Applicability of La 2Mo 2− yW yO 9 materials as solid electrolyte for SOFCs

Abstract

Electrolyte materials with composition La 2Mo 2− y W y O 9 ( y = 0–1.5) exhibit high ionic conductivity ranging from 0.11 S cm − 1 ( y = 0) to 0.05 S cm − 1 (y = 1.5) at 1023 K, which is comparable to gadolinia doped ceria. The ionic conductivity is predominant in a wide range of oxygen partial pressures from 0.21 to 10 − 20 atm at 973 K with ionic transport numbers higher than 0.95 under humidified 5%H 2–Ar. Above this temperature, a significant increase of the n-type electronic conductivity and degradation of the ceramic microstructure are observed as a consequence of the cell volume expansion in addition to the formation of new phases upon reduction. Considering the potential use of these materials as solid electrolytes, the chemical compatibility with most of the typical electrodes commonly used in SOFCs (e.g. cobaltites, ferrites, chromites and manganites) was investigated. Severe chemical reaction was found between La 2Mo 2− y W y O 9 and most of the studied electrodes, mainly due to molybdenum migration. Furthermore, the high thermal expansion coefficients of La 2Mo 2O 9 based materials (∼ 15 · 10 − 6 and ∼ 20 · 10 − 6 K − 1 in the low and high temperature range, respectively) restrict the choice of compatible materials as cell components. Moreover, electrolyte and electrodes present poor contact due to the low fixing temperature, necessary to prevent excessive chemical reaction between the materials, and also the large thermal expansion coefficients of lanthanum molybdate materials, resulting in the separation of both materials after several consecutive thermal cycles. Therefore, the use of these materials as SOFC electrolyte seems to be rather limited.