Direct access to metal or metal oxide nanocrystals integrated with one-dimensional nanoporous carbons for electrochemical energy storage

Article abstract of DOI:10.1021/ja106612d

Metal and metal oxide nanocrystals have sparked great interest due to their excellent catalytic, magnetic, and electronic properties. Particularly, the integration of metallic nanocrystals and one-dimensional (1D) electronically conducting carbons to form

Full text of DOI:10.1021/ja106612d

Published on Web 10/01/2010
Direct Access to Metal or Metal Oxide Nanocrystals Integrated
with One-Dimensional Nanoporous Carbons for
Electrochemical Energy Storage
Yanyu Liang,† Matthias Georg Schwab,† Linjie Zhi,‡ Enrico Mugnaioli,§ Ute Kolb,§
Xinliang Feng,*,† and Klaus Mu¨llen*,†
Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128, Mainz, Germany,
National Center for Nanoscience and Technology of China, Zhongguancun, Beiyitiao 11,
100190, Beijing, P. R. China, and Institute of Physical Chemistry, Johannes Gutenberg
UniVersita¨t, Welderweg 11, D-55099, Mainz, Germany
Received July 26, 2010; E-mail: feng@mpip-mainz.mpg.de; muellen@mpip-mainz.mpg.de
Abstract: Metal and metal oxide nanocrystals have sparked great interest due to their excellent catalytic,
magnetic, and electronic properties. Particularly, the integration of metallic nanocrystals and one-dimensional
(1D) electronically conducting carbons to form metal-carbon hybrids can lead to enhanced physical and
chemical properties or even the creation of new properties with respect to single component materials.
However, direct access to thermally stable and structurally ordered 1D metal-carbon hybrids remains a
primary challenge. We report an in situ fabrication of Co3O4 or Pt nanocrystals incorporated into 1D
nanoporous carbons (NPCs) via an organometallic precursor-controlled thermolysis approach. The AB2-
type (one diene and two dienophile) 3,4-bis(4-dodecynylphenyl)-substituted cyclopentadienone and its
relevant cobalt or platinum complex are first impregnated into the nanochannels of AAO (anodic alumina
oxide) membranes. The intermolecular Diels-Alder reaction of these precursor molecules affords the
formation of cobalt or platinum functionalized polyphenylene skeletons. Subsequent thermolysis transforms
the polyphenylene backbones into 1D nanoporous carbonaceous frameworks, while the metallic moieties
are reduced into Co or Pt nanocrystals, respectively. After removal of the AAO template, 1D NPCs/Co3O4
or NPCs/Pt are obtained, for which structural characterizations reveal that high-quality Co3O4 or Pt
nanocrystals are distributed homogeneously within carbon frameworks. These unique 1D metal-carbon
hybrids exhibit a promising potential in electrochemical energy storage. NPCs/Co3O4 is evaluated as an
electrode material in a supercapacitor, for which Co3O4 nanocrystals contribute an exceptionally high
gravimetric capacitance value of 1066 F g-1. NPCs/Pt is applied as an electrocatalyst showing excellent
catalytic efficiency toward methanol oxidation in comparison to commercial E-TEK (Pt/C) catalyst.
or nanoelectronics.3-6 With respect to other substrate materials,
1D carbons such as carbon nanotubes (CNTs) and nanofibers
show advantages for growing and anchoring functional metallic
nanocrystals in view of their superior physical features including
Introduction
Metal and metal oxide nanocrystals have been extensively
studied owing to their unique magnetic, catalytic, and electronic
properties across a broad range of fundamental and technological
potential.1,2 In particular, the integration of metallic nanocrystals
and one-dimensional (1D) substrate materials to form nanoscale
heterostructures has been explored for a variety of applications
including biolabeling, chemical sensing, catalysis, and micro-
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† Max Planck Institute for Polymer Research.
‡ National Center for Nanoscience and Technology of China.
§ Johannes Gutenberg Universita¨t.
