Long Term Testing of Short Stacks with Solid Oxide Cells for Water Electrolysis

Results from a 4000 h test of a solid oxide fuel cell (SOFC) stack operated in the H2O electrolyser mode are reported. The stack, which consisted of 5 cells with electrolyte supported cells, each of 100 cm(2) active area, was operated at 820 C under a current density of -0.4 Acm(-2), an absolute feed gas humidity of 90%, and a steam conversion of 40%. Faradaic efficiencies close to 100% and initial cell voltages of similar to 1.2 V were achieved, which translates to high electrical-to-chemical energy conversion efficiency. Average (cell + interconnect) voltage degradation under constant current operation amounted to 5.6% per 1000 h. The cell specific, partly reversible, voltage evolution with time together with impedance spectroscopic results indicates that a significant part of degradation arises from ohmic losses at the cell interconnects. The response of the stack to a loss of steam supply under current flow is described and post-mortem analysis is reported.