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Electrochemical behavior and underpotential deposition of Sm on reactive electrodes (Al, Ni, Cu, and Zn) in a LiCl–KCl melt

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  • Corresponding authors:

    Yun Xue    E-mail: yunxue@hrbeu.edu.cn

    Yong-de Yan    E-mail: y5d2006@hrbeu.edu.cn

  • Received: 26 April 2020Revised: 30 May 2020Accepted: 1 June 2020Available online: 3 June 2020
  • Sm extraction from a LiCl–KCl melt was carried out by forming alloys on various electrodes, including Al, Ni, Cu, and liquid Zn, and the electrochemical behaviors of the resultant metal products were investigated using different electrochemical techniques. While Sm metal deposition via the conventional two-step reaction process was not noted on the inert electrode, underpotential deposition was observed on the reactive electrodes because of the latter’s depolarization effect. The depolarization effects of the reactive electrodes on Sm showed the order Zn > Al > Ni > Cu. Sm–M (M = Al, Ni, Cu, or Zn) alloys were deposited by galvanostatic and potentiostatic electrolysis. The products were fully characterized by XRD and SEM–EDS, and the stability of the obtained M-rich compounds was determined. Finally, the relationship between the electrode potential and type of Sm–M intermetallic compounds formed was assessed on the basis of the observed electrochemical properties and electrodeposits.
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Electrochemical behavior and underpotential deposition of Sm on reactive electrodes (Al, Ni, Cu, and Zn) in a LiCl–KCl melt

  • Corresponding authors:

    Yun Xue    E-mail: yunxue@hrbeu.edu.cn

    Yong-de Yan    E-mail: y5d2006@hrbeu.edu.cn

  • 1. Harbin Engineering University, Harbin 150001, China
  • 2. College of Science, Heihe university, Heihe 164300, China
  • 3. Geely Automobile Research Institute, Ningbo 315336, China

Abstract: Sm extraction from a LiCl–KCl melt was carried out by forming alloys on various electrodes, including Al, Ni, Cu, and liquid Zn, and the electrochemical behaviors of the resultant metal products were investigated using different electrochemical techniques. While Sm metal deposition via the conventional two-step reaction process was not noted on the inert electrode, underpotential deposition was observed on the reactive electrodes because of the latter’s depolarization effect. The depolarization effects of the reactive electrodes on Sm showed the order Zn > Al > Ni > Cu. Sm–M (M = Al, Ni, Cu, or Zn) alloys were deposited by galvanostatic and potentiostatic electrolysis. The products were fully characterized by XRD and SEM–EDS, and the stability of the obtained M-rich compounds was determined. Finally, the relationship between the electrode potential and type of Sm–M intermetallic compounds formed was assessed on the basis of the observed electrochemical properties and electrodeposits.

    • Reprocessing of spent nuclear fuel (SNF) is considered a key problem related to the sustainability of nuclear power generation [1]. During pyroprocessing, large amounts of U or U–Pu products are electrodeposited on cathodes for recovery, and fission products left in molten salts are extracted for salt decontamination [2]. When the molten salt reactor is operated, the major fission products, which often include rare earth elements, can form stable fluoride compounds that may corrode the structural materials and affect the properties of the primary salt obtained. Thus, these fission products must be eliminated from the melt to optimize reactor operations [3].

