The chemical stabilities of modified NiO cathodes doped with 1.5 mol% CoO and 1.5 mol% LiCoO2 fabricated by a conventional tape casting method were evaluated through the real MCFC single cell operation. The heat-treated samples before oxidation had proper porosities and microstructures for a NICK cathode. At 150 mA cm(-2) in current density, the MCFC single cell using a CoO-doped NiO cathode showed stable cell voltages in the range of 0.833-0.843 V for 1000 h. In contrast, the cell using a LiCoO2-doped NiO cathode with a maximum of 0.836 V at 500 h degraded to 0.826 V at 1000 It due to a wet seal breakdown at the cathode side. The amounts of nickel precipitated in the electrolytes of the cells using modified NiO cathodes doped with Coo and LiCoO2 after the operation for 1000 h were 1.2 and 1.4 wt.%, respectively, which were about 60% lower than that of the standard cells using pure NiO cathodes. The enhanced chemical stability of modified NiO cathodes seems to be attributed to the fact that the presence of cobalt increases the lithium content in the cathodes by converting Ni2+ to Ni3+, resulting in stabilizing the layered crystal structure. (C) 2004 Elsevier Ltd. All rights reserved.