The effect of organic additives in electrodeposition of Co from an amide-type ionic liquid

Article abstract of DOI:10.1016/j.electacta.2010.10.074

The effect of organic additives, coumarin and thiourea (TU), on the electrodeposition of cobalt (Co) was investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA), containing Co(TFSA)₂ as a Co source. In case of addition of coumarin, the deposition potential of Co shifted to the positive side while there was no change in the coordination environment of Co²⁺. However, the surface morphology was changed by addition of coumarin, suggesting the specific adsorption of coumarin on the surface of electrode. On the other hand, the deposition potential of Co shifted to the more positive side by addition of TU, probably due to the change in the coordination environment of Co²⁺ from [Co(TFSA)₃]^- to [Co(TU)₄]²⁺. The diffusion coefficient of [Co(TU)₄]²⁺ was determined as 7.2 × 10^-8 cm² s^-1 at 27 °C by chronoamperometric method. The surface morphology of electrodeposit obtained from the electrolyte with TU was flat and less granular as compared to that without TU.

Full text of DOI:10.1016/j.electacta.2010.10.074

Electrochimica Acta 56 (2011) 1190–1196
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
The effect of organic additives in electrodeposition of Co from an amide-type
ionic liquid
Ryuta Fukui, Yasushi Katayama∗, Takashi Miura
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1, Kohoku-Ku, Yokohama, Kanagawa 223-8522, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 1 October 2010
Received in revised form 19 October 2010
Accepted 23 October 2010
Available online 4 November 2010
The effect of organic additives, coumarin and thiourea (TU), on the electrodeposition of
cobalt (Co) was investigated in an amide-type ionic liquid, 1-butyl-1-methylpyrrolidinium
bis(trifluoromethylsulfonyl)amide (BMPTFSA), containing Co(TFSA)2 as a Co source. In case of addition
of coumarin, the deposition potential of Co shifted to the positive side while there was no change in
the coordination environment of Co2+. However, the surface morphology was changed by addition of
coumarin, suggesting the specific adsorption of coumarin on the surface of electrode. On the other hand,
the deposition potential of Co shifted to the more positive side by addition of TU, probably due to the
change in the coordination environment of Co2+ from [Co(TFSA)3]− to [Co(TU)4]2+. The diffusion coeffi-
cient of [Co(TU)4]2+ was determined as 7.2 × 10−8 cm2 s−1 at 27 ◦C by chronoamperometric method. The
surface morphology of electrodeposit obtained from the electrolyte with TU was flat and less granular as
compared to that without TU.
Keywords:
Ionic liquid
Cobalt
Electrodeposition
Coumarin
Thiourea
© 2010 Elsevier Ltd. All rights reserved.
1. Introduction
bis(trifluoromethylsulfonyl)amide (BMPTFSA). The overpotential
for the electrodeposition of Co was as large as about 1.5 V and the
Various kinds of room-temperature ionic liquids have been
expected as a new electroplating bath because they have many
advantages in general such as wide electrochemical potential win-
dow, negligibly low vapor pressure, high ionic conductivity [1].
Aprotic ionic liquids often have wider electrochemical potential
window than aqueous media. Thus, it is expected to be possible to
deposit less noble metals and their alloys, which cannot be obtained
in conventional aqueous media. In addition, the ionic liquids con-
sisting of bis(trifluoromethylsulfonyl)amide (TFSA−) are not only
stable against moisture but also immiscible with water [2]. Since
the amide-type ionic liquids can be handled more easily than other
hydrophilic ionic liquids, electrodeposition of several metals has
been reported [3–11].
Cobalt (Co) is one of the important materials as a constituent
of magnetic, corrosion-resistant alloys. The electrodeposition of
Co alloy in the aqueous solution is generally difficult because
of the narrow electrochemical potential window of water.
Thus, the ionic liquids are expected as the new electrolytes
for electrodeposition of Co or its alloys. Several studies about
the electrodeposition of Co have been already reported in
ionic liquids [12–24]. We have already reported the possibil-
ity of Co electrodeposition from 1-butyl-1-methylpyrrolidinium
crystallinity of the electrodeposit obtained at 25 ◦C was very low.
