Core/shell Ni@Pd nanoparticles supported on MWCNTs at improved electrocatalytic performance for alcohol oxidation in alkaline media

Article abstract of DOI:10.1016/j.electacta.2012.05.098

Ni@Pd nanoparticles with core/shell structure uniformly dispersed on multi-walled carbon nanotubes (Ni@Pd/MWCNTs) is successfully prepared via a two-step strategy: impregnation-reduction method and replacement method. The Ni@Pd/MWCNTs composite was characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis. It shows a uniform dispersion of Ni@Pd nanoparticles with core/shell structure on MWCNTs with the average particle size of 3.4 nm. The Ni@Pd/MWCNTs composite was used as electrocatalyst for alcohol oxidation in alkaline media for fuel cells. The electrocatalytic activity of ethanol oxidation on Ni@Pd/MWCNTs is 2.3 times higher than that of Pd/MWCNTs electrocatalyst at the same Pd loadings. The enhanced electrocatalytic properties could be attributed to not only the electric synergistic effect between Pd and Ni, but also the high use ratio of Pd for its shell structure.

Full text of DOI:10.1016/j.electacta.2012.05.098

Electrochimica Acta 77 (2012) 237–243
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Core/shell Ni@Pd nanoparticles supported on MWCNTs at improved
electrocatalytic performance for alcohol oxidation in alkaline media
Mingmei Zhang, Zaoxue Yan, Jimin Xie∗
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Ni@Pd nanoparticles with core/shell structure uniformly dispersed on multi-walled carbon nanotubes
(Ni@Pd/MWCNTs) is successfully prepared via a two-step strategy: impregnation–reduction method
and replacement method. The Ni@Pd/MWCNTs composite was characterized by transmission electron
microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analysis.
It shows a uniform dispersion of Ni@Pd nanoparticles with core/shell structure on MWCNTs with the
average particle size of 3.4 nm. The Ni@Pd/MWCNTs composite was used as electrocatalyst for alco-
hol oxidation in alkaline media for fuel cells. The electrocatalytic activity of ethanol oxidation on
Ni@Pd/MWCNTs is 2.3 times higher than that of Pd/MWCNTs electrocatalyst at the same Pd loadings.
The enhanced electrocatalytic properties could be attributed to not only the electric synergistic effect
between Pd and Ni, but also the high use ratio of Pd for its shell structure.
Received 25 April 2012
Received in revised form 30 May 2012
Accepted 30 May 2012
Available online 7 June 2012
Keywords:
Carbon nanotube
Core/shell Ni@Pd electrocatalyst
Alcohol oxidation
Alkaline media
© 2012 Elsevier Ltd. All rights reserved.
Fuel cell
1. Introduction
Another solution is to develop cheap catalyst promoter which
could remarkably improve the overall catalytic activity through the
Fuel cell technology has been considered as an effective way to
solve the problem of the exhaustion trend for fossil fuel. However,
the high cost has been remaining the choke point in the popular
application of fuel cells, especially the high cost of the noble metal
supported electrocatalyst, which is one of the most critical units in
fuel cell systems [1]. To decrease the use level of noble metal at the
condition of remaining or increasing the electrocatalytic activity is
of great significance.
One solution is to disperse the noble metal nanoparticles with
less size by using various supports which could greatly improve the
use ratio of noble metal and electrocatalytic activity [2–6]. Carbon
materials with various shapes and structures have been adopted as
electrocatalst support due to its low weight, high specific surface
area and chemical inertia. Yan et al. [7] synthesized three kinds of
hollow carbon hemispheres, which showed highly improved dis-
persion, use ratio and mass transportation for Pd electrocatalyst.
