The electrochemical properties of an Sm0.5Sr0.5CoO3-delta/Co3O (SSC/Co3O4) composite cathode were investigated as a function of the cathode-firing temperature, SSC/Co3O4 composition, oxygen partial pressure and CO2 treatment. The results showed that the composite cathodes had an optimal microstructure at a firing temperature of about 1100 degrees C, while the optimum Co3O4 content in the composite cathode was about40 wt.%. A single cell with this optimized C-40-1100 cathode exhibited a very low polarization resistance of 0.058 Omega cm(2), and yielded a maximum power density of 1092 mW cm(-2) with humidified hydrogen fuel and air oxidant at 600 degrees C. The maximum power density reached 1452 mW cm(-2) when pure oxygen was used as the oxidant for a cell with a C-30-1100 Cathode operating at 600 degrees C due to the enhanced open-circuit voltage and accelerated oxygen surface-exchange rate. X-ray diffraction and thermogravimetric analyses, as well as the electrochemical properties of a CO2-treated cathode, also implied promising applications of Such highly efficient SSC/Co3O4 composite cathodes in single-chamber fuel cells with direct hydrocarbon fuels operating at temperatures below 500 degrees C. (c) 2008 Elsevier B.V. All rights reserved.