Electrodeposition of palladium in a hydrophobic 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide room-temperature ionic liquid

Article abstract of DOI:10.1016/j.electacta.2007.02.074

The electrochemical reduction of palladium halide complexes such as PdBr₄^2- and PdCl₄^2- was investigated in a hydrophobic room-temperature ionic liquid (RTIL), 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI). The irreversible electrode reaction between Pd(II) and Pd(0) was observed in BMPTFSI containing PdBr₄^2- or PdCl₄^2- by cyclic voltammetry. The diffusion coefficient of PdBr₄^2- was estimated to be (1-2) × 10^-7 cm² s^-1 by choronopotentiometry and chronoamperometry. The deposition of crystalline Pd metal was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy. It was suggested by the chronoamperometric measurements on PdBr₄^2- in BMPTFSI that the initial stage of the electrodeposition of Pd on the polycrystalline Pt electrode surface involves three-dimensional progressive nucleation under diffusion control. The reduction potential of PdCl₄^2- was more negative than that of PdBr₄^2-, reflecting the difference in the donor property between chloride and bromide.

Full text of DOI:10.1016/j.electacta.2007.02.074

Electrochimica Acta 53 (2007) 87–91
Electrodeposition of palladium in a hydrophobic
1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)
imide room-temperature ionic liquid
Yuri Bando, Yasushi Katayama∗, Takashi Miura
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3-14-1,
Kohoku-Ku, Yokohama 223-8522, Japan
Received 21 December 2005; received in revised form 27 February 2007; accepted 27 February 2007
Available online 4 March 2007
Abstract
The electrochemical reduction of palladium halide complexes such as PdBr42− and PdCl42− was investigated in a hydrophobic room-temperature
ionic liquid (RTIL), 1-n-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide (BMPTFSI). The irreversible electrode reaction between
2−
2−
2−
Pd(II) and Pd(0) was observed in BMPTFSI containing PdBr4 or PdCl4 by cyclic voltammetry. The diffusion coefficient of PdBr4 was
estimated to be (1–2) × 10−7 cm2 s−1 by choronopotentiometry and chronoamperometry. The deposition of crystalline Pd metal was confirmed by
2−
X-ray diffraction and X-ray photoelectron spectroscopy. It was suggested by the chronoamperometric measurements on PdBr4 in BMPTFSI
that the initial stage of the electrodeposition of Pd on the polycrystalline Pt electrode surface involves three-dimensional progressive nucleation
under diffusion control. The reduction potential of PdCl42− was more negative than that of PdBr42−, reflecting the difference in the donor property
between chloride and bromide.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Ionic liquid; 1-n-Butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide; Palladium; Electrodeposition; Electroplating
1. Introduction
vapor pressure, high chemical and thermal stability, acceptable
intrinsic ionic conductivity, and wide potential window. Elec-
trochemical deposition of Pd or Pd alloy was studied previously
in some RTILs, such as 1-ethyl-3-methylimidazolium (EMI+)
chloride–AlCl3 [2,3], EMICl–ZnCl2 [4], and EMIBF4 [5]. How-
ever, these RTILs are not suitable for practical use because of
their high sensitivity against moisture.
It has been known that the RTILs composed of
bis(trifluoromethylsulfonyl)imide anion (TFSI−) are not only
stable against moisture but also immiscible with water. Although
the solubility of water in TFSI−-based RTILs is not negli-
gible, it is easy to eliminate water from the TFSI−-based
RTILs compared with other ionic liquids miscible with water.
Therefore, theseTFSI−-basedRTILsareexpectedtobethealter-
native media for various practical electrochemical processes.