The electrodeposited Co showed poor adhesion to the substrate
[25]. The similar results have been reported for the electrodepo-
sition of iron (Fe) [26] and nickel (Ni) [27] in BMPTFSA. The large
overpotential for electrodepostion of these metals is considered
related to the structure of electric double layer at the interface of
the electrode and the ionic liquid. In particular, the layer formed by
adsorption of BMP+ on the electrode surface in the negative poten-
tial region may inhibit not only the charge transfer but also such
processes as nucleation, crystallization, and surface diffusion. On
the other hand, we also reported that addition of acetone changed
the coordination environment of Co2+ and reduced the overpoten-
tial for electrodeposition of Co [28]. Thus, electrodeposition of Co
is considered to be affected strongly by the charge and coordinat-
ing ligands of Co(II) species in the ionic liquid. It has been reported
that the morphology of aluminum (Al) [29], silicon (Si) [5] and Ni
[27] electrodeposited in BMPTFSA is influenced by the adsorption
of BMP+ on substrates.
In the present paper, the effect of additives, coumarin and
thiourea (TU), on the electrochemical behavior of Co electrodepo-
sition has been investigated in BMPTFSA. Although both coumarin
and TU are known as brightener or leveler, which adsorbs onto the
surface and change the surface morphology of deposit in aqueous
solutions [30–35], there are only a few reports about the effects
of such additives in ionic liquids [36,37]. On the other hand, there
have been several reports about the additives in electrodeposition
∗
Corresponding author.
E-mail address: katayama@applc.keio.ac.jp (Y. Katayama).
0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.10.074

R. Fukui et al. / Electrochimica Acta 56 (2011) 1190–1196
1191
from ionic liquids, such as propylene carbonate [14,18] and ethyl
alcohol [14] in Co electrodeposition, and toluene [38], lithium chlo-
ride (LiCl) [38] in Al electrodeposition. According to these reports,
the additives change the electrochemical behavior, surface mor-
phology and grain size of deposits. However, these effects may be
caused by the dilution of the ionic liquids since the amount of the
additives is too much against that of the ionic liquids in some cases.
2. Experimental
BMPBr was prepared by the reaction of 1-methylpyrrolidine
(Tokyo Kasei) and butyl bromide (Tokyo Kasei) in acetonitrile at
room temperature, then purified by recrystallization and finally
dried under vacuum at 120 ◦C for 24 h. BMPTFSA was prepared by
interacting BMPBr with LiTFSA (Kanto Denkakougyou) in deionized
water, extracted into dichloromethane, separated by evaporation
and finally dried under vacuum at 120 ◦C for 24 h. Co(TFSA)2 was
obtained by reacting CoCO3 with HTFSA aqueous solution at 60 ◦C
and dried under vacuum at 130 ◦C for 24 h. Coumarin (Tokyo Kasei)
and TU (Tokyo Kasei) were dried under vacuum at 40 ◦C for 24 h
before use.
All the electrochemical measurements were conducted with a
potentio/galvanostat (Hokuto Denko, HABF-501) combined with
a digital recorder. A platinum (Pt) or copper (Cu) disk (Nilaco,
7.08 × 10−2 cm2) was employed as a working electrode after mirror
polishing, electrolytic degreasing in alkali solution and wash-
ing with 10 vol.% H2SO4 and distilled water. Co (Nilaco) was
used as a counter electrode. A silver (Ag) wire (Sanwa kinzoku)
immersed in a 0.1 mol dm−3 AgCF3SO3/EMITFSA (EMI+ = 1-ethyl-
3-methylimidazolium) was used as the reference electrode. The
inner liquid was separated from the main electrolytes by a porous
glass (Vycor). All the potentials in this paper were referred to this
Ag|Ag(I) electrode, of which the potential was +0.44 V against fer-
rocene/ferrocenium couple [25].
The electrodeposits were washed with ethanol (Wako, >99.8%)
and dried in the argon (Ar) atmosphere. The samples were char-
acterized by a scanning electron microscope (SEM, FEI Sirion),
energy dispersive X-ray analysis (EDX, EDAX Phoenix), X-ray pho-
toelectron spectrometer (XPS, JOEL JPPS-9000 MC) and X-ray
diffractometer (XRD, Rigaku RAD-C, CuK␣).
Handling of hygroscopic reagents and all the electrochemical
measurements were conducted in an Ar-filled glovebox with a
continuous gas purification apparatus (Miwa Seisakujyo, DBO-1K-
SH). The concentrations of H2O and O2 in the gas were kept under
800 ppb and 1 ppm, respectively.
The absorption spectra of the ionic liquids were measured using
an air-tight quartz cell (light path length 1 mm) with the aid of
UV-vis spectrometer (JASCO, V-530).
