Bimetallic and trimetallic catalyzed carbon nanotubes for aqueous H2, Cl2 fuel cell electrodes

Article abstract of DOI:10.1016/j.jallcom.2008.09.085

Chemical vapour deposition (CVD) based synthesis of carbon nanotubes (CNTs) within the pores of porous ceramic substrates, using turpentine oil as a precursor, has been examined. Composites of Pt-Sn, Pt-Ni, Ni-Sn and Pt-Sn-Ni catalysts are tried for growi

Full text of DOI:10.1016/j.jallcom.2008.09.085

Journal of Alloys and Compounds 476 (2009) 697–704
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Bimetallic and trimetallic catalyzed carbon nanotubes for aqueous H2,
Cl2 fuel cell electrodes
U.B. Suryavanshi, C.H. Bhosale∗
Electrochemical Material Laboratory, Department of Physics, Shivaji University, Kolhapur (MH), India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 25 June 2008
Received in revised form 25 August 2008
Accepted 14 September 2008
Available online 29 November 2008
Chemical vapour deposition (CVD) based synthesis of carbon nanotubes (CNTs) within the pores of porous
ceramic substrates, using turpentine oil as a precursor, has been examined. Composites of Pt–Sn, Pt–Ni,
Ni–Sn and Pt–Sn–Ni catalysts are tried for growing CNTs. In the CVD process, a reactor temperature of
1100 ◦C is maintained for the carbon deposition. Pt, Sn, and Ni catalysts are deposited with sputter coater,
vacuum deposition and electrodeposition techniques respectively. The materials are characterized by
X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, Fourier transform
infrared spectroscopy (FTIR) and resistivity measurements by the Van der Pauw method. The prepared
electrodes are tested for aqueous H2, Cl2 fuel cells. The Linear Sweep Voltametry (LSV) characteristics
and chronoamperometry (CA) of a half cell (i.e. hydrogen electrode coated with different electrocatalysts)
with 40% KCl solution are measured. Composite of Pt + Ni + Sn coated carbon nanotubes shows better
performance than other tried catalysts.
Keywords:
Nanostructures
Vapour deposition
X-ray diffraction
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
tris of pinaceae family. A simple template synthesis method has
been explored [21–24] for preparing micro- and nanostructured
Carbon nanotubes are unique whose outstanding properties
have sparked an abundance of research since their discovery in
1991 [1]. These nanomaterials are very promising for the develop-
ment of novel, technological applications such as batteries [2], tips
for scanning probe microscopy [3], electrochemical actuators [4],
sensors [5] etc. In addition to initially developed laser furnace [6]
and arc discharge [7] techniques, catalytic chemical vapour depo-
sition (CCVD) method [8–11] has been contrived for a scalable,
large scale production of carbon nanotubes, with various carbon
source molecules tested such as carbon monoxide, methane, ben-
zene, xylene, toluene etc. These carbon sources are related to fossil
fuels which may not be sufficient in near future; so researchers from
IIT Bombay switched over to reproducible natural carbon source:
camphor and grown various kind of nano-carbons [12–15]. Kumar
and Ando reported condition of growing multi-walled carbon nan-
otubes (MWNTs) [16,17] single-walled carbon nanotubes (SWNTs)
[18] and vertically aligned carbon nanotubes [19] with well-valued
material camphor [20]. This gives new idea to develop nanotechnol-
ogy with ecofriendly carbon source: turpentine oil, which is derived
from the oleoresin of Pinus species particularly from Pinus Palus-
materials. This method entails synthesizing the desired material
within the pores of porous ceramic membrane and has been used
to make tubules and fibrils composed of polymers, metals [25],
semiconductors [26], metal oxides [27], carbon [28] and compos-
ite materials [5,29]. Primary interest is in the production of carbon
nanotubes that could be used as anode in aqueous H2, Cl2 fuel cells.
In caustic soda industries chlorine releases as byproduct and pur-
pose is to utilize it electrochemically and produce power as well
as HCl. Initial cost of fuel cell due to platinum electrodes is the
main challenge in commercialization of fuel cells. CNT may solve
this problem as it has high conductivity and porosity for gas dif-
fusion. This approach includes CVD based synthesis of carbon thin
film within the pores of ceramic membrane with catalyst deposited
in the pores. In the present work, different catalysts like Pt–Sn,
Pt–Ni, Ni–Sn and Pt–Sn–Ni have been deposited with electrode-
position, sputter coating and vacuum deposition techniques. The
formed electrodes have been further used for structural and elec-
trical characterization.
2. Experimental
2.1. Carbon deposition
Porous ceramic were cut into small size of 2 cm × 2 cm × 0.2 cm and were used
as a substrate for growing carbon nanotubes using CVD method. The CVD unit
consisted of two furnaces joined side by side with a quartz tube traversing hori-
zontally along the axis of both of them. The vapourizing furnace (A) was used to
∗
Corresponding author. Tel.: +91 231 2609228; fax: +91 231 2691533.
E-mail addresses: ubsuryavanshi@yahoo.com (U.B. Suryavanshi),
chb phy@unishivaji.ac.in (C.H. Bhosale).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.09.085

