A Promising Crystalline KCl: Electrolyte Material for Studying the Electrochemical Properties of Cerium on Liquid Indium Electrodes

Abstract

Potassium chloride crystals are good conductors in the UV to mid-infrared broadband and can be widely used as materials for high-power CO₂ lasers. However, the most important use of potassium chloride crystals has been overlooked; their stable chemistry and wide electrochemical window allow them to form molten eutectic salts with other chlorides for the dry reprocessing of spent fuel. The aim of this work is to investigate the electrochemical properties of the actinide substitute cerium metal on liquid indium media in order to provide basic data for further realization of lanthanides (Ln)/actinides (An) separation. In this paper, we investigated the electrochemical behavior of 3LiCl-2KCl-CeCl₃ melt at liquid metal media indium (In) electrode using various transient electrochemical analysis methods, such as cyclic voltammetry, square wave voltammetry and open circuit potential technique. The quasi-equilibrium potentials of Ce³⁺ at different temperatures on the liquid metal In electrode were determined. On the redox process, the reaction kinetics of the oxidation–reduction process in the 3LiCl-2KCl-CeCl₃ system is studied. It was determined that the reduction of Ce³⁺ at the liquid metal In electrode is an irreversible single-step process with three electron transfers. The relationship between the diffusion coefficient and the reduction process of Ce³⁺ ions on liquid metal indium electrodes at different temperatures was determined. The expression for the diffusion coefficient and temperature of Ce³⁺ at the liquid metal indium electrode is: lnD = 1.43 − 7974.69/T.

1. Introduction

The continuous exploitation and use of fossil fuels have caused serious and difficult to treat environmental pollution problems. In recent years, efforts have been made to develop environmentally friendly and renewable clean energy sources to replace traditional fossil fuels, such as wind [1,2], solar-based (photovoltaics [3,4,5,6] or photocatalysis [7,8,9,10,11,12,13]), hydrogen-based (fuel cell [14,15] and electrocatalysis [16,17,18]), batteries [19,20], supercapacitors [21,22], thermoelectric [23,24,25,26,27] and nuclear energy [28,29]. More realistically, nuclear energy remains the most promising clean energy source for efficient energy supply and large-scale utilization [30,31]. Given the historical implementation of nuclear energy, we have full confidence in its use to replace organic fuels such as coal, oil and natural gas [32]. However, with the rapid development of nuclear energy, the radioactive materials in spent reactor fuel have increased, so the effective reprocessing of spent fuel has become the most difficult issue globally [33,34]. Therefore, finding and developing promising chloride and fluoride molten salt systems and determining the basic physicochemical properties of molten salt systems is one of the important ways to achieve breakthroughs [33,35]. It is widely believed that KCl crystals are good conductors in the broadband from ultraviolet to mid-infrared, and are broadly used for infrared spectroscopy and for manufacturing ultraviolet and infrared optical components [35]. At the same time, these crystals are also an important material for thin film substrates, especially for “film-substrate” separation applications, where these water-soluble substrates can be easily dissolved with water, leaving only the thin film [36]. More interestingly, KCl crystals have been found to be an extremely promising material for the formation of molten eutectic salt systems with other chlorides and for dry reprocessing of spent fuel [35,36,37].

Recently, molten salts have attracted considerable attention as the most promising novel technology among the many reaction media for separating Ln and An, and most importantly for their ability to efficiently process radioactive waste and spent nuclear fuel [38,39,40]. High-temperature thermochemical methods using molten salts and liquid metals in the reprocessing of spent nuclear reactor fuel are being investigated [41]. Researchers use liquid metals to selectively extract fissile elements dissolved in molten salts for the purpose of separation [42]. The chemical properties of the molten salt and the liquid metal determine the selectivity of the thermochemical separation process occurring between the interfaces of the two phases [41,43,44,45,46,47,48,49,50]. In addition, the nature and composition of the studied melt systems play a decisive role in the selective separation process of the fission products. Understanding the thermodynamic properties of fission elements in SNFs is essential for their applicability in practical separation processes [32,51,52,53]. The currently proposed method for the separation of fission elements at liquid electrodes is effective and novel, but previous work on the electrode separation of indium in liquid metal is scarce [54]. In this work, the electrochemical properties of the fissile element cerium (Ce) in spent fuel are emphasized. Ce is the most important fissile element in the lanthanide family and is chemically similar to An. Therefore, Ce has been widely used as a substitute for the behavior of the element An [55]. More importantly, despite its chemical similarity to many fissile elements, cerium metal is one of the most dangerous elements among fission products and, in addition, it is a neutron poison [56,57].

