A polyoxometalate-assisted approach for synthesis of Pd nanoparticles on graphene nanosheets: Synergistic behaviour for enhanced electrocatalytic activity

Article abstract of DOI:10.1039/c5ra01084e

A polyoxometalate (POM) assisted approach has been employed to prepare a nanohybrid of Pd nanoparticles (PdNPs) and graphene nanosheets (GNSs). The Keggin-type POM, phosphomolybdic acid (PMo₁₂), was applied to serve as both reducing and stabilising agent. The as-prepared nanohybrid (Pd/PMo₁₂/GNSs) was comprehensively characterised using transmission electron microscopy and X-ray diffraction analysis. The synergistic behaviour of PdNPs, PMo₁₂ and GNSs in the nanohybrid leads to elevated electrocatalytic property for ethanol oxidation. Moreover, the Pd/PMo₁₂/GNSs nanohybrid was activated by applying a sufficiently negative potential which plays a key role in promoting the electrocatalytic activity. The activated catalyst presents a superior performance towards ethanol electrooxidation reaction and shows better tolerance to poisoning species compared to Pd and Pt nanoparticles. The outstanding electrocatalytic activity of the tri-component (Pd/PMo₁₂/GNSs) nanohybrid is discussed with relevance to its application in direct alcohol fuel cells (DAFCs).

Full text of DOI:10.1039/c5ra01084e

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A polyoxometalate-assisted approach for synthesis
of Pd nanoparticles on graphene nanosheets:
synergistic behaviour for enhanced electrocatalytic
activity†
Cite this: RSC Adv., 2015, 5, 24319
a
a
a
b
Sara Khadempir, Ali Ahmadpour,* Mohammad T. Hamed Mosavian, Narges Ashraf,
c
d
de
Fatemeh F. Bamoharram, Scott G. Mitchell and Jesus M. de la Fuente
´
A polyoxometalate (POM) assisted approach has been employed to prepare a nanohybrid of Pd
nanoparticles (PdNPs) and graphene nanosheets (GNSs). The Keggin-type POM, phosphomolybdic acid
(PMo12), was applied to serve as both reducing and stabilising agent. The as-prepared nanohybrid (Pd/
PMo12/GNSs) was comprehensively characterised using transmission electron microscopy and X-ray
diffraction analysis. The synergistic behaviour of PdNPs, PMo12 and GNSs in the nanohybrid leads to
elevated electrocatalytic property for ethanol oxidation. Moreover, the Pd/PMo12/GNSs nanohybrid was
activated by applying a sufficiently negative potential which plays a key role in promoting the
electrocatalytic activity. The activated catalyst presents a superior performance towards ethanol
electrooxidation reaction and shows better tolerance to poisoning species compared to Pd and Pt
nanoparticles. The outstanding electrocatalytic activity of the tri-component (Pd/PMo12/GNSs)
nanohybrid is discussed with relevance to its application in direct alcohol fuel cells (DAFCs).
Received 19th January 2015
Accepted 23rd February 2015
DOI: 10.1039/c5ra01084e
www.rsc.org/advances
can be improved signicantly in alkaline media; therefore, the
quest for a suitable Pt-free catalyst to be used in this media is
Introduction
1,11–13
Direct alcohol fuel cells (DAFCs) are regarded as attractive still progressing.
future power sources due to their high energy density, low
pollutant emission, ease of handling, portable applications and as an alternative choice. Application of palladium nanoparticles
As a result, Pd-based catalysts have recently been introduced
1–4
low operating temperature. Various alcohols such as meth- (PdNPs) – as opposed to bulk Pd metal – allows synthesis of a
anol, ethanol, ethylene glycol, glycerol and so forth have been highly efficient catalyst using only a very small amount of this
5
–8
widely investigated for their potential applications as fuels.
material. PdNPs are one of the most useful catalysts for oxida-
Among the different types of DAFCs, there is a special interest in tion of small organic molecules and they have reportedly higher
direct ethanol fuel cells, since ethanol is not toxic and it can electrocatalytic activity towards ethanol oxidation compared
4
,14–16
easily be prepared in large amounts by fermentation of sugar- with that of PtNPs; especially in alkaline media.
Besides,
9–11
containing raw materials.
