Nickel core-palladium shell nanoparticles grown on nitrogen-doped graphene with enhanced electrocatalytic performance for ethanol oxidation

Article abstract of DOI:10.1039/c6ra06416g

Herein, we report a facile two-step strategy for green synthesis of nickel core-palladium shell nanoclusters on nitrogen-doped graphene (Ni@Pd/NG) without any surfactant and additional reducing agent. During the synthesis, nitrogen-doped graphene acted as both the active substance and support by taking advantage of its moderate reducing and highly dispersing capacities. Characterization indicated that a uniform dispersion of Ni@Pd nanoparticles on nitrogen-doped reduced graphene oxide had a 2.8 nm average particle size. Unexpectedly, the as-prepared Ni@Pd/NG hybrid exhibited much greater activity and stability than that of Pd/graphene and commercial Pd/C electrocatalyst at the same Pd loadings. Possible mechanisms for the enhanced electrocatalytic performance of nitrogen-doped reduced graphene oxide after combining with Ni@Pd are proposed. The present study provides an efficient strategy to synthesize highly efficient electrocatalysts.

Full text of DOI:10.1039/c6ra06416g

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Page 1 of 33
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Nickel core-Palladium shell nanoparticles growing on nitrogen-doped
graphene with enhanced electrocatalytic performance for ethanol
oxidation
∗
Mingmei Zhang , Yuan Li, Denghui Pan, Zaoxue Yan, Suci Meng, Jimin Xie
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang
2
12013, P. R. China
Abstract
Here we report a facile twoꢀstep strategy for green synthesis of nickel coreꢀ
palladium shell nanoclusters on nitrogenꢀdoped graphene (Ni@Pd/NG) without any
surfactant and additional reducing agent. During the synthesis, nitrogenꢀdoped
graphene acts as both the active substance and support by taking advantage of its
moderate reducing and highly dispersing capacities. Characterization indicated that a
uniform dispersion of Ni@Pd nanoparticles on nitrogenꢀdoped reduced graphene
oxide have the average particle size of 2.8 nm. Unexpectedly, the asꢀprepared
Ni@Pd/NG hybrid exhibits much greater activity and stability than that of
Pd/graphene and commercial Pd/C electrocatalyst at the same Pd loadings. Possible
mechanisms for the enhanced electrocatalytic performance of nitrogenꢀdoped reduced
graphene oxide after combining with Ni@Pd were proposed. The present study
provides an efficient strategy to synthesize highly efficient electrocatalyst.
Key words: nitrogenꢀdoped graphene; core/shell nanoelectrocatalyst; alcohol
∗
Correspondence author Tel: +86 11 88791708; fax: +86 11 88791800.
Eꢀmail address: jdzmm@163.com
1

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oxidation; fuel cell
1
. Introduction
Among all types of fuel cells, direct ethanol fuel cells (DEFCs) based on liquid
fuels is a promising biomassꢀderived power sources and widely investigated due to its
unique properties including higher theoretical mass energy density, lower toxicity and
operating temperature, easier in handling and transportation than other types of fuel
1
ꢀ3
cells . Moreover, ethanol can be easily obtained on a large scale from chemical
4
industry and fermentation of agricultural products or biomass .In the development of
DEFCs technology, the highly active electroꢀcatalysts are important and necessary in
order to completely oxidize each ethanol. Therefore, it has become a hot topic
research to design and develop efficient ethanol electroꢀoxidation anode catalysts for
5
DEFCs, especially in alkaline media . Nowadays, the electrocatalysts of anode
catalyst in DEFCs have predominantly relied on Pt based catalysts which have been
6
ꢀ9
extensively investigated . Nevertheless, the high cost has been remaining the choke
point in the popular application of fuel cells for a long time, especially the high cost of
the noble metal supported electrocatalyst, which is one of the most critical units in
fuel cell systems. It is necessary to develop new catalyst materials or improve the
efficiency of existing catalysts and supports for alcohol oxidation. Thus, there is a
strong motivation to increase the utilization of catalysts via their dispersion as small
particles on a support material. Recently, Pdꢀbased nanoꢀelectrocatalysts are emerging
2

