Electrochemical analysis of an electrolyte-supported solid oxide cell-based MK35x stack during long-term electrolysis operation

Kurzfassung

The currently ongoing scale-up of the high-temperature solid oxide electrolysis (SOEL) technology requires the understanding of the underlying dominant degradation mechanisms to set base lines for the development of accelerated stress tests and to enable continuous progress in increasing stack durability. In the present study, the degradation behavior of SOEL stacks of the type ‘MK35x’ with chromium-based interconnects (CFY) and electrolyte-supported cells (ESC) developed at Fraunhofer IKTS was investigated. For this purpose, the initial electrochemical performance of a 10-cell stack was extensively characterized in various operating conditions in both fuel cell and electrolysis mode and showed varying performance gradients depending on the operating conditions. Degradation was evaluated during galvanostatic steady-state steam electrolysis operation for more than 3000 h at an oxygen side outlet temperature of 816°C and a current density of -0.6 A cm-2 and showed an average voltage evolution rate of -0.3 %/kh demonstrating the high stability of this stack technology. By means of electrochemical impedance spectroscopy analysis it was demonstrated that the stack’s ohmic resistance increase was over-compensated by a decrease in polarization resistance most likely due to an enhancement in LSMM’/ScSZ oxygen electrode performance.