Structure–property relationship in layered perovskite cathode LnBa0.5Sr0.5Co2O5+δ (Ln = Pr, Nd) for solid oxide fuel cells

The layered cobaltites LnBa0.5Sr0.5Co2O5+δ (Ln = Pr, Nd) have been prepared by solid state reaction technique and structure–property relationships were investigated by means of neutron diffraction, ac impedance and dc conductivity measurements. Room temperature neutron diffraction shows the ordered distribution of oxygen vacancies in [PrOδ] planes which doubles the lattice parameters from the perovskite cell parameter as a = b ≈ 2ap, and c ≈ 2ap (ap is the cell parameter of the simple perovskite) yielding tetragonal symmetry in the P4/mmm space group. On heating, the oxygen vacancy ordering disappears and the structure can be defined as a = b ≈ ap and c ≈ 2ap in the same space group. Oxygen occupancies have been determined as a function of temperature from neutron diffraction. It was found that from 573 K to 973 K the total oxygen loss is about 0.265 O/formula unit and 0.366 O/formula unit for Pr and Nd containing materials, respectively. The oxygen occupancy decreases and cell volume increases with increasing temperature. Electrical conductivity measurements in air show that conductivity decreases with temperature, and at 873 K the conductivity is 493 S cm−1 and 255 S cm−1 for Pr and Nd containing samples, respectively. AC impedance measurements in symmetrical cell arrangement with CGO electrolyte shows that area specific resistance decreases with increasing temperature. At 873 K the ASR is 0.286 Ω cm2 and 1.15 Ω cm2 for Pr and Nd containing samples, respectively.