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<p> This thesis demonstrates two concepts, one a viable fabrication process for an FeCr alloy supported solid oxide fuel cell (SOFC), and second, the use of CozMn04 (spinel)as the cathode material. Ni/YSZ and YSZ layers were used as anode and electrolyte respectively. The fabrication process consisted of tape casting of iron and chromium oxide powders for the support, dip coating of NiO-YSZ-Fe30 4-Crz03-C and YSZ as anode and electrolyte respectively, synthesis of CozMn04 from Co304 and MnOz as the , cathode material and finally screen printing of the CozMn04 cathode. The support, the anode, and the electrolyte were co-fired at 1350°C in air for 10 hours, then CozMn04 was screen printed and the cell was again fired at 1250°C for 4 hours in air. The complete cell was reduced in pure Hz at 950°C for 10 hours to convert the major part of support into Fe-Cr alloy, leaving approximately 20% unreduced FeCrz04. </p> <p> The fully fabricated cell was tested at 820°C using 7% Hz, 93% Nz as the fuel and air as the oxidant. The Co2MnO4 cathode which reduced to MnO + Co during the final processing stage was recovered in-situ at the start of the test. Pt mesh was used for current collection. The power density was in the range of 80-120 mW/cm2. </p> / Thesis / Master of Applied Science (MASc)

No / Traditionally materials with the fluorite and perovskite structures have dominated the research in the area of oxide ion/proton conducting solid electrolytes. In such cases, the key defects are oxide ion vacancies, and conduction proceeds via a simple vacancy hopping mechanism. In recent years, there has been growing interest in alternative structure types, many of which contain tetrahedral moieties. For these new systems, an understanding of the accommodation of defects and the nature of the conduction mechanism is important for the optimisation of their conductivities, as well as to help target related structures that may display high oxide ion/proton conduction. Computer modelling studies on a range of systems containing tetrahedral moieties are presented, including apatite-type La9.33+xGe6O26+3x/2, cuspidine-type La4Ga2-xTixO9+x/2 and La1-xBa1+xGaO4-x/2. The type of anion defect (vacancy or interstitial), their location and the factors affecting their incorporation are discussed. In addition, modelling data to help to understand their conduction mechanisms are presented, showing novel aspects including the important role of the tetrahedral moieties in the conduction processes.

No / Traditionally materials with the fluorite and perovskite structures have dominated the research in the area of oxide ion/proton conducting solid electrolytes. In such cases, the key defects are oxide ion vacancies, and conduction proceeds via a simple vacancy hopping mechanism. In recent years, there has been growing interest in alternative structure types, many of which contain tetrahedral moieties. For these new systems, an understanding of the accommodation of defects and the nature of the conduction mechanism is important for the optimisation of their conductivities, as well as to help target related structures that may display high oxide ion/proton conduction. Computer modelling studies on a range of systems containing tetrahedral moieties are presented, including apatite-type La9.33+xGe6O26+3x/2, cuspidine-type La4Ga2¿xTixO9+x/2 and La1¿xBa1+xGaO4¿x/2. The type of anion defect (vacancy or interstitial), their location and the factors affecting their incorporation are discussed. In addition, modelling data to help to understand their conduction mechanisms are presented, showing novel aspects including the important role of the tetrahedral moieties in the conduction processes

Hybrid solid oxide fuel cell / gas turbine (SOFC/GT) systems offer high efficiency power generation, but face numerous integration and operability challenges. This dissertation addresses the application of hardware-in-the-loop simulation (HILS) to explore the performance of a solid oxide fuel cell stack and gas turbine when combined into a hybrid system. Specifically, this project entailed developing and demonstrating a methodology for coupling a numerical SOFC subsystem model with a gas turbine that has been modified with supplemental process flow and control paths to mimic a hybrid system. This HILS approach was implemented with the U.S. Department of Energy Hybrid Performance Project (HyPer) located at the National Energy Technology Laboratory. By utilizing HILS the facility provides a cost effective and capable platform for characterizing the response of hybrid systems to dynamic variations in operating conditions. HILS of a hybrid system was accomplished by first interfacing a numerical model with operating gas turbine hardware. The real-time SOFC stack model responds to operating turbine flow conditions in order to predict the level of thermal effluent from the SOFC stack. This simulated level of heating then dynamically sets the turbine's "firing" rate to reflect the stack output heat rate. Second, a high-speed computer system with data acquisition capabilities was integrated with the existing controls and sensors of the turbine facility. In the future, this will allow for the utilization of high-fidelity fuel cell models that infer cell performance parameters while still computing the simulation in real-time. Once the integration of the numeric and the hardware simulation components was completed, HILS experiments were conducted to evaluate hybrid system performance. The testing identified non-intuitive transient responses arising from the large thermal capacitance of the stack that are inherent to hybrid systems. Furthermore, the tests demonstrated the capabilities of HILS as a research tool for investigating the dynamic behavior of SOFC/GT hybrid power generation systems.

