This PhD thesis presents investigations of perovskite-related compounds in systems of interest for applications in components in solid oxide fuel cells. The compound compositions derive from substitutions in the parent compounds LaCoO3, LaCrO3 and SrFeO3. Novel phases La2Co1+z(MgxTi1-x)1-zO6 were synthesized and investigated with regard to structure, thermal expansion, electronic and magnetic properties. The study focused on the composition lines La2Co(MgxTi1-x)O6 (z=0), where the oxidation state of Co nominally changes from +2 (x=0.0) to +3 (x=0.5), and La2Co1+z(Mg0.5Ti0.5)1-zO6, with a varying fraction of Co3+ ions. XANES data show that the Co ions in the system have discrete oxidation states of +2 and +3. The TEC increases with increasing x due to an increasing contribution from spin state transitions of the Co3+ ions. Novel compounds La2Cr(M2/3Nb1/3)O6 with M=Mg, Ni, Cu were synthesized and characterized with respect to structure and magnetic properties. XRPD and NPD data indicate Pbnm symmetry; however, SAED patterns and HREM images indicate a P21/n symmetry for M=Mg, and Cu. The magnetic measurements results were rationalized using the Goodenough-Kanamori rules. Oxygen-deficient phases with x≥0.63 in SrxY1-xFeO3-δ and Sr0.75Y0.25Fe1-yMyO3-δ (M=Cr, Mn, Ni and y=0.2, 0.33, 0.5), were synthesized and characterized with respect to structure, oxygen content, thermogravimetry, TEC, conductivity and magnetic properties. Powder patterns of phases agree with cubic  perovskite structures. NPD data for x=0.75 reveal anisotropic displacement for the O atom, related to local effects from Fe3+/Fe4+ ions. SAED patterns for x=0.75 reveal the presence of an incommensurate modulation. The compounds start to lose oxygen in air at ~ 400°C. The TEC up to ~400°C for x=0.75 is ~10.5 ppm/K and increase to ~17.5 ppm/K at higher temperatures. The conductivity for x=0.91 is 164 S/cm at 400°C. Partial substitution of Fe by Cr, Mn or Ni does not increase the conductivity or decrease TEC.

A novel 2D hybrid with MoS₂ nanocrystals strongly coupled on nitrogen-enriched graphene (MoS₂/NGg-C₃N₄) is realized by mild temperature pyrolysis (550 °C) of a self-assembled precursor (MoS₃/g-C₃N₄–H⁺/GO). With rich active sites, the boosted electronic conductivity and the coupled structure, MoS₂/NGg₋C₃N₄ achieves superior lithium storage performance.

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In this thesis, the focus is on studying BaZrO3-BaCeO3 based proton conductors due to that they represent very promising proton conductors to be used for Intermediate Temperature Solid Oxide Fuel Cells (ITSOFCs). Here, dense BaZr0.5Ce0.3Y0.2O3-δ (BZCY532) ceramics were selected as the major studied materials. These ceramics were prepared by different sintering methods and doping strategies. Based on achieved results, the thesis work can simply be divided into the following parts: 1) An improved synthesis method, which included a water-based milling procedure followed by a freeze-drying post-processing, was presented. A lowered calcination and sintering temperature for a Hf0.7Y0.3O2-δ (YSH) compound was achieved. The value of the relative density in this work was higher than previously reported data. It is also concluded that this improved method can be used for mass-production of ceramics. 2) As the solid-state reactive sintering (SSRS) represent a cost-effective sintering method, the sintering behaviors of proton conductors BaZrxCe0.8-xLn0.2O3-δ (x = 0.8, 0.5, 0.1; Ln = Y, Sm, Gd, Dy) during the SSRS process were investigated. According to the obtained results, it was found that the sintering temperature will decrease, when the Ce content increases from 0 (BZCLn802) to 0.3 (BZCLn532) and 0.7 (BZCLn172). Moreover, the radii of the dopant ions similar to the radii of Zr4+ or Ce4+ ions show a better sinterability. This means that it is possible to obtain dense ceramics at a lower temperature. Moreover, the conductivities of dense BZCLn532 ceramics were determined. The conductivity data indicate that dense BZCY532 ceramics are good candidates as either oxygen ion conductors or proton conductors used for ITSOFCs. 