Electrochemical codeposition of copper and manganese from room-temperature N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid

Article abstract of DOI:10.1016/j.electacta.2009.07.016

The voltammetric behavior of N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid (BMP-TFSI) containing Cu(I), Mn(II), or mixtures of Cu(I) and Mn(II) as well as the electrodeposition of copper-manganese alloy coatings (Cu-Mn alloy

Full text of DOI:10.1016/j.electacta.2009.07.016

Electrochimica Acta 54 (2009) 6935–6940
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemical codeposition of copper and manganese from room-temperature
N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid
a,∗,1
b
a
Po-Yu Chen
, Ming-Jay Deng , Ding-Xuan Zhuang
a
Department of Medicinal and Applied Chemistry, Kaohsiung Medical University, 100 Shi-Chuan 1st Road, Kaohsiung City 807, Taiwan
Department of Chemistry, National Cheng-Kung University, Tainan 701, Taiwan
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The voltammetric behavior of N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic
liquid (BMP-TFSI) containing Cu(I), Mn(II), or mixtures of Cu(I) and Mn(II) as well as the electrodeposition
of copper–manganese alloy coatings (Cu–Mn alloy coatings) was studied at 323 K. The Cu(I) and Mn(II)
species required to prepare these coatings were introduced into the BMP-TFSI by anodic dissolution of the
relevant metallic electrodes. Electrodeposits of Cu, Mn, and Cu–Mn with various contents of Mn can be
obtained by controlled-potential electrolysis. It was found that the compositions and surface morphology
of the electrodeposited Cu–Mn alloy coatings depend on the deposition potentials and the concentration
ratio of [Cu(I)]/[Mn(II)] in BMP-TFSI. The Cu–Mn alloy coatings obtained in this study were compact
and adherent. They did not show any significant X-ray diffraction signal that could be assigned to the
crystalline structures of Cu, Mn, or Cu–Mn alloys. In the aqueous solution containing 0.1 M NaCl, the
Cu–Mn alloy coatings demonstrated passive behavior—no continuous oxidation was observed. However,
a significant oxidation current was observed at the electrodes deposited with Cu or Mn.
Received 23 April 2009
Received in revised form 1 July 2009
Accepted 4 July 2009
Available online 15 July 2009
Keywords:
Cu–Mn codeposition
Ionic liquid
Electrodeposition
Room temperature
Bis(trifluoromethylsulfonyl)imide
©
2009 Elsevier Ltd. All rights reserved.
1. Introduction
Copper–manganese (Cu–Mn) alloy coatings have been prepared
by using electrodeposition in ammonium sulfate ((NH ) SO ) solu-
4
2
4
The electrodeposition of sacrificial coatings on steel is a promis-
ing method of protecting steel parts from corrosion. Manganese
tions [15–17]. A higher solution pH is more favorable for the
electrodeposition of Cu–Mn alloy coatings because hydrogen evolu-
2+
(Mn) has a very negative redox potential. Therefore, it is a very
tion is suppressed and [Cu(NH3)n=1–4
]
forms [17]. The formation
promising sacrificial material. Compared with other frequently
used sacrificial metals, such as Zn and Cd, Mn is more environmen-
tally friendly. Cadmium ion dissolved from the sacrificial coatings
is toxic to humans and to organisms [1–5]. Zinc ion, however, is a
necessary nutrient but it is toxic and hazardous if its concentration
is high [6–8]. Although Mn has superior sacrificial properties, pure
Mn coatings on steel are not very practical because of their high
reactivity and fast dissolution. Electrodeposited Mn is metastable
and is in a centered tetragonal (ct) ␥ phase [9,10] that is ductile.
