Electrochemical behavior of dysprosium(III) in eutectic LiF-DyF3 at tungsten and copper electrodes

Journal Pre-proof Electrochemical behavior of dysprosium(III) in eutectic LiF-DyF3 at tungsten and ☆ copper electrodes Chunfa Liao, BoqingCai, Xu Wang, Shumei Chen, Gong Chen, Jueyuan Lin PII:

S1002-0721(19)30194-2

DOI:

https://doi.org/10.1016/j.jre.2019.07.016

Reference:

JRE 601

To appear in:

Journal of Rare Earths

Received Date: 2 March 2019 Revised Date:

24 July 2019

Accepted Date: 25 July 2019

Please cite this article as: Liao C, BoqingCai Wang X, Chen S, Chen G, Lin J, Electrochemical behavior ☆ of dysprosium(III) in eutectic LiF-DyF3 at tungsten and copper electrodes , Journal of Rare Earths, https://doi.org/10.1016/j.jre.2019.07.016. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © [Copyright year] Published by Elsevier B.V. on behalf of Chinese Society of Rare Earths.

Electrochemical behavior of dysprosium( ) in eutectic ☆

LiF-DyF3 at tungsten and copper electrodes☆ Chunfa Liao*, BoqingCai, Xu Wang, Shumei Chen, Gong Chen, Jueyuan Lin Faculty of Materials, Metallurgy and Chemistry, Jiangxi University of Science and Technology, Ganzhou 341000, China

ABSTRACT

Electrochemical behavior of dysprosium(Dy) ions in LiF-DyF3 (24 mol%) was investigated by cyclic voltammetry, chronoamperometry and chronopotentiometry. Dy-Cu alloy samples were prepared by constant-potential electrolysis in LiF-DyF3 (24 mol%) at the Cu electrode. The Cu5Dy and Cu phases were characterized by an X-ray diffractometer and a scanning electron microscope equipped with an energy dispersive spectrometer. The results showed that the reduction of Dy(Ⅲ) ions in a LiF-DyF3 (24 mol%) molten salt system is found to be a quasi-reversible diffusion-controlled process which occurs via a one-step reaction involving the transfer of three electrons. The electro-crystallization processes of the Dy metal at the W electrode and the mode of nucleation confirmed that progressive nucleation was dominant at high concentrations of Dy ions in the LiF-DyF3 salt. At lower concentrations, the instantaneous nucleation of Dy with three-dimensional growth of the nuclei was dominated. Key Words：Molten salt electrolysis, Dy-Cu alloy, Electrochemical mechanism, Alloy preparation, Rare earths 1.

Introduction

Dysprosium (Dy) is found at approximately 6 ppm in Earth’s crust, lower only than yttrium among heavy rare earth elements. Dy-containing materials have wide applications including magnetostriction,1 magnetorefrigeration,2 hydrogen storage3, and phosphor.4 Dy is especially useful as an additive in permanent magnetic materials (Nd-Fe-B) because of its favorable magnetic properties, thermal stability, and fluorescence. It was reported that Nd-Fe-B is sufficiently coercive over 453 K to ensure that the motor of an electric or hybrid electric vehicle can run normally in the high-temperature environment.5 A crystalline grain-boundary phase with a low melting point is formed by adding an appropriate amount of copper (Cu), resulting in sintering and grain refinement effects that improve the microstructure of the grain boundary. However, the remanence (Br) is almost unaffected.6,7 Indeed, previous studies8,9 have demonstrated that, adding the proper amount of Dy to Cu-based alloys can enhance their initial susceptibility and magnetic scattering of superconducting electrons. Differently from additions of other elements such as Al, Mn, Co, addition of Dy will not degrade the low-temperature resistivity of the alloys. Dy is therefore expected to be an idea matrix material to overcome the proximity coupling in multi-core superconductors and also to retain the stability. At present, the preparation methods of rare earth alloys include melting,10 calciothermic reduction, 11 and molten salt electrolysis.12 At least 95% of rare earth alloys have been prepared by ☆

Foundation item: Project supported by the National Natural Science Foundation of China (5167041092, 51564015) * Corresponding author E-mail address :[email protected] (C.F. Liao)

molten salt electrolysis which is a simple and environmentally friendly process and allows easy control of the alloy phase.13 However, the exact reaction mechanism at the electrode surface remains unclear, and few works investigated fluoride-based molten salts. In this study, LiF-DyF3 and Dy2O3 were used as the raw materials to prepare Dy-Cu alloys that were then characterized by XRD, SEM and EDS. Electrochemical methods such as cyclic voltammetry, chronoamperometry, and chronopotentiometry were used to study the formation mechanisms of the alloys at the electrode surface. 2.

