In order to investigate the influences of the additive (KF) on electrochemistry and deposit morphology of Pr, various electrochemical techniques were used to comparative investigate the electroreduction potential and diffusion coefficient of Pr3+ and kinetic properties of Pr3+/Pr in LiCl-KCl-PrCl3 before and after the addition of KF at different molar concentration ratio of F− to Pr3+ (k). Cyclic voltammetry, square wave voltammetry and reverse chronopotentiometry results showed that the value of k (k = 0, 1, 2, 3 and 4) had no effect on reduction mechanism of Pr3+. With the increase of k, the reduction peak potential moved in the negative direction, the diffusion coefficient decreased, and diffusion activate energy increased. Meanwhile, the exchanged current densities (j0), charge transfer resistances (Rct), and activate energies (Ea) were measured at different k by linear polarization technique, which illustrated that with the augment of k, j0 gradually reduced, and Ea and Rct increased. Furthermore, the electrochemical preparation of Pr aided by KF was explored by potentiostatic electrolysis at different k, and the products were characterized by XRD, SEM and EDS, which indicated that with the increase of k, the morphology of metallic Pr changed from slender needles to granular.
Research Article
Electrochemical formation of Pr aided by additive (KF) in LiCl-KCl molten salt
https://doi.org/10.21203/rs.3.rs-4199388/v1
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Additive
KF
diffusion coefficient
exchanged current density
morphology of deposit
The utilization of nuclear energy to generate electricity can reduce the greenhouse effect, harmful gas emissions and energy consumption. Therefore, nuclear energy plays a very important role in the energy development process [1, 2]. In order to assure the sustainable development of nuclear energy, the reprocessing of spent fuel is of great important step in the utilization of nuclear resource. Dry reprocessing technologies using molten salt as medium show a great of application advantages [3]. At present, dry reprocessing technology includes molten salt electrorefining [4–6], oxide electrodeposition [7–9], reduction extraction using molten salt and liquid metal [10, 11], as well as fluoride volatilization[12]. Among them, molten salt electrorefining, involving in anodic dissolution [13–15] and cathodic electrodeposition processes [16–20], has the advantages of short process flow, small amount of radioactive waste, and low critical risk, and is considered to be the most promising dry reprocessing technology for spent fuel. In order to improve uranium utilization and reduce the volume of nuclear waste, the electrochemical extraction of uranium and rare earth elements (REs) from LiCl-KCl molten salt was studied [16–21]. Since Pr is a typical fission product, in order to extract and separate Pr from molten salt, some scholars have studied the electrochemical reduction of Pr. For example, Li et al. studied the electrode process dynamics of Pr3+ on W electrode and calculated the diffusion coefficient of Pr3+ in eutectic LiCl-KCl was 1.04×10− 5 [20]. Liu et al. explored the electrochemical behavior of Pr3+ on W and Al-Zn electrodes and estimated the diffusion coefficient of Pr3+ on W electrode and Al-Zn electrode was 2.02×10− 5 and 4.53×10− 5, respectively [21]. Yin et al. studied the electrochemical behavior of Pr3+ on Mo electrode and Ni electrode at different temperatures. The diffusion activation energy was computed to be 34.35 kJ mol− 1[22]. Castrillejo et al. explored the electrochemical properties of Pr3+ in the molten mixture of LiCl-KCl and CaCl2-NaCl by using different electrochemical methods. The diffusion coefficient of Pr(III) ions was measured to be on the order of 10− 5 [23].
