Pulse reverse electrodeposition of Pt-Co alloys onto carbon cloth electrodes

Article abstract of DOI:10.1016/j.jallcom.2013.01.079

Galvanostatic pulse reverse (PR) electrodeposition of Pt-Co alloys onto pretreated carbon cloth from sulfate solutions has been conducted. The aim is to utilize the anodic current applied during the reverse time to manipulate the morphology and composition of the Pt-Co alloys through a dealloying process. The cathodic current density was found to have a strong effect on the amount and the morphology of the deposited Pt-Co, but not on its composition, over the tested range (20-200 mA cm^-2). A higher current density enhances the Pt-Co nucleation process leading to smaller and finer grained Pt-Co alloy electrodeposits, and ultimately to a higher electroactive surface area. Applying the anodic current over a current density range of 50-200 mA cm^-2 does not substantially alter the amount, morphology or composition of the deposited Pt-Co alloys, with the Pt contents remaining between 91 and 94 mol%. For the PR electrodeposition under the conditions studied, regardless of the magnitude applied, the anodic current does not indicate any dissolution of the Pt-Co alloy deposits, which is supported by the anodic dissolution experiment. Thus, the morphology and composition of the Pt-Co alloy deposits produced by PR electrodeposition are not significantly different from those produced by pulse current electrodeposition at the same cathodic current density.

Full text of DOI:10.1016/j.jallcom.2013.01.079

Journal of Alloys and Compounds 559 (2013) 69–75
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Pulse reverse electrodeposition of Pt–Co alloys onto carbon cloth electrodes
Napapat Chaisubanan a, Nisit Tantavichet a,b,
⇑
a Department of Chemical Technology, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
b Center of Excellence on Petrochemical and Materials Technology, Chulalongkorn University, Bangkok 10330, Thailand
a r t i c l e i n f o
a b s t r a c t
Article history:
Galvanostatic pulse reverse (PR) electrodeposition of Pt–Co alloys onto pretreated carbon cloth from
sulfate solutions has been conducted. The aim is to utilize the anodic current applied during the reverse
time to manipulate the morphology and composition of the Pt–Co alloys through a dealloying process.
The cathodic current density was found to have a strong effect on the amount and the morphology of
the deposited Pt–Co, but not on its composition, over the tested range (20–200 mA cmꢀ2). A higher
current density enhances the Pt–Co nucleation process leading to smaller and finer grained Pt–Co alloy
electrodeposits, and ultimately to a higher electroactive surface area. Applying the anodic current over
a current density range of 50–200 mA cmꢀ2 does not substantially alter the amount, morphology or
composition of the deposited Pt–Co alloys, with the Pt contents remaining between 91 and 94 mol%.
For the PR electrodeposition under the conditions studied, regardless of the magnitude applied, the
anodic current does not indicate any dissolution of the Pt–Co alloy deposits, which is supported by the
anodic dissolution experiment. Thus, the morphology and composition of the Pt–Co alloy deposits
produced by PR electrodeposition are not significantly different from those produced by pulse current
electrodeposition at the same cathodic current density.
Received 13 November 2012
Received in revised form 16 January 2013
Accepted 18 January 2013
Available online 29 January 2013
Keywords:
Pt–Co alloy
Pulse reverse electrodeposition
Electrocatalyst
Dissolution
Proton exchange membrane fuel cell
Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction
in contrast, can fabricate the Pt-alloy catalyst electrodes in a single
step without the need for high temperature heat-treatment
There has been a fairly recent vast increase in the application of
metal or metal alloy coatings by electrodeposition in diverse areas,
such as decoration, corrosion protection [1], ultra large scale inte-
grated circuit fabrication [2], sensors [3–5] and electrode fabrica-
tions to use in electrochemical reactions [6,7]. In addition,
electrodeposition can also be used to prepare Pt electrocatalyst
layers for use in proton exchange membrane fuel cells (PEMFCs)
[8–17]. In conventional methods, Pt has to be synthesized and
loaded on the carbon (C) supports first (Pt/C). The Pt/C is then
mixed with ionomers to form a colloidal solution and subsequently
painted or sprayed onto the carbon paper or carbon cloth electrode
to form the Pt catalyst electrode. On the other hand, the fabrication
of Pt catalyst electrodes by electrodeposition is a single step pro-
cess where the synthesis of Pt particles and their application onto
the electrode take place simultaneously. In addition, the typical
methods for Pt-based alloy catalyst electrode fabrication for use
in PEMFCs may require a high temperature heat-treatment for
the alloying process, which adds more steps and can lead to the
formation of larger Pt-alloy particles [18–24]. Electrodeposition,
[25–30].
