Verification of electrodeposition behavior of Ce(III) in terms of its empirical formula in high temperature molten LiCl-KCl

Abstract

In this research, the electrodeposition behavior of Ce(III) in high temperature molten LiCl-KCl was verified with empirically derived correlation formulae for the prediction of charge transfer during the electrodeposition and structural/morphological visualization of electrodeposit on the electrode. A new empirical approach was introduced in order to solve critical and inherent problems raised from relatively large discrepancies and uncertainties of electrochemical properties and a random growth of electrodeposit on the electrode. Electrochemical conditions were defined with experimental parameters, which are explicitly controllable by the user: time, current, concentration, overpotential, and initial surface area of electrode. The empirical correlation successfully described and predicted different behaviors of electrodeposition in terms of the rate of charge transfer at various electrochemical conditions.

In the course of deriving empirical correlations for two different electrochemical techniques, chronopotentiometry (CP) and chronoamperometry (CA), four significant electrochemical features were determined to characterize and predict the electrodeposition: a maximum achievable current in CP, a ratio of current at cathodic peak potential to minimum current in CA, a slope of CA, and a concentration for limiting current condition. The empirical correlation formulae were then derived by tracing electrochemical plots during the electrodeposition and by investigating collection efficiencies. The correlation successfully predicted the electrodeposition with high precisions for concentrations less than 1.3 wt%. Electrodepositions at concentrations larger than 1.3 wt% showed relatively larger errors, because this region was not in limiting current condition and was highly influenced by the growth of surface area due to a largely induced current in the beginning.

Different types of electrodeposition were defined with boundary conditions of concentration and overpotential. The distinct characteristics during the growth of electrodeposits were clearly visualized in a real-time, for the first time, with an optically accessible window. The transient state of electrode was seen and matched with electrochemical signals during electrodeposition to identify critical electrochemical conditions for different structures of electrodeposits. $80 mA/cm^2$ of current density was a boundary condition for CP experiments differentiating electrodeposits between mossy structure and dendritic structure. 0.4 wt% and 1.3 wt% and the location of overpotential were boundary conditions of concentrations for different structures of electrodeposits (mossy, dendritic, and large hat-like) in CA experiments. In addition, as a state of the art technique, x-ray and computed tomography were applied to morphological analysis of electrodeposit enabling the selective analysis of pure metal deposit from the mixture of metal and co-captured salt. Reconstructed image from the radiography and computed tomography was then successfully utilized to quantitatively analyze the volume of target deposit structure in a non-destructive way.

In addition, the applicability of this empirical approach was tested with other similar lanthanides (La(III), Pr(III), Gd(III)) and uranium(U(III)). It was clarified in the research that the electrodeposition highly depends on the limiting current condition and diffusion coefficients. However, it was not able to clearly define the relationship between electrodeposition behaviors and electrochemical properties due to large uncertainties and discrepancies in reported data. Instead, four lanthanides exhibited similar electrochemical characteristics, while U(III) showed totally different characteristics, which make the relative prediction of electrodeposition behavior more complex. Therefore, it is recommended to apply this approach only for lanthanides in a qualitative way, because it is hard to define a definite value of the electrochemical properties.

This study aims at the development of empirical correlations with an experimental approach to understand and verify the electrodeposition behavior of Ce(III). Experimental parameters were selected to concisely describe various electrochemical conditions, and based on those parameters, the empirical correlations were successfully developed. The two states-of-the-art techniques were utilized for structural and morphological analysis, and various types of electrodeposits and required electrochemical conditions for their formations were confirmed. It was also verified that this empirical approach is possible to predict electrodeposition behaviors of other elements with their unique properties. Finally, it is expected to refer to or to widely utilize for the new probable prospects in determining the surface area of the transient electrode and in updating the working electrode information in simulations in the high temperature molten salt electrodeposition.