      Sm is a fission product of SNF and has been widely studied on various inert and reactive electrodes [2]. Sm may transition from the trivalent state to the divalent state in a molten salt to induce cyclic electrolysis, which could affect the current efficiency of the system [4]. Sm extraction on inert electrodes in chloride [56] and fluoride [78] melts was first investigated via electrochemical studies and thermodynamic analysis, and a two-step mechanism was discovered. In this mechanism, the second step occurs outside of the electrochemical window. Therefore, Sm extraction can be only realized on reactive electrodes in molten salts. The shifting of the potential of Sm toward positive values is attributed to the lowering of the activity of the element in the reactive metal phase to form stable Sm-based alloys [9]. Al presents excellent properties and has been widely applied to different fields, such as the aviation industry [1013] and batteries [1417]. Some studies on Sm extraction have focused on this metal’s combination with Al. Electrochemical studies on Sm have also been conducted in LiF–CaF2–AlF3–Sm2O3/SmF3 melts on an inert electrode [7,18], as well as in LiCl–KCl–SmCl3/Sm2O3–AlCl3 on an Al electrode [1921]; in these studies, the main products were usually Al3Sm or Al2Sm. Ni, as a remarkable reactive metal, could be employed in LiF–CaF2 melts [8,22]; in this case, a mixture of Ni2Sm and Ni3Sm may be obtained under different current densities. NiSm, Ni2Sm, and Ni5Sm could be prepared from LiCl–KCl melts by using different potentials [23]. Taxil et al. [24] introduced Cu as a reactive electrode to extract Sm from the LiF–CaF2 system; here, a mixture of Cu4Sm, Cu5Sm, and Cu6Sm was formed by galvanostatic electrolysis at 1113 K. Besides solid reactive electrodes, liquid Zn electrodes may also be employed in LiCl–KCl–SmCl3 systems. For example, previous investigations clearly showed the formation of Zn12Sm and Zn17Sm2 on Zn electrodes by potentiostatic electrolysis at 873 K [25].

      Sm may be combined with other metals to improve the properties of the primary material. Indeed, a number of physical properties, including micro-hardness and corrosion resistance, could be remarkably improved by the metal’s ability to form whiskers [26]. Rare-earth elements may be combined with a reactive metal as a transition alloy to form a new material. Iida et al. [27], for instance, studied Ni–Sm alloy films. While only Ni2Sm could be obtained under various applied potentials, Ni3Sm and Ni5Sm could be formed on Ni2Sm electrodes by anodic potentiostatic electrolysis. Magnetic Ni–Sm alloys could be prepared from LiCl–KCl–SmCl3–NiCl2 melts on inert electrodes [28]; here, different intermetallic compounds, including NiSm, Ni2Sm, and Ni5Sm, may be obtained by coreduction. Electrochemical synthesis of Cu–Sm dendritic alloys for use as catalysts may be carried out by coreduction in LiCl–KCl–SmCl3–CuCl2 melts [29] to produce CuSm, Cu5Sm, and Cu6Sm under various applied potentials.

      Herein, we examined the electrochemical behavior and underpotential deposition of Sm on various reactive electrodes in a molten salt. The extraction efficiency of Sm strongly depends on the deposition potential [30], depolarization value [31], and stability of the cathodic products [3233]. In this work, we studied the deposition potential of Sm on reactive electrodes and prepared Sm–M (M = Al, Ni, Cu, Zn) alloys by potentiostatic and galvanostatic electrolysis. The electrochemical behavior of the metal products formed was then determined to provide a feasible method to extract Sm from contaminated salts.

    2.   Experimental
    • The electrochemical cell used in this work was designed as a three-electrode system with molten salt, as shown in Fig. 1. The molten salt was composed of LiCl–KCl eutectic salt (>99.0% anhydrous, Xilong Scientific Co., Ltd.) and SmCl3 (>99.9% anhydrous, Alfa-Aesar). Prior to the experiments, LiCl–KCl blank salt was mixed in an alumina crucible and then heated in a vacuum drying oven at 473 K for over 24 h. Thereafter, the crucible was transferred to a furnace for heating up to the experimental temperature of 773 K. Cyclic voltammetry (CV) was conducted on the blank molten salt to detect impurities in the system. If any impurity was found, pre-electrolysis by potentiostatic electrolysis was conducted at −2.1 V vs. Ag/AgCl until these impurities could no longer be detected. Next, SmCl3 was added to the melt, and the actual concentration of Sm was measured using inductively coupled plasma optical emission spectrometry (HORIBA).

      Figure 1.  Illustration of the electrochemical cell.

