More details about the 40000h stack test

On the 1st June I mentioned about an interesting publication from Juelich, where they have successfully completed a 40000h stack test. As this is a very important publication, I think it deserves a bit more comments here.

Together with DTU Energyconversion, group at FZ Juelich are at the forefront of the fuel cell technology, especially when it comes to the stack technology.  In the available literature, no long term testing of stacks were reported so far. With the required lifetime of at least 40000h (US Department of Energy requirements), no one has so far presented such a long experiment. FZJ in the framework of the Real-SOFC project started long term stact test some time ago. This year the test reached over 40000 hours of operation and some conclusions might be drawn.

Technical details of the stack:

• stack F1002-97 is under operation from August 2007
• operating conditions:
  
  • operation temperature: 700C
  • current density: 500 mA cm-2
  • fuel: humidified (20% steam) H2, fuel utilization 40%
  • oxidant: compressed air, oxygen utilization: 25%
  • cathode: LSCF with CGO diffusion barrier layer
  • interconnect: ITM-Plansee with MnO protective coating
    

As the interconnect with MnO coating cannot be too protective, in the next stack (F1004-21), Crofer 22 APU with a Mn,Co,Fe protective spinel was used deposited by atmospheric plasma spraying. This stack has so far reached 15000 hours of operation. It's performance has been far superior (degradation of only 0.12%/kh) to the one with MnO coating (degradation of 0.9%/kh).

The critical degradation phenomena are therefore Cr poisoning from the interconnect. Developing of a successful coating is therefore a direct need. Other degradation phenomena has been identified as the Mn diffusion from the cathode to the electrolyte. This weakens the electrolyte and can cause cracking of the cell.