- 1. Electrochemistry Communications 7 (2005) 905912
www.elsevier.com/locate/elecomNitrogen containing carbon nanotubes
as supports forPt Alternate anodes for fuel cell applicationsT.
Maiyalagan, B. Viswanathan *, U.V. VaradarajuDepartment of
Chemistry, Indian Institute of Technology Madras, Chennai 600 036,
India Received 7 June 2005; accepted 7 July 2005 Available online 8
August 2005Abstract Aligned nitrogen containing carbon nanotubes
have been synthesized using Anodisc alumina membrane as template.
Highly dis-persed platinum nanoparticles have been supported on the
nitrogen containing carbon nanotubes. Nitrogen containing
carbonnanotubes as platinum catalyst supports were characterized by
electron microscopic technique and electrochemical analysis. TheEDX
patterns show the presence of Pt and the micrograph of TEM shows
that the Pt particles are uniformly distributed on thesurface of
the nitrogen containing carbon nanotube with an average particle
size of 3 nm. Cyclic voltammetry studies revealed ahigher catalytic
activity of the nitrogen containing carbon nanotube supported Pt
catalysts. 2005 Elsevier B.V. All rights reserved.Keywords:
Nitrogen containing carbon nanotubes; Template synthesis; Alumina
template; Catalyst support; Methanol oxidation1. IntroductionPt
based catalysts. High noble metal loadings on the electrode [4,5]
and the use of peruorosulfonic acid Since the last decade, fuel
cells have been receiving anmembranes signicantly contribute to the
cost of the de-increased attention due to the depletion of fossil
fuels vices. An ecient way to decrease the loadings of pre-and
rising environmental pollution. Fuel cells have been cious platinum
metal catalysts and higher utilization ofdemonstrated as
interesting and very promising alterna-Pt particles is by better
dispersion of the desired metaltives to solve the problem of clean
electric power gener-on the suitable support [6]. In general, small
particle sizeation with high eciency. Among the dierent types ofand
high dispersion of platinum on the support will re-fuel cells,
direct methanol fuel cells (DMFCs) are excel-sult in high
electrocatalytic activity. Carbon materialslent power sources for
portable applications owing to itspossess suitable properties for
the design of electrodeshigh energy density, ease of handling
liquid fuel, low in electrochemical devices. Carbon is an ideal
materialoperating temperatures (60100 C) and quick start upfor
supporting nano-sized metallic particles in the elec-[1,2].
Furthermore, methanol fuel cell seems to be highlytrode for fuel
cell applications. No other material exceptpromising for
large-scale commercialization in contrastcarbon material has the
essential properties of electronicto hydrogen-fed cells, especially
in transportation [3]. conductivity, corrosion resistance, surface
properties,The limitation of methanol fuel cell system is due toand
the low cost required for the commercialization oflow catalytic
activity of the electrodes, especially the an- fuel cells. In
general, the conventional supports namelyodes and at present, there
is no practical alternative tocarbon black is used for the
dispersion of Pt particles [7].The appearance of novel carbon
support materials, * Corresponding author. Tel.: +91 044 22574200;
fax: +91 44 such as graphite nanobers (GNFs) [8,9], carbon
nano-22574202.tubes (CNTs) [1017], carbon nanohorns [18], and car-
E-mail address: [email protected] (B. Viswanathan). bon nanocoils
[1922], provides new opportunities of1388-2481/$ - see front matter
2005 Elsevier B.V. All rights
reserved.doi:10.1016/j.elecom.2005.07.007
2. 906T. Maiyalagan et al. / Electrochemistry Communications 7
(2005) 905912carbon supports for fuel cell applications. Bessel et
al. [8]alumina template by wetting method [28]. Afterand
Steigerwalt et al. [9] used GNFs as supports for Ptcomplete solvent
evaporation, the membrane was placedand PtRu alloy electrocatalysts
and observed better in a quartz tube (30 cm length, 3.0 cm
diameter), kept inactivity for methanol oxidation. The high
electronic a tubular furnace and carbonized at 1173 K under
Arconductivity of GNFs and the specic crystallographicgas ow. After
3 h of carbonization, the quartz tubeorientation of the metal
particles resulting from well-was cooled to room temperature. The
resulting templateordered GNF support were believed to be the
importantwith carbonnitrogen composite was immersed in 48%factors