      The working electrode (WE) was made up of various electrodes, including W wire (99.99%, ϕ1 mm), Al wire (99.99%, ϕ1 mm), Ni wire (99.99%, ϕ1 mm), and Cu wire (99.99%, ϕ1 mm), and liquid Zn granules (99.99%; Sinopharm Chemical Reagent Co., Ltd.). The reactive Zn electrode, which presented a liquid state at the experimental temperature, was placed in an alumina crucible (ϕ16 mm × 16 mm). Mo wire was used as the conductor for the Zn metal and electrode clamp. The reference electrode was made of Ag wire (99.99%, ϕ1 mm), 1wt% AgCl (99.99%, Alfa-Aesar), and an alundum tube (ϕ6 mm) containing LiCl–KCl eutectic molten salt. The counter electrode was a graphite rod (ϕ4 mm) of spectral purity. All experimental tests, including CV, square wave voltammetry (SWV), and open-circuit chronopotentiometry (OCP), were carried out on an Autolab PGSTAT302N potentiostat/galvanostat.

      The samples were prepared by galvanostatic or potentiostatic electrolysis. After sonicleaning with distilled water, the alloy samples were placed in a cylinder by using a metallographic sample mounting press, smoothed with sand paper, and then polished with a polisher for further analysis. X-ray diffractometry (XRD; Bruker, D8 Advance) was performed to identify the phase composition of the samples. Scanning electron microscopy (SEM)–energy dispersive spectrometry (EDS; JSM-6480A; JEOL Co., Ltd.) was also carried out to examine the morphology and elementary composition of the samples.

    3.   Results and discussion
    • A typical CV curve obtained on the W electrode after addition of SmCl3 to the LiCl–KCl eutectic melt is shown in Fig. 2. The A0/A0′ peak couple is known to indicate the reduction/oxidation of Li(I)/Li. The B/B′ redox signal is characteristic of a soluble–soluble system, which corresponds to the reaction of Sm(III)/Sm(II) [34]. The soluble/insoluble reaction corresponding to the Sm(II)/Sm(0) peak couple could not be determined because the reaction potential is beyond the electrochemical window of the primary salt [6]. The standard reduction potentials of LiCl/Li, KCl/K, and SmCl2/Sm were determined according to previous thermochemical data [35] and are shown in Table 1. However, the reduction potential of SmCl2/Sm under the condition of supercooling could not be calculated because of a lack of data. The standard potential of SmCl3/SmCl2 has been reported in the literature [6].

      Figure 2.  CV plot obtained after addition of 1wt% SmCl3 to the LiCl–KCl melt at 773 K. Working electrode: W; scan rate: 0.1 V/s.

      Redox coupleStandard reduction potential $ {E}^{\ominus} $ /
      (V vs. Cl2/Cl)
      LiCl/Li−3.55
      KCl/K−3.69
      SmCl2/Sm−3.59
      SmCl3/SmCl2−2.00a
      Note: a—The value is obtained from Cordoba and Caravaca [6].

      Table 1.  Standard reduction potentials of LiCl/Li, KCl/K, SmCl2/Sm, and SmCl3/SmCl2

      Fig. 3 shows four groups of typical CV curves obtained on the reactive solid (Al, Ni, and Cu) and liquid Zn electrodes in the LiCl–KCl–1wt%SmCl3 melt at 773 K. The Ai/Ai′ (i = 1, 2, 3, 4) peak couple in Fig. 3 represents the reaction of Li(I)/Li on the different reactive electrodes. The positive shift of potentials shown in Figs. 3(a) and 3(d) is attributed to the underpotential deposition of Al–Li [26] and Zn–Li [25] alloys. The most positive peak couples, namely, C/C′, D/D′, E/E′, and F/F′, in the different CV curves correspond to the redox signals of Al, Ni, Cu, and Zn, respectively. Besides the redox signals Ai/Ai′ and B/B′ (Sm(III)/Sm(II)) and the peak couples of the reactive metals, several other pairs of signals could be attributed to the formation of Sm–M intermetallic compounds. The phase diagram of the Al–Sm system, for example, reveals that five intermetallic compounds could be obtained [36]. However, only one redox signal, G/G′, is observed in Fig. 3(a). Moreover, two pairs of signals, namely, H/H′ and I/I′, which correspond to the formation of Ni–Sm intermetallic compounds, are shown in Fig. 3(b), but eight intermetallic compounds should exist according to the Ni–Sm phase diagram [37]. Thermodynamic evaluation of the Cu–Sm system [38] reveals that five intermetallic compounds should be formed. However, only two pairs of signals, J/J′ and K/K′, were correlated with Cu–Sm intermetallic compounds. These results may be attributed to the slow growth of other phases [39]. The redox signal of Sm(III)/Sm(II) on the liquid Zn electrode cannot usually be observed due to the very closed potential (B/B′ and N/N′). The redox signals M/M′ and N/N′ in Fig. 3(d) indicate the presence of two of the eight expected Zn–Sm intermetallic compounds [40], which could be attributed to the low solubility of Sm in Zn [41]. Thus, the extraction or separation of Sm from molten salts may be accomplished by applying different potentials.

