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

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.