Effect of Alkali Metal Cations on Electrochemical Behavior of Ti(III) Ions in Fluoride–Chloride Mixed Melts
Authors: Norikawa Yutaro , Yasuda Kouji , Numata Koma , Awazu Tomoyuki , Majima Masatoshi , Nohira Toshiyuki 
Published: February 2020

1．
Introduction

Titanium metal has superior properties such as high strength-to-weight ratio, heat resistance, and corrosion resistance, etc. However, its high production cost and poor workability have been preventing the more widespread use of Ti metal. To utilize the superior properties and resolve the problems of Ti metal, electrdeposition of Ti from molten salt has been studied [1–5]. In previous studies, chloride [1,2], fluoride [3,4], and fluoride–chloride [5] molten salts were used as electrolytes. Generally, compact and smooth Ti films were obtained from fluoride melts such as LiF–NaF–KF [3,4] or fluoride-chloride melts such as NaCl–KCl–NaF [5]. The effect of F− ions on the electrodeposition reaction of Ti was also reported [6,7]. In these reports, the potential of Ti(III)/Ti(0) became negative when F/Cl or F/Ti ratio became higher. However, to the best of our knowledge, there have been no reports on the effect of cations on the electrochemical behavior of Ti ions in fluoride–chloride mixed melts. Recently, we reported the electrodeposition of Ti and the electrochemical behavior of Ti(III) ions in KF–KCl (45:55 mol%) melt at 923 K [8,9]. In the present study, we conducted the similar investigation in LiF–LiCl (45:55 mol%) melt at 923 K in order to reveal the effect of cations on the electrochemical behavior of Ti ions.

2.
Experimental

The experiments were conducted in LiF–LiCl (45:55 mol%) melt in an Ar-filled glove box at 923 K. Li2TiF6 (0.5 mol%) and Ti sponge (0.33 mol%) were added to the melt as Ti sources, and Ti (IV) ions were converted to Ti (III) ions by proportionation reaction. Mo flag and plate electrodes were used as working electrodes. The counter and reference electrodes were Ti rods. The potential of the reference electrode was calibrated by the deposition potential of Li metal on a Mo electrode. Samples produced by the galvanostatic electrolysis were analyzed using XRD after washing with water to remove the adhered salts.

3．
Result and discussion

Fig. 1 shows a square wave voltammogram (SWV) measured at frequency of 10 Hz at a Mo flag electrode in the LiF–LiCl melt after the addition of Li­2TiF6 (0.50 mol%) and Ti sponge (0.33 mol%). A single peak with a half width of 96 mV is observed at 1.22 V vs. Li+/Li. A small shoulder at 1.3 V is assumed to correspond to the formation of Ti–Mo alloy. The number of transferred electrons is calculated to be 2.9 from the peak half width. To confirm the electrodeposition of Ti, galvanostatic electrolysis was carried out at a current density of −50 mA cm-2 for 20 min. The potential during electrolysis was around 1.2 V. Fig. 2 shows an optical image of the sample. The deposit exhibits metallic luster. XRD analysis confirmed that the deposit is metallic Ti. From these results, the peak at 1.22 V in the SWV is confirmed to correspond to the reduction of Ti(III) ions to Ti metal. Since the potential of Ti(III)/Ti(0) is 0.32 V vs K+/K in KF–KCl (45:55 mol%) [9], the Ti(III)/Ti(0) potential in LiF–LiCl is much more positive even considering the difference between K+/K and Li+/Li potentials. This result indicates that the alkali cation species have an influence on the reduction stability of Ti(III) complex ions.

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Figure 1