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vapourize turpentine oil (C10 H16 ) and that of pyrolysing furnace (B) was used to
pyrolyse the turpentine oil vapours. Nitrogen gas was flushed at the flow rate
160 ml/min through the tube for 15 min before starting the deposition to remove
the air completely. A quartz boat containing 3 ml of turpentine oil was placed in
the furnace A, maintained at optimized temperature i.e. at 250 ◦C with temperature
controller (Aplab make, Model No. 9601). The temperature of furnace (B) containing
ceramic substrates was maintained at optimized 1100 ◦C with the help of temper-
ature controller (SELEC make, Model No. DTC324). At this temperature, conducting
carbon was deposited which is the basic need of fuel cell electrodes. The deposi-
tion was carried out for 1 h under the nitrogen flow. Nitrogen gas flushed until the
furnaces were cooled down to room temperature and then samples were collected.
electrode cell consisting of the electrode to be tested (working electrode), a plat-
inum foil of dimensions 2 cm × 2 cm × 0.1 cm (counter electrode) and saturated
calomel electrode (SCE) as a reference electrode. 40% KCl solution was used as
electrolyte at room temperature. The actual cell configuration was ‘hydrogen elec-
trode, H2|40% KCl (SCE)|Pt’. All electrochemical measurements were performed with
potentiostat, EG and G make VersaStat - II model, controlled by electrochemistry
software M270. The same flow rate of hydrogen was maintained in all experi-
ments.
2.7.1. Linear Sweep Voltametry (LSV)
LSV was employed to study the reduction of hydrogen with respect to catalyst
activated carbon nanomaterials. The potential of working electrode was scanned
from initial potential to final potential (−500 to +700 mV vs SCE) with scan rate
(20 mV/s) as a ramp.
2.2. Electrodeposition of nickel
Catalyst loading is the important part in growing carbon nanotubes. Nickel films
were deposited onto carbon coated ceramic from an electrolytic bath containing 1N
concentration of Ni(NO3)2·6H2O for 30 min. Films were deposited at a potential of
0.55 V with respect to saturated calomel electrode (SCE), the reference electrode.
Electrodeposition was carried out on the 2 cm × 2 cm area of the substrates at poten-
tiostatic mode in an unstirred bath at room temperature. After deposition, the films
were rinsed in double distilled water and were used for further deposition of carbon.
2.7.2. Chronoamperometry (CA)
In chronoamperometry, the time dependent deposition current density I(t) is
recorded for measuring half cell performance of hydrogen on particular working
electrode at constant. The CA was measured at reduction potential for each elec-
trode calculated with LSV and by applying potential step of 0.2 V. The performance
was firstly measured for 10 s only and the electrodes, which were giving notice-
able performance, measured for 3600 s. I–V characteristics was studied for the
2.3. Deposition of platinum
Pt was deposited with JEOL JFC-1600 auto fine magnetron type sputter coater
consisting of a basic unit (350 mm wide × 340 mm deep × 282 mm high, 12 kg) and
rotary pump unit (280 mm wide × 120 mm deep × 215 mm high, 10 kg). Standard Pt
