Electrochemical performance of Ni-yttria stabilized zirconia (YSZ) electrode, which is widely used as the anode for solid oxide fuel cells (SOFCs), is evaluated for H-2 production in solid oxide electrolysis cells (SOECs). The impedance spectra of Ni-YSZ electrode are composed of two major depressed arcs. The high-frequency impedance (>10(4) Hz) can be assigned to transfer of the charged species across the Ni/YSZ interface, and the mid-frequency arc (10(2)similar to 10(3) Hz) is possibly associated with the gas-solid interaction such as adsorption, dissociation, desorption, etc. The impedance spectra are strongly influenced by the gas composition in both fuel cell (H-2 oxidation) and electrolysis (H2O reduction) reactions; polarization resistance decreases with increasing concentration of the reactant species. In fuel cell mode, both high- and mid-frequency arcs increase with decreasing H-2 concentration, while the impedance spectra are dominated by the mid-frequency arc and the high-frequency arc remains negligible even at low H2O concentration in electrolysis mode. Reaction mechanisms and elementary reaction pathways for H-2 oxidation and H2O reduction are suggested based on impedance spectra. The minimum impedance is observed at 50% H2O in electrolysis mode, and further increase in H2O concentration causes degradation of the electrode performance, possibly due to local oxidation of Ni. (C) 2013 Elsevier B.V. All rights reserved.