Electrodeposited Pd-Co catalyst for direct methanol fuel cell electrodes: Preparation and characterization

Article abstract of DOI:10.1016/j.electacta.2008.01.069

Pd-Co alloy has been recently proposed as a catalyst for the cathode of direct methanol fuel cells with both excellent oxygen reduction activity and methanol tolerance, hence electrodeposition of this alloy is an attractive approach for synthesizing porous metal electrodes with high methanol tolerance in direct methanol fuel cells. In this study, we electrodeposited two types of Pd-Co films onto Au substrates by applying different current density (-10 or -200 mA cm^-2); and then characterized them in terms of morphology, composition, crystal structure, and catalytic activity. Pd-Co deposited at -10 mA cm^-2 was smooth and possessed smaller particles (ca. 10 nm), while that at -200 mA cm^-2 was dendritic (or rough) and possessed larger particles (ca. 50 nm). Both the Pd-Co alloys were found to be almost the same structure, i.e. a solid solution of ca. Pd₇Co₃ with Pd-skin, and also confirmed to possess comparable activity in oxygen reduction to Pt (potential difference at 1.0 μA cm^-2 was 0.05 V). As for methanol tolerance, cell-voltage was not influenced by addition of 1 mol dm^-3 methanol to the oxidant solution. Our approach provides fundamental technique for synthesizing Pd-Co porous metal electrodes by electrodeposition.

Full text of DOI:10.1016/j.electacta.2008.01.069

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 4679–4686
Electrodeposited Pd-Co catalyst for direct methanol fuel
cell electrodes: Preparation and characterization
Satoshi Tominakaa,1, Toshiyuki Mommab,1, Tetsuya Osakaa,∗,2
a Department of Applied Chemistry, Graduate School of Science and Engineering, Waseda University,
3-4-1 Okubo, Shinjuku, Tokyo 169-8555, Japan
b Waseda Institute for Advanced Study, Nishi-Waseda 1-6-1, Shinjuku, Tokyo 169-8050, Japan
Received 4 November 2007; received in revised form 14 January 2008; accepted 21 January 2008
Available online 3 February 2008
Abstract
Pd-Co alloy has been recently proposed as a catalyst for the cathode of direct methanol fuel cells with both excellent oxygen reduction activity
and methanol tolerance, hence electrodeposition of this alloy is an attractive approach for synthesizing porous metal electrodes with high methanol
tolerance in direct methanol fuel cells. In this study, we electrodeposited two types of Pd-Co films onto Au substrates by applying different current
density (−10 or −200 mA cm−2); and then characterized them in terms of morphology, composition, crystal structure, and catalytic activity. Pd-Co
deposited at −10 mA cm−2 was smooth and possessed smaller particles (ca. 10 nm), while that at −200 mA cm−2 was dendritic (or rough) and
possessed larger particles (ca. 50 nm). Both the Pd-Co alloys were found to be almost the same structure, i.e. a solid solution of ca. Pd7Co3 with
Pd-skin, and also confirmed to possess comparable activity in oxygen reduction to Pt (potential difference at 1.0 ␮A cm−2 was 0.05 V). As for
methanol tolerance, cell-voltage was not influenced by addition of 1 mol dm−3 methanol to the oxidant solution. Our approach provides fundamental
technique for synthesizing Pd-Co porous metal electrodes by electrodeposition.
© 2008 Elsevier Ltd. All rights reserved.
Keywords: Pd-Co alloy; Electrodeposition; Oxygen reduction catalyst; Direct methanol fuel cells; Methanol tolerance
1. Introduction
simply casting a mixture of catalyst particles and binder onto
current collectors [8]. Recently, some sophisticatedly structured
current-collectors, e.g. metal-meshelectrodes[9]andmicroelec-
trodes [10–13], have been studied for achieving better fuel-cell
performance. They require catalysts to be synthesized directly
and selectively onto the current collectors, thus, electrodeposi-
tion has been applied for preparing electrodes. Electrodeposition
is also appropriate to synthesizing ordered porous materials with
templates, e.g. mesoporous metals [14–16] and macroporous
metals [15]. Therefore, electrodeposition is an attractive method
for preparing electrodes for fuel cells.
