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Cette thèse vise à étudier la diffusion protonique, et dans une moindre mesure ionique, au sein d’un matériau électrolyte pour pile à combustible BaCeO3 dopé Gd, en adoptant une démarche multi-échelle. Tout d’abord, des calculs ab initio ont été réalisés afin de déterminer les positions stables des défauts protoniques OH_O et des lacunes d’oxygène VO dans le matériau. Puis, en utilisant toujours le formalisme de la théorie de la fonctionnelle de la densité, les barrières d’énergies pour les deux types de défauts entre deux positions stables ont été calculées. Enfin, ces barrières ont été utilisées dans un algorithme de Monte-Carlo cinétique afin de simuler des trajectoires de protons et de lacunes d’oxygène. Cette méthode permet d’accéder à des grandeurs macroscopiques, accessibles expérimentalement, telles que l’énergie d’activation, le coefficient de diffusion ou la mobilité, en se basant uniquement sur des données atomiques issues de simulations ab initio. Le gadolinium semble être un dopant intéressant pour le cérate de barium au vu de son faible pouvoir attractif sur le proton : il permet ainsi la création de nombreuses lacunes d’oxygène, qui pourront incorporer des molécules d’eau, sans toutefois piéger l’hydrogène. Ces deux conditions sont nécessaires pour obtenir un bon électrolyte pour les oxides solides conducteurs de protons. / This thesis deals with the study of protonic diffusion, and to a lesser extent ionic, inside a Gd-doped BaCeO3, a possible electrolyte for fuel cell, using a multi-scale approach. First of all, first principles calculations have been made to determine stable positions for protonic defects OH_O and oxygen vacancies V O in the material. Then, using the same formalism of density functional theory, energy barriers for both kinds of defects have been computed between two stable positions. Finally, these barrier heights have been used in a Kinetic Monte-Carlo algorithm to simulate trajectories of protons or oxygen vacancies. This method allows to access macroscopic values that can be found by experiments, such as activation energy, diffusion coefficient or mobility, using only atomic data coming from ab initio simulations. Gadolinium seems to be an interesting dopant in barium cerate considering its weak power of attraction on the proton: the introduction of gadolinium creates lots of oxygen vacancies that will be able to incorporate water molecules, without trapping the hydrogen. Both these conditions are necessary to get a good electrolyte for protonic ceramic fuel cell.

Cette thèse porte sur l'étude des effets quantiques en dynamique moléculaire (DM). La DM est une méthode numérique qui permet l'étude des propriétés de la matière condensée. Cependant, la méthode étant basée sur la mécanique classique, les effets quantiques associés à la dynamique des noyaux, tels que l'énergie de point zéro ou l'effet tunnel, ne sont pas pris en compte. Ces effets quantiques nucléaires peuvent cependant jouer un rôle majeur, en particulier aux basses températures et/ou dans les systèmes contenant des atomes légers comme l'hydrogène. La dynamique moléculaire par intégrales de chemins (PIMD) est souvent utilisée, dans ce cas, pour tenir compte de la nature quantique des noyaux. Cette approche fournit des résultats quantiques exacts, mais son coût en temps de calcul élevé limite son domaine d'application. La méthode du bain thermique quantique (QTB) a été proposée comme une alternative à la PIMD. L'approche QTB est particulièrement intéressante car son coût en temps de calcul est équivalent à celui de la DM standard permettant ainsi l'étude de systèmes complexes et de plus grande taille.La première partie de cette thèse est consacrée à l'étude de la méthode QTB. Nous avons étudié le comportement de la méthode sur différents systèmes modèles afin d'étudier ses limites. En particulier, le problème du "zero point energy leakage" est étudié en détail et nous montrons que l'augmentation du coefficient de friction du QTB permet de limiter ce problème. Nous avons également développé une combinaison de la méthode QTB avec la méthode PIMD. Cette méthode combinée QTB-PIMD permet de réduire le coût en temps de calcul des simulations PIMD standards.Dans une deuxième partie, nous avons utilisé ces méthodes pour étudier la conduction de