The Department of Energy and the Idaho National Laboratory are developing a Next Generation Nuclear Plant (NGNP) to serve as a demonstration of state-of-the-art nuclear technology. The purpose of the demonstration is two fold: 1) efficient low cost energy generation and 2) hydrogen production. Although a next generation plant could be developed as a single-purpose facility, early designs are expected to be dual-purpose. While hydrogen production and advanced energy cycles are still in their early stages of development, research towards coupling a high temperature reactor, electrical generation and hydrogen production is under way. Many aspects of the NGNP must be researched and developed to make recommendations on the final design of the plant. Parameters such as working conditions, cycle components, working fluids, and power conversion unit configurations must be understood. Three configurations of the power conversion unit were modeled using the process code HYSYS; a three-shaft design with 3 turbines and 4 compressors, a combined cycle with a Brayton top cycle and a Rankine bottoming cycle, and a reheated cycle with 3 stages of reheat were investigated. A high temperature steam electrolysis hydrogen production plant was coupled to the reactor and power conversion unit by means of an intermediate heat transport loop. Helium, CO2, and an 80% nitrogen, 20% helium mixture (by weight) were studied to determine the best working fluid in terms cycle efficiency and development cost. In each of these configurations the relative heat exchanger size and turbomachinery work were estimated for the different working fluids. Parametric studies away from the baseline values of the three-shaft and combined cycles were performed to determine the effect of varying conditions in the cycle. Recommendations on the optimal working fluid for each configuration were made. The helium working fluid produced the highest overall plant efficiency for the three-shaft and reheat cycle; however, the nitrogen-helium mixture produced similar efficiency with smaller component sizes. The CO2 working fluid is recommend in the combined cycle configuration.

Cette thèse valide le concept de matériaux céramiques conducteurs mixtes protoniques/électroniques et leur utilisation en tant qu.anode pour électrolyseur de la vapeur d.eau à température intermédiaire. Les matériaux développés sont des cobaltites d.alcalino-terreux et de terre-rares couramment utilisés pour leur forte conductivité électronique dans la gamme de températures 300-600°C. La stabilité de chaque matériau a été vérifiée pendant 350h sous air et sous vapeur d.eau. Après analyse de la compatibilité chimique avec le matériau d.électrolyte BaZr0.9Y0.1O3, huit compositions ont été sélectionnées représentatives de deux structures pérovskites, classique et lacunaire : BaCoO3, LaCoO3, Sr0.5La0.5CoO3, Ba0.5La0.5CoO3, GdBaCo2O5, NdBaCo2O5, SmBaCo2O5 et PrBaCo2O5. L.évolution thermique de la st.chiométrie en oxygène de chaque matériau a été déterminée en couplant le titrage par iodométrie et ATG sous air sec. Une série d.ATG sous air humidifié a permis de sélectionner la gamme de températures optimale dans laquelle l.incorporation des protons est possible et maximale. Les profils d.incorporation des protons ont été réalisés par SIMS microanalyse nucléaire en géométrie ERDA sur deux échantillons. Les coefficients de diffusion du deutérium ont pu être déterminés confirmant la mobilité des protons dans les cobaltites. Sous air humide, le composé lacunaire NdBaCo2O5 incorpore rapidement, un nombre significatif de protons qui se distribuent de façon homogène au c.ur de l.échantillon. L.optimisation microstructurale des anodes a permis d.obtenir à 450°C et 600°C des résistances totales de cellule symétrique très prometteuses. / This thesis validates the concept of mixed electron/proton ceramic conductors to be used as anode materials for intermediate temperature steam electrolyzer. The materials developed are based on cobaltites of alkaline-earth metals and rare earth elements commonly used for their high electronic conductivity in the temperature range of 300-600°C. The stability of each material has been assessed during 350h in air and moist air. After checking the chemical compatibility with the BaZr0.9Y0.1O3 electrolyte material, eight compositions have been selected: BaCoO3, LaCoO3, Sr0.5La0.5CoO3, Ba0.5La0.5CoO3, GdBaCo2O5, NdBaCo2O5, SmBaCo2O5 and PrBaCo2O5. The thermal evolution of the oxygen stoichiometry of each material was determined by coupling iodometric titration and TGA in dry air. TGA in moist air has allowed determining the optimum temperature range for which proton incorporation is possible and maximized. Proton incorporation profiles have been determined on two cobaltites using SIMS and nuclear microanalysis in the ERDA configuration. Deuterium diffusion coefficients have been determined confirming the proton mobility in these materials. Under moist air, NdBaCo2O5 is shown to incorporate rapidly a significant number of protons that spread homogeneously within the material bulk. Anode microstructure optimization has allowed reaching at 450°C and 600°C total resistance values on symmetrical cell highly promising.

