New Route of Acetylene Synthesis via Electrochemical Formation of Metal Carbides from CO2 in Chloride Melts

Electrochemical conversion of CO2 in high-temperature molten salts provides a unique process for synthesizing materials that cannot be obtained in low-temperature aqueous systems. In this study, we propose a novel route to produce acetylene utilizing the electrochemical reduction of CO2 in chloride melts. Acetylene is generated by the reaction between water and metal carbides, which are formed by the reduction of CO2 and cations in the melts. We demonstrated the proposed process by electrochemical measurements and quantitative gas analysis. To investigate the electrolytic system suitable for forming Li2C2 and CaC2 as the metal carbides at high current efficiency, two types of melts: LiCl–KCl–CaCl2–CaO melt at 723 K and NaCl–KCl–CaCl2–CaO melt at 823 K, and two types of electrodes: metallic electrodes (Fe, SUS304, SUS316, Mo, Ta and Ti) and carbon electrodes (graphite, glassy carbon, and highly oriented pyrolytic graphite) were used. It was found that acetylene was obtained with a current efficiency of 68% by galvanostatic electrolysis at −200 mA cm–2 on a Fe electrode in the NaCl–KCl–CaCl2–CaO melt mixed with 7.0 mol % CaC2 under CO2 atmosphere. The CaC2 played a key role in preventing the dissolution of electrodeposited metal carbides into the melts.