Pt-based catalysts are employed extensively for oxidation of more than that of Pt.
the abundance of Pd on the earth crust is at least y times
11,15
Furthermore, the application of PdNP
alcohols in DAFCs; however, the high cost, nite resources and hybrids is of considerable interest, since the dispersion of
poisoning of Pt catalysts – particularly in alkaline media – PdNPs in a proper conductive support such as carbon would
restrict commercialisation of DAFCs. The efficiency of fuel cells minimise the aggregation of PdNPs; while at the same time
maximise their electrocatalytic activity.
Graphene is a two-dimensional carbon material that shows
Department of Chemical Engineering, Faculty of Engineering, Ferdowsi University of some exceptional properties such as high electrical conduc-
a
Mashhad, Mashhad, Iran. E-mail: ahmadpour@um.ac.ir
b
tivity, large specic surface area and superior electron mobility,
Department of Chemistry, Faculty of Sciences, Ferdowsi University of Mashhad,
making it an ideal support material for catalytically active metal
Mashhad, Iran
17,18
NPs.
Although there are several approaches to load metal
c
Department of Chemistry, Mashhad Branch, Islamic Azad University, Mashhad, Iran
d
NPs on graphene nanosheets (GNSs), developing simple and
Instituto de Nanociencia de Aragon (INA), Universidad de Zaragoza, 50018-Zaragoza,
effective methods that produce well-dispersed NPs without
Instituto de Ciencia de Materiales de Arag ´o n (ICMA), CSIC-Universidad de Zaragoza, using hazardous/toxic organic reagents are most desirable.
Spain
e
C/Pedro Cerbuna 12, 50009-Zaragoza, Spain
Polyoxometalates (POMs) are early-row transition metal
oxygen anionic clusters with a remarkable and diverse range of
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra01084e
1
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19–22
redox and photo-electrochemical properties.
POMs have to be stable for several months. The resulting homogenous
been utilised in the preparation of metal NPs/POM/GNSs brown GO was dried in air.
nanohybrids, since they can act as both reducing and stabilis-
2
3–25
ing molecules for metal NPs.
reported the synthesis of Au, Pd and PtNPs on GNSs using
phosphotungstic acid, H 40, for biosensing applications
and the electrooxidation of formic acid and methanol in acidic
Liu and co-workers have
Synthesis of Pd/PMo /GNSs
1
2
3
PW12O
2
In a typical synthesis, 3.6 mg PdCl and 28.4 mg PMo12 were
26–29
added to 100 mL DI water and completely dissolved. Then, 9.6
mL of the as-prepared solution was transformed to a spectro-
photometer cell, 460 mg GO was added to the mixture and
ultrasonicated for 20 min. Thereaer, the mixture was deoxy-
media, respectively.
Herein, we report how the Keggin-type POM phosphomo-
lybdic acid (H PMo O , herein PMo ) was employed to
3
12 40
12
prepare well-dispersed POM-stabilised PdNPs on GNSs. The
electrocatalytic activity of the tri-component nanohybrid was
then tested and was found to be an efficient catalyst for ethanol
electrooxidation. Also, an activation step has been proposed to
enhance the electrocatalytic activity of the tri-component Pd/
PMo12/GNSs nanohybrid. In summary, this easy-to-prepare
nanohybrid shows to be a promising catalyst for ethanol elec-
trooxidation in alkaline media.
2
genated with N gas, and 0.4 mL isopropyl alcohol were added
to the mixture and it was irradiated under a UV lamp (125 W
high pressure mercury vapour lamp) for 2 h under continuous
stirring. The reaction was performed at a constant room
temperature using water circulating system. Aer the photo-
catalytic Pd reduction process, the mixture was centrifuged at
ꢀ
1
4 000 rpm for 10 min, and then dried at 50 C overnight. By
adjusting the amount of GO, different loadings of PdNPs on
GNSs (15, 30 and 45 wt%) were obtained. The Pd/PMo12/CNTs
nanohybrid was synthesised with the same procedure by
substituting of GO with CNTs. The Pd/GNSs were prepared
Experimental
Chemicals and reagents
32
using a procedure from the literature.