For the galvanostatic electrodeposition, different current wave-
forms, including direct current (DC) electrodeposition, pulse cur-
rent (PC) or pulse reverse current (PR) electrodeposition, can be
applied. Pulse electrodeposition (PC and PR) is widely accepted
to be superior to DC electrodeposition since the metal coatings pre-
pared by PC electrodeposition consist of smaller and finer grained
structures [8,10,12,29,31–37], which lead to a higher surface area
[10]. In addition, unlike DC electrodeposition which has the ap-
plied current density as the only controlling parameter, PC electro-
deposition has various operating parameters, namely the cathodic
current density (iC), on-time (Ton), off-time (Toff), duty cycle (ratio
of on-time to the pulse cycle period) and frequency, to control
the electrodeposition process, to produce deposits with desired
properties. PR electrodeposition has the capability to improve the
deposit uniformity over that achieved by DC or PC plating by pref-
erentially dissolving overplated areas during the reverse-time
(Trev) where an anodic current density (iA) is applied. For metal
alloy deposition of some systems, such as Ni–Fe and Co–Fe, the
use of PR electrodeposition can also affect the deposit composition
differently than that of PC electrodeposition [38–40]. The benefit of
PR electrodeposition of noble metals, such as Pt, may be minimal
since they may be stable and may not dissolve during the
reverse-time. However, the bi-metal electrodeposition of Pt and a
⇑
Corresponding author at: Department of Chemical Technology, Faculty of
Science, Chulalongkorn University, Bangkok 10330, Thailand. Tel.: +66 2 218 7677;
fax: +66 2 255 5831.
E-mail address: Nisit.T@chula.ac.th (N. Tantavichet).
0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2013.01.079

70
N. Chaisubanan, N. Tantavichet / Journal of Alloys and Compounds 559 (2013) 69–75
purged 0.5 M H2SO4 solution with a Pt gauze counter electrode and a Ag/AgCl ref-
erence electrode. The cyclic voltammetry (CV) was scanned between ꢀ0.4 and
ꢀ1.4 V (Ag/AgCl) at a scan rate of 20 mV sꢀ1 using an Autolab PGSTAT 10 Potentio-
stat. The EAS (in m2 gPtꢀ1) was estimated from the coulometric charge of the hydro-
gen desorption peak using the following equation [49–52]:
non-noble metal, such as Co, may benefit from the use of PR elec-
trodeposition to manipulate the physical and chemical properties
of the Pt-non noble metal alloy through the electrochemical deal-
loying process of the less noble metal. The preferential dissolution
of the non-noble metal from the Pt alloy structure can lead to a
more porous and higher Pt content deposit, which can improve
its catalytic performance.
In this present work, we investigate the PR electrodeposition of
Pt–Co alloys onto pretreated carbon cloths in sulfate solutions. Pt–
Co alloys are widely considered as possible candidates for cathode
electrocatalysts in PEMFCs due to their higher electrocatalytic
activity towards the oxygen reduction reaction (ORR) compared
to pure Pt or other Pt-based alloys [19,20,41–48]. The morpholog-
ical and structural characteristics and compositions of the Pt–Co
electrodeposited catalyst layers were studied by scanning electron
microscopy (SEM), energy-dispersive spectroscopy (EDX), and X-
ray diffractometry (XRD). The main focus of the research reported
here is on the effect of the reverse current applied during the re-
verse-time where the dealloying of Co from the electrodeposited
Pt–Co alloys is expected to take place, since this can result in the
altered morphology and composition of the Pt–Co alloy deposits
from those prepared by PC electrodeposition.
qH
EAS ¼ 0:1
ð1Þ
WPt ꢂ qH;ref
where qH is the total charge density (mC cmꢀ2) associated with the hydrogen under-
potential desorption (calculated by integrating the hydrogen desorption peak of the
CV curve with the subtraction of the baseline due to the double layer capacity), WPt is
the Pt loading (mg cmꢀ2) and qH,ref is the charge density (0.21 mC cmꢀ2), corre-
sponding to the adsorption of a monolayer of hydrogen on a polycrystalline Pt
surface.
