CONICET | Buscador de Institutos y Recursos Humanos

The layered cobaltite GdBaCoO5+δ, displaying mixed conductivity, has recently obtained great attention because its potential use as IT-SOFCs cathode [1-4]. This attention is mainly due to its excellent property for surface oxygen exchange [5] and ionic conductivity [6], which does promote the oxygen reduction reaction. However, at high temperature this oxide presents a phase transition from orthorhombic to tetragonal symmetry [7]. The question is: How this phase transition affects the long term stability of cell considering, for example, the effects associated to thermal cycling?. This work presents a detailed study of the O/T phase transition at high temperature in order to clarify the relationship between temperature and oxygen content with phase transition; and the effect of this in long term stability. We can observe from thermo-gravimetric (TG) with controlled pO2, differential scanning calorimetric (DSC), dilatometry and high temperature X-ray diffraction (HT-XRD) that the O/T phase transition shift to higher temperature when oxygen partial pressure increases. The thermal expansion coefficient (TEC) and the change of lattice parameter, studied by dilatometry (see Fig. 1) and HT-XRD, show a slight increment after phase transition (TEC= 17.6 and 18.4 10-6 K-1 between 20-450°C and 20-900°C, respectively). The equilibrium TG data, showing a slight two-phase field around 5+δ≈5.45 (see Fig 2), together with DSC measurements, support that this phase transition is a first-order type, with a change of enthalpy DH≈4 J/g (see Fig. 3). [1] J-H. Kim et al. J. of the Electrochem Soc. 155 (4), 2008, B385-B390 [2] Na Li et al. J. of Alloy and Compounds 454 (2008) 274-279. [3] A. Chang et al. Solid State ionics 177 (2006) 2009-2011 [4] A. Tarancón et al. J. Power Sources 174 (2007) 255-263 [5] A. Tarancón, et al. J. of Materials Chem. 14 (2007) 3175-3181

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