The research presented is based on alternative anode materials for high temperature steam electrolysis. The key to commercially viable renewable energy economy is based on energy storage of intermittent sources. Hydrogen is the preferred form of energy storage for solid oxide electrolysis cells. However, conventional anode material lanthanum strontium manganite (LSM), suffers from poor ionic conductivity, thus prohibiting much of the bulk electrode from providing an enhanced electrochemical performance. This study explores the use of a double-layered perovskite system with mixed electronic and ionic conductivity for use as anode material. Specifically, the SmBa₁₋ₓSrₓCo₂O[sub](5+δ) system (SBSCO) is analyzed for characteristics that may enhance the performance and feasibility of SBSCO as an alternative anode material to LSM. Previous in-house work showed SmBa₀.₅Sr₀.₅Co₂O[sub](5+δ) had the lowest area specific resistance of any double- layered material reported. Here the system is further explored by studying the full range of compositions. From X-ray diffraction analysis, increased Sr substitution leads to a tetragonal phase change in SBSCO. High temperature x-ray diffraction of compositions showed thermal stability of structure. Magnetization measurements are reported for selected compositions. The stability of SBSCO was examined in CO₂ containing atmospheres. Despite containing alkaline earth metals, the system offers limited CO₂ tolerance. A set of thermodynamic parameters is presented based on CO₂ partial pressure and temperature. Model indicates SBSCO is a stable electrode material for both electrolysis and fuel cell modes. Compositions were tested for steam electrolysis performance with the use of YSZ electrolyte, and Ni-YSZ and La₀.₄Sr₀.₄Ni₀.₀₆Ti₀.₉₄O₂.₉₄ cathodes. SmBa₀.₃Sr₀.₇Co₂O[sub](5+δ) had the highest performance for compositions (0≤x≤1) based on I-V curves and impedance measurements. Stability tests were conducted in potentiostatic mode and no delamination was observed for SBSCO in microstructural analysis after testing. From these studies, SBSCO is demonstrated to be a suitable for application in electrolysis and an alternative for LSM as anode material.

The development of technologies that enable efficient and reliable energy inter-conversion and storage is of key importance for tempering the intermittent availability of renewable energy sources, and thus for developing an energy economy based on sustainable, clean energy production. Solid oxide electrolysis cells (SOECs) may be used to store excess electrical energy as hydrogen, while solid oxide fuel cells (SOFCs) could convert back hydrogen into electricity, thus balancing energy availability and demand. However, the current state-of-the-art hydrogen electrode used in both SOECs and SOFCs, the Ni-yttria-stabilised zirconia cermet (Ni-YSZ), is unreliable in conjunction with intermittent energy sources, in particular due to its innate redox instability. This thesis explores the fundamental properties of various inherently redox stable A-site deficient titanate perovskite systems (A1-αBO3, B = Ti), seeking to uncover the principles that enhance their properties so that they may be used to replace Ni-YSZ. In particular, this work demonstrates that the versatility of perovskites with respect to the introduction of lattice defects such as vacancies and cation substitutions enables considerable improvements in the extent of reduction, electronic conductivity and overall electrochemical activity. Most importantly, the defect chemistry context set by the presence of A-site vacancies was found to trigger the exsolution of electrocatalytically active nanoparticles from the parent perovskite, upon reduction. This is an entirely new phenomenon which was explored and exploited throughout this study to produce perovskite surfaces decorated with uniformly distributed catalytically active nanoparticles. As demonstrated in this study, the exsolution phenomenon excels in terms of producing nanoparticles with uniform size, distribution, diverse composition and ‘unconventional' surface anchorage. The resulting enhanced properties, and especially the exsolution phenomenon, contributed coherently towards improving the suitability of the perovskites developed here towards their application as hydrogen electrode materials. Consequently, when integrated into SOEC button cells as hydrogen electrodes, they exhibited a step-change increase in performance compared to other perovskites considered to date. Many of the principles and perovskite defect chemistry explored and exemplified in this study on perovskite titanates may be extended to other perovskites as well. In particular the advanced control and understanding achieved in this work over the exsolution phenomenon may inspire the formulation of new and sophisticated oxide materials with advanced functionality.

