Electrochemistry of thorium in LiCl-KCl eutectic melts

Article abstract of DOI:10.1016/j.electacta.2007.06.022

This work presents a study of the electrochemical properties of Th chloride ions dissolved in a molten LiCl-KCl eutectic, in a temperature range of 693-823 K. Transient electrochemical techniques such as cyclic voltammetry, chronopotentiommetry and chronoamperometry have been used in order to investigate the reduction mechanism on a tungsten electrode and the diffusion coefficient of dissolved Th ions. All techniques showed that only one valence state was stable in the melt. The reduction into Th metal was found to occur according to a one-step mechanism, through a nucleation-controlled process which requires an overpotential of several 100 mV. At 723 K, the diffusion coefficient is D_{Th(723 K)} = 3.15 ± 0.15 × 10^-5 cm² s^-1. EMF measurements indicated that, at 723 K, the standard apparent potential is E_{ThC l4 / Th}^{* 0} (723 K) = -2.582 V versus Cl₂/Cl^-, and the activity coefficient γ_{ThC l4} (723 K) = 4.6 × 10^-4 on the mole fraction scale (based on a pure liquid reference state).

Full text of DOI:10.1016/j.electacta.2007.06.022

Electrochimica Acta 52 (2007) 7432–7437
Electrochemistry of thorium in LiCl–KCl eutectic melts
L. Cassayre, J. Serp, P. Soucek, R. Malmbeck∗, J. Rebizant, J.-P. Glatz
European Commission, JRC, Institute for Transuranium Elements, P.O. 2340, 76125 Karlsruhe, Germany
Received 28 February 2007; received in revised form 24 May 2007; accepted 9 June 2007
Available online 15 June 2007
Abstract
This work presents a study of the electrochemical properties of Th chloride ions dissolved in a molten LiCl–KCl eutectic, in a temper-
ature range of 693–823 K. Transient electrochemical techniques such as cyclic voltammetry, chronopotentiommetry and chronoamperometry
have been used in order to investigate the reduction mechanism on a tungsten electrode and the diffusion coefficient of dissolved Th ions. All
techniques showed that only one valence state was stable in the melt. The reduction into Th metal was found to occur according to a one-
step mechanism, through a nucleation-controlled process which requires an overpotential of several 100 mV. At 723 K, the diffusion coefficient is
∗0
DTh(723 K) = 3.15 0.15 × 10−5 cm2 s−1. EMF measurements indicated that, at 723 K, the standard apparent potential is E
(723 K) = −2.582 V
ThCl /Th
versus Cl2/Cl−, and the activity coefficient γThCl (723 K) = 4.6 × 10−4 on the mole fraction scale (based on a pure liquid r4eference state).
4
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Molten chlorides; Thorium; Transient electrochemistry; EMF measurements
1. Introduction
[4], while others mentioned that the only stable valence state is
Th(IV) [5–7].
In the objective of the reprocessing of spent nuclear fuels,
pyrochemical techniques are investigated world-wide in molten
chlorides and fluorides salts to set-up processes for grouped
separation of actinides from the fission products. In this con-
text, basic electrochemical properties of actinides (An) were
reassessed at the Institute for Transuranium Elements, and up to
now the behaviour of U, Pu, Np and Am cations was studied in
the LiCl–KCl eutectic molten salt [1–3]. Transient electrochem-
ical techniques were used to determine the diffusion coefficient
of the dissolved actinide ions, as well as their redox potentials.
Basic thermochemical properties (Gibbs energy and activity
coefficient) were also determined in the 673–823 K temperature
range.
The electrochemical behaviour of Th ions dissolved in the
same chloride melt is reported in the present paper. Although
Th is not a fission product, this work aims at completing the
thorough study of An in LiCl–KCl eutectic melt. A literature
review indicated that the electrochemical behaviour of dissolved
Th was not yet fully established in the LiCl–KCl eutectic, mostly
because some authors proposed the existence of Th(II) cations
An electrochemical study was then performed at different
temperatures and concentrations in order to determine the basic
properties of Th ions in the chloride melt, and to evaluate the
reduction mechanism into Th metal on an inert W electrode.
2. Experimental
The experiments, storage and handling of all chemicals were
carried out in a glove box in purified Ar atmosphere, equipped
with a well-type oven.
Almost 1 g of metallic Th was alloyed with 26.5 g of Bi and
placed in an Al2O3 crucible together with 43.9 g of LiCl–KCl
eutectic (Aldrich 99.99%). The electrolytic bath was prepared
by oxidising the Th with a known amount of BiCl3 dissolved
in the salt, as described in [1]. Five different Th concentrations
were prepared by successive additions of BiCl3.
Two or three salt samples of about 100 mg were drawn out at
each concentration, dissolved and diluted in nitric acid. The con-
centrations of Th were determined both by inductively coupled
plasma mass spectrometry (ICP-MS) and X-ray fluorescence
(XRF) analysis. The analysed concentrations were: 0.20, 0.43,
0.74, 0.96 and 1.46 wt.%.
∗
The electrochemical measurements (cyclic voltamme-
try, chronopotentiommetry, chronoamperommetry, open-circuit
Corresponding author. Tel.: +49 7247 951 376; fax: +49 7247 951 99611.
E-mail address: rikard.malmbeck@ec.europa.eu (R. Malmbeck).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.06.022

