Three-dimensional nitrogen-doped graphene nanoribbons aerogel as a highly efficient catalyst for the oxygen reduction reaction

Article abstract of DOI:10.1002/smll.201402472

A highly conductive, ultralight, neat and versatile nitrogen-doped GNRs aerogel has been fabricated by a new hydrothermal method for the first time. The newly developed aerogel shows a very promising performance when used as a novel ORR catalyst in both alkaline and acidic solutions.

Full text of DOI:10.1002/smll.201402472

Catalysts
Three-Dimensional Nitrogen-Doped Graphene
Nanoribbons Aerogel as a Highly Efficient Catalyst
for the Oxygen Reduction Reaction
Liang Chen, Ran Du, Jinghan Zhu, Yueyuan Mao, Cheng Xue, Na Zhang,
Yanglong Hou, Jin Zhang,* and Tao Yi*
Efficient catalysts for the cathodic oxygen reduction reac- sheets, such as their light weight, large surface area, and good
tion (ORR) play a crucial role in the development of fuel conductivity, but they also possess special properties, such as
[
1]
cell technology. Platinum-based materials have long been a high length-to-width ratio, straight edges, and ideal surface
used as active catalysts for the ORR. However, due to the regularity with few defects on the basal plane. These fea-
high cost, sluggish ORR process, susceptibility to methanol tures make them more suitable for functional materials than
[
9]
crossover, and limited stability of the platinum-based cata- random graphene sheets. On the other hand, unlike gra-
[
1]
lysts, great efforts have been devoted to search for more phene sheets that feature a zero bandgap, GNRs possess an
[
10]
efficient, durable, and inexpensive alternative ORR catalysts open bandgap that varies with the ribbon width. Moreover,
[
2]
by employing non-precious metals (Fe, Co, etc.), metal chemical doping of GNRs with heteroatoms, such as N and
[
3]
oxides (Fe O , Co O , MnCo O , etc.), as well as metal-free B, can effectively realize further modulation of the electronic
2
3
3
4
2
4
heteroatom-doped carbon nanomaterials (porous carbon, energy gap, thus enhancing the reactivity and electrocatalytic
[
4]
[10]
carbon nanotubes, graphene, etc.). Among them, carbon ability of the materials when used as ORR catalysts.
nanotubes (CNTs) and graphene-based materials have out- Developing new graphene-based catalysts with good
performed many of their counterparts in the past decades ORR performance not just in alkaline but also in acidic
because of their unique features, including their high thermal environments is desired because most fuel cells operate in
conductivity, giant electron mobility, large surface area, and acidic electrolytes. However, to the best of our knowledge,
[
5,6]
excellent stability.
As a rising star of the emerging carbon most graphene-based materials exhibit a good ORR activity
family, graphene nanoribbons (GNRs) have recently inspired in alkaline electrolytes, but poor activity in acids due to the
[
11–15]
a large amount of research in materials science for various relatively few catalytic sites formed within the materials.
applications in the fields of catalysis and nanoscale devices To conquer the aforementioned drawbacks of these materials
[
7,8]
due to their unique structure and properties.
Compared to and acquire high-performance catalysts for fuel cells, a pos-
graphene nanosheets, which have irregular sizes and shapes, sible approach is to introduce a large number of active sites
GNRs not only retain the unique characteristics of graphene onto the graphene sheets without destroying their conjugated
structures. However, conventional graphene-based materials
are generally prepared by reduction from their oxidated pre-
cursor, namely graphene oxide (GO), which tends to lose its
structural integrity and high electrical conductivity, a cru-
cial property for electron transfer during electrocatalysis,
L. Chen, Y. Mao, C. Xue, Prof. T. Yi
Department of Chemistry and Concerted
Innovation Center of Chemistry for Energy Materials
Fudan University
2
20 Handan Road, Shanghai 200433, P.R. China
[11]
because of the harsh oxidation and exfoliation. Reduction
E-mail: yitao@fudan.edu.cn
through chemical and thermal approaches removes most of
the oxygen-containing groups, but the defects usually remain,
which inevitably breaks the regularity of the resulting
R. Du, N. Zhang, Prof. J. Zhang
Center for Nanochemistry
Beijing National Laboratory for Molecular Sciences
Key Laboratory for the Physics and Chemistry of Nanodevices
State Key Laboratory for Structural
Chemistry of Unstable and Stable Species
College of Chemistry and Molecular Engineering
Peking University
[
16]
reduced graphene samples.