3. Results and discussion
The XRD results of the Pt–Co deposits prepared from the PR
electrodeposition at a cathodic current density of 20 mA cmꢀ2
and different anodic current densities, along with that of the Pt
electrodeposit prepared by DC electrodeposition at 10 mA cmꢀ2
,
are shown in Fig. 1. The inset shows the lattice parameter and
Pt–Pt mean atomic distance of the Pt and Pt–Co electrodeposits,
as estimated from the XRD patterns. The XRD patterns of the
Pt–Co deposits prepared from PR electrodeposition using other
cathodic current densities are similar to those shown in Fig. 1
and so are not shown here. The XRD patterns of the Pt–Co depos-
its prepared by PR electrodeposition show noticeable shifts of the
Pt(111), Pt(200), Pt(220), Pt(311) and Pt(222) peaks to higher
2h values compared to those in the Pt electrodeposited catalyst.
No additional peaks appear on the XRD patterns of the Pt–Co
deposits from that of the Pt deposit. The lattice parameter and
Pt–Pt mean atomic distance of the Pt–Co electrodeposited cata-
lysts become smaller than that of the Pt electrodeposited catalyst.
These results indicate the contraction of Pt lattices due to the
incorporation of smaller Co atoms into the face cubic center
(fcc) structure of the larger Pt structure [19,42,53,54] during
Pt–Co electrodeposition. Also, there is no evidence of metallic
2. Experimental
2.1. Preparation of pretreated carbon cloth
The pretreated carbon cloth was prepared by applying two sub-layers, one
hydrophobic and one hydrophilic, based on previously reported work [17]. The
hydrophobic layer was prepared from a mixture of a carbon black (Vulcan XC-
72), PTFE (60 wt.%) (Aldrich) and isopropanol (Fluka) with a PTFE to carbon black
weight ratio of 30:70 and was applied onto a 5 cm2-carbon cloth (Electrochem,
Inc.) until having the loading of 1.9 mg cmꢀ2. Then, a hydrophilic layer consisting
of a Nafion 117 ionomer (Fluka) and glycerol (Fluka) with a Nafion to glycerol
weight ratio of 50:50 was painted on top to have the total hydrophilic loading of
0.8 mg cmꢀ2. Finally, the prepared non-catalyst electrode was dried at 300 °C for
2 h.
2.2. Electrodeposition of Pt–Co on the pretreated carbon cloth
Co (the reflection of Co(111) is expected to appear at
a
2h = 44.8) or Co oxides found from the XRD patterns, which
implies that the Co atoms are incorporated into the Pt structure
in the form of an alloy.
Electrodeposition of the Pt–Co alloys was conducted in a two compartment
electrochemical cell, separated by
a Nafion 115 membrane, using a standard
three-electrode system. The pretreated carbon cloth was placed in a compartment
containing a solution of 0.01 M H2PtCl6ꢁ6H2O (Fluka), 0.1 M CoSO4ꢁ7H2O (Fluka) and
0.5 M H2SO4. The titanium gauze used as the counter electrode was placed in the
other compartment containing 0.5 M H2SO4. The Ag/AgCl reference electrode (3 M
KCl, Metrohm) was placed in the same compartment as the working electrode to re-
duce the voltage drop when monitoring the electrode potential during the electrol-
ysis. During the electrodeposition the solution was stirred by a magnetic stirrer at
300 rpm. All electrodeposition experiments were carried out using an Autolab
PGSTAT 10 Potentiostat (Eco Chemie) equipped with voltage multiplier to widen
the monitored potential range. The electrode responses recorded during the electro-
deposition are reported in the Ag/AgCl scale.
The cathodic and anodic current densities were evaluated over the range of 20–
200 mA cmꢀ2 and 50–200 mA cmꢀ2, respectively. Each cathodic pulse was set to
have 10 mC cmꢀ2 of charge density. Unless otherwise stated, all electrodeposition
experiments were carried out until a total net charge density of 2 C cmꢀ2 was
passed where the total cathodic and anodic charge densities for the PR electrode-
position were fixed at 4 and 2 C cmꢀ2, respectively. After the electrodeposition,
the Pt–Co electrodeposited carbon cloth was removed from the solution and dried
at 80 °C for 2 h. The total amount of the Pt–Co alloy catalyst electrodeposited on the
electrode was estimated from the weight difference between that before and after
the electrodeposition.