Recently, Pd-based alloys have been reported to be oxygen
reduction reaction (ORR) catalysts with excellent methanol tol-
erance [3,17–26]. The tolerance is important to DMFCs, because
methanol permeating through the electrolyte membrane from
anode is oxidized on ORR catalysts at cathode, and thus volt-
age as well as fuel-efficiency is degraded [27,28]. So far, the
Pd-based ORR catalysts have been synthesized as nanoparticles
or smooth films, not as porous metal catalysts to the best of our
knowledge [3,17–26].
In polymer electrolyte fuel cells, more effective catalysts are
strongly required for commercialization, hence the materials
and also their preparation methods have been studied exten-
sively. Metal alloys [1–3], metal porphyrins [4,5], and other
metal compounds [6,7] as well as their preparation methods
based on thermal, chemical, and/or electrochemical treatment
have been developed. Those materials are generally synthesized
as nanoparticles for obtaining higher specific surface area, hence
metal-alloy nanoparticles have been regarded as the most active
catalyst so far [1–3].
Those nanoparticles are required to be formed into porous
electrodes for effectively promoting reactions of fuel cells [8].
Conventionally, the porous structures have been prepared by
∗
Corresponding author. Tel.: +81 3 5286 3202; fax: +81 3 3205 2074.
E-mail address: osakatets@waseda.jp (T. Osaka).
ISE member.
ISE fellow.
1
2
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.01.069

4680
S. Tominaka et al. / Electrochimica Acta 53 (2008) 4679–4686
Among the alloys, we focused on Pd-Co alloy, because they
2.2. Characterization of the Pd-Co films
are possibly synthesized by electrodeposition as reported in
the field of magnetic materials [29,30] or thin-coating mate-
rials [31]. Nonetheless, as oxygen reduction catalysts, only a
preliminary evaluation on ORR activity was reported on elec-
trodeposited Pd-Co alloy [21].
In this study, we electrodeposited and characterized Pd-Co
alloys. Two different current densities were applied to obtain
different surface morphology: rough and smooth; rough surface
is directly applicable as porous electrode; smooth surface is suit-
able for template method. Compositions in the deposits should
beless-dependentoncurrentdensitiesforobtaininghighlyactive
composition (i.e. Pd7Co3 [3,17–26]) easily; hence we applied
an electrodeposition bath where the deposition potentials of Pd
and Co are almost the same according to a literature [31]. Our
approach is expected to extend application of Pd-Co catalyst to
porous metal electrodes.
The Pd-Co films (as-deposited) were characterized in terms
of morphology, composition, and crystal structure. Morphology
was observed with a field emission-scanning electron micro-
scope (FE-SEM, S-5500, Hitachi) at 5.0 kV. Bulk composition
was determined by inductively coupled plasma-atomic emission
spectroscopy (ICP-AES, IRIS-AP, Thermo Fisher Scientific)
after dissolving the samples in aqua regia. Surface was investi-
gated by X-ray photoelectron spectroscopy (XPS, JPS-9010TR,
JEOL) applying MgK radiation (10 kV, 10 mA). Elemental
depth profile was evaluated by glow discharge-optical emission
spectrometry (GD-OES, GDA750, Rigaku). Crystal structures
were analyzed by means of X-ray diffractmetry (XRD, in-
plane, RINT-TTRIII, Rigaku) applying Cu K␣ radiation (50 kV,
300 mA).
2.3. Electrochemical properties of the Pd-Co films
2. Experimental
Electrochemical properties of the deposits were evaluated
with respect to oxygen reduction activity, methanol tolerance,
and response in acid media. Oxygen reduction activity was
evaluated by linear sweep voltammetry (LSV) from the open
circuit potential to 0.2 V vs. Ag/AgCl at 1 mV s−1 in an oxygen-
saturated sulfuric acid (0.5 mol dm−3). Methanol tolerance was
evaluated by LSV in the same conditions as oxygen reduc-
tion activity measurements except for addition of methanol
(1 mol dm−3) to the solution. Electrochemical responses were
Two types of Pd-Co films were electrodeposited onto
Au substrates by applying different current densities, and
then, characterized with respect to morphology, composition,
crystal structure, and catalytic activity. In addition, perfor-
mance of the Pd-Co during fuel cell operation was also
evaluated.
