Effects of Temperature and Different Electrolysis Processes on Mg Metal Deposition in Molten Salt Electrolysis
Authors: Nishi Ayana , Sasaki Tatsuya , Takenaka Toshihide , Matsumoto Toshiharu , Nagayasu Katsushi 
Published: December 2024

Metallic Mg has excellent mechanical properties, and its demand is growing worldwide. However, the supply risk and high greenhouse gas emission in its production became non-negligible nowadays. Our laboratory has been studying the Mg production process by molten salt electrolysis using the raw material extracted from seawater. In this study, Mg metal deposition was attempted by potentio-static and galvano-static electrolysis, and the influence of electrolysis temperature was discussed.
Molten MgCl2-NaCl-CaCl2 was used as the electrolytic bath. Mo wire was used as the working electrode, and carbon rod was used as the counter electrode. The reference electrode was a Ag/AgCl couple in the same bath in a mullite tube, and its potential of the reference electrode was calibrated against the Mg deposition potential. Experiments were conducted in an Ar atmosphere, and the electrolysis temperature was 680°C~740°C. After observing the cathodic behavior by cyclic voltammetry, potentio-static electrolysis or galvano-static electrolysis was performed. The obtained electrodeposits were evaluated by XRD and XRF, and the cathodic current efficiency was calculated from the weight of the deposit and the quantity of electricity.
The current in the potentio-static electrolysis became almost stable after the initial change in electrolysis. The stable current was 2.0A/cm2 at -0.2V (vs. Mg dept.), and depended on the applied potential. The potential in the galvano-static electrolysis also became stable after the initial change. The stable potential was -0.3V (vs. Mg dept.) at 1.0A/cm2. The stable potential was slightly affected by the applied current, but this potential was usually equivalent to that of Mg metal deposition. The values by the potentio-static electrolysis and those by the galvano-static electrolysis were almost consistent. Metallic Mg was obtained with either potentio-static or galvano-static electrolysis. The purity was generally higher than 99.9%, and rarely depended on the electrolysis method. The cathodic current efficiency was about 90% usually, and not influenced by the electrolysis method significantly.
Figure 1 was shown the relationship between the electrolysis temperature and Mg purity of the electrodeposits. A clear dependence of the Mg purity on electrolysis temperature was not seen, and the purity was usually better than 99.8%. The current efficiency was not affected significantly by electrolysis temperature. In the case that the galvano-static electrolysis where the temperature changed during electrolysis was performed, the Mg purity and the current efficiency were not changed so much.
The influences of electrolysis temperature and electrolysis method on the Mg purity and the current efficiency were studied in this study, and their influences were shown limited. Since the change in electrolytic current causes the change in the bath temperature in the actual Mg metal production by molten salt electrolysis, the results above indicate that the Mg deposition with variable temperature was possible under the actual electrolysis condition:1.0A/cm2 at 680~740°C.
This work was performed under the support of the commissioned work (JPNP14004) of the New Energy and Industrial Technology Development Organization (NEDO).

Figure 1