Experimental

2.1. Preparation and purification of the melt All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd. (AR grade ＞99.5 wt%). The experimental salt was prepared by mixing the chemicals required for a given ratio. As shown in Table 1, there is one eutectic point and two peritectic points in LiF-DyF3 molten salt system. The eutectic temperature is 973 K at which the least energy is required to melt the salt. In turn, the crystallization is more stable, thereby meeting the requirements of the industrial production. Thus, the eutectic composition (24 mol% DyF3) was selected for this study. The anhydrous LiF-DyF3 (76:24 mol%) mixture was dried under vacuum for more than 12 hours at 473 K to eliminate water. The resulting supporting electrolyte was then placed in a high-purity graphite crucible in an electric furnace which was heated to 1243 K. The Dy (Ⅲ) ions were introduced into the electrolytic bath in the form of either DyF3 or Dy2O3. All the experiments were carried out in a glove box from Inert Technology, with a controlled argon (Ar) atmosphere to maintain a low oxygen concentration (≤ 4 ppm). 2.2 Electrochemical apparatus and electrodes Fig. 1 shows the electrochemical equipment. All the electrochemical experiments in this work were performed with an Autolab PGSTAT 302N potentiostat/galvanostat controlled by a Nova 1.9 software package. The reference electrode was a platinum wire (0.5-mm diameter, 99.99 wt%) and a tungsten (W) rod (10-mm diameter, 99.99 wt%) was used as the auxiliary electrode. In this study, the working electrodes were a W wire (1-mm diameter, 99.99 wt%) and Cu wire (5-mm diameter, 99.99 wt%), respectively. According to the alloy phase, Dy3+ or Li+ does not react with W to form W-Dy or W-Li alloy. W electrode can be used to study the reduction, oxidation, and alloying processes of Dy3+ and Cu2+ on the surface of the inert electrode. The electrodes were thoroughly polished using SiC paper, and the electrode surfaces were cleaned thrice with ethanol and dried in a vacuum oven prior to use. The electrochemical experiments were carried out in an airtight furnace with a high-purity Ar atmosphere. 2.3 Preparation and characterization of Dy-Cu alloy Fig. 2 shows the electrolysis apparatus. The Dy-Cu alloy samples were prepared by potentiostatic electrolysis and the furnace body was heated to 1243 K. The Cu electrode surface area was calculated by measuring its immersion depth in the molten salt. The sample was washed with distilled water to remove the conglutinating salts, and then the sample was abraded and polished by emery paper and polishing cloths, followed by ultrasonic cleaning in ethanol and storing in vacuum drying oven before analysis. The alloy samples were analyzed by X-ray

diffraction (XRD, SMART APEX-Ⅱ) with a monochromatic Cu-Kα line (λ = 0.15418 nm) irradiation at 40 kV and 40 mA. The surface and cross sections of the Cu rob were imaged after Dy deposition using a scanning electron microscope (SEM, MIRA3 LMH) with energy dispersive spectrometry (EDS, UltraDry) to analyze the surface morphology and chemical distribution. 3.