Therefore, acquiring novel
graphene-based catalysts with structural integrity as well as
abundant active sites still remains a great challenge. GNRs
prepared by longitudinally unzipping multi-walled carbon
nanotubes do cater for these requirements. Assembling these
GNRs into macroscopic bulk materials with desired struc-
tures and performances is one of the most powerful strategies
for expanding the structural features of GNRs and creating
novel physicochemical properties that differ from those of
the individual components, thus, enriching the practical appli-
cations of GNRs. The assembly of GNRs into macroscopic
aerogels, materials with an advanced three-dimensional (3D)
Beijing 100871, P.R. China
E-mail: jinzhang@pku.edu.cn
J. Zhu, Prof. Y. Hou
Department of Materials Science and Engineering
College of Engineering
Peking University
Beijing 100871, P.R. China
DOI: 10.1002/smll.201402472
small 2014,
DOI: 10.1002/smll.201402472
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communications
L. Chen et al.
Figure 1. a) Illustration of the synthetic route for N-GNRs-A. b) Photographs of as-prepared 3D graphene nanoribbons aerogels derived from Py-free
left) and 5 vol% Py-containing (middle) GONRs suspension (10 mg mL−1) after hydrothermal treatment. c) A monolith of ultralight N-GNRs-A
standing on a dandelion. d) Photograph of N-GNRs-A in a hot flame of an alcohol burner. e) XRD patterns of 1) pristine MWCNTs, 2) GONRs,
) GNRs-A, and 4) N-GNRs-A. f) SEM image of the resulting N-GNRs-A. g,h) Typical TEM and high-resolution TEM images of the as-prepared N-GNRs-A.
(
3
Inset in (g) shows three interwoven curly nanoribbons. Inset in (h) is the selected-area electron diffraction (SAED) pattern of the aerogel.
architecture,[17] can not only integrate the intriguing proper- Information. For the preparation of ultralight, versatile
ties of GNRs but also endow the material with the special N-GNRs-A, GONRs prepared by longitudinally unzipping
features of aerogels. However, to the best of our knowledge, multi-walled carbon nanotubes at a certain concentration
−1
the assembly of GNRs into macroscopic 3D aerogels in a (1–10mgmL )weremixedwith5vol%pyrrole(Py)monomer.
controlled manner coupled with doping of the gels with het- Hydrothermal treatment of the mixture at 180 °C for 12 h
[
22–24]
eroatoms has not been explored.
resulted in a mechanically robust monolithic hydrogel,
In the present study, we have fabricated a highly con- which was then freeze-dried and annealed at 1030 °C for
ductive, ultralight, neat and versatile nitrogen-doped GNRs 2 h under Ar atmosphere (see Figure 1a). Here, with a
aerogel (denoted as N-GNRs-A) through a facile hydro- π-conjugated structure containing an electron-rich N atom,
thermal method for the first time. The N-GNRs-A possesses Py acts as the N source as it can be easily adsorbed onto the
−3
a very low density (2.5–22 mg cm ), large surface area surface of the GONRs through hydrogen bonding and π−π
2
−1
−1
(
480–617 m g ), and excellent conductivity (0.5–35.6 S m ) interactions during the hydrothermal process. As a result, N
that depend on the initial concentration of the graphene atoms can be in-situ doped into the lattice of GNRs in the
oxide nanoribbons (GONRs). These properties allow the following high-temperature pyrolysis procedure. Moreover,
resultant aerogel to be used as an ORR electrocatalyst with the Py adsorbed on the GONRs can also act as a swelling
a quite positive onset potential, high cathodic current density, agent or inhibitor to restrain the aggregation of GNRs in
[
12]
and excellent durability, which are comparable or better than the process of gel formation. Therefore, the amount of Py
those of commercial Pt/C catalysts in alkaline medium. More monomer is an important determinant of the character and
interestingly, the N-GNRs-A catalyst also shows pronounced quality of the gel. A low content of Py (<5 vol%) causes a
ORR electrocatalytic activity under acidic conditions.
