Ionic Conduction Mechanism Analysis for Neodymium Complex in Phosphonium Ionic Liquids and Potassium Bis(trifluoromethylsulfonyl)amide Melts
Authors: Nishimura Atsuki , Matsumiya Masahiko , Tsunashima Katsuhiko 
Published: February 2020

Nd-Fe-B magnets are used in variety of high-tech industries because of owning most powerful magnetic force. The demand for Nd-Fe-B magnets has gradually increased because these magnets play an important role in applications that require energy saving and efficiency. Hence, the recovery of rare earth metals from spent Nd–Fe–B magnets is necessary to secure a stable supply of these resources. As one of effective recovery method for Nd metals, the electrodeposition using ionic liquids (ILs) and low-temperature molten salts (LTMSs) is attractive from a viewpoint of total saving energy. Moreover, it is important to evaluate the ionic conduction mechanism analysis for Nd(III) in IL and LTMS, because the ionic conductivity and the mobility of Nd(III) would be influenced on the current efficiency of the electrodeposition process. In recent, we have already revealed that the electrodeposition behaviors for Nd(III) in [P2225][TFSA] ILs [1] and the electrochemical and nucleation behaviors for Nd(III) in potassium bis(trifluoromethyl-sulfonyl) amide (KTFSA) melts [2]. In this study, ionic conductivities for Nd(III) in both media were evaluated and ionic conduction mechanism will be revealed from a standpoint of theoretical aspects.
[P2225][TFSA] was prepared from [P2225]Br and LiTFSA by a metathesis reaction according to the reference [3]. The resulting crude IL was purified by washing several times with distilled water and bromide anion contamination was not present in the purified IL, as confirmed by AgNO3 check. [P2225][TFSA] was dried in vacuo at 393 K for 72 h. The average yield of IL was confirmed to be 96.9%. KTFSA was dried in vacuo at 373 K for 24 h and the purity of KTFSA was confirmed by a differential thermal analysis (DSC8230, Rigaku) and thermogravimetry differential thermal analysis (TG8120, Rigaku) at a heating rate of 5.0 K min-1. The measured melting point of KTFSA was 461.5±0.5 K, which was consistent with the reference [4]. Nd(TFSA)3 was prepared by reacting Nd2O3 with 1,1,1-trifluoro-N-[(trifluoromethyl)sulfonyl] methanesulfone-amide (HTFSA) at 373 K under agitation. This solution was evaporated at 423 K in order to remove the unreacted acid components. The purified Nd(TFSA)3 salt was obtained as a purplish fine powder and dried at 373 K in a vacuum chamber (<1.0 × 10−6 atm) for 24 h. The yield of the synthesized Nd(TFSA)3 was 97.2%. Then, [P2225][TFSA] ILs dissolved 0.50M Nd(III) and KTFSA melts dissolved 0.01M Nd(III) were dried at 373 K in vacuo for 120 h and 24 h, respectively. The water contents of [P2225][TFSA] and KTFSA media dissolved Nd(III) were 12.6 ppm and <50 ppm, respectively. Platinum wires with 0.7 mm inside diameter were employed as working and counter electrodes. Then, platinum wire with 0.5 mm inside diameter was used as a quasi-reference electrode, because the frequency exhibited good reproducibility. The ionic conductivity with temperature dependence was measured by AC impedance method in the frequency region of 0.1Hz-1.0MHz. The cell constant was determined from the measurement of a KCl standard solution at 298 K.
Arrhenius plot of ionic conductivities for [P2225][TFSA] and KTFSA media dissolving Nd(III) was shown in Fig. 1. The ionic conductivity of [P2225][TFSA] was higher than that of KTFSA due to low viscosity of IL. In addition, the ionic conductivity of both media dissolving Nd(III) was altered from the corresponding neat solution and the slope with temperature dependence is also different each other. This result indicates that the ionic conduction was mainly caused from the transportation of metallic complexes and ionic species. The temperature dependence for each medium was evaluated from VTF equation and theoretical aspects will be discussed in this study.
References
[1] M. Matsumiya, “Electrodeposition of rare earth metal in ionic liquids”, Application of Ionic Liquids on Rare Earth Green Separation and Utilization, (2016) 117-153.
[2] A. Nishimura, M. Matsumiya, H. Ota, S. Kawakami, Electrochem. Commun., 65 (2016) 23-26.
[3] K. Tsunashima, M. Sugiya, Electrochem. Commun., 9 (2007) 2353-2358.
[4] R. Hagiwara, K. Tamaki, K. Kubota, T. Goto, T. Nohira, J. Chem. Eng. Data, 53 (2008) 355-358.

Figure 1