Fig. 1. UV–vis spectra obtained in BMPTFSA containing (a) 100 mmol dm−3
Co(TFSA)2, (b) 100 mmol dm−3 Co(TFSA)2 and 10 mmol dm−3 coumarin and (c)
10 mmol dm−3 coumarin at room temperature. Light path length: 1 mm.
3.1.2. Electrochemical behavior and electrodeposition in the
presence of coumarin
The cyclic voltammograms of a Pt electrode in BMPTFSA con-
taining 100 mmol dm−3 Co(TFSA)2 and 10 mmol dm−3 coumarin
are shown in Fig. 2. These cyclic voltammograms were scanned
cathodically from the rest potential. The cathodic potential limit
of Fig. 2a was set to −2.2 V since the cathodic current assignable
to the reductive decomposition of coumarin was observed around
−2.3 V as shown in Fig. 2c. The electrodeposition and stripping of
Co were observed in the absence of coumarin (Fig. 2b), as the same
as reported previously [25]. When 10 mmol dm−3 coumarin was
added to BMPTFSA containing Co(TFSA)2, the cathodic current peak
shifted slightly to the more positive side, suggesting the electrode-
position of Co was affected by the addition of coumarin although
the coordination environment of Co2+ is expected to be identical in
both absence and presence of coumarin.
The electric charges for anodic dissolution of Co were smaller
than those for electrodeposition regardless of the presence of
3. Results and discussion
3.1. Effects of addition of coumarin
3.1.1. Coordination environment of Co2+
The coordination environment of metal species dissolved in
the ionic liquid is quite important to its electrochemical behav-
ior. Fig. 1 shows the absorption spectra of BMPTFSA containing
100 mmol dm−3 Co(TFSA)2 and 10 mmol dm−3 coumarin at room
temperature. There was no difference between the spectra before
and after addition of coumarin, indicating addition of coumarin
does not change the coordination environment of Co2+. It has been
reported that the divalent cobalt species is dissolved by forming
[Co(TFSA)3]− in EMITFSA ionic liquid [39]. Thus, the divalent cobalt
species is considered as [Co(TFSA)3]− in BMPTFSA regardless of the
presence of coumarin.
Fig. 2. Cyclic voltammograms of
100 mmol dm−3 Co(TFSA)2 and 10 mmol dm−3 coumarin, (b) 100 mmol dm−3
Co(TFSA)2 and (c) 10 mmol dm−3 coumarin at 27 ◦C. Scan rate: 50 mV s−1
a Pt electrode in BMPTFSA containing (a)
.

1192
R. Fukui et al. / Electrochimica Acta 56 (2011) 1190–1196
additives probably because electrodeposited Co was not dissolved
completely in the successive anodic dissolution. This would be
explained by the solubility of Co(TFSA)2 in the ionic liquid and/or
the change of local viscosity of the ionic liquid. During the anodic
dissolution of Co, the concentration of Co(II) near the electrode sur-
face increases rapidly since the diffusion of Co(II) in the ionic liquid
is very slow compared with aqueous solutions. When the concen-
tration of Co(II) reaches to the solubility of Co(TFSA)2, the anodic
dissolution will be suppressed by deposition of Co(TFSA)2. On the
other hand, an increase in the concentration of Co(II) causes an
increase in the viscosity of the ionic liquid. Thus, anodic dissolution
of Co leads to an increase in the viscosity of the ionic liquid near the
electrode surface and a decrease in the diffusivity of Co(II) or TFSA−.
The change of local viscosity near the electrode has been studied
in electrochemical deposition and dissolution of silver in the same
ionic liquid with electrochemical quartz crystal microbalance [56].
In addition, poor adhesion of Co on the electrode surface is also
considered as the reason for the incomplete anodic dissolution of
deposited Co.
Fig. 3. Potential during the galvanostatic cathodic reduction on a Cu substrate
in BMPTFSA containing 100 mmol dm−3 Co(TFSA)2 with (a) 0, (b) 0.1, (c) 1, (d)
10 mmol dm−3 of coumarin at 27 ◦C. Current density: −0.05 mA cm−2
.
Thus, galvanostatic cathodic reduction on a Cu substrate was
carried out in order to examine the effects of addition of coumarin
on the electrodeposits. The steady-state potentials during elec-
trodeposition were measured for examining the effects of additives
on the overpotentials of electrodeposition since the cyclic voltam-
mograms are often affected by such miscellaneous processes as
nucleation and diffusion. Fig. 3 shows the potential profile observed
during the galvanostatic cathodic reduction on a Cu substrate in
BMPTFSA containing 100 mmol dm−3 Co(TFSA)2 with 0.1, 1, and
10 mmol dm−3 coumarin. The potential during the galvanostatic
cathodic reduction with coumarin was more positive than that
without coumarin even when the concentration of coumarin was
as small as 0.1 mmol dm−3. A dark-gray and shiny deposit was
obtained on the substrate after the galvanostatic electrolysis in the
electrolytes with coumarin while a black and lusterless deposit was
obtained in the absence of coumarin. The adhesion of the deposit
obtained from the electrolyte with coumarin was always better
than that without coumarin.
