Comparison of oxygen exchange kinetics of the IT-SOFC cathode materials La0.5Sr0.5CoO3 − δ and La0.6Sr0.4CoO3 − δ

Abstract La 0.5 Sr 0.5 CoO 3  −  δ and La 0.6 Sr 0.4 CoO 3  −  δ are characterized with respect to application as cathode materials in intermediate temperature solid oxide fuel cells (IT-SOFCs). Surface exchange and transport parameters of oxygen are determined by the conductivity relaxation technique between 525 °C and 725 °C at oxygen partial pressures of 0.1, 0.01 and 0.001 bar. Electrical conductivities of both compounds range between 1000 and 2400 S cm - 1 and are slightly higher for La 0.6 Sr 0.4 CoO 3  −  δ . However, La 0.5 Sr 0.5 CoO 3  −  δ shows superior performance with regard to oxygen diffusion, ionic conductivity and oxygen surface exchange within the investigated range of temperatures and oxygen partial pressures. At 725 °C the chemical surface exchange coefficient of oxygen is 2 × 10 - 3  cm s - 1 for La 0.5 Sr 0.5 CoO 3  −  δ and 4 × 10 - 4  cm s - 1 for La 0.6 Sr 0.4 CoO 3  −  δ . At lower oxygen partial pressures a strong decrease in the surface exchange coefficient is observed and diffusion data cannot be obtained from relaxation measurements. Oxygen vacancy diffusion coefficients are similar for both compounds and range between 10 - 6 and 10 - 8  cm 2  s - 1 with activation energies around 100 kJmol - 1 . At 725 °C and 0.1 bar oxygen partial pressure, the ionic conductivity of La 0.5 Sr 0.5 CoO 3  −  δ is 1 × 10 - 2  S cm - 1 with an activation energy of 118 kJmol - 1 . The results show that both compositions meet the requirements for the application as IT-SOFC cathodes in the short-term range.