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as excellent substitute to Ptꢀbased catalysts and have been proved to be a promising
candidate for direct ethanol fuel cells because of more abundance, much cheaper,
higher electrocatalytic activity and greater resistance to intermediate products for
10
ethanol electroꢀoxidation in alkaline medium . However, there are also a number of
challenges and obstacles for Pd based nanoꢀelectrocatalysts, such as low utilization
efficiency of Pd, which hinder the practical application in commercialization of
portable fuel cell technology. Till today, it is still necessary to develop lowꢀcost,
effective catalysts for DEFCs. Noticeably, most noble metal NPs catalytic reactions
occur only on the surface of the NPs and a large fraction of atoms in the core are
catalytically inactive. Therefore, to make a large percentage of noble metal atoms
available for catalysis and to reduce their consumption, the inner noble metal atoms
1
1ꢀ12
should be replaced by other nonꢀnoble metals
. A variety of coreꢀshell
nanoparticles have also been reported as catalysts in DMFCs. Density functional
theory studies suggested that the enhanced catalytic activity for the coreꢀshell
nanoparticle originates from a combination of an increased availability of COꢀfree Pt
surface sites on the Ru@Pt nanoparticles and a hydrogenꢀmediated lowꢀtemperature
CO oxidation process that is clearly distinct from the traditional bifunctional CO
1
3
oxidation mechanism . Currently, more research focus on the core/shell structure
14ꢀ16
with noble metal as shell and base transition metal (Ni、Co 、Cu、Fe) as core
.
Consequently, a noble metal at outer surface and base metal second atomic layer
instead of noble metal as electrocatalyst promoters changes the catalytic activity,
17
selectivity, and stability owing to synergistic effects . Besides the active metal
3

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regulation, another alternative effective approach to enhance electrocatalytic activity
1
8ꢀ20
is to seek and develop novel catalyst support
. Graphene, as an atomicꢀlayerꢀthick
two dimensional material, displays intriguing potential benefit as a support material
for DEFCs due to its many unique chemical and physical properties such as superior
electrical conductivity, high surfaceꢀtoꢀvolume ratio, ultrathin thickness, structural
2
1ꢀ23
flexibility and chemical stability
. Recently, heteroatom doped graphene materials
have received much attention as support for electrocatalysts, paving the way for the
growth of catalytically active metals with controlled morphology and dispersion on
24ꢀ25
the surface of graphene support
. To further tailor the catalytic support properties
of graphene, the nitrogen doping is important and perhaps the most frequently chosen
method which can enhance the conductivity of graphene and induces nꢀtype
semiꢀconductor behavior, because of the nitrogen atom is of comparable atomic size
and contains five valence electrons available to form strong valence bonds with
26, 27
carbon atoms
. More importantly, the nitrogenꢀdoped graphene (NG) with more
functional groups for property design would be provided by the incorporating
28, 29
different types of nitrogen into the carbon network of graphene
. Recent studies
show that nitrogen doping of catalyst supporting materials can provide small particle
sizes and a high dispersion of the catalyst nanoparticles, strong bonding between the
support and catalyst, electronic structure modification of the catalysts and an increase
in the electronic conductivity of the supports. In addition, studies also show that,
nitrogen doped graphene can act as good electrocatalysts even in the absence of
3
0
precious metals . Inspired by our previous studies, it is of great interest to develop
4

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highly active Nicore–Pdshell supported on N dopedꢀgraphene catalysts for the ethanol
oxidation reaction. Since metal nanoꢀparticles interact strongly with the
nitrogenꢀdoped graphene surface, it can highly be dispersed and show good stability
with the graphene surface, and the interaction between Ni core and Pd shell which
make Ni@Pd/NG setting up a fairly conductive network to facile chargeꢀtransfer and
massꢀtransfer processes. Therefore, the combination of Ni@Pd nanoparticles (NPs)
and NG may open up a new avenue for designing the next generation catalysts for
DEFCs.
In this article, Ni@Pd core/shell nanoparticles supported on nitrogenꢀdoped
graphene (Ni@Pd/NG) and its electrocatalytic activity and stability for alcohol
oxidation were presented. The preparation of Ni@Pd core/shell nanostructure could
not only reduce the consumption of Pd, but also take advantage of the interaction
between Ni core and Pd shell such as the ligand effect, downshift in dꢀband energy
center which are favourable for the promotion of the electrochemical activity and
31
stability of Ni@Pd/NG nanostructure . At the same time, Nꢀdoped graphene not only
serve as support, but also act as a secondary catalyst, which could increase the
accessibility of the active sites. The immobilization of Ni@Pd NPs embed on the N
doped graphene to fabricate the Ni@Pd/NG nanocatalyst would be promising for the
purpose of preventing the aggregation of Ni@Pd. We applied the Ni@Pd/NG to
catalyzing alcohol oxidation, where the possible origin of the large surface area and
excellent chemical stability of Ni@Pd/ NG was discussed.
5

