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

Abstract

Electrochemical conversion of CO 2 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 CO 2 in chloride melts. Acetylene is generated by the reaction between water and metal carbides, which are formed by the reduction of CO 2 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 Li 2 C 2 and CaC 2 as the metal carbides at high current efficiency, two types of melts: LiCl−KCl−CaCl 2 −CaO melt at 723 K and NaCl−KCl−CaCl 2 −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−CaCl 2 −CaO melt mixed with 7.0 mol % CaC 2 under CO 2 atmosphere. The CaC 2 played a key role in preventing the dissolution of electrodeposited metal carbides into the melts.

Relationship between the Thermodynamic Quantities and Coordination Structure of Oxide Ions in CaCl2-Based Chloride Melts

“…For example, a potential-pO 2– diagram can be used to predict the phase state of materials in the high-temperature molten salt in studies on corrosion science utilizing the NaCl–CaCl 2 -based melt. , The μ O 2– o′ identified in this study will be helpful for constructing the potential-pO 2– diagram to predict the corrosion behavior of the material more accurately. These values can also be used to thermodynamically predict the electrodeposition potential in the Si deposition process and the electrochemical formation of CaC 2 . In addition, microscopic interactions between ions have critical effects on the elementary processes of the phase transition of materials in high-temperature molten salts.…”

“…These values can also be used to thermodynamically predict the electrodeposition potential in the Si deposition process 6 and the electrochemical formation of CaC 2 . 5 In addition, microscopic interactions between ions have critical effects on the elementary processes of the phase transition of materials in high-temperature molten salts. The reported data will contribute to realizing these applications using the melts as reaction media.…”

“…The composition of NaCl–CaCl 2 and LiCl–CaCl 2 is eutectic, and that of NaCl–KCl–CaCl 2 is not eutectic. Each composition has been reported to be useful for engineering purposes. ,,, Each mixture in an alumina crucible was kept under a vacuum for more than 48 h at 473 K to expel its water content. The treatment of the salts was the same as in our previous report, and the effects of moisture and salt decomposition were almost negligible.…”

“…The potential of the electrode was calibrated with reference to that of Na(I) and Ca(II)/Na–Ca alloy for the NaCl–CaCl 2 melt, the Na(I), K(I), and Ca(II)/Na–K–Ca alloy for the NaCl–KCl–CaCl 2 melt, and Li(I) and Ca(II)/Li–Ca alloy for the LiCl–KCl–CaCl 2 melt, which was prepared by electrodepositing each alloy on a nickel wire in each melt. The reaction at the cathodic limit in the LiCl–KCl–CaCl 2 melt and the NaCl–KCl–CaCl 2 melt at 723 K has been reported to the Li–Ca alloy formation 14 and Na–K–Ca alloy formation, 5 respectively. The reaction at the cathodic limit in the NaCl–CaCl 2 melt was analyzed by inductively coupled plasma atomic emission spectroscopy (ICP-AES; ICP-8100, Shimadzu Corporation) for the sample obtained by potentiostatic electrolysis.…”

“…High-temperature chloride melts containing CaCl 2 have been attracting attention as reaction media in diverse engineering processes, such as thermal storage medium, , fuels and coolants for nuclear power, and electrolytes for metal electrowinning from metallic oxides . More specifically, the NaCl–CaCl 2 melt has been reported to be useful as a heat storage medium because of its low cost and high heat storage density. , Moreover, the formation process of metal carbides from CO 2 in the NaCl–KCl–CaCl 2 melt and the Si electrodeposition process from SiO 2 in a LiCl–KCl–CaCl 2 melt have been reported. Because the CaCl 2 -based melt exhibits high solubility of metallic oxides, it is essential to evaluate both the thermodynamic behavior of oxide ions, such as chemical potential and microscopic behavior, such as interactions between oxide ions and constituent ions in the melt, to understand and control the compatibility of materials and melts when assuming such applications as reaction media.…”