Large-Area Aminated-Graphdiyne Thin Films for Direct Methanol Fuel Cells

Article abstract of DOI:10.1002/anie.201910588

A two-dimensional (2D) carbon nanofilm with uniform artificial nanopores is an ideal material to ultimately suppress the fuel permeation in the proton exchange membrane fuel cells. Graphdiyne has great mechanical strength, high dimensional stability, and

Full text of DOI:10.1002/anie.201910588

Carbon 154 (2019) 13e23
Contents lists available at ScienceDirect
Carbon
journal homepage: www.elsevier.com/locate/carbon
Nitrogen-containing graphene networks with high volumetric
capacitance and exceptional rate capability
,
,
, *
Deping Wang a 1, Wenming Hu b 1, Qian Ma b, Xuefang Zhang b, Xiaohong Xia b
,
Hui Chen b, Hongbo Liu b
,
*
a College of Chemistry and Materials Science, Hengyang Normal University, Hengyang, 421008, China
b College of Material Science and Engineering, Hunan University, Changsha, Hunan, 410082, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Graphene has attracted more attention as advanced electrodes for supercapacitors due to its unique
geometry structure and outstanding physicochemical property. But its low specific capacitance, espe-
cially low volumetric capacitance, has greatly restricted the practical application of graphene electrode
materials. Herein, we synthesized nitrogen-containing graphene networks by using 2, 3-diaminopyridine
(O-DAP) as functional agent in a facile hydrothermal route. During the hydrothermal process, not only
the pyrrolic-N, but also the pyrazine-N is produced in the graphene lattice at the edge/defect site of
graphene because of the double eNH2 in DAP reactant. Owing to the high nitrogen content (17.5 at%),
special nitrogen configuration, and high density (1.66 g cmꢀ3), the N-containing graphene networks
present high gravimetric capacitances up to 353 F gꢀ1 and high volumetric capacitances over 586 F cmꢀ3
in 1 M H2SO4 electrolyte. More remarkably, the N-containing graphene electrodes exhibit exceptional
rate capability with a capacitance retention of 80.6% at a high current density of 20 A gꢀ1 and good
cycling stability of 91.5% retention after 5000 cycles in a symmetrical two-electrode configuration.
© 2019 Elsevier Ltd. All rights reserved.
Received 21 May 2019
Received in revised form
13 July 2019
Accepted 25 July 2019
Available online 26 July 2019
Keywords:
Graphene networks
Nitrogen-containing
Volumetric capacitance
Rate capability
1. Introduction
as mobile electronics and electric vehicles, the volumetric capacity
is a more important metric than gravimetric ones for practical
Graphene is an attractive electrode material for supercapacitors
due to its high theoretical specific capacitance (550 F gꢀ1), superior
electrical conductivity (107 S mꢀ1), and good mechanical stability
[1]. However, most pure graphene-based materials deliver small
specific capacitances (below 200 F gꢀ1) because of irreversible
agglomeration during the fabrication process [2]. Creating “porous
graphene” has been recognized as a promising strategy to improve
the mass specific capacity of graphene electrode materials [3]. The
porosity of graphene could either come from the holes or pores in
the graphene mesh surface [4e7], or from the three-dimensional
network structure of graphene lamellae, such as aerosols [8e10],
hydrogels and foams [11e13]. Unfortunately, the density of those
porous graphene is generally less than 0.5 g cmꢀ3 [14] (some even
lower than 0.1 g cmꢀ3 [9,12]), leading to a low volumetric capacity.
Since supercapacitors are usually employed in a limited space, such
application of supercapacitors.
Recently, numerous experimental studies [15e23] have shown
that functionalizing and/or doping graphene with nitrogen can not
only modulate local electronic structures and improve the electrical
conductivity, but also facilitate the access of electrode surface area
to the electrolyte ions. Taking advantage of abundant functional-
ities on graphene oxide (GO) surface, N-containing graphene can be
easily synthesized in large scale by hydrothermal or solvothermal
treatment of GO with nitrogen containing precursors. Various ni-
trogen containing molecules, such as non-aromatic diamine/tri-
amine [15], phenylenediamine [16,17], hydroxylamine [18],
formamide [2], ammonium bicarbonate [19], urea [20,21], hexa-
methylenetetramine [22], amitrole [23] et al., have been utilized as
the nitrogen source for N-containing graphene. Despite its
improved gravimetric performance, N-containing graphenes, in
most cases, remains have low volumetric performance because of
their porous structure as a result of well-spaced graphene frame-
work or activation. Depressing the porosity of N-containing gra-
phenes is expected to assure high density and high volumetric
capacitance, but the rate performance would be limited due to the
* Corresponding authors.
E-mail addresses: xxh@hnu.edu.cn (X. Xia), hndxlhb@163.com (H. Liu).
Contributed equally to this work.
1
https://doi.org/10.1016/j.carbon.2019.07.085
0008-6223/© 2019 Elsevier Ltd. All rights reserved.

14
D. Wang et al. / Carbon 154 (2019) 13e23
long ion diffusion path. Hence, it is still a big challenge to obtain
graphene-based electrode materials with both high volumetric
capacity and good rate capability.
characterized using an ESCALAB 250Xi spectrometer (USA). The
electrical conductivity was measured by a four-probe method (RTS-
8 Four-Point probe meter, China).
It has been found that nitrogen characteristic configuration,
such as pyrrolic-N, pyridinic-N, and quaternary-N, has significant
influence on the electrochemical performance of N-containing
graphene [20]. For examples, pyridinic-N and pyrrolic-N have been
proven to improve the charge mobility and increase the capacitance
through its reversible redox reaction especially in aqueous elec-
trolytes, whereas quaternary-N could increase the electrical con-
ductivity of graphene. Moreover, the nitrogen content and
functional sites also play an important role in their performance,
especially in terms of improving the rate capability [24]. In this
work, N-containing graphene network (up to 17.5 at%) was suc-
cessfully fabricated by employing 2, 3-diaminopyridine (O-DAP) as
functional agent. The chemical interaction between monomers and
GO and nitrogen configurations in graphene on the charge trans-
port properties were experimentally studied. The specific surface of
N-containing graphene was significantly reduced due to an accel-
erated restacking phenomenon with functionalizing process [25].
