Partial nickel plating using organic gel electrolyte

Article abstract of DOI:10.1016/j.electacta.2007.04.051

A partial nickel plating method was developed by using the organic gel electrolyte. The gel electrolyte was made from polyvinylchloride as a gelling agent and tetrahydrofuran as an organic solvent. The suitable conditions for nickel plating were investigated by the measurements of polarization curve and electrochemical impedance. The pattern electroplating using thin layer of gel electrolyte was carried out on the copper substrate. The substrate surface could be plated by nickel with high uniformity and sharp edge line. The present method will be widely used for the surface patterning without masking.

Full text of DOI:10.1016/j.electacta.2007.04.051

Electrochimica Acta 52 (2007) 6421–6426
Partial nickel plating using organic gel electrolyte
Masayuki Itagaki∗, Isao Shitanda, Wataru Nakamura, Kunihiro Watanabe
Department of Pure and Applied Chemistry, Faculty of Science and Technology,
Tokyo University of Science, Noda, Chiba 278-8510, Japan
Received 23 December 2006; received in revised form 16 April 2007; accepted 16 April 2007
Available online 25 April 2007
Abstract
A partial nickel plating method was developed by using the organic gel electrolyte. The gel electrolyte was made from polyvinylchloride as
a gelling agent and tetrahydrofuran as an organic solvent. The suitable conditions for nickel plating were investigated by the measurements of
polarization curve and electrochemical impedance. The pattern electroplating using thin layer of gel electrolyte was carried out on the copper
substrate. The substrate surface could be plated by nickel with high uniformity and sharp edge line. The present method will be widely used for
the surface patterning without masking.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Organic solvent; Gel electrolyte; Nickel plating; Partial plating; Electrochemical impedance
1. Introduction
In the present study, we developed a new partial nickel plating
method by using the organic gel electrolyte. Nickel electroplat-
ing has been widely used for the industrial products because
of their excellent corrosion resistance, hardness, and beautiful
externals with silver-white color. However, nickel electroplating
from an aqueous electrolyte solution, like Watt solution [8], is
usually accompanied by hydrogen generation. The cathode cur-
rent efficiency of some aqueous nickel electroplating solutions
may vary from 90% to 97% [8]. If an aqueous electrolyte solu-
tion was used in the partial plating using the gel electrolyte, the
electrodeposition of nickel was inhibited by absorption of the
hydrogen on the substrate. Therefore, we use the organic sol-
vent for the electroplating solution because of the wide potential
window.
Partial plating is a technique for coating only in the specific
part on a substrate. Partial plating has been used for the repairing
damaged part of the film and available for the surface patterning.
Brush plating is one of the useful and portable methods of the
partial plating without masking [1–4]. A brush attached to anode
is dipped in a plating solution and then contacted on the specific
part of a cathode substrate. Itisdifficultforthismethod to control
the surface area precisely with good reproducibility. Besides,
laser plating has been examined for partial plating [5], but it
requireslaserequipment, whichisgenerallylargeandexpensive.
There are some reports regarding the research for the partial
plating with the aqueous gel electrolyte as follows: (a) plating
used agar as gelling agent to manufacture probe pin [6] and (b)
copper plating from copper sulfate solution gelated by gelatin
[7]. The authors [7] reported various electrochemical measure-
ments for copper plating with gel electrolyte, and clarified the
reaction mechanisms and the roles of additives. In this method,
the pattern plating can be carried out with the use of a small
amountoftheelectrolytebythinningitsthicknessandthepattern
can be drawn with the gel easily and precisely without masking.
An aim of this research is development of the nickel partial
plating with organic gel electrolyte and analysis of the nickel
electrodeposition in the gel electrolyte by using the electrochem-
ical impedance spectroscopy (EIS).
2. Experimental method
2.1. Gel electrolyte
Ni(ClO4)2·6H2O, LiCl and LiClO4 were used for the
electrolytes. LiCl and LiClO4 act as the complexing agent
and supporting electrolyte, respectively. Tetrahydrofuran (THF,
∗
Corresponding author. Tel.: +81 4 7122 9492; fax: +81 4 7123 9890.
E-mail address: itagaki@rs.noda.tus.ac.jp (M. Itagaki).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.04.051