Owing to the better electrical conductivity and chemical stabil-
ity, carbon nanotubes (CNTs) have attracted numerous studies in
electrocatalyst support. Wang et al. [8] reported a facile room tem-
perature wet chemical approach toward single-crystal platinum
nanocubes/CNTs heterostructures with high dispersion and the as-
synthesized CNTs supported Pt electrocatalyst displayed excellent
activity toward the oxygen reduction reaction.
way of synergistic effect between the promoter and noble metal
[9–11]. The alloy that composed of noble metal and transition
metal has been found higher electrocatalytic activity than simple
noble metal electrocatalyst, which is explained as synergistic effect
between the two metals [12–17]. Therein, the core/shell structure
with noble metal as shell and base transition metal as core has
more advantages. As catalytic reactions occur on the surface of the
nanoparticles, a large fraction of metal in the core of the nanopar-
ticle is wasted. Consequently, a noble metal at outer surface and
base metal second atomic layer instead of noble metal as electro-
catalyst are of great significance for the use level improvement of
noble metal.
There have been lots of methods for preparing core/shell
nanoparticles [18–22,14,23–35]. Wang et al. [16] synthesized
PdCo@Pd/C nanoparticles with core/shell structure using H2-
induced surface segregation effect. They found that the PdCo@Pd/C
electrocatalyst showed much higher methanol tolerances than Pt/C
when being used in oxygen reduction reaction. Lu et al. [24] synthe-
sized Au–Pd core/shell nanocrystals with tetrahexahedral structure
by using Au nanocubes as structure-directing cores and found high
electrocatalytic activity. Another well known method is the spon-
taneous galvanic replacement technique that was proposed by
Adzic and Kokkinidis [25–35]. According to this method, core/shell
bimetallic electrocatalysts with monolayer noble metal shell on
noble metal or non-precious metal core can be prepared [36–38].
However, most of the synthesized noble metal based core/shell
nanoparticles have thicker noble metal shell than that of the simple
∗
Corresponding author. Tel.: +86 11 88791708; fax: +86 11 88791800.
E-mail address: Xiejm391@sohu.com (J. Xie).
0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.05.098

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M. Zhang et al. / Electrochimica Acta 77 (2012) 237–243
noble metal particles reported. Zhao et al. [15] have successfully
synthesized Ni@Pd nanoparticles on multi-walled carbon nano-
tubes by one-step, but the Ni@Pd nanoparticles they synthesized
show an uneven distribution on MWCNTs, which in fact could not
increase the use ratio of noble metal.
Recently, Pd-based electrocatalysts have aroused much atten-
tion because they can be highly active for the alcohol oxidation
in alkaline media where many non-noble metals are stable for
electrochemical applications [39,40]. The mechanistic study on the
electrooxidation of ethanol on Pd has also been reported based on
an in situ FTIR spectroelectrochemical study [41].
Herein, Ni@Pd core/shell nanoparticles supported on MWCNTs
(Ni@Pd/MWCNTs) and its electrocatalytic properties for alcohol
oxidation were studied. The Ni@Pd/MWCNTs has uniform dis-
persion of Ni@Pd nanoparticles with the average particle size
of 3.4 nm indicating less thickness of Pd shell. We applied the
Ni@Pd/MWCNTs to catalyzing alcohol oxidation, where the pos-
sible origin of the high activity of Ni@Pd/MWCNTs was discussed.
Fig. 1. Schematic diagram of the synthesis of Ni@Pd nanocatalysts on MWCNTs.
analysis gave the actual Pd contents as 9.7 wt.% for Ni@Pd/MWCNTs
and 9.2 wt.% for Pd/MWCNTs, respectively.
2.4. Preparation of catalyst electrode
For electrode preparation, 20.6 mg Ni@Pd/MWCNTs or 21.7 mg
Pd/MWCNTs 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 loadings were controlled at
0.02 mg cm−2. All chemicals were of analytical grade and used as
received.
