A program of study has been established for the quantitative study of electrode reactions in solid oxide fuel cells. The initial focus of the program is the mixed conducting cathode material strontium-doped lanthanum manganate (LSM). A formalism was established treating reactions taking place at the gas-exposed surface of mixed conducting electrodes. This formalism was incorporated into a phenomenological model for oxygen reduction in LSM, which treats the phenomenon of sheet resistance. Patterned electrodes were designed that reduce the dimensionality of the appropriate model, and these electrodes were successfully fabricated using DC sputtering and photolithography. A new model for the bulk defect equilibrium in LSM was proposed and shown to be a better fit to nonstoichiometry data at low temperatures. The fitting was carried out with a particle swarm optimizer and a rigorous method for identification. It was shown that a model for the interface structure between LSM and yttria-stabilized zirconia (YSZ) that assumes free oxygen vacancies in YSZ does not accord with experimental observations. Cluster variation method (CVM) was adapted for analysis of the problem, and a new analytical method combining CVM and electrical contributions to the free energy was proposed.

Reproduction de : Thèse de doctorat : Science et génie des matériaux : Toulouse 3 : 2008. / Titre provenant de l'écran-titre. Bibliogr. à la fin des chapitres.

Thesis (Ph. D.)--State University of New York at Binghamton, Materials Science and Engineering Program, 2009. / Includes bibliographical references.

Not available / text

Les micro-plasmas constituent une technologie d'avenir pour des applications aussi nombreuses que diverses : dépollution, traitement de surface, applications bio-médicales, accélération aérodynamique... Nous avons étudié ces micro-plasmas dans des gaz inertes (hélium ou argon), en les alimentant en courant continu dans des structures de type micro-cathode creuse. Afin de comprendre les mécanismes physiques régissant leur comportement, nous les avons caractérisés par plusieurs diagnostics, notamment par caméra ICCD et par spectrométrie d'émission optique. Ce dernier diagnostic nous a permis de déterminer la température du gaz des micro-plasmas, par l'analyse de la structure rovibrationnelle des raies du second système positif de l'azote (présent à l'état de traces), mais aussi d'effectuer des mesures de densité électronique, en analysant l'élargissement Stark de la raie H béta. Ces paramètres physiques obtenus expérimentalement, ont été comparés à leurs équivalents obtenus par simulation (logiciel GdSIM du laboratoire Laplace). Ce travail de thèse a également permis de montrer la possibilité d'atteindre le régime luminescent anormal d'un micro-plasma, en réduisant l'aire de la surface cathodique extérieure de la micro-cathode creuse. Ce régime de fonctionnement s'accompagne d'une hausse rapide de la température du gaz, ainsi que d'un phénomène d'hystérésis qui apparaît sur une courbe I-V, pour une rampe d'alimentation en courant linéairement croissante puis décroissante. Dans le cas de plusieurs micro-plasmas fonctionnant en parallèle, nous avons mis à jour un nouveau mécanisme, expliquant l'allumage des cavités de proche en proche.

We have developed a thermionic cathode capable of high emission currents. The structure of this cathode is oxide coated carbon nanotubes (CNTs) on a tungsten (W) substrate. This cathode was superior in emission due to the combination of the field enhancement effect from the CNTs and the lowered work function from the semiconducting oxide surface. Such oxide coated CNTs were excellent electron emitters. Conventional electron emission theories, such as Richardson's and Fowler-Nordheim's, did not accurately describe the field enhanced thermionic emission from such emitters. A unified electron emission theory was adopted and numerical simulations were performed to explain the deviation of electron emission from conventional field and thermionic emission theories. Also, the thermionic measurement system and measurement methods were improved in order to measure and characterize the strong electron emission from this new cathode. Large electron emission current from such structures also made a new thermionic cooling device a possibility. Cooling due to the electron emission was measured in terms of temperature drop, and a large temperature drop was observed from this cathode structure. Finally, applications of this cathode in plasma discharge devices were explored. This new cathode was tested in a plasma environment and initial results were obtained. / Department of Physics and Astronomy

