Structural and Transport Properties of the Blc-Pbco System; Effects of Cation Ordering

Abstract

The drive to reduce the operating temperatures of solid oxide fuel cells (SOFCs) and electrolysers (SOECs) requires the discovery of new materials showing enhanced oxygen transport and electrocatalytic properties. As well as modifying the composition of materials with the simple perovskite structure, lately there has also been interest in materials with other, often, layered structures including the Ruddlesden-Popper type materials, such as La2NiO4, and double perovskites such as GdBaCo2O5 (GBCO) and PrBaCo2O5 (PBCO) [1]. Whilst Sr-substituted lanthanum cobaltite (La0.6Sr0.4CoO3-d, LSC64) is widely used, the Ba-substituted analogue La1-xBaxCoO3-d (BLC) is attracting recent interest showing competitive electrode performances [2,3]. On the other hand, the double perovskite PrBaCo2O5+d (PBCO) has high diffusivity and surface exchange activity, which is often attributed to the anisotropic diffusion of oxygen due to the ordering of Pr and Ba cations on the A-site. However, experimental measurements suggest that the degree of anisotropy between diffusion in along the c axis and in the ab plane is only around half an order of magnitude [4]. Therefore, the family of compositions between single perovskite BLC and double perovskite PBCO offers an interesting system to study the effects of cation ordering on transport behaviour. In this work, we have studied the structural and oxygen transport properties of materials in the La0.5-x/2Prx/2Ba0.5CoO3-d compositional space. The materials were synthesised by a conventional solid state reaction route, and X-ray diffraction revealed that Pr-rich compositions (0.7 < x < 1) retain the same double perovskite structure as PBCO, with cation ordering on the A-site, and hence more properly written as La1-xPrxBaCo2O5+d. The La-rich compositions (0 < x <0.7) showed a cubic symmetry, similar to the Sr-analogue LSC64. Oxygen isotope exchange studies on the Ba0.5La0.5CoO3 end member show that the oxygen tracer diffusion (D*) and exchange (k*) coefficients are similar to those of PBCO, and of Sm0.5Sr0.5CoO3, which are some of the highest reported [5], explaining the high electrochemical performance of the single perovskite material [2,3]. Preliminary results indicate these values change only slightly across the composition field, with no significant increase as the structure changes from single to double perovskite. These findings seem to suggest that it is not the A-cation ordering in the crystal structure of PBCO that provides per se its high performance, but the defect chemistry, namely the high concentration of oxygen vacancies in the material.