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

Abstract

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 La2-xSrxCoTiO6-δ produces Co2+ to Co3+ oxidation while vacancies in La2-xCoTiO6-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 La2-xSrxCoTiO6 system in a wide pO2 range is dominated by p-type electronic conduction mechanism while for La2-xCoTiO6-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 La2-xSrxCoTiO6-δ, which is fairly similar to the values obtained for LSM-based cathodes. However much higher polarization resistances are found for the La2-xCoTiO6-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 La2-xSrxCoTiO6-δ as the cathode are now in progress.