Porous SnO2 hexagonal prism-attached Pd/rGO with enhanced electrocatalytic activity for methanol oxidation

Article abstract of DOI:10.1039/c7ra03659k

Porous SnO₂ hexagonal prisms, as a new promoter, were attached to Pd-based systems held by a reduced graphene oxide (rGO) support (Pd-SnO₂/rGO) for the catalysis of the electrooxidation reaction of methanol. Cyclic voltammetry (CV) tests revealed that the electrocatalytic activity and stability were substantially improved by SnO₂ with a special morphology. The specific activity (SA, j_k, area) and mass activity (MA, j_k, area) of Pd-SnO₂/rGO were enhanced 1.31 and 3.3 times those of the Pd/rGO catalyst, respectively. Moreover, the CO-tolerance was also remarkably enhanced due to the presence of SnO₂. It is believed that higher surface areas, more active sites, which are offered by the porous architecture of SnO₂, as well as the synergetic effect between all components contribute to the improvement of the catalytic activities of the Pd-SnO₂/rGO catalysts. Cost savings and the CO-poisoning obstacle being surmounted, which are the two main probing directions for elevating the overall performance of direct methanol fuel cells, makes the as-prepared Pd-SnO₂/rGO a promising electrocatalyst.

Full text of DOI:10.1039/c7ra03659k

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PAPER
Porous SnO2 hexagonal prism-attached Pd/rGO
with enhanced electrocatalytic activity for
methanol oxidation
Cite this: RSC Adv., 2017, 7, 29909
*
Yiran Hu, Tao Mei, Jinhua Li,
Jianying Wang and Xianbao Wang
Porous SnO2 hexagonal prisms, as a new promoter, were attached to Pd-based systems held by a reduced
graphene oxide (rGO) support (Pd–SnO2/rGO) for the catalysis of the electrooxidation reaction of
methanol. Cyclic voltammetry (CV) tests revealed that the electrocatalytic activity and stability were
substantially improved by SnO2 with a special morphology. The specific activity (SA, jk, area) and mass
activity (MA, jk, area) of Pd–SnO2/rGO were enhanced 1.31 and 3.3 times those of the Pd/rGO catalyst,
respectively. Moreover, the CO-tolerance was also remarkably enhanced due to the presence of SnO2. It
is believed that higher surface areas, more active sites, which are offered by the porous architecture of
SnO2, as well as the synergetic effect between all components contribute to the improvement of the
catalytic activities of the Pd–SnO2/rGO catalysts. Cost savings and the CO-poisoning obstacle being
surmounted, which are the two main probing directions for elevating the overall performance of direct
methanol fuel cells, makes the as-prepared Pd–SnO2/rGO a promising electrocatalyst.
Received 30th March 2017
Accepted 25th May 2017
DOI: 10.1039/c7ra03659k
rsc.li/rsc-advances
a satisfactory effectiveness of the Pd-based electrocatalysts. The
most widely accepted strategy is to use less-precious metals to
combine alloys, such as Pd–Cu,7,8 Pd–Ni,9 and Pd–Ag,10,11 or
1 Introduction
Direct methanol fuel cells (DMFCs) possess signicant potential
for portable devices and transportation utilization since the
possibility of directly using fuel (methanol) at low operating
temperatures and the interoperability of the existing distribu-
tion and consumption pattern of liquid fuel.1–4 For methanol
electrooxidation at the anode, palladium is regarded as an eye-
catching alternative to platinum as a catalyst. Besides their
lower price as compared to the Pt-based electrocatalysts, Pd-
based electrocatalysts can achieve appreciable activity for the
oxidation of methanol in a high pH environment, where non-
precious metals are adequately stable for electrochemical
applications.5 The stability of base metals in an alkaline envi-
ronment provides us more choices to cut down the expenditure
of the catalysts. In addition, the better CO-type intermediates
tolerance was shown in Pd-based electrocatalysts than in Pt-
based electrocatalysts, especially when the pH is high.6 The
intermediates during the electrooxidation of methanol, espe-
cially carbon monoxide, are strongly absorbed onto the active
site of pure noble metals, such as Pd, and limit the catalytic
efficiency.
introduce metal oxides, such as NiO,12 CeO2 (ref. 13), MnO2 (ref.
