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.

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.

Electrolysis is the technology, which provides the key for sector coupling. It enables the conversion of renewable energy to material value generation. Obviously, it is of utmost importance, that the energy is used in the most efficient way. High-temperature electrolysis on solid oxide cells provides high efficiency combined with the possibility to convert both water (H₂O) and carbon dioxide (CO₂) to hydrogen (H₂) and carbon monoxide (CO) at the same time. In this contribution, we show the summary and conclusions of multiple investigations of the high-temperature electrolysis. A series of detailed analysis has been performed using current-voltage characteristics (IV curves), electrochemical impedance spectroscopy (EIS), and theoretical calculations.

In this work, the transition boundary of high-temperature co-electrolysis towards pure CO₂-electrolysis was investigated. AC and DC measurements with H₂O concentrations from 0.0%^eq to 39.7%^eq led to the identification of the underlying electrochemical processes. Comparison of CO₂ conversion in the (R)WGS reaction with electrochemical CO₂ reduction by means of area specific resistance (ASR) showed that gas composition determines the dominant reduction reaction (H₂O or CO₂ reduction). The cell performance of CO₂-electrolysis was found to be comparable to co-electrolysis in the boundary region up to around 4.7 %^eq H₂O. The threshold for the perception of CO₂-electrolysis during co-electrolysis has been established to be around 31.7 %^eq H₂O. The hypothesis that pre-domination of CO₂-electrolysis increases with decreasing H₂O concentrations below this boundary limit was underlined by measurement results.