Evaluation of Ba deficient NdBaCo2O5+δ oxide as cathode material for IT-SOFC

Neodymium- and cobalt-based layered perovskite oxides with increasing Ba deficiency (NdBa1-xCo2O5+δ, x = 0.0 ÷ 0.2) are prepared via solid state firing. The chemical and electrochemical properties of these materials are characterized via XRD, SEM, TG-DTA, 4-probe conductivity measurement and EIS tests on symmetric cells. The maximum tolerated Ba deficiency is slightly below 10%. The compounds crystallize in an ordered layered structure with orthorhombic Pmmm (for x = 0 and 0.5) and tetragonal P4/mmm (for x = 0.10) crystal lattice. All the compounds show high total conductivity (between 400 and 600 S/cm at 700 °C). A remarkable tenfold decrease of the area specific resistance is observed at decreasing the Ba content from 0 to 10%, with the compound at 10% Ba deficiency reaching the best performance (0.1 Ω∗cm2 at 700 °C). Detailed equivalent circuit analysis on EIS tests at varying temperature (550 to 700 °C) and O2 content (100% to 5% v/v) reveals that the limiting steps are the transport of oxygen ions within the cathodic phase and across the electrolyte interface at high frequency, and the formation of an oxygen ion at intermediate frequency. The effect of Ba on the electrochemical mechanism is associated with a promotion of the bulk diffusion steps at high frequency.