Chemical industries rely heavily on fossil resources for the production of carbon-based chemicals. A possible transformation towards sustainability is the usage of carbon dioxide as a source of carbon. Carbon dioxide is activated for follow-up reactions by its conversion to carbon monoxide. This can be accomplished by electrochemical reduction in solid oxide cells. In this work, we investigate the process performance of the direct high-temperature CO₂ electrolysis by current-voltage characteristics (iV) and Electrochemical Impedance Spectroscopy (EIS) experiments. Variations of the operation parameters temperature, load, fuel utilization, feed gas ratio and flow rate show the versatility of the procedure with maintaining high current densities of 0.75 up to 1.5 A·cm⁻², therefore resulting in high conversion rates. The potential of the high-temperature carbon dioxide electrolysis as a suitable enabler for the activation of CO₂ as a chemical feedstock is therefore appointed and shown.

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.