of 57 mm in diameter was used as a target. The specimen stage size was adjusted
at the height of 35 mm and pressure at 10−1 Pa. Coating time was set for 30 s and
sputtering current was about 40 mA. The films have been deposited with 15 nm
thickness.
2.4. Deposition of tin
Sn was deposited with ‘HINDHIVAC vacuum coater model – 12A4D consisting
of vacuum chamber and pumping system. The chamber is evacuated by HIND-
HIVAC diffpack pump model-114D and backed by 200 l/min, double stage, direct
driven, rotary pump, model ED-12 with an overload protection. The films of Sn were
vacuum deposited by Sn foil (99% pure, Merck) with resistive heating using molyb-
denum boat. The deposition was carried out on porous ceramic substrates under
vacuum more than 10−5 mbar. The thickness and deposition rates were monitored
with quartz crystal monitor (DTM 101 operated at 6 MHz). The films with 15 nm
thickness were deposited at deposition rate 10 Å/s.
Different combinations of catalysts like Pt–Sn, Pt–Ni, Ni–Sn and Pt–Sn–Ni have
been tried for growing carbon nanotubes.
2.5. Growing of carbon nanotubes
Carbon nanotubes were grown onto the catalyst coated carbon thin films by CVD
with turpentine oil (3 ml) as a precursor kept at 250 ◦C and pyrolysed at 1100 ◦C.
2.6. Characterization of grown carbon nanotubes
Structural properties of prepared carbon electrodes were studied by XRD
technique. XRD studies were carried out with Phillips analytical PW 1710 X-ray
diffractometer. X-ray machine was operated at 40 kV, 30 mA, using Cu K␣1 and
K␣2 radiation having wavelengths ꢀ = 1.54056 and 1.54439 Å respectively. The XRD
patterns for these materials were recorded within the span of angles between 10◦
and 100◦ for nickel catalyzed carbon films. Micro-structure investigation has been
studied with SEM (JEOL JSM 6360, Japan). For this purpose platinum (15 nm) was
coated by Polaron sputter coater unit E-2500. Raman spectra of these samples were
taken with cofocal Raman microscope (CRM 200, WiTec, USA). Raman spectra of as
grown CNTs were measured by green laser with wavelength of 532 nm and power
of 14.4 mW. Each spectrum was performed with 30 s acquisition time, an illumina-
tion spot size of 1 ␮m. FTIR spectra were recorded on a Perkin-Elmer spectrum–1
spectrophotometer. The samples were made according to the technique of Stim-
son and Schiedt. Sample powder is mixed with KBr and a homogeneous mixture is
formed with a mortar and pestle. This mixture was placed in a cylindrical dye of
13 mm diameter and pressed for about 5 min at 200 kg/cm2 to make pellet. These
pellets were then kept in a dry box for 10 min. Then these pellets were scanned in
the frequency range of 450–4000 cm−1. FTIR patterns of the porous ceramic were
subtracted from the FTIR pattern of samples with a carbon deposit to get the actual
pattern of deposited carbon. Resistivity of the samples was measured with Van der
Pauw technique.
2.7. Half cell performance
The electrochemical performance of the hydrogen electrode was studied with
half cell configuration. Half cell tests were carried out in conventional three
Fig. 1. XRD patterns of the carbon nanotubes catalyzed with (a) Pt–Sn; (b) Pt–Ni;
(c) Ni–Sn; (d) Pt–Sn–Ni.

