In this work, the durability of Ni-YSZ based solid oxide cells was investigated during co-electrolysis of steam and carbon dioxide (45% H2O + 45% CO2 + 10% H-2) at current density of -1.5 or -2.0 A cm(2). The cell consists of similar to 300 mu m Ni-YSZ support, similar to 10 mu m Ni-YSZ electrode, similar to 10 mu m YSZ electrolyte and similar to 15 mu m LSM-YSZ oxygen electrode. The gas conversion was 45% at -1.5 A cm(-2) and 60% at -2.0 A cm(-2), and the operating durations were up to 700 h. The detailed electrochemical analysis revealed significant increase of the ohmic resistance, oxide ion transport resistance in the Ni-YSZ composite electrodes and the electrochemical reaction resistance at the Ni-YSZ triple-phase boundaries. The performance degradation is mainly ascribed to the microstructural change in the Ni-YSZ electrode close to the YSZ electrolyte, including the percolation loss of Ni, the contact loss between Ni and YSZ electrolyte and the decomposition of YSZ close to Ni-YSZ vertical bar YSZ interface. The electrochemical performance and the microstructure of the oxygen electrode were found to be relatively stable. (C) 2016 Elsevier B.V. All rights reserved.