Self-Supported Interconnected Pt Nanoassemblies as Highly Stable
Electrocatalysts for Low-Temperature Fuel Cells
Bao Yu Xia, Wan Theng Ng, Hao Bin Wu, Xin Wang,* and Xiong Wen (David) Lou*
Low-temperature fuel cells have attracted considerable
attention because of their high power density, low operating
temperature (<1208 8C), and reduced pollution as a new
power source for automobiles and portable electronic devi-
ces.[1,2]However, their commercialization is impeded by the
poor durability and activity of electrocatalysts.[2–4]State-of-
the-art electrocatalysts primarily consist of Pt nanoparticles
(NPs) supported on carbon black (Pt/CB).The poor
durability of Pt/CB catalysts is reflected in a fast and
significant loss of electrochemical surface area (ECSA) and
thus degradation of the fuel cells performance. It has been
argued that the loss of ECSA is mainly ascribed to the
corrosion of carbon supports, which further results in migra-
tion, aggregation, and Ostwald ripening of Pt NPs because of
their high surface energy and zero-dimensional (0D) struc-
One effective strategy to improve the durability of
electrocatalysts is to use one-dimensional (1D) Pt nano-
structures, including nanowires (NWs),[8,9]nanorods,[10,11]and
nanotubes,[12,13]owing to their inherent anisotropic morphol-
ogy and unique structure compared to the isotropic 0D
Pt NPs.[14–16]Meanwhile, the 1D Pt nanostructures have
a unique combination of dimensions in multiple length
scales, and they do not require a high surface-area support
(e.g. carbon black). Thus, they have the potential to avoid the
carbon-corrosion problem and further improve mass-trans-
port characteristics.[13,17]Nevertheless, most of the reported
1D Pt nanocatalysts are in the form of freestanding nano-
crystals, which are similar to the Pt/CB catalyst.Recently,
assembly of 1D nanostructured Pt into two-dimensional (2D)
membranesand even three-dimensional (3D) nano-net-
works[19,20]has attracted remarkable attention because of
their many unique structural characteristics, including high
porosity, good flexibility, large surface area per unit volume,
and interconnected open-pore structures.[20,21]Thus, the syn-
thesis of controlled NWassembly would be an important new
development, because each individual NW can be connected
with a number of NWs in different ways to provide a large
diversity of interconnectivity.[19,22,23]Consequently, the many
efforts devoted to this area have led to 3D NW super-
structures, such as arrays,[24–26]networks,and hierarchical
structures.Compared with template-assisted strategies for
nanoassemblies,[19,20,28]however, it is significantly more chal-
lenging to develop a direct synthesis approach for 3D
Herein, we report the synthesis, characterization, and
electrochemical evaluation of Pt nanoassemblies prepared by
a one-pot method (see Supporting Information for the
experimental details). We found that each Pt nanoassembly
contained more than ten interconnected Pt NWs. This
structure could maximize the surface area to volume ratio
and therefore decrease the amount of catalytically inactive
support material, while simultaneously minimizing the load-
ing of the precious metal.[7,32]Moreover, the interconnected
3D nanoassemblies, consisting of long nanowires, make the Pt
less vulnerable to dissolution, migration, Ostwald ripening,
and aggregation compared to the 0D Pt nanoparticles.
Furthermore, the mass transfer within the electrode can be
facilitated by building 3D porous structures with the aniso-
tropic, interconnected Pt NWs. Because of their many advan-
tages, such 3D Pt-nanoassembly catalysts exhibit higher
durability and activity than commercial electrocatalysts
made of CB-supported 0D Pt nanoparticles.
Figure 1a shows a representative field-emission scanning
electron microscopy (FESEM) image of Pt nanoassemblies.
The as-prepared Pt nanoassemblies were of high uniformity
and each nanoassembly was composed of many long Pt NWs.
