Reduced graphene oxide grafted by the polymer of polybromopyrroles for nanocomposites with superior performance for supercapacitors

Article abstract of DOI:10.1039/c5ta06751k

An integrated structure has been designed by grafting the polymer of polybromopyrroles (PPBP) onto reduced graphene oxide (RGO) to produce RGO/PPBP nanocomposites with superior electrochemical performance for supercapacitors. The RGO/PPBP nanocomposites are featured with a high nitrogen content (>9 at%), enhanced degree of graphitization, improved specific surface area, abundant micropores, and a tunable hierarchical structure on the basis of sample characterization by XRD, Raman, FT-IR, XPS, SEM, high-resolution TEM, BET, and scanning probe microscopy (SPM) techniques. The grafting of PPBP onto RGO not only suppresses agglomeration and restacking of RGO but also tailors the growth of PPBP on RGO, producing a developed hierarchical structure beneficial for mass/charge transfer. The synergistic effect between RGO and PPBP ensures superior electrochemical performance of RGO/PPBP. In a three-electrode mode, the typical RGO/PPBP electrode presents a galvanostatic capacitance (C_g) of 256 F g^-1 at a current density of 10 A g^-1, with a capacitance retention of 99.2% after 10000 cycles in 1 mol L^-1 H₂SO₄. More significantly, the typical RGO/PPBP&3Verbar;RGO/PPBP supercapacitor cell exhibits a high C_cell value of 68 F g^-1 at 5 A g^-1, with a capacitance retention of 91.9% after 10000 cycles. Also, relatively high energy density values of 13.6, 9.4, and 6.7 W h kg^-1 with the corresponding power density of 0.5, 2.5, and 10 kW kg^-1 are achieved, enabling the tested cell to stay at the high level for carbon-based supercapacitors with an aqueous electrolyte.

Full text of DOI:10.1039/c5ta06751k

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ARTICLE
Reduced Graphene Oxide Grafted by Polymer of
Polybromopyrroles to Nanocomposites with Superior
Performance for Supercapacitors
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Shouzhi Wang, Ligang Gai*, Haihui Jiang, Zhenzhen Guo, Nana Bai, and Jianhua Zhou
An integrated structure has been designed by grafting polymer of polybromopyrroles (PPBP) onto reduced graphene oxide
(RGO) to produce RGO/PPBP nanocomposites with superior electrochemical performance for supercapacitors. The
RGO/PPBP nanocomposites are featured with high nitrogen content (> 9 at.%), enhanced degree of graphitization,
improved specific surface area, abundant micropores, and a tunable hierarchical structure on the basis of sample
characterizations with XRD, Raman, FT-IR, XPS, SEM, high-resolution TEM, BET, and scanning probe microscopy (SPM)
techniques. The grafting of PPBP onto RGO not only suppresses agglomeration and restacking of RGO but also tailors the
growth of PPBP on RGO, producing a developed hierarchical structure beneficial for mass/charge transfer. The synergistic
effect between RGO and PPBP ensures superior electrochemical performance of RGO/PPBP. In a three-electrode mode,
the typical RGO/PPBP electrode presents a galvanostatic capacitance (Cg) of 256 F g–1 at a current density of 10 A g–1, with
capacitance retention of 99.2% after 10,000 cycles in 1 mol L‒1 H2SO4. More significantly, the typical RGO/PPBPǁRGO/PPBP
supercapacitor cell exhibits a high Cg value of 272 F g–1 at 5 A g–1, with capacitance retention of 91.9% after 10,000 cycles.
Also, relatively high energy density values of 13.6, 9.4, and 6.7 Wh kg–1 with corresponding power density of 0.5, 2.5, and
10 kW kg–1 are achieved, enabling the tested cell to stay at the high level for carbon-based supercapacitors with aqueous
electrolyte.
leaving behind intersheet pores that are not sufficient for
accessibility to the electrolyte.3ꢀ8
1. Introduction
To enhance the electrochemical performance of grapheneꢀ
Supercapacitors have attracted increasing interest in the
electrochemical energy conversion/storage systems because
they can deliver high power density and extremely long cycle
life and complement or even replace batteries in the energy
storage field, especially when high power delivery or uptake is
needed.1 In recent years, graphene has been considered a
promising candidate as a supercapacitor electrode material due
to its attractive characteristics such as large specific surface
based supercapacitors, many approaches have been developed:
(i) creating large SBET and/or tuning pore size distribution
(PSD) through chemical activation of graphene or exfoliated
graphite oxide;8,9 (ii) introducing spacers to suppress
restacking of graphene sheets;10,11 (iii) designing corrugated or
threeꢀdimensional (3D) graphene;12,13 (iv) doping sp2 carbon
with heteroatoms to modulate the electronic structure and
increase the wettability of graphene;14ꢀ16 and (v) constructing
grapheneꢀbased nanocomposites to maximize the synergistic
effect between the components.1ꢀ3,17,18 For construction of
grapheneꢀ and RGOꢀbased nanocomposites, graphene oxide
(GO) obtained through chemical exfoliation of graphite is
universally employed as a matrix to tailor the growth of
electroactive species that, in turn, hinder restacking of the
sheets.18 This is because the functional oxygenꢀcontaining
groups on GO sheets can serve as nucleation sites to anchor
foreign species.19 Meanwhile, GO is reduced to graphene or
RGO in chemical or thermal conditions, resulting in the
formation of grapheneꢀ or RGOꢀbased nanocomposites.