      Figure 3.  CV curves obtained on the reactive solid electrodes (a) Al, (b) Ni, and (c) Cu and the liquid electrode (d) Zn in LiCl–KCl–1wt%SmCl3 melt at 773 K.

    • SWV generally shows greater sensitivity than CV [42]. As shown in Fig. 4, SWV curves were obtained on the four reactive electrodes in the LiCl–KCl eutectic melt after addition of 1wt% SmCl3. Signals corresponding to Sm–M intermetallic compounds, namely, G at −1.67 V, H at −2.15 V, I at −1.78 V, and J(K) at −2.07 V, closely corresponded to the CV results. However, in the red curve of Fig. 4, a new signal O was noted at approximately −1.59 V; this signal could be attributed to the formation of a certain Al–Sm intermetallic compound. Signals obtained during underpotential deposition on the Al electrode were relatively more positive than the potential of Li metal. No Ni–Li or Cu–Li intermetallic compounds were observed; therefore, the signal of Li(I) on the Ni and Cu electrodes is identical to that on the W electrode.

      Figure 4.  SWV plots of 1wt% SmCl3 in LiCl–KCl melt on W, Al, Ni, and Cu electrodes at 773 K and 20 Hz.

      The signals described above were analyzed in detail, and the reactions of Sm on the various reactive electrodes are summarized in Table 2. Due to the underpotential deposition of Sm on the reactive electrodes, the depolarization value (ΔE) is written as [31]:

      Redox coupleReaction
      A0/A0′, A2/A2′, A3/A3′$ {\rm{L}}{{\rm{i}}^ + } + {{\rm{e}}^ - } \leftrightarrow {\rm{Li}} $
      A1/A1′$ x{\rm{L}}{{\rm{i}}^ + } + y{\rm{Al}} + x{e^ - } \leftrightarrow {\rm{L}}{{\rm{i}}_x}{\rm{A}}{{\rm{l}}_y}$
      A4/A4′$ x{\rm{L}}{{\rm{i}}^ + } + y{\rm{Zn}} + x{{\rm{e}}^ - } \leftrightarrow {\rm{L}}{{\rm{i}}_x}{\rm{Z}}{{\rm{n}}_y}$
      B/B′$ {\rm{S}}{{\rm{m}}^{3 + }} + {{\rm{e}}^ - } \leftrightarrow {\rm{S}}{{\rm{m}}^{2 + }}$
      C/C′$ {\rm{A}}{{\rm{l}}^{3 + }} + 3{{\rm{e}}^ - } \leftrightarrow {\rm{Al}}$
      G/G′, O/O′$ x{\rm{S}}{{\rm{m}}^{3 + }} + y{\rm{Al}} + 3x{{\rm{e}}^ - } \leftrightarrow {\rm{S}}{{\rm{m}}_x}{\rm{A}}{{\rm{l}}_y}$
      D/D′$ {\rm{N}}{{\rm{i}}^{2 + }} + 2{{\rm{e}}^ - } \leftrightarrow {\rm{Ni}}$
      H/H′, I/I′$ x{\rm{S}}{{\rm{m}}^{3 + }} + y{\rm{Ni}} + 3x{{\rm{e}}^ - } \leftrightarrow {\rm{S}}{{\rm{m}}_x}{\rm{N}}{{\rm{i}}_y}$
      E/E′$ {\rm{C}}{{\rm{u}}^{2 + }} + 2{{\rm{e}}^ - } \leftrightarrow {\rm{Cu}}$
      J/J′, K/K′$ x{\rm{S}}{{\rm{m}}^{3 + }} + y{\rm{Cu}} + 3x{{\rm{e}}^ - } \leftrightarrow {\rm{S}}{{\rm{m}}_x}{\rm{C}}{{\rm{u}}_y}$
      F/F′$ {\rm{Z}}{{\rm{n}}^{2 + }} + 2{{\rm{e}}^ - } \leftrightarrow {\rm{Zn}} $
      M/M′, N/N′$ x{\rm{S}}{{\rm{m}}^{3 + }} + y{\rm{Zn}} + 3x{{\rm{e}}^ - } \leftrightarrow {\rm{S}}{{\rm{m}}_x}{\rm{Z}}{{\rm{n}}_y}$

