Detailed Study of Degradation Behavior of Solid Oxide Cells in Electrolysis and Co-Electrolysis Mode

Kurzfassung

High temperature steam and co-electrolysis has a high potential for the efficient production of hydrogen or syngas. 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 behaviour by elucidating the relevant degradation mechanisms. Fuel electrode supported cells containing perovskite-type air electrodes were fabricated by ceramic processing and sintering techniques to be electrochemically characterized in electrolysis and co-electrolysis operating mode. I-V curves and electrochemical impedance spectra have been recorded for cell characterization. For a systematic investigation of the influence of relevant operating parameters such as temperature, current density and fuel gas humidification on long-term degradation a special test bench has been established which allows electrochemical characterization of 4 cells simultaneously under relevant SOEC conditions. This arrangement allows for variation of one distinct operating parameter while keeping other parameters strictly constant. A series of measurements over 1000 hours each in the temperature range 750-850 °C with different fuel gas humidity (40-80 mol%) and different current densities between 0 and 1.5 A/cm2 has been performed in steam electrolysis mode. Additionally, a second series of measurements has started in co-electrolysis mode at different operating temperatures and different steam-to-CO2 ratios. The progress of degradation was monitored in-operando approximately every 150 h by impedance spectroscopy. It was possible to differentiate different electrode processes, a mass transport limitation on the fuel electrode and the electrolyte resistance. Post-mortem investigations have been conducted to localize and identify the limiting processes and to clarify the correlation between degradation processes and operational parameters. In this paper the selection and preparation of electrode materials and the process of cell manufacturing as well as the experimental setup for cell characterization and long-term measurements are described. Results of electrochemical cell characterization performed at different operational conditions in electrolysis and co-electrolysis mode are shown and degradation phenomena observed and their underlying mechanisms based on different electrochemical processes are explained.