Results and discussion

3.1 Cyclic voltammetry (CV) studies Fig. 3 shows representative CV curves obtained with a W electrode between LiF-DyF3 and LiF-DyF3-Dy2O3 molten salt systems at the scan rate range from 100 to 300 mV/s at 1243 K. Fig 3(a) shows the signals A/A' and B/B' that are observed in the curve and defined as follows. Peak A with the reduction potential approximately –2.0 V (versus Pt) most likely corresponded to the reduction of Li ions. The corresponding anodic peak A' at approximately –1.0 V (versus Pt) is ascribed to the dissolution of Li metal. The cathodic peak (B) at approximately –0.13 V (versus Pt) may have been caused by Dy metal deposited at the W electrode by direct reduction of Dy(Ⅱ) ion into Dy(0). Table 2 shows that the reduction potential of Dy ion has been previously reported to be below –1.0 V.15-17 Additional peaks are observed within the potential range from –1.6 to –0.15 V (versus Pt). The fact that no reduction or oxidation reactions occurred in this electrochemical window confirmed the suitability of the LiF-DyF3 molten salt system for our investigation. As shown in Fig. 3(b), two pairs of cathodic/anodic signals are observed. The cathodic current peak A/B and band anodic current peak A’/B’ were also ascribed to the formation and oxidation of the Li/Dy metal. The reduction potentials are approximately –2.0 and –0.4 V (versus Pt), respectively. In the reverse scan, the oxidation potentials ware approximately –1.5 and –0.08 V (versus Pt), respectively. Fig. 4 plots the cathodic current peak (Ip) versus the square root of sweep rates. The linear relationship indicates that the reduction process of Dy(Ⅱ)/Dy(0) is controlled by diffusion of Dy (Ⅱ) in the LiF-DyF3 and LiF-DyF3-Dy2O3 molten salt systems. The relationship between the cathodic potential peak and logarithmic scan rate v (lgv) also showed a linear correlation, suggesting the reaction Dy(Ⅱ)/Dy(0) is a typical soluble-reversible process.18 Following the correlation in Fig. 4(a, b), the electrochemical reduction process of Dy(Ⅱ) at platinum can be concluded to be diffusion-controlled and quasi-reversible. The addition of Dy2O3 can increase the concentration of Dy(Ⅱ) in the LiF-DyF3 system, thereby enhancing the reduction and oxidation reactions of Dy(Ⅱ)/Dy(0). Fig. 5 reveals the measured shift in the positive direction of these peaks, which further confirmed that the reaction is controlled by diffusion. The diffusion coefficient of Dy(Ⅱ) can be calculated by the Randles-Serlik equation (Eq. (1))19,20which is applicable for a soluble-insoluble system:
 =

   0.4463      



 

(1)

Where, Ip represents the cathodic current peak (A), A the surface area of the electrode (cm2), C0 the bulk concentration of the electroactive species (mol/cm3), n the number of exchanged electrons of the reaction, F Faraday’s constant (96500 C/mol), R the gas constant (8.314 J/(mol·K)), D0 the

diffusion coefficient (cm2/s), v the potential sweep rate (V/s), and T the absolute temperature (K). Assuming n = 3, the measured value of A is 31.416×10–3 cm2; the value of C0 is 1.07×10–4 mol/cm3; and the Dy(Ⅱ) diffusion coefficient is calculated to be 1.122×10-4 cm2/s at 1243 K. Figs. 6 and 7 show the CV curves at 1243 K for the LiF-DyF3 and LiF-DyF3-Dy2O3 (1 wt%) systems, respectively. The reduction peak A and oxidation peak A' corresponded to the redox of Li (Ⅱ). In Fig. 7, the peak shape of the redox couples labeled B/B' can be seen to be essentially the same in the two molten salt systems, indicating that the addition of Dy2O3 had little effect on the system. It seems that the Dy3+ participated in the reaction mainly originated from DyF3. Fig. 8 directly analyzes the CV curve of the LiF-DyF3-Dy2O3 molten salt in the potential range from -0.7 to 0.3 V. The reduction peak B and the oxidation peak B' were treated by Eq. (2) separately.17,21 An obvious linear relationship was found between the potential value of the oxidation peak B' and lg[(Id–I)/I]. The n value is 5.18 obtained from the line slope. Therefore, Dy(0) and Cu(0) can be expected to transition into Dy3+ and Cu2+ ions near the oxidation peak, which further indicates the adhesion of the Cu-Dy alloy phase at the Cu electrode.  = ⁄ +

. !

