of the cell. Tests were performed on electrode supported solid oxide electrolysis cells at the Idaho National Laboratory. These cells were subjected to various operating temperatures and inlet steam mole fractions. Voltage vs. current density experimental data were collected and compared to computational data in order to validate the model.

Tese (doutorado) - Universidade Federal de Santa Catarina, Centro Tecnológico, Programa de Pós-Graduação em Ciência e Engenharia de Materiais, Florianópolis, 2015.
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As células a combustível de óxido sólido (SOFC) são células para conversão de hidrogênio em energia elétrica, altamente eficientes e limpas, já que produzem eletricidade, calor, e unicamente agua como gás de exhaustão do processo eletroquímico. A célula eletrolítica de óxido sólido (SOEC) corresponde à operação inversa da SOFC. Células a combustível de óxido sólido reversíveis (RSOFC) são dispositivos para produzir energia e armazená-la empregando hidrogênio como portador da energia, atuando de forma reversível como células combustível ou eletrolisador. Um breve análise econômico mostra RSOFC como una alternativa viável para sistemas híbridos de energias renováveis. O estado-da-arte dos materiais dos componentes da célula foi analisado e discutido em detalhe, comparando pontos comuns desenvolvidos neste trabalho. Materiais como Perovskitas, fases Ruddlesden-Popper e Perovskitas Duplas mostram-se como potencialmente mais eficientes em comparação com o material mais utilizado para eletrodos de oxigênio, manganita de estrôncio lantânio (LSM). Uma microestrutura nanocristalina pode melhorar propriedades chave, por exemplo para o eletrólito mais utilizado, zircônia estabilizada com yttria (YSZ), pode-se obter um aumento do >95% em condutividade iônica comparando tamanhos de grão de 300 nm e 2.15 µm. Para atingir as estructuras nanocristalinas, devem-se usar pós com tamanhos de partícula pequenos e técnicas de sinterização que inibam o crescimento de grão. Fases Ruddlesden-Popper como as baseadas em La2NiO4+d, apresentam alta permeabilidade ao oxigênio, e condutividade iônica entre outras propriedades vantajosas para RSOFC. Neste trabalho é reportada a síntese e as propriedades de transporte de oxigênio para um material novo (La2-ySryNi1-xMoxO4+d, 0.0=y=0.4 e 0.0=x=0.1). A análise das fases revelou um limite de solubilidade baixo de Mo no sítio B da estrutura A2BO4+d. O material La1.8Sr0.2Ni0.95Mo0.05O4+d (LSMN) fase pura foi sintetizado com sucesso. Amostras de LSMN foram conformadas por compressão isostática e sinterizadas a 1500ºC durante 4 h. As densidades atingidas foram maiores do que 95% e o tamanho de grão foi de 14±8 µm. Um modelo simples de defeitos foi usado para explicar o comportamento da condutividade elétrica. O coeficiente de superfície (kchem) e os coeficientes de difusão (Dchem) em termos de temperatura e PO2 foram avaliados por relaxamento da condutividade elétrica (ECR) e comparados a materiais semelhantes, mostrando que dopagem no sitio B de La2NiO4+d com Mo, pode melhorar as propriedades de transporte de oxigênio. Para melhorar a condutividade iônica, e desejável manufaturar o eletrólito com pós de pequeno tamanho e planejar a sinterização com os perfis de tempo-temperatura, para obter microestruturas com tamanhos de grão menores. Há falta de modelos que possam prever as densidades atingidas durante sinterização. Neste trabalho foi desenvolvido um modelo para predizer a densificação, como função da temperatura, tempo e tamanho de partícula. O modelo foi capaz de predizer densidades atingidas em diferentes condições de sinterização. A sinterização acarreta densificação e crescimento de tamanho de grão simultaneamente, particularmente para materiais nanocristalinos. Atualmente, métodos como spark plasma sintering (SPS), prensagem a quente (HP), sinterização em duas etapas (TSS) e queima rápida (FF) são empregados para inibir o crescimento de grão enquanto é mantida alta densificação. Neste trabalho, foi comparado experimentalmente FF e sinterização convencional (RH), em compactos de yttria estabilizada com zircônia (3YSZ e 8YSZ). Foram feitos experimentos em um forno tubular com taxas de aquecimento de ~500 °C/min (FF) e 10ºC/min (RH), analisando a mudança contínua da densidade dos compactos e a distribuição de tamanhos de grão da estrutura densa. As amostras sinterizadas pela rota convencional apresentam maior crescimento de grão por um fator de ~4 e ~2 respeito do tamanho dos pós. Por outro lado, as amostras sinterizadas pela rota de queima rápida suprimiram o crescimento com um fator de ~1 para os dois materiais. Esses resultados indicam que altas taxas de aquecimento minimizam o crescimento de tamanho de grão.