Even with a reduced specific surface area (43 m2 gꢀ1), the N-con-
2.3. Electrochemical measurements
The working electrode was fabricated by loading a mixture of
active material (80 wt%), acetylene black (10 wt%) and polytetra-
fluoroethylene (10 wt%) on a stainless-steel mesh (as current col-
lector, 1 cm ꢂ 1 cm). Then, the electrodes were dried at 80 ꢁC under
vacuum overnight. The mass loading of the active material on the
current collector was measured to be 2 mg cmꢀ2. The electro-
chemical performance was evaluated on a CHI660E electrochemical
workstation (China) at 25 ꢁC, including galvanostatic charging/
discharging (GCD), cyclic voltammetry (CV), and electrochemical
impedance spectroscopy (EIS). In a three-electrode configuration,
platinum sheet and Ag/AgCl electrode were used as the counter and
the reference electrode, respectively. GCD and CV tests were
recorded between ꢀ0.2 and 0.8 V (vs. Ag/AgCl) in 1 mol Lꢀ1 H2SO4
aqueous electrolyte. EIS was carried out at an open circuit potential
of a 5.0 mV amplitude in a frequency range of 0.01 Hze100 kHz. In
the symmetrical two-electrode configuration, two N-containing
graphene electrodes with the same mass were used as electrodes
and separated by a glassy fibrous separator. CV and GCD tests of the
two-electrode system were recorded between 0 and 1 V.
taining graphene presents
a considerably improved electro-
chemical performances with a high specific capacitance (up to
353 F gꢀ1 and 586 F cmꢀ3 at 0.1 A gꢀ1 in 1 M H2SO4 electrolyte
measured in three-electrode system, 263 F gꢀ1 at 0.5 A gꢀ1
measured in two-electrode system), exceptional rate capability
(212 F gꢀ1 at 20 A gꢀ1 in two-electrode system), and good cycling
stability (91.5% retention after 5000 cycles).
The specific gravimetric capacitance (Cg, F gꢀ1) in the three-
electrode system was calculated by equation (1) [27]:
ð
2I Vdt
2. Experimental section
Cg ¼
(1)
ꢀ
ꢀ
ꢀ
Vf
Vi
mV2
2.1. Synthesis of N-containing graphene networks
GO was prepared through a modified Hummers process with
where C (F gꢀ1) is the specific capacitance, I (A) is the discharge
R
natural graphite powder (average particle size of 20 mm) [26]. The
current, t (s) is the discharge time, Vdt is the integral current area,
V (V) is the potential with initial and final values of Vi and Vf,
respectively, and m (g) is the mass of active materials loaded in
working electrode.
N-containing graphene was synthesized as follows. First, a homo-
geneous GO solution was prepared by dispersing 160 mg GO and
0.2 mL ammonia solution in 40 mL deionized water after ultra-
sonication in a water bath at 100 W for 3 h. Then, 3.67 mmol O-DAP
were dissolved completely in 40 mL deionized water by stirring.
Next, the precursor O-DAP solution was mixed with the GO
dispersion under ultrasonication in a water bath at 100 W for 2 h
and transferred to autoclaves to perform a one-step hydrothermal
process at 165 ꢁC for 12 h. Then, the hydrothermal products were
washed with diluted ethanol and water sufficiently to remove re-
sidual precursor and freeze-dried. The obtained products were
denoted as O-DAP-NG. In order to figure out the influence of ni-
trogen configuration on the electrochemical performance, 1,2-
diamino cyclohexane (O-DACH) was also used as functional agent
at the same procedure and denoted as O-DACH-NG. For compari-
son, the reduced GO was prepared under the same hydrothermal
procedure without adding nitrogen precursor and denoted as RG.
The density of electrode materials was determined by pressed
200 mg sample into a thick molds with a 10 mm diameter under
10 MPa for 2 min [28]. The press density was calculated by equation
(2):
r
¼ M/(
p
*r2*T)
(2)
where M (g) is weight of sample, r (cm) is the diameter of the mold
and T (cm) is the thickness.
The volumetric capacitance (Cv, F cmꢀ3) was calculated from the
following equation [28]:
Cv ¼
r
Cg
(3)
where
r
is the density of electrode materials (g cmꢀ3).
2.2. Characterizations
Specific capacitances of a single electrode derived from galva-
nostatic tests in the two-electrode system can be calculated from
equation (4) [29,30]:
The morphology characteristics of samples were observed by
scanning electron microscope (SEM, JSM-6700F, Japan) and trans-
mission electron microscopy (TEM, JEM3010, Japan), respectively.
The surface area and pore structure of samples were analyzed on
aN2 adsorption/desorption apparatus (Micromeritics ASAP Tristar
3020, USA) after degassing at 300 ꢁC for 3 h. Raman spectra were
obtained on a spectrophotometer (LABRAM-010, France) using a
wave length of 633 nm. Fourier transform infrared spectroscopy
(FT-IR) spectra were recorded by a TENSOR27 spectrometer. X-ray
4I
Cm
¼
(4)
ðdU=dtÞm
where Cm (F gꢀ1) is the specific capacitance, I (A) is the constant
current, m (g) is the total mass of active materials loaded in the two
working electrodes, U (V) is the voltage.
Specific gravimetric and volumetric energy density (Em, Wh
photoelectron
spectroscopy
measurements
(XPS)
were
kgꢀ1 and Ev, Wh Lꢀ1), as well as power density (Pm, W kgꢀ1 and Pv,

D. Wang et al. / Carbon 154 (2019) 13e23
15
W Lꢀ1) of the symmetric supercapacitors were obtained from
equations (5)e(8) [28e30]:
basal plane aggregation of the nanosheets was observed in the O-
DACH-NG (Fig. 2f). These results indicate that the overlapping and
coalescing of the reduced graphene sheets may be accelerated due
to nitrogen functionalization [33]. Meanwhile, by comparing the
TEM images of O-DAP-NG and O-DACH-NG (Fig. 2g and h), it can be
seen that the sheets of O-DAP-NG is thinner and more stretchable
than that of O-DACH-NG.
ð
I
UðtÞdt
Em
¼
(5)
3:6 ꢂ m
Em
In order to investigate the effect of nitrogen functionalization on
the specific surface area and pore structure of samples, N2
adsorption/desorption measurements were performed and pre-
sented in Fig. 3. As shown in Fig. 3a, RG displays a typical Type-IV
isotherm with a large hysteresis loop (H4) in the middle pressure
range (P/P0 ¼ 0.45e0.95), suggesting the mesoporous characteris-
tics. The porosity parameters of the samples are listed in Table 1.
Pm
EV
¼
(6)
(7)
D
t
¼
r
Em
Ev
PV
¼
(8)
Dt
The BET specific surface area of RG was calculated to be 282 m2 gꢀ1
,
where I (A) is the applied current, m (g) is the total mass of elec-
whereas the O-DAP-NG and O-DACH-NG show a much lower spe-
cific surface area, which is 43 m2 gꢀ1 and 9 m2 gꢀ1, respectively.
Possible reasons for the diminished surface area are the high
density of nitrogen functional moieties blocking the pores and the
accelerated restacking and coalescing phenomenon of reduced
graphene sheets induced by nitrogen functionalization [25,34]. The
pore size distribution (determined by the density functional theory,
DFT) is presented in Fig. 3b. The RG sample possesses a mesopore
size within the range of 2e10 nm. However, the O-DAP-NG and O-
DACH NG only have a weak and broad meso-to macroporous size
distribution in the range from 20 to 100 nm.