It is interesting that the Pt NW subunits point out in various
directions to form the 3D hierarchical structure. X-ray
diffraction (XRD) and energy dispersive X-ray (EDX)
analyses confirmed that the as-prepared samples consisted
exclusively of Pt (Supporting Information, Figure S1 and S2)
with a face-centered cubic (fcc) structure (JCPDS card no. 04-
0802). The morphology and structure of the nanoassemblies
were further characterized by transmission electron micros-
copy (TEM). An overview TEM image of Pt nanoassemblies
is shown in Figure 1b, displaying that each 3D Pt nano-
assembly contained about 8–15 Pt NWs. The diameter and
length of the Pt NWs were in the range of 5–10 nm and 100–
200 nm, respectively, corresponding to an aspect ratio of
about 20. The selected-area electron-diffraction (SAED)
pattern (inset of Figure 1b) of Pt nanoassemblies showed
concentric rings, composed of bright discrete diffraction spots,
that were indexed to (111), (220), (311), and (331) crystal
planes of fcc Pt, indicating the high degree of crystallinity of
individual NWs. A TEM image with high magnification of
a single Pt nanoassembly is shown in Figure 1c. We clearly
[*] Dr. B. Y. Xia, W. T. Ng, H. B. Wu, Prof. X. Wang, Prof. X. W. Lou
School of Chemical and Biomedical Engineering, Nanyang Techno-
70 Nanyang Drive, Singapore 637457 (Singapore)
Dr. B. Y. Xia, Prof. X. W. Lou
Energy Research Institute @ NTU, Nanyang Technological University
50 Nanyang Drive, Singapore 637553 (Singapore)
Supporting information for this article (experimental details) is
available on the WWW under http://dx.doi.org/10.1002/anie.
Angew. Chem. Int. Ed. 2012, 51, 1–5 ? 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
These are not the final page numbers! ??
observed that the Pt nanoassembly consists of about
15 Pt NWs that are approximately 10 nm in diameter and
200 nm in length. More interestingly, these NWs are inter-
connected at the center. This pattern suggests that the
formation of a Pt nanoassembly might involve growth from
a common point rather than the simple aggregation of
preformed particles.The detailed structural features of
the NWs were characterized by high-resolution TEM
(HRTEM), as shown in Figure 1d. HRTEM observation of
a Pt NW revealed the single-crystal nature of the NW, with
a growth direction along the h111i axis. The lattice spacing of
0.23 nm (Figure 1e) corresponded well to the interplane
spacing of Pt (111) planes. Furthermore, the Pt NW had a step
or concave surface topology and high-index exposed facets as
marked in Figure 1 f, as well as in regions 1 and 3 in Figure 1d.
The fast Fourier transform (FFT) patterns of the atomic
lattice fringes were identical, as displayed in the insets of
Figure 1e,f, further showing the single crystallinity of the NW
along the h111i direction. These highly crystalline Pt nano-
assemblies, consisting of interconnected Pt NWs, are believed
to be a good candidate for fuel-cell catalysts.
Tostudythe mechanism of3DPtnanoassemblyformation
from interconnected 1D Pt NWs, the roles of synthesis
reagents were investigated in detail. It was previously
reported that oleylamine could facilitate the formation and
growth of Pt NWs/nanorods.In the present synthesis
system, oleylamine serves simultaneously as the solvent,
reducing agent, and stabilizer for the formation of metal
nanostructures.Oleylamine might act as a ligand to form
stable Pt4+complexes and lead to the reduction of Pt4+
complexes to a metallic state at an elevated temperature.
Additionally, cetyltrimethylammonium bromide (CTAB)
might play an important role as the structure-directing
agent in the formation of multi-wire structures.Only
irregular multipods were obtained without the addition of
CTAB (Figure 2a). When the amount of CTAB was increased
to 50 mg, the desired 3D Pt nanoassemblies containing
interconnected Pt NWs were formed. Further increasing the
amount of CTAB (100 mg) resulted in the formation of Pt
nanoassemblies composed of many short nanorods. This
result suggests that the interconnected Pt networks are likely
to form only in the presence of CTAB.The evolution of the
morphology with reaction time has also been studied to
understand the formation of Pt nanoassemblies. As revealed
by the FESEM and TEM images in Figure S3 (see Supporting
Information), the length of constituent Pt NWs increased as
the reaction progressed. This observation suggests that the
slow reduction of Pt4+ions in oleylamine solution, in the
presence of CTAB, favors the growth of Pt nanocrystals along
the h111i direction and thereafter the formation of hierarch-
We next evaluated the electrocatalytic activity of the Pt
nanoassemblies for the oxygen-reduction reaction (ORR).
