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Oxygen reduction reaction of (La,Sr)(Co,Fe)O_3-δ(LSCF) cathode on La_9.5Si₆O_26.25apatite electrolyte is studied by electrochemical impedance spectroscopy over the temperature range 700–850℃ and the oxygen partial pressure range 0.01–0.21 atm. The impedance responses show three separable arcs and are analyzed by the equivalent circuit method. For O₂ reduction reaction on LSCF cathode on GDC, the impedance responses were characterized by a single and reasonably well-defined semicircle at low frequency. Different to the O₂ reduction reaction on the LSCF electrode on GDC, for the reaction on the LSCF electrode on La_9.5Si₆O_26.25 apatite electrolyte the impedance behavior was characterized by clearly separable arcs at high, intermediate and low frequencies. The electrode process associated with the high frequency arc is basically independent of oxygen partial pressure, and the activation energy is 231±7 kJ mol^-1. On the other hand, for the electrode process associated with the intermediate and low frequency arcs, the reaction orders with respect to partial pressure of oxygen are in the range of －0.136～0.218 and 0.037～0.161 and the activation energies are 124±28 kJ mol^-1 and 192±12 kJ mol^-1, respectively. Within the O₂ reduction processes on LSCF cathode on La_9.5Si₆O_26.25 apatite electrolyte, the resistive process at low frequency arc is theoxygen diffusion through the porous LSCF electrode from the bulk to the reaction sites. The resistive component of the intermediate frequency feature corresponds to thedissociative adsorption of oxygen on the surface of LSCF electrode. Finally, the high frequency arc is attributed to the oxygen diffusion through the electrode bulk and migrationfrom the three phase boundary (tpb) into La_9.5Si₆O_26.25 apatite electrolyte lattice. The results show that the oxygen ion migration from the tpb into La_9.5Si₆O_26.25 apatite electrolyte lattice would be much more difficult than that into GDC electrolyte. The electrode resistance for the O₂ reduction on LSCF cathode on La_9.5Si₆O_26.25 apatite electrolyte increases with the cathodic polarization time. The effect of the activation process of the cathodic polarization treatment was primarily on the impedance responses at high and intermediate frequencies.

LSCF cathode; apatite electrolyte; solid oxide fuel cells; oxygen reduction