Transport and kinetics properties of LaF3 in FLiNaK molten salt determined by electrochemical methods

Article abstract of DOI:10.1016/j.jfluchem.2020.109502

It is found that lanthanum species can exist in the form of LaF₆^3? in FLiNaK (46.5 LiF-11.5 NaF-42 KF, mol%) molten salt in previous studies. The understanding of transport and reaction kinetics parameters of LaF₆^3? ions in FLiNaK molten salt is essential for the electrochemical separation of lanthanum from FLiNaK melt to achieve the coolant clean-up in fluoride salt cooled high-temperature reactor (FHR). Through conducting chronopotentiometry tests, the diffusion coefficients of LaF₆^3? ions in FLiNaK molten salt at the temperature range of 923 K–1023 K were determined. By conducting potentiodynamic polarization measurements at different concentrations and temperatures, the exchange current density, reaction rate constant, and charge transfer coefficient were obtained. The experiment data of potentiodynamic polarization were analyzed using a non-simplified electrode kinetics equation which incorporates both mass transfer and reaction kinetics.

Full text of DOI:10.1016/j.jfluchem.2020.109502

Journal Pre-proof
Transport and Kinetics Properties of LaF in FLiNaK Molten Salt
3
Determined by Electrochemical Methods
Yafei Wang, Qiufeng Yang, Jinsuo Zhang
PII:
S0022-1139(20)30053-1
DOI:
https://doi.org/10.1016/j.jfluchem.2020.109502
FLUOR 109502
Reference:
To appear in:
Journal of Fluorine Chemistry
Received Date:
Revised Date:
Accepted Date:
14 October 2019
24 February 2020
25 February 2020
Please cite this article as: Wang Y, Yang Q, Zhang J, Transport and Kinetics Properties of
LaF in FLiNaK Molten Salt Determined by Electrochemical Methods, Journal of Fluorine
3
Chemistry (2020), doi: https://doi.org/10.1016/j.jfluchem.2020.109502
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Transport and Kinetics Properties of LaF
3
in FLiNaK Molten Salt Determined by
Electrochemical Methods
a
a
a*
Yafei Wang , Qiufeng Yang , Jinsuo Zhang
a
Nuclear Engineering Program, Department of Mechanical Engineering, Virginia Polytechnic
Institute and State University, Blacksburg, Virginia 24061, USA
*
Corresponding author: zjinsuo5@vt.edu
Graphical abstract
Highlights
1
2
. Fundamental properties of LaF 3-
in FLiNaK molten salt was explored.
6
3
-
. Diffusion coefficient of LaF
chronopotentiometry measurements.
6
in FLiNaK molten salt was obtained from

3
4
. The relationship between diffusion coefficient and temperature was derived for the
temperature range of 973 to 1023 K.
3
-
. Kinetic parameters of LaF
6
in FLiNaK molten salt was achieved by potentiodynamic
polarization through employing an electrode kinetics equation.
Abstract
It is found that lanthanum species can exist in the form of LaF 3-
6
in FLiNaK (46.5 LiF-11.5 NaF-
4
2 KF, mol%) molten salt in previous studies. The understanding of transport and reaction kinetics
3
-
parameters of LaF
6
ions in FLiNaK molten salt is essential for the electrochemical separation of
lanthanum from FLiNaK melt to achieve the coolant clean-up in fluoride salt cooled high-
temperature reactor (FHR). Through conducting chronopotentiometry tests, the diffusion
3
-
coefficients of LaF
6
ions in FLiNaK molten salt at the temperature range of 923 K to 1023 K
were determined. By conducting potentiodynamic polarization measurements at different
concentrations and temperatures, the exchange current density, reaction rate constant, and charge
transfer coefficient were obtained. The experiment data of potentiodynamic polarization were
analyzed using a non-simplified electrode kinetics equation which incorporates both mass transfer
and reaction kinetics.
Keywords: Lanthanum; FLiNaK molten salt; diffusion coefficient; exchange current density;
reaction rate constant
1
. Introduction
FHR is a promising GEN IV reactor concept with a potential to be manufactured as a commercial
power reactor [1]. FHR combines liquid molten salt coolant and graphite matrix coated-particle
fuel originally proposed for High Temperature Gas-cooled Reactor (HTGR). Because of the most
favorable nuclear properties such as negative void coefficient [2] and low neutron absorption
cross-section [3], FLiBe (67LiF-33BeF , mol%) is selected as the primary coolant. FLiNaK is a
2
candidate salt mixture for the intermediate loop and a good surrogate of FLiBe [4]. FHR has the
advantages of low operating pressure and high-temperature power cycle which will enhance safety
and increase output for the electricity generation of a reactor, however, one challenge presented
for the application of molten fluoride salt in FHR is the acquisition of fission products from fuel
leakage into the coolant during normal operation. The circulating fission products within the
coolant would be a potential radioactive source for both the maintenance worker and the public
[
5]. Therefore, to ensure the continuous operation of the reactor and minimize the volume of salt
coolant waste, it is necessary to have an online coolant clean-up system installed in FHR [6,7].
As an ionic fluid, molten salt lends itself to various chemical control applications due to its
electrochemical properties. Several different chemical control methods for molten salt coolant
clean-up have been proposed. For instance, applying reducing electrical currents by

