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A Quartz Tube Based Ag/Ag+ Reference Electrode Bull. Korean Chem. Soc. 2009, Vol. 30, No. 1 133
A Quartz Tube Based A
g
/A
g
+
Reference Electrode with a Tun
g
sten Tip Junction
for an Electrochemical Study in Molten Salts
Y. J. Pa rk , * Y. J. Jung, S. K. Min, Y. H. Cho, H.-J. Im, J.-W. Yeon, and K. Song
Nuclear Chemistry Research Division, Korea Atomic Energy Research Institute, Daejeon 305-353, Korea
*E-mail: jparky@kaeri.re.kr
Received July 15, 2008, Accepted November 18, 2008
A newly designed Ag/Ag+ reference electrode in a quartz tube with a tungsten tip junction (W-tip-Quartz- REF)
was fabricated and its electrochemical performance was compared with a conventional Pyrex tube-based Ag/Ag+
reference electrode (Py-REF). The results of the electrochemical potential measurements with the W-tip-Quartz-
REF and the Py-REF in the LiCl-KCl eutectic melts for a wide temperature range proved that the oxide layer on
the surface of the tungsten metal tip provided a high ionic conduction. Stability of our newly designed W-tip-
Quartz-REF was tested by measuring a junction potential for 12 hours at 700oC. The results of the cyclic voltam-
metric measurement indicated that the Ag/Ag+ reference electrode in the quartz tube with a tungsten tip junction
can provide a good performance for a wide temperature range.
Key Words: Reference electrode, Quartz tube, Electrochemical, Eutectic melts
Gamry
Potentiostat
Keithley
Electrometer
Temperature
Controller
T/C Quartz Cell
LiCl-KCI Eutectic Melt
Quartz-REF
W-tip-Quartz-REFPyrex-REF
MO electrode MO electrode Ar-filled
Glove Box
Fig ure 1. Schematic diagram of the electrochemical measurement
system in the molten salts media.
Introduction
Pyrochemical processing of nuclear fuels using a molten
salt as a solvent is regarded as one of the promising options for
a future spent nuclear fuel management.1 Molten salts are
known as suitable media for electrorefining and electro-
winning of metal. In order to reach a better understanding and
control of these metal deposition processes, accurate know-
ledge of the electrochemical deposition mechanism is essential.
Therefore, many electrochemical studies of actinides and lan-
thanides in various molten salts have been carried out in the
past decade.2,3 However, electrochemical studies of a molten
salts system, specially in the temperature range of 450 to 90
0℃, can incur many difficulties due to the problems encoun-
tered in selecting a reference electrode with all the desired
characteristics i.e stability, durability, reproducibility and also
fast response.
In electrochemical studies, a three-electrode cell incorpo-
rating a stable reference electrode is essential to avoid an un-
certainty of the electrode reactions. An Ag/Ag+ electrode is a
conventional reference electrode used for a molten salts me-
dia such as LiCl and LiCl/KCl eutectics. Pyrex, so called so-
dium glass, is commonly used as a reference electrode at a
molten salts temperature between 450 and 600oC, since it can
be fabricated easily into a thin-wall tube and thus provide a
sufficient ionic conductivity.4-6 It cannot be used at the tem-
peratures above 600oC, since it can be deformed easily due to
its low melting point (820oC).
Quartz is commonly used in various research fields espe-
cially at a high temperature up to 1200oC, since it becomes
soft at a temperature around 1400oC. Considering that a silver
wire in an electrode has a melting point at around 962oC, a
quartz tube-based electrode with the combination of a quartz
tube and a silver wire could only be used for a working tem-
perature up to ~ 950oC. P. Gao et al.7 tested a quartz sealed
Ag/AgCl reference electrode for a CaCl2 molten salt system.
This reference electrode revealed a successful performance
from an electrochemical analysis in CaCl2 based molten salts
in the temperature range between 700 and 950oC. This quartz
based electrode, however, had a major drawback due to its
high electric resistance at a temperature below 700oC. For ex-
ample, the resistance of the quartz electrode measured at 600oC
was 4×105 ohm.
In this study, an Ag/Ag+ reference electrode in a quartz tube
was fabricated with a tungsten tip junction which contains a
porous metal oxide layer for an ionic conduction, and then its
performance for an electrochemical measurement was as-
sessed in high temperature molten salts between 450 and 600oC.
