Effect of the a cation size disorder on the properties of an iron perovskite series for their use as cathodes for SOFCs

In order to develop a cathode that can be used in intermediate temperature solid oxide fuel cells, a family of iron perovskites with the general formula Ln0.5M0.5FeO3-δ (A=Ln1-xMx; Ln=La, Nd and/or Pr; M=Sr, Ca or/and Ba) with fixed divalent dopant concentration x to 0.5, mean A cation radius rA to 1.25Å but with different A cation size disorder σ2(r A) has been prepared by the conventional solid state reaction. These compounds have been characterised by power X-ray diffraction (XRD), high-resolution scanning electron microscopy (SEM), electrical conductivity, I-V measurements and AC impedance spectroscopy. All the samples have a rhombohedral crystal structure (space group: R-3c). SEM images show that the average grain size increases with increasing σ2(rA) in the bulk samples. The DC four-probe measurements indicate that electrical conductivity is strongly dependent on the σ2(rA). Electrochemical impedance spectra at 700 and 800°C revealed that better electrochemical cathode performances are for samples with smaller values of σ 2(rA). © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.