A composite oxygen electrode based on Co3O4-loaded La0.8Sr0.2MnO3 (LSM)-BaCe0.5Zr0.3Y0.16Zn0.04O3-delta (BCZYZ) is investigated for steam electrolysis in a proton-conducting solid oxide electrolyzer. The conductivity of LSM is studied with respect to temperature and oxygen partial pressure and correlated to the electrochemical properties of the composite oxygen electrodes in symmetric cells and solid oxide electrolyzers at 800 degrees C. The optimal Co3O4 loading in the composite oxygen electrode is systematically investigated in symmetric cells; loading catalytically active Co3O4 significantly enhances the electrode performance, unlike the bare LSM-BCZYZ electrode. Steam electrolysis was then performed using LSM-BCZYZ and 6 wt.% Co3O4-loaded LSM-BCZYZ oxygen electrodes, respectively. The Co3O4-loaded catalyst significantly improves the electrode process and enhances the current density below a certain applied voltage. The current efficiencies reach approximately 46% with a 10% H2O/air feed for the oxygen electrode. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.