Anode-supported planar solid oxide fuel cells (SOFCs) were successfully fabricated employing a single-step cofiring process. The cells were comprised of a Ni + yttria-stabilized zirconia (YSZ) anode, a YSZ electrolyte, a Ca-doped LaMnO3 (LCM) + YSZ cathode active layer, and an LCM cathode current collector layer. The fabrication process involved tape casting of the anode, screen printing of the electrolyte and the cathode, and single-step cofiring of the green-state cell in the temperature range of 1300-1330 C for 2 h. The maximum power densities were 1.50 W/cm(2) at 800 degrees C, 1.20 W/cm(2) at 750 degrees C, and 0.87 W/cm(2) at 700 degrees C, with humidified hydrogen (97%H-2-3% H2O) as fuel and air as oxidant. The experimentally measured voltage-current density (V-i) curves were fitted into a polarization model to obtain the area specific ohmic resistance, exchange current density (anodic and cathodic), anodic limiting current density, cathodic limiting current density, and effective binary diffusivity of hydrogen and water vapor in the anode as well as that of oxygen and nitrogen in the cathode. The cell was also tested at 800 S C with various compositions of humidified hydrogen to simulate the effect of practical fuel utilization on the performance of single cells. The V-i curves obtained in various fuel compositions were successfully modeled by fitting only the exchange current density. Anodic and cathodic activation polarizations and the exchange current densities at various fuel compositions were determined. An analytical model describing H-2-H2O reaction at the anode triple-phase boundaries was postulated based on the relationship between the anodic exchange current density and the hydrogen partial pressure in the fuel. The model predicted that the formation of water molecules from adsorbed hydrogen and hydroxyl radical was the rate-determining step in the anodic reaction. (c) 2007 The Electrochemical Society.