Designing Optimal Perovskite Structure for High Ionic Conduction.

Solid-oxide fuel/electrolyzer cells are limited by a dearth of electrolyte materials with low ohmic loss and an incomplete understanding of the structure-property relationships that would enable the rational design of better materials. Here, using epitaxial thin-film growth, synchrotron radiation, impedance spectroscopy, and density-functional theory, the impact of structural parameters (i.e., unit-cell volume and octahedral rotations) on ionic conductivity is delineated in La0.9 Sr0.1 Ga0.95 Mg0.05 O3- δ . As compared to the zero-strain state, compressive strain reduces the unit-cell volume while maintaining large octahedral rotations, resulting in a strong reduction of ionic conductivity, while tensile strain increases the unit-cell volume while quenching octahedral rotations, resulting in a negligible effect on the ionic conductivity. Calculations reveal that larger unit-cell volumes and octahedral rotations decrease migration barriers and create low-energy migration pathways, respectively. The desired combination of large unit-cell volume and octahedral rotations is normally contraindicated, but through the creation of superlattice structures both expanded unit-cell volume and large octahedral rotations are experimentally realized, which result in an enhancement of the ionic conductivity. All told, the potential to tune ionic conductivity with structure alone by a factor of ≈2.5 at around 600 °C is observed, which sheds new light on the rational design of ion-conducting perovskite electrolytes

Oxygen transport porperties in the Ba2Co9O14 and Ca3Co4O9+δ cobaltites : contribution of SIMS and LEIS

Ce travail porte principalement sur la caractérisation des propriétés de transport de l'oxygène dans deux cobaltites, Ba2Co9O14 et Ca3Co4O9+δ, matériaux prometteurs comme cathode de pile à combustible à oxyde solide ou anode d’électrolyseur haute-température. Une grosse partie du travail a concerné la mise en place de la mesure de profils de diffusion de l'oxygène par échange isotopique et analyse SIMS. L'étude a ainsi démontré que ces deux matériaux sont des conducteurs mixtes ionique/électronique.Alors que les paramètres de transport mesurés sur Ba2Co9O14sont relativement faibles, les phases dérivées de Ca3Co4O9+δ présentent des coefficients d'échange en surface du même ordre de grandeur que ceux des matériaux de cathode les plus performants aujourd'hui. La structure de Ca3Co4O9+δ est constituée de l'alternance de couches Ca2CoO3-δ de type NaCl et de couches hexagonales CoO2. L'étude de céramiques texturées a démontré une diffusion facilitée parallèlement aux couches, probablement au sein des couches de type NaCl, lacunaires en oxygène. Par ailleurs, les premières mesures par LEIS ont montré la présence préférentielle de calcium à l'extrême surface du matériau.The main objective of this work was the characterization of the oxygen transport properties of oxygen in two cobaltite materials, Ba2Co9O14 et Ca3Co4O9+δ, promising as SOFC cathode or SOEC anode. A significant part of this work was devoted to the set-up of oxygen diffusion profiles measurement by combining isotopic exchange and SIMS analysis. It has been demonstrated that these ceramics are mixed ionic–electronic conducting (MIEC) materials. Even though Ba2Co9O14's oxygen transport coefficients are relatively low, Ca3Co4O9+δ and derivatives show surface exchange coefficients close to those encountered in the today's most promising cathode materials.Ca3Co4O9+δ is built upon the stacking of Ca2CoO3-δ rock-salt layers and CoO2 hexagonal layers. The study of textured ceramics showed a preferential diffusion along the layers, probably inside the rock-salt layers which contain oxygen vacancies. In addition, first LEIS measurements showed that the uppermost atomic layer of the structure is mainly made up of calcium atoms

Microstructural Stability of Tailored CaMn0.875−xFexTi0.125O3−δ Perovskite Oxygen Carrier Materials for Chemical Looping Combustion

acceptedVersio

Manufacturing of perovskite oxygen carriers by spray granulation for chemical looping combustion

acceptedVersio

Structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1−x−yTixFeyO3−δ

Calcium manganite-based perovskite-type oxides hold promise for application in chemical looping combustion processes and oxygen transport membranes. In this study, we have investigated the structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1−x−yTixFeyO3−δ. Distinct from previous work, data of high-temperature X-ray diffraction (HT-XRD) in the temperature range 600–1000 °C (with intervals of 25 °C) demonstrates that CaMnO3−δ (CM) transforms from orthorhombic to a mixture of orthorhombic and tetragonal phases between 875 °C and 900 °C. Rietveld refinements show the formation of a pure tetragonal phase at 975 °C and of a pure cubic phase at 1000 °C. Partial substitution of manganese by iron and/or titanium to yield CaMn0.875Ti0.125O3−δ (CMT), CaMn0.85Fe0.15O3−δ (CMF) or CaMn0.725Ti0.125Fe0.15O3−δ (CMTF) leads to different phase behaviours. While CMT remains orthorhombic up to the highest temperature covered by the HT-XRD experiments, CMF and CMTF undergo an orthorhombic → tetragonal → cubic sequence of phase transitions. Electrical conductivity relaxation measurements are conducted to determine the chemical diffusion coefficient (Dchem) and the surface exchange coefficient (kchem) of the materials. The results demonstrate that oxygen transport is hindered in the tetragonal phase, when occurring, which is attributed to a possible ordering of oxygen vacancies. The small polaron electrical conductivity of CM in the cited temperature range is lowered upon partial manganese substitution, by about 10% for CMF and up to half an order of magnitude for CMT and CMTF

Structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1−x−yTixFeyO3−δ

Calcium manganite-based perovskite-type oxides hold promise for application in chemical looping combustion processes and oxygen transport membranes. In this study, we have investigated the structure, electrical conductivity and oxygen transport properties of perovskite-type oxides CaMn1−x−yTixFeyO3−δ. Distinct from previous work, data of high-temperature X-ray diffraction (HT-XRD) in the temperature range 600–1000 °C (with intervals of 25 °C) demonstrates that CaMnO3−δ (CM) transforms from orthorhombic to a mixture of orthorhombic and tetragonal phases between 875 °C and 900 °C. Rietveld refinements show the formation of a pure tetragonal phase at 975 °C and of a pure cubic phase at 1000 °C. Partial substitution of manganese by iron and/or titanium to yield CaMn0.875Ti0.125O3−δ (CMT), CaMn0.85Fe0.15O3−δ (CMF) or CaMn0.725Ti0.125Fe0.15O3−δ (CMTF) leads to different phase behaviours. While CMT remains orthorhombic up to the highest temperature covered by the HT-XRD experiments, CMF and CMTF undergo an orthorhombic → tetragonal → cubic sequence of phase transitions. Electrical conductivity relaxation measurements are conducted to determine the chemical diffusion coefficient (Dchem) and the surface exchange coefficient (kchem) of the materials. The results demonstrate that oxygen transport is hindered in the tetragonal phase, when occurring, which is attributed to a possible ordering of oxygen vacancies. The small polaron electrical conductivity of CM in the cited temperature range is lowered upon partial manganese substitution, by about 10% for CMF and up to half an order of magnitude for CMT and CMTF

Manufacturing of perovskite oxygen carriers by spray granulation for chemical looping combustion

acceptedVersio

Correlating surface crystal orientation and gas rinetics in perovskite oxide electrodes

Solid-gas interactions at electrode surfaces determine the efficiency of solid-oxide fuel cells and electrolyzers. Here, the correlation between surface-gas kinetics and the crystal orientation of perovskite electrodes is studied in the model system LaSrCoFeO. The gas-exchange kinetics are characterized by synthesizing epitaxial half-cell geometries where three single-variant surfaces are produced [i.e., LaSrCoFeO/LaSrGaMgO/SrRuO/SrTiO (001), (110), and (111)]. Electrochemical impedance spectroscopy and electrical conductivity relaxation measurements reveal a strong surface-orientation dependency of the gas-exchange kinetics, wherein (111)-oriented surfaces exhibit an activity >3-times higher as compared to (001)-oriented surfaces. Oxygen partial pressure ((Formula presented.))-dependent electrochemical impedance spectroscopy studies reveal that while the three surfaces have different gas-exchange kinetics, the reaction mechanisms and rate-limiting steps are the same (i.e., charge-transfer to the diatomic oxygen species). First-principles calculations suggest that the formation energy of vacancies and adsorption at the various surfaces is different and influenced by the surface polarity. Finally, synchrotron-based, ambient-pressure X-ray spectroscopies reveal distinct electronic changes and surface chemistry among the different surface orientations. Taken together, thin-film epitaxy provides an efficient approach to control and understand the electrode reactivity ultimately demonstrating that the (111)-surface exhibits a high density of active surface sites which leads to higher activity

Oxide Ion Transport in Promising Cobaltites for SOC

International audienceTwo cobaltites were studied as air electrodes for Solid Oxide Cells with a Cerium Gadolinium Oxide electrolyte (CGO): Ca3Co4O9+δ, well known for its thermoelectric properties, and Ba2Co9O14. After optimisation of composition and thickness, a very good ASR of only 0.08 Ω.cm2 was obtained for the 50 wt% Ba2Co9O14 - 50 wt% 50CGO composite. In contrast, a value of 0.5 Ω.cm2 was reached for the 50 wt% Ca3Co4O9+δ - 50 wt% CGO composite. Although, Ba2Co9O14 sample contained 18O after annealing, oxide ion diffusion in this compound still has to be confirmed. In contrast, high surface exchange kinetics were measured for both Ca3Co4O9+δ and NdBaCo2O5+δ, for which mainly calcium and barium/neodymium were evidenced at the uppermost surface of the samples, whose atoms may play a key role in the mechanism of oxygen molecules dissociation

Correlating Surface Crystal Orientation and Gas Kinetics in Perovskite Oxide Electrodes.

Solid-gas interactions at electrode surfaces determine the efficiency of solid-oxide fuel cells and electrolyzers. Here, the correlation between surface-gas kinetics and the crystal orientation of perovskite electrodes is studied in the model system La0.8 Sr0.2 Co0.2 Fe0.8 O3 . The gas-exchange kinetics are characterized by synthesizing epitaxial half-cell geometries where three single-variant surfaces are produced [i.e., La0.8 Sr0.2 Co0.2 Fe0.8 O3 /La0.9 Sr0.1 Ga0.95 Mg0.05 O3-δ /SrRuO3 /SrTiO3 (001), (110), and (111)]. Electrochemical impedance spectroscopy and electrical conductivity relaxation measurements reveal a strong surface-orientation dependency of the gas-exchange kinetics, wherein (111)-oriented surfaces exhibit an activity >3-times higher as compared to (001)-oriented surfaces. Oxygen partial pressure ( pO2 )-dependent electrochemical impedance spectroscopy studies reveal that while the three surfaces have different gas-exchange kinetics, the reaction mechanisms and rate-limiting steps are the same (i.e., charge-transfer to the diatomic oxygen species). First-principles calculations suggest that the formation energy of vacancies and adsorption at the various surfaces is different and influenced by the surface polarity. Finally, synchrotron-based, ambient-pressure X-ray spectroscopies reveal distinct electronic changes and surface chemistry among the different surface orientations. Taken together, thin-film epitaxy provides an efficient approach to control and understand the electrode reactivity ultimately demonstrating that the (111)-surface exhibits a high density of active surface sites which leads to higher activity