This test is conducted in an O2-saturated 0.5m H2SO4solution
using a glassy carbon (GC) rotation disk electrode (RDE) at
room temperature with a sweep rate of 10 mVs?1. Figure 3a
shows the ORR polarization curves for Pt nanoassemblies,
commercial Pt electrocatalyst (E-TEK, 50 wt% Pt), and self-
prepared Pt/CB (20 wt% Pt; see Supporting Information,
Figure S4 and S5 for XRD/TEM characterizations of com-
mercial Pt and Pt/CB). The half-wave potentials of Pt
nanoassemblies, commercial Pt, and Pt/CB were 0.839,
0.829, and 0.818 V, respectively, showing that the activity of
Figure 1. a) Representative FESEM and b) TEM images of the intercon-
nected Pt nanoassemblies. Inset of (b): SAED pattern of Pt nano-
assemblies. c) TEM image of a single Pt nanoassembly consisting of
NWs. d) HRTEM micrograph of a single-crystalline Pt NW, with the
direction of growth along the h111i axis; e,f) the magnified HRTEM
micrographs recorded from regions 1 and 2 marked in (d), respectively.
Insets of (e,f): corresponding FFT patterns.
Figure 2. TEM images of Pt nanoassemblies prepared with or without
CTAB: a) 0; b) 100 mg CTAB.
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the Pt nanoassemblies was higher than that of commercial Pt
and Pt/CB catalysts. Mass activity and specific activity are
important parameters of electrocatalysts. As shown in Fig-
ure 3b, Pt nanoassemblies exhibited a mass activity of
12.4 Ag?1Pt at 0.85 V (versus a reversible hydrogen electrode
(RHE)), which was about 1.5- and 2.1-times of that of
commercial Pt and Pt/CB, respectively. Despite having
a lower Pt ECSA than commercial Pt (Figure 3c), the specific
ORR activity of Pt nanoassemblies was 2.2- and 1.7-times of
that of commercial Pt and Pt/CB catalysts, respectively
(Figure 3d). This improvement in activity agreed well with
the results for Pt NWs.[13,18]Furthermore, an ORR durability
test was carried out by accelerated cyclic voltammetry (CV).
After 3000 cycles, the polarization curves of the commercial
Pt catalyst showed a significant drop in the half-wave
potential and an associated decrease in the diffusion-limited
current (Figure 3e). On the contrary, Pt nanoassemblies
showed only a slight decrease in the diffusion-limited current,
and the half-wave potential remains almost unchanged over
3000cycles (Figure 3 f). It is therefore apparent that the Pt
nanoassemblies possessed superior durability in terms of both
ECSA retention and, more importantly, activity retention,
when compared with the commercial Pt electrocatalyst.
