Microwave-assisted synthesis of carbon supported Pt nanoparticles for fuel cell applications

Article abstract of DOI:10.1039/b208600j

Spherical and uniform Pt nanoparticles 3.5-4.0 nm supported on carbon were prepared by microwave irradiation and exhibited very high electrocatalytic activity in the room-temperature oxidation of liquid methanol.

Full text of DOI:10.1039/b208600j

Microwave-assisted synthesis of carbon supported Pt nanoparticles for
fuel cell applications
Wei Xiang Chen,a Jim Yang Lee*abc and Zhaolin Liuc
a Singapore-MIT Alliance, National University of Singapore, 4 Engineering Drive 3, Singapore 117576
b Department of Chemical and Environmental Engineering, National University of Singapore, 10 Kent Ridge
Crescent, Singapore 119260. E-mail: cheleejy@nus.edu.sg
c
Institute of Materials Research and Engineering, 3 Research Link, 117602
Received (in Cambridge, UK) 15th August 2002, Accepted 27th September 2002
First published as an Advance Article on the web 8th October 2002
Spherical and uniform Pt nanoparticles 3.5-4.0 nm sup-
ported on carbon were prepared by microwave irradiation
and exhibited very high electrocatalytic activity in the room-
temperature oxidation of liquid methanol.
JEM 2010). The Pt contents were determined by EDX (JEOL
JSM-5600LV).
Electroactivities were measured by cyclic voltammetric and
chronoamperometry using an EG&G model 273 potentiostat/
galvanostat and a three-electrode test cell at room temperature.
The working electrode was a thin layer of Nafion-impregnated
Pt/C composite cast on a vitreous carbon disk electrode.2 A Pt
gauze and a saturated calomel electrode (SCE) were used as the
counter and reference electrode, respectively. The electrolyte
was a solution of 2 M CH3OH (Tedia, HPLC) in 1 M H2SO4 (J.
T. Baker, A.C.S. Reagent).
Pt and Pt alloys are catalytically active in several room-
temperature electro-oxidation reactions of interest to fuel cell
applications. It is well known that the metal catalytic activity is
strongly dependent on the particle shape, size and the particle
size distribution.1 Conventional preparation techniques based
on wet impregnation and the chemical reduction of metal
precursors do not provide satisfactory control of particle shape
and size.1 Consequently there has been continuing effort to
develop alternative synthesis methods based on microemul-
sions,2 sonochemistry,3,4 and microwave irradiation,5–8 all of
which are in principle more conducive to produce colloids and
clusters on the nanoscale, and with greater uniformity.
Microwave heating through dielectric losses is fast and
simple, uniform, energy efficient, and has been used in
preparative chemistry and materials synthesis.9 Recently, there
has been more reported successes in using microwave irradia-
tion to prepare high purity nanoparticles with narrow particle
size distributions. For example, polymer stabilized Pt, Ru, Ag
and Pd colloids were prepared from the microwave heating of
ethylene glycol solutions of dissolved metal salts.5–8 Metal
oxide nanoparticles (e.g. CeO2, ZrO2) could likewise be
obtained using microwave irriadation.10,11
In this communication, a simple microwave procedure for
preparing Pt metal nanoparticles supported on carbon is
reported. Pt/carbon nanocomposites containing 10, 15 and 20
wt% of Pt were successfully prepared by microwave irradiation.
TEM imaging showed a uniform dispersion of spherical Pt
nanoparticles 3.5–4.0 nm in diameter and with a narrow particle
size distribution on the carbon surface. Laboratory tests showed
that these Pt/carbon catalysts were more electrochemically
active in the room-temperature oxidation of liquid methanol
than commercially available catalysts. To the best of our
knowledge, we are not aware of any other report on the rapid
and direct synthesis of electrochemically active Pt nanoparticles
supported on carbon using microwave irradiation.
