Electroextraction of gadolinium from Gd2O3 in LiCl–KCl–AlCl3 molten salts

Electrochimica Acta 109 (2013) 732–740

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Electroextraction of gadolinium from Gd2 O3 in LiCl–KCl–AlCl3 molten salts Kui Liu a,b , Ya-Lan Liu a , Li-Yong Yuan a , Xiu-Liang Zhao b , Zhi-Fang Chai a,c,∗ , Wei-Qun Shi a,∗∗ a Group of Nuclear Energy Nano-Chemistry, Key Laboratory of Nuclear Analytical Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China b School of Nuclear Science and Technology, University of South China, Hengyang 421000, China c School of Radiological & Interdisciplinary Sciences, Soochow University, Soochow 215123, China

a r t i c l e

i n f o

Article history: Received 28 April 2013 Received in revised form 16 July 2013 Accepted 16 July 2013 Available online 1 August 2013 Keywords: Molten chloride Gadolinium AlCl3 Intermetallic compounds Co-reduction

a b s t r a c t This work concerns directly the extraction of gadolinium from Gd2 O3 by co-reduction with Al3+ in the LiCl–KCl–AlCl3 molten salts on the molybdenum and aluminum electrode, respectively, at temperature of 773 K. Gibbs energy calculation shows that AlCl3 can favorably chloridize Gd2 O3 and release Gd3+ ions. The electrochemical behaviors of Al3+ , Gd3+ and the mechanisms of alloy formation were investigated by conducting a series of electrochemical techniques. Four typical signals were observed in cyclic voltammetry, square wave voltammetry and chronopotentiometry, while five plateaus were observed in open circuit chronopotentiometry, which might correspond to different kinds of Gd–Al intermetallic compounds. Potentiostatic and galvanostatic electrolysis were conducted to extract gadolinium from the LiCl–KCl–AlCl3 melts by the co-reduction method, and it was found that if the molybdenum cathode was used, GdAl2 particles would be deposited and adhere to a large amount of salt electrolytes. In contrast, when the aluminum plate was used as the cathode, a layer formed by potentiostatic electrolysis corresponding to an intermetallic compound (GdAl3 ) and three layers formed by galvanostatic electrolysis corresponding to three different Gd–Al intermetallic compounds such as GdAl3 , GdAl2 and GdAl could be collected and identified through scanning electron microscopy (SEM)–energy dispersive X-ray (EDX) and X-ray diffraction (XRD) analysis. Besides, electroextractions of gadolinium by co-reduction with Al3+ on the aluminum electrode have been performed with extraction efficiency of 89.7% for potentiostatic electrolysis and 96.5% for galvanostatic electrolysis, respectively. © 2013 Published by Elsevier Ltd.

1. Introduction The implementation of partitioning and transmutation (P&T) technology is intended to reduce the inventories of long-lived radiotoxic minor actinides (MA) and fission products (FP) in nuclear wastes through spent fuel reprocessing, in which high efficient separations of actinides (An) over lanthanides (Ln) are always expected [1]. The necessity for the separation of An from Ln could be ascribed to the high level of Ln which constitutes nearly 25% of FPs in weight [2], and also the notorious large neutron absorption cross-section of Ln which would reduce the transmutation

∗ Corresponding author at: Group of Nuclear Energy Nano-Chemistry, Key Laboratory of Nuclear Analytical Techniques, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China. Tel.: +86 10 88233968; fax: +86 10 88235294. ∗∗ Corresponding author. Tel.: +86 10 88233968; fax: +86 10 88235294. E-mail addresses: [email protected] (Z.-F. Chai), [email protected] (W.-Q. Shi). 0013-4686/$ – see front matter © 2013 Published by Elsevier Ltd. http://dx.doi.org/10.1016/j.electacta.2013.07.084

efficiency. Moreover, Ln can react with fuel claddings and the reaction products (alloys) would lower the cladding melting point [3]. However, because of the similar chemical properties between Ln and An, acquiring a desirable achievement to separate An from Ln is one of the most challenging tasks in the spent fuel reprocessing [4]. Pyrochemical reprocessing has been considered as a promising alternative to the traditional hydrometallurgical process for the separation of actinides from FP due to its compactness, resistance to radiation effects, criticality control benefits, compatibility with advanced fuel types, and the ability to produce low purity products to resist nuclear proliferation [5–8]. In the pyrochemical reprocessing, molten salts are used as the solvent in which the separation of An from Ln is normally carried out by reductive extraction and/or electrolysis [9]. Basically, actinides and Al can interact strongly and readily form intermetallic compounds, which would allow the extraction of An at more anodic potential [10]. In this regard, production of An–Al alloys would be a plausible approach to achieve the efficient separation of actinides from lanthanides.

