Preparation and electrochemical characterization of nitrogen doped graphene by microwave as supporting materials for fuel cell catalysts

Article abstract of DOI:10.1016/j.electacta.2011.11.062

A quick and efficient approach to prepare nitrogen doped graphene (NG) is proposed in this paper via microwave heating in NH₃ atmosphere. Results show that graphene, as an allotrope of carbon, is a good microwave-absorbing material and can reach a high temperature in minutes, facilitating nitrogen incorporation into the structure under NH₃. Elemental analysis and X-ray photoelectron spectroscope (XPS) verified the success of N-doping with the nitrogen content of 5.04 wt%. For comparison, both plain grapheme (G) and the NG were used as supporting materials for platinum to investigate their potential application in fuel cells. Transmission electron microscope (TEM) images showed that the NG improved the distribution of Pt particles. Themogravimetry (TG) and differential scanning calorimeters (DSC) revealed better thermal stability of the Pt/NG than that of the Pt/G. Furthermore, the Pt/NG catalysts exhibited higher electrochemical active surface area, methanol catalytic activity, and tolerance to CO poisoning than those of the Pt/G under fuel cell conditions. It suggests that the NG prepared by microwave synthesis has provided a new way to improve electrocatalytic activity in fuel cells.

Full text of DOI:10.1016/j.electacta.2011.11.062

Electrochimica Acta 60 (2012) 354–358
Contents lists available at SciVerse ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Preparation and electrochemical characterization of nitrogen doped graphene
by microwave as supporting materials for fuel cell catalysts
Yuchen Xina,b, Jian-guo Liua,b,∗, Xiao Jiea, Wenming Liua, Fuqiang Liuc,∗∗, Ying Yinb, Jun Gua,
Zhigang Zoua,b
a Eco-materials and Renewable Energy Research Center, Department of Physics and National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China
b Department of Materials Science and Engineering, Nanjing University, Nanjing 210093, China
c Electrochemical Energy Lab, Department of Material Science and Engineering, University of Texas at Arlington, Arlington, TX 76019, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
A quick and efficient approach to prepare nitrogen doped graphene (NG) is proposed in this paper
via microwave heating in NH3 atmosphere. Results show that graphene, as an allotrope of carbon, is
a good microwave-absorbing material and can reach a high temperature in minutes, facilitating nitrogen
incorporation into the structure under NH3. Elemental analysis and X-ray photoelectron spectroscope
(XPS) verified the success of N-doping with the nitrogen content of 5.04 wt%. For comparison, both
plain grapheme (G) and the NG were used as supporting materials for platinum to investigate their
potential application in fuel cells. Transmission electron microscope (TEM) images showed that the NG
improved the distribution of Pt particles. Themogravimetry (TG) and differential scanning calorimeters
(DSC) revealed better thermal stability of the Pt/NG than that of the Pt/G. Furthermore, the Pt/NG cat-
alysts exhibited higher electrochemical active surface area, methanol catalytic activity, and tolerance
to CO poisoning than those of the Pt/G under fuel cell conditions. It suggests that the NG prepared by
microwave synthesis has provided a new way to improve electrocatalytic activity in fuel cells.
© 2011 Elsevier Ltd. All rights reserved.
Received 7 November 2011
Accepted 16 November 2011
Available online 25 November 2011
Keywords:
Nitrogen doped graphene
Microwave
Fuel cell
Pt/C
Catalyst stability
1. Introduction
[7–12]. In particular, the doping of nitrogen into carbon struc-
tures has received great attention. For graphene sheets, N-doping
Since graphene was first discovered in 2004 [1,2], it has attracted
great interest from both experimental and theoretical scientific
communities [3]. Graphene, with a two-dimensional array of sp2-
bonded carbon atoms, exhibits high surface areas (theoretical
specific surface area of 2620 m2 g−1), superior electrical and ther-
mal conductivities, excellent mechanical strength and elasticity,
and ease of modification [4]. Due to the unique physical and chem-
ical properties, graphene provides an ideal base for electronics,
sensors, composites, batteries, supercapacitors and hydrogen stor-
age [5,6].
can enhance the electrical and thermal conductivities, and is also
a leading potential strategy to enrich free charge-carrier den-
sity [6,13,14]. More importantly, the incorporation of nitrogen
functional groups into carbon network can serve as active sites
for anchoring metal particles, strengthening the metal-graphene
interaction [15]. Several approaches have been successfully
demonstrated to dope nitrogen into graphene sheets, such as joule
annealing in NH3 [16], nitrogen plasma [17], chemical vapor depo-
sition [12,18], arc-discharge of carbon electrode [19], hydrothermal
reduction in the presence of hydrazine and ammonia [20], and sur-
prisingly by hydrazine sonication [21]. However, convenient and
effective doping of graphene is still needed.