All chemicals were of analytical grade purity and used as
received without any purication. Palladium(II) chloride
(
PdCl ), isopropyl alcohol, sodium hydroxide (NaOH), potas-
2
Apparatus
sium permanganate (KMnO ), hydrogen peroxide (H O ) and
4
2 2
graphite powder (<50 mm) were purchased from Merck (Darm- High-resolution transmission electron microscopy (HRTEM)
stadt, Germany). Functionalised Multi Walled Carbon Nano- and high-angle annular dark eld-scanning transmission elec-
tubes (CNTs, >95%, OD: 5–15 nm, US NANO). H
3
PMo12O
40 tron microscopy (HAADF-STEM) images were acquired using a
(PMo12) was purchased from Sigma-Aldrich. Phosphate buffer Tecnai S-Twin30, 300 keV, GIF-TRIDIEM. Energy-dispersive X-
solution (0.1 M) was prepared by dissolving the proper amount ray spectroscopy (EDS) data were obtained on the same instru-
of NaH PO $2H O (Merck, Darmstadt, Germany) in deionised ment. Scanning electron microscopy (SEM) images were
2
4
2
water and adjusting the pH to 8.5 by 0.1 M NaOH solution. acquired using a eld emission SEM Inspect F50 with an EDX
Deionised (DI) water (18.3 MU cm) was used throughout the system INCA PentaFETx3 (FEI Company, Eindhoven, Nether-
entire work.
lands) in an energy range between 0–30 keV. TEM observation
was obtained on a CM120, Philips transmission electron
microscope and X-ray diffraction (XRD) analysis was carried out
using the X'Pert Pro MPD diffractometer.
All the electrochemical experiments were carried out using a
m-Autolab type III electrochemical workstation with a conven-
tional three-electrode setup. A glassy carbon electrode (GC, 2
mm in diameter, Azar electrode Co.) was used as working
electrode. A platinum wire and a saturated Ag/AgCl electrode
Synthesis of graphene oxide (GO)
GO was prepared by pre-oxidation of graphite which was fol-
30,31
lowed by post-oxidation with Hummers method.
In the pre-oxidation step, the graphite powder (2.0 g) was
added to the mixed solution of H SO 99.97% (20 mL), K
1.0 g) and P (1.0 g). The mixture was reacting at 80 C for
.5 h; aer which, it was cooled and diluted with 1 L of DI water
2
4
2 2 8
S O
ꢀ
(
4
2 5
O
(both from Azar electrode Co.) were used as counter and refer-
and le overnight. Then, it was ltered, washed and dried in air.
In the post-oxidation step, the pre-oxidised graphite (0.5 g) was
dispersed into a chilled mixture of NaNO (0.5 g) and H SO (23
ence electrodes, respectively.
3
2
4
mL) in an ice bath. Thereaer, KMnO
4
(10 g) was added slowly
Electrochemical experiments
while the solution was stirring and the temperature maintained
ꢀ
ꢀ
down to 20 C. The mixture was kept at 35 C for 2 h, aer which The modied electrode was prepared by the following proce-
DI water (140 mL) was added with controlling the temperature dures: (a) GC electrode was polished thoroughly by 0.5 mm
ꢀ
below 50 C to give a dark brown solution. In order to stop the alumina slurry and was washed with water and acetone; (b) a
ꢁ
1
reaction, 2 mL of H O were added and the reaction was allowed catalyst solution (1.0 mg mL ) was prepared by sonicating the
2
2
to settle overnight. Aer centrifuging and washing with hydro- as-prepared catalyst and proper amount of DI water; and (c) 2 mL
chloric acid and DI water, the resulting solid was air-dried. DI of catalyst solution was dropped onto the surface of the pre-
water was then added and the GO was resuspended using polished GC electrode and dried in air. All experiments were
ultrasonication. It is notable that the GO dispersion was found performed at room temperature.
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Fig. 1 shows typical TEM images of the 30% Pd/PMo12/GNSs
with different magnications. From Fig. 1a it can be observed
that the GNSs were covered homogeneously with PdNPs. From
Results and discussion
Structural analysis
The morphology and microstructure of the Pd/PMo12/GNSs Fig. 1b, it is seen that small well-dened spherical PdNPs are
nanohybrids were investigated by TEM and SEM.
dispersed homogeneously on the surface of the GNSs, with the
Fig. 1 (a and b) TEM images of Pd/PMo12/GNSs with different magnifications; (c) HAADF-STEM image of 30% Pd/PMo12/GNSs (d) the corre-
sponding EDS analysis.