13) and SnO2 (ref. 13), as co-catalysts. Compared to pure metals,
metal oxides provide more possibilities of forming a construc-
tion that is more complicated and stable. Among all types of
metal oxide promoters, SnO2 attracts signicant attention due
to its great stability as well as particular electrochemical prop-
erties. It has been extensively used in protection against corro-
sion, catalysts, and even been considered as a substitute for
commercial graphite as an anode of a lithium-ion battery
(LIB).1,14 Moreover, SnO2 can be a promoter of water displace-
ment, which is the step that determines the rate of methanol
oxidation.15
Furthermore, a proper support should be selected for the
reason that the inevitable aggregation of nanosized catalysts may
reduce the activity of the materials. The large surface areas and
numerous active sites of the nanoparticles are crucial to the
catalytic characteristics, on account of the surface where the
catalytic reaction usually proceeds. Among the carbon materials,
graphene is appropriate for Pd particles to disperse on it. Its
ultrahigh electrical conductivity, large surface area, and abun-
dant functional groups (hydroxyls, epoxides, carbonyls, etc.),
which can immobilize and x nanoparticles, can theoretically
enhance the properties of the Pd-based electrocatalysts.16–19
Apart from the characteristics of each component, some
outstanding superior and synergetic properties can be man-
ifested when they are smartly integrated together. For instance,
the electronic conguration, stability, and exibility of the
To save costs as well as overcome the CO-poisoning obstacle,
a
number of studies have been performed to achieve
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Key
Laboratory for the Green Preparation and Application of Functional Materials,
Ministry of Education, Hubei Key Laboratory of Polymer Materials, School of
Materials Science and Engineering, Hubei University, Wuhan 430062, China.
E-mail: wangxb68@aliyun.com; Fax: +86 2788661729; Tel: +86 2788661729
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structures and the efficiency of electron transfer might be resulting in a black powder. The black powder was soaked in 50
enhanced, as well as the number of electroactive sites can be mL of 3.0 M HCl for 30 min at room temperature under stirring
increased.20–22
until the powder totally turned off-white. Finally, the off-white
Herein, porous SnO2 hexagonal prism-attached Pd/rGO was powder was washed with ultrapure water until the pH of the
2ꢀ
prepared by the co-reduction of PdCl4 and GO with porous upper clear water was neutral.
SnO2 hexagonal prisms, which was pre-prepared with water
soluble chitosan as a dispersing agent in deionized pure water 2.3 Preparation of the porous SnO2 hexagonal prism-
and excess NaBH4 as a reductant. The oil-bath reaction under attached Pd/rGO
constant pressure and relatively low temperature conserves
energy and is more practicable for large-scale production.
Although SnO2-enhanced Pd-based electrocatalysts with carbon
supports have been reported, to the best of our knowledge, SnO2
Porous SnO2 hexagonal prism-attached Pd/rGO catalyst (deno-
ted as SnO2–Pd/rGO) was obtained via a reduction method in
only one step. In the standard preparation, 60 mg of GO powder
was rst ultrasonically dispersed in 50 mL of distilled water for
with a specic morphology has never been used in anodic
1 h. The mixture was obtained by blending 10 mg of the porous
catalysts for methanol oxidation. The porous SnO2 hexagonal
SnO2 hexagonal prisms and was homogenously dispersed in 20
prism obtained in advance was not destroyed during the entire
mL water aer ultrasonication for 10 min, 20 mL PdCl2 solution
(5 mM), 2 mL solution of water soluble chitosan (0.1%) together
reduction reaction and provided more active sites. High-
performance catalysts always have high surface areas and
with the obtained GO homogeneous solution. The above-
some of them have a porous construction. The interior hollow
mentioned mixture was transferred into a 150 mL round-
parts of the porous SnO2 decrease the quantity of the buried
bottomed ask and constantly stirred for 30 min. Aer this,
the uniform solution was placed in an oil bath. When the
temperature reached 100 ꢁC, a 20 mL solution containing 1.2 g
nonfunctional Pd atoms, and the uncommon geometry offers
a great possibility to trim physical and chemical properties.23
Compared to pure Pd supported on rGO or on porous SnO2
NaBH4 was added dropwise into the ask under continuous
hexagon, the as-prepared porous SnO2 hexagonal prism-
stirring. The oil bath was controlled at 100 ꢁC for 4 h to obtain
attached Pd/rGO exhibits superior activity towards the oxida-
the SnO2–Pd/rGO catalyst.
tion of methanol that benets from higher surface areas, more
active sites, which were offered by the individual components,
as well as the synergetic effect between all of them.
