A solid oxide fuel cell (SOFC) test unit was constructed with yttria-stabilized zirconia (YSZ) as the solid electrolyte, Ni-YSZ as the anode and Pt as the cathode. The SOFC unit was tested by varying hydrogen supply time, voltage, and temperature at 700-900 degrees C. The phenomena of electrochemical promotion of bulk lattice-oxygen extraction and fuel-free current have been observed. The amount of lattice oxygen additionally extracted by electrochemical promotion is limited. This limitation is more pronounced as the operating temperature increases. The interstitial hydrogen species are formed as either the interstitial hydrogen atoms in the bulk of Ni electrode or the interstitial protons in the bulk of YSZ. The amount of the interstitial hydrogen species is also limited. Oxygen transport in YSZ to the YSZ-Ni interface is rate limiting for H2O formation over the anode. These new characteristics on the electrochemical behavior can optimize the operating conditions and the designing features of hydrogen-fed SOFCs. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.