Proton exchange membrane (PEM) fuel cells are competitive with other emerging technologies that are being considered for automotive transportation. Commercialization of PEM fuel cells would decrease emissions of criteria pollutants and greenhouse gases and reduce US dependence on foreign oil. However, many challenges exist that prevent this technology from being realized, including power requirements, durability, on-board fuel storage, fuel distribution, and cost. This dissertation focuses on fuel-cell durability, or more specifically catalyst stability. New techniques to comprehensively observe and pin-point degradation mechanisms are needed to identify stable catalysts. In this text, an in operando method to measure changes in catalyst particle size at the cathode of a PEM fuel cell is demonstrated. The pair distribution function analysis of X-ray diffraction patterns, generated from an operating fuel cell exposed to accelerated degradation conditions, was used to observe the growth of catalyst particles. The stability of Pt/C and PtCo/C electrodes, with different initial particle sizes, was monitored over 3000 potential cycles. The increase in particle size was fit to a linear trend as a function of cycle number for symmetric linear sweeps of potential. The most stable electrocatalyst was found to be alloyed PtCo with a larger initial particle size. A better understanding of oxide growth kinetics and its role in platinum dissolution is needed to develop a comprehensive fuel-cell performance model. There is an ongoing debate present in the current literature regarding which oxide species are involved in the oxide growth mechanism. This dissertation discusses the results of in operando X-ray absorption spectroscopy studies, where it was found that PtO2 is present at longer hold times. A new method to quantify EXAFS data is presented, and the extent of oxidation is directly compared to electrochemical data. This comparison indicated that PtO2 was formed at the expense of an initial oxide species, and these steps were included in a proposed mechanism for platinum oxidation. Simulations of platinum oxidation in literature have yet to fully replicate an experimental cyclic voltammogram. A modified Butler-Volmer rate equation is presented in this thesis. The effect of including an extra parameter, χ, in the rate equations was explored. It was found that while the χ-parameter allowed the cathodic peak width to be decoupled from the Tafel slope for the platinum-oxide reduction, its inclusion could not address all observed experimental characteristics. Exploration of this concept concluded that current is not a function of only potential and coverage. To that end, a heterogeneous oxide layer was introduced. In this model, place-exchanged PtO2 structures of varying energy states are formed through a single transition state. This treatment allowed, for the first time, the simulation of the correct current-potential behavior under varying scan rates and upper potential limits. Particle size plays a critical role in catalysts stability. The properties of nanoparticles can differ significantly from bulk values, yet few tools exist to measure these values at the nanoscale. Surface stress and surface energy are diagnostic criterion that can be used to differentiate nano from bulk properties. The pair distribution function technique was used to measure lattice strain and particle size of platinum nanoparticles supported on carbon. The effect of adsorbates on surface stress was examined and compared to previous literature studies. Furthermore, a methodology for measuring the surface energy of supported platinum nanoparticles has been developed. While the results of this work are significant, many more challenges need to be addressed before fuel-cell vehicles are marketed. Recommendations for future work in the field of catalyst durability are addressed.

Solid oxide fuel cell (SOFC) research is currently underway to improve performance, cost and durability by lowering the operating temperature to ~600°C. One approach is to design fabrication processes capable of tailoring desirable cathode microstructures to enhance mass and charge transfer properties through the porous medium. The aim of this study is to develop a cost effective fabrication technique for deposition of novel microstructures, specifically, functionally graded thin films of LSM oxide with porosity graded structure for use as IT- SOFCs cathode. Spray pyrolysis method was chosen as a low-temperature processing technique for deposition of porous LSM films onto dense YSZ substrates. The effort was directed toward the optimization of the processing conditions for deposition of high quality LSM films with variety of morphologies in the range of dense to porous microstructures. Results of optimization studies on spray parameters revealed that the substrate surface temperature is the most critical parameter influencing the roughness and morphology, porosity, cracking and crystallinity of the film. Physical and chemical properties of deposited thin films such as porosity, morphology, phase crystallinity and compositional homogeneity have shown to be extensively dependent on the deposition temperature as well as solution flow rate and the type of precursor solution among other parameters. The LSM film prepared from organo-metallic precursor and organic solvent showed a homogeneous crack-free microstructure before and after heat treatment as opposed to aqueous solution. Also, increasing the deposition temperature and the solution flow rate, in the specific range of 520-580 ℃ and 0.73-1.58 ml/min, respectively, leads to change the microstructure from a dense to a highly porous film. Taking the advantage of simplicity of spray pyrolysis technique combined with using metal-organic compounds, the conventional ultrasonic spray system was modified to a novel system whereby highly crystalline multi-layered porosity graded LSM cathode with columnar morphology and good electrical conductivity in the range of 500-700 °C was fabricated through a multi-step spray and via applying optimum combination of spray parameters. This achievement for the current graded LSM cathode would allow its use in IT-SOFCs.

Thesis (Ph.D.)--University of Wollongong, 2006. / Typescript. Includes bibliographical references: leaf 188-189.

Thesis(M.S.)--Auburn University, 2006. / Abstract. Vita. Includes bibliographic references.