Moreover, the graphene or RGO matrix not only improves the
area (SBET), good flexibility, excellent electrical conductivity,
good thermal and chemical stability, extended potential
window, and abundant surface functional groups.2,3 However,
the capacitance of supercapacitors based on individual
graphene or reduced graphene oxide (RGO) is usually limited
by the easy agglomeration and restacking of graphene sheets,
a.Institute of Advanced Energy Materials and Chemistry, School of Chemistry and
Pharmaceutical Engineering, Qilu University of Technology, Jinan 250353,
People’s Republic of China.
b.*Corresponding author. E-mail: liganggai@qlu.edu.cn
† Electronic Supplementary Informaꢀon (ESI) available: Synthesis of GO and PBPs,
SEM images of RGO, PPBP, and the electrode film, SPM images of RGO/PPBP-2,
and additional characterization data. See DOI: 10.1039/x0xx00000x
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electrical conductivity of the composites but also buffers the
volume change of electroactive species in electrodes during
cycling.18
H
Br
N
O
Br
COOH
(a)
Br
OH
Br
Br
+
H
PBPs anchored to GO
through hydrogen bonding
Br
Br
N
HN
Br
PBPs
Br
O
OH
GO
O
H
O
O
PPBP
C
O
OHNR
O
PPBP
Br
NH
(b)
Br
O
formation of RGO/PPBP
through covalent bonding
Br
Br
NH
PPBP
O
H
N
H
*
*
accompanying with release
of H2O, CO, CO2 etc.
O
H
*
N
N
Br
Br
*
Br
Br
*
N
N
*
*
Br
*
*
Scheme 1. Illustration of the formation mechanism of RGO/PPBP nanocomposite. For simplicity, the C=C bonds in GO and RGO are omitted.
Recently, we have reported on a new type of polymer with
hierarchical architecture and crystalline CNx ≤ 0.25)
formation of RGO/PPBP. Moreover, PPBP can combine with
RGO through covalent C–N bonding, accompanying with
release of poreꢀmaking agents such as H2O, CO, and CO2 etc.,
during the microwave irradiation and anneal treatment
(Scheme 1b). The anneal treatment at 500 °C is to optimize
the electrochemical performance of the nanocomposites for
supercapacitors, based on the result that PPBP obtained after
anneal treatment at 500 °C for 12 h exhibits the optimum
electrochemical properties.20 As expected, the resulting
RGO/PPBP nanocomposites exhibit enhanced electrochemical
performance, compared with individual RGO,4ꢀ8, graphene,23,24
PPBP,20 and many other carbonꢀbased composites using
graphene or RGO as the matrix in recent reports.25ꢀ32
(
x
domains through deep thermal cyclodebromination of
polybromopyrroles (PBPs).20 This newꢀtype polymer exhibits
high volumetric capacitance and specific surface capacitance,
based on its features with high tap density, a low SBET, a high
degree of graphitization, and a microporous characteristic.20
However, there is
a large space for enhancement in
electrochemical performance of the polymer of PBPs
(hereafter referred to as PPBP), especially in rate capability
and cycling performance. Inspired by the fact that GO can
tailor the growth of electroactive species to make them wellꢀ
dispersed on the nanosheets,1ꢀ3,17ꢀ19 we designed a strategy to
produce RGO/PPBP nanocomposites with enhanced
electrochemical performance for supercapacitors, as illustrated
in Scheme 1. First, GO obtained through modified Hummers
2. Experimental section
method21 is dissolved in
Nꢀmethylꢀ2ꢀpyrrolidone (NMP)
2.1 Preparation of RGO/PPBP Nanocomposites
solution by ultrasonic treatment, followed by cooling the NMP
solution to 0 °C. Then, newlyꢀformed PBP ethanol solution is
mixed with the NMP solution. The mixture is vigorously
stirred for 2 h under Ar atmosphere. During the agitation
process, GO sheets anchor PBP molecules through hydrogen
bonding in the form of O–HꢁꢁꢁN or OꢁꢁꢁH–N (Scheme 1a). This
is based on the facts that PBPs contain amino groups and GO
is wealthy in oxygenꢀcontaining functional groups such as
hydroxyl, carbonyl, carboxyl, and ether linkage.2,16
Subsequently, the mixture is subjected to a microwaveꢀ
assisted solvothermal process, followed by annealing the
resulting precipitate at 500 °C under Ar atmosphere to
produce GRO/PPBP nanocomposites. During microwave
irradiation, PPBP is formed through thermal condensation of
PBPs,20 because halogenated pyrroles are not stable even at
