Study of Detailed Degradation Behavior of Solid Oxide Electrolyzer Cells (SOEC)

Kurzfassung

High temperature 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. Within a joint project in the frame of the “Helmholtz Energy Alliance” German Aerospace Center (DLR) and Forschungszentrum Jülich (JÜLICH) started a detailed study on the performance and degradation behaviour of planar SOE cells. Cathode-supported cells containing novel perovskite-type air electrodes were fabricated by ceramic processing and sintering for electrochemical characterization in electrolysis operating mode. A special test bench has been installed which allows measuring polarization curves of 4 cells simultaneously under relevant SOEC conditions. For a systematic investigation of the influence of the operating parameters temperature, fuel gas humidification and current density on SOEC long-term degradation a series of measurements over 1000 hours 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. 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 rate 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 are shown and observed degradation phenomena and their underlying mechanisms based on different electrochemical processes are explained.