STUDY OF ELECTROCHEMICAL DEPOSITION OF IRON (II)
IONS ON A SOLID TUNGSTEN ELECTRODE IN CHLORIDE
MELTS AT 700-750OC
Alex Lugovskoya,b*, Michael Zinigrad,a‡ Doron Aurbachb§ and Anatoly Fishmanc¶
a
Materials Research Center, College of Judea and Smaria, Ariel 44837, Israel
b
Department of Chemistry, Bar-Ilan University, Ramat-Gan 52900, Israel
cInstitute of Mettallurgy, Ural Branch of Russian Academy of Science, Amundsen
101, 620016, Ekaterinburg, Russia
‡
§
*lugovsa@yosh.ac.il,
zinigrad@yosh.ac.il,
aurbach@mail.biu.ac.il,
¶
fishman@ural.ru
Abstract
The process of electrodeposition of iron (II) ions reacting on solid tungsten electrodes
in molten chloride systems at about 700oC to form metal iron has been studied by the
conventional cyclic voltammetry and convolution cyclic voltammetry. The deposition
occurs under diffusion control for all the concentrations and temperatures studied.
Ferromagnetic iron needles and polyhedrons having only poor adhesion to the
tungsten substrate, are found on the tungsten electrode.
Diffusion coefficients of iron (II) were calculated according to Randles-Sevcik and
Berzins-Delahay equations for the conventional cyclic voltammetry and by the
limiting convoluted current for the convolution cyclic voltammetry. Convolution
cyclic voltammetry is believed to be superior to conventional cyclic voltammetry for
the quantitative evaluation of diffusion coefficients. The values of the diffusion
coefficients lay in the range 1-4010-5 cm2 / sec for the temperature range of 700750oC. The Arrhenius temperature dependence of the diffusion coefficients is
characterized by the value of Ea=31.2 kJ/mol.
Introduction
Whereas close attention has been attracted to the electrochemistry of high-temperature
halide systems due to their interest both in the applied and theoretic aspects, the
behavior of some elements frequently used in this field, remains poorly studied. So,
only a few researches have been devoted to the electrochemistry of iron in chloride
melts. Bjerrum and Andreasen have performed a potentiostatic study of the system
KCl-FeC13 at about 300oC and found that the main component of the system depends
on the molar fraction of KCl and may vary in the range of KFeCl4 to K2Fe2Cl7 and
FeCl3 [1]. Inman et al studied the chronopotentiometry of Fe(II) and Fe(III) in LiClKCl melt at 400-500oC and found that Fe+2 is reduced in a two-electron transfer into
the metal iron [2]. Only unclear peaks insufficient for being analyzed were obtained
for the oxidation of Fe+2 into Fe+3, which was explained by the thermodynamic
instability of Fe+3 at the temperatures of the experiments [2].2 Shuzhen et al
performed a profound voltammetric study of Fe(II) in molten MgCl2-NaCl-KCl at
about 500oC and confirmed the two-electron character of reduction of Fe(II) [3].
Castrillejo et al studied the electrochemical reduction of Fe(II) in fused ZnCl2-2NaCl
at 450oC by different techniques and obtained the results consistent with the
previously cited groups [4].
While the above-cited works dealt with relatively low temperatures, the behavior of
iron ions can be of both mechanistic and applied interest at higher temperatures,
typical for molten NaCl-KCl system (above ca. 700oC). This system is frequently
77
used in many methods of separation, purification and synthesis for a wide range of
metals. In spite of some difficulties connected with the high reactivity and low
stability of iron, a mechanistic study of its electrochemistry at above 1000K can
provide one with some information useful for pyrochemical engineering and applied
electrochemistry of fused salt systems.