The SEM images of the Pt–Co deposits prepared from different
electrodeposition conditions are shown in Fig. 2. For comparison
purposes, the Pt–Co electrodeposits prepared by DC (current den-
sity and charge density of 10 mA cmꢀ2 and 2 C cmꢀ2, respectively)
and PC (cathodic current density of 20 mA cmꢀ2 and charge density
of 2 and 4 C cmꢀ2) electrodeposition were also conducted and their
SEM images are shown in Fig. 3. The amount and composition of
the Pt–Co deposits prepared under different conditions are showed
in Table 1. SEM images (Fig. 2) indicate that the catalyst layers pre-
pared by PR electrodeposition at the lowest cathodic current den-
sity (20 mA cmꢀ2) consist of large Pt–Co particles. The size of the
Pt–Co particles is reduced at higher cathodic current densities. In
addition, compared to the deposit produced by the DC electrode-
position at 10 mA cmꢀ2 (Fig. 3a), the deposits obtained from the
pulse electrodeposition under the conditions studied (Figs. 2 and
3b and c) have smaller grains and finer structures due to the ability
to apply a significantly larger current density during the pulse elec-
trodeposition without violation of mass transfer limitations [55]. A
higher applied current density generally leads to more energy
being available to enhance the nucleation process during electro-
crystalization and so produce smaller-grained particles [56]. How-
ever, when comparing the morphologies of the Pt–Co alloy
deposits obtained under different anodic current densities, the
SEM results reveal no significant difference in the Pt–Co deposited
structure, even though anodic current densities as low as
20 mA cmꢀ2 and as high as 200 mA cmꢀ2 were used. In fact, the
2.3. Analysis of the Pt–Co deposit
The morphology and composition of the electrodeposited Pt–Co catalyst layers
were studied by SEM (JEOL: JSM 6400) equipped with EDX. The structural charac-
teristics of the electrodeposited catalyst layers were examined by XRD (Bruker
AXS: D8 Discover).
The electrochemical active surface area (EAS) of each Pt–Co alloy deposit was
studied based on underpotential of hydrogen desorption using the electrochemical
technique. The carbon cloth coated with the Pt–Co electrodeposited catalyst layer
was placed in
a single compartment electrochemical cell containing nitrogen

N. Chaisubanan, N. Tantavichet / Journal of Alloys and Compounds 559 (2013) 69–75
71
(111)
(200)
(311)
(220)
(222)
(a)
(b)
(c)
(d)
30
40
50
60
70
80
90
2theta (degree)
Fig. 1. X-ray diffraction patterns of the (a) Pt deposit prepared by DC electrodeposition at 10 mA cmꢀ2 and (b–d) Pt–Co alloy deposits prepared by PR electrodeposition at a
cathodic current density of 20 mA cmꢀ2 and anodic current densities of (b) 50, (c) 100 and (d) 200 mA cmꢀ2. The inset presents the lattice parameter and the Pt–Pt mean
atomic distance of the Pt and Pt–Co deposits prepared under the above conditions.
anodic current density
100 mA cm-2
cathodic
current
density
50 mA cm-2
200 mA cm-2
(a)
(d)
(g)
(b)
(e)
(h)
(c)
(f)
(i)
20 mA cm-2
100 mA cm-2
200 mA cm-2
1 µm
Fig. 2. SEM images (ꢂ5000) of the Pt–Co alloy deposits prepared by PR electrodeposition at different cathodic current densities of (a–c) 20, (d–f) 100 and (g–i) 200 mA cmꢀ2
and anodic current densities of (a, d, and g) 50, (b, e, and h) 100 and (c, f, and i) 200 A cmꢀ2 for cathodic and anodic charge densities of 4 and 2 C cmꢀ2, respectively.
morphologies of the Pt–Co deposits prepared by the PR electrode-
position (Fig. 2a–c) and PC electrodeposition (Fig. 3b), where no
anodic current was applied, are relatively similar. These results im-
ply that the magnitude of the applied anodic current or the pulse
mode may not have any profound effect on the deposit morphol-
ogy of the Pt–Co deposits.