room temperature.22 Meanwhile, GO is reduced to RGO in a
microwaveꢀassisted solvothermal system,23 leading to the
GO was prepared by a modified Hummers method,21 and
PBPs were synthesized through electrophilic bromination of
pyrrole with bromine (Br2) in absolute ethanol.20 Detailed
synthetic procedures with respect to GO and PBPs were
provided in Electronic Supplementary Information (ESI). In a
typical synthetic procedure for preparation of RGO/PPBP
nanocomposites, 40 mg of GO were dissolved into 60 mL of
NMP by ultrasonic treatment for 0.5 h, followed by cooling
the NMP solution in a lowꢀtemperature reaction bath with
temperature kept at 0 °C. Then, quantitative newlyꢀformed
PBP ethanol solution was mixed with the NMP solution. The
mixture was vigorously stirred for 2 h under Ar atmosphere,
followed by a microwaveꢀassisted heat treatment at 180 °C for
0.5 h. The microwave power was set at 200 W. The resulting
precipitate was collected by filtration, dried in a vacuum oven,
and finally annealed at 500 °C for 12 h under Ar atmosphere
2 | J. Name., 2012, 00, 1-3
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to produce RGO/PPBPꢀ
n
samples, where
n
denotes the mass
In a threeꢀelectrode system, a platinum sheet was used as the
counter electrode, Hg/Hg2SO4 electrode as the reference
electrode, and 1 mol L‒1 H2SO4 aqueous solution as the
electrolyte. The working electrode was prepared by bladeꢀ
coating wellꢀblended slurry onto a stainless steel cloth, which
served as the current collector. The slurry consists of 80 wt.%
active material, 10 wt.% SuperꢀP (conducting carbon), and 10
wt.% PVDF in NMP. The asꢀmade electrodes were dried in a
vacuum oven at 80 °C for 12 h, and then subjected to double
rolling to ensure close contact between the active material and
the current collector. The mass of active material upon
stainless steel cloth was 2–3 mg, and the coating area was 1
cm2.
ratio of PPBP to RGO. For example, to obtain RGO/PPBPꢀ1
sample, 2.6 mL of newlyꢀformed PBP ethanol solution was
mixed with the NMP solution containing 40 mg of GO. For
comparison, individual PPBP and RGO were also prepared
under otherwise identical conditions.
2.2 Characterization
Xꢀray diffraction (XRD) patterns were recorded on a Bruker
D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406
Å). Raman spectra were collected at room temperature on a
Renishaw inVia plus laser Raman spectrometer with a 514 nm
Ar+ ion laser. Elemental analysis was performed on an
Elementar Analysensysteme Vario EL III element analyzer.
Scanning electron microscopy (SEM) images and energyꢀ
dispersive spectroscopy (EDS) mapping images were taken on
a Hitachi Sꢀ4800 fieldꢀemission scanning electron microscope.
Transmission electron microscopy (TEM) images were taken
on a Philips Tecnai Twinꢀ20U highꢀresolution transmission
electron microscope, operating at an accelerating voltage of
200 kV. Scanning probe microscopy (SPM) images were
taken on a Multimode 8 Nanoscope V system scanning probe
microscope. Fourier transform infrared (FTꢀIR) spectra were
collected on a Shimadzu IRPrestigeꢀ21 infrared spectrometer
using pressed KBr discs. The FTꢀIR spectra were recorded
with resolution of 4 cm‒1 over the range of 4000–400 cm‒1. Xꢀ
ray photoelectron spectroscopy (XPS) measurements were
performed on a Thermo Fisher Scientific ESCALAB 250 Xꢀ
ray photoelectron spectrometer, using monochromatic Al Kα
radiation (1486.6 eV). To record C1s, N1s, and O1s highꢀ
resolution spectra, pass energy of 50 eV and step size of 0.05
eV were adopted. Binding energies for the highꢀresolution
spectra were calibrated by setting C1s at 284.8 eV.
In a twoꢀelectrode system, two symmetrical electrodes were
soaked with 1 mol L‒1 H2SO4, separated by a sulfonation film
(BH5510, Shenzhen Gebang Co., China), and then assembled
into
a layered structure and sandwiched between two
polytetrafluoroethylene (PTFE) sheets with parafilm. The
active material on each electrode was ca. 2 mg. The thickness
of the electrode film lies in the range of 40–62 ꢂm by SEM,
which is larger than the recommended value of 15 ꢂm for
commercial supercapacitors (Figure S1, ESI).33
CV and GCD curves were collected at ‒0.7–0.55 V vs.