2.4 Material characterization
The morphology and microstructure of the catalyst were
observed using transmission electron microscopy (TEM, Tecnai
2 Experimental
2.1 Chemicals
F20) and eld emission scanning electron microscopy (FE-SEM,
JSM7100F). Energy dispersive X-ray spectroscopy (EDX) was
used to analyze the mass distribution of the elements using an
Every reagent was maintained as received and no further puri-
analyzer attached to the FE-SEM. Powder XRD was carried out
cation was carried out before use. Analytical grade SnCl4-
using a D8-Advance diffractometer (Bruker, Germany) with a Cu
$5H2O, NaOH, CuCl2$H2O, NaBH4, PdCl2, and CH3OH were
Ka radiation source (l ¼ 0.15418 nm). The surface elemental
obtained from Sinopharm Chemical Reagent Co. Water soluble
composition of the sample was investigated via X-ray photo-
electron spectroscopy (XPS; Thermo Fisher Scientic Escalab
250Xi).
chitosan was bought from Zhejiang Aoxing Biotechnology Co.
Ltd. The few-layered GO was purchased from Hunan Fenghua
Chemical Co. The experimental deionized water was freshly
processed by a P60-CY Kertone Ultrapure Water System (Ker-
tone Water Treatment Co. Ltd, resistivity ¼ 18.25 MU cm).
2.5 Electrochemical measurements
A CHI760E electrochemical workstation (Shanghai Chenhua,
China) was employed for the electrochemical studies at room
temperature (ꢂ25 ꢁC). The measurements were performed
2.2 Synthesis of the porous SnO2 hexagonal prisms
The porous SnO2 hexagonal prisms were synthesized using using a three-electrode conguration that was composed of
a water-bath method with some modication.24 In a typical a modied glass carbon electrode (GCE) (3 mm in diameter),
synthesis of the porous SnO2 hexagonal prisms, rst 0.79 g of a platinum electrode, and a saturated calomel electrode (SCE) as
SnCl4$5H2O was dissolved in 100 mL of distilled water under the working, counter, and reference electrodes, respectively.
stirring at room temperature, and then, 0.76 g of NaOH was Before the surface coating, the GCE was polished to a mirror-
added under continuous vigorous stirring. Aer the solution like nish with 0.3 and 0.05 mm alumina powder and rinsed
became clear, 50 mL solution of 0.375 g CuCl2$H2O was poured with nitric acid solution, ethanol, and doubly distilled water
into the abovementioned uniform solution at 30 ꢁC and was using an ultrasonic cleaner and dried in air. Each specimen was
stirred for 15 min, resulting in a milky bꢁlue sediment. The ultrasonically dispersed in 500 mL of ethanol to obtain
resultant blend was allowed to stand at 30 C for 6 h. The ob- a uniform catalyst ink of 2 mg mLꢀ1. Aer at least 60 min of
tained sediment was centrifuged, ultrasonically cleaned using ultrasonic cleaning, a 3 mL droplet of the resulting mixture was
ultrapure water and absolute ethyl alcohol three times each and pressed onto the polished GCE and dried at 25 ꢁC for 1 h. High
kept in a drying oven at 50 ꢁC for 12 h. Subsequently, the purity N2 was utilized to deaerate the electrolyte for at least
product was annealed at 620 ꢁC for 2 h in air atmosphere, 30 min before all the measurements. For the methanol
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electrooxidation tests, an alkaline solution with 0.5 M KOH and
1 M CH3OH was used as the electrolyte.
3 Results and discussion
The porous SnO2 hexagonal prism was prepared using a special
method without any template. The solid hexagonal prisms of
copper tin hydroxide were rst obtained through the reaction of
SnCl4$5H2O and CuCl2$H2O with NaOH via a water-bath
method. Aer the basic morphology was successfully attained
with the help of CuCl2$H2O, the hydroxide was separated into
CuO and SnO2 aer the annealing process with the morphology
being retained. Taking the stability of SnO2 under an acidic
condition into consideration, 3 M HCl was used to remove the
CuO part and the porous nanostructures of SnO2 were obtained
from the remaining vacancies of CuO. The oil bath with NaBH4
as a reducing agent provides an alkaline environment, which is
appropriate for GO reduction. Water-soluble chitosan as
a dispersing agent prevents the rGO assembling to graphite to
a great extent. The formation strategy for preparing Pd–SnO2/
rGO is demonstrated in Scheme 1.
