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ISSN : 1738-1894(Print)
ISSN : 2288-5471(Online)
Journal of Nuclear Fuel Cycle and Waste Technology Vol.19 No.2 pp.161-176
DOI : https://doi.org/10.7733/jnfcwt.2021.19.2.161

Electrochemical Behavior of Sm(III) on the Aluminium-Gallium Alloy Electrode in LiCl-KCl Eutectic

Chang-Mei Ye1,2,a, Shi-Lin Jiang1,a, Ya-Lan Liu1, Kai Xu3, Shao-Hua Yang2*, Ke-Ke Chang3, Hao Ren1, Zhi-Fang Chai1,3, Wei-Qun Shi1*
1Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
2Jiangxi University of Science and Technology, Jiangxi 341000, China
3Ningbo Institute of Industrial Technology, Chinese Academy of Sciences, Ningbo 315201, China
a

The first two authors contributed equally to this work.


* Corresponding Author. Shao-Hua Yang, Jiangxi University of Science and Technology, E-mail: xizi527@163.com, Tel: +86-15970085173

* Wei-Qun Shi, Institute of High Energy Physics, E-mail: shiwq@ihep.ac.cn, Tel: +86-10-88233968

November 11, 2020 ; December 7, 2020 ; January 18, 2021

Abstract


In this study, the electrochemical behavior of Sm on the binary liquid Al-Ga cathode in the LiCl-KCl molten salt system is investigated. First, the co-reduction process of Sm(III)-Al(III), Sm(III)-Ga(III), and Sm(III)-Ga(III)-Al(III) on the W electrode (inert) were studied using cyclic voltammetry (CV), square-wave voltammetry (SWV) and open circuit potential (OCP) methods, respectively. It was identified that Sm(III) can be co-reduced with Al(III) or Ga(III) to form AlzSmy or GaxSmy intermetallic compounds. Subsequently, the under-potential deposition of Sm(III) at the Al, Ga, and Al-Ga active cathode was performed to confirm the formation of Sm-based intermetallic compounds. The X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analyses indicated that Ga3Sm and Ga6Sm intermetallic compounds were formed on the Mo grid electrode (inert) during the potentiostatic electrolysis in LiCl-KCl-SmCl3-AlCl3- GaCl3 melt, while only Ga6Sm intermetallic compound was generated on the Al-Ga alloy electrode during the galvanostatic electrolysis in LiCl-KCl-SmCl3 melt. The electrolysis results revealed that the interaction between Sm and Ga was predominant in the Al-Ga alloy electrode, with Al only acting as an additive to lower the melting point.



초록


    1. Introduction

    Electrorefining process in molten salt is a promising technology to separate and recover actinides (An) from advanced spent nuclear fuels (SNF) [1-5]. Traditionally, in the electrorefining process, uranium is mainly recovered on the stainless steel cathode by electrodeposition, while plutonium and other minor actinides (MA) along with some residual uranium [6] are deposited on a liquid cadmium cathode (LCC). However, since the deposition potentials of An and Ln are very close on the LCC, the deposition products often contain significant amount of lanthanides, leading to poor An/Ln separation factor [7]. To achieve a higher separation efficiency of An/Ln, various active cathode materials have been explored [8-17]. Up to now, the electrochemical behaviors and thermodynamic properties (such as solubility and activity coefficient) of An and Ln on these cathodes have been systematically studied, and the relative theoretical separation factors of An/Ln have been also evaluated [18-23]. According to the findings of the Institute of Transuranium Elements (ITU), the deposition potential gaps of An and Ln on solid Al cathodes are larger than that on other active cathodes, so the most favorable cathode material for An/Ln separation should be Al [24, 25]. In recent years, our group studied the electrochemical properties of An and Ln on the Al electrode in an in-depth manner, and carried out the separation experiments of An/Ln. We found that the Al electrode indeed showed excellent An/Ln separation performance [26]. Nevertheless, at the experimental temperature, the Al cathode is in solid form, and the electrodeposition process is controlled by the tardy solid-solid diffusion, resulting in a slow separation rate. Generally, the melting point of an aluminum alloy is lower than that of a pure aluminum metal. Herein, it is designed to incorporate Ga into aluminum to form an Al-Ga alloy to lower the melting point of aluminum. In fact, our group has also studied the electrochemical behavior of U and Lns on a Ga electrode [11, 27-29], and the Ga electrode also showed impressive An/Ln separation performance. Lambertin et al. [18], also believed that Ga could perform An/Ln separation efficiently and selectively. Therefore, it is feasible to believe that the addition of Ga into Al electrode may afford new opportunities on the An/Ln separation. Actually, there are a few reports on the applications of Al-Ga for An/Ln separation. For instance, Volkovich et al. [30] found that the activity and solubility of uranium in Al-Ga alloy increased with the increase of Al content. In addition, according to the report of Smolenski et al., Al-Ga alloy should be a forward-looking medium for nuclear waste treatment [31].

    In this work, to investigate the application of liquid Al- Ga alloy electrode in electrorefining process, the electrochemical behavior of Sm(III) on the Al-Ga alloy electrode in LiCl-KCl eutectic was studied. Samarium is a typical multivalent fission product with high content, which could be recovered only by forming alloys with reactive electrodes in electrorefining process. Hence, Sm was chosen as a surrogate to evaluate the performance of liquid Al-Ga alloy electrode. Moreover, the co-reduction behavior of Sm(III) with Ga(III) and Al(III) on a W electrode was also determined to provide basic data for further explaining the reduction mechanism.

    2. Experimental

    2.1 Experimental Materials and Melt Preparation

    Anhydrous potassium chloride ( > 99.9%), lithium chloride ( > 99.9%), and bismuth metal granule ( > 99.9%) were purchased from Sinopharm Chemical Reagent Co. Ltd. Samarium chloride ( > 99.9%) and aluminum chloride ( > 99.9%) were obtained from Alfa Aesar, while gallium chloride ( > 99.9%) was obtained from Energy Chemical. The lithium metal ( > 99.9%) came from Sigam-Aldrich. First, the LiCl-KCl eutectic salt mixture (LiCl : KCl = 59 : 41 mol%) was dried in a vacuum oven for 24 h at 473 K to remove residual moisture, then was transferred into the glove box. Subsequently, the mixture was placed in a 217 cm3 alumina crucible and was melted in furnace at 773 K. Afterwards, impurities in the melt were removed by pre-electrolysis at the potential of −0.7 V (vs. Li/Li3Bi) for 5 h. In the experiment, the Sm(III), Ga(III) and Al(III) ions were introduced into the melt by adding SmCl3, GaCl3 or AlCl3 powders and their concentrations were determined by the inductively coupled plasma atomic emission spectrometer (ICP-AES, JY 2000-2).