L. Cassayre et al. / Electrochimica Acta 52 (2007) 7432–7437
7433
potential) were carried out with a PAR 273 potentiostat. Inert
working electrodes were prepared using 1 mm W wire. The ref-
erence electrode was composed of an Ag wire dipped into a
LiCl–KCl–AgCl (1 wt.%) mixture, contained in a Pyrex glass
tube. The auxiliary electrode was a 1 mm Mo spiral-wire.
The ohmic resistance of the electrolyte was determined by
impedance spectroscopy at 733 K: it was found to be 0.41 ꢀ, and
this value was considered constant on the whole range of temper-
ature (693–823 K). The ohmic drop correction was performed
using the relation:
Ecorr = Emeas − ReI
(1)
Fig. 2. Cyclic voltammograms (corrected for ohmic drop) at scan rates from 50
to 300 mV s−1 obtained on a W electrode (S = 0.31 cm2) in a melt containing
0.43 wt.% of ThCl4 in the LiCl–KCl eutectic, T = 733 K.
where Ecorr is the IR-corrected potential (V), Emeas the measured
potential (V), Re the resistance between the reference electrode
and the working electrode (ꢀ) and I is the applied current (A).
For each concentration, chronopotentiograms and cyclic
voltammograms were plotted at five temperatures (693, 733,
773, 798 and 823 K). The W working electrode was cleaned
between each measurement by applying a short anodic polariza-
tion at a current of 10 mA, until reaching the chlorine evolution
potential.
The effect of the scan rate on the shape of the cyclic voltam-
mograms is shown in Fig. 2. In the whole scan rate range
(from 50 to 300 mV s−1), the cathodic peak potential is slightly
shifted towards more cathodic potentials with increasing scan
rate, which is usually attributed to the irreversibility of an elec-
trochemical reaction [8]. The analysis of the peak potential
evolution versus the logarithm of the scan rate at five tempera-
tures, presented in Fig. 3, shows a linear relationship between
the cathodic peak potential and the logarithm of the scan rate. It
is however difficult to conclude about the reversibility of the sys-
tem, since the decrease of the cathodic peak potential (a few mV)
could be due to a small error provoked by ohmic drop corrections
and the uncertainty of the Re measurement. It can be men-
tioned that most of the other actinides have shown a reversible
behaviour at low scan range, typically below 200 mV s−1 [1–3].
A close-up view of the cathodic peak (Fig. 4) shows a cross-
over between the direct and the reverse scan. This cross-over
is indicative of a nucleation-controlled deposition process. A
prerequisite for the growth of Th on the W substrate is the
formation of thermodynamically stable nuclei on the surface.
The formation of such nuclei requires a potential more negative
3. Results
3.1. Reduction mechanism
A cyclic voltammogram of the whole electrochemical win-
dow recorded on an inert W electrode (S = 0.31 cm2) is shown in
Fig. 1. The solvent decomposition occurs at about −2.40 V ver-
sus Ag/AgCl on the cathodic side (Li+ reduction) and +1.25 V
versus Ag/AgCl on the anodic side (Cl− oxidation). It appears
that there is only one redox system due to the presence of Th
ions in the melt, attributed to the reduction of Th ions into
Th metal. It shows that the reduction of Th ions into metal is
a one-step process, and that dissolved Th has only one stable
electro-active form in the large electrochemical window of the
molten LiCl–KCl eutectic.
Fig. 3. Evolution of the potential of the cathodic peak Ec (corrected for ohmic
drop) with the logarithm of the scan rate for cyclic voltammograms obtained on a
W electrode (S = 0.31 cm2), at five temperatures, in a melt containing 0.43 wt.%
of ThCl4 in the LiCl–KCl eutectic.
Fig. 1. Cyclic voltammogram (corrected for ohmic drop) obtained on a W elec-
trode (S = 0.31 cm2) in a melt containing 0.43 wt.% of ThCl4 in the LiCl–KCl
eutectic, T = 773 K, scan rate = 100 mV s−1
.