Afin d’améliorer les performances électrochimiques de cellules d’électrolyse de la vapeur d’eau à haute température (EVHT), de nouveaux matériaux d’électrode à oxygène de typeLn2NiO4+δ (Ln = La, Pr ou Nd), Pr4Ni3O10±δ et La0,6Sr0,4Fe0,8Co0,2O3-δ ont été étudiés. Ces composés ont été sélectionnés pour leurs propriétés de conduction mixte électronique et ionique. Après la caractérisation de leurs propriétés physico-chimiques, les matériaux ont été mis en forme au sein de demi-cellules symétriques, en intercalant une couche d’interface fine à base de cérine entre l’électrode et l’électrolyte de zircone yttriée. Cette architecture contribue à la diminution de la résistance de polarisation de l’électrode (RP <0,1 Ω.cm2 à 800°C) et de la surtension anodique. Un modèle électrochimique a été développé afin de décrire et d’analyser les courbes de polarisation expérimentales.L’électrode présentant les plus faibles surtensions, Pr2NiO4+δ, a été sélectionnée et caractérisée au sein de cellules complètes à cermet support. En fonctionnement EVHT à800°C, une densité de courant élevée a été obtenue, de l’ordre de i = -0,9 A.cm-2 pour une tension de cellule de 1,3V et un taux de conversion d’environ 60%. / In order to improve the electrochemical performances of cells for high temperature steam electrolysis (HTSE), innovative oxygen electrode materials have been studied. The compounds Ln2NiO4+δ (Ln = La, Pr or Nd), Pr4Ni3O10±δ and La0.6Sr0.4Fe0.8Co0.2O3-δ have been selected for their mixed electronic and ionic conductivity. First, their physical and chemical properties have been investigated. Then, the electrodes were shaped on symmetrical half cells,adding a thin ceria-based interlayer between the electrode and the yttria doped zirconia-based electrolyte. These architectured cells lead to low polarization resistances (RP< 0.1 Ω.cm2 at 800°C) as well as reduced anodic over potentials . An electrochemical model has been developed in order to describe and analyze the experimental polarization curves.The electrode with the lower overpotential, i.e. Pr2NiO4+δ, has been selected and characterized into complete cermet-supported cells. Under HTSE operation, at 800°C, a high current density was measured, close to i = -0.9 A.cm-2 for a cell voltage equals to 1.3 V, the conversion rate being about 60%.

