EQCM measurement of Ag (I) /Ag reaction in an amide-type room-temperature ionic liquid

Article abstract of DOI:10.1149/1.3223669

Electrodeposition of silver was investigated using an impedance technique (separately excited, passive technique) electrochemical quartz crystal microbalance (EQCM) in a room-temperature ionic liquid. The mass changes during silver deposition and dissolut

Full text of DOI:10.1149/1.3223669

Journal of The Electrochemical Society, 156 ͑11͒ D503-D507 ͑2009͒
D503
0
013-4651/2009/156͑11͒/D503/5/$25.00 © The Electrochemical Society
EQCM Measurement of Ag„I…ÕAg Reaction in an Amide-Type
Room-Temperature Ionic Liquid
,
z
Nobuyuki Serizawa, Yasushi Katayama,* and Takashi Miura
Department of Applied Chemistry, Faculty of Science and Technology, Keio University,
Yokohama 223-8522, Japan
Electrodeposition of silver was investigated using an impedance technique ͑separately excited, passive technique͒ electrochemical
quartz crystal microbalance ͑EQCM͒ in a room-temperature ionic liquid. The mass changes during silver deposition and dissolu-
tion were observed with the current efficiencies of nearly 100% during potential sweep, constant potential step, and constant
current step experiments. The product of the viscosity and density of the electrolyte near the electrode, ␩ ␳ , can be estimated by
L
L
the resonance resistance, which can be monitored simultaneously with the resonance frequency. The change in the ␩L␳L value
during silver deposition was consistent with the change in the calculated concentration of Ag͑I͒ near the electrode. During the
outer-sphere electron-transfer reaction between ferrocenium and ferrocene, no significant changes in the mass and the ␩L␳L value
were observed.
©
2009 The Electrochemical Society. ͓DOI: 10.1149/1.3223669͔ All rights reserved.
Manuscript submitted March 23, 2009; revised manuscript received August 18, 2009. Published September 30, 2009. This was
Paper 3026 presented at the Honolulu, Hawaii Meeting of the Society, October 12–17, 2008.
Metal deposition in aqueous solutions has been utilized for vari-
ous industrial processes for a long time. However, there are several
limitations, such as the restriction of metal species for the narrow
Experimental
BMPBr was prepared by the reaction of 1-methylpyrrolidine ͑To-
kyo Kasei, purity of 98.0%͒ with bromobutane ͑Tokyo Kasei,
8.0%͒ in acetonitrile ͑Junsei Kagaku, 99.5%͒ at room temperature,
electrochemical potential window of water, the competitive H evo-
2
9
lution which sometimes leads to embrittlement of deposits, and the
concentration change due to the vaporization of water. Recently,
room-temperature ionic liquids have been intensively expected and
investigated as the alternative electrolytes for batteries and electro-
plating baths because they generally have high cathodic stability, no
proton source, negligible vapor pressure, high thermal stability, and
then purified by recrystallization for 3 times and finally dried under
vacuum at 120°C for 1 day. BMPTFSA was obtained by interacting
LiTFSA ͑Kanto Denka Kogyo, Ͼ99.9%͒ with the equimolar amount
of BMPBr in distilled water at room temperature, separated by ex-
traction into dichloromethane ͑Junsei Kagaku, Ͼ99.5%͒ and dried
under vacuum at 120°C for 1 day. No significant current attributable
to such impurities as water, bromide, and oxygen was detected by
cyclic voltammetry using a Pt electrode in the as-prepared ionic
liquid.
AgTFSA salt was prepared by reacting Ag2O ͑Wako Pure
Chemical Industries, Ͼ99.0%͒ with HTFSA ͑Morita Kagaku Ko-
gyo, 99.9%͒ aqueous solution at room temperature, followed by
evaporation, and dried under vacuum at 90°C for 1 day. Almost all
of these procedures were carried out in darkness because AgTFSA is
sensitive to light. The composition of Ag in AgTFSA was verified by
quantitative titration. The dehydration temperature of hydrated
AgTFSA was estimated to be ca. 45°C by thermal gravimetry
differential thermal analysis ͑Rigaku, Thermo Plus TG8120͒. Then,
1
acceptable ionic conductivity. Ionic liquids composed of bis͑trifluo-
−
romethylsulfonyl͒amide ͑TFSA ͒ have high stability against mois-
ture and low solubility of water compared with those composed of
2
,3
other anions. For these advantages, electrodeposition of metals
−
and alloys has been studied widely especially in the TFSA -based
4
,5
ionic liquids.
