Systematic Parameter Study on the Influence of Humidification and Current Density on SOEC Degradation

Friday, 31 July 2015: 15:00
Alsh (Scottish Exhibition and Conference Centre)
M. P. Hoerlein (German Aerospace Center), G. Schiller (German Aerospace Center (DLR)), F. Tietz (Forschungszentrum Jülich GmbH (IEK-1)), and K. A. Friedrich (German Aerospace Center (DLR))
A special measurement setup with the ability to measure four cells simultaneously in reversible SOEC/SOFC mode was implemented. As preliminary experiment extensive characterization of anode supported Ni-YSZ|YSZ|CGO|LSCF cells provided by Forschungszentrum Jülich was conducted including polarization curves and impedance measurements in the range from 750 °C to 850 °C and fuel gas humidifications from 40 mol% H2O to 80 mol% H2O in SOFC and SOEC mode.

Core of this work is the systematic investigation of the influence of the operating parameters temperature, fuel gas humidification and current density on SOEC long-term degradation and its underlying individual processes. In order to obtain this information a series of five 1000 h experiments with an operating temperature between 750 °C to 850 °C and fuel gas humidification between 40 mol% H2O to 80 mol% H2O was devised. During each measurement over 1000 h four cells are measured simultaneously under identical conditions with the exception of current density where each cell runs galvanostatically with a fixed value between 0 A/cm2 and 1.5 A/cm2.  The progress of degradation was monitored in-situ approximately every 150 h by impedance spectroscopy. It was possible to isolate fuel electrode processes R1+2, an oxygen electrode process R3, a mass transport limitation on the fuel electrode R4 and the electrolyte resistance R0. It could be shown that the processes R0 and R1+2 show a strong correlation between current density and degradation; however the processes R3 and R4 exhibit degradation behavior completely independent of the applied current. Furthermore it could be shown that the oxygen electrode process R3 demonstrates rapid degradation during the first approximately 500 h dominating the overall cell degradation, however slowing down to be negligible after 1000 h.

Post-mortem investigations are being conducted in order to localize and identify the rate limiting processes R0 to R4. In this way we will be able to explain the observed degradation for each process and also validate the observed dependencies ex-situ. Additionally, the remaining long-term experiments are being conducted in order to complete this study and clarify the correlation between degradation processes and fuel gas humidity as well as operating temperature.