Rapid, general synthesis of pdpt bimetallic alloy nanosponges and their enhanced catalytic performance for ethanol/methanol electrooxidation in an alkaline medium

Article abstract of DOI:10.1002/chem.201202909

We have demonstrated a rapid and general strategy to synthesize novel three-dimensional PdPt bimetallic alloy nanosponges in the absence of a capping agent. Significantly, the as-prepared PdPt bimetallic alloy nanosponges exhibited greatly enhanced activity and stability towards ethanol/methanol electrooxidation in an alkaline medium, which demonstrates the potential of applying these PdPt bimetallic alloy nanosponges as effective electrocatalysts for direct alcohol fuel cells. In addition, this simple method has also been applied for the synthesis of AuPt, AuPd bimetallic, and AuPtPd trimetallic alloy nanosponges. The as-synthesized three-dimensional bimetallic/trimetallic alloy nanosponges, because of their convenient preparation, well-defined sponge-like network, large-scale production, and high electrocatalytic performance for ethanol/methanol electrooxidation, may find promising potential applications in various fields, such as formic acid oxidation or oxygen reduction reactions, electrochemical sensors, and hydrogen-gas sensors. Soak it up: A rapid and general strategy is demonstrated for the preparation of novel three-dimensional PdPt bimetallic alloy nanosponges (see figure) that exhibit greatly enhanced activity and stability towards ethanol/methanol electrooxidation in an alkaline medium. Copyright

Full text of DOI:10.1002/chem.201202909

DOI: 10.1002/chem.201202909
Rapid, General Synthesis of PdPt Bimetallic Alloy Nanosponges and Their
Enhanced Catalytic Performance for Ethanol/Methanol Electrooxidation in
an Alkaline Medium
[
a, b]
[a, b]
[a, b]
Chengzhou Zhu,
Shaojun Guo,
and Shaojun Dong*
Abstract: We have demonstrated a
rapid and general strategy to synthesize
novel three-dimensional PdPt bimetal-
lic alloy nanosponges in the absence of
a capping agent. Significantly, the as-
prepared PdPt bimetallic alloy nano-
sponges exhibited greatly enhanced ac-
tivity and stability towards ethanol/
methanol electrooxidation in an alka-
line medium, which demonstrates the
potential of applying these PdPt bimet-
allic alloy nanosponges as effective
electrocatalysts for direct alcohol fuel
cells. In addition, this simple method
has also been applied for the synthesis
of AuPt, AuPd bimetallic, and AuPtPd
trimetallic alloy nanosponges. The as-
synthesized three-dimensional bimetal-
lic/trimetallic alloy nanosponges, be-
cause of their convenient preparation,
well-defined
sponge-like
network,
large-scale production, and high elec-
trocatalytic performance for ethanol/
methanol electrooxidation, may find
promising potential applications in var-
ious fields, such as formic acid oxida-
tion or oxygen reduction reactions,
electrochemical sensors, and hydrogen-
gas sensors.
Keywords: alloys · fuel cells · palla-
dium
· platinum · mesoporous
AHCTUNGTRmENNNUG aterials
Introduction
macroporous walls and nanoparticles by using hydrogen
[6a]
bubbles as a dynamic template. Jiang and co-workers re-
ported the fabrication of metal nanoporous films by using a
self-organization of ultrathin nanowires induced by a cap-
Porous metal architectures (PMAs) induced by various as-
semblies and self-assemblies are considered to be a signifi-
cant subject in the field of nanochemistry and nanotechnolo-
[12]
ping-agent replacement. Though considerable progress in
the synthesis of PMAs has been made, the approaches re-
ported previously are often time-consuming, complicated,
and/or carried out at high temperatures. Therefore, further
exploration of efficient synthetic methods and new perspec-
tives for their applications are highly desirable.