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2
. Experimental details
2
.1. Materials
Natural flake graphite was obtained from Qingdao Guyu graphite Co., Ltd. with
a particle size of 150 nm. Nickel acetate tetrahydrate (Ni(ac) ·4H O), Ethylene glycol,
ammonia and Palladium chloride (PdCl ) were purchased from Sinopharm Chemical
2
2
2
Reagent Co., Ltd., China and used as received without any further purification.
Distilled water was also used throughout the experiment.
2
.2. Synthesis of GO and NRGO
Graphite oxide (GO) was prepared from purified natural graphite using a
3
2
modified Hummers’ method . A mixture of 3.0 g of graphite powder, 150 mL of
SO and 6.0 g of NaNO was prepared and immersed into ice bath. Then, KMnO
8.0 g) were added gradually under stirring and the temperature of the mixture was
H
2
4
3
4
(
maintained below 10◦C. After stabilization of the reaction temperature, the reaction
media was heated at 80°C and stirred for 2h until it became pasty brownish, and then
diluted with distilled water. Successively, the mixture was stirred for 30 min and 25
2 2
mL of 30 wt. % H O solution was slowly added to the mixture to reduce the residual
KMnO , after which the color of the mixture changed to brilliant yellow. The solid
4
product was separated by centrifugation and washed repeatedly with 5% HCl solution
to remove metal ions followed by 1.5 L of distilled water to remove the acid. For
further purification, the resulting solid was reꢀdispersed in distilled water and dialyzed
for one week to remove any residual salts and acids. Finally, the solid was separated
6

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by sintered discs and freezeꢀdried.
To synthesize NRGO material, 80 mg of GO were dispersed into 60 mL water
and magnetically stirred for 3 h. GO was exfoliated by sonication for 2 h. After that,
certain amount of urea was also added in and the solution was ultrasonicated for
another 0.5 h. Finally, the mixture was transferred into a 50 mL Teflon lined stainless
steel autoclave and was heated at 180°C for 12 h. The resulting product was filtered,
washed with distilled water for several times and freezeꢀdried.
2
.3 Preparation of Ni/ NG composite
For synthesizing the Ni/ NG compounds, 25 mg of NRGO was dispersed in 100
mL of ethylene glycol (EG) by sonication for 60 min. Then the solution was added
with 15 mg of Nickel acetate under vigorously stirring, and then subjected to
microwave (MW) oven (900 W) for intermittent microwave heating at a 20 s on
and 10 s off procedure for 15 times in a microwave oven temperature 180 °C. The pH
of the entire solution was adjusted to 10 by adding NaOH (2.0 M). After cooling to
room temperature, the product was washed with distilled water for 5ꢀ6 times to
remove the excess EG and subsequent separation by sintered discs and dried in
vacuum at 80 °C for 24 h. Finally the nickel impregnated nitrogenꢀdoped graphene
(Ni/NG) was obtained. The Ni contents of were analyzed by inductively coupled
plasma spectroscopy (ICP, Optima2000DV, USA) analysis, which showed 26.5 wt. %
of Ni in Ni/NG compound.
2
.4 Preparation of Ni@Pd/ NG electrocatalyst
Ni@Pd/NG nanosized electrocatalysts were synthesized using replacement
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2 4
method. Typically, 20 mL of 0.093 mol/L H PdCl and 1.65 g Ni/NG were dispersed
in 50mL distilled water in beaker. The resulting solution was uniformly dispersed by
sonification for 10 min, and then vigorously stirred for 24 h at room temperature. So
that spontaneous Ni replacement by Pd occurred:
2
ꢀ
2+
ꢀ
2
Ni/NG + PdCl
4
→ Pd(Ni)/NG + 2Ni + 6Cl
These reactions are thermodynamically favorable since the standard potentials of
2+
the Ni /Ni couples are ꢀ0.257 V vs. SHE, they are lower than the standard potential
2
ꢀ
4
of the Pd (II) Cl /Pd couple (+0.951 V vs. SHE). The black solid was separated by
sintered discs, washed with deioned water for several times, and finally dried in a
vacuum oven at 60 °C. For comparison, Pd nanoparticles supported on
nitrogenꢀdoped reduced graphene oxide (Pd/NG) as electrocatalyst was also obtained
2 4
directly by reducing the H PdCl in graphene suspension using ethylene glycol
microwave reduction. The theoretical Pd contents in both Ni@Pd/ NG and Pd/ NG
were targeted at 15 wt. %. The ICP analysis gave the actual Pd contents as 13.8wt. %
for Ni@Pd/ NG and 13.5 wt. % for Pd/ NG, respectively.
2
.5 Preparation of catalyst electrode
For electrode preparation, 25.1 mg Ni@Pd/NG were dispersed in 1 mL ethanol
and 1 mL 0.5 wt. % Nafion suspension (DuPont, USA) under ultrasonic agitation to
form the electrocatalyst ink. The electrocatalyst ink (40 ꢁL) was then deposited on the
surface of the glassy carbon rod and dried at room temperature overnight. The total Pd
ꢀ2
loadings were controlled at 0.02 mg cm . All chemicals were of analytical grade and
used as received.
8

