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Nitrogen-doped graphene by ball-milling graphite with melamine for energy conversion and
storage
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2015 2D Mater. 2 044001
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2D Mater. 2 (2015) 044001
doi:10.1088/2053-1583/2/4/044001
PAPER
RECEIVED
1 June 2015
Nitrogen-doped graphene by ball-milling graphite with melamine for
energy conversion and storage
REVISED
19 August 2015
ACCEPTED FOR PUBLICATION
15 September 2015
Yuhua Xue1,2, Hao Chen1, Jia Qu1 and Liming Dai1,2
1
PUBLISHED
13 October 2015
2
Institute of Advanced Materials for Nano-Bio Applications, School of Ophthalmology & Optometry, Wenzhou Medical University, 270
Xueyuan Xi Road, Wenzhou, Zhejiang 325027, People’s Republic of China
Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering, Case
Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, USA
E-mail: [email protected], [email protected] and [email protected]
Keywords: graphene, N-doping, ball milling, supercapacitor, fuel cell
Abstract
N-doped graphene was prepared by ball milling of graphite with melamine. It was found that ballmilling reduced the size of graphite particles from 30 to 1 μm and facilitated the exfoliation of the
resultant small particles into few-layer N-doped graphene nanosheets under ultrasonication. The asprepared N-doped graphene nanoplatelets (NGnPs) exhibited a nitrogen content as high as 11.4 at.%,
making them attractive as efficient electrode materials in supercapacitors for energy storage and as
highly-active metal-free catalysts for oxygen reduction in fuel cells for energy conversion.
Introduction
Own to its high surface area and excellent electrical,
mechanical and thermal properties [1–3], the singleatom-thick graphene has attracted a great deal of
attention for various potential applications. Consequently, graphene materials have been widely studied
for energy conversion and storage in fuel cells [4],
supercapacitors [5], solar cells [6] and batteries [7, 8].
Several approaches, including mechanical exfoliation
[2], reduction of graphene oxide [9, 10], and chemical
vapor deposition (CVD) [11–13], have been developed
for producing graphene materials. Of particular interest, a ball-milling method has been recently devised to
eco-friendly produce edge-doped graphene sheets in
large quantity and at low cost [14, 15]. N-doped
graphene materials generated by ball-milling of graphite with N-containing inorganic molecules (e.g., N2
and NH3 gases) have been demonstrated to show good
electrocatalytic activities for oxygen reduction reaction (ORR) [16]—an important electrochemical reaction that controls the performance of fuels and metalair batteries [17–21]. The observed ORR electrocatalytic activity for N-doped graphene is attributable to the
N-doping induced charge redistribution, which
changes the absorption mode of O2 on the N-doped
graphitic carbon surface to facilitate the ORR [4, 21].
Ball-milling with gases often requires complicate,
expensive capsules and extremely careful fabrication
© 2015 IOP Publishing Ltd
process. In this study, we prepared N-doped graphene
by ball milling of graphite with melamine—a nitrogen-rich solid organic compound. We found that ballmilling with N-containing solid organic compounds
(e.g., melamine), unlike ball-milling with
N-containing inorganic gases, had not only greatly
simplified the material fabrication process but also
enhanced the doping efficiency. The resultant
N-doped graphene was shown to possess a nitrogen
content as high as 11.4 at.% as well as good electrical
and electrochemical properties attractive for energy
storage and conversion. As far as we are aware, the preparation of N-doped graphene by ball-milling graphite with N-containing organic compounds has not
been previously reported.
Results and discussion
Scheme 1(a) schematically shows the preparation
procedure for producing N-doped graphene by ballmilling graphite with melamine. In a typical experiment, graphite was mixed with melamine at a weight
ratio of 1:10 prior to the ball milling. As can be seen in
Scheme 1(b), the resulted N-doped graphene is highly
disperseable in water.
Figure 1(a) reproduces a typical SEM image of the
pristine graphite before ball milling, which shows an
average particle size of about 30 μm. In comparison
2D Mater. 2 (2015) 044001
Y Xue et al
Scheme 1. (a) The formation of N-doped graphene by ball milling graphite with melamine, and (b) the resulting N-doped graphene
dispersed in water.
Figure 1. SEM images of graphite (a) before ball milling and (b) after ball milling with melamine, followed by ultrasonication.
with figures 1(a), (b) shows that the ball-milling
caused a significant particle-size reduction down to
about 1 μm.
Figure 2(a) shows XRD spectra of the graphite and
the resultant N-doped graphene. As expected, the graphite shows a very sharp peak at 2θ=26°, indicating a
high graphitization degree with a graphitic interlayer
distance of 0.334 nm. As also shown in figure 2(a), the
resultant N-doped graphene shows a broad peak at
2θ=24°. The observed downshift in the diffraction
peak, together with the concomitant peak broadening,
indicates the occurrence of the ball-milling-induced
edge-doping of graphite/graphene [14]. Figure 2(b)
shows Raman spectra of the graphite before and
after ball milling. As can be seen, ball-milling dramatically increased the D band with respect to the G
band, indicating a significantly increased number of
defect sites induced by the ball-milling and heteroatom-doping. The introduction of defects significantly
reduced the thermal stability of graphite (figure 2(c)).
