Synthesis of Pt75Sn25/SnO2/CNT nanoscaled electrode: Low onset potential of ethanol electrooxidation

Article abstract of DOI:10.1016/j.jallcom.2012.11.103

With the objective of lowering the potential oxidation of ethanol at PtSn nanocatalyst, we present the synthesis of free-standing catalyst layer comprising a current collector/carbon nanotubes (catalyst support)/SnO ₂/Pt₇₅Sn₂₅ (catalyst) nanostructured layers, each layer constructed upon the one below it. The CNTs are grown by chemical vapor deposition (CVD), whereas SnO₂ and Pt₇₅Sn ₂₅ are synthesized by pulsed laser deposition and cross-beam laser deposition, respectively. FESEM revealed that Pt₇₅Sn₂₅ nanoparticles assemble into cauliflower-like arrangement. TEM and HR-TEM showed that the Pt₇₅Sn₂₅ layer thickness is of ca. 25 nm with a particle mean diameter of 4.3 nm. It was found that addition of SnO₂ to Pt₇₅Sn₂₅ promotes significantly the oxidation of ethanol at Pt₇₅Sn₂₅ nanoparticles relative to a carbon nanotubes support. Indeed, the electrooxidation of ethanol at CNTs/SnO ₂/Pt₇₅Sn₂₅ electrode starts at about 100 mV negative with respect to that at CNT/Pt₇₅Sn₂₅. This decreased overpotential required to oxidize ethanol is very significant and has profound implications to developing high performing anodes for direct ethanol fuel cells technology.

Full text of DOI:10.1016/j.jallcom.2012.11.103

Accepted Manuscript
Synthesis of Pt75Sn25/SnO2/CNT nanoscaled electrode: low onset potential of
ethanol electrooxidation
Amel Tabet-Aoul, Mohamed Mohamedi
PII:
S0925-8388(12)02087-7
DOI:
http://dx.doi.org/10.1016/j.jallcom.2012.11.103
JALCOM 27338
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9 November 2012
10 November 2012
Please cite this article as: A. Tabet-Aoul, M. Mohamedi, Synthesis of Pt75Sn25/SnO2/CNT nanoscaled electrode: low
onset potential of ethanol electrooxidation, (2012), doi: http://dx.doi.org/10.1016/j.jallcom.2012.11.103
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Synthesis of Pt75Sn25/SnO2/CNT nanoscaled electrode: low onset potential of
ethanol electrooxidation
Amel Tabet-Aoul and Mohamed Mohamedi*
Institut National de la Recherche Scientifique (INRS) -Énergie, Matériaux et
Télécommunications (EMT), 1650 Boulevard Lionel Boulet, Varennes, Québec, J3X 1S2,
Canada
Abstract
With the objective of lowering the potential oxidation of ethanol at PtSn nanocatalyst, we
present the synthesis of free-standing catalyst layer comprising a current collector/carbon
nanotubes (catalyst support)/SnO2/Pt75Sn25 (catalyst) nanostructured layers, each layer
constructed upon the one below it. The CNTs are grown by chemical vapor deposition
(CVD), whereas SnO2 and Pt75Sn25 are synthesized by pulsed laser deposition and cross-
beam laser deposition, respectively. FESEM revealed that Pt75Sn25 nanoparticles assemble
into cauliflower-like arrangement. TEM and HR-TEM showed that the Pt75Sn25 layer
thickness is of ca. 25 nm with a particle mean diameter of 4.3 nm. It was found that addition
of SnO2 to Pt75Sn25 promotes significantly the oxidation of ethanol at Pt75Sn25 nanoparticles
relative to a carbon nanotubes support. Indeed, the electrooxidation of ethanol at
CNTs/SnO2/Pt75Sn25 electrode starts at about 100 mV negative with respect to that at
CNT/Pt75Sn25. This decreased overpotential required to oxidize ethanol is very significant
and has profound implications to developing high performing anodes for direct ethanol fuel
cells technology.
Keywords: Tin dioxide; Platinum; Platinum-tin alloy; Carbon nanotubes; Scaled nanofilms;
Ethanol electro-oxidation
* Corresponding author. Tel.: +1-450-929-8231; fax: +1-450-929-8102.
E-mail address: mohamedi@emt.inrs.ca
1