for the observed enhanced electrocatalytic activ-HF at room
temperature for 24 h to remove the aluminaity. The morphology and
the nature of the functional template and the nitrogen containing
CNTs weregroups of the support inuence the activity of fuel cell
obtained as an insoluble fraction. The nanotubes
wereelectrocatalyts [2326]. Carbon with sulphur or nitrogen then
washed with distilled water to remove the residualbased
functionality [25], can inuence the activity of the HF and dried at
393 K.catalyst. The present report focuses on the eorts undertaken
2.3. Loading of Pt catalyst inside nanotubeto develop
unconventional supports based platinumcatalysts for methanol
oxidation. Nitrogen containing Platinum nanoclusters were loaded
inside the N-CNTcarbon nanotubes were used to disperse the
platinumas follows; the C/alumina composite obtained
(beforeparticles eectively without sintering and to increase the
dissolution of template membrane) was immersedthe catalytic
activity for methanol oxidation. The tubu- in 73 mM H2PtCl6 (aq)
for 12 h. After immersion, thelar morphology and the nitrogen
functionality of the membrane was dried in air and the ions were
reducedsupport have inuence on the dispersion as well as to the
corresponding metal(s) by a 3 h exposure to ow-the stability of the
electrode. In this communicationing H2 gas at 823 K. The underlying
alumina was thenthe preparation of highly dispersed platinum
supported dissolved by immersing the composite in 48% HF foron
nitrogen containing carbon nanotubes, the evalua- 24 h. This
procedure resulted in the formation of Pttion of the activity for
the methanol oxidation of these nanocluster loaded N-CNT and the
complete removalelectrodes and comparison with the activity of
conven- of uorine and aluminum was conrmed by EDXtional electrodes
are reported.analysis. 2.4. Preparation of working electrode2.
ExperimentalGlassy carbon (GC) (Bas electrode, 0.07 cm2) was2.1.
Materials polished to a mirror nish with 0.05 m alumina suspen-
sions before each experiment and served as an underly- All the
chemicals used were of analytical grade. Poly-ing substrate of the
working electrode. In order tovinyl pyrrolidone (Sisco Research
Laboratories, India),prepare the composite electrode, the nanotubes
weredichloromethane and concentrated HF (both from dispersed
ultrasonically in water at a concentration ofMerck) were used.
Hexachloroplatinic acid was ob-1 mg ml1 and 20 ll aliquot was
transferred on to atained from Aldrich. 20 wt% Pt/Vulcan carbons
were polished glassy carbon substrate. After the
evaporationprocured from E-TEK. Methanol and sulphuric acid of
water, the resulting thin catalyst lm was coveredwere obtained from
Fischer chemicals. The aluminawith 5 wt% Naon solution. Then the
electrode wastemplate membranes (Anodisc 47) with 200 nm diameter
dried at 353 K and used as the working electrode.pores were
obtained from Whatman Corp. Naon5 wt% solution was obtained from
Dupont and was used 2.5. Characterization methodsas received.The
chemical composition of the nanotubes was2.2. Synthesis of nitrogen
containing carbon nanotubes determined by elemental analysis using
Hereaus CHN analyzer after the removal of alumina template. The
Pyrolysis of nitrogen containing polymers is a facile scanning
electron micrographs were obtained usingmethod for the preparation
of carbon nanotube materi-JEOL JSM-840 model, working at 15 keV.
The nano-als containing nitrogen substitution in the carbon
frame-tubes were sonicated in acetone for 20 min and thenwork.
Nitrogen containing carbon nanotubes werewere dropped on the
cleaned Si substrates. The AFMsynthesized by impregnating
polyvinylpyrrolidone imaging was performed in air using the
Nanoscope IIIA(PVP) inside the alumina membrane template and
subse-atomic force microscope (Digital Instruments, St. Bar-quent
carbonization of the polymer [27]. Polyvinylpyr- bara, CA) operated
in contact mode. For transmissionrolidone (PVP 5 g) was dissolved
in dichloromethaneelectron microscopic studies, the nanotubes
dispersed(20 ml) and impregnated directly in the pores of the in
ethanol were placed on the copper grid and the 3. T. Maiyalagan et
al. / Electrochemistry Communications 7 (2005) 905912 907images
were obtained using Phillips 420 model, operat- nanotubes is shown
in Fig. 1(a). Fig. 1(b) shows lateraling at 120 keV.view of the
nitrogen containing carbon nanotubes with low magnication and Fig.