The porous SnO2 hexagonal prism was synthesized with
a length of 0.4–1.0 mm and a width of 0.1–0.4 mm (Fig. 1a–d). The
evenly distributed porous structures can be clearly observed
from the TEM images. On amplifying the images of the shape of
one end of the obtained copper tin hydroxide and porous SnO2
hexagonal prism (inset in Fig. 1a and b), a special six-prism
shape was observed. Fig. 1a and b are the SEM images of the
solid hexagonal prisms of copper tin hydroxide and porous
hexagonal prisms of SnO2, respectively, and they show a similar
morphology. Fig. 1c and d are the TEM images of the porous
SnO2 and Pd/SnO2 hexagonal prisms. Fig. 1f illustrates the TEM
image of Pd–SnO2/rGO, which indicates that the porous
hexagonal prisms of SnO2 maintained the basic morphology
aer the reduction. Theoretically, SnO2 would turn into stan-
niferous salts in a strong alkali environment only when the
temperature is above 750 ꢁC; thus, its components would not be
affected during the entire reaction. The structure of the SnO2
hexagonal prisms was not broken by the airow caused by the
relatively moderate reaction mode of an oil bath, as shown in
Fig. 1 (a) SEM images of the solid hexagonal prisms of copper tin
hydroxide; (b) SEM images of the porous hexagonal prisms of SnO2; (c),
and (d) TEM images of the porous SnO2 and Pd/SnO2 hexagonal
prisms. (e) TEM image of GO and (f) TEM image of Pd–SnO2/rGO.
Fig. 1f. Pd nanoparticles on the surface of rGO were uniformly
distributed. The TEM image of a few-layered GO is presented in
Fig. 1e as a reference.
Fig. 2 shows the XRD patterns for the as-synthesized samples
over the 2q range of 5–85ꢁ. It can be clearly seen from Fig. 2a
that the XRD patterns of the as-prepared and the purchased
SnO2 have the same diffraction peaks of SnO2 (JCPDS: 41-1445),
which indicates that SnO2 has been successfully synthesized.
Pure Pd NPs with face-centered cubic (fcc) crystalline structures
show 2q values of (111), (200), and (220) that are consistent with
the standard values of Pd (JCPDS: 87-0643). The XRD diffraction
peaks of the (111), (200), and (220) facets of the fcc crystalline
structures can be observed at 2q ¼ 39ꢁ, 45ꢁ, and 67ꢁ from
Fig. 2b. The plane characteristic peak of Pd (111) was adopted to
estimate the average crystallite size D of the Pd NPs following
the Scherrer's equation:
D ¼ 0.9l/b cos q
(a)
In the abovementioned formula, l (nm) is the X-ray wave-
length (l ¼ 0.15418 nm for Cu Ka), b (rad) is the full width at
half-maximum (FWHM) of the peak in the diffraction pattern,
and q (rad) is the Bragg diffraction peak. According to Fig. 2b,
the FWHM are 0.830, 0.795, and 0.745 for Pd/rGO, Pd/SnO2, and
Pd–SnO2/rGO catalysts, respectively. On calculating using eqn
(a), the crystallite sizes of the Pd NPs were found to 10.05, 10.50,
Scheme 1 Schematic for the preparation of porous SnO2 hexagonal
prism-attached Pd/rGO.
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Taken together with the results of XRD and the XPS spectra, it
was observed that GO was highly reduced by NaBH4. Spectral
peaks of Pd 3d and Sn 3d were observed in Pd–SnO2/rGO (Fig. 3c
and d), validating the existence of Pd and SnO2 in the obtained
sample. The Pd 3d spectrum, as shown in Fig. 2c, is formed by
the Pd 3d3/2 and Pd 3d5/2 states, and the two signals can be
further deconvoluted into two components of Pd2+ and Pb0,
respectively. The percentage of Pb0 is about 77.8%, as measured
by the relative peak areas, indicating that aer the synthesis
process, Pd–SnO2/rGO mainly contain Pb0 with a few oxidation
states of palladium from the un-reacted ions.17,26,27 Pd2+ was
inevitably adsorbed onto the hollow portion of SnO2 owing to
the large surface areas of the porous construction and was not
totally exposed for the reduction reaction to occur. The Sn 3d
spectrum, as seen in Fig. 2d, has two peaks of Sn 3d3/2 at
495.5 eV and Sn 3d5/2 at 487.0 eV, which is in good agreement
with the energy splitting reported for SnO2.28 Fig. 2e presents
the survey spectra of the porous SnO2 hexagonal prism-attached
Pd/rGO, which reveals that there are no other heteroelements
apart from Pd, Sn, C, and O. The O 1s was detected at 530.8 eV,
where the peak of the oxygen species appeared in SnO2.