R
trodes, Udt is the area under the GCD curve,
time.
DtðsÞ is the discharge
3. Results and discussions
3.1. Structural and morphological characterization of samples
The possible formation mechanism of the hydrothermal reac-
tion of GO and O-DAP is shown in Fig. 1. GO sheets have their basal
planes decorated mostly with hydroxyl and epoxide groups, in
addition to carboxyl, hydroxyl and carbonyl groups located pre-
sumably at the edges. Therefore, GO reacted with DAP in the hy-
drothermal process can lead to the incorporation of pyridinic
nitrogen moieties into graphene sheets. Imine (-C¼N-) group was
assembled by the reaction of 2,3-diaminopyridine with mono-
ketone (C¼O), while the reaction of 2,3-diaminopyridine with o-
diketone (O¼CeC¼O) can produce more stable pyrazine ring [31].
In the meanwhile, DAP reacted with carboxyl (COOH) to form
amide and imidazole ring [16]. DACH reacted with GO in the hy-
drothermal process is similar to DAP with GO.
FT-IR spectroscopy were performed to explore the chemical
changes of the starting GO during the hydrothermal reaction, as
shown in Fig. 4a. The characteristic peaks of GO at 1054, 1120, and
1730 cmꢀ1 can be assigned to the stretching vibration of alkoxyl
(CeO), epoxy (CeOeC) and carbonyl (C¼O) groups, respectively
[35]. A broad adsorption band appeared at 3000e3500 cmꢀ1 is
related to the hydroxyl (eOH) groups on the GO structure. In the FT-
IR spectra of RG, residual oxygen functional groups were observed
such as the symmetrical stretching vibration of CeOeC at
1120 cmꢀ1, C¼O stretching at 1722 cmꢀ1, eOH group at 3423 cmꢀ1
,
suggesting partial reduction of GO under hydrothermal condition.
For O-DAP-NG, the signal intensity of oxygen groups was remark-
ably decreased as most of them were removed during the hydro-
thermal process [36]. New peaks at 772 cmꢀ1, 1192 cmꢀ1 and
1566 cmꢀ1 were observed. The peak at 772 cmꢀ1 is attributed to
NeH in plane bending vibrations. The strong peak at 1566 cmꢀ1 can
be attributed to the C¼C/C¼N stretching vibration mode of benzoid
rings in pyridine and pyrazine [16,37]. And the peak at 1192 cmꢀ1
also can be assigned to the characteristic peak of pyrazine [16,38].
The results indicate that pyrazine structures formed in hydrother-
mal process, as well as the pyridinic-N can be efficiently incorpo-
rated at the edge of graphene through them. It can be seen that the
The morphologies and microstructures of N-containing gra-
phene networks and reduced graphene oxide (RG) were observed
by scanning electron microscopy (SEM) and transmission electron
microscopy (TEM). It can be discerned that the RG displays a typical
three-dimensional crumpled sheet-linked morphology (Fig. 2a and
b) [8]. O-DAP-NG and O-DACH-NG possess a similar morphology
but has much larger sheet size (Fig. 2c and e) than that of RG. This
might be due to the accelerated coalescing behavior ascribe to the
deoxygenation during hydrothermal reduction of GO with O-DAP
and O-DACH molecules [32,33]. The O-DAP-NG displays a flexible
graphene-like nanosheets with abundant wrinkles from the high
magnification SEM observation in Fig. 2d. Severe basal plane-to-
Fig. 1. The possible formation mechanism of the hydrothermal process between GO and DAP

16
D. Wang et al. / Carbon 154 (2019) 13e23
Fig. 2. SEM images of (aeb) RG, (ced) O-DAP-NG, (eef) O-DACH-NG and TEM images of (g) O-DAP-NG, (h) O-DACH-NG.
Fig. 3. N2 absorption-desorption isotherms (a) and the corresponding DFT pore size distribution (b) of RG, O-DAP-NG and O-DACH NG. (A colour version of this figure can be viewed
online.)
1325 cmꢀ1 (D) and 1584 cmꢀ1 (G) were observed for O-DACH-NG
and O-DAP-NG. The intensity ratio of D band to G band (ID/IG) tends
Table 1
Pore characteristics of RG and N-containing graphene networks.
to increase in the order of RG, O-DACH-NG and O-DAP-NG, which is
1.30, 1.34 and 1.51, respectively. The highest ID/IG ratio of O-DAP-NG
reveals more defects generating from nitrogen incorporation dur-
ing hydrothermal process than in O-DACH-NG [6]. In addition, a
small peak emerged at 1501 cmꢀ1 in O-DAP-NG further confirms
the formation of pyrazine structure by reaction between the two
amino groups on 2,3-DAP and two neighborhood oxygen functional
groups at the edge of graphene [16].
XPS were performed to further elucidate the chemical states and
surface composition of the N-containing graphene (Fig. 5, Table 2).
The survey spectrum (Fig. 5a) exhibits the presence of carbon (C),
nitrogen (N) and oxygen (O) elements in O-DAP-NG and O-DACH-
NG, whereas RG only has C and O elements. The oxygen content of
RG is 13.7%, which is much lower than that of graphene oxide,
suggesting a successful removal of oxygen functional groups. The
content of nitrogen in O-DAP-NG is up to 17.5 at%, which is much
higher than that of O-DACH-NG (6.9 at%), and also higher than most
other N-containing carbon materials (see Table 3). Furthermore, the
high-resolution N 1s spectrum of O-DAP-NG and O-DACH-NG
(Fig. 5b and c) can be deconvoluted into four peaks locating at
398.8 eV, 399.4 eV, 400.4 eV and 401.5 eV, which can be assigned to
Samples
RG
O-DAP-NG
O-DACH-NG
SBET (m2 gꢀ1
)
282
0.29
4.1
43
9
Pore volume (cm3 gꢀ1
)
0.12
11.0
1.66
0.036
15.7
1.87
Average pore width (nm)
r
(g cmꢀ3
)
1.29
spectrum of O-DACH-NG also shows CeN bonds (1189 cmꢀ1) and
C¼N bonds (1572 cmꢀ1), implying the successful incorporation of
nitrogen into the reduced graphene oxide. Fig. 4b compares the
Raman spectrum of GO, RG, O-DAP-NG and O-DACH-NG. The
Raman spectrum of GO shows a broad G band at 1594 cmꢀ1 and a D
band at 1359 cmꢀ1, which correspond to the D (related to the
disordered crystal structure) and G (associated with graphitic
structure) bands, respectively. This D band is ascribed to the sp3
amorphous carbons originated by oxidation. The Raman spectrum
of RG also displays the existence of D and G bands at 1345 and
1589 cmꢀ1, but with an increased D/G intensity ratio (1.30)
compared to that in GO (0.86). This change suggests a greater
number of sp2 domains with smaller size created in RG upon
reduction of the exfoliated GO [39]. Two major peaks at around

D. Wang et al. / Carbon 154 (2019) 13e23
17
Fig. 4. FT-IR spectra (a), Raman spectra (b) of GO, RG, O-DAP-NG and O-DACH NG. (A colour version of this figure can be viewed online.)