    2.2 Electrochemical Apparatus and Electrodes

    All experiments were carried out in a glove box filled with high purity argon gas, where the concentrations of oxygen and moisture were less than 1 ppm, and electrochemical measurements were done by using an Autolab PGSTAT 302 N potentiostat-galvanostat controlled with the Nova 1.11 software package from Metrohm. In this work, a threeelectrode electrochemical cell were assembled in an alumina crucible (Φ 48×120 mm), and the experimental electrolytic cell device is shown in Fig. 1. Working electrode 1 (WE1) was a W wire with 1 mm in diameter for the electrochemical behavior measurements. Working electrode 3 (WE3) was a molybdenum grid (S = 1 cm2, 300 mesh) fixed by a W wire for electrolysis experiments, and working electrode 2 (WE2) was the reactive electrodes including Al rod, liquid Ga metal and Al-Ga alloy (Ga-1.67wt% Al). Al-Ga alloy was performed by uniformly mixing pure aluminum (99.999%) and gallium (99.99%) metals into a corundum crucible (Φ 16×16 mm), and was melted in an electric resistance furnace for 24 h. For assembling liquid working electrodes, liquid Ga metal and Al-Ga alloy were loaded in corundum crucibles, and molybdenum wires were inserted as conductors. The counter electrode (CE) was a graphite rod (Φ = 6 mm) in spectral purity. The reference electrode (RE) was a binary Li-Bi alloy (xLi = 5wt%) sealed in a BN tube, as shown in Fig. 1. The design and fabrication of the reference electrode has been explained in our previous work [32].

    2.3 Preparation and Characterization of Cathodic Deposits

    Both potentiostatic electrolysis and galvanostatic electrolysis were employed to preparing Sm-Ga, Sm-Ga-Al alloys. The potentiostatic electrolysis was carried out on the molybdenum grid in molten LiCl-KCl-SmCl3-AlCl3-GaCl3 melt, while the galvanostatic electrolysis was performed on the liquid Ga and Al-Ga alloy electrodes in LiCl-KCl-SmCl3 melt. Alloy samples were washed with ethylene glycol in an ultrasonic bath to remove the residual salt, then were dried and stored in the glove box for further analyses. Then, these samples were analyzed by SEM (Hitachi S-4800) with EDS (GENESIS 2000) and XRD (Bruker, D8 Advance).

    2.4 Calculation of Phase Diagrams

    Although the thermodynamic parameters of the phases in the binary Al-Ga [33], Al-Sm [34] and Sm-Ga [35] systems are available from literatures, the Sm-Ga-Al phase diagram has not been reported. Therefore, the CALPHAD (CALculation of PHAse Diagram) [36] was employed to calculate the Sm-Ga-Al phase diagram. The formation of the SmGaAl ternary intermetallic compound was obtained by ab initio calculation [37] and used to determine its Gibbs energy. The isothermal section of 773 K was calculated using the Gibbs energy function in the dataset of the Sm- Ga-Al system using Thermo-Calc software. The SmGaAl Gibbs free energy was obtained by modeling as follows:

    Δ G f , S m G a A 1 = G S m G a A 1 1 3 G S m r h o 1 3 G A 1 f c c 1 3 G G a o r t
    (1)

    Where Δ G f , S m G a A 1 is the Gibbs free energy of formation for SmGaAl. G S m r h o , G G a o r t and G A 1 f c c , are the Gibbs energy of the pure elements Sm, Ga and Al in their reference states (rhombohedral_C19, orthorhombic_A11 and fcc_A1), respectively.

    3. Results and Discussion

    3.1 The Sm-Ga-Al Ternary Phase Diagram

    Fig. 2 displays the isothermal cross-section phase diagram of the Sm-Ga-Al system at 773 K. Six kinds of Sm-Ga and four kinds of Al-Sm intermetallic compounds are displayed in the phase diagram. Nevertheless, there was no Ga-Al intermetallic compound observed. Ideally, the SmGaAl ternary intermetallic compound should be in Al2Sm + Ga2Sm two-phase region with orange dot. However, the area for SmGaAl ternary intermetallic compound formation was much smaller than that for CeGaAl which we prepared via the co-reduction of Ce, Al and Ga on the W electrode [38], suggesting the conditions for forming SmGaAl intermetallic compound should be more difficult.

    3.2 Electrochemical Behavior Analysis

    3.2.1 Co-reduction Behaviors of Sm(III) with Ga(III), Sm(III) with Al(III) on the W Electrode

    To fundamentally investigate the electrochemical behavior of Sm(III) on the Al-Ga alloy electrode, the electrochemical behavior of Sm(III) on a W electrode was first studied in LiCl-KCl-SmCl3 melt. As shown in Fig. 3(a) (black curve), only two redox couples A/A' and F/F', which represent the reduction/oxidation of Li(I)/Li and Sm(III)/Sm(II) respectively, are observed in the electrochemical window of LiCl-KCl-SmCl3 melt [39, 40]. When GaCl3 was added into the LiCl-KCl-SmCl3 system, two redox couples H/H' and G/G' attributed to Ga(III)/Ga(I) and Ga(I)/Ga(0) respectively are found on the red curve in Fig. 3(a), which is agreed with the result of Liu et al [11]. Moreover, three pairs of peaks I/I', J/J' and M/M' are clearly observed between A/A' and F/F', and therefore ascribed to the formation/dissolution of Ga6Sm, Ga3Sm and GaxSmy intermetallic compounds (determined from XRD analyses), respectively. There are six different Sm-Ga intermetallic compounds based on the phase diagram [41]. However, only three kinds of Sm-Ga intermetallic compounds were formed during CV measurement, which may be related to the relative concentrations of Sm(III) and Ga(III) in the melt. Moreover, as displayed in Fig. 3(b), a weak oxidation signal I'' was determined, which might be correlated with the dissolution of an unstable Sm-Ga intermetallic compound. The electrochemical signals mentioned above and their corresponding reactions are summarized in Table 1.

