Electrochemical Synthesis of Diamond in Molten LiCl–KCl–K₂CO₃–KOH

Reagent-grade LiCl (>99.0%; FUJIFILM Wako Pure Chemical Corp., Osaka, Japan) and KCl (>99.5%; FUJIFILM Wako Pure Chemical Corp., Osaka, Japan) were mixed in the eutectic composition (molar ratio of LiCl: KCl = 58.5:41.5, 300 g), and the mixture was loaded in an alumina crucible (AS ONE Corp., Osaka, Japan; outer diameter: 90 mm; inner diameter: 84 mm; height: 120 mm). The mixed salt was then dried under vacuum at 453 K for over 72 h. The crucible was placed at the bottom of an inner carbon vessel in an airtight Kanthal container. Electrochemical measurements were conducted in a dry Ar atmosphere at 973 K. After blank measurements in molten LiCl–KCl, K₂CO₃ (>99.5%; FUJIFILM Wako Pure Chemical Corp., Osaka, Japan) as the carbon source and/or KOH (>85.0%; FUJIFILM Wako Pure Chemical Corp., Osaka, Japan) as the hydrogen source were added to the molten salt.

Electrochemical measurements and potentiostatic electrolysis were performed using a three-electrode method with an electrochemical measurement system (Hokuto Denko Corp., HZ-3000, Tokyo, Japan). The working electrodes were a Ni flag (99.95%, Nilaco Corp., Tokyo, Japan; diameter: 3.0 mm, thickness: 0.1 mm) and Ni plate (99.95%, Nilaco Corp., Tokyo, Japan, 10 mm × 10 mm, thickness: 0.2 mm). The structure of the flag electrodes was reported in our previous study. ³⁸ An Ag wire immersed in molten LiCl–KCl containing 0.5 mol% AgCl (99.5; FUJIFILM Wako Pure Chemical Corp., Osaka, Japan), set in a mullite tube (Nikkato Corp., o.d. 6 mm × i.d. 4 mm × length 500 mm, 56% Al₂O₃–40% SiO₂; HB grade) was used as the reference electrode. A glass-like carbon rod (99.5%, Nilaco Corp., Tokyo, Japan; diameter: 3.0 mm) was used as the counter electrode. The potential of the reference electrodes was calibrated with respect to the dynamic Li⁺/Li potential, as determined by cyclic voltammetry on a Ni electrode. The melt temperature was measured using a type K thermocouple. Salt adhering to the deposits was removed by washing with distilled water.

Before electrolysis, diamond seeding was conducted on Ni plate substrates. Ni plates were immersed in a mixture of 100 mg of nanodiamond powder (Tokyo Chemical Industry Co., Ltd, particle size <10 nm), 5.0 ml of acetone (FUJIFILM Wako Pure Chemical Corp., Osaka, Japan, super dehydrated), and 5.0 ml of dimethyl sulfoxide (FUJIFILM Wako Pure Chemical Corp., Osaka, Japan, super dehydrated). The transducer tip of the ultrasonic homogenizer (Q125; Qsonica Corp., Newtown, CT, USA) was placed in the mixed solution and sonicated for 60 min.

The samples prepared by electrolysis were analyzed using scanning electron microscopy (SEM, Phenom Pro Generation 5; Thermo Fisher Scientific Inc., Waltham, MA), energy-dispersive X-ray spectroscopy (EDX, SE1200–8001; Thermo Fisher Scientific Inc., Waltham, MA), and micro-Raman spectroscopy (Nanofinder 30; Tokyo Instruments, Inc., Tokyo, Japan) using a laser source with an excitation wavelength of 633 nm.