The XPS spectrum of electrodeposit obtained from BMPTFSA
containing 100 mmol dm−3 Co(TFSA)2 and 10 mmol dm−3 of
coumarin gave the peaks corresponding to Co 2p 3/2 and 2p 1/2 at
793.0 and 777.8 eV after Ar+ etching for 100 s. Therefore, the elec-
trodeposit was confirmed to be metallic Co. However, no diffraction
peak assignable to metallic Co was obtained by XRD, probably due
to the grain size of deposited Co was too small, as reported in the
ionic liquid without coumarin [25].
The SEM images of electrodeposits are shown in Fig. 4. The
appearance of the deposits obtained with coumarin was smoother
and less granular than that without coumarin. Coumarin is well
known as the typical leveling agent, which alters the morphology of
electrodeposits effectively at a very low concentration. The effects
of leveling agents on the morphology of electrodeposits were com-
monly explained by the adsorption of the leveling agents on the
surface of the substrate and deposits [30]. The arrival rate of the
leveling agent at the projecting parts is greater than that at the
caved parts. Since the adsorption of the leveling agent hinders the
Fig. 4. SEM images of the electrodeposit obtained by a galvanostatic cathodic reduction on a Cu substrate in BMPTFSA containing 100 mmol dm−3 Co(TFSA)2 with (a) 0, (b)
0.1, (c) 1 and (d) 10 mmol dm−3 coumarin at 27 ◦C. Current density: −0.05 mA cm−2
.

R. Fukui et al. / Electrochimica Acta 56 (2011) 1190–1196
1193
Fig. 6. Absorption spectra of BMPTFSA containing 5 mmol dm−3 Co(TFSA)2 with 0,
10, 20 and 30 mmol dm−3 TU. The inset shows the dependence of the absorbance
of the bands at 625, 685 and 720 nm on the concentration of TU (CTU). Light path
length: 1 mm.
and TU at room temperature. A major absorption band around
550 nm in Fig. 5a is assigned to the d–d transition of octahe-
drally coordinated Co2+, [Co(TFSA)3]−. When 10 mmol dm−3 TU
was added to the ionic liquid, several absorption bands, 307, 348,
625(sh), 686 and 725 nm, appeared as shown in Fig. 5b. These bands
were not derived from TU itself as shown in Fig. 5c. There have been
reported several complexes of Co2+ and TU, such as [Co(TU)2]2+
[49], [Co(TU)3]2+ [50], [Co(TU)4]2+ [50,51] and [Co(TU)6]2+ [52].
Fig. 6 shows the dependence of the absorbance at 625, 685 and
720 nm on the concentration of TU. The absorbance of the bands
increased with an increase in the concentration of TU up to
20 mmol dm−3, as shown in the inset of Fig. 6. Since the similar
spectra have been reported for Co(TU)4(ClO4)2 and Co(TU)4(NO3)2
[51], the absorption bands can be assigned to [Co(TU)4]2+. The for-
mation of [Co(TU)4]2+ in this ionic liquid can also be confirmed
by the saturation of the absorbance when the concentration of TU
exceeded 20 mmol dm−3, which is four times of the concentration
of Co2+. The assignments of the absorption bands are summarized
in Table 1.
Fig. 5. UV–vis spectra of BMPTFSA containing (a) 100 mmol dm−3 Co(TFSA)2, (b)
100 mmol dm−3 Co(TFSA)2 + 10 mmol dm−3 TU, and (c) 10 mmol dm−3 TU at room-
temperature. Light path length: 1 mm.
electrodeposition of metals, the deposition rate at the projecting
parts is slower than that at the caved parts, resulting in a decrease
in the roughness of the surface.
In aqueous solutions, there are a few reports about the effects of
coumarin on electrodeposition of Co and its alloys while the change
in the deposition potentials of Co was observed by the addition of
coumarin [40]. Since the coordination environment of Co2+ in this
ionic liquid was unchanged by addition of coumarin as described
above, the change in the morphology of electrodeposits is consid-
ered ascribed to the adsorption of coumarin.