In this study, graphene (G) was first synthesized by microwave
exfoliation of graphitic oxide, and N-doped graphene (NG) was
then prepared by microwave-heating of the graphene in NH3 atmo-
sphere. It was demonstrated that graphene can absorb microwave
easily and reach a very high temperature in few seconds, quickly
incorporating nitrogen atoms. The N-doping method through
microwave in NH3 was efficient and fast with a high nitrogen con-
tent. Moreover, the Pt/G and Pt/NG catalysts were prepared by
microwave-assisted polyol to investigate their application in half-
cell reaction of direct methanol fuel cells (DMFCs). Compared to
the Pt/G catalysts, Pt/NG catalysts showed better thermal stability
Many efforts, such as chemical doping, have been made to
further modulate the properties of graphene over past several
years. Devising chemical doping method for this two dimen-
sional material is key to its widespread potential applications
∗
Corresponding author at: Department of Materials Science and Engineering,
Nanjing University, 22 Hankou Road, Nanjing 210093, China. Tel.: +86 25 83621219;
fax: +86 25 83686632.
∗∗
Co-corresponding author at: Department of Material Science and Engineering,
University of Texas at Arlington, Arlington, TX 76019, United States.
Tel.: +1 817 272 2704; fax: +1 817 272 2538.
E-mail addresses: jianguoliu@nju.edu.cn (J.-g. Liu), fuqiang@uta.edu (F. Liu).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.11.062

Y. Xin et al. / Electrochimica Acta 60 (2012) 354–358
355
Table 1
and exhibited excellent performance under fuel cell operation. It
suggests that the NG is a promising supporting material for noble
metal catalysts in DMFCs.
Elemental analysis of G and NG.
Samples
C (wt. %)
H (wt. %)
O (wt. %)
N (wt. %)
C/O (at. %)
G
NG
82.72
82.64
1.57
2.08
15.71
10.25
–
5.04
7.02
10.75
2. Experimental
2.1. Preparation of G and NG
5 × 10−10 mbar and 5 × 10−9 mbar during the measurements. Spe-
cific surface area collected on a Micromeritics Tristar-3000 surface
area by using the Brunauer–Emmett–Teller (BET) model after the
samples had been degassed in a flow of N2 for 3 h. Elemental
analysis was conducted on a Elementar Vario MICRO at 980 ◦C
and 1174 mbar. Thermogravimetry (TG) and differential scanning
calorimetry (DSC) were performed on a STA 449C thermogravi-
metric analyzer from room temperature to 800 ◦C at a rate of
10 ◦C min−1 under air flow.
Electrochemical measurements were performed at an electro-
chemical station (PARSTAT 2273, Princeton Applied Research). Pt
foil and a saturated calomel electrode (SCE) were served as a
counter electrode and reference electrode, respectively. A glassy
carbon disk with a diameter of 5 mm was employed as working
electrode. About 10 ␮g Pt of the catalyst was deposited onto the
glassy carbon disk with an area of 0.196 cm2, corresponding to Pt
loading of 51.02 ␮g cm−2. For cyclic voltammetry (CV) measure-
ments, the working electrode was immersed in 0.5 M H2SO4 and
0.5 M CH3OH solution saturated by high purified nitrogen with the
potential between −0.241 and 1.0 V versus SCE. In the CO stripping
experiment, the pre-adsorption of CO was achieved by bubbling CO
into the solution and holding the potential at −0.15 versus SCE for
20 min. After that, the solution was purged with N2 gas for another
20 min at −0.15 V to remove the dissolved CO in the solution. All
potentials were measured and quoted against the SCE. Durability
investigation of catalysts was carried out by accelerated durabil-
ity test (ADT) method. ADT was conducted by potential cycling
between 0.6 V and 1.2 V (NHE) at 50 mV s−1 in N2 saturated 0.5 M
H2SO4.