electrochemical separation technique or introducing reducing agent in liquid cathode (such as Li
in Bi). Among them, the electrochemical separation technique has been demonstrated as a
promising option for the clean-up of molten salt coolant in FHR. In the electrochemical separation
technique, a proper electrical potential or current can be applied, some of soluble fluorides of
fission products in molten salt coolant will be electrodeposited as metals or metal compounds on
the cathode. In that way, the fission products can be extracted out with cathode from the molten
salt to realize the clean-up of the coolant in FHR.
Lanthanum is one of the abundant lanthanide fission products in nuclear reactors [8] and which is
commonly taken as a model system for the electrochemical separations of fission products in
molten salt. To manage the electrochemical separation of lanthanum, the understanding of its
fundamental properties is significant. Massive studies have been performed for the fundamental
properties of lanthanum in LiCl-KCl molten salt [9,10,11]. However, the investigation on the
fundamental properties of lanthanum in molten fluoride salt is still rare. Chesser et al. [12] recently
studied the diffusion coefficient of lanthanum in FLiNaK molten salt using cyclic voltammetry
technique, and reported that the diffusion coefficient is much less than that in chloride salt.
Based on available studies, the deposition of La metal in FLiNaK-LaF
inert electrode such as W is due to the deposition reaction of LaF
3
molten salt system on an
3
-
6
/La [6]. In addition,
3
+
Dracopoulos et al [13] found the dipole-induced-dipole interaction between the La partially
+
shielded by the fluorides and the mobile K was enhanced, and reported La ions would
3
-
predominantly exist in the form of LaF
Rollet [14] pointed out the local structure of molten fluoride salt could change when LaF
and the fundamental properties of lanthanum species would change accordingly as well. Therefore,
6
in the KF-rich molten salt mixtures. At the same time,
3
exists
3
-
it is interesting to know the transport and kinetics properties of LaF
6
in molten salt. In the present
in FLiNaK molten salt were studied at different
concentrations and temperatures by the method of chronopotentiometry. Additionally,
3
-
work, the diffusion coefficients of LaF
6
3
-
potentiodynamic polarization measurements at different LaF
6
concentrations and temperatures
were carried out. Through using a non-simplified electrode kinetics equation as reported by Guo
et al [15] to nonlinearly fit the obtained potentiodynamic polarization curves, the kinetics
parameters such as exchange current density, reaction rate constant, and charge transfer coefficient
3
-
for the redox reaction of LaF /La were determined.
6
2
. Experimental
All the electrochemical experiments and salt preparations were performed in an argon-filled glove
box manufactured by Inert Technology. The oxygen and moisture inside the glovebox were
maintained below 5 ppm and 0.5 ppm respectively during the normal operation. The chemicals
used in this study include lithium fluoride (LiF, >99.0% purity, Sigma Aldrich), sodium fluoride
(NaF, >99.0% purity, Sigma Aldrich), and potassium fluoride (KF, >99.5% purity, Sigma Aldrich).
To provide lanthanum ions, lanthanum fluoride (LaF ) with the purity of ≥99.99% was procured
3
from Sigma Aldrich. 50 g FLiNaK constituents were prepared by weighing LiF, NaF, and KF
chemicals according to the FLiNaK composition of 46.5 mol% LiF, 11.5 mol% NaF, and 42 mol%