2. Experimental details
2.1. Preparation of acid-functionalization of MWCNTs
MWCNTs used in this work were purchased from Shenzhen
Nanotechnologies Port Co. Ltd. (Shenzhen, China) with the diam-
eter of 20–40 nm, lengths of 6–15 mm, and purity of 98%. The
MWCNTs were purified in nitric acid solution by the ultrasonic
treatment for 30 min, refluxed with concentrated nitric acid at
100 ◦C for 12 h, then washed and filtered with deionized water
[42,43]. In order to generate lots of functional groups on the sur-
face of MWCNTs, the typical treatment was as follows [44,45]:
H2SO4/HNO3 (3:1) and MWCNTs were mixed with sonication for
10 min, then placed in an oil bath at 80 ◦C for 1 h. The resulting
product was filtered and washed with deionized water for several
times, and dried in a vacuum oven at 60 ◦C for 12 h.
2.5. Characterization of the supports and the electrocatalysts
The morphology of the as-prepared samples were examined by
a field emission scanning electron microscopy instrument (SEM,
Hitachi S-4800 II, Japan) and transmission electron microscopy
(TEM, JEOL-JEM-2010, Japan) operating at 120 kV. The structures
of the obtained samples were examined by X-ray diffraction (XRD)
using D8 Advance X-ray diffraction (Bruker axs company, Germany)
equipped with Cu-KR radiation (ꢀ) 1.5406 (Å), employing a scan-
ning rate of 0.02◦ s−1 in the 2ꢁ range from 10◦ to 80◦.
All electrochemical measurements were performed in a three-
electrode cell on an IM6e potentiastat (Zahner-Electrik, Germany)
at 30 ◦C controlled by a water-bath thermostat. A platinum foil
(1.0 cm2) and Hg/HgO (1.0 mol dm−3 KOH) were used as counter
and reference electrodes, respectively.
2.2. Preparation of Ni/MWCNTs composite
Nickel nanocrystals deposited on MWCNTs were prepared by
impregnation–reduction method using a vacuum quartz tube fur-
nace (OTF-1200X system) at the temperature of 600 ◦C [46–48].
Briefly, MWCNTs were thoroughly dispersed into a saturated
nickel nitrate aqueous solution and settled for two days at room
temperature. Then the product was dried at 130 ◦C, and subse-
quently calcined at 600 ◦C for 30 min under nitrogen atmosphere,
and finally the nickel impregnated MWCNTs (Ni/MWCNTs) were
obtained. The Ni contents of were analyzed by inductively coupled
plasma spectroscopy (ICP, Optima2000DV, USA) analysis, which
showed 16.4 wt.% of Ni in Ni/MWCNTs compound.
3. Results and discussion
3.1. The formation mechanism of Ni@Pd/MWCNTs
The schematic illustration for the formation of Ni@Pd/MWCNTs
nanocomposite is shown in Fig. 1. Once acid-MWCNTs were
thoroughly dispersed into a saturated nickel nitrate aqueous
solution and settled for two days at room temperature, the
nickel-impregnated MWCNTs were formed. The calcination at
600 ◦C under nitrogen atmosphere could make Ni-impregnated
MWCNTs transformed into monocrystalline nickel nanoparticles
(Ni/MWCNTs). After the Ni/MWCNTs being added in H2PdCl4
solution and continuously stirred for 14 h, well dispersed Ni@Pd
nanoparticles with core/shell structure on MWCNTs were obtained.