3) The effect of NiO on the sintering behaviors, morphologies and conductivities of BZCY532 based electrolytes were systematically investigated. According to the achieved results, it can be concluded that the dense BZCY532B ceramics (NiO was added during ball-milling before a powder mixture calcination) show an enhanced oxygen and proton conductivity. Also, that BZCY532A (NiO was added after a powder mixture calcination) and BZCY532N (No NiO was added in the whole preparation procedures) showed lower values. In addition, dense BZCY532B and BZCY532N ceramics showed only small electronic conductivities, when the testing temperature was lower than 800 ℃. However, the BZCY532A ceramics revealed an obvious electronic conduction, when they were tested in the range of 600 ℃ to 800 ℃. Therefore, it is preferable to add the NiO powder during the BZCY532 powder preparation, which can lower the sintering temperature and also increase the conductivity. 4) Dense BZCY532 ceramics were successfully prepared by using the Spark Plasma Sintering (SPS) method at a temperature of 1350 ℃ with a holding time of 5 min. It was found that a lower sintering temperature (&lt; 1400 ℃) and a very fast cooling rate (&gt; 200 ℃/min) are two key parameters to prepare dense BZCY532 ceramics. These results confirm that the SPS technique represents a feasible and cost-effective sintering method to prepare dense Ce-containing BaZrO3-BaCeO3 based proton conductors. 5) Finally, a preliminary study for preparation of Ce0.8Sm0.2O2-δ (SDC) and BZCY532 basedcomposite electrolytes was carried out. The novel SDC-BZCY532 based composite electrolytes were prepared by using the powder mixing and co-sintering method. The sintering behaviors, morphologies and ionic conductivities of the composite electrolytes were investigated. The obtained results show that the composite electrolyte with a composition of 60SDC-40BZCY532 has the highest conductivity. In contrast, the composite electrolyte with a composition of 40SDC-60BZCY532 shows the lowest conductivity. In summary, the results show that BaZrO3-BaCeO3 based proton-conducting ceramic materials represent very promising materials for future ITSOFCs electrolyte applications. / <p>QC 20150423</p>

Ces travaux de thèse ont pour vocation d'étudier des oxydes mixtes Ce1-x-yZrxPryO2-z afin de corréler composition chimique-structure avec les propriétés de réductibilité/mobilité ionique de l'oxygène associées aux performances catalytiques pour l'oxydation du CO et du propane, en conditions essence et Diesel. La connaissance de la quantité de terres rares au degré d'oxydation +4 a été déterminante. Notamment, plus le taux de Pr4+ est important, plus le composé est réductible et plus la mobilité de l'oxygène s'accroît. L'évaluation du coefficient de diffusion de l'oxygène D* a montré que sa valeur devenait élevée, rivalisant ainsi avec les meilleurs conducteurs ioniques de l'oxygène à basse température. Les échanges isotopiques de l'oxygène permettent de montrer que la vitesse d'échange croît quand le taux de Pr augmente. L'étude de la combustion du propane sur ces oxydes a montré qu'une bonne mobilité de l'oxygène permettait d'améliorer les performances catalytiques. Ainsi, l'oxyde de composition Ce0,45Zr0,1Pr0,45O2-x (CZP45) présente les meilleures propriétés pré-citées et l'activité catalytique la plus élevée. L'oxyde CZP45 a donc été imprégné de Pd afin d'être comparé à deux catalyseurs utilisés industriellement et à iso-teneur en Pd, dispersée sur ?-Al2O3 (Diesel) et Ce0,5Zr0,5O2-x (essence). En régime Diesel, Pd-CZP45, après une étape initiale de réduction, présente une meilleure activité catalytique à basse température que Pd-?