Chemical and structural stability of La0.6Sr0.4Co0.2Fe0.8O3− ceramic vs. medium/high water vapor pressure

Abstract

During the last decades, perovskite-type oxides have received large attention as potential electrolytes and electrodes for Solid Oxide Fuel Cells (SOFC), including Proton Ceramic Fuel Cells (PCFC), gas separation membranes and High Temperature Steam Electrolysers (HTSE). A thermal treatment in an autoclave, at a temperature close to an operating temperature, was used to measure the chemical stability of La 0.6 Sr 0.4 Co 0.2 Fe 0.8 O 3 À δ (LSCF6428) ceramic under medium and high water pressure ( $20 and 40 bar). This mixed ionicÀ electronic conductor (MIEC) exhibits interesting properties as cathode of fuel cell materials. The reactivity rate of the investigated LSCF6428 sample under the protonation process conditions (several weeks at 550 1C using CO 2 -free and CO 2 -saturated water) was studied in order to evaluate a potential use of this compound. Bulk and surface structural/chemical changes were characterized by optical microscopy, TGA, dilatometry, Raman and ATR-FTIR spectroscopy. The results revealed only minor surface modifications in the case of ceramic treated under medium vapor pressure (20 bar) using CO 2 -free water. On the contrary, under higher pressure (40 bar) and CO 2 -saturated water several second phases were detected, namely strontianite, cobalt oxides and hematite. The chemical/structural stability of LSCF6428 is compared with previously investigated RareEarth nickelate ceramics: La 2 NiO 4 þ δ / Pr 2 NiO 4 þ δ / Nd 2 NiO 4 þ δ .

Orbital facilitated charge transfer originated phonon mode in Cr‐substituted PrFeO3: A brief Raman study

“…These results are in good conformance with the earlier published literature. [13,18,21,23,38] The power-dependent Raman spectra for 30% Cr-substituted PrFeO 3 is displayed in Figure S5c. From the figure, it is clear that with increase in the power of laser, the relative intensity of Raman mode around 660 cm −1 scales significantly as compared with that of other modes, which further confirms the CT speculation.…”

“…[ 14–16 ] It is now well established that RS is one of the versatile technique not only to investigate the crystal structure but also to study the orbital mediated CT mechanism in doped rare earth ortho‐ferrites. [ 3,17,18 ]…”

“…[14][15][16] It is now well established that RS is one of the versatile technique not only to investigate the crystal structure but also to study the orbital mediated CT mechanism in doped rare earth orthoferrites. [3,17,18] It is well known that the atomic arrangement with orthorhombic symmetry decompose into Raman active (7A g + 7B 1g + 5 B 2g + 5B 3g ), eight inactive modes (8 A u ), three acoustic translational modes (1 B 1u + 1 B 2u + 1 B 3u ), and 25 IR-active modes (7 B 1u + 9 B 2u + 9 B 3u ). [17,19,20] The Raman modes at lower wavenumber arising around 100 cm −1 are assigned as the soft A g modes.…”

Evaluación del cerámico con estructura tipo perovskita LaFe0,2Co0,8O3 / Evaluation of the perovskite LaFe0.2Co0.8O3

“…Se observa también una señal exotérmica cercana a los 248 • C relacionada con transformaciones de los compuestos de coordinación metal-citrato, y otro pico exotérmico a 620 • C asociado con transformaciones de carbonatos y oxo-carbonatos [27]. El análisis estructural, desarrollado mediante difracción de rayos X en la muestra calcinada en aire a 850 • C durante 12 horas, y mostrado en la Figura 5, demuestra la obtención de una única fase predominante, y que concuerdan con el espectro de la estructura de referencia LaCoO 3 (JCPDS = 00-048-0123) que presenta como parámetros cristalográficos: sistema cristalino romboédrico, grupo espacial R-3c (167), a = 5.4445 Å, b = 5.4445 Å, c = 13.0936 Å y volumen de celda = 336 Å 3 , con orientación cristalográfica principal hacia los planos (1 1 0) y (1 0 4), acorde con reportes preliminares relacionados con la obtención de este tipo de composiciones, evidenciando la efectividad del método en la síntesis de una composición de una fase pura [28][29][30]. La determinación del tamaño de cristal, utilizando la ecuación de Debye-Scherrer y una constante de 0,89 como referencia, permitió establecer un tamaño de 31 nm [31].…”

“…Los espectros Raman de la Figura 6, confirman la formación del óxido LaCoO 3 , gracias a la presencia de dos señales intensas localizada a 570 y 650 cm −1 que concuerdan con modos vibracionales de tensión E g para materiales con contenidos de lantano y cobalto en los sitios catiónicos A y B respectivamente, dentro de la estructura tipo ABO 3 romboédrica de espacio grupal R-3c, cuyos modos activos para esta técnica son A 1g + 4E g [28,[32][33][34][35]. La Figura 7 muestra las imágenes obtenidas a través de microscopia electrónica de barrido de la superficie de la muestra del material a diferentes magnificaciones.…”