      Table 2.  Summary of the CV and SWV results of Sm on various reactive electrodes at 773 K.

      where ${E_{\rm{R}}}$ and ${E_{\rm{W}}}$ correspond to the reduction potential of Sm(III) on reactive and inert electrodes, respectively. However, ${E_{\rm{W}}}$ could not be determined in the present experiment because Sm(II)/Sm(0) occurs outside of the electrochemical window. Therefore, the real depolarization values could not be obtained. The depolarization effects of the reactive electrodes on Sm showed the order Zn > Al > Ni > Cu. Depolarization on reactive electrodes facilitates the deposition of Sm(III) and improves the current efficiency of the system [31]. Reduction potential data can provide a fundamental basis for the extraction of Sm from molten salts on reactive electrodes and preparation of alloy materials.

    • OCP curves were recorded after each electrolytic process, and the equilibrium potentials of the Sm–M intermetallic compounds were determined [4344]. First, thin layers of the Sm–M alloys were fabricated on the reactive electrodes by potentiostatic electrolysis for a certain period of time. Different SmxMy phases were formed following the diffusion of the Sm metal into the M metals by underpotential deposition. The potentiostatic control was then disconnected, and discharging phenomena were recorded as plots of potential versus time. Potential plateaus due to the different states of coexisting phases were also determined [8,45]. The red line in Fig. 5 shows several of these potential plateaus. For example, plateaus 9 at −1.85 V, 11 at −1.6 V, and 14 at −1.40 V correspond to the presence of Al–Sm intermetallic compounds, which is in accordance with the previous literature [26]. The blue line in the same figure shows four plateaus (4 at −2.11 V, 7 at −1.87 V, 10 at −1.74 V, and 15 at −1.36 V) corresponding to Ni–Sm intermetallic compounds. Plateau 7 corresponds to a new Ni–Sm alloy [27]. Sm extraction on the Cu electrode, which is represented by the green line in Fig. 5, shows four plateaus, namely, 2 at −2.24 V, 5 at −2.03 V, 6 at −1.94 V and 13, at −1.46 V, which are attributed to Cu–Sm intermetallic compounds. Plateaus 6 and 13 indicate new phases compared with those shown in the CV and SWV curves. When Sm metal is deposited on liquid Zn metal, as demonstrated by the pink line in Fig. 5, only plateau 12 is attributed to Zn–Sm intermetallic compound. Plateaus 1, 3, and 8 reflect the equilibrium potentials of Li, Al–Li alloy, and Zn–Li alloy, respectively. The change in potential of Li on the reactive electrodes is in accordance with the SWV findings. The dotted black line 18 corresponds to the reaction Sm(II)/Sm(III), and plateaus 16, 17, 19, and 20 correspond to the open circuit potentials of metallic Zn, Al, Cu, and Ni, respectively.

