Understanding SOEC Degradation Processes by means of a Systematic Parameter Study

Kurzfassung

Solid Oxide Electrolysis Cell (SOEC) technology is a promising approach for storing large quantities of electrical energy. One major obstacle for wide spread implementation is the cell’s limited durability due to a number of degradation phenomena. Therefore, understanding the origin and evolution of degradation processes is essential for developing long-term operating SOEC technology. The present study aims at identifying and characterizing the predominant degradation phenomena of planar Ni/YSZ|YSZ|CGO|LSCF anode supported cells from CeramTec, Germany, operating on H2/H2O mixtures, by investigating the influence of operating parameters on cell deterioration. In order to systematically study the influence of temperature in a range between 750°C and 850°C and fuel gas humidity in a range between 40 mol.-% and 80 mol.-% on SOEC degradation, a matrix of experiments over 1000 hours each was devised. Furthermore, the influence of the current density was determined in a range between OCV and 1.5 A·cm‒2 for each investigated combination of temperature and humidity, thereby gaining insight into coupled correlations between operating parameters and SOEC degradation. In order to shed light on underlying physico-chemical processes and performance limiting factors a detailed physico-chemical modeling approach was employed. A more detailed description of the model and the results are given in a second contribution to this conference. Periodic in-situ impedance measurements were used to track the development of each individual process over the duration of the experiment. The observed degradation characteristics of each process vary greatly including constant degradation, decreasing degradation and stable behavior, depending on the nature of the processes as well as the operating parameters applied. Finally the in-situ degradation observations were substantiated by post mortem analyses. In order to identify microstructural changes especially those appeared in electrolyte and fuel electrode, SEM measurements were conducted while EDX measurements were used to monitor elemental enrichment or depletion as well as impurity deposition. Furthermore, changes in surface and bulk crystallography were investigated by XPS and XRD measurements. Hence, this systematic experimental study on SOEC degradation coupled with the physico-chemical understanding from the modeling approach allows not only the development of strategies for increasing cell’s lifetime but could also be used to determine experimental protocols for accelerated degradation.