In this study, the electrochemical reduction of some palla-
dium halide complexes have been investigated in a hydrophobic
RTIL, BMPTFSI (BMP+ = 1-n-butyl-1-methylpyrrolidinium),
under dry atmosphere, and the possibility of using this media
for Pd electrodeposition was examined in order to obtain the
Palladium has excellent physical and chemical properties
such as corrosion and wear resistance, thermal stability and
high catalytic activity for various chemical reactions, so that
it is mainly used for the industrial applications like catalysis
and electrical devices. However, it is not easy to obtain the Pd
films without hydrogen embrittlement by electrodeposition from
conventional aqueous plating baths since Pd has high catalytic
activity against hydrogen evolution and absorbs great amount of
hydrogen.
Recently, room-temperature ionic liquids (RTILs) have been
drawn great attention and studied in many academic fields
[1]. In electrochemistry, their applications for batteries and
electroplating baths have been strongly expected due to the
interesting characteristics of the ionic liquids, such as negligible
∗
Corresponding author. Tel.: +81 45 566 1561; fax: +81 45 566 1561.
E-mail address: katayama@applc.keio.ac.jp (Y. Katayama).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.02.074

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Y. Bando et al. / Electrochimica Acta 53 (2007) 87–91
Pd deposits without hydrogen embrittlement. BMPTFSI was
selected because of its low melting point (−18 ◦C) and high
electrochemical stability [6].
3. Results and discussion
The addition of BMPX (X− = Br− or Cl−) was necessary to
dissolve PdX2 in BMPTFSI. Since divalent palladium is known
2−
to form a square planer tetracoordinated complex, PdX4
,
2. Experimental
the molar ratio of BMPX to PdX2 was set to 2:1. Fig. 1
shows the UV–vis absorption spectra of BMPTFSI contain-
ing either 0.625 mM PdBr2 and 1.25 mM BMPBr or 2.5 mM
PdCl2 and 5.0 mM BMPCl. Since these spectra were consis-
BMPBr was prepared by the reaction of 1-methylpyrrolidine
(Tokyo Kasei, purity > 95%) and n-butyl bromide (Tokyo
Kasei > 98%) in dehydrated acetonitrile (JUNSEI, 99.5%) at
room-temperature under dry Ar atmosphere, followed by recrys-
tallization and vacuum drying at 120 ◦C for 24 h. BMPTFSI
was obtained as a separated phase by the metathesis of BMPBr
and LiTFSI (Kanto Denka Kogyo, purity > 99.98%) in deion-
ized water. Then, BMPTFSI was extracted by dichloromethane
(JUNSEI, purity 99.0%), separated by evaporation and finally
dried under vacuum at 120 ◦C for 24 h to eliminate residual
dichloromethane and water. PdBr2 (Aldrich, purity 99%) or
PdCl2 (Aldrich, purity 99.0%) was used as supplied and dis-
solved into BMPTFSI with the addition of BMPBr or BMPCl
(prepared similarly as BMPBr), respectively. The molar ratio
of PdBr2 and BMPBr (or PdCl2 and BMPCl) was 1:2. The
dissolved Pd species in BMPTFSI were examined by UV–vis
spectroscopy (JASCO, V-530). The handling of all the hygro-
scopic reagents was carried out in an argon-filled glove box
with a continuous gas purification instrument (Miwa Seisakujyo,
1ADB-2 + MM2-P15S).
2−
tent with those of PdX4 in aqueous solution [7,8] as listed
in Table 1, it was confirmed that the dissolved species in
BMPTFSI is PdX42−, which forms according to the following
reaction:
2−
BMPX + PdX2 → BMP+ + PdX4
(X = Br− or Cl−).
(1)
Fig. 2 shows the cyclic voltammograms of a Pt electrode
in BMPTFSI containing 10 mM PdBr42−. A cathodic current
2−
peak was observed at about −1.6 V in the presence of PdBr4
.
