ZINC IONS REDUCTION ON SOLID METAL ELECTRODES IN
CHLORIDE MELTS
Alex Lugovskoy *1a, Zeev Unger 1,2b, Michael Zinigrad 1c,
Doron Aurbach 2d
1
Material and Chemical Engineering Department, Ariel University
Center of Samaria, Ariel, 40700, Israel
2
Department of Chemistry, Bar-Ilan University, Ramat-Gan, 52900,
Israel
a
lugovsa@ariel.ac.il, bzevikito@ariel.ac.il, сzinigrad@ariel.ac.il,
d
aurbach@mail.biu.ac.il
keywords: electrodeposition, chloride melts, cyclic voltammetry, hightemperature electrochemistry
Abstract
The reduction of zinc ions on solid tungsten and platinum
electrodes in chloride melts at the temperatures 700 – 750 °C was
studied by cyclic voltammetry, chronoamperometry and energy
dispersion spectroscopy. It was established that no zinc is reduced on
platinum electrodes. As for the reduction of zinc ions on tungsten
electrodes, the process has a complex character: it starts as an
irreversible two-electron zinc ion reduction and, after the new phase is
formed, the process of saturation of the electrode surface with lithium or
sodium begins. As the second process develops, the alkaline metal
becomes essentially the only constituent on the electrode surface.
General
Since zinc is industrially recovered from sulfate solutions rather
than from melts and because its melting temperature (419.5 °C) is lower
than the temperatures of most molten chloride compositions, the
reduction of zinc ions on solid electrodes in chloride melts has been
investigated relatively poorly. There are quite a few papers devoted to
the electrolysis of zinc containing chloride melts (1, 2) and these cover
only some details of the electrochemistry of this metal. However, zinc is
not only an engineering metal. It can often be a component of molten
chloride systems, in which various processes of synthesis or purification
104
are performed. Therefore, the detailed electrochemical behavior of zinc
can be of great importance.
The study of electro-reduction processes of zinc ions on solid tungsten
and platinum electrodes in eutectic NaCl – KCl and LiCl – KCl melts in
the temperature range of 700 – 750 °C is presented in this work. These
temperatures are somewhat higher than the eutectic points of NaCl –
KCl (646 °C ) and LiCl – KCl (628 °C) and the melts are therefore
"liquid enough" to be used in technologically important processes of
lanthanides and actinides separation, reduction and rectification. On the
other hand, these temperatures are significantly lower than the boiling
point of zinc (907 °C) and there is essentially no loss of the metal due to
evaporation.
Experimental
The electrochemical experiments were performed using a threeelectrode cell, made of sintered alumina, placed in an alumina crucible
under nitrogen atmosphere. Tungsten (99.95%, 1 mm diameter) and
platinum wires (99.95%, 0.5mm diameter) were used as the working
electrodes and their surface area was controlled by immersion depth
(typically, 6–12mm) and by measuring their diameter before and after
each experiment. A 1mm tungsten wire served as a pseudo-reference
electrode, and a flat spiral tungsten wire, set perpendicular to the
working and reference electrodes close to the bottom of the cell, served
as the counter electrode. The area of the counter electrode was ~ 20 fold
as large as that of the working electrode. ZnCl2, LiCl, NaCl and KCl
(99.0% +ACS grade, Alfa Aesar) were used for the preparation melts
without further purification.
Zinc chloride was mixed with alkaline metals chlorides using
mortar and pestle in a glove-bag in dry nitrogen atmosphere. The
mixture was then placed into a crucible, the electrode cell was mounted
and transferred into the furnace (single-zone Carbolite 1600 °C STF tube
furnace). In the furnace the mixture was first dried under vacuum at 40–
50 °C for an hour. After completing the drying, dry nitrogen was
bubbled through the electrolyte during its heating up to the temperature
of the experiments (700–750 ◦C) for another hour. The temperature was
controlled by a type S thermocouple, placed next to the cell and
protected by an alumina capillary, thus maintaining a precision of ±1 °C
in measuring and controlling the temperature. Dry nitrogen atmosphere
105
(1 bar) was maintained in the furnace during the measurements and the
post-experimental cooling. The electrochemical measurements were
carried out using an Autolab PGStat-12 potentiostat. SEM images and
element analysis by EDS were performed with a SEM system from
JEOL Inc., Model JSM 7000F.