Diffusive mass transport within porous SOFC components is explored using two modeling approaches that can better inform the SOFC electrode design process. These approaches include performance metrics for electrode cross-sectional design and a fractal approach for modeling mass transport within the pore structure of the electrode reaction zone. The performance metrics presented are based on existing analytical models for transport within SOFC electrodes. These metrics include a correction factor for button-cell partial pressure predictions and two forms of dimensionless reactant depletion current density. The performance impacts of multi-dimensional transport phenomena are addressed through the development of design maps that capture the trade-offs inherent in the reduction of mass transport losses within SOFC electrode cross-sections. As a complement to these bulk electrode models, a fractal model is presented for modeling diffusion within the electrochemically active region of an SOFC electrode. The porous electrode is separated into bulk and reaction zone regions, with the bulk electrode modeled in one-dimension based on the dusty-gas formalism. The reaction zone is modeled in detail with a two-dimensional finite element model using a regular Koch pore cross-section as a fractal template for the pore structure. Drawing on concepts from the analysis of porous catalysts, this model leads to a straightforward means of assessing the performance impacts of reaction zone microstructure. Together, the modeling approaches presented provide key insights into the impacts of bulk and microstructural geometry on the performance of porous SOFC components.

Mixed ionic electronic conductors (MIEC) have gained importance recently due to their roles in energy conversion devices such as solid oxide fuel cells (SOFC). Recent experimental data have shown that an increased vacancy concentration in a MIEC changes its elastic modulus and causes volumetric expansion. Since the MIEC in a device is constrained mechanically, the volumetric changes can induce substantial mechanical stresses. Such stresses not only lead to premature failure but can also alter the electrochemical performance of the device. In order to fully understand the interactions between stresses and defect transport a coupled theory is needed. This thesis develops a framework to study stress-defect transport interactions. The framework is based on a proper construction of the stress dependent electrochemical potential by introducing two material properties, namely the coefficient of chemical expansion (CCE) and the open system elastic constants (OSEC). The CCE characterizes the strains due to non-stoichiometry while the OSEC represents the stoichiometry dependent elastic stiffness. In this work these parameters are determined using atomistic simulations. The system of equations that govern the coupled electrochemical and mechanical fields is solved using a combination of numerical and analytical techniques. The developed solutions are analyzed to provide insights into the nature and the extent of the interactions. It is found that the non-stoichiometry-induced stress is in the same order of magnitude or even higher than that induced by thermal mismatch in a typical SOFC. In the vicinity of material flaws (cracks, voids, etc.), such stresses are further intensified which may cause fracture of the MIEC. In addition, non-stoichiometry-induced stresses can significantly alter the distribution of point defects, thus affecting the electrochemical performance of the ionic device. Furthermore, the non-stoichiometry induced stresses increases the thickness of the surface charge layer. The thermodynamic framework and the computational algorithms developed in this work provides effective methodologies and tools to analyze stress-defect transport interactions in ionic solids for designing and reliability analysis of ionic devices such as fuel cells, oxygen pumps, chemical sensors, etc.