[
1]
gy. PMAs combine the properties characteristic of metals,
such as good electrical and thermal conductivity and catalyt-
ic activity, with the most distinctive properties characteristic
of nanoarchitectures, such as aerogels, which include high
surface area, low density, and high gas permeability, find
[2]
[3]
[4]
broad applications in catalysis, sensors, membranes, and
Pd-based nanomaterials represent a new class of advanced
nanocatalysts for various chemical processes, including fuel
[5]
hydrogen storage. To date, great efforts are being directed
toward the synthesis of functional PMAs, including templat-
[
13]
[14]
[15]
cells,
sis.
sensors,
water treatment,
and organic cataly-
[
6]
[7]
[8]
[16]
ing and dealloying approaches, sol–gel approaches, car-
Recent advances revealed that rationally controlling
[9]
[2a]
bothermal methods, and so on. For example, Eychmꢀller
et al. have developed unsupported monometallic (Pt, Au,
the size, shape, and composition allows the properties of var-
ious noble metals to be varied and provides enormous op-
portunities for tailoring their properties, thus enhancing
[8,10]
Pd, and Ag) and bimetallic (PtAg and AuAg) aerogels.
Composite aerogels of both semiconductor and metal nano-
particles have also been successfully realized.
[17]
their functions and performance in applications. Besides
the synthesis of monodisperse Pd nanocrystals with different
morphologies, constructing three-dimensional (3D) Pd-
based PMAs by using nanostructures as building blocks has
become one of the most attractive research topics because
these nanomaterials are highly promising for catalytic appli-
[11]
Xia et al.
sculpted a variety of copper films with open interconnected
[
a] C. Zhu, S. Guo, Prof. S. Dong
State Key Laboratory of Electroanalytical Chemistry
Changchun Institute of Applied Chemistry
Chinese Academy of Sciences
Changchun 130022 (P.R. China)
Fax : (+86)431-85689711
[18]
cation.
Additionally, aside from morphology and struc-
ture, the compositions of noble metal nanostructures were
also accurately controlled to further enhance their functions
and performance in applications. Of these nanostructures,
metal alloy nanostructures provide an effective strategy for
E-mail: dongsj@ciac.jl.cn
[
b] C. Zhu, S. Guo, Prof. S. Dong
[19]
enhancing the functionality of materials. Specifically, large
efforts were made to synthesize Pd-based alloy nanocata-
Graduate School of the Chinese Academy of Sciences
Beijing 100039 (P.R. China)
[20]
lysts for applications in direct alcohol fuel cells (DAFCs).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201202909.
In comparison with monometallic nanostructures, Pd-based
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FULL PAPER
alloy nanostructures show enhanced electrocatalytic activi-
ties due to synergistic and electronic effects. Prominent ex-
amples include the synthesis of hexaoctahedral AuPd with
ring was continued for about 5 min until the entire solution
became colorless. The PdPt BANs (denoted as Pd Pt
50
50
BANs) were obtained after filtering and washing. It should
be noted that no additional capping agent was added during
the whole process, with the exception of the metal precur-
sors and reducing agent. The morphology and structure of
the as-prepared Pd Pt BANs were first characterized by
[21]
[22]
high-index facets, PtPd alloy nanodendrites, and PdAg
[
23]
alloy nanowires.
also investigated as substitutes for noble metals to prepare
Note that non-noble metals have been
[24]
promising Pd-based alloy electrocatalysts. In particular, to
the best of our knowledge, there is no successful demonstra-
tion of the construction of 3D Pd-based alloy nanostructures
by using a facile and general method.
5
0
50
using scanning electron microscopy (SEM) and transmission
electron microscopy (TEM). Figure 1A, B shows representa-
tive SEM images, demonstrating that a sample with a
sponge-like structure could be fabricated on a large scale.