Expe rimental and Methods
Chemicals and instrumentation. Lithium chloride (LiCl)/
potassium chloride (KCl) eutectic salts (anhydrous beads)
were obtained from Aldrich Co. Ltd. (purity ≥ 99.999%).
Sliver chloride (AgCl) was purchased from Alfa Aesar (purity
≥ 99.998%). All the chemicals were used without further puri-
fication.
The electrochemical setup used for the voltammetric
studies is shown in Fig. 1. The electrochemical reaction vessel
134 Bull. Korean Chem. Soc. 2009, Vol. 30, No. 1 Y. J. Park et al.
WxOy layer
Tungsten tip
Sliver wire
Quartz Cell
LiCl-KCl eutectic
WxOy layer
Tungsten tip
Sliver wire
Quartz Cell
LiCl-KCl eutectic
WxOy layer
Tungsten tipTungsten tip
Sliver wireSliver wire
Quartz CellQuartz Cell
LiCl-KCl eutecticLiCl-KCl eutectic
Figure 2. Conceptual diagram and photo of the quartz tube‐
b
ased
reference electrode with a tungsten tip junction.
W-ti
p
-Quartz tube-based A
g
/A
g
+
Quartz tube-based A
g
/A
g
+
P
y
rex tube-bas ed A
g
/A
g
+
Figure 3. Shapes of the W-tip-Quartz-REF, Quartz-REF and Pyrex-
REF after the electrochemical experiments at 750oC.
was made of a quartz tube (350 mm in length, 40 mm outer
diameter and 2 mm in wall thickness) by using a glass blowing
technique. Two molybdenum wires of 2.0 mm in diameter
were used as a working and counter electrode, respectively.
These molybdenum wires were encased in quartz tubes to
prevent an electrical contact with other electrodes. One end of
the molybdenum wires were polished thoroughly by using
sand paper prior to every use. The Ag/Ag+ reference electrode
contains 1.00 mol% of AgCl in the LiCl-KCl eutectic melts
and the Ag wire is inserted into the tube allowing the Ag wire
being immersed in the molten salts. Finally, the top of the
reference electrode was sealed with a Teflon tape to prevent
an evaporation of the molten salts at high temperature.
The temperature of the molten salt was measured to ± 1oC
using a calibrated K type Chromel-Alumel thermocouple wire.
Cyclic voltammograms were obtained using a voltametric an-
alyzer (Gamry Instruments Reference 600 potentiostat/galva-
nostat) interfaced with a PC at a scan rate of 50.0 mVs-1.
Electrochemical potentials were measured by using a electro-
meter (Keithley Model 6514, input impedance = 2×1014 Ω).
All the performance and sample preparations were handled
inside a glove box with argon atmosphere in which the oxygen
content and moisture levels were less than 1 ppm.
Fabrication of Ag/Ag+ reference electrodes. Three tube-
based Ag/Ag+ reference electrodes, (1) Pyrex tube with a thin
wall-end (Py-REF), (2) quartz tube with a thin wall-end (Quartz-
REF), (3) quartz tube with a tungsten tip junction (W-tip-
Quartz-REF) were fabricated to test their performance for an
ionic conduction and electrochemical measurement in LiCl-
KCl eutectic melts in the temperature range between 400 to
700oC.
Pyrex and quartz tubes with a thin wall-end were prepared
by using a general glass blowing technique. A W-tip-Quartz-
REF was prepared by inserting a small piece of tungsten wire
(1.0 mm diameter, 7.0 mm length) into the bottom wall of the
quartz tube as shown in Fig. 2. Prior to the insertion, the sur-
face of the tungsten metal tip was oxidized by using a torch
flame in order to obtain a high ionic conduction through the
porous tungsten oxide layer.
Res ults and disc ussi on
The electrochemical reaction of the Ag/Ag+ reference elec-
trode in molten salts can be described as follows:
Ag+ (molten salt) + e- (Ag wire) = Ag (Ag wire) (1)
A conventional Pyrex tube-based reference electrode has
been widely used since it has advantages such as a low cost
and easy fabrication by a glass blowing technique. Since one
end of the Pyrex electrode is inserted into a thin-wall, at ap-
proximately less than 0.5 mm, it provides a high ionic con-
duction due to a diffusion of the sodium ion in the Pyrex.
