Quantifying Galvanostatic Degradation of Solid Oxide Electrolysis Cells: The onset of accelerated degradation of Ni-yttria stabilized zirconia electrode

Galvanostatic operation of solid oxide electrolysis cells (SOECs) at high current densities and low temperatures would enable faster implementation and enhance the competitiveness of SOEC technology. However, the fuel electrode of Ni-yttria stabilized zirconia-based cells (Ni-YSZ) experiences considerable degradation at high current densities. This study investigates the long-term durability of Ni-YSZ fuel-electrode supported SOECs operated galvanostatically for steam electrolysis at different current densities and temperatures. Detailed electrochemical evaluation of the cells reveals that: The short-term degradation of the fuel electrode is reflected on the rapid increase of the fuel electrode polarization resistance (R-Fuel) while in the long-term, its degradation primarily stems from a continuous increase of ohmic resistance (R-Ohmic). Microstructure analysis suggests that physical detachment of Ni from YSZ is not a prerequisite for Ni migration but constitutes an additional degradation phenomenon. More specifically, the Ni-YSZ electrode exhibits accelerated degradation, when the fuel electrode overpotential (eta(Fuel)) exceeds -205 mV (at 800 degrees C) and -285 mV (at 750 degrees C). Upon surpassing these overpotential thresholds, a low-frequency inductive contribution emerges in the Nyquist plots of EIS. Consequently, this operando emergence should be related to the accelerated degradation of the Ni-YSZ electrode, initiated by the Ni/YSZ detachment and impurities inclusion phenomena.