Electrodeposition of Ti in Molten Alkali Metal Fluoride–Chloride Mixtures: Comparative Study of Li, Na, K, and Cs Systems
Authors: Norikawa Yutaro , Unoki Makoto , Nohira Toshiyuki 
Published: September 2023

1. Introduction
Titanium (Ti) is a metal with excellent properties such as high corrosion resistance and biocompatibility. However, it is still not widely used due to its high production cost and poor workability. If Ti films can be plated on a variety of substrates, the excellent surface properties of Ti can be imparted, and it is expected to be applied in a wider range of fields.
We have previously studied Ti electrodeposition using KF–KCl [1-3] and LiF–LiCl [4,5] melts and reported that smooth Ti films were obtained. Here, fluoride–chloride melts containing a single cation, such as KF–KCl and LiF–LiCl, were used so that differences due to cations could be compared. As a result, we have already reported that the electrodeposition potential of Ti differs significantly between the K- and Li-based systems [4]. Therefore, in this study, we investigated the electrodeposition of Ti and the electrochemical behavior of Ti(III) ions in the CsF-CsCl and NaF-NaCl melts. This allowed us to systematically compare the effects of cations on titanium electrodeposition for the Li-, Na-, K-, and Cs-based systems.
2. Experimental
The experiments were conducted in eutectic CsF–CsCl and NaF–NaCl melts in an Ar glove box. A2TiF6 (A = Cs, Na) and sponge Ti were added to the melts and Ti(IV) ions were converted to Ti(III) ions by comproportionation reaction. Ni plate, Mo and Au flag electrodes were used as the working electrodes. The counter and reference electrodes were Ti rods. The potential of the reference electrode was calibrated by A+/A (A = Cs, Na) potential measured at a Mo electrode. The electrochemical behavior of Ti(III) was studied by cyclic voltammetry. Samples prepared by galvanostatic electrolysis of Ni plate substrates were analyzed by X-ray diffraction (XRD) after washing with distilled water.
3. Result and Discussion
Figure 1 (a) shows cyclic voltammograms measured at a Mo flag electrode in the negative potential region in molten CsF–CsCl at 823 K. After the addition of Ti(III) ions, cathodic currents around 0.3 V were larger than those before the addition, and a current shoulder was observed at 0.3 V. Furthermore, a sample prepared by galvanostatic electrolysis at a cathodic current density of 25 mA cm−2 was confirmed to be Ti metal by XRD. Considering that the potential during the electrodeposition was around 0.3 V, the cathodic currents around 0.3 V in the CV were confirmed to be
Ti(III) + 3e− → Ti(0).
Figure 1 (b) shows cyclic voltammograms measured at a Au flag electrode in the positive potential region in molten CsF–CsCl containing Ti(III) ions at 823 K. A pair of redox currents were observed around 1.7 V. The number of electrons transferred was 0.86 which was calculated from the difference in the cathodic and anodic peak potentials. This result indicates that the redox reaction around 1.7 V is
Ti(IV) + e− ⇄ Ti(III).
Here, the formal potential of Ti(IV)/Ti(III) was calculated to be 1.63 V from the average values of the cathodic and anodic peak potentials.
We compared the obtained potentials of Ti(III)/Ti(0) and Ti(IV)/Ti(III) to those previously reported in KF–KCl and LiF–LiCl melts [2,4]. Figure 2 shows the comparison of the Ti(III)/Ti(0) and Ti(IV)/Ti(III) potentials in CsF–CsCl, KF–KCl, and LiF–LiCl melts. All potentials were calibrated using F2/F− potential for comparison because Ti(III) ions exist as fluoro-complexes in the melts. Both Ti(III)/Ti(0) and Ti(IV)/Ti(III) potentials were negative in the order of Li-, K-, Cs-based systems. Such potential differences are explained by the interactions of alkaline cations (A+) and fluoride ions (F−); smaller cations interact more strongly with F−, resulting in reduced stability of Ti fluoro-complexes. At the meeting, we will compare and discuss all the results, including those obtained with NaF-NaCl melts.
Acknowledgment
A part of this work was supported by JSPS KAKENHI Grant Number 22K14507. A part of this study was conducted as a collaboration with Sumitomo Electric Industries, Ltd.
References
[1] Y. Norikawa, K. Yasuda, and T. Nohira, Mater. Trans., 58, 390 (2017).

[2] Y. Norikawa, K. Yasuda, and T. Nohira, Electrochemistry,
86, 99 (2018).

[3] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc., 166, D755 (2019).

[4] Y. Norikawa, K. Yasuda, and T. Nohira, J. Electrochem. Soc., 167, 082502 (2020).

[5] Y. Norikawa, M. Unoki, K. Yasuda, and T. Nohira, J. Electrochem. Soc., 169, 092523 (2022).

Figure 1