      Figure 5.  OCP curves recorded on various reactive electrodes (solid Al, Ni, and Cu and liquid Zn) after potentiostatic electrolysis for a certain period of time in LiCl–KCl–1wt%SmCl3 melt at 773 K.

      Taken together, the results of different electrochemical techniques reveal that the electrochemical signals of all Sm–M intermetallic compounds may be difficult to obtain because of the influence of different thermodynamic and kinetic effects. The phase composition of these compounds was further studied by electrochemical deposition.

    • Sm extraction was carried out by galvanostatic and potentiostatic electrolysis on the Al, Ni, Cu, and liquid Zn electrodes to fabricate Sm–M intermetallic compounds.

    • Electroextraction of Sm was carried out on the Al electrode by potentiostatic electrolysis at –1.7 V for 4 h at 773 K. The XRD pattern shown in Fig. 6 reveals the presence of the Al3Sm phase only. The phase diagram of Al–Sm [46] reveals that the Al-richest intermetallic compound formed above 1351 K is Al11Sm3 while that formed below 1408 K is Al3Sm. Therefore, the signal G in Fig. 3 and Fig. 4 is attributed to Al3Sm. Compared with the results obtained by CV and SWV, the potential of signal O is positive than G, in this case, peak O might be related to Al2Sm. Unmarked peaks in the XRD pattern are attributed to the presence of impurities.

      Figure 6.  XRD pattern of the sample prepared by potentiostatic electrolysis at −1.7 V. Working electrode: Al; duration: 4 h; temperature: 773 K.

      The product was further analyzed by cross-sectional SEM and EDS, and the results are shown in Fig. 7. The SEM image obtained exhibited distinct light and dark gray zones, which were labeled 001 and 002, respectively. EDS analysis then revealed that these zones correspond to the Al–Sm intermetallic compound and Al matrix, respectively. The ratio of Al to Sm in the deposit is approximately 3/1, which is in accordance with the XRD analysis.

      Figure 7.  SEM–EDS analysis results of the sample prepared by potentiostatic electrolysis at −1.7 V. Working electrode: Al; duration: 4 h; temperature: 773 K.

    • Galvanostatic and potentiostatic electrolysis was conducted to electroextract Sm on the Ni electrode. Different intermetallic compounds were detected. Fig. 8 shows the XRD pattern of the product formed by potentiostatic electrolysis at –1.5 V; the pattern reveals the formation of Ni and Ni5Sm phases. Therefore, signal I in the CV and SWV curves may indicate the formation of Ni5Sm. Galvanostatic electrolysis at −0.62 A·cm−2 was carried out on the Ni electrode for 4 h at 773 K. As can be seen in Fig. 9, another intermetallic compound, namely, Ni2Sm, was formed. This product is different from the product obtained by potentiostatic electrolysis in Fig. 8.

      Figure 8.  XRD pattern of the sample prepared by potentiostatic electrolysis at −1.5 V. Working electrode: Ni; duration: 12 h; temperature: 773 K.

      Figure 9.  XRD pattern of the sample prepared by galvanostatic electrolysis at −0.62 A·cm−2. Working electrode: Ni; duration: 4 h; temperature: 773 K.

      Fig. 10 shows the SEM–EDS results of the sample in Fig. 8; here, the dark gray (001) and light gray (002) zones correspond to the Ni matrix and alloy phase, respectively. A clear boundary between the Ni matrix and alloy phase could also be observed in the SEM image; this boundary reveals that the alloy phase grows uniformly. Liu et al. [28] investigated the coreduction of Ni–Sm alloys in a LiCl–KCl–NiCl2–SmCl3 melt by potentiostatic electrolysis and found that Ni5Sm, Ni2Sm, and NiSm are the main products of this process.

      Figure 10.  SEM–EDS analysis results of the sample prepared by galvanostatic electrolysis at −0.62 A·cm−2. Working electrode: Ni; duration: 4 h; temperature: 773 K.

