Structure and high-temperature properties of the (Sr,Ca,Y)(Co, Mn)O3−y perovskites — perspective cathode materials for IT-SOFC

Oxygen deficient perovskites Sr0.75Y0.25Co1−xMnxO3−y, x=0.5 and 0.75, were prepared by using the citrate route at 1373–1573 K for 48 h. The cubic Pm-3m perovskite structure for x=0.5 was confirmed by electron diffraction study and refined using neutron powder diffraction (NPD) data. For x=0.75, the superstructure corresponding to a  =2×aper, b=2×aper, c  =2×aper (a0b−b− tilt system, space group Imma) was revealed by electron diffraction. The solid solution Sr0.75−xCaxY0.25Co0.25Mn0.75O3−y, 0.1≤x≤0.6 and compound Ca0.75Y0.25Mn0.85Co0.15O2.92 were prepared in air at 1573 K for 48 h. The crystal structure of Ca0.75Y0.25Mn0.85Co0.15O2.92 was refined using NPD data (S.G. Pnma, a=5.36595(4), b=7.5091(6), c=5.2992(4) Å, Rp=0.057, Rwp=0.056, χ2=4.26). High-temperature thermal expansion properties of the prepared compounds were studied in air using both dilatometry and high-temperature X-ray powder diffraction data (HTXRPD). They expanding non-linearly at 298–1073 K due to the loss of oxygen at high temperatures. Calculated average thermal expansion coefficients (TECs) for Sr0.75Y0.25Co1−xMnxO3−y, x=0.5, 0.75 and Ca0.75Y0.25Mn0.85Co0.15O2.92(1) are 15.5, 15.1, and 13.8 ppm K−1, respectively. Anisotropy of the thermal expansion along different unit cell axes was observed for Sr0.15Ca0.6Y0.25Co0.25Mn0.75O3−y and Ca0.75Y0.25Mn0.85Co0.15O2.92. Conductivity of Sr0.75Y0.25Co1−xMnxO3−y, x=0.5 and 0.75 increases with the temperature reaching 110 S/cm for x=0.5 and 44 S/cm for x=0.75 at 1173 K. Samples of Sr0.75−xCaxY0.25Co0.25Mn0.75O3−y, 0.1≤y≤0.6 were found to be n-type conductors at room temperature with the similar temperature dependence of the conductivity and demonstrated the increase of the σ value from ∼1 to ∼50 S/cm as the temperature increases from 300 to 1173 K. Their conductivity is described in terms of the small polaron charge transport with the activation energy (Ep) increasing from 340 to 430 meV with an increase of the calcium content from x=0 to x=0.6.

Graphical abstractThe substitution of cobalt in the 314-phase Sr0.75Y0.25CoO2.62 by manganese leads to the formation of oxygen deficient compounds Sr0.75Y0.25Co1−xMnxO3−y, x=0.5 and 0.75, with the cubic and orthorhombic perovskite structures. The conductivity of the x=0.5 sample (110 S/cm) at 1173 K is substantially higher in comparison with x=0.75 (44 S/cm). Together with comparable TEC values for x=0.5 (15.5 ppm K−1) and 0.75 (15.1 ppm K−1) samples this makes the former one a promising cathode material for IT-SOFC. Partial replacement of Sr by Ca in Sr0.75Y0.25Co0.25Mn0.75O3−y leads to the formation of the solid solution with the orthorhombic perovskite structure, Sr0.75−xCaxY0.25Co0.25Mn0.75O3−y, 0.1≤y≤0.6. Along with Sr0.75Y0.25Co0.5Mn0.5O3−y, novel perovskite Ca0.75Y0.25Mn0.85Co0.15O3−y represent promising cathode material for IT-SOFC due to low TEC value (13.8 ppm K−1) and high conductivity of 135 S/cm at 1173 K.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► Mn-doped 314-phases Sr0.75Y0.25Co1−xMnxO3−y, x=0.5, 0.75 were synthesized. ► Sr0.75−xCaxY0.25Co0.25Mn0.75O3−y, x=0.1-–0.6, Ca0.75Y0.25Mn0.85Co0.15O3−y were made. ► High-temperature conductivity properties and crystal structure were studied. ► High-temperature thermal expansion behavior was investigated.