Fig. 5. XPS of RG and N-containing graphene. (a) XPS survey scan, (b) N 1s XPS spectra of O-DAP-NG, (c) N 1s XPS spectra of O-DACH-NG. (A colour version of this figure can be
viewed online.)
Table 2
The percentage mass content of elements, relative nitrogen content of samples determined by high-resolution XPS survey and the electronic conductivity.
Sample
C (at O (at N (at N distribution (%)
Electronic conductivity (S
%)
%)
%)
cmꢀ1
)
Pyridinic-N
Amine-N
Pyrrolic-N
Quaternary-N
(398.7 0.1 eV)
(399.4 0.1 eV)
(400.4 0.1 eV)
(401.5 0.1 eV)
RG
86.3 13.7 0.0
e
e
e
e
5.5
7.3
6.8
O-DAP-NG 77.2 5.4
O-DACH- 86.4 6.7
NG
17.5 44.6
6.9 31.5
21.4
38.9
26.8
26.5
7.1
3.1

18
D. Wang et al. / Carbon 154 (2019) 13e23
Table 3
Comparison of volumetric capacitance and rate capability of different carbon materials.
Carbon sample
SBET (m2 gꢀ1
)
N content (at%)
Cv* (F cmꢀ3
)
Cs (F gꢀ1
)
Rate (F gꢀ1
)
Electrolyte
Ref.
N-doped graphene
N-doped porous carbon
N-doping carbon
532
1569
1004
29
627
439
605
197
489
485
e
7.7
6.5
8.7
5.3
9.2
5.3
3.4
7.7
5.3
4.4
13.9
0.3
1.2
17.5
387 (1)*
377 (0.2)
180 (1)
234 (10)*
232 (20)
140 (20)
96 (5) *
255 (10)*
200 (20)*
196 (100)*
226.0 (20)*
152 (20)
225 (20) *
443 (20)
195 (20)
120 (10)
212 (20)
6 M KOH
1 M H2SO4
6 M KOH
1 M H2SO4
1 M H2SO4
6 M KOH
6 M KOH
6 M KOH
1 M H2SO4
6 M KOH
1 M H2SO4
6 M KOH
6 M KOH
1 M H2SO4
[20]
[45]
[53]
[31]
[49]
[43]
[28]
[54]
[55]
[2]
N-doped graphene
214 (0.1) *
408 (1)*
231 (1)*
375 (0.1) *
334 (0.5)*
190 (0.1)
255 (1)*
458 (0.5)
339 (0.05)
151 (0.5)
261 (1)
3D N-doped graphene
N-doped mesoporous carbon sphere
N-doped holey graphene
N-doped graphene
N-doped mesoporous carbons
N-doped graphene
N-doped graphene film
N-enriched porous carbon/graphene
Graphene hydrogel
439 (0.1)
438 (0.5)
200 (0.1)
390 (0.5)
711 (0.5)
365 (0.05)
294 (0.5)
586 (0.1)
[50]
[47]
[48]
This work
915
39
43
N-containing graphene
Cv: the volumetric specific capacitance.
Cs: the gravimetric specific capacitance.
*: the specific capacitance in three-electrode systems.
The value in the brackets means the current density (A gꢀ1).
pyridinic-N/pyrazine-N/imines [40], amines, pyrrolic-N, and qua-
ternary nitrogen, respectively. Notably, the pyridinic-N content
reaches 7.8 at% in O-DAP-NG and constitutes 44.6% of the total ni-
trogen content, while the pyridinic-N content in O-DACH-NG re-
mains at relatively low levels (2.2 at%). Besides improving the
wettability of electrode, the incorporation of pyridinic-N/pyrazine-
N and pyrrolic-N could offer additional capacitance through the
Faradaic redox reactions, especially in acidic aqueous electrolytes
[41]. Furthermore, the presence of graphitic-N could improve the
conductivity of the N-containing graphene [33], which can be
confirmed by the measurements of intrinsic electronic conductiv-
ity, as shown in Table 2.
materials at the same current density (See Table 3). EIS was carried
out to gain a deep understanding of the capacitive property of N-
containing graphene electrodes, as shown in Fig. 6c. The resulting
Nyquist plots can be well fitted by the equivalent circuit model
(inset). The x-intercept of the semicircle in the high frequency re-
gion corresponds to the bulk solution resistance (Rs) [43]. The Rs of
all three electrodes are similar (0.85 U). The diameter of the semi-
circle represents the charge-transfer resistance (Rct) [43]. The Rct
values for the O-DAP-NG, O-DACH-NG and RG electrodes are 0.85,
1.63 and 1.96 U, respectively. The smallest diameter of O-DAP-NG
electrode indicates fast charge transfer between the electrode
material and electrolyte, which may be ascribed to the enriched N
atoms and proper nitrogen configuration. Moreover, the more
vertical line of the O-DAP-NG electrode in the low frequency region
suggests better capacitive behavior and smaller diffusion resis-
tance, which is desirable for high-rate capability. In addition, the
stability of newly generated nitrogen-containing bonds in O-DAP-
NG and O-DACH-NG were also investigated by a long-term cyclic
voltammetry (CV) measurements at a fixed scan rate (10 mV sꢀ1) in
a three-electrode configuration, as shown in Fig. 6d and e,
respectively. CV curves of the first and 500 th cycles are almost
identical in shape of the two electrodes, indicating a high revers-
ibility of the pseudocapacitive interactions and excellent electro-
chemical stability of nitrogen containing graphene electrode.
