Electrochemical Nucleation and Growth of Actinides on Tungsten Electrode from Molten Salts

Chronomamperometry (CA) was used to study the nucleation and growth mechanism of actinides (e.g. U, Pu) on a tungsten electrode. CA is a method widely used to study nucleation phenomena in aqueous or molten media. The objective of this work is to fundamentally characterize the initial stages of metal phase formation during electrodeposition of actinides from molten salt at inert electrodes in order to enhance the understanding of the interfacial electrochemistry at solid electrodes and to obtain kinetic parameters. 

Current-time (i-t ) curves measured at different fixed potentials were used to study the mechanism of the nucleation process and to determine quantitatively magnitude of many of its parameters (e.g. nucleation and growth rates, nucleation sites, nuclei density, diffusion coefficient, etc.). The shape of the current transients is associated with the nucleation and growth of a metal covering a foreign substrate. An example of current transients for various potential pulses in molten LiCl/KCl containing U³⁺ ions is shown in Figure 1. Each transient can be divided into three zones. The first zone (1) represents the double-layer charging current and the formation of the first nuclei on the electrode. This is followed by an increase in the current due to the total number of nuclei and growth of independent nuclei (2). This rise culminates in a maximum, the position of which depends on the magnitude of the potential step and is a characteristic of nucleation processes. Finally, in part 3, the slow decrease in current corresponds to limitation by semi-infinite linear diffusion of the metal ions to the electrode surface. From the experimental i-t curves recorded at different overpotentials, dimensionless i-t curves were constructed by plotting the current parameters (I/I_m; I_m≡ current maximum) vs. the time parameter (t/t_m; t_m≡ the time at which the maximum is reached).

These curves were compared to theoretical models for the instantaneous and progressive limiting cases for three-dimensional nucleation and growth described by Scharifker and Hills.^1,2 In the instantaneous case, all nuclei are created at the same moment at the beginning of the electrolysis. In the progressive case, the rate of nucleation is slow and continues to take place at the surface while other clusters are growing².

 It was found that the nucleation mode of actinides depends on concentration of their ions in the electrolyte and changes from progressive to instantaneous as their concentration increases. At lower concentrations the nucleation is progressive, with nuclei continuing to form as U and Pu deposit on the cathode. At higher concentration, however, the nucleation is instantaneous, with an initial rapid formation of nuclei followed by continued deposition only at those initial nucleation sites.

In addition to defining the nucleation mode, additional parameters were found from the current-time transients: (diffusion coefficient, number of nuclei created, nuclei density, nucleation rates, etc). These parameters were obtained at different salt compositions and different magnitudes of the overpotentials and the results were compared.

References:

1. G. Hills and B.J. Scharifker, J. Electroanal. Chem. 130 (1981) 81.
2. B. Scharifker and G. Hills, Electrochim. Acta 28 (1983) 879.

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See more of: Physical and Analytical Electrochemistry General Session V
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