This sponge-like feature was further confirmed by TEM
(Figure 1C, D) and HRTEM (Figure 1E) imaging. It is
noted that these 3D networks are not loosely aggregated
nanoparticle structures but are fused architecture. We found
that the interconnected PdPt BANs are composed of fused
nanoparticles of about 5 nm in diameter. As a control,
Herein, we demonstrate for the first time a rapid and gen-
eral way to synthesize 3D PdPt bimetallic alloy nanosponges
(
PdPt BANs), which represent a new type of unsupported
alloy PMAs with several important benefits: 1) Construction
of 3D architectures through rapid self-assembly and forma-
tion of the alloy can be completed simultaneously within
5
min; 2) significantly, the as-prepared 3D PdPt BANs ex-
hibit enhanced electrocatalytic activity toward ethanol/
methanol electrooxidation in an alkaline medium; 3) this
facile method could be also applicable to synthesize Pt- and
Au-based alloy nanosponges (NSs).
mono AHCTUNGTERUNNNmG etallic Pd and Pt NSs were also prepared. As expect-
ed, both of them were composed of porous, interconnected
networks (Figure S1 in the Supporting Information), and the
formation of the alloy had no effect on the morphology of
the final products. The polycrystalline nature of the sponge,
observed in the electron diffraction (ED) pattern, also sug-
gests that the obtained PdPt BANs originated from the
fusion of nanoparticles (Figure 1D, inset). The HRTEM
image of an individual PdPt alloy nanoparticle (Figure 1E)
shows its single-crystalline structure with highly ordered
continuous fringe patterns. The measured interplanar spac-
ing for the lattice fringes is 0.22 nm, which corresponds to
the (111) lattice plane of face-centered-cubic (fcc) PdPt
alloy nanostructures, whereas the fast Fourier transform
(FFT) pattern (Figure 1E, inset) of the HRTEM image fur-
ther indicates that the individual PdPt alloy nanoparticle is
a single crystal. Furthermore, the formation of PdPt BANs
was characterized by energy-dispersive X-ray (EDX) spec-
Results and Discussion
By using a strong reducing agent, metal precursors can be
reduced simultaneously at the nucleation and growth stages,
which is regarded as one feasible way to synthesize metallic
[18,25]
alloy nanostructures.
Additionally, the fusion of bare
metal nuclei formed during the rapid reduction process can
[26]
be induced to form 3D BANs instantaneously. For a typi-
cal synthesis of PdPt BANs, a metal precursor aqueous solu-
tion that contained H PdCl (2.5 mL, 0.1m) and H PtCl
6
2
4
2
(
(
2.5 mL, 0.1m) was quickly injected into aqueous NaBH4
25 mL, 0.1m) with stirring at room temperature. The stir-
Figure 1. A,B) SEM, C,D) TEM, and E) HRTEM images of the as-obtained Pd50Pt50 BANs. D) Inset shows the ED pattern. E) Inset shows the FFT pat-
tern of the HRTEM image (squared part). F) EDX pattern of Pd50Pt50 BANs.
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S. Dong et al.
troscopy (Figure 1F). The corresponding EDX spectrum of
PdPt BANs shows peaks that correspond to Pd and Pt ele-
ments, which confirms the existence of Pd and Pt on the sur-
face of PdPt BANs.
The alloy formation was further characterized by elemen-
tal mapping. Figure 2 shows the high-angle annular dark-
could be easily obtained in powder form after a simple
drying treatment (Figure 3B).
A typical feature of these PdPt BANs is that the composi-
tion involved can be tuned easily by changing the amount of
metal precursors added during the synthesis, to obtain an
optimized ratio and further enhance the functions and appli-
[27]
cation performance of the PdPt BANs. To vary the Pd/Pt
atomic ratio, precursor solutions (5 mL) that contained 3.75,
3
.33, or 2.50 mL of H PdCl were used. The formed particles
2 4
are denoted as Pd Pt , Pd Pt and Pd Pt and were char-
7
5
25
67
33
50
50
acterized by using inductively coupled plasma optical emis-
sion spectrometry (ICP-OES). As expected, the molar ratio
of the two metal precursors is carried over to the final PdPt
BANs due to the addition of excess NaBH , which has
4
strong reduction ability. The effect of the Pt content on the
morphology of the PdPt BANs was investigated. Figure S2
in the Supporting Information shows typical SEM and TEM
images of the obtained Pd Pt and Pd Pt BANs. It is
Figure 2. HAADF-STEM-EDS mapping images of A) Pd50Pt50 BANs,
B) Pd-K, and C) Pt-M.