2.3. Preparation of Ni@Pd/MWCNTs electrocatalyst
Ni@Pd/MWCNTs nanosized electrocatalysts were synthesized
using replacement method. Typically, 10 mL of 0.186 mol/L H2PdCl4
and 2 g Ni/MWCNTs were dispersed in 50 mL distilled water in
beaker. The resulting solution was uniformly dispersed by soni-
fication for 10 min, and then vigorously stirred for 14 h at room
temperature. 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 MWCNTs (Pd/MWCNTs) as electrocatalyst was also obtained
directly by reducing the H2PdCl4 in MWCNTs suspension using
formic acid as reducing agent. The theoretical Pd contents in both
Ni@Pd/MWCNTs and Pd/MWCNTs were targeted at 10 wt.%. The ICP
3.2. Morphology and element analysis
The morphology and element distribution of Ni@Pd nanopar-
ticles were examined by TEM and EDS as shown in Fig. 2. A
large number of extremely small nanoparticles dispersed homoge-
neously on the surface of the MWCNTs (Fig. 2B) comparing with the
smooth surface of none supported MWCNTs (Fig. 2A). The Ni@Pd
nanoparticles on the MWCNTs have a narrow diameter range from
2 to 5 nm. The HRTEM image describing the crystalline nature of
Ni@Pd nanoparticles is shown in Fig. 2C. The single crystalline

M. Zhang et al. / Electrochimica Acta 77 (2012) 237–243
239
Fig. 2. TEM images of acid-treated MWCNTs (A), Ni@Pd/MWCNTs (B), HRTEM images of Ni/MWCNTs (C), EDS spectrum of Ni@Pd/MWCNTs composites (D), TEM images of
Pd/MWCNTs (E) and EDS spectrum of Pd/MWCNTs composites (F).
Ni@Pd particles are confirmed, the lattice planes with a inter-
layer 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 (1 1 1) crystal planes. Besides, Pd nanoparticles
supported on MWCNTs with the diameter range of 3–4 nm could be
observed from Fig. 2D. The elemental analysis by EDS proved that
the Ni@Pd/MWCNTs is composed of C, Ni and Pd (Fig. 2E, the Cu
signal comes from the sample holder, and the O signal comes from
the oxidation of surfacial metal atom), however, the Pd/MWCNTs
has no Ni signal (Fig. 2F).
the graphite-like structure of the MWCNTs. The peaks at 2ꢁ of 44.8◦,
52.0◦ and 76.6◦ correspond to the Ni (1 1 1), (2 0 0) and (2 2 0) crystal
planes, and the peaks at 2ꢁ of 40.3, 46.8 and 68.3 correspond to Pd
(1 1 1), (2 0 0) and (2 2 0) crystal planes, respectively. It is important
that no diffraction peaks of their oxides were detectable and the
high and sharp peaks indicted their well crystallinity. The Scherer’s
equation was used to calculate the crystal size of Pd:
Kꢀ
D =
B cos ꢁ
where D presents the average diameter in nm, K presents the
Scherrer constant (0.89), ꢀ presents the wavelength of X-ray
(ꢀ = 0.154056 nm), B presents the corresponding full width at half
maximum (FWHM) of the (2 2 0) diffraction peak, and ꢁ presents the
Bragg’s diffraction angle. The Pd particle size was calculated from
all the Pd (1 1 1), (2 0 0) and (2 2 0) crystal plane parameters and
averaged as 3.4 nm and 3.8 nm for Ni@Pd/MWCNTs and
3.3. XRD analysis
The crystal structure of MWCNTs, Ni/MWCNTs, Pd/MWCNTs
and Ni@Pd/MWCNTs were determined by XRD as shown in Fig. 3A.
The broad peaks at 2ꢁ = 26.1◦ are associated with C (0 0 2) planes of

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M. Zhang et al. / Electrochimica Acta 77 (2012) 237–243
A
Pd (111)
B
C (002)
MWCNTs
Ni (111)
Ni (200)
Pd/MWCNTs
Ni (220)
Ni/MWCNTs
Pd (111)
Pd (200)
Pd (220)
Ni@Pd/MWCNTs
Pd/MWCNTs
Ni@Pd/MWCNTs
20
30
40
50
60
70
80
40.1
40.2
2 theta(deg.)
40.3
40.4
40.5
2 theta(deg.)
D
Pd (200)
C
Pd (220)
Pd/MWCNTs
Pd/MWCNTs
Ni@Pd/MWCNTs
Ni@Pd/MWCNTs
46.4
46.6
46.8
2 theta(deg.)
47.0
47.2
67.8
68.0
2 theta(deg.)