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N. Chaisubanan, N. Tantavichet / Journal of Alloys and Compounds 559 (2013) 69–75
Fig. 3. SEM images (ꢂ5000) of the Pt–Co alloy deposits prepared by (a) DC electrodeposition at 10 mA cmꢀ2 and 2 C cmꢀ2 and (b and c) PC electrodeposition at a cathodic
current density of 20 mA cmꢀ2 and charge densities of (b) 2 and (c) 4 C cmꢀ2
.
Table 1
Deposit loading, chemical composition, current efficiency and electrochemical surface area of the as-deposited Pt–Co alloys prepared under different electrodeposition conditions.
Mode iC
(mA cmꢀ2
Ton
(s)
iA
Trev
(s)
qon
qrev
Deposit loading
Pt:Co
(mol%)
Current efficiencya
(%)
EAS
)
(mA cmꢀ2
)
(C cmꢀ2
)
(C cmꢀ2
)
(mg cmꢀ2
)
(m2 gPt)
DC
PC
10
20
200
0.5
–
–
–
50
100
200
50
100
200
50
–
2
2
4
4
4
4
4
4
4
4
4
4
–
–
–
2
2
2
2
2
2
2
2
2
0.3768
0.1739
0.4493
0.4493
0.4278
0.4928
0.2464
0.2029
0.2029
0.0579
0.0539
0.0579
92:8
93:7
93:7
93:7
93:7
92:8
91:9
93:7
93:7
91:9
94:6
94:6
37.9
17.5
22.6
22.6
21.5
24.8
12.4
10.2
10.2
2.9
54
–
69
72
72
0.03
0.03
0.1
0.05
0.025
0.1
0.05
0.025
0.1
0.05
0.025
PR
20
100
200
0.5
0.2
0.1
71
221
271
200
821
799
820
100
200
2.7
2.9
a
Current efficiency is calculated based on the applied cathodic charge density.
Like the morphology, the amount of Pt–Co deposited on the
area for the HER of the Pt–Co catalysts produced at the higher
cathodic pulse amplitudes, since Pt is a good catalyst for the HER
and the small-grained catalysts (Fig. 2) lead to the higher active
area for the HER (this will be discussed in detail later).
electrode was found to be strongly affected by the magnitude of
the cathodic current density applied (Table 1). The amount of Pt–
Co deposited on the carbon cloth tends to decrease as the cathodic
current density is raised, falling from about 0.45 mg at 20 mA cmꢀ2
down to 0.20 and 0.054 mg at 100 and 200 mA cmꢀ2, respectively.
The decrease in the amount of Pt–Co catalyst deposited by an
increase in the cathodic pulse amplitude is likely to be due to an
increase in the hydrogen evolution reaction (HER). Thermodynam-
ically, Pt preferentially deposits much easier than Co since the
standard reduction potentials of PtCl26ꢀ and Co2+ ions to metallic
metals are approximately at 0.76 and ꢀ0.28 V (SHE), respectively.
However, the reduction potentials of these two metals are sepa-
rated by the hydrogen evolution reaction. That means at high
applied current densities, like ones used in this study, the electro-
deposition of Pt–Co alloy is normally accompanied by the HER.
During electrodeposition at the high applied current density, the
surface concentration of Pt ions reaches low level and approaches
the mass transfer limitation, while surface concentration of Co ions
remains relatively high, so that the Co reduction and the HER can
proceed at the higher rates. Although the HER is thermodynami-
cally easier to proceed compared to the Co deposition reaction, un-
der the appropriate conditions (i.e., at high negative potentials and
in a plating bath with a limited amount of H+ ions) the Co deposi-
tion can proceed at a faster rate than the HER [57]. However, at the
very high negative cathodic potentials and in the high acidic plat-
ing bath (Fig. 4), the HER is the predominant cathodic reaction so
that the amount and the current efficiency of Pt–Co deposits are
decreased by an increase in the cathodic pulse amplitude (Table 1).