During the electrolytic recovery of uranium and REs, a dendrite morphology of electrodeposits was observed on the cathode, which may cause a short circuit between the grown dendrite and the anode or part of the dendrites falling into the molten salt, resulting in the decrease of recovery efficiency of uranium and REs [19]. It is reported that the deposits morphology can be changed by changing the composition of the electrolyte salt, adding additives and changing the ratio of additives to metal ions [24]. Yao et al. investigated the effect of NaI as an additive on the electrodeposition of Al coating in AlCl3-NaCl-KCl molten salt. It is found that that the addition of NaI can promote the formation of Al coating and improve the morphology of electrodeposition [25]. Kong et al. reported that the introducing F− ion into the molten salt, with the continuous increase of fluoride ions, the nucleation mode of Ti changed from progressive nucleation to instantaneous nucleation, and the overpotential of Ti deposition to shift to the negative direction [26]. The presence of F− might stabilize the high oxidation state Ti(III) in the fluoro-titanium complex state. Polyakova et al. found that the reduction mechanism of Zr(IV) was changed from Zr(IV)→Zr(II)→Zr(0) to Zr(IV) + 4e− =Zr in NaCl-KCl-ZrCl4 molten salt with the introduction of F−[27]. Wang et al. explored the impacts of F− on electrochemical reduction mechanism of Zr(IV) and nucleation mechanism of Zr. They found that the nucleation mechanism of Zr was changed, and with increase of the molar ratio of F− to Zr(IV), the diffusion coefficients of Zr(IV) decreased[28]. Jiang et al. investigated the competitive coordination of Cl− and F− towards La3+, Nd3+ and Sm3+ in LiCl-KCl-LnCl3(Ln = La, Nd, Sm)-LiF molten salts by using spectroscopic and computational approaches. They found that four distinct species, [LnCl6]3−, [LnCl5F]3−, [LnCl4F2]3− and [LnCl4F3]4−, were formed, resulting in the reduction potential of Ln3+ negatively shift[29, 30]. Liu et al. investigated the influence of LiF on the reduction mechanism and coordination properties of uranium in LiCl-KCl molten salt. They found that F− participated in coordination to make the U(IV) complex more stable, resulting in that the equilibrium potential of U(IV)/U(III) moved negatively, closer to that of U(III)/U[31]. However, there have been few studies on finding suitable additives to improve the electrodeposition of U in LiCl-KCl molten salts.
In order to avoid the formation of dendrite and increase of the recovery efficiency, it is necessary to study the impact of additives on the electro-reduction mechanism and morphology of deposits. In order to avoid the introduction of other hetero ions besides F− into LiCL-KCl, LiF and KF can be used as candidate additives. Therefore, in this study, KF was selected as additives to investigate the effect of F− on electrochemistry and electrodeposition morphology of Pr. Cyclic voltammetry, square wave voltammetry, reverse chronopotentiometry, linear polarization techniques were employed to compare the change of electroreduction potential, equilibrium potential, diffusion coefficient of Pr3+ and kinetic properties of Pr3+/Pr with the successive addition of KF. Further, the potentiostatic electrolysis was conducted in LiCl-KCl-PrCl3-KF molten salts at different molar concentration ratio of F− to Pr3+, the products were characterized by SEM-EDS and XRD to study the electrochemical formation of Pr aided by additive KF.
2.1 Molten salt preparation
LiCl and KCl (anhydrous, > 99.9%) were bought from Tianjin Kemiou Chemical Reagent Co. Ltd., and PrCl3 (99.9%) and KF (anhydrous, 99.9%) were bought from High Purity Chemical Co., Ltd. And Shanghai Aladdin Practical Co. Ltd., respectively. Due to the hygroscopicity of LiCl, KCl and LiCl were vacuum dried at 673 K for 24 hours and then put into a drying oven for later use. According to the molar ratio of KCl to LiCl was 2:3, KCl and LiCl were mixed and placed in an Al2O3 crucible to melt. In order to reduce the influence of water and impurities during the experiment, pre-electrolysis was used to remove water and impurities before the experiment. PrCl3 and KF were added in the melt as Pr3+ and F− ion sources, respectively. In each experiment, KF was directly added to the melt to obtain different concentration ratios of F− to Pr3+. For convenience of expression, the concentration ratio of F− to Pr3+ is defined as k. Due to the hygroscopicity of the salt, all operations were conducted in a glove box filled with Ar (< 10 ppm H2O and O2).