In order to better illustrate the potential applications of the N-
containing graphene, we assembled a symmetrical supercapacitor
based on the N-containing graphene and tested in 1 M H2SO4
electrolyte. Fig. 7a compares the CV curves of the electrodes at a
scan rate of 5 mV sꢀ1 in the symmetrical system. There are obvious
redox humps in both anodic and cathodic curves of O-DAP-NG and
O-DACH-NG electrodes, further demonstrating the pseudocapaci-
tance is originated from the Faradaic redox reactions of pyridinic-N
and pyrrolic-N atoms involving protons [44]. As shown in Fig. 7b,
the CV curves of as-assembled O-DACH-NG supercapacitor exhibit
evident distortion when the scan rate increased, indicative of large
polarization for O-DACH-NG. In comparison, CV curves of O-DAP-
NG (Fig. 7c) show a rectangle-like shape even at a high scan rate up
to 200 mV sꢀ1, suggesting the low internal resistance and high rate
delivery with rapid charging-discharging characteristic. Typical
GCD curves of the O-DAP-NG at different current densities from 1 to
20 A gꢀ1 are illustrated in Fig. 7d. It can be seen that all the GCD
curves in the two-electrode system are not strictly symmetrical, but
slightly distorted due to the nitrogen incorporation. However, the
IR drop is almost negligible even at the high current density of
3.2. Electrochemical characterization of samples
Electrochemical performances of N-containing graphene and RG
for supercapacitors were first estimated using a three-electrode
system in 1 M H2SO4 aqueous solution. Fig. 6a compares the CV
curves of the electrodes at a scan rate of 5 mV sꢀ1. The RG electrode
shows a nearly rectangular CV shape, indicative of a typical electric
double-layer capacitive (EDLC) behavior. Obviously, much larger
capacitive response with distinct redox peaks can be seen for the O-
DAP-NG and O-DACH-NG electrodes, indicating the presence of a
strong pseudocapacitive behavior in addition to EDLC. These redox
reactions probably based on the protonation of pyridinic-N and
pyrrolic-N atoms in acidic medium [42]. Furthermore, the O-DAP-
NG electrode exhibits a stronger capacitive response than O-DACH-
NG, indicating that the high nitrogen content and suitable nitrogen
configuration have greatly positive effect on enhancing the specific
capacitance of graphene materials. The charge/discharge curves of
electrode (Fig. 6b) were also recorded. The RG exhibits nearly linear
in shape, again indicative of the ideal EDLC behavior. In contrast,
the O-DAP-NG and O-DACH-NG electrodes show distorted trian-
gular shape, further demonstrating the combination effect of
pseudocapacitance and EDLC. The O-DAP-NG electrodes exhibit the
longest charge-discharge time, in accordance with the CV results.
The specific capacitance of O-DAP-NG, O-DACH-NG and RG calcu-
lated from the GCD curves is 353 F gꢀ1, 244 F gꢀ1 and 176 F gꢀ1 at a
current density of 0.1 A gꢀ1, respectively. Taking the very small
specific surface area of the N-containing graphene into consider-
ation, the capacitance should mainly come from pseudocapaci-
tance. More importantly, the volumetric specific capacitance of O-
DAP-NG is as high as 586 F cmꢀ3 (353 F gꢀ1,1.66 g cmꢀ3) at a current
density of 0.1 A gꢀ1, which is superior to most N-containing carbon

D. Wang et al. / Carbon 154 (2019) 13e23
19
Fig. 6. (a) CV curves of RG, O-DAP-NG and O-DACH-NG electrodes at 5 mV sꢀ1 in a three-electrode system, (b) Galvanostatic charge/discharge (GCD) curves at 0.1 A gꢀ1, (c) Nyquist
plots, inset shows the enlarged part and the corresponding equivalent circuit model, (d) the long-term CV measurements of O-DAP-NG electrode (500 cycles) at 10 mV sꢀ1, (e) the
long-term CV measurements of O-DACH-NG electrode (500 cycles) at 10 mV sꢀ1. (A colour version of this figure can be viewed online.)
20 A gꢀ1 (insert of Fig. 7d), further confirming the low internal
resistance of the O-DAP-NG supercapacitor material. Fig. 7e dis-
plays the calculated specific capacitances from GCD curves of the
(217 F gꢀ1) and RG (117 F gꢀ1). This value is lower than the capaci-
tance (353 F gꢀ1) of three-electrode system due to electrode po-
larization [45]. Significantly, the volumetric capacitance and
gravimetric capacitances (Fig. 7f) still retain 352 F cmꢀ3 and
212 F gꢀ1 at the current density of 20 A gꢀ1, respectively, much
higher than those of nitrogen-doped graphene and porous carbons,
meaning excellent rate capability (see Table 3). The exceptional rate
capability of O-DAP-NG could be explained by the increased
three devices at different current densities from 0.1 to 20 A gꢀ1
.
Both O-DAP-NG and O-DACH-NG have higher specific capacitance
than RG even though they have much lower specific surface area
and pore volume. Notably, the maximum gravimetric capacitances
of O-DAP-NG is 263 F gꢀ1
, higher than that of O-DACH-NG

20
D. Wang et al. / Carbon 154 (2019) 13e23

D. Wang et al. / Carbon 154 (2019) 13e23
21
Fig. 8. The schematic illustration of the effect of DAP functionalization on the electrochemical performances of graphene based materials. (A colour version of this figure can be
viewed online.)
electrical conductivity [45] and strong affinity for Hþ in the elec-
trolyte [33] by abundant pyridinic-N/pyrazine-N at the edges and
defect sites of graphitic domains. Ragone plots in Fig. 7g again
display the highest energy density of O-DAP-NG, which delivers an
energy density of 9.1 and 7.3 W h kgꢀ1 at a power density of 0.12
and 4.8 kW kgꢀ1 at 0.5 and 20 A gꢀ1, respectively, demonstrating an
ultrafast ion/electron exchange/transport. More importantly, volu-
metric Ragone plot of O-DAP-NG symmetric supercapacitor shows
a maximum volumetric energy density of 15.1 W h Lꢀ1 with the
corresponding power density of 0.2 kW Lꢀ1 and retains
12.2 W h Lꢀ1 at 7.9 kW Lꢀ1. It is noteworthy that such high volu-
metric capacitive performance of N-containing graphene out-
performs to the behaviors of other previously reported N-
containing carbon-based electrodes for supercapacitors in aqueous
electrolytes, such as heteroatom-doped porous carbon-tube
(12.15 W h Lꢀ1 at 0.7 kW Lꢀ1) [46], N-enriched porous carbon/gra-
phene composites (6.7 W h Lꢀ1 at 5.0 kW Lꢀ1) [47], functionalized
graphene hydrogel [48] (11.2 W h Lꢀ1 at 0.125 kW Lꢀ1) etc. The cycle
stability was performed at a current density of 2 A gꢀ1 (Fig. 7h). In
addition to the high volumetric capacitance and excellent rate
capability, O-DAP-NG based symmetric supercapacitor demon-
strates good cycling stability. Approximately 91.5% of its initial
capacitance is retained after 5000 cycles, which is comparable to
many pseudocapacitive electrodes [20,49,50] and some commer-
cial activated carbons [51,52].
high electrochemical activity in the acidic electrolyte, which is
beneficial for the penetration of electrolyte and fast ion transport.
Due to the massive electroactive sites, strong affinity for electrolyte
ions, together with high packing density, the as-prepared N-con-
taining graphene electrodes exhibit an ultrahigh volumetric
capacitance, superior rate capability, and outstanding cycle
stability.
4. Conclusion
The nitrogen-containing graphene framework was successfully
synthesized through a simple one-step hydrothermal reaction by
employing DAP as functional agent. Specific pyridinic-N configu-
ration can be effectively incorporated into graphitic network at
edge/defect sites through newly formed pyrazine linkages. Because
of its high nitrogen content, specific nitrogen configuration,
appropriate functional sites and high density, the O-DAP-NG ma-
terials display a high specific capacitance up to 353 F gꢀ1 and an
ultrahigh volumetric capacitance up to 586 F cmꢀ3 at a current
density of 0.1 A gꢀ1. More importantly, the two-electrode sym-
metric supercapacitor made of O-DAP-NG displays a high volu-
metric energy density of 12.2 Wh Lꢀ1 at 7.9 kW Lꢀ1, exceptional rate
capability (212 F gꢀ1 at a current density of 20 A gꢀ1), and good
cycling stability, indicative of their potential application for high-
performance supercapacitors.
