Effect of Co substitution for Mn on Y_1−x Sr _x MnO₃ properties for SOFC cathode material

Abstract

Y_0.6Sr_0.4Mn_1−y Co _y O₃ (0 ≤ y ≤ 0.4) perovskite oxides were prepared by the coprecipitation method. The effect of Co substitution for Mn on the crystal structure, electrical conductivity and thermal expansion properties were investigated. By X-ray powder diffraction, the crystal structure was found to change from hexagonal symmetry of Y_0.8Sr_0.2MnO₃ to orthorhombic of Y_0.6Sr_0.4Mn_1−y Co _y O₃. The differences in the structure of the unsubstituted Y_1−x Sr _x MnO₃ (0.2 ≤ x ≤ 0.4) are attributed to the average ionic radii of the cations and the amounts of Mn⁴⁺ present. The results of electrical conductivity analysis can be described by the small polaron hopping conductivity model. With Co substitution, the activation energy increases, possibly due to an increase of Jahn–Teller distortion, at an extent higher than the increase of the concentration of charge carriers; thus, the electrical conductivity decreases. In addition, the relative densities of the materials reached ∼94% with sintering at 1350°C for 12 h and had higher concentration of the available lattice sites, thus showing higher conductivity, than that with sintering at 1300°C for 6 h, which achieved ∼70% relative density. It is also found that the thermal expansion coefficient (TEC) increases as the Sr and Co content of Y_1−x Sr _x Mn_1−y Co _y O₃ increases and those with Co content of y = 0.2 exhibit TEC compatibility with YSZ.