Fig. 2 (a) XRD patterns of the prepared and purchased SnO2 and (b)
XRD patterns of the purchased GO and prepared Pd/rGO, Pd/SnO2,
and Pd–SnO2/rGO.
The as-synthesized samples were further characterized via
their electrochemical properties as a promising catalyst for the
methanol oxidation reaction (MOR). The electrocatalytic
behaviors of the porous SnO2 hexagonal prism-attached Pd/rGO
catalyst were tested; moreover, for comparison, the same tests
were carried out for graphene-based Pd (denoted as Pd/rGO), Pd
based on porous the SnO2 hexagonal prisms (denoted as Pd/
SnO2), and Pd prepared by reducing 5 mM PdCl2 aqueous
and 11.20 nm for the Pd/rGO, Pd/SnO2, and Pd–SnO2/rGO
catalysts, respectively. Compared to the peaks of SnO2 shown in
Fig. 2a, there are fewer peaks that can be clearly observed. The
diffraction characteristic peaks at 2q ¼ 26.6ꢁ, 33.9ꢁ, and 51.8ꢁ
correspond to the (100), (101), and (211) facets, respectively, of
tetragonal SnO2, as shown in Fig. 2b; however, other peaks are
weak and cannot even be observed. The faint peaks of SnO2
were hardly observed since the main Pd diffraction peaks and
the sharp peaks of SnO2 were comparatively stronger. The
crystallinity of SnO2 becomes pronounced in Pd–SnO2/rGO as
evidenced by the increased intensity of the diffraction peaks.25
Moreover, as shown in the XRD patterns of GO, a sharp peak at
2q ¼ 11ꢁ of the C (002) facets can be observed. The fact that no
obvious peaks can be found at the same positions in the
patterns of other samples illustrates that GO is indeed partially
reduced and rGO is formed. The XRD pattern of Pd/rGO has
a wide peak at around 2q ¼ 23ꢁ, whereas the pattern of Pd–SnO2/
rGO does not show this peak. This indicates that the reduction
degree of GO among Pd–SnO2/rGO was better than that of Pd/
rGO. Moreover, the peak of SnO2 could also hinder the obser-
vation of this diffraction peak.
XPS analysis was used for investigating the surface compo-
sition and electron conguration of the prepared catalysts.
Fig. 3a and b reveal the C 1s regions of the purchased GO and
Pd–SnO2/rGO. From the C 1s spectra of GO and Pd–SnO2/rGO,
four components corresponding to C–C, C–OH, C–O, and OH–
C]O species can be further separated out. On the basis of the
correlation of the C 1s spectrum of graphene oxide (Fig. 3a) and
that of the Pd–SnO2/rGO (Fig. 3b), the reduction in the quantity
of oxygenated functional groups was observed, also indicating
the high reduction of GO via thermal and chemical reactions.
Fig. 3 (a) XPS spectrum for C 1s regions of purchased GO; (b) XPS
spectrum for C 1s regions, (c) Pd 3d regions and (d) Sn 3d regions of
prepared Pd–SnO2/rGO; and (e) XPS survey spectrum for prepared
Pd–SnO2/rGO.
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Table 1 Catalysts actual loading and electrochemical characteristics
of the samples during cyclic voltammograms
solution with NaBH4 to gure out the structural advantages of
Pd–SnO2/rGO and the enhancement from porous SnO2 hexag-
onal prisms and rGO. The catalytic activities of the specimens
were acquired through CV tests in a N2-saturated alkaline
solution of 0.5 M KOH + 1.0 M CH3OH. The currents of the
catalytic activities of different catalysts were standardized,
which was realized by the Pd mass, to make a comparison.