Fig. 8 is provided to illustrate the effect of nitrogen incorpora-
tion on the electrochemical performances of O-DAP-NG electrode.
Under hydrothermal conditions, DAP molecules tend to function-
alize at the edge of graphene through the formation of the pyrazine
structure or imidazole structure, as confirmed by Raman and XPS
results, rather than intercalate into the interlayer of the graphene
sheets as molecular spacers. On the other hand, the epoxy and
hydroxyl groups on the basal planes can be significantly removed
during hydrothermal reduction at DAP-induced basic pH. The
overlapping and coalescing of graphene sheets are accelerated due
Acknowledgments
This work was supported financially by the National Natural
Science Foundation of China (517720835, 51402101 and 51472083),
Hunan Provincial Natural Science Foundation of China (14JJ3059
and 2017JJ2008), Science and Technology Planning Project of Hu-
nan Province (2018GK1030) and Growth Scheme for Young
Teachers of Hunan University (No. 531107040185).
to the promoted pep attractive interactions and result in a very low
References
specific area and pore volume. Benefiting from the N-enriched
(pyridinic, pyrazine or imidazole-N) edge/defects sites, the
agglomerated graphene sheets have a strong affinity for Hþ and
[1] R. Raccichini, A. Varzi, S. Passerini, B. Scrosati, The role of graphene for elec-
trochemical energy storage, Nat. Mater. 14 (2015) 271e279, https://doi.org/
10.1038/NMAT4170.
Fig. 7. Capacitive behaviors of O-DAP-NG, O-DACH-NG and RG electrodes measured in the symmetric supercapacitor in 1 M H2SO4. (a) CV curves of samples at 5 mV sꢀ1, (b) CV
curves of O-DACH-NG electrode at scan rates from 5 to 200 mV sꢀ1, (c) CV curves of O-DAP-NG electrode at the scan rates from 5 to 200 mV sꢀ1, (d) GCD curves of O-DAP-NG
electrode at different current densities from 0.2 to 20 A gꢀ1, (e) Gravimetric capacitances of O-DAP-NG, O-DACH-NG and RG at different current densities, (f) Volumetric capacitance
of O-DAP-NG electrode at different current densities, (g) Ragone plots of O-DAP-NG electrode and (h) Cycling stability of O-DAP-NG electrode at a current density of 2 A gꢀ1 over
5000 cycles. (A colour version of this figure can be viewed online.)

22
D. Wang et al. / Carbon 154 (2019) 13e23
[2] Y. Wang, Z. Wei, Y. Nie, Y. Zhang, Generation of three dimensional pore-
[25] J. Yang, M.R. Jo, M. Kang, Y.S. Huh, H. Jung, Y.-M. Kang, Rapid and controllable
synthesis of nitrogen doped reduced graphene oxide using microwave-
assisted hydrothermal reaction for high power-density supercapacitors, Car-
bon 73 (2014) 106e113. https://doi.org/10.1016/j.carbon.2014.02.045.
[26] W.S. Hummers Jr., R.E. Offeman, Preparation of graphitic oxide, J. Am. Chem.
Soc. 80 (1958) 1339, https://doi.org/10.1021/ja01539a017.
[27] L.Q. Mai, A. Minhas-Khan, X. Tian, K.M. Hercule, Y.-L. Zhao, X. Lin, et al.,
Synergistic interaction between redox-active electrolyte and binder-free
functionalized carbon for ultrahigh supercapacitor performance, Nat. Com-
mun. 4 (1e7) (2013) 2923. https://doi.org/10.1038/ncomms3923.
[28] Y. Zhang, L. Ji, W. Li, Z. Zhang, L. Lu, L. Zhou, et al., Highly defective graphite for
scalable synthesis of nitrogen doped holey graphene with high volumetric
capacitance, J. Power Sources 334 (2016) 104e111. https://doi.org/10.1016/j.
jpowsour.2016.10.018.
[29] Y.F. Nie, Q. Wang, X.Y. Chen, Z.J. Zhang, Nitrogen and oxygen functionalized
hollow carbon materials: the capacitive enhancement by simply incorpo-
rating novel redox additives into H2SO4 electrolyte, J. Power Sources 320
(2016) 140e152. https://doi.org/10.1016/j.jpowsour.2016.04.093.
controlled nitrogen-doped graphene hydrogels for high performance super-
capacitors by employing formamide as the modulator, J. Mater. Chem. 5
(2017) 1442e1445, https://doi.org/10.1039/c6ta09734k.
[3] Y. Chen, Z. Liu, L. Sun, Z. Lu, K. Zhuo, Nitrogen and sulfur co-doped porous
graphene aerogel as an efficient electrode material for high performance
supercapacitor in ionic liquid electrolyte, J. Power Sources 390 (2018)
215e223. https://doi.org/10.1016/j.jpowsour.2018.04.057.
[4] Y. Xu, C.Y. Chen, Z. Zhao, Z. Lin, C. Lee, X. Xu, et al., Solution processable holey
graphene oxide and its derived macrostructures for high-performance
supercapacitors, Nano Lett. 15 (2015) 4605e4610, https://doi.org/10.1021/
acs.nanolett. 5b01212.
[5] Y. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W. Cai, P.J. Ferreira, et al., Carbon-
based supercapacitors produced by activation of graphene, Science 332 (2011)
1537e1541, https://doi.org/10.1126/science. 1200770.
[6] H.-K. Kim, S.-M. Bak, S.W. Lee, M.-S. Kim, B. Park, S.C. Lee, et al., Scalable
fabrication of micron-scale graphene nanomeshes for high-performance
supercapacitor applications, Energy Environ. Sci.
https://doi.org/10.1039/c5ee03580e.
9 (2016) 1270e1281,
€€
ꢀ
[30] A. Laheaar, P. Przygocki, Q. Abbas, F. Beguin, Appropriate methods for eval-
uating the efficiency and capacitive behavior of different types of super-
capacitors, Electrochem. Commun. 60 (2015) 21e25. https://doi.org/10.1016/
j.elecom.2015.07.022.
[7] X.-L. Su, L. Fu, M.-Y. Cheng, J.-H. Yang, X.-X. Guan, X.-C. Zheng, 3D nitrogen-
doped graphene aerogel nanomesh: facile synthesis and electrochemical
properties as the electrode materials for supercapacitors, Appl. Surf. Sci. 426
(2017) 924e932. https://doi.org/10.1016/j.apsusc.2017.07.251.