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2
.6 Characterization of the supports and the electrocatalysts
The particle sizes and shapes of the asꢀprepared samples were examined by a
transmission electron microscopy (TEM, JEOLꢀJEMꢀ2010, Japan) operating at 200
kV. The asꢀsynthesized products were washed with absolute ethyl alcohol to remove
impurities, and then dispersed in absolute ethyl alcohol. The 2ꢀ3 drops of the
dispersion were dripped onto a carbonꢀcoated TEM grid. The crystalline phase and
phase purity of the Pd nanoparticles and Ni@Pd particles were analyzed by Xꢀray
diffraction (XRD) using D8 Advance Xꢀray diffraction (Bruker axs company,
Germany) equipped with CuꢀKR radiation (λ) 1.5406 (Å), employing a scanning rate
–1
of 0.02° s in the 2θ range from 10° to 80°. Xꢀray photoelectron spectroscopy (XPS)
was recorded on a PHIꢀ5702, and the C1S line at 291.4 eV was used as the binding
energy reference. The graphitization degree of N dopedꢀgraphene was determined by
Laser MicroꢀRaman Spectrometer (Renishaw inVia, Renishaw plc, UK). All
electrochemical measurements were performed in a threeꢀelectrode cell on an IM6e
potentiastat (ZahnerꢀElectrik, Germany) at 30°C controlled by a waterꢀbath thermostat.
2
ꢀ3
A platinum foil (1.0 cm ) and Hg/HgO (1.0 mol dm KOH) were used as counter and
reference electrodes, respectively.
3
. Results and discussion
The schematic illustration for the formation of Ni@Pd/NG nanocomposite is
shown in Fig. 1. Once graphite oxide was synthesized, it was dispersed thoroughly in
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urea solution by sonication and was heated at 180°C for 12 h. Then，it was subjected
to microwave (MW) heating in a microwave oven after nickel nitrate was added.
2 4
Finally the Ni/NG being added in H PdCl solution and continuously stirred for 14 h
at room temperature, well dispersed Ni@Pd nanoparticles with core/shell structure on
Nꢀdoped graphene were obtained.
Fig. 2 shows typical XRD patterns of the as prepared of GO(a), NRGO(b),
Pd/NG(c), Ni@Pd/NG (d) composites. All peaks can be indexed with face centered
cubic (fcc) structures. The original graphite oxide sample shows a strong diffraction
peak centered at 2θ= 9.8°, corresponding to the (001) reflection of graphene oxide, far
33
smaller than that of pristine graphite (Fig. 2(a)). After surface doped, the Nꢀdoped
graphene revealed negligible C (001) peaks compared to GO (Fig. 2(b)). The
weakening of the crystalline C (001) peak suggested that urea corrosion and defects
resulted from nitrogen doping made the interlayer spacing larger. In addition, as seen
from the XRD pattern of Fig. 2(c), the diffraction peaks at the 2θ of 40.1°, 46.5° and
6
8.2°can be indexed to the characteristic (111), (200) and (220) cubic crystalline
structured Pd. Besides, as it is displayed in Fig. 2(d), positive shift of the Pd peaks
occurs obviously on the Ni@Pd/NG comparing with the Pd/NG, indicating the Ni
atoms enter into the Pd crystals which caused a very small change of the Pd crystal
34
lattice distance. Jiang et al reported that the variation of crystal lattice parameters of
noble metal could obviously improve its catalytic activities. This can also be
attributed to the decorating effect of the Pd atomic shell layer on the alloyed
nanoparticles. The Pd particle size was calculated from all the Pd crystal plane
1
0

