Electrochemical Impedance Spectroscopy (EIS) as well as Galvanostatic measurements were conducted on a BioLogic VMP-300 potentiostat. Galvanostatic charging and cycling was conducted in Swagelok-type cells which are assembled in an argon-filled glovebox and sealed using PTFE seals which have been confirmed via helium-leak testing to show a leak rate smaller than 10 -9 mbar l s -1 . Cycling with stack pressure was conducted in a hot-press as described elsewhere . Inert atmosphere for these pressure cells was not ensured by sealing, but by operation inside an argon-glovebox.