In order to improve the separation effect regarding An and Ln elements, some experts have conducted basic research on the application of Al, Bi and Cd cathodes; however, the high melting point of metal Al (660 °C) renders it difficult to realize the application of liquid cathodes [58,59,60,61]. Therefore, considering that indium metal is a liquid at low temperatures and located in the same main group as Al, it should have similar chemical properties and is expected to replace the Al electrode as a liquid metal cathode. The content of this chapter proposes to study the electrochemical behavior of Ce³⁺ on liquid metal indium in eutectic salt LiCl-KCl melt by using transient electrochemical methods such as cyclic voltammetry, square wave voltammetry and open circuit potential technique to provide basic parameters for the application of In as liquid metal in molten salt dry reprocessing process.

2. Experimental Section

A low eutectic mixture of LiCl-KCl (2:3 wt.%) was first dehydrated under vacuum drying oven at 473 K for more than 24 h to remove adsorbed water and then melted under a dry alumina crucible in a quartz glass capsule. Pre-electrolysis at 2.10 V for 4 h was used to remove impurities. CeCl₃ (99.9 wt.%) after chlorination by CCl₄ was used as a source of Ce³⁺ and added directly to the crucible containing the LiCl-KCl eutectic salt. All experiments were performed under argon atmosphere to prevent environmental O₂ or H₂O interference. The electrolytic cell is an alumina or glassy carbon crucible placed in a cylindrical quartz vessel with a placed eutectic salt. The experiments were performed under an inert argon atmosphere (99.999%). The cells are heated using a vertical heater connected to the programmable device to control the temperature of the heater to ±8 K. The operating temperature is measured using a NiCr thermocouple.

The experiments in this chapter employ a three-electrode system to study the electrochemical behavior of Ce³⁺ on indium cathode, the active liquid indium electrode is used as the working electrode, and the liquid In electrode is made by placing the liquid indium in a 15 mm × 20 mm alumina crucible with a mass of 5 g of indium. The specific electrochemical redox process is shown schematically in Figure 1. The reference electrode is composed of corundum tube containing silver wire (d = 1 mm, 99.99% pure silver) and AgCl (1 wt.%, molar fraction of 0.0039) of corundum tube, and the ratio of salt in the reference electrode was KCl-LiCl (3:2 wt.%). The silver wire was polished with SiC sandpaper and inserted into the melt to obtain a more accurate chemical reaction. CeCl₃ with a concentration of about (2.80 wt.%) and In 4 g were simultaneously placed in a crucible containing KCl-LiCl eutectic salt and the furnace temperature was raised to 823 K. The auxiliary electrode was composed of a 3 mm glassy carbon rod (SU-2000). The electrode potentials during the experiments were used (vs. Ag/AgCl) as a reference.

Electrochemical measurements including CV, SWV, and OCP methods were used to study the electrochemical behavior of Ce on liquid metal indium electrodes. The electrochemical data obtained were measured and analyzed using a PGSTAT 302N electrochemical workstation (Autolab, Metrohm) and controlled by Nova 1.8 software package. The Ce-In alloy was obtained by constant potential electrolysis at the indium electrode at a selected applied potential. After the experiment, a small amount of sample was dissolved in nitric acid, and the cerium (uranium) concentration in the molten salt was determined by ICP-MS test. The indium containing the deposited cerium metal was washed and quantita-tively dissolved in nitric acid or perchloric acid. An ICP-MS test was subsequently con-ducted to obtain the alloy composition and determine the concentration of cerium on the metal indium.

