High-Temperature Co-Electrolysis and Degradation by Oxidized Trace Contaminants in Feed Gases
Authors: Schäfer Dominik , Kunz Felix , Schierholz Roland , Eichel Rudiger-A 
Published: September 2023

The transformation of our energy system will likely require the indirect electrification of the sectors heating, chemical industry and mobility by power-to-X processes, because not all processes can easily use electrical power directly. The high-temperature co-electrolysis using solid-oxide cells (SOCs) can produce syngas compositions desired by the downstream power-to-X process without the need of a reverse water-gas shift reactor and is therefore particularly well suited to produce synthesis gas as feedstock for various power-to-fuel and power-to-chemicals processes. Besides, a coupling of heat-producing downstream processes would increase system efficiency significantly.1

Besides the intrinsic degradation processes of the cells (e.g. nickel migration) and stacks (e.g. contact deterioration) any additional processes triggered by impurities in the feed gases are of relevance for any technical applications. Under certain circumstances, impurities can be introduced by Balance of Plant (BoP) components like the steam generators or preheaters that can release silicon species.2 Also, the feed gases themselves may contain harmful substances. This poisoning effect must be taken into account, because economically gases of lower qualities are preferred as purification steps increase the costs of the syngas. Depending on the exact source of carbon dioxide (CO2) the supplier uses, it contains different secondary components, of which sulfur species are of particular concern. In contrast to fuel-cell mode operation, in electrolysis mode operation hydrogen sulfide (H2S) is not the most likely candidate, but sulfur dioxide (SO2). The reasons for this is that CO2 may be obtained from combustion processes and that hydrogen sulfide is not stable in the atmosphere but has a short half-life time.
Therefore, understanding the influence of SO2 contained in the supplied carbon dioxide on the performance of the co-electrolysis is of great importance. There is substantial knowledge about the impact of H2S in feed gases in fuel-cell mode, but the impact of sulfur dioxide in electrolysis mode is not as well investigated. Skafte et al. reported a severe effect in CO2 electrolysis with as low as 20 ppb H2S.3 In the context of this application, the relevant sulfur species present in the feed gases will be SO2. It should be noted, that the heterogenous reduction of SO2 with hydrogen or carbon monoxide is catalyzed by oxides with oxygen vacancies and mobility like CeO2 or YSZ and facilitated by the presence of transition metals.4 Intermediary steps and products beside H2S can include significant amounts of elemental S and COS and the formation of sulfided nickel seems likely. The formation of these products depends on the exact reaction conditions, e.g. temperature, H2/SO2 ratio, steam content, which makes the situation more complex as in fuel-cell mode with hydrogen as fuel. Jeanmonod et al. came to the conclusion, that even concentrations of SO2 as low as 0.5 ppm caused irreversible degradation in Ni/YSZ electrodes in co-electrolysis mode.5

This contribution will report about investigations performed on the degradation caused by sulfur dioxide and silicon species in sub-ppm concentrations in SOCs with Ni/YSZ and Ni/GDC fuel electrodes operated in co-electrolysis mode, using stacks in the Jülich F10 and F20 design. These targeted their impact, potential recovery, possible damage mechanisms and safe thresholds of sulfur dioxide in carbon dioxide. For this the authors performed long-term stationary operation, monitoring of poisoning experiments by electrochemical impedance spectrocopy and post-test analysis of cells, including SEM/EDX and chemical trace analyses.

Acknowledgement

The authors would like to thank their colleagues at Forschungszentrum Jülich GmbH for their great support and the Helmholtz Society as well as the the German Federal Ministry of Education and Research for financing these activities as part of the Kopernikus P2X-2 project (03SFK2Z0-2).

References

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