K. Liu et al. / Electrochimica Acta 109 (2013) 732–740

Actually, a typical molten chloride salt-based electrorefining process using solid aluminum cathodes to separate actinide has been proposed by Serp et al. [9]. A relatively high separation efficiency of An over Ln [11,12] and excellent extraction and recovery of An has been demonstrated [13–15]. Castrillejo et al. [16–22] and Bermejo et al. [4,6] also successfully extracted Ln by forming Al–Ln intermetallic compounds on the aluminum cathode in LiCl–KCl–LnCl3 melts. In addition, Conocar et al. [10] found that the separation efficiency was highly dependent on the metallic phase and the most promising media for An–Ln separation would be aluminum. On the other hand, as a typical element of the fission products, the electrochemical behaviors of gadolinium in molten salts have been studied by some researchers. Bermejo et al. [23] extracted gadolinium in LiCl–KCl eutectic by underpotential deposition on the Al electrode and determined the standard Gibbs energy, enthalpy and entropy of formation of the Al3 Gd intermetallic compound. In addition, Nourry et al. [2,24,25] investigated the electrochemical behaviors of gadolinium on the inert molybdenum electrode and the extraction of gadolinium on reactive cathodes (Cu, Ni) in LiF–CaF2 melts. They concluded that the depolarization of the cathodic reaction to form alloys (Cux Gdy and Nix Gdy ) on the active electrode could lead to an extraction rate of nearly 100%. In all, previous works showed that the extraction of gadolinium from molten salts was conceivable on an active electrode. However, to the best of our knowledge, the investigation about the co-deposition of Gd3+ and Al3+ in molten salts is still lacking, which is of obvious significance not only to the expansion of the electrochemistry of these two metal ions but also to the preparation of more novel Gd–Al alloys. Hence, this work predominantly focused on the co-reduction mechanism of Gd3+ , Al3+ by conducting a series of electrochemical techniques and directly extracted gadolinium from Gd2 O3 in the LiCl–KCl–AlCl3 molten salts at temperature of 773 K by using inert molybdenum and active aluminum electrode. Our results might provide useful information for the well understanding of extraction and separation of gadolinium from FPs in the LiCl–KCl molten salt. 2. Experimental 2.1. Chemicals Anhydrous LiCl, KCl, AlCl3 and Gd2 O3 (analytical grade) were purchased from Sinopharm Chemical Reagent Co., Ltd. All operations of reagents were carried out in a glove box in which the concentrations of oxygen and moisture were controlled to be less than 2 ppm. The LiCl–KCl eutectic mixture (LiCl:KCl = 58.8:41.2 mol%) was first dried under vacuum for more than 24 h at 473 K to remove residual water, and then melted under a dry argon atmosphere in an alumina crucible placed in a sealed stainless steel cell inside a programmable electric furnace, the temperature deviation of the furnace can be maintained below ±2 K. The working temperature was measured using a nickel–chromium thermocouple protected by an alumina tube inserted into the solution. The anhydrous AlCl3 and Gd2 O3 powders were directly introduced into the eutectic. 2.2. Electrochemical apparatus and electrodes All electrochemical measurements were carried out by using an Autolab PGSTAT 302 N potentiostat–galvanostat controlled with the Nova 1.9 software package from Metrohm. Transient electrochemical techniques, i.e. cyclicvoltammetry, chronopotentiometry, square wave voltammetry and open circuit chronopotentiometry were used to explore the electrochemical properties of gadolinium

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in the molten salt systems in this experiment. The reference electrode (RE) was made of a 1 mm silver wire (Alfa, 99.9+%) immersed into the solution of LiCl–KCl–AgCl (1 wt.%) molten salts, which was placed in an closed-end alundum tube with a diameter of 4 mm. An inert molybdenum wire (Alfa, 99.9+%) with the diameter of 0.5 mm was polished by sandpaper, washed with alcohol using ultrasound, inserted into a 2 mm alundum tube, and then was used as the working electrode (WE). Before each measurement, the working electrode was cleaned by galvanostatic anodic polarization. The active electrode surface area was determined after each experiment by measuring the immersion depth of the electrode in the molten salts. As for the counter electrode (CE), a 1 mm molybdenum wire (Alfa, 99.9+%) and a 6 mm graphite rod were used. For the electrolysis, a 1 mm molybdenum wire (Alfa, 99.9+%) and a 2 mm thick aluminum plate (Alfa, 99.999%) were used as cathodes and a 6 mm graphite rod was used as the anode.