On the other hand, the adsorption of the leveling agent usually
increases the overpotential for electrodeposition [33,41]. The slight
decrease in the overpotential in this ionic liquid may be related
to the structure of electric double layer. The structure of electric
double layer in ionic liquid is considered quite different from that
in aqueous solutions. There are lots of reports about the structure
of electric double layer in ionic liquids [42–46] and the adsorp-
tion of cation is often pointed out [4,29,47]. The influence of the
adsorption of cations against the electrode reaction and/or surface
morphology of electrodeposits has been also reported in ionic liq-
uids [27,48]. According to these reports, the adsorption of bulky
cations is expected to prevent the electrode reaction processes
(electron transfer, surface diffusion, crystalline growth). Therefore,
there is a possibility that the electrode reaction becomes more
favorable by replacement of adsorbed bulky cations with smaller
molecules, which have higher adsorbing ability than the cations.
The decrease in the overpotential of electrodeposition of Co may
be explained by a decrease in the coverage of BMP+ cations on the
substrate and deposits by adsorption of coumarin.
3.2.2. Electrochemical behavior of [Co(TU)4]2+ complex
Fig. 7 shows the cyclic voltammogram of a Pt electrode
in BMPTFSA containing 100 mmol dm−3 [Co(TU)4]2+/BMPTFSA
(100 mmol dm−3 Co(TFSA)2 + 400 mmol dm−3 TU). The cathodic
current in the ionic liquid with TU appeared at a more positive
potential than that without TU.
The change in the deposition potential can be confirmed more
obviously in galvanostatic cathodic reduction, as shown in Fig. 8.
The steady-state potential observed for the reduction of [Co(TU)4]2+
was more positive than that for [Co(TFSA)3]−, indicating the over-
potential of Co electrodeposition for [Co(TU)4]2+ is smaller than
that for [Co(TFSA)3]−. However, the overpotential for Co electrode-
position has been reported to become larger by the formation of
cobalt–TU complexes [53,54] in aqueous solutions. The reduction
potential of a metal species is related to the Gibbs energy of the
metal species. In aqueous solutions without TU, Co2+ is solvated
by water molecules to form an aqua-complex. When TU is added
Table 1
3.2. Effects of addition of TU
Absorption bands and molar absorption coefficients (mol−1 dm3 cm−1 in parenthe-
sis) for [Co(TU)4]2+ in several systems.
3.2.1. Coordination environment of Co2+
The addition of TU to BMPTFSA containing Co2+ resulted in the
change in the color of the ionic liquid from purple to blue-green,
System
Adsorption band/nm
Reference
BMPTFSA
Butanol
Acetone
625 (424)
625 sh (−)
641 sh (−)
685 (737)
688 (585)
686 (690)
720 (686)
725 (535)
720 (632)
This study
49, 50
51
indicating the change in the coordination environment of Co2+
Fig. 5 shows the UV–vis spectra of BMPTFSA containing Co(TFSA)2
.

1194
R. Fukui et al. / Electrochimica Acta 56 (2011) 1190–1196
Fig. 10. Cyclic voltammograms of
100 mmol dm−3 Co(TFSA)2 and 10 mmol dm−3 TU, (b) 100 mmol dm−3 Co(TFSA)2,
and (c) 10 mmol dm−3 TU at 27 ◦C. Scan rate: 50 mV s−1
a Pt electrode in BMPTFSA containing (a)
.
Fig. 7. Cyclic voltammograms of
a Pt electrode in BMPTFSA containing (a)
100 mmol dm−3 [Co(TU)4]2+ and (b) 100 mmol dm−3 [Co(TFSA)3]− at 27 ◦C. Scan rate:
10 mV s−1
.
Fig. 11. Potential observed during the galvanostatic cathodic reduction on a Cu sub-
strate in BMPTFSA containing (a) 100 mmol dm−3 Co(TFSA)2 and (b) 100 mmol dm−3
Co(TFSA)2 and 10 mmol dm−3 TU at 27 ◦C. Current density: −0.05 mA cm−2
.