% '%

"# $ & ( %

(2)

Where, E is the observed anodic potential (V), I the observed anodic current (A), E1/2 the potential of the half peak (V), Ip the anodic current peak (A); R, T, F, and n have the same meanings and units as indicated in Eq. (1). Fig. 7 shows no redox peak in the LiF-DyF3 system, and a new redox peak appears at B/B' after the addition of Dy2O3 (1 wt%). The emerging peak at B/B' corresponds to the deposition and dissolution of the Dy-Cu alloy. The addition of Dy2O3 is beneficial to the alloying of Dy3+ and Cu2+ ions, which explains that rare earth oxides (RE2O3) are always used for electrolysis in industrial operations. 3.2 Chronoamperometry (CA) studies CA is a sensitive technique for evaluating nucleation modes and growth phenomena.22 In this method, the metal is deposited on a foreign substrate to form active sites. Initial response of the transient currents is assumed to occur at these active sites and quickly characterized by peak identification. Fig. 9 shows the CA curve obtained to identify the nucleation and growth phenomena of Dy3+ ions deposited at the W electrode at various potentials. The initial region Ⅱ of each transient current rapidly decreased over time, which corresponds to the formation of the first nuclei after the charging of the double layer. The current then increased gradually in region Ⅱ, which is associated with crystal nucleus growth and the formation of a diffusion zone around the new nucleus at the electrode. Finally, the transient current slowly decayed in region Ⅱ, which is typical in a diffusion-controlled process. The ascending segment of the transient current curve corresponding to the initial nucleation zone can be used to characterize the dynamic process of electro-crystallization. In this study, the current transients exhibit the characteristic shape of diffusion-controlled three-dimensional nucleation. Based on the nucleation growth theory, 23,24 three-dimensional nucleation can be divided into two types: instantaneous and progressive nucleation. In instantaneous nucleation, all initial nuclei

emerge at the same time at the beginning of the electrolysis process. In progressive nucleation, a crystal nucleus gradually forms during the electrolysis process. In this study the Dy nucleation mode at the W electrode was determined from the relationship between the transient current and time according to Allongue and Souteyrand25 and non-dimensional CA plots according to Scharifker and Hill.26 The Scharifker and Hill formulas for instantaneous and progressive nucleation are represented by the following equations, respectively:
) =
) =

/ *!+, -. 0 12/0  )  2 4 0

(3)

/ *!56 +, 7 -. 0 12/0  )  2 4 0

(4)

Where, N0 is the initial nucleation number, Z the valence state, M the deposited atomic mass (g/mol), D the diffusion coefficient of the deposited atom (mol/cm3), C the concentration of deposited ions (mol/cm3), I(t) polarization current (A), t the polarization time (s) and Kn the nucleation constant. Fig. 10 shows the proportional relationship between I and t1/2 at different overvoltages. It can be seen that the initial stage of the electrochemical deposition of Dy at the W electrode can be explained by instantaneous and three-dimensional nucleation models and crystal growth is controlled by hemispherical or linear diffusion. Fig. 9 shows that when a negative voltage is applied, both the current density of electrolysis and the nucleation rate are increased. Region Ⅱ is also diffusion-controlled, as indicated by the slower current density following the more negative voltage applied. These results show that region Ⅱ is more vulnerable to the ion diffusion rate as a limiting factor. The Scharifker and Hill formulas provide an appropriate non-dimensional model for all the CA results. Fig. 11 shows the transient current normalized to (I/Im)2 versus (t/tm)2 and it compares the results with the typical Scharifker-Hill model for instantaneous and progressive nucleation, presented by Eq. (5) and Eq. (6), respectively. %



%



8% : = 9

8% : = 9

.;<= $1 >⁄>9

.<= $1 >⁄>9

>



(5)

>0



(6)

− exp 8−1.2564 > :( 9

− exp 8−2.3367 > 0 :( 9

As seen in Fig. 9, a platform appeared after the peak current Im when a low voltage was applied, and then the transient current decreased gradually. The current and current density at the surface of the W electrode decreased together with the nucleation rate, delaying the diffusion-controlled region in the melt. The Scharifker-Hill model failed because of the emergence of the transient peak plateau, and therefore this paper presents further investigations of only the fitting curves with higher applied voltages (from –0.40 to –0.42 V). As shown in Fig. 11, the curve of t/tm and (I/Im)2 at the rising current stage (region Ⅱ) more closely approximated the theoretical progressive curve, indicating that the nucleation mode of Dy3+ at the surface of the W electrode obeyed progressive nucleation in the current rising stage. Subsequently, the concentration of Dy3+ decreased due to the ions were controlled by diffusion. Then, the experimental curves seem to be close to the theoretical