    On the other hand, Fig. 4 shows cyclic voltammograms measured on a W electrode before (black curve) and after (red curve) the addition of AlCl3 into LiCl-KCl-SmCl3 melt at 773 K. In the red curve of Fig. 4, besides F/F', two pairs of new redox signals K/K' and N/N' appear, which are ascribed to the deposition/dissolution of Al-Sm intermetallic compounds in the LiCl-KCl-SmCl3-AlCl3 molten salt system according to Liu et al [42].

    Similar phenomenon was observed in the co-reduction process of Al(III) and Ga(III) ions as well. As presented in Fig. 5, the redox couple C/C' corresponds to the deposition/ dissolution of solid Al metal. The potential for redox peaks C/C' was more positive than that in Fig. 4 because of the under-potential deposition of Al(III) on the liquid Ga film electrode, which was discussed in previous works [38, 43]. Redox couples B/B' and E/E' in Fig. 5 are respectively related to the formation/dissolution of Al-Li and Ga-Li intermetallic compounds in accordance with previous reports [11, 44, 45]. Furthermore, no electrochemical signal related to the Al-Ga intermetallic compound was found. This is consistent with the phase diagram of the Al-Ga system [33].

    3.2.2 Co-reduction Behaviors of Sm(III) with both Ga(III) and Al(III) on the W Electrode

    CV test performed on a W electrode in LiCl-KCl- SmCl3-GaCl3-AlCl3 melt at 773 K is shown on the red curve of Fig. 6(a). According to the discussion above, signals K/K' and N/N' on the black curve should correspond to the deposition/dissolution of two Al-Sm intermetallic compounds. It is worth noting that the potential of peak K was slightly shifted due to the under-potential deposition of Sm on the Al electrode. In addition, a new redox couple O/O' was found compared with Fig. 4, and should correspond to the deposition/dissolution of another Al-Sm intermetallic compound, which might be also related with the concentration of Al(III) [42]. Fig. 6(b) shows the CV curves obtained in the LiCl-KCl-SmCl3-GaCl3 and LiCl-KCl-SmCl3-GaCl3- AlCl3 systems. As discussed in Fig. 3(a), the redox signals I/I', J/J' and M/M' should be related to Sm-Ga intermetallic compounds. In CV curves from LiCl-KCl-SmCl3-GaCl3- AlCl3 melt (the red one in Fig. 6), a pair of new redox signals X1/X1' was observed. According to the isothermal section phase diagram of Al-Ga-Sm system at 773K in Fig. 2, the formation of intermetallic compound SmAlGa should be theoretically reasonable, so X1/X1' might be related to the formation/dissolution of Sm-Ga-Al intermetallic compounds. Further investigation is conducted in section 3.3 to identify this pair.

    To get more detail about the co-reduction behaviors of Sm(III), Al(III) and Ga(III), square wave voltammetry was conducted on the W electrode in different molten salt systems at 773 K, as shown in Fig. 7. The signal C2 obtained in the LiCl-KCl-SmCl3-AlCl3-GaCl3 melt was located at a more anodic position than C1 in the LiCl-KCl-SmCl3-AlCl3 system, due to C2 correspond to the deposition of Al on the liquid Ga film electrode. The other signals in Fig. 7 were identical to the signals in Fig. 6 except for signals C1 and C2.

    Besides, open circuit chronopotentiometry, as a suitable technique, was used to analyze the dissolution of Sm- Ga, Al-Sm and Sm-Ga-Al intermetallic compounds in this work. Fig. 8 shows a series of OCP curves recorded in different molten salt systems after applying an electrodepositing potential of −0.85 V (vs. Li/Li3Bi) at 773 K, respectively. According to CV and SWV analyses, the platforms C2 and C1 are attributed to the two-phase equilibrium of Al(0)/Al(III) on the Ga film electrode and the W electrode respectively, and the platforms A, G and H refer to twophase equilibrium of Li(0)/Li(I), Ga(0)/Ga(I) and Ga(I)/ Ga(III) redox couples, respectively. Platforms I and J on the red curve and K, N and O on the black one are respectively associated with two-phases coexisting states of Sm-Ga and Al-Sm intermetallic compounds. In addition, the plateau X1 might be attributed to the dissolution process of Sm-Ga-Al ternary intermetallic compound. Further investigation was needed to verify. The reactions involved in the platforms on these OCP curves are consistent with previous analysis from CV and SWV curves.

    3.2.3 Electrochemical Behavior of Sm(III) on the Al Cathode

    Fig. 9 illustrates the comparison of the CVs collected on Al electrodes. The electrochemical window on the Al electrode was smaller than that on the W electrode in the LiCl- KCl-SmCl3 melt (Fig. 9 red curve). Then, the redox couples of A/A' and B/B' represent the reduction/dissolution of metallic lithium and Al-Li alloy, respectively. The anodic oxidation based on the Al electrode occurs at a smaller anode potential, thus the anodic limit at 1.0 V corresponds to the anodic dissolution process of the Al electrode. In the red curve of Fig. 9, the cathodic peak V/V' corresponding to the reduction/dissolution of Al(III) could be observed, and according to a study by Castrilejo et al., it was confirmed that 3Sm(III)+Al→3Sm(II)+Al(III) reaction occurred [46]. Furthermore, when the curve continued to scan in the negative direction, the redox signal S/S' could be observed, which corresponds to the formation/dissolution of Al-Sm alloy (coincide with N/N' in the red curve of Fig. 4). Finally, the electrochemical signals and corresponding reactions on the Al, liquid gallium and Ga-1.67wt% Al alloy electrodes are summarized in Table 2.