[31] M.S. Lee, H.-J. Choi, J.-B. Baek, D.W. Chang, Simple solution-based synthesis of
pyridinic-rich nitrogen-doped graphene nanoplatelets for supercapacitors,
Appl. Energy 195 (2017) 1071e1078. https://doi.org/10.1016/j.apenergy.
2016.07.107.
[8] Y. Xu, K. Sheng, C. Li, G. Shi, Self-assembled graphene hydrogel via a one-step
hydrothermal process, ACS Nano
10.1021/nn101187z.
4 (2010) 4324e4330, https://doi.org/
[9] Z.-S. Wu, Y. Sun, Y.-Z. Tan, S. Yang, X. Feng, K. Müllen, Three-dimensional
graphene-based macro-and mesoporous frameworks for high-performance
electrochemical capacitive energy storage, J. Am. Chem. Soc. 134 (2012)
19532e19535. https://doi.org/10.1021/ja308676h.
[32] X. Sun, P. Cheng, H. Wang, H. Xu, L. Dang, Z. Liu, Z. Lei, et al., Activation of
graphene aerogel with phosphoric acid for enhanced electrocapacitive per-
formance, Carbon 92 (2015) 1e10. https://doi.org/10.1016/j.carbon.2015.02.
052.
[10] X. Yu, Y. Kang, H.S. Park, Sulfur and phosphorus co-doping of hierarchically
porous graphene aerogels for enhancing supercapacitor performance, Carbon
101 (2016) 49e56. https://doi.org/10.1016/j.carbon.2016.01.073.
[11] J. Li, G. Zhang, C. Fu, Facile preparation of nitrogen/sulfur co-doped and hi-
erarchical porous graphene hydrogel for high-performance electrochemical
capacitor, J. Power Sources 345 (2017) 146e155. https://doi.org/10.1016/j.
jpowsour.2017.02.011.
[12] C.-M. Chen, Q. Zhang, C.-H. Huang, X.-C. Zhao, B.-S. Zhang, Q.-Q. Kong, et al.,
Macroporous ‘bubble’ graphene film via template-directed ordered-assembly
for high rate supercapacitors, Chem. Commun. 48 (2012) 7149e7151, https://
doi.org/10.1039/c2cc32189k.
[33] J. Yang, J. Shin, M. Park, G.-H. Lee, M. Amedzo-Adore, Y.-M. Kang, The syn-
ergistic effect of nitrogen doping and paraphenylenediamine functionaliza-
tion on the physicochemical properties of reduced graphene oxide for electric
double layer supercapacitors in organic electrolytes, J. Mater. Chem. 5 (2017)
12426e12434, https://doi.org/10.1039/c7ta03194g.
[34] D. Hulicova-Jurcakova, M. Kodama, S. Shiraishi, H. Hatori, Z.H. Zhu, G.Q. Lu,
Nitrogen-enriched nonporous carbon electrodes with extraordinary super-
capacitance, Adv. Funct. Mater. 19 (2009) 1800e1809, https://doi.org/
10.1002/adfm.200801100.
[35] J. Oh, J.H. Lee, J.C. Koo, H.R. Choi, Y.K. Lee, T.S. Kim, et al., Graphene oxide
porous paper from amine-functionalized poly(glycidyl methacrylate)/gra-
phene oxide core-shell microspheres, J. Mater. Chem. 20 (2010) 9200e9204,
https://doi.org/10.1039/C0JM00107D.
[36] K.C. Wasalathilake, D.G.D. Galpaya, G.A. Ayoko, C. Yan, Understanding the
structure-property relationships in hydrothermally reduced graphene oxide
hydrogels, Carbon 137 (2018) 282e290. https://doi.org/10.1016/j.carbon.
2018.05.036.
[13] W. Zhang, C. Xu, C. Ma, G. Li, Y. Wang, K. Zhang, et al., Nitrogen-superdoped
3D graphene networks for high-performance supercapacitors, Adv. Mater. 29
(1e9) (2017) 1701677, https://doi.org/10.1002/adma.201701677.
[14] Y. Gogotsi, P. Simon, True performance metrics in electro-chemical energy
storage,
Science
334
(2011)
917e918,
https://doi.org/10.1126/
science.1213003.
[15] B. Song, J. Zhao, M. Wang, J. Mullavey, Y. Zhu, Z. Geng, et al., Systematic study
on structural and electronic properties of diamine/triamine functionalized
graphene networks for supercapacitor application, Nano Energy 31 (2017)
183e193. https://doi.org/10.1016/j.nanoen.2016.10.057.
[16] Y. Lu, F. Zhang, T. Zhang, K. Leng, L. Zhang, X. Yang, et al., Synthesis and
supercapacitor performance studies of N-doped graphene materials using o-
phenylenediamine as the double-N precursor, Carbon 63 (2013) 508e516.
https://doi.org/10.1016/j.carbon. 2013.07.026.
[17] B. Song, J. Choi, Y. Zhu, Z. Geng, L. Zhang, Z. Lin, et al., Molecular level study of
graphene networks functionalized with phenylenediamine monomers for
supercapacitor electrodes, Chem. Mater. 28 (2016) 9110e9121, https://
doi.org/10.1021/acs.chemmater. 6b04214.
[18] Y. Chang, G. Han, J. Yuan, D. Fu, F. Liu, S. Li, Using hydroxylamine as a reducer
to prepare N-doped graphene hydrogels used in high-performance energy
storage, J. Power Sources 238 (2013) 492e500. https://doi.org/10.1016/j.
jpowsour.2013.04.074.
[37] D. Dinda, A. Gupta, S.K. Saha, Removal of toxic Cr(VI) by UV-active function-
alized graphene oxide for water purification, J. Mater. Chem.
11221e11228, https://doi.org/10.1039/C3TA 12504A.
1 (2013)
[38] R.H. Sestrem, D.C. Ferreira, R. Landers, M.L.A. Temperini, G.M. Nascimento,
Synthesis and spectroscopic characterization of polymer and oligomers of
ortho-phenylenediamine, Eur. Polym. J. 6 (2010) 484e493, https://doi.org/
10.1016/j.eurpolymj.2009.12.007.
[39] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, et
al., Synthesis of graphene-based nanosheets via chemical reduction of exfo-
liated graphite oxide, Carbon 45 (2007) 1558e1565, https://doi.org/10.1016/
j.carbon.2007.02.034.
[40] D.W. Chang, H.-J. Choi, J.-B. Baek, Wet-chemical nitrogen-doping of graphene
nanoplatelets as electrocatalysts for the oxygen reduction reaction, J. Mater.
Chem. 3 (2015) 7659e7665, https://doi.org/10.1039/c4ta07035f.
[41] J. Zhu, T. Feng, X. Du, J. Wang, J. Hu, L. Wei, High performance asymmetric
supercapacitor based on polypyrrole/graphene composite and its derived
nitrogen-doped carbon nanosheets, J. Power Sources 346 (2017) 120e127.
https://doi.org/10.1016/j.jpowsour.2017.02.034.
