High-temperature co-electrolysis of carbon dioxide and steam is a promising method to produce ‘white’ syngas by making use of renewable energy and carbon dioxide as sustainable feedstock. The technological key advantage is the possibility to tailor syngas compositions over a broad range. This paper presents a systematic investigation of the syngas tailoring process by establishing relationships between feed gas compositions and flow rates to the syngas ratio. A linear dependence between the H₂O:CO₂ ratio in the feed gas and the H₂:CO ratio in the output gas was observed. Furthermore, the syngas ratio remains mostly invariant upon variations in electrochemical potential and fluctuating gas utilizations/flow rates during operation of a co-electrolysis cell. Most importantly, the co-electrolysis performance was demonstrated to operate at high current densities of up to 3.2 A·cm⁻² over a broad range of feed gas compositions with faradaic efficiencies of nearly 100%. The possibility to operate co-electrolysis under transient load conditions renders this method particularly suitable in future scenarios of intermittent availability of renewables. The results described here illustrate the versatility of co-electrolysis, which can produce all relevant syngas compositions in a single-step process at constantly high performance.

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.

The concept of an integrated power‐to‐gas (P2G) process was demonstrated for renewable energy storage by converting renewable electrical energy to synthetic fuels. Such a dynamically integrated process enables direct production of synthetic natural gas (SNG) from CO₂ and H₂O. The produced SNG can be stored or directly injected into the existing natural gas network. To study process integration, operating parameters of the high‐temperature solid oxide electrolysis cell (SOEC) producing syngas (H₂+CO) mixtures through co‐electrolysis and a fixed bed reactor for syngas methanation of such gas mixtures were first optimized individually. Reactor design, operating conditions, and enhanced SNG selectivity were the main targets of the study. SOEC experiments were performed on state‐of‐the‐art button cells. Varying operating conditions (temperature, flow rate, gas mixture and current density) emphasized the capability of the system to produce tailor‐made syngas mixtures for downstream methanation. Catalytic syngas methanation was performed using hydrotalcite‐derived 20 %Ni‐2 %Fe/(Mg,Al)O_x catalyst and commercial methanation catalyst (Ni/Al₂O₃) as reference. Despite water in the feed mixture, SNG with high selectivity (≥90 %) was produced at 300 °C and atmospheric pressure. An adequate rate of syngas conversion was obtained with H₂O contents up to 30 %, decreasing significantly for 50 % H₂O in the feed. Compared to the commercial catalyst, 20 %Ni‐2 %Fe/(Mg,Al)O_x enabled a higher rate of CO_x conversion.

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.

Pure carbon dioxide (CO₂) electrolysis presents a sustainable method for the production of carbon monoxide (CO). CO is an important feedstock for producing base chemicals for the chemical industry. High-temperature CO₂ electrolysis in solid oxide electrolysis cells (SOECs) make it possible to recycle climate-damaging CO₂ and utilize renewable energy sources for the conversion to CO. We thoroughly investigated the direct CO₂ electrolysis electrochemically with respect to the process parameters temperature, fuel utilization, load, flow rates, and CO₂:CO feed gas ratio. With faradaic efficiencies of about 100% and high current densities of up to 1.5 A∙cm,^-2 the results show the high potential of pure CO₂ electrolysis.