Results and discussion
Deposition of zinc on a tungsten electrode
Some typical voltammograms for the electrochemical reduction of
Zn(II) are shown in Fig. 1.
0.4
A
, V/sec
E ,V
p
0.3
0.05
0.2
0.5
-0.600
-0.650
-0.680
1.10
1.41
1.64
1.13
1.50
1.77
(peak C)
Q , C/cm
2
Q , C/cm
2
c
0.2
i, A/cm
2
a
0.1
Q /Q ~ 1
a
c
0
-0.1
C
-0.2
-1
-0.5
0
E, V vs. W
Fig. 1. Cyclic voltammograms related to the electrochemistry of Zn2+ ions
(0.163 mol / L) in equimolar NaCl-KCl melt on a W electrode at 700°C. Scan
rates are 50 mV / sec (solid line), 200 mV /sec (slashed line) and 500 mV / sec
(dotted line). Each charge density was calculated as the sum of areas limited by
the baseline and the appropriate current density curves for the forward and
backward semi-cycles.
106
As follows from Fig. 1, a single cathodic peak C corresponds to
one anodic peak A. The potential, shape and behavior of the cathodic
peak are typical for the metal deposition on a solid electrode (2-4). No
difference is observed between the reduction of zinc ions in NaCl – KCl
and in LiCl – KCl melts. Peak A is assigned to the reoxidation of zinc.
Both peaks are clearly not independent on the scan rate. Rather, peak C
is shifted to more negative potentials and peak A moves to more positive
potentials as the scan rate increases. The dependence of the cathodic
peak potential on the scan rate is shown in Fig. 2. Such voltammetric
response is typical for irreversible processes.
0.75
p
-E , V
0.7
0.65
0.6
0.55
0
0.1
0.2
0.3
0.4
0.5
0.6
, V/s
Fig. 2. Dependence of the cathodic peak potential on the scan rate for the
reduction of Zn2+ (0.163 mol / L) at 710°C on a W electrode.
The cathodic peak C appears at about -0.6 V vs. tungsten electrode
for the scan rate of 50 mV/sec and at -0.7 V for 500 mV/sec. Such a
significant shift is a clear indication that the process is irreversible. The
cathodic peak not only is shifted as the scan rate grows, but it becomes
107
broader so that the difference |Ep – Ep/2| grows from 0.1 V for 0.05 V/sec
to 0.15 V for 0.5 V/sec. Values of n calculated by equation 23, are in
the range of 1.56 for low scan rates to 1.04 for high scan rates. The most
logical interpretation of this finding is that the charge-transfer is of twoelectrons, which is not surprising in the case of Zn2+ ions reduction. The
value of  is then 0.78 for 0.05 V/sec and 0.52 for 0.5 V/sec. This is
evident that the rate determining step is the Faradaic process
Zn2+ + 2e- Zn,
when the system is close to the steady state. Note that at low enough
potential scanning rates diffusion limitations may be less influencing
while at higher scan rates the diffusion limitations are more important.
Randles-Sevcik dependencies for the zinc (II) ions reduction
demonstrate linearity, but their intercepts are apparently non-zero (Fig.
3).
0.7
0.6
p
i , A/cm
2
0.5
0.4
0.3
0.2
0.1
0
0
0.2
0.4
0.6
0.8
1
 ,V s
1/2
1/2 -1/2
Fig. 3. Randles-Sevcik plots for Zn2+ ions reduction on W in a NaCl-KCl melt
at 700 °C, different concentration of the ions (peak C in Figure 39). ○ 9.00x10-5
mol/mL Zn2+;  1.63x10-4 mol/mL Zn2+; ▲ 1.77x10-4 mol/mL Zn2+.