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L. Cassayre et al. / Electrochimica Acta 52 (2007) 7432–7437
Fig. 4. Cyclic voltammogram (corrected for ohmic drop) illustrating the ‘nucle-
ation cross-over effect’ on the reverse scan for Th deposition on a W electrode
(S = 0.31 cm2) in a melt containing 0.96 wt.% of ThCl4 in the LiCl–KCl eutectic,
Fig. 6. Plotting of I/t3/2 in the rising part of the chronoamperograms of Fig. 5 at
two potentials ((♦) −1.60 V; (ꢀ) −1.54 V).
T = 733 K, scan rate = 100 mV s−1
.
than that required to reduce Th(IV) cations, and leads to what
is known as a nucleation overpotential, i.e. the potential has to
be more negative to deposit Th on W than on Th itself. Thus, on
the forward sweep a potential significantly more negative than
the equilibrium potential of the Th(IV)/Th couple is required
before deposition commences. On the reverse sweep, however,
the deposition is occurring on a Th surface and thus continues
until the equilibrium potential is reached.
The nucleation process taking place at the tungsten electrode
can further be evidenced by potential-step measurements. The
shape of the curves presented in Fig. 5 is indeed characteristic
of a nucleation process. After charging the double layer, the
initial regime of each transient is characterized by a decrease
in current associated to the formation of the first nuclei on the
tungsten electrode (zone I). The current then increases, which
correspond to an increase of the active area due to crystal growth
(zone II), until reaching a maximum with coordinates Im and tm.
Finally, the current decreases with time due to diffusion control
(zone III).
Chronoamperometry is widely used to study nucleation
mechanisms because it is proven that the shape of the I = f(t)
curves is typical of the mode of nucleus formation and growth
[9,10]. Zone II of these chronoamperograms obeys indeed an
equation of the following type [10]:
I = αtx
(2)
The value of x depends on the mechanism of the nucleation
process: if instantaneous nucleation (during which all the nuclei
are created at the same moment at the beginning of electrol-
ysis) takes place, x is equal to 1/2; if progressive nucleation
(where new Th nuclei are continuously generated throughout
electrolysis) takes place, x is equal to 3/2.
The coefficient α depends on the geometry of the nuclei and
of the growth control (diffusion or kinetics of electrochemical
reaction control).
According to the proportionality between I and t3/2 shown in
Fig. 6 for two overvoltages, it can be proposed that the nucleation
of Th is a progressive, three-dimensional process.
It is possible to confirm this progressive nucleation mode
by comparing data extracted from the chronoamperograms
with non-dimensional theoretical models [9,10] in the case
of three-dimensional nucleation of the metal. As detailed
by Massot et al. [11] in the case of the progressive nucle-
ation of Ta on Ag in fluoride melts, the experimental curves
(I/Im)2 = f(t/tm) can be compared to theoretical curves obtained
from the models. As shown in Fig. 7, the experimental data
for Th deposition fit quite well with the progressive nucleation
mode.
3.2. Diffusion coefficient
Chronopotentiograms obtained at 733 K are plotted in
Fig. 8. Again, there was only one redox system evidenced by
chronopotentiommetry. The diffusion coefficient of Th ions was
calculated in the 693–823 K temperature range by application of
the Sand law, which does not depend on the reversibility of the
system [8]. Assuming a 4-electron transfer reaction, the relation
Fig. 5. Chronoamperograms corresponding to Th deposition on a W electrode
(S = 0.31 cm2) in a melt containing 0.20 wt.% of ThCl4 in the LiCl–KCl eutectic,
T = 733 K, at two potentials (1: −1.60 V, 2: −1.54 V).