2.2 Electrode preparation and electrochemical equipment
A three-electrode system was assembled in a LiCl-KCl-PrCl3-KF molten salt electrolyte for electrochemical testing with Autolab (PGSTAT 302N, Metrohm, Switzerland) and Nova 1.10 software. The working electrode is tungsten wire (Φ = 1 mm, 99.99%), and the surface area of the tungsten wire was computed by the depth of insertion into the molten salt. A silver (Ag) wire (Φ = 1 mm, 99.99%) inserted in LiCl-KCl-AgCl (CAgCl=1wt%) molten salt contained in a corundum tube was served as reference electrode. To prevent the presence of F− from interfering with Ag/AgCl electrode, all potentials in this work reference to Li(I)/Li couple. The counter electrode was graphite electrode (Φ = 6mm, 99.9%).
2.3 Products characterization
After potentiostatic electrolysis at -2.2 V, the obtained samples were successively polished with 1000# and 3000# SiC sandpaper, and then placed in a glove box for further analysis. SEM-EDS (scanning electron microscope, JEOL Co., Ltd., JSM-6480A) with an energy-dispersive spectrometer (Bruker XFlash Detector 6/60, Germany) and XRD (X-ray diffraction, Rigaku RAXIS-RAPID IP) were used to characterize and analyze the electrolysis products.
3.1 Electrochemical behavior of Pr3+ after the addition of KF
3.1.1 Electroreduction mechanism of Pr3+ after the addition of KF
Figure 1(a) exhibits the cyclic voltammograms recorded in LiCl-KCl molten salt before and after adding PrCl3 (6.60×10− 5 mol cm− 3) at 773 K and scan rate of 0.10 V s− 1. In the dotted line measured in LiCl-KCl blank salt, a couple of peak of R1/O1 can be observed at -0.04 V/0 V, which pertains to the formation and dissolution of metallic Li. After adding PrCl3 into the molten salt (blue solid line), in addition to the reduction/oxidation peak R1/O1, a new couple of reduction/oxidation peaks R2/O2 appears at 0.4 V/0.49 V, pertaining to the formation and dissolution of Pr metal. It can be found that only a pair of new reduction and oxidation peaks appears after the addition of PrCl3, it is inferred that the formation and oxidation reactions of Pr on W electrode are a one-step three-electron reaction.
$$P{r^{3+}}+3{e^ - } \rightleftharpoons Pr$$ 1
After adding KF (k = 1) in the molten salt (red line), obviously, the cyclic voltammogram shows a reduction/oxidation peak, which is consistent with blue line. It is inferred that the reduction mechanism of Pr3+ ions is a one-step process with exchanged 3-electron.
To investigate the impact of KF on the reduction mechanism at different molar concentration ratio of F− to Pr3+(k), the number of exchanged electrons (n) for reversible system can be estimated by SWV technique as follows [32]:
n = 3.52RT/FW1/2 (2)
where R represents the gas constant, J mol− 1 K− 1; T denotes the temperature, K; F is the Faraday constant, C mol− 1; W1/2 designates the peak width at half, mV.
Figure 2 shows the square wave voltammograms measured at frequency of 30 Hz and different k (k = 0, 1, 2, 3, 4). Clearly, the reduction peak potential shifts negatively with the increase of k. The calculated n is listed in Table 1, which shows that n is close to 3, indicating that after the addition of KF in the molten salt, the reduction of Pr3+ ions is still one-step 3-electron process.
Figure 3(a) shows the reverse chronopotentiogram of Pr3+ recorded in LiCl-KCl-PrCl3 molten salts at ± 30 mA and 773 K. There are two potential plateaus in the reverse chronopotentiogram appear at 0.391 V and 0.467 V, respectively, ascribed to the reduction and oxidation of Pr3+ ions. The transition times related to the reduction and oxidation reactions are obtained to be τ1 = 1.11 s and τ2 = 1.13 s by making a tangent, as shown in Fig. 3(a). The transition times of the reduction and the oxidation processes were almost equal, indicating that the reduction process of Pr3+ ions is a one-step 3-electron transfer process. Figure 3(b) shows the reverse chronopotentiogram of Pr3+ ion recorded in LiCl-KCl-PrCl3-KF molten salts at k = 1 and ± 30 mA. The transition times related to the reduction and oxidation processes are obtained to be τ1 = 1.08 s and τ2 = 1.13 s, which indicates that the reduction process of Pr3+ is a one-step three-electron transfer process. The result can further confirm that the addition of KF has no impact on electrochemical reduction mechanism of Pr3+.