    3.2.4 Electrochemical Behaviors of Sm(III) on Liquid Ga and Al-Ga Alloy Electrodes

    Subsequently, the deposition of Sm was then conducted at the Al-Ga alloy electrode. Fig. 10 presents the CV curves recorded on W, liquid gallium and Al-Ga alloy electrodes in the LiCl-KCl-SmCl3 melt at 773 K. As for the gray curve in Fig. 10, the redox peaks of Sm(III)/Sm(II) on the W electrode was determined at 0.8/0.96 V. However, when the working electrode was replaced by a liquid gallium one, the electrochemical window (about −0.1 to 1.5 V) (black curve) became much narrower, where the anodic limit and cathodic limit correspond to the oxidation of gallium metal and reduction of Li metal, respectively. In contrast, the black curve shows a large cathodic signal starting at about 1.05 V, which corresponds to the reduction of Sm(III) in the bulk liquid Ga. The redox couple U/U' observed at 0.11/0.41 V may be related to the deposition/dissolution of Sm(III) on the liquid Ga electrode. Subsequently, when the Ga-1.67wt% Al alloy served as the working electrode, the electrochemical window (about −0.1 to 1.1 V) (red curve) was much reduced again, where the anodic limit and the cathodic limit correspond to the oxidation of Al-Ga alloy electrode and the reduction of lithium metal, respectively. The redox couple W/W' might be related to the deposition/ dissolution of Sm(III) on the Al-Ga alloy electrode.

    3.3 Preparation and Characterization of the Sm-Ga and Sm-Ga-Al Alloys

    3.3.1 Potentiostatic Electrolysis on the Molybdenum Grid

    Preparation of Sm-Ga and SmGaAl alloys via potentiostatic electrolysis on the Mo grid in LiCl-KCl-SmCl3-GaCl3 melt was carried out at 773 K. Fig. 11(a, d, e) display the XRD and SEM-EDS patterns of the deposit electrolyzed at 0.1 V in LiCl-KCl-SmCl3-GaCl3 melt for 3.5 h at 773 K. Based on XRD characterizations, the Sm-Ga alloys were composed of Ga3Sm and Ga6Sm phases (Fig. 11(a)). The deposited products were shown to be irregular block according to SEM images (Fig. 11(e)), and EDS analysis showed that these particles contained Sm and Ga (Fig. 11(d)). Similarly, for the electrolytic potential at −0.3 V, the XRD figure showed that the Sm-Ga alloy was only composed of Ga6Sm phases (Fig. 11(b)). The SEM pattern presented heterogeneous particles in Fig. 11(g), mainly composed of Sm and Ga according to the EDS scan (Fig. 11(f)). In addition, when the electrolytic potential of −0.72 V was employed, it could be found that these particles consisted of two intermetallic compounds Ga3Sm and Ga6Sm, respectively (Fig. 11(c)). However, in Fig. 11(h), the presence of oxygen is due to oxidation during sample processing. Similarly, the presence of chlorine is due to a small amount of residual salt in the sample. Since the related articles was reported the preparation of Al-Sm alloy [42, 47], no further discussion was needed here.

    Subsequently, in order to analyze the electrodeposition product on the Mo grid, potentiostatic electrolysis was carried out in the LiCl-KCl-SmCl3-GaCl3-AlCl3 molten salt at a potential of −0.2 and −0.72 V, respectively, based on the CV and SWV results. At first, the electrolytic potential of −0.2 V was used, which was around of the reduction peak X1 in the CV curve of Fig. 6. Fig. 12(a, c, d) display the XRD and SEM-EDS analyses of the sample, which was obtained by potentiostatic electrolysis at −0.2 V on the molybdenum grid for 3.5 h at 773 K. The intermetallic compounds Ga6Sm and Ga3Sm with compact spherical particle were identified (Fig. 12(a, d)). And EDS (Fig. 12(c)) was shown as mainly Ga, Sm and Al. Afterwards, the electrolytic potential was carried out at −0.72 V, which was around of the signal K in CV curve in Fig. 6. Based on the results of Fig. 12(b), Ga6Sm, Al3Sm and Ga phases were obtained. According to the SEM image in Fig. 12(f), the form showed an irregular massive grain structure. And EDS (Fig. 12(e)) was shown as mainly Ga, Sm and Al. As shown in Fig. 12(a, b), there was no new phase AlGaSm characterized by XRD at different electrolytic potentials, indicating that AlGaSm should be an unstable intermetallic compound. Furthermore, it was found that the Al-Sm alloy formed in all of the samples were less than the Ga-Sm alloy. A reasonable explanation is that the interaction between Ga and Sm is stronger than Al and Sm, making the formation of Ga-Sm alloy much easier than Al-Sm alloy. According to these comprehensive analyses, the following conclusions can be drawn: (1) two kinds of binary Sm-Ga intermetallic compound (Ga6Sm and Ga3Sm) can be formed by potentiostatic electrolysis in LiCl-KCl-SmCl3-GaCl3 melt and LiCl-KCl-SmCl3-GaCl3- AlCl3 melt; (2) At a relatively negative potential (−0.72 V), the electrolysis product contained Al3Sm, manifesting that the reduction signal K corresponds to the Al-Sm intermetallic compound (see Fig. 6); (3) According to Fig. 12(a, b), the pure single phase of Sm-Ga-Al is not obtained, indicating that the ternary intermetallic compound SmGaAl is difficult to form and unstable.

