This work investigates the activation and delamination of La0.6Sr0.4Co0.2Fe0.8O3-delta (LSCF) air electrode of solid oxide electrolyzer cells sintered on yttria-stabilized zirconia (YSZ) electrolyte. After polarization with an electrolysis current of 1 A cm(-2) for 24 h at 800 degrees C, the LSCF electrode delaminates accompanied by an increase of ohmic and polarization resistance. Notably, polarization resistance decreases at the beginning. By scanning electron microscopy (SEM), a thin but dense layer is observed at the LSCF YSZ interface of an as prepared sample, which is identified as SrZrO3 phase by X-ray diffractometry. This layer causes the initial high polarization resistance due to retarded ionic and electronic conductivity. After the test, SEM reveals that the SrZrO3 layer delaminates from YSZ electrolyte. Moreover, energy dispersive X-ray tests confirm that Co diffuses to the SrZrO3 layer and SrZrO3 YSZ interface. Later, the LSCF electrode with Co-containing SrZrO3 layer is shown to perform better than that with pure SrZrO3 layer. Thus, Co diffusion can be the reason for the initial decrease of polarization resistance and renders the generation of oxygen at SrZrO3-YSZ interface during the electrolysis. Owing to its limited porosity, the SrZrO3 layer traps the generated oxygen. High pressure eventually builds up at the SrZrO3 YSZ interface driving the delamination of SrZrO3 layer, and hence the entire LSCF electrode. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.