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exception of some large PdNPs. It looks like there may be two intensity indicating high crystallinity of Pd. Besides, since there
size dispersions of the PdNPs. It is noted that the PdNPs pref- are no noticeable diffraction peaks in the measurement that
erentially attached to the surfaces of GNSs, rather than in the correspond to the crystallised PMo12 or Mo oxides, it can be
regions without GNSs (Fig. S1†). A typical HAADF-STEM image concluded that the PMo12 is adsorbed in a highly dispersed
35
(
Fig. 1c) and coupled EDS analysis (Fig. 1d) showed that strong manner on the catalyst with no agglomeration.
molybdenum peaks exist except the palladium peaks, conrm-
ing the existence of the PMo12 in the hybrid. The as-prepared Electrochemical characterisation of Pd/PMo /GNSs nano-
1
2
nanohybrid consists mainly of carbon with an amount of hybrid
oxygen from PMo12 frame and probably due to the presence of
To investigate the electrochemical performance of Pd/PMo12
/
traced unreduced oxygen-containing groups on the surface of
GNSs. The strong peaks of Cu originate from the copper TEM
grid. Fig. S2 and S3† represent the TEM and HAADF-STEM
images of samples with 15 and 45% Pd loadings. These
images indicate good contrast for the PdNPs in 15% loading.
For 45% loading a signicant increase in the amount of PdNPs
on GNSs can be observed; these particles are much more
aggregated (i.e. less discrete particles) and they appear to be of
more irregular shape.
GNSs nanohybrid and its catalytic behaviour towards electro-
oxidation of ethanol, cyclic voltammetry (CV) experiments were
performed.
Electrocatalytic oxidation of ethanol
Electrocatalytic oxidation of ethanol was studied at the pure
PdNPs and the Pd/PMo12/GNSs modied GC with different
loadings (15, 25, 30 and 45%) of Pd on GNSs in the solution of
1.0 M NaOH and 1.0 M ethanol at room temperature (Fig. 3).
SEM images of samples with different loadings of PdNPs
Each CV curve was characterised by two well-dened anodic
(Fig. S4†) show the dispersion of PdNPs on the surface of
(oxidation) current peaks. The peak in the forward scan direc-
transparent thin and sleek crumpled graphene sheets and
conrm the TEM results.
tion was assigned to the oxidation of freshly chemisorbed
ethanol and the peak in reverse scan direction corresponds to
the oxidation of residual carbonaceous species formed in the
The XRD patterns of the GO and 30% Pd/PMo12/GNSs are
shown in Fig. 2. The graphite pattern (not shown) reveals a
36
ꢀ
forward scan. As shown in Fig. 3, the electrocatalytic oxidation
of the as-prepared catalysts is enhanced by increasing the
PdNPs loading up to 30% and then declined with further
increasing in loading due to the agglomerations of PdNPs. As a
result, 30% loading of PdNPs on GNSs and CNTs were used for
further studies.
sharp diffraction peak at 2q ¼ 26.7 corresponding to (002)
˚
planes with d-spacing of 3.34 A. As shown in the XRD pattern of
GO, the disappearance of the graphite diffraction peak at about
ꢀ
ꢀ
2
6.7 and appearance of new peak in 9.27 corresponding to the
˚
interlayer spacing of 9.53 A revealed the successful oxidation of
the graphite. In addition, the increases in d value of GO
compared to graphite shows that the distance between the
carbon sheets has increased due to the insertion of interplanar
oxygen-containing functional groups. Aer the reduction of GO
The electrocatalytic oxidation of ethanol was studied by Pd/
PMo /GNSs and Pd/PMo /CNTs (Fig. 4) and the results are
1
2
12
shown in Table 1.
It should be noted that ethanol electrooxidation peak in
and PdCl2 by PMo -assisted photoreduction process, the
12
ꢀ
back-ward scan was observed at ꢁ0.4 V for Pd/PMo /GNSs,
1
2
disappearance of the peak at 2q ¼ 9.27 conrms that oxygen
which indeed occurs at the potential near the reduction peak
potential for oxygenated species on Pd surface (i.e. ꢁ0.4 V) as
shown by the insert in Fig. 4.
groups have been removed and GO has been aked and reduced
3
3
ꢀ
to GNSs; also the strong diffraction peaks at 2q ¼ 40.02 ,
ꢀ
ꢀ
4
6.59 and 68.08 indicate PdNPs formation and assigned to the
(
111), (200) and (220) crystalline planes of the face-centered
34
cubic (fcc) structures Pd (JCPDS no. 46-1043), respectively.