Electrochemical quartz crystal microbalance ͑EQCM͒ can detect
small mass change in a quartz crystal electrode during electrochemi-
cal experiments. Thus, it has been utilized for analysis of elec-
trodeposition in aqueous solutions. However, as a conventional os-
cillator technique ͑self-excited, active technique͒ EQCM was
6
inoperative in some ionic liquids due to their high viscosity. As an
impedance technique ͑separately excited, passive technique͒ EQCM
can be used in highly viscous fluids including some ionic liquids.
Recently, EQCM measurements during metal deposition in ionic
liquids have been studied successfully with the impedance
−
3
BMPTFSA containing 0.025 or 0.050 mol dm AgTFSA was pre-
pared by dissolving AgTFSA and further dried under vacuum at
0°C for 1 day. Ferrocene ͑Kanto Kagaku, Ͼ98%͒ was used as
supplied.
Electrochemical measurements were conducted using a potentio/
9
7
-13
technique.
The resonance resistance and frequency can be mea-
sured simultaneously using this type of EQCM. The resonance re-
sistance is known to depend on the product of the viscosity and
galvanostat ͑Hokuto Denko, HABF-501, 1510͒ with an EQCM sys-
tem ͑Seiko EG&G, QCA922͒. A Au-coated quartz crystal electrode
1
4
density of the media near the quartz crystal electrode. In the
present study, the changes in the mass and the product of the density
2
͑0.196 cm ͒ was employed as a working electrode. Ag wire ͑Sanwa,
99.5%͒ or Ag wire immersed in 0.1 mol dm− AgCF SO ͑Aldrich,
3
␳L and viscosity ␩ of the electrolyte near the electrode during elec-
L
3
3
trodeposition were examined using the impedance technique EQCM
in an ionic liquid, BMPTFSA ͑BMP+ denotes 1-butyl-1-
methylpyrrolidinium͒. The electrodeposition and dissolution of sil-
ver were used as a model system for examining the EQCM mea-
Ͼ99%͒ in BMPTFSA was used as a reference electrode, and they
are denoted as “Ag wire” and “Ag/Ag͑I͒,” respectively, in this pa-
per. Ag or Pt wire ͑Tokuyama Kagaku, 99.9%͒ was used as a counter
electrode. Handling of hygroscopic materials and electrochemical
measurements was carried out in an argon-filled glove box with a
continuous gas purification apparatus ͑Miwa Seisakusyo, DBO-
1KP-KOI͒. The temperature inside the glove box was controlled at
surements because there are 1 several reports on silver
5-17
electrodeposition in ionic liquids.
An outer-sphere electron-
−
transfer reaction between ferrocene ͑Fe͑Cp͒ , Cp denotes cyclo-
pentadienyl͒ and ferrocenium ͓͑Fe͑Cp͒ ͔ ͒ was also studied using
2
+
25°C. The concentration of H O in the gas was kept under 1 ppm.
2
2
the EQCM.
The ␩L␳L value of the electrolyte was measured by an oscillating
viscometer ͑Yamaichi, VM-1G-L͒ for the purpose of cross-
checking. Electrodeposits were analyzed by an X-ray photoelectron
spectroscopy ͑XPS, JEOL JPS-9000MC͒ and a scanning electron
microscopy ͑SEM, Keyence VE-9800͒.
*
Electrochemical Society Active Member.
E-mail: katayama@applc.keio.ac.jp
z
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D504
Journal of The Electrochemical Society, 156 ͑11͒ D503-D507 ͑2009͒
Theory
(a)
EQCM can be used for measuring the mass change in a quartz
crystal electrode by the piezoelectric effect of the quartz crystal.
