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Electrochemical impedance, cyclic voltammetry, and chronoamperometry have been used for characterization of the medium-temperature half-cells Ce 0.8 Gd 0.2 O 1.9 ͉La 0.6 Sr 0.4 CoO 3−␦ ͑Sys 1͒, Ce 0.8 Gd 0.2 O 1.9 ͉Pr 0.6 Sr 0.4 CoO 3−␦ ͑Sys 2͒, and Ce 0.8 Gd 0.2 O 1.9 ͉Gd 0.6 Sr 0.4 CoO 3−␦ ͑Sys 3͒ at various electrode potentials and temperatures. Analysis of the impedance data shows that the kinetically mixed process is probable, characterized by the slow electron transfer to an adsorbed and thereafter dissociated oxygen atom O ads , as well as by slow mass transfer of electroactive species inside the cathode or O ads at the internal surface of the porous cathode. The total polarization resistance increases with rising the atom mass of the A-site cation in the porous perovskite structure. The values of activation energy decrease slightly with increasing negative polarization and in the order of half-cells Sys 3 Ͼ Sys 2 Ͼ Sys 1. The transfer coefficient ␣ c over 0.5 indicates the deviation of the mainly charge-transfer-limited process toward the mass-transfer-limited process in the porous cathode with decreasing temperature. The electrochemical characteristics of half-cells Sys 2 and Sys 1 are stable under repetitive potential and thermocycling during long operation times ͑t Ͼ 1200 h for Sys 2 and t Ͼ 4800 h for Sys 1͒.

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