As can be distinctly seen in Fig. 4a, highest MA of the porous
SnO2 hexagonal prism-attached Pd/rGO has a peak current of
1032.8 mA mgPdꢀ1, which is around 1.3, 3.3, and 21.5 times that
of Pd/SnO2 (780.5 mA mgPdꢀ1), Pd/rGO (311.6 mA mgPdꢀ1), and
Pd (48.1 mA mgPdꢀ1), respectively. Aer being scattered around
the carbon material, in this case rGO, the mass activity of the
Pd catalyst dramatically improved. The same enhancement
appeared in the anchoring of Pd on porous SnO2 hexagonal
prisms and rGO for increasing amount of transferred charge
from the loaded Pd nanoparticals to the rGO funds. Moreover,
the lower onset potential of Pd–SnO2/rGO than that of other
reference materials (shown in Table 1) shows a substantially
improved electrocatalytic activity of the porous SnO2 hexagonal
prism-attached Pd/rGO towards methanol oxidation.
Pd mass
content
(wt%)
Peak current
density
(mA mgPd
Eonset
(V vs. SCE)
If/Ib
ratio
ꢀ1
Electrocatalyst
)
Pd
100.00
12.12
49.37
12.89
48.1
311.6
780.5
ꢀ0.45
ꢀ0.48
ꢀ0.50
ꢀ0.52
1.06
1.79
1.38
2.13
Pd/rGO
Pd/SnO2
Pd–SnO2/rGO
1032.8
Pd + OHꢀ / Pd–(OH)ads + eꢀ
(2)
Pd–(CH3OH)ads + OHꢀ / Pd–(CH3O)ads + H2O + eꢀ (3)
Pd–(CH3O)ads + OHꢀ / Pd–(CH2O)ads + H2O + eꢀ
Pd–(CH2O)ads + OHꢀ / Pd–(CHO)ads + H2O + eꢀ
Pd–(CHO)ads + OHꢀ / Pd–(CO)ads + H2O + eꢀ
Pd–(CO)ads + Pd–(OH)ads / Pd–(COOH)ads + Pd
Pd–(COOH)ads + OHꢀ / Pd + CO2 + H2O + eꢀ
(4)
(5)
(6)
(7)
(8)
As for Pd-based catalysts, the mechanism of MOR under an
alkaline condition is as follows:29,30
Pd + CH3OH / Pd–(CH3OH)ads
(1)
CO, which is the main poisoning intermediate species of the
catalyst, infects the active sites and signicantly impedes the
access of methanol, and the activity of the Pd-based catalysts thus
drops. CO will be further oxidized to the nal product CO2, which
corresponds to the backward current density. Therefore, the ratio
of the forward and backward oxidation current density If/Ib is
a vital indicator to measure the endurance of the catalyst to
poisoning types that are dominated by CO.31 From Fig. 4a, the If/Ib
value of Pd–SnO2/rGO was detected to be 2.13, higher than that of
Pd/rGO (shown in Table 1). The signicant increase in If/Ib
showed that on the surface of Pd–SnO2/rGO, methanol was more
effectively oxidized and generated less poisoning species than on
Pd/rGO. The reason for this variation is that the addition of SnO2
can promote the rate of methanol electrooxidation, which is
closely related to water displacement. The adsorbed hydroxyl
groups generated from the water displacement step reacted with
the adsorbed CO and timely removed it, surmounting the CO-
poisoning obstacle of Pd to a certain extent as follows:
SnO2 + H2O / SnO2–(OH)ads + H+ + eꢀ
(9)
Pd–(CO)ads + SnO2–(OH)ads / Pd + SnO2 + CO2 + H+ + eꢀ
(10)
Stability measurements of the catalysts were achieved via
chronoamperometry (CA) in a N2-saturated alkaline solution
with 0.5 M KOH and 1.0 M CH3OH and conducted at ꢀ0.1 V vs.
SCE for 1000 s. As shown in Fig. 4b, for all the catalysts, the
curves show an ultrafast fall-off at the beginning and then
gradually reach a state that is approximately steady. The pre-
sented initial current was higher for the double layer charge as
well as the abundant active sites held by the surface of the
Fig. 4 (a) Cyclic voltammograms and (b) chronoamperometric curve
for MOR catalyzed by Pd–SnO2/rGO, Pd/SnO2, Pd/rGO, and Pd in an
alkaline solution of 0.5 M KOH and 1 M CH3OH. The CVs were obtained
at a scan rate of 50 mV sꢀ1 and the chronoamperometric curves were
obtained at ꢀ0.1 V.
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catalysts. Then, the intermediate products poison the samples lattice of palladium simultaneously proceed in the anodic
and block the surface active sites, leading to a quick decline.32 sweep.
