Anode-supported planar solid oxide fuel cells (SOFCs) with and without the anode active layer were fabricated by a single-step cofiring process. The cells were comprised of a porous Ni+yittria-stabilized zirconia (YSZ) anode support, a porous fine-grained Ni+YSZ anode active layer for some experiments, a dense YSZ electrolyte, a porous fine-grained Ca-doped LaMnO3 (LCM)+YSZ cathode active layer, and a porous LCM cathode current collector layer. The fabrication process involved tape casting of the anode support followed by screen printing of the remaining component layers. The cells were then cofired at 1300 degrees C for 2 h. Sintered cells were electrochemically tested between 700 and 800 degrees C with air as oxidant and various compositions of humidified hydrogen as fuel to simulate the effect of fuel utilization on cell performance. Experimentally measured current density-voltage (I-V) characteristics of the cells were fitted into a polarization model, and the cell parameters, including the area-specific ohmic resistance, exchange current density, anodic limiting current density, cathodic limiting current density, effective binary diffusivity of hydrogen and water vapor in the anode, and that of oxygen and nitrogen in the cathode, were obtained. Evaluation of the electrochemical performance and the polarization modeling results indicated that the cell performance is dominated by the cathodic activation polarization at low fuel utilization condition. However, the cell performance loss due to the anodic activation polarization increases as the fuel utilization increases. The anode active layer significantly improves the cell performance at high fuel utilization by lowering the anodic activation polarization under H2O-rich fuel because the anode active layer has finer and less porous microstructures and increases the number of the reaction sites near the anode-electrolyte interface. Cell performance at high fuel utilization and the effect of the anode active layer on the cell performance were discussed in detail. (C) 2008 The Electrochemical Society.