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M. Itagaki et al. / Electrochimica Acta 52 (2007) 6421–6426
Table 1
Wako Chemical) was used for an organic solvent. Polyvinylchlo-
ride (PVC, degree of polymerization: 1100, Wako Chemical)
was used for a gelling agent. Gel electrolyte was prepared by
adding PVC in the THF electrolyte. All chemicals used in the
present paper were the analytical grade. Molecular sieve was
used for the purification of THF electrolyte. All experiments
were carried out under a dry argon atmosphere in a glove box.
Viscosity of the THF solution containing various concentration of PVC
Viscosity (Pa s)
THF (PVC 0 wt%)
PVC 5 wt%
PVC 10 wt%
5.9 × 10−4
1.3 × 10−2
8.2 × 10−2
2.2. Measurement of cathodic polarization curve and
electrochemical impedance
3. Results and discussion
3.1. The preparation of gel electrolyte
Cathodic polarization curve and electrochemical impedance
were measured for the analysis of the nickel electrodeposition in
the gel electrolyte. These measurements were performed using
the three electrodes method. Pure copper rod (Nilaco, grade:
99.5%) was used for working electrode whose diameter was
5.0 mm. Nickel plate was used for the counter electrode. The ref-
erence electrode was Ni wire. In the present paper, the potential
is represented relative to the Ni reference electrode. Before the
measurement, the surface of the working electrode was polished
with emery papers down to grid size #2000. After polishing, the
working electrode was cleaned in methanol and distilled water
successively.
The i–E curve was measured by using the potentiostat
(Hokuto, HG-501G) connected with the potential sweeper
(Hokuto, HB-111). The potential was scanned at 0.1 V/min, and
the response current was recorded by the personal computer.
The electrochemical impedance was measured by using
the high response potentiostat (Hokuto, HG-501G) and the
frequency response analyzer (FRA, NF circuit block 5020)
controlled by a personal computer through GP-IB interface.
The impedance measurement was carried out in a frequency
range from 10 mHz to 10 kHz. The amplitude of the potential
modulation of the working electrode was 10 mV. The mea-
surement of electrochemical impedance was started at 30 min
after polarization at an arbitrary potential to obtain the steady
state.
We chose THF as the solvent because the THF dissolved
many kinds of electrolytes. THF has been applied for electro-
plating with organic solvent, especially the electrodeposition of
alminum with high throwing power [9–11]. The PVC was cho-
sen as a gelling agent. PVC has been used as the gelling agent in
a wide variety of fields. PVC is quite stable in THF. In addition,
PVC is employed as a brightener in THF electroplating bath
[11].
Table 1 shows the viscosity of THF electrolyte containing
various concentrations of PVC. The viscosity of the solution
increased with the increase of PVC concentration. The THF
electrolyte containing 0–10 wt% PVC behaved as the Newton
fluid. On the other hand, the electrolyte containing 15–20 wt%
PVC was the non-Newton fluid and behaved as a gel. The viscos-
ity cannot be measured more than 20 wt% PVC, since 105 Pa s
is upper limit of the present measurement. In these results, we
have decided that THF electrolyte containing 20 wt% PVC was
used for the gel electrolyte.
3.2. Measurement of cathodic polarization curve
The cathodic polarization curves of copper electrode in a gel
electrolytecontainingvariousconcentrationofNi(ClO4)2·6H2O
and LiCl were shown in Fig. 1. The gel electrolyte contained
equimolar amounts of Ni(ClO4)2·6H2O and LiCl because LiCl
wasusedasthecomplexingagentfornickelousion. Thecathodic
The surface of the electroplated film was observed by using
the scanning laser microscope (Keyence, VK-8500) and ana-
lyzed by GD-OES (Horiba, JY-5000RF).
2.3. Partial plating of nickel on the copper surface
Partial plating was performed on a copper plate in galvanos-
tatic electrolysis using thetwo electrodesmethod. Acopper plate
(Nilaco, grade: 99.9%) was used for cathode. A nickel plate was
used for the anode. To mold the thin film of gel electrolyte,
the electrolyte was injected into the various size of the template.
Then, the gel layer thickness was equal to the template thickness.
The anode was put on the gel electrolyte. The current density
was set at −1 mA/cm2. The electrode surface after plating was
photographed with a digital camera. Before the measurement,
the surfaces of the cathode and anode were soaked for 5 min
by the degreasing agent (Okuno Seiyaku, Ace Clean) followed
by polishing with alumina powder (diameters: 0.05 ␮m). After
polishing, the electrode was cleaned in methanol and distilled
water successively.
Fig. 1. The cathodic polarization curves of copper in the gel electrolyte (20 wt%
PVC) containing NiClO4·6H2O and LiCl: (1) 0 mol/dm3, (2) 0.05 mol/dm3, (3)
0.1 mol/dm3, and(4)0.5 mol/dm3. Allthegelelectrolytescontained0.1 mol/dm3
LiClO4 as a supporting electrolyte. The concentration of polyvinylchloride
(PVC) was 20 wt%. The potential sweep rate was 0.1 V/min.

