Fuel cells are promising devices for clean energy conversion. In particular, Solid Oxide Fuel Cells (SOFC) convert natural gas and biogas into electricity and cogenerated heat with high efficiency. Their working temperature (700-800°C) may induce performance degradation in the cells and other components of the stack (series connection of cells). Chromium contamination of the cathode (oxygen electrode) is one of these degradation processes in SOFC. This chromium evaporates from the steel bipolar plates which act as cell-to-cell interconnections. To suppress the Cr evaporation, the steel plates must be coated. Understanding the degradation phenomena in the stack and their evolution helps to improve the device lifetime. Post test analyses are not always sufficient but already give good insight. On the other hand, the possibility to reach operating temperature (up to 900°C) and to mimic operating conditions of oxidising and reducing atmospheres (up to 30 Pa of O₂ or H₂) in environmental SEM allow to observe the evolution in near real condition. In this study, Sandvik Material Technology’s cerium-cobalt protective PVD coating on top of the Sandvik SSHT steel (FeCr-base) is analysed. Several movies were recorded during Cr-oxidation growth underneath the coating for 48 hours in an ESEM in oxidising conditions (900°C, 30Pa O₂). Growth observation of the oxide layer allows a better understanding of the behaviour of the steel with this protective layer. Pore stability, elemental diffusion through layers and the probable formation of new phases were observed by SEM and EDX. FIB was used for cross section observations and will be used for TEM lamella preparation in future. TEM observation will allow to analyse the formed crystals by diffraction and establish a correlation with  the observations from the ESEM experiments. 

Acknowledgements

Max Planck Institute – EPFL centre for the funding of this project.
Dr Marco Cantoni(EPFL-CIME) for FIB preparation.

Figures:

Fig. 1: SE images of surface oxidation. Crystal growth and evolution is visible.

Fig. 2: SE images of surface oxidation from 42 hours under oxidising conditions, crystal coarsening. Layer growth can be seen in the red circle.

Fig. 3: SE image (in-lens detector) of FIB cross section of oxidised sample (48 hours, 900°C, 30Pa O2).

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EMC Abstracts - https://emc-proceedings.com/abstract/cobalt-cerium-coating-formation-from-esem-to-tem-analyses/