Electro deposition of Zinc on solid Tungsten and Platinum in high
temperatures molten salts
Unger Zeev (a)
1
2
Ariel University Center of Samaria, Ariel 44837, Israel
Bar-Ilan University, Ramat-Gan, Israel
(a) zevikito@ariel.ac.il
Abstract:
When approaching molten salts systems we are encountered with what could be
described as "abnormal" chemistry. We will demonstrate the basic principles of
kinetics in such systems and will discuss two of those systems. They will shed light
on the chemistry involved in molten salts and will allow an overlook of that
fascinating chemistry world. The results will be analyzed and two theories of kinetics
in such systems will be proposed.
Introduction:
In this research, an explanation of the possible kinetics and behavior for the electro
deposition of zinc from zinc chloride in different systems of molten salts will be
described.
Some basics kinetic models that will enable a systematic analysis of the experimental
data and hence to further understand the chemical reactions that occur inside the
sealed system, which contains the molten salts and the reactant, will be given. When
concerning the effect of high temperature it is necessary to take into account that often
such high temperature influences and even changes the very nature of reactions and
metals as to transform minor Chemical reactions, at room temperature, into crucial
reactions at high temperatures. It is important to further note that, in this project, all
the work that was done, was done in un aqueous solutions and there for, all the
interactions between the solvent and the electro active species differs from the
"normal" electrochemistry. Here, we concentrated our efforts on electro deposition of
Zinc on solid tungsten and platinum electrodes in a mixture of NaCl and KCl (being a
mixture that is in high use in the electrochemical world, even though it was not
intensively researched). This environment, of molten salts, gives a number of crucial
advantages, in regarding to electrochemistry reactions. The first is an excellent
electric conductivity and the second is a large electrochemical window in which we
can perform measurements without interfering with the solvent. Hence, working with
only the desirable electro specie and avoiding problematic issues like side reactions
and contaminating reagents, as known from aqueous chemistry.
In this work, a ternary salt system was used in order to investigate the chemistry
involved but also as a mean to lower the temperature inside the system as we
encounter the eutectics point, giving us the ability to work only at around 7000C
instead of around 8000C (the melting point of pure NaCl). Given that, we where able
to melt the mixture of salts at around 6600C . Temperatures started at 7000C and
ended at 7500C as above that point the fumes of chlorides and the evaporation of the
salt will change the concentration of our electrochemically specie in the vessel (where
all the mixture is kept and the reactions occur) and will cause a sever problem when
calculating physical values from the voltammogram. (Such as extracting the diffusion
coefficient, for example).
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Our theory was that the whole process is controlled by diffusion. And if so, finding
the reaction limiting step we will give us the tools to determine the kinetics and
mechanism for the reactions. (Hence the mathematical analysis, in which we can
calculate the diffusion coefficient according to the known. Randles – Sevcik equation:
ip = 0.4463 (nF)3/2/(RT)1/2D1/2C*1/2 and in accordance to Berzins and Delahay
equation : ip = 0.6105 (nF)3/2/(RT)1/2D1/2C*1/2 where D stands for the diffusion
coefficient, and  is the scan rate).
We will demonstrate the differences between the two systems and the regarding
results and ideas concerning them both. As will be explained later in the text, the two
differs in the rate determining step and therefore, the aspect toward each of them will
be different. An explanation of the difference in the systems will count on both
mathematical solutions and on analytical review and spectral analysis. This theory, of
a diffusion controlled system, will not answer questions regarding the second system
and there for, a second theory will be given and proven (regarding to the system in
which we electrodeposited Zinc on a solid Pt electrode). When comparing the two
systems we will in fact compare two very different types of reactions, and when
placed along side to each other, will give us a perspective of molten salts chemistry.
Experimental Section
We employ cyclic voltammetry (CV) in order to investigate and perform the redox
and the oxidation reactions upon the solid working electrode. Using the CV is highly
useful as an analysis of the kinetics in the system which is being investigated and is
happening at the same time with the actual reaction. A three electrodes cell was in
use, in which in all systems, the counter electrode was a spiral tungsten wire of 1mm
diameter and the reference electrode was a plain 1mm width tungsten wire. In the
second system a Pt wire was taken as the working electrode and in the first system a
tungsten wire was used also as the working electrode. The two salts were mixed (KCl
and NaCl) with the electrochemical specie (ZnCl2) and inserted into a ceramic vessel,
following insertion to a special alloy container and into the inert furnace.
Pic. 1 the three electrodes are stuck
in the melt (the mixture of salts
after the work with CV at high
temperature).
