Steam electrolysis (H(2)O -> H(2) + 0.5O(2)) and carbon dioxide electrolysis (CO(2) -> CO + 0.5O(2)) have been investigated in solid oxide electrolysis cells (SOECs). Due to similarity to solid oxide fuel cells (SOFCs), advances have been made in the development of high temperature SOECs based on cell assemblies composed of nickel-yttria stabilised zirconia (Ni-YSZ) hydrogen electrode/YSZ electrolyte/lanthanum strontium manganite-YSZ (LSM-YSZ) oxygen electrode; however, the cell operated under SOEC mode was found to exhibit lower performance than SOFC mode. Research is therefore driven towards the selection of materials for SOEC compartments. The work reported here seeks to investigate the performance of SOEC electrolytes. The use of bi-layered gadolinium doped ceria (GDC)/YSZ electrolyte was investigated compared to GDC and YSZ electrolyte. Ni-GDC was used as an H(2) electrode while LSM-YSZ was used as an O(2) electrode. The study was carried out over a range of SOEC operating conditions. This included controlling the partial pressure of H(2)O, H(2), CO(2), and CO in the process gas, operating temperature of 800 degrees C, applied voltage (OCV to 1.5 V), and electrolysis current. The bi-layered YSZ/GDC electrolyte cell exhibited significantly higher performance when compared to the cell using YSZ or GDC electrolyte. The EIS response showed that the ASR of the cell was lower in SOEC mode when compared to SOFC mode. However, the cell exhibited performance decay during steam electrolysis (H(2)O:H(2) = 50:50, 800 degrees C) over 93 h (1 mV h(-1)). The cell was also investigated toward CO(2) electrolysis. Comparable performance under steam electrolysis and coelectrolysis was observed while the performance was lower when operating in the CO(2) electrolysis mode. (c) 2010 Elsevier B.V. All rights reserved.