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The overpotential of Ni/yttria-stabilized zirconia cermet anodes used in H-2-H2O solid oxide fuel cells (SOFCs) was studied. We measured the current-density dependence of the anodic overpotential at 850 degrees C operating temperature using the inlet fuel,oases with compositions of 100% H-2, 80% H-2 + 20% H2O, and 20% H-2 + 80% Ar. It was found that the overpotential was lower under wet-anode conditions than under dry conditions. We also showed that under wet-anode conditions the overpotential gradually increased with the increase of the current density, whereas under dry anode conditions, the overpotential increased steeply with the increase of the current density up to 20 mA/cm(2) and then showed a gradual increase with the current density. We related the overpotential of H-2-H2O SOFCs to the reaction rate constants and to the operating conditions by using a reaction model and found that the surface reaction model can successfully explain the experimental results. (C) 2000 The Electrochemical Society. S0013-4651(00)02-001-2. All rights reserved.