Abstract : Solid oxide fuel cells (SOFC) are cells for conversion of hydrogen into electrical power, highly efficient and clean, since produces electricity, heat, and solely water as exhaust gas by electrochemical processes. Solid Oxide Electrolyser Cells (SOEC) correspond to the reverse operation of SOFC. Reversible solid oxide fuel cells (RSOFC) are devices to produce energy and store it employing hydrogen as energy carrier, acting reversibly as fuel or electrolyser cells. A brief financial review shows RSOFC as a viable alternative for hybrid renewal energy systems. Current state of electrolyte, hydrogen and oxygen electrodes materials has been reviewed and discussed in detail, comparing common points between SOFC and SOEC developed here. Perovskites, Ruddlesden-Popper series and Double Perovskites materials show to have lower resistance, therefore, potentially more efficiency than the state-of-art oxygen electrode, lanthanum strontium manganite (LSM). A fine-grained microstructure can enhance key properties, for instance the state-of-art electrolyte yttria stabilized zirconia (YSZ) increase >95% the ionic conductivity comparing grain sizes 300 nm and 2.15 µm. To achieve fine-grained structure, must be employed powders with small particle sizes and sintering techniques to hinder the grain growth. Ruddlesden-Popper series as La2NiO4+d-based materials exhibit high oxygen permeability, ionic conductivity among other properties advantageous for RSOFC. In this work, the synthesis and oxygen transport properties for a novel material (La2-ySryNi1-xMoxO4+d 0.0=y=0.4 and 0.0=x=0.1) are reported. The phase relations analysis disclose low Mo solubility limit on the B-site of the A2BO4+d structure. Single phase La1.8Sr0.2Ni0.95Mo0.05O4+d bar shape samples were cold-isostatically pressed and pressureless sintered at 1500ºC for 4 h. Sintered densities above 95% and grain size of 14.3±8 µm were obtained. A simple defect model was applied to explain electrical conductivity. Surface exchange coefficients (kchem) and bulk diffusion coefficients (Dchem) in terms of temperature and PO2 were assessed by electrical conductivity relaxation (ECR) and discussed comparing with similar compounds, showing that doping the B site of lanthanum nickelate with Mo can enhance the oxygen transport properties.To enhance the materials ionic conductivity, is desirable to manufacture the electrolyte using powders with small particle size and plan the sintering technique with the time-temperature profile to obtain fine-grained microstructures. There is a lack of accurate models to predict the compacts density during sintering. Here a densification model was developed to predict densification, as a function of temperature, time and particle size. The model was able to predicting the achieved density using different sintering conditions. Sintering of powders leads to simultaneous densification and grain growth, particularly for nanocrystalline materials. Currently, methods such as spark plasma sintering (SPS), hot pressing (HP), two-step sintering (TSS) and fast firing (FF) are employed to hinder grain growth while maintaining a high densification. In this work, FF consisting in thermal treatments with high heating rates (>500º/min) and conventional sintering (RH) approaches were experimentally compared for yttria-stabilized zirconia (3YSZ and 8YSZ) compacts. Experiments were carried out in a tube furnace with a heating rate of ~500 °C/min (FF) and 10 °C/min (RH), analyzing the continuous density change and the grain size distribution of the dense structure. RH-samples present grain size bigger by a factor of ~4 and ~2 in comparison to raw powder for 8YSZ and 3YSZ respectively. Conversely, FF method completely suppresses grain growth at the experimental conditions with a growth factor of ~1 for both materials. Those results indicate that high heat inputs minimize grain growth.