    • Sm electroextraction was carried out on the Cu electrode. The Cu–Sm alloy was fabricated by potentiostatic electrolysis at −1.6 V. XRD analysis revealed the presence of Cu6Sm, which is the Cu-richest alloy phase that could be formed from this system (Fig. 11). SEM also revealed a homogeneous alloy phase.

      Figure 11.  XRD pattern of the sample prepared by potentiostatic electrolysis at −1.6 V. Working electrode: Cu; duration: 10 h; temperature: 773 K.

      Figure 12.  SEM image of the sample prepared by potentiostatic electrolysis at −1.6 V. Working electrode: Cu; duration: 10 h; temperature: 773 K.

    • Sm extraction on the liquid Zn electrode was conducted by galvanostatic electrolysis at −0.15 A·cm−2 and 773 K. A Zn–Sm intermetallic compound could be formed by the saturation of Sm in liquid Zn. Sm2Zn17 but not SmZn12 was detected by XRD, as shown in Fig. 13. SEM–EDS revealed that the gray zones marked 001 and 002 are related to the Zn–Sm alloy phase, as shown in Fig. 14.

      Figure 13.  XRD pattern of the sample prepared by galvanostatic electrolysis at −0.15 A·cm−2. Working electrode: liquid Zn; duration: 8 h; temperature: 773 K.

      Figure 14.  SEM–EDS analysis results of the sample prepared by galvanostatic electrolysis at −0.15 A·cm−2. Working electrode: liquid Zn; duration: 8 h; temperature: 773 K.

      The preparation of Sm–M alloys on different reactive electrodes was achieved by galvanostatic or potentiostatic electrolysis in LiCl–KCl–SmCl3 melt. During electrolysis, the reactive electrode could be considered the metal-rich material. Thus, M-rich intermetallic compounds may be expected to form prior to other alloy phases. The formation mechanism of these compounds is affected by the thermodynamic, kinetic, and inherent properties of the system components. The M-rich intermetallic compounds Al3Sm and Cu6Sm were observed. During extraction of Sm on the Ni electrode, Ni5Sm and not the expected Ni-richest compound Ni17Sm2 was obtained, likely because the former is more stable than the latter according to the Ni–Sm phase diagram. Ni2Sm could also be obtained by galvanostatic electrolysis, during which alloys with the cubic body-centered structure of Fd-3m, such as Ni2Pr, Ni2Dy, and Ni2Yb, are also easily formed [47]. On the Zn electrode, Zn17Sm2, which is more stable than the Zn-richest compound SmZn11, was obtained. Therefore, while not all possible Sm–M intermetallic compounds are produced, M-rich compounds with the highest stability can be obtained on reactive electrodes.

    4.   Conclusion
    • In summary, CV, SWV, and OCP were carried out to investigate the electrochemical behaviors of Sm(III) on four reactive electrodes, including Al, Ni, Cu, and liquid Zn. Extraction of Sm was then conducted by galvanostatic/potentiostatic electrolysis to form Sm–M alloys. The depolarization effect of the reactive electrodes on Sm showed the order Zn > Al > Ni > Cu. Single-phase intermetallic compounds were prepared via potentiostatic or galvanostatic electrolysis on the four reactive electrodes. The XRD patterns of the deposits obtained by underpotential electrolysis revealed that Al3Sm, Ni5Sm, and Cu6Sm could be formed at the applied potentials of –1.7, –1.5, and –1.6 V, respectively. Ni2Sm and Zn17Sm2 phases could be fabricated by galvanostatic electrolysis. While not all possible Sm–M intermetallic compounds are formed by this process, M-rich compounds with high stability could be obtained on the reactive electrodes. The results reveal the electrochemical behavior and main products of Sm on reactive electrodes and provide an important reference for the decontamination of Sm in molten salts.

    Acknowledgements
    • This work was financially supported by the National Natural Science Foundation of China (Nos. 21976047, 21790373, and 51774104), the Ph.D. Student Research and Innovation Fund of the Fundamental Research Funds for the Central Universities, China (No. 3072019GIP1011), and the Fundamental Research Funds for the Central Universities, China (No. 3072020CFT1008).

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