In this thesis different materials have been developed to use them in electrochemical cells. The electrochemical cells studied can be divided into two material big groups: solids oxides and acid salts materials. In the first group, materials to use them in electrodes for fuel cells an electrolyzer based on oxygen ion conductor electrolytes were optimized. Pertaining to this group, the influence of doping the Ba0.5Sr0.5Co0.8Fe0.2O3-d perovskite with 3% of Y, Zr and Sc in B position (ABO3-d) was checked. That optimization could reduce the polarization resistance of electrodes and improve the stability with time. Additionally, the limiting mechanisms in the oxygen reduction reaction were determined, and the influence of CO2 containing atmospheres was checked. La2NiO4+d;, pertaining to the Ruddlesden-Popper serie, is a mixed conductor of electron and oxygen ions. This compound was doped in La position (with Nd and Pr) and in Ni position (with Co). The dopants introduced were able to produce structural change and improve the cell performance, reducing in more than one order of magnitude the La1.5Pr0.5Ni0.8Co0.2O4+d; polarization resistance respect to the reference material (La2NiO4+d). In addition, the properties of an electrode based on the pure electronic conductor, La0.8Sr0.2MnO3-d; (LSM), were optimized. The triple phase boundary was enlarged by the addition of a second phase with ionic conductivity. That strategy made possible to reduce the electrode polarization resistance. In order to improve the oxygen reduction reaction, the addition of different catalysts by infiltration was studied. The different infiltrated oxides changed the electrochemistry properties, being the praseodymium oxide the catalyst which made possible a reduction in two orders of magnitude the electrode polarization resistance respects to the composite without infiltration. Furthermore, the efficiency of the cell working in fuel cell and electrolyzer mode was improved. Concerning the materials selected to use as electrodes on proton conductor electrolytes, the efficiency of electrodes based on LSM was optimized by using a second phase with protonic conductivity (La5.5WO12-d) and varying the sintering temperature of the electrode. Finally, the catalytic activity of the cell was boosted by infiltrating samaria doped ceria nanoparticles, achieving higher power densities for the fuel cell. The materials pertaining to the Ruddlesden-Popper series and studied for ionic conductor electrolytes were also used for cathodes in proton conductor fuel cells. After checking the compatibility with the electrolyte material, the influence of different electrode sintering temperatures and air containing atmospheres (dry, H2O y D2O) on the cathode performance was studied. Finally, the electrochemical cells based on acid salts (CsH2PO4) were designed and optimized. In that way, different cell configurations were studied, enabling to obtain thin and dense electrolytes and active electrodes for the hydrogen reduction/oxidation reactions. The thickness of the electrolyte was reduced by using steel and nickel porous supports. Furthermore, an epoxy resin type was added to the electrolyte material to enhance the mechanical properties. The electrodes configuration was modified from pure electronic conductors to composite electrodes. Moreover, copper was selected as an alternative of the expensive platinum working at high operation pressures. The cells developed were able to work with high pressures and with high content of water steam in fuel cell and electrolyzer modes. / En la presente tesis doctoral se han desarrollado materiales para su uso en celdas electroquímicas. Las celdas electroquímicas estudiadas, se podrían separar en dos grandes grupos: materiales de óxido sólido y sales ácidas. En el primer grupo, se optimizaron materiales para su uso como electrodos en pilas de combustible y electrolizadores, basados en electrolitos con conducción puramente iónica. Dentro de este grupo, se comprobó la influencia de dopar la perovskita Ba0.5Sr0.5Co0.8Fe0.2O3-d, con un 3% de Y, Zr y Sc en la posición B (ABO3-d). Esta optimización llevó a la reducción de la resistencia de polarización así como a una mejora de la estabilidad con el tiempo. Así mismo, se determinaron los mecanismos limitantes en la reacción de reducción de oxígeno, y se comprobó la influencia de la presencia de CO2 en condiciones de operación. El La2NiO4+d perteneciente a la serie de Ruddlesden-Popper, es