Figure 3(c) presents the chronopotentiograms of Pr3+ ions recorded when KF was added in the molten salt at different k (k = 0, 1, 2, 3, 4). Clearly, with the increase of k, the reduction potentials shift negatively, which is in line with results shown in Fig. 2. The reason for the negative potential shift is that the radius of F− is smaller than Cl−, and the complexing ability of F− with Pr3+ ions is stronger than Cl−, so F− can replace Cl− around Pr3+ ions and form a Pr-F bond. Since the bond energy of the Pr-F bond is larger than that of the Pr-Cl bond, the formation of Pr requires greater energy, resulting in a negative shift in the peak potential of the deposition.
3.1.2 Kinetic properties of Pr3+/Pr couple after the addition of KF
The effect of KF on diffusion coefficient of Pr3+ was investigated. Figure 4 shows cyclic voltammograms of LiCl-KCl-PrCl3-KF molten salts at different molar concentration ratio of F− to Pr3+ (k = 0, 1, 2, 3, 4) and scan rates. With the increase of scanning rate, the anodic/cathodic peak current increases and peak potential moves to positive/negative direction. As the increase of k, peak current decreases, and peak potential moves negatively.
Figure 5 shows the changes of the peak current and potential of Pr3+/Pr at different scanning rates and k. As can be senn from Fig. 5a that with the increase of k, the peak potential of R2 moves negatively; when the scanning rate less than 1.0 V s− 1, the peak potential of R2 hardly changes with logv, indicating the reduction of Pr3+ is reversible process. When the scanning rate is larger than 1.0 V s− 1, the peak potential moves negatively, indicating the reduction of Pr3+ is quasi-reversible process at different k (k = 0, 1, 2, 3, 4). The cathodic peak current (Ipc) decreases with the increase of k, and Ipc has a good linear relationship with v1/2, showing that the control step is diffusion-controlled process (see Fig. 5b).
Since the reduction of Pr3+ is reversible process at scanning rate less than 1.0 V s− 1, the diffusion coefficient can be measured employing the Berzinse-Delahay equation [33].
$${I_{\text{p}\text{c}}}= - 0.611{\left( {\frac{{{F^3}}}{{RT}}} \right)^{1/2}}{n^{3/2}}{D^{1/2}}CS{v^{1/2}}$$ 3
Where Ipc is the cathode peak current, A; v is the scanning speed, V s− 1, other physical quantities have the same meaning as mentioned above.
The diffusion coefficients of Pr3+ ions at different k are computed according to the formula (3) and presented in Table 1. The results indicate that as the increase of k, the diffusion coefficient of Pr3+ decreases. The reason can be explained as follows: According to formula 3, when other conditions are constant, the diffusion coefficient is proportional to Ipc. Based on the above discussion, Ipc decreases with the increase of k, thus, the diffusion coefficient decreases accordingly.
Cyclic voltammetry was also conducted in LiCl-KCl-PrCl3 molten salt at different k on W electrode in the temperature range of 803–863 K, as shown in S1. The diffusion coefficients of Pr3+ at different k and temperatures are computed and listed in Table 1. The results show that under the same temperature, the diffusion coefficient gradually decreases with augmenting k; and under the same k, the diffusion coefficient gradually increases with the elevating temperature. Compared with the diffusion coefficient of Pr3+ in LiCl-KCl molten salt without adding F−, our results are of the same order of magnitude 10− 5 [20–23]. Moreover, the relationships between lnD and reciprocal temperature at different k are plotted and presented in Fig. 6. The linear relationships at different k are observed and follow Arrhenius's law.
4Where D0 is the pre-exponential factor, Ea denotes the diffusion activation energy.
