The mixed ionic and electronic conductors (MIECs) of Sm0.5Sr0.5Co0.4Ni0.6O3-delta (SSCN)-Sm0.2Ce0.8O1.9 (SDC) were investigated for potential application as a cathode material for intermediate-temperature solid oxide fuel cells (IT-SOFCs) based on an SDC electrolyte. Electrochemical impedance spectroscopy (EIS) technique was performed over the temperature range of 600-850 degrees C to determine the cathode polarization resistance which is represented by area specific resistance (ASR). To investigate the ORR mechanism, the impedance diagram for 70SSCN-30SDC was measured under applied cathodic voltage from E = 0.0 to E = -0.3 V. It indicated that the charge transfer dominated the rate-determining step at the temperature of 600 degrees C; whereas the diffusion or dissociative adsorption of oxygen dominated the rate-determining step at the temperature of 800 degrees C. In this study, the exchange current density (i(0)) for oxygen reduction reaction (ORR) was determined from the EIS data. The i(0) value of 70SSCN-30SDC/SDC was 187.6 mA cm(-2) which is larger than the i(0) value of 160 mA cm(-2) for traditional cathode/electrolyte, i.e. LSM/YSZ at 800 degrees C, indicating that the 70SSCN-30SDC composite cathode with a high catalytically active surface area could provide the oxygen reduction reaction areas not only at the triple-phase boundaries but also in the whole composite cathode. (C) 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.