7
5
25
67
33
field scanning TEM (HAADF-STEM) image of as-prepared
Pd Pt BANs (Figure 2A) and the related mapping with Pt
and Pd (Figure 2B, C). The uniform color distribution in the
nanosponges reveals that the alloy structure is indeed
formed after simultaneous reduction with NaBH4.
clear that on decreasing the Pt content from 50 to 25 at.%,
the morphology of the final PdPt BANs is almost unchanged
and similar sponge-like networks were obtained.
Information on the surface area and porosity properties
of the PdPt BANs was characterized by using nitrogen ad-
50
50
The effect of precursor concentration on the formation of
the alloy nanosponges was also investigated (Figure 3A).
Sodium borohydride reduction is an age-old method to
obtain metal nanoparticles from their precursors in the pres-
sorption measurements (Figure 4). The N physisorption iso-
2
therm is essentially a type IV curve with a H3 hysteresis
loop (Figure 4A), which demonstrates the typical porous
characteristic of Pd Pt BANs. The slope of the curves rises
5
0
50
significantly for relative pressures higher than 0.95. Addi-
tionally, there is no plateau at high relative P/P pressures in
0
the adsorption isotherm, which implies the simultaneous
Figure 3. A) Digital photographs of the PdPt alloy products formed for
equimolar concentrations of precursors (a: 0.1m; b: 20 mm; c: 10 mm;
d: 5 mm) and NaBH solutions in a 1:5 v/v ratio. B) Photograph of the ob-
4
tained Pd50Pt50 BANs in powder form.
ence of a capping agent, but is seldom used to obtain porous
noble metal nanostructures. If the concentration of metal
precursors (equimolar concentrations of H PtCl and
2
6
H PdCl ) was low, alloy metal nanoparticles instead of nano-
2
4
sponges were formed (Figure 3A, tubes b–d). Further in-
creasing the metal precursors to 0.1m resulted in the forma-
tion of alloy nanosponges (Figure 3A, tube a). It is clear
from our studies that the formation of PdPt BANs depends
on the initial concentration of metal precursors. Addition of
aqueous NaBH4 solution to the metal precursor solution
created many alloy nuclei that acted as nucleation centers
for further growth. Rapid fusion and growth of the bare
nuclei led to interconnected nanosponges if the concentra-
tion of metal precursors was increased to around 0.1m.
Therefore, the nuclei formed in large quantities at a specific
concentration of metal precursors play an important role in
Figure 4. A) Nitrogen adsorption–desorption isotherms and B) pore size
distribution of the as-obtained Pd50Pt50 BANs.
[26]
synthesizing PdPt BANs. It is noted that the final product
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PdPt Bimetallic Alloy Nanosponges
FULL PAPER
[
8,10]
presence of macropores (pore diameter >50 nm).
Thus,
and their storage and transport are much easier than hydro-
gen. Great progress has been achieved in fuel-cell science
and technology, especially in constructing state-of-the-art Pt-
the hysteresis loop of Figure 4A can be assigned to the tex-
tural mesopores of fused nanoparticles. The strong adsorp-
tion at P/P values of close to 1.0 should be a result of the
[30]
based nanocatalysts.
However, the commercialization of
0
accessible macropores of the prepared Pd Pt BANs, which
fuel cells is severely hampered by the high costs, sluggish ki-
netics, and low stability of conventional platinum catalysts.