When the potentiostatic cathodic reduction was carried out on
a Ni electrode at −1.7 V, a black electrodeposit without bright-
ness was obtained on the electrode. Both planar and granular
deposits were observed in the SEM image of the electrode-
posit, as shown in Fig. 3. The deposit was identified as metallic
Pd by XRD (Fig. 4) and XPS (Fig. 5). Therefore, it was con-
firmed that the cathodic reaction observed at −1.6 V in the cyclic
A three-electrode cell placed in the glove box was used for
all the electrochemical measurements with the aid of a potentio-
stat/galvanostat (Hokuto Denko, HABF-501). Polycrystalline Pt
disk electrodes (Nilaco, 7.85 × 10−3 or 7.07 × 10−2 cm2) were
used as the working electrodes. These electrodes were mirror-
polished, treated in nitric acid, rinsed with distilled water and
acetone, and dried before use. A Pt wire was used as a counter
electrode. A reference electrode was consisted of a silver wire
immersed in 0.1 mol dm−3 AgCF3SO3/EMITFSI (EMI+ = 1-
ethyl-3-methylimidazolium) separated from the bulk solution
by porous glass (Vycor). The potential of this Ag/Ag(I) ref-
erence electrode was +0.44 V against a ferrocene/ferrocenium
redox couple.
Potentiostatic and galvanostatic electrodeposition experi-
ments were conducted with a two-compartment cell. A Ni
plate (Nilaco, 1 or 0.25 cm2) was used as a substrate. The
Ni plate was mirror-polished using 0.1 ␮m alumina slurry
(MARUTO), washed with distilled water and acetone, and dried
in the air before use. An Au plate was used as the counter
electrode and placed in the separated compartment with a
glass filter (G4). The same reference electrode was used as
described above. The obtained electrodeposits were charac-
terized by X-ray diffraction (XRD, Rigaku RAD-C, Cu K␣
radiation with a HOPG monochrometer), X-ray photoelectron
spectroscopy (XPS, JEOL JPS-9000MC), scanning electron
microscopy (SEM, NIKON XL-30 ESEM), and energy disper-
sive X-ray analysis (EDX, EDAX Phoenix).
Fig. 1. UV–vis absorption spectra of the BMPTFSI containing 0.625 mM (—)
or 2.5 mM (- - - -) Pd(II)/BMPTFSI.
Table 1
Comparison of UV–vis spectra of PdX42− (X = Br− and Cl−) in BMPTFSI and
aqueous solution
Observed bands (nm)
BMPTFSI
Assignmenta
Aqueous solutionb
2−
2−
2−
2−
PdCl4
PdBr4
PdCl4
PdBr4
1
522
437
390sh
350
480
350sh
498
426
371sh
331
480
332
1A1g → Eg (d–d)
1
1A1g → Eu (LMCT)
1
280
1A1g → A2u + 1Eu(␲) (LMCT)
All the electrochemical measurements including electrode-
position experiments were conducted at 25 ◦C in the Ar filled
globe box.
a
According to Ref. [8].
Data taken from Ref. [7].
b

Y. Bando et al. / Electrochimica Acta 53 (2007) 87–91
89
Fig. 5. XPS spectrum of the deposit obtained on the Ni substrate by the poten-
tiostatic electrodeposition in 10 mM PdBr42−/BMPTFSI at −1.7 V (electric
charge: 18 C, electrolysis time: 42 h).
Fig. 2. Cyclic voltammograms of a Pt electrode in BMPTFSI with (—) and
2−
without (- - - -) 10 mM PdBr4 at 25 ◦C. Scan rate: 50 mV s−1
.
because part of Pd deposited during the preceding cathodic
scan dropped off from the electrode. The peak potential of the
cathodic current peak was shifted toward negative potential with
an increase in the sweep rate. Although the iR compensation was
not employed in this cyclic voltammetry, the iR drop could be
estimated to be less than 10 mV assuming the resistance between
the working and reference electrode is about 500 ꢀ, which is
typical for our cell. The cathodic peak potential depended on
the logarithm of the sweep rate as shown in Fig. 6, suggesting
the electrode reaction is regarded as electrochemically irre-
versible [9]. The product of the transference coefficient, α, and
the number of electrons in the rate-determining process, na, was
estimated to be 0.34 from the slope. This αna value may imply
the reduction of PdBr42− to Pd(0) involves 2 one-electron trans-
fer steps since the value of α is normally in the range from 0.3
to 0.7. However, further investigation is necessary in order to
2−
Fig. 3. SEM image of the deposit obtained on a Ni substrate by the poten-
tiostatic electrodeposition in 10 mM PdBr42−/BMPTFSI at −1.7 V (electric
charge: 18 C, electrolysis time: 42 h).
clarify the detailed mechanism of the reduction of PdBr4 to
Pd(0).