These diffraction peaks were observed to be sharp with a high
Fig. 3 Cyclic voltammograms (CV) in 1.0 M NaOH and 1.0 M ethanol
ꢁ
1
solution, scan rate: 50 mV s for Pd/PMo12/GNSs at different loading
Fig. 2 XRD pattern of GO (bottom) and 30% Pd/PMo12/GNSs (top).
and for pure PdNPs.
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affinity of Pd towards hydrogen adsorption, it is difficult to
determine a monolayer of adsorbed hydrogen. Therefore, ECSA
is measured through the reduction peak of palladium oxide in
4
NaOH solution. The CV of the as-prepared catalyst in N -satu-
2
rated 1.0 M NaOH solution is shown in Fig. 5.
The ECSA is calculated from eqn (1):
Q
ECSA ¼
(1)
Sl
where “S” is the proportionally constant used to relate charge
with area, “l” is the catalyst loading in gram, and “Q” is the
columbic charge for the reduction of palladium oxide, which
was determined by the integration of area under the oxide
ꢁ2
reduction peak. A charge value of 4.05 cm was assumed for
the reduction of palladium oxide monolayer The ECSA not only
Fig. 4 Cyclic voltammograms (CV) in 1.0 M NaOH with 1.0 M ethanol, provides the data about active surface area but also gives
ꢁ
1
scan rate: 50 mV s for Pd/PMo12/GNSs (solid line) and for Pd/PMo12
/
important information about the selection of electrocatalytic
support. The calculated values of ECSA for Pd/PMo /GNSs and
CNTs (dashed line). Insert: (CV) curve in the absence of ethanol.
1
2
ꢁ1
2
Pd/PMo /CNTs catalysts were 38.67 and 26.3 m g , respec-
1
2
tively. The results conrm the better performance of GNSs
support compared to CNTs.
f
The forward peak mass current density (J ) for the Pd/PMo12/
GNSs is considerably higher than that of the Pd/PMo12/CNTs
which shows the superior performance of the GNSs as support.
In comparison with CNTs, GNSs have a larger specic surface
area, thereby providing much more accessible sites for uniform
dispersion of PdNPs and improving catalytic activity.
Electrochemical behaviour of immobilized PMo12
To prove the presence of PMo12 in the structure of the as-
prepared catalyst, the CVs of Pd/PMo12/GNSs and Pd/GNSs
were recorded in N -saturated 1.0 M H SO solution which
2 2 4
have been indicated in Fig. 6. Comparison of the CVs of Pd/
PMo12/GNSs and Pd/GNSs shows that oxidation and reduction
peaks appearing at about 0.00 and 0.1 V could be attributed to
In addition, the ratio of the forward oxidation peak current
(
I
f
) to the backward oxidation peak current (I
important criterion for comparing the performance of the
catalysts. The higher value of I /I , implies that the catalyst has
b f b
), I /I , is an
f
b
greater catalytic activity and better tolerance to carbonaceous
species accumulation. The I /I for Pd/PMo /GNSs (i.e. 4.1) is
f
b
12
higher than that of Pd/PMo /CNTs (3.7) and even signicantly
1
2
4,12,37
greater than the values reported in the literatures.
The other considerable evidence is that the onset potential
for Pd/PMo12/GNSs is more negative than Pd/PMo12/CNTs. It
reveals that electrooxidation of ethanol with GNSs support is
more facile than CNTs.
For comparison, the activity of graphene oxide toward
ethanol electrooxidation is presented in Fig. S5.† As shown in
this gure, the graphene oxide has no catalytic activity.
Electrochemical active surface area (ECSA)
It is well known that electrochemical active surface area (ECSA)
is measured using cyclic voltammogram of the catalysts in
NaOH or H SO solution. In the case of Pt-based catalysts, the
2 4
charge of a monolayer of adsorbed hydrogen is used to evaluate
the ECSA. However, for Pd-based catalysts, due to the strong solution, scan rate: 50 mV s
Fig. 5 Cyclic voltammogram (CV) of Pd/PMo12/GNSs in 1.0 M NaOH
ꢁ
1
.