Figure 2(d) shows a UV–vis spectrum of the N-doped
graphene.
Chemical composition of the newly-produced
N-doped graphene was investigated by x-ray photoelectron spectroscopy (XPS). As shown in figure 3(a),
the XPS survey spectrum shows the C, N and O peaks
with an atomic content of 84.7%, 11.4% and 3.9%,
2
respectively. The corresponding curve-fitted highresolution XPS N1s spectrum in figure 3(b) reveals the
presence of three different nitrogen species in the
N-doped graphene, namely pyridinic N at 398.6 eV,
pyrrolic N at 400.5 eV, and graphitic N at 401.3 eV.
The high N-content (11.4 at.%) with pyridinic N
as a dominate component makes the N-doped graphene produced by ball milling graphite and melamine attractive for energy conversion and storage [22].
In this context, we used the N-doped graphene as electrode materials for supercapacitors. Figure 4(a) shows
the CV curves measured over a wide range of scanning
rates from 50 to 500 mV s−1 in a three-electrode cell
with 1 M H2SO4 electrolyte. The corresponding galvanostatic charging-discharging curves at the current
densities from 0.2 to 2.5 A/g are given in figure 4(b),
from which the specific capacitance was calculated as a
function of the current density by C=IΔt/(MΔV)
[23], where I is the applied current, Δt is the discharge
time, M is the mass of N-doped graphene electrodes
and ΔV is the potential range. Figure 4(c) shows the
dependence of the specific capacitance on the current
density. The electrochemical impedance spectra
shown in figure 4(d) reveals a series resistance of the
capacitor as low as 2.787 ohms. Figures 4(e) and (f)
show the cycling stability measured from the galvanostatic charging-discharging cycles at 0.2 A/g,
2D Mater. 2 (2015) 044001
Y Xue et al
Figure 2. (a) XRD spectra of the pristine graphite and N-doped graphene, (b) Raman spectra of graphite and N-doped graphene, (c)
TGA curves of graphite before and after ball-milling, and (d) UV spectrum of the N-doped graphene in water (cf Scheme 1(b)).
Figure 3. (a) A survey XPS spectrum and (b) high-resolution XPS N1s spectrum of N-doped graphene generated by ball milling
graphite with melamine.
indicating an excellent operation stability with an
almost 100% retention of the capacitance over 2000
cycles (figure 4(f)).
The N-doped graphene was further tested as a
metal-free catalyst for ORR. Figures 5(a) and (b)
reproduce the CV curves for the graphite before and
after ball milling with melamine measured in an aqueous solution of N2- or O2-saturated 0.1 M KOH solution, respectively, at a scanning rate of 50 mV s−1,
which show a substantial reduction current in the presence of oxygen, but not under nitrogen. Graphite has
3
been known to possess certain ORR activities via a 2e−
pathway [24]. Compared to graphite, the N-doped
graphene produced by ball-milling graphite with melamine exhibited a significantly improved electrocatalytic activity towards ORR in terms of both the
onset/peak potentials and the peak current
(figure 5(b)).
Figure 5(c) shows the linear scan voltammetry
(LSV) curves measured on a rotating disk electrode
(RDE) for the pristine, N-doped graphene, and commercially available Pt/C electrode (C2-20, 20%
2D Mater. 2 (2015) 044001
Y Xue et al
Figure 4. The supercapacitor properties of the N-doped graphene measured in 1 M H2SO4 solution. (a) CV curves at various scanning
rates from 50 to 500 mV s−1. (b) Galvanostatic charging-discharging curves at the current densities from 0.2 to 2.5 A/g. (c) Specific
capacitances at the current densities from 0.2 to 2.5 A/g. (d) Electrochemical impedance plots. (e) Galvanostatic charging-discharging
curves 2.5 A/g. (f) The cycling stability at 2.5 A/g over 2000 cycles.
platinum on Vulcan XC-72R; E-TEK). As can be seen,
the on-set potentials for the pristine graphite and
N-doped graphene are about −0.35 and −0.15 respectively. Besides, the pristine graphite showed a two-step
2e- ORR process while the N-doped graphene, like Pt/
C, exhibited a one-step LSV curve (figure 5(c)).
Figure 5(d) shows the LSV curves measured at different rotation speeds on the RDE for the N-doped graphene. As expected, the steady-state current density
increased as the rotation rate increased from 400 to
1600 rpm. The transferred electron number per oxygen molecule involved in the ORR process was determined by Koutecky–Levich equation, which relates
the current density j to the rotation rate ω of the electrode:
4
1
1
1
= +
j
jk
Bw 0.5
(1)
where jk is the kinetic current density and B is
expressed by the following expression:
2/ 3 -1/ 6
n
C
( )
B = 0.2nF DO2
O2
(2)
where n represents the number of electrons transferred per oxygen molecule; F is the Faraday constant
(F=96 485 C mol−1); DO2 is the diffusion coefficient
of O2 in 0.1 M KOH (1.9×10−5 cm2 s−1); ν is the
kinematic viscosity of the electrolyte solution
(0.01 cm2 s−1); CO2 is the concentration of dissolved
O2 (1.2×10−3 mol L−3). The constant 0.2 is adopted
when the rotation speed is expressed in rpm.
