#
Electrochemical Properties of NdCl_{3} and CeCl_{3} in Molten LiCl-KCl Eutectic Salt

^{*}

## Abstract

**:**

_{3}and CeCl

_{3}are characterized by the very close proximity of their standard reduction potentials. Thus, they represent the most challenging salt mixtures with respect to decoupling electrochemical responses and calculating concentrations. The goals of this study include the determination of the apparent standard potentials of both Nd

^{3+/}Nd

^{2+}and Ce

^{3+}/Ce in LiCl-KCl-CeCl

_{3}or NdCl

_{3}mixtures. Matrix and concentration effects on fundamental electrochemical properties are of particular interest, as these thermodynamic properties will impact the ability to use voltammetry to measure concentrations in unknown samples. Several LiCl-KCl-CeCl

_{3}or NdCl

_{3}mixtures with NdCl

_{3}/CeCl

_{3}concentrations ranging from 4 to 10 wt% at 753 to 793 K have been explored. For apparent standard potential values of CeCl

_{3}, a good agreement was obtained with the literature data, except above 773 K.

## 1. Introduction

_{3}[8]. Thermodynamic and transport properties of binary LnX3-MX systems have been investigated widely [9,10,11], where Ln = Ce, Nd, La, Pr, Gd; M = Li, Na, Cs, and X = F, Cl, I, and Br. The addition of lanthanide chloride to alkali metal chloride results in the formation of a variety of stoichiometric compounds. The formation of compounds and complexes in the molten salt system affects the electrical conductivity and other thermo-physical properties [12]. Recently, Castrillejo and coworkers reported the electrochemical behavior of a series of lanthanide elements in LiCl-KCl eutectic melt in the temperature range of 400–550 °C [7]. Most of the reports available in the open literature pertain to the electrochemical behavior of individual element of the lanthanide series at dilute concentrations [12]. Iizuka investigated chronopotentionmetry to determine diffusion coefficient of CeCl

_{3}at various temperature. Marsden and Pesic measured apparent standard potentials and diffusion coefficients of CeCl

_{3}by cyclic voltammetry methods [13,14]. Additionally, Novoselova conducted the emf method to determine the apparent standard redox potential of NdCl

_{3}in fused LiCl-KCl eutectic at a different temperature [15,16,17]. However, the effect of the presence of multiple components along with moderate to high concentrations in the same electrolyte has not been largely investigated in detail. Furthermore, the thermodynamic properties of lanthanides in the LiCl-KCl are not known at various temperatures. In this report, we present results of an investigation of individual species electro-and thermo-physical properties of Nd

^{3+}/Nd and Ce

^{3+}/Ce in LiCl-KCl eutectic molten salt. The effects of NdCl

_{3}and CeCl

_{3}concentrations on the diffusion coefficient, activity coefficient and activation energy are currently being studied to enhance the fundamental knowledge of how electrochemical behavior, specifically for CeCl

_{3}or NdCl

_{3}contents ranging from 4 wt% to 10 wt% in LiCl-KCl eutectic salt at 753 to 793 K. Moreover, in ternary systems, there are various concentration-dependent effects that have not been previously reported in literature. Therefore, the main objective in this study is to explore electrochemical and thermodynamic behaviors of cerium (Ce) and neodymium (Nd) in high concentration ranges (4 wt% to 10 wt%) of LiCl-KCl eutectic salt by utilizing electrochemical transition techniques, such as cyclic voltammetry (CV) and chronopotentionmetry (CP).

## 2. Experimental

#### 2.1. Electrochemical Apparatus and Electrodes

_{3}and CeCl

_{3}with concentrations ranging from 4 to 10 wt%. All the electrochemical measurements were carried out with Autolab/PGSTAT128N potentiostat. The data acquisition was controlled by NOVA 2.0 software for cyclic voltammetry. A three-electrode electrochemical cell was assembled in an alumina crucible, which was positioned in a graphite vessel and heated in an electric furnace. A chromel–alumel thermocouple was used for temperature measurement with an accuracy of ±1 °C. The reference electrode (RE) was self-assembled by a silver wire (1 mm diameter, 99.99% purity). The reference electrode consisted of a closed-end porous quartz tube, in which the LiCl-KCl eutectic salt containing 5 mol% AgCl was placed, and an Ag wire of 1.0 mm diameter was immersed into the molten bath. A glassy carbon rod of 2.0-mm diameter was used as a counter electrode (CE). The surface area of the glassy carbon counter electrode in contact with the salt phase was about 43.1 mm

^{2}. The working electrode (WE) consisted of a metallic tungsten (W) wire (1.0-mm diameter, 99.9% purity) dipped into a molten salt bath (Figure 1E).