instantaneous curve, indicating that the system progressed further via instantaneous nucleation when the concentration of ions was lower. 3.3 Open circuit chronopotentiometry (OCP) studies Fig. 12 shows the reduction behavior of Dy3+ at the W electrode as obtained by OCP. In Fig. 3, two CV peaks corresponding to the reduction of Li and Dy3+ at the W electrode in the LiF-DyF3 and LiF-DyF3-Dy2O3 molten salt systems showed that Li+ had more negative reduction potential than Dy3+. Fig. 12(a) shows two potential plateaus in the OCP curve, at –0.31 and –1.10 V, corresponding to the reduction of Dy3+ and Li+, respectively, at the W electrode. The transition time (τ) representing the amount of time elapsed before the OCP curves indicated that, the active ions in the diffusion layer of the electrolyte on the surface of the electrode are completely depleted by reduction and deposition. In addition, the CV curves further support the diffusion-controlled process of Dy3+ to Dy metal and the validity of the Sand’s Law, 17 as expressed by the following formula: GH

 

= −0.5 I π

 

(7)

Where, i is the applied current (A), τ the transition time (s), n the number of transferred electrons, S the surface area of the electrode (cm2); F, C, and D have the same meanings and units as in the previous equations. According to the CV curves shown in Fig. 5, it can be calculated that, C = 1.07×10-4 mol/cm3 and Do= 1.122×10-4 cm2/s. The number of exchanged electrons n = 3.26 ≈ 3, confirming that the reduction of Dy3+ in LiF-DyF3 melt is a one-step process. The reaction is Dy3+ + 3e → Dy(0). Fig. 12(b) shows three potential plateaus in the OCP curve: –0.37, –0.88, and –1.4 V. Because the precipitation potential of Li+ is relatively high in molten salt systems, the third plateau was attributed to the reduction of Li+ to Li metal. Compared with Fig. 12(a), the precipitation potential of Li+ was polarized to a more negative potential. However, because of the Dy3+ and Cu2+ alloying process, the initial precipitation potential of Dy3+ did not deviate obviously, and no obvious reduction peak emerged near –0.88 V. 3.4 Preparation and characterization of Dy-Cu alloys Based on the above electrochemical study and the binary phase diagram of the Dy-Cu alloy shown in Fig. 13, six intermetallic compounds (Cu7Dy, Cu5Dy, Cu9Dy2, Cu7Dy2, Cu2Dy and CuDy) were identified in the Dy-Cu system. The solubility of rare earth oxides in fluoride melts was reported to be approximately 7–8 wt%.27In order to avoid uncomplete dissolution of the oxides, which would result in the deterioration of electrolytic conditions due to the adhesion of the rare earth oxides to the Cu rod or the suspension of the molten salt system, the electrolysis system used for this study was LiF-DyF3 (400 g at 24 mol% DyF3) and Dy2O3 (5 wt%). The electrolyte was heated to 1243 K in a graphite crucible. A copper rod (10-mm diameter) and graphite were used as the cathode and anode, respectively. The electrode depth was 10±0.1 mm. The potentiostatic electrolysis at the Cu electrode was conducted sequentially at 4.5 V and 20–25 A for 60 min. The salt attached to the cathode substrate was removed, and the liquid alloy at the bottom of the graphite crucible was introduced into a mold to cool and solidify. Finally, the alloy sample was polished as shown in Fig. 14.