3. Results and Discussion

The comparative cyclic voltammetry on the molybdenum electrode and the liquid indium metal electrode after the addition of Ce³⁺ to the molten eutectic LiCl-KCl melt at 723 K are shown in Figure 2, which clearly reflects the redox processes occurring at the different electrodes. Regarding the cyclic voltammetry of redox reactions occurring at the molybdenum electrode, the potential control ranges from −2.7 to 0 V, approximately at potentials of −2.6 V and −2.4 V representing the reduction and dissolution of lithium. When scanning in the positive direction, the Cl⁻ undergoes oxidation to form Cl₂ around +0.8 V. However, in this experiment, we chose to avoid scanning in the positive direction in order to prevent Cl₂ from affecting the normal operation of the working electrode, so as not to produce a large amount of Cl₂ on the surface of the working electrode. In addition to the precipitation of lithium metal in the black curve, a new pair of redox peaks appears in the curve, corresponding to the redox process of cerium, where Ce is also precipitated at a potential near −2.10 V and the oxidation peak potential is near −1.95 V, as shown in Figure 2. On the other hand, no other peak signals were observed before the precipitation of lithium metal, indicating that the redox process was a one-step reaction and that no co-deposition of Ce³⁺ and Li⁺ ions occurred. It can be seen that the peak shape corresponding to the redox of cerium metal on a molybdenum electrode is characterized by a typical electrochemical process of first generating solid-phase material on a heterogeneous inert electrode and then stripping and dissolving the deposited material into solution [61]. For the CV occurring at the indium metal electrode, redox peaks appear at −1.472 V and −1.183 V. This phenomenon is mainly attributed to the deposition of Ce³⁺ on the liquid metal indium electrode forming an In alloy and the oxidation of the alloy to Ce³⁺. In addition, the CV of Ce³⁺ at the liquid metal indium electrode observed only a pair of redox peaks, so the reduction of Ce³⁺ at the liquid metal indium electrode is a one-step reduction reaction with the number of transferred electrons (n) of 3. The calculated values of αn are listed in Table 1.

The peak potential and peak current measured by CV versus sweep rate are important techniques to determine the reversibility of the reaction process, and systems in which the peak potential does not vary with the sweep rate usually exhibit reversibility. In this paper, the mechanism of the Ce³⁺/Ce reaction in an insoluble–soluble system is investigated by analyzing the CV at different potential scanning rates. The natural logarithmic function of the peak cathode potential and the scan rate showed regular changes, and the type of reaction control could be judged. The CV on the liquid metal indium electrode in LiCl-KCl-CeCl₃ (3.0 wt.%) melt at different scan rates are shown in Figure 3a. We clearly find that the cyclic voltammetry in the figure gives mainly three pairs of redox peaks, where peaks M and M¹ correspond to the redox potential of the In electrode. Peaks F and F¹ correspond to the redox potential of Ce on the liquid metal In electrode. L and L¹ correspond to the deposition and dissolution processes of Li on the In electrode. Here, we focus on the redox process of Ce on a liquid metal In electrode. As the scan rate increases from 50 mV·s⁻¹ to 200 mV·s⁻¹, the cathode peak potential changes significantly, with the potential moving from −1.472 V to a more negative −1.502 V. At the same time, the peak current shifts gradually in a more negative direction with the increase in scan rate. Figure 3b shows the curve of the cathode peak potential (E_P,C) versus the logarithm of the sweep speed (lgv), from which it can be seen that the cathode peak potential and the sweep speed are linearly related, so it can be determined that the redox reaction of Ce³⁺ on the liquid metal indium electrode is irreversible. Moreover, both the results we obtained and those reported in the literature exhibit irreversible reactions at liquid metal gallium electrodes [62].

For irreversible reactions, the value of the electron transfer coefficient (α) can be calculated from Equation (1).

(1)$E_{P,C}-E_{P/2,C}=-1.86\frac{RT}{\alpha nF}$

• where: E_P,C—peak potential, V

• E_P/2,C—half-peak potential, V

• n—number of electrons transferred during the reaction.

The transfer of Ce³⁺ ions in the melt to the indium electrode occurs solely via diffusion, and under the conditions where no other chemical reactions occur, it can be determined that the peak cathode current is proportional to the flux through the surface of the liquid metal electrode. In order to calculate the diffusion coefficient of Ce³⁺ at the liquid metal indium electrode in the molten system of LiCl-KCl, this chapter is calculated by derivation of equation.

