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 (x_Li = 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].

Fig. 1

Schematic diagram of experimental electrolytic cell.

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:

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.

Fig. 2

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

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 Ga_xSm_y 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.

Fig. 3

(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.

Table 1

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

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].

Fig. 4

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.

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].

Fig. 5

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.

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.

Fig. 6

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.

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.

Fig. 7

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.

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.

Fig. 8

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.

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.

Fig. 9

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.

Table 2

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

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.

Fig. 10

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.

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.

Fig. 11

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.

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.

Fig. 12

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.

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.

Fig. 13

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.

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.