In a 100 mL beaker, 1.0 mL of an aqueous solution of 0.05 M
H2PtCl6·6H2O (Alrdich, A.C.S. Reagent) was mixed with 50
mL of ethylene glycol (Mallinckrodt, AR), and 0.5 mL of 0.8 M
KOH was added dropwise. 0.040 g of carbon XC-72 with a
specific surface area (BET) of 250 m2 g21 and an average
particle size of 40 nm were uniformly dispersed in the mixed
solution by ultrasound. The beaker was placed in the center of
a microwave oven (National NN-S327WF, 2450 MHz, 700 W)
and heated for 60 s. The resulting suspension was filtered and
the residue was washed with acetone. The solid product was
dried at 393 K overnight in a vacuum oven. Catalysts with
varying amounts of Pt from 10–20 wt% were prepared by
varying the H2PtCl6 content in the ethylene glycol solution. A
mole ratio of KOH/Pt = 8 was used throughout to induce small
and uniform Pt nanopartcle formation.5 The size and morphol-
ogy of samples were characterized by TEM imaging (JEOL
EDX measurements indicated Pt loadings of 9.5, 13.6 and
18.6 wt% for three samples prepared for the nominal loadings of
10, 15 and 20 wt%, respectively. A commercially available Pt/C
catalyst from E-TEK with a nominal Pt loading of 20 wt% (18.8
wt% by EDX) was used as a benchmark. Repeated TEM
examinations showed that the Pt nanoparticles are uniformly
distributed on the carbon surface. All samples have about the
same Pt particle size and the difference between them is the
particle density on the carbon surface. Fig. 1 shows the TEM
image of the 18.8 wt% sample in comparison with the
commercial E-TEK Pt/C catalyst. The microwave assisted
heating of H2PtCl6/KOH/H2O in ethylene glycol had evidently
facilitated a more uniform dispersion of Pt nanoparticles on the
carbon surface. When poly(N-vinyl-2-perrolidone) (PVP) was
used to substitute for carbon XC-72 in the preparation of
nanoparticles (an established methodology to stabilize nano-
particles against agglomeration5–7), the resulting polymer-
stabilized nanoparticles were hardly dispersible on the carbon.
It is suspected that the strong affinity between PVP and Pt
nanoparticles had resulted in poor adhesion property of either of
them with a foreign surface. More importantly, Fig. 1 shows
that the microwave-synthesized Pt nanoparticles are spherical
with significantly smaller mean diameters and a narrower
particle size distribution than those in the E-TEK catalyst. The
diameters for the majority of the particles are between 3.5 and
4.0 nm, with only a few particles larger than 5.0 nm. The E-TEK
catalyst, by comparison, has Pt particles about 5.1 nm in
diameter, and a broad particle size distribution ranging from
about 2–10 nm. It is generally agreed that the size of metal
nanoparticles is determined by the rate of reduction of the metal
Fig. 1 TEM images of (a) microwave-synthesized Pt nanoparticles
supported on Vulcan carbon XC-72 and (b) commercially available E-TEK
Pt/C catalyst (nominal Pt loading 20 wt%).
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precursor. The dielectric constant (41.4 at 298 K) and the
dielectric loss of ethylene glycol are high, and hence rapid
heating occurs easily under microwave irradiation. In ethylene
glycol mediated reactions (the ‘polyol’ process), ethylene
glycol also acts as a reducing agent to reduce the metal ion to
metal powders.12
The fast heating by microwave accelerates the reduction of
the metal precursor and the nucleation of the metal clusters. The
easing of the nucleation-limited process greatly assists in small
particle formation. Additionally the homogeneous microwave
heating of liquid samples reduces the temperature and concen-
tration gradients in the reaction medium, thus providing a more
uniform environment for the nucleation and growth of metal
particles. The carbon surface may contain sites suitable for
heterogeneous nucleation and the presence of a carbon surface
interrupts particle growth. The smaller and nearly single
dispersed Pt nanoparticles on carbon XC-72 prepared by
microwave irradiation can be rationalized in terms of these
general principles.
Fig. 2 shows the cyclic voltammograms of methanol
oxidization under acidic conditions (2 M CH3OH/1 M H2SO4)
catalyzed by the microwave-synthesized Pt/C catalyst and the
E-TEK Pt/C catalyst, respectively. The voltammetric features
are in good agreement with most published work.13,14 The
current peak at about 0.70 V (vs. SCE) in the forward scan is
attributed to methanol electrooxidation on the Pt/C catalyst. Fig.