The graphitic oxide (GO) was synthesized from natural graphite
powder by a modification of Hummers and Offeman’s method
[22,23]. Graphene was prepared by microwave-thermal expansion
using GO as precursor. GO was put into a three-neck flask purged
with N2 at a rate of 100 ml min−1 for 5 min through a mass flow
controller. Next, the flask was placed in a household microwave
oven (Galanz, 2450 MHz, 700 W) under high power for 60 s to
obtain graphene via the microwave-thermal expansion procedure
[24–26]. To prepare the NG, the flask with graphene was purged
with NH3 at a rate of 200 ml min−1 for 5 min and then treated by
microwave. Graphene can absorb microwave easily thus reaching a
very high temperature (beyond 500 ◦C) in a short time. The power
was on for 60 s/off for 120 s and repeated for 4 times with the flow
of NH3 at 200 ml min−1 in the whole process.
2.2. Preparation of Pt/G and Pt/NG composites
The Pt/G and Pt/NG catalysts were synthesized by microwave-
assisted polyol process. The NG with a weight of 80 mg was
dispersed in 30 ml ethylene glycol by sonication for 30 min. Then,
2.7 ml of H2PtCl6 solution (20 mg H2PtCl6·6H2O ml−1) was slowly
added and the pH value of this mixture was adjusted to 12 using
1 M NaOH. The solution was placed in the microwave oven and
intermittently heated in the pattern of 10 s on and 20 s off for 10
times. Next, the pH value of the solution was adjusted to 3 using
1 M HCl and subsided for 3 h. The final solution was filtered and fur-
ther washed with hot water. The residue was re-dispersed in water
by sonication and dried by freezing drying, which can prevent irre-
versible aggregation of graphene during the drying process [4]. Pt/G
catalysts were synthesized by the same method for comparison.
3. Results and discussion
2.3. Physical and electrochemical characterizations
Microwave treatment of the graphene under NH3 atmosphere
resulted in N doping in the G sheets. Most of the oxygenated groups
in the NG were removed during the treatment [27]. Table 1 listed
the elemental analysis of G and NG. Upon N-doping, the nitrogen
content of NG was 5.04 wt%, while the oxygen content decreased to
10.25 wt% with the C:O atom ratio of 10.75. The results verified that
the oxygen-containing functional groups were removed during the
N-doping process.
The morphology of Pt nanoparticles on different carbon sup-
ports was explored by transmission electron microscope (TEM)
on a JEM-200CX at 200 kV. X-ray diffraction (XRD) was conducted
on a Rigaku D/MAX-Ultima III X-ray diffractometer using Cu K␣
radiation (ꢀ = 0.15406 nm). The 2ꢁ angular range from 10◦ to 80◦
was explored at a scan rate of 10◦ min−1. The X-ray photoelec-
tron spectroscopy (XPS) was performed by ESCALAB 250 apparatus,
using monochromated Al K␣ radiation at 150 W, in the pass energy
(PE) mode (PE = 20 eV). The pressure of the spectrometer was
XPS analysis (Fig. 1) further revealed that the surface com-
positions of the NG. Only carbon, oxygen and nitrogen were
detected, and the content of N was slightly lower than that showed
C1s
Pyridine N
(a)
(b)
Pyrrole or amino N
Quaternary N
O1s
N1s
NG
G
408
404
400
396
392
1000 800
600
400
200
0
Binding Energy (eV)
Binding energy (ev)
Fig. 1. The XPS survey spectra of G and NG (a) and N(1s) XPS spectra of NG (b).

356
Y. Xin et al. / Electrochimica Acta 60 (2012) 354–358
Fig. 3 displayed the TEM images of the Pt/G and Pt/NG. The size
Graphene
Pt
Pt/G
Pt/NG
distributions were obtained by measuring the sizes of 200 ran-
domly selected particles. The Pt particles deposited on the plain
graphene exhibited some agglomeration with the average parti-
cle size of 2.9 nm. The smaller Pt particle size (2.5 nm) on the NG,
with a slightly narrower distribution, revealed that the NG was
more capable of forming well-dispersed metal particle [31]. The
smaller Pt particles of the Pt/NG suggested larger Pt surface area,
which can be beneficial to electrochemical reaction. It had been
reported that the nitrogen species could attract metal particle,
since the introduction of nitrogen into the carbon structure was
responsible for the presence of electronegative species on carbon
surface [32,33]. As shown in the XPS results in Fig. 1, the NG had
abundant N elements in the forms of pyridine, pyrrole/amino, and
quaternary N. These electron-rich sites can facilitate metal precur-
sor reduction and dispersion of metal particles. On the other hand,
as mentioned above, most of Pt particles were easily deposited on
the nitrogen species as nitrogen species were electronegative, the
well dispersed Pt particles in the Pt/NG thus implied that the nitro-
gen species were all evenly distributed on the surface of the doped
graphene.