68.2
68.4
68.6
Fig. 3. (A) The XRD patterns of CNTs, Ni/CNTs, Pd/CNTs and Ni@Pd/CNTs. (B)–(D) The enlarged XRD patterns of Pd/MWCNTs and Ni@Pd/MWCNTs at different diffraction
angles.
Pd/MWCNTs respectively, which are very close to the TEM
results.
Moreover, the Ni@Pd/MWCNTs has about 80 mV negative onset
potential than that on Pd/MWCNTs. The results indicated that the
Ni@Pd core/shell and alloy structure significantly improved the
activity and the output when used in fuel cells. This is extremely
important for the efficiency of fuel cells. The above results were
further evidenced by comparing the electrochemical active sur-
face area (EASA) as shown in Fig. 4D. The EASAs of Ni@Pd/MWCNTs
and Pd/MWCNTs were calculated according to the reduction region
of PdO to Pd [58], they are 91.3 and 78.2 m2 g−1, respectively.
The EASA on Ni@Pd/MWCNTs is 1.2 times higher than that on
Pd/MWCNTs, confirming that the core/shell catalyst could improve
the use ratio of the Pt metal. However, the peak current density on
Ni@Pd/MWCNTs is 2.3 times higher than that on Pd/MWCNTs. The
times in EASA (1.2) is less than that in peak current density (2.3),
indicating that the Ni has synergistic effect on Pd electrocatalyst.
This result is consistent with the literature [58].
The chronopotentiometric testing was carried out as shown
in Fig. 5. The results indicated that the Ni@Pd/MWCNTs electro-
catalyst could sustain larger current densities for stable ethanol
oxidation than that of Pd/MWCNTs electrocatalyst. The electrode
potential was polarized to higher potentials at higher current
densities than 10 mA cm−2 on Ni@Pd/MWCNTs due to the loss
of the catalytic activity for ethanol oxidation. However, the
Pd/MWCNTs electrode could not sustain the constant current den-
sity of 6 mA cm−2 due to the lower active surface area and lower
utilization of Pd even at the same Pd loadings.
Besides, as it is displayed in Fig. 3B–D, positive shift of the Pd
peaks occurs obviously on the Ni@Pd/MWCNTs comparing with
the Pd/MWCNTs, indicating the Ni atoms enter into the Pd crystals
which caused the narrow transformation of the Pd crystal lattice
distance. Kadirgan et al. [49] reported that the variation of crys-
tal lattice parameters of noble metal could obviously improve its
catalytic activities.
3.4. Ni@Pd/MWCNTs as electrocatalysts for alcohol oxidation
Fig. 4A shows the cyclic voltammograms of ethanol, methanol
and isopropanol oxidation on Ni@Pd/MWCNTs electrodes. The
Ni@Pd/MWCNTs electrocatalyst shows extremely high activ-
ity for the ethanol oxidation in terms of the onset potential
and peak current density in comparison with methanol and
isopropanol in alkaline solution. For ethanol oxidation, the
Ni@Pd/MWCNTs shows 2.3 times peak current density as high
as that on Pd/MWCNTs electrocatalyst at the same Pd loadings
as shown in Fig. 4B. Fig. 4C indicated that the mass activ-
ity of the ethanol oxidation in 1 mol dm−3 ethanol solution on
−1
Ni@Pd/MWCNTs was 3495 mA mgPd which is much higher than
−1
that of 1539 mA mgPd
on Pd/MWCNTs electrode. The activities
of the Ni@Pd/MWCNTs electrocatalyst were compared to others
in literature. We found that the peak and mass current density
of the Ni@Pd/MWCNTs is 1.4–20 times higher than the reported
Pd/MWCNTs electrocatalysts [50,51], Ni–Pd alloy supported elec-
trocatalysts [52,53], and Pd supported carbon and metal oxides
electrocatalysts [54–57] at the similar electrochemical reaction
conditions, displaying obvious commercial competition.
Based on the previous studies, the mechanism of ethanol oxi-
dation on Pd electrocatalyst is described though the following Eqs.