-Al2O3 pour l'oxydation du propane. Nous avons expliqué ces excellentes performances par la présence de clusters de Pd2+/Pd° finement dispersés sur l'oxyde (CZP45) et par la capacité du catalyseur à adsorber et conduire l'oxygène / This work describes characterizations of Ce1-x-yZrxPryO2-z mixed oxides to link their chemical composition-structure with their reducibility/oxygen mobility. These latter properties were correlated with their catalytic activity for the oxidation of CO and propane, both in stoichiometric (gasoline) and lean-burn (Diesel) conditions. The determination of the quantity of rare earth at the +4 oxidation degree, especially for Pr4+, has been of paramount importance. Actually, the higher the Pr4+ rate, the more reducible is the oxide and the better is the oxygen mobility. The evaluation of the diffusion coefficient of the oxygen D* highlighted high values, similar to those of the best reported oxygen ionic conductors at low temperature. Isotopic exchanges showed that the oxygen exchange rate increases with the Pr loading in the oxide. The catalytic activity for propane combustion of these oxides increases with their oxygen mobility. Finally, CZP45 (Ce0,45Zr0,1Pr0,45O2-x) exhibits the best pre-quoted properties as well as the highest catalytic activity. Therefore, CZP45 has been impregnated with Pd. The activity of Pd-CZP45 catalyst was compared with two industrially catalysts containing the same Pd content supported either on ?-Al2O3 (Diesel) or Ce0,5Zr0,5O2-x (gasoline). In Diesel conditions, Pd-CZP45 shows a better low-temperature catalytic activity than Pd-?-Al2O3 for propane oxidation after an initial step of reduction. We have attributed these remarkable performances to the presence of Pd2+/Pd° clusters finely dispersed on the oxide CZP45 and to its ability to adsorb and conduct oxygen

Perovskite based materials have great potentials for various energy applications and the search for new materials for uses in SOFCs has largely been concentrated to this class of compounds. In this search, we have studied perovskite phases in the system La2Co1+z(Ti1-xMgx)1-zO6, with 0  x 0.9 and z = 0.0, 0.2, 0.4, 0.6. Crystal structures were characterized by XRD and, for selected compositions, also by NPD and SAED. They exhibit with increasing x, as well as increasing z, a progressive increase in symmetry from monoclinic to orthorhombic to rhombohedral. The main focus in this work has been on the investigation of structure-property relations for compositions with 0.0 x 0.5 and z = 0. The nominal oxidation state of Co increases for these with increasing x, from Co2+ for x = 0 to Co3+ for x = 0.5. Magnetic measurements and XANES studies showed that the average spin state of Co changes linearly with increasing x, up to x = 0.5, in accordance with varying proportions of Co with two fixed oxidation states, i.e. Co2+ and Co3+. The data suggests that the Co3+ ions have an IS spin state or a mixture of LS and HS spin states for all compositions with nominally only Co2+ and Co3+ ions, possibly with the exception of the composition with x = 0.1, 0.2 and z = 0, for which the data indicate that the spin state might be HS. The XANES data indicate furthermore that for the perovskite phases with z = 0 and x &gt; 0.5, which in the absence of O atom vacancies contain formally Co4+, the highest oxidation state of Co is Co3+, implying that the substitution of Ti4+ by Mg2+ for x ³ 0.5 effects an oxidation of O2- ions rather than an oxidation of Co3+ ions. The thermal expansion was found to increase nearly linearly with increasing oxidation state of Co. This agrees well with findings in previous studies and is attributable to an increase in the ionic radius of Co3+ ions with increasing temperature, due to a thermal excitation from a LS to IS or LS/HS spin states. High temperature electronic conductivity measurements indicate that the electronic conductivity increases with an increase of both relative and absolute amount of Co3+. The latter can be attributed to an increase in the number of Co-O-Co connections. Additional high temperature magnetic measurements for selected samples, whose susceptibilities did not follow a Curie law behaviour up to room temperature, showed effective magnetic moments that did approach plateaus even at high temperatures (900 K). Interpretations of these data are, however, hindered by the samples losing oxygen during the applied heating-cooling cycle. The present study has shown that the investigated system is suitable for further studies, of more fundamental character, which could provide further insight of the structure-property relationships that depend on the oxidation state of Co. / Studies of cobalt based perovskites for cathode materials in solid oxide fuel cells.