1(c) shows lateral view of2.6. Electrochemical measurementsthe
nitrogen containing carbon nanotubes with high magnication. The
hollow structure and well alignment All electrochemical studies
were carried out using aof the nitrogen containing carbon nanotubes
have beenBAS 100 electrochemical analyzer. A conventional identied
by SEM.three-electrode cell consisting of the GC (0.07 cm2)AFM
images of the nitrogen containing carbon nano-working electrode, Pt
plate (5 cm2) as counter electrode tubes deposited on a silicon
substrate are shown inand Ag/AgCl reference electrode were used for
the cyclic Fig. 2. The AFM tip was carefully scanned across
thevoltammetry (CV) studies. The CV experiments weretube surface in
a direction perpendicular to the tubeperformed using 1 M H2SO4
solution in the absenceaxis. From the AFM images, it is inferred
that a partand presence of 1 M CH3OH at a scan rate ofof the long
nanotube is appearing to be cylindrical in50 mV s1. All the
solutions were prepared by using ul-shape and is found to be
terminated by a symmetrictra pure water (Millipore, 18 MX). The
electrolytes were hemispherical cap. Because of the nite size of
thedegassed with nitrogen gas before the electrochemicalAFM tip,
convolution between the blunt AFM tip andmeasurements.the tube body
will be giving rise to an apparently greater lateral dimension than
the actual diameter of the tube [29].3. Results and discussion The
TEM images are shown in Fig. 3(a)(c). The open end of the tubes
observed by TEM shows that Elemental analysis was conducted to
examine the nanotubes are hollow and the outer diameter ofwhether
nitrogen has really entered the carbon nanotubethe nanotube closely
match with the pore diameter offramework. It has been found that
the samples prepared template used, with a diameter of 200 nm and a
lengthcontained about 87.2% carbon and 6.6% nitrogen (w/w).of
approx. 4050 lm. It is evident from the micrographsThe SEM images
of the nitrogen containing carbon that there is no amorphous
material present in the nano-nanotubes support are shown in Fig.
1(a)(c). Top view tube. Fig. 3(c) shows the TEM image of Pt
nanoparticlesof the vertically aligned nitrogen containing carbon
lled carbon nanotubes. TEM pictures reveal that the PtFig. 1. SEM
images of the nitrogen containing carbon nanotubes: (a) the top
view of the nanotubes; (b) side view of the vertically aligned
nanotubesand (c) high magnication lateral view of the nanotubes. 4.
908 T. Maiyalagan et al. / Electrochemistry Communications 7 (2005)
905912Fig. 2. AFM image of the nitrogen containing carbon
nanotubes.Fig. 3. TEM images of the nitrogen containing carbon
nanotubes: (a) at lower magnication; (b) at higher magnication
image of the individualnanotube (an arrow indicating the open end
of the tube) and (c) Pt lled nitrogen containing carbon nanotubes.
5. T. Maiyalagan et al. / Electrochemistry Communications 7 (2005)
905912909particles have been homogeneously dispersed on the the
current increases quickly due to dehydrogenationnanotubes and
particle sizes were found to be around of methanol followed by the
oxidation of absorbed3 nm. The optimal Pt particle size for
reactions in themethanol residues and reaches a maximum in the
poten-H2/O2 fuel cell is 3 nm [30]. The importance of the Pt tial
range between 0.8 and 1.0 V vs. Ag/AgCl. In theparticle size on the
activity for methanol oxidation iscathodic scan, the re-oxidation
of methanol is clearlydue to the structure sensitive nature of the
reactionobserved due to the reduction of oxide of platinum.and the
fact that particles with dierent sizes will Electrocatalytic
activity of methanol oxidation has beenhave dierent dominant
crystal planes and hence the dif- found to be strongly inuenced by
the metal dispersion.ferent intercrystallite distances, which might
inuencePure Pt electrode shows an activity of 0.167 mA cm2.methanol
adsorption. The commercial Pt/C has a veryThe Pt/N-CNT shows a
higher activity of 13.3 mA cm2high specic surface area but
contributed mostly bywhere as conventional 20% Pt/Vulcan
(E-TEK)micropores less than 1 nm and are therefore more
di-electrode shows less activity of 1.3 mA cm2 comparedcult to be
fully accessible. It has been reported that the to nitrogen
containing carbon nanotube supportedmean value of particle size for
20% Pt/Vulcanelectrode. The nitrogen containing carbon
nanotube(E-TEK) catalyst was 2.6 nm [31]. The EDX pattern of
supported electrodes shows a ten fold increase in thethe as
synthesized catalyst shows the presence of Pt catalytic activity
compared to the E-TEK electrode.particles in the carbon nanotubes
and also the completeThe Pt/N-CNT electrode showed higher
electrocatalyticremoval of uorine and aluminum has also
beenactivity for methanol oxidation than commercialconrmed in Fig.