The comparative electrocatalytic performances of the
three Pt materials were further evaluated using the CV
technique. First, the CV curves of the three catalysts were
recorded at room temperature in a 0.5m H2SO4solution at
a sweep rate of 100 mVs?1. As shown in Figure 4a–c, all CV
curves exhibited strong peaks associated with hydrogen
adsorption/desorption (HAD) between ?0.24 and 0.2 V, and
Pt oxide formation/reduction in the range of 0.2–1.0 V. The
ECSA was calculated by integrating the charge passing the
electrode during the HAD, after correction for double-layer
formation. The charge required to oxidize a hydrogen mono-
layer was measured as 0.21 mCcm?2, which corresponded to
a surface density of 1.3?1015Pt atoms per cm2. As shown in
Figure 3c, the specific ECSA of Pt nanoassemblies was
40.8 m2g?1, which was about 65% of that of the commercial
E-TEK Pt catalyst (62.3 m2g?1). The lower ECSA for the Pt-
nanoassembly catalyst was most likely because of the large
size of the Pt nanoassemblies (about 10 nm in diameter)
compared with that of Pt nanoparticles (about 3 nm) in
commercial catalysts. However, it was similar to that of Pt/CB
(40.1 m2g?1) because of the interconnected 3D hierarchical
assembly and anisotropic NW building blocks, which can
improve mass transport and catalyst utilization. The Pt
nanoassemblies were also evaluated as an electrocatalyst for
the methanol oxidation reaction (MOR), and exhibited
improved catalytic activity and stability, as shown in CV and
chronoamperometry (CA) curves (Supporting Information,
The durability of the electrocatalysts remains one of the
most important issues to be addressed before the widespread
application of fuel cells becomes feasible. Accelerated
durability tests (ADT) of the catalysts were conducted by
cycling the potential between ?0.24 and 1.0 V in a 0.5m
H2SO4solution at room temperature, for evaluation of the
long-term electrochemical stability of the catalysts.[13,17,18]For
Figure 3. a) Polarization curves for ORR of Pt nanoassemblies (NAs),
commercial Pt and Pt/CB in an O2saturated 0.5m H2SO4solution at
room temperature (1600 rpm; sweep rate: 10 mVs?1). j=current
density. b) Mass activity (at 0.85 V vs. RHE), c) ECSA, and d) specific
activity for the three catalysts. The polarization curves were obtained
after different numbers of cycles for commercial Pt (e) and Pt nano-
Figure 4. Comparison of the electrochemical durability of Pt nano-
assemblies, commercial Pt catalysts (E-TEK), and self-prepared
20 wt% Pt/CB. Cyclic voltammetry (CV) curves for a) Pt nanoassem-
blies, b) commercial Pt catalyst, and c) Pt/CB catalyst after prolonged
cycles of CV. The durability tests were carried out at room temperature
in 0.5m H2SO4solution with a sweep rate of 100 mVs?1. SCE=satu-
rated calomel electrode. d) Loss of ECSA of Pt nanoassemblies (NAs),
commercial Pt catalyst, Pt/CB catalyst with the number of CV cycles.
Angew. Chem. Int. Ed. 2012, 51, 1–5? 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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comparison, the stabilities of commercial Pt catalyst (E-TEK,
50 wt% Pt) and self-prepared 20 wt% Pt/CB were also
studied under identical conditions. Figure 4a–c shows the
CV curves of the three catalysts after 1000–5000 cycles. The
current densities of the peaks in HAD potential region
(?0.24–0.2 V) for the commercial Pt catalyst (Figure 4b) and
Pt/CB (Figure 4c) dropped dramatically with the increase of
the number of CV cycles. In contrast, the Pt nanoassemblies
catalyst (Figure 4a) exhibited only a slight drop in the current
densities of the peaks upon cycling in the same potential
range. The loss of ECSA with cycling was plotted in Fig-
ure 4d. After 5000 cycles, the Pt nanoassembly catalyst lost
only 16% of the initialPt ECSA, while the commercial Pt and
Pt/CB catalysts lost 85% and 95% of their initial ECSA,
respectively. Even when the ADT test for self-supported Pt-
nanoassembly catalysts was further prolonged to 10000
cycles, the ECSA only dropped about 27.5%. Moreover, in
the CV profiles (Figure 4b,c), a decrease in the double-layer
capacitance by about 75% for commercial Pt and Pt/CB
catalysts was recorded after the Pt degradation test, which is
related to the corrosion of the amorphous carbon support.
However, no significant change in double-layer capacitance
was recorded for Pt nanoassemblies owing to the absence of
a carbon support. The ADT results demonstrated that Pt
nanoassemblies have significantly higher stability than the
commercial Pt and Pt/CB catalysts. To understand the loss of
ECSA, a post-cyclization TEM investigation was carried out
for the commercial Pt and Pt nanoassembly electrocatalysts
(Supporting Information, Figure S7).