2.3. Preparation and characterization of cathodic deposits Gadolinium electro-deposits were firstly prepared in the LiCl–KCl–AlCl3 –Gd2 O3 molten salt on molybdenum and aluminum plates by potentiostatic electrolysis with a cathode potential previously defined by cyclic voltammetry and open circuit chronopotentiometry at 773 K for 12 h. Galvanostatic electrolysis was carried out in the molten salt to compare the deposits with those preformed at constant potential. Electro-deposits were prepared by applying a constant cathodic current (∼−30 mA) on the aluminum plate at 773 K for 8 h. After the electrolysis, the electrodes were removed from the bath and layer of deposits were found adhering on the surface of the electrodes. Afterward, the aluminum electrode was cut out and washed with ethylene glycol (Sinopharm, 99.8%) by ultrasound to remove residual salt. The deposits on the molybdenum electrode containing many salts were removed and repeatedly washed with ethanol. Then, all the samples were dried and especially stored in a vacuum drying oven before subjected to further analysis. Scanning electron microscopy (SEM) (Hitachi S-4800 and JEOL JSM 6360LV)–energy dispersive X-ray (EDX) (GENESIS 2000) techniques were employed to analyze the surface morphology and micro composition of gadolinium and aluminum deposits. Surface analysis of the samples was also performed by X-ray diffusion (XRD) (Bruker, D8 Advance) to identify the formation of Gd–Al intermetallic compounds.

2.4. Gadolinium extraction process As described in previous works [26,27], the extraction experiments were carried out on two kinds of working electrodes in the LiCl–KCl–AlCl3 –Gd2 O3 molten salt at 773 K. A molybdenum electrode (0.64 cm2 ) and an aluminum plate (5 cm2 ) were used to extract gadolinium by potentiostatic or/and galvanostatic electrolysis, respectively. The applying potentials and currents were based on the previous cathode potential cyclic voltammetry and open circuit chronopotentiometry. Another molybdenum electrode was applied to monitor the remaining Gd in the melt during the electrolytic process. Salt samples were obtained by taking the supernatant of the melts before and after the extraction process to calculate the extraction efficiency. After the extraction process, deposit and salt samples were dissolved in diluted hydrochloric acid and ultrapure water, respectively, and then the Gd contents of these samples were analyzed using an inductively coupled plasma atomic emission spectrometer (ICP-MS, Perkin Elemer NEXION 300D).

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K. Liu et al. / Electrochimica Acta 109 (2013) 732–740

0.10

at ∼−2.21 V and its corresponding anodic peak O at ∼−1.99 V. The sharp anodic striping peak O is typical of the dissolution of metal deposited in the cathodic scan. According to references [23,28], Caravaca et al and Bermejo et al. concluded that gadolinium metal could be deposited in a single step through reduction of Gd3+ directly into Gd with 3 electrons exchanged at the inert electrode. As the Gd2 O3 powder are introduced into the melts, Fig. 1(b) illustrates the comparison of CVs of the LiCl–KCl–Gd2 O3 (1.5 wt.%) melts (black curve) and the LiCl–KCl–AlCl3 (2 wt%)–Gd2 O3 (1.5 wt.%) melts (red curve) at 773 K with a scan rate of 100 mV/s. The black curve just with Gd2 O3 added is compatible with the curve of blank LiCl–KCl eutectic. This phenomenon indicates that the Gd2 O3 powder almost does not dissolve at all in the LiCl–KCl molten salts. Actually, Hayashi and Minato [29] also found that Gd2 O3 is very stable in LiCl–KCl melts. So there is no electroactive species of Gd ions available in the melts to induce any redox peak in the CV. After the introduction of AlCl3 , a series of redox peaks are observed between the reduction of Al3+ and Li+ , as shown in Fig. 1(b) (red curve). Since the potentials of some of these peaks are more positive than that of pure gadolinium metal, those peaks could be associated with some Gd–Al intermetallic compounds. These results prove that the Gd2 O3 powder has been chloridized by AlCl3 and the Gd3+ ions have been released into the LiCl–KCl eutectic. The reaction can be represented as:

0.05

Gd2 O3 (s) + 2AlCl3 (s) → 2GdCl3 (s) + Al2 O3 (s)

Current / A

0.14 0.12 0.10 0.08 0.06 0.04 0.02 0.00 -0.02 -0.04 -0.06 -0.08 -0.10 -0.12

O

A'

C' B'

D' A

B D R

a

C -2.5

-2.0

-1.5

-1.0

-0.5

0.0

Potential vs. (Ag/AgCl) / V 0.25 0.20

Current / A

0.15

0.00

-0.05 -0.10 -0.15 -0.20 -0.25

b -2.5

-2.0

-1.5

-1.0

-0.5

0.0

Potential vs. (Ag/AgCl) / V Fig. 1. (a) Comparison of the CVs of the LiCl–KCl–AlCl3 (1 wt%) melts (black curve) and the LiCl–KCl–GdCl3 (1 wt%) melts (red curve); (b) comparison of the CVs of the LiCl–KCl–Gd2 O3 (1.5 wt%) melts (black curve) and the LiCl–KCl–AlCl3 (2 wt%)–Gd2 O3 (1.5 wt%) melts (red curve) at the molybdenum electrode at 100 mV/s. Apparent electrode area: 3.14 × 10−3 cm2 ; temperature: 773 K. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

