Characterization and Long-Term Testing of Solid Oxide Electrolyzer Cells

Kurzfassung

A reliable energy supply which is based on increasing shares of sustainable and renewable energy sources, such as wind power and solar energy, requires appropriate storage technologies. Hydrogen as energy carrier, produced by water electrolysis using electric current from regenerative energy sources, offers a high potential in this respect. A very efficient option to produce hydrogen in this way is high-temperature steam electrolysis based on solid oxide electrolyzer cells (SOEC). This technology requires operating temperatures in the range of 700-1000 °C and offers some additional advantages compared to low temperature electrolysis techniques. The higher operating temperature results in faster reaction kinetics thus enabling potentially high energy efficiency. From a thermodynamic point of view, part of the energy demand for the endothermic water splitting reaction can be obtained from heat produced within the cell. The electric energy demand can be further significantly reduced if high temperature heat from renewable energy sources such as geothermal or solar thermal power or waste heat from industrial processes is available. Furthermore, it is possible with high temperature electrolysis to not only split water but also carbon dioxide or a mixture of both to produce synthesis gas (syngas) or other energy carriers such as methane or methanol by subsequent catalytic conversion. For a further development of this promising technology, development work on materials and cells as well as extensive operational experience is still needed. A main objective is to develop highly efficient and long-term stable cells and stacks using novel electrode materials and to improve the degradation behavior by elucidating the relevant degradation mechanisms. To this aim, German Aerospace Center (DLR) and Forschungszentrum Jülich (JÜLICH) who have both long experience in the development of SOFC/SOEC technology started a joint project in the frame of the “Helmholtz Energy Alliance” on electrochemical energy storage and conversion. Cathode-supported cells containing novel perovskite-type air electrodes were fabricated by ceramic processing and sintering for electrochemical characterization in electrolysis operating mode. The selection and preparation of electrode materials and the process of cell manufacturing is described. A new test bench has been installed which allows measuring polarization curves of 4 cells simultaneously under relevant SOFC and SOEC conditions as well as performing long-term durability measurements. Results of electrochemical measurements performed at different operational conditions, such as different steam content and operating temperature, are presented. After operation the cells were investigated by post-test analytical methods; hereby special emphasis is put on the detailed investigation of degradation phenomena and mechanisms by applying numerous characterization techniques.