certain shrinkage in the gel, whereas superfluous Py signifi-
Several approaches have been developed to prepare cantly increases the density of the gel. Using 5 vol% of the
[
18]
high-quality GNRs, including gas-phase oxidization,
Py monomer, almost no shrinkage of the hydrogel occurs.
chemical vapor deposi- Figure 1b indicates that, compared to the Py-free pure GNRs
techniques as well as longitudinally unzipping multi- aerogel (denoted as GNRs-A), the N-GNRs-A possessed an
[
19]
[20]
lithographic,
sonochemical,
[
21]
tion
walled carbon nanotubes. Among these approaches, only obviously larger volume, leading to a quite low density (2.5 ±
the last one can be utilized to prepare GNRs with a defined 0.5 mg cm ) when the initial concentration of GONRs was
structure on a large scale. Thus, this approach was used to 1 mg mL . This value was slightly higher than some of the
prepare the homogeneous GONRs in the present work. carbon-based aerogels (0.16–2.1 mg cm ),
[
7]
−
3
−1
−
3 [12,25,26]
but much
The experimental details are provided in the Supporting lower than that of other 3D carbon-based architectures
2
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DOI: 10.1002/smll.201402472

Three-Dimensional Nitrogen-Doped Graphene Nanoribbons Aerogel as a Highly Efficient Catalyst for the Oxygen
(
Table S1 in the Supporting Information).[27–31] The ultralight
X-ray photoelectron spectroscopy (XPS) was used to
aerogel can be placed on top of a dandelion without breaking analyze the chemical attributes of N-GNRs-A and GONRs
it (Figure 1c). Furthermore, the aerogel is thermostable and (Figure 2a and Figure S6, Supporting Information). As
fire-resistant. A piece of N-GNRs-A emits red light on an expected, the XPS spectrum of the GONRs showed only
alcohol burner instead of catching fire, possibly because of C and O peaks, whereas additional N peaks were observed
the quick removal of heat during combustion because of its for the N-GNRs-A, which confirmed the doping of N atoms
high porosity (Figure 1d). Thermogravimetric analysis (TGA) within the aerogel (2.8%, N/C atomic ratio). The signifi-
curves indicate that the as-prepared N-GNRs-A shows a cant decrease in O content from 32.3% for the GONRs to
weight loss of about 2.6 wt% at a temperature of up to 2.07% for the N-GNRs-A can be attributed to the reduc-
8
00 °C in N and it can bear a temperature of approximately tion of the GONRs during the high-temperature pyrolysis
2
4
00 °C in air without significant weight loss (Figure S1, Sup- process, which was further affirmed by comparing the high-
porting Information).
resolution C 1s spectra of the two samples (Figure S7, Sup-
The structure and morphology of the as-prepared porting Information). The high-resolution N 1s spectrum of
N-GNRs-A samples were investigated by X-ray diffraction N-GNRs-A reveals the presence of both pyridinic (398.5 eV)
(
XRD) (Figure 1e), Raman spectroscopy (Figure S2, Sup- and pyrrolic (401 eV) N atoms (Figure 2b), indicating that
[
32]
porting Information), scanning electron microscopy (SEM) the N atoms have been doped into the lattice of the GNRs.
