Electrochemical Synthesis of Carbon Materials from Carbon Dioxide in Molten Chloride Systems

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1. IntroductionTo achieve carbon neutrality by 2050, significant attention has been paid to the development of CO2 capture and utilization (CCU) technologies in addition to CO2 capture and storage (CCS) technologies. Here, electrochemical CO2 conversion technology in molten salt is considered one of the promising candidates for CCU because it can reduce CO2 to elemental carbon [1,2]. In recent years, we have been working on the conversion of CO2 into a variety of valuable carbon materials. For example, we reported the electrochemical synthesis of diamond from molten LiCl-KCl-K2CO3-KOH [3]. In the present study, we report recent results on the electrochemical synthesis of various carbon allotropes, including diamond, in molten chlorides.2.ExperimentalAs a typical molten salt among the chloride systems, we used eutectic LiCl-KCl. Cyclic voltammetry and potentiostatic electrolysis were performed after adding K2CO3 to the molten salt. For diamond synthesis, KOH was also added to the molten salt. The working electrode was a Ni flag (Φ 3 × 0.1 mm) or Ni plate (5 mm × 10 mm × 0.1 mm), the counter electrode was a glass-like carbon rod, and the reference electrode was an Ag+/Ag electrode. The potential was calibrated with Li+/Li potential. After electrochemical measurements were performed, samples were prepared by potentiostatic electrolysis using the Ni plate electrodes. In the case where CO2 was used as a raw material, molten LiCl-KCl containing Li2O was prepared, and CO2 and H2O were bubbled into it in predetermined amounts, respectively. After electrochemical measurements, potentiostatic electrolysis was performed to prepare samples. The obtained samples were analyzed by SEM, EDX, and micro-Raman spectroscopy. In addition to the LiCl-KCl, we used several other chloride-based molten salts for use at higher temperatures.3. Results and DiscussionFor diamond synthesis, CO2 and H2O were introduced into molten LiCl-KCl-Li2O at 973 K. Then, CO2 was converted into CO3 2− and H2O into OH−. Samples were prepared by potentiostatic electrolysis using Ni plate electrodes at 1.0 to 1.2 V (vs. Li+/Li). The charge density was unified to 10 C cm−2. As an example, optical images of the sample obtained at 1.2 V is shown in Fig. 1. The electrode surfaces were covered with black deposits, but most of the deposits, except for the edges, were detached during the water rinsing. Raman spectra were measured at the points indicated by the crosses in Fig. 1. As shown in Fig. 2, sharp peaks were observed at 1332 cm−1, overlapping the spectra characteristic of amorphous carbon. The results indicate that the majority of the product was amorphous carbon, but a small amount of diamond was synthesized electrochemically.On the day of the meeting, we will also present the results obtained in other chloride-based molten salts.AcknowledgmentA part of this work was supported by JSPS KAKENHI Grant Number 21K19024. A part of this study was conducted in collaboration with Cosmo Oil Co., Ltd.