Quartz fabrication into a thin end-wall is performed by using
a hydrogen-oxygen flame as well as a glass blowing technique
in our laboratory. Even though a precise mechanism of ionic
conduction for a quartz tube-based reference electrode has not
been established yet, the resistance of the quartz end-wall,
which is mainly affected by the thickness of the end-wall, is
too high to provide a sufficient ionic conduction at temper-
atures lower than 750oC. The required ionic conduction can be
achieved by using a porous diaphragm or a salt bridge. For the
same logical connection, a porous metal oxide layer was ap-
plied to our new design of a quartz tube-based electrode for an
ionic conduction. Surface of a small piece of tungsten wire
was oxidized by using a torch and then inserted into the bot-
tom end of the quartz tube. Fig. 2 shows a conceptual drawing
of the quartz tube-based Ag/Ag+ reference electrode with a
metal tip junction which contains a porous metal oxide layer
for an ionic conduction as well as a photo of our final product.
Performance of three types of tube-based Ag/Ag+ reference
electrodes, a pyrex tube type, a quartz tube type, and a quartz
tube with a tungsten tip junction were tested for their stability,
and also their electrochemical properties. Fig. 3 shows the
photos of three types of tube-based electrodes after a 12 hours
experiment in LiCl-KCl eutectic melts at 750℃. The Pyrex
tube-based electrode was severely damaged and finally bro-
ken to pieces probably due to its low melting point (820℃),
while quartz tube-based electrodes revealed a good stability in
a molten salt experiment at a high temperature.
Performance of the quartz tube-based reference electrode
was tested by measuring the potential difference by using the
Pyrex tube-based reference electrode as a primary reference.
The potential difference between the quartz tube-based
W-tip-Quartz-REF and the Pyrex tube-based Py-REF in the
A Quartz Tube Based Ag/Ag+ Reference Electrode Bull. Korean Chem. Soc. 2009, Vol. 30, No. 1 135
024681012
-0.01
0.00
0.01
0.02
0.03
Time (hour)
Time (hour)
0.03
0.02
0.01
0.00
-0.01
Potential between W-tip-Q-REF and Py-REF
0 2 4 6 8 10 12
Figure 5. Electrochemical junction potential changes between the
quartz tube-based Ag/Ag+ reference electrodes with a tungsten tip
junction and the pyrex tube-based Ag/Ag+reference electrode in
the LiCl-KCl eutectic melt at 700 oC for various time durations.
400 450 500 550 600 650 700 750
-0.03
-0.02
-0.01
0.00
0.01
0.02
0.03
0.04
0.05
Temperature (oC)
0.05
0.04
0.03
0.02
0.01
0.00
-0.01
-0.02
-0.03
Potential between W-tip-Q-REF and Py-REF
400 450 500 550 600 650 700 750
Temperature (℃)
Figure 4. Electrochemical junction potentials between the quartz
tube-based Ag/Ag+ reference electrodes with a tungsten tip junction
and the pyrex tube-based Ag/Ag+reference electrode in the
LiCl-KCl eutectic melt at various temperatures.
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
0.5
Current(A)
Potential(V vs. Ref.)
545℃
Potential(V vs. Ref.)
-2.5 -2.0 -1.5 -1 .0 -0.5 0 .0 0.5 1.0
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
A
Potential(V vs. Ref. )
558℃
Potential(V vs. Ref.)
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
Current(A)
V vs. Ref.
578℃
Potential(V vs. Ref.)
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
Current(A)
V vs. Ref.
595℃
Potential(V vs. Ref.)
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
A
Potential(V vs. Ref.)
608℃
Potential(V vs. Ref.)
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-0.4
-0.2
0.0
0.2
0.4
0.6
Current(A)
V vs. Ref.
619℃
Potential(V vs. Ref.)
Figure 6. Cyclic voltammograms at the molybdenum electrode using a quartz tube-based Ag/Ag+ reference electrode in LiCl-KCl eutectic
melts at various temperatures.
LiCl-KCl eutectic melts were measured in the temperature
range between 440 and 750oC. As shown in Fig. 4, a low po-
tential difference of less than 10 mV was observed.
Stability of our newly designed W-tip-Quartz-REF was
tested for 12 hours at 700oC. As shown in Fig. 5, the potential
difference between the W-tip-Quartz-REF and the Py-REF
one only varies within 1.4 mV, which indicates a good
stability. In General, tungsten metal is oxidized to WO3 in an
oxygen atmosphere.8 In this oxidation process, the surface of
the tungsten wire turns to a porous structure due to the density
difference of W metal (19.35 g/cm3) and WO3 (7.16 g/cm3).9
Therefore, it can be concluded that a good stability of the
W-tip-Quartz-REF is mainly due to the porous structure of
WO3 formed on the surface of the tungsten wire, which pro-
vides additional ion conducting path.
