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. The Rietveld refinements show 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

Oxide diffusion in innovative SOFC cathode materials

International audienceOxide diffusion was studied in two innovative SOFC cathode materials, Ba2Co9O14 and Ca3Co4O9+δ derivatives. Although oxygen diffusion was confirmed in the promising material Ba2Co9O14, it was not possible to derive accurate transport parameters because of an oxidation process at the sample surface which has still to be clarified. In contrast, oxygen diffusion in the well-known Ca3Co4O9+δ thermoelectric material was improved when calcium was partly substituted with strontium, likely due to an increase of the volume of the rock salt layers in which the conduction process takes place. Although the diffusion coefficient remains low, interestingly, fast kinetics towards the oxygen molecule dissociation reaction were shown with surface exchange coefficients higher than those reported for the best cathode materials in the field. They increased with the strontium content; the Sr atoms potentially play a key role in the mechanism of oxygen molecule dissociation at the solid surface

Oxygen transport kinetics of the misfit layered oxide Ca3Co4O9+d

The oxygen transport kinetics of the misfit-layered cobaltite, Ca3Co4O9+d, known for its thermoelectric properties, was investigated by combined application of 18O/16O isotope exchange and electrical conductivity relaxation techniques. Although oxygen diffusion is found to be two orders of magnitude lower than in well-investigated lanthanum nickelates, e.g., La2NiO4+d, the mixed ionic–electronic conductor Ca3Co4O9+d is found to exhibit fast surface exchange kinetics (k* ¼ 1.6 10 7 cm s 1 at 700 C to be compared to 1.3 10 7 cm s 1 for the nickelate), rendering it a promising electrode for application as an air electrode in solid oxide cells. In parallel, the chemical nature of the outermost surface of Ca3Co4O9+d was characterized by means of Low Energy Ion Scattering (LEIS) spectroscopy. The absence of cobalt at the sample's outermost surface suggests that the Ca2CoO3 d rock salt layers in the structure may play a key role in the oxygen exchange mechanism

Growth of Ca3Co4O9+δ thin film on sapphire substrate and CGO dense pellet by pulsed laser deposition. Structural, microstructural, surface and electrochemical characterizations

International audienceno abstrac

«Oxygen transport and electrochemical performances in Ca₃Co₄O₉ derivatives as SOFC cathode»

International audienc

«Growth of Ca₃Co₄O₉ on CGO dense pellet by Pulsed Laser Deposition and XRD, TEM and Ellipsometry Characterizations»

International audienceno abstrac