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10.1021/ja106612d  2010 American Chemical Society

Integration of Metallic Nanocrystals and 1D Carbons
A R T I C L E S
Scheme 1. Chemical Structure of the Four Precursors for the Fabrication of NPCs/Co3O4 and NPCs/Pt respectively: Cp, Co-Cp, BpCp, and
Pt-BpCp
high surface area, mechanical strength, and thermal stability.7
When appropriate metallic nanocrystals are incorporated into
1D electronically conducting carbons to form metal-carbon
hybrids, unprecedented physical and chemical properties become
available due to the effects of spatial confinement and synergetic
electronic interactions between metallic and carbonaceous
components.5,6,8
solid-state thermal treatment. The AB2-type (one diene and two
dienophile) 3,4-bis(4-dodecynylphenyl)-2,5-bis(4-dodecylphe-
nyl) cyclopentadienone (Cp, Scheme 1) and its corresponding
dicobalt hexacarbonyl complex (Co-Cp, Scheme 1) serve as
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In general, two well-established synthetic protocols have been
put forward for the fabrication of 1D metal-carbon hybrids:
(1) preformed metallic nanocrystals are connected to function-
alized 1D carbon frameworks via either covalent or noncovalent
interactions; and (2) naked metallic nanocrystals are directly
grown or deposited onto the surface of 1D carbons through
chemical reduction processes. For the first strategy, strong acid
assisted functionalization treatments are generally involved
which unavoidably introduce a large number of defect sites into
1D carbons, leading to the disruption of their intrinsic electronic
and mechanical properties.5,9 The second approach is attractive
toward creating 1D metal-carbon hybrids; however, the absence
of functional units renders it impossible to uniformly distribute
metallic nanocrystals onto 1D carbons.6,10 In addition, the
involvement of multiple and tedious synthetic steps in both
methods limits experimental simplicity, not to mention that the
agglomeration of metallic nanocrystals often occurs in their
synthesis. Although few reports on thermal decomposition of
commercially available organometallic complexes have been
demonstrated to achieve 1D metal-carbon hybrids, they suffer
from poor structural order owing to the difficulties in controlling
the nucleation and growth of metallic nanocrystals.11,12 There-
fore, establishing an efficient and facile synthesis protocol for
the homogeneous distribution of metallic nanocrystals into 1D
carbon frameworks remains a great challenge.13
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In this article, we present a novel strategy for in situ
fabrication of metallic nanocrystals integrated with 1D nanopo-
rous carbons (NPCs) via an organometallic precursor-controlled
thermolysis. The synthetic procedure typically involves the
impregnation of precursor molecules into inorganic nanochan-
nels of anodic alumina oxide (AAO) membranes followed by
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A R T I C L E S
Liang et al.
Characterization. For transmission electron microscopy (TEM)
and high-resolution transmission electron microscopy (HRTEM)
investigations, the samples were sprayed onto holey carbon coated
copper grids using the procedure described elsewhere.17 TEM and
HRTEM characterizations were carried out on Tecnai F30 and
Tecnai F20 transmission electron microscopes (Philips, FEI, TEC-
NAI), both equipped with a field emission gun and working at 200
and 300 kV respectively. Nanoelectron diffraction (NED) patterns
were acquired with a 10 C2 aperture, and the beam diameter was
approximately 30 nm. TIA (TEM Imaging & Analysis) software
was used for Energy dispersive X-ray (EDX) analysis. For scanning
transmission electron microscopy (STEM), a high angular annular
dark field (HAADF) detector was applied to enhance the contrast.