Hg/Hg2SO4 in a threeꢀelectrode mode and at 0‒1 V in a twoꢀ
electrode mode by varying scan rate from 1 to 200 mV s–1 and
current density from 0.1 to 20 A g–1, respectively. Alternating
current (AC) EIS spectra were collected in a frequency range
of 10–2–105 Hz at the open circuit voltage with AC amplitude
of 5 mV.
For threeꢀelectrode cells, gravimetric capacitance (Cg) derived
from galvanostatic discharge curves was calculated from eq 1,
represented as4
Nitrogen adsorption/desorption isotherms were collected at
Iꢀt
mꢀV
(1)
Cg =
77.1
K using an America Micromeritics ASAP 2020
sorptometer. Before recording the nitrogen sorption isotherms,
the samples were degassed at 250 °C for 6 h. The Brunauer–
Emmett–Teller (BET) method was utilized to calculate the
specific surface areas (SBET). The PSD was determined
according to BJH desorption dV/dlog(r) pore volume plot and
HorvathꢀKawazoe differential pore volume plot, respectively.
The conductivity of the samples were measured on a four
probe resistivity tester (Guangzhou Four Probe Tech., China).
Sample powders were separately mixed with polyvinylidene
fluoride (PVDF) in a mass ratio of 9:1 and then subjected to a
pressure of 20 MPa to produce discs for measurement because
where Cg is the gravimetric capacitance (farad per gram),
constant discharge current (ampere), ꢃ the discharge time
(second), ꢃ the voltage window (volts), and the mass of
active material on the working electrode (gram).
In a twoꢀelectrode system, the capacitance for a single
I the
t
V
m
electrode
(Csingle) was calculated according to eq 2,
represented as12,16
4Iꢀt
mꢀV
(2)
Csingle
=
individual RGO/PPBPꢀ
unsuitable for measurement.
n samples produced fragile discs
where
m is the total mass of active material on the two
electrodes (gram). The capacitance for the measured
supercapacitor cell
represented as20
(Ccell) was calculated from eq 3,
2.3 Fabrication of Supercapacitors and Electrochemical Tests
The electrochemical properties of the samples were
characterized at room temperature with cyclic voltammetry
Iꢀt
mꢀV
(3)
Ccell
=
(CV),
galvanostatic
charge/discharge
(GCD),
and
electrochemical impedance spectroscopy (EIS) techniques,
using a CHI 660E electrochemical workstation (Shanghai CH
Instruments Co., China).
where
m
is the total mass of active material on the two
electrodes (gram).
Cg derived from cyclic voltammograms was calculated from
eq 4, represented as24
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Figure 1. (a) XRD spectra and (b) Raman spectra of the samples.
and the other is attributed to turbostratic interlayer stacking of
PPBP which is dominant in RGO/PPBPꢀ3.20 Also, a weak and
broad peak characteristic of the (100) plane of graphitic
carbon can be discerned at 43°. This result implies that the
degree of graphitization for the composite samples can be
improved by integrating PPBP with RGO, compared with
individual RGO.
Vb
1
(4)
Cg =
IdV
∫
Va
mv(Vb −Va )
where
m
is the mass of active material on the working
the scan rate (volts per second), the
electrode (gram),
v
I
discharge current (ampere), and Vb and Va are the high and
low voltage limit of the CV curves (volts).
Figure 1b shows the Raman spectra of the samples. For each
spectrum, there are two broad peaks centered around 1347 and
1583 cm–1, which can be fitted into four or five bands through
Gaussian/Lorentzian functions to provide more structural
information (Table S1, ESI).20,34 Taking RGO/PPBPꢀ2
spectrum as an example, the two peaks can be fitted by
Lorentzian function into four bands at 1243, 1344, 1473, and
1584 cm–1, corresponding separately to disordered graphitic
lattice (D4, 1243 cm–1, 36.4% in total peak areas; D1, 1344
cm–1, 22.0%), amorphous carbon (D3, 1473 cm–1, 20.7%), and
ordered graphitic lattice (G + D2, 1584 cm–1, 20.9%). Usually,
The energy density (E) and power density (P) of the
supercapacitor cell were estimated according to eqs 5 and 6,
respectively, represented as26
CcellꢀV 2
(5)
E =
2
E
(6)
P =
ꢀt
where ꢃV is the cellꢀoperation voltage window (volts) and ꢃt
the discharge time (second).