Table 1 Ethanol electrooxidation properties of Pd/PMo12/GNSs and Pd/PMo12/CNTs
a
Pdꢁ1
ꢁ2
b
f
Pdꢁ1
ꢁ2
c
b
E
op (V)
J
f
(mA mg
cm
)
E
(V)
J
b
(mA mg
cm
)
E
(V)
f b
I /I
Pd/PMo12/GNSs
Pd/PMo12/CNTs
ꢁ0.68
ꢁ0.57
31 668
11 154
ꢁ0.275
ꢁ0.223
7722
2967
ꢁ0.409
ꢁ0.392
4.1
3.7
a
b
c
Eop ¼ onset potential.
E
f
¼ forward peak current potential.
E
b
¼ backward peak current potential.
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Fig. 6 Cyclic voltammograms (CV) of Pd/PMo12/GNSs (solid line) and
Pd/GNSs (dashed line) in 1.0 M H SO solution, scan rate: 50 mV s .
2 4
ꢁ1
Fig. 8 Cyclic voltammograms (CV) of the Pd/PMo12/GNSs in 1.0 M
NaOH solution containing different fuels: ethanol, acetaldehyde and
potassium acetate (scan rate: 50 mV s ).
ꢁ1
the presence of PMo12. Obviously, the peaks at potentials more
negative than ꢁ0.05 V correspond to hydrogen evolution,
proton discharge.
40
3
involves the formation of Pd-(CH CO)ads upon C–H cleavage.
The successive reactions can be expressed as follows:
2 4
Furthermore, the CVs of Pd/PMo12/GNSs in 1.0 M H SO
solution at different scan rates and the insert shows that the ip,a
and the ip,c for the attributed oxidation and reduction peaks of
PMo , are linear versus scan rates, indicating a surface-
Pd + CH CH OH 4 Pd-(CH CH OH)
ads
(2)
3
2
3
2
ꢁ
Pd-(CH
3
CH
2
OH)ads + 3OH
1
2
ꢁ
controlled redox process for PMo12 adsorbed on the surface of
3 2
/ Pd-(CH CO)ads + 3H O + 3e (3)
38,39
PdNPs and GNSs (Fig. 7).
Pd-(CH CO) + Pd-OHads / Pd-CH COOH + Pd
(4)
(5)
To investigate the mechanism of the electrocatalytic oxida-
tion of ethanol, cyclic voltammograms of the prepared catalyst
were recorded in three separate alkaline (1.0 M NaOH) solutions
containing ethanol, acetaldehyde and potassium acetate
3
ads
3
ꢁ ꢁ
3 2
COOH + OH / Pd + CH COO + H O
Pd-CH
3
(
Fig. 8). No oxidation current was observable in the acetate
It is noteworthy that the reaction of the Pd-(CH CO) with
the activated Pd-OHads species is the rate determining step, so
the efficiency of the reaction is determined by the degree of
3
CH COads and OHads coverage. Formation of Pd-OHads would
3
ads
solution; while acetaldehyde showed a considerable oxidation
peak. This fact implies that the acetate is the nal product and
acetaldehyde is an active intermediate which is consistent with
the mechanism reported previously in the literatures.
Based on this mechanism, ethanol electrooxidation on Pd
36,40,41
take place through the adsorption of hydroxyl groups at the
surface of PdNPs at negative potentials as shown by the
41
following equation.
ꢁ
ꢁ
Pd + OH / Pd-OHads + e
(6)
ꢁ
This implies that the mass transfer of OH to the surface of
PdNPs plays an essential role in the efficiency of Pd-OHads
ꢁ
formation. Electrolytic generation of OH species adjacent to
the surface of PdNPs would promote the reaction efficiency.
Electrolysis of water in basic media results in the formation of
ꢁ
OH species which are readily adsorbed at the surface of
neighboring PdNPs. To verify this concept, an activation step
has been performed to the surface of electrode by applying a
constant potential step of ꢁ1.5 V for 300 s duration in phos-
phate buffer solution (pH ¼ 8.50). As a result, the following
reaction would take place at the electrode–electrolyte interface
and the Pd-OHads sites create at the surface of PdNPs.