Special precautions had to be taken as the ZnCl2 is very hygroscopic and so, a
procedure was plotted in order to rid all the moisture in the air, as we did not want to
let the metal oxidize by the water in the air: 1. Vacuum the furnace for about half an
hour 2. Slowly raise the pressure up to a single atmosphere while pumping in an inert
gas. 3. In order to make sure that the system is dry, the temperature was raised slowly
up to about 1500C. 4. Followed by another heating, this time almost to the verge of
melting the mixture of salts and ZnCl2. 5. Heat the system to 7000C.
It is worth noticing that there has been a great effort trying to complete as much work
as possible inside a glove bag as all the reagents involved in this research are
hygroscopic, especially the ZnCl2. The gas used in order to keep the system inert was
normally nitrogen (5\9) but there has been some use in Argon as well. All the work
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was done in a Carbolite inert furnace and the measurements were taken using an
Autolab potentiostat\ galvanostat
Measurements where taken at different sets of temperatures and different sets of
ingredients (different concentrations of the ZnCl2) in order to be able to compare
different settings of the same system. Each arrangement was prepared as described
earlier and was set of using the cyclic voltammetry.
In order to make sure that the results have derived from the actual reaction that we
wanted to study and were not influenced by a nucleation process and that the
nucleation is not worth noticing, we also employed a chronoamperometry analytical
procedure (as known, in voltammetry, the peak of the current, as showed in the
voltammogram, may be influenced by the nucleation step and as we are dealing with
high temperature chemistry we could not over look that question).
Analysis was not done strictly by mathematical tools, but also using a S.A.M which
gave us an exact look of our working electrodes and was of a crucial proof that our
theory was, in fact, true. Our initial concept of how the system should probably work
was accurate for the system in which we performed electro deposition of Zinc on solid
W electrode. When we analyzed the second system, where the Zinc was electro
deposited on solid Pt electrode we had built another kinetics theory, which is in high
stages of work, as this article is being written.
Result and discussion
We now have to assume a logic kinetic model for this behavior, which means that we
have to find what the rate determining step is and what is the reaction that takes place
there.
1. The first system that will be analyzed is in which our original idea was proven,
accurate. This being the system in which Zinc was electrodeposited on a solid W as a
working electrode. Concentration of the Zinc chloride that was added was averaging at
around 3.16E-05 mol\ml and was deposited at 7000C, 7100C, 7300C, 7500C with scan
rates rating from 20 mV\S to 1200 mV\S.. A typical voltammogram of that system is
shown is fig 1. Where the X axis stands for the Voltage (measured in volts) and the Y
axis stands for the current (measured in amp).
Fig1. 200 mV\S
at 7000C
As can very clearly seen in fig. 1 the current is very high (if compared to "normal"
aqueous chemistry) and the catodic peak is clear. We could easily extract the peak
current and divide it by the area of working electrode that was sank is the mixture in
order to receive the peak current density. As shown before, using the Randles –
Sevcik equation and the Berzins Delahay equation we calculated the diffusion
coefficient. The Randles – Sevcik model is considered to be true in the case of ideal
systems. Clearly, this is not the case here, as we are talking about molten salts, and as
that being the case, there are certainly numerous interactions inside the solvent and
between the electro active specie and the solvent itself. Because of that fact the
Berzins and Delahay model (5) gave what might be considered the real coefficient
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constant. A S.A.M was employed on the working electrode sample and clearly
showed that the Zinc was layered nicely on the working electrode, as we expected.
The sample of the working electrode was taken by means of chrono amperometry at
the peak voltage and was left for an hour; until we had no doubt that the reaction has
ended (the electro deposition on the solid working electrode). In order to conclude
that this electro deposition is, in fact, controlled by diffusion, we extracted the root of
the scan rate versus the peak current density and encountered an almost linear graph,
as shown in fig. 2 (Randles-Sevcik plot). This also proves the reversibility of the
process and is a strong conformation to our initial theory. Combined with the results
of the map and line scanning of the S.A.M. we can conclude that our kinetic model is
the right one and that the system is indeed, diffusion controlled. Further more,
According to all the information we collected we can assume in good confidences that
the reaction is carried out in the following manner: there are two electrons moving at
once from the metal to the reactant back and forth (depending on the voltage we
employ on the system using the CV) and the whole process is relatively simple and is
controlled by diffusion. Meaning that the rate of the reaction is determined by how
fast the reactant can move toward the working electrode from the depth of the phase.