L. Cassayre et al. / Electrochimica Acta 52 (2007) 7432–7437
7435
Fig. 7. Comparison of the dimensionless experimental data obtained from
current-time transients at E = −1.60 V with the theoretical models for instan-
taneous and progressive nucleation in a melt containing 0.20 wt.% of ThCl4 in
the LiCl–KCl eutectic, T = 733 K.
Fig. 9. Verification of the Sand equation for chronopotentiograms obtained on
a W electrode (S = 0.31 cm2) in a melt containing 0.43 wt.% of ThCl4 in the
LiCl–KCl eutectic.
of the diffusion coefficient, according to the Arrhenius law:
1719
is:
log DTh = −2.126 −
(4)
√
S ΠDTh
T
√
nFcTh
I
τ =
(3)
At 723 K, the diffusion coefficient is: DTh(723 K)
=
2
3.15 0.15 × 10−5 cm2 s−1
.
where I is the applied current (A), τ the transition time (s),
n the number of exchanged electrons, F the Faraday constant
(C mol−1), cTh the Th concentration (mol cm−3), S the electrode
surface (cm2) and DTh is the diffusion coefficient (cm2 s−1).
For each concentration, the Sand equation was verified
through the linearity of the plot of I versus τ−1/2. An exam-
ple is given in Fig. 9. Using the slope of each line, the diffusion
coefficient was determined according to Eq. (3).
The temperature dependence of the diffusion coefficient is
plotted in Fig. 10 in the coordinates log(DTh) − 1/T. The data
obtained for four concentrations are in fairly good agreement,
with an average relative standard deviation of 8%. A linear
regression of the data was performed to determine the evolution
This value is in the same order of magnitude as others deter-
mined for actinides (U, Pu, Np, Am) in the LiCl–KCl eutectic
melt [12], but one order of magnitude higher than published by
Martinot [5] in the case of Th.
3.3. Apparent standard potential and activity coefficient
Due to the nucleation overpotential, the transient electro-
chemical techniques are not suitable for the determination of
the apparent standard potential of the system. EMF measure-
ments were then performed to determine the apparent standard
potential. For each temperature and concentration, the tung-
sten electrode was coated with Th metal by application of a
Fig. 8. Chronopotentiograms (corrected for ohmic drop) obtained on a W elec-
trode (S = 0.31 cm2) in a melt containing 0.43 wt.% of ThCl4 in the LiCl–KCl
eutectic, T = 733 K.
Fig. 10. Diffusion coefficient of Th(IV) dissolved in LiCl–KCl as function of
inverse temperature, determined according to the Sand equation.