3. Results and discussion 3.1. Cyclic voltammetry Cyclic voltammetry measurements were carried out by using an inert molybdenum electrode in LiCl–KCl eutectic at 773 K. Fig. 1(a) shows the cyclic voltammograms (CVs) of the LiCl–KCl eutectic containing AlCl3 (1 wt%) (black curve) and GdCl3 (1 wt%) (red curve) with a scan rate of 100 mV/s. In the black curve, a small cathodic peak A is observed at the potential of ∼−1.0 V vs. Ag/AgCl and its corresponding large anodic peak A (potential ∼ −0.84 V) are attributed to the deposition and dissolution of aluminum. Two cathodic peaks B and D at around −2.24 V and −2.4 V are also observed, which could be related to the formation of Li–Al alloys. Their corresponding anodic peaks B and D are found at around −2.13 V and −2.28 V. The Li–Al alloys are produced by the underpotential deposition of lithium on the solid pre-deposited aluminum film covering on the molybdenum electrode. At more negative potential, peak C and peak C are observed due to the reduction and oxidation of lithium metal. These results are similar to those of Castrillejo et al. [21]. The red curve shows a single cathode peak R

(1)

From the thermodynamic analysis [30], the value of Gibbs energy of this reaction is calculated to be −420.665 kJ mol−1 at 773 K, which means that reaction (1) is energetically favorable in our case. Therefore, when the sweep potential was applied, the simultaneous co-reduction of Al3+ and Gd3+ on the molybdenum electrode produced a noticeable change in the electrochemical signal on the CV. To get more details about the formation of these intermetallic compounds, CVs of the LiCl–KCl–AlCl3 (2 wt.%)–Gd2 O3 (1.5 wt.%) melts at various scan ranges were collected as shown in Fig. 2(a). Besides the redox couple (A/A ) of deposition and dissolution of aluminum, five new redox couples are clearly observed in the CVs. The anodic peak A1 is attributed to the oxidation of Al–Mo alloy underpotentially deposited just prior to the reduction of Al3+ in the forward scan [21,31,32]. Four redox couples E/E , F/F , G/G and H/H are observed at potential between −2.17 V and −1.0 V, which should be associated with the formation of different Gd–Al intermetallic compounds. Due to the so-called “depolarization effect”, the co-reduction of Gd3+ and Al3+ occurred at more anodic potential than compared to the pure Gd metal deposition, which is similar to the under potential deposition of Gd at the Al [23] and Ni cathode [25]. In this process, Gd is the more reactive metal, Al is the less reactive one and the intermetallic compounds Gdx Aly are formed by co-reduction. The formation mechanism can be described as the following: yAl3+ + 3ye− = yAl;

(2)

xGd3+ + 3xe− = xGd;

(3)

xGd + yAl = Gdx Aly .

(4)

The overall process can be defined as: xGd3+ + yAl3+ + 3(x + y)e− = Gdx Aly .

(5)

According to the Nernst law, the equilibrium potential of the system Gd/Gdx Aly can be expressed as: EGd3+ /Gd

x Aly

= EGd3+ /Gd −

RT ln(aGd(in nF

Gdx Aly ) )

(6)

K. Liu et al. / Electrochimica Acta 109 (2013) 732–740

0.10

A'

0 - -1.2V 0 - -1.5V 0 - -1.8V 0 - -2.3V

0.08

0.04

H' I'

0.02

F'

0.00

A1'

G'

0.00 -0.02 -0.04 -0.06

I H -2.4

0.06 0.05 0.04

Current / A

A

F

0.03 0.02

G

-2.0

a

E -1.6

-1.2

-0.8

-0.4

0 - -1.15V 0 - -1.55V 0 - -1.85V 0 - -2.05V

0.0 -0.1 -0.2 -1.5

-1.0

-0.5

0.0

0.01 0.00

-0.01 -0.02

H

G F

A

E

b

-0.03 -2.2 -2.0 -1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0

Potential vs. (Ag/AgCl) / V

where aGd(in GdX Aly ) denotes the activity of Gd in the Gdx Aly intermetallic compound; T is the absolute temperature in K, n corresponds to the number of exchanged electrons, F designates the Faraday constant. EGd3+ /Gd represents the equilibrium potential of Gdx Aly )

is less than one, which pro-

motes the shift of the electroreduction potential of Gd3+ toward a more anodic value (EGd3+ /Gd Al ) as shown in the CVs. According x

-0.06

C -2.0

-1.5

-1.0

-0.5

0.0

Fig. 3. Comparison of the SWVs of the LiCl–KCl–AlCl3 (1 wt%) melts (dotted curve) and the LiCl–KCl–AlCl3 (2 wt%)–Gd2 O3 (1.5 wt%) melts (black curve) at the molybdenum electrode. Step potential: 1 mV; frequency: 20 Hz; apparent electrode area: 3.14 × 10−3 cm2 ; temperature: 773 K.

surface. It cannot take place when the surface of the molybdenum electrode was fully covered by Gd–Al intermetallic compounds. In addition, Fig. 2(b) shows the CVs of the LiCl–KCl–AlCl3 (1 wt.%)–Gd2 O3 (1.23 wt.%) melts at various scan ranges at 773 K. The current signal of the redox peaks G/G and H/H is much weaker than those couples in Fig. 2(a). This phenomenon might result from the lower concentration of Gd3+ in the melts, and there is not enough Gd3+ to further form Gd-rich intermetallic compound in the co-reduction process. 3.2. Square wave voltammetry

