(Invited) Electrochemical Synthesis of Diamond from Carbon Dioxide in Molten Lithium Chloride-Potassium Chloride Systems
Authors: Nohira Toshiyuki , Norikawa Yutaro , Horiba Yurina , Matsuyama Ren 
Published: September 2023

Introduction
In order to achieve carbon neutrality by 2050, not only the CO2 capture and storage (CCS) but also the CO2 capture and utilization (CCU) is necessary. Here, electrochemical CO2 conversion technology in molten salt is considered to be one of the promising candidates for CCU [1,2]. In this study, we aimed to convert CO2 into diamond, which is one of the most valuable carbon materials. As a first step, we investigated the electrochemical synthesis of diamond in molten LiCl-KCl systems containing K2CO3 and KOH, based on the premise that CO2 dissolves as CO3
2- in molten salts containing O2-. As a result, micro-Raman spectroscopy and scanning electron microscopy (SEM) confirmed that a part of the deposits obtained by electrolysis were diamonds. As a second step, we attempted the electrochemical synthesis of diamond using CO2 as the actual raw material. Here, H2O was also bubbled in the molten salt in order to produce KOH. The deposits obtained by electrolysis were then analyzed by micro-Raman spectroscopy and SEM, which confirmed the synthesis of diamond.
Experimental
Cyclic voltammetry and potentiostatic electrolysis were performed after adding K2CO3 and KOH to LiCl-KCl eutectic melts under an Ar atmosphere at 973 K. 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, LiCl-KCl eutectic molten salt 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.
Result and Discussion
First, electrochemical measurements in baths containing only K2CO3, only KOH, or both indicated that carbon deposition and hydrogen evolution proceed simultaneously in the potential range more negative than 1.2 V. Then, 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 at 10 C cm-2. As an example, the surface SEM image of a deposit obtained at 1.1 V in a bath containing 0.05 mol% K2CO3 and 1.0 mol% KOH is shown in Fig. 1(a). The EDX analysis of this area showed that only C and Ni from the substrate were detected. The result of micro-Raman spectroscopy of the sample is shown in Fig. 1(b), which shows a spectrum characteristic of diamond with a sharp peak at 1332 cm-1. Based on the results of these three analyses, the electrochemically synthesized angular particles were identified as diamond.
On the day of the meeting, we will also present the results of electrolysis using CO2 as the raw material.
Acknowledgment
A part of this work was supported by JSPS KAKENHI Grant Number 21K19024.
References
[1] H. Yin, X. Mao, D. Tang, W. Xiao, L. Xing, H. Zhu, D. Wang, D. R. Sadoway, Energy Environ. Sci., 6, 1538 (2013).
[2] Y. Chen, M. Wang, J. Zhang, J. Tu, J. Ge, and S. Jiao, J. Mater. Chem. A, 9, 14119 (2021)

Figure 1