A study of the cathodic deposition of a metal has to consider at least three main
factors influencing the process mechanism: (1) mass transport from the solution to the
electrode; (2) electron transfer, which can be conjugated with some chemical
processes; and (3) the formation of the deposit of the metal. The role of each process
in the overall reaction varies in a very wide range and depends on many factors, such
as chemical and phase composition of a solution and an electrode, temperature,
electrode potentials and others.
This work’s purpose is to examine the contribution of those factors to the process
mechanism, to determine its limiting stage and the type of reaction control and to
compare the mechanism with the determined for lower temperatures [1-4]. Another
objective of the study was to try to decrease the strict demand to the purity of the
materials limiting the technological applicability of the process because of its extreme
expensiveness, keeping at the same time the character of the process unchanged.
Experimental
Detailed description of the experimental setup is given elsewhere [5]. All experiments
were carried out in equimolar NaCl-KCl (melting temperature 664oC [6] confirmed
by DSC). Tungsten rod was used as working electrode and its surface area was
controlled by the immersion depth. Counter and reference electrodes were also made
of tungsten wire. The shape of voltammograms typical for the reduction-reoxidation
of a metal on a solid electrode (Fig. 1) evidenced that the edge effect of the working
electrode can be neglected and the current distribution can be considered homogenous
and normal to the side surface.
78
0.12
A'
0.1
0.08
0.06
0
i (A)
-0.8
0.6
0.04
0.02
0
B
-0.02
A
-0.04
-0.6
-0.5
-0.4
-0.3
-0.2
-0.1
0
E (V) vs. W
Fig. 1. Typical cyclic voltammograms for the reduction
of Fe+2 (0.09 mol / kg) at 750 oC on a W electrode.
On insert: extended cathodic sweep.
The concentrations of Fe(II) ions varied in the range 10-5 – 10-4 mol / L so that in the
course of an experiment much less that 1% of iron could have been consumed even if
the process would have been entirely irreversible.
The electrochemical window for the equimolar NaCl-KCl melt was cathodically
limited by alkali metal deposition at -1.1V vs. tungsten pseudo-reference electrode.
The anodic limit of the system was about +0.8V, where the evolution of chlorine
starts. Hence, the width of the electrochemical window E is about 1.9V, which is
typical for such systems [4,7].
Results and discussion
1. Electrodeposition of Iron on the Tungsten Electrode
Some typical voltammograms for the electrochemical reduction of Fe(II) are shown in
Fig. 1. Cathodic peak A and anodic peak A' are similar to those obtained for lower
temperatures [3,4]. Crossover B demonstrating a change of the electrode surface
typical for a phase formation is seen in a part of voltammograms. Attempts to extend
the cathodic sweep to more negative potentials were not informative, because a broad
cathodic wave appears at potentials lower than that of peak A and monotonically rises
until the window limit. Metal ferromagnetic iron is found in the melt after the
completion of each experiment.
Randles-Sevcik plots for the dependences of the reduction peak current ip on the
square root of scan rate () are clearly linear up to at least 2 V / sec (Fig. 2).
79
0.3
0.01 mol / kg
0.25
0.027 mol / kg
0.065 mol / kg
0.15
p
i , A/cm
2
0.2
0.1
0.05
0
0
0.5
1
 ,V
1/2
1/2
sec
1.5
-1/2
o
Fig. 2. Randles-Sevcik Plots for Fe(II) Deposition on W electrode at 700 C
Therefore, the process of iron (II) reduction can be considered to be diffusion
controlled under these conditions, which is consistent with the observations at lower
temperatures [3,4].
2. The structure and composition of the deposits on the tungsten electrode
Black fine needles and polyhedrons having distinct ferromagnetic properties are found
on the tungsten electrode after the deposition at -0.5V. The adhesion of the deposits to
tungsten is poor and individual crystals are seen in SEM and optic microscope images
(Fig. 3).