108
It is evident that the process Zn2+ + 2e- Zn is complicated by
something else. Despite the irreversible character of the deposition
process, it is still reasonable to roughly evaluate the diffusion coefficient
of Zn2+ according equation 1:
ip = 0.6105 (nF)3/2/(RT)1/2D1/2C*1/2
(11)
where ip is the peal current density (A / cm2), n is the number of
electrons, F is Faraday constant (96500 C), R is the gas constant (8.314
J/mol∙K), T is the absolute temperature (K), D is the diffusion coefficient
(cm2 / sec), C* is the bulk concentration of a Red (Ox) species (mol /
cm3) and  is the scan rate (V / sec).
Thus calculated diffusion coefficients are shown in Table 1.
Table 1. Diffusion coefficients of Zn2+ to a tungsten electrode in NaClKCl melt.
C*105, mol / L
9.00
16.3
17.7
D 105, cm2 / sec
9.55/n
10.20/n
13.64/n
Given that the value of n for the reduction of Zn2+ cannot exceed 2 and
0 ≤  ≤ 1 ( ≈ 0.5 for most cases), reasonable values of n must be
close to 1-2. Therefore, the values of the diffusion coefficients from
Table 2 lie in the range of 1-6∙10-4 cm2/sec. Available literature data for
the diffusion coefficients of most metal ions lie in the range 10-5-10-4
cm2/sec. Particularly, T. Støre, G. M. Haarberg and R. Tunold found that
the values of the diffusion coefficients for Zn2+ in KCl-LiCl melts at
400°C lie in the range 0.6 – 1.06∙10-5 cm2/sec (2). Delimarski provides
the value of the diffusion coefficient of Zn2+ in NaCl-KCl at 710°C,
which is 2.3∙10-5 cm2/sec (5). The deviation of our results from the
literature data can hint that that the process cannot be treated as simple
zinc ion reduction on the surface of tungsten.
109
It is worth to mention that the fact that the diffusion coefficient
for zinc ions in the chloride melt lay in the range 10-4 – 10-5 cm2/sec may
serve as an indirect argument in the discussion about the existence of
complex species described by the general formula [ZnxCly]z+ in chloride
melts. While some authors argue in favor of the formation of complex
ions (6 – 10), other studies give evidence for the existence of individual
zinc ions as the key reacting species (11 – 12). The relatively high
values of the diffusion coefficients found in our experiments hint that the
charge is transferred by individual ions rather than by more massive
complex moieties.
0.4
0.35
0.25
p
i , A/cm
2
0.3
0.2
o
700 C
o
750 C
0.15
o
740 C
o
720 C
0.1
0.05
0.2
0.3
0.4
0.5
0.6
0.7
 ,V
1/2
1/2
0.8
0.9
1
-1/2
s
Fig. 4. Randles-Sevcik plots for Zn2+ reduction on W in a NaCl-KCl melt for
different temperatures. [Zn2+] = 9.00x10-5 mol/mL
Another intriguing aspect of the zinc ions deposition process on
a tungsten electrode can be seen in the temperature dependence of
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Randles-Sevcik plots (Fig. 4). As seen from Fig. 4, Randles-Sevcik plots
do not change (to the accuracy of the experiment) as the temperature
rises from 700°C to 750°C.
The lack of dependence of Randles-Sevcik plots on the
temperature is really surprising. A plausible explanation to this could be
an additional process in the system, which occurs simultaneously with
the observed process, but does not involve charge-transfer and cannot be
detected electrochemically. Such a process could compensate for the
expected increase of the slope of Randles-Sevcik plots as the
temperature grows and thus distort the temperature dependence.
The most probable candidates for such competing processes are
a coupled chemical (not charge-transfer) reaction or a process of phaseformation. However, cyclic voltammetry alone cannot discriminate
between these two possibilities.
Fig. 5. A chronoamperometric plot for the deposition of Zn2+ on a tungsten
electrode. Temperature 725°C, [Zn2+] = 9.00x10-5 mol/mL. The potential was
stepped from OCV to -0.55 V.