Fig. 2. CVs of (a), the LiCl–KCl–AlCl3 (2 wt%)–Gd2 O3 (1.5 wt%) melts and (b) the LiCl–KCl–AlCl3 (1.5 wt%)–Gd2 O3 (1 wt%) melts at the molybdenum electrode at 100 mV/s at various scan ranges. Apparent electrode area: 3.14 × 10−3 cm2 ; temperature: 773 K.

pure Gd metal. Generally, aGd(in

A

B

Potential vs. (Ag/AgCl) / V

0.1

-2.0

E

D

-0.04

-2.5

0.2

-2.5

H

-0.08

0.0

Potential vs. (Ag/AgCl) / V

G

-0.02

E'

A1

F

I Current / A

Current / A

0.06

735

y

to the Gd–Al phase diagram [33], five intermetallic compounds are available, i.e. GdAl3 , GdAl2 , GdAl, Gd3 Al2 and Gd2 Al. Correspondingly, four reduction peaks (E, F, G, H) in Fig. 2(a) should represent at least four different Gd–Al intermetallic compounds. The reduction and oxidation potential of these peaks are summarized in Table 1. The cathodic peak I at around −2.17 V and its corresponding anodic I at around −1.87 V might be attributed to the reduction of Gd3+ and the oxidation of gadolinium metal. As one can see, the potential of peak I is similar with that of the formation of Li–Al alloy (peak B in Fig. 1(a)). Actually, according to Castrillejo et al. [16], underpotential deposition of Li to form Li–Al alloy only occurs on the Al

Table 1 The reduction and oxidation potentials (V) of peaks in Fig. 2(a). Redox-couple

A/A

E/E

F/F

G/G

H/H

I/I

Reduction Oxidation

−1.0 −0.9

−1.44 −1.31

−1.75 −1.33

−1.93 −1.42

−2.0 −1.69

−2.17 −1.87

Square wave voltammetry, as a more sensitive transient method than cyclic voltammetry, was employed to further investigate the electrochemical behavior of Gd3+ in the LiCl–KCl–AlCl3 –Gd2 O3 melts. Fig. 3 shows the comparison of square wave voltammograms (SWVs) obtained in the LiCl–KCl eutectic containing AlCl3 (1 wt.%) (dotted curve) and AlCl3 (2 wt.%)–Gd2 O3 (1.5 wt.%) (black curve) at a signal frequency of 20 Hz, scanning from 0 to −2.5 V at the molybdenum electrode at 773 K. In the dotted curve, three large cathodic signals at −0.996, −2.189, −2.3 V can be observed, which corresponds to the formation of Al metal (peak A) and underpotential deposition of Al–Li alloys (peak B and D), respectively. The results fit well with those collected from CVs in Fig. 1(a). Moreover, one small signal A1 , prior to the reduction of Al3+ , is also expectedly observed in the curve, which could be attribute to the formation of Al–Mo alloys [21,31,32]. In the black curve, three new signals are clearly identified at −1.43, −1.75and −2.2 V, which are also in accordance with the results obtained from CVs in Fig. 2(a). In addition, a small shoulder G at around −1.94 V is also observed in the curve. The four signals E, F, G and H might be corresponding to the formation of four different Al–Gd intermetallic compounds. A small signal I at −2.2 V might be associated with the formation of Gd elementary substance deposited on the Gd–Al alloy covered molybdenum electrode. The potential of signal I is quite different with signal B and D in the SWV, which further emphasizes that the underpotential deposition of Li only occurs on the Al surface, but not on the Al–Gd intermetallic compounds. 3.3. Chronopotentiometry In this section, Fig. 4 illustrates the evolution of a group of chronopotentiograms (CPs) of LiCl–KCl–AlCl3 (1 wt.%)–Gd2 O3

K. Liu et al. / Electrochimica Acta 109 (2013) 732–740

-0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4 -1.6 -1.8 -2.0 -2.2 -2.4 -2.6

12mA 14mA 16mA 18mA 20mA

A E

E F

F

F

G

H I

0

2

4

6

H C

8

I

10 12 14 16 18 20 22

Time / s Fig. 4. CPs of the LiCl–KCl–AlCl3 (1 wt%)–Gd2 O3 (1.23 wt%) melts at the molybdenum electrode at various current densities. Apparent electrode area: 3.14 × 10−3 cm2 ; temperature: 773 K.