(111)
(200)
(220)
0
10
20
30
40
50
60
70
80
90
2θ(degrees)
Fig. 2. XRD patterns Pt/G and Pt/NG.
in elemental analysis. The nitrogen signal of the NG in the XPS
spectra (Fig. 1a) indicated that nitrogen atoms were introduced to
the graphene network. The peaks located at 388.1 eV, 399.9 eV and
401.1 eV were attributed to pyridine N, pyrrole or amino N and
quaternary N, respectively [28]. The dominant peak at 398.1 eV
present in the spectra suggested that pyridine N was the major
state in NG. Based on the detail analysis of N1s, the contents of
three nitrogen species were 73.04%, 20.61% and 6.35% for pyridine
N, pyrrole/amino N and quaternary N, respectively. Zhang et al. [16]
reported that 500 ◦C was an optimal temperature for maximum
N-doping, where all three species were stable. During the N-doping
procedure, ammonia formed amino free radicals, which can attack
the oxygenated groups [29]. The amino species diminished quickly
when the temperature was higher than 500 ◦C, while pyridine
N and quaternary N remained relatively stable. It suggested that
the graphene might reach a temperature beyond 500 ◦C during
N-doping reaction in a short time since it absorbs microwave easily.
Fig. 2 showed the XRD patterns of 20 wt% Pt/G and Pt/NG cata-
lysts. The diffraction peaks at 39.8◦, 46.3◦, and 67.5◦ can be indexed
to the (1 1 1), (2 0 0), and (2 2 0) planes of the face-centered cubic
(fcc) phase of Pt, while those at 26.5◦ corresponding to graphene
[4]. From the full width half maximum of the (1 1 1) peak (the peak
of Pt (2 2 0) was too weak to calculate the particle size), the volume-
averaged particles size was calculated from the Scherrer equation
[30]. The average Pt particle sizes were 3.0 nm and 2.5 nm for Pt/G
and Pt/NG respectively, suggesting that NG can be helpful for the
distribution of Pt particles.
Fig. 4 showed the TG and DSC curves for all samples. The heating
rate in all thermal scanning measurements was 10 ◦C min−1 start-
ing at room temperature and increasing to 800 ◦C under air flow.
Above 300 ◦C, the Pt/G started to decompose gradually and a sig-
nificant drop in mass occurred at 407 ◦C, which was assigned to the
combustion of the carbon skeleton of graphene. The weight change
of the Pt/NG had a similar trend. However, no significant mass
loss was observed for the Pt/NG up to 497 ◦C. This indicated that
the Pt/NG was more thermally stable than the Pt/G. The enhanced
thermal stability may be explained by the enhanced interaction
between the Pt and graphene, and possibly decrease of defects on
graphene sheets. The Pt loading, determined from the ending TG
curves, was 20.6% and 21.7% for Pt/G and Pt/NG respectively, which
was close to the theoretical Pt loading (20%) in experiment.
Fig. 5 illustrated the cyclic voltammetry (CV) curves of the sam-
ples in nitrogen-saturated 0.5 M H2SO4 solution with a potential
range from −0.241 V to 1 V vs. SCE. The electrochemical active sur-
face area (ECSA) was estimated by hydrogen adsorption/desorption
from the electrode surface [34]. The ECSAs for the Pt/G and Pt/NG
were estimated to be 37.62 m2 g−1 and 80.45 m2 g−1 respectively,
which suggested that the Pt/NG was more favorable toward electro-
chemical reaction. The oxidation/reduction (hydroquinone) peaks
(around 0.3–0.4 V) were clearly observed for the Pt/G. The peaks
are possibly associated with the oxygen-containing groups on the
graphene, such as carboxyl groups, hydroxyl groups and epoxy
groups [4]. However, there were little or no hydroquinone peaks for
Fig. 3. TEM images and histogram of Pt particles of (a) Pt/G and (b) Pt/NG.