A further insight on the nature of the deposition process can be
provided by chronoamperometry. As seen from Fig. 5, the current falls
in the course of the first 11 seconds of the experiment and then rises,
reaches a peak and gradually declines as expected with time until the
end of the experiment (300 seconds).
The initial falling and rising of the current can be attributed to
the nucleation of the deposits, fluctuations of current for more advanced
111
reaction times as seen in Fig. 5, may indicate to a very active chargetransfer process, which cannot be explained by a simple zinc deposition
process.
Even more surprising information is provided by EDS analysis
of the working electrode after a 3000 second deposition experiment at –
0.55 V (Fig. 6, Table 2). The most striking result of the analysis is the
unexpectedly high content of sodium on the electrode surface. This
amount of sodium cannot be accounted for melt adhesion or penetration,
because the percentage of potassium and chlorine is much smaller. In
fact, the working electrode looks as it was made of sodium with
moderate inclusions of tungsten and zinc, rather of tungsten.
Fig. 6. An EDS spectrum of tungsten working electrode after 3000 second
deposition at – 0.55 V. Temperature 725°C, [Zn2+] = 1.38x10-4 mol/mL.
Table 2. Element composition of the tungsten working electrode surface
calculated from the EDS spectrum after 3000 second deposition at –
0.55 V. Temperature 725°C, [Zn2+] = 1.38x10-4 mol/mL.
Element
At. %
Na
60.84
K
5.80
Cl
28.61
112
W
2.24
Zn
1.91
A somewhat similar phenomenon was reported by Thus T. Støre,
G. M. Haarberg and R. Tunold for the deposition of Zn2+ on a glassy
carbon electrode in KCl-LiCl melts at 400°C (2). They observed a
“substantial residual current observed prior to the Zn(II) reduction
peak”. This current was attributed by them to lithium intercalation into
the lattice of the glassy carbon electrode.
Unfortunately, the data about standard reduction potentials of
many important ions in molten chlorides are lacking. The only source, in
which suitable potentials were found, is the book of Yu. Delimarski
“Electrochemistry of Ionic Melts” (5). The values of standard potentials
tabulated in this book, were calculated on the base a few assumptions
and are far from being strictly thermodynamical. However, they are
helpful from the practical point of view. The potentials relevant for this
discussion are summarized in Table 3.
Table 3. Standard reduction potentials in molten chlorides (adopted from
ref. [5]).
Half-Element
*EH2 (700°C), V
Li+|Li
- 2.39
Na+|Na
- 2.36
K+|K
- 2.50
Zn2+|Zn
- 0.40
Fe2+|Fe
- 0.07
As seen from Table 3, the standard potentials of lithium and
sodium are very close to each other. Therefore, it is not surprising that
the interference from sodium in the deposition of zinc ions is similar to
that of lithium, as reported by T. Støre, G. M. Haarberg and R. Tunold.
Of course, it is not intercalation that serves as the moving force of the
process of sodium penetration into the surface layers of zinc deposit on
the tungsten electrode.
The large amounts of sodium in the deposits obtained in the study
of the Zn2+ ions reduction on tungsten electrodes cannot be explained as
the formation of a W-Na alloy, because such a process is not observed
by the cyclic voltammograms of NaCl-KCl on tungsten electrodes in the
absence of zinc ions (3). Therefore, it is zinc, which triggers the
deposition of sodium. Moreover, the data obtained by
chronoamperometry at E = – 0.55 V vs. W (Fig. 5) indicate that there are
two sequential faradaic processes. The first of them is relatively weak
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and is completed after ~ 11 seconds. Then the second process starts and
its current only grows with time. The first process can be related to the
reduction of zinc ions and the formation of zinc deposits. As the
electrode surface is covered by a layer of zinc, the interaction of this
layer with Na+ ions begins. Apparently sodium ions are absorbed by the
liquid zinc (Tm = 419 °C) and this facilitates their reduction at the
potential so much more positive than the sodium reduction potential in
the absence of zinc ( - 1.1 V vs. W). Both lithium and sodium are liquid
at the temperature of the experiment and these two metals form on the
electrode surface a liquid solution with zinc, which continues to absorb
new portions of the lithium or sodium ions.
