Steam Electrolysis in Reversibly Operated SOFC: Long-Term Cell Testing Beyond 1000 h

Abstract

Long-term operation of both electrolyte and (hydrogen) electrode supported solid oxide cells as steam electrolyser is reported. The electrode supported cell was operated during 6100 h under a current density of j = -0.75 Acm -2 at 778-780°C, the electrolyte supported cell during 2200 h under j = -0.5 Acm -2 at 834°C. Steam conversion was 40 %. Cell voltages were in the range of 1.0 to 1.1 V during the entire tests, largely below those of alkaline or polymer electrolyte membrane cells. Voltage degradation was below 1 %/1000 h (<13 mV/1000 h) in both tests, which is sufficient for practical application. Periodically taken OCVs remained constant, i.e. there was no increase in non-ionic electrolyte conduction. Impedance spectroscopic measurements showed that the increase in the cell resistance with time was both due to increasing ohmic and reactive terms at the electrode supported cell. At the electrolyte supported cell the ohmic rise prevailed.