Fig. 8. Potential during the galvanostatic cathodic reduction on a Cu substrate
in BMPTFSA containing (a) 100 mmol dm−3 [Co(TU)4]2+ and (b) 100 mmol dm−3
[Co(TFSA)3]− at 27 ◦C. Current density: −0.05 mA cm−2
.
sidered affected also by the structure of electric double layer. The
reduction of the cobalt species occurs in a comparatively negative
potential region, where the electrode is charged negatively. Under
such a situation, the electrode surface is covered with cationic
species in order to compensate the negative excess charge on the
electrode. In case of the absence of TU, the cationic species in the
ionic liquid is BMP+ only. Thus, the electrode surface is expected
to be covered with bulky BMP+ to form a cation layer, which may
prevent the electron transfer and/or surface reaction, leading to an
to the aqueous solution, water molecules coordinating to Co2+ is
replaced with TU to form a TU-complex, which is expected to
be more stable thermodynamically than aqua-complex. Thus, the
reduction potential of the TU-complex becomes more negative than
that of the aqua-complex. In case of the ionic liquid, the thermody-
namic stability of [Co(TU)4]2+ is expected to be higher than that of
[Co(TFSA)3]−. However, the overpotential for Co deposition is con-
Fig. 9. SEM images of the electrodeposit obtained by the galvanostatic cathodic reduction on a Cu substrate in BMPTFSA containing (a) 100 mmol dm−3 [Co(TU)4]2+ and (b)
100 mmol dm−3 [Co(TFSA)3]− at 27 ◦C. Current density: −0.05 mA cm−2
.

R. Fukui et al. / Electrochimica Acta 56 (2011) 1190–1196
1195
Fig. 12. SEM image of the electrodeposit obtained by the galvanostatic cathodic reduction on a Cu substrate in BMPTFSA containing (a) 100 mmol dm−3 Co(TFSA)2 and
10 mmol dm−3 TU and (b) 100 mmol dm−3 Co(TFSA)2 (without TU) at 27 ◦C. Current density: −0.05 mA cm−2
.
increase in the overpotential. On the other hand, [Co(TU)4]2+ has a
positive charge and can participate in the cation layer on the very
surface of the electrode. Since [Co(TU)4]2+ can access to the surface
of the electrode in this way, the overpotential for the reduction
of [Co(TU)4]2+ is considered smaller than that of [Co(TFSA)3]−. The
similar effect has been reported for BMPTFSA containing Co(TFSA)2
and acetone [28].
The electrodeposit was identified as metallic cobalt by XPS.
However, no diffraction peak corresponding to metallic Co was
observed by XRD. The surface morphology of the electrodeposit
obtained from BMPTFSA containing 100 mmol dm−3 [Co(TU)4]2+
was shown in Fig. 9. The deposit obtained from the ionic liquid with
TU was flatter and less granular than that without TU, as observed
for addition of coumarin. The leveling effect of TU has been well
known for Co electrodeposition in aqueous solution [55].
The diffusion coefficient of [Co(TU)4]2+ in BMPTFSA was esti-
mated to be 7.2 × 10−8 cm2 s−1 by chronoamperometry on a Pt
electrode at 27 ◦C. This value is close to that of [Co(TFSA)3]− in
BMPTFSA (1.0 × 10−7 cm2 s−1) [25].
small amount of TU is effective for reducing the overpotential of Co
deposition and improving the surface morphology.
4. Conclusion
The electrochemical behavior of [Co(TFSA)3]− was changed by
the addition of coumarin and TU. In both cases, the deposition
potential shifted to the more positive side and the surface mor-
phology of deposits became flat and less granular. The adhesion of
the deposits also became better. In case of coumarin, since the coor-
dination environment of Co2+ was unchanged, the electrochemical
behavior was expected to be affected mainly by the adsorption of
coumarin to the electrode surface. In case of TU, the dissolved cobalt
species was changed from [Co(TFSA)3]− to [Co(TU)4]2+. The deposi-
tion potential of Co from [Co(TU)4]2+ was more positive than that of
[Co(TFSA)3]−, and the surface morphology of deposit obtained from
[Co(TU)4]2+ was flat and less granular. These effects are consid-
ered due to the formation of the positively charged cobalt species,
[Co(TU)4]2+, which can access to the negatively charged electrode
surface.