M. Itagaki et al. / Electrochimica Acta 52 (2007) 6421–6426
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3.3. Measurement of the electrochemical impedance in the
gel electrolyte
Fig. 3 shows the Nyquist plots of electrochemical impedance
measured at various potentials (−0.4, −0.6, −0.8, −1.0 V) in
the gel electrolyte containing 0.5 mol/dm3 Ni(ClO4)2·6H2O,
and 0.5 mol/dm3 LiCl. A small inductive loop was observed
in Fig. 3(1) and (2) The inductive loop observed at the low
frequency range was found to be the Faradaic impedance due
to the adsorbed intermediate [12,13]. The reaction mechanism
of the nickel electrodeposition was proposed as the two-step
reaction (Ni(II) → Ni(I)ad → Ni) [14], and the adsorbed inter-
mediate would be Ni(I)ad. Therefore, the nickel would deposit
at the two-step reaction in the organic gel electrolyte. The val-
ues of solution resistance (Rsol) evaluated from the intersection
of real axis at high frequency side were about 50 ꢀ cm2 in
Fig. 3.
Fig. 2. The depth profile of the electroplated film performed on a copper sheet
in the electrolyte containing 0.5 mol/dm3 NiClO4·6H2O and 0.5 mol/dm3 LiCl.
The film was electroplated by the potentiostatic electrolysis at −1.0 V vs. Ni,
50 ◦C, and analyzed by GD-OES measurement.
In Fig. 3(3) and (4), Warburg impedance was observed in
low frequency range. From the results of Fig. 3(3) and (4), it
was found that the contribution of diffusion to the electrodepo-
sition mechanism was small since the Warburg impedance was
observed at only narrow potential range.
currents were observed at the potential region between −0.5
and −2 V. The blank current ((1) in Fig. 1) would be caused
by a non-Faraday process. The cathodic current increased with
the increase of the concentrations of Ni(ClO4)2·6H2O and LiCl.
Afterthemeasurement, itwasobservedthattheelectrodesurface
was coated with the silver-white film.
3.4. Influence of the gel electrolyte thickness for the
electrodeposition of nickel
The GD-OES measurement was carried out to obtain the
depth profile of the silver-white film (Fig. 2). It was found that
the film contained only Ni. The impurities such as other metals
are not detected. In addition, the current efficiency of the elec-
troplating evaluated from the mass change between before and
after the measurement was approximately 99%. The hydrogen
gas generation was not observed in the potential area between 0
and −2.5 V. These results indicate that the nickel plating from
the organic gel electrolyte can be performed with high purify
and high current efficiency.
It is important for high quality plating to keep the nickel con-
centration in the gel electrolyte. The surface concentration of
nickelous ion is concerned with the mass transfer of the nick-
elous ion from the anode to the cathode. However, the diffusion
rate of the nickel ion might be smaller than that in aqueous solu-
tion. Therefore, it is necessary to investigate whether the gel
electrolyte thickness influences on the cathodic reaction or not.
The influence of the gel electrolyte thickness was investi-
gated by using two electrodes method. The electroplating was
Fig. 3. Experimental results of the electrochemical impedance of copper in the gel electrolyte containing 0.5 mol/dm3 Ni(ClO4)2·6H2O, 0.5 mol/dm3 LiCl. The
potentials were (1) −0.4 V, (2) −0.6 V, (3) −0.8 V, and (4) −1.0 V.