Fig. 3. Iron deposits on the tungsten electrode. From left to right: optic (x1000), SEM (x720),
SEM (x7800)
The element composition of the deposits was analyzed by EDS, and according to it
these, contain 77.5% W (substract), 17.6% Fe, 3.8% K and 3.5% Cl. Surprisingly, no
Na is detected. Apparently, potassium ions are selectively solvated by iron in the
course of deposition.
3. Diffusion coefficients of Fe(II)
Since the rate determining step for the iron deposition process is the diffusion of Fe+2
to the electrode, the appropriate diffusion coefficients can be calculated from the
electrochemical measurements. These were calculated by three different methods.
(1) Since the CV results demonstrate quasi-reversible character, Randles-Sevcik
80
equation can be applied (5.1) [8]:
ip = 0.4463 (nF)3/2/(RT)1/2  D1/2 C*1/2,
(5.1)
where ip is the cathodic peak current density from the CV (A), n=2 is the number of
electrons transferred, F is Faraday constant, R is the gas constant, T is the temperature
(K), D is the diffusion coefficient (cm2 / sec), C* is the bulk concentration of Fe+2
(mol / cm3) and  is the scan rate (V / sec).
(2) Another model for the process of an electrodeposition on a solid electrode was
proposed by Berzins and Delahay [9] and is based on the assumption of a constant
activity of metal deposit equal to unity. This model is considered to best describe the
processes of metal deposition on solid electrodes in fused salt systems according to
equation (5.2):
ip = 0.6105 (nF)3/2/(RT)1/2D1/2C*1/2,
(5.2)
Because of the lack of an unambiguous method for treating cyclic voltammetry in
order to calculate diffusion coefficients for the electrodeposition processes and
because of the intrinsic limitations of the CV technique, an alternative approach to
obtaining such information is needed.
Chronopotentiometry and chronoamperometry are normally used as more accurate
techniques for the elucidation of kinetic parameters [8].
However, these techniques require special experimental conditions in which
occasional stirring of the solution is completely avoided. Construction of such cells
designed for high temperature melts is complicated and this decreases the usefulness
of chrono-techniques for finding the diffusion coefficients in high-temperature molten
salt systems.
(3) The method of semi-integral convolution is successfully used for solving that
0.06
0.04
i (A)
0.02
0
m
L
-0.02
-0.6
-0.4
-0.2
0
0.2
E (V)
Fig. 4. A semi-integral convolution (solid line) to the conventional CV
o
(dotted line) for the deposition of Fe(II) on a tungsten electrode at 700 C.
m is the convoluted limiting current density
L
81
problem and is considered the most convenient and reliable for high-temperature
systems [4,7]. A typical semi-integral convolution voltammogram is shown in Fig. 4.
As seen from Fig. 4, limiting convoluted current mL is achieved, in contrast to the
peak of the cyclic voltammogram. The value of the limiting convoluted current is
given in equation (5.3) [7]:
mL = nFD1/2C*
(5.3)
Therefore, the diffusion coefficient can be calculated from equation (5.3). The values
of the diffusion coefficients calculated by the abovementioned three methods, are
given in Table 1.
C*105, mol / L
T,
Method of calculation
1.45
3.92
4.36
7.07
Randles-Sevcik
D 105, cm2 / sec
Berzins-Delahay
32.0
40.3
50.8
12.4
7.3
14.7
4.6
3.4
5.2
5.7
6.1
6.6
6.7
17.1
21.6
27.1
6.6
3.9
7.8
2.4
1.8
2.8
3.1
3.3
3.5
3.6
K
973
983
1003
973
983
1003
973
1023
963
973
983
993
1003
Convolution
22.6
23.7
24.8
8.8
9.8
13.2
3.7
4.1
4.6
5.4
5.7
5.6
Table 1. Diffusion coefficient of Fe+2 to the tungsten electrode in NaCl-KCl melt
Some peculiarities are clearly seen in Table 1. First of all, three different methods
provide rather different results. The values of diffusion coefficients calculated by the
convolution technique monotonously rise with the temperature for all the measured
concentrations, while the values obtained by the other methods demonstrate less
regular behavior. This is consistent with the literature data [4,7], where convolution
cyclic voltammetry is considered to provide more reliable results as compared to
conventional voltammetry. Very roughly, the diffusivities calculated from the
convolution method are higher than those obtained from Berzins-Delahay equation
(which is, again, consistent with the literature [4,7]), but lower than those calculated
by Randles-Sevcik equation.