Y. Xin et al. / Electrochimica Acta 60 (2012) 354–358
357
Pt/G
Pt/NG
Pt/G
Pt/NG
(b)
100
80
(a)
4
0
60
-4
40
-8
20
-12
0
200
400
600
800
0
200
400
600
800
Temperature (oC)
Temperature ( oC)
Fig. 4. TG (a) and DSC (b) curves of Pt/G and Pt/NG.
3
2
The electrocatalytic activities toward methanol oxidation of the
catalysts in 0.5 M CH3OH and 0.5 M H2SO4 were shown in Fig. 7.
The peak current density in the forward sweep for the Pt/NG was
24.94 mA cm−2, which was almost 2 times of that of the Pt/G at
13.36 mA cm−2. The enhanced methanol catalytic activity of the
Pt/NG may be attributed to the following two effects induced by
N-doping. On the one hand, the incorporation of nitrogen in the car-
bon network can be favorable to the dispersion of Pt particles and
alter the electronic structures of the graphene, thus strengthening
the interaction between metal and support and introducing more
Pt active sites [15]. On the other hand, the conductivity plays a vital
role in the electrochemical methanol oxidation reaction. Remov-
ing of oxygen functional groups during N-doping using microwave
heating would result in less defect sites, enhancing its electronic
conductivity [16]. This is demonstrated by a lower onset potential
of the Pt/NG than that of the Pt/G.
Pt/G
Pt/NG
1
0
-1
-2
-3
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential (V vs. SCE)
To test the effect of N-doping on the stability, the catalysts were
evaluated under the ADT of DMFC conditions. Fig. 8 showed the
normalized ECSAs as a function of the CV cycle number obtained
for the Pt/G and Pt/NG. ADT investigation was performed from 0.6
to 1.2 V vs. NHE at 50 mV s−1 in N2 saturated 0.5 M H2SO4. It is
generally believed that the decrease of electrode performance in
DMFCs is mainly due to ECSA loss of the catalysts [35]. The ECSA
loss appeared on both catalysts under the potential cycling due
to Pt dissolution or agglomeration. The normalized ECSA of the
Fig. 5. Cyclic voltammograms of Pt/G and Pt/NG in 0.5 M H2SO4. Scan
rate = 20 mV s−1 at 25 ◦C.
the Pt/NG, due to significant decrease of oxygenated groups during
N-doping procedure.
CO stripping experiment was performed to investigate the CO
tolerance of the catalysts and the results were shown in Fig. 6. The
electro-oxidation current of the COads species on the Pt/NG was
around 6.37 mA cm−2, which almost doubles that of the Pt/G, indi-
cating that the Pt/NG catalysts were more tolerant to CO poisoning
than the Pt/G catalysts. The higher ECSA of the Pt/NG according to
CO striping (as also shown in CV results) indicated the higher Pt
utilization in Pt/NG [31]. Therefore, Pt/NG could contribute to the
enhanced activity toward CO oxidation.
Pt/NG decreased from 100% (80.45 m2 g−1) to 34.2% (27.54 m2 g−1
)
after 1000 cycles degradation, while that of the Pt/G decreased
from 100% (37.62 m2 g−1) to 8.8% (3.23 m2 g−1). The results obvi-
ously revealed that the Pt/NG was more electrochemically stable
compared to the Pt/G.
It was well known that graphene contains some defects such as
the oxygen-containing functional groups on the surface. Graphene
7
Pt/G
Pt/NG
28
6
Pt/G
5
4
Pt/NG
24
20
16
12
8
3
2
1
0
-1
-2
-3
4
0
-4
-0.4
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
Potential (V vs. SCE)
Potential (V vs. SCE)
Fig. 6. CO stripping voltammetry of Pt/G and Pt/NG in 0.5 M H2SO4.
Fig. 7. Cyclic voltammograms of methanol oxidation on Pt/G and Pt/NG.