According to the linear slope, the evaluated diffusion activation energies at different k are also presented in Table 1. Obviously, with the continuous addition of KF, the diffusion activation energy of Pr3+ gradually increases. Compared with the diffusion activation energy of Pr3+ in LiCl-KCl molten salt without adding F−, our result is slightly larger than that estimated by Yin et al. [22].
|
T/K |
D×10− 5/ cm2 s− 1 |
Activation energies /kJ mol− 1 |
||||||
|---|---|---|---|---|---|---|---|---|
|
773 |
803 |
833 |
863 |
|||||
|
0 |
3.60 |
5.33 |
6.32 |
6.55 |
40.944 |
|||
|
1 |
2.58 |
3.99 |
4.53 |
5.9 |
47.575 |
|||
|
2 |
1.54 |
2.26 |
2.84 |
4.33 |
57.247 |
|||
|
3 |
1.49 |
1.83 |
2.35 |
3.99 |
62.908 |
|||
|
4 |
1.22 |
1.79 |
2.33 |
3.95 |
71.621 |
|||
The impact of KF on dynamic properties of Pr3+/Pr couple was investigated using the Butler-Volmer formula[34] when the overpotential ƞ is very small (|η|<10 mV).
5Where j is the current density, j0 represents the exchange current density, A cm− 2; n, F, R and T were mentioned above.
The LP technique based on formula (5) displays a linear relationship of j with ƞ in a small overpotential near the equilibrium potential of Pr3+/Pr couple. Therefore, the exchange current density (j0) can be computed by the linear slope. Meantime, the charge transfer resistance (Rct) can be obtained employing the following expression:
$${R_{{\text{ct}}}}=\frac{{RT}}{{nF{j_0}}}$$ 6
Therefore, LP method was applied to measure j0 of Pr3+/Pr couple. Firstly, metallic Pr was deposited on W electrode at 773 K by applying potential 0.32 V for 30 s at different k. Then, the power was turned out, the electrode was still in the molten salt, and the metallic Pr deposited on W electrode starts to dissolve. During this process, when the composition of the electrode surface is within the range of a two-phase coexisting state, a potential plateau is observed. Thus, the equilibrium potentials of Pr3+/Pr at different k and different temperatures were measured and are listed in Table 2. Obviously, the equilibrium potentials shift to negative direction with the increase of k and to positive direction with the rising temperature. Then, in the over-potential range of ± 5 mV, LP measurement was conducted at scan rate of 5 mV s− 1 and is presents in Fig. 7. The linear relationship of j with ƞ can be observed. Using the linear slope of j with ƞ, j0 can be obtained, and charge transfer resistance is computed employing formula (6). They are listed in Table 2. With the increase of k, the j0 reduces, the charge transfer resistance increases, while with the rising temperature, j0 increases, the charge transfer resistance decreases.
The change of ln(j0) with temperature shows a good linear relationship, as shown in the inset of Fig. 7. According to formula (8), the activation energy Ea of Pr3+/Pr couple at different k on W electrode can be computed. They are also listed in Table 2.
$$\text{l}\text{n}{j_0}=-{E_\text{a}}/RT+\text{l}\text{n}A$$ 8
Where Ea denotes the activation energy, kJ mol− 1; A represents the pre-exponential factor.
|
k |
773 K |
803 K |
833 K |
863 K |
Activation energies /kJ mol− 1 |
||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
|
Eeq/V |
j0/A cm− 2 |
Rct/Ω |
Eeq/V |
j0/A cm− 2 |
Rct/Ω |
Eeq/V |
j0/A cm− 2 |
Rct/Ω |
Eeq/V |
j0/A cm− 2 |
Rct/Ω |
||||||
|
0 |
0.400 |
0.031 |
0.720 |
0.413 |
0.031 |
0.730 |
0.423 |
0.032 |
0.760 |
0.434 |
0.033 |
0.740 |
8.37 |
||||
|
1 |
0.398 |
0.027 |
0.800 |
0.410 |
0.030 |
0.770 |
0.422 |
0.031 |
0.760 |
0.430 |
0.033 |
0.760 |
10.32 |
||||
|
2 |
0.390 |
0.084 |
0.840 |
0.400 |
0.029 |
0.800 |
0.417 |
0.024 |
0.990 |
0.420 |
0.024 |
1.020 |
15.98 |
||||
|
3 |
0.383 |
0.019 |
1.180 |
0.394 |
0.019 |
1.210 |
0.409 |
0.021 |
1.140 |
0.423 |
0.023 |
1.070 |
17.47 |
||||
|
4 |
0.376 |
0.018 |
1.240 |
0.385 |
0.018 |
1.270 |
0.399 |
0.019 |
1.290 |
0.406 |
0.019 |
1.900 |
21.34 |
||||
As can be seen that with the continuous addition of KF, the activation energy of the reaction gradually increases, resulting in a decrease in the reaction rate. The reason why the reaction activation energy gradually increases is that Pr-F bond is more stable than the Pr-Cl bond. The electrodeposition of Pr require higher energy to break Pr-F bonds.