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parameters and averaged as 2.8 for Ni@Pd/NG, which are very close to the TEM
results.
Raman spectroscopy is a powerfuland widely used tool for identifying
grapheneꢀbased materials and detecting the doping effect of GO, such as defect
structures and disordered crystal structures of carbon and carbonꢀheteroatoms from
3
5
carbonꢀcarbon bonds
Fig. 3 compared the Raman spectra of GO, NRGO and
−1
Ni@Pd/NG. All samples displayed one intense D band at 1347.27 cm due to the
3
6
indicative of the expected crystallite structure of graphite and one relatively weak G
band due to the optically allowed E mode. A blue shift of the G band is usually a
2
g
result of the effects of disorder and the presence of isolated short doubleꢀbond
37
−1
segments . It is noteworthy that of the NGO (1583.03cm , Fig. 3c) and Ni@Pd/NG
−1
composites (1578.18cm , Fig. 3a) exhibit blue shifts compared with that of GO (Fig.
b). It reveals that the partial N heteroatoms may be introduced on these structural
sites to form substitutional N species during nitridation and can increase the numbers
3
3
8
of structural defects on grapheme surface . These indicate the electronic interaction
between NRGO and Ni@Pd for electron transportation between NRGO and Ni@Pd.
In addition, the intensity ratio of the D bandand G band (ID/IG) is an indication of the
number of structural defects and a quantitative measure of edge plane exposure, which
is most commonly being used as the level of doping and the chemical modification of
39
graphitic carbon sample . It is found that ID/IG values of Ni@Pd/NG (1.13) and
NGO (1.10) are higher than that of GO (1.02), indicating the successful nitrogen
4
0
doping into RGO that can induce higher concentration of structure defect . Such a
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higher ratio of D peak and G peak in Ni@Pd/NG is attributable to the decrease in the
2
average size of the sp domains upon the formation of nanoparticles on the graphene
and the structural defects and edge plane exposure caused by nitrogen atom
41
incorporation into the graphene layers .
The morphology and element distribution of Ni@Pd nanoparticles supported on
NG were investigated by TEM and EDS as shown in Fig. 4. Compared with GO (Fig.
4
a), the NRGO (Fig. 4b) appeared to obviously aggregate with more layers. It has
been proved experimentally that the defects created by nitrogen doping act as the
anchoring sites for the uniform dispersion of small metal nanoparticles
homogeneously on the surface even without any additional protective reagents or
surfactants in the system(Fig. 4c). The HRTEM image describing the crystalline
nature of Ni@Pd nanoparticles is shown in Fig. 4d. The single crystalline Ni@Pd
particles are confirmed, the lattice planes with an interlayer distance of 0.203 nm in
the core are indexed to Ni (111) crystal planes, the outer layer with the lattice space of
0
.224 nm corresponds to Pd (111) crystal planes. The elemental analysis by EDS
proves that the Ni@Pd/graphene is composed of C, Ni and Pd (Fig. 4e, the Cu signal
comes from the sample holder). The particle size distribution of Ni@Pd/NG
particles derived from TEM image are a narrow diameter range from 1.5 to 4.5 nm
and the average diameter is about 2.8nm (Fig. 4f). Clearly, nickel nanoparticles
interacting strongly with the Nꢀdoped graphene surface plays a key role in keeping a
similar and highly disperses Ni@Pd particle size on the supports. It is known that
Nꢀdoped graphene tends to interact with metal species. Density functional theory
1
2

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calculation was used to show that extra NiꢀC bonds was formed at the interface and
4
2
more electrons transferd from the interfacial CꢀC bonds to the NiꢀC bonds . It can
imagine that the Ni particles were supported on N doped graphene firstly, then the Pd
particle coating on the Ni core. At the same time Pd particles were fixed on the
graphene because the bottom of Pd also contact with the graphene. Possibly,
palladium and nickel nanoꢀparticles have strong interaction with the N doped
graphene surface, which restrains the aggregation and the growth of the nanoparticles
to form larger particles, resulting in the uniform distribution of these nanoparticles.
XPS were further performed to analyze the chemical compositions, oxidation
states and nitrogen bonding configurations of Ni@Pd/NG catalysts. The high
resolution XPS spectra of C 1s (Fig. 5a) show that there were three peaks (CꢀC, 283.9
eV; CꢀN, 285.5 eV; C=O, 287.9 eV). The peak centered at 283.9 eV can be assigned
2
to the adventitious hydrocarbon from the defectꢀcontaining sp hybridized carbon
atoms present in graphitic domains, and the CꢀN bond (285.5 eV) is suggesting that
43
the N was successfully doped in graphene nanosheets . Moreover, the signal of the
high resolution N1s peak (Fig. 5b) could be deconvoluted into three characteristic
peaks corresponding to the presence of pyridinic N, pyrrolic N and quaternary N in
44
the Ni@Pd/NG . As previous reports, the pyridinic N (398.3 eV) contributed to
nitrogen atoms at the edge of graphene planes that are bonded to two carbon atoms
and donate one p electron to the aromatic π system, the pyridinic edge sites can also
provide a stronger interaction between catalyst nanoparticles and carbon support and
this reduces the agglomeration of a coreꢀshell nanophase. pyrrolic N (399.4 eV)
1
3

