Abstract
Perovskites of the ABO3 type, such as LaMnO3, can be used as air electrodes in solid oxide fuel cells and electrolyzers. Their properties can be tuned by A- and B-site substitutions. The influence of La substitution by Ca on the oxygen nonstoichiometry has been investigated frequently, but the results depend highly on the synthesis and atmospheric conditions. In this work, a series of La1–xCaxMnO3+δ (x = 0–0.5) was synthesized using conventional solid-state synthesis under an air atmosphere. The structures of the materials were studied in detail with powder X-ray diffraction. The initial oxygen nonstoichiometries were determined using thermogravimetric reduction. The samples were subsequently analyzed in terms of defect chemistry in dependence of temperature, atmosphere, and Ca content via thermogravimetric analysis. The changes in the manganese charge states were investigated by X-ray absorption near-edge spectroscopy experiments. The influence of intrinsic and extrinsic effects on the Mn-valence state of the differently Ca-substituted samples as calculated from thermogravimetric analysis and as determined directly from X-ray absorption near-edge spectroscopy is presented.
Highlights
Lanthanum−manganese oxides are frequently discussed for application as oxygen electrodes, for example, in solid oxide cells, as catalysts or as oxygen sensors and supercapacitors.[1−5] Some of them are well known for their colossal magnetoresistive effect, such as Ca-doped LaMnO3.6−8 Substitutions at the lanthanum site, for example, by calcium, revealed the possibility to change the material structure and defect chemistry
Defect chemistry is mainly governed by oxygen nonstoichiometry and manganese charge state and plays an important role for the catalytic activity of the material.[9−12] Material synthesis methods and conditions affect the defect chemistry.[12−17] As they are exposed to varying atmospheres at high temperatures during their application, it is important to investigate how the defect chemistry changes under varying temperature and atmosphere
We present the results of the systematic investigation of the La1−xCaxMnO3+δ system with respect to the changes in its oxygen stoichiometry and Mn-oxidation state in dependence of Ca content, atmosphere, and temperature by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and X-ray absorption near edge spectroscopy (XANES)
Summary
Lanthanum−manganese oxides are frequently discussed for application as oxygen electrodes, for example, in solid oxide cells, as catalysts or as oxygen sensors and supercapacitors.[1−5] Some of them are well known for their colossal magnetoresistive effect, such as Ca-doped LaMnO3.6−8 Substitutions at the lanthanum site, for example, by calcium, revealed the possibility to change the material structure and defect chemistry. Oxygen uptake (change in the δ-value) and simultaneously partial change in the Mn-oxidation state from Mn3+ to Mn4+ lead to a reduced distortion These changes within stoichiometric LaMnO3 are not influenced by substitution, and as such, we define them as intrinsic. We present the results of the systematic investigation of the La1−xCaxMnO3+δ system with respect to the changes in its oxygen stoichiometry and Mn-oxidation state in dependence of Ca content, atmosphere, and temperature by powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA), and X-ray absorption near edge spectroscopy (XANES). Aircalcined La1−xCaxMnO3+δ (x = 0−0.5) and nitrogen-tempered LaMnO3 and La0.9Ca0.1MnO3+δ were measured in oxygen (99.9992%), nitrogen (99.9992%), and synthetic air (80% N2, 20% O2) in a thermogravimetric analyzer (STA 449 F1 Jupiter from Netzsch) to determine changes in their oxygen stoichiometry. These measurements were performed in O2, N2, and synthetic air
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