Composite electrodes made of Pt nanoparticles deposited on carbon nanotubes grown on fuel cell backings

Article abstract of DOI:10.1016/j.cplett.2003.08.021

Multiwalled carbon nanotubes (MWCNTs), with typical lengths of 20 μm and diameters of 40 nm, have been grown directly on carbon paper backing. A sulfonic acid-silicate intermediate was used to deposit Pt particles on the MWCNTs in order to obtain an electrode that could be used in electrocatalysis. The electrical path between the Pt nanoparticles (1.2±0.3 nm in size) and the carbon paper, through the MWCNTs, was demonstrated by cyclic voltammetry. The Pt surface density of a typical MWCNT composite electrode is estimated to be about 25% of that of an ELAT electrode from E-TEK, containing 0.4 mg Pt/cm².

Full text of DOI:10.1016/j.cplett.2003.08.021

Chemical Physics Letters 379 (2003) 99–104
www.elsevier.com/locate/cplett
Composite electrodes made of Pt nanoparticles deposited
on carbon nanotubes grown on fuel cell backings
a
X. Sun , R. Li , D. Villers , J.P. Dodelet , S. Desilets
a
a
a,
b
*
ꢀ
a
ꢀ
INRS-Energie et Mateꢀriaux, 1650 boul. Lionel Boulet, Varennes, Qc, Canada, J3X 1S2
b
R&D pour la Deꢀfense Canada-Valcartier, 2459 boul. Pie-XI Nord, Val-Belair, Qc, Canada, G3J 1X5
Received 22 July 2003; in final form 22 July 2003
Published online: 4 September 2003
Abstract
Multiwalled carbon nanotubes (MWCNTs), with typical lengths of 20 lm and diameters of 40 nm, have been grown
directly on carbon paper backing. A sulfonic acid–silicate intermediate was used to deposit Pt particles on the
MWCNTs in order to obtain an electrode that could be used in electrocatalysis. The electrical path between the Pt
nanoparticles (1.2 ꢀ 0.3 nm in size) and the carbon paper, through the MWCNTs, was demonstrated by cyclic vol-
tammetry. The Pt surface density of a typical MWCNT composite electrode is estimated to be about 25% of that of an
ELAT electrode from E-TEK, containing 0.4 mg Pt/cm2.
Ó 2003 Elsevier B.V. All rights reserved.
1. Introduction
established. Our interest in MWCNTs is to use
them as electrocatalyst supports in polymer elec-
trolyte membrane (PEM) and in direct methanol
fuel cells. Indeed, the use of carbon nanostructures
as metal supports in electrochemistry and fuel cells
has been reported by several authors [6–11].
However, in these reports, the catalysts were de-
posited onto loose nanotubes, or nanohorns, just
as on carbon black. Our strategy for the electrode
fabrication in this work is different.
We recently developed a unique CVD proce-
dure permitting us to grow MWCNTs on the
carbon fibers of a fuel cell backing. In the present
work, we will show that it is possible to deposit Pt
particles on these MWCNTs and, therefore, to
obtain a composite electrode for which an electri-
cal path exists between the Pt catalyst and the
Carbon nanotubes are of significant interest,
due to their unique structure, properties and po-
tential applications [1–3]. If the attention today is
mainly focused on the synthesis, purification,
functionalization and possible applications of sin-
gle walled carbon nanotubes (SWCNTs), multi-
walled carbon nanotubes (MWCNTs) are
certainly closer to commercial applications. For
instance, the use of MWCNTs in composite ma-
terials [4], or as electrical field emitters [5], is well
* Corresponding author. Fax: +450-929-8198.
E-mail address: dodelet@inrs-emt.uquebec.ca (J.P. Dode-
let).
0009-2614/$ - see front matter Ó 2003 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2003.08.021