358
Y. Xin et al. / Electrochimica Acta 60 (2012) 354–358
(2011AA11A271), Natural Science Foundation of China (21176111,
100
80
60
40
20
0
Pt/G
Pt/NG
20906045), Fundamental Research Funds for the Central Universi-
ties (1106021341), and Priority Academic Program Development
of Jiangsu Higher Education Institutions.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.electacta.2011.11.062.
References
[1] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V.
Grigorieva, A.A. Firsov, Science 306 (2004) 666.
[2] K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, M.I. Katsnelson, I.V. Grigorieva,
S.V. Dubonos, A.A. Firsov, Nature 438 (2005) 197.
[3] A.K. Geim, K.S. Novoselov, Nat. Mater. 6 (2007) 183.
[4] Y.C. Xin, J.G. Liu, Y. Zhou, W.M. Liu, J. Gao, Y. Xie, Y. Yin, Z.G. Zou, J. Power Sources
196 (2011) 1012.
0
200
400
600
800
1000
Number of cycles
Fig. 8. Normalized electrochemical active surface area based on cycle numbers
[5] D. Li, M.B. Müller, S. Gilje, R.B. Kaner, G.G. Wallace, Nanotechnology 3 (2008)
101.
during accelerated durability test.
[6] Y. Wang, Y.Y. Shao, D.W. Matson, J.H. Li, Y.H. Lin, ACS Nano
1790.
[7] X.R. Wang, X.L. Li, L. Zhang, Y.K. Yoon, P.K. Weber, H.L. Wang, J. Guo, H.J. Dai,
Science 324 (2009) 768.
[8] D. Gunlycke, J. Li, J.W. Mintmire, C.T. White, Appl. Phys. Lett. 91 (2007) 112108.
[9] F. Cervantes-Sodi, G. Csanyi, S. Piscanec, A.C. Ferrari, Phys. Rev. B 77 (2008)
165427.
[10] Y.P. Wu, S.B. Fang, Y.Y. Jiang, Solid State Ionics 120 (1999) 117.
[11] D. Hulicova, M. Kodama, H. Hatori, Chem. Mater. 18 (2006) 2318.
[12] D.C. Wei, Y.Q. Liu, Y. Wang, H.L. Zhang, L.P. Huang, G. Yu, Nano Lett. 9 (2009)
1752.
4 (2010)
sheets with these defects are easily to be oxidized and are unstable
under fuel cell operation [36]. N-doping of graphene can fix these
defects by removing these oxygenated groups, thereby enhancing
the stability and conductivity of graphene. In addition, N-doping
not only modifies the graphene surface by forming the nitrogen
functional groups, but also can changes its electronic structures
[37,38]. Moreover, N-doping is also contributing to the effective
immobilization of metal nanoparticles by strengthening the metal-
support interaction. Some literature [39] reported that the nitrogen
elements in pyridine N and quaternary N would contribute one
[13] Y.C. Ma, A.S. Foster, A.V. Krasheninnikov, R.M. Nieminen, Phys. Rev. B 72 (2005)
205416.
[14] C.W. Zhou, J. Kong, E. Yenilmez, H.J. Dai, Science 290 (2000) 1552.
[15] R.T. Lv, T.X. Cui, M.S. Jun, Q. Zhang, A.Y. Cao, D.S. Su, Z.J. Zhang, S.H. Yoon, J.
Miyawaki, I. Mochida, F.Y. Kang, Adv. Funct. Mater. 21 (2011) 999.
[16] L.S. Zhang, X.Q. Liang, W.G. Song, Z.Y. Wu, Phys. Chem. Chem. Phys. 12 (2010)
12055.
[17] R. Imran Jafri, N. Rajalakshmib, S. Ramaprabhu, J. Mater. Chem. 20 (2010)
7114.
[18] S. Maldonado, K.J. Stevenson, J. Phys. Chem. B 109 (2005) 4707.
[19] N. Li, Z.Y. Wang, K.K. Zhao, Z.J. Shi, Z.N. Gu, S.K. Xu, Carbon 48 (2010)
255.
and two P electrons, respectively, in the graphitic ␲ system. And
␲
the nitrogen functional groups can also serve as active sites for
anchoring metal ions because of the high electron density gen-
erated from the incorporation of nitrogen [15]. Nitrogen doping
can thus improve the dispersion state of metal nanoparticles on
graphene surface.
[20] D.H. Long, W. Li, L.C. Ling, J. Miyawaki, I. Mochida, S.H. Yoon, Langmuir 26 (2010)
16096.