At room temperature for several weeks, it transforms into a brittle
body-centered cubic (bcc) ␣ phase [11], which easily cracks upon
impact. It has been shown that this phase transformation can be
suppressed by codepositing ␥-Mn with face-centered cubic (fcc)
metals, especially Cu [10,12]. In addition to the corrosion resistance
of Cu–Mn codeposits, several papers have pointed out that Cu–Mn
coatings are good electrocatalysts for carbohydrates [13] and amino
acids [14].
of copper complexes leads to a negative shift in the reduction poten-
tial of Cu(II), moving it closer to the reduction potential of Mn(II).
However, the electroplating bath is only stable for a limited time
under higher pH conditions because hydrolysis and/or oxidation
of Mn(II) induces turbidity in the solution. Although Cu–Mn alloy
coatings can be obtained from the aforementioned solutions, low
current efficiency is usually encountered and high current density
must be applied.
It has been reported that the electrodeposition of Mn is fea-
sible in room-temperature air- and water-stable ionic liquids.
Extremely high current efficiencies (∼100%) have been reported
[18–20]. High efficiencies were obtained because no hydrogen
evolution involved in the electrodeposition process. In addition,
Mn(II) species dissolved in the ionic liquids is very stable. Pre-
cipitate was never observed. The characteristics of ionic liquids
are well known. They include a wide electrochemical window,
undetectable vapor pressure, large temperature ranges of the liq-
uid, and no hydrogen evolution for aprotic ionic liquids. Ionic
liquids are therefore promising electrolytes for the electrodeposi-
tion of metals and alloys. They are especially useful for reactive
elements because no hydrogen evolution will be involved in the
electrodeposition. The undetectable vapor pressure of ionic liq-
∗ Corresponding author. Tel.: +886 7 3121101x2587; fax: +886 7 3125339.
E-mail address: pyc@kmu.edu.tw (P.-Y. Chen).
ISE member.
1
0
013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.07.016

6
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P.-Y. Chen et al. / Electrochimica Acta 54 (2009) 6935–6940
uids implies that ionic liquids are inflammable and can be used
under high temperatures without vaporization. Because the elec-
trodeposition of Mn in ionic liquids has been proven to be feasible,
it is possible to electrodeposit Cu–Mn alloy coatings in ionic
liquids. Higher current efficiencies can be expected. However,
there have been no reports of the electrodeposition of Cu–Mn
alloy coatings in ionic liquids. In the present study, Cu–Mn alloy
coatings were electrodeposited on tungsten (W) substrate in
N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide
The surface morphologies and elemental compositions of the
electrodeposits were investigated using a Philips XL-40FEG field
emission scanning electron microscope (FE-SEM) coupled with an
energy dispersive spectrometer (EDS) operating at 15 kV. The EDS
was employed to semiquantitatively determine the compositions
of the obtained Cu–Mn alloy coatings. The crystalline structure of
the Cu, Mn, or Cu–Mn alloy coatings was also analyzed with the
Shimadzu Model XD-D1 powder X-ray diffractometer (XRD).
(
BMP-TFSI) ionic liquid to demonstrate the feasibility using ionic
3. Results and discussion
liquids in the electrodeposition of Cu–Mn alloy coatings. The
voltammetric behaviors of Cu(I), Mn(II), or mixtures of Cu(I) and
Mn(II) were also studied. The Cu–Mn layers were characterized
with a scanning electron microscope (SEM) and an energy dis-
persive spectrometer (EDS). An anodic potentiodynamic scan was
carried out in deaerated 0.1 M NaCl solution to assess the corrosion
performance of the obtained Cu–Mn coatings.