un conductor mixto de iones oxígeno y electrones. Éste, fue dopado tanto en la posición del La (con Nd y Pr) como en la posición del Ni (con Co). Los dopantes introducidos además de producir cambios estructurales, provocaron mejoras en el rendimiento de la celda, reduciendo para alguno de ellos, como el La1.5Pr0.5Ni0.8Co0.2O4+d, en casi un orden de magnitud la resistencia de polarización del electrodo de referencia (La2NiO4+d). De la misma manera, se optimizaron las propiedades del electrodo basado en el conductor electrónico puro La0.8Sr0.2MnO3-d (LSM). La adición de una segunda fase, con conductividad iónica, permitió aumentar los puntos triples (TPB) en los que la reacción de reducción de oxígeno tiene lugar y reducir la resistencia de polarización. Con el fin de mejorar la reacción de reducción de oxígeno, se estudió la adición de nanocatalizadores mediante la técnica de infiltración. Los diferentes óxidos infiltrados produjeron el cambio de las propiedades electroquímicas del electrodo, siendo el óxido de praseodimio el catalizador que consiguió disminuir en dos órdenes de magnitud la resistencia de polarización del composite no infiltrado. De la misma manera, la mejora de la eficiencia del electrodo infiltrado con Pr, mejoró los resultados de la celda electroquímica trabajando como pila (mayores densidades de potencia) y como electrolizador (menores voltajes). En lo que respecta a los materiales seleccionados para su uso como electrodos en electrolitos con conductividad protónica, se optimizó la eficiencia del cátodo basado en LSM, mediante el uso de una segunda fase conductora protónica (La5.5WO12-d) y variando la temperatura de sinterización del electrodo. Finalmente, se mejoró la actividad catalítica mediante la infiltración de nanopartículas de ceria dopada con samario, produciendo mayores densidades de corriente de la pila de combustible. Los materiales pertenecientes a la serie de Ruddlesden-Popper y usados para cátodos en pilas iónicas, fueron empleados también para cátodos en pilas protónicas. Después de comprobar que el material electrolítico (LWO) era compatible con los compuestos de la serie de Ruddlesden-Popper, se estudió la influencia de la temperatura de sinterización de los electrodos en el rendimiento, así como de la composición de la atmosfera de aire (seca, H2O y D2O). Finalmente, se diseñó y optimizó las celdas electroquímicas basadas en sales ácidas (CsH2PO4). En este sentido, se estudiaron diferentes configuraciones de celda, que permitieran obtener un electrolito denso con el menor espesor posible y unos electrodos activos a la reacción de reducción/oxidación de hidrógeno. Se consiguió reducir el espesor del electrolito soportando la celda en discos de acero y níquel porosos. Se añadió una resina tipo epoxi al material electrolítico para aumentar sus propiedades mecánicas. De la misma manera, se cambió la configuración de los electrodos pasando por conductores electrónicos puros a electrodos compuestos por conductores / En la present tesis doctoral es van desenvolupar materials per al seu ús en cel·les electroquímiques. Les cel·les electroquímiques estudiades poden ser dividides en dos grans grups: materials d'òxid sòlid i sals àcides. En el primer grup, es van optimitzar materials per al seu ús com a elèctrodes en piles de combustible i electrolitzadors, basats en electròlits amb conducció purament iònica. Dins d'este grup, es va comprovar la influència de dopar la perovskita Ba0.5Sr0.5Co0.8Fe0.2O3-d amb un 3% de Y, Zr i Sc en la posició B (ABO3-d;). Esta optimització va portar a la reducció de la resistència de polarització així com a una millora de l'estabilitat amb el temps. Així mateix, es van determinar els mecanismes limitants en la reacció de reducció d'oxigen, i es va comprovar la influència de la presència de CO2 en condicions d'operació. El La2NiO4+d pertanyent a la sèrie de Ruddlesden-Popper, és un conductor mixt d'ions oxigen i electrons. Este, va ser dopat tant en la posició del La (amb Nd i Pr) com en la posició del Ni (amb Co). Els dopants introduïts a més de produir canvis estructurals, van provocar millores en el rendiment de la cel·la, reduint per a algun d'ells, com el La1.5Pr0.5Ni0.8Co0.2O4+d, en quasi un ordre de magnitud la resistència de polarització de l'elèctrode de referència (La2NiO4+d). De la mateixa manera, es van optimitzar les propietats de l'elèctrode basat en el conductor electrònic pur La0.8Sr0.2MnO3-d (LSM). L'addició d'una segona