第7回 東北イオニクス研究会
Carlos Bernuy-Lopez研究員による特別講演
題目：Chemical Stability of the Layered LaBaCo2O5+δ Double Perovskite SOFC Cathode
開催日：2016年6月16日 13:30~15:00
開催場所：多元研 事務部棟二階 大会議室
講演概要：
Layered double perovskite with general chemical formula LnBaCo2O5+δ (Ln=Lanthanide or Y) are promising cathodes for proton conducting fuel cells due to their outstanding mixed conducting properties1. In our recent work2, LnBaCo2O5+δ was obtained by annealing disordered La0.5Ba0.5CoO3-δ perovskite at 1100 °C in N2. Thermogravimetric analysis (TGA) demonstrated that a critical level of oxygen vacancies is required in La0.5Ba0.5CoO3-δ in order to stabilize the ordered LaBaCo2O5+δ. It was concluded that LaBaCo2O5+δ is metastable with respect to La0.5Ba0.5CoO3-δ in oxidizing atmosphere. High temperature X-ray diffraction (HT-XRD) revealed no structural changes for tetragonal ordered LaBaCo2O5+δ in air. However, a change to orthorhombic structure due to ordering of oxygen vacancies was observed between 475 °C and 650 °C in inert atmosphere. Combining TGA and HT-XRD data also showed that the ordering of oxygen vacancies depends strongly on the level of oxygen deficiency and temperature. An anisotropic chemical expansion was shown for LaBaCo2O5+δ (Figure 1). A high electronic conductivity for both ordered and disordered materials in both O2 (~1000 and 500 S cm−1 at 500 °C in pO2 = 1 bar for La0.5Ba0.5CoO3-δ and LaBaCo2O5+δ, respectively) and N2 (~300 and 175 S cm−1 at 500 °C for La0.5Ba0.5CoO3-δ and LaBaCo2O5+δ, respectively) was evidenced and this was higher for La0.5Ba0.5CoO3-δ. than LaBaCo2O5+δ. This shows that cation ordering can be detrimental for electronic conduction. Here, the chemical stability and oxygen non-stoichiometry of the two materials are investigated by coulometric titration. It is confirmed that La0.5Ba0.5CoO3-δ is more stable than LaBaCo2O5+δ and that the materials can tolerate a high oxygen vacancy concentration before they decompose. Cation order/disorder as a function of temperature and atmosphere is further investigated by high temperature X-ray diffraction. The data will be discussed and correlated with the order/disorder effects on the A-site and oxygen sub-lattices.
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1. R. Strandbakke, V. A. Cherepanov, A. Y. Zuev, D. S. Tsvetkov, C. Argirusis, G. Sourkouni, S. Prünte and T. Norby, Solid State Ionics, 2015, 278, 120-132.
2. C. Bernuy-Lopez, K. Høydalsvik, M.-A. Einarsrud and T. Grande, Materials, 2016, 9, 154.