KF using a precision analytical balance with an accuracy of 0.0001 g (MS 105DU Mettler Toledo).
The prepared FLiNaK constituents and LaF chemicals were mixed inside a plastic sample bottle
3
and then transferred to a nickel crucible (55 ml, Sigma Aldrich). The salt powder with the weight
of 0.003 g was taken out for Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) sample,
which was firstly dissolved into 65 wt% nitric acid. Once the salt was completely dissolved, an
amount of deionized water was added to obtain a 2 v/v% nitric acid based ICP-MS sample solution.
5
identical ICP-MS samples were prepared in total and the average quantity of the salt impurity is
shown in Table 1.
The present study assumed that the dominant ions of La is LaF
6
3-, therefore, the concentration
added into FLiNaK salt.
3
3-
(
mol/cm ) of LaF
6
in FLiNaK is obtained based on the amount of LaF
3
3
The unit of mol/cm was adopted for the concentration in this study for the convenience of
calculation. The nickel crucible with an external alumina safety crucible was then placed in a
muffle furnace (Thermofisher Scientific FB1315M) for which the temperature can be maintained
within ±1℃ during the experiment. The muffle furnace was customized with a lid which was
equipped with a quartz assembly to fix the electrodes. The salt mixture was heated at 473 K for 24
hours to eliminate the oxygen and moisture and subsequently heated up to the target temperature
for further electrochemical tests.
The electrochemical tests in this study utilized a three-electrode electrochemical cell in the furnace.
The working electrodes used was a 1.6 mm diameter tungsten rod (Midwest Tungsten
Service, >99.5% purity). The counter electrode was a 3.05 mm diameter graphite rod (99.9995%
purity, Alfa Aesar). The reference electrode utilized was a 0.81 mm diameter platinum wire
2
-
(99.99%, Surepure Chemetals) with potential associated to Pt/PtO
x
/O [16]. The bottoms of the
electrodes were immersed in the FLiNaK-LaF salt bath while their top ends were connected to
3
the Gamry Interface 1000 poteniostat to perform the electrochemical measurements. The surface
area of the working electrode was measured according to the submerged depth into the salt melt.
Open circuit potential test was conducted before the chronopotentiometry and potentiodynamic
polarization tests to check the stability of the system. The current interrupt method which is
programmed in the Gamry Framework software was chosen to perform IR compensation in all the
chronopotentiometry measurements.
3
. Results and discussion
The electrochemical behavior of LaF
3
in FLiNaK molten salt has been studied in the previous
studies [12,17] by cyclic voltammetry test as shown in Figure 1.Generally, it is believed that the
electrodeposition of lanthanum is impossible in FLiNaK molten salt because of the more cathodic
3
+
+
standard reduction potential of La /La than that of K /K. However, Lanthanum predominantly
3
-
3+
3-
exists in the species of LaF
6
instead of La in FLiNaK molten salt [6] and the existence of LaF
6
helps shift the electrodeposition potential to a more anodic value. As a result, the electrochemical
3
-
-
reduction of LaF
La+6F , as reported in the previous study [6].
3
in FLiNaK molten salt is a one-step charge transfer process, i.e., LaF
6
+3e →
-