Thus, it has been often utilized for the in situ analysis of plating,
-2
0.1 mA cm
1
8-24
corrosion, modified electrodes, and chemical sensors.
The fre-
quency change related to the mass change ͑⌬m͒ in the quartz crystal
2
5
electrode, ⌬f , is expressed by the Sauerbrey equation
m
2
2
f ⌬m
0
⌬
fm = −
͓1͔
1/2
A͑␮ ␳ ͒
q
q
where f0 is the resonance frequency, A is the surface area of the
electrode, is the shear modulus of quartz ͑2.95
(b)
␮
q
1
0
−1 −2
ϫ 10 kg m s ͒, and ␳q is the density of quartz ͑2.65
3
−3
ϫ 10 kg m ͒. However, the total shift of frequency ͑⌬f͒ is given
by the sum of ⌬fm and ⌬f␩␳, which is related to the product of the
2
6
viscosity ␩ and density ␳ of the electrolyte near the electrode
L
L
5 µg
⌬
f = ⌬fm + ⌬f␩␳
͓2͔
͓3͔
␩L␳L
3
0
/2
⌬
f␩␳ = − f
ͱ
␲
␮q␳q
(
c)
Only a small change in the ␩L␳ value was observed during the
electrode reaction between ͓Fe͑CN͒ ͔ and ͓Fe͑CN͒6͔ in an
L
2
-4 -1
ρ
η
4
−
3−
0.1 g cm
s
6
27
aqueous solution. In the impedance technique EQCM, the quartz
crystal is forcedly oscillated by the externally applied electric field,
and the resonance frequency and resistance can be obtained simul-
taneously by seeking the minimum of the resonance resistance with
scanning the frequency of electric field applied for oscillating the
quartz crystal. In contrast, only the resonance frequency is obtained
with the oscillator technique EQCM, in which the quartz crystal is
-
1.0
-0.5
0.0
0.5
1.0
Potential / V vs. Ag wire
oscillated by a resonance circuit. The resonance resistance R is
1
1
/2
14
proportional to ͑␩ ␳ ͒ within a wide range of the ␩ ␳ value
L
L
L L
Figure 1. ͑a͒ Cyclic voltammogram, ͑b͒ the mass change, and ͑c͒ the change
in the ␩L␳L value in 0.050 M Ag͑I͒/BMPTFSA on a Au electrode at a scan
rate of 2 mV s .
1
/2
A͑2␲f ␩ ␳ ͒
0
L L
R1 =
͓4͔
−1
2
k
where k is an electromechanical coupling factor. The k value can be
estimated from the changes in the resonance frequency and resis-
tance before and after contacting a liquid sample with the quartz
electrode according to Eq. 3 and 4. ⌬fm can also be estimated by the
change in the half-value width of admittance plots, which is some-
initial value after the anodic current stemmed. The ␩L␳L value is
expected to depend on the concentration of Ag͑I͒ because the vis-
cosity and density of the ionic liquid are largely dependent on the
concentration of AgTFSA. On starting silver deposition, a diffusion
layer is formed and the Ag͑I͒ concentration near the electrode drops.
At the negative potential enough to attain a diffusion limiting con-
dition, the Ag͑I͒ concentration reaches zero at the electrode surface.
In the anodic stripping, the Ag͑I͒ concentration increases rapidly at
the electrode surface. After all of the deposits are oxidized, Ag͑I͒
near the electrode diffuses toward the bulk of the electrolyte. Figure
28
times called “damping.” The sensitivity of EQCM against the vis-
cosity and density of the liquid decays exponentially through the
distance from the electrode with the damping of the shear wave of
2
6
the quartz crystal. The decay length ␦ is defined by
␩L
␦
=
ͱ
͓5͔
␲
␳ f
L 0
2
shows the ␩L␳L values during cyclic voltammetry in BMPTFSA
In BMPTFSA, the ␦ value is estimated to be 1.3 ␮m using the
containing 0.025 M Ag͑I͒. The difference in the ␩L␳L values before
and after silver deposition in the cathodic sweep in 0.025 M Ag͑I͒/
BMPTFSA was a half smaller than that in 0.050 M Ag͑I͒/
BMPTFSA, verifying that the ␩ ␳ value is dependent on the con-
viscosity ͑0.68 g cm−1 s ͒ and density ͑1.47 g cm ͒ of neat
−1
−3
BMPTFSA.