Moreover, these drawbacks, together with the relative toxic-
ity of methanol, are boosting research aimed at using other
alcohols as fuels in DAFCs. Therefore, the design of novel
catalytic structures that possess high activities and, most of
all, are able to oxidize primary and secondary alcohols with
fast kinetics and tolerable deactivation is highly desirable
and remains a great challenge. Compared with Pt-based cat-
alysts, the less expensive and widely available Pd has been
exploited as a substitute for Pt in DAFCs. Significantly, the
real attraction for Pd-based electrocatalysts originates in the
fact that, unlike Pt-based electrocatalysts, they can be highly
active for the oxidation of a large variety of substrates in an
alkaline medium. Although the formation of carbonates
may damage performance, DAFCs operating in alkaline
conditions also have some other obvious advantages, such as
improved reaction kinetics and a less corrosive environment
5
0
50
in turn result from the cavities formed between the fused
nanoparticles. The Brunauer–Emmett–Teller (BET) surface
2
À1
area of Pd Pt BANs was calculated to be 19.04 m g
(
5
0
À1
50
2
2885 m mol ). The surface area of the Pd Pt BANs is
50 50
comparable with those of nanoporous AuPd bimetallic
[
2b]
foams
and interconnected hierarchical porous palladium
[28]
nanostructures. Pore size distribution curves indicated the
presence of a broad range of pores from micropores (<
2
nm) to mesopores (2–50 nm; Figure 4B). In additional, the
absence of a plateau at high relative pressure (P/P ) in the
0
adsorption isotherm implies the simultaneous presence of
macropores (pore diameters of between 50 and 138 nm),
which was also observed in the above SEM and TEM
images. As well as the featured alloy properties, the hier-
archical porous structure of the PdPt BANs is also advanta-
geous for the transport and diffusion of electrolyte ions if
the as-prepared materials are used as catalytic materials.
The wide-angle XRD patterns (Figure 5) for all the PdPt
BANs showed several intense peaks that were assigned to
[20]
to the electrodes. Moreover, Pd bimetallic alloys, in place
of pure Pd, possess enhanced electrochemical activity be-
cause the bimetallic alloys can facilitate desorption of mole-
cule species and result in greater surface-site availability and
[31]
thus higher catalytic activity. Based on these advantages,
we expect our newly synthesized PdPt BANs could be used
as advanced nanoelectrocatalysts for different alcohols
(
ethanol/methanol) in an alkaline medium.
It is reported that Pd-based catalysts are remarkably at-
tractive for ethanol electrochemical oxidation in an alkaline
medium, and represent an important alternative to Pt-based
[32]
catalysts for direct ethanol alkaline fuel cells. Figure 6A
shows the typical cyclic voltammograms (CVs) of Pd Pt
33
67
BANs, Pd NSs, and Pt NSs in N -saturated 0.5m H SO . All
2
2
4
samples exhibited a strong peak related to proton reduction/
hydrogen oxidation below 0.1 V. In the positive scan,
metals/alloys were oxidized between 0.5 and 0.6 V. In the
negative scan, the oxidized metals/alloys were reduced be-
tween 0.55 and 0.47 V. It is observed that the reduction peak
potential of the Pd Pt BANs is between those of Pt and
Figure 5. XRD patterns of Pd, Pt and PdPt BANs with various composi-
tions; a) Pt, b Pd50Pt50, c) PD67Pt33, d) Pd75Pt25, e) Pd.
fcc structures. In the case of the Pd NSs, the XRD peaks at
40.16, 46.58, 68.03, 82.02, and 86.488 correspond to (111),
6
7
33
(
200), (220), (311), and (222) planes, respectively. It is worth
Pd, which indicates that Pd Pt BANs are formed as an
67 33
[24b]
noting that as the Pd content was increased, the diffraction
peaks of these PdPt BANs (e.g., (220) peaks) were slightly
shifted to a larger angle range owing to the difference in
atomic size between Pt and Pd. Peak positions of PdPt
BANs are between those of Pd NSs and Pt NSs, which sug-
alloy, not as a core/shell structure.