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referred to the N atom contributing two pꢀelectrons to the p system due to the
contribution of pyridine and pyrrol functionalities; and quaternary N (400.8 eV) was
derived from the graphene layers by replacing a carbon atom within a graphene plane
4
5,46
and contributes two p electrons to the π system
.The detailed Pd3d spectra of
catalysts (Fig. 5c) displayed a doublet consisting of a highꢀenergy band (Pd3d3/2) and
a lowꢀenergy band (Pd3d5/2) at 339.98 and 334.65 eV for Ni@Pd/NG, such binding
energy values are in accordance with those reported for entered into the Pd lattice and
a core/shell of Ni@Pd was formed. The peaks of Ni 2p3/2 and Ni 2p1/2, which were
located at around 855.5 and 873.4 eV (Fig. 5d), were assigned that nickel is composed
of two surface chemical states on N dopedꢀgraphene. Remarkably, there are some
extra peaks labeled as satellite peaks which are present around the expected Ni 2p3/2
4
7
and Ni 2p1/2 signals in the Ni 2p region .
As a highly useful technique, cyclic voltammograms (CVs) is frequently used to
investigate the heterogeneous the electron transfer ability occurring on the electrode
surface. Fig.6 (a) shows the cyclic voltammograms (CVs) of alcohol electroꢀoxidation
on above prepared electrodes in 1.0 M KOH + 1.0 M ethanol solution at 303 K and
−1
electrodes at a scan rate of 50 mV s . Two featuring wellꢀstrong anodic current peaks
were observed in the CV curves of the different catalysts (Fig.6 (a)). The oxidation
peak in the forward scan were reminiscent the amount of ethanol electroꢀoxidation
48, 49
process at the Pdꢀbased electrocatalysts and consistent with the literature reports.
The reverse scan peak is primarily associated with the removal of carbonaceous
5
0
species which are not completely oxidized in the forward scan . More explicitly, the
1
4

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−
3
mass activity of the ethanol oxidation in 1 mol dm ethanol solution on Ni@Pd/NG
−1
−1
was 3650 mA mgPd , which is higher than that of 2435 mA mgPd on Pd/NG and
−
1
also higher than that of 1528 mA mgPd on Pd/G electrode. The Ni@Pd/ NG
electrode shows 4.5 times peak current density as high as that on Pd/C electrocatalyst
at the same Pd loadings during the similar electrochemical reaction conditions. The
activities of the Ni@Pd/ NG electrocatalyst were most likely a result of the particular
structure of bimetallic nanodendrites and the highly surface area of Nꢀdoped graphene
as support being beneficial for the uniform dispersion of the Ni@Pd nanoparticles to
make them highly used for the easier mass transfer, which display mast commercial
competition. When the Ni was added to form the NiꢀPd alloy, according to Zhang’s
51
2
report , the Ni would transform to Ni(OH) in alkaline media at the reaction potential
and consequently increase the coverage of OHads on Pd surface, which would
ultimately accelerate the reaction rate through the Eq.:
Ni(OH)2
ꢀ
ꢀ
Pdꢀ(CH CO) + OH
→
PdꢀCH COOH + e
3
3
ads
The above results were further evidenced by comparing the electrochemical active
surface area (EASA) as shown in Fig.6 (b) in the KOH solution without ethanol. As
shown in 6(b), the peaks between ꢀ0.9 and ꢀ0.5 V originated from the hydrogen
52,53
adsorptionꢀdesorption . On the reverse sweep, the defined peak near ꢀ0.25 V was
characteristic of the reduction of Pd oxide formed during the positiveꢀgoing sweep to
Pd. No other obvious characteristic peaks appear in the CV curves in the KOH
5
4
solution with or without alcohol, which is consistent with the literature reports .
The EASA of the above catalysts were studied by CV tests from −0.70 to 0.20 V in
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5

