Investigation of the Long-term Stability of Solid Oxide Electrolysis Stacks under Pressurized Conditions in Exothermic Steam and Co-electrolysis Mode

In this study three identically constructed ten-layer stacks with electrolyte supported cells were tested in exothermic steam and co-electrolysis mode at elevated pressures of 1.4 and 8 bar. Investigations during constant-current operation at a current density of -0.5 A cm(-2)and a reactant conversion of 70% over 1,000-2,000 h were carried out. The inlet gas composition for steam electrolysis was 90/10 (H2O/H-2) and 63.7/31.3/3.3/1.7 (H2O/CO2/H-2/CO) for co-electrolysis operation. All stacks showed highly similar resistances at the beginning of the tests indicating a high level of accuracy and repeatability during manufacturing. The stack operated in steam electrolysis mode at 1.4 bar showed comparably low degradation of 8 mV kh(-1)cell(-1), whereas the stack operated at 8 bar showed an approximately four times higher degradation. The third stack was operated in co-electrolysis mode at 1.4 and 8 bar and showed noticeably higher degradation rates than during steam electrolysis mode. The predominant increase of the ohmic resistance during operation was identified to be mainly responsible for the observed degradation of all three stacks, whereas the increase of the polarization resistances played a subordinate role. Within the post-test analysis, noticeably high nickel depletion was observed for the stack operated at the highest pressure in steam electrolysis mode. Furthermore, partial delamination of electrodes was observed. The degradation is discussed with relation to phenomena and experimental parameters during operation.