Electronic Conduction of Yttria-Stabilized Zirconia Electrolyte in Solid Oxide Cells Operated in High Temperature Water Electrolysis

Anode-supported solid oxide fuel cells with yttria-stabilized zirconia (YSZ) electrolytes, both commercial and research, are used at 800-900 degrees C as solid oxide H2O electrolyzer cells (SOECs). When the operation is extended to current densities j corresponding to a steam-conversion rate above 100%, or when the steam supply is interrupted under constant current conditions, cell voltages saturate at similar to 1.9 V at 810 degrees C. A cell survives 64 h polarization at j=-0.34 A cm(-2) without any steam supply. The mechanism limiting the cell voltage is attributed to electronic conduction in the YSZ electrolyte. No indications are found for electrolyte decomposition. Because the saturation voltage exceeds typical operation voltages by several hundred millivolts, electrolyte conduction in the normal SOEC mode remains predominantly ionic. The rise in the cell voltage to the value determined by electronic conduction occurs when steam transport in the hydrogen/steam electrode becomes limiting. As a consequence of H2O back-diffusion from the cell exhaust in the used unsealed test configuration, the rise occurs at nonzero current density under zero steam supply. Impedance spectroscopic data are compared with and without steam supply and are qualitatively interpreted with an equivalent circuit model.