Solid electrolyte membrane reactor (SEMR) based on the Ni-YSZ (yttria-stabilizecl zirconia) supported YSZ electrolyte membrane was developed and used in the high-temperature electrochemical reduction of steam, CO2 and a mixture of steam-CO2. Effect of different operating parameters such as temperature, Iced gas composition, and operating voltage on the performance of SEMR in a high steam and/or CO2 environment was investigated. Experiments were performed in the temperature range of 750-850 degrees C with 20% H-2-80% oxidant as a feed to the Ni-YSZ electrode and air to the LSM electrode. The electrochemical performance was investigated using the current density-voltage (i-V) curves and electrochemical impedance spectra (EIS). A high area-specific resistance (ASR) was observed in a fuel cell mode as compared to the electrolysis mode because of very low fuel content and consequently, a higher concentration polarization. At 850 degrees C and 1.5 V, a very high current density of -1.9 A/cm(2), -1.5 A/cm(2) and -1.2 A/cm(2) is observed for steam, steam-CO2 (co-electrolysis) and CO2 electrolysis respectively. The decrease in electrochemical performance with increasing CO2 content was found to be related to the mass transport limitation of the fuel electrode. An equivalent circuit model was used to fit the impedance data and separate the various polarization losses in the cell. EIS results showed that different performance limiting step is involved depending on the operating conditions. At low operating temperature and/or cell voltage, the polarization losses at LSM oxygen electrode contributed significantly to total ASR of the cell, whereas at high operating temperature and cell voltage, the concentration polarization at fuel electrode led to a decrease in the cell performance. (C) 2014 Elsevier B.V. All rights reserved.