The obtained PdPt
BANs are electrocatalytically active for ethanol oxidation
reaction (EOR) and the electro-oxidation curves of these
PdPt BANs in aqueous 0.5m NaOH+1m ethanol solution
are shown in Figure 6B. Of the three kinds of PdPt BANs
investigated, Pd Pt BANs have the highest activity and
[29]
gests that an alloy of Pd and Pt was formed.
However,
6
7
33
unlike PdAg and PdAu alloy nanostructures, it was very dif-
ficult to resolve Pt and Pd in the XRD patterns because the
lattice mismatch ratio of Pd/Pt was 0.77%.
display a less positive anodic onset potential and high mass
current density, which resembles the behavior found for
[27b]
PdAg alloys.
Additionally, the peak current of the
DAFCs based on liquid fuels have attracted enormous at-
tention as power sources for portable electronic devices and
fuel-cell vehicles owing to the much higher energy density
of liquid fuels. Alcohols, such as methanol, ethanol, ethylene
glycol, and glycerol, exhibit high volumetric energy density,
Pd Pt BANs electrode towards EOR is almost hundred
6
7
33
times larger than that in an N -saturated 0.5m NaOH aque-
2
ous solution (Figure S3 in the Supporting Information),
which indicates the excellent electrocatalytic activity of
PdPt BANs. Chronoamperometry, a useful method for eval-
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S. Dong et al.
À1
2 2 4
Figure 6. A) CVs of a) Pt, b) Pd67Pt33 BANs, and c) Pd in N -saturated H SO solution (0.5m ) at a scan rate of 50 mVs . B) CVs of a) Pd67Pt33,
À1
b) Pd75Pt25, and c) Pd50Pt50 BANs electrodes in an aqueous 0.5m NaOH+1m ethanol solution at a scan rate of 50 mVs . C) Current density–time curves
of a) Pd67Pt33, b) Pd75Pt25, and c) Pd50Pt50 BANs electrodes in aqueous 0.5m NaOH+1m ethanol solution at À0.2 V. D) CVs of a) Pd67Pt33 BANs, b) Pd,
À1
c) Pt, and d) E-TEK Pd/C electrodes in aqueous 0.5m NaOH+1m ethanol solution at a scan rate of 50 mVs . E) Current density–time curves of
a) Pd67Pt33 BANs, b) Pd, c) Pt, and d) E-TEK Pd/C electrodes in aqueous 0.5m NaOH+1m ethanol solution at À0.2 V. F) CVs of Pd67Pt33 BANs elec-
À2
trode in aqueous 0.5m NaOH+1m ethanol solution a) before and b) after stability test. The loading amount of noble metal was 71.4 mgcm for each cat-
alyst.
uation of electrocatalyst stability in fuel cells, was also em-
ployed to investigate the electrochemical activity and stabili-
ty of different catalysts. Current–time responses were moni-
tored at À0.25 V for 200 s in an aqueous solution of 0.5m
NaOH+1m ethanol, and the typical results are shown in
Figure 6C. As expected, it is observed that of these three
electrocatalysts, the Pd Pt BANs exhibited best perform-
the EOR activities for the Pd Pt BANs before and after
67 33
stability test. There is little change in the CVs after the sta-
bility test, which illustrates that the PdPt BANs are a class
of stable electrocatalysts for EOR. Furthermore, there is
also no visible morphology change in the Pd Pt BANs
6
7
33
after the stability test (Figure S4 in the Supporting Informa-
tion). It is generally accepted that a CH CO intermediate
6
7
33
3
ads
ance for EOR, which is consistent with the CVs described
above. The catalytic performance of Pd Pt BANs was fur-
is formed on the Pd catalyst in the alkaline medium and its
[32c,33]
reaction with OHads is the rate-determining step.
Given
6
7
33
ther compared with Pd NSs, Pt NSs, and commercial Pd/C.