#### 2.2. Determination of Active Electrode Area

^{2}. This active surface was determined by the following equation [18].

^{2}) [19], density (𝜌) of eutectic LiCl-KCl (1.62 g/cm

^{3}) [19] and the equation derived for a meniscus on a small cylinder. In order to maintain the same electrode area, the electrode was kept at a constant depth to the LiCl-KCl eutectic salt after each experiment. All of the electrochemical experiments were carried out in a glovebox with a high-purity argon gas atmosphere (H

_{2}O, O

_{2}< 0.1 ppm) in the set up shown in Figure 1.

## 3. Results and Discussion

#### 3.1. Determination of the Diffusion Coefficient of CeCl_{3} and NdCl_{3} in LiCl-KCl

^{3+}|Ce were calculated based on the modified Randles–sevcik equation according to the work of Hamel and co-workers [20]. Figure 2A shows representative cyclic voltammograms of a LiCl-KCl-4 wt% CeCl

_{3}solution using a tungsten working electrode. A single, large peak is observed with the anodic peak being slightly larger and steeper than the cathodic peak, which is typical of a soluble-insoluble transition. As expected, peaks increase in magnitude as the scan rate increases. The electrochemical reversibility of the CeCl

_{3}redox reaction was confirmed using CV. The reversibility of the redox reaction can be confirmed by plotting the cathodic peak currents as a function of the square root of the scan rate. In such a plot, a linear relationship between the peak currents vs. square root of the scan rate establishes reversibility. This relationship can be seen in Figure 2B. Figure 3A shows the diffusion coefficient of CeCl

_{3}as a function of temperature, as calculated based on the Randles–sevcik equation. The diffusion coefficient varies 1.53% based on concentration and 3.2% based on temperature. Figure 3B. shows the diffusion coefficients match well with literature values at 753–763 K.

^{2}/s and T in K for the 4 wt% CeCl

_{3}case. From this relationship, the value of the activation energy is found to be 76.15 kJ/mol, and, as expected, the activation energy decreased as the CeCl

_{3}concentration increased. It is expected that at a high concentration of CeCl

_{3}, there is enough analyte in solution to fully cover the working electrode. In the case of 4 wt% NdCl

_{3}in LiCl-KCl solution, Figure 5 shows a series of CVs at different scan rates. While the large, sharp reduction and oxidation peaks look similar to the soluble–insoluble transition peaks seen in the CeCl

_{3}studies, several broader peaks just to the right of the large peaks can be seen.

^{3+}/Nd

^{2+}soluble-soluble redox transition. The sharper peaks should be correlated with the potential scanning rates via the Berzins–Delahay relationship, reliable for a reversible soluble/insoluble systems [21,22,23].

^{2}, C

_{0}is the solute concentration in moles/cm

^{3}, D is the diffusion coefficient in cm

^{2}s

^{−1}, F is the Faraday constant (96,485 C), n is the number of exchanged electrons, v is the potential scanning rate in Vs

^{−1}and T is the temperature in K. The linearity of I

_{p}versus v

^{1/2}proves that the electrochemical reduction process is controlled by the diffusion of Nd

^{3+}ions in the solution. Figure 6 indicates the influence of the temperature on the value of diffusion coefficient from performing a series of cyclic voltammograms at several concentrations. At least 2.2% of the difference in the diffusion coefficient as a function of NdCl

_{3}concentration and temperature, which can be up to 3.6%, can be seen in this graph. The resulting plot in Figure 7 obeys Arrhenius’ law and can be further expressed as follows for the 4 wt% NdCl

_{3}case:

