# Electrochemical Properties of NdCl3 and CeCl3 in Molten LiCl-KCl Eutectic Salt
## 3 Results and Discussion  
### Determination of the Diffusion Coefficient of CeCl\({}_{3}\) and NdCl\({}_{3}\) in LiCl-KCl  
The diffusion coefficient of Ce\({}^{3+}\)|Ce were calculated based on the modified Randles-sevcik equation according to the work of [PERSON] 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.  
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\({}_{\text{CeCl}3}=8.626\times 10^{-3}\)). These results obey the following the Arrhenius' law, which is:  
\[D=D_{0}\;exp(-\frac{E_{a}}{RT}) \tag{2}\]  
where \(E_{a}\) is the activation energy. The linearity of the evolution of \(\ln(D)\) versus \(1/T\) is observed in Figure 4 with these results, which can be expressed as:  
\[\ln D=\frac{-9.16}{T}+4.1394 \tag{3}\]  
where \(D\) is in cm\({}^{2}\)/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. 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 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. These peaks can likely be associated with the Nd3+/Nd2+ 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]. \[I_{p}=-0.6105 nFSC_{0}(\frac{nF}{RT}^{1/2})D^{1/2}v^{1/2} \tag{4}\]  
where \(S\) is the electrode area in cm2, \(C_{0}\) is the solute concentration in moles/cm3, \(D\) is the diffusion coefficient in cm2 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 Nd3+ ions in the solution.