Electrochemistry Communications 11 (2009) 1195–1198
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Electrochemistry Communications
journal homepage: www.elsevier.com/locate/elecom
Supportless PdFe nanorods as highly active electrocatalyst
for proton exchange membrane fuel cell
Wenzhen Li a,*, Pradeep Haldar b
a
b
Department of Chemical Engineering, Michigan Technological University, Houghton, MI 49931, USA
College of Nanoscale Science and Engineering (CNSE), State University of New York (SUNY) at Albany, NY 12206, USA
a r t i c l e
i n f o
Article history:
Received 11 March 2009
Received in revised form 29 March 2009
Accepted 31 March 2009
Available online 9 April 2009
Keywords:
Nanorod
Oxygen reduction
Catalyst
Palladium
Fuel cells
a b s t r a c t
The PdFe nanorods (PdFe-NRs) with tunable length were synthesized by an organic phase reaction of
[Pd(acac)2] and thermal decomposition of [Fe(CO)5] in a mixture of oleyamine and octadecene at
160 °C. They show a better proton exchange membrane fuel cell (PEMFC) performance than commercial
Pt/C in working voltage region of 0.80–0.65 V, due to their high intrinsic activity to oxygen reduction
reaction (ORR), reduced cell inner resistance, and improved mass transport.
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
Highly active and low cost electrocatalysts are of great importance for large-scale commercial applications of PEMFCs [1,2].
One-dimensional (1-D) carbon nanotubes (CNTs) supported electrocatalysts have shown improved activity towards oxygen reduction reaction (ORR), durability and mass transport [3–6],
however, still suffer the carbon corrosion issue [7]. Supportless
1-D metallic and multi-metallic electrocatalysts have recently attracted enormous attention [8–10], since they can offer unique
benefits, such as (1) anisotropic morphology and (2) thin and dense
metal catalyst layer, which lead to higher mass transport of reactants, (3) high aspect ratio, which is immune to surface energy driven coalescence via crystal migration. Pt and PtPd nanotubes with
a diameter of 50 nm and length of 5–20 lm were reported to have
improved ORR specific activities [9]. However, their relatively
small electrochemical surface areas (ECSA) (<15 m2/g), due to its
large wall thickness of 4–7 nm and inaccessible inner tube surface
area, greatly limit the mass activity enhancement. In order to meet
the fuel cell cost requirements, the research on decreasing Pt content in electrodes and replacing Pt by other inexpensive metals,
such as Pd, Fe, Co, etc. were extensively studied [11–15]. Pd is considered less expensive than Pt (the price of Pd is 1/4–1/5 as that of
Pt), but less active for the ORR. In an acid electrolyte the ORR exchange current density of Pd is 10 10 A/cm2, which is one magni* Corresponding author. Tel.: +1 906 487 2298.
E-mail address: wzli@mtu.edu (W. Li).
1388-2481/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.elecom.2009.03.046
tude lower than that of Pt (10 9 A/cm2) [16]. Carbon supported
Pd–M alloy nanoparticles [12,13], such as PdFe, PdCo and PdCoAu,
have been demonstrated a comparable ORR activity as Pt. In this
paper, we present a convenient synthesis method for preparing
PdFe nanorods with a large ECSA of ca. 60 m2/g, and they show
high performance in both three-electrode-cell and PEM single cell
tests.
2. Experimental
The synthesis procedures of PdFe-NR are described as follows: a
mixture of 153 mg Pd(acac)2 (0.5 mmol) and 20 mL oleylamine
(OAm) under a blanket of nitrogen was quickly heated to 110 °C
and held for 30 min. After 120 lL Fe(CO)5 was injected into the
synthesis system, the temperature was raised to 160 °C and held
for 30 min. The solution was then cooled down to room temperature by removing the mantle heater. A mixture of 10 mL hexane
and 50 mL ethanol was added and the product was separated by
centrifugation (6000 rpm for 10 min). The product was cleaned
by redispersing in a mixture of 10 mL hexane and 15 mL ethanol
and separating by centrifugation for three times. The final PdFeNR product was dispersed in 10 mL hexane. 20 ml mixtures of
OAm and octadecene (ODE) with volumetric ratio of 1:1 and 3:1
were used for synthesis of PdFe-NR(b) and PdFe nanoparticles
(PdFe-NP), respectively. Pd nanoparticles (Pd-NP) with a diameter
of 2–3 nm were synthesized by reduction of Pd(acac)2 in the presence of OAm and oleic acid at 200 °C for 30 min. These Pd-based NR
and NP powders were obtained by filtrating their hexane solutions
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W. Li, P. Haldar / Electrochemistry Communications 11 (2009) 1195–1198
through a Nylon filter (pore size: 0.2 lm) and being dried at 60 °C
in a N2-saturated oven.
