Study on Degradation of Solid Oxide Cells during Electrolysis and Co-Electrolysis Operation

High temperature steam and co-electrolysis have a high potential for the efficient production of hydrogen or syngas as energy carriers for a sustainable energy supply. For a further development of this promising technology, development work on materials and cells as well as extensive operational experience is still needed. A main objective is to develop highly efficient and long-term stable cells and stacks using novel electrode materials and to improve the degradation behavior by elucidating the relevant degradation mechanisms. In this paper we report a systematic investigation under relevant operating conditions such as temperature, fuel gas composition and current density on the long-term behavior of solid oxide cells. A special test bench allowing the electrochemical characterization of four cells simultaneously has been established. This arrangement allows for variation of one distinct operating parameter while keeping other parameters strictly constant. Fuel electrode and metal supported cells containing perovskite-type air electrodes fabricated by ceramic processing and sintering techniques have been electrochemically characterized in electrolysis and co-electrolysis operating mode by monitoring I-V curves and electrochemical impedance spectra. A series of measurements over at least 1000 hours each in the temperature range of 750-850 °C with different fuel gas compositions including different steam-to-carbon ratios and different current densities between 0 and 1.5 A/cm2 has been performed in steam electrolysis and co-electrolysis operating mode. The progress of degradation was monitored in-operando approximately every 150 h by impedance spectroscopy measurements. Post-test investigations have been conducted to link the changes in electrochemical behavior with the microstructural changes and to correlate them with operational pa-rameters. Results of electrochemical cell characterization performed at different operational conditions in electrolysis and co-electrolysis mode as well as on degradation phenomena observed are presented. Underlying mechanisms based on different electrochemical processes will be discussed