2.1. Electrodeposition of the catalysts
evaluated by cyclic voltammetry (CV, −0.2 to 1.0 V, 50 mV s−1
)
in a nitrogen-saturated sulfuric acid (0.5 mol dm−3). All the
properties were evaluated in a conventional three-electrode cell
(the reference electrode: Ag/AgCl, the counter electrode: a Au
wire) using a computer-controlled potentiostat (PARSTAT 2273,
Princeton Applied Research) at 30 ◦C.
The bath composition for electrodepositing Pd-Co and detail
conditions are tabulated in Table 1. Electrodeposition was
carried out on a flat Au electrode (deposited on a silicon
substrate, 0.13 cm2) by applying constant current for control-
ling deposition rate to produce different films: low current
density, −10 mA cm−2, for smooth film; high current den-
sity, −200 mA cm−2, for rough film. The former and the
latter are named Pd-Co-10 and Pd-Co-200, respectively. Poten-
tials shown in this paper are referred to a Ag/AgCl (KCl
saturated).
For comparison, pure Pd and pure Pt were also electrode-
posited. The Pd was electrodeposited at −1 mA cm−2 (600 s) in
a bath containing only the palladium salt and the NH4Cl shown
in Table 1. The Pt was electrodeposited at −1 mA cm−2 (600 s)
in a bath containing H2PtCl6 (0.02 mol dm−3).
2.4. Activity of the Pd-Co during fuel cell operation
Using our ␮-DMFC [10–12] as an example device, we
investigate activity of the Pd-Co during fuel cell operation
[32]. The ␮-DMFC was fabricated on a silicon substrate with
micro electromechanical systems (MEMS) technology, follow-
ing our previous reports [10–12]. The ␮-DMFC consists of
two parallel microchannels with Au current collectors and
thus catalysts should be selectively deposited onto the Au
electrode (0.0091 cm2) in micrometer size. Onto the cathode
current collector Pd-Co or Pt was electrodeposited, and onto the
anode current collector Pt-Ru was electrodeposited. Pd-Co was
deposited by applying constant current (−200 mA cm−2, 5 s).
The time, 5 s, was the loading amount optimized in terms of mass
flow in the microchannel. Pt or Pt-Ru was deposited by apply-
Table 1
Bath composition and conditions for electrodepositing Pd-Co alloy
Reagent
Concentration (mol dm−3
)
[Pd(NH3)4]Cl2
CoCl2·6H2O
NH4Cl
Malonic acid
pH
Temperature
Current density
Deposition time
0.076
0.034
0.078
0.120
ing pulse current (on: −50 mA cm−2, 0.1 s; off: 0.5 s; 2 C cm−2
)
for uniform deposition onto the microchannel in bathes reported
previously [12]. For Pt-Ru preparation, 1 C cm−2 of Pt-Ru was
deposited onto 1 C cm−2 of Pt because this process possibly
reproduces an active Pt-Ru catalyst for the microchannel.
The cell performance was evaluated after the microchannels
were covered up with a Nafion 112 (DuPont) membrane for
9.1 (adjusted by NH4OH)
Room temperature
−10 or −200 mA cm−2
60 s

S. Tominaka et al. / Electrochimica Acta 53 (2008) 4679–4686
4681
enabling ion transport between the electrodes. Current–voltage
curves were recorded on a computer-controlled galvanostat with
supplying a methanol solution (1 mol dm−3) containing sulfu-
ric acid (0.5 mol dm−3) to the anode channel and an oxygen
saturated sulfuric acid solution (0.5 mol dm−3) to the cathode
channel using syringe pumps. To evaluate methanol tolerance of
the Pd-Co catalyst during cell operation, current–voltage curves
with adding 1 mol dm−3 methanol to the cathode solution was
also evaluated.
3. Results and discussion
3.1. Electrodeposition of Pd-Co
We successfully obtained a shiny silver-colored film (Pd-Co-
10) and a matt black film (Pd-Co-200) on the Au substrates.
As preliminary test, we checked the color of deposits changed
around −23 mA cm−2. To obtain sufficiently rough surface, we
Fig. 2. GD-OES of Pd-Co-10: Pd (black line), Co (grey line), and Au (dotted
line). The inset shows depth profiles of molar fraction on Pd and Co.
applied excessively high current density, −200 mA cm−2
.
Current efficiency in the depositions of Pd-Co-10 and Pd-
Co-200 are respectively calculated at 87% and 74% by means of
ICP-AES. The decreases of the efficiency are probably attributed
to hydride generation, hydrogen evolution, and/or oxygen reduc-
tion according to a literature on the Pd electrodeposition [33].