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−
1
−1
1
.0 mol L KOH at a scan rate of 50 mV s , and were calculated based on the PdO
−
2
reduction peak adapting, and the assumption of 212 uC cm of the electrode surface
5
5
2
−1
2
−1
2
−1
2
−1
.
They are 158.7 m g , 103.6 m g , 85.8 m g and 59.4 m g for the Ni@Pd/
NG, Pd/NG, Pd/G and Pd/C, respectively. The EASA on Ni@Pd/ NG is 2.7 times
higher than that on Pd/C. The largest EASAs of Ni@Pd/ NG catalyst is mainly due to
the replacement of Pd active sites on the surface by Nꢀdoped active center, also due to
its high specific surface area for shell structure. Furthermore, theoretical study has
shown that nitrogenꢀdoped reduced graphene oxide leads to a higher positive charge
on a carbon atom adjacent to the nitrogen atoms, and a positive shift of Fermi energy
at the apex of the Brillouin zone of graphene readily attracts electrons from the anode
56
to facilitate the ethanol oxidation process . Moreover, the Ni@Pd/ NG has about 40
mV negative onset potential than that on Pd/NG. The negative shift indicated that
ethanol was more easily oxidized by Ni@Pd/ NG, which was evidence that the
coreꢀshell structure could tremendously improve the kinetics of the ethanol
5
7
electroꢀoxidation . The results further indicated that the Ni@Pd significantly
improved the activity and the output when used in fuel cells.
The stabilities of the Ni@Pd/ NG and Pd/ G electrodes for ethanol oxidation are
shown in Fig.7. The dotted lines are the cycling difference from the 1st cycle to the
3
8
,000th cycle. It is clear that the EASA of the Pd/ G electrodes reduced 17.6% from
2
−1
2
−1
5.8 m g to 70.7 m g for ethanol oxidation after the 3,000th cycle. However, the
2
−1
2
−1
EASA of the Ni@Pd/ NG reduced 8.9 % from 158.7 m g to 144.6 m g . It is
evident that the Ni@Pd/ NG electrocatalyst has a higher stability than the Pd/ G. The
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promoted electrochemical stability may be due to the stronger interaction force
between Ni@Pd and N dopedꢀgraphene than the force between Pd and graphene.
The improved mass transfer property of the Ni@Pd/ NG electrocatalyst was
evidenced by performing various scan rates for ethanol oxidation (Fig. 8). The
deflection from the linear line of the data at lower scan rates in the relationship of
peak current density and square root of scan rate on electrode indicates the
improvement in the mass transfer. The straight line appears over the scan rate of 49
ꢀ1
ꢀ1
mV s on Ni@Pd/ NG comparing with Pd/G (20 mV s ), indicating N
dopedꢀgraphene improved mass transfer, however, it shows a straight line at any scan
rate on Pd/C electrode, indicating the concentration polarization.
The chronoamperometric curves on the three electrodes at a fixed potential of
ꢀ0.18V were shown in Fig. 9. As can be seen that the initial current densities decrease
rapidly for all the three catalysts owing to the formation of reactive intermediates such
58
3 3
as COads, CH COOHads and CH CHOads during the electrochemical reaction process ,
but the Ni@Pd/NG shows the highest initial and highest steadyꢀstate current density
than the Pd/NG and PD/C, indicating that the Ni@Pd/NG hybrid possessed a good
catalytic durability and excellent catalytic activity.
The data of CVs and chronoamperometric measurement suggested that the
Ni@Pd/NG exhibited an enhanced catalytic activity and good stability for ethanol
electroꢀoxidation. The superior electrocatalytic performance of the proposed catalyst
(Ni@Pd/NG) could be attributed to the following factors. Firstly, it is approved that
the catalyst surface structure strongly affected catalytic activity. The core/shell
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structure catalysts have different intermetallic surface structure or the downshift in
dꢀband energy center of the noble metal, which will certainly affect the catalytic
activity. Researchers called it electric “ligand effect” that between the two metal
layers of the core/shell catalyst, which could promote desorption of the toxic
intermediate products such as CO from the noble metal, and consequently improve the
59,60
anodic catalyst activity and stability
. Secondly, the N doped on the graphene
sheets could not only help the formation of small, highly concentrated and uniformly
dispersed Ni@Pd nanoparticles, but also the doped N, acted as a secondary catalyst
increasing more catalytic activity center and the rapidly removal of intermediate
poisoning species. In addition, the Ni@Pd/NG was possessed of a larger EASA
compared with the other electrocatalysts because of more catalytic activity center of
Nꢀdoped graphene, a narrower size distribution, more uniform distribution and more
perfect crystal structure on Nꢀdoped graphene than those of other asꢀprepared
composites.
4
. Conclusions
In summary, Ni@Pd/NG nanocomposites have been successfully fabricated
through an environmentally friendly two step strategy, and the catalytic properties
toward the ethanol oxidation are investigated. As a new catalyst, the Ni@Pd/NG
exhibits better kinetics, large specific surface areas, higher tolerance and
electrochemical stability than the other catalysts for ethanol electroꢀoxidation in
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DEFCs, which was correlated with the core shell architecture, nitrogen doping of
graphene, and the synergistic effects between Ni and Pd. Importantly, the Ni cores of
the Ni@Pd NPs reduced the consumption of Pd, Nꢀdoped grapheme had a large
specific surface area and more active site. This remarkable improvement of the
electrocatalytic performance may strongly provides new insights into the other
grapheneꢀbased metallic systems and prepare good electrocatalyst materials in alcohol
oxidation for fuel cells.
Acknowledgements
This work was supported by the financial supports from Natural Science
Foundations of Jiangsu (BK20140531), Zhenjiang Industry Supporting Plan
(GY2014040), China Postdoctoral Science Foundation (2015M570410) and Research
Foundation for Talented Scholars of Jiangsu University (14JDG187); Dr. Suci Meng
thanks National Natural Science Foundation of China (21103073)
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Figure captions
Fig.1 Schematic diagram of the synthesis of Ni@Pd nanocatalysts on N doped
graphene.
Fig.2 shows typical XRD patterns of the as prepared of GO(a), NRGO(b), Pd/NG(c),
Ni@Pd/NG (d) composites.
Fig.3 Raman spectra of a (Ni@Pd/NG), b (GO), and c (NRGO)
Fig.4 TEM images of GO (a), NRGO (b), Ni@Pd/NG (c) ,HRTEM images of
Ni@Pd/NG (d), EDS spectrum of Ni@Pd/NG (e) and Size distribution of mental
particles of Ni@Pd/NG derived from TEM images (f).
Fig.5 XPS spectra of (a) C 1s, (b) N 1s, (c) Pd 3d and (d) Ni 2p of the sample
Ni@Pd/NG.
Fig.6 (a) cyclic voltammograms of ethanol oxidation on Ni@Pd/NG, Pd/NG, Pd/G
and Pd/C electrodes and (b) cyclic voltammograms of Ni@Pd/NG, Pd/NG, Pd/G and
ꢀ3
ꢀ1
Pd/C electrodes in 1.0 mol dm KOH solution at 303 K, scan rate: 50 mV s .
Fig. 7 (A) the cyclic voltammograms of the Ni@Pd/NG electrocatalysts and (B) the
ꢀ3
cyclic voltammograms on Pd/G in 1.0 mol dm KOH solution at 303 K, scan rate: 50
ꢀ1
mV s . The dotted line shows the difference from the 1st cycle to the 3000th cycle.
Fig. 8 plots of the peak current density against the square root of the scan rate for
ꢀ3
ꢀ3
Ni@Pd/NG, Pd/G and Pd/C electrodes in 1.0 mol dm ethanol/1.0 mol dm KOH
solution, 303 K.
Fig. 9. Chronoamperometric curves of Ni@Pd/NG, Pd/NG and Pd/C at ꢀ0.18 V (vs
SCE) in 1.0 M KOH + 1.0 M ethanol solution.
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4