3
.1. Voltammetric study of Mn(II) and Cu(I) species in BMP-TFSI
ionic liquid
The electrochemical behavior of Mn(II) in BMP-TFSI is not dis-
cussed in detail here because it was carefully studied in our previous
experiments [19]. The voltammetric behavior of Mn(II) is shown for
readers’ convenience. Because anodic dissolution of the Mn elec-
trode was employed to introduce Mn(II) into the BMP-TFSI ionic
liquid, cyclic voltammetry was carried out at the Mn electrode. The
relevant cyclic voltammogram is shown in the inset of Fig. 1. The
potential was initially scanned from −1.0 V towards the positive
direction. A sudden increase of the anodic current was observed
when the potential reached −0.3 V, indicating that Mn can be oxi-
dized in the ionic liquid. A current loop was observed during the
reverse scan, implying that a fresh surface of Mn might be produced
during the anodic scan [19]. The anodic current of Mn dissolution
did not continuously increase. Instead, it reached a plateau, indicat-
ing that the oxidation reaction may be kinetically hindered. During
2. Experimental
N-Butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)-
imide (BMP-TFSI) ionic liquid was prepared and purified by a
previously published procedure [21]. However, in this study,
chlorobutane was employed in the procedure instead of iodobu-
tane. The ionic liquid was dried under vacuum for at least 1 day
while being heated to 393 K. The water content of the ionic liquid
was determined with a coulometric Karl Fischer titrator (Metrohm
−
7
56 KF) and the value was approximately 70 ppm. No Cl was
detected with a platinum electrode by using cyclic voltammetry.
Although the BMP-TFSI ionic liquid is air- and water-stable, all
electrochemical experiments were performed inside a glove box
the bulk electrolysis of the Mn electrode, the reaction occurring
at the counter electrode was the oxidation of BMP+ (the cation of
(
MBRAUN, UNI LAB-B) filled with purified nitrogen in order to
the BMP-TFSI ionic liquid). This reaction did not interfere with the
system because the counter electrode was separated from the bulk
solution. Fig. 1 shows the cyclic voltammogram of Mn(II) recorded
at the W electrode. The potential was initially scanned from 0.0 V
towards the negative direction and then back to 0.0 V. The reduction
wave of Mn(II) is very close to the cathodic limit of the BMP-TFSI
ionicliquid. However, no interference was observedduring theelec-
trodeposition of Mn, as long as the applied potential was below the
prevent the interference produced from oxygen and moisture
that are absorbed into the ionic liquid if it is exposed to a normal
atmosphere. The concentrations of O and H O inside the glove box
2
2
were maintained under 1 ppm. Manganese block (99.9%), copper
wire (99.9%), and tungsten wire (99.95%) were purchased from
Alfa Aesar and used after being cleaned in concentrated nitric acid.
Mn(II) and Cu(I) were introduced into the ionic liquid by electrolyz-
ing a Mn anode and a Cu anode, respectively. Anodic dissolution was
employed because the Mn and Cu compounds tested in this study
cannot dissolve in BMP-TFSI. In addition, the effect of counter ions
from Mn and Cu compounds on the electrochemical behavior of
Cu(I) and Mn(II), respectively, can be avoided by employing anodic
dissolution. Other compounds used in this study were at least
reagent grade. Deionized water produced from a Milli-Q Gradient
system (Millipore) was used to prepare all the aqueous solutions.
All electrochemical experiments were carried out with a Prince-
ton Applied Research 263A potentiostat/galvanostat (PAR 263A).
A conventional three-electrode electrochemical cell was used. The
detailed constituents were reported in a previous study [19]. For the
voltammetricstudy, atungsten(W)diskelectrodeoraplatinum(Pt)
disk electrode was used as the working electrode. However, a piece
of tungsten wire was employed as the working electrode for the
electrodeposition of the Cu, Mn, or Cu–Mn alloy coatings. A plat-
inum wire (Alfa Aesar, 99.95%) immersed in ferrocene/ferrocenium
+
(
Fc/Fc = 50/50 mol.%) BMP-TFSI solution contained in a glass tube
with porous Vycor tip (Bioanalytical Systems, MF-2042) was used
as a reference electrode; therefore, all values of potential are auto-
+
matically reported with respect to the redox potential of Fc/Fc . The
counter electrode consisting of a platinum wire spiral immersed
in BMP-TFSI was separated from the bulk ionic liquid by a poros-
ity E glass frit. For aqueous solutions, Ag/AgCl saturated with NaCl
was used as the reference electrode and a platinum wire that was
directly immersed in the aqueous solutions was used as the counter
electrode.