The composition, morphology and structure of these
Pd-based nanocatalysts were analyzed by transmission elec-
180
Pd (111)
P
e
140
120
Pd (220)
Pd (200)
160
Intensity / a.u.
tron microscopy (TEM, JOEL2010, 200 kV), X-ray diffraction
(XRD, SCINTAG-XDS2000, Cu Ka radiation, k = 1.5418 Å) and
Induced
couple
plasma–atomic
emission
spectroscopy
(ICP–AES).
100
80
Pd-NP
2.9 nm
PdF NR
PdFe-NR
2.7 nm
60
40
20
0
30
35
40
45
50
55
60
2 theta degree /
65
70
75
80
o
Fig. 1. TEM images of PdFe-NP (a) and NR with a length of 10 nm (b), 50 nm (c), and 50 nm in one bundle (d), and XRD patterns of Pd-NP and PdFe-NR (e).
1197
W. Li, P. Haldar / Electrochemistry Communications 11 (2009) 1195–1198
distance corresponding to the highest ORR activity is around
0.273 nm for PdFe/C catalyst [13]. In our work, the Pd–Pd bond distance of the PdFe-NR is very close to this optimum Pd–Pd bond distance (0.273 nm).
Theoretically, the chemical surface area (CSA) of a metal nanorod is inversely proportional to its diameter, but is independent
with its length. The Pd-NR with a diameter of 3 nm has a CSA of
110 m2/g. The ECSA of PdFe-NR obtained from CV test (based on
the hydrogen desorption peaks) is 60 m2/g, which gives a Pd utilization of 55%. The ORR activity for PdFe-NR and Pt/C catalysts measured using RDE technique in the three-electrode-cell is shown in
Fig. 2. At 0.9 V (RHE), the PdFe-NR exhibits a lower ORR activity
than Pt/C, however, as the voltage is low to 0.85 V, the mass activity (MA) for PdFe-NR is 284 mA/mgPd, which is slightly higher than
265 mA/mgPt for Pt/C, this trend is even larger when the voltage is
decreased to 0.8 V: 1290 mA/mgPd for PdFe-NR vs. 805 mA/mgPt
for Pt/C. The ORR curve of PdFe-NR is steeper than that of Pt/C, this
is attributed to thin layer of PdFe-NR (no carbon, dense and thinner
catalyst on GC electrode), which can facilitate oxygen transport to
PdFe-NR surface.
A 5-cm2-MEA with the PdFe-NR as cathode catalyst (0.22 mgPt/
2
cm ) was fabricated by a self-developed screen-printing technique.
The PdFe-NR based cathode catalyst layer is only around 1 lm. The
H2–air PEMFC performance comparison of this MEA and the commercial MEA with Pt/C cathode catalyst of 0.5 mgPt/cm2 is shown
in Fig. 3. At a high working voltage of 0.9 V, the mass activity of Pt/
C is 36 mA/mgPt, which is two times higher than PdFe-NR (18 mA/
mgPd). However, as the working voltage moves to the practical
operation region (0.8–0.65 V), the cell performance of PdFe-NR is
a
PdFe-NR
Pt/C
2
-2
0.1 M HClO4, 99.999% O2
o
10 mV/s, 65 C, 1600 rpm, anodic scan
2
2
25 µgPd/cm and 20 µgPt/cm
-3
-4
-5
-6
-7
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
b
300
-1
E / V [RHE]
250
0.85 V
0.5
MA SA
PdFe-NR
Pt/C
0.4
200
0.3
150
0.2
100
0.9 V
0.1
0
0.0
-2
50
Specific Activity / mA.cm
TEM images of PdFe NP and NR are shown in Fig. 1a–d. Using an
OAm and ODE ratio of 1/3, uniformly dispersed PdFe nanoparticles
with an average particle size of 2–4 nm are observed (Fig. 1a). With
the ratio of OAm to ODE increasing to 1:1, PdFe-NR(b) with a diameter of 3 nm and short length of 10 nm were synthesized (Fig. 1b).