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L. Cassayre et al. / Electrochimica Acta 52 (2007) 7432–7437
Table 1
Apparent standard potential E*◦ derived from OCP measurements, standard
potential E◦ calculated from tabulated data [12] and calculated activity coeffi-
cient of ThCl4 in the LiCl–KCl eutectic
Potential (V)
Temperature (K)
693 733
773
798
823
∗0
E
vs. Ag/AgCl −1.380 −1.350 −1.325 −1.296 −1.287
Th(IV)/Th(0)
E
vs. Cl2/Cl−
−2.599 −2.577 −2.559 −2.534 −2.530
−2.481 −2.457 −2.433 −2.419 −2.404
∗0
Th(IV)/Th(0)
ET0h(IV)/Th(0) vs. Cl2/Cl−
104γ(ThCl4)
3.7
5.0
6.8
8.1
9.6
electrode as detailed in [1], according to following equation:
2.303RT
EAg/AgCl(V versus Cl2/Cl−) = EA0 gCl
+
log XAgCl
(8)
Fig. 11. Evolution of the equilibrium potential Eeq with the molar fraction of
Th(IV) for five temperatures in the LiCl–KCl eutectic.
F
with XAgCl = 0.0039 (1 wt.%)
cathodic current for 60 s, and the open-circuit potential (OCP)
was recorded versus the Ag/AgCl reference electrode. A very
stable plateau was obtained each time, allowing the measure-
ment of the corresponding equilibrium potential Eeq.
The measured equilibrium potential for a pure metal in equi-
librium with its chloride is given by the Nernst law:
EA0 gCl = −1.0910 + 2.924 × 10−4 T(K) according to
[6,13].
The data of apparent standard potential are plotted versus
temperature in Fig. 12 and compared to tabulated data from
Smirnov [14] and Yang et al. [6]. The agreement is fairly good,
with a discrepancy of less than 40 mV between the different set
of data.
eq
E
(V versus Ag/AgCl)
Th(IV)/Th(0)
A linear fit of the apparent standard potentials established in
this work gives:
2.303RT
nF
aTh(IV)
= ET0h(IV)/Th(0)
+
log
(5)
aTh(0)
∗0
E
(V versus Cl2/Cl−)
Th(IV)/Th(0)
where ET0h(IV)/Th(0) is the standard potential at a hypothetical
supercooled liquid reference state of unit mole fraction and unit
activity, aTh(IV) and aTh(0) are the activity of Th ions and of
metallic Th, respectively. Unit activity was assumed for pure
= −2.983 + 5.540 × 10−4 T(K)
(9)
The activity coefficient γ(ThCl4) was determined according
metal.
∗0
to Eq. (6), using our experimental data for E
and tab-
∗0
Th(IV)/Th(0)
The apparent standard potential, E
of the Th redox
Th(IV)/Th(0)
ulated data for ET0h(IV)/Th(0). The standard potential ET0h(IV)/Th(0)
was calculated from tabulated Gibbs energy data from [15]
according to Eq. (10):
couple is defined as
2.303RT
nF
∗0
E
= ET0h(IV)/Th(0)
+
log γTh(IV)
(6)
Th(IV)/Th(0)
ꢁG◦f(sc)
ET0h(IV)/Th(0)
=
(10)
where γTh(IV) = aTh(IV)/XTh(IV) is the activity coefficient of Th
and XTh(IV) is the molar fraction of Th in the salt.
4F
eq
Combining Eqs. (5) and (6), E
can be expressed
Th(IV)/Th(0)
as
2.303RT
nF
eq
∗0
E
= E
+
log XTh(IV)
(7)
Th(IV)/Th(0)
Th(IV)/Th(0)
A linear fit of the plots of Eeq versus log(∗X0Th) (Fig. 11)
identified to Eq. (7) allows then to determine ETh(IV)/Th(0) and
n.
Firstly, the number of electrons of the redox system was cal-
culated according to the slope of the lines: an average value
of n = 4.17 was obtained, which confirms that there is only one
stable state of Th chloride in the melt, i.e. Th(IV).
Secondly, the zero-intercept of the lines (log(XTh) = 0) gives
∗0
a value of E
for each temperature, as summarized in
Th(IV)/Th(0)
Table 1. For the purpose of comparison with literature data, the
Fig. 12. Comparison of the apparent standard potential of Th(IV)/Th(0) in the
LiCl–KCl eutectic: (ꢁ) this work; (ꢀ) Smirnov [14]; (♦) Yang [6].
potential data have been calculated versus the Cl2/Cl− reference

L. Cassayre et al. / Electrochimica Acta 52 (2007) 7432–7437
7437
where ꢁG◦f(sc) is the standard Gibbs energy of formation of
sion financial support in the 6th Framework Program, under the
contract FI6W-CT-2003-408854, “EUROPART”.
ThCl4 in the liquid state at the relevant temperature (J mol−1).
In the 693–823 K temperature range, the activity coefficient
is rather low (∼5 × 10−4, see Table 1), which confirms recent
Raman spectra studies [16] showing a strong tendency of Th(IV)
to form chloride complexes in chloride melts.
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4. Conclusions
This electrochemical study of Th ions in a LiCl–KCl eutectic
melt indicates that there is only one stable electro-active specie
in the 693–823 K temperature range and in the 0.20–1.46 wt.%
concentration range. The temperature dependence of the poten-
tial of metallic Th in equilibrium with its ions indicates that this
stable species is Th(IV).
The electrochemical reduction of Th(IV) to Th metal is a
reaction controlled by a nucleation process inducing an overpo-
tential of several 100 mV. The analysis of chronoamperograms
showed that the deposition of Th occurs according to a progres-
sive nucleation mode.
By using EMF measurements, the standard apparent poten-
∗0
tial was found to be E
(723 K) = −2.582 V versus
ThCl4/Th
Cl2/Cl−, and the activity coefficient on the mole fraction scale
γThCl (723 K) = 4.6 × 10−4
.
4
At 723 K, the diffusion coefficient is DTh(723 K)
=
3.15 0.15 × 10−5 cm2 s−1, and this coefficient dependence
on temperature follows the Arrhenius law.
[14] M.V. Smirnov, Electrode Potentials in Fused Chlorides (Elektrodnye
Potentsialy v Rasplavlennykh Khloridakh), Nauka, Moscow, 1973.
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dynamics of Actinide Elements Compounds. Part 8: The Actinide Halides,
IAEA, Vienna, 1983.
Acknowledgments
The authors wish to thank B. Lynch for XRF measurements
as well as C. Scheppler, S. Birck and S.V. Winckel for ICP-MS
analysis. This work was carried out with the European Commis-
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