(2)$I_P=-0.495 8nFSc{D}^{\frac{1}{2}}{v}^{\frac{1}{2}}{(\frac{\alpha nF}{RT})}^{1/2}$

• where: I_P—peak current of cathode, A

• S—Surface area of liquid metal indium electrode, cm²

• F—Faraday constant, 96,485 C-mol⁻¹

• c—Molar concentration of Ce³⁺ in LiCl-KCl melt, mol-cm⁻³

• R—Ideal gas constant, 8.314 J/(mol·K)

• T—Thermodynamic temperature of the reaction in the LiCl-KCl molten salt system, K

• D—Diffusion coefficient, cm² s⁻¹.

Figure 4a represents the relationship curves between the peak cathode current and the square root of the sweep rate at different temperatures. From Figure 4a, it can be seen that the peak current value of the cathode is linearly related to the square root of the sweep rate, and this result can indicate that the mass transfer during the reduction of Ce³⁺ on the surface of the liquid metal indium electrode to form the Ce-In alloy is controlled by diffusion. The obtained experimental numbers were substituted into Equation (2), and the diffusion coefficients of Ce³⁺ on the liquid metal indium electrode in LiCl-KCl melts at different temperatures were calculated and included in Table 2.

The activation energy of Ce³⁺ diffusion in the LiCl-KCl melt into the liquid metal indium electrode is calculated according to the Arrhenius formula.

(3)${\ln D=\ln D}_0=-E_a/RT$

The relationship equation between diffusion coefficient and activation energy can be derived from Equation (3), and the corresponding results are obtained by fitting the calculated data using the software OriginPro 9.64, and the relationship curve between diffusion coefficient and temperature is depicted by Figure 4b.

According to Figure 4b, the expression of the diffusion coefficient of Ce³⁺ at the metal indium electrode versus temperature is:

(4)$\ln D=1.43 - 7974.69/T$

The results show that the diffusion coefficient of Ce³⁺ in the LiCl-KCl melt at the liquid indium metal electrode increases with an increase in the experimental temperature within the temperature range of 723–823 K.

We also measured the equilibrium electrode potential of the Ce-In alloy using the OCP technique method. Figure 5a shows the equilibrium electrode potential versus time for a liquid indium working electrode (S = 0.38 cm²) in a molten eutectic salt 3LiCl-2KCl-CeCl₃ (2.4 wt.%) melt after a short-time polarization in the temperature range of 723–823 K.

The plateau that appears on the curve can be clearly seen in Figure 5a. The potential plateau corresponds to the composition of the electrode surface in a two-phase coexistence state, i.e., the quasi-equilibrium potential of the formed Ce-In alloy. Our clear hairline shows a plateau near −1.4 V. The diffusion of the deposited metal generates stable compounds as it reacts with the deposit, representing the formation of a Ce-In metal compound. In addition, the equilibrium potential of the solid Ce and liquid-metal-indium alloys shifts in a positive direction as the temperature increases. Finally, we find that the plateau is reproduced around −0.5 V, which is attributed to the residual potential of the indium electrode. Our results share similar characteristics with those reported in the literature for the generation of intermetallic compounds by Ce-Ga. The difference in potential is due to the difference in equilibrium potential generated by different intermetallic compounds [62].

The square wave voltammetry potential waveform is a superposition of a step wave reference potential and a bidirectional potential pulse. In addition, it constitutes the background current of the impurity redox reaction Faraday current, which is effectively deducted by differential subtraction. SWV is one of the most sensitive methods for transient electrochemical testing. With the property of eliminating capacitance and residual current, square wave voltammetry has been widely used in studies exploring the electrochemical behavior of lanthanides and actinides. In this paper, the SWV of Ce³⁺ in LiCl-KCl melt over liquid metal indium electrode were tested using different frequencies. We also confirmed the electrochemical process of intermetallic compound formation by the co-reduction of Ce³⁺ ions in the LiCl-KCl-CeCl₃ molten salt system containing liquid metal indium electrodes from electrochemical methods such as CV and OCP. Figure 5b shows the SWV of Ce³⁺ at the liquid indium electrode at different frequencies (5 to 30 Hz). We found only one reduction peak in the range of −1.37 V to −1.4 V, which further indicates that the reduction of Ce³⁺ to αIn₆Ce is a one-step process of transferring three electrons. Meanwhile, the reduction peak potential shifts from −1.389 V to −1.394 V as the frequency increases from 5 Hz to 30 Hz, indicating that the reduction of Ce³⁺ in this system is irreversible.