    3.3.2 Galvanostatic Electrolysis on Al, Liquid Gallium and Al-Ga Alloy Electrodes

    To further study the electrodeposition of Sm on the Al-Ga alloy cathode, the electrodeposition of Sm on an Al electrode was first studied. Galvanostatic electrolysis was carried out at −0.1 A for 10 h on the Al electrode in LiCl- KCl-SmCl3 melt, as shown in Fig. 13(a), the diffraction peaks in pattern were identified as Al, Al3Sm and Al2Sm phases. Our galvanostatic electrolysis products are consistent with the results of Castrilejo et al. [46] . According to the SEM image in Fig. 13(e), this sample showed a more uniform particle structure. And EDS (Fig. 13(d)) was mainly shown as Sm and Al elements. Theoretically, Ga2Sm is known to be one of the rich Ga phase intermetallic compounds based on the Sm-Ga phase diagram [35]. And for the Ga electrode, as shown in Fig. 13(b), a large amount of Ga2Sm was mainly observed, which indicates that the Sm element can be saturated in the liquid Ga to form a Sm-Ga intermetallic compound (here Ga2Sm is formed), which is related to signals U/U' in Fig. 10. Since the gallium was used as a liquid cathode, it was inevitable to mix some gallium metal when processing an electrolytic sample. Then the EDS in Fig. 13(f) was mainly shown as Sm and Ga elements, and the sample in Fig. 13(g) showed an irregular block structure. Further, in order to compare the above deposition relationship, as shown in Fig. 13(c), galvanostatic electrolysis was carried out at −0.1 A for 10 h on Ga- 1.67wt% Al alloy electrode in LiCl-KCl-SmCl3 melt. The precipitate deposited on the Ga-1.67wt% Al alloy electrode was rinsed with ultrapure water, then dried for SEM-EDS detection. Surface analysis was performed by XRD. In Fig. 13(c) XRD image showed the inclusion of Al and Ga6Sm. The reason for this phenomenon may be that the concentration of Al in the Al-Ga electrode is not high enough, which makes it easier for Sm to combine with Ga in the Al-Ga alloy electrode to form Ga-Sm alloy. It is also the reason why the ternary SmGaAl is not easily formed. According to the SEM image in Fig. 13(i), the sample showed a bulk particles structure. The EDS (Fig. 13(h)) was mainly expressed as Sm, Ga, and Al elements.

    4. Conclusions

    The co-reduction behaviors of Sm(III), Al(III) and Ga(III) ions and the electrochemistry of Sm(III) on different active electrode materials (solid Al, liquid Ga and Ga-1.67wt% Al alloy electrodes) were systematically studied. The redox signals related to SmGaAl ternary intermetallic compound was detected (Fig. 6), however there was no SmGaAl intermetallic compound obtained by potentiostatic or galvanostatic. It could be inferred that SmGaAl is an unstable intermetallic compound and difficult to prepare. In addition, Ga3Sm and Ga6Sm intermetallic compounds could be formed by the potentiostatic electrolysis in LiCl-KCl-SmCl3-GaCl3-AlCl3 melt. On the other hand, in the LiCl-KCl-SmCl3 melt, Ga2Sm could be formed by the galvanostatic electrolysis on the liquid Ga electrode and Ga6Sm was obtained by the galvanostatic electrolysis on the Al-Ga alloy electrode. Therefore, we believe that Sm is more likely to interact with Ga to form Sm-Ga intermetallic compounds, and Al only serves as an additive which is difficult to interact with Sm during the electrolysis. In the next step, further works on adjusting the content of Al and Ga in the liquid electrode, and the application prospects of the binary liquid Al-Ga cathode will be conducted in detail.

    Acknowledgements

    This work was supported by the Major Program of the National Natural Science Foundation of China (Grant No. 21790373), and the Young Scientists Fund of the National Natural Science Foundation of China (Grant No.51604252), and Guangxi Key Laboratory of Processing for Non-ferrous Metallic and Featured Materials (Grant No. GXYSOF1815).

    Figures

    JNFCWT-19-2-161_F1.gif

    Schematic diagram of experimental electrolytic cell.

    JNFCWT-19-2-161_F2.gif

    Isothermal section phase diagram of Al-Ga-Sm at 773 K.

    JNFCWT-19-2-161_F3.gif

    (a) CV curves obtained on the W electrode (0.41 cm2) in LiCl-KCl-SmCl3 (1.17×10−4 mol∙cm−3) melt (black curve) and LiCl-KCl-SmCl3 (1.17×10−4 mol∙cm−3)-GaCl3 (2.82×10−5 mol∙cm−3) melt (red curve); (b) CV curves of different scan ranges obtained on the W electrode (0.41 cm2). Scan rate: 0.1 V/s; temperature: 773 K.

    JNFCWT-19-2-161_F4.gif

    CV curves obtained on the W electrode (0.41 cm2) in LiCl-KCl- SmCl3 (9.72×10−5 mol∙cm−3) melt (black curve) and LiCl-KCl-SmCl3 (9.72×10−5 mol∙cm−3)-AlCl3 (2.47×10−5 mol∙cm−3) melt (red curve). Scan rate: 0.1 V/s; temperature: 773 K.

    JNFCWT-19-2-161_F5.gif

    CV curves obtained on the W electrode (0.41 cm2) in LiCl-KCl- AlCl3 (2.07×10−5 mol∙cm−3) (black curve) and LiCl-KCl-AlCl3 (2.07×10−5 mol∙cm−3)-GaCl3 (6.45×10−6 mol∙cm−3) melt (red curve). Scan rate: 0.1 V/s; temperature: 773 K.

    JNFCWT-19-2-161_F6.gif

    Comparison of the CV curves obtained on W electrode (0.41 cm2) in different systems: (a) black curve: LiCl-KCl-SmCl3 (9.15×10−5 mol∙cm−3)- AlCl3 (2.9×10−5 mol∙cm−3) melt; (b) black curve: LiCl-KCl-SmCl3 (1.09×10−4 mol∙cm−3)-GaCl3 (2.11×10−5 mol∙cm−3) melt, red curve: LiCl-KCl-SmCl3 (1.01×10−4 mol∙cm−3)-GaCl3 (2.87×10−5 mol∙cm−3)-AlCl3 (2.39×10−5 mol∙cm−3) melt; scan rate: 0.1 V/s; temperature: 773 K.

    JNFCWT-19-2-161_F7.gif

    SWV obtained on W electrode (0.41cm2) in LiCl-KCl-SmCl3 (9.15×10−5 mol∙cm−3)-AlCl3 (2.9×10−5 mol∙cm−3) melt (black curve), LiCl-KCl-SmCl3 (1.09×10−4 mol∙cm−3)-GaCl3 (2.11×10−5 mol∙cm−3) melt (red curve), LiCl-KCl-SmCl3 (1.01×10−4 mol∙cm−3)-AlCl3 (2.39×10−5 mol∙cm−3)-GaCl3 (2.87×10−5 mol∙cm−3) melt (blue curve) at 773 K, with condition of amplitude: 25 mV; potential step: 1 mV; frequency: 20 Hz.