1. Introduction
Ethanol is particularly interesting as a green, nontoxic and has high theoretical energy density
(8.0 kWh kg-1) [1] and could make direct ethanol fuel cells (DEFCs) advantageous low-emission
power sources. The entire ethanol oxidation reaction (EOR) involves 12 electrons per ethanol
molecule. Nevertheless, ethanol suffers from difficulties associated with its complete oxidation
to CO2, and a performing electrocatalyst must activate C-H, CO, and C-C bonds. The breaking of
the C-C bond is not simply realized at low temperature and has a crucial effect on the fuel cell
efficiency and the theoretical energy yield [2]. For that endeavor, Pt was often tailored with other
metals like Ru [3-4], Re [5-6] or Sn [7-11]. These investigations showed that Pt/Sn catalysts
somehow lower the onset potential of oxidation of ethanol compared to Pt electrodes. In recent
years, we have been involved in the development of PtSn catalysts for the EOR and found that
Pt75Sn25 synthesized onto carbon nanotubes (CNTs) displayed outstanding catalytic activity [12]
compared to Pt75Sn25 deposited onto graphite [13] or to Pt prepared onto CNTs [12].
Nevertheless, at the Pt/Sn catalysts, the complete oxidation of ethanol to CO2 at low potentials
was not attained, instead producing only acetic acid and acetaldehyde as C-C bond cleavage is
probably the most complicated step. The multifunctional requirements of catalysts for the direct
oxidation of ethanol which include the ability to activate C-H, CO and C-C bonds, suggest that
optimum performance will require ternary or even quaternary catalysts that not only oxidize
ethanol at low potentials but also demonstrate higher current density than pure Pt. This may be
achieved by incorporating a third component in the Pt/Sn catalytic structure. Among the
candidates, there is SnO2 which showed very high catalytic properties towards methanol
oxidation when combined with noble metal nanoparticles such as Pt or PtRu [14-16]. Similar
effect has been observed with Pt/SnO2 for the EOR [17-23] who showed improvement in the
2

EOR compared to Pt alone. The SnO2’s role is believed to provide OH to oxidize strongly bound
intermediates, such as CO [17-18].
In our continuous effort to improve the catalytic performance of Pt75Sn25/CNT electrode
with the principal objective of lowering further the onset potential of the EOR, this work was
undertaken to synthesize for the first time and characterize a binderless hierarchically
organized layers comprising a current collector (CP)/CNTs/SnO2/Pt75Sn25 nanostructured
catalyst layer.
The CNTs are grown by chemical vapor deposition (CVD) onto a an electrically conductive
carbon paper substrate (CP) that acts as the current collector, whereas SnO2 and Pt75Sn25 are
synthesized by pulsed laser deposition (PLD) and cross-beam laser deposition (CBLD),
respectively. A multitude of techniques such as field emission scanning electron microscopy
(FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) were
employed to characterize the structural surface properties of the synthesized materials.
Finally, the electrocatalytic activity in ethanol solution over the prepared
CP/CNT/SnO2/Pt75Sn25 structures in comparison to that of non-containing SnO2 (CP/CNT/Pt
and CP/CNT/Pt75Sn25) structures are reported.
2. Experimental section
2.1 Carbon nanotubes synthesis
CNTs were grown at 700oC by CVD using Ni as catalyst deposited by PLD onto a carbon
paper (CP, Toray), acetylene (carbon source), hydrogen and argon (gas carrier) gases at flow
rates of 30, 140 and 100 sccm, respectively. Full details regarding the CNTs synthesis can be
found in our previous publications [12, 24].
3