4. Conclusions

In this paper, the electrochemical behavior of cerium on liquid cathode indium in a molten eutectic salt 3LiCl-2KCl-CeCl₃ system was studied, and the redox processes on liquid metal indium electrode were investigated using CV, SWV and OCP techniques, respectively. The quasi-equilibrium potentials of Ce³⁺ at different temperatures on the liquid metal In electrode were determined. The results show that the reduction of Ce³⁺ at the liquid In metal electrode in the 3LiCl-2KCl-CeCl₃ system at a temperature of 773 K occurs at about −1.4 V and is a one-step reduction process with three electron transfers. The electrochemical reduction of Ce³⁺ at the liquid indium metal electrode is irreversible and controlled by mass transfer. In addition, the diffusion coefficients at 723–823 K were calculated as 0.6512 × 10⁻⁵, 0.9105 × 10⁻⁵, 1.2032 × 10⁻⁵, 1.4517 × 10⁻⁵, and 1.7326 × 10⁻⁵ cm² s⁻¹, respectively. The diffusion coefficient and temperature of Ce³⁺ at the liquid metal In electrode are: lnD = 1.43 − 7974.69/T.

Author Contributions

L.Q. and W.H. conceived the idea and supervised the project. M.X. Investigation, Writing—original draft, Formal analysis. M.X. and Y.D. Investigation. Y.Z. and Y.J. Software with the help of H.L., X.L. and L.Q. wrote the manuscript with input from all authors. All authors have read and agreed to the published version of the manuscript.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The supporting data are available from the corresponding authors on reasonable request.

Conflicts of Interest

The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Figures and Tables

Figure 1. Schematic diagram of the equilibrium potential determination of Ce3+ at liquid indium metal (In) electrode in the LiCl-KCl eutectic salt system.

Figure 2. Comparison between cyclic voltammetry of LiCl-KCl-CeCl3 melt at 723 K at molybdenum and liquid indium metal electrodes, respectively; sweep rate: 100 mV s−1.

Figure 3. (a) Cyclic voltammetry of LiCl-KCl-CeCl3 (3.0 wt.%) molten salt system at liquid metal indium electrode at 773 K at different sweep rates. Scan rate: 0.05–0.2 V-s−1, (b) Relationship between the peak cathode potential and the logarithm of the scan rate.

Figure 4. (a) Relationship between the peak cathode current and the square root of the scan rate in the cyclic voltammetry of 3LiCl-2KCl-CeCl3 across different temperatures at the liquid metal indium electrode in LiCl-KCl melts. Scanning speed: 0.05–0.2 V-s−1, (b) Diffusion coefficient of Ce3+ on liquid metal indium electrode versus temperature in LiCl-KCl melt at different temperatures.

Figure 5. (a) Potential energy (vs. Ag/AgCl) versus time in 3LiCl-2KCl-CeCl3 melts at different temperatures on liquid metal indium electrodes after short-time polarization under inert atmosphere conditions. Current—80 mA; Temperature: 1—723 K; 2—753 K; 3—773 K; 4—803 K; 5—823 K. (b) SWV curves of Ce3+ on liquid metal indium electrode.

Table 1

Values of peak cathodic potential, half-front potential and αn of Ce³⁺ on liquid metal indium electrode in the molten salt system of LiCl-KCl-CeCl₃ (3.0 wt.%) at 773 K.

v/(mV·s−1)	EP,C/V	EP/2,C/V	EP/2,C/V − EP,C/V	α
50	−1.472	−1.342	0.130	0.953
75	−1.480	−1.347	0.133	0.931
100	−1.487	−1.351	0.136	0.912
150	−1.492	−1.353	0.139	0.892
200	−1.503	−1.360	0.143	0.867

Table 2

Diffusion coefficients of Ce³⁺ ions in LiCl-KCl melt at different temperatures at liquid metal indium electrodes.

T(K)	723	753	773	803	823
D × 10−5/cm2 s−1	0.6512	0.9105	1.2032	1.4517	1.7326

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