The other factor that also facilitates the HER for the Pt–Co deposi-
tion at the higher cathodic current densities is the higher active
Unlike the magnitude of the cathodic current density, the mag-
nitude of the anodic current density (iA) does not seem to affect the
amount of Pt–Co deposited on the electrodes. Interestingly, the
amounts of Pt–Co alloys electrodeposited by PR electrodeposition
are similar to those produced by PC electrodeposition using the
same cathodic charge density of 4 C cmꢀ2 (i.e., about 0.44 mg cmꢀ2
for an iC of 20 mA cmꢀ2). The amount and morphology of the Pt–Co
deposited by different anodic current density amplitudes indicate
that the pulse plating mode and the magnitude of the anodic cur-
rent density do not substantially affect the physical properties of
the Pt–Co alloy deposits.
The composition of the Pt–Co alloys prepared by PR electrode-
position is not significantly affected by the magnitude of the catho-
dic and anodic current densities, since the Pt content remains
between 91% and 94% for all electrodeposition conditions evalu-
ated (Table 1). As discussed earlier, the Pt–Co alloy electrodeposit-
ion at the high cathodic current densities takes place under the
mass transfer limitation of Pt and the excess current is used for
the Co deposition and the HER. If the amount of H+ ions in the plat-
ing baht is limited, the HER is diffusion limited and Co deposition
makes up the deficit so that the Pt–Co alloy with the higher Co con-
tent is expected [57–59]. On the other hand, for the Pt–Co electro-
deposition in the high acidic plating baht, the HER proceeds at the
faster rate than the Co deposition reaction. Thus, the excess part of
the cathodic current is used for the HER and the composition of the
Pt–Co alloys is unaffected by the cathodic pulse amplitude.
Interestingly, the Pt content of the Pt–Co alloy obtained from PC

N. Chaisubanan, N. Tantavichet / Journal of Alloys and Compounds 559 (2013) 69–75
73
10
5
10
(a)
(b)
5
0
0
-5
-5
0
1
2
3
0
1
2
3
time / s
time / s
10
10
5
(c)
(d)
5
0
0
-5
-5
0
1
2
3
0
1
2
3
time / s
time / s
Fig. 4. Electrode response during (a) PC electrodeposition at a cathodic current density of 20 mA cmꢀ2 and (b–d) PR electrodeposition at a cathodic current density of
20 mA cmꢀ2 and anodic current densities of (b) 50, (c) 100 and (d) 200 mA cmꢀ2
.
electrodeposition (93%) also falls in the range of those obtained
by PR electrodeposition. It has been reported previously that
the applied current density for DC and PC electrodeposition has
only a small effect on the composition of the Pt–Co alloys [29].
It has been reported that decreasing the Pt content in the Pt–Co
deposit could be achieved more easily by reducing the amount
of Pt present in the electrolyte [26,29,60,61]. However, little re-
search has focused on the application of PR electrodeposition
for Pt–Co electrodeposition.
place on the Pt–Co deposit surface during the reverse-time is an
oxygen evolution reaction (H2O ? ½O2 + 2H+ + 2eꢀ).
To test this hypothesis, PR electrodeposition was used to pro-
duce Pt–Co alloy coatings on three pretreated carbon clothes at a
fixed cathodic current density of 20 mA cmꢀ2 under the same three
anodic current densities as performed previously. Then, the Pt–Co
alloy coated carbon clothes were dried (80 °C for 2 h), weighed and
analyzed by SEM and EDX. Each Pt–Co alloy coated carbon cloth
was then placed in the working electrode compartment of the
two compartment electrochemical cell containing the same solu-
tion in each compartment as used for the electrodeposition. The
dissolution tests were done by applying anodic pulse waveforms
with anodic current densities of 200 mA cmꢀ2 for 0.025 s,
100 mA cmꢀ2 for 0.05 s and 50 mA cmꢀ2 for 0.1 s, (same wave-
forms used for the electrodeposition, but without the cathodic
pulse) on the Pt–Co coated carbon clothes produced by the PR elec-
trodeposition. After the total anodic charge density of 2 C cmꢀ2
was passed, the Pt–Co coated carbon cloths were removed, dried,
weighed and subjected to morphological and chemical analysis
to monitor any dissolution of the Pt–Co deposits or changes in
the morphology or Pt–Co alloy composition after the anodic puls-
ing, similar to when the PR electrodeposition was carried out.