100
X. Sun et al. / Chemical Physics Letters 379 (2003) 99–104
carbon backing. The deposition of Pt on
MWCNTs has been chosen because Pt and its
alloys are currently the most extensively used
catalysts in PEM and direct methanol fuel cells
[12–14].
ploit the same idea to deposit Pt particles onto
MWCNTs grown on carbon paper. The present
work demonstrates that it is possible to obtain
very small Pt particles deposited onto MWCNTs
and that these metal particles are in electrical
contact, through the MWCNT, with the carbon
backing, enabling the composite structure to be
used as an electrode.
Carbon nanotubes are hydrophobic materials,
rendering difficult the adhesion of metal deposits.
It has been reported that carbon nanotubes cannot
be wet by liquids with surface tension higher than
100–200 mN/m [15], indicating that most metals
would not adhere to them. In order to improve
metal deposition onto nanotubes, two main ap-
proaches were previously developed: (i) surface
modification and (ii) sensitization activation. The
former is associated with the oxidation of the
nanotube surface, in order to create functional
groups and increase metal nucleation [16,17]. The
latter involves the generation of small nuclei (e.g.,
of Pd–Sn) to further promote metal deposits on
carbon nanotubes [18,19].
2. Experimental
MWCNTs were synthesized by heating Co–Ni
particles deposited on the fibers of a carbon paper
in a specifically designed CVD reactor [22]. This
reactor takes advantage of the fact that the carbon
fuel cell backing (E-TEK, Division of DeNora
with 81% porosity) is the most resistive part of the
electrical circuit. It can therefore be heated by
Joule effect, up to 800 °C, a temperature high en-
ough to decompose ethylene and grow nanotubes.
Prior to Co–Ni deposition, the carbon backings
were pretreated with methanol for 30 min, in order
to improve the homogeneity of the Co–Ni particle
sizes and their distributions on the carbon fibers of
the backing. Details of Co–Ni particle deposition
have been described previously [20,21] but the
important step in their fabrication is the use
of a commercially available silane derivative
(2(4-chlorosulfonylphenyl) ethyl trichlorosilane,
from United Chemical Technologies) which, upon
hydrolysis, forms a sulfonic acid–silicate, permit-
ting the exchange of Hþ for Co2þ and Ni2þ ions
(see Fig. 1). The latter ions are the precursors of
the Co–Ni particles. Pt deposition was carried out
using the same silane derivative but, this time,
using Pt2þ ions as Pt particle precursors. The de-
position of Pt particles was carried out by im-
mersing the MWCNT/carbon paper composite
structure in a solution containing 0.04 M PtCl2, 1
vol% of silane derivative and 6 vol% of water in
ethanol. The composite structure was then dried at
room temperature and reduced in a flow of
H2 + Ar, at 580 °C for 15 min. The methanol
treatment, which was used with success to improve
the distribution of Co–Ni particles on the carbon
paper, was also applied to the nanotubes before
adsorbing the Pt sulfonated-silicate (see Fig. 1).
In our previous studies on the growth of
MWCNTs [20,21], we successfully used a sulfonic
acid–silicate intermediate to disperse Co–Ni par-
ticles, used in the catalytic decomposition of eth-
ylene, to promote the growth of nanotubes on the
carbon backing (see Fig. 1). In this work, we ex-
Fig. 1. Schematic reaction of the silane derivative, which hy-
drolyzes and polymerizes to become a sulfonic acid–silicate
layer able to exchange protons with Ni and Co ions, as a first
step in the production of catalytic sites on carbon paper. The
surface polarity of this support may be modified by methanol
pretreatment. The same procedure may be used with Pt ions to
obtain a Pt sulfonate–silicate, which is the precursor of the Pt
particles.