3.2.3. Effect of addition of TU at a low concentration
Fig. 10 shows the cyclic voltammograms of
a Pt elec-
Acknowledgements
trode in BMPTFSA containing 100 mmol dm−3 Co(TFSA)2 and
10 mmol dm−3 TU. As described in the previous section, TU is
expected to coordinate Co2+ to form [Co(TU)4]2+. Thus, the cobalt
species in this ionic liquid are considered as 2.5 mmol dm−3
[Co(TU)4]2+ and 97.5 mmol dm−3 [Co(TFSA)3]− from Fig. 7. It should
be noted that the onset potential for Co deposition in the ionic liquid
with 10 mmol dm−3 TU was much more positive than that without
TU. The steady-state deposition potential of Co in the ionic liquid
with 10 mmol dm−3 TU was also more positive than that without
TU in galvanostatic cathodic reduction, as shown in Fig. 11. Thus,
the addition of a small amount of TU is effective for reduction of the
overpotential of Co deposition in this ionic liquid. This phenomenon
may be explained by the “TU cycle” or “catalytic effect of TU” at the
electrode–electrolyte interface. As mentioned above, [Co(TU)4]2+
can access to the surface of the electrode and can be reduced to
metallic cobalt. In this electrode reaction, the local concentration
The present work was financially supported by Grant-in-Aid for
Scientific Research on Priority Areas of “Science of Ionic Liquids”
(No. 17073016) from the Ministry of Education, Culture, Sports,
Science, and Technology, Japan (MEXT).
References
[1] A.P. Abbott, K.J. McKenzie, Phys. Chem. Chem. Phys. 8 (2006) 4265.
[2] F. Endres, S. Zein El Abedin, Phys. Chem. Chem. Phys. 8 (2006) 2101.
[3] S. Zein El Abedin, E.M. Moustafa, R. Hempelmann, H. Natter, F. Endres, Elec-
trochem. Commun. 7 (2005) 1111.
[4] F. Bebensee, L. Klarhofer, W. Maus-Friedrichs, F. Endres, Surf. Sci. 601 (2007)
3769.
[5] N. Borisenko, S. Zein El Abedin, F. Endres, J. Phys. Chem. B 110 (2006) 6250.
[6] S. Zein El Abedin, N. Borisenko, F. Endres, Electrochem. Commun. 6 (2004) 510.
[7] S. Zein El Abedin, U.W. Biermann, F. Endres, Electrochem. Commun. 7 (2005)
941.
[8] S. Zein El Abedin, H.K. Farag, E.M. Moustafa, U. Welz-Biermanny, F. Endres, Phys.
Chem. Chem. Phys. 7 (2005) 2333.
[9] S. Zein El Abedin, A.Y. Saad, H.K. Farag, N. Borisenko, Q.X. Liu, F. Endres, Elec-
trochim. Acta 52 (2007) 2746.
[10] M.-J. Deng, P.-Y. Chen, I.-W. Sun, Electrochim. Acta 53 (2007) 1931.
[11] J.-K. Chang, C.-H. Huang, W.-T. Tsai, M.-J. Deng, I.-W. Sun, P.-Y. Chen, Elec-
trochim. Acta 53 (2008) 4447.
[12] C.L. Hussey, T.M. Laher, Inorg. Chem. 20 (1981) 4201.
[13] R.T. Carlin, P.C. Trulove, H.C. De Long, J. Electrochem. Soc. 143 (1996) 2747.
[14] N. Koura, T. Endo, Y. Idemoto, J. Non-Cryst. Solids 205 (1996) 650.
of TU is expected to increase by liberation of TU from [Co(TU)4]2+
.
The liberated TU will interact with [Co(TFSA)3]− existing close to
the electrode to form [Co(TU)4]2+. This “TU cycle” near the electrode
seems to require a certain amount of TU in the electrolyte since this
effect was not observed when the concentration of TU was set to 0.1
and 1 mmol dm−3. The appearance of the electrodeposit obtained
in the ionic liquid with 10 mmol dm−3 TU was flat and less gran-
ular, as shown in Fig. 12. These results suggest the addition of a

1196
R. Fukui et al. / Electrochimica Acta 56 (2011) 1190–1196
[15] J.A. Mitchell, W.R. Pitner, C.L. Hussey, G.R. Stafford, J. Electrochem. Soc. 143
(1996) 3448.
[36] F. Endres, M. Bukowski, R. Hempelmann, H. Natter, Angew. Chem. Int. Ed. 42
(2003) 3428.
[16] M.R. Ali, A. Nishikita, T. Tsuru, Electrochim. Acta 42 (1997) 1819.
[17] R.T. Carlin, H.C. De Long, J. Fuller, P.C. Trulove, J. Electrochem. Soc. 145 (1998)
1598.
[18] P.-Y. Chen, I.-W. Sun, Electrochim. Acta 46 (2001) 1169.
[19] H.-Y. Hsu, C.-C. Yang, Z. Naturforsch. B: Chem. Sci. 58B (2003) 139.
[20] C.A. Zell, W. Freyland, Langmuir 19 (2003) 7445.
[21] W. Freyland, C.A. Zell, S. Zein El Abedin, F. Endres, Electrochim. Acta 48 (2003)
3053.