Fig. 7 shows the chronopotentiograms of the Pt elec-
trode in 10 mM PdBr42−/BMPTFSI. The plateau potentials
shifted negatively as the current density increased, which
is particular behavior of the irreversible electrode reaction.
From the transition time, τ of each chronopotentiogram, the
diffusion coefficient of PdBr42−, D, was calculated to be
2−
voltammogram was ascribed to the reduction from Pd(II)Br4
to Pd(0):
PdBr42− + 2e− = Pd + 4Br−.
(2)
An anodic current peak at −0.4 V can be assigned to the
anodic dissolution of Pd deposited during the preceding cathodic
potential sweep. The electric charge for the anodic dissolu-
tion was smaller than that for the cathodic deposition probably
Fig. 4. XRD pattern of the deposit obtained on the Ni substrate by the poten-
tiostatic electrodeposition in 10 mM PdBr42−/BMPTFSI at −1.7 V (electric
charge: 18 C, electrolysis time: 42 h).
Fig. 6. A relationship between cathodic peak potentials and the logarithms of
scan rates for the cyclic voltammograms in 10 mM PdBr42−/BMPTFSI at 25 ◦C.

90
Y. Bando et al. / Electrochimica Acta 53 (2007) 87–91
The chronoamperometry was attempted to obtain the
information about the initial stage of Pd deposition on Pt
polycrystalline electrode surface. Fig. 8 shows the chronoam-
perograms of a Pt electrode in 10 mM PdBr42−/BMPTFSI. A
current peak characteristic of nucleation process was observed
just after the charging current for each potential step. These
peaks were analyzed according to three-dimensional nucleation
model under diffusion control [11]. Fig. 9 shows the relationship
between the non-dimensional parameters, j/jm and t/tm, where
jm is the peak current density, t the time and tm is the time at jm.
Scharifker and Hills investigated that the 3D nucleation on an
Fig. 7. Chronopotentiograms of a Pt electrode in 10 mM PdBr42−/BMPTFSI at
25 ◦C.
Fig. 8. Chronoamperogram of a Pt electrode in 10 mM PdBr42−/BMPTFSI at
25 ◦C.
1.3 × 10−7 cm s−1 by the Sand equation:
jτ1/2
C∗
nFπ1/2D1/2
=
(3)
2
where j is the current density, C* the bulk concentration of
PdBr42−, n the number of electrons and F is Faraday con-
2−
stant. The diffusion coefficient of PdBr4 is close to those of
FeCl4− and FeBr4− (1.1 and 0.97 × 10−7 cm s−1, respectively)
in BMPTFSI [10].
Fig. 10. SEM images of the deposits on the Ni substrates by the galvanostatic
electrodeposition in 10 mM PdBr42−/BMPTFSI at 25 ◦C. The current densities
were (a) −0.05 mA cm−2, (b) −0.02 mA cm−2 and (c) −0.01 mA cm−2 (electric
charge: 1 C).
Fig. 9. Non-dimensional plots of j2/jm2 vs. t/tm using the data of Fig. 8. Instan-
taneous (—), progressive (- - - -), −1.6 V (ꢀ) and −1.7 V (ꢀ).

Y. Bando et al. / Electrochimica Acta 53 (2007) 87–91
91
Table 2
2−
Comparison of the reduction current peaks of PdCl4 in the cyclic voltammo-
grams of some RTILs.
