Experimental analysis of the influence of elevated operating pressures on Solid Oxide Cell stacks during steam, co- and CO2 electrolysis

Kurzfassung

Moving towards a renewable energy powered society, the solid oxide electrolysis cell (SOEC) technology can be an essential constituent for the efficient supply of molecules like hydrogen and carbon monoxide for various synthesis routes in the chemical industry from renewable electricity. However, downstream synthesis reactors for the production of chemicals like methane, ammonia, dimethyl ether (DME), methanol or jet fuel are typically operated at elevated pressures in the range of 10-60 bar to achieve high conversion or high yield. Furthermore, storage and transportation of gaseous products also require a certain pressurization of the electrolysis products. The operation of the electrolyzer at an elevated pressure can therefore be highly beneficial since additional compression work of the produced molecules can be significantly reduced or omitted. As for polymer electrolyte membrane and alkaline electrolyzers pressurized operation is already state of the art, the research and developments associated with the operation of pressurized solid oxide electrolyzers is rare. However, SOECs have the potential to be operated significantly more efficient than the low-temperature electrolyzers. In this thesis, the pressurized operation of SOEC stacks during steam, co- and CO2 electrolysis is investigated experimentally. The experiments were carried out using a test rig that was adapted for the electrolysis operation of different stack types. It enables stack characterizations in an operating pressure range of 1.4 to 8 bar. Thus, the influence of increased partial pressures as additional experimental parameters can provide scientific knowledge. 10-layer stacks having either electrolyte supported cells (ESC) or fuel electrode supported cells (in electrolysis: cathode supported cell, CSC) were used for the scientific investigations. The ohmic resistances of both stack types were quantified over a wide temperature range and provided to the literature via parameterized mathematical expressions. Since the ohmic resistance was found to be the dominant cell resistance of the ESC, it was theoretically investigated with a simplified model. A deviation of 15-20 % between the theoretically and experimentally obtained ohmic resistance value was found which indicates a considerable amount of unaccounted resistance that can be attributed to contact resistances occurring within the stack. The overall performance and the influence of an elevated operating pressure were found to be significantly different by operating a stack with electrolyte supported or fuel electrode supported cells. The stack using the latter cell concept showed a significantly lower ohmic resistance and almost twice as high achievable current densities compared to the stack with electrolyte supported cells. A significantly decreased area specific resistance (ASR) at higher pressures was observed for the CSC stack that was attributed to reduced activation and diffusion resistances. This phenomenon led to a crossing of the current-voltage characteristics within the endothermic operating regime during the studied steam, co- and CO2 electrolysis operations. Hence, a noticeable performance gain was observed for the pressurized operation of the CSC stack. In contrast, the performance of the ESC stack was hardly influenced by an increased operating pressure. Rather, it was found to be primarily dictated by the operating temperature due to pre-dominant contribution of the ohmic resistance to the overall performance. Gas analysis was used during the co-electrolysis operations and showed that the thermodynamic equilibrium is reached with both stack types. Consequently, both the reverse water-gas shift (rWGS) and the methanation reaction occur stack-internally which indicates that the chemical reactions are fast and the catalytic surfaces of the stacks are sufficiently active and available in relation to the gas hourly space velocity. For high operating pressures, methane contents of up to 7 % were found. The ASR of both stack types was quantified with the help of several steady-state and dynamically recorded current-voltage curves at different pressures. Consequently, detailed relations about the temperature and pressure dependent performance characteristics of both the CSC and the ESC stacks were provided to the literature by this thesis. Electrochemical impedance spectroscopy (EIS) analyses were used to identify the characteristic frequencies of the electrochemical processes that occur within both stack types. The influence of an elevated operating pressure on the electrochemical processes was closely investigated for both steam and CO2 electrolysis operation. By investigating the pressurized co-electrolysis operation, it could be shown that the main reaction path for the CO production can be attributed to the rWGS reaction. The amount of CO that is generated by the rWGS reaction or by the electrochemical CO2 reduction process during co-electrolysis operation represents still a scientific gap to which the current thesis contributes. Moreover, this work presents an in-depth analysis of the long-term degradation of three ESC stacks operated under pressurized steam and co-electrolysis mode. The operatingdurations were 1,000-2,000 hours at a constant current with high conversions of 70 %, thus representing the first published experimental analysis of the SOEC stack degradation behavior under relevant pressurized operations for more than 200 hours. It was observed that the degradation is predominantly dictated by a time dependent increase of the ohmic resistance. The experimental results furthermore revealed that an increased operating pressure leads to increased performance loss and that co-electrolysis operation has an additionally worsening effect on the long-term stability of the stacks. Detailed post-test analyses of the electrodes and the bipolar plates were carried out. A noticeably higher degree of nickel depletion was found within the fuel electrode of the stack that was operated at the highest pressure. Observed delamination, contamination and oxide layer formation within the post-test analyses of the stacks are discussed. Based on an existing numerical model of one repeating unit, a 1D+1D stack model was developed and parameterized with experimental data to predict the performance and temperature gradients during the ressurized stack operation of the ESC. The validity of the model was shown for several operating conditions at different pressures. The relations of the predominant resistance contribution and the pressure dependent activation energy of the electrochemical steam reduction process were used as input parameters for the mathematical equations. Beside the electrochemistry, the model includes a proper heat transfer model according to the structure of the experimental test setup.