Dense BaCe0.8Sm0.2O2.90 (BCSO) thin films were successfully fabricated on porous NiO-BCSO substrates by dry pressing process. As characterized by scanning electron microscope, the BCSO films were about 50 gm. With Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF) as cathodes, single cells were tested at 600 and 700 degrees C with humidified (3% HB2O) hydrogen as fuel and oxygen as oxidant. The open circuit voltage of 1.049 V at 600 degrees C and 1.032 V at 700 degrees C were achieved, indicating negligible gas permeation through the BCSO thin films. Maximum power densities of 132 and 340 mW/cm(2) were obtained at 600 and 700 degrees C, respectively. The impedance measurements at open circuit conditions showed that there were two rate-limiting processes for the electrode reactions and that the cell performances were essentially determined by the electrode polarization resistances at temperature below 650 degrees C, which implied that it was essential to reduce the electrode polarization by developing novel electrode materials to improve the performance of ITSOFC based on BCSO electrolyte. Conductivities of BCSO under the cell operating circumstances were obtained as 0.00416, 0.00662 and 0.00938 Scm(-1) at 500, 600 and 700 degrees C, respectively. The activation energy of BCSO conductivity was calculated as 29.5 and 43.8 kJ/mol for the temperature range of 550-700 degrees C and of 400-550 degrees C, respectively. Endurance test was firstly carried out with 75 mu m BCSO electrolyte at 650 degrees C at the operating voltage of 0.7 V and current density about 0.12 A/cm(2). Both voltage and current density remained stable for 1000 min. (c) 2005 Elsevier B.V. All rights reserved.