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M. Itagaki et al. / Electrochimica Acta 52 (2007) 6421–6426
3.5. Measurement of the electrochemical impedance using
the two electrode method
From the results of the V–t curve in the Section 3.4, it was
found that the voltage between two electrodes increased with
the increase of gel layer thickness. The voltage (V) is expressed
by
V = Vꢀ + iRsol
(1)
where Vꢀ, i and Rsol were voltage induced by the concentra-
tion polarization between two electrodes, imposed current and
solution resistance, respectively. According to Eq. (1), the Rsol
increase with the increase of V. Consequently, the values of Rsol
and other reaction parameter at the various thickness of the gel
electrolyte were estimated by the electrochemical impedance
method.
Fig. 4. The cell voltage and time curves during nickel electrodeposition at var-
ious thicknesses of gel electrolyte (0.5 mol/dm3 Ni(ClO4)2·6H2O, 0.5 mol/dm3
LiCl). The thicknesses of gel electrolyte were (1) 3 mm, (2) 5 mm, (3) 7 mm,
and (4) 12 mm. The current density was −1 mA/cm2.
Fig. 6 shows the Nyquist plots of electrochemical impedance
measured at various thickness of the gel electrolyte. A capaci-
tive loop was observed in the fourth quadrant and a part of an
inductivity loop was observed in the first quadrant in Fig. 6.
The equivalent circuit for an electrode/solution interfaces at two
electrodes cell is shown in Fig. 7. In Fig. 7, Rct1 and Rct2 repre-
sent the charge transfer resistance. C1 and C2 are the capacitance
of the electric double layer at two electrodes. RL and L repre-
sent the resistance and inductance, respectively to represent the
inductive loop. The electrochemical parameters evaluated by the
curve-fitting were shown in Table 2. The Rsol value rises with the
increase of the gel layer thickness linearly. On the other hand,
the values of Rct were constant without depending on a gel layer
thickness. From these results, it was confirmed that the voltage
(in Fig. 4) increased with the increase of the Rsol value depend-
ing on the gel layer thickness. Therefore, it was found that the
dissolution/electrodeposition reactions were not influenced by a
performed at −1 mA/cm2. The gel electrolyte thickness was
varied between 3 and 12 mm. The V–t curves were shown
in Fig. 4. The plating time was set at 2000 s. The voltage
arises in the early stage, and became constant after 500 s. The
rapid increase of voltage which caused degradation of elec-
trolyte was not observed at any sample. The voltage between
two electrodes aroused as the gel layer thickness increased.
Fig. 5 shows the photographs of electrode surface observed
by confocal laser scanning microscope after nickel deposition
at various thicknesses of gel electrolyte. The smooth surface
with luster was observed regardless of the gel layer thickness.
From the results above-mentioned, it was found that the gel
layer thickness between 3 and 12 mm had no effect for the
cathodic electrodeposition and the anodic dissolution. There-
fore, it makes possible to plate with a small volume of the gel
electrolyte.
Fig. 5. Photographs of electrode surface after nickel deposition at various thicknesses of gel electrolyte (0.5 mol/dm3 Ni(ClO4)2·6H2O, 0.5 mol/dm3 LiCl) by confocal
laser scanning microscope. The thicknesses of gel electrolyte were (1) 3 mm, (2) 5 mm, (3) 7 mm, and (4) 12 mm. The applied current density was −1 mA/cm2. The
plating time was set at 2000 s.

M. Itagaki et al. / Electrochimica Acta 52 (2007) 6421–6426
6425
Fig. 6. Experimental results of the electrochemical impedance of copper in the gel electrolyte containing 0.5 mol/dm3 Ni(ClO4)2·6H2O, 0.5 mol/dm3 LiCl at various
thicknesses of gel electrolyte. The thicknesses of gel electrolyte were (1) 3 mm, (2) 5 mm, (3) 7 mm, and (4) 12 mm.
Fig. 7. Equivalent circuit for an electrode/solution interface at two electrode
cell.
gel layer thickness. From these results, it makes possible to plate
with high current efficiency by thinning down the gel electrolyte
thickness.
3.6. Pattern plating of nickel
Fig. 8. The photographs of (a) cathode and (b) anode after the pattern elec-
troplating using gel electrolyte. The gel electrolyte contained 0.5 mol/dm3
Ni(ClO4)2·6H2O, 0.5 mol/dm3 LiCl. The cathode substrate was copper. The
current density was −1 mA/cm2. The average thickness of gel electrolyte was
about 0.55 mm. The plating time was set at 2000 s.
The pattern plating of nickel was performed on a copper sub-
strate by the galvanostatic electrolysis in a gel electrolyte. The
current density was −1 mA/cm2. The plating time was set at
2000 s. The average thickness of the gel electrolyte was about
0.55 mm. Fig. 8 shows photographs of the substrate and the
counter electrode surface after plating observed by using the
digital camera. The substrate surface plated had high uniformity
and sharp edge line. In addition, the counter electrode was dis-
solved stably. It makes possible to perform the gel pattern plating
of nickel using organic solvent.
Table 2
4. Conclusion
Electrochemical parameter derived by curve fittings
Membrane thickness
The electroplating of nickel with the organic gel electrolyte
has been studied. THF and PVC were chosen as the organic
solvent and the gelling agent, respectively.
The influences of the nickelous ion concentration on the
reaction mechanisms of nickel electroplating were investigated
by electrochemical impedance spectroscopy. A small inductive
loop was observed at high and low frequency ranges. The reac-
3 mm
5 mm
7 mm
21.4
3.03
262
97.1
8.29
12 mm
Rct1 (ꢀ cm2)
C1 (×10−4 F)
Rsol (ꢀ cm2)
Rct2 (ꢀ cm2)
C2 (×10−6 F)
19.3
24.1
19.0
4.02
411
97.6
8.82
4.51
125
103
1.93
185
93.78
8.19
7.16

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M. Itagaki et al. / Electrochimica Acta 52 (2007) 6421–6426
tion mechanism of the nickel electrodeposition was proposed as
the two-step reaction (Ni(II) → Ni(I)ad → Ni).
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The electroplating in the gel electrolyte was performed by
using two electrodes method. By means of the adjustment of
the gel electrolyte thickness, the pattern plating of nickel can
be performed to a copper substrate by the galvanostatic elec-
trolysis in a gel electrolyte. The nickel can be plated on the
copper substrate surface with high uniformity and sharp edge
line.
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