Fig. 1. Cyclic voltammograms recorded at W electrode in BMP-TFSI containing
0
.025 M Mn(II). The inset shows the cyclic voltammogram of a Mn electrode in
−
1
BMP-TFSI. Scan rate: 50 mV s . Temperature: 323 K.

P.-Y. Chen et al. / Electrochimica Acta 54 (2009) 6935–6940
6937
Table 1
Results of anodic dissolution of Cu electrode in BMP-TFSI ionic liquid.
4
na
ꢀM (g)
10 m (mol)
Qtheory (n = 1) (C)
Qexp (C)
0
0
0
0
0
.0013
.0012
.0012
.0011
.0011
0.2046
0.1888
0.1888
0.1731
0.1731
1.97
1.82
1.82
1.67
1.67
1.91
1.81
1.80
1.80
1.70
0.97
0.99
0.99
1.08
1.02
a
Average value of n is 1.01 ± 0.04.
observed, regardless of the initial scan direction. The conversion
between Cu(I) and Cu(II) is a quasi-reversible reaction at the W elec-
trode, indicated by the large separation of peak potentials (ꢀEp).
Thisbehaviorindicatesthatthisconversioniskineticallycontrolled,
resulting from the change in the number of anions complexed to the
Cu centers. On the other side of the voltammogram, a typical strip-
ping peak was observed. It indicates that Cu(I) can be reduced to Cu
metal and deposited onto the electrode surface during the cathodic
scan and that the deposited Cu can be reoxidized from the electrode
surface in the anodic scan, indicating that electrodeposition of Cu
is feasible in this ionic liquid.
Because the codeposition of Cu and Mn would occur in BMP-
TFSI, the voltammetric behavior of the Cu(I)/Cu(0) redox couple
was studied in more detail. Fig. 3 shows the cyclic voltammo-
grams of the Cu(I)/Cu(0) redox couple recorded at the Pt and
W electrodes, respectively, in BMP-TFSI containing 0.1 M Cu(I).
Fig. 2. Cyclic voltammograms recorded at W electrode in BMP-TFSI containing 0.1 M
Cu(I). The inset shows the cyclic voltammogram of a Cu electrode in BMP-TFSI. Scan
−1
rate: 50 mV s . Temperature: 323 K.
limit. Our previous study pointed out that the current efficiency
of Mn electrodeposition carried out in BMP-TFSI was very high
(
∼100%) [19]. This indicates that decomposition of the ionic liq-
uid did not happen at the potential of the Mn electrodeposition. In
addition to the reduction wave of Mn(II), a typical stripping peak
can be observed in the reverse scan, as shown in Fig. 1, indicat-
ing the reoxidation of the Mn deposited on the electrode surface
during the cathodic scan. The deposited Mn, however, cannot be
completely reoxidized from the electrode surface due to the short-
age of the stripping peak area compared with the reduction wave
area. This behavior is caused by the kinetically hindered oxida-
tion of deposited Mn [18–20] and has been observed in several
electrodeposition processes in air- and water-stable ionic liquids
[
23,24].
The voltammetric behavior of the Cu electrode and the
Cu(I)/Cu(0) redox couple has also been studied in BMP-TFSI [22].