l'hydrogène dans des matériaux pérovskites. Nous nous intéressons d'abord à l'impact des effets quantiques sur la diffusion de l'hydrogène dans BaZrO3, un matériau d'électrolyte potentiel pour piles à hydrogène. L'hydrogène étant l'élément le plus léger, un impact important des effets quantiques est attendu. Nous trouvons que les effets quantiques sont effectivement importants à basse température, mais leur impact sur la diffusion reste faible aux températures de fonctionnement typiques des piles à hydrogène. Enfin, nous avons étudié les mécanismes de diffusion de l'hydrogène dans GdBaCo2O5.5. Nous mettons en évidence une diffusion anisotrope dans ce matériau et deux mécanismes principaux de diffusion. / This thesis deals with the study of quantum effects in molecular dynamics (MD). MD is a powerful numerical method to investigate the properties of condensed matter systems. However, since the method is based on classical mechanics, quantum effects associated with the dynamics of the nuclei, such as zero-point energy or tunnelling, are not taken into account. These nuclear quantum effects can, however, play a major role in particular at low temperatures and/or in systems containing light atoms such as hydrogen. In these cases, a standard way to account for the quantum nature of the nuclei is to use path integral molecular dynamics (PIMD). This method provides exact quantum results however its high computational cost limits its range of applicability. The quantum thermal bath (QTB) method has been proposed as an alternative to PIMD. The QTB method is particularly appealing because of its computational cost that is equivalent to standard MD thus allowing to study large and complex systems.The first part of this thesis is devoted to the study of the QTB method. We have studied the behavior of the method in selected model systems in order to investigate its limitations. We have focused, in particular, on the zero-point energy leakage problem and found that increasing the friction coefficient of the QTB can significantly limit this problem. We also have developed another way to use the QTB method by combining it with PIMD. This combined QTB-PIMD method allows, in particular, to decrease the computational cost of standard PIMD simulations.In a second part, we have used these methods to study hydrogen conduction in perovskite materials. We have first investigated the impact of quantum effects on the diffusion of hydrogen in BaZrO3, a potential electrolyte material for hydrogen fuel cells. Since hydrogen is the lightest element, we expect quantum effects to have a significant impact on its dynamics. We find that quantum effects are indeed significant at low temperatures although their impact on the diffusion remains low at the typical working temperatures of hydrogen fuel cells. Finally, we have investigated the diffusion mechanisms of hydrogen in GdBaCo2O5.5. We evidence that the diffusion is anisotropic in this material and two main diffusion mechanisms.

Ce travail de thèse présente les résultats obtenus lors de l'étude des mécanismes élémentaires à l'origine de la forte conduction protonique mesurée dans le cas de clathrates hydrates d'acides forts. Une méthodologie combinant diffusion neutronique, résonance magnétique nucléaire et simulation de dynamique moléculaire "ab-initio" a permis de modéliser les différents processus dynamiques impliqués, se produisant sur des temps allant de la nanoseconde à la femtoseconde. Le modèle proposé explique la forte conduction de ces systèmes aqueux par la délocalisation à longue distance de leurs protons résultant d'un mécanisme de type Grotthuss gouverné par la relaxation des molécules aqueuses environnant les protons en excès. / This work shows the results obtain about the study of elementary mechanisms behind the high protonic conduction of strong acids clathrate hydrate. A method using quasiélastic neutron scattering and pulse field gradient NMR experiments both with DFT molecular dynamic simulations allowed to establish a model which describe the several dynamical processes involve occuring on timescales from nanosecond to femtosecond. The model deduced explain the high conduction property of ionic clathrate hydrate by a delocalization of their protons following a grotthuss type mecanism managed by the relaxation of water molecules around the excess protons.

