In this work, Co-doped NiFe2O4 spinels (NFCO-x) are successfully fabricated and characterized as possible cathode materials for the intermediate-temperature solid oxide fuel cells (SOFC). Results of the binding energy calculations using the density functional theory suggest that the reverse spinel structure is stable when Co3+ occupies the octahedral interstitial sites. Total and ionic-only conductivities indicate that NFCO-x are a kind of mixed electronic-ionic conductors. Ionic transferring numbers are approximately 0.049 and 0.006 for NFCO-0.1 and NFCO-0.5, respectively, measured at 700 degrees C in air. Co dopant in the NFCO-x improves the electronic conductivity at the expense of the ionic conductivity. For NFCO-0.5, electronic and ionic conductivities are approximately 0.24 and 9.6 x 10(-4) S cm(-1), respectively, measured also at 700 degrees C in air. Unlike behaviour of the conductivities, the polarization resistance of symmetric cells with NFCO-x electrodes decreases when increasing the Co content (x) to a certain level, and then increases. The cell containing the NFCO-0.5 electrode exhibits the lowest polarization resistance (R-p), which is approximately 1.51 Omega cm(2) at 650 degrees C. For single cells, the maximum power density is 320 mW cm(-2) measured at 650 degrees C using a 38-mu m-thick SDC electrolyte and an NFCO-0.5 cathode. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.