the intensity ratio of ID
degree of graphitization in partially graphitic carbons, and the
ID
IG value decreases as the sp2 domain size increases.35 In the
present case, we use ID1 I(G+D2) to reflect the degree of
graphitization of the samples (Table 1). It is found that the
ID1 I(G+D2) value for each composite sample is lower than that
/IG is considered a measure of the
/
3. Results and discussion
/
3.1 Structure, Composition, and Morphology
/
Figure 1a shows the XRD spectra of the samples. The GO
spectrum exhibits a strong peak at 11.2°, corresponding to the
(002) plane with d
ꢀspacing of ca. 0.79 nm for GO.5,7,12,24 It
should be pointed out that the interlayer distance of GO is
solvationꢀdependent because oxygenꢀcontaining solvent can
function as an intercalation species due to the strong
interaction between solvent molecules and oxygenꢀcontaining
functional groups of GO.12,24 After thermal reduction, the peak
characteristic of GO disappears while a broad band peaked at
for individual RGO, and slightly decreases as the PPBP
content increases. This result is indicative of a relatively
higher degree of graphitization for the composite samples
compared with that for individual RGO, in accordance with
the XRD result. A relatively higher degree of graphitization
with respect to RGO/PPBP is beneficial for improvement in
electronic conductivity, which plays a crucial role in enhanced
rate capability However, the conductivity of the samples
decreases as the mass ratio of PPBP/RGO increases (Table 1).
This is due to the bad contact between PPBP granules, in
which RGO is coated.
25.6° occurs corresponding to the (002) plane with
of ca. 0.35 nm for RGO (Figure 1).5 The decrease in
d
d
ꢀspacing
ꢀspacing
for RGO is due to the removal of solvent and oxygenꢀ
containing functional groups.24
For the composite samples, the broad peak characteristic of
the (002) plane of graphitic carbon is centered at 24–25.6°.
The surface physics and chemistry of the samples were
examined by the FTꢀIR and XPS techniques. Figure 2a shows
the FTꢀIR spectra of the samples. The peaks in GO spectrum
correspond separately to the
v
(O–H) stretching vibration
(C=O) stretching vibration (1710 cm–1),
(O–H)
(C=C) stretching
The left shift in position of the broad peak with increasing dꢀ
spacing arises from two reasons. One reason is that the
grafting of PPBP onto RGO suppresses restacking of RGO,
(3427 cm–1),
bending and
v
v
δ
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Table 1. Physicochemical properties of the samples.
sample
N content (at.%)
ID1/I(G+D2)
conductivity
BET analysis
(S m–1
)
CHN
XPS
SBET
Smicro/SBET
–
RGO/PPBP-1
RGO/PPBP-2
RGO/PPBP-3
RGO
43.3
10.8
4.2
378.4
390.6
259.6
173.1
5.57
7.63
8.80
–
9.06
9.66
11.66
–
0.798
0.797
0.783
0.866
44.6%
40.1%
–
357.1
Figure 2. (a) FT-IR spectra; (b) XPS survey spectra; (c) C 1s for GRO/PPBP-2; (d) N 1s for GRO/PPBP-2; (e) O 1s for GRO/PPBP-2; (f) Illustration of different N and O components in
PPBP.
vibrations (1632 cm–1),
cm–1), and
(C–O)/ (C–O–C) stretching vibrations (1219,
1091, 1054, 980, and 830 cm–1),7,25,29 indicating a wealth of
oxygenꢀcontaining functional groups upon the GO sheets.
However, there are mainly two broad bands centered at 1567
and 1183 cm–1 in RGO spectrum, corresponding separately to
the v(C=C) and v(C–OH)/v
(C–O–C) stretching vibrations.25
This result reveals that oxygenꢀcontaining functional groups
still remains upon the carbon network after thermal reduction
of GO at 500 °C for 12 h under Ar atmosphere, producing a
γ
(O–H) deforming vibration (1398
as the mass ratio of PPBP/RGO increases, in accordance with
the CHN elemental analysis (Table 1). For Nꢀ and/or Oꢀ
containing carbonꢀbased materials, the electrochemical
performance is highly dependent on the component ratio of
heteroatoms.20,36 In the present case, the components of C, N,
and O are resolved by deconvolution of the highꢀresolution
C1s, N1s, and O1s spectra (Figure 2c–e), and listed in Table
S2 (ESI). Taking RGO/PPBPꢀ2 as an example, the peaks at
284.5, 285.1, 286.7, and 290.9 eV correspond separately to
sp2ꢀhybridized C=C (12.8%, in total area), sp3ꢀhybridized C–C
(43.3%), C–O/C–N (32.0%), and O–C=O (11.9%) component
(Figure 2c, green lines).16,24,36 The low percentage of C=C
(12.8%) is indicative of a highly functionalized surface. The
fitted N1s spectrum exhibits four components at 398.4, 400.4,
401.9, and 403.9 eV (Figure 2d, red lines), corresponding
separately to pyridinic N (Nꢀ6, 22.4%), pyrrolic/pyridone N
(Nꢀ5, 59.5%), quaternary N (NꢀQ, 4.4%), and Nꢀoxides (NꢀX,
13.7%).16,20,37 It is well accepted that Nꢀ5 and Nꢀ6 have a high
binding ability with electrolyte cations and NꢀQ improves the
electronic conductivity of the material.37,38 Therefore, high
capacitive and rate capabilities are expected with respected to
the Nꢀcontaining composite samples. For the O1s spectrum,
three components resolved by Gaussian fitting are centered at
v
v
RGO sample. The red shift in peak position of v(C=C) for
RGO compared with that for GO is due to the removal of
oxygenꢀcontaining groups linked with sp2ꢀhybridized carbon.