The electrochemistry of Cu(I) is discussed here in more detail. Cyclic
voltammetry was carried out at the Cu electrode. The relevant cyclic
voltammogram is shown in the inset of Fig. 2. The potential was
initially scanned from −0.6 V in the anodic direction towards the
switching potential and then back to −0.6 V. The anodic dissolution
of Cu was apparent at +0.5 V, as indicated by the sudden increase
in anodic current. The small reduction wave around +0.1 V is due
to redeposition of dissolved Cu formed during the anodic scan. In
order to determine the oxidation state of the dissolved Cu species,
changes in the mass of the Cu electrode were determined after each
passage of a given charge, Qexp. The data obtained from several
such controlled-potential coulometric experiments are collected in
Table 1. In this table, ꢀM is the change in the mass of the Cu elec-
trode, m is the number of moles related to this mass change, and
Qtheory (n = 1) is the charge based on m for n = 1 (where n is the num-
ber of electrons transferred during the anodic dissolution process).
The experimental results shown in Table 1 imply that Cu(I) species
were produced during the process of anodizing a Cu electrode in
BMP-TFSI.
Fig. 2 shows the cyclic voltammograms recorded at the W elec-
trode in BMP-TFSI containing 0.1 M Cu(I). The arrows indicate the
initial direction of the potential scan. Two redox couples were
Fig. 3. Cyclic voltammograms recorded at Pt and W electrodes, respectively, in BMP-
TFSI containing 0.1 M Cu(I). Scan rate: 50 mV s . Temperature: 323 K.
−
1

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P.-Y. Chen et al. / Electrochimica Acta 54 (2009) 6935–6940
These cyclic voltammograms show that the potential was initially
scanned towards the negative direction and then reversed at vari-
ous potentials. As can be seen, the reduction of Cu(I) needs a higher
overpotential at the W electrode than that observed at the Pt elec-
trode because the reduction wave of Cu(I) was observed at a more
negative region. In addition, a current loop was observed at the W
electrode when the scan was reversed at more positive potentials,
indicating that a nucleation process was involved in the reduction of
Cu(I) at the W electrode. No current loop, however, was observed in
thecyclicvoltammogramsrecordedatthePtelectrode, eventhough
the scan was reversed ahead of the peak potential. Fig. 3 also shows
that the stripping peak increased when the scan was reversed at
more negative potentials. The charge corresponding to the stripping
peaks is almost identical to that of the reductive waves, indicating
that the electrodeposits of Cu adhered on the electrode surface well.
potentials, as indicated by the positive shift in the reduction wave
of Mn(II). This behavior implies that Cu(I) and Mn(II) may have cer-
tain interactions or even form some Cu–Mn compounds, resulting
in a smaller separation of the reduction potentials. This behavior is
beneficial to Cu–Mn codeposition. Similar behavior was observed
for the mixture of Zn(II) and Mn(II), in which the electrodeposition
of Zn-Mn coatings was carried out [20]. In that solution, the reduc-
tion wave of Zn(II) was not observed at the potential, in which Zn(II)
should be reduced to zinc metal if the solution contains Zn(II) alone.
In the mixture of Zn(II) and Mn(II), Zn(II) was reduced to zinc metal
at the potentials where Mn(II) was reduced. Therefore, the reduc-
tion wave of Zn(II) seems to be disappeared. The reduction wave
of Cu(I), however, was still observed in the mixture of Cu(I) and
Mn(II). Fig. 4a also shows that the oxidative stripping peak of the
deposited Cu shifted to a more positive potential. The separation of
the peak potentials for the redox couple Cu(I)/Cu(II) increased when
the cyclic voltammogram was performed in the mixture of Cu(I) and
Mn(II). Again, this behavior indicates that Cu(I) may interact with
Mn(II) in this solution.
3.2. Voltammetric study of BMP-TFSI containing mixtures of Cu(I)
and Mn(II)
Fig. 4b shows the cyclic voltammograms recorded in the mix-
tures of Cu(I) and Mn(II) with concentration ratios of Cu(I)/Mn(II)
ranging from 0.5 to 2. In general, the reductive and oxidative waves
of Cu(I) shifted to more negative and more positive potentials,
respectively, with decreasing molar ratios of Cu(I)/Mn(II). Because
the oxidation of deposited Mn is kinetically hindered, we believe
that the stripping peak that occurred between 0 and 1 V was mostly
from the oxidation of Cu deposited during the cathodic scan per-
formed in the mixtures of Cu(I) and Mn(II).