    JNFCWT-19-2-161_F8.gif

    OCP curves obtained on a W (0.41 cm2) electrode at 773 K in LiCl-KCl-SmCl3 (9.15×10−5 mol∙cm−3)-AlCl3 (2.9×10−5 mol∙cm−3) melt (black curve), LiCl-KCl-SmCl3 (1.09×10−4 mol∙cm−3)-GaCl3 (2.11×10−5 mol∙cm−3) melt (red curve), LiCl-KCl-SmCl3 (1.01×10−4 mol∙cm−3)-AlCl3 (2.39×10−5 mol∙cm−3)-GaCl3 (2.87×10−5 mol∙cm−3) melts (red curve) after deposition at −0.8 V for 10 s.

    JNFCWT-19-2-161_F9.gif

    Comparison of the CV curves collected on the W (gray curve) and Al electrodes (blue curve) in LiCl-KCl-SmCl3 (2.41×10−4 mol∙cm−3) melt at 773 K. Scan rate: W electrode, 100 mV∙s−1; Al electrode, 50 mV∙s−1.

    JNFCWT-19-2-161_F10.gif

    Comparison of the CV curves collected at W (gray curve), liquid Ga (black curve) and Al-Ga alloy (red curve) electrodes in LiCl-KCl- SmCl3 (2.15×10−4 mol∙cm−3) melt at 773 K. Scan rate: W electrode, 100 mV∙s−1; Ga electrode, 50 mV∙s−1; Al-Ga electrode, 50 mV∙s−1.

    JNFCWT-19-2-161_F11.gif

    XRD and SEM-EDS analyses of the cathodic deposits obtained by potentiostatic electrolysis on molybdenum grid. ( a, d, e ) at 0.1 V in the LiCl-KCl-SmCl3 (2.31×10−4 mol∙cm−3)-GaCl3 (1.58×10−4 mol∙cm−3) melt, for 3.5 h, at 773 K; ( b, f, g ) at −0.3 V in the LiCl-KCl-SmCl3 (1.92×10−4 mol∙cm−3)-GaCl3 (4.45×10−5 mol∙cm−3) melt, for 3.5 h, at 773 K; ( c, h, i ) at −0.72 V in the LiCl-KCl-SmCl3 (3.33×10−4 mol∙cm−3)-GaCl3 (4.41×10−5 mol∙cm−3) melt, for 3.5 h, at 773 K.

    JNFCWT-19-2-161_F12.gif

    XRD and SEM-EDS analyses of the deposits obtained by potentiostatic electrolysis on molybdenum grid: ( a, c, d ) at −0.2 V in the LiCl-KCl- SmCl3 (4.79×10−4 mol∙cm−3)-GaCl3 (3.67×10−5 mol∙cm−3)-AlCl3 (1.47×10−4 mol∙cm−3) melt, for 3.5 h, at 773 K; ( b, e, f ) at −0.72 V in the LiCl-KCl- SmCl3 (1.06×10−4 mol∙cm−3)-GaCl3 (2.42×10−5 mol∙cm−3)-AlCl3 (1.23×10−5 mol∙cm−3) melt, for 3.5 h, at 773 K.

    JNFCWT-19-2-161_F13.gif

    XRD and SEM-EDS analyses of deposits obtained by galvanostatic electrolysis on different electrodes: ( a, d, e ) at −0.1 A in the LiCl-KCl- SmCl3 (2.70×10−4 mol∙cm−3) melt on Al electrode(S = 1.884 cm2), for 10 h, at 773 K; ( b, f, g ) at −0.1 A in the LiCl-KCl-SmCl3 (1.46×10−4 mol∙cm−3) melt on liquid Ga electrode (3.0 g, 2.0 cm3), for 10 h, at 773 K; ( c, h, i ) at −0.1 A in the LiCl-KCl-SmCl3 (2.15×10−4 mol∙cm−3) melt on Ga-1.67% Al alloy electrode (3.0 g, 2.0 cm3), for 10 h, at 773 K.

    Tables

    Summary of electrochemical signals and the corresponding reactions on a W electrode

    Summary of electrochemical signals and the corresponding reactions on Al, liquid Ga and Al-Ga alloy electrodes

