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.