Fourier transform infrared (FTIR) spectra were recorded with a
Nicolet 730 FTIR spectrometer. Thermal gravimetric analysis
(TGA) was performed on a Mettler TGA/SDTA 851e thermobal-
Figure 1. Schematic illustration of the preparation of 1D NPCs/Co3O4 or
NPCs/Pt.
ance at a heating rate of 10 K min-1
.
molecular precursors for the buildup of cobalt functionalized
polyphenylene skeletons via Diels-Alder polymerization, which
are transformed into NPCs with the incorporation of Co3O4
nanocrystals (NPCs/Co3O4) after further thermolysis. Similarly,
3,4-bis(4-dodecynylphenyl)-2,5-bis(4-(2,2′-bipyridyl)phenyl) cy-
clopentadienone (BpCp, Scheme 1) and its corresponding
platinum dichloride complex (Pt-BpCp, Scheme 1) are chosen
for the synthesis of NPCs integrated with Pt nanocrystals (NPCs/
Pt). After removal of the AAO template, these novel 1D
metal-carbon hybrids are further evaluated in electrochemical
energy storage. Remarkably, the as-prepared NPCs/Co3O4 and
NPCs/Pt show superior electrochemical performances as active
electrode materials in supercapacitor and fuel cell devices,
respectively.
Electrochemical experiments were conducted on an EG&G
potentiostat/galvanostat Model 2273 instrument. A conventional cell
with a three-electrode configuration was used throughout the study.
For a supercapacitor device, a working electrode was prepared by
mixing NPCs/Co3O4 with carbon black (Mitsubishi Chemicals, Inc.)
and polytetrafluoroethylene (PTFE) binder. The weight ratio of these
three components was 80:10:10. For a fuel cell device, a working
electrode was prepared by a procedure described elsewhere.18
Briefly, 5 mg of as-prepared NPCs/Pt or commercially available
E-TEK (Pt/C) were dispersed in 1 mL of 0.05 wt % Nafion solution
by ultrasonication treatment for 5 min. Then a 25 µL aliquot of
the dispersion was transferred onto a mirror-polished glassy carbon
electrode. In these two devices, a platinum foil was applied as a
counter electrode with a standard calomel electrode (SCE) as a
reference electrode.
Experimental Section
Results and Discussion
Synthesis. The synthesis of Cp bearing two dodecynyl groups
has been reported previously.14 It has been shown that the
intermolecular Diels-Alder cycloaddition reaction between alkynyl
and cyclopentadienone moieties in the skeleton of Cp is an efficient
method for the generation of polyphenylene structures.14 One of
the two alkynyl functions was further transformed into a dicobalt
hexacarbonyl metal complex by treatment with dicobalt octacar-
bonyl Co2(CO)8, giving rise to the formation of Co-Cp (Figure
S1).14b Another precursor compound, BpCp containing 2,2′-
bipyridyl moieties, was prepared according to the synthetic route
shown in Scheme S1. A complex of Pt-BpCp was then obtained
by the coordination reaction of BpCp with potassium tetrachloro-
platinate K2PtCl4 (Figure S2).15
First, the precursors of Cp and Co-Cp or BpCp and Pt-BpCp
were impregnated into the nanochannels of AAO membranes
(Whatman International Ltd. with an average pore diameter of 200
nm). Heat treatment of the precursors in the confined channels at
150 °C generated cobalt or platinum functionalized polyphenylene
frameworks.14a,16 Subsequently, the samples were treated at 350
°C for 5 h under an argon atmosphere followed by thermolysis at
700 °C for 1 h. After the removal of the AAO template in a 3 M
NaOH solution, NPCs/Co3O4 or NPCs/Pt was obtained respectively
(Figure 1).
Structural Characterizations. FTIR spectroscopy and dif-
ferential scanning calorimetry (DSC, measured from 20 to 200
°C) were employed to investigate intermolecular Diels-Alder
cycloaddition reactions for Cp and Co-Cp or BpCp and Pt-BpCp.