fase, amb conductivitat iònica, va permetre augmentar els punts triples (TPB), en els que la reacció de reducció d'oxigen té lloc, i reduir la resistència de polarització. A fi de millorar la reacció de reducció d'oxigen, es va estudiar l'adició de nanocatalitzadors per mitjà de la tècnica d'infiltració. Els diferents òxids infiltrats van produir el canvi de les propietats electroquímiques de l'elèctrode, sent l'òxid de praseodimi el catalitzador que va aconseguir disminuir en dos ordres de magnitud la resistència de polarització del composite no infiltrat. De la mateixa manera, la millora de l'eficiència de l'elèctrode infiltrat amb Pr, va millorar els resultats de la cel·la electroquímica treballant com a pila (majors densitats de potència) i com a electrolitzador (menors voltatges). Pel que fa als materials seleccionats per al seu ús com a elèctrodes en electròlits amb conductivitat protònica, es va optimitzar l'eficiència del càtode basat en LSM, per mitjà de l'ús d'una segona fase conductora protònica (La5.5WO12-d;) i variant la temperatura de sinterització de l'elèctrode. Finalment, es va millorar l'activitat catalítica mitjançant la infiltració de nanopartícules de ceria dopada amb samari, produint majors densitats de corrent de la pila de combustible. Els materials pertanyents a la sèrie de Ruddlesden-Popper i usats per a càtodes en piles iòniques, van ser empleats també per a càtodes en piles protòniques. Després de comprovar que el material electrolític (LWO) era compatible amb els compostos de la sèrie de Ruddlesden-Popper, es va estudiar la influència de la temperatura de sinterització dels elèctrodes en el rendiment, així com de la composició de l'atmosfera d'aire (seca, H2O i D2O). Finalment, es van dissenyar i optimitzar les cel·les electroquímiques basades en sals àcides (CsH2PO4). En este sentit, es van estudiar diferents configuracions de cel·la, que permeteren obtindre un electròlit dens amb el menor espessor possible i uns elèctrodes actius a la reacció de reducció/oxidació d'hidrogen. Es va aconseguir reduir l'espessor de l'electròlit suportant la cel·la en discos d'acer i níquel porosos. Es va afegir una resina tipus epoxi al material electrolític per a augmentar les seues propietats mecàniques. De la mateixa manera, es va canviar la configuració dels elèctrodes passant per conductors electrònics purs a elèctrodes compostos per conductors protònics / Navarrete Algaba, L. (2017). New electrochemical cells for energy conversion and storage [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/78458 / TESIS

A reversible solid oxide fuel cell (RSOFC) system could buffer intermittent electrical generation, e.g. wind, wave power by storing electrical energy as hydrogen and heat. RSOFC were fabricated by thermoplastic extrusion of (La₀.₈Sr₀.₂)₀.₉₅MnO[subscript(3−δ)] (LSM) ceramic support tubes, which were microstructurally stable with 55% porosity at 1350°C. A composite oxygen electrode of LSM-YSZ was applied, providing a homogeneous substrate for a 20 μm - 30 μm thick YSZ electrolyte. A dip-coated 8YSZ slurry, and a painted commercial 3YSZ ink gave sintered densities of 90% and nearly 100% at 1350°C, respectively. A porous NiO/YSZ fuel electrode was also painted on. A Ag/Cu reactive air braze was unsuccessful at forming a void-free joint between the RSOFC and a 316 stainless steel gas delivery tube, as the braze did not penetrate the oxidation layer on the steel. Two alumina-based ceramic cements failed to fully seal the cell to an alumina gas delivery tube, due to thermal expansion coefficient mismatches and porosity after curing. Therefore, the maximum open circuit voltage (OCV) obtained during RSOFC testing was 0.8 V at 440°C. LSM-YSZ symmetrical cell performance measurements with oxygen pressure showed a diffusion polarisation, which was assigned to dissociative adsorption and surface diffusion of oxygen species. A collaborative RSOFC system software model showed ohmic and activation losses dominated the RSOFC, and diffusion losses were insignificant. Pressurisation from 1 to 70 bar increased the RSOFC Nernst voltage by 11% at 900°C, and reduced the entropy of the gases, reducing heat production and increasing electrical efficiency. A 500 kg Sn/Cu phase change heat store prevented the system overheating. Over a 16 h discharge-charge RSOFC cycle in the range 5 mol.% - 95 mol.% hydrogen in steam, at 20.4 A per cell or 3250 A m⁻², the electrical energy storage efficiency was 64.4%.