Potentials during deposition (shown in Fig. 1) of Pd-Co-10 (ca.
−0.75 V) and Pd-Co-200 (ca. −2.0 V, fluctuated) were lower
than the Nernst potential of hydrogen evolution (−0.74 V) of
the bath (pH 9.1). These results suggest the relatively low effi-
ciency of Pd-Co-200 is attributed to hydrogen evolution, indeed,
we observed bubbles during Pd-Co-200 deposition.
GD-OES for Pd-Co-10 are shown in Fig. 2. The spectra
indicate that Pd and Co were deposited in a constant ratio
except for the Co-rich region near the Au electrode (the pre-
peaks within 0.2 s is attributed to the system of GD-OES
[34]). Elemental depth profile (inset) was calculated using
the thickness of 190 nm (FE-SEM) and the bulk composition,
3.2. Characterization
3.2.1. Compositions
Bulk compositions of Pd-Co-10 and Pd-Co-200 were deter-
mined by ICP-AES as Pd66Co34 and Pd75Co25, respectively.
Thus, we successfully obtained the highly active composition
(Pd7Co3) in oxygen reduction [3,17–26].
Fig. 1. Deposition potentials of Pd-Co-10 (at 10 mA cm−2) and Pd-Co-200 (at
200 mA cm−2).
Fig. 3. XPS of Pd-Co-10: Pd-3d (a) and Co-2p (b). The vertical dotted lines
refer to Pd-3d5/2 of pure Pd and Co-2p3/2 of pure Co [40].

4682
S. Tominaka et al. / Electrochimica Acta 53 (2008) 4679–4686
assuming that the etching rate was constant, and also that
the interaction among elements was negligible [35]. Compo-
sitions are roughly estimated as about Pd7Co3 for the depth
from 10 to 100 nm. Pd composition within 10 nm increases to
the surface, hence we investigate the surface composition by
XPS.
XPS of Pd-Co-10 are shown in Fig. 3 (XPS of Pd-Co-200
were almost the same). The peak area of Pd is clearly larger
than that of Co, that is, the surface is considered to be Pd rich
even taking sensitivity factors into consideration (the sensitivity
factor of Pd-3d is 2–3 times higher than that of Co-2p [36]).
This result probably indicates the formation of Pd-skin reported
in the case of Pt alloys [37,38]. The Pd skin is probably ratio-
nalized in terms of thermodynamics with the alloy segregation
theory [39] or replacement of surface Co by Pd in the bath after
electrodeposition.
Fig. 4. XRD patterns of PdCo-10 and Pd-Co-200. The vertical dotted lines refer
to the diffraction angles of pure Pd. The weak diffraction peak marked with *
in Pd-Co-200 is attributed to the Au substrate.
The surface Pd and Co are found to be metallic, not oxide, by
comparing experimental binding energies (Pd-3d5/2: 334.7 eV,
Co-2p3/2: 777.4 eV) to those of a literature (Pd-3d5/2: 334.4 eV,
Co-2p3/2: 781.5 eV) [40]. As for the binding energy of Co, the
shift of 2p3/2 peak to lower energy suggests alloying of Co with
Pd [40].
3.2.2. Crystal structure
XRD patterns of Pd-Co-10 and Pd-Co-200 are shown in
Fig. 4. The vertical dotted lines show the diffraction angles of
the fcc Pd lattice referred to JCPDS-ICDD: 40.2◦ for (1 1 1),
46.7◦ for (2 0 0), and 68.1◦ for (2 2 0). Peaks of Pd-Co-10 (40.6◦,
47.1◦, and 69.0◦) and Pd-Co-200 (40.8◦, 47.3◦, and 69.6◦) are
Fig. 5. SEMimagesofPd-Co-10(as-deposited):top-view(a–c), cross-sectionalview(d). TheinsetsshowtheimagesofPd-Co-10afterelectrochemicalmeasurements.
The arrow in “d” refers to the Pd-Co deposit on the Au substrate.

S. Tominaka et al. / Electrochimica Acta 53 (2008) 4679–4686
4683
similar to those of the fcc Pd, except for slight shifts to higher
angles. Those shifts indicate that the Pd lattice was contracted
by replacing fcc Pd atoms with Co atoms [41], thus, our Pd-
Co were found to be substitutional solid solutions. These solid
solutions are considered as stable in acid media according to
previous reports (some ordered phases, e.g. L10 and L12, were
reported to dissolve in acid) [42–44].