L
L
Results and Discussion
centration of Ag͑I͒ in the ionic liquid. Similar behavior was
1
3
+
observed in tin deposition and dissolution, and Na doping and
EQCM measurements in the ionic liquid.— The resonance fre-
quency dropped immediately about 19 kHz after BMPTFSA con-
taining 0.050 M AgTFSA was injected into the cell. According to
Eq. 3, this frequency change corresponded to the ␩ ␳ value of
29
dedoping in a Nafion film.
EQCM measurements were also carried out in BMPTFSA con-
taining 0.010 M Fe͑Cp͒2 for comparison. Figure 3 shows the
L
L
changes in the mass and the ␩ ␳ value during cyclic voltammetry.
2
−4 −1
L L
1
.2 g cm s , which was close to that measured by the oscillating
In contrast to the results of silver deposition, as shown in Fig. 1, no
2
−4 −1
viscometer ͑1.0 g cm s ͒. Figure 1b shows the mass change in a
Au electrode during cyclic voltammetry in the ionic liquid. As
shown in the cyclic voltammogram ͑Fig. 1a͒, the mass increased
with the cathodic current assigned to deposition of silver and de-
creased with the anodic one assigned to stripping of silver. Figure 1c
shows the change in the ␩L␳L value during potential sweep. The
␩L␳L value decreased when deposition started and reached a con-
stant value, which was close to that of neat BMPTFSA, then in-
creased sharply with anodic dissolution, and finally decayed to the
significant changes in the mass and the ␩ ␳ value were observed.
L
L
+
In this case, ͓Fe͑Cp͒ ͔ and Fe͑Cp͒ are both soluble in the ionic
2
2
liquid. Thus, no mass change in the electrode during the reaction is
expected. Because the total amount of soluble species is constant, it
is expected that the viscosity and density near the electrode do not
change significantly.
The change in the ␩ ␳ value.— EQCM measurements during
L
L
constant potential step and constant current step experiments were
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Journal of The Electrochemical Society, 156 ͑11͒ D503-D507 ͑2009͒
D505
0
0
0
0
0
.20
.15
.10
.05
.00
0.00
-0.05
ρ
η
ρ
η
-0.10
-
-
-
0.05
0.10
-0.15
0.15
-
0
20
40
60
80
100
1.0
-0.5
0.0
0.5
1.0
Time / s
Potential / V vs. Ag wire
Figure 4. The changes in the ␩L␳L value on a Au electrode during constant
potential step experiments in 0.050 M Ag͑I͒/BMPTFSA.
Figure 2. The change in the ␩L␳L value on a Au electrode during the poten-
tial sweep experiment in 0.025 M Ag͑I͒/BMPTFSA.
also carried out in BMPTFSA containing 0.050 M Ag͑I͒. Figure 4
shows the changes in the ␩L␳L value during potential step experi-
ments. In the cathodic potential step to −500 mV vs Ag wire, which
was negative enough to attain a diffusion limiting condition, the
␩L␳L value decreased rapidly and reached a constant value, which
was consistent with that of neat BMPTFSA and close to that ob-
served during cyclic voltammetry ͑Fig. 1c͒. This behavior is also
interpretable by the change in the Ag͑I͒ concentration. Figure 5
shows the changes in the ␩L␳L value during constant current step
experiments. The ␩ ␳ value decreased with the elapse of time. The
rates of decrease increased with an increase in the current densities.