4. Conclusions
[21] Da-Wei Wang, I.R. Gentle, G.Q. Lu, Electrochem. Commun. 12 (2010)
1423.
In summary, we demonstrated an efficient and convenient
route to synthesize N-doped graphene with high nitrogen content
through microwave-heating of graphene in NH3 atmosphere. The
nitrogen concentration doped in the graphene was 5.04 wt%. The
thermal stability of the Pt/NG (prepared by microwave-assisted
polyol) was significantly enhanced by N-doping. Moreover, the
Pt/NG exhibited higher electrochemical active surface area, higher
methanol catalytic activity, stronger tolerance to CO poisoning,
and better long term operation stability under fuel cell condition.
The enhanced performance of Pt/NG can be attributed to follow-
ing effects induced by nitrogen doping: (1) the decrease of defects
on graphene enhances stability and conductivity of supports; (2)
incorporation of nitrogen functional groups aids in anchoring Pt
seed and improving the dispersion of Pt particles, as well as intro-
ducing more Pt active sites; (3) N-doping modifies the electronic
structures of graphene to become more electronegative, which
could strengthen the metal-support interaction. It indicates that
N-doped graphene is a promising support for electrocatalysts for
advanced fuel cells.
[22] W.S. Hummers, R.E. Offenman, J. Am. Chem. Soc. 80 (1958) 1339.
[23] L.J. Cote, K. franklin, J.X. huang, J. Am. Chem. Soc. 131 (2009) 1043.
[24] H.T. Pejman, P.S. George, Carbon 48 (2010) 3993.
[25] S.H. Park, S.M. Bak, K.H. Kim, J.P. Jegal, S.I. Lee, J. Lee, K.B. Kim, J. Mater. Chem.
(21) (2011) 680.
[26] V. Sridhar, J.H. Jeon, I.K. Oh, Carbon 48 (2010) 2953.
[27] Y.F. Li, Z. Zhou, P.W. Shen, Z.F. Chen, ACS Nano 3 (2009) 1952.
[28] D.S. Geng, Y. Chen, Y.G. Chen, Y.L. Li, R.Y. Li, X.L. Sun, S.Y. Ye, S.N. Knights, Energy
Environ. Sci. 4 (2011) 760.
[29] X.L. Li, H.L. Wang, J.T. Robinson, H. Sanchez, G. Diankov, H.J. Dai, J. Am. Chem.
Soc. 131 (2009) 15939.
[30] W. Li, W.J. Zhou, H.Q. Li, Z.H. Zhou, B. Zhou, G.Q. Sun, X. Qin, Electrochim. Acta
49 (2004) 1045–1055.
[31] G. Wu, R. Swaidan, D.Y. Lia, N. Li, Electrochim. Acta 53 (2008) 7622.
[32] S.F. Zheng, J.S. Hu, L.S. Zhong, L.J. Wan, W.G. Song, J. Phys. Chem. C 111 (2007)
11174.
[33] B. Zheng, W.T. Zheng, K. Zhang, Q.B.Wen, J.Q. Zhu, S.H. Meng, X.D. He, J.C. Han,
Carbon 44 (2006) 962.
[34] A. Pozio, M.D. Francesco, A. Cemmi, F. Cardellini, L. Giorgi, J. Power Sources 105
(2002) 13.
[35] J.J. Wang, G.P. Yin, Y.Y. Shao, Z.B. Wang, Y.Z. Gao, J. Phys. Chem. C 112 (2008)
5784.
[36] Y.Y. Shao, S. Zhang, C.M. Wang, Z.M. Niea, J. Liu, Y. Wang, Y.H. Lin, J. Power
Sources 195 (2010) 4600.
[37] Y.Y. Shao, J.H. Sui, G.P. Yin, Y.Z. Gao, Appl. Catal. B 79 (2008) 89.
[38] B.G. Sumpter, V. Meunier, J.M. Romo-Herrera, E. Cruz-Silva, D.A. Cullen, H. Ter-
rones, D.J. Smith, M. Terrones, ACS Nano 1 (2007) 369.
[39] J. Lahaye, G. Nanse, A. Bagreev, V. Strelko, Carbon 37 (1999) 585.
Acknowledgements
This work was financially supported by National High
Technology Research and Development Program of China