In addition, compositions in the crystalline parts of Pd-Co-
10 and Pd-Co-200 are calculated as Pd87Co13 and Pd82Co18,
respectively, on the basis of Vegard’s rule using the angles of
(1 1 1). These compositions are different from the bulk compo-
sitions by ICP-AES, thus, we speculate that our Pd-Co possibly
contains amorphous parts, small crystals, and/or anisotropic
crystals [45].
in the macroscopic structure (Fig. 5a, b and d). Thus, we confirm
Pd-Co is stable to some extent even in acid media.
SEM images of Pd-Co-200 after electrochemical measure-
ments are shown in Fig. 6. Macroscopically, Pd-Co-200 is rough,
and the deposits are dendritic, hence we confirm that porous Pd-
Co films were successfully prepared by applying high current
density. Particle size is larger (ca. 50 nm) than that of Pd-Co-10
(ca. 10 nm).
3.3. Electrochemical response
CV of Pd-Co-10 is compared with that of Pd-Co-200 and
that of Pd in Fig. 7a. The CVs show redox responses in two
potential regions: hydrogen region (−0.2 to 0.1 V) and oxide
region (0.4–1.0 V). The CVs of Pd-Co is similar to that of Pd,
while some sharp peaks are observed in the hydrogen region.
The peaks of Pd-Co are similar to those of a Pd-black reported
by Chevillot et al. [46], thus we identify the peaks of Pd-Co
according to the peaks of Pd-black as follows:
3.2.3. Morphology
SEM images of Pd-Co-10 before electrochemical mea-
surements are shown in Fig. 5. To investigate influence of
electrochemical measurements, images after electrochemical
measurements are shown in the insets. Surface of Pd-Co-10 is
smooth, and the thickness is uniform (ca. 190 nm). The influence
ofelectrochemicalmeasurementsisonlyincreaseof particlesize
to ca. 10 nm (Fig. 5c), while obvious changes are not observed
• Peak 1 (0.05 V): Hydrogen adsorption/desorption (strongly
bound) for surface Pd.
Fig. 6. SEM images of Pd-Co-200 deposited after electrochemical measurements: top-view (a–c), cross-sectional view (d).

4684
S. Tominaka et al. / Electrochimica Acta 53 (2008) 4679–4686
the alloying effect. In the hydrogen region, current densities of
Pd-Co alloys are smaller than that of Pd. Peak 2, that is hydrogen
absorption into Pd-Co, is obviously smaller than that of Pd. The
results indicate that alloying with Co prevents hydrogen absorp-
tion into Pd-Co. In the oxide region, current densities of Pd-Co
are almost the same as that of Pd, while the potential of oxide
formation/deformation of Pd-Co was shifted to lower potential.
Thus, the Pd-skin of Pd-Co is found to be easily oxidized com-
pared with pure Pd. This result is different from higher potential
shift reported on Pt alloys [48,49].
3.4. Catalytic activity
3.4.1. Oxygen reduction activity
LSV of oxygen reduction on Pd-Co-10 and the corresponding
Tafel plots based on Aec are compared with that of Pt and that
of Pd in Fig. 8. By comparison of Pd-Co-10 and Pd-Co-200 in
Fig. 8a, current of Pd-Co-200 near onset potential (∼0.7 V) is
significantly larger than that of Pd-Co-10 as expected from the
larger surface area. The specific activity of Pd-Co-200 and Pd-
Fig. 7. CVs of Pd-Co-10 (black line), Pd-Co-200 (grey line), and Pd (dotted line)
in 0.5 mol dm−3 H2SO4: current density based on geometrical area (a), electro-
chemically active area (b). The insets indicate that oxide formation/deformation
of Pd-Co occurs at lower potential than Pd. Sweep rate was 50 mV s−1
.
• Peak 2 (−0.05 V): Hydrogen absorption/desorption for inside
Pd (␣ and ␤ phases).
• Peak 3 (−0.2 V): Hydrogen adsorption/desorption (less-
strongly bound) for surface Pd.
Sharp peaks of Pd-Co (Peaks 1 and 3) are found to be
attributed to the response from surface Pd, or Pd-skin.