(1)–(5) [58–63]:
Pd + OH− → Pd-OHads + e−
(1)
(2)
Pd + CH3CH2OH → Pd-(CH3CH2OH)ads

M. Zhang et al. / Electrochimica Acta 77 (2012) 237–243
241
80
80
60
40
20
0
A
B
Pd/MWCNTs
Ni@Pd/MWCNTs
Ethanol
Methanol
Isopropanol
60
40
20
0
-0.6
-0.4
-0.2
0.0
0.2
-0.6
-0.4
-0.2
0.0
0.2
E / V vs. Hg/HgO
E / V vs. Hg/HgO
4000
3000
2000
1000
0
D
C
Pd/MWCNTs
Ni@Pd/MWCNTs
2
0
Pd/MWCNTs
Ni@Pd/MWCNTs
-2
-0.6
-0.4
-0.2
0.0
0.2
-0.6
-0.4
-0.2
0.0
0.2
E / V vs. Hg/HgO
E / V vs. Hg/HgO
Fig. 4. (A) Cyclic voltammograms of different alcohol oxidation on Ni@Pd/MWCNTs electrode, in 1.0 mol dm−3 KOH/1.0 mol dm−3 alcohol solution at 303 K, scan rate:
50 mV s−1; (B) and (C) cyclic voltammograms of ethanol oxidation on Ni@Pd/MWCNTs and Pd/MWCNTs electrodes in 1.0 mol dm−3 KOH/1.0 mol dm−3 ethanol solution at
303 K, scan rate: 50 mV s−1 and (D) cyclic voltammograms of Ni@Pd/MWCNTs and Pd/MWCNTs in 1.0 mol dm−3 KOH solution at 303 K, scan rate: 50 mV s−1
.
Pd-(CH3CH2OH)ads + 3OH− → Pd-(CH3CO)ads + 3H2O + 3e− (3)
potential and consequently increase the coverage of OHads on
Pd surface, which would ultimately accelerate the reaction rate
through the Eq. (6):
Pd-(CH3CO)ads + Pd-OHads → Pd-CH3COOH + Pd
Pd-CH3COOH + OH− → Pd + CH3COO− + H2O
(4)
Pd-(CH3CO)ads + OH−Ni(OH)
→
2Pd-CH3COOH + e−
(6)
(5)
So the final product would be the acetate. When the Ni was
added to form the Ni–Pd alloy, according to Zhang’s report [58], the
Ni would transform to Ni(OH)2 in alkaline media at the reaction
Liang et al. [62] reported from the cyclic voltammetry studies
that the rate determining step is the removal of the adsorbed acyl by
the adsorbed hydroxyl, while the dissociative adsorption of ethanol
proceeds quickly. Fang et al. [59] found that the catalytic perfor-
mance of Pd electrode depend on the pH of the fuel solution. The
best performance was observed in 1 M NaOH solution (pH = 14),
while the electrochemical activity decreased by either increasing
or decreasing the NaOH concentration. Spendelow and Wieckowski
[64] attributed this “pH effect” to enhanced ion transport and facile
charge transfer.
Ni@Pd/MWCNTs 10 mA cm-2
0.1
Ni@Pd/MWCNTs 12 mA cm-2
Pd/MWCNTs 6 mA cm-2
0.0
For the purpose of further comparing the kinetic and mass trans-
port properties of the two electrodes, Tafel polarization analysis
[58] and the relation between the peak current density and the
scan rate [65] were provided. The Tafel plots are showed in Fig. 6A,
the slopes for Ni@Pd/MWCNTs and Pd/MWCNTs electrocatalysts
are 212 and 195 mV dec−1 at 303 K, respectively. By extrapolating
the Tafel line, to the point where the overpotential equals zero,
the exchange current density can be obtained. The exchange cur-
rent density for Ni@Pd/MWCNTs is 2.14 × 10−2 mA cm−2, which is
4.7 times higher than that of Pd/MWCNTs (4.57 × 10−3 mA cm−2).