In order to verify the phase composition of the Dy-Cu alloy, the polished sample is analyzed by XRD; the results are shown in Fig. 15. The sample is mainly composed of Cu and Cu5Dy phases, and the following reaction between Cu and Dy3+ could be inferred： 5Cu + Dy O + 3e' = Cu< Dy（8） Fig. 16 shows the crystal phase of the alloy as observed by SEM, which indicated that the alloy is composed of two types of crystal phases: the matrix phase (gray) and the boundary phase (black). The grain boundary between the two crystal phases is clear and the microstructure of the prepared alloy is uniform. The EDS mapping of the visible zone in Fig. 16 showed that the alloy contained Dy and Cu elements, along with a small amount of oxygen (≤ 4.57 mol%). Combining the XRD and EDS results, it can be concluded that the matrix phase (gray) is Cu5Dy and the boundary phase (black) is Cu. There is a small amount of Dy present in the Cu phase which was not reported in the Dy-Cu phase diagram. In conjunction with the CV curves shown in Fig.7, Dy3+ could be inferred to have diffused and enriched on the electrode surface during electrolysis following a two-step process: (1) Dy3+ directly enriched on the surface of the Cu electrode and then alloyed with the Cu electrode under the action of the reduction current at high temperature. (2) At high temperature, the Cu electrode was bound with strong oxidizing elements such as oxygen and fluorine to form complex compounds such as CuOF, which tends to separate out Cu2+ in a strong reduction current. The reduction potential of Cu2+ is similar to that of Dy3+ in a Dy-rich system, resulting in codeposition to form the Dy-Cu alloy. The EDS analysis of the visible microstructure zone of the alloy is shown in Fig. 17. The results indicated that the black zone (point 1) is Cu phase mixed with a small amount of Dy. The region around point 1 is considered to be a Cu-rich phase based on the combined XRD and EDS analyses. The region around point 2 (gray zone) is found to be a Dy-rich matrix phase. In combination with the XRD pattern, the gray zone is considered to be a Cu5Dy phase. At point 2, the atomic ratio of Dy to Cu is close to 5:1, and the data in Table 3 further confirmed that the gray phase is Cu5Dy.The bulk composition of the alloy is provided in table 3. Conclusions The electrochemical behavior of Dy3+ ions at W and Cu electrodes was investigated by different electrochemical techniques in the LiF-DyF3 (24 mol%) molten salts system. The results indicate that the reduction potential of Dy3+at the W electrode is approximately –0.31 V, which occurred via a one-step reduction with the transfer of 3 electrons. The electrochemical reduction of Dy3+ to Dy metal is a quasi-reversible reaction controlled by diffusion. The diffusion coefficient of Dy3+ in LiF-DyF3 (24 mol%) melts is 1.122×10-4 cm2/s at 1243 K. The nucleation is continuous when the concentration of Dy3+ on the surface of W electrode is high, and the nucleation is instantaneous when the concentration of Dy3+ is lower. The Dy3+ ions are directly reduced by one step reaction and alloyed to transfer 5 electrons with Cu2+ ions at the Cu electrode, with an alloying potential of –0.88 V. The Dy-Cu binary alloy is prepared by potentiostatic electrolysis in a LiF-DyF3 eutectic system. EDS mapping identified that the Dy-Cu alloy occurs as both Cu and Cu5Dy phases. The separation of the alloy from the electrolyte and carbon compounds is sufficient to satisfy the quality requirements of downstream enterprises. 4.

5.

References

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Tables and Figtures Table 1 Invariant equilibria and singular points in LiF-DyF3 systems14 Composition (DyF3 mol%)

Temperature (K)

Type of equilibrium

Equilibrium reaction

24

973

Eutectic

L↔LiF+LiDyF4

46

1071

Peritectic

L+Orth.DyF3↔L+ LiDyF4

81

1343

Peritectic

L+Hex.DyF3↔L+ LiDyF4

Fig. 1 Electrochemical apparatus

Fig. 2 Schematic diagram of the electrolysis apparatus Table 2 Reduction potentials and diffusion coefficients of different molten salt systems15-17

Reduction

Reduction potential

Diffusion

Measurin

potential (Li+/Li)

(Dy3+/Dy)

coefficient (Dy3+)

g method

LiF-DyF3

–2.0 vs (Pt)/V

–0.75 vs (Pt)/V

1.159×10-4 cm2/s

CP

LiF-NaF-KF-DyF3

–1.27 vs (Pt)/V

–0.45 vs (Pt)/V

3.69×10-4 cm2/s

CV

LiF-CaF2-DyF3

0.15 vs (Li/Li+)/V

0.13 vs (Li/Li+)/V

1.55×10-5 cm2/s

CV

Molten salt system

1.0

3

(a)

A'

(b)

100 mV/s 150 mV/s 200 mV/s 250 mV/s 300 mV/s

2

0.5 B'

Current/A

1

Current/A

A'

B 0 B

0.0 B -0.5

-1

100 mV/s 150 mV/s 200 mV/s 250 mV/s 300 mV/s

A -2

A

-1.0 -3

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

-2.5

1.0

-2.0

-1.5

-1.0

-0.5

0.0

0.5

Potential/V(vs.Pt)

Potential/V(vs.Pt)