In summary, novel 3D Pt nanoassemblies consisting of 1D
single-crystal Pt nanowires were prepared by a one-pot
method. The presence of CTAB was found to be crucial for
the formation of a 3D interconnected structure. The inter-
connected 3D Pt nanoassemblies showed a higher resistance
to dissolution, migration, Ostwald ripening, and aggregation
compared to the 0D Pt nanoparticles. Benefitting from
unique structural features, these interconnected 3D Pt nano-
structures, when evaluated as electrocatalysts for low-temper-
ature fuel cells, manifest high electrochemical surface area
and significantly improved long-term stability compared to
the 0D Pt nanoparticles (commercial Pt catalyst and self-
prepared Pt/CB nanoparticles catalyst).
Received: February 26, 2012
Published online: && &&, &&&&
nanowires · platinum
Keywords: cyclic voltammetry · electrocatalysts · fuel cells ·
 R. F. Service, Science 2002, 296, 1222.
 R. Borup, J. Meyers, B. Pivovar, Y. S. Kim, R. Mukundan, N.
Garland, D. Myers, M. Wilson, F. Garzon, D. Wood, P. Zelenay,
K. More, K. Stroh, T. Zawodzinski, J. Boncella, J. E. McGrath,
M. Inaba, K. Miyatake, M. Hori, K. Ota, Z. Ogumi, S. Miyata, A.
Nishikata, Z. Siroma, Y. Uchimoto, K. Yasuda, K.-i. Kimijima,
N. Iwashita, Chem. Rev. 2007, 107, 3904.
 B. C. H. Steele, A. Heinzel, Nature 2001, 414, 345.
 J. Zhang, K. Sasaki, E. Sutter, R. R. Adzic, Science 2007, 315,
 Y. Bing, H. Liu, L. Zhang, D. Ghosh, J. Zhang, Chem. Soc. Rev.
2010, 39, 2184.
 X. Wang, W. Z. Li, Z. W. Chen, M. Waje, Y. S. Yan, J. Power
Sources 2006, 158, 154.
 L. Cademartiri, G. A. Ozin, Adv. Mater. 2009, 21, 1013.
 Z. Xu, C. Shen, Y. Hou, H. Gao, S. Sun, Chem. Mater. 2009, 21,
 C. Koenigsmann, W.-p. Zhou, R. R. Adzic, E. Sutter, S. S. Wong,
Nano Lett. 2010, 10, 2806.
 S. Maksimuk, S. Yang, Z. Peng,H. Yang, J. Am. Chem. Soc. 2007,
 J. Mao, X. Cao, J. Zhen, H. Shao, H. Gu, J. Lu, J. Y. Ying, J.
Mater. Chem. 2011, 21, 11478.
 T. Kijima, T. Yoshimura, M. Uota, T. Ikeda, D. Fujikawa, S.
Mouri, S. Uoyama, Angew. Chem. 2004, 116, 230; Angew. Chem.
Int. Ed. 2004, 43, 228.
 Z. Chen, M. Waje, W. Li, Y. Yan, Angew. Chem. 2007, 119, 4138;
Angew. Chem. Int. Ed. 2007, 46, 4060.
 Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F.
Kim, H. Yan, Adv. Mater. 2003, 15, 353.
 H. Lee, S. E. Habas, S. Kweskin, D. Butcher, G. A. Somorjai, P.
Yang, Angew. Chem. 2006, 118, 7988; Angew. Chem. Int. Ed.
2006, 45, 7824.
 T. K. Sau, A. L. Rogach, Adv. Mater. 2010, 22, 1781.
 Y. Liu, D. Li, S. Sun, J. Mater. Chem. 2011, 21, 12579.
 H.-W. Liang, X. Cao, F. Zhou, C.-H. Cui, W.-J. Zhang, S.-H. Yu,
Adv. Mater. 2011, 23, 1467.