L
L
Because no significant change in the ␩ ␳ value was observed in
L
L
+
͓
Fe͑Cp͒ ͔ /Fe͑Cp͒ reaction during both the potential step and the
2 2
constant current step measurements, it is suggested that these behav-
iors are characteristic for the deposition process, in which only the
oxidant is a soluble species.
Concentration profile.— Because it is inferred that the change in
the ␩ ␳ value reflects the concentration profile near the electrode,
the concentration near the electrode was calculated assuming a one-
dimensional linear diffusion for a planar electrode. In this silver
deposition system, the concentration of the oxidant, Ag͑I͒, was only
considered because the reductant is metallic silver.
-
2
L
L
0
.01 mA cm
The concentration profile is the function of the time and distance
from the electrode. During a potential step experiment, when the
potential is negative enough to attain a diffusion limiting condition,
2
2
the concentration is expressed by
0.00
-
-
-
-
0.02
0.04
0.06
0.08
1
0
µg
2
-4 -1
.01 g cm
s
-0.10
ρ
η
ρ
η
j
-
-
-
0.12
0.14
0.16
-
0.8
-0.6
-0.4
-0.2
0.0
0
10
20
30
40
50
Potential / V vs. Ag / Ag(I)
Time / s
Figure 3. ͑a͒ Cyclic voltammogram, ͑b͒ the mass change, and ͑c͒ the change
in the ␩ ␳ value in 0.010 M Fe͑Cp͒ /BMPTFSA on a Au electrode at a scan
Figure 5. The changes in the ␩L␳L value on a Au electrode during constant
L
L
2
current step experiments in 0.050 M Ag͑I͒/BMPTFSA. The current densities
−
1
−2
rate of 1 mV s
.
were −50, −75, −100, −125, and −150 ␮A cm .
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D506
Journal of The Electrochemical Society, 156 ͑11͒ D503-D507 ͑2009͒
1.0
0.8
0.6
0.4
0.2
0.0
0
5
10
15
20
Time / s
Figure 6. The calculation result of the average concentration in the region
x = 0–3.9 ␮m during a constant potential step experiment. The step potential
is negative enough to attain a diffusion limiting condition.
Figure 7. The calculation results of the average concentration in the region
x = 0–3.9 ␮m during constant current step experiments. The current densi-
−2
ties were −50, −75, −100, −125, and −150 ␮A cm
.
x
c͑x,t͒ = c* erfͩ ͪ
͓6͔
Current efficiency.— The mass change during electrolysis is es-
timated by Faraday’s law
0
.5 0.5
2
D t
where D is the diffusion coefficient of the oxidant and x is the
distance from the electrode. During the constant current step experi-
QM
22
⌬m =
͓9͔
ment, the concentration is given by
nF
0
.5
2
j
Dt
− x
x
c͑x,t͒ = c* +
2
ͩ ͪ expͩ ͪ − x erfcͩ ͪ
where Q is the charge passed during electrolysis, M is the molecular
ͭ
ͮ
0
.5 0.5
nFD
␲
4Dt
2D
t
−1
weight ͑in this case 107.9 g mol ͒, n is the number of electrons,
͓
7͔
and F is the Faraday constant. The comparison of the mass change
observed by EQCM with that calculated from Eq. 9 during potential
sweep, constant potential step, and constant current step experiments
is shown in Fig. 8. Because the EQCM results were consistent with
the mass changes estimated from the electric charges during these
three electrochemical experiments, the current efficiency of Ag
deposition in BMPTFSA is regarded as almost 100%. The electrodes
after the measurements were analyzed by XPS and SEM. The par-
ticle size of Ag deposits was in the range of 0.2–0.5 ␮m from the
SEM image, as shown in Fig. 9. The XPS spectrum gave no signal
assignable to nitrogen, fluorine, and sulfur, which are constituents of
where j is the current density.
The sensitivity of EQCM against the ␩L␳L value decays expo-
nentially with the distance from the electrode, and the decay length
in BMPTFSA is estimated to be about 1.3 ␮m. Therefore, the
concentration of Ag͑I͒ was calculated at each 100 nm from
to 3.9 ␮m ͑3␦͒ from the electrode. The sensitivity decays into
under 5% in the region farther than 3␦. The average concentration in
this region was calculated from the following equation
␦
0
−
x
+
−
c͑x͒expͩ ͪdx
the ionic liquid, indicating that the inclusion of BMP or TFSA into
the deposits did not occur in the ionic liquid.