Ionic liquid
Electrode
Reduction potential
(V) vs. Fc/Fc+
Reference
BMPTFSI
EMI–Cl–BF4
AlCl3–EMIC (neutral)
Pt
GC
Pd
−1.4
−1.2
−1.5
This work
[4]
[3]
close to those of the two other RTIL, suggesting that the outer
sphere effect on the redox potential of these complexes is not
so significant in these media.
Fig. 11. Cyclic voltammograms of a Pt electrode in BMPTFSI containing
10 mM PdCl42− (—) and 10 mM PdBr42− (- - - -) at 25 ◦C. Scan rate: 50 mV s−1
4. Conclusions
.
PdBr2 and PdCl2 dissolved in BMPTFSI as square planer
complexes and gave red and orange-yellow solutions, respec-
tively. The cathodic current peak attributed to the reduction
of Pd(II) to Pd(0) was observed in the cyclic voltammogram
electrode surface is classified into instantaneous and progressive
nucleation process. The relationship between jm and tm is given
by the following equation for each nucleation process:
2−
for PdBr42−/BMPTFSI. The electrode reaction of PdBr4 to
jm2 tm = 0.1629(nFC)2D (instantaneous)
jm2 tm = 0.2598(nFC)2D (progressive)
(4)
(5)
metallic Pd was irreversible and the diffusion coefficient of
2−
PdBr4 was about (1–2) × 10−7 cm2 s−1 at 25 ◦C. It was sug-
gested that the initial stage of the electrodeposition of Pd from
PdBr42−/BMPTFSI on the polycrystalline Pt electrode surface
involves three-dimensional progressive nucleation under diffu-
sion control. The morphology of the deposits obtained by the
galvanostatic electrodeposition depended on the current density.
In addition, PdCl42− showed a similar electrochemical behavior
It is apparent from the comparison of the data obtained in this
study with the simulated curves in Fig. 9 that the initial stage of
Pd deposition on the polycrystalline Pt electrode can be regard
as the progressive nucleation. Furthermore, using the equation
for the progressive nucleation shown above, the diffusion coef-
ficient is calculated to be 1.6 × 10−7 cm s−1, which is close to
that estimated by the chronopotentiometry.
2−
as PdBr4 in BMPTFSI, while the reduction potential shifted
negatively by 0.2 V.
Galvanostatic electrodeposition of Pd on a Ni substrate was
also carried out at various current densities. Fig. 10 shows the
SEM images of the deposits. Black and powdery deposits were
obtained at higher current densities (Fig. 10a). However, smooth
deposits with brightness could be obtained at lower current den-
sities (Fig. 10c). The further investigation on controlling the
morphologies of the deposit is under way.
Acknowledgements
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). This
work was also supported by Grant-in-Aid for the 21st Century
COE program “KEIO Life Conjugate Chemistry” from MEXT.
PdCl2 is commonly used for industrial Pd electroplating
baths. Also, as mentioned in Section 1, some electrochemical
2−
measurements on the PdCl4 in other RTILs were performed
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examine the electrochemical behavior of PdCl42− and compare
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gram of the Pt electrode in 10 mM PdCl42−/BMPTFSI. The
2−
reduction peak of PdCl4
appeared at −1.8 V, which was
2−
more negative than that for PdBr4 by about 0.2 V as shown
in Fig. 1. The deposition of metallic Pd was also possible
from PdCl42−/BMPTFSI. The shift in the peak potential can
be attributed to the difference in donor property of the ligand.
Since the donor number of Cl− is larger than that of Br−,
the chlorocomplex is expected be more stabilized than the
bromocomplex. The reduction peak potentials of the Pd chloro-
complexes in other RTILs are listed in Table 2 [3,5]. All the
peak potentials are obtained from the cyclic voltammograms at
scan rate of 100 mV s−1, and referred to Fc/Fc+ redox potential
in each media for comparison [12]. As shown in this table,
2−
the reduction potential of PdCl4
in BMPTFSI was very