(1.23 wt.%) melts at the molybdenum electrode with different current densities at 773 K. Firstly, the cathodic current density is applied to −3.82 A cm−2 (−12 mA). Three potential plateaus (A, E, F) are observed in the black curve, corresponding to the deposition of aluminum and the co-deposition of gadolinium and aluminum on the pre-deposited Al to form two Al-rich Gd-Al intermetallic compounds, respectively. When the cathodic current density is applied to −4.46 A cm−2 (−14 mA), the curves exhibit three new potential plateaus (G–I), which could be associated with the formation of another two Gd–Al intermetallic compounds and the Gd metal, respectively. Meanwhile, when the current density is applied up to −5.1 A cm−2 (16 mA), the electrode potential (plateau I) reaches a limiting value corresponding to the deposition of the lithium metal. It’s obvious that all potential plateaus in Fig. 4 completely coincide with the potential range obtained in the CVs and SWVs, respectively. 3.4. Open circuit chronopotentiometry Open circuit chronopotentiometry was also carried out to investigate the mechanism of formation and dissolution of alloys and to ensure the formation of the maximum number of intermetallic compounds. The measurement process was conducted as follows. Firstly, a thin layer of Gd–Al intermetallic compounds was electrodeposited on a molybdenum electrode by potentiostatic electrolysis for short periods in the melts. Then, the open-circuit potential of the molybdenum electrode was measured as a function of time after switching off the potentiostatic control [22]. During this process, the potential plateaus corresponding to biphasic equilibrium of coexisting state (Gdx Aly ) at the molybdenum electrode surface will be observed. Fig. 5 illustrates a group of open circuit chronopotentiograms (OCPs) of AlCl3 (x wt.%)–Gd2 O3 (y wt.%) in the LiCl–KCl eutectic after applying a potential of −2.50 V vs. Ag/AgCl for 5 s or 10 s at the molybdenum electrode at 773 K. These curves exhibit several plateaus typical of the successive formation of various solid phases on the molybdenum electrode. In order to confirm the reproducibility of the experiment, the same measurement was repeated for several times for each molten system. Curve (I) shows the OCP curve obtained in the LiCl–KCl–AlCl3 (1 wt.%) melts after applying a potential of −2.50 V for 10 s. Originally, a very stable plateau maintaining at a potential close to −2.43 V (plateau a) is observed, which could be interpreted as the Li+ /Li equilibrium potential in the melts. Afterwards, the potential plateau b emerge at ∼−2.16 V and the inflexion point located at ∼−2.25 V appear which

-0.8

Potential vs. (Ag/AgCl) / V

Potential vs. (Ag/AgCl) / V

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a

-1.0 -1.2 -1.4

e1

e

e

e1 -2.02

-1.6

-2.04

g

-1.8

-2.08 -2.10

b

-2.2

d

-2.4 -2.6

-2.06

f

h

h

-2.0

g

-2.16 -2.18 -2.20

c 0

i

-2.12 -2.14

22.0

22.5

20 40 60 80 100 120 140 160 180 200 220

Time / s Fig. 5. OCPs of (I) AlCl3 (1 wt%), (II) AlCl3 (1 wt%)–Gd2 O3 (1 wt%), (III) AlCl3 (1 wt%)–Gd2 O3 (1.23 wt%) for 10 s and (IV) AlCl3 (2 wt%)–Gd2 O3 (1.5 wt%) for 5 s in the LiCl–KCl eutectic at the molybdenum electrode after electrodepositing at −2.50 V (vs. Ag/AgCl). Apparent electrode area: 3.14 × 10−3 cm2 ; temperature: 773 K.

are both correlated with the formation of Al–Li alloys. Finally, the potential plateau maintaining at −0.99 V (plateau h) is associated with the equilibrium potential of Al3+ /Al redox couple. Curves (III) and (IV) show the OCPs of AlCl3 (1 wt.%)–Gd2 O3 (1.23 wt.%) and AlCl3 (2 wt.%)–Gd2 O3 (1.5 wt.%) in the LiCl–KCl eutectic after electrodepositing at −2.50 V for 10 s and 5 s, respectively. Apart from those two identified potential plateaus (a, c) observed in curve (I), four new plateaus (e1 , e, g, h) are also observed at ∼−1.37, −1.43, −1.83, −1.98 V in curve (IV). It is in particularly noteworthy that at more positive potential than plateau e (∼−1.43 V), the plateau e1 (∼−1.37 V) is unexpectedly observed, which could be also due to the presence of a Gd-Al intermetallic compound (probably GdAl3 ). Interestingly, only one signal appears at ∼−1.44 V in other electrochemical methods (CV, SWV and CP). This could be interpreted by the different formation rate for each intermetallic compound. In these measurements, some intermetallic compounds forming rapidly are present while other phases forming much slower are absent in these curves. In addition, plateau f at −1.75 V corresponding to the presence of another Gd–Al intermetallic compound is absent in curve (IV), whereas it is observed very clearly in curve (III). The transformation of one phase into another and the difference of concentrations of Gd3+ and Al3+ in the melts also could be the possible explanation for these phenomena. Herein, we suggest that plateaus e1 –h in curve (III), should correspond to the five biphasic coexisting states, i.e. GdAl3 , GdAl2 , GdAl, Gd2 Al and Gd3 Al2 , respectively. In the inserted figure, a small plateau at −2.12 V is observed, which could correspond to the formation of pure Gd metal. Curve (II) shows the OCP obtained in the LiCl–KCl–AlCl3 (1 wt.%)–Gd2 O3 (1 wt.%) melts electrodepositing at −2.50 V for 10 s. It is clear that only two potential plateaus (e1 and e) appear in the curve could be associated with the GdAl3 and GdAl2 intermetallic compounds. Due to the shortage of Gd3+ diffused into the clusters of Al atoms at the stage of nucleation, and then only Al-rich intermetallic compounds were formed in the co-reduction process. 3.5. Characterization of the cathodic deposits To confirm the co-reduction of Gd3+ and Al3+ and examine the formation of Gd–Al intermetallic compounds at the equilibrium potential determined by CV and OCP measurements, potentiostatic electrolysis was firstly carried out at potential −1.5 V vs. Ag/AgCl melts on the molybdenum electrode for 12 h at 773 K. Fig. 6 shows the SEM micrograph and EDS analysis of cathodic deposits after