͵
͵
␦
˜
c =
͓8͔
−
x
expͩ ͪdx
Conclusion
␦
The mass change was observed during Ag͑I͒/Ag reaction in the
ionic liquid using an impedance technique EQCM. The current ef-
ficiency of silver deposition in BMPTFSA was nearly 100%, and no
The diffusion coefficient of Ag͑I͒ in BMPTFSA was assumed to be
−
8
2
−1
9
ϫ 10 cm s from the result of the Cottrell plots.
The results of calculation of average concentration during a con-
+
−
stant potential step and constant current step experiments are shown
in Fig. 6 and 7, respectively. As shown in Fig. 6, during the constant
potential step experiment, the concentration decreased rapidly when
the deposition started. Although without consideration of convec-
tion, the change in concentration with the elapse of time was similar
to that of the ␩L␳L value in Fig. 4. In the constant current step
experiments, the rate of decrease increased with an increase in the
current density ͑Fig. 7͒, as seen in Fig. 5. Although the relationship
between the concentration and the ␩ ␳ value has not been investi-
inclusion of BMP and TFSA into deposits was observed by XPS.
The change in the product of the viscosity and density near the
electrode, ␩L␳L, could be measured by monitoring the resonance
resistance R1. Because the changes in the ␩L␳L values during a
potential step and constant current step experiments were consistent
with the calculation of the average Ag͑I͒ concentration, the ␩L␳L
value is suggested to reflect the concentration of Ag͑I͒ near the
electrode. There was no change in the mass and the ␩L␳L value
during the outer-sphere electron-transfer reaction like
L
L
+
gated quantitatively, the changes in the ␩ ␳ value in Fig. 4 and 5
͓Fe͑Cp͒2͔ /Fe͑Cp͒2 because they are both soluble species in the
L
L
are considered, reflecting the changes in the concentration of Ag͑I͒
near the electrode, as shown in Fig. 6 and 7, respectively. Thus, it
seems possible to clarify the concentration of the electroactive spe-
cies near the electrode by monitoring the ␩ ␳ value when the de-
pendence of the viscosity and density on the concentration is avail-
able.
ionic liquid. Thus, it is considered possible to estimate the concen-
tration change in the electroactive species near the electrode during
the electrode reaction when either oxidant or reductant is soluble.
The impedance technique EQCM may be a useful tool for the in-
vestigation of batteries and plating baths using ionic liquids from a
viewpoint of the in situ measurement of the ␩ ␳ value.
L
L
L
L
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Journal of The Electrochemical Society, 156 ͑11͒ D503-D507 ͑2009͒
D507
1
1
1
4
2
0
8
6
4
2
0
2
4.0
(
a)
(
b)
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Q-base
EQCM
Q-base
EQCM
-
-1.0
-0.5
0.0
0.5
1.0
0
20
40
60
80 100 120 140
Figure 8. The mass changes estimated by
EQCM and the electric charge Q during
Potential / V vs. Ag / Ag(I)
Time / s
͑
a͒ potential sweep, ͑b͒ constant potential
step, and ͑c͒ constant current step experi-
ments on a Au electrode in 0.050 M Ag͑I͒/
BMPTFSA.
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
(c)
Q-base
EQCM
0
100
200
300
Time / s
2
3
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Acknowledgments
͑
2004͒.
The present work was financially supported by a Grant-in-Aid
for Scientific Research on Priority Areas of “Science of Ionic Liq-
uids” ͑no. 17073016͒ from the Ministry of Education, Culture,
Sports, Science, and Technology, Japan ͑MEXT͒ and by the Kato
Foundation for Promotion of Science. The authors also thank Kanto
Denka Kogyo and Morita Kagaku Kogyo for supplying LiTFSA and
HTFSA, respectively.
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