Electrodeposition of Ti in molten AF–ACl–A3TiF6 (A = Li, Na, K)
Authors: Norikawa Yutaro , Nohira Toshiyuki 
Published: December 2024

Introduction
Titanium (Ti) has excellent properties such as high corrosion resistance and biocompatibility. However, it is still not widely used due to its high production cost and poor processability. If it is possible to plate Ti films on various materials, the excellent surface properties of Ti can be imparted, and it is expected to be applied to a wider range of fields.
We have already reported Ti film electrodeposition in molten KF–KCl [1-3] and LiF–LiCl [4,5] and smooth Ti films were obtained. Here, fluoride–chloride melts consisting of a single cation, such as KF–KCl and LiF–LiCl, were used so that differences due to cations could be compared. In this study, we investigated the electrochemical behavior of Ti(III) ions and the electrodeposition of Ti in molten AF–ACl–A3TiF6 (A = Li, Na, K, Cs), and compared the results in each melt.
Experimental
The experiments were conducted in molten AF–ACl–A3TiF6 (A = Li, Na, K, Cs). For the Li, Na, and K systems, the composition of AF–ACl was 33:67 mol% and the temperature was 1023 K. For the Cs system, the composition was 50:50 mol% and the temperature was 823 K. A2TiF6 (A = Li, Na, K, Cs) and sponge Ti were added to the melts and Ti(IV) ions were converted to Ti(III) ions by comproportionation reaction. The concentrations of A3TiF6 were 0.50 mol% for the Li, Na, and K systems and 0.67 mol% for the Cs system. 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 = Li, Na, K, Cs) potential measured at a Mo electrode. The electrochemical behavior of Ti(III) was studied by cyclic voltammetry(CV). Samples prepared by galvanostatic electrolysis of Ni plate substrates were analyzed by X-ray diffraction (XRD) after washing with distilled water.
Results and Discussion
Electrochemical behavior of Ti(III) ions
In the negative potential region, CVs were measured at a Mo flag electrode. The cathodic currents were observed for all systems. These currents are attributed to Ti electrodeposition from Ti(III) ions.
Ti(III) + 3e− → Ti(0).
The standard potential (E
0) of Ti(III)/Ti(0) in each melt was calculated from the potential of the immersed Ti rod and the molar fraction of A3TiF6. The obtained E
0 of Ti(III)/Ti(0) were 1.41, 0.69, 0.52, and 0.53 V vs. A+/A for Li, Na, K, and Cs systems, respectively.
In the positive potential region, CVs were measured at an Au flag electrode. The redox currents were observed for all systems. These currents are attributed to the redox reaction of Ti(III) and Ti(IV).
Ti(IV) + e− ⇄ Ti(III).
The peak potentials were constant at various scan rates, indicating that the redox reactions are reversible. Then, the formal potentials (E
0’) of Ti(IV)/Ti(III) were calculated from the anodic and cathodic peak potentials. The obtained E
0’of Ti(IV)/Ti(III) were 2.83, 2.04, 1.77, and 1.62 V vs. A+/A for Li, Na, K, and Cs systems, respectively.
We compared the obtained E
0 of Ti(III)/Ti(0) and E
0’of Ti(IV)/Ti(III) in molten AF–ACl–A3TiF6 (A = Li, Na, K, Cs), as shown in Fig. 1. All potentials were calibrated using Cl2/Cl− potential for comparison. Both Ti(III)/Ti(0) and Ti(IV)/Ti(III) potentials were negative in the order of Li > Na > K > Cs systems.
Electrodeposition of Ti
Figure 2 shows the optical images and XRD patterns of electrodeposited Ti in Li, Na, and K systems. Although the crystal orientation was different, well-crystalized Ti deposits were obtained in these systems. However, in the Cs system, only powder-like Ti was obtained because the Cs metal fog was co-deposited during electrodeposition.
Acknowledgment
A part of this work was supported by JSPS KAKENHI Grant Number 22K14507 and a project, JPNP20004, subsidized by NEDO. A part of this study was conducted in collaboration with Sumitomo Electric Industries, Ltd.
References
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[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