Electrochemical Behavior of Dysprosium Complex in Potassium Bis(trifluoromethylsulfonyl)Amide Melts

Abstract

1. Introduction From the standpoint of energy conservation, the development of a recovery process for rare earth metals with reduced energy consumption is desired. In previous investigations, we demonstrated the recovery of Nd metal using low-temperature molten salts (LTMSs) [1,2], because an LTMS has many useful physicochemical properties [3] such as a wide electrochemical window, low liquid-phase temperature, and high ionic conductivity. In our findings, the reduction process of Dy(III) proceeded in two steps by way of Dy(II), [Dy(III)+e-→Dy(II), Dy(II)+2e-→Dy(0)] in [NTf2 -] based ionic liquids (ILs), because [Dy(III)(NTf2)5]2- and [Dy(II)(NTf2)4]2- in ILs were confirmed by Raman spectroscopy and DFT calculations [4]. However, there were no information about the detailed solvation conformations of Dy(II) and Dy(III) in K[NTf2] melts. Therefore, we have investigated the electrochemical behavior of Dy(II) and Dy(III) in K[NTf2] melts. 2. Experimental The K[NTf2] melts including Dy(III) (molar fraction: x Dy=0.1) were applied as the electrolytic for the electrochemical measurement and this solution was dried at 373 K in a vacuum chamber (<0.1 MPa) for 24 h. Cyclic voltammetry (CV) at 493±1.0 K was carried out using a cylindrical cell constituted from three-electrode system under Ar flow. The Pt electrode was used as a working electrode that was mirror polished using alumina paste. Platinum wires (φ=0.7 mm) were employed as a counter electrode and a quasi-reference electrode (Q.R.E), because the potential obtained using a Pt Q.R.E was stable and exhibited a good reproducibility at medium temperature. All the potentials were compensated for the redox reaction of K/K+ couple. As for the electrodeposition process, a prismatic Dy rod and a Cu substrate were employed as an anode and a cathode, respectively. The anode was surrounded by a soda lime tube with a Vycor glass filter at the bottom to prevent the diffusion of dissolution components from the anode into electrolyte. The potentiostatic electrolysis was carried out while stirring the electrolyte at 500 rpm in order to increase the current density of the electrodeposition process. An in-depth analysis of the electrodeposits was conducted with Al-Kα radiation by XPS. The sputtering rate was 27.2 nm min-1 estimated from the sputtering rate of the Si standard. 3. Results and discussion From the voltammetric analysis of Dy(III) in K[NTf2] melts, there were no apparent anodic peaks corresponding to an oxidation of Dy(0) in voltammogram. This result indicated that the reduction of Dy(III) was an irreversible process. It was also consistent with the reference [5], which was reported that no oxidation peak of Dy(0) in DMF and DMPT. There were two reduction peaks around +1.0 V and +2.3 V vs. K/K+ in voltammogram. The reduction peak around +2.3 V would be based on Dy(III)/Dy(II) charge transfer reaction. The reduction peak around +1.0 V would be corresponded to Dy(II)/Dy(0) deposition process. Regarding the charge transfer reaction, it was confirmed that the plot of the cathodic peak of the current density vs. the square root of the scan rate showed a good linear relation. It indicated that the charge transfer reaction was controlled by the diffusion process.In the electrodeposition process, the blackish-brown electrodeposits had a relatively strong adhesion on the Cu substrate. The Dy3d 5/2 peak of the electrodeposits was shown in Fig. 1. The Dy3d 5/2 peaks for Dy metal and Dy2O3 were theoretically positioned at 1295.8 eV and 1289.0 eV, respectively [6]. The Dy3d 5/2 spectra of the top surface and the middle layer showed a relatively good agreement with the theoretical value. This study enabled us to conclude that Dy metal was able to electrodeposit in a metallic state from the K[NTf2] melts. Acknowledgement This research was partially supported by the Grant-in-Aid for Scientific Research (No. 18H03404) from the Ministry of Education, Culture, Sports, Science and Technology, Japan.