The review article covers all state-of-the art materials related to high-temperature electrolyzers based on oxygen-ion conductors. The focus lies on the cell materials, materials of additional components like interconnects and sealants are briefly described.

High-temperature electrolysis using solid oxide electrolysis cells (SOECs) is an innovative technology to temporarily store unused electrical energy from renewable energy sources. However, they show continuous performance loss during long-term operation, which is the main issue preventing their widespread use. In this work, we have performed the long-term stability tests up to 1000 h under steam and co-electrolysis conditions using commercial NiO-YSZ/YSZ/GDC/LSC single cells in order to understand the degradation process. The electrolysis tests were carried out at different temperatures and fuel gas compositions. Intermittent AC- and DC- measurements were performed to characterize the single cells and to determine the responsible electrode processes for the degradation during long-term operation. An increased degradation rate is observed at 800 °C compared to 750 °C under steam electrolysis conditions. Moreover, a lower degradation rate is noticed under co-electrolysis operation in comparison to steam electrolysis operation. Finally, the post-test analyses using SEM-EDX and XRD were carried out in order to understand the degradation mechanism. The delamination of LSC is observed under steam electrolysis conditions at 800 °C, however, such delamination is not observed during co-electrolysis operation. In addition, Ni-depletion and agglomeration are observed on the fuel electrode side for all the cells.

In the temperature range of high temperature co-electrolysis of both steam and carbon dioxide, the reverse water-gas shift reaction (RWGS) takes place. Prior studies were conducted with a narrow gas composition range to investigate the role of RWGS during co-electrolysis. The results for steam electrolysis, CO₂ electrolysis, and co-electrolysis caused different conclusions regarding the role of electrochemical CO₂ and H₂O conversion compared to RWGS during co-electrolysis. This work aims to resolve the role of CO₂ conversion as part of RWGS in co-electrolysis. The boundary is characterized by AC and DC measurements over a broad gas composition range from CO₂ electrolysis towards co-electrolysis with nearly 50%^eq H₂O. Especially, the electrochemical CO₂ reduction and CO₂ conversion in the RWGS are compared to clarify their role during co-electrolysis. The results revealed that gas composition determined the predominant reaction (H₂O or CO₂ reduction). The cell performance of co-electrolysis in the boundary region up to 5%^eq H₂O was similar to the performance of CO₂ electrolysis. Up to 30%^eq H₂O, the performance increases with H₂O concentration. Here, both CO₂ and H₂O electrolysis occur. Above 30%^eq H₂O, steam electrolysis and the RWGS reaction both dominate the co-electrolysis process.

The Distribution of Relaxation Times (DRT) is an important analytical tool that is capable of giving initial information from Electrochemical Impedance Spectra (EIS) with respect to the number of relaxation processes occurring in the system and their corresponding relaxation frequencies. The DRT transformation with the Tikhonov regularization is used for analysis of EIS data obtained from the characterization Solid Oxide Fuel and Electrolysis Cells (SOFC/SOEC) operating at high temperatures. The effects of this transformation together with occurring pitfalls on the most commonly implemented circuit elements used to describe EIS data was investigated to gain a better understanding. The behavior of the DRT transformation as a function of the individual circuit elements is reported, the regularization parameter λ is taken as a sweep parameter to investigate its influence and optimal ranges for the selection of λ are presented.

SFM-based single cells show high performance but poor long-term stability under steam electrolysis due to a dense interlayer formation at the SFM/GDC interface. In contrast, the cells with composite SFM-GDC electrodes exhibit exceptional durability.

Severe performance loss of state-of-the-art Ni-cermet fuel electrodes has been observed due to Ni particle agglomeration and Ni migration away from the triple-phase boundary at the active electrode layer. Investigation of Ni-free perovskite Sr₂Fe_2-xMo_xO_6-δ (SFM)–30 wt% Ce_0.8Gd_0.2O_2-δ (GDC) as fuel electrode material showed good redox stability in oxidizing and reducing atmospheres. These characteristics meet exactly the targeted requirements for new solid oxide electrolyzer materials. A comparison of SFM and SFM-GDC fuel electrodes showed similar performance for H₂O, CO₂, and co-electrolysis between 750 °C and 900 °C. Long-term degradation measurements and post-test SEM-EDX analysis clarified that SFM-GDC exhibits good phase stability in comparison to pure SFM fuel electrodes. The composite fuel electrode Sr₂Fe_1.1Mo_1.0O_6-δ–30 wt% Ce_0.8Gd_0.2O_2-δ (SFM15-GDC) reached polarization resistances (R_P) in the same range as Sr₂Fe_1.0Mo_1.0O_6-δ with around 127 mΩ∙cm² and 137 mΩ∙cm at 900 °C in steam electrolysis.