X. Sun et al. / Chemical Physics Letters 379 (2003) 99–104
101
A Hitachi-9000 NAR high-resolution trans-
mission electron microscope (HRTEM), with en-
ergy dispersive X-ray spectroscopy (EDX),
operated at 300 kV, and a Hitachi S-4700 field-
emission scanning electron microscope (FE-SEM),
operated at 15 kV, were employed to characterize
the structure, composition and morphology of the
carbon nanotubes and Pt particles.
The electrochemical properties of the electrode
materials were investigated by cyclic voltammetry
in a standard three-electrode cell. A Pt foil served as
counter electrode, and a saturated calomel elec-
trode (SCE) was used as reference electrode. All
working electrodes had the same 1 cm2 of geomet-
rical surface. Cyclic voltammetry measurements
were carried out using a 273A EGG Potentiostat
with a 10 mV/s sweep rate. Fe(CN)36ꢁ=4ꢁ were used
as the oxydo–reduction couple (as a 10 mM solu-
tion of K3 Fe(CN)6 in 0.5 M K2SO4) for the bare
carbon paper and for the MWCNT/carbon paper
electrodes. A H2SO4 solution, at, pH 1, purged with
Ar, was used to perform the voltammetry of Pt
particles deposited on MWCNT/carbon paper. For
all those experiments, the voltammograms were
recorded after cycling the working electrode at least
20 times, at 50 mV/s, in the same potential range as
that used to record the voltammetry. The amount
of Pt in contact with the electrolyte was mea-
sured by integration of the charge related to the
H-adsoption in the cyclic voltammogram re-
corded at 10 mV/s. In order to be sure that the
electrolyte wets the electrodes, they are soaked
under stirring during 15 min in methanol, then in
H2SO4 at, pH 1, before being placed in the cell
under Ar atmosphere.
Fig. 2. (a) Scanning electron micrograph and (b) transmission
electron micrograph of the MWCNTs on a carbon fiber. Inset:
high resolution micrograph of the wall of a typical nanotube.
nanotubes [20,21]. These tubes are about 20 lm in
length and are characterized by a diameter of
30–50 nm. They are tip-grown, as indicated by the
presence of the Co–Ni alloy particles at the tip of
the tubes (Fig. 2b). These particles are completely
enclosed in the nanotube and are therefore pro-
tected from dissolution when the MWCNT/carbon
paper electrode is immersed in an acid solution. A
closer observation of the walls of a typical tube is
shown in the inset of Fig. 2b. The high resolution
TEM micrograph reveals that the tube is com-
posed of 22 well organized concentric carbon lay-
ers, separated by 0.34 nm.
3. Results and discussion
A typical SEM micrograph of MWCNTs,
grown on the fibers of a carbon paper, is shown in
Fig. 2a. Fig. 2b displays a TEM close-up of the
same MWCNTs. A high density of tubes, firmly
anchored on the carbon paper, resulted from the
methanol pretreatment of the carbon paper before
adsorbtion of the Co–Ni sulfonate–silicate. The
latter is used as precursor of the Co–Ni particles,
which are the catalysts used for the growth of the
It is possible to deposit Pt particles on these
nanotubes by simply adsorbing the Pt salt on
methanol pretreated MWCNT/carbon paper. This
procedure yields large (ꢂ4–10 nm) Pt particles
after heat-treatment in a reducing atmosphere, as

102
X. Sun et al. / Chemical Physics Letters 379 (2003) 99–104
shown in Fig. 3a. However, much smaller Pt par-
ticles (Fig. 3b) are obtained when Pt ions are dis-
persed on MWCNT/carbon paper using the silane
derivative procedure (Fig. 1) to adsorb Pt ions
onto the nanotubes. A magnified view of the ho-
mogeneous dispersion of the small Pt particles is
presented in Fig. 3c, which also displays the gra-
phitic structure of the nanotube walls, comprising
15 concentric graphene sheets. This tube has an
outer diameter of nearly 35 nm with a hollow
center of about 25 nm. The size of about 100 Pt
particles was measured on a high resolution TEM
micrography and an average Pt particle size of
1.2 ꢀ 0.3 nm was determined. The EDX composi-
tional analysis of several Pt nanoparticles confirms
the presence of Pt, along with a small amount of
Co and Ni, which have their origin in the metals
used as catalysts for the nanotube growth. An
important point to stress is that neither S (a poison
for Pt) nor Si signals were detected by EDX, with
the Pt signal. It should also be noted that all
Pt particles are located on the external surface of
the tubes because the tubes were not opened. The
density of the Pt particles, based on averaging the
measurements performed from HRTEM microg-
raphies on five tubes, is nearly one particle/
100 nm2. Accordingly, if one typical nanotube is
20 lm long and 40 nm in diameter, it will be
decorated with 2.5 ꢃ 104 Pt particles on its outer
wall.
It is of practical importance to demonstrate that
this composite structure is really an electrode, i.e.,
that:
(i) the MWCNTs are in electrical contact with the
fuel cell backing,
(ii) the Pt particles are in electrical contact with
MWCNTs.
The electrical contact between MWCNTs and
the carbon fibers of the fuel cell backing is dem-
onstrated in Fig. 4. This figure shows the voltam-
metry of the Fe(CN)36ꢁ=4ꢁ couple on a bare carbon
paper and on MWCNTs grown on the carbon
paper. A typical voltammogram with graphite
features [23] and a higher available electroactive
surface is obtained on MWCNTs, strongly sug-
gesting that these nanotubes are electrically con-
2
MWCNTs on Carbon Paper
1
0
Bare Carbon Paper
-1
-2
-800 -600 -400 -200
0
200 400 600 800 1000
Potential (mV vs SCE)
Fig. 3. Transmission electron micrographs of Pt nanoparticles
deposited on MWCNTs. (a) In the absence of the silane pre-
cursor; (b) in the presence of the silane precursor; (c) higher
magnification TEM of (b).
Fig. 4. Cyclic voltammograms in a K3Fe(CN)6 aqueous solu-
tion of: bare carbon paper substrate; MWCNTs deposited on
the carbon paper. Geometrical area of both electrodes: 1 cm2.