ꢁ
ꢁ
Fig. 7 Cyclic voltammograms (CV) of Pd/PMo12/GNSs in 1.0 M H
solution at different scan rate. Insert: corresponding dependence of
the peaks for PMo12
2
SO
4
2H
O + 2e / H
(g) + 2OH
(7)
2
2
.
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Table 2 Effect of activation step on ethanol electrooxidation
Pdꢁ
1
ꢁ2
Pdꢁ1
ꢁ2
f b
I /I
Eop (V)
J
f
(mA mg
cm
)
E
f
(V)
J
b
(mA mg
cm
)
E
b
(V)
Activated Pd/PMo12/GNSs
Pd/PMo12/GNSs
ꢁ0.68
ꢁ0.53
31 668
2341
ꢁ0.275
ꢁ0.210
7722
836
ꢁ0.409
ꢁ0.300
4.1
2.8
Table 2 shows the effect of activation step on the perfor-
mance of the catalyst towards the catalytic electrooxidation of
ethanol. Comparison of the CVs indicates the excellent perfor-
mance of the activated Pd/PMo /GNSs catalyst.
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8
Conclusion
We have shown that the tri-component of Pd/PMo12/GNSs
nanohybrid can be used as an efficient electrocatalyst for
oxidation of ethanol. This material has been prepared using the
Keggin-type polyoxometalate phosphomolybdic acid (PMo ) as
7 J. L. Dongyao Wang, Z. Wu, J. Zhang, Y. Su, Z. Liu and C. Xu,
Int. J. Electrochem. Sci., 2009, 4, 1672–1678.
1
2
both a reducing and stabilising agent. A novel method for
activation of the catalyst has also been described and the elec-
trochemical investigations here within indicate that this acti-
vation process substantially enhances the electrocatalytic
8 S. C. Si-Wen Xie, Z.-Q. Liu and C.-W. Xu, Int. J. Electrochem.
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9 Z. Cui, P. J. Kulesza, C. M. Li, W. Xing and S. P. Jiang, Int. J.
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activity of the catalyst. Consequently, the as-prepared tri- 10 X.-M. Chen, Z.-J. Lin, T.-T. Jia, Z.-M. Cai, X.-L. Huang,
component nanohybrid catalyst showed a considerably higher
electrocatalytic activity towards ethanol oxidation. The superior
Y.-Q. Jiang, X. Chen and G.-N. Chen, Anal. Chim. Acta,
2009, 650, 54–58.
electrocatalytic performance is a synergic effect resulting prin- 11 R. N. Singh, A. Singh and Anintida, Carbon, 2009, 47, 271–
cipally from: (i) the superior electron mobility of GNSs and ionic
278.
conductivity of PMo12; (ii) the small size and high dispersion of 12 R. N. Singh and R. Awasthi, Catal. Sci. Technol., 2011, 1, 778–
PdNPs on the GNS support; (iii) a possible synergic and
783.
42
bifunctional effect between PdNPs and PMo12
;
and (iv) 13 N. Tian, Z.-Y. Zhou, N.-F. Yu, L.-Y. Wang and S.-G. Sun, J. Am.
formation of abundant Pd-OHads sites in the activation step.
Moreover, because of the higher ECSA, better tolerance towards 14 C. W. Xu, H. Wang, P. K. Shen and S. P. Jiang, Adv. Mater.,
poisoning carbonaceous species and improve onset potential, 2007, 19, 4256–4259.
the Pd/PMo /GNSs shows an enhanced catalytic activity 15 P. K. Shen and C. Xu, Electrochem. Commun., 2006, 8, 184–
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1
2
compared to Pd/PMo12/CNTs. In summary, the activated
188.
Pd/PMo12/GNSs nanohybrid showed highly promising features 16 H. Wang, C. Xu, F. Cheng and S. Jiang, Electrochem.
for development of Pd-based catalysts to be used in alkaline
direct ethanol fuel cells.
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17 S. Park and R. S. Ruoff, Nat. Nanotechnol., 2009, 4, 217–224.
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19 A. Ahmadpour, A. Ayati, H. Rashidi and B. Tanhaei, Synth.
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