(
(
Figure 1f), and transmission electron microscopy (TEM) Brunauer−Emmett−Teller (BET) analysis showed that the
Figure 1g,h). The XRD pattern clearly confirmed the lon- specific surface area of N-GNRs-A can reach up to 617 m g
2
−1
gitudinal unzipping of the multi-walled carbon nanotubes (Figure 2c), which is much larger than that of previously
−1
[12,25]
(
MWCNTs) to GONRs and the subsequent formation of the reported graphene-based aerogels (270–280 m2 g ),
[
7]
GNRs aerogel. Generally, it can be seen from Figure 1e GNR or GNR–CNT hybrid aerogels obtained by in-situ
2
−1 [28]
that the pristine MWCNTs have a broad peak at 26°, which chemical unzipping (113 m g )
corresponds to the (002) reflection of the carbon nanotubes. based frameworks,
and other 3D carbon-
suggesting large open pores
[
12,18,27–36]
Upon unzipping, the obtained GONRs shared a broad peak within the aerogel, which is consistent with the SEM observa-
at 10.2° corresponding to the (001) reflection, which is very tions. The surface areas of both N-GNRs-A and GNRs-A are
[
17]
2
−1
similar to that of GO,
indicating that the interlayer space much higher than that of the precursors GONRs (31 m g ,
of the GONRs was much larger than that of the pristine Figure S8 and S9, Supporting Information), indicating that
MWCNTs due to the unzipping process and further introduc- the porous structure of N-GNRs-A mainly comes from the
tion of oxygen-containing functional groups into the GNRs. hydrothermal assembly process. The specific surface area of
Following the hydrothermal procedure and heat treatment, a N-GNRs-A was further confirmed by using a standard meth-
broad peak located at approximately 26° reappeared in the ylene-blue absorption method (Figure S10, Supporting Infor-
spectrum for the N-GNRs-A. To further verify the above mation). The values obtained via this method are slightly
results, UV-visible spectra, SEM, and TEM images of the pris- higher than that of the BET values, which may be caused by
tine MWCNTs and as-obtained GONRs are also provided the fact that methylene-blue absorption measures not only
in the Supporting Information. The pristine CNTs exhibit the mesopores but also the macropores within the aerogels,
a strong absorption peak at 248 nm that can be ascribed to whereas the nitrogen adsorption–desorption test can only
π→π* transitions of the aromatic C-C bonds. After unzipping, measure the mesopores. Despite its porous structure, the
the absorption of the resulting GONRs blue-shifted from resulting N-doped aerogel also possessed a high conductivity
−
1
2
45 to 229 nm (Figure S3, Supporting Information), similar of up to 35.6 S m (Movie S1, Supporting Information, and
to that when peeling off GO, further confirming the com- Figure 2d), which is much higher than (or comparable to)
plete unzipping of the CNTs to GONRs. The SEM images those of pure graphene aerogels and other graphene-based
[
24,37]
show that the N-GNRs-A exhibits an interconnected, porous materials, as has been reported previously.
The high con-
structure with hierarchical pores ranging from hundreds of ductivity of N-GNRs-A facilitates electron transfer through
nanometers to tens of micrometers (Figure S4, Supporting the entire material to the surface of the electrode and guar-
Information). A similar morphology can also be observed in antees a high electrocatalytic activity for ORR, as described
the SEM images of Py-free GNRs-A and N-doped aerogel below.