The cyclic voltammograms recorded at the Pt electrode in BMP-
TFSI containing Cu(I), Mn(II), and a mixture of Cu(I) and Mn(II) with
a concentration ratio of [Cu(I)]/[Mn(II)] = 0.5 (0.033 M/0.067 M) are
shown in Fig. 4a. By carefully examining these voltammograms, it
can be found that the reduction wave of Cu(I) in the mixture is
more negative than that observed in the solution containing Cu(I)
alone. Mn(II) in the mixture was reduced to Mn(0) at more positive
Based on the cyclic voltammograms shown in Fig. 4, the codepo-
sition of Cu–Mn should be possible if the applied potential is more
negative than −1.5 V. The electrodeposition of Cu–Mn coatings is
discussed in the following section.
3.3. Electrodeposition of Cu–Mn alloy coatings in BMP-TFSI
The electrodeposition of Cu–Mn alloy coatings was investigated
by bulk controlled-potential electrolysis on tungsten wires in BMP-
TFSI containing various [Cu(I)/[Mn(II)] concentration ratios. The
reduction current of the electrodeposition was very stable as long
as the solution was steadily stirred with a magnetic stirring bar
derived by a magnetic stirrer. Good adherent coatings of Cu–Mn
codeposits were obtained. The atomic ratios of Mn in the Cu–Mn
alloy coatings were determined by EDS, and the results are shown in
Fig. 5. It must be emphasized that EDS can only provide semiquan-
titative measurements and the results are not absolutely accurate
Fig. 4. Cyclic voltammograms recorded at Pt electrode in BMP-TFSI containing Cu(I),
Fig. 5. Dependence of Mn contents contained in selected Cu–Mn coatings on the
deposition potential. W wire was used as the substrate. The concentration ratios of
[Cu(I)]/[Mn(II)] are shown in the inset. Electrodeposition temperature: 323 K.
Mn(II), and mixtures of Cu(I) and Mn(II), respectively. The concentrations of the
−1
metal ions are indicated. Scan rate: 50 mV s . Temperature: 323 K.

P.-Y. Chen et al. / Electrochimica Acta 54 (2009) 6935–6940
6939
Fig. 6. SEM micrographs of Mn, Cu, and Cu–Mn coatings electrodeposited at W wire in BMP-TFSI. The compositions of the Cu–Mn coatings are indicated as atomic ratios on
each plot.
if the sample does not have a flat surface. However, the results still
can provide a trend for us. As can be seen, the content of Mn in
the Cu–Mn alloy coatings depends on the applied potential and
the concentration ratio of [Cu(I)]/[Mn(II)]. The atomic ratio of Mn
coatings had a totally different surface morphology. Their morphol-
ogy was dependent on the compositions of the Cu–Mn codeposits.
In general, the grain size of the Cu–Mn codeposits decreased as
the Mn content in the Cu–Mn coatings increased. Regardless of the
compositions of the Cu–Mn alloy coatings, the Cu–Mn codeposits
exhibited good adherence on the substrate. The Cu–Mn coatings
were also analyzed with XRD. However, no significant signal was
observed that could be assigned to the crystalline structure of the
Cu, Mn, or Cu–Mn alloys. This finding indicated that the Cu–Mn
alloy coatings obtained in this study could be metallic glasses
or amorphous. Amorphous Cu–Mn coatings have also been pre-
pared by electrodeposition in sulfate-based electrolytes [16,17]. The
authors indicated that the amorphous Cu–Mn alloy coatings did not
show any structural transformation.