L'objectif de la thèse est de fabriquer un conducteur protonique à base de phyllosilicates, fonctionnant à une température comprise entre 200°C et 300°C pour la production d'hydrogène d'une part, et l'hydrogénation du CO2 issu de la biomasse d'autre part. Le dépôt sera effectué sur un substrat en acier fritté, mis au point spécifiquement en collaboration avec le fournisseur.Afin d'appréhender les phénomènes régissant la croissance du dépôt et donc, pouvoir optimiser les performances du procédé, un suivi in-situ par spectroscopie d'impédance électrochimique sera effectué pour chaque essai. Ces analyses seront couplées avec d'autres analyses ex-situ (MEB,XPS,...). L'objectif étant de pouvoir contrôler minutieusement les caractéristiques de la couche formée, et reproduire rapidement le procédé à l'échelle industrielle.A la fin de la thèse, les résultats seront intégrés dans le programme SOLARVI, qui vise à stocker l'énergie issue de sources renouvelables pour la production d'hydrogène d'une part, et la transformation du CO2 issu de la biomasse en produits valorisables dans le domaine de l'énergie et dans la chaîne du carbone d'autre part. / This thesis aims at the elaboration of a phyllosilicate-based protonic conductor working at temperatures between 200°C and 300°C for hydrogen production and hydrogenation of CO2 coming from biomass. The deposit will be carried out on a sintered steel alloy, developed with the supplier.In order to understand the phenomena governing the growth of the deposit, an in-situ monitoring will be performed for each test, by electrochemical impedance spectroscopy. These analyses will be coupled with other ex-situ analyses (SEM, XPS...). That will allow us to obtain the good final properties of the protonic layer and quickly bring the process at industrial scale.At the end of the thesis, all results will be integrated within SOLARVI program, which aims at the energy storage coming from renewable sources, by hydrogen prduction and transformation of CO2 into products which could be valorized in energy and chemistry fields.

Compostos condutores protônicos de zirconato de bário dopado com ítrio, BaZr0,8Y0,2O3-δ, preparados por síntese de estado sólido, foram compactados e sinter izados com ZnO e B2O3 como aditivos. Os corpos cerâmicos sinter izados foram analisados por difração de raios X e espectroscopia de impedância. Superfícies polidas e atacadas termicamente foram observadas em microscópio de varredura por sonda. As medidas de densidade mostraram que a maior densificação foi obtida com óxido de zinco nas proporções de 2 e 5 peso%, atingindo aproximadamente 95% da densidade teórica. As medidas de resistividade elétrica evidenciaram a menor resistividade elétrica do composto cerâmico BaZr0,8Y0,2O3-δ, com 5 peso% de ZnO. Os aditivos de sinter ização, óxido de boro e óxido de zinco, foram eficientes para se obter compostos com menores valores de resistividade elétrica que os obtidos em compostos sinter izados sem aditivos. / BaZr0.8Y0.2O3-δ, protonic conductors, prepared by the ceramic route, were pressed and sintered with ZnO and B2O3 sinter ing aids. The sintered pellets were analyzed by X-ra y diffraction and impedance spectroscopy. Polished and thermally etched surfaces of the pellets were observed in a scanning probe microscope. The highest values of apparent densit y, 95%T.D., were obtained with 2 and 5 wt.% ZnO. The lowest value of electr ical resistivit y was obtained in BaZr0.8Y0.2O3-δ, compounds with 5 wt.% ZnO. Boron oxide and zinc oxide sinter ing aids were efficient to improve the apparent densit y as well as the electr ical conductivit y of BaZr0.8Y0.2O3-δ, protonic conductors.

