A nanostructured cathode is fabricated by incorporating a mixed ionic and electronic conducting (MIEC) perovskite, Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF), via ion impregnation into the most common, highly electronic conducting, and structurally stable La0.8Sr0.2MnO3-delta (LSM) porous cathode skeleton. The introduction of nanosized MIEC BSCF particles significantly improves the electrocatalytic activity of the LSM for the oxygen reduction reaction of solid oxide fuel cells at an intermediate temperature range of 600-800 degrees C. The electrode polarization resistance of a 1.8 mg cm(-2) BSCF-impregnated LSM cathode is 0.18 Omega cm(2) at 800 degrees C, which is similar to 12 times lower than that of pure LSM. A single cell with an yttria-stabilized zirconia (YSZ) electrolyte film and the nanostructured BSCF/LSM cathode exhibits maximum power densities of 1.21 and 0.32 W cm(-2) at 800 and 650 degrees C, respectively. The atomic force microscopy (AFM) studies of the electrode/electrolyte interface before and after polarization indicate that the impregnation of BSCF extends the three-phase boundary area for the oxygen reduction reaction from the electrode/electrolyte interface to the electrode bulk. The formation of a second phase was also observed by AFM for the BSCF-impregnated LSM after being heat-treated at 800 degrees C though its phase could not be identified due to the extremely small amount of the second phase particles on the YSZ electrolyte surface. The initial structure and polarization performance stability of the nanostructured BSCF-impregnated LSM composite cathodes are also investigated. (C) 2010 The Electrochemical Society. [DOI: 10.1149/1.3428366] All rights reserved.