X. Sun et al. / Chemical Physics Letters 379 (2003) 99–104
103
nected to the carbon backing. The electrical con-
tact of Pt particles with MWCNTs is demon-
strated in Fig. 5. Figs. 5a and b display the
voltammograms obtained in an Ar purged H2SO4
solution at pH 1, for the carbon paper alone and
for the MWCNTs on carbon paper, respectively.
The full curve in Fig. 5c is the voltammogram
occurring on Pt particles on MWCNTs in the same
solution, while the dotted curve in Fig. 5c is the
voltammogram occurring on Pt particles on car-
bon black in an ELAT electrode from E-TEK (0.4
mg Pt/cm2; 20 wt% Pt/C). The characteristic fea-
tures of Pt surface electrochemistry [24], superim-
posed on a capacitive current, are seen in both
voltammograms of Fig. 5c, suggesting that Pt
particles are in electrical contact with MWCNTs in
the composite electrode as they are with carbon
black in the ELAT electrode.
the H-adsorption in both cyclic voltammograms
of Fig. 5c. A typical charge of 15.2 mC/cm2 is
detected for Pt on MWCNTs. For comparison,
the charge measured for the E-TEK electrode is
66.4 mC/cm2. Therefore, the charge ratio: QE-TEK
/
QMWCNT ¼ 4.4. Based on the findings that a
smooth polycrystalline Pt surface shows a H-
adsoprtion charge of 0.210 mC/cm2 for 1.3 ꢃ 1015
Pt atoms/cm2 [25,26], the Pt surface density is
therefore 4.1 ꢃ 1017 Pt/cm2 for ELAT, while it is
0.9 ꢃ 1017 Pt/cm2 for the MWCNT composite
electrode.
A short-time stability test for Pt particles on
MWCNTs was performed by immersing the elec-
trode in H2SO4 (pH 1) for 18 h. TEM observation
of this electrode indicates that Pt particles are not
removed by this treatment and that they maintain
their small size. The MWCNTs remain also
strongly bond to the carbon paper substrate.
It is of interest now to determine how many Pt
atoms of the MWCNT and of the ELAT elec-
trodes are in contact with the electrolyte. This may
be evaluated by measuring the charge related to
4. Conclusions
It has been demonstrated that highly dispersed,
and very small (1.2 ꢀ 0.3 nm), Pt particles have
been successfully deposited on the surface of
MWCNTs grown on the fibers of carbon paper
used as fuel cell backing. Cyclic voltammetry
strongly suggests that there is an electrical contact
through MWCNTs between the Pt particles and
the carbon backing, enabling the use of these
composite structures as electrodes. The Pt surface
in contact with the electrolyte is estimated to be
about 25% of that of a typical 0.4 mg Pt/cm2
ELAT electrode from E-TEK. The silane deriva-
tive procedure used to obtain Pt (or Co–Ni) par-
ticles on MWCNTs (or on carbon paper [20,21] ) is
very versatile and may well be adapted to obtain
any Pt alloy (e.g., Pt–Ru) at any composition, or
to obtain electrodes with nanoparticles made of
several other transition metals and alloys.
(a)
Carbon paper
(Current x 15)
5
0
-5
-10
(b)
MWCNT on Carbon paper
5
0
(Current x 6)
-5
-10
5
(c)
ELAT (0.4 mg Pt/cm2)
0
-5
-10
Pt Particles on MWCNT
400 800 1200 1600
-800
-400
0
Acknowledgements
Fig. 5. Cyclic voltammograms in H2SO4 (pH 1) of: (a) a bare
carbon paper; (b) MWCNTs grown on carbon paper; (c) small
Pt particles on MWCNTs and Pt on carbon black in ELAT
electrode (0.4 mg Pt/cm2) from E-TEK. Geometrical area of all
electrodes: 1 cm2.
ꢁ
This work was supported by Le Ministere de la
Defense Nationale et les Ministeres des Ressources
ꢀ
ꢁ
ꢀ
Naturelles du Canada et du Quebec.