Ce travail s’articule autour de la mesure de la résistance protonique des électrodes de PEMFC. Un modèle d’électrode volumique permet d’ajuster les spectres d’impédance électrochimique expérimentaux d’une cathode balayée à l’azote et d’estimer la résistance protonique de l’électrode ainsi que celle de la membrane, dans des conditions d’humidité contrôlées. De même, des mesures sont effectuées sur une cathode alimentée en oxygène pur produisant du courant et de l’eau. Elles permettent d’estimer les résistances protoniques de l’électrode et de la membrane dans différentes conditions de stoechiométrie et de courant. L’humidité effective dans la membrane, ainsi que dans l’électrode sont alors estimées à l’aide des corrélations obtenues en conditions d’humidité contrôlées. L’auto-humidification d’un assemblage membrane-électrode est suivie au moyen d’une cellule segmentée apportant à la mesure une résolution spatiale. L’humidité effective est plus élevée dans l’électrode que dans la membrane. Un deuxième volet concerne l’étude de la dégradation au moyen de tests de vieillissement accélérés. Une compaction de l’électrode due à la corrosion du support carboné est détectable par spectroscopie d’impédance lors des tests de démarrage. En revanche, aucune dégradation significative du ionomère dans l’électrode n’a pu être détectée au cours de ce travail / This work focuses on the development of electrochemical impedance spectroscopy (EIS) methods to measure the protonic resistance of PEMFC active layers. Experimental spectra of a cathode fed with nitrogen are fitted to a volumetric electrode impedance model to yield the protonic resistance of the electrode and that of the membrane in controlled humidity conditions. In addition, EIS measurements are performed on a cathode fed with oxygen, delivering current and producing water. The protonic resistances of the membrane and of the electrode are obtained in several conditions of gas stoichiometry and of current density. The effective humidity within the membrane and within the electrode is estimated using the calibration obtained in controlled humidity conditions. Thus, the monitoring of a MEA self humidification is achieved with spatial resolution using a segmented cell designed in our lab. The effective humidity is higher in the catalyst layer than in the membrane. A second part of this work is dedicated to the catalyst layer degradations. Accelerated stress tests consisting in a membrane chemical degradation protocol, a carbon degradation protocol, a start-up protocol and a dry operation regime are performed. A compaction of the electrode due to carbon corrosion is detected during start-up protocols. A strong chemical attack of the ionomer has been observed within the membrane, close to the cathode side. However, no degradation of the ionomer within the cathode has been measured

Submitted by Aelson Maciera (aelsoncm@terra.com.br) on 2017-06-05T20:20:51Z No. of bitstreams: 1 TeseELP.pdf: 16549429 bytes, checksum: 8564c0e7a4dbc3f95cb21b3270edf754 (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-06-06T18:43:55Z (GMT) No. of bitstreams: 1 TeseELP.pdf: 16549429 bytes, checksum: 8564c0e7a4dbc3f95cb21b3270edf754 (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-06-06T18:44:56Z (GMT) No. of bitstreams: 1 TeseELP.pdf: 16549429 bytes, checksum: 8564c0e7a4dbc3f95cb21b3270edf754 (MD5) / Made available in DSpace on 2017-06-08T19:45:20Z (GMT). No. of bitstreams: 1 TeseELP.pdf: 16549429 bytes, checksum: 8564c0e7a4dbc3f95cb21b3270edf754 (MD5) Previous issue date: 2016-12-19 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES) / The study of solid electrolytes (SE) is important to the scientific and technological development of materials used in application related to clean energy generation, such as Solid Oxide Fuel Cells (SOFCs). Yttrium-doped Barium Cerate is a SE with perovskite structure and great potential for this application because of its high values of protonic conductivity in temperatures between 350 and 600°C, which would allow the replacement of the currently used Zirconia based SE that operate above 800 °C, reducing the manufacturing cost of SOFCs. In the present work, eighteen compositions of BaCe1-xYxO3-δ system with x ranging from 0 to 0.2 were synthesized via modified citrate process and for half of the compositions an addition of 1 wt% ZnO as sintering aid was made. With the goal of establishing a correlation between structure, microstructure and electrical conductivity with the yttrium concentration and in the presence or absence of the sintering aid, all