An excellent SOFC performance in a mixture of methane and air is achieved by constructing a one-chamber cell from a YSZ solid electrolyte with a 25 wt.% Ce0.8Gd0.2O1.9 (GDC)-added Ni anode and 15 wt.% MnO2-added La0.8Sr0.2MnO3 (LSM) cathode, which are deposited on opposite surfaces (A-type cell) or the same face (B-type cell) of the solid electrolyte. Experiments are carried out in a mixture of methane and air having a volume ratio of methane to oxygen of 1/1 at a flow rate of 300 ml min(-1) (methane 52 ml min(-1); oxygen 52 ml min(-1); nitrogen 196 ml min(-1)) and at an operating temperature of 950 degrees C. The A-type cell shows an increasing power density with decreasing thickness of the solid electrolyte. The maximum power density is 204 mW cm(-2) at a solid electrolyte thickness of 0.3 mm. The B-type cell shows an increasing power density with decreasing gap between the two electrodes. The maximum power density is 102 mW cm(-1) for a gap between the two electrodes of 1 mm. The working mechanism of the one-chamber SOFC and the addition effects of GDC and MnO2 are studied using a two-chamber cell which permits independent evaluations of the Ni-and LSM-based electrodes. (C) 2000 Elsevier Science B.V. All rights reserved.