Reverse fuel cell operation, i.e. electrolysis, is interesting for the storage of excess electricity from renewables (hydro, PV, wind,..) into fuel. Low temperature reverse PEFC split water into hydrogen and oxygen, whereas high temperature reverse SOFC (called SOEC – solid oxide electrolysis cells) split steam and CO2 into synthesis gas (H2-CO-mixture), which is then further upgraded to methane (for injection into the natural gas grid) or to other valuable transportable and storable fuels.