4. It has been reported that thePt/Vulcan (E-TEK) electrode. The
anodic currentelectronic and physical structures of a Pt
particledensity of Pt/N-CNT electrode is found to be
higherdeposited on carbon dier from those of the bulk Pt. than that
of Pt/Vulcan (E-TEK) electrode, which indi-The electronic change in
Pt/C is considered as a resultcates that the catalyst prepared with
nitrogen containingof functional groups of the carbon support that
mightcarbon nanotubes as the support has excellent catalyticinuence
the electronic structure of Pt particulateactivity on methanol
electrooxidation.[3236]. The nitrogen functional group on the
carbonThe onset potential and the forward-scan peak
currentnanotubes surface intensies the electron withdrawingdensity
for the dierent electrodes are given in Table 1.eect against Pt and
the decreased electron density of The onset potential of methanol
oxidation at nitrogenplatinum facilitate oxidation of methanol.
containing carbon nanotube supported catalysts occurs Fig. 5(a)(c)
shows the cyclic voltammogram ofat 0.22 V, which is relatively more
negative to that ofmethanol oxidation. Fig. 5(c) shows the cyclic
voltam- the other catalysts. This may be attributed to the high
dis-mogram of Pt/N-CNT electrode in 1 M H2SO4/1 Mpersion of
platinum catalysts and the nitrogen functionalCH3OH run at a scan
rate of 50 mV s1. The electrocat- groups on its surface. The higher
electrocatalytic activityalytic activity of methanol oxidation at
the Pt/N-CNTof the nitrogen containing carbon nanotube
supportedelectrodes was evaluated and compared with that of
electrode is due to higher dispersion and a good interac-the
conventional electrodes. During the anodic scan, tion between the
support and the Pt particles The VulcanFig. 4. EDX pattern of
Pt/N-CNT electrode. 6. 910 T. Maiyalagan et al. / Electrochemistry
Communications 7 (2005) 905912Fig. 5. Cyclic voltammograms of (a)
pure Pt; (b) Pt/Vulcan (E-TEK) and (c) Pt/N-CNT in 1 M H2SO4/1 M
CH3OH run at 50 mV s1.Table 1 also been reported [38]. All these
results indicate thatElectrochemical parameters for methanol
oxidation on the variousthe nitrogen functionality on CNT inuences
theelectrodescatalytic activity of the catalyst. The enhanced
electrocat-Electrocatalyst Methanol oxidation Forward peak current
alytic eect of the nitrogen containing carbon nanotubeonset
potential (V)density (mA cm2)vs. Ag/AgClsupported electrodes could
also be partly due to thefollowing factors which require further
investigation:Bulk Pt 0.4 0.16720% Pt/C (E-TEK)0.451.3(1) higher
dispersion on the nitrogen containing carbonPt/N-CNT0.22 13.3
nanotube support increases the availability of anenhanced
electrochemically active surface area, (2)appearance of the specic
active sites at the metalsupport boundary and, (3) strong
metalsupportcarbon support has randomly distributed pores of
vary-interaction.ing sizes which may make fuel and product diusion
Long-term stability is important for practical applica-dicult
whereas the tubular three-dimensional morphol-tions. Fig. 6 shows
the current densitytime plots of var-ogy of the nitrogen containing
carbon nanotubes makes ious electrodes in 1 M H2SO4 and 1 M CH3OH
at 0.6 V.the fuel diusion easier. The Vulcan carbon contains high
The performance of Pt electrodes was found to be poorlevels of
sulfur (ca. 5000 ppm or greater), which could compared to the E-TEK
and Pt/N-CNT electrode. Thepotentially poison the fuel-cell
electrocatalysts [37]. nitrogen containing carbon nanotube
electrodes are theNitrogen containing carbon nanotubes used in this
study most stable for direct methanol oxidation. The
increas-contains heterocyclic nitrogen so that it preferentially
at-ing order of stability of various electrodes is; Pt <
Pt/taches the Pt particles. The selective attachment of Au Vulcan
(E-TEK) < Pt/N-CNT. We are currently investi-nanoparticles on
nitrogen doped carbon nanotubes hasgating whether nitrogen has a
catalytic role that contrib- 7. T. Maiyalagan et al. /
Electrochemistry Communications 7 (2005) 905912 911 [6] T.
