Correlation between thermal expansion and oxide ion transport in mixed conducting perovskite-type oxides for SOFC cathodes

Oxygen deficiency, electrical conductivity, ionic transport and thermal expansion of alternative cathode materials of the A1−aA′aBO3 perovskite-type oxides (A=La, Pr, Ce; A′=Sr; B=Mn, Fe, Co, Ni, Ga, Mg) were measured as a function of temperature and oxygen partial pressure. Additionally, lanthanum strontium gallates were investigated as electrolyte materials. On the basis of the results a general picture of the A- and B-site influences on the thermal and electrical properties of the perovskite-type oxides is discussed. From lanthanum manganite to strontium ferrite and from lanthanum ferrite to strontium cobaltite the thermodynamic stabilities decrease and thermal expansion coefficients increase. The increase of the ionic conductivity is more influenced by Sr concentration on the A-site, and the increase of the electronic conductivity more by Fe and Co concentration on the B-site, respectively. The ionic conductivities at 800°C calculated from permeation measurements are correlated with the thermal expansion coefficients in the form of log σO=−7.08+2.9×105×TEC (σO in S cm−1, TEC in 10−6 K−1). The correlation is discussed on the basis of the dependence of both quantities on oxygen vacancy concentration.