L’électrolyse de la vapeur d’eau à haute température (EHT) est une voie permettant de produire de l’hydrogène d’une grande pureté et avec un fort rendement, ceci sans émission de CO2. Un des verrous actuels de cette technologie est la forte surtension associée à la réaction d’oxydation des ions O2- qui se déroule à l’électrode à oxygène (anode). L’objectif de ce travail était de concevoir de nouveaux matériaux d’anode possédant des propriétés de conductivité mixte (i.e. électronique et ionique), dans le but d’obtenir des surfaces de réaction plus importantes afin de diminuer cette surtension. A cet effet, une étude comparative a été réalisée sur huit oxydes (ferrites et nickelates de terres rares). Après synthèse et mise en forme, ces matériaux ont fait l’objet de caractérisations physico-chimiques puis électrochimiques en demi-cellules symétriques sous atmosphère unique afin de déterminer ceux présentant les meilleures propriétés sous courant nul et sous polarisation anodique. Quatre composés de structure dérivée de type K2NiF4 ont ainsi été sélectionnés pour être caractérisés de façon plus approfondie en cellules complètes à électrolyte support en conditions EHT (750 - 850°C). Il a ainsi été possible d’obtenir, pour une tension de cellule de 1,3 V une densité de courant de 0,9 A/cm² à 850°C, soit près de deux fois plus qu’avec une cellule identique comportant comme matériau d’anode un composite commercial optimisé à base de LaMnO3 substitué au strontium. / High temperature steam electrolysis (HTSE) is a way to produce hydrogen with a high purity, with noteworthy efficiency and without CO2 emission. Nowadays, a blocking point concerning this technology is the high overvoltage related to the oxidation of the O2- ions occurring at the oxygen electrode (anode). The aim of this work was to design new anode materials with mixed conducting properties (i.e. electronic and ionic), in order to obtain larger reaction areas and to lessen this overvoltage. In this aim, eight compounds (ferrites and rare earth nickelates) were investigated. After synthesis and shaping, these compounds were characterized using physical, chemical and electrochemical analyses in symmetrical half cells, under single atmosphere, in order to determine which ones have the best properties under zero current and under anodic polarization. Four compounds of structure derived from K2NiF4-type were then selected to be more accurately characterized in complete electrolyte supported cells, under HTSE conditions (750 - 850°C). It was then possible to obtain, for a 1.3 V cell voltage, a current density of 0.9 A/cm² at 850°C, which is nearly two times larger than the one obtained with a same cell including a commercial composite material based on strontium substituted LaMnO3 as anode.

L'Electrolyse de la Vapeur d'eau à Haute Température (EVHT) est l'un des procédésde production d'hydrogène les plus prometteurs. Dans l'optique d'une économie del'hydrogène produit par EVHT, de nombreux verrous restent à lever. L'un d'entre euxporte sur l'étanchéité. En effet, dans un EVHT, la gestion des gaz est primordiale. Ilfaut pouvoir gérer et prévoir dans le temps le comportement des joints afin d'éviter unedégradation des performances. Or, en EVHT, les températures de fonctionnement sontélevées (classiquement autour de 800 °C), des phénomènes de fluage ou de relaxationapparaissent, le différentiel de dilatation thermique entre les cellules électrochimiques encéramique et les interconnecteurs métalliques doit être pris en compte. Enfin, il convientde maintenir l'étanchéité de l'empilement à faible niveau d'effort pour ne pas risquerd'endommager la partie céramique.L'objet du travail de cette thèse démarre par un constat simple : nous ne disposons pasd'outils de prédimensionnement des joints à haute température permettant de prévoirun débit de fuite. Dès lors que l'on est amené à changer un paramètre de fonctionnement,comme la température, la pression, la stratégie de chargement, la géométrie ou la naturedu joint, une nouvelle expérience doit être menée.A partir d'essais d'étanchéité et de simulations numériques aux éléments finis, un modèleoriginal est proposé. Ce modèle qui a été validé en fonction de différents paramètresexpérimentaux permet d'estimer le débit de fuite associé à un joint en Fecralloy (Fe-CrAl) selon sa forme, ses conditions de serrage et du temps de maintien. Offrant ainsila possibilité de concevoir à moindre coût des joints spécifiques pour l'application visée. / High Temperature Steam Electrolysis (HTSE), is one of the most promising processfor hydrogen production. In a hydrogen economy produced via HTSE, many problemshave to be overcome. One of them is related to sealing. Actually, in a HTSE, gasmanagement is very important. The behavior of the seal has to be predicted in time toavoid a deterioration of the performances. But, in a HTSE, the fuctioning temperaturesare important (typically around 800 °C), creep or relaxation may occur, the differencebetween the thermal expansion of the ceramic cells and the metallic interconnectorsmust be taken into account. Finally, the sealing has to be maintain with low effortsprotect the ceramic.This thesis started from the noticing that we don't have any designing tool for hightemperature seals that may help to foresee a leak rate. Since we have to change anyexperimental parameter, such as the temperature, the pressure, the loading strategy, thegeometry or the material of the seal, another experiment has to be done.From sealing tests and finite element modelisation, an original model is presented.This model that has been validated for different experimental parameters allows toestimate the leak rate of a Fecralloy (FeCrAl) seal depending on its shape, the loadingconditions and tightening time. This may help to design specific low cost seals for thedesired applications.