3
.1.Determination of diffusion coefficient by chronopotentiometry
Based on the known redox reaction of LaF occurring in FLiNaK molten salt, the reduction process
3
of LaF in FLiNaK melt was studied by chronopotentiometry test in the present study and then the
diffusion coefficients at several temperatures were determined. Figure 2 illustrates the typical
3
-4
3
chronopotentiograms of LaF
3
(1.05×10 mol/cm ) in FLiNaK molten salt at the temperatures of
9
23, 973, and 1023 K when applying different currents. The first plateau shown with time
3
-
evolution corresponds to the reduction of LaF
6
into La metal. After this plateau, the cathodic
potential rapidly decreases to reach a limiting value which corresponds to the reduction of
3
-
potassium. The transition time for the reduction of LaF
6
can be determined from
chronopotentiograms using the method described in the literature [18]. Figure 3 (a) shows the
relationship between the applied current and the obtained transition time for the
chronopotentiometry data given in Figure 2. A fine proportional relation can be observed between
-
1/2
I and τ , therefore, the Sand’s equation is applicable for the reduction of LaF in FLiNaK molten
3
3
-
-
salt. The diffusion coefficient of LaF
6
can be calculated from the slope of the plot of I versus τ
1
/2
based on Sand’s equation [19] shown below
퐼휏1/2 = 0.5푛퐹퐶 푆휋1/2퐷1/2
∗
(1)
표
∗
where I is the applied current (A), τ is the transition time (s), 퐶 is the bulk concentration
표
3
2
(
mol/cm ), D is the diffusion coefficient (cm /s), F is the Faraday’s constant, S is the working
2
electrode surface area (cm ) and n is the electron transfer number.
In addition to the chronopotentiometry test of LaF in FLiNaK molten salt at the concentration of
3
-
4
3
-4
3
1
.05×10 mol/cm , the same test was performed at a higher concentration (2.25×10 mol/cm ).
The relationship between the applied current I and the obtained transition time τ was plotted
Figure 3 (b)). The diffusion coefficients at these two different concentrations were calculated
based on equation (1) and summarized in Table 2. These values are slightly smaller than the
(
-
7
2
-6
2
-6
2
reported 9.16×10 cm /s at 923K, 2.15×10 cm /s at 973 K, and 4.62×10 cm /s at 1023 K by
Chesser et al [12]. However, if considering all of these values are extremely small, this discrepancy
is acceptable. When comparing with the diffusion coefficient of lanthanum ion in chloride salt
3
-
[
9,10,11], it is found the one obtained for LaF
6
in FLiNaK molten salt is much smaller. The
reason for this phenomenon was still not well understood. However, the smaller diffusion
3
-
coefficient of LaF
6
could be attributed to the difference on the physical properties of molten
fluoride salt and chloride salt, for example, the higher viscosity of molten fluoride salt, or different
chemical forms of La in molten chloride and fluoride salts.
Based on simulations in molten salt system [20], it is reasonable to assume that the diffusion
coefficient is independent of the species concentration. The average values based on the
measurement values for the two concentrations at 923, 973, and 1023 K were calculated and given
in Figure 4 in the Arrhenius plot. A correlation was obtained:

푙푛퐷 = −ꢀ.5945 − ꢀꢁ487/푇
(2)
(3)
which can be written in the form of Arrhenius equation:
퐸푎
퐷 = 퐷 exp⁡(−
)
ꢂ
푅푇
where 퐸 is the activation energy (J/mol), R is the gas constant and T is the temperature (K), the
푎
activation energy 퐸 was acquired with a value of 103.8 kJ/mol which is close to the value of 127
푎
kJ/mol reported in the previous study [12]. The small difference could be attributed to the
difference on experimental methods used for the estimation of diffusion coefficient and the
inevitable errors existing in the experimental tests, for example, the measurement of the working
electrode area.
3
.2.Determination of kinetic parameters by potentiodynamic polarization
Kinetics parameters such as exchange current density and charge transfer coefficient are important
3
-
to understand the reaction kinetics of LaF
6
to La metal in molten FLiNaK salt. Potentiodynamic
polarization can be used to determine the values of these kinetics parameters. In potentiodynamic
polarization, the electrode at zero-current potential is in fact under cathodic polarization since the
measured current is the sum of charging current and faradic current [21]. Charging current comes
from the continuous change of the potential during the scan process and is proportional to scan
rate. Based on that the magnitude of charging current decreases with the decreasing scan rate, low
scan rate is usually employed in potentiodynamic polarization to minimize the influence of
charging current. The zero-current potentials at the scan rates of 0.5 and 1 mV/s are commonly
believed to nearly equal to the equilibrium potential [21]. However, the potentiodynamic
polarizations at 0.5 and 1 mV/s will take longer time, electrode surface may change greatly during
this process and bring more noises to the curve obtained. In the present study, the potentiodynamic
polarizations were first carried out at four different scan rates: 0.5, 1, 2, and 5 mV/s. Figure 5
indicates the potentiodynamic polarization curves at these four different scan rates highly
coincident on the curve during each scan except the distortion part. Therefore, the scan rate of 5
mV/s is sufficiently low to approach the steady state in the investigated system and will be adopted
for all the following tests.
In Figure 5, the potential applied on the inert working electrode (W) was scanned from the negative
overpotential to the anodic direction in the potentiodynamic polarization test. In the negative
3
-
overpotential section, LaF
6
was reduced to La metal and formed a layer on the tungsten working
electrode surface. A distortion on the curve, where the initial potential scan started was observed
in this section, could be attributed to the initial development of the diffusion layer. In the positive
overpotential section, the dissolution of the deposited La occurs and the current increased with the
increase of overpotential. The irregular variation was observed when overpotential η becomes

more positive. This is due to the heterogeneous dissolution of deposited La layer which might not
be uniform after negative overpotential scan [15]. Some spots of the deposited La layer might
dissolve faster, which will cause the exposure of some parts of the working electrode during the
anodic polarization. Thus, the coverage area of La and the measured current will drop suddenly at
some points. The dissolution of La layer and the decrease of La coverage area on the electrode
with time might be different at different scan rates, as a result, there is an obvious deviation on the
distortion area in the positive overpotential scan section for each scan.
Figure 6 illustrates the potentiodynamic polarization curves obtained at the scan rate of 5 mV/s for
FLiNaK molten salt containing different concentrations of LaF at the temperatures of 923, 973,
3
and 1023 K, respectively. The figure shows that the measured current density increases with
concentration for each temperature. The surface area of the working electrode was used to convert
the current to current density based on the assumption that the current is uniformly distributed on
electrode surface.
In order to determine the kinetics parameters, an analysis method based on the non-simplified
electrode kinetics equation was applied here. This method is verified to be more reliable than the
conventional Tafel and linear polarization methods [15]. The non-simplified electrode kinetics
equation for the redox reaction of lanthanum ion can be expressed as follows [15]
ꢃꢀ − 훼ꢄ푛퐹ꢄ
−훼푛퐹
푅푇
푖ꢂ {푒푥푝 [
휂] − 푒푥푝 ꢅ
−훼푛퐹
휂ꢆ}
푅푇
푖ꢂ
푖 =
(4)
ꢀ
−
푒푥푝 ꢅ
휂ꢆ
푖L
푅푇
2
2
Where i is the current density (A/cm ), i
0
is the exchange current density (A/cm ), n is the electron
2
transfer number, α is the charge transfer coefficient, η is the overpotential (V). i (A/cm ) is the
L
limiting diffusion current density.
Equation (4) was applied to fit the experimental potentiodynamic polarization curves. The values
of the various parameters that best fit the experimental data can be extracted by the least squares
2
fitting procedure through minimizing χ with respect to the 3 fitting parameters: i0,
i
L, and α. Thus,
if we refer to the parameters by a vector m= [ i0,
iL, α], then the Chi-square function to be minimized
can be expressed by
휒2⁡ꢃ푚ꢄ = |푖푠ꢇꢈꢃ푚ꢄ − 푖ꢉꢊꢋꢃ푚ꢄ|2
(5)