#### 3.2. Determination of the Apparent Standard Potential of CeCl_{3} and NdCl_{3} in LiCl-KCl

_{0}= −1.90132 V vs. Ag

^{+}|Ag reference scale (E

_{0}= −3.0475 V vs. Cl

^{−}|Cl

_{2}). The same methods were used to create a Nernst plot as a function of temperature. Previous studies are shown for comparison, showing good agreement between these data sets, as shown Figure 8B. Using the slope from this plot, n was calculated to be 2.93, which is very close to the expected value of 3.0. In the case of Nd, due to the coexisting Nd

^{2+}and Nd

^{3+}oxidation states in LiCl-KCl eutectic molten salt, the usual method of calculating the apparent standard potential of the Nd

^{3+}+ 3e

^{−}$\overline{)\leftrightarrow}$ Nd redox couple can no longer be applied since the exact amount of Nd

^{3+}in the melt is unknown. However, a method applied to americium, which has +2 and +3 oxidation states, in a journal article by Fusselman and colleagues [22], can be used to provide a route to estimate the standard potential of both the +2 and +3 redox couple. The derivation is as follows:

^{2+}+ 2e

^{−}$\overline{)\leftrightarrow}$ Nd, $\Delta {G}_{N{d}^{2+}}^{{0}^{\prime}}$ + RTln (${X}_{N{d}^{2+}})+2\Delta {G}_{{e}^{-}}=\Delta {G}_{Nd(0)}$

^{3+}+ e $\overline{)\leftrightarrow}$ Nd

^{2+}, $\Delta {G}_{N{d}^{3+}}^{{0}^{\prime}}$ + RTln (${X}_{N{d}^{3+}})+\Delta {G}_{{e}^{-}}=\Delta {G}_{N{d}^{2+}}$

^{3+}+ 3e

^{−}$\overline{)\leftrightarrow}$ Nd, $\Delta {G}_{N{d}^{3+}}^{{0}^{\prime}}$ + RTln (${X}_{N{d}^{3+}})+3\Delta {G}_{{e}^{-}}=\Delta {G}_{Nd(0)}$

^{+}Oxidation: ln (${X}_{N{d}^{2+}})=\frac{2F(E-{E}_{N{d}^{2+}}^{{0}^{\prime}})}{RT}$

^{2+}and Nd

^{3+}are unknown, these variables must be written in terms of total Nd ions, Nd

^{Ox}, which is known:

^{+}ions to the total oxidized ions in the molten salt. Combining the equations:

^{+}state is found by measuring the equilibrium potential of the cyclic voltammograms attained at each concentration and making a Nernst equation, as shown in Figure 9. The intercept of this plot is the apparent standard potential of the 2

^{+}state, ${E}_{N{d}^{2+}}^{{0}^{\prime}}$ = −1.8582 V vs. Ag+|Ag reference (−3.0042 vs. Cl

^{−}|Cl

_{2}), which can then be used to calculate the $\mathsf{\epsilon}$.

#### 3.3. Determination of the Activity Coefficient of CeCl_{3} and NdCl_{3} in LiCl-KCl

^{−}|Cl

_{2}reference scale, the above equation can be written as:

_{3}, and NdCl

_{3}were found in a paper by Patrick Masset and coworkers [24]. The Gibbs energy of formation of MClx in the supercooled states was derived from the crystal data (cr) of MClx (cr), the enthalpy of fusion (Δ

_{fus}H) and the heat capacity of the liquid phase (C

_{p}) [25] according to the below Equation.

_{3}and NdCl

_{3}as a function of temperature are then given in Table 1.

## 4. Conclusions

_{3}and NdCl

_{3}in the molten LiCl-KCl eutectic for pyroprocessing technology. The determination of the reaction mechanism was primarily investigated with cyclic voltammetry and was confirmed using Nernst plots. The results include the apparent standard potential, diffusion coefficient and the activity coefficient in the molten salt considered. The free Gibbs energies of formation, and the activity coefficients were derived from the electrochemical measurements. Apparent standard potential was determined through plotting the equilibrium potential, determined via cyclic voltammetry, of each concentration measurement against the natural logarithm of the concentration. The intercept of this Nernst plot was defined as the apparent standard potential. For apparent standard potential values of CeCl

_{3}, a good agreement was obtained with the literature data, except above 773 K. The activity coefficient can then be calculate based on the experimentally determined apparent standard potential and standard-state Gibb’s free energy.

