Iron doping and Ba deficiency in NdBaCo2O5+δ Cathodes for IT-SOFCs

Co-based double perovskites (LnBaCo2O5+δ Ln=Lanthanides or Y) have been recently proposed as cathode materials for IT-SOFCS (1, 2). These compounds crystallize in a double perovskite structure (LnCoO3-δ–BaCoO3-δ), where Ln and Ba ions sit on alternated single perovskite layers. Pr, Nd and Gd based compounds show the best performance, likely due to their optimum ion size. The effect of Fe-doping on the B-site and of Ba deficiency on the A-site have been investigated: XRD, TG-DTA, ICPMS and cerimetric titrations were performed to assess the phase composition, crystal structure, chemical composition and oxygen content. The electrochemical properties were investigated via 4-probe electrical conductivity measurements and Electrochemical Impedance Spectroscopy on symmetrical cells with GDC electrolyte. All the compounds show high total electrical conductivity (between 400 and 600 S/cm at 700 °C). A tenfold decrease of the area specific resistance is observed at increasing Ba deficiency to 10%. Detailed equivalent circuit analysis reveals that the effect of Ba is associated with a promotion of the bulk diffusion steps at high frequency. Iron substitution triggers a structural change from orthorhombic to tetragonal, and lowers the electrical conductivity. The compound with 20% of iron shows the lowest polarization resistance (0.17 Ω*cm2 at 700°C). The ORR mechanism investigated by ECM and physical 1D modeling shows that the first electronation of the oxygen atom on the MIEC surface and the bulk diffusion of the oxygen vacancies in the MIEC lattice are the rate determining steps.