The non-linear least square algorithm can search this 3- dimensional parameter space to find the
2
optimum values that minimize χ . In detailed, the optimization process was performed through
2
2
iterating the values of i0,
i
L, and α in the ranges of 0≤ i
0
≤0.1 A/cm , -0.1 A/cm ≤ i ≤0, and 0≤ α≤1
L
by the step sizes of 0.00001, 0.0001, and 0.01, respectively. Due to the distortion at the both ends
of the experimental polarization curve, the data in the overpotential range of -0.2 to 0.05 V was
used to do the fitting. Through this way, these three parameters at which the calculated
potentiodynamic polarization best fits the experimental one can be obtained. The solid lines in
Figure 7 are the calculated polarization curves at the optimized values. It can be seen that the
experimental data at the range (-0.2~ 0.1 V) are fitted well by the method. The optimized values
for the parameters are included in Table 3. In addition to the parameters discussed previously,
ꢂ
reaction rate constant 푘 is another key kinetics parameter for the understanding of the kinetics of
the redox reaction. It is related to the exchange current density i and charge transfer coefficient α
0
by the following correlation [15,19]:
∗
ꢃ1ꢌꢍꢄ
ꢂ
(6)
푖 = 푛퐹푘 퐶
ꢂ
표
The values of exchange current density 푖 listed in Table 3 were plotted against its corresponded
ꢂ
3
concentration of LaF
3
in mol/cm which can be seen in Figure 8. Equation (6) was used to fit the
data points in Figure 8 from which it can be known all data points agree well with the fitted power
law trend line. The values of charge transfer coefficient 훼 from the fitted process using equation
(6) are 0.47 at 923 K, 0.51 at 973 K, and 0.47 at 1023 K, respectively. These numbers in fact are
close to the values of 훼 obtained from the optimization fitting of individual test results (Table 3).
ꢂ
Table 4 lists the values of reaction rate constant 푘 determined from the fitted expression by
ꢂ
equation (6), from which it can be known reaction rate constant 푘 increases with temperature.
4
. Conclusions
3
-
The present study explored the fundamental properties of LaF
6
in FLiNaK molten salt. The
in FLiNaK molten salt was obtained from chronopotentiometry
measurements. It is found the diffusion coefficient of lanthanum ion in fluoride salt is much less
3
-
diffusion coefficient of LaF
6
3
-
than that in chloride salt. The diffusion coefficients of LaF
6
in FLiNaK molten salt at the
temperature range of 923 to 1023 K was correlated by 푙푛퐷 = −ꢀ.5945 − ꢀꢁ487/푇 with
temperature. Other fundamental parameters including exchange current density, charge transfer
3
-
coefficient, and reaction rate constant of LaF
potentiodynamic polarization tests at three concentrations in the temperature range of 923 K to
023 K. A non-simplified electrode kinetics equation was employed as the analysis method to
6
/La reaction were measured via performing
1
nonlinearly fit the obtained potentiodynamic polarization curves through which the values of
exchange current density and charge transfer coefficient were obtained. Based on the obtained
exchange current density at different concentrations and temperatures, the values of reaction rate
constant and charge transfer coefficient were fitted by a correlation equation. The derived reaction

state constant at different temperatures shows that they are positively related. The derived charge
transfer coefficient is close to the ones achieved from non-simplified electrode kinetics equation,
which indicates the reliability of the obtained results.
Declaration of interests
The authors declare that they have no known competing financial interests or personal relationships
that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by NEUP Award Number DE-NE0008306. Any opinions, findings,
conclusions, or recommendations expressed in this publication are those of the authors and do not
necessarily reflect the views of the Department of Energy Office of Nuclear Energy.