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Fig.1. Schematic diagram of the synthesis of Ni@Pd nanocatalysts on N doped
graphene.
GO
NRGO
OH
O
O
OH
O
O
OH
O
O
O
OH
urea
OH
OH
O
OH
180℃/12 h
O
C
OH
OH
Ni(Ac)2
microwave
H PdCl4
2
Ni@Pd/NG
Ni/NG
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Fig.2 shows typical XRD patterns of the as prepared of GO(a), NRGO(b), Pd/NG(c),
Ni@Pd/NG (d) composites.
Ni(111)
Ni(200)
Ni(220)
(d) Ni@Pd/NG
Pd(111)
(c) Pd/NG
Pd(200)
Pd (220)
(
b) NRGO
(a) GO
10
20
30
40
50
60
70
80
2
theta(deg.)
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Fig.3 Raman spectra of a (Ni@Pd/NG), b (GO), and c (NRGO)
D
(
(
(
c)NRGO
b)GO
a)Ni@Pd/NG
1347.27
G
1
578.18
583.03
598.78
1
1
1
000
1500
Raman Shift
2000
2500
-1
(
cm )
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Fig.4 TEM images of GO (a), NRGO (b), Ni@Pd/NG (c) ,HRTEM images of
Ni@Pd/NG (d), EDS spectrum of Ni@Pd/NG (e) and Size distribution of mental
particles of Ni@Pd/NG derived from TEM images (f).
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Fig.5 XPS spectra of (a) C 1s, (b) N 1s, (c) Pd 3d and (d) Ni 2p of the sample
Ni@Pd/NG.
C-C
pyridinic N
a
b
graphitic N
pyrrolic N
C-N
C=O
2
90
288
286
284
282
280
408 406 404 402 400 398 396 394 392
Binding Energy (eV)
Binding Energy (eV)
Pd3d5/2
Ni2p3/2
d
c
Pd3d3/2
Ni2p1/2
3
48 346 344 342 340 338 336 334 332 330
880
870
860
850
Binding Energy (eV)
Binding Energy (eV)
2
9