The role of the Co2+/Co3+ redox-pair in the properties of La2-xSrxCoTiO6 (0 < x <0.5) perovskites as components for solid oxide fuel cells

Abstract

Substitution of La3+ by Sr2+ in the perovskite La2CoTiO6 yields materials of the La2−xSrxCoTiO6 series. The dominant charge-compensating mechanism is oxidation of Co2+ if they are prepared at air. The as prepared oxides can be reduced inducing a large amount of oxygen vacancies while keeping the perovskite structure. The electrical behaviour of the La2−xSrxCoTiO6 series is dominated by p-type electronic conduction in a wide pO2 range through non-adiabatic hopping of small-polarons. The electrical conductivity increases with x, except for the x = 0.5 material which shows an unexpectedly low conductivity due to microstructural and short-range ordering effects. The highest conductivity material, La1.60Sr0.40CoTiO6, is selected to study the electrochemical properties of the series. This compound is chemically compatible with YSZ up to 1373 K, in both oxidizing and reducing atmospheres. The preliminary evaluation of the electrode performance reveals that La1.60Sr0.40CoTiO6-based electrodes exhibit polarization resistances of typically 0.8 Ω cm2 at 1073 K in oxygen, which are close to the values obtained for LSM-based cathodes. Thus, the electrochemical behaviour of this oxide as cathode is particularly encouraging since the electrode microstructure is not optimized; it is expected that an improved microstructure will perform at least similarly to the state-of-art in SOFCs materials.