2 clearly shows that this peak is significantly higher in the
microwave-synthesized Pt/C catalyst than in the E-TEK Pt/C
catalyst. Fig. 2 also shows that the electro-oxidation of
methanol began at 0.3–0.4 V vs. SCE where the current density
was 0.18 A (mg Pt)21 for the microwave-synthesized Pt/C, and
0.12 A (mg Pt)21 for the E-TEK Pt/C catalyst. The difference is
significant enough that the current density at 0.4 V can be used
as an indicator of the catalytic activity of the catalysts under
moderate polarization conditions. The Pt/C catalysts were
therefore biased at 0.4 V vs. SCE and the changes in their
polarization currents with time were recorded (Fig. 3). Besides
a higher activity, the microwave-synthesized Pt/C catalyst also
displays the same, if not better, fade rate (indicating catalyst
deactivation) in comparison with the E-TEK catalyst. These
experimental measurements prove unmistakably a greatly
improved Pt/C electrocatalyst for the room temperature electro-
oxidation of liquid methanol. The significant improvement in
the catalyst performance derives directly from the better
Fig. 3 Polarization current vs. time plots for the electrooxidation of
methanol in 2 M CH3OH/1 M H2SO4 electrolyte at 0.4 V (vs. SCE) at room
temperature.
utilization of Pt when the latter exists as uniform small particles
stabilized on a carbon substrate.
In conclusion, a microwave assisted rapid heating method has
been successfully developed for the preparation of Pt/C
catalysts with high electrocatalytic activities in direct methanol
fuel cell applications. The Pt nanoparticles, which were
uniformly dispersed on carbon, were 3.5–4.0 nm in diameter
and had a very narrow particle size distribution. The preparation
method is simple, fast and energy efficient. It can be used as a
general method to prepare other supported metal particles from
metal precursors which are susceptive to the polyol proc-
ess.15,16
This work was supported by the MEBCS programme of the
Singapore-MIT Alliance.
Notes and references
1 I. S. Armadi, Z. L. Wang, T. C. Green, A. Henglein and M. A. El-Sayed,
Science, 1996, 272, 1924.
2 Z. L. Liu, J. Y. Lee, M. Han, W. X. Chen and L. M. Gan, J. Mater.
Chem., 2002, 12, 2453.
3 K. Okitsu, A. Yue, S. Tanabe and H. Matsumoto, Chem. Mater., 2000,
12, 3006.
4 T. Fujimoto, S. Teraushi, H. Umehara, I. Kojima and W. Henderson,
Chem. Mater., 2001, 13, 1057.
5 W. Y. Yu, W. X. Tu and H. F. Liu, Langmuir, 1999, 15, 6.
6 W. X. Yu and H. Y. Liu, Chem. Mater., 2000, 12, 564.
7 W. X. Tu and H. F. Liu, J. Mater. Chem., 2000, 10, 2207.
8 S. Komarneni, D. S. Li, B. Newalkar, H. Katsuki and A. S. Bhalla,
Langmuir, 2002, 18, 5959.
9 S. A. Galema, Chem. Soc. Rev., 1997, 26, 233.
10 X. H. Liao, J. M. Zhu, J. J. Zhu, J. Z. Xu and H. Y. Chen, Chem.
Commun., 2001, 937.
11 J. H. Liang, Z. X. Deng, X. Jiang, F. Li and Y. Li, Inorg. Chem., 2002,
41, 3602.
12 F. Fievert, J. P. Lagier and M. Figlarz, MRS Bull., 1989, 24, 29.
13 M. L. Andersen, R. M. Storud and D. R. Rolison, NanoLett, 2002, 2,
235.
14 J.-F. Drillet, A. E. J. Friedemann, R. Kotz, B. Schnyder and V. M.
Schmidt, Electrochim. Acta, 2002, 47, 1983.
15 L. K. Kurihara, G. M. Chow and P. E. Schoen, Nanostruct. Mater.,
1995, 5, 607.
Fig. 2 Cyclic voltammograms of Pt/C electrode in 2 M CH3OH/1 M H2SO4
electrolyte at 20 mV s21 at room temperature.
16 A. Miyazaki, I. Balint, K. I. Aika and Y. Nakano, J. Catal., 2001, 203,
364.
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