2.2 SnO2 and Pt75Sn25 synthesis
SnO2 was deposited onto CNTs by PLD by means of a pulsed KrF excimer laser (λ = 248
nm, pulse width = 17 ns, and repetition rate = 50 Hz). Pure SnO2 target (99.99%, Kurt J.
Lesker Co.) was used to deposit SnO2 using a laser fluence of 3.5 J cm-2 and 20000 laser
pulses under 2 Torr of He background pressure.
Pt75Sn25 was deposited onto SnO2 layer by Cross-beam laser deposition (CBLD) (dual beam)
by means of a pulsed KrF excimer laser (λ = 248 nm, pulse width = 17 ns, and repetition rate
= 50 Hz). Pure platinum (99.99%, Kurt J. Lesker Co.) and tin (99.998-99.999%) (Kurt J.
Lesker Co.) targets were used using a laser fluence of 4 J cm-2 and 50000 laser pulses under
2 Torr of He background pressure.
During SnO2 or Pt75Sn25 deposition, the targets are moved continuously across the laser beam
(via a dual rotation and translation motion) to obtain a uniform ablation over the entire
surface of the target. Prior to deposition, the chamber was evacuated by means of a turbo
pump (4x10-5 Torr). Helium was then introduced in the deposited chamber. In all cases, the
substrate-to-target distance was fixed at 5 cm and all experiments were performed at room
temperature. Full details about the PLD technique can be found elsewhere [25-26].
2.3 Material characterization
The surface morphology of the as-prepared samples was examined by means of a field
emission scanning electron microscope FESEM, JEOL-JSM-7401F apparatus and a
transmission electron microscopy (JEOL-JEM-2100F operating at 200 kV).
The crystalline structure of all samples was determined by X-ray diffraction (XRD) using
Bruker D8 Advance diffractometer equipped with a Cu K source. The diffractometer was
operated at 40 kV and 40 mA. All diffractograms were acquired in the Grazing Incidence
4

Diffraction (GID) scan mode with a low incident angle of 2° and a 2θ angular step size of
0.04° with an acquisition time of 4s per step.
X-ray photoelectron spectroscopy (XPS) measurements were carried out via a VG Escalab
220i-XL set with an Al K source (1486.6 eV). The anode was operated at 10 kV and 20
mA. The pass energy of the analyzer was fixed at 20 eV. All samples were analyzed with a
spot size of 250×1000 μm located approximately in the center of the sample. A survey
spectra ranging from 0 to 1000 eV was first acquired, and then higher resolution multiplex
scan spectra (Pt 4f, Sn 3d, O1s, and C1s core levels) were obtained. The C1s core level peak
at 284.6 eV, resulting from hydrocarbon contaminants at the surface, was used as an internal
reference. All spectra have been recalibrated with respect to the C1s core level peak of
adventitious carbon contamination.
2.4 Electrochemical measurements
The electrocatalytic properties were studied by cyclic voltammetry (CV) in a mixture of 1 M
C2H5OH+0.5 M H2SO4 deaerated solutions. Prior to the electrochemical measurements, the
surface of the working electrode was cleaned electrochemically by potential cycling in 0.5 M
H2SO4. Before each test, dissolved oxygen was removed from the solution by bubbling argon for
20 to 30 min. All electrochemical measurements were conducted at room temperature using a
three compartments electrochemical cell with the reference electrode and counter electrode being
an Ag/AgCl, 3M NaCl and a platinum coil, respectively. The reference electrode was separated
from the analyte solution by a Luggin capillary that is very close to the working electrode to
minimize the ohmic drop. Data acquisition was conducted with a potentiostat/galvanostat
Autolab from EcoChemie.
5

3. Results and discussion
3.1 Characterization
FESEM and TEM images of CP/CNT, CP/CNT/SnO2 and CP/CNT/Pt75Sn25 are reported
in references [12], [23], and [12] respectively. Fig. 1 shows FESEM images with increasing
magnifications of Pt75Sn25 layer prepared onto SnO2/CNT. The images reveal that Pt75Sn25
particles assemble into cauliflower-like arrangements. This is similar to Pt75Sn25 deposited
onto CNTs [12] and indicates that the beneath SnO2 layer had no effect on the morphology of
Pt75Sn25. The TEM and HR-TEM analyses of the Pt75Sn25 layer deposited onto SnO2/CNT
are reported in Fig. 2a and Fig. 2b, respectively, which show that the Pt75Sn25 layer is made
of high density of fine interconnected nanoparticles. An estimation of the Pt75Sn25 layer
thickness gives a value of ca. 25 nm as depicted in Fig. 2b. The particle size distribution is
very narrow ranging between 3 and 6 nm with a predominance around 4 nm and the average
particle diameter of 4.3 ± 0.6 nm (Fig. 2c) which is close to 4.6 nm of Pt75Sn25 deposited
onto CNTs [12].
XRD profile of the Pt75Sn25/SnO2/CNT/ layers is very complex and shows the presence of
several strong and weak diffraction peaks corresponding to Sn, SnO2, C and Pt (Fig. 3). In
the case of Pt, the most intense diffraction peaks are the (111) and (200) peaks that occur at
2θ) 39.8° and 46.4°, respectively. In addition, it should be noted that the Sn diffraction peaks
do not interfere with the Pt(111) peak. Thus, the Pt(111) diffraction peak was chosen to
evaluate the lattice parameter of the fcc phase. The Pt lattice parameter gives a 3.942 Å fcc
lattice constant larger than that of Pt (3.923 Å) deposited onto CNTs. The lattice parameter
for the Pt75Sn25 catalysts is larger than that of pure Pt, due to a lattice expansion after
alloying.
6

