Structure and properties of perovskites for SOFC cathodes as a function of the A-site cation size disorder

Abstract

The structure, electronic conductivity and electrochemical properties of the oxide perovskites with the Ln0.5M0.5FeO3−δ composition (Ln=La, Sm; M=Ba, Sr) are studied in the context of their possible use as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFCs). The main objective of the work is to analyse the influence of the A-site cation size disorder (σ2(rA)) on the properties of these compounds. The A-site cation size disorder of these samples varies from 0.0286 to 0.089Å2, whilst the doping (x) and the average ionic radius of the A-site cations (<rA>) are fixed to 0.5 and ~1.30Å, respectively.The samples are prepared using the glycine-nitrate method. X-ray powder diffraction (XRD) shows that all of the compounds have rhombohedral symmetry (space group: R-3c). The structure is highly dependent on σ2(rA); the lattice parameters and unit cell volume decrease with decreasing σ2(rA). All of the samples have a porous microstructure with fine grain sizes (100–250nm).The electrical conductivity (σ) measurements on the bulk samples shows a strong dependence on the A-site cation size disorder. Electrochemical impedance spectroscopy (EIS) measurements of the cathodes on the yttria-stabilised zirconia (YSZ) electrolyte-based symmetrical cells indicate that the lowest polarisation resistance value is observed for the sample with the lowest σ2(rA). The effect of introducing a samarium doped ceria (SDC) interlayer between the best performing cathode and the electrolyte is also studied.