Matsumoto, T. Komatsu, K. Arai, T. Yamazaki, M. Kijima, H. Shimizu,
Y. Takasawa, J. Nakamura, Chem. Commun. 7 (2004) 840. [7] K.
Kinoshita, Carbon: Electrochemical and Physicochemical Properties,
John Wiley, New York, 1988. [8] C.A. Bessel, K. Laubernds, N.M.
Rodriguez, R.T.K. Baker, J. Phys. Chem. B 105 (6) (2001) 1115. [9]
E.S. Steigerwalt, G.A. Deluga, D.E. Cliel, C.M. Lukehart, J. Phys.
Chem. B 105 (34) (2001) 8097.[10] B. Rajesh, V. Karthik, S.
Karthikeyan, K.R. Thampi, J.M. Bonard, B. Viswanathan, Fuel 81
(2002) 2177.[11] Z.L. Liu, X.H. Lin, J.Y. Lee, W.D. Zhang, M. Han,
L.M. Gan, Langmuir 18 (2002) 4054.[12] W.Z. Li, C.H. Liang, W.J.
Zhou, J.S. Qiu, Z.H. Zhou, G.Q. Sun, J. Phys. Chem. B 107 (2003)
6292.[13] T. Matsumoto, T. Komatsu, H. Nakano, K. Arai, Y.
Nagashima, E. Yoo, T. Yamazaki, M. Kijima, H. Shimizu, Y. Takasawa,
J. Nakamura, Catal. Today 90 (2004) 277.[14] C. Kim, Y.J. Kim, Y.A.
Kim, T. Yanagisawa, K.C. Park, M. Endo, M.S. Dresselhaus, J. Appl.
Phys. 96 (2004) 5903.Fig. 6. Current density vs. time curves at (a)
Pt/N-CNT; (b) Pt/Vulcan[15] Yangchuan Xing, J. Phys. Chem. B 108
(50) (2004) 19255.(E-TEK) and (c) Pt measured in 1 M H2SO4 + 1 M
CH3OH. The[16] C. Wang, M. Waje, X. Wang, J.M. Tang, C.R. Haddon,
Y. Yan,potential was stepped from the rest potential to 0.6 V vs.
Ag/AgCl. Nano Lett. 4 (2) (2004) 345.[17] M. Carmo, V.A. Paganin,
J.M. Rosolen, E.R. Gonzalez, J. Power Sources 142 (2005) 169.utes
to the observed enhancement in the methanol oxi- [18] T. Yoshitake,
Y. Shimakawa, S. Kuroshima, H. Kimura, T. Ichihashi, Y. Kubo, D.
Kasuya, K. Takahashi, F. Kokai, M.dation. Recent experiments
conducted in our laboratory Yudasaka, S. Iijima, Physica B 323
(2002) 124.on Pt supported N-doped and undoped CNTs reveal the [19]
T. Hyeon, S. Han, Y.E. Sung, K.W. Park, Y.W. Kim, Angew.importance
of nitrogen functionalities on methanol oxi-Chem. Int. Ed. 42
(2003) 4352.dation activity [39]. [20] K.W. Park, Y.E. Sung, S.
Han, Y. Yun, T. Hyeon, J. Phys. Chem. In summary, it is reported
platinum catalysts are B 108 (2004) 939.[21] G.S. Chai, S.B. Yoon,
J.S. Yu, J.H. Choi, Y.E. Sung, J. Phys.highly dispersed on the
surface of well-aligned Chem. B 108 (2004) 7074.nitrogen containing
carbon nanotube. The tubular[22] S.H. Joo, S.J. Choi, I. Oh, J.