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Fig. 6. SEM (a)–EDS (b) and XRD (c) analysis of the cathodic deposits obtained by potentiostatic electrolysis in LiCl–KCl–AlCl3 (2 wt.%)–Gd2 O3 (1.5 wt.%) melts at the molybdenum electrode. Temperature: 773 K; potential: −1.5 V; time: 12 h.

washed repeatedly with ethanol. Obviously, the SEM–EDS reveals that the deposits are predominantly composed of salts and very small amount of Gd–Al intermetallic compound particles are inlaid in the deposits. In addition, XRD analysis of this sample identified that the components of the deposits are KCl salt and intermetallic compound GdAl2 . In addition, samples were also prepared by potentiostatic electrolysis for 12 h at 773 K. The SEM micrograph of the intersecting surface of the sample obtained at potential −1.5 V is shown in Fig. 7(a), which displays the formation of a coherent layer on the aluminum plate. One unique intermetallic compound of GdAl3 is confirmed by XRD analysis Fig. 7(b). Since the presence of GdAl2 in the potentiostatic electrolysis at −1.5 V on the molybdenum electrode, so at ∼−1.44 V, the co-reduction of Gd3+ and Al3+ on the aluminum plate might involve two processes. Firstly, the co-reduction is initiated on the electrode surface, which can be expressed as: Gd3+ + 2Al3+ + 9e−1 → GdAl2

(7)

Then, the diffusion of gadolinium into the aluminum electrode follows, which can be described as: GdAl2 + Al → GdAl3

(8)

The overall process can be represented as: Gd3+ + 2Al3+ + 9e−1 → GdAl3

(9)

Fig. 7(c) and (d) shows the SEM image of the intersecting surface and the XRD spectrum of the deposit obtained at −2.0 V on the aluminum plate. The SEM image also shows a layer on the electrode and the formation of one intermetallic compound GdAl3 is also identified by XRD analysis. The facts that only one intermetallic compound is formed in the potentiostatic electrolysis at −2.0 V could be due to a lower current density. Other Gd–Al phases

formed by co-reduction fast diffuse into the Al metal and the formation rate is much slower than the diffusion rate, which leads to the transformation of these Gd–Al phases into the more stable Alrich phase (GdAl3 ) on the Al electrode. In all, the obtained alloys in our case are similar to those obtained by under potential deposition with the Al cathode [23], the basic formation mechanism of alloys still has some differences. To compare the deposits prepared at constant potential, galvanostatic electrolysis was then carried out with the current of ∼30 mA for 8 h. The cathode potential evolution was collected during the electrolysis, which was almost maintaining at the range of ∼−1.7 V to ∼−2.25 V. From the inside out, three uniform and compact layers are clearly observed on the Al plate in the SEM image of the sample, as shown in Fig. 7(e). Three intermetallic compounds GdAl3 , GdAl2 , GdAl are also identified by XRD analyses (Fig. 7(f)). Unlike potentiostatic electrolysis, galvanostatic electrolysis provides an efficiently method to plate Gd–Al intermetallic compounds on the Al substrate and a stable current to equably form more Gd–Al intermetallic compounds by a successive co-reduction process. The GdAl3 phase is preferentially formed to cover the whole electrode. When the inter-diffusion of gadolinium and aluminum reaches equilibrium, the GdAl2 phase begins to grow on the GdAl3 surface, and then subsequently reach other equilibriums to form other phases until the depletion of Gd3+ in the melts. Nevertheless, due to the variation of potential during the electrolysis process, galvanostatic electrolysis still not be a suitable method for the real separation process. 3.6. Gadolinium extraction Extraction experiments were firstly carried out on a molybdenum electrode at potential −1.5 V vs. Ag/AgCl in the LiCl–KCl–AlCl3

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Fig. 7. SEM and XRD analyses of the cathodic deposits obtained in LiCl–KCl–AlCl3 (2 wt.%)–Gd2 O3 (1.5 wt.%) melts at an aluminum electrode at 773 K. (a, b) Potentiostatic electrolysis at −1.5 V; time: 12 h; (c, d) potentiostatic electrolysis at −2.0 V; time: 12 h; (e, f) galvanostatic electrolysis at −30 mA; temperature: 773 K; time: 8 h.