Next, the surface constituent phases of the as-synthesized Pt75Sn25/SnO2/CNT/ layers was
further confirmed by XPS results shown in Fig. 4. The XPS survey scan (not shown) presented
peaks only for C, O, Pt and Sn with a surface composition of 44.05, 27.26, 23.32 and 5.37 at%,
respectively. The high-resolution XPS Pt 4f core level spectrum exhibits two peaks whose
maximum intensities are positioned between 71.27 and 74.61 eV binding energy range (Fig. 4a)
in accordance with Pt being in a metallic state [27]. The high-resolution XPS Sn 3d and O1s core
level spectra are shown in Fig. 4b and 4c, respectively. The Sn 3d core level spectra could be
deconvoluted into two components, which are ascribed to elemental Sn(0) and Sn (+4) oxide.
The BE of the elemental Sn(0) and Sn(+4) oxide are 485.6 eV and 486. 4 eV, respectively [28].
The O 1s spectrum is composed of one component positioned at around 531.2 eV, which is
attributed to O in the SnO2 phase.
3.2. Electrocatalytic properties
Fig. 5 shows linear scan voltammograms (LSVs) with 5 mV s-1 potential scan rate recorded at
CNT/SnO2/Pt75Sn25 compared to CNT/Pt75Sn25 and CNT/Pt in a 0.5 M H2SO4+ 1 M ethanol
solution. All the LSVs exhibit well defined characteristic ethanol oxidation waves in agreement
with the literature [29-31]. From Fig. 5, it can be seen clearly that higher electrocatalytic activity
towards ethanol oxidation is obtained with the CNT/Pt75Sn25 and CNT/SnO2/Pt75Sn25 electrodes
relative to CNT/Pt electrode. It can be however further noticed that addition of SnO2 to Pt75Sn25
did not affect the current density of the later, i.e., both CNT/Pt75Sn25 and CNT/SnO2/Pt75Sn25
electrodes delivered about 40 mA/cm2 of peak current density, which is high at such low
potential scan rate.
7

Further important parameter towards the development of anode catalyst is the onset potential
of ethanol oxidation reaction, which is defined as the potential value at which the anodic current
(current due to ethanol oxidation) starts to flow. The insert of Fig. 5 shows that the onset
potential delivered by CNT/SnO2/Pt75Sn25 catalyst is about 170 mV and 100 mV negative with
respect to those of CNT/Pt and CNT/Pt75Sn25, respectively. It means that the electron-transfer
kinetics has been greatly fostered by the addition of SnO2 to CNT/Pt75Sn25 catalyst. This
decreased overpotential required to oxidize ethanol is very significant and has profound
implications to developing high performing anodes for direct ethanol fuel cells technology.
In order to understand the effect of SnO2 in the electrooxidation of ethanol, Fig. 6 resumes
the onset potential extracted from CVs recorded with 5 mV/s potential scan rate for ethanol
oxidation at SnO2-containing electrodes compared with these of SnO2-free containing electrodes.
A first observation shows that in all cases, the electrodes containing the CNTs exhibit onset
potentials lower than those containing smooth graphite (Gr) as the catalyst support. This is due to
the high electrical properties of CNTs which has fostered the electron-transfer kinetics of ethanol
at Pt and PtSn catalysts. Addition of Sn to Pt provides further enhancement in the kinetics as
demonstrated by the decreased overpotential required to oxidize ethanol, i.e., the onset potential
at CNT/PtSn is 70 mV negative than that recorded at CNT/Pt electrode. This results is in line
with those reported in the literature which showed that PtSn catalysts are the most active binary
systems in acidic media. It was suggested that the significant enhancement provided by alloying
Pt with Sn atoms can be attributed to a combination of both bifunctional and ligand (electronic
effects) [32-33]. However, these systems promote the incomplete oxidation of ethanol
acetaldehyde and acetic acid rather than its complete oxidation to carbon dioxide [33-35].
8