The RGO/PPBP spectra are similar to but not identical with
the RGO spectrum, lying in that the band peaked at 1183 cm–1
for RGO blueꢀshifts to 1206 cm–1 for RGO/PPBPꢀ3 (Figure 2a,
inset). This is due to the existence of C=N and C–N bonds in
the composite samples. Therefore, the two broad bands peaked
at 1576 and 1183–1206 cm–1 are separately assigned to
v(C=C)/v(C=N) and v(C–O)/v(C–N) stretching vibrations.
The existence of elemental N and O in the composite samples
is confirmed by the XPS survey spectra (Figure 2b). The
atomic percentage of N derived from the XPS data increases
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Figure 3. SEM images of the samples: (a,d) RGO/PPBP-1; (b,e) RGO/PPBP-2; (c,f) RGO/PPBP-3. d, e, and f are the high-magnification images corresponding separately to a, b, and c.
Figure 4. TEM images of RGO/PPBP-2: (a) low magnification; (b and inset in a) high-resolution images corresponding to the squared area in a.
530.2, 532.3, and 536.6 eV (Figure 2e, magenta lines),
corresponding to C=O/C–N=O (OꢀI, 30.6%), C–OH/C–O–C
(OꢀII, 62.2%), and chemisorbed oxygen and/or water (OꢀIII,
7.2%), respectively.37 The relatively higher percentage of C–
O/C–N species revealed by the C1s and O1s spectra is
consistent with the FTꢀIR result (Figure 2a). As demonstrated
in literature,36 OꢀI species contributes positively to reversible
pseudocapacitance while OꢀII and OꢀIII species offer
irreversible pseudocapacitance for the oxygenꢀcontaining
materials. Considering that RGO is coated by PPBP, the
detected N and O in the XPS spectra mainly arise from the
surface of PPBP. The components of N and O species in PPBP
are illustrated in Figure 2f.
1 (Figure 3a,d), the sample well retains the sheet morphology
of RGO. It is worth noting that the both sides of an exfoliated
RGO nanosheet are coated by a thin film of PPBP. This is
confirmed by the SPM images of RGO/PPBPꢀ2 (Figure S3,
ESI) and the Nꢀmapping image of RGO/PPBPꢀ1 with
homogeneous nitrogen distribution (Figure 2d, inset). As the
mass ratio of PPBP/RGO increases to 2, the PPBP coating
grows into a 2D network structure (Figure 3b,e) with thickness
ranging from 1 to 6 nm (Figure S3, ESI). Further increase in
the mass ratio of PPBP/RGO to 3 results in a featureless PPBP
coating constituted by aggregated nanoparticles (Figure 3c,f),
a situation that is similar to the growth of individual PPBP
during thermal treatment (Figure S2c,d).
Figure 3 shows the SEM images of the samples. Compared
with those of RGO (Figure S2a,b), the images of the
composite samples exhibit rougher surfaces. For RGO/PPBPꢀ
The network structure of RGO/PPBPꢀ2 is also confirmed by
the TEM images (Figure 4). The nanosheet with corrugated
structure can be clearly observed in Figure 4a. Further
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Scheme 2. Proposed formation mechanism of RGO/PPBP nanosheet with network structure: (a) PBPs combine with GO (yellow) through hydrogen bonding; (b) PPBP (dark green)
grows on the both sides of RGO (gray); (c) formation of network-structured RGO/PPBP.
Figure 5. (a) Nitrogen sorption isotherms of the samples; (b) PSD plots of the samples.
examination shows the RGO nanosheet is coated by a 2D
networkꢀstructured PPBP film, which can be discerned by the
discrepancy in contrast of the image (Figure 4a, inset). Highꢀ
resolution image reveals a partially graphitic PPBP film upon
RGO (Figure 4b), in concert with the Raman result (Figure
1b).
resulting products in NMP are determined to be pyrrolidone (
w 85), 1ꢀmethylꢀ1Hꢀpyrrolꢀ2(5H)ꢀone ( w 97), and NMS
w 113) (Table S3, ESI). This result indicates that the
reducibility of NMP arises from both the oxygenꢀscavenging
property and the produced H2 during the formation of
1,
M
(3, M
2, M
compound 2. On the other hand, as a byꢀproduct released from
both electrophilic substitution of pyrrole with Br2 and
polymerization of PBPs, HBr can function as a reducing agent
to reduce GO into RGO.42 With increasing the volume of PBP
ethanol solution in reaction medium, the PPBP coating grows
into a network structure (Scheme 2c and Figure 3b). This is
attributed to the corrugated RGO matrix and the anisotropic
growth behavior of the polymer.20 In the case of a large
amount of PBPs in reaction medium, except a portion of PPBP
grows on the surface of the matrix to form RGO/PPBP
nanosheet, considerable portion of PPBP individually
nucleates in solution. These individually nucleated PPBP
nanoparticles agglomerate and adhere to the surface of the
newlyꢀformed RGO/PPBP nanosheet through van der Waals
interactions, leading to the formation of RGO/PPBP
composite with featureless characteristic (Figure 3c).