To discuss quantitatively, CVs based on electrochemically
active area (Aec) are shown in Fig. 7b. Aec was estimated
using double layer capacitance (Cdl), assuming that values of
Cdl per Aec of different electrodes are the same. The value
of Cdl per Aec was calculated at 8.6 × 10−2 mF cm−2 using a
flat Pt electrode according to the integral double layer capaci-
tance (3.1 × 10−1 mF, 0.2–0.25 V)and Aec (15.5 cm2) calculated
according to hydrogen desorption charge [47]. According to
the Cdl values, Aec of Pd-Co-10, Pd-Co-200, and Pd were cal-
culated at 1.5 cm2 (0.13 mF), 6.1 cm2 (0.52 mF), and 0.4 cm2
(0.034 mF), respectively. Here we find that the larger response
of Pd-Co-200 compared with Pd-Co-10 in Fig. 7a is attributed
mainly to the increase of surface area because the CVs in Fig. 7b
appears to be almost the same.
Fig. 8. Oxygen reduction activity of Pd-Co compared with Pt and Pd. (a) LSVs
of oxygen reduction on Pd-Co-10 (black line), Pd-Co-200 (grey line), Pt (dashed
line), and Pd (dotted line). Sweep rate was 1 mV s−1. (b) Tafel plots based on
electrochemically active area on Pd-Co-10, Pd-Co-200 (deposited for 5 s to
obtain linear behavior [32]), Pt, and Pd.
Electrochemical response of Pd-Co electrode is compared
with that of Pd using the CVs based on Aec (Fig. 7b) to clarify

S. Tominaka et al. / Electrochimica Acta 53 (2008) 4679–4686
4685
Co-10 is almost the same as shown in Fig. 8b. By comparison of
Pd-Co and Pt, the limiting current densities (ca. −0.4 mA cm−2
,
Fig. 8a) and the potential gradients (ca. 60 mV dec.−1, Fig. 8b)
are almost the same, thus, oxygen reduction of our Pd-Co films
probably follows the similar mechanism to Pt as reported by Ota
et al. [18,50]. Potentials at low current density, 1.0 ␮A cm−2
(Aec), are 0.68 V for Pd-Co, 0.73 V for Pt, and 0.58 V for Pd
(Fig. 8b). Oxygen reduction activity of Pd-Co was confirmed to
be higher than Pd and lower than Pt. The increase of catalytic
activity of our Pd-Co compared with Pd is partly attributed to the
electronic effect; because Pd O bond of the Pd-skin should be
more stable than Pd as mentioned above. Moreover, geometric
effect possibly contributes to the increase as reported on the
Pt-alloy systems [48,49].
3.4.2. Methanol tolerance
LSV of oxygen reduction in the presence of 1 mol dm−3
methanolonPd-Co-10andthatonPd-Co-200arecomparedwith
that on Pt and that on Pd in Fig. 9a. Potentials at 1.0 ␮A cm−2
(Aec) are 0.62 V for Pd-Co-10, 0.66 V for Pd-Co-200, 0.27 V
for Pt, and 0.51 V for Pd. These values are compared with
those in the absence of methanol in Fig. 9b. From the compar-
ison, significantly high methanol tolerance of our Pd-Co films
compared with Pt is clearly confirmed. The slight decrease of
Fig. 10. Cell performance of a micro-DMFC with different cathode cat-
alyst: (a) Pt and (b) Pd-Co catalyst. Fuel solution: 1 mol dm−3 methanol
solution containing 0.5 mol dm−3 sulfuric acid. Oxidant solution: oxygen sat-
urated 0.5 mol dm−3 sulfuric acid w/o or w/1 mol dm−3 methanol. Flow rate:
80 ␮L min−1. Temperature: room temperature.
potential caused by addition of methanol is probably attributed
to methanol adsorption on the surface as reported by Ota and
co-workers [18].
As for activity during fuel cell operation, current–voltage
(current–power) curves in the presence or the absence of
methanol are shown in Fig. 10. In the case of Pt cath-
ode (Fig. 10a), the voltage significantly decreases by adding
1 mol dm−3 methanol to the cathode channel, e.g. open circuit
voltage decreases from 0.55 to 0.19 V. However, in the case of
Pt-Co cathode (Fig. 10b), the current–voltage curve does not
show any remarkable changes even in the presence of methanol,
thus we confirmed the excellent methanol tolerance of our Pd-Co
during fuel cell operation.