The following speculation may account for the phenomenon
observed in our system:
1. Zinc ions are discharged on the surface of the tungsten
electrode. As the surface concentration of zinc atoms grows, nucleation
overpotential starts to dump the overall process. This dumping is
observed in the course of the first 11 seconds in Fig. 5.
2. Zinc (or zinc-tungsten) phase is formed. This phase triggers the
process of sodium-zinc exchange:
Zn + Na+  Zn+ + Na or Zn + 2Na+  Zn2+ + 2Na
3. The process (2) becomes the main process on the electrode
surface.
Deposition of zinc on a platinum electrode
Some typical voltammograms for the electrochemical reduction
of Zn(II) are shown in Fig. 7. Again, no difference is observed between
the processes in NaCl – KCl and in LiCl – KCl melts and two melts are
further described on the instance of in NaCl – KCl alone.
As seen from Fig. 7, the voltammogram is completely anomalous
as compared to the other studied systems. No cathodic peaks are
observed in the range -1.1V to + 0.9V, i.e. in the limits of the
electrochemical window. The peaks – 1.25V and at +0.9 V are the same
as for the “blank” melt NaCl-KCl. These are the limits of the
electrochemical window.
A very poorly pronounced anodic peak A at about – 0.28 V is
similar to the anodic peak A, which appears for the zinc deposition on a
tungsten electrode (Fig. 1). However, the cathodic branch of the
voltammogram contains a continuous transition to the cathodic limit of
114
the windows rather than a peak. It is obvious that zinc deposition is
masked by another process, whose nature cannot be studied in the
framework of this research.
Fig. 7. Cyclic voltammograms related to the electrochemistry of Zn2+ ions
(0.176 mol / L) in equimolar NaCl-KCl melt on a Pt electrode at 700°C. Scan
rate is 300 mV/sec
Fig. 8. An EDS spectrum of a platinum working electrode after 3000 second
cathodic polarization at – 0.7 V vs. W at 725°C in equimolar NaClKCl melt containing 1.76x10-4 mol/mL of Zn2+ ions.
115
An attempt of obtaining a sample of zinc deposit by holding the
system at – 0.7 V (that is, at such a potential, which is considerably more
positive than the cathodic limit, but more negative than the potential, at
which zinc is deposited on a tungsten electrode) for 3000 seconds, was
made. However, the analysis (Fig. 8) demonstrated that essentially no
zinc is found on the surface of the electrode (Table 4), since the value
0.98 At. % is comparable with the sensitivity of the method. The rich
content of potassium (58.57 At. %) in the surface layers can hint that
potassium sorption is the process, which masks the deposition of zinc.
However, this information alone is not sufficient for making positive
conclusions.
To try to understand the essence of the process, other molten
chloride systems containing no potassium could be studied. However,
such a study is far beyond the framework of the current work.
Table 4. Element composition of the platinum working electrode surface
calculated from the EDS spectrum after 3000 second deposition at –
0.55 V. Temperature 725°C, [Zn2+] = 1.76x10-4 mol/mL.
Element
At. %
Na
5.55
K
58.57
Cl
34.26
Pt
6.18
Zn
0.98
Conclusions
The deposition of zinc on a tungsten electrode starts as an
irreversible two-electron zinc ion reduction: Zn2+ + 2e- Zn. After an
obvious initial nucleation step, a new phase is formed. This phase
catalytically launches the process of saturating the electrode surface with
sodium. After the onset of the process of sodium deposition the latter
becomes essentially the only constituent on the electrode surface.
The attempts of studying the deposition of zinc ions on a platinum
electrode were unsuccessful because this process is masked by another
process, which can result in the saturation of the electrode by potassium.
The exact nature of the latter process demands a separate study.
116
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