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17] Wang, Yafei, et al. "Electrochemical separation study of LaF
of Nuclear Materials 518 (2019): 162-165.
[
3
in molten FLiNaK salt." Journal

[
18] Laity, Richard W., and James DE McIntyre. "Chronopotentiometric diffusion coefficients in
fused salts I. Theory1a." Journal of the American Chemical Society 87.17 (1965): 3806-3812.
19] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Fundamental and Applications, Wiley,
New York, 2001.
20] Wang, Yafei, Wentao Zhou, and Jinsuo Zhang. "Investigation of concentration-dependence
[
[
of thermodynamic properties of lanthanum, yttrium, scandium and terbium in eutectic LiCl-KCl
molten salt." Journal of Nuclear Materials 478 (2016): 61-73.
[
21] Guo, Shaoqiang, Evan Wu, and Jinsuo Zhang. "Investigation of electrochemical kinetics for
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Nuclear Materials 510 (2018): 414-420.

Figure 1. Cyclic voltammograms obtained in FLiNaK-LaF
and pure FLiNaK molten salts at the scan rate of 100 mV/s reported in the previous study [17].
3
(working electrode area=0.673 cm2)

Figure 2. Chronopotentiograms of LaF
3
in FLiNaK molten salt at the temperatures of (a) 923 K,
-4
3
2
(
b) 973K, and (c) 1023 K. LaF =1.05×10 mol/cm ; electrode area=0.296 cm .
3

Figure 3. Linear relationship of I versus τ-1/2 for the chronopotentiometry test of LaF
in FLiNaK
3
-4
3
-4
3
molten salt with the concentration of (a) LaF
3
=1.05×10 mol/cm and (b) LaF =2.25×10 mol/cm .
3
Figure 4. Arrhenius plot of diffusion coefficient as a function of temperature.

Figure 5. Potentiodynamic polarization curves measured at different scan rates for LaF in FLiNaK
3
-4
3
molten salt. LaF =1.58×10 mol/cm ; T=923K.
3

Figure 6. Potentiodynamic polarization curves at different concentrations for the temperatures of
a) 923 K, (b) 973 K, and (c) 1023K. Scan rate: 5 mV/s.
(

Figure 7. Experimental and fitted potentiodynamic polarization curves at the scan rate of 5 mV/s
5
for LaF
mol/cm ; (b) LaF
3
3
in FLiNaK molten salt at the temperatures of 923, 973, and 1023 K. (a) LaF
=6.31×10-
3
-4
3
-4
3
3
=1.58×10 mol/cm ; (c) LaF =2.59×10 mol/cm .
3
Figure 8. The plot of 푖 from Table 3 versus LaF concentration.
3
ꢂ

Table 1. Trace impurities in the FLiNaK-LaF
3
salt and blank solution.
Impurities (wt%)
Solutions
Mg
Al
Si
P
Fe
Ni
Se
Ba
0
Blank
solution
0
.0005 0.0092 0.1126 0.0015 0.0014 0.0001 0.0009
FLiNaK-LaF
3
0.0021 0.0094 0.1677 0.0025 0.0026 0.0114 0.0015 0.0007

Table 2. Diffusion coefficients of LaF 3-
in FLiNaK molten salt determined using equation (1).
6
∗
3
1.05×10-4
2.25×10-4
973
퐶 (mol/cm )
표
T (K)
923
3.47
973
4.92
1023
8.14
923
2.24
1023
13.4
7
2
D×10 (cm /s)
4.76

Table 3. Optimized parameters determined by non-simplified electrode kinetics equation.
Concentration(mol/cm3) T (K)
23
973
2
2
푖 (A/cm )
훼
0.47
0.52
0.53
0.48
0.50
0.37
0.45
0.52
0.44
푖 (A/cm )
ꢂ
퐿
9
0.00013
0.00069
0.00250
0.00025
0.00120
0.00380
0.00060
0.00180
0.00460
-0.0058
-0.0065
-0.0102
-0.0049
-0.0070
-0.0169
-0.0010
-0.0109
-0.0177
6
1
2
.31×10-5
.58×10-4
.59×10-4
1
9
023
23
973
1
9
023
23
973
023
1

ꢂ
Table 4. The obtained values of reaction rate constant 푘 at different temperatures using equation
6).
(
ꢂ
T (K)
023
푘 (cm/s)
7
1
7.6×10-
-7
9
73
23
3.8×10
9
5.9×10-8