104
X. Sun et al. / Chemical Physics Letters 379 (2003) 99–104
[14] M.P. Hogarth, T.R. Ralph, Platinum Met. Rev. 46 (2002)
146.
References
[15] E. Dujardin, T.W. Ebbesen, H. Hiura, K. Tanigaki,
Science 265 (1994) 1850.
[1] S. Subramoney, Adv. Mater. 10 (1998) 1157.
[2] H. Dai, Acc. Chem. Res. 35 (2002) 1035.
[3] O. Zhou, H. Shimoda, B. Gao, S. Oh, L. Feming, G. Yue,
Acc. Chem. Res. 35 (2002) 1045.
[16] Y.P. Sun, K. Fu, Yi Lin, W. Huang, Acc. Chem. Res. 35
(2002) 1096.
[17] T.W. Ebbesen, H. Hiura, M.E. Bischer, Adv. Mater. 8
(1996) 155.
[4] R. Andrews, D. Jacques, D. Qian, T. Rantell, Acc. Chem.
Res. 35 (2002) 1008.
[18] Z. Liu, X. Lin, J. Lee, W. Zhang, M. Han, L. Gan,
Langmuir 18 (2002) 4054.
[5] Q.H. Wang, M. Yan, R.P.H. Chang, Appl. Phys. Lett. 78
(2001) 1294.
[19] L.M. Ang, T.S.A. Hor, G.Q. Xu, C.H. Yung, S.P. Zhao,
J.L.S. Wang, Carbon 38 (2000) 363.
ꢀ
[20] X. Sun, B. Stansfield, J.P. Dodelet, S. Desilets, Chem.
[6] B.S. Sherigara, W. Kutner, F. DÕSouza, Electroanalysis 15
(2003) 753.
[7] H. Baughman, A.A. Zakhidov, W.A. de Heer, Science 297
(2002) 787.
Phys. Lett. 363 (2002) 415.
[21] X. Sun, R. Li, G. Lebrun, B. Stansfield, J.P. Dodelet,
[8] Z. Liu, X. Lin, J.Y. Lee, W. Zhang, M. Han, L.M. Gan,
Langmuir 18 (2002) 4054.
ꢀ
S. Desilets, Int. J. Nanosci. 1 (2002) 223.
[22] O. Smiljanic, T. Dellero, A. Serventi, G. Lebrun, B.L.
[9] W. Li, C. Liang, J. Qiu, W. Zhou, H. Han, Z. Wei, G. Sun,
Q. Xin, Carbon 40 (2002) 787.
ꢀ
Stansfield, J.P. Dodelet, M. Trudeau, S. Desilets, Chem.
Phys. Lett. 342 (2001) 503.
[10] E.S. Steigerwalt, G.A. Deluga, C.M. Lukehart, J. Phys.
Chem. B 106 (2002) 760.
[23] K. Kinoshitam, Carbon Electrochemical and Physico-
chemical Properties, Wiley, New York, 1988, p. 227.
[24] H. Angerstin-Kozlowska, in: E.B. Yeager, J.OÕM. Bockris,
B.E. Conway, S. Sarangapani (Eds.), Comprehensive
Treatise of Electrochemistry, Plenum Pub. Corp., New
York, 1984, (Chapter 2).
[11] T. Yoshitake, Y. Shimakawa, S. Kuroshima, H. Kimura,
T. Ichihashi, Y. Kubo, D. Kasuya, K. Takahashi, F.
Kokai, M. Yudasaka, S. Iijima, Phys. B 323 (2002)
124.
[12] T.R. Ralph, M.P. Hogarth, Platinum Met. Rev. 46
(2002) 3.
[25] V.S. Bagotsky, Y.B. Vassileyev, Electrochim. Acta 12
(1967) 1323.
[26] R. Woods, Electroanal. Chem. 9 (1976) 1.
[13] T.R. Ralph, M.P. Hogarth, Platinum Met. Rev. 46 (2002)
117.