Oxygen nonstoichiometry of LaNi0.6Co0.4O 3-δ (LNC64) was measured by thermogravimetry, and electrochemical properties of thin film electrode were evaluated by ac impedance measurement around OCV and applied dc biases. At 873 and 973 K, δ in LaNi0.6Co0.4O3-δ was below 0.003 in all measured p(O2) range while δ increased to 0.011 at p(O 2) = 10-4 bar at 1073K. From the comparison between chemical capacitance which was estimated from the oxygen nonstoichiometry and evaluated from ac impedance measurement, it was suggested that the ratedetermining step was surface reaction or bulk diffusion. The area specific interfacial conductivity σE for LNC64 film electrode was c.a.5.6 Scm -2 at 1073 K in air. Since the σE tendency in LNC64 film electrode well accorded with the oxygen nonstoichiometry and the capacitance for electrode reaction, it is suggested that ionic conduction was enhanced at 1073 K.
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