The initial current of the porous SnO2 hexagonal prism-
Therefore, the ECSAs were measured by the area of peak IV
attached Pd/rGO catalyst was highest as well as steady stage generated from the reduction of Pd(II) during backward
current. Together with the minimal rate of current decay, Pd– sweeping. Moreover, peak I can be due to the reaction of
SnO2/rGO denitely had the best poisoning tolerance and hydrogen oxidation, which emerges about between ꢀ0.2 and
catalytic stability. The stronger metal–substrate interaction 0 V, and peak III is related to the generation of palladium(II)
provides Pd/rGO with a higher steady current than that of Pd/ oxide on the surface of the Pd-based catalysts.6 The ECSAs were
SnO2, and it agrees with the results of the If/Ib value calculated calculated by integrating the part of the CV curves that pre-
by the CVs curves.
sented the reduction charge of the newly formed Pd(OH)2
To further explore and compare the activities of the electrode layer:34
materials, the electrochemical active surface areas (ECSAs) were
tested. Fig. 5a presents the CVs of the Pd/rGO and porous SnO2
hexagonal prism-attached Pd/rGO catalysts from the fourth
ꢀ
SPdðOHÞ
0:42MPd
V
2
ECSA ¼
(b)
cycle. The CV scanning rate was 50 mV sꢀ1 with a measuring
potential ranging from ꢀ0.2 to 1 V (vs. SCE) in a solution of O2-
removed 0.5 M H2SO4. A total of four portions can be seen in the
sweeping curves corresponding to the four electrochemical
redox processes that occur on the surface of three electrodes.
The ECSAs of the catalysts based on Pt are normally calculated
by obtaining the part of the CV curves corresponding to the
coulombic charge for hydrogen desorption integrated in the
CV.33 However, the cyclic voltammogram curves of Pd are way
too far from those of Pt. Hydrogen is absorbed not only on the
surface but strongly enough to be partly absorbed into the
lattice of palladium. The H2 absorption started even before the
underpotential deposition (UPD) of the absorbed hydrogen
atoms and it led to an additional ux of faradaic current.
Accordingly, two pathways for the desorption processes of
hydrogen to the Pd surface or into the electrolyte from the
where SPd(OH) /V is the charge for the reduction of Pd(II), and
2
0.42 mC cmꢀ2 is the charge density during the process of
generating a thin Pd(OH)2 layer with full coverage, which is the
value for a representative single-crystal palladium surface. The
MPd are obtained by the EDX analysis (shown in Fig. 6). The
calculated ECSAs of the catalysts were 25.18 and 63.66 m2 gꢀ1
for Pd/rGO before and aer enhancement of the porous SnO2
hexagonal prisms, respectively, in an acid solution. Therefore,
our as-prepared Pd–SnO2/rGO catalysts had a more efficient
Fig. 5 (a) CV curves for the Pd–SnO2/rGO and Pd/rGO catalyst-
modified electrodes in O2-removed 0.5 M H2SO4. (b) The specific
activity and mass activity at 0.50 V.
Fig. 6 EDX spectra of (a) Pd–SnO2/rGO; (b) Pd/SnO2; and (c) Pd/rGO.
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surface structure for MOR. Fig. 5b reveals the comparison of the
specic activity (SA, jk, area) and mass activity (MA, jk, mass,
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4 Conclusion
In summary, a simple oil-bath method to prepare porous SnO2
hexagonal prism-attached Pd/rGO composite material was
described; the composite material was characterized and tested
as a catalyst for methanol electrooxidation. We chose SnO2 as
a promoter of the Pd-based electrocatalyst to simultaneously cut
the cost and accelerate the rate of the pivotal step of MOR. The
step, which is water displacement, produces more hydroxyls to
combine with the absorbed CO, thus easing the poisoning
phenomenon of catalysts of MOR and strengthening the
stability. Furthermore, the high surface areas of the porous
structure of the SnO2 hexagonal prisms increase the number of
active sites. Hollow parts of SnO2 together with the rGO support
enable the Pd nanoparticles to be separated instead of being
aggregated. The composite's electronic conductivity was also
enhanced by the rGO support. Accordingly, in relation to Pd/
SnO2, Pd/rGO, and Pd, Pd–SnO2/rGO has a substantially
upgraded electrocatalytic activity and durability for methanol
oxidation. This study is a new attempt of using SnO2 with
a special morphology for MOR.
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Acknowledgements
This work was nancially supported by the National Key R&D
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