The results indicate that the Ni@Pd/MWCNTs electrocatalyst is
more efficient than the Pd/MWCNTs. The relation between the
peak current density and the scan rate is showed in Fig. 6B. It is
reported that the deflection from the linear line of the data at lower
scan rates in the relationship of peak current density and square
-0.1
-0.2
-0.3
0
10
40
50
60
2t0/ mi3n0
Fig. 5. Chronopotentiometric curves of ethanol oxidation on Ni@Pd/MWCNTs and
Pd/MWCNTs at different current densities in 1.0 mol dm−3 ethanol/1.0 mol dm−3
KOH solution, 303 K.

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M. Zhang et al. / Electrochimica Acta 77 (2012) 237–243
0.6
0.4
0.2
0.0
120
A
B
100
Ni@Pd/MWCNTs
80
60
40
20
0
Pd/MWCNTs
Ni@Pd/MWCNTs
Pd/MWCNTs
0
2
4
6
8
10 12 14 16 18
-2.5 -2.0 -1.5 -1.0 -0.5 0.0
0.5
1.0
log (i/mA cm-2)
v1/2/ (mV s-1)1/2
Fig. 6. (A) Tafel plots of the Ni@Pd/MWCNTs and Pd/MWCNTs electrodes in 1.0 mol dm−3 ethanol/1.0 mol dm−3 KOH solution with a scan rate of 1 mVs−1, 303 K and (B) plots
of the peak current density against the square root of the scan rate for both electrodes.
root of scan rate on electrode indicates the improvement in the
mass transfer [65]. The straight lines appear over the scan rate of
20 mV s−1 on both Ni@Pd/MWCNTs and Pd/MWCNTs electrodes.
The results display that the ethanol electro-oxidation reaction on
both Ni@Pd/MWCNTs and Pd/MWCNTs electrodes is kinetic con-
trolled at low scan rate (>20 mV s−1), and mass transport controlled
at high scan rate (>20 mV s−1).
oxidation are significantly enhanced. This might be attributed to
crystal nickel entered into palladium crystal lattice leading to syn-
ergistic effect. This also implies that Ni@Pd/MWCNTs may be good
candidates for electrocatalyst because improving of the precious
metal catalyst (Pd) effect in alcohol oxidation for fuel cells.
Acknowledgements
There have been several interpretations about the improved cat-
alytic activity and stability on the alloyed catalysts [66–68]. Firstly,
it is approved that the catalytic activity is strongly affected by the
catalyst surface structure [69,70]. The alloyed catalysts have dif-
ferent intermetallic surface structure or interaction comparing to
the single metal catalysts, which will certainly affect the catalytic
activity. Researchers called it electric “ligand effect” that between
the two metal layers of the alloy catalyst, which could promote des-
orption of the toxic intermediate products such as CO [71] from the
noble metal, and consequently improve the anodic catalyst activity
and stability [72]. Secondly, the downshift in d-band energy cen-
ter of the noble metal that induced by the presence of Fe, Co or Ni
has been taken in account to interpret the unusual catalytic per-
formance on the alloyed catalysts [73–75]. The thin layer of noble
meta would become compressive in atom arrangement when sup-
ported on Fe, Co or Ni substrate. Hammer and Nørskov revealed that
the narrow down of the d-band weakens CO adsorption because of
decreased back-donation from Pt to anti-bonding CO orbitals [76].
Thus, the anodic catalyst activity and stability will be improved.
Thirdly, the core/shell structure of the catalyst is also an important
factor in improving the electrochemical activity, which results from
the improvement in the use ratio of the noble metal due to its shell
structure, as discussed in the “EASA” part.
This work was supported by the financial supports of Jiangsu Sci-
ence and Technology Support Program (BE2010144) and Research
Foundation for Talented Scholars of Jiangsu University (11JDG149).
Dr. Z.X. Yan thanks the support by Research Foundation for Talented
Scholars of Jiangsu University (11JDG142).
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