3.3 The morphology of electrodeposited Pr after the addition of KF
The morphology of electrodeposited Pr was explored by potentiostatic electrolysis at -2.2 V for 1800 s on Mo electrode at 773 K before and after KF addition at different k (k = 0, 1, 2, 3, 4) in a LiCl-KCl-PrCl3 molten salt. The deposits were characterized by XRD and SEM-EDS, as shown in Figs. 8 and 9. As can be seen from Fig. 8, the diffraction patterns matches the PDF#02-0692, which means that metallic Pr is formed before and after the addition of different contents of KF. Since the salt attached to the samples was not removed by sandpaper, KCl diffraction peaks are detected in XRD. The diffraction peaks marked with “?” in Fig. 8 were analyzed by JADE 5.0 and its attribution cannot be confirmed.
Figure 9a and b display the SEM and EDS images of the deposit obtained when k = 0. It was observed that the product prepared without the addition of KF shows clear slender needles and accompanied by some small granules. EDS indicates the formation of Pr on Mo electrode, which is in line with XRD patterns (Fig. 9a). After adding KF into the molten salt (k = 1), as shown in Fig. 9c, compared with Fig. 9a, the slender needles decrease and the granular shape appears. EDS result (see Fig. 9d) confirms the formation of metallic Pr. When k = 2, the morphology of Pr is mainly granular, and a small number of needles appear (see Fig. 9e and f). With the increase of k (k = 3–5), as shown in Fig. 9g-j, slender needles gradually disappear, and the size of granules gradually decreases. Obviously, with the continuous addition of KF, the morphology of electrodeposited Pr changes from slender needle-like to granular.
The electrochemical preparation of Pr aided by KF were investigated by a series of electrochemical techniques. The results showed that the addition of KF had impact on the electrochemical deposition potential and kinetic properties of Pr3+/Pr couple, and the reduction mechanism of Pr3+ was still one-step 3-electron process. With the continuous addition of KF, the reduction and equilibrium potentials of Pr3+/Pr(0) continuously shifted to negative direction, and the diffusion coefficient of Pr3+ decreased gradually and the diffusion activation energy increased; and the exchange current density decreased, the reaction resistance and activate energy increased. However, as temperature elevated, the exchange current density increased, the reaction resistance and activate energy decreased. The impact of KF on the morphology of electrodeposited Pr was studied by potentiostatic electrolysis at different k and the deposits were characterized by SEM-EDS and XRD. The morphology of the prepared Pr showed needle-like without adding KF. With the increase of k, the morphology of Pr changed from slender needles to granular. The addition of KF may be inhibit the formation of dendrites.
Author Contribution
Mei Li: Supervision, Writing - review & editing, Investigation.Rui Du: Writing - original draft, Investigation Validation.Hedi Wei: Investigation, Validation.Jiayi Chen: Data curation, Validation.Meng Zhang: Data curation.Rugeng Liu: Formal analysis, Data curation.Wei Han: Resources, Methodology, Supervision, Writing-review & editing.All authors reviewed the manuscript
Acknowledgements
The work was financially supported by the National Natural Science foundation of China (U2167215 and 22076035) and the Fundamental Research Funds for the Central Universities (3072022JC1501).
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No competing interests reported.