before pyrolysis (Figure S5, Supporting Information). More-
over, the TEM images further indicated that the three- nanotubes,
GNRs, a kind of 1D carbon material derived from carbon
[
6,7]
can serve as an ideal bridge combining the
dimensional structure of the aerogel was constructed by features of CNTs and graphene sheets. The one-atom thick
ribbon-like sheets, namely GNRs (marked by the arrows in GNRs with quasi-one-dimensional (1D) planar geometry
Figure 1g). High-resolution TEM revealed that these ribbon- does not only provide a large specific surface area but also
like walls typically consist of double layers or multilayers of facilitates electron transport, and hence it can be more effec-
graphene sheets (Figure 1h). However, because of the strong tive than CNTs and graphene sheets for catalysis. Inspired by
oxidation effect, which inevitably gives rise to imperfections the superb chemical and physical properties of the N-GNRs-
in the product precursor (GONRs), the as-prepared nanor- A demonstrated above, we expected that the aerogel might
ibbons show a slightly narrow and curly morphology, as can have great potential for ORR catalysis. The ORR catalytic
−
3
be seen from the SEM and high-resolution TEM (HRTEM). performance of N-GNRs-A (ρ = 22 mg cm ) was investi-
Similar morphologies and interpretations have been pub- gated using cyclic voltammetry (CV) in a N - and O -satu-
2
2
[
7,10]
lished before.
rated 0.1 m aqueous KOH electrolyte solution at a scan rate
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communications
L. Chen et al.
Figure 2. a) XPS survey profiles of the resulting GONRs powder and N-GNRs-A. b) High-resolution N 1s peak of as-prepared N-GNRs-A. c) Nitrogen
sorption isotherms of as-obtained N-GNRs-A with different densities. d) Typical I–V curves of N-GNRs-A with different densities.
of 10 mV s− (Figure 3a). Unlike the Py-free GNRs aerogel, as the interconnected porous structure, large specific surface
which has no obvious redox peak (Figure S11, Supporting area, and good conductivity. Thus, oxygen could be easily
Information), the N-GNRs-A exhibited a significant redox adsorbed onto the catalyst surface with preferred configura-
1
peak at approximately −0.19 V, representing the O reduc- tion in a large quantity, and the electrons required for ORR
2
[
38,39]
tion process in O -saturated KOH solution.
For compar- could be transferred into the catalyst without difficulty.
ison, the related tests of N-GNRs-A catalysts with different It is well known that conventional electrocatalysts gener-
mass loading are provided in the Supporting Information ally possess sluggish ORR kinetics and often suffer from poor
Figure S12,S13). The results show that with an increase in electron transfer to the adsorbed oxygen molecules, which
2
(
mass loading of catalyst per area the onset potential of the generally results in the occurrence of a two-electron reduc-
catalyst does not change significantly, however, the cathodic tion of oxygen as well as the desired four-electron reduc-
current density presents a marked increase.To gain additional tion. The two-electron process is less favorable because of
insight into the role of N-GNRs-A in the ORR, linear sweep its lower efficiency and the corrosive nature of the resulting
voltammetry (LSV) curves were recorded on a rotating ring- hydrogen peroxide (H O ). To further confirm the electron
2
2
disk electrode (RRDE) in an O -saturated 0.1 m KOH solu- transfer number (n) and the H O yield of N-GNRs-A during
2
2
2
tion at a scan rate of 10 mV s− (Figure 3b). The N-GNRs-A the ORR process, RRDE curve of N-GNRs-A at 1600 rpm
sample has a high onset potential of around −0.05 V, which were performed as shown in Figure 3c. The n values of the
is only 0.08 V more negative than that of a Pt/C electrode N-GNRs-A electrode at different potentials were calculated
1
(
20 wt% Pt on carbon black has an onset potential of ca. to be 3.71–3.96. Moreover, the measured H O yield on the
2 2
0
.03 V), but much more positive than that of GNRs-A and N-GNRs-A electrode was found to be less than 14.7% over
MWCNTs (<−0.15 V, Figure 3b). This onset potential is also the entire potential range (Figure 3d and Figure S14, Sup-
much more positive than that of most heteroatom-doped porting Information), which is similar to that of the recently
graphene-based materials and typical porous carbon-based reported N,S-codoped graphene nanomesh (N-S-GMF) and
[
12,13,15]
[14,15]
materials (Table S2, Supporting Information).