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Tese (Doutoramento)
IPEN/T
Instituto de Pesquisas Energeticas e Nucleares - IPEN-CNEN/SP

Dans ce travail, nous avons établi un protocole de fabrication industrielle pour réaliser des cellules de piles à combustible avec interfaces électrode/électrolyte architecturées, ou planes. Nous avons démontré que pour deux types d'échantillons, différents par les matériaux, la microstructure, le nombre de couches et l'emplacement de l'architecture, l'architecture de l'interface électrode/électrolyte entraîne une augmentation très significative des performances. Les mesures de polarisation et l'EIS sont utilisées pour étudier les performances électrochimiques des cellules, ainsi que pour comparer les cellules architecturées et les cellules planes. Nous isolons l'influence de l'architecture sur les spectres d'impédance globaux en utilisant une méthode de comparaison innovante basée sur l'étude des écarts relatifs des parties de résistance dépendantes de la fréquence. Ainsi, l'architecture a une influence favorable sur les performances électrochimiques en améliorant les capacités catalytiques des électrodes ainsi que le transfert de charges (et en particulier le transfert d'ions) dans la cellule. L'architecture induit une augmentation de 60 % de la densité de puissance maximale pour les cellules de Type I et de 75 % pour les cellules de Type II
In this work, we have established an industrial fabrication protocol for single fuel cells with either architectured or planar electrode/electrolyte interfaces. We have demonstrated that in two types of samples, differing in materials, microstructure, number of layers, and architecture location, the architecturation of the electrode/electrolyte interface results in a highly significant performance increase. Polarization measurements and EIS are used to study the electrochemical performances of the cells, to compare the architectured and planar ones. We isolate the influence of the architecturation on global impedance spectra by using an innovative comparison method based on the study of the relative gaps of the frequency-dependent resistance parts. Thus, the architecturation has a strongly favorable influence on the electrochemical performances by enhancing the catalytic capabilities of the electrodes as well as the charge transfer (and in particular the ion transfer) within the cell. The architecturation induces a 60 % increase of the maximum power density for the Type I cells and 75% for the Type II cells

In this work, a broad range of analytical methods was applied to the study of the following three materials systems: yttria-stabilised zirconia (YSZ), samarium-doped ceria (SDC) and SDC-supported metal catalysts. YSZ and SDC were studied in the light of their application as solid electrolytes in Solid Oxide Fuel Cells. The SDC-supported metal catalysts were evaluated for application in the reforming of methanol. The conductive properties of YSZ pellets derived from powders of different Y contents and particle size ranges were investigated using Impedance Spectroscopy (IS). Comparative studies of the crystallography (by X-ray Powder Diffraction (XRD)), morphology (by Scanning and Transmission Electron Microscopy (SEM, TEM)), chemical composition (by Energy Dispersive X-ray Spectroscopy (EDX) and Inductively Coupled Plasma Mass Spectroscopy (ICP-MS)) and sintering behaviour (dilatometry) were employed in the overall assessment of the conductivity results collected. Detailed studies of three SDC compositions were performed on nanopowders prepared by a low temperature method developed in the Baker group. Modifications led to a simple and reliable method for producing high quality materials with crystallites of ~10 nm diameter. The products were confirmed by XRD and TEM to be single-phase materials. Thermogravimetric analysis, dilatometry, specific surface area determination, elemental analysis and IS were carried out on these SDC powders. The relationships between particle size, chemical composition, sintering conditions and conductivity were studied in detail allowing optimum sintering conditions to be identified and ionic migration and defect association enthalpies to be calculated. Finally, the interesting results obtained for the SDC nanopowders were a driving force for the preparation of SDC-supported metal catalysts. These were prepared by three different methods and characterised in terms of crystallographic phase, specific surface area and bulk and surface chemical composition. Isothermal catalytic tests showed that all catalysts had some activity for the reforming of methanol and that some compositions showed both very high conversions and high selectivities to hydrogen. These catalysts are of interest for further study and possibly for commercial application.