Both DSC curves (Cp and Co-Cp or BpCp and Pt-BpCp)
disclose an exothermal peak at around 150 °C, attributable to
the Diels-Alder reaction between cyclopentadienone and alkyne
moieties (Figure S3). Figure 2a shows the FTIR spectra of the
Cp and Co-Cp mixture before and after the DSC measurement,
respectively. Three sharp peaks centered at 2084, 2045, and 2013
cm-1 can be assigned to the terminal carbonyl groups presented
in the skeleton of Co-Cp, in agreement with values reported in
the literature.19 The peak at the wavenumber 1711 cm-1 can be
assigned to the stretching vibration of the CdO group in the
cyclopentadienone moiety.20 After the heat treatment, the bands
in the range of 2013-2084 cm-1 are no longer detected, while
the peak corresponding to the carbonyl vibration significantly
decreases in intensity. A similar result can be observed in the
FTIR spectra of BpCp and Pt-BpCp before and after the DSC
measurement (Figure 2b). In fact, after the heating procedure
described above, the products are no longer soluble in normal
organic solvents. In association with the MALDI-TOF mass
spectra showing no detectable peaks, these results suggest a
successful intermolecular cross-linking process14a,16 of Cp and
(14) (a) Zhi, L.; Wu, J.; Li, J.; Stepputat, M.; Kolb, U.; Mu¨llen, K. AdV.
Mater. 2005, 17, 1492–1496. (b) Hamaoui, B. E.; Zhi, L.; Wu, J.; Li,
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(15) Smith, A. P.; Fraser, C. L. Macromolecules 2002, 35, 594–596.
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K. Angew. Chem., Int. Ed. 2007, 46, 3464–3467. (b) Zhi, L. J.; Wang,
J. J.; Cui, G. L.; Kastler, M.; Schmaltz, B.; Kolb, U.; Jonas, U.; Mu¨llen,
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Integration of Metallic Nanocrystals and 1D Carbons
A R T I C L E S
Figure 3. (a and b) TEM and STEM (dark field) images of NPCs/Co3O4
show 1D nanoporous carbons and well-dispersed Co3O4 nanocrystals with
sharp edges. (c) HRTEM image of a Co3O4 nanocrystal. (d) HRTEM image
of a Co3O4 nanocrystal oriented along [001]; the orthogonal (220) and
(2-20) lattice fringes are drawn. (e) NED pattern of a Co3O4 nanocrystal
oriented along [11-4] indicates that it is single crystalline cubic Co3O4. (f)
EDX shows the presence of both O and Co peaks.
average diameter of 200-500 nm and a length of a few
micrometers. On the basis of Brunauer-Emmett-Teller
(BET)16 and TEM studies (Figure 3a and b), the carbonaceous
frameworks possess a nanoporous structure with a pore diameter
ranging from 1 to 20 nm. High-contrast Co3O4 nanoparticles
with a size of 10 to 30 nm and sharp edges are homogeneously
distributed within these carbon textures (Figure 3a and b).
HRTEM and NED experiments further disclose that these cobalt
nanoparticles are single crystalline face-centered cubic Co3O4
j
(a ) 8.1 Å, space group of Fd3m, No. 227, Figure 3c-e).
Figure 2. FTIR spectra of (a) Cp and Co-Cp before and after the DSC
measurement; (b) BpCp and Pt-BpCp before and after the DSC measure-
ment. Stars indicate the terminal carbonyl, alkynyl, and CdO group of the
cyclopentadienone, respectively. For better comparison of the intensities,
Common lattice distances of 4.7, 4.1, 2.9, and 2.4 Å correspond
to {111}, {200}, {220}, and {311} planes, respectively. EDX
spectroscopy detects both the peaks of O and Co (Figure 3f).
In fact, the initially synthesized metallic nanocrystals are cobalt
as indicated by the X-ray diffraction (Figure S6). The formation
of cobalt oxide after the removal of the AAO template can be
attributed to the long-time impregnation in concentrated NaOH
solution which results in the external surface oxidation of Co
into Co3O4.22
To enhance the degree of crystallinity of the above synthe-
sized Co3O4 nanocrystals, the as-prepared NPCs/Co3O4 is further
annealed at 400 °C in air for 10 min. The morphology of NPCs
is preserved, whereas the optical contrast shown in Figure 4a
is lower than that of the nonoxidized form. The generated Co3O4
all spectra were normalized to the band at 2930 cm-1
.
