Solid oxide electrolyser cells (SOEC) operated for 6100 h and 9000 h with a current density of -0.75 and -1 A/cm(2), respectively, were analyzed and compared with a non-operated cell. The cathode-supported cells consisted of an 8 mu m thick Ni/8YSZ cermet as hydrogen cathode, a 10 mu m thick 8YSZ electrolyte, a screen-printed Ce0.8Gd0.2O1.9 diffusion barrier with a thickness of 5 mu m and a 30 mu m thick La0.58Sr0.4Co0.2Fe0.8O3 layer as oxygen anode. The cells were investigated by various electron microscopy as well as microanalytical techniques. The post-test analyses showed several degradation phenomena such as formation of nano-sized pores at grain boundaries, formation of SrZrO3 at the interface electrolyte/anode and agglomeration of nickel particles in the cathode. Comparisons of the operated cells with the non-operated cell indicate that nickel depletion in the cathode is responsible for a loss of performance since this depletion leads to a significant enlargement of electrolyte thickness. Analysis of these cells leads to the conclusion that two mass transport processes in the electrolyte caused by different driving forces are the main reason of the cell performance loss: at the cathode side, the electrical potential gradient appears to govern the formation of pores, while at the anode side, formation of SrZrO3 generates a chemical potential gradient causing the cations to migrate against the direction of the electric field. (C) 2014 Elsevier B.V. All rights reserved.