Abstract: The kinetics of oxygen reduction over Sm0.5Sr0.5CoO3- La0.8Sr0.2Ga0.8Mg0.15Co0.05O3 (SSC-LSGMC5) cathode was investigated using polarization and AC(alternating current) impedance. Two arcs were observed in the impedance spectra of SSC-LSGMC5 at near equilibrium conditions. The conductivities of the high frequency arcs had no dependency on oxygen partial pressures (pO2), and showed an activation energy about (1.0～1.1)×102 kJ•mol－1. The dependency of the conductivities of the low frequency arcs decreased from 0.6 to 0.4 with the decrease in temperature 1073～873 K，accordingly,the activation energies increased from 1.3×102 to 1.6×102 kJ•mol－1 with the increase in (2×104～1×105 Pa). The high frequency arcs could be related to the transfer of oxygen ions across the electrode/electrolyte interface, while the low frequency arcs could correspond to the diffusion of oxygen atoms from the electrode surface to the three-phase boundary. The polarization curves of SSC-LSGMC5 agreed well with the Butler-Volmer equation. The anodic and cathodic charge transfer coefficients were all about 1 and the pO2 dependency of the exchange current density was around 0.25, suggesting a rate-determining step of charge transfer. The mechanism of oxygen reduction over SSC-LSGMC5 depended strongly on the over-potential and reaction temperature.

Key words: Sm_0.5Sr_0.5CoO₃, Kinetics, Polarization, AC impedance, Charge transfer