Before electrolysis, the electrochemical behavior of CO₃ ²⁻ and OH⁻ was investigated. Figure 2a shows cyclic voltammograms at a Ni flag electrode in a molten LiCl–KCl before and after adding 0.05 mol% of K₂CO₃ and 1.0 mol% of KOH at 973 K. Enlarged voltammograms at 1.0 V–1.4 V vs Li⁺/Li are also shown in Fig. 2b. Only the currents due to Li metal deposition and dissolution were observed in the blank LiCl–KCl melts (black curve). After adding K₂CO₃ (red curve), cathodic currents were observed to flow from 1.3 V, a phenomenon that was not observed before the addition, indicating that the currents correspond to carbon deposition. Furthermore, after adding KOH (blue curve), cathodic currents from 1.3 V were observed to increase, indicating that H₂ generation occurs at potentials nearly identical to those of carbon deposition. These results indicate that carbon and hydrogen can be simultaneously generated at potentials more negative than 1.3 V.

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Based on the electrochemical measurement results, potentiostatic electrolysis was performed in molten LiCl–KCl–K₂CO₃ (0.05 mol%)–KOH (1.0 mol%) at 1.10 V with a charge density of 10 C cm⁻². For comparison, electrolysis was also performed under the same conditions except that KOH was not added. Figures 3a and 3b show the appearance of the Ni plate substrate before electrolysis and the Raman spectrum of the substrate, respectively. The Ni plate exhibited a metallic luster, and no Raman signal was detectable. Although diamond nanoparticles were seeded onto the Ni plate, Raman spectroscopy could not detect the diamond nanoparticles because the particle size was small and not agglomerated. Figures 3c and 3d show the appearance of the Ni plate substrate after electrolysis in molten LiCl–KCl–K₂CO₃ (0.05 mol%) and the Raman spectrum of the deposit, respectively. Although some of the deposits peeled off from the plates, black deposits were observed, indicating successful carbon electrodeposition. The obtained Raman spectrum indicated that the black deposits were amorphous carbon. Figure 3e shows the appearance of the Ni plate after electrolysis in molten LiCl–KCl–K₂CO₃ (0.05 mol%)–KOH (1.0 mol%). Different from the case where KOH was not added, brown deposits were obtained. Although data is not shown, Raman spectroscopy showed that the brown deposits were amorphous carbon. However, certain parts of the sample exhibited a sharp peak at 1332 cm⁻¹, which is unique to diamonds. Figures 3f shows the Raman spectrum of the sample at the point indicated by the white arrow. A sharp peak at 1332 cm⁻¹ was observed, superimposed on the Raman spectrum of amorphous carbon.

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Figure 4a shows an SEM image of the area where a sharp peak was observed. A magnified image is shown in Fig. 4b. Angular deposits with grain sizes of 2–5 μm were observed. Furthermore, only C and Ni were detected by the EDX analysis of the angular deposits. Considering the depth of EDX detection, the detected Ni is considered to derive from the substrate, and the angular deposits should be composed of carbon only. In general, a diamond is identified through the presence of a sharp Raman peak at 1332 cm⁻¹ and an angular shape. ³⁶ Therefore, two analysis results of the deposits–the sharp Raman peak at 1332 cm⁻¹ and angular carbon–indicate that the diamonds were electrodeposited. On the other hand, only a very small amount of diamond was found, suggesting that most of the current was used for amorphous carbon deposition and hydrogen generation. Further optimization of electrolysis conditions is needed to electrodeposit diamonds with high current efficiency.

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We have proposed a novel electrochemical process for synthesizing diamonds from CO₂ in molten salt. The unique feature of our approach is that carbon and hydrogen are generated simultaneously at the cathode. Subsequently, C(sp², sp) reacts with hydrogen and is removed as CH₄ gas. As an initial study of the method, we conducted electrochemical measurements and electrolysis in molten LiCl–KCl–K₂CO₃–KOH at 973 K. The cyclic voltammetry analysis showed that carbon deposition and hydrogen generation occurred simultaneously at the potential more negative than 1.3 V. Raman spectroscopy and SEM observations of the sample prepared at 1.1 V vs Li⁺/Li with a 10 C cm⁻² charge density indicated that the diamonds were electrodeposited on the Ni substrate.

This work was supported by JSPS KAKENHI [Grant Number JP21K19024]. We thank Koji Hidaka and Seigo Maruyama for their contributions to establishing the foundation for this study.