[22] C.-C. Yang, T.-H. Wu, M.-F. Shu, Z. Naturforsch. 59b (2004) 519.
[23] S. Schaltin, P. Nockemann, B. Thijs, K. Binnemans, J. Fransaera, Electrochem.
Solid-State Lett. 10 (2007) D104.
[24] M.-Z. An, P.-X. Yang, C.-N. Su, A. Nishikata, T. Tsuru, Chin. J. Chem. 26 (2008)
1219.
[37] A.P. Abbott, K. El Ttaib, K.S. Ryder, E.L. Smith, Trans. Inst. Met. Finish. 86 (2008)
234.
[38] A.P. Abbott, F. Qiu, H.M.A. Abood, M.R. Aliac, K.S. Ryder, Phys. Chem. Chem.
Phys. 12 (2010) 1862.
[39] K. Fujii, T. Nonaka, Y. Akimoto, Y. Umebayashi, S. Ishiguro, Anal. Sci. 24 (2008)
1377.
[40] M. Mounaga, L. Ricq, P. Berc¸ ot, Surf. Coat. Technol. 202 (2008) 1645.
[41] T. Mimani, M. Mayanna, N. Munichandraiah, J. Appl. Electrochem. 23 (1993)
339.
[42] R. Parsons, Chem. Rev. 90 (1990) 813.
[43] C. Nanjundiah, S.F. McDevitt, V.R. Koch, J. Electrochem. Soc. 144 (1997) 3392.
[44] T. Iwahashi, T. Nishi, H. Yamane, T. Miyamae, K. Kanai, K. Seki, D. Kim, Y. Ouchi,
J. Phys. Chem. C 113 (2009) 19237.
[25] R. Fukui, Y. Katayama, T. Miura, Electrochemistry 73 (2005) 567.
[26] M. Yamagata, N. Tachikawa, Y. Katayama, T. Miura, Electrochemistry 73 (2005)
564.
[27] Y.-L. Zhu, Y. Kozuma, Y. Katayama, T. Miura, Electrochim. Acta 54 (2009)
7502.
[45] M.T. Alam, M.M. Islam, T. Okajima, T. Ohsaka, J. Phys. Chem. C 112 (2008) 16600.
[46] S. Baldelli, Acc. Chem. Res. 41 (2008) 421.
[47] S. Rivera-Rubero, S. Baldelli, J. Phys. Chem. B 108 (2004) 15133.
[48] H. Valencia, M. Kohyama, S. Tanaka, H. Matsumoto, J. Chem. Phys. 131 (2009)
244705.
[28] Y. Katayama, R. Fukui, T. Miura, J. Electrochem. Soc. 154 (2007) D534.
[29] S. Zein El Abedin, E.M. Moustafa, R. Hempelmann, H. Natter, F. Endres, Chem.
Phys. Chem. 7 (2006) 1535.
[30] S.A. Watson, J. Edwards, Trans. Inst. Met. Finish. 34 (1957) 167.
[31] T. Mimani, M. Mayanna, J. Electrochem. Soc. 140 (1993) 984.
[32] C. Madore, D. Landolt, J. Electrochem. Soc. 143 (1996) 3936.
[33] C.-C. Chang, A.C. West, J. Electrochem. Soc. 144 (1997) 3050.
[34] F.E. Luborsky, IEEE Trans. Magn. 5 (1969) 106.
[35] M. Mounaga, L. Ricq, P. Berc¸ ot, J. Appl. Electrochem. 38 (2008) 231.
[49] N. Meneces, C.R. Piriz Mac-coll, J. Inorg. Nucl. Chem. 34 (1972) 3711.
[50] C.D. Flint, M. Goodgame, J. Chem. Soc. A (1967) 1718.
[51] F.A. Cotton, O.D. Faut, J.T. Mague, Inorg. Chem. 3 (1964) 17.
[52] G.M.S. El-Bahy, B.A. El-Sayed, A.A. Shabana, Vib. Spectrosc. 31 (2003) 101.
[53] J. García-Torres, E. Gómez, E. Vallés, J. Appl. Electrochem. 39 (2000) 233.
[54] I. Tabakovic, S. Riemer, V. Inturi, P. Jallen, A. Thayer, J. Electrochem. Soc. 147
(2000) 219.
[55] R.F. Renner, K.C. Liddell, J. Appl. Electrochem. 32 (2002) 621.
[56] N. Serizawa, Y. Katayama, T. Miura, J. Electrochem. Soc. 156 (2009) D503.