That rate is measurable, and indeed was calculated, as said before. We calculated the
diffusion coefficient 750C
and showed that it is consistent with logic – the constant raised
as temperature rose. Conclusion of that calculation is shown in fig. 3
30
25
20
15
10
0
0 -0.1
5
-0.2
y = -0.0222x - 0.0329
R2 = 0.9982
-0.3
SR^0.5
-0.4
)SR^0.5( ‫ליניארי‬
ip
35
Fig 2.showes the
Randles-Sevcik plot
-0.5
-0.6
-0.7
-0.8
‫סיכום‬
SR^0.5
3.50E-06
3.00E-06
1‫סידרה‬
)1‫ליניארי (סידרה‬
D
2.50E-06
Fig. 3 shows the
conclude constant (D)
versus the
temperature.
2.00E-06
1.50E-06
1.00E-06
5.00E-07
0.00E+00
690
700
710
720
730
740
750
760
Temp
2. When coming to work with the second system - electro deposition of Zinc on solid
Pt we anticipated to see similar results, only with probably two catodic peaks, as we
are referring to Pt . And indeed, the scan showed two peaks. However, as it turned
out, we had difficulty determining that this system was in fact diffusion controlled.
We employed a S.A.M on the sample (the working electrode) and the result of the
mapping scan showed that the electro deposition of Zinc on top of the Pt working
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electrode was random and scarce. As of now, we can not explain with out a shadow of
a doubt why that is. We suggest two alternative kinetics models.
2.1. It is reasonable to assume that Zinc is not likely to react with the platinum solid
working electrode with ease and we are required to adjust our system in such a
manner that the reactions will yield the solid Zinc on top of the solid Pt in a clear
deposit. It is possible that the absorption to another element, in our case, the Pt, can
interfere with the desired reaction and hence the result. Since that being the case, the
Zinc is then "reluctant" to further accept another electron and hence the low rate of the
reaction. As evidence we can clearly see the random dispersion of the Zinc on top of
the Pt electrode when examining the working electrode in S.A.M. (using map and line
scanning) and the small amount of Zinc that actually went through the reaction and
was deposited on the working electrode. If in fact, it was a diffusion controlled
reaction then we were supposed to see a nicely layer of deposited Zinc on top of the
working electrode. But, as said before, the Zinc was scarcely seen on the working
electrode and even then, was in a random order .If that being the case, then we will
see the Zinc on top of the counter and reference electrodes. This is being prepared for
spectroscopic view as this paper is written and results have not yet received. If that is
the case then there is no reaction happening in this system on the Pt electrode.
2.2. On the other hand it is reasonable to assume that because we are talking about Pt
then there is some kind of absorption happening on the surface of the solid Pt metal,
but only after the Zn2+ receives a single electron. There are two peaks after employing
CV and need to be explained. One can easily belong to the actual electron transfer and
the second to the absorption. That gives the second hypothesis; meaning that there is a
reaction and that it is preformed in the following manner: the Zn2+ receives only one
electron. It is then absorbed on the Pt surface and we are encountered with a fairly
stable transition state where the Zinc is still Zn+1. Hence, there is a large energetic
barrier and this causes for the deceleration of the reaction. As this article is written,
the work to prove either theory is in progress, mainly by mathematical tools. The
sample here was achieved as described earlier in section 1. A typical voltammogram
of that system is showed in fig.3 showing the two peaks.
Fig3. 200 mV\S at
7000C
Where the X axis stands for the Voltage (measured in volts) and the Y axis stands for
the current (measured in amp).
It will be interesting to prove that point by using the same system, but this time with
Pt also as counter and as reference electrode and compare those reactions with what
was received when the counter and the reference electrode were solid W. If we will
see Zinc on top of the electrode there, then the Zinc is reactive with Pt and in that
case, it is likely that the kinetics is what was described in section 2.2.
With that, we have a good overview of the electrochemistry world, concerning molten
salts and a good grasp of what is actually happening inside those systems. We have
encountered two very different systems and where able to analyze in good precision
what happens in one and give a logical theory as to the other.
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Acknowledgements
To A. Lugovskoy whom with out, this work would have never came into light, to
Prof. M. Zinigrad for the council and all the members of the Advanced Material
Research Centre of the university centre of Samaria in Ariel.
Reference
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Potentiometric and Spectrophotometric - Study of the System KCl-FeC13 in the
'Temperature Range 275-350
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and Electro deposition clusters
3. Steven Christian LANS The Limiting Phenomena at the Anode of the Electrowinning
of Zinc from Zinc Chloride in a Molten Chloride Electrolyte.
4. Alex Lugovskoy Electrochemical Determination of Diffusion Coefficients of Iron
(II) Ions in Chloride Melts at 700-750oC
5. T Berzins, P Delahay, J. Am. Chem. Soc., 1953, 75(3) , 555
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