The electrode responses during the anodic pulse tests (data not
shown) are similar to those for the anodic electrode responses dur-
ing the PR electrodeposition (Fig. 4b–f).
The electrode responses during PR and PC electrodeposition
were recorded (shown in Fig. 4 for PR and PC electrodeposition
at a cathodic current density of 20 mA cmꢀ2). The open circuit
potential of the Pt–Co deposited carbon cloth in the plating solu-
tion was found to be around 1.1 V. During the reverse-time, the
anodic potentials rise to 4, 8 and 9 V for anodic current densities
of 50, 100 and 200 mA cmꢀ2, respectively, which are much higher
than the open circuit potential or the electrode potential during
the off-time of the PC electrodeposition (Fig. 4a). Based on a pour-
baix diagram of Pt, Pt is expected to be stable due to the forma-
tion of the passive film at high positive potentials. Accordingly,
one would expect a significant dissolution of Co from the Pt–Co
alloys deposited by the PR electrodeposition, and so should yield
an alloy containing a higher Pt content than 93 mol%. However,
the Pt content in the Pt–Co alloys produced by PR electrodeposit-
ion remains essentially unchanged regardless of the magnitude of
the anodic current density applied and again is similar to that
produced by PC electrodeposition, as seen in the composition
analysis together with the morphological analysis and the
amount of Pt–Co alloys deposited. Thus, an applied anodic current
density as high as 200 mA cmꢀ2 for the total anodic charge
density of 2 C cmꢀ2 may not dissolve any significant amount of
Co from the Pt–Co alloy structure and the only reaction taking
The % weight change and composition analysis results (Table 2)
and corresponding SEM images (Fig. 5) indicate no (or very little)
dissolution of the Pt–Co alloy deposits during the anodic pulse
since there is less than a 2% weight change, the compositions re-
main at ꢃ93 mol% Pt and the morphologies show no visible signs
of dissolution or morphological change (Fig. 5 vs. Fig. 2a–c) of
the Pt–Co deposits before and after the anodic pulse was applied.

74
N. Chaisubanan, N. Tantavichet / Journal of Alloys and Compounds 559 (2013) 69–75
Table 2
Weight change and chemical composition of Pt–Co alloys before and after the dissolution tests.
PR electrodeposition
Dissolution pulse current density
wt. Change (%)
Pt:Co before dissolution test
Pt:Co after dissolution test
iC = 20 mA cmꢀ2; iA = 50 mA cmꢀ2
iC = 20 mA cmꢀ2; iA = 100 mA cmꢀ2
iC = 20 mA cmꢀ2; iA = 200 mA cmꢀ2
50 mA cmꢀ2
100 mA cmꢀ2
200 mA cmꢀ2
ꢀ0.24
ꢀ1.8
ꢀ1.7
93:7
93:7
92:8
93:7
93:7
92:8
Fig. 5. SEM images (ꢂ5000) of the Pt–Co alloy deposits prepared by PR electrodeposition at a cathodic current density of 20 mA cmꢀ2 and anodic current densities of (a) 50,
(b) 100 and (c) 200 mA cmꢀ2, after the dissolution tests.
20
Thus, applying an anodic current in the form of PR electrodeposit-
ion may not be of benefit in manipulating the morphology and
15
10
5
composition of the Pt–Co deposited alloys compared to the con-
ventional PC electrodeposition, since no significant dissolution of
the Co from the Pt–Co alloys occurred during the anodic pulse.