## Author Contributions

## Funding

## Conflicts of Interest

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**Figure 1.**Experimental setup and instruments. (

**A**) Ar atmosphere glove box. (

**B**) Schematic diagram of experimental apparatus. (

**C**) Oxygen and moisture monitoring. (

**D**) Autolab/PGSTAT128N potentiostat (

**E**) Electrode setup in glove box.

**Figure 2.**(

**A**) Cyclic voltammogram of 4 wt% Ce

^{3+}/Ce in LiCl-KCl eutectic salt at 773K. (

**B**) Dependence of the current versus the square root of the scan rate according to the changing temperature in LiCl-KCl-CeCl

_{3}(X

_{CeCl3}= 8.626 × 10

^{−3}).

**Figure 3.**(

**A**) Diffusion coefficient of Ce

^{3+}as a function of CeCl

_{3}concentration and temperature in LiCl-KCl. (

**B**) Comparison of values from literature.

**Figure 4.**(

**A**) Linear relationship of the logarithm of CeCl

_{3}diffusion coefficient vs. the reciprocal of the absolute temperature. (

**B**) Activation energy as a function of CeCl

_{3}concentration.

**Figure 5.**Cyclic Voltammograms of 4 wt% NdCl

_{3}in LiCl-KCl eutectic molten salt with scan rate; 50, 100, 200, 300 mV/s at 773 K.

**Figure 7.**(

**A**) Linear relationship of the logarithm of NdCl

_{3}diffusion coefficient vs. the reciprocal of absolute temperature. (

**B**) Activation energy as a function of NdCl

_{3}concentration.

Elements | T/K | E^{0}/V vs. Ag/AgCl | E^{0}/V vs. Cl_{2}/Cl^{−} | $\Delta {G}_{MC{l}_{x}}^{{0}^{\prime}}$/kJmol^{−1} [24,25,26] | Activity Coefficient |
---|---|---|---|---|---|

Ce^{3+}lCe | 753 | −1.9246 | −3.0708 | −890.1 | 3.012 × 10^{−3} |

763 | −1.9183 | −3.0645 | −887.9 | 3.179 × 10^{−3} | |

773 | −1.9013 | −3.0475 | −884.2 | 3.222 × 10^{−3} | |

783 | −1.8739 | −3.0201 | −877.3 | 3.474 × 10^{−3} | |

793 | −1.8621 | −3.0083 | −871.4 | 3.7013 × 10^{−3} | |

Nd^{2+}lNd | 753 | −1.895 | −3.0412 | − | − |

763 | −1.882 | −3.0282 | − | − | |

773 | −1.858 | −3.0042 | − | − | |

783 | −1.851 | −2.9972 | − | − | |

793 | −1.843 | −2.9892 | − | − | |

Nd^{3+}lNd | 753 | −1.989 | −3.1352 | −923.9 | 8.973 × 10^{−5} |

763 | −1.983 | −3.1302 | −914.7 | 9.732 × 10^{−5} | |

773 | −1.978 | −3.1242 | −901.2 | 2.142 × 10^{−4} | |

783 | −1.956 | −3.1022 | −898.0 | 3.245 × 10^{−4} | |

793 | −1.945 | −3.0912 | −879.5 | 3.843 × 10^{−4} |

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**MDPI and ACS Style**

Kim, S.; Lee, S.-h.
Electrochemical Properties of NdCl_{3} and CeCl_{3} in Molten LiCl-KCl Eutectic Salt. *Appl. Sci.* **2020**, *10*, 7252.
https://doi.org/10.3390/app10207252

**AMA Style**

Kim S, Lee S-h.
Electrochemical Properties of NdCl_{3} and CeCl_{3} in Molten LiCl-KCl Eutectic Salt. *Applied Sciences*. 2020; 10(20):7252.
https://doi.org/10.3390/app10207252

**Chicago/Turabian Style**

Kim, Seunghyun, and Sang-hwan Lee.
2020. "Electrochemical Properties of NdCl_{3} and CeCl_{3} in Molten LiCl-KCl Eutectic Salt" *Applied Sciences* 10, no. 20: 7252.
https://doi.org/10.3390/app10207252