3.2 Formation Mechanism
In fact, PPBPꢀcoated RGO is formed during the microwaveꢀ
assisted solvothermal process. As mentioned before, PBPs
with amino groups combine with GO wealthy in oxygenꢀ
containing groups through hydrogen bonding during the
mixing process (Schemes 1a and 2a). Under microwave
irradiation, GO is reduced into RGO and PBPs polymerize
into PPBP film on the both sides of RGO through selfꢀ
propagation condensation (Scheme 2b and Figure 3d),39
accompanying with release of Br2 and HBr. The combination
of RGO with PPBP is probably in the form of C–N bonding
by releasing H2O, CO, and CO2 (Scheme 1b). The formation
of RGO is due to the reducibility of NMP under microwave
irradiation. As reported in literature,40 NMP can be easily
oxidized into
Nꢀmethylsuccinimide (NMS) under thermal
3.3 BET Analysis
treatment in air. Therefore, NMP has been utilized as both
solvent and an oxygenꢀscavenger for GO to produce exfoliated
RGO at the boiling temperature.41 To verify the role of NMP
in the microwaveꢀassisted solvothermal reduction process,
individual NMP was subjected to microwave irradiation (200
W) at 180 °C for 30 min. It is found that the uncolored solvent
slowly turns brown during irradiation. Using the gas
chromatography/mass spectrometry (GC/MS) technique, the
The SBET and PSD of the samples have also been evaluated
because they are closely related to the electrochemical
performance of electrode materials. Detailed data from BET
analysis are provided in Table S4 (ESI). Compared with that
for RGO/PPBPꢀ1, the nitrogen sorption isotherms for
RGO/PPBPꢀ2 and RGO/PPBPꢀ3 (Figure 5a) exhibit an
apparent microporous characteristic reflected by the typeꢀI
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Figure 6. Electrochemical performance of the samples tested in a three-electrode mode: (a) Galvanostatic charge/discharge curves at 1 A g–1; (b) cyclic voltammograms at 50 mV s–
1; (c) plots of Cg as a function of current density; (d) cycling performance at 10 A g–1; inset is the GCD curves at the initial stage and after 10,000 cycles.
sorption curves with abrupt adsorption plateau at the low
relativeꢀpressure region (P/Po ˂ 0.4).43 Also, the observed
hysteresis loops at high relativeꢀpressure region (P/Po > 0.4)
reveal the coexistence of mesoꢀ and macropores.12,27 The
micropores are mainly caused by the small molecules (e.g.
HBr, Br2, H2O, CO, CO2, NH4Br, and C2HxN) released during
the microwave irradiation and thermal treatment.20 The
mesopores are attributed to stacking of both RGO/PPBP
nanosheets and PPBP nanoparticles. Both microꢀ and
mesopores can function as electrolyte container allowing
quick ion motion during charge/discharge processes to
improve rate performance of the electrode materials.44
Compared with that for individual RGO, the SBET for
RGO/PPBP is greatly improved (Table 1). This result
indicates that combination of PPBP with RGO can prevent
restacking of RGO sheets, consolidating the suggestion that
PPBP grows on the both sides of RGO (Scheme 2b).
Compared with that for RGO/PPBPꢀ1 and RGO/PPBPꢀ2, the
SBET for RGO/PPBPꢀ3 is decreased, due to the agglomeration
of PPBP granules.
RGO/PPBPꢀ1, RGO/PPBPꢀ2, and RGO/PPBPꢀ3, respectively.
As demonstrated in literature,45 electrode materials with
relatively high SBET and abundant subnanopores exhibit high
electrochemical double layer capacitance (EDLC) according
to equation represented as
εrε0 A
(7)
d
C =
where εr is the electrolyte dielectric constant, ε0 is the
dielectric constant of the vacuum,
area, and the charge separation distance.
On the basis of the above analysis, we conclude that
RGO/PPBP nanocomposites can be obtained through
A is the electrode surface
d
a
microwaveꢀassisted solvothermal approach followed by
anneal treatment. The composites are featured with high
nitrogen content, enhanced degree of graphitization, improved
SBET and abundant micropores, and a tunable hierarchical
structure. These features ensure superior electrochemical
performance of the samples when tested in the role of
electrode materials for supercapacitors.