 D. Wang, H. Luo, R. Kou, M. P. Gil, S. Xiao, V. O. Golub, Z.
Yang, C. J. Brinker, Y. Lu, Angew. Chem. 2004, 116, 6295;
Angew. Chem. Int. Ed. 2004, 43, 6169.
 M. Rauber, I. Alber, S. M?ller, R. Neumann, O. Picht, C. Roth,
A. Schçkel, M. E. Toimil-Molares, W. Ensinger, Nano Lett. 2011,
 J. N. Tiwari, R. N. Tiwari, K. S. Kim, Prog. Mater. Sci. 2012, 57,
 S. H. Sun, D. Q. Yang, D. Villers, G. X. Zhang, E. Sacher, J. P.
Dodelet, Adv. Mater. 2008, 20, 571.
 E. P. Lee, J. Chen, Y. Yin, C. T. Campbell, Y. Xia, Adv. Mater.
2006, 18, 3271.
 J. H. Yuan, K. Wang, X. H. Xia, Adv. Funct. Mater. 2005, 15, 803.
 U. H. Lee, J. H. Lee, D. Y. Jung, Y. U. Kwon, Adv. Mater. 2006,
 X. Zhang, W. Lu, J. Da, H. Wang, D. Zhao, P. A. Webley, Chem.
Commun. 2009, 195.
 M. J. Bierman, Y. K. A. Lau, A. V. Kvit, A. L. Schmitt, S. Jin,
Science 2008, 320, 1060.
 A. Takai, H. Ataee-Esfahani, Y. Doi, M. Fuziwara, Y. Yamauchi,
K. Kuroda, Chem. Commun. 2011, 47, 7701.
 V. Tzitzios, D. Niarchos, M. Gjoka, N. Boukos, D. Petridis, J. Am.
Chem. Soc. 2005, 127, 13756.
 Y. Song, Y. Yang, C. J. Medforth, E. Pereira, A. K. Singh, H. Xu,
Y. Jiang, C. J. Brinker, F. van Swol, J. A. Shelnutt, J. Am. Chem.
Soc. 2004, 126, 635.
 T. K. Sau, A. L. Rogach, F. J?ckel, T. A. Klar, J. Feldmann, Adv.
Mater. 2010, 22, 1805.
 A. Kloke, F. von Stetten, R. Zengerle, S. Kerzenmacher, Adv.
Mater. 2011, 23, 4976.
 M. A. Mahmoud, C. E. Tabor, M. A. El-Sayed, Y. Ding, Z. L.
Wang, J. Am. Chem. Soc. 2008, 130, 4590.
 Z. Niu, Q. Peng, M. Gong, H. Rong, Y. Li, Angew. Chem. 2011,
123, 6439; Angew. Chem. Int. Ed. 2011, 50, 6315.
 B. Lim, Y. Xia, Angew. Chem. 2011, 123, 78; Angew. Chem. Int.
Ed. 2011, 50, 76.
 Y. Song, R. M. Garcia, R. M. Dorin, H. Wang, Y. Qiu, E. N.
Coker, W. A. Steen, J. E. Miller, J. A. Shelnutt, Nano Lett. 2007,
These are not the final page numbers!
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Communications Download full-text
B. Y. Xia, W. T. Ng, H. B. Wu, X. Wang,*
X. W. Lou*
Self-Supported Interconnected Pt
Nanoassemblies as Highly Stable
Electrocatalysts for Low-Temperature
In it for the long haul: Clusters of Pt
nanowires (3D Pt nanoassemblies,
Pt NA) serve as an electrocatalyst for low-
temperature fuel cells. These Pt nano-
assemblies exhibit remarkably high sta-
bility following thousands of voltage
cycles (see graph) and good catalytic
activity, when compared with a commer-
cial Pt catalyst and 20% wt Pt catalyst
supported on carbon black (20% Pt/CB).
Angew. Chem. Int. Ed. 2012, 51, 1–5 ? 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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