[19] D. Liu, C. Fu, N. Zhang, H. Zhou, Y. Kuang, Three-dimensional porous nitrogen
doped graphene hydrogel for high energy density supercapacitors, Electro-
chim. Acta 213 (2016) 291e297. https://doi.org/10.1016/j.electacta.2016.07.
131.
[42] A.M. Khattak, Z.A. Ghazi, B. Liang, N.A. Khan, A. Iqbal, L. Li, et al., A redox-active
2D covalent organic framework with pyridine moieties capable of faradaic
[20] Y. Liao, Y. Huang, D. Shu, Y. Zhong, J. Hao, C. He, et al., Three-dimensional
nitrogen-doped graphene hydrogels prepared via hydrothermal synthesis as
high-performance supercapacitor materials, Electrochim. Acta 194 (2016)
136e142. https://doi.org/10.1016/j.electacta. 2016.02.067.
[21] B. Xie, Y. Chen, M. Yu, T. Sun, L. Lu, T. Xie, et al., Hydrothermal synthesis of
layered molybdenum sulfide/N-doped graphene hybrid with enhanced
supercapacitor performance, Carbon 99 (2016) 35e42. https://doi.org/10.
1016/j.carbon.2015.11.077.
[22] J.W. Lee, J.M. Ko, J.D. Kim, Hydrothermal preparation of nitrogen-doped gra-
phene sheets via hexamethylenetetramine for application as supercapacitor
electrodes, Electrochim. Acta 85 (2012) 459e466. https://doi.org/10.1016/j.
electacta.2012.08.070.
[23] A.S. ' liwak, B. Grzyb, N. Díez, G. Gryglewicz, Nitrogen-doped reduced gra-
phene oxide as electrode material for high rate supercapacitors, Appl. Surf. Sci.
399 (2017) 265e271. https://doi.org/10.1016/j.apsusc.2016.12.060.
[24] J.-M. Seo, L.-S. Tan, J.-B. Baek, Defect/edge-selective functionalization of car-
bon materials by “direct” friedelecrafts acylation reaction, Adv. Mater. 29
(1e8) (2017) 1606317, https://doi.org/10.1002/adma.201606317.
energy storage, J. Mater. Chem.
10.1039/c6ta05784e.
[43] J. Wang, H. Liu, H. Sun, W. Hua, H. Wang, X. Liu, et al., One-pot synthesis of
nitrogen-doped ordered mesoporous carbon spheres for high-rate and long-
cycle life supercapacitors, Carbon 127 (2018) 85e92. https://doi.org/10.
1016/j.carbon.2017.10.084.
[44] Y.H. Lee, K.H. Chang, C.C. Hu, Differentiate the pseudocapacitance and dou-
blelayer capacitance contributions for nitrogen-doped reduced graphene ox-
ide in acidic and alkaline electrolytes, J. Power Sources 227 (2013) 300e308.
https://doi.org/10.1016/j.jpowsour. 2012.11.026.
[45] Y. Song, J. Yang, K. Wang, S. Haller, Y. Wang, C. Wang, et al., In-situ synthesis of
graphene/nitrogen-doped ordered mesoporous carbon nanosheet for super-
capacitor application, Carbon 96 (2016) 955e964. https://doi.org/10.1016/j.
carbon.2015.10.060.
4 (2016) 16312e16317, https://doi.org/
[46] J. Zhao, Y. Li, G. Wang, T. Wei, Z. Liu, K. Cheng, et al., Enabling high-volumetric-
energy-density supercapacitors: designing open, low-tortuosity heteroatom-
doped porous carbon-tube bundle electrodes, J. Mater. Chem.
23085e23093, https://doi.org/10.1039/c7ta07010a.
5 (2017)

D. Wang et al. / Carbon 154 (2019) 13e23
1032e1039, https://doi.org/10.1002/adfm.200801057.
23
[47] Q. Xie, G. Chen, R. Bao, Y. Zhang, S. Wu, Polystyrene foam derived nitrogen-
enriched porous carbon/graphene composites with high volumetric capaci-
tances for aqueous supercapacitors, Microporous Mesoporous Mater. 239
(2017) 130e137. https://doi.org/10.1016/j.micromeso.2016. 10.007.
[48] Y. Tan, D. Wu, T. Wang, P. Liu, J. Guo, D. Jia, Facile synthesis of functionalized
graphene hydrogel for high performance supercapacitor with high volumetric
capacitance and ultralong cycling stability, Appl. Surf. Sci. 455 (2018)
683e695. https://doi.org/10.1016/j.apsusc.2018.05.161.
[49] B. Wang, Y. Qin, W. Tan, Y. Tao, Y. Kong, Smartly designed 3D N-doped
mesoporous graphene for high-performance supercapacitor electrodes, Elec-
trochim. Acta 241 (2017) 1e9. https://doi.org/10.1016/j.electacta.2017.04.
120.
[52] G. Lota, T.A. Centeno, E. Frackowiak, F. Stoeckli, Improvement of the structural
and chemical properties of a commercial activated carbon for its application
in electrochemical capacitors, Electrochim. Acta 53 (2008) 2210e2216,
https://doi.org/10.1016/j.electacta.2007.09.028.
[53] Y. An, Y. Yang, Z. Hu, B. Guo, X. Wang, X. Yang, et al., High-performance
symmetric supercapacitors based on carbon nanosheets framework with
graphene hydrogel architecture derived from cellulose acetate, J. Power
Sources 337 (2017) 45e53. https://doi.org/10.1016/j.jpowsour.2016.10.112.
[54] X. Zhao, H. Dong, Y. Xiao, H. Hua, Y. Cai, Y. Liang, et al., Three-dimensional
Nitrogen-doped graphene as binder-free electrode materials for super-
capacitors with high volumetric capacitance and the synergistic effect be-
tween nitrogen configuration and supercapacitive performance, Electrochim.
Acta 218 (2016) 32e40. https://doi.org/10.1016/j.electacta.2016.09.096.
[55] X. Yang, H. Ma, G. Zhang, Nitrogen-doped mesoporous carbons for super-
capacitor electrodes with high specific volumetric capacitance, Langmuir 33
(2017) 3975e3981, https://doi.org/10.1021/acs.langmuir.7b00489.
[50] G. Lian, C.C. Tuan, L. Li, S. Jiao, K.S. Moon, Q. Wang, et al., Ultrafast molecular
stitching of graphene films at the ethanol/water interface for high volumetric
capacitance, Nano Lett. 17 (2017) 1365e1370, https://doi.org/10.1021/
acs.nanolett.6b04035.
~
ꢀ
[51] E. Raymundo-Pinero, M. Cadek, F. Beguin, Tuning carbon materials for
supercapacitors by direct pyrolysis of seaweeds, Adv. Funct. Mater. 19 (2009)