Thermodynamic perspective of Sr-related degradation issues in SOFCs

(La,Sr)(Co,Fe)O3-delta is very common as cathode material in SOFC applications. Sr in this type of cathode material is very reactive to form secondary phases with other oxides, which affect micro-structures and properties of the cathode materials, GDC layers and ZrO2-based electrolytes. The Sr-related degradation issues, Cr poisoning and volatile Sr species formation, are studied. As supplement to existing experimental knowledge on Cr poisoning, specific thermodynamic aspects for Cr poisoning are discussed. The thermodynamic calculations show that the partial pressure pCrO(3) has a stronger temperature dependence than pCrO(2)(OH)(2), and when considering the reaction between SrO and CrO3(g), dependent on different pCrO(3) and pO(2), different Sr-Cr-O compounds SrCrO4, SrCrO3, Sr3Cr2O8 or Sr2CrO4 could be formed. In addition, thermodynamic calculations show that in the presence of water vapor, formation of volatile Sr(OH)(2) is possible as well. pSr(OH)(2) depends on temperature, pH(2)O and SrO activity and can be of the same order of magnitude as pCrO(2)(OH)(2). Volatile Sr(OH)(2) can diffuse through the porous GDC layer and react with ZrO2-based electrolytes to form SrZrO3 precipitates. The reaction between gaseous Sr species and 8YSZ sheet is studied experimentally. The surface of the 8YSZ sheet is investigated by SEM coupled with EDS, confirming the deposition of Sr.