More- other heteroatom-doped carbon-based materials,
fur-
over, the ORR kinetic current density of the N-GNRs-A is ther confirming the one-step, 4-electron ORR process of the
also much higher than other reference catalysts. The more N-GNRs-A electrode. The RDE measurements as well as the
positive onset potential as well as the higher current density corresponding K–L plots also confirmed the efficient 4-elec-
of N-GNRs-A may be attributed to the perfect synergistic tron dominated ORR process (Figure 3e-f, Figure S15, Sup-
effect of the doped pyridinic and pyrrolic N species as well porting Information).
4
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Three-Dimensional Nitrogen-Doped Graphene Nanoribbons Aerogel as a Highly Efficient Catalyst for the Oxygen
Figure 3. The ORR performance for N-GNRs-A in alkaline solution. a) CV curves of N-GNRs-A in a N - and O -saturated 0.1 M KOH solution at a
2
2
−
1
scan rate of 10 mV s . b) LSV curves of pristine MWCNTs, GNRs-A, N-GNRs-A, and Pt/C in an O -saturated 0.1 M KOH solution at a scan rate of
0 mV s and a rotation speed of 1600 rpm. c) RRDE curves of N-GNRs-A at 1600 rpm. d) Peroxide percentage and electron transfer number (n)
2
−
1
1
of N-GNRs-A within the potential range of −1.0 V to −0.2 V. e) RDE curves of N-GNRs-A in an O -saturated 0.1 M KOH solution with various rotation
2
−1
−1
−1/2
speeds at a scan rate of 10 mV s . f) K–L plots of J vs. ω
at different electrode potentials derived from RDE measurements (with mass loading
−
2
of 0.08 mg cm
)
In light of practical applications in fuel cells it is well higher than that of expensive Pt/C catalysts (approximately
known that fuel molecules (e.g., methanol) may pass through 60%) (Figure 4b), indicating that the N-GNRs-A electrocata-
the membrane from anode to cathode and poison the cathode lyst is much more stable than commercial Pt/C in an alkaline
catalyst. The N-GNRs-A electrode was therefore subjected medium.
to further testing with regards to the crossover effect and its
In addition, the electrocatalytic performance of N-GNRs-
long-term stability for the ORR. Figure 4a shows the current– A for ORR in acidic medium was also investigated. The CV
time (i–t) chronoamperometric responses for the ORR at curve indicated a significant reduction process for N-GNRs-
N-GNRs-A and Pt/C electrodes. After adding 3 m methanol A with a pronounced cathodic ORR peak at 0.20 V versus
to the electrolyte an obvious inversion of the current density Ag/AgCl electrode when the electrolyte (0.5 m H SO ) was
2
4
was observed for the Pt/C electrode, whereas the N-GNRs- saturated with O2 (Figure S16a, Supporting Information),
A electrode remained almost unchanged, suggesting the whereas no obvious response was observed at the same
excellent tolerance of the N-GNRs-A electrode to methanol. potential range under N . To further quantify the ORR elec-
2
Lastly, the durability of N-GNRs-A was also compared with tron-transfer pathway, we employed an RRDE technique as
that of a commercial Pt/C catalyst. Nearly 78% of the current shown in Figure S16b. The N-GNRs-A catalyst exhibited an
density was retained after continuous work for 20 000 s for ORR process approximating a 4-electron transfer pathway
the N-GNRs-A electrode, which was similar to that of other (n = 3.66–3.92 over the potential range of −0.2 V to 0.4 V,
[
15,29]
heteroatom-doped graphene nanomaterials
but well Figure S16c, Supporting Information) with a low H O yield
2 2
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communications
L. Chen et al.
N species further enhances the reactivity and electrocatalytic
[
11–15]
ability of the GNRs.