Fig. 3 CV results of LiF-DyF3 (a) and LiF-DyF3-Dy2O3 (1 wt%) (b) at 1243 K, with scan rates of 0.1, 0.15, 0.2, 0.25, and 0.3 V/s; Working electrode (WE): W wire; Auxiliary electrode (AE): W rod; Quasi-reference electrode RE): Pt (b)

(a)

-0.08

-0.04

-0.10

-0.06

-0.12

Ip/A

Ip/A

-0.08 -0.10 -0.12

-0.14 -0.16 -0.18

-0.14

-0.20

-0.16 0.25

-0.22 0.30

0.35

0.40

0.45

0.50

0.55

0.30

0.60

0.35

0.40

0.45

0.50

0.55

Square root of scan rate (V/s)1/2

Square root of scan rate (V/s)1/2 -0.05

-0.33

-0.06

-0.34

-0.07

-0.35

-0.08 -0.36

Ep/V

Ep/V

-0.09 -0.10 -0.11 -0.12 -0.13

-0.38 -0.39 -0.40

-0.14 -0.15 -1.1

-0.37

-0.41 -1.0

-0.9

-0.8

-0.7

-0.6

-0.5

-0.4

-1.0

-0.9

log(v/(V/s))

-0.8

-0.7

-0.6

-0.5

log(v/(V/s))

-0.7

-0.65 -0.70

-0.8

-0.75

log(|Ip|/A)

log(|Ip|/A)

-0.9 -1.0 -1.1 -1.2 -1.3

-0.80 -0.85 -0.90 -0.95 -1.00 -1.05 -1.10

-1.4 -0.14

-0.12

-0.10

E/V

-0.08

-0.06

-1.15 -0.41

-0.40

-0.39

-0.38

-0.37

-0.36

-0.35

-0.34

E/V

Fig. 4 (a) Cathodic current peak, Ip, vs the square root of sweep rates, v1/2, and cathodic potential peak, Ep, vs logarithmic scan rate v (lgv) derived from the cyclic voltammograms of LiF-DyF3 melts with different scan rates (b) Ip vs v1/2 and Ep vs lgv derived from the CV curves of LiF-DyF3–Dy2O3 (1 wt%) melts with different scan rates

3

A'

LiF-DyF3 LiF-DyF3-Dy2O3

2

Current/A

1

B' 0

B

0.3

LiF-DyF3-Dy2O3

(-0.050V,0.233A) B

0.2

-1 I/A

0.1

0.0

-2

-0.1

A

-0.2 -1.0

B' (-0.355V， -0.045A) -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

E/V

-3 -2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

Potential/V(vs.Pt)

Fig. 5 CV results for pure LiF-DyF3 melts and LiF-DyF3-Dy2O3 (1 wt%) melts. WE: W; temperature: 1243 K; scan rate: 0.1V/s 1.0

A' 0.5

Current/A

B' 0.0

B -0.5

100 mV/s 150 mV/s -1.0

200 mV/s

A

250 mV/s 300 mV/s -1.5 -1.6

-1.2

-0.8

-0.4

0.0

0.4

Potential/V(vs.Pt)

Fig. 6 CV curves of the LiF-DyF3 system with different scanning rates at 1243 K, WE: Cu LiF-DyF3

A' 0.4 C' B'

0.0

Current/A

LiF-DyF3-Dy2O3

B

C

-0.4

A

-0.8

-1.2 -1.6

-1.2

-0.8

-0.4

0.0

0.4

Potential/V(vs.Pt)

Fig. 7 CV curves for pure LiF-DyF3 melts and LiF-DyF3-Dy2O3 (1wt%) melts. WE: Cu; Temperature: 1243 K; Scan rate: 0.1 V/s

0.5 -0.184

100 mV/s 150 mV/s 200 mV/s 250 mV/s 300 mV/s 350 mV/s

0.3

Current/A

-0.188

Potential/V

0.4

0.2

-0.192

-0.196 y = a + b*x

Equation

Intercept Slope

B

-0.20147 -0.0475

-0.200 -0.4

-0.3

-0.2

-0.1

0.0

lg((Ip-I)/I)

0.1 0.0

-0.1 -0.2 -0.3 -0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

Potential/V(vs.Pt)