4. Effect of concentration
Another feature of the obtained values of the diffusion coefficients is that they clearly
are not independent on the concentration of iron ions. This was also the case in the
work of Shuzhen et al in spite of the opposite conclusion made by that group [3]. The
values of diffusion coefficients are inversely correlated to the concentration of iron.
This can apparently be an evidence of structure reorganization of the molten chloride
82
systems and demands further studies.
5. Effect of temperature
The Arrhenius temperature dependence of the diffusion coefficients calculated by the
convolution technique is described with equation (5.4) and the appropriate value of Ea
is 31.2 kJ/mol:
 31246 
DFe  1.225  102 exp  

RT 

(5.4)
2
Thus calculated value of the diffusion activation energy is typical for metal ions in
chloride melts [10, 11]. The Arrhenius dependence of the diffusion coefficients found
in this work shows only qualitative agreement to the literature data for lower
temperatures [3, 4]. This is not surprising if one considers the differences in the
molten salt systems compositions used and large temperature “gap” between our
experiments and those performed by other authors.
Conclusions
The process of electrodeposition of iron (II) ions reacting on solid tungsten electrodes
in molten chloride systems at about 700oC to form metal iron has been studied by
electrochemical techniques. The process occurs under diffusion control for all the
concentrations and temperatures studied.
The adhesion of the deposits to tungsten is poor and ferromagnetic iron crystals are
found on the tungsten electrode.
Convolution cyclic voltammetry is believed to be superior to conventional cyclic
voltammetry for the quantitative evaluation of diffusion coefficients in these systems.
The values of the diffusion coefficients lay in the range 3-2210-5 cm2 / sec. The
values of diffusion coefficients found in this research are inversely correlated to the
concentration of iron. The Arrhenius temperature dependence of the diffusion
coefficients is linear and characterized by the value of Ea=31.2 kJ/mol.
Acknowledgements
To B. Markovsky (Bar-Ilan university) for his kind consultations on numerous aspects
of this research.
References and notes
1. Andreasen, H. A.; Bjerrum, J. Inorganic Chemistry, 1975, 14(8), 1807.
2. Inman, D.; Legey, J. C.; Spencer, R., J. Appl. Electrochem., 1978, 8, 269.
3. Shuzhen, D.; Dudley, P.; Inman, D., J. Electroanal. Chem., 1982, 142, 215
4. Castrillejo, Y., Martinez, A. M., Vega, M., Sanchez Batanero, P., J. Appl.
Electrochem., 1996, 26, 1279.
5. Lugovskoy A., Zinigrad, M. and Aurbach, D. submitted to Israel Journal of
Chemistry
6. Nguen-Ba-Chanh, Bastide, J.-P, J. Chim. Phys. et Phys.-Chim.-Biol, 1968, 65,
1425.
7. Støre, T.; Haarberg, G.M.; Tunold, R. J. Appl. Electrochem. 30, 2000, 1351.
83
8. Bard, A. J.; Faulkner, L.R.: Electrochemical Methods: Fundamentals and
Applications, Wiley, 2nd edition, 2000.
9. Berzins, T.; Delahay, J. J. Am. Chem. Soc. 75 (1953) 555.
10. Holliday, A. K. ; Chambers, C., Modern Inorganic Chemistry, Butterworth,
1975, p. 394.
11. Molten Salts, Characterisation and Analysis, edit. by G. Mamantov, Marcel
Dekker, N-Y., 1969, p. 519.
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