4. Conclusions
Electrodeposited Pd-Co was found to be a solid solution of
ca. Pd7Co3 with Pd-skin. As for catalytic activity in oxygen
reduction, the Pd-Co possessed comparable activity to Pt (poten-
tial difference at 1.0 ␮A cm−2 (Aec) was 0.05 V). Moreover, the
Pd-Co possessed significantly higher methanol tolerance than
Pt.
Fig. 9. Methanol tolerance of Pd-Co compared with Pt and Pd during oxy-
gen reduction in the presence of 1 mol dm−3 methanol. (a) LSVs for Pd-Co-10
(black line), Pd-Co-200 (grey line), Pt (dashed line), and Pd (dotted line). Sweep
rate was 1 mV s−1. (b) Comparison of the potentials of oxygen reduction at
1 ␮A cm−2 (Aec) in the condition w/o or w/methanol.
Furthermore, we obtained different morphology (smooth or
dendritic) with almost the same structures and catalytic activ-

4686
S. Tominaka et al. / Electrochimica Acta 53 (2008) 4679–4686
ity. Thus, we conclude that electrodeposition is applicable for
preparing Pd-Co alloy as ORR catalyst for fuel cells. Our
approach will extend application of Pd-Co catalyst to porous
metal electrodes.
[18] K. Lee, O. Savadogo, A. Ishihara, S. Mitsushima, N. Kamiya, K. Ota, J.
Electrochem. Soc. 153 (2006) A20.
[19] V. Raghuveer, A. Manthiram, A.J. Bard, J. Phys. Chem. B 109 (2005)
22909.
[20] J.L. Ferna´ndez, D.A. Walsh, A.J. Bard, J. Am. Chem. Soc. 127 (2005) 357.
[21] W.E. Mustain, K. Kepler, J. Prakash, Electrochem. Commun. 8 (2006) 406.
[22] W.E. Mustain, K. Kepler, J. Prakash, Electrochim. Acta 52 (2007) 2102.
[23] Y. Wang, P.B. Balbuena, J. Phys. Chem. B 109 (2005) 18902.
[24] M.H. Shao, T. Huang, P. Liu, J. Zhang, K. Sasaki, M.B. Vukmirovic, R.R.
Adzic, Langmuir 22 (2006) 10409.
[25] L. Zhang, K. Lee, J. Zhang, Electrochim. Acta 52 (2007) 3088.
[26] V. Raghuveer, P.J. Ferreira, A. Manthiram, Electrochem. Commun. 8
(2006) 807.
Acknowledgements
We thank H. Obata, S. Ohta, T. Aida, and S. Enomoto for
technical supports for the measurements. This work was partly
supported by Encouraging Development Strategic Research
Centers Program “Establishment of Consolidated Research
Institute for Advanced Science and Medical Care” from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy (MEXT), Japan, and partly carried out at the Global COE
program of MEXT “Center for Practical Chemical Wisdom”.
[27] V.A. Paganin, E. Sitta, T. Iwasita, W. Vielstich, J. Appl. Eletrochem. 35
(2005) 1239.
[28] B. Gurau, E.S. Smotkin, J. Power Sources 112 (2002) 339.
[29] R.D. Noce, N. Barelli, P.T.A. Sumodjo, D.R. Cornejo, A.V. Benedetti, J.
Magn. Magn. Mater. 306 (2006) 199.
[30] R. Gontarz, L. Smardz, B. Szymanski, P. Juzikis, J. Magn. Magn. Mater.
120 (1993) 278.
[31] J. A. Abys, I. Boguslavsky, H. K. Straschil, US Patent 5976344, Nov. 2,
1999.
[32] S. Tominaka, S. Ohta, T. Momma, T. Osaka, ECS Trans. 11 (2007) 1369.
[33] R.L. Penven, W. Levason, D. Pletcher, J. Appl. Electrochem. 20 (1990)
399.
[34] M.R. Winchester, R. Payling, Spectrochim. Acta B 59 (2004) 607.
[35] J. Pisonero, B. Ferna´ndez, R. Pereiro, N. Bordel, A.S. Medel, Trac-Trends
Anal. Chem. 25 (2006) 11.
References
[1] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Appl. Catal. B:
Environ. 56 (2005) 9.
[2] H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang, D.P. Wilkinson, J. Power
Sources 155 (2006) 95.