(2 wt.%)–Gd2 O3 (1.5 wt.%) melts at 773 K. The initial concentrations of the Gd3+ and Al3+ were calculated to be 1.24 wt% and 0.29 wt% of the melts, respectively, based on the ICP-MS analysis before the electrolysis. However, due to the evaporation of AlCl3 , the concentration of Al3+ in the melts decreased rapidly during the electrolysis process. It seems difficult to extract gadolinium efficiently on the inert molybdenum electrode by potentiostatic electrolysis at −1.5 V in this case. Actually, according to the ICP-MS analysis, a poor extraction efficiency of 44.3% was obtained after 36 h of electrolysis. Therefore, considering the economic efficiency of this experiment, an aluminum plate was used as the cathode to improve the extraction efficiency. The extraction experiments were then performed on an aluminum plate at 773 K. In order to facilitate the comparison, both potentiostatic electrolysis at the potential of −1.5 V and galvanostatic electrolysis at the current density of −30 mA were

both carried out in the molten salts. The electrolysis processes were monitored by the in situ recording SWVs, as shown in Fig. 8(a) and (b). Significant decrease of current density are observed in both of the graphs, corresponding to the decrease of Al3+ and Gd3+ concentrations in the melts during the extraction process. In addition, for the potentiostatic extraction, the concentration decreased rapidly at the initial stage of the electrolysis. Actually, the similar phenomenon has been observed by Gibilaro et al. [26,27]. Interestingly, we noticed that the quick decrease of the concentration for the galvanostatic extraction occurred in middle stage of the electrolysis process. To completely extract gadolinium, electrolyses were performed for enough time to obtain a cathodic current density approaching zero on the SWV. According to the ICP-MS results of the salt samples collected before and after the electrolysis, the

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Current / A

extraction of gadolinium from Gd2 O3 in the LiCl–KCl–AlCl3 melts by co-reduction at more anodic potential on an aluminum electrode is effective.

26h

0.00 -0.01

18 h

-0.02

Acknowledgements This work was supported by the Major Research Plan “Breeding and Transmutation of Nuclear Fuel in Advanced Nuclear Fission Energy System” of the Natural Science Foundation of China (Grant 91226201) and the “Strategic Priority Research program” of the Chinese Academy of Sciences (Grants XDA03010401, XDA03010403 and XDA03010404).

8h

-0.03

0h

-0.04

a -2.5

-2.0

-1.5

-1.0

Potential vs. (Ag/AgCl) / V

-0.5

0.0

24 h

0.00

Current / A

739

-0.01

16 h

-0.02

8h

-0.03

0h

-0.04

b -2.5

-2.0

-1.5

-1.0

-0.5

0.0

Potential vs. (Ag/AgCl) / V Fig. 8. Current density evolution during the electrolysis exhibited by SWVs of the LiCl–KCl–AlCl3 (2 wt%)–Gd2 O3 (1.5 wt%) melts. Step potential: 1 mV; frequency: 20 Hz; temperature: 773 K. (a) Potentiostatic electrolysis at −1.5 V; (b) galvanostatic electrolysis at −30 mA.

extraction efficiencies of the both potentiostatic and galvanostatic electrolysis were calculated to be 89.7% and 96.5% for gadolinium, respectively. 4. Conclusions The electrochemical co-reductive behaviors of Al3+ , Gd3+ and the mechanisms of alloy formation were investigated in the LiCl–KCl–AlCl3 –Gd2 O3 melts on the inert molybdenum electrode at 773 K by CV, SWV, CP and OCP techniques. Four typical signals are shown in CV, SWV and CP while five plateaus are observed in OCP corresponding to different kinds of Gd–Al intermetallic compounds. Gd–Al intermetallic compounds are prepared by potentiostatic and galvanostatic electrolysis melts at molybdenum and aluminum electrode. According to the analyses of SEM-EDX and XRD, only GdAl2 intermetallic compound particles adhering to a significant amount of the salt electrolyte are observed on the molybdenum electrode deposit by potentiostatic electrolysis. Only one intermetallic compound (GdAl3 ) is formed by potentiostatic electrolysis after applying a potential at −2.2 V and −1.5 V, while three intermetallic compounds (GdAl3 , GdAl2 , GdAl) are observed by galvanostatic electrolysis at −30 mA on the aluminum plate cathode, respectively. Extractions of gadolinium by co-reduction with Al3+ on an aluminum electrode yield extraction efficiency of 89.7% for potentiostatic electrolysis at −1.5 V and 96.5% for galvanostatic electrolysis at −30 mA. These prove that the

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