Co-Cp or BpCp and Pt-BpCp leading to the formation of cobalt
or platinum functionalized polyphenylene backbones (Figure
S3).
Further heat treatment at 350 °C for 5 h is expected to promote
completion of the Diels-Alder reactions, affording a more rigid
polyphenylene skeleton within the AAO membranes.14a,16 Sub-
sequently, the samples are treated at 700 °C under an argon
atmosphere, allowing the transformation of the polyphenylene
backbones into carbonaceous frameworks in the membrane
channels, while the metallic moieties in the precursor of Co-
Cp or Pt-BpCp can be reduced into Co or Pt nanocrystals,
respectively. After the removal of the AAO template in a 3 M
NaOH solution, NPCs/Co3O421 and NPCs/Pt are achieved (see
below discussion for detailed structural characterizations).
As demonstrated in Figure S4, a large-scale STEM image
reveals that NPCs/Co3O4 comprises 1D materials with an
(21) The external metallic cobalt nanoparticles can be oxidized into cobalt
oxide during the removal of the AAO template process; see detailed
discussion and Figure S6.
(22) (a) Tzvetkoff, T.; Girginov, A.; Bojinov, M. J. Mater. Sci. 1995, 30,
5561–5575. (b) Gencheva, Tzvetkoff, R. T.; Bojinov, M. Appl. Surf.
Sci. 2004, 241, 459–470.
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A R T I C L E S
Liang et al.
Figure 5. (a) STEM image of NPCs/Pt (dark field) overview shows the
typical tubular feature of the carbons. (b) TEM image of NPCs/Pt discloses
the homogeneously dispersed platinum nanocrystals on 1D carbon substrates
with rounded shape. (c-e) HRTEM images of different oriented platinum
nanocrystals demonstrate that each particle is a single crystal. (f) NED
patterns show that the only crystalline phase is metallic Pt. (g) EDX reveals
both C and Pt peaks and a small O peak possibly coming from the Cu grid.
Figure 4. (a and b) TEM and STEM (dark field) images of NPCs/Co3O4
after oxidation treatment show 1D nanoporous carbons and the well-
dispersed Co3O4 nanocrystals with rounded shape. (c) HRTEM image of a
Co3O4 nanocrystal. (d) HRTEM image of a Co3O4 nanocrystal oriented along
[01-1]; the two lattice fringe sets (111) and (1-1-1) are drawn. (e) NED
pattern of a Co3O4 nanocrystal oriented along [01-1] indicates that it is
single crystalline cubic Co3O4. (f) EDX shows the presence of both O and
Co peaks.
Electrochemical Properties. The good dispersion of metallic
nanocrystals24 within 1D nanoporous carbon frameworks indi-
cates that the as-synthesized NPCs/Co3O4 and NPCs/Pt are
promising electrode materials for electrochemical energy stor-
age.25 As a typical example, NPCs/Co3O4 is evaluated in an
electrochemical capacitor device after additional oxidation. For
comparison, conventional multiwalled carbon nanotube sup-
ported polycrystalline Co3O4 nanoparticles (CNTs/Co3O4, prepa-
ration procedure can be found in the Supporting Information)
with the same mass loading of cobalt species were also
investigated under the same experimental conditions. Figure 6a
presents representative cyclic voltammograms (CV) measured
at a scan rate of 10 and 50 mV s-1 for both NPCs/Co3O4 and
CNTs/Co3O4, respectively. None of the CV curves exhibits a
symmetrical rectangular shape, indicating that pesudocapacitance
originating from the electrochemically active Co3O4 component
is dominant in the whole capacitance. This asymmetrical CV
curve is more prominent in the case of NPCs/Co3O4 at a scan
rate of 50 mV s-1, revealing that single-crystalline Co3O4
exhibits a better high-rate response than that of the polycrys-
nanocrystals exhibit smooth edges with a size of 20 to 40 nm,
which is larger than that of the nonoxidized counterpart. It is
noted that most Co3O4 nanocrystals (more than 90%) reveal a
well-ordered single crystalline nature with a weak contrast at
the core of the nanoparticles, suggesting the formation of a
hollow structure after additional oxidation treatment (Figure 4b
and c). Such a novel morphological evolution can be rationally
ascribed to the well-known Kirkendall effect.23 All of the NED
patterns are well consistent with the face-centered cubic Co3O4
single crystals with a ) 8.1 Å (Figure 4d, e). The oxygen content
in NPCs/Co3O4 is greatly increased relative to that of the
nonoxidized counterpart (Figure 4f). It is therefore reasonable
to conclude that the additional thermal annealing in air leads to
a complete oxidation of the Co species with the formation of
larger hollow Co3O4 nanocrystals.