Fig. 6 and Fig. 7 show representative examples of the cyclic
voltammograms for the LiCl-KCl eutectic melts at the mo-
lybdenum electrode obtained with the W-tip-Quartz-REF and
the quartz tube-based Ag/Ag+ electrode (Quartz-REF), re-
spectively, with in a temperature range between 450 and
700oC. Quartz-REF did not provide a stable and reproducible
current response with a severe noise for the applied potential
range between -2.5 and 0.5 V on the Mo working electrode at
the temperature below 600oC as shown in Fig. 6. Even though
this noise phenomenon became lower as the temperature in-
creased, some noises were still observed at the temperature
higher than 600oC. This result indicates that the Quartz-REF
cannot be recommended as a reference electrode in the tem-
perature range between 450 and 700oC.
It was found that the current increase at the beginning of po-
tential scanning (0.5 V) and the vertex potential (-2.5 V) was
increased as shown in Fig. 7. This phenomena happens prob-
136 Bull. Korean Chem. Soc. 2009, Vol. 30, No. 1 Y. J. Park et al.
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-0.09
-0.06
-0.03
0.00
0.03
0.06
0.09
Current(A)
Potential(V vs. Ref.)
468℃
Potential(V vs. Ref.)
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-0.20
-0.15
-0.10
-0.05
0.00
0.05
0.10
0.15
0.20
Current(A)
Potential(V vs. Ref.)
530℃
Potential(V vs. Ref.)
-2.5 -2.0 -1.5 -1.0 -0 .5 0.0 0.5 1.0
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
Current(A)
Potential(V vs. Ref.)
578℃
Potential(V vs. Ref.)
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-0.4
-0.3
-0.2
-0.1
0.0
0.1
0.2
0.3
0.4
Current(A)
Potential(V vs. Re f.)
616℃
Potential(V vs. Ref.)
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-0.4
-0.2
0.0
0.2
0.4
Current(A)
Potential(V vs. Ref.)
646℃
Potential(V vs. Ref.)
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
Current(A)
Potential(V vs. Ref.)
700℃
Potential(V vs. Ref.)
Fig ure 7. Cyclic voltammograms at the molybdenum electrode using a quartztube-based Ag/Ag+ reference electrode with a tungsten tip junc-
tion in LiCl-KCl eutectic melts at various temperatures.
ably due to that the reaction rate of electrolyte oxidation (Li+ +
e → Li+) and reduction (Cl‑ → 1/2 Cl2 + e) reactions were
increased. The rate of electrochemical reaction, generally, in-
creases exponentially rate with overpotential as well as the
slope of the current gain.10 Therefore, the polarization behav-
ior shown in Fig. 7 seems to correspond to the normal electro-
chemical behavior of overpotential and temperature. However,
when using the Quartz-REF, relatively smaller current re-
sponses, thus loss of current, were observed from the begin-
ning (0.5 V) to the vertex (-2.5 V) potential under the same ex-
perimental condition. As the IR drop causes the loss of cur-
rent, this result indicates a superiority of the W-tip-Quartz-
REF with a point junction compared with the Quartz-REF
from the IR drop of view.
Conclusions
In this study, we investigated the performance of a newly
designed Ag/Ag+ reference electrode in a quartz tube with a
tungsten tip junction. The oxide layer on the surface of the
tungsten metal tip provided a high ionic conduction. The
results of the electrochemical potential measurements between
the W-tip-Quartz-REF and the Py-REF one in the LiCl-KCl
eutectic melts for a wide temperature range, and also a 12 hour
experiment at 700oC, revealed a good performance and
stability of our W-tip-Quartz-REF. The results of the cyclic
voltammetry measurement indicated that the Ag/Ag+ reference
electrode in the quartz tube with a tungsten tip junction can
provide a good performance for a wide temperature range,
especially at a high temperature up to 700oC. Consequently, a
quartz-tube based Ag/Ag+ reference electrode with a tungsten
tip junction can be a good solution for an electrochemical
measurement for a wide temperature range in molten salts.
Ac know ledgments . This work was supported under the mid-
and long-term nuclear research and development program of
the Korean Ministry of Education, Science and Technology.
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