    References

    1. V. Smolenski, A. Novoselova, and V.A. Volkovich, “Thermodynamics of La and U and the Separation Factor of U/La in Fused Me(Ga-40wt% In)/3LiCl-2KCl System”, J. Nucl. Mater., 495, 285-290 (2017).
    2. S.X. Li, D. Vaden, B.R. Westphal, G.L. Frederickson, R.W. Benedict, and T.A. Johnson, “Integrated Efficiency Test for Pyrochemical Fuel Cycles”, Nucl. Technol., 166(2), 180-186 (2009).
    3. D. Vaden, S.X. Li, B.R. Westphal, K.B. Davies, T.A. Johnson, and D.M. Pace, “Engineering-Scale Liquid Cadmium Cathode Experiments”, Nucl. Technol., 162(2), 124-128 (2008).
    4. B.P. Reddy, S. Vandarkuzhali, T. Subramanian, and P. Venkatesh, “Electrochemical Studies on the Redox Mechanism of Uranium Chloride in Molten LiCl-KCl Eutectic”, Electrochim. Acta, 49(15), 2471-2478 (2004).
    5. Y. Liu, K. Liu, L. Luo, L. Yuan, Z. Chai, and W. Shi, “Direct Separation of Uranium From Lanthanides (La, Nd, Ce, Sm) in Oxide Mixture in LiCl-KCl Eutectic Melt”, Electrochim. Acta, 275, 100-109 (2018).
    6. T. Koyama, M. Iizuka, N. Kondo, R. Fujita, and H. Tanaka, “Electrodeposition of Uranium in Stirred Liquid Cadmium Cathode”, J. Nucl. Mater., 247, 227-231 (1997).
    7. C.C. McPheeters, R.D. Pierce, and T.P. Mulcahey, “Application of the Pyrochemical Process to Recycle of Actinides From LWR Spent Fuel”, Prog. Nucl. Energy, 31(1-2), 175-186 (1997).
    8. J. Serp, P. Lefebvre, R. Malmbeck, J. Rebizant, P. Vallet, and J.P. Glatz, “Separation of Plutonium From Lanthanum by Electrolysis in LiCl-KCl Onto Molten Bismuth Electrode”, J. Nucl. Mater., 340(2-3), 266-270 (2005).
    9. F. Lantelme, T. Cartailler, Y. Berghoute, and M. Hamdani, “Physicochemical Properties of Lanthanide and Yttrium Solutions in Fused Salts and Alloy Formation With Nickel”, J. Electrochem. Soc., 148(9), C604-C613 (2001).
    10. S.Y. Melchakov, D.S. Maltsev, V.A. Volkovich, L.F. Yamshchikov, and A.G. Osipenko, “Separation of Uranium and Lanthanides in a Fused Salt - Liquid Gallium Based Alloy System”, 19th Int. Symp. on Molten Salts and Ion Liquids, ECS Trans., 64, 369-375, Oct. 05-09, Cancun (2014).
    11. K. Liu, H.B. Tang, J.W. Pang, Y.L. Liu, Y.X. Feng, Z.F. Chai, and W.Q. Shi, “Electrochemical Properties of Uranium on the Liquid Gallium Electrode in LiCl-KCl Eutectic”, J. Electrochem. Soc., 163(9), D554-D561 (2016).
    12. O. Conocar, N. Douyere, and J. Lacquement, “Extraction Behavior of Actinides and Lanthanides in a Molten Fluoride/Liquid Aluminum System”, J. Nucl. Mater., 344(1-3), 136-141 (2005).
    13. K. Liu, L.Y. Yuan, Y.L. Liu, X.L. Zhao, H. He, G.A. Ye, Z.F. Chai, and W.Q. Shi, “Electrochemical Reactions of the Th4+/Th Couple on the Tungsten, Aluminum and Bismuth Electrodes in Chloride Molten Salt”, Electrochim. Acta, 130, 650-659 (2014).
    14. Y. Castrillejo, M.R. Bermejo, P.D. Arocas, A.M. Martínez, and E. Barrado, “The Electrochemical Behaviour of the Pr(III)/Pr Redox System at Bi and Cd Liquid Electrodes in Molten Eutectic LiCl-KCl”, J. Electroanal. Chem., 579(2), 343-358 (2005).
    15. M. Kurata, Y. Sakamura, T. Hijikata, and K. Kinoshita, “Distribution Behavior of Uranium, Neptunium, Rare- Earth Elements (Y, La, Ce, Nd, Sm, Eu, Gd) and Alkaline-Earth Metals (Sr,Ba) Between Molten LiClChang- KCI Eutectic Salt and Liquid Cadmium or Bismuth”, J. Nucl. Mater., 227(1-2), 110-121 (1995).
    16. L.X. Luo, Y.L. Liu, N. Liu, L. Wang, L.Y. Yuan, Z.F. Chai, and W.Q. Shi, “Electrochemical and Thermodynamic Properties of Nd (III)/Nd (0) Couple at Liquid Zn Electrode in LiCl-KCl Melt”, Electrochim. Acta, 191, 1026-1036 (2016).
    17. Y.L. Liu, W. Zhou, H.B. Tang, Z.R. Liu, K. Liu, L.Y. Yuan, Y.X. Feng, Z.F. Chai, and W.Q. Shi, “Diffusion Coefficient of Ho3+ at Liquid Zinc Electrode and Coreduction Behaviors of Ho3+ and Zn2+ on W Electrode in the LiCl-KCl Eutectic”, Electrochim. Acta, 211, 313-321 (2016).
    18. D. Lambertin, S. Ched’homme, G. Bourges, S. Sanchez, and G.S. Picard, “Activity Coefficients of Plutonium and Cerium in Liquid Gallium at 1073 K: Application to a Molten Salt/Solvent Metal Separation Concept”, J. Nucl. Mater., 341(2-3), 131-140 (2005).
    19. T. Toda, T. Maruyama, K. Moritani, H. Moriyama, and H. Hayashi, “Separation Factor of Americium From Cerium in Molten Chloride - Liquid Gallium Reductive Extraction System”, Electrochemistry, 77(8), 649- 651 (2009).
    20. H. Moriyama, H. Yamana, S. Nishikawa, S. Shibata, N. Wakayama, Y. Miyashita, K. Moritani, and T. Mitsugashira, “Thermodynamics of Reductive Extraction of Actinides and Lanthanides From Molten Chloride Salt Into Liquid Metal”, J. Alloys Compd., 271-273, 587- 591 (1998).
    21. V.A. Volkovich, D.S. Maltsev, L.F. Yamshchikov, and A.G. Osipenko, “Thermodynamic Properties of Uranium in Liquid Gallium, Indium and Their Alloys”, J. Nucl. Mater., 464, 263-269 (2015).
    22. A. Novoselova, V. Smolenski, V.A. Volkovich, A.B. Ivanov, A. Osipenko, and T.R. Griffiths, “Thermodynamic Properties of La-Ga-Al and U-Ga-Al Alloys and the Separation Factor of U/La Couple in the Molten Salt-Liquid Metal System”, J. Nucl. Mater., 466, 373- 378 (2015).
    23. S. Ghosh, R. Ganesan, R. Sridharan, and T. Gnanasekaran, “Measurement of Chemical Activities of Rare Earths (RE: Ce, Pr, Sm and Eu) in Cadmium Alloy”, J. Nucl. Mater., 467, Part 1, 280-285 (2015).
