Surface Change of Liquid Ga Electrode By Na Electrodeposition in NaTf2N-TEATf2N Ionic Liquid

A liquid metal electrode has interesting property such as phase change from a liquid to a solid depend on alloys composition. There are many reports of solid metal deposition on a liquid electrode, there are few papers of liquid metal deposition on a liquid electrode. In this study, Ga was used as the liquid electrode and Na electrodeposition was carried out in ionic liquid electrolyte at 160 ^oC. Since the melting point of Ga and Na are 29.8 and the 97.8 ^oC, at the moment when Na is electrodeposited, the liquid Na presents on the liquid Ga. In this experiment, we focused on the surface change of the Ga electrode before and after the electrodeposition of liquid Na.

The experiment was conducted in a glove box under Ar atmosphere. NaTf₂N (Sodium bis (trifluoromethanesulfonyl) imide) -TEATf₂N (Tetraethylammonium bis (trifluoromethanesulfonyl) imide) mixed ionic liquid with molar ratio of 1 : 4 was used as an electrolyte at 160 ^oC. Liquid Ga or solid Ag plate was used as a working electrode and liquid Na was used as counter electrode and reference electrode. The voltammogram measurement was performed in the range from 2.5 V to -0.5 V (vs. Na/Na⁺) with sweep rate of 10 mV s^-1. Constant current electrolysis was carried out at 5 mAcm^-2 for 3 minutes. The liquid Ga electrode surface was taken by a digital camera. The time change of the open circuit potential of the Ga electrode after constant current electrolysis was measured by potentiostat.

In voltammogram measurement, cathodic current on Ag working electrode flowed at lower than 0.0 V (vs. Na / Na ⁺). In the liquid Ga electrode, cathodic current started at 0.6 V, then the cathodic current increased again at lower than about 0 V. The color of the liquid Ga surface changed slightly from bright metallic to black and gray. When the potential was reversed to anodic direction, the color of the Ga surface changed to gray, black and bright metallic. In liquid Ga electrode, start of cathodic current was more positive potential compared with solid Ag electrode. It is considered the cathodic current at range from 0.0 to 0.6 V may relate to formation of Na-Ga intermetallic compound. In constant current electrolysis, the potential change showed in two steps with similar the voltammogram measurement, color of the Ga surface changed to black in the first step and gray in the second step.

In the open-circuit potential measurement, 3 potential plateaus of 1.9, 0.58, and 0.38 V (vs. Na / Na ⁺) were observed. The potential plateau at 1.9 V corresponds to pure Ga, the potential plateaus 0.58 and 0.38 V may correspond to Na-Ga intermetallic compounds with different composition. It is known from the Na-Ga binary phase diagram that there are two intermetallic compounds, Ga₄Na and Ga₃₉Na₂₂. In the surface color change of Ga electrode to black, it is considered that formation of Ga₄Na on the surface because small amount of Na liquid was electrodeposited on the Ga electrode. Further, by depositing Na on the surface, Ga₃₉Na₂₂ may be formed on the surface, and even if more Na is deposited on Ga₃₉Na₂₂, the intermetallic compound does not change any more, and metal Na is deposited on the Ga₃₉Na₂₂. Therefore, the color of the electrode surface may have changed from black to gray.