Similarly, NPCs/Pt is first examined by TEM characterization
after the removal of the AAO template. In Figure 5a and b, Pt
nanoparticles with an average diameter of 2 to 6 nm are
homogeneously distributed into carbon frameworks. HRTEM
images (Figure 5c-e) reveal that the metallic phase is composed
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j
of Pt single crystals (a ) 3.9 Å, space group Fm3m, No. 225).
In accordance with NED patterns (Figure 5f), the only crystalline
phase presented in NPCs/Pt is metallic platinum showing high
resolution lattice fringes of {111}, {200}, {220}, and {311},
respectively. EDX analysis confirms both the C and Pt
components (Figure 5g). Additional peaks for Cu and O can be
attributed to the underlying copper grid.
(23) Yin, Y. D.; Rioux, R. M.; Erdonmez, C. K.; Hughes, S.; Somorjai,
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Integration of Metallic Nanocrystals and 1D Carbons
A R T I C L E S
(126 F g-1). Even under high current rates, both the initial
specific capacitance and the rate response of NPCs/Co3O4 are
superior to those of CNTs/Co3O4 (Figure 6b). Assuming that a
specific capacitance of NPCs ranges from 232 to 139 F g-1 16c
and the cobalt mass content is around 21% based on the TGA
analysis (Figure S7), the corresponding specific capacitance
value of Co3O4 nanocrystals at a current rate of 3, 6, 12, 30,
and 60 Ag-1 is 1066, 1028, 951, 879, and 721 F g-1, respectively
(Figure 6b). To our knowledge, these results are among the best
electrochemical capacitive values for cobalt oxide electrode
materials.26,27 More importantly, the specific capacitive value
is preserved at 721 F g-1 for a high current density of 60 A
g-1, equivalent to 67.6% retention of the initial value of 1066
F g-1, suggesting that such a prominent capacitive performance
can be maintained under a high-power density operation. With
a maximum voltage of 0.6 V, a high capacitance value of 333
F g-1 can be obtained for NPCs/Co3O4 even after 4500 cycles
at a current density of 6 A g-1 (Figure 6c). Apparently, the
superior electrochemical properties of NPCs/Co3O4, which
include outstanding capacitive activity and excellent stability,
can be reasonably ascribed to the unique composition and
structure as discussed above.
The electrochemical results described above unambiguously
demonstrate that NPCs/Co3O4 is attractive for application in
electrochemical energy storage. Here, NPCs/Pt is exploited to
catalyze the electro-oxidation of methanol in alkaline medium,
which represents a promising alternative for direct fuel cells.28
The primary effect of KOH concentration on methanol electro-
oxidation illustrated as a linear scan is shown in Figure 7a. For
these experiments, the concentration of methanol is set to a
constant value of 1 M. In the forward scan region, the anodic
peak potential shifts negatively, and the current density, which
has been normalized to the electroactive Pt surface area (j, Figure
S8), increases greatly when the concentration of KOH is
increased from 0.1 to 5 M. With the assumption that the
electrochemical process is mainly dominated by the diffusion
control,29 the higher concentration of KOH can facilitate the
adsorption of OH- ions and methanol on Pt active sites, thereby
leading to the negative shift of peak potential and the increase
of j. Furthermore, the cyclic voltammograms of NPCs/Pt in a 5
M KOH solution with a scan rate varying between 2 and 50
mV s-1 are shown in Figure 7b. The well-defined forward and
backward scan peaks indicate the formation of the adsorbed
hydroxyl and acetyl groups on the active sites of Pt as well
as the rapid removal of the surplus carbonaceous species during
(26) (a) Shan, Y.; Gao, L. Mater. Chem. Phys. 2007, 103, 206–210. (b)
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Figure 6. (a) Cyclic voltammograms of NPCs/Co3O4 and CNTs/Co3O4 at
sweep rates of 10 and 50 mV s-1 in 1 M KOH electrolyte. (b) The specific
capacitance versus the current density for NPCs/Co3O4, Co3O4, and CNTs/
Co3O4, respectively (inset: the charge and discharge curves of NPCs/Co3O4
at the density of 6 A g-1). (c) The evolution of the specific capacitance
versus the number of cycles for NPCs/Co3O4.
