Electrochemistry of Iodide in LiCl-KCl Molten Salts and Anionic Chemla Effect: An Overview
Authors: Shrestha Nikunja , Day Brandon , Utgikar Vivek , Raja Krishnan S. , Fredrickson Guy , Frank Steven 
Published: February 2020

Several radioactive iodine isotopes are formed as fission products in the nuclear fuel, and retained within the fuel matrix by the fuel cladding as a containment. During reprocessing of the used fuels by the pyroprocessing route, a significant fraction of iodine was reported to be retained in the molten salt as iodide (I-). High concentration of iodine leads to significant deterioration of the Pt anode during electrolytic reduction of the oxide spent fuels. [[i]] The electrochemistry of iodine/iodide couple has been extensively studied in aqueous and non-aqueous conditions because of its application in dye-sensitized solar cells, and synthetic chemistry.

The physical and electrical properties of the molten salt systems depend on the structure and interactions of the constituents. The structural arrangements that are present in molten salts can be viewed as intermediates between discrete chemical bonds and periodic crystalline lattices[[ii]]. The inter-atomic interactions determine the local ordering of the molten salt. The electrochemical properties are significantly influenced by the structural characteristics of the molten salt. The ion size and type in a molten salt system such as KX-LiX would affect the electrochemical stability. The ordering and structure of the melt could be described based on the entropy. The entropy (ΔS) of the electrolysis of melt can be determined using the relation:

ΔS = nF(∂E/∂T)P (1)

Where, n = number of electrons, (∂E/∂T)P = change in electrochemical window with temperature at constant pressure. Low entropy values indicate higher order and enhanced attractive interaction of the species in the molten salt. Generally, an increase in the entropy was observed with increase in the anion size of unary molten salts[[iii]]. The electrolysis potential turns out to be more anodic as the anion size increases due to weaker Coulombic interaction between the anion-cation pairs as well as increased repulsion between the larger anions[[iv]]. When two types of cations are present in molten salt with different sizes and different charge densities, an asymmetric polarization of anions is anticipated which may result in the electrostatic stability of the mixture. When two different types of anions such as chloride and iodide are present along with different cations, the asymmetric polarizations of the ions and their effect on the physical and electrochemical properties are not well documented.

The Chemla effect[v] refers to a phenomenon where the internal mobility of a larger cation is higher than that of a smaller cation at high concentrations of the larger cation in a binary salt mixture such as LiCl-KCl that consists of large and small cations with a common anion. This effect could lead to compositional gradient of ions in an electrochemical cell where a local shift in the composition from liquidus range may cause solid precipitation of salt[[vi]]. Chemla effect has been reported for ternary cation systems such as (Na,K,Cs)Cl [[vii]] and also for anions in Li(Cl,NO3)[[viii]]. Presence of iodide at above a threshold concentration could result in anionic Chemla effect where the internal mobility of iodide could be higher than that of chloride. The other possible scenario could be at low concentration of chloride, the mobility of chloride could be larger than that of iodide, as observed in the Li(Cl,NO3). However, in normal pyroprocessing condition, the chloride concentration will not be lower than that of iodide.

The redox reactions of iodide are given as:

2I- ↔ I2 + 2e- (2)

3I- ↔ I3
- + 2e- (3)

With the chloride addition, the redox reactions are given as:

I- + 2Cl- ↔ [ICl2]- + 2e- (4)

I3
- + 6Cl- ↔ 3[ICl2]- + 4e- (5)

This presentation will give an overview of the electrochemistry of iodide in the LiCl-KCl molten salt system that is relevant to the pyroprocessing of used nuclear fuels based on the existing database from the published literature, and new preliminary results obtained at the University of Idaho in collaboration with the Idaho National Laboratory.

[i] S. M. Frank, P.K. Tripathy, S.D. Herrman, Global 2013, Salt Lake City, Sep.29 – Oct. 3, 2013

[ii] J.D. Martin, S.J. Goettler, N. Fosse, L. Iton, Nature, 419 (2002) 381

[iii] M. Chemla, I. Okada, Electrochimica Acta 35 (1990) 1761

[iv] I.K. Delimarskii, B.F. Markov, Electrochemistry of Fused Salts, The Sigma Press Publishing, Washington, DC, 1961

[v] J. Pdrie' and M. Chemla, C. R. Acad. Sci. 250, 3986 (1960).

[vi] R. Takagi, H. Shimotake, K. J. Jensen, J. Electrochem. Soc., 131 (1984) 1280

[vii] M. Matsumiya, H. Matsuura, R.Takagi, Y. Okamoto, Journal of The Electrochemical Society,
147 (11) 4206-4211 (2000)

[viii] A. Endoh, I.Okada, J. Electrochem. Soc., Vol. 137, No. 3, March 1990