When pure OAm was used for the synthesis, 50 nm-length PdFeNRs with a similar diameter of 3 nm were produced (Fig. 1c).
Therefore, we demonstrated an easy approach to control the length
of PdFe-NR by altering the volumetric ratio of OAm to ODE. It has
been reported that the length of 1-D PtFe nanowire can be controlled by altering the surfactants ratio [17]. In our experiment, it
is interesting to find that around 20–40 PdFe nanorods were selfassembled to ‘flower’-like bundles (Fig. 1d), and the distance between two nanorods is around 2–4 nm. This bundle structure is
likely to form a thin and dense catalyst layer in a MEA, which
can facilitate the mass transport of reactants. The composition of
the PdFe-NR was controlled by injecting different amount of
Fe(CO)5. For example, 0.12 mL gave a Pd–Fe ratio of 52:48 (determined by ICP–AES). XRD patterns of PdFe-NR and Pd-NP are shown
in Fig. 1e. Broad Pd diffraction peaks are observed for both samples.
The particle size based on calculation of Pd (2 2 0) diffraction peak
by Debye–Sherrer formula is 2.7 nm for PdFe-NR and 2.9 nm for
Pd-NP. This confirms the PdFe-NR has a small diameter. It is also
observed that all Pd diffraction peaks of PdFe-NR shift to right,
compared to Pd-NP. The Pd–Pd bond (inter-atomic) distance is
0.2729 nm for PdFe-NR, which is smaller than the bond distance
of Pd-NP (0.2747 nm). It was reported the optimum Pd–Pd bond
Mass Activity / mA.mg
3. Results and discussion
0
-1
i [mA/cm ]
A conventional three-electrode-cell setup (electrolyte cell AFCELL3, Pine) with a glassy carbon working electrode, a reversible
hydrogen reference electrode (HydroflexÒ) and a Pt wire counter
electrode, was used for cyclic voltammetry (CV) and rotating disk
electrode (RDE) tests of the PdFe-NR and Pt/C (20 wt%, BASF-FuelCell) samples at 65 °C. The working electrode was prepared by
dropping 10 lL of the ink on the glassy carbon electrode. The ink
was prepared by ultrasonically mixing 3 mg of PdFe-NR in 5 mL
ethanol. 10 lL of 0.05 wt% Nafion was added on top to fix the electrocatalyst. For the Pt/C, 20 lL of the ink with 1 mg Pt/C/ml ethanol
was dropped on the surface of glassy carbon and covered by 10 lL
of 0.05 wt% Nafion. The CV test was conducted at 50 mV/s from
0.05 V to 1.0 V (vs. RHE). The ORR polarization curve was obtained
at 10 mV/s from 0.35 V to 0.95 V (vs. RHE), using a rotating speed of
1600 rpm under oxygen (99.999%) bubbling. The working electrolyte is 0.5 M H2SO4 and 0.1 M HClO4 for CV and ORR tests,
respectively.
The MEA fabrication is based on a self-developed screen-printing technique. Briefly, the screen printing paste was prepared by
adding PdFe-NR, solubilized Nafion, cyclohexanol and ethylene
glycol to a small vial, and then was coated directly onto K+-form
Nafion membrane (50 lm, DuPont, NRE212) to make cathode catalyst coated membrane (CCM). The thickness of PdFe-NR catalyst
layer was determined by a digital micrometer, five points of the
CCM was randomly measured and the average thickness was reported. Pt/C was screen-printed on the other side of the CCM as anode catalyst. After the K+-form CCM was converted to H+-form, it
was assembled into a 5-cm2-PEMFC-hardware. A commercial
MEA (BASF-FuelCell) with 0.5 mgPt/cm2 for both anode and cathode was also tested as a controlled sample. The cell temperature
was kept at 65 °C and the relative humidity (RH) of both anode
and cathode are 100%. After the MEA was activated at 0.7 V for
8 h, its PEMFC performance was tested by applying a constant voltage and collecting the corresponding current density and in situ inner resistance (based on current interruption technique).
