Electrochemical characterization of Ni-yttria stabilized zirconia electrode for hydrogen production in solid oxide electrolysis cells

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 H2 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 (>104 Hz) can be assigned to transfer of the charged species across the Ni/YSZ interface, and the mid-frequency arc (102∼103 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 (H2 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 H2 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 H2 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.