(27) (a) Liang, Y. Y.; Cao, L.; Kong, L. B.; Li, H. L. J. Power Sources
2004, 136, 197–200. (b) Cao, L.; Xu, F.; Liang, Y. Y.; Li, H. L. AdV.
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Xu, C. L.; Li, H. L. Microporous Mesoporous Mater. 2009, 117, 55–
60.
tallinecounterpartinCNTs/Co3O4.Galvanostaticcharge-discharge
measurements were applied to investigate the capacitive per-
formance at a set of current densities ranging from 3 to 60 A
g-1. The calculated specific capacitance of NPCs/Co3O4 at a
current rate of 3, 6, 12, 30, and 60 A g-1 is 382, 374, 342, 311,
and 278 F g-1, respectively (Figure 6b). Taking the current
density of 3 A g-1 as an example, the specific capacitance of
NPCs/Co3O4 is 382 F g-1, much higher than that of CNTs/Co3O4
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Bianchini, C.; Shen, P. K. Chem. ReV. 2009, 109, 4183–4206.
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Zhang, J.; Marinkovic, N. S.; Liu, P.; Frenkel, A. I.; Adzic, R. R.
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A R T I C L E S
Liang et al.
Figure 8. (a) Linear scan (left) for the electro-oxidation of methanol on
the NPCs/Pt and E-TEK in a 1 M methanol + 5 M KOH solution at a scan
rate of 5 mV s-1, and the potential-dependent current density ratio R (right)
for NPCs/Pt and E-TEK. (b) Chronoamperometry of methanol oxidation at
-0.3 V for the NPCs/Pt and E-TEK in a 1 M methanol + 5 M KOH
solution.
to the accumulated intermediate carbonaceous species.31 As
illustrated in Figure 7c, high values of If/Ib ranging from 3.5 to
5.8 are achieved, in contrast to the published data of 1.5 to 2
for Pt/C catalysts.31 This finding confirms that NPCs/Pt exhibits
excellent catalytic efficiency and good tolerance toward poison-
ing species for the electro-oxidation of methanol.
The linear scans of NPCs/Pt and commercially available
E-TEK catalysts with the same Pt mass loading, which have
been normalized to the electro-active Pt surface area (j, Figure
S8), are compared in Figure 8a. In the forward scan region, the
value of j toward methanol oxidation of NPCs/Pt is higher than
that of E-TEK. The relative enhancement factor R, which is
defined as the ratio of the j value measured on NPCs/Pt versus
on E-TEK, varies from 4.3 to 9.0 depending on the applied
electrode potential. Additionally, under a fixed oxidation current
density, the corresponding potential value of NPCs/Pt is much
lower than that of E-TEK. The typical chronoamperometric
Figure 7. (a) Linear scan for the electro-oxidation of methanol on NPCs/
Pt in 1 M methanol solution with different KOH concentrations at a scan
rate of 5 mV s-1. (b) Cyclic voltammograms for the electro-oxidation of
methanol on NPCs/Pt in a 1 M methanol + 5 M KOH solution at a scan
rate range of 2 to 50 mV s-1. (c) Scan rate dependent current density (j)
and the value of If/Ib for the electro-oxidation of methanol on NPCs/Pt,
respectively.
