The performance of a proton-conducting ceramic-electrolyte fuel cell (PCFC) with BaCe0.85Y0.15O3-delta (BCY15) electrolyte, NiO-BCY15 anode and La0.8Sr0.2MnO3 (LSM) cathode is analyzed in different gas conditions in 600-750 degrees C range. The distribution of relaxation time (DRT) analysis of the electrochemical impedance spectroscopy (EIS) data obtained by varying the gas-flow rate. water vapor pressure (pH(2)O) and oxygen partial pressure (pO(2)) is performed to identify the major electrode sub-processes involved in overall electrode reactions. The study of variations in cathodic pO(2) shows that the cathodic polarization due to the charge transfer reaction is the major contributor toward the overall electrode polarization resistance. The effect of electrolyte thickness on the fuel cell performance is also studied and a peak power density of similar to 0.76 W.cm(-2) at a current density of similar to 1.95 A.cm(-2) is achieved at 750 degrees C with 8 mu m thick electrolyte. The PCFC shows stable performance without any sign of material degradation during the long term operation at a fixed input current for 100 h. The fuel cell performance is compared with those of similar PCFCs with Ba0.5Sr0.5Co0.8Fe0.2O3-delta (BSCF5582), LSM-GDC, and LSM-BCY15 as cathodes, and possible reasons for the observed difference is discussed in terms of the nature of their conductivity. (C) 2015 The Electrochemical Society. All rights reserved.