Theoretical study of protonic conduction in Gd-doped BaCeO3 : an electrolyte for fuel cell

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

Conduction protonique au sein d'un électrolyte pour pile à combustible : BaCeO3 dopé Gd

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 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

Theoretical study of protonic conduction in Gd-doped BaCeO3 (an electrolyte for fuel cell)

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.CHATENAY MALABRY-Ecole centrale (920192301) / SudocSudocFranceF

Effects of biaxial strain on bulk 8\% yttria-stabilised zirconia ion conduction through molecular dynamics

Tensile strain is thought to give rise to enhanced conduction properties in ion conducting compounds. However, most experimental studies in the field involve simultaneous presence of interface structures and strain, thus complicating separation of the individual effects. Here, we present molecular dynamics calculations that clarify the influence of biaxial strain in bulk yttria-stabilised zirconia. Such a study mimics what may be experimentally observed in epitaxially deposited films. We show that, as expected, tensile strain leads to enhanced ion conduction properties. The maximum enhancement is observed for a 2-3\% tensile strain. We show that the increase of bulk diffusion is in part due to an opening of the Zr-Zr and Zr-Y distances induced by tensile strain, leading to a smaller oxygen migration energy. Above a 3\% tensile strain, the diffusion coefficient of oxygen is strongly reduced, reaching values even lower than without strain. This decrease is associated with important structural changes of the cation and oxygen network. Also, we show that the diffusion coefficient increases by less than a factor 2 at 833 K for the optimal strain value. This confirms that the great increase of conductivity observed in zirconia/strontium titanate multilayers was due either to an electron contribution from strontium titanate or to the presence of interfaces, but not to the direct influence of strain on the oxygen diffusion coefficient in zirconia. Copyright \textcopyright 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved

Simulations of REBaCo2_2O5.5_{5.5} (REGd, La, Y) cathode materials through energy minimisation and molecular dynamics

The GdBaCo2O5+x oxide has been presented as a promising cathode material for solid oxide fuel cells. It presents very high oxygen exchange and diffusion coefficients, two characteristics of utmost importance for an efficient cathode material. Yet the understanding at atomic scale of these two properties is rather limited. Here, we performed calculations to understand the influence of rare-earth nature in REBaCo2O5.5 (REGd, La, Y) on material stability and oxygen diffusion properties. Through energy minimisation, we determined the most energetically favourable distribution of A-site cations and oxygen vacancies. We also investigated with Molecular Dynamics simulations the mechanisms of oxygen diffusion in A-site ordered REBaCo2O5.5. The results confirm that oxygen vacancies essentially lie in the RE-plane and that diffusion is mainly two-dimensional with oxygen moving in the (a,b) plane while diffusion along the c axis is strongly hindered. Between 1300 and 1900K, the activation energy for oxygen diffusion lies in the range 0.69–0.83eV depending on the RE cation nature, values in good agreement with the experimental ones. We show that, in the double perovskite structure, the replacement of Gd by a larger rare-earth ion enhances oxygen diffusion properties but also reduces the stability of the double perovskite structure

Reply to the ‘Comment on “Proton transport in barium stannate: classical, semi-classical and quantum regime”’

International audienceWe respond to the erroneous criticisms about our modeling of proton transport in barium stannate [G. Geneste et al., Phys. Chem. Chem. Phys., 2015, 17, 19104]. In this previous work, we described, on the basis of density-functional calculations, proton transport in the classical and semi-classical regimes, and provided arguments in favor of an adiabatic picture for proton transfer at low temperature. We re-explain here our article (with more detail and precision), the content of which has been distorted in the Comment, and reiterate our arguments in this reply. We refute all criticisms. They are completely wrong in the context of our article. Even though a few of them are based on considerations probably true in some metals, they make no sense here since they do not correspond to the content of our work. It has not been understood in the Comment that two competitive configurations, associated with radically different transfer mechanisms, have been studied in our work. It has also not been understood in the Comment that the adiabatic regime described for transfer occurs in the protonic ground state, in a very-low barrier configuration with the protonic ground state energy larger than the barrier. Serious confusion has been made in the Comment with the case of H in metals like Nb or Ta, leading to the introduction of the notion of (protonic) “excited-state proton transfer”, relevant for H in some metals, but (i) that does not correspond to the (ground state) adiabatic transfers here described, and (ii) that does not correspond to what is commonly described as the “adiabatic limit for proton transfer” in the scientific literature. We emphasize, accordingly, the large differences between proton transfer in the present oxide and hydrogen jumps in metals like Nb or Ta, and the similarities between proton transfer in the present oxide and in acid–base solutions. We finally describe a scenario for proton transfer in the present oxide regardless of the temperature regime

Proton transport in barium stannate: classical, semi-classical and quantum regimes

International audienceDensity-functional theory calculations are performed to investigate proton transport in BaSnO 3. Structural optimizations in the stable and saddle point configurations for transfer (hopping) and reorientation allow description of the high-temperature classical and semi-classical regimes, in which diffusion occurs by over-barrier motion. At lower temperature (typically below 300 K), we describe the thermally-assisted quantum regime, in which protonic motion is of quantum nature and occurs in ''coincidence'' configurations favored by thermal fluctuations of the surrounding atoms. Both the non-adiabatic and the adiabatic limits are examined. In the adiabatic limit, the protonic energy landscape in the coincidence configuration is very flat. Path-integral molecular dynamics simulations of the proton in the coincidence potential reveal, in the transfer case, that the density of probability of H + has its maximum at the saddle point, because the zero-point energy exceeds the coincidence barrier. Arguments are given that support the adiabatic picture for the transfer mechanism. In the case of reorientation, the time scales for the existence of the coincidence and for protonic motion, as estimated from the time-energy uncertainty principle by using a simple one-dimensional model, are of the same order of magnitude, suggesting that the adiabatic limit is not reached. Protonic transfer and reorientation in this oxide are therefore governed by different mechanisms below room temperature