compositions were characterized with X-ray Powder Diffraction, Raman Spectroscopy, Scanning and Transmission Electron Microscopy (TEM), Energy-dispersive X-ray Spectroscopy (EDS) and Impedance Spectroscopy. Secondary phases were characterized using TEM, EDS and Electron Diffraction. All samples presented relative bulk density values above 95%. The bulk electrical conductivity is proportional to the yttrium concentration in the grain and, in general, the ZnO addition promoted grain growth, increasing its average size up to five times in some compositions. The ZnO acts mainly at the grain boundary region and it is effective as sintering aid only when the composition has some yttrium content. Among all synthesized compositions, BaCe0.8Y0.2O3-δ without ZnO addition showed the highest electrical conductivity value at 600°C (≈ 31.5 mS/cm). In the group of samples with ZnO, the highest values (close to 18.4 mS/cm), were obtained for compositions with Yttrium content above 14 at.%. / O estudo de eletrólitos sólidos (ES) é importante para o desenvolvimento científico e técnológico de materiais para aplicações relacionadas à geração de energia limpa, como por exemplo, células a combustível de óxido sólido (CaCOS). O cerato de bário dopado com ítrio é um ES com estrutura perovskita com grande potencial de aplicação por apresentar valores altos de condutividade protônica em temperatura entre 350 e 600 °C, o que possibilitaria substituir os atuais ES à base de Zircônia que operam acima de 800 °C, reduzindo o custo de fabricação das CaCOS. Neste trabalho, dezoito composições do sistema BaCe1-xYxO3-δ com x variando entre 0 e 0,2 foram sintetizadas via processo citrato modificado e para metade das composições, uma adição de 1% em massa de ZnO como aditivo de sinterização foi feita. Visando estabelecer uma correlação entre estrutura, microestrura e condutividade elétrica com a concentração de ítrio e a presença ou não de aditivo de sinterização, todas as composições foram caracterizadas por meio de difração de raios X, espectroscopia Raman, microscopia eletrônica de varredura e transmissão (MET), espectroscopia de raios X por dispersão em energia (EDS) e espectroscopia de impedância. Fases secundárias foram caracterizadas por MET, EDS e difração de elétrons. Todas as composições apresentaram valores de densidade relativa acima de 95%. A condutividade elétrica do grão é proporcional a concentração de ítrio na matriz e, no geral, a adição de ZnO favoreceu o crescimento de grão, aumentando o seu tamanho médio em até cinco vezes. O ZnO atua principalmente na região de contorno de grão e é eficiente como aditivo apenas na presença ítrio. Dentre as composições sintetizadas, a BaCe0,8Y0,2O3-δ sem ZnO apresentou o maior valor de condutividade a 600°C (≈ 31,5 mS/cm). No grupo das amostras com ZnO, os valores mais altos, próximos a 18,4 mS/cm, foram obtidos para composições com teor de Y acima de 14% at. / CNPq: 207073/2014-7 / CNPq: 160534/2012-7

Compostos condutores protônicos de zirconato de bário dopado com ítrio, BaZr0,8Y0,2O3-δ, preparados por síntese de estado sólido, foram compactados e sinter izados com ZnO e B2O3 como aditivos. Os corpos cerâmicos sinter izados foram analisados por difração de raios X e espectroscopia de impedância. Superfícies polidas e atacadas termicamente foram observadas em microscópio de varredura por sonda. As medidas de densidade mostraram que a maior densificação foi obtida com óxido de zinco nas proporções de 2 e 5 peso%, atingindo aproximadamente 95% da densidade teórica. As medidas de resistividade elétrica evidenciaram a menor resistividade elétrica do composto cerâmico BaZr0,8Y0,2O3-δ, com 5 peso% de ZnO. Os aditivos de sinter ização, óxido de boro e óxido de zinco, foram eficientes para se obter compostos com menores valores de resistividade elétrica que os obtidos em compostos sinter izados sem aditivos. / BaZr0.8Y0.2O3-δ, protonic conductors, prepared by the ceramic route, were pressed and sintered with ZnO and B2O3 sinter ing aids. The sintered pellets were analyzed by X-ra y diffraction and impedance spectroscopy. Polished and thermally etched surfaces of the pellets were observed in a scanning probe microscope. The highest values of apparent densit y, 95%T.D., were obtained with 2 and 5 wt.% ZnO. The lowest value of electr ical resistivit y was obtained in BaZr0.8Y0.2O3-δ, compounds with 5 wt.% ZnO. Boron oxide and zinc oxide sinter ing aids were efficient to improve the apparent densit y as well as the electr ical conductivit y of BaZr0.8Y0.2O3-δ, protonic conductors.

No description available.