Figure 6D shows the CVs of EOR catalyzed by the above-
mentioned catalysts in 0.5m NaOH+1m ethanol solution. It
can be seen that the optimized Pd Pt BANs show a signifi-
the high electrocatalytic activity and stability towards EOR,
it is postulated that both electronic and synergistic effects
between Pd and Pt are beneficial to promote the absorption
À
of OH and consequently to desorb the intermediate prod-
6
7
33
À1
cant increase in peak current density (1.62 Amg ) for EOR
ucts formed during the EOR. However, the very high elec-
trocatalytic activity of the PdPt BANs can also be ascribed
to their 3D porous architectures with a high BET surface
area and a clean surface, which favor the exposure of cata-
lytic active sites and transport and diffusion of reactants due
to the existence of many mesopores and micropores. Taken
together, these factors clearly demonstrate that the obtained
3D PdPt BANs possess advantageous intrinsic structural as-
pects that impart improved catalytic performance towards
EOR.
À1
À1
compared with Pt (0.35 Amg ), Pd (1.35 Amg ), and Pd/C
(
À1
0.80 Amg ) catalysts. Obviously, the EOR activity of Pd
and PdPt alloy in alkaline media is significantly higher than
[20b,32c]
that of Pt, which agrees well with previous reports.
The Pd Pt BANs catalysts are less likely to be poisoned
67
33
and have significantly higher activity, which indicates the
promoting effects of alloying with Pt. At the same time, the
electrochemical stability of EOR was confirmed by a subse-
quent chronoamperometric test (Figure 6E). Similarly,
Pd Pt BANs have higher current density at both the start
Of the pure metals, platinum has the highest catalytic ac-
tivity for the methanol oxidation reaction (MOR) in both
67
33
and the end of the stability test, which indicates that the al-
loyed Pt can enhance the electrochemical stability of
Pd Pt BANs for EOR in alkaline media. Figure 6F shows
[34]
acid and in alkaline media.
A variety of alloyed metals
have been investigated as MOR promoters on Pt in alkaline
67
33
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PdPt Bimetallic Alloy Nanosponges
FULL PAPER
for Pt-based electrocatalyst-
[20a]
s.
On the basis of the excel-
lent electrocatalytic activity
toward ethanol and methanol
in an alkaline medium, it is be-
lieved that the unsupported
PdPt BANs can be used as ad-
vanced
electrocatalysts
in
DAFCs.
Interestingly, we further ex-
tended the method to success-
fully prepare PdAu and AuPt
BANs. Figure 8 shows the
SEM images of the as-pre-
pared PdAu (Figure 8A, B)
and AuPt BANs (Figure 8C,
D). It is noted that both of
them possess the same sponge-
like morphology, similar to
that of PdPt BANs. Aside from
these BANs, AuPtPd trimetal-
lic alloy nanosponges (TANs)
were also facilely synthesized
through alternation of the
metal precursors in the syn-
thetic process. The more de-
tailed molar composition of
noble metal species used to
synthesize BANs/TANs are
Figure 7. A) CVs of a) Pd50Pt50, b) Pd67Pt33, and c) Pd75Pt25 BANs electrodes in aqueous 1m KOH +1m metha-
nol solution at a scan rate of 50 mVs . B) Current density–time curves of a) Pd75Pt25, b) Pd67Pt33, and
À1
c) Pd50Pt50 BANs electrodes in aqueous 0.5m NaOH+1m ethanol solution at À0.2 V. C) CVs of a) Pd50Pt50
BANs, b) E-TEK Pt/C, c) Pt, and d) Pd electrodes in aqueous 1m KOH +1m methanol solution at a scan rate
À1
of 50 mVs . D) Current density–time curves of a) Pd50Pt50 BANs, b) E-TEK Pt/C, c) Pt, and d) Pd electrodes
À2
in aqueous 1m KOH +1m methanol solution at À0.2 V. The loading amount of noble metal was 71.4 mgcm
for each catalyst.
media. Under the circumstances, we expect that our newly
designed 3D PdPt BANs could also be used as an advanced
catalyst to enhance the electrocatalytic activity towards
MOR, further extending their application field in DAFCs.