The potential corrosion resistance of two selected Cu–Mn coat-
ings was assessed in a 0.1 M NaCl solution. Potentiodynamic anodic
polarization curves recorded in a deaerated 0.1 M NaCl solution
using the Cu–Mn alloy coatings electrodeposited on tungsten wire
with BMP-TFSI ionic liquid are shown in Fig. 7. The behaviors of
tungsten wires coated with Mn and Cu alone are also provided for
comparison. It was found that the oxidation current of deposited
Mn suddenly increased when the potential was more positive than
−1.1 V (vs. Ag/AgCl saturated with NaCl). The current change was
very rapid, indicating that the anodic dissolution of deposited
Mn is thermodynamically and kinetically favorable in the chlo-
ride solution. The deposited Cu dissolved into the solution when
the potential was more positive than 0 V (vs. Ag/AgCl saturated
with NaCl). However, the oxidation current changed in a slower
manner, indicating that the rate of anodic Cu dissolution in the chlo-
ride solution was slower than that of the deposited Mn. Compared
with the Mn and Cu coatings, the Cu–Mn alloy coatings demon-
strated unusual voltammetric behavior, as shown in Fig. 7. Unlike
the deposited Cu or Mn, the coatings of the Cu–Mn codeposits
increased as the deposition potential (E ) became more negative
d
with each electroplating bath. At the same E , the content of Mn
d
was richer when the Cu–Mn alloy coatings were prepared in the
solution containing a higher concentration of Mn(II). This behavior
is reasonable due to the different reduction potentials of Cu(I) and
Mn(II). At the potentials at which the codeposition of Cu and Mn
occurs, the reduction of Cu(I) is completely under the control of dif-
fusion. The reduction rate of Mn(II), however, is controlled by the
applied potentials and concentrations of Mn(II). Therefore, the con-
tent of Mn in the Cu–Mn codeposits increased with more negative
applied potentials and higher concentrations of Mn(II). Fig. 5 shows
that the content of Mn in the Cu–Mn coatings can be adjusted to any
amount by regulating the applied potential and the concentration
ratio of [Cu(I)]/[Mn(II)].
3.4. Surface morphology and corrosion resistance of Cu–Mn
coatings
Several selected samples, whose compositions were determined
with EDS and shown in Fig. 5, were studied with SEM. The surface
morphology of these samples is shown in Fig. 6. For comparison, the
coatings containing Mn and Cu alone, respectively, are also shown
in the figure. The compositions determined with EDS for the coat-
ings of Cu–Mn codeposits are shown on each micrograph. The SEM
micrographs indicate that the Mn coating primarily consisted of
spherical grains. The smaller grains aggregated together to form
islands. The grain size of Cu coatings, however, is much smaller
than that of Mn coatings. Therefore, the surface of the Cu coat-
ing is very smooth, even on a micrometer scale. The Cu–Mn alloy

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P.-Y. Chen et al. / Electrochimica Acta 54 (2009) 6935–6940
of Mn in the Cu–Mn codeposits can be controlled by the applied
potential and the concentration ratios of [Cu(I)]/[Mn(II)]. Based on
the analysis of XRD, the coatings of Cu–Mn codeposits have no
crystalline structure and are perhaps amorphous. The Cu–Mn alloy
coatings prepared in this study displayed passive behavior during
the anodic scan of potential in NaCl solutions. The passive behavior
may be a result of oxide formation. The oxide layer may inhibit the
anodic dissolution of the Cu–Mn alloy coatings, resulting in better
corrosion resistance.
Acknowledgment
This research was supported by the National Science Council of
Taiwan under grant number NSC96-2113-M-037-014-MY2.
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(
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[
[
[
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D575.
[
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4
. Conclusions
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[
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This study showed that the electrodeposition of Cu–Mn alloy
(1999) 4164.
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coatings is feasible in the air- and water-stable room-temperature
ionic liquid BMP-TFSI. The Cu(I) and Mn(II) species that are neces-
sary to prepare the coatings must be introduced into the ionic liquid
by anodic dissolution of the relevant metal electrodes. The content
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