    24. J. Serp, M. Allibert, A.L. Terrier, R. Malmbeck, M. Ougier, J. Rebizant, and J.P. Glatz, “Electroseparation of Actinides From Lanthanides on Solid Aluminum Electrode in LiCl-KCl Eutectic Melts”, J. Electrochem. Soc., 152(3), C167-C172 (2005).
    25. P. Soucek, R. Malmbeck, C. Nourry, and J.P. Glatz, “Pyrochemical Reprocessing of Spent Fuel by Electrochemical Techniques Using Solid Aluminium Cathodes”, Energy Proedia, 7, 396-404 (2011).
    26. O. Conocar, N. Douyere, J.P. Glatz, J. Lacquement, R. Malmbeck, and J. Serp, “Promising Pyrochemical Actinide/ Lanthanide Separation Processes Using Aluminum”, Nucl. Sci. Eng., 153(3), 253-261 (2006).
    27. J.W. Pang, K. Liu, Y.L. Liu, C.M. Nie, L.X. Luo, L.Y. Yuan, Z.F. Chai, and W.Q. Shi, “Electrochemical Properties of Lanthanum on the Liquid Gallium Electrode in LiCl-KCl Eutectic”, J. Electrochem. Soc., 163(14), D750-D756 (2016).
    28. B. Li, K. Liu, J. Pang, L. Yuan, Y. Liu, and M. Lin, “Electrochemical Properties of Gadolinium on Liquid Gallium Electrode in LiCl KCl Eutectic”, J. Rare Earths, 36(6), 656-661 (2018).
    29. K. Liu, Y.L. Liu, Z.F. Chai, and W.Q. Shi, “Evaluation of the Electroextractions of Ce and Nd From LiCl-KCl Molten Salt Using Liquid Ga Electrode”, J. Electrochem. Soc., 164(4), D169-D178 (2017).
    30. V.A. Volkovich, D.S. Maltsev, L.F. Yamshchikov, A.V. Chukin, V.V. Smolenski, A.V. Novoselova, and A.G. Osipenko, “Thermodynamic Properties of Uranium in Gallium-Aluminium Based Alloys”, J. Nucl. Mater., 465, 153-160 (2015).
    31. V. Smolenski, A. Novoselova, V. Volkovich, Y. Luk’ yanova, A. Osipenko, A. Bychkov, and T.R. Griffiths, “The Effect of Al Concentration on Thermodynamic Properties of Nd and U in Ga-Al-Based Alloys and the Separation Factor of Nd/U Couple in a “Molten Salt- Liquid Metal System” ”, J. Radioanal. Nucl. Chem., 311(1), 687-693 (2017).
    32. T. Yin, K. Liu, Y. Liu, Y. Yan, G. Wang, Z. Chai, and W. Shi, “Electrochemical and Thermodynamic Properties of Uranium on the Liquid Bismuth Electrode in LiCl- KCl Eutectic”, J. Electrochem. Soc., 165(14), D722- D730 (2018).
    33. A. Watson, “Re-assessment of Phase Diagram and Thermodynamic Properties of the Al-Ga System”, Calphad, 16(2), 207-217 (1992).
    34. S.H. Zhou and R.E. Napolitano, “Modeling of Thermodynamic Properties and Phase Equilibria for the Al-Sm Binary System”, Metall. and Mater. Trans. A, 39(3), 502-512 (2008).
    35. S.P. Yatsenko, A.A. Semyannikov, B.G. Semenov, and K.A. Chuntonov, “Phase Diagrams of Rare Earth Metals With Gallium”, J. Less-Common Metals, 64(2), 185-199 (1979).
    36. N. Saunders and A.P. Miodownik, CALPHAD (Calculation of Phase Diagrams): A Comprehensive Guide, in: Pergamon Materials Series, R.W. Cahn, ed., 1st ed., 1-479, Pergamon Press, Oxford (1998).
    37. K. Chang, B. Hallstedt, and D. Music, “Thermodynamic and Electrochemical Properties of the Li-Co-O and Li-Ni-O Systems”, Chem. Mater., 24(1), 97-105 (2012).
    38. D.W. Yang, Y.L. Liu, T.Q. Yin, M. Li, W. Han, K.K. Chang, Z.F. Chai, and W.Q. Shi, “Co-reduction Behaviors of Ce (III), Al (III) and Ga (III) on a W Electrode: An Exploration for Liquid Binary Al-Ga Cathode”, Electrochim. Acta, 319, 869-877 (2019).
    39. Y. Castrillejo, C. de la Fuente, M. Vega, F. de la Rosa, R. Pardo, and E. Barrado, “Cathodic Behaviour and Oxoacidity Reactions of Samarium (III) in Two Molten Chlorides With Different Acidity Properties: The Eutectic LiCl–KCl and the Equimolar CaCl2–NaCl Melt”, Electrochim. Acta, 97, 120-131 (2013).
    40. Y. Gao, Y. Shi, X. Liu, C. Huang, and B. Li, “Cathodic Behavior of Samarium(III) and Sm-Al Alloys Preparation in Fluorides Melts”, Electrochim. Acta, 190, 208- 214 (2016).
    41. S.P. Yatsenko, A.A. Semyannikov, B.G. Semenov, and K.A. Chuntonov, “Phase Diagrams of Rare Earth Metals With Gallium”, J. Less-Common Metals, 64(2), 185-199 (1979).
    42. K. Liu, Y.L. Liu, L.Y. Yuan, H. He, Z.Y. Yang, X.L. Zhao, Z.F. Chai, and W.Q. Shi, “Electroextraction of Samarium From Sm2O3 in Chloride Melts”, Electrochim. Acta, 129, 401-409 (2014).
    43. D.O. Flamini, S.B. Saidman, and J.B. Bessone, “Aluminium Activation Produced by Gallium”, Corros. Sci., 48(6), 1413-1425 (2006).
    44. Y. Sun, M.L. Zhang, W. Han, M. Li, and Y.S. Yang, “Electrochemical Formation of Al-Li Alloys by Codeposition of Al and Li From LiCl-KCl-AlF3 Melts at 853 K”, Chem. Res. Chin. Univ., 29(2), 324-328 (2013).
    45. T. Toda, T. Maruyama, K. Moritani, H. Moriyama, and H. Hayashi, “Thermodynamic Properties of Lanthanides and Actinides for Reductive Extraction of Minor Actinides”, J. Nucl. Sci. Technol., 46(1), 18-25 (2009).
    46. Y. Castrillejo, P. Fernández, J. Medina, P. Hernández, and E. Barrado, “Electrochemical Extraction of Samarium From Molten Chlorides in Pyrochemical Processes”, Electrochim. Acta, 56(24), 8638-8644 (2011).
    47. D.B. Ji, Y.D. Yan, M.L. Zhang, X. Li, X.Y. Jing, W. Han, Y. Xue, Z.J. Zhang, and T. Hartmann, “Electrochemical Preparation of Al-Sm Intermetallic Compound Whisker in LiCl-KCl Eutectic Melts”, Electrochim. Acta, 165, 211-220 (2015).
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