Moreover, aerogels with large sur-
face areas can inevitably lead to more efficient access to the
electrolyte compared to other doped powder or film counter-
parts. The good conductivity of N-GNRs-A also plays a vital
role in the ORR by facilitating the electron transfer between
the N-GNRs-A and oxygen. All of the results mentioned
above support the conclusion that N-GNRs-A is an efficient
catalyst for ORR both in alkaline and acidic solutions.
In summary, a highly conductive, ultralight, neat and ver-
satile nitrogen-doped GNRs aerogel has been fabricated via a
facile hydrothermal method and exhibits an ultra-low density,
large surface area, and good conductivity.The obtained aerogel
can act as a novel ORR catalyst with superb electrocatalytic
activity and better stability than commercial Pt/C catalysts
in alkaline medium. Interestingly, the N-GNRs-A electrode
also has a remarkable ORR performance in acidic solutions.
Its excellent catalytic properties for ORR may be attributed
to the perfect synergistic effect of the N-doped, structural
integrity of the GNRs, its nanoporous structure, high surface
area, and good conductivity. We believe that the ORR perfor-
mance of the GNRs-based materials can be further optimized
by introducing other heteroatoms or realizing co-doping with
atoms such as B, P, and S. Our future work will focus on this
direction. The aerogel presented here may open up a signifi-
cant avenue for the fabrication of a series of high-performance
GNR-based porous materials with promising applications.
Experimental Section
Figure 4. a) Methanol crossover effect on N-GNRs-A and Pt/C upon
experimental details see the Supporting Information.
addition of 3 M methanol after about 10 min in an O -saturated 0.1 M
2
KOH solution at −0.4 V. b) Current–time chronoamperometric response
of N-GNRs-A and Pt/C catalysts at −0.4 V in O2 saturated 0.1 M KOH
aqueous solution at a rotation rate of 1600 rpm.
Supporting Information
of 4.2–17.1% over the measured potential range.These results
[
14]
are similar to that of the recently reported N-S-GMF and
[
15]
Supporting Information is available from the Wiley Online Library
or from the author.
nitrogen-doped carbon nanosheets (NDCNs),
inferior to previously reported work.
but is still
[
40,41]
The LSV curves
of the N-GNRs-A at different electrode rotation speeds also
confirmed the above RRDE results (Figure S16d, Supporting
Information).
Acknowledgements
The excellent electrocatalytic performance of N-GNRs-
A for the ORR both in alkaline and acidic solutions may be
derived from the unique 1D regular structure of GNR that
differs from random graphene sheets. The conventional pre-
cursors of graphene (namely, graphene oxide) always lose
their structural integrity and high electrical conductivity,
whereas GNRs prepared by longitudinally unzipping of
multi-walled carbon nanotubes can preserve their structural
integrity and high electrical conductivity as well as endow
extra catalytic sites to the electrode by incorporating oxygen-
containing functional groups during the unzipping process.
Additionally, Py serves as a swelling agent to suppress the
shrinking of the gel. As a result, the as-prepared aerogel pos-
sesses a large amount of pores in the 3D gel network, thus
insuring the efficient mass transfer to the active sites and
markedly facilitating the ORR diffusion kinetics. The doped
The authors acknowledge the financial support from the National
Basic Research Program of China (2013CB733700), the China
National Funds for Distinguished Young Scientists (21125104), the
National Natural Science Foundation of China (51373039), the Spe-
cialized Research Fund for the Doctoral Program of Higher Education
(20120071130008), the Program for Innovative Research Team in
University (IRT1117), the Program of Shanghai Subject Chief Scien-
tist (12XD1405900), and the Shanghai Leading Academic Discipline
Project (B108). We also thank Chenyu Xu and Prof. Liangti Qu from
the Beijing Institute of Technology for the RRDE tests.
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DOI: 10.1002/smll.201402472

Three-Dimensional Nitrogen-Doped Graphene Nanoribbons Aerogel as a Highly Efficient Catalyst for the Oxygen
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small 2014,
DOI: 10.1002/smll.201402472
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