Fig. 8 CV results from the LiF-DyF3-Dy2O3 (1 wt%) system with different scanning rates at 1243 K, scanning potential runs from 0.1 to –0.7 V and back again; WE: Cu 0.035

﹣0.38V ﹣0.39V ﹣0.40V ﹣0.41V ﹣0.42V

0.030

﹣i/(A·cm2)

0.025 0.020 0.015

I

0.010

Jm

Ⅱ

0.005

Ⅱ

0.000

tm

-0.005 0

1

2

3

4

5

Time/s

Fig. 9 Potentiostatic current-time transients of LiF-DyF3 at different overpotentials ﹣0.38V ﹣0.39V ﹣0.40V ﹣0.41V ﹣0.42V

0.15

Current/A

0.10

0.05

0.00

-0.05

-0.10 -0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

t1/2/s1/2

Fig. 10 I vs i1/2 in the region Ⅱ shown in Fig. 9

1.0

(I/Im)2

0.8

0.6

0.4 ﹣ 0.40V ﹣ 0.41V

0.2

﹣ 0.42V

Instantaneous 0.0

Progress 0.0

0.5

1.0

1.5

2.0

2.5

t/tm

Fig. 11 Non-dimensional plots of the corrected (I/Im)2 vs t/tm at different overpotentials in region Ⅱ and region Ⅱ -0.2

(a)

﹣0.124A

-0.2

(b)

﹣0.34A

A -0.4

﹣0.31V

-0.4 -0.6

Potential/V

-0.6

Potential/V

﹣0.37V

-0.8 -1.0 ﹣1.10V

-1.2

-0.8

B ﹣0.88V

-1.0 -1.2

C -1.4

-1.4

-1.6 0.0

-1.6 0.0

0.5

1.0

1.5

2.0

2.5

Time/s

3.0

﹣1.4V

0.5

1.0

1.5

2.0

2.5

Time/s

Fig. 12 OCP curves for W (a) and Cu (b) electrodes in the LiF-DyF3 melts at 1243 K

Fig. 13 Binary phase diagram for the Dy-Cu alloy28

Fig. 14 Digital image of alloy sample

3.0

Sample XRD Cu5Dy(PDF00-043-1393)

Intensity/a.u

Cu(PDF00-001-1241)

20

40

60

80

100

2θ / (°)

Fig. 15 XRD pattern of the sample alloy

Fig. 16 SEM images of grain boundary region at different magnifications

Fig. 17 SEM images of grain boundary region (a) and element distributions of Cu (b), Dy (c), O (d), and C (e) in the Cu-Dy alloy sample

Point 1 Point 2

1000

Cu 800

Cu

cps / eV

600

400

O Dy

200

Dy

Dy Dy

0 0

5

10

15

20

Energy / keV

Fig. 18 EDS spectra at points 1 and 2 Table 3 Composition at points 1, 2 (EDS spectra) and bulk for the Dy-Cu alloy sample Molar ratio (%) Position Cu

Dy

C

O

Point 1

94.09

1.34

0

4.57

Point 2

15.35

77.40

0

7.25

Bulk

48.85

45.42

0.04

4.39

Graphical abstract: 3 A' 2

LiF-DyF3

Sample XRD

LiF-DyF3-Dy2O3

Cu5Dy(PDF00-043-1393)

Relative Intensity/a.u

Cu(PDF00-001-1241)

Current/A

1 B' 0 B (-0.050V,0.233A) B

LiF-DyF3-Dy2O3 0.2

I/A

-1

0.0

-2

A B' (-0.355V，， -0.045A)

-0.2 -1.0

-3 -2.5

-0.5

0.0

0.5

E/V

-2.0

-1.5

-1.0

-0.5

Potential/V(vs.Pt)

0.0

0.5

20

40

60

80

100

2θ / (°)

Electrochemical behavior of dysprosium (Dy) ions in LiF-DyF3 (24 mol%) was investigated by different electrochemical methods at W and Cu electrodes. Dy-Cu alloy samples were prepared by constant-potential electrolysis in LiF-DyF3 (24 mol%) at the Cu electrode.

Conflict of interest statement We declare that we have no financial and personal relationships with other people or organizations that can inappropriately influence our work, there is no professional or other personal interest of any nature or kind in any product, service and/or company that could be construed as influencing the position presented in, or the review of, the manuscript entitled