Long-term stability of the oxygen exchange properties of (La,Sr) 1 − z(Co,Fe)O 3 − δ in dry and wet atmospheres

Abstract

The long-term degradation of the surface oxygen exchange kinetics of the IT-SOFC cathode material La 0.58Sr 0.4Co 0.2Fe 0.8O 3 − δ (LSCF) is investigated in dry and wet O 2–Ar atmospheres at 600–700 °C. Conductivity relaxation measurements indicate high chemical surface exchange coefficients ( k chem) in dry O 2–Ar atmosphere at 600–800 °C with E a( k chem) ∼ 200 kJ mol − 1 . However, during 1 kh an exponential decrease of k chem is observed at 600 °C in dry atmosphere. After switching to a H 2O-containing atmosphere a further step-wise decrease of k chem by a factor of 2 occurs, followed by an approximately linear long-term degradation with a rate of ∼ 10% kh − 1 . At 700 °C a pronounced decrease of the surface exchange coefficient by 2 orders of magnitude is found after switching from dry to wet atmospheres. Post-test XPS analysis indicates that the deterioration of the catalytic activity of LSCF originates in significant changes of the surface cation composition (especially Sr enrichment) and silicon poisoning.