[3] J.L. Ferna´ndez, V. Raghuveer, A. Manthiram, A.J. Bard, J. Am. Chem. Soc.
127 (2005) 13100.
[4] R. Bashyam, P. Zelenay, Nature 443 (2006) 63.
[5] P. Convert, C. Coutanceau, P. Crouigneau, F. Gloaguen, C. Lamy, J. Appl.
Electrochem. 31 (2001) 945.
[36] M.P. Seah, I.S. Gilmore, S.J. Spencer, J. Electron Spectrosc. Relat. Phenom.
120 (2001) 93.
[37] T. Toda, H. Igarashi, M. Watanabe, J. Electrochem. Soc. 145 (1998) 4185.
[38] T. Toda, H. Igarashi, M. Watanabe, J. Electroanal. Chem. 460 (1999) 258.
[39] F.L. Williams, D. Nason, Surf. Sci. 45 (1974) 377.
[40] L. Guczi, Z. Shay, G. Stefler, F. Mizukami, J. Mol. Catal. A: Chem. 141
(1999) 177.
[41] V. Jalan, E.J. Taylor, J. Electrochem. Soc. 130 (1983) 2299.
[42] F.R. de Boer, R. Boom, W.C.M. Mattens, A.R. Miedema, A.K. Niessen,
Cohesion in Metals: Transition Metal Alloys, vol. 1, North-Holland, New
York, 1988.
[6] T.J. Shcmidt, U.A. Paulus, H.A. Gasteiger, N. Alonso-Vante, R.J. Behm,
J. Electrochem. Soc. 147 (2000) 2620.
[7] H. Tributsch, M. Bron, M. Hilgendorff, H. Schulenburg, I. Dorbandt, V.
Eyert, P. Bogdanoff, S. Fiechter, J. Appl. Electrochem. 31 (2001) 739.
[8] S.S. Kocha, in: W. Vielstich, A. Lamm, H.A. Gasteiger (Eds.), Handbook
of Fuel Cells, vol. 3, Wiley, England, 2003 (Chapter 43).
[9] E.H. Yu, K. Scott, R.W. Reeve, L. Yang, R.G. Allen, Electrochim. Acta 49
(2004) 2443.
[10] S. Motokawa, M. Mohamedi, T. Momma, S. Shoji, T. Osaka, Electrochem.
Commun. 6 (2004) 562.
[11] S. Motokawa, M. Mohamedi, T. Momma, S. Shoji, T. Osaka, Electrochem-
istry 73 (2005) 346.
[12] S. Motokawa, M. Mohamedi, T. Momma, S. Shoji, T. Osaka, Electrochem-
istry 73 (2005) 352.
[13] S.C. Kelley, G.A. Deluga, W.H. Smyrl, Electrochem. Solid-State Lett. 3
(2000) 407.
[14] J. Jiang, A. Kucernak, Chem. Mater. 16 (2004) 1362.
[15] Y. Yamauchi, K. Kuroda, Electrochem. Commun. 8 (2006) 1677.
[16] Y. Yamauchi, T. Ohsuna, K. Kuroda, Chem. Mater. 19 (2007) 1335.
[17] O. Savadogo, K. Lee, K. Oishi, S. Mitsushima, N. Kamiya, K. Ota, Elec-
trochem. Commun. 6 (2004) 105.
[43] Y. Matsuo, J. Phys. Soc. Jpn. 32 (1972) 972.
[44] T. Malla´t, J. Petro´, J. Electroanal. Chem. 208 (1986) 169.
[45] L.I. Kveglis, S.M. Jarkov, I.V. Saroverova, Phys. Solid State 43 (2001)
1543.
[46] J.P. Chevillot, J. Farcy, C. Hinnen, A. Rousseau, J. Electroanal. Chem. 64
(1975) 39.
[47] T. Biegler, D.A.J. Rand, R. Woods, J. Electroanal. Chem. 29 (1971) 269.
[48] J.F. Drillet, A. Ee, J. Friedemann, R. Ko¨tz, B. Schnyder, V.M. Schmidt,
Electrochim. Acta 47 (2002) 1983.
[49] R.K. Raman, G. Murgia, A.K. Shukla, J. Appl. Electrochem. 34 (2004)
1029.
[50] E. Yeager, Electrochim. Acta 29 (1984) 1527.