Fig. 2. (a) Polarization curves of the ORR on PdFe-NR and Pt/C (BASF-FuelCell), and
(b) mass activity and specific activity of PdFe-NR and Pt/C in three-electrode-celltest.
1198
W. Li, P. Haldar / Electrochemistry Communications 11 (2009) 1195–1198
shows a performance drop when the operation voltage is lower
than 0.65 V, this could be attributed to ‘water flooding’ inside the
catalyst layer. Future work focused on optimizing the catalyst layer
compositions and the gas diffusion layer structures will be expected to improve its performance in high current density region,
and this is currently under study in my group.
0.9
2
PdFe-NR, 0.22mgPd/cm
0.8
0.7
2
Pt/C, 0.50mgPt/cm
E/ V
0.6
0.5
4. Conclusions
0.4
0.3
2
Anode catalyst: Pt/C (40wt%, BASF-FuelCell), 0.5 mgPt/cm
o
Cell: 65 C, 100% RH
Anode/cathode: H2/air, 0.1/0.4 slpm, 0/0 psi
0.2
0.1
0.0
0
100
200
300
400
500
600
700
800
2
i [mA/cm ]
Fig. 3. Polarization curves of single PEMFCs (5-cm2-MEA) with PdFe-NR and Pt/C
(BASF-FuelCell) cathode catalysts.
In this study, uniform PdFe-NR with a diameter of 3 nm and
controllable length of 10–50 nm were synthesized through a single-step organic phase reduction process. The ratio of two surfactants (OAm and ODE) was found to effectively control the length
of nanorods in range of 10–50 nm. The PdFe-NR demonstrates a
better PEMFC performance than commercial Pt/C in practical
working voltage region (0.80–0.65 V), which can be attributed to
their high intrinsic activity towards ORR, reduced cell inner resistance, and improved mass transport.
Acknowledgements
higher than that of Pt/C e.g. at 0.75 V, the current density and the
mass power density of PdFe-NR is 1.65 times higher than Pt/C
(245 mA/cm2 vs. 148 mA/cm2, 184 mW/cm2 vs. 111 mW/cm2),
even on half precious metal loading (0.50 mgPt vs. 0.22 mgPd).
When the working voltage is in the range of 0.8–0.65 V, the polarization curve is a combined effect of catalyst activity, inner cell
resistance and mass transport. Therefore, the I–V curves show
the PdFe-NR catalyst has a better overall PEMFC performance in
this working voltage region.
We attributed the improved PEMFC performance to: (1) the
advantageous Pd–Pd bond distance (0.2729 nm) for PdFe-NR,
which favors oxygen double bond dissociation [13], thus improves
the ORR intrinsic activity; (2) the unique 1-D nanorod structure
and the super thin catalyst layer (1 lm vs. 25 lm for Pt/C catalyst
layer), which can reduce the overall internal resistance of PdFe-NRbased MEA (6 mX vs. 9 mX for Pt/C-based MEA) and improve the
mass transport. It was reported that a 60–80% mass activity (power
density) benefit could be gained if the ohm resistance and mass
transport were effectively reduced [2]. Here we clearly demonstrated this benefit through using super-thin 1-D PdFe-NR-based
catalyst layer. The reported supportless nanocatalyst synthesis
and MEA fabrication strategies have a great potential to be extended to other highly active bi-, tri-metallic 1-D electrocatalysts,
such as PdCo/Ni-NR for ORR and PtRuCo-NR for methanol oxidation reaction. It should be noted that the PdFe-NR-based MEA
We thank Seth Knupp, Dr. Thomas Murray and Dr. Wentao
Wang for their helps with XRD and TEM analysis and MEA
fabrication.
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