Ln1−xSrxMnO3 (Ln=Pr, Nd, Sm and Gd) as the cathode material for solid oxide fuel cells

The perovskite type manganite systems, Ln1−xSrxMnO3 (Ln=La, Pr, Nd, Sm, and Gd; 0 ≤ x ≤ 0.5 ) were studied as the electrode materials for solid oxide fuel cells (SOFC) from the view point of applications to the co-firing process of the electrolyte and electrode at a higher temperature. The reactivity tests with yttria-stabilized zirconia (YSZ) showed that the formation of the pyrochlore Ln2Zr2O7 was suppressed for the perovskites having smaller lanthanoids than La, especially for the Pr1−xSrxMnO3 and Nd1−xSrxMnO3 systems. No reaction product appeared between the Ln1−xSrxMnO3 perovskite and YSZ even at a high annealing temperature of 1400°C when the lanthanoid elements were Pr, Nd and Sm and the Sr content was around 30% (x=0.3). The adjustment of the thermal expansion rate of Ln1−xSrxMnO3 to YSZ needed more Sr substitution with an increase of the atomic number of lanthanoids. The Sr contents, x, showing comparable values with 8m/o YSZ were around x=0.1–0.2 in La1−xSrxMnO3, 0.3 in Pr1−xSrxMnO3 and Nd1−xSrxMnO3 and more than 0.4 in Sm1−xSrxMnO3 and Gd1−xSrxMnO3. The electrical conductivity, which may strongly relate with the polarization of electrode reaction, showed about 200 S/cm at 1000°C for all Ln0.7Sr0.3MnO3 systems. The polarization measurements as an air electrode showed no significant dependence on the kind of lanthanoid elements. From the viewpoint of the electrode activity, high electrical conductivity, low reactivity and good compatibility with YSZ, Pr0.7Sr0.3MnO3 and Nd0.7Sr0.3MnO3 are considered to be the most suitable electrode materials for the co-firing process.