In contrast, several studies have previously reported the use of
dealloying process to change the morphology and composition of
Pt–Co alloys even when no (or a small magnitude of) anodic poten-
tial was applied [48,52,62,63]. It was reported that the level of Pt
present in the Pt–Co alloy is an important factor in the dissolution
process [63]. For very Pt-rich (>90% Pt) Pt–Co alloys, based on the
Co dissolution model presented by Hoshi and Co-workers [63], all
Co atoms in the second layer are partially covered with Pt atoms or
most of the Co atoms present in the Pt–Co alloy structure may form
strong bonds directly to Pt (i.e., Co–Pt bonding). It is, then, more
difficult to separate Co atoms from the Pt hosted structure. For
Pt–Co alloys with a lower Pt content, a number of Co atoms in
the second layer are exposed completely without the coverage of
Pt atoms present in the first layer [63] or some Co atoms may form
Co–Co bonding that is easier to dissolve out of the Pt–Co alloy
structure than the Co–Pt bonded atoms [62]. As a result, it is easier
for Co atoms to dissolve out of the Pt–Co alloy structure. Thus, it
may be possible that the use of the PR electrodeposition can, in
fact, manipulate the morphology and composition of the deposited
Pt–Co alloys by dissolving parts of the Pt–Co alloys when applying
the anodic pulse, but the Pt content in the Pt–Co alloys has to be
low enough. As shown above, although the deposition mode and
the magnitude of the cathodic current density used in this study
have a substantial effect on the deposit morphology, they do not
have any profound effect on the composition of the deposited
Pt–Co alloy. To decrease the amount of Pt deposited in the Pt–Co
alloys, the composition of the plating solution would have to be ad-
justed in a way to enhance the deposited amount of Co in the Pt–Co
alloys. Currently, we are investigating the possibility to utilize the
dissolution process in PR electrodeposition to control the structure
and composition of the Pt–Co alloys in different plating bath
compositions.
0
-5
-10
-15
-20
-0.5
0
0.5
1
1.5
V / Ag/AgCl
Fig. 6. Cyclic voltammograms (CVs) of Pt–Co deposits prepared by DC electrode-
position at 10 mA cmꢀ2, PC electrodeposition at a cathodic current density of
20 mA cmꢀ2 and PR electrodeposition at a cathodic current density of 20 mA cmꢀ2
and anodic current densities of 50, 100 and 200 mA cmꢀ2, in N2-purged 0.5 M
H2SO4.
electrodeposition have a significantly higher EAS than that pro-
duced by DC electrodeposition. In addition, the Pt–Co deposits pre-
pared under the same cathodic current density have relatively
similar EAS regardless of the magnitude of the applied anodic cur-
rent density, whilst increasing the cathodic current density leads to
a higher EAS. These results support previous findings that the
cathodic current density has a profound effect, whereas the anodic
current density shows no effect, on the morphology of the Pt–Co
deposits, and those with finer and smaller grained structures yield
a higher EAS.
Fig. 6 shows the CVs of some Pt–Co coated carbon clothes
obtained from DC and pulse (PC and PR) electrodeposition in a
N2-purged 0.5 M H2SO4 solution. The calculated EAS values
are listed in Table 1. The Pt–Co deposits produced by pulse
4. Conclusion
The electrodeposition of Pt–Co alloys onto pretreated carbon
cloths by PR electrodeposition reveals that the anodic current pulse

N. Chaisubanan, N. Tantavichet / Journal of Alloys and Compounds 559 (2013) 69–75
75
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does not significantly dissolve the deposited Co part from the Pt–
Co alloys and so cannot manipulate the physical and chemical
properties of the deposited Pt–Co alloys through dealloying of
the less noble Co metal. Overall, the Pt–Co alloys prepared by PC
or PR electrodeposition have smaller and finer grained structures
with higher EAS than that prepared by DC electrodeposition. The
Pt contents were found to be between 91 and 94 mol% for all
electrodeposition conditions studied, regardless of the deposition
mode and the magnitudes of the cathodic and anodic current
densities applied. The cathodic current density shows a signifi-
cant effect on the amount, morphology and ultimately EAS of
the deposited Pt–Co alloys. The applied anodic current, on the
other hand, shows no substantial effect on the amount and mor-
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and chemical properties of the Pt–Co alloys produced by the PR
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trodeposition. A separate dissolution experiment has shown that
the morphology, weight and composition of Pt–Co alloys remain
unchanged when only an anodic pulse is applied on the Pt–Co
alloy coated carbon cloths. The results of this study indicate that
most of the Co atoms in high Pt content Pt–Co alloys are rela-
tively inert towards anodic dissolution when alloyed to Pt so that
no or only a very small amount of Co dissolves during the PR
electrodeposition. Accordingly, manipulating the morphology
and composition of the Pt–Co alloy deposits cannot be achieved
by PR electrodeposition.
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Acknowledgments
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The authors gratefully express their gratitude to the Higher
Education Research Promotion and National Research University
Project of Thailand, Office of the Higher Education Commission
(EN276B), and to the Graduate School of Chulalongkorn University,
for financial support during the course of this study.
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