On the Defect Chemistry, Electrical Properties and Electrochemical Performances As Solid Oxide Fuel Cell Cathode Materials of New La-(Sr/Vac)-Co-Ti-O Perovskites

Perovskite-type oxides are well known materials that have been proposed as electrodes and electrolytes for solid oxide fuel cells (SOFCs). The structure, which is referred to the ABO₃ stoichiometry_, can accommodate many different transition metal ions in the B-site; its electronic conductivity will vary depending upon the nature and the oxidation state of these ions. On the other hand, the creation of anion vacancies or aliovalent substitution in the A position will affect to oxide ion conductivity. A proper combination of B cations and doping or vacancy creation in A-site may provide new materials with valuable properties for SOFCs.

We have analysed the effect of La³⁺ by Sr²⁺ substitution and vacancies creation in several double perovskites, La₂MTiO₆ (M = Co, Ni, Cu). Defect chemistry and electrical behavior have been investigated in order to unveil the nature of charge carriers. Electrochemical performances have been assessed through polarization resistance measurements. In this communication we present the results regarding La₂SrTiO₆ perovskites.  La/Sr substitution in La_2-xSr_xCoTiO_6-δ produces Co²⁺ to Co³⁺ oxidation while vacancies in La_2-xCoTiO_6-δ yield Co²⁺ oxidation for low A-vacancy concentration. Interestingly, oxygen vacancies are predominant for higher A-vacancies concentration. These oxygen vacancies seem to be associated to La vacancies likely inhibiting oxide ion mobility. Accordingly to operating defect chemistry the electrical behavior of the La_2-xSr_xCoTiO₆ system in a wide pO₂ range is dominated by p-type electronic conduction mechanism while for La_2-xCoTiO_6-δ the p-type behavior is only observed at high pO₂ and changes to n-type at low pO₂. The contribution of oxide ion conductivity could not be unveiled due to the larger contribution of electronic component to total conductivity in the pO2 range analyzed. The preliminary evaluation of the electrodes performance reveals polarization resistances in the 0.6-0.9 Ωcm² range at 1073 K in oxygen for La_2-xSr_xCoTiO_6-δ, which is fairly similar to the values obtained for LSM-based cathodes. However much higher polarization resistances are found for the La_2-xCoTiO_6-δ  with values between 2.6-9.6 Ωcm² in air at 1073 K.  Additional electrochemical experiments to determine performances of planar (1-2 cm²) single SOFC bearing La_2-xSr_xCoTiO_6-δ as the cathode are now in progress.