Kwak, Z. Liu, O. Terasaki, R. Ryoo,morphology and the nitrogen
functionality favour Nature 412 (2001) 169.dispersion of the Pt
particles. The electrocatalytic[23] M. Uchida, Y. Aoyama, M.
Tanabe, N. Yanagihara, N. Eda, A.properties of Pt particles
supported on the three- Ohta, J. Electrochem. Soc. 142 (1995)
2572.[24] S.C. Roy, P.A. Christensen, A. Hamnett, K.M. Thomas,
V.dimensional nitrogen containing carbon nanotube Trapp, J.
Electrochem. Soc. 143 (1996) 3073.electrode shows higher catalytic
activity for methanol[25] A.K. Shukla, M.K. Ravikumar, A. Roy, S.R.
Barman,oxidation than the commercial E-TEK electrode, D.D. Sarma,
A.S. Arico, J. Electrochem. Soc. 141 (1994)which implies that the
well-aligned nitrogen containing1517.carbon nanotube arrays have
good potential[26] S. Ye, A.K. Vijh, L.H. Dao, J. Electrochem. Soc.
144 (1997) 90.application as a catalyst support in direct
methanol[27] T. Maiyalagan, B. Viswanathan, Mater. Chem. Phys. 93
(2005)fuel cells.291.[28] M. Steinhart, J.H. Wendor, A. Greiner,
R.B. Wehrspohn, K. Nielsch, J. Schilling, J. Choi, U. Goesele,
Science 296 (2002) 1997.Acknowledgements[29] S.C. Tsang, P. de
Oliveira, J.J. Davis, M.L.H. Green, H.A.O. Hill, Chem. Phys. Lett.
249 (1996) 413. We thank the Council of Scientic and Industrial Re-
[30] K. Kinoshita, J. Electrochem. Soc. 137 (1990) 845.[31] E.
Antolini, L. Giorgi, F. Cardellini, E. Passalacqua, J. Solid
Statesearch (CSIR), India, for a senior research fellowship to
Electrochem. 5 (2001) 131.one of the authors T. Maiyalagan. [32]
S.C. Hall, V. Subramanian, G. Teeter, B. Rambabu, Solid State
Ionics 175 (2004) 809.[33] P.L. Antonucci, V. Alderucci, N.
Giordano, D.L. Cocke, H. Kim, J. Appl. Electrochem. 24 (1994)
58.References[34] M.C. Roman-Martinez, D. Cazorla-Amoros, A.
Linares- Solano, C. Salinas-Martinez de Lecea, Curr. Top. Catal. 1
[1] M.P. Hogarth, G.A. Hards, Platinum Met. Rev. 40 (1996) 150.
(1997) 17. [2] T.R. Ralph, Platinum Met. Rev. 41 (1997) 102.[35]
M.C. Roman-Martinez, D. Cazorla-Amoros, A. Linares-Solano, [3] B.D.
McNicol, D.A.J. Rand, K.R. Williams, J. Power Sources 83 C.
Salinas-Martinez de Lecea, H. Yamashita, M. Anpo, Carbon (2001)
47.33 (1) (1995) 3. [4] A. Hamnett, Catal. Today 38 (1997) 445.[36]
C.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. Tsiplakides,
[5] S. Wasmus, A. Kuver, J. Electroanal. Chem. 461 (1999) 14.
Electrochemical Activation of Catalysis, Promotion, Electrochemical
8. 912 T. Maiyalagan et al. / Electrochemistry Communications 7
(2005) 905912 Promotion, and Metalsupport Interactions, Kluwer, New
York, [38] K. Jiang, A. Eitan, L.S. Schadler, P.M. Ajayan, R.W.
Siegel, N. 2001. Grobert, M. Mayne, M. Reyes-Reyes, H. Terrones, M.
Terrones,[37] K.E. Swider, D.R. Rolison, J. Electrochem. Soc. 143
(3) (1996)Nano Lett. 3 (3) (2003) 275. 813. [39] T. Maiyalagan, B.
Viswanathan, U. Varadaraju, in preparation.