the forward scan.30 Very interestingly, the current density of
the forward scan (If) is much higher than that of the backward
scan (Ib). In general, the current density ratio (If/Ib) is regarded
as a critical parameter to estimate the tolerance of the catalyst
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Integration of Metallic Nanocrystals and 1D Carbons
A R T I C L E S
curve of NPCs/Pt and E-TEK in a mixed solution of 1 M
methanol and 5 M KOH solution is recorded, respectively
(Figure 8b). Under a given potential of -0.3 V, for the
continuous electro-oxidation of methanol on the electrode
surface, tenacious reaction intermediates such as CO and HCHO
would begin to accumulate if the kinetics of the removal reaction
could not keep pace with the methanol oxidation.28b,32 A slow
decay of the peak current for NPCs/Pt with the time scale
implies that it has better poisoning tolerance in the alkaline
medium than E-TEK (Figure 7c), well consistent with the cyclic
voltammogram results. At 3000 s, the corresponding oxidation
current value of j remains at 0.13 mA cm-2 for NPCs/Pt, much
higher than 0.052 mA cm-2 for E-TEK.
On the basis of the above results and discussions, controlled
thermolysis of organometallic precursors has been shown to be
an efficient approach for the fabrication of 1D metal-carbon
hybrids. Structural characterizations disclose that high-quality
Co3O4 or Pt nanocrystals are well distributed within the 1D
nanoporous carbon frameworks. This can be due to the
preformation of metallic functionalized polyphenylene back-
bones that enable control of the nanoarchitectures of carbon-
aceous and metallic components at the molecular level during
the thermal treatment. As a consequence, the obtained 1D NPCs/
Co3O4 and NPCs/Pt materials exhibit superior electrochemical
performance in supercapacitors and fuel cells, respectively. This
can be rationally attributed to their unique composition and
structure. First, the combination of individual metallic nano-
crystals with electronically conducting NPCs provides electron
superhighways,33 allowing for rapid and efficient charge trans-
port and leading to an increase in the overall electronic
conductivity. Second, the homogeneously distributed metallic
nanocrystals on the nanoporous carbon framework of NPCs offer
high-surface-area access to the active sites, thus leading to
efficient utilization of the metallic phase for electrochemical
energy storage. Third, grafting these metallic nanocrystals onto
the mechanically stable NPCs can effectively prevent the
agglomeration of electroactive metallic nanocrystals during the
continuous electrochemical process.
Conclusion
In summary, we have successfully demonstrated an in situ
fabrication of 1D metal-carbon hybrids incorporating high-
quality Co3O4 or Pt nanocrystals with nanoporous carbon
frameworks via a controlled thermolysis of organometallic
precursors. Due to their unique composition and architecture,
NPCs/Co3O4 and NPCs/Pt display excellent electrochemical
performances in supercapacitor and fuel cell devices, respec-
tively. Our findings provide a deeper understanding of the
concept of organometallic precursor-controlled thermolysis as
a straightforward processing protocol for the buildup of
thermally stable and structurally ordered 1D metal-carbon
hybrids. This approach enables the promising prospect that a
variety of functional hybrid materials with intriguing properties
will be accessible for widespread applications in a range of
lithium ion batteries, micro- or nanotransistors, and magnetic
memory devices.
Acknowledgment. This work was financially supported by the
Max Planck Society through the program ENERCHEM, National
Natural Science Foundation of China (No. 50701023). Y.L.
gratefully acknowledges the Alexander von Humboldt Stiftung for
a research fellowship. M.G.S. thanks the “Graduate School Materi-
als Science in Mainz” for a scholarship.
Supporting Information Available: Synthetic route and
MALDI-TOF mass spectra of Co-Cp and Pt-BpCp; XRD, DSC,
TGA, and electrochemical active surface spectra. These materi-
als are available free of charge via the Internet at http://
pubs.acs.org.
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