2D Mater. 2 (2015) 044001
Y Xue et al
Figure 5. Typical cyclic voltammograms for the ORR at (a) the graphite electrode and (b) the N-doped graphene electrode in a N2saturated (black curve) or O2-saturated (red curve) 0.10 M KOH solution. Scan rate: 50 mV s−1. (c) RDE voltammograms of the Pt,
graphite and N-doped graphene electrodes in an O2-saturated 0.1 M KOH solution at a scan rate of 10 mV s−1 and rotation speed of
1600 rpm. (d) LSV curves of the N-doped graphene electrode at different rotation speeds. (e) Koutecky–Levich plots of j−1 versus ω−1/
2
at different electrode potentials of −0.30, −0.35, −0.4, −0.45, and −0.50 V. (f) The dependence of the transferred electron number
(n) on the potential deduced from (e).
From the LSV curves shown in figure 5(d), the corresponding Koutecky–Levich plots (j−1 versus ω−1/2)
at various electrode potentials were constructed
and shown in figure 5(e), indicating a first-order
reaction kinetics with respect to the concentration of
dissolved O2. The n value for the N-doped graphene
was derived to be 3.3–3.8 at potentials ranging
from −0.3 to −0.5 V (figure 5(f)), suggesting a fourelectron process for ORR on the N-doped graphene
electrode.
5
The stability of the N-doped graphene and Pt catalysts were evaluated at a constant voltage of −0.3 V
over continuous chronoamperometric measurements
for 20 000 s in a 0.1 M O2-saturated KOH solution at a
rotation rate of 1600 rpm. Figure 6(a) shows the current-time (i-t) chronoamperometric response of the
N-doped graphene electrode at −0.3 V in O2-saturated 0.1 M KOH, along with the corresponding curve
from the Pt/C for comparison. As can be seen, the
relative current densities for the N-doped graphene
2D Mater. 2 (2015) 044001
Y Xue et al
Figure 6. (a) Current–time (i–t) chronoamperometric responses of the N-doped graphene and the Pt/C electrodes at −0.3 V in O2saturated 0.1 M KOH. (b) Current–time (i–t) chronoamperometric responses of the N-doped graphene and the Pt–C electrodes upon
introduction of methanol after about 300 s at −0.3 V.
and Pt/C electrodes reduced to about 92% and 72%,
respectively, at 20 000 s, indicating that the N-doped
graphene is much more stable than Pt as an ORR catalyst. Figure 6(b) shows that the N-doped graphene is
almost free from the methanol cross-over effect while
the Pt/C exhibits a dramatic current reduction upon
the addition of methanol [4].
Conclusions
We have developed an eco-friendly and scalable
method for production of N-doped graphene in a large
quantity at low cost by ball milling graphite with
melamine. The resultant N-doped graphene possesses
a nitrogen content as high as 11.4 at.%, attractive as an
effective electrode in supercapacitors for energy storage and as an efficient metal-free catalyst for oxygen
reduction in fuel cells for energy conversion. Furthermore, the methodology developed in this study is
applicable to nitrogen-doping of other materials as
well as low-cost, large-scale production of graphene
materials doped with heteroatoms other than nitrogen
by ball-milling graphite with other appropriate
organic compounds.
Methods
Preparation of N-doped graphene by ball milling
The N-doped graphene was prepared by ball milling of
graphite and melamine in a planetary ball-mill
machine (Pulverisette 6, Fritsch). In a typical experiment, 1 g of the graphite and 10 g of melamine were
put into a stainless steel grinding bowl (80 mL)
containing 200 stainless steel grinding ball (5 mm).
The bowl was sealed followed by fixing it in the
planetary ball-mill machine. The mixture was ball
milled at 500 rpm for 48 h. After the ball milling, the
as-prepared product was washed with hot water
6
(80 °C) for 5 times, followed by dispersing in water
(1 mg mL−1) and ultrasonicated for 2 h (Scheme 1(b))
for subsequent use.
Characterization
X-ray diffraction (XRD) was performed on a Miniflex
II Desktop x-ray diffractometer. Raman spectra
were collected using a Raman spectrometer
(Renishaw) with a 514 nm laser. The thermogravimetric analysis (TGA) was carried out on a
TA instrument with a heating rate of 10 °C min−1 in
nitrogen. X-ray photoelectron spectroscopic (XPS)
measurements were carried out on a PHI 5000
VersaProbe. Scanning electron microscopic (SEM)
images were taken on JEOL JSM-6510LV SEM. The
electrochemical measure were measured on a computer-controlled potentiostat (CHI 760C, CH Instrument, USA).
Acknowledgments
This work was supported financially by NSFC
(51202167), NSF (CMMI-1400274, IIP-1343270),
NSFC-NSF
(DMR-1106160),
CWRU-WMU
(CON115346) and the ‘Thousand Talents Program’ of
China.
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