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699
Fig. 2. SEM images of the carbon nanotubes catalyzed with (a) Pt–Sn; (b) Pt–Ni; (c) Ni–Sn; (d) Pt–Sn–Ni.
Fig. 3. Raman spectra of the carbon nanotubes catalyzed with (a) Pt–Sn; (b) Pt–Ni; (c) Ni–Sn; (d) Pt–Sn–Ni.

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electrodes which give remarkable performance while scanning chronoamperom-
etry.
3.2. Scanning electron microscopy studies
Fig. 2(a)–(d) shows SEM micrographs for Pt–Sn, Pt–Ni, Ni–Sn and
Pt–Sn–Ni catalyzed carbon thin films deposited on porous ceramic.
All micrographs show well-grown carbon nanotubes. In SEM image
of Pt–Sn catalyzed carbon nanotubes (Fig. 2(a)) coiled structure of
carbon nanotubes have been observed. The average diameter of
these carbon nanotubes is nearly 200 nm. Pt–Ni catalyzed carbon
nanotubes (Fig. 2(b)) show randomly oriented carbon nanotubes
of diameter nearly 100 nm only. Ni–Sn catalyzed carbon nanotubes
(Fig. 2(c)) show blurred image of formed carbon nanotubes hav-
ing diameter less than that of 100 nm. Pt–Sn–Ni catalyzed carbon
nanotubes (Fig. 2(d)) show coiled structure of carbon nanotubes of
diameter 200–300 nm. So each tried catalyst enhances the growth
of carbon nanotubes while Pt–Ni and Ni–Sn catalyzed carbon nan-
otubes are of low diameter (∼100 nm). In both case nickel is the
common catalyst, which may catalyze the reaction in such a way
that the formed carbon nanotubes are of lower diameter than that
of other catalyst.
3. Results and discussion
When a volatile compound of the substance to be deposited is
vapourized and the vapour is thermally decomposed or reacted
with other gases, vapours or liquids at the substrate to yield non-
volatile reaction products which deposit atomistically (atom by
atom) on the substrate, the process is called chemical vapour depo-
sition [30,31].
3.1. X-ray diffraction studies
Fig. 1(a)–(d) shows the XRD patterns for Pt–Sn, Pt–Ni, Ni–Sn and
Pt–Sn–Ni catalyzed carbon nanotubes. All the XRD patterns show
microcrystalline nature of the films with characteristic graphite
peaks. Conducting carbon i.e. graphitic carbon is the basic need
for the fuel cell electrodes which gets fulfilled in these prepared
electrodes.
Fig. 4. FTIR patterns of the carbon nanotubes catalyzed with (a) Pt–Sn; (b) Pt–Ni; (c) Ni–Sn; (d) Pt–Sn–Ni.

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701
Fig. 4. (Continued ).
3.3. Raman spectroscopy studies
sity is too low. This indicates the formation of single-walled CNTs
but it might be of less in number.
Raman spectroscopy is a sensitive method for CNT characteriza-
tion. Due to specific combination of strong Van Hove singularities
of the phonon density and Raman resonance effect it is possible to
measure Raman scattering spectra of small bundles or even of sin-
gle nanotube. Fig. 3(a), (b), (c) and (d) shows the Raman spectra of
the films catalyzed with Pt–Sn, Pt–Ni, Ni–Sn and Pt–Sn–Ni respec-
tively. The well-separated two peaks (G and D) are clearly observed
for all samples. G-peak (around 1600 cm−1) corresponds to tan-
gential stretching mode (E2g) of highly oriented pyrolytic graphite
and suggests CNTs are composed of crystalline graphitic carbon
and D-peak (around 1300 cm−1) originates from disorder in the
sp2 hybridized carbon and indicates lattice distortion in the curved
graphene sheets, tube ends etc. [32]. From Fig. 3 it is observed that
G-peak intensity is maximum for Pt + Ni + Sn catalyzed CNTs while
for Ni + Sn catalysed CNTs D-peak intensity is more than that of
G-peak intensity. This is because of defects in the tube such as pen-
tagons, curvature etc. The lower frequency Radial Breathing Mode
(RBM) peaks are observed for Pt + Sn catalyzed CNTs but its inten-
3.4. Fourier transform infrared spectroscopy
To study the nature of carbon nanotubes, FTIR were taken.
Fig. 4(a), (b), (c) and (d) shows the FTIR spectrum of the films
catalyzed with Pt–Sn, Pt–Ni, Ni–Sn and Pt–Sn–Ni respectively. All
three graphs show three major peaks of O–H, C–H stretch and C C.
The peak of 1019–1020 cm−1 is a substrate peak. C C shows the
graphitic nature of the carbon nanotubes. Both graphs shows two
major peaks in the range of 2850–2920 cm−1 corresponding to sp3
and sp2 carbon atoms respectively, because it has been reported
that the absorption for C–H stretching vibrations on sp2 bonded
carbons is found in the range 2920–3060 cm−1 while for sp3 car-
bons in the range 2850–2915 cm−1 [13]. From FTIR, calculated ratio
of sp2 to sp3 is nearly equal to 2 for Pt + Sn catalyzed CNTs and it is
greater than 2 for other tried catalysts. The percentage of sp2 carbon
atoms is around 86% for Pt + Sn and for others nearly 90% indicating
more graphite in the deposited material. This calculation suggests