As such, the CVs displayed in Figure 7A reveal that the cat-
alytic activity towards MOR is sensitive to the Pt content of
the alloy and that the Pd Pt BANs exhibits the highest ac-
shown in Table 1. Until now, this is the most convenient
method to construct 3D BANs/TANs and we strongly be-
lieve that these newly designed nanomaterials could find
wide applications in various fields.
Conclusion
5
0
50
tivity. The enhanced electrochemical stability of Pd Pt
50
5
0
BANs was also confirmed by the current–time response
Figure 7B). This composition-tuned trend of electrocatalyt-
We have demonstrated a simple and general strategy for the
synthesis of novel 3D PdPt BANs in the absence of a cap-
(
ic activity is different from the discussion of EOR above,
which can be attributed to the different electrocatalytic
ping agent. By addition of aqueous NaBH solution to the
4
metal precursors, fusion of bare metal nuclei formed during
the rapid reduction process can be induced to form 3D
BANs instantaneously. Significantly, the as-prepared PdPt
BANs exhibited greatly enhanced activity and stability to-
wards ethanol/methanol electrooxidation in an alkaline
medium, which demonstrates the potential application of
these PdPt BANs as effective electrocatalysts for DAFCs.
The findings in this report are important not only for the
synthesis of 3D BANs with large BET surface areas, but
also the development of advanced nanoelectrocatalysts to
improve the efficiency of DAFCs. In addition, this simple
method has also been applied for the synthesis of AuPt,
AuPd BANs, and AuPtPd TANs through alternation of the
precursors in the synthetic process. Because of their conven-
ient preparation, the as-synthesized 3D BANs/TANs well-
defined sponge-like network, large-scale production, and
high electrocatalytic performance for EOR and MOR may
[20a]
mechanism between ethanol and methanol.
It can be
seen that whereas the current density of Pd NSs is little
higher than at Pt NSs, the anodic onset potential and peak
potential at Pt NSs were negative, which represents im-
proved kinetics at Pt NSs. Compared with the monometallic
catalysts (Pt and Pd NSs) and commercial Pt/C catalyst, the
unsupported Pd Pt BANs also show higher electrocatalytic
50
50
activity and stability (Figure 7C, D). In detail, the mass ac-
tivity on the Pd Pt BANs electrode is about 2.84, 3.24, and
50
50
3.14 times greater than on the Pd NSs, Pt/C, and Pt NSs
electrode. According to mechanistic studies of MOR on Pd-
based electrocatalysts, this enhanced MOR electroactivity in
alkaline media at the obtained Pd Pt BANs can be ex-
5
0
50
plained on the basis of a more effective promoting mecha-
nism for COads removal, which thus decreases electrode poi-
soning, similar to the general bifunctional mechanism theory
Chem. Eur. J. 2013, 19, 1104 – 1111
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1109

S. Dong et al.
Figure 8. SEM images of the as-obtained AuPt (A, B) and AuPd BANs (C, D) and AuPtPd TANs (E, F).
surface of the modified GCE and dried before the electrochemical ex-
periments took place.
Table 1. Compositional molar ratios of Pd and Pt species in precursor
solutions.
H
5
2
PdCl
4
[mL]
H
–
2
PtCl
6
[mL]
4
HAuCl [mL]
Pd NSs
–
Pd75Pt25 BANs
Pd67Pt33 BANs
Pd50Pt50 BANs
Pt NSs
3.75
3.33
2.5
–
1.25
1.67
2.5
5
–
–
–
–
Acknowledgements
AuPt BANs
AuPd BANs
AuPtPd TANs
–
2.5
1.67
2.5
–
1.67
2.5
2.5
1.67
This work was supported by the National Natural Science Foundation of
China (nos. 20935003 and 20820102037) and the 973 Project (nos.
2009CB930100 and 2010CB933603).
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1110
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ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2013, 19, 1104 – 1111

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Published online: November 23, 2012
Chem. Eur. J. 2013, 19, 1104 – 1111
ꢁ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
1111