Next, the addition of SnO2 to CNT/Pt electrode further affects the onset potential. Indeed, the
onset potential is shifted negatively by 20 mV at CNT/SnO2/Pt electrode. These results clearly
indicate that SnO2 facilitates the decomposition of ethanol and/or has a synergetic/cocatalytic
effect on Pt in the oxidation of ethanol as it has been proposed in the literature [36]. As to PtSn
catalyst, the addition of SnO2 in the support for PtSn nanoparticles greatly enhanced their
activity for ethanol oxidation with the greatest effect seen for the CNT/SnO2/PtSn electrode (Fig.
6).
4. Conclusions
Nanostructured catalysts are under study to enhance the performance in the low-potential
regions for direct alcohol fuel cells applications. We synthesize for the first time a binderless
hierarchically organized layers comprising a current collector (CP)/CNTs/SnO2/Pt75Sn25
nanostructured electrode with the objective of lowering the onset potential of ethanol
oxidation reaction.
Thus CNTs/SnO2/Pt75Sn25 electrode developed here responded well when used as an
anode. The results presented here show unambiguously that SnO2 supports promote
significantly the oxidation of ethanol at PtSn nanoparticles relative to a carbon nanotubes
support. Indeed, it was found that the electrooxidation of ethanol at CNTs/SnO2/Pt75Sn25
electrode starts at about 100 mV negative with respect to that at CNT/Pt75Sn25. This
decreased overpotential required to oxidize ethanol is very benefical to anodes for direct
ethanol fuel cells technology.
It is also very important to point out the effect of the catalyst support which played a non-
trivial role in the electrode performance. Indeed, our results show that for a given catalyst,
the electroactivities are higher with the carbon nanotubes than with graphite.
9

Work is underway to identify reaction products and quantify CO2 formation in situ
spectroscopic methods such as Differential Electrochemical Mass Spectroscopy (DEMS). Such
studies will further shed light on the promotional effect of SnO2 in our developed catalyst
structures with respect to ethanol electro-oxidation reaction mechanism at low potentials.
Acknowledgements
This work was supported by the Natural Sciences Engineering Research Council of Canada
(NSERC), the Centre Québécois sur les Matériaux Fonctionnels (CQMF) and the FQRNT.
10

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Figures captions
Fig. 1. FESEM images with increasing magnification of PLD-synthesized Pt75Sn25 layer onto
SnO2.
Fig. 2. (a) and (b) TEM and HR-TEM images of Pt75Sn25 and (c) represents Pt75Sn25 size
distribution histogram.
Fig. 3. XRD patterns of CNT/SnO2/Pt75Sn25.
Fig. 4. High resolution X-ray photoelectron spectra of PLD-synthesized SnO2 film onto CNTs
(a) Pt 4f core level, (b) Sn 3d core level peaks of Sn and (c) O1s core level peak of oxygen.
Fig. 5. Linear scan voltammetry in 0.5 M H2SO4+1 M C2H5OH-argon purged solution with the
potential scan rate of 5 mV/s recorded at CNT/Pt (black curve), CNT/Pt75Sn25 (red curve) and
CNT/SnO2/Pt75Sn25 (blue curve) electrodes. Insert shows the onset potential of ethanol
electrooxidation.
Fig. 6. Comparative onset potential of ethanol oxidation extracted from voltammograms
recorded in 0.5 M H2SO4+1 M C2H5OH-argon purged solution with the potential scan rate of 5
mV s-1. The nature of the electrodes are labeled on the figure.
14


A pulsed laser synthesis is used for the deposition of Pt, SnO2 and PtSn alloy thin films
onto carbon nanotubes.


These nanoscaled materials were characterized by FESEM, TEM, XRD and XPS.
Enhanced electrocatalytic properties towards ethanol oxidation.

Figure 1

Figure 2

Figure 3

Figure 4a

Figure 4b

Figure 4c

Figure 5

Figure 6