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Ni–Sn show lowest resistivity i.e. 8.083 × 10−6 ꢁ cm. That means
formed carbon nanotubes are highly conducting i.e. of graphitic
nature. The same conclusion has been drawn from X-ray diffraction
pattern.
3.6. Half cell performance
When hydrogen gas passes through catalyst activated carbon
nanomaterials, which are coated on porous ceramic substrates,
hydrogen dissociates into hydrogen ions and electrons. The ions
pass through 40% KCl solution while that of electrons through outer
circuit. When two phases are brought together to form a boundary,
differences of chemical and electrochemical potential cause mobile
charge carriers i.e. ions, to migrate until free energy equilibrium is
reached and a separation of charge results. The force of electro-
static attraction limits the distances to which the charges can be
separated; resulting double layer formation. The charges in the two
layers are equal and opposite. At such boundaries, tendency of ions
to move across such double layers contribute importantly to E, the
driving force of the overall cell reaction [19].
Fig. 5. Resistivity measurement by Van der Pauw technique for carbon nanotubes
catalyzed with Pt–Sn, Pt–Ni, Ni–Sn and Pt–Sn–Ni.
that presence of relatively small percentage of sp2 carbon atoms
in deposited CNTs. From results it can be observed that turpentine
oil having formula C10H16 decomposes at 1100 ◦C and forms carbon
nanotubes which are graphitic in nature.
3.6.1. Linear Sweep Voltametry
Fig. 6(a), (b), (c) and (d) shows LSV for the CNTs catalyzed
with Pt–Sn, Pt–Ni, Ni–Sn and Pt–Sn–Ni respectively. Fig. 6(a)
shows the I region in which appreciable potential drop occurs
due to electrode–electrolyte interface resulting from the bending
of bands. The starting value of the I region current density is
−375 ␮A/cm2. When scanning potential enters in the region II,
the current curve is becoming flat due to equilibrium anodic and
cathodic current. In region III, cathodic current starts to increase.
It represents the starting of reduction of hydrogen and further
3.5. Resistivity by Van der Pauw technique
Fig. 5 shows the resistivity of carbon nanotubes catalyzed with
Pt–Sn, Pt–Ni, Ni–Sn and Pt–Sn–Ni. All carbon films show low resis-
tivity (in the range of 10−6 ꢁ cm) where the CNT catalyzed with
Fig. 6. Linear Sweep Voltametry (LSV) of the carbon nanotubes catalyzed with (a) Pt–Sn; (b) Pt–Ni; (c) Ni–Sn; (d) Pt–Sn–Ni.

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703
Fig. 7. Chronoamperometry of the carbon nanotubes catalyzed with (a) Pt–Sn; (b) Pt–Ni; (c) Ni–Sn; (d) Pt–Sn–Ni.
sharp increase in current up to 400 ␮A/cm2. Fig. 6(b) shows the
reduction potential of hydrogen on Ni + Pt in positive region with
respect to SCE. While hydrogen reduces on Ni + Sn and Pt + Sn + Ni
catalyzed CNTs at −140 and −75 mV respectively with respect
to SCE [Fig. 6(c) and (d)]. That means Pt + Sn + Ni catalyzed CNTs
dissociates hydrogen at low potential.
explored. XRD, SEM, Raman spectroscopy, FTIR and resistivity
measurements confirms the deposition of graphitic CNTs of low
resistivity (10−6) which is essential for fuel cell electrode. From LSV
and CA curves, Pt + Ni + Sn coated CNTs yield high current densi-
ties. This means Pt + Ni + Sn coated CNTs can replace pure platinum
electrode and reduce initial cost of fuel cell.
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3.6.2. Chronoamperometry
In a chronoamperometry, fuel cell performance shows variation
of current density with respect to time at constant potential i.e.
at reduction potential of hydrogen. Fig. 7(a), (b), (c) and (d) shows
CA curve CNTs catalyzed with Pt–Sn, Pt–Ni, Ni–Sn and Pt–Sn–Ni
respectively. Fig. 7(a) and (b) shows very less current density (1.5
and 4 mA/cm2 respectively) may be because of less catalytic activ-
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to the catalyst cone. A delicate balance must be maintained among
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trode structure. Fig. 7(c) shows CA curve for Ni + Sn catalyzed CNTs.
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ble at 110 mA/cm2. As it is good performance it has been checked
for 3600-s time period. But after 2500 s some degradation in per-
formance is seen and it becomes stable at 100 mA/cm2 (Fig. 7(d)).
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