This study concerned the preparation, characterisation and evaluation of different proton conductors for the Reverse Water Gas Shift Reaction (RWGS) and their evaluation as electrolytes for Solid Oxide Fuel Cells (SOFC) under H₂ and O₂. Materials with both catalytic and conductive properties are of a great interest, as their use in electrocatalytical systems may be very important. Sr₃CaZr₀.₅Ta₁.₅O₈.₇₅ (SCZT), BaCe₀.₉Y₀.₁O₂.₉₅ (BCY10) and Ba₃Ca₁.₁₈Nb₁.₈₂O₈.₇₃ (BCN18), were the initial materials in this study. Thermogravimetric analysis under different atmospheres (5%H₂/Ar, Ar, 5%CO₂, etc), were performed on SCZT and BCN18; with both materials being shown to be stable under reducing and oxidising conditions. Conductivity measurements by two terminal a.c. impedance were also conducted on SCZT and BCN18 under oxidising and reducing atmospheres. As found in literature, BCN18 showed mixed conductivity; with electronic conductivity at high temperatures and pure ionic conductivity below 550ºC, This behaviour was shown in chapter 3, where the change on conduction process was observed upon different gas feeds. Its maximum conductivity values for the different atmospheres were: 8.50x10⁻⁵ S/cm (Dry 5%H₂/Ar at 200ºC), 4.24x10⁻⁴ S/cm (Wet 5%H₂/Ar at 500ºC) and 4.48x10⁻³ S/cm (Air at 900ºC). SCZT proton conducting behaviour was also measured (wet and dry 5%H₂/Ar). Exhibiting an order of magnitude higher in total conductivity upon hydration of the gas feed (σdry=1.01x10⁻⁶ and σwet=1.07x10⁻⁵ at 450ºC). The doping of barium cerate with Zr and Zn by Tao and Irvine, lead to a more stable and easily sinterable BaCe₀.₅Zr₀.₃Y₀.₁₆Zn₀.₀₄O₃ (BCZYZ). Following this work, the introduction of ZnO as a sintering aid to SCZT and BCN18 resulted in Sr₃CaZr₀.₄Ta₁.₅Zn₀.₁O₈.₇₅ (SCZTZ), and Ba₃(Ca₁.₁₈Nb₁.₇₀Zn₀.₁₂)O₈.₅₅ (BCNZ); with higher final densities (above 90% dense). As with BCN18, BCNZ also exhibited mixed conductivity; with higher total conductivity values than BCN18. A maximum of total conductivity of 1.85x10⁻³ S/cm at 900ºC for BCNZ was measured against 6.99x10⁻⁴ S/cm at 900ºC for BCN18. A change in conductivity process was observed when using air or wet 5%H₂/Ar, achieving a maximum of 3.85x10⁻⁴ S/cm at 400ºC when under wet hydrogen. All materials (as powders) have shown catalytic activity for the reverse water gas shift (RWGS) reaction, with the lowest conversion temperature onset at 400ºC for SCZT and a maximum conversion of CO₂ to CO of 42%, with circa 0.52 and 0.59 mmol/s.m² of CO produced at 900ºC by BCN18 and BCZYZ, respectively. No relation between mechanisms for the RWGS and σ for these materials were expected below 10% conversion, as no correlation was found between their activation energies. BCY10 as shown a partial decomposition when exposed to the RWGS reaction, for what BCZYZ After fuel cell testing under H₂ and O₂ both SCZTZ and BCNZ showed mixed conductivity. SCZTZ under different hydrogen partial pressures, exhibited a behaviour close to a pure proton conductor, however, when exposed to both reducing and oxidising conditions, its behaviour did not follow the theoretical values. On the other hand, BCNZ behaves as a pure ionic conductor below 500ºC; with increasing influence of the electronic conductivity on temperature increase. However, as seen for BCNZ conductivity data from 2 terminal a.c. impedance, below 650ºC wet 5%H₂ exhibited the highest conductivity values. This, in additions to the pure ionic conductive behaviour below 400ºC (from the effective ionic transport number), suggests that BCNZ becomes closer to a pure proton conductor with temperature decrease.