Figure 5b shows the PSD plot of RGO/PPBP samples, where
micropores and mesopores coexist and micropores with pore
width centered at 1.74–1.98 nm are abundant. The
microporous characteristic of the samples is further reflected
by the HorvathꢀKawazoe differential pore volume plot (Figure
5b, inset), where subnanopores with pore width centered at
0.53, 0.59, and 0.57 nm dominate the subnanopores in
3.4 Electrochemical Properties
Figure 6a shows the typical GCD curves of the samples in
threeꢀelectrode cells at a current density of 1 A g–1. It is found
that RGO/PPBP electrodes exhibit longer discharge times than
individual PPBP and RGO. This result indicates superior
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electrochemical performance of the composite samples in
accordance with the result of CV measurements (Figure 6b).
The capacitive performance of the samples was also examined
as a function of the current density (Figure 6c). Although
individual PPBP exhibits a comparable Cg value (543 F g–1) to
that of RGO/PPBP (438, 550, 507 F g–1) at 0.1 A g–1, its rate
capability is inferior to that for the composite samples. For
example, the Cg values for RGO/PPBPꢀ1, RGO/PPBPꢀ2,
RGO/PPBPꢀ3, and PPBP are 288, 375, 337, and 277 F g–1,
respectively, as the current density increases to 1 A g–1. In the
case of individual RGO, it exhibits good rate capability but
bad capacitive performance. As a result, the RGO/PPBP
nanocomposites exhibit enhanced capacitive and rate
performance compared with individual PPBP and RGO. This
indicates a synergistic effect between PPBP and RGO, where
PPBP anchoring on RGO suppresses agglomeration and
restacking of RGO, thereby resulting in an increased available
surface area for RGO and, hence, enhanced EDLC. On the
other hand, GO serves as a support to anchor PBPs so that
PPBP nucleates and grows with good dispersion and tunable
structure on the both sides of RGO, maximizing the EDLC
and pseudocapacitance contributed from PPBP. In other words,
the integrated RGO/PPBP structure with developed electron
conductive network and shortened ion transport paths is
responsible for the enhanced capacitance. We infer that the
integrated RGO/PPBP structure is through the covalent C–N
bonding between the two components. To confirm this
inference, individual RGO and PPBP were mechanically
blended to form a RGO–PPBP mixture with mass ratio of
PPBP/RGO equaling 2, hereafter referred to as RGO–PPBPꢀ
PPBPꢀ2m exhibits much inferior performance to RGO/PPBP,
counterꢀevidencing a strong combination rather a simple
blending between the two components.
The inflections at 0–0.1 V in discharge curves (Figure 6a) are
attributed to the pseudocapacitive contribution from
heteroatom functional groups,46 corresponding well to the
Faradaic humps in the anodic sweeps of CV with quasiꢀ
rectangular shape (Figure 6b). Among the RGO/PPBP
electrodes, RGO/PPBPꢀ2 exhibits the highest electrochemical
(Figure 6c). In view of the similar degree of graphitization
(Table 1) and nitrogen component distribution (Table S2, ESI)
among RGO/PPBP samples, the superior electrochemical
performance of RGO/PPBPꢀ2 is attributed to its corrugated 2D
network structure and relatively higher SBET and Smicro/SBET.
The cycling stability of the electrodes was tested at 10 A g–1 in
a threeꢀelectrode mode. After 10,000 cycles, the RGO/PPBP
electrodes exhibit high Cg and capacitance retention (Table 2),
especially for RGO/PPBPꢀ2 (Figure 6d) whose GCD curves
after 10,000 cycles resemble those of the initial cycles (Figure
6d, inset). Also, the Coulombic efficiency is 100% after the
first cycle. This result indicates excellent capacitive
reversibility, rate capability, and cycling performance of
RGO/PPBPꢀ2.
To evaluate possible practical application in supercapacitors,
the
electrochemical
performance
of
RGO/PPBPꢀ
2ǁRGO/PPBPꢀ2 supercapacitor cell was further tested, using 1
mol L–1 H2SO4 as the electrolyte. The discharge curves are
almost symmetric with the charge curves at the current density
range of 0.1–5 A g–1 (Figure 7a) , indicating a high capacitive
reversibility.12 The quasiꢀrectangular shape of the CV (Figure
7b) at the scan rate range of 1‒100 mV s–1 reflects
2
m. The electrochemical performance of RGO–PPBPꢀ2
m was
also examined as shown in Figure 6c. It is apparent that RGO–
Table 2. Comparison in the electrochemical performance of the samples with other advanced electrode materials in recent reports.
sample
initial Cg
current density/
cycle number
electrolyte
Cg retention
(%)
E (Wh kg–1)/
refs.
(F g–1
)
P (kW kg–1
)
RGO/PPBPꢀ1a
RGO/PPBPꢀ2a
232
256
10 A g–1/10,000
10 A g–1/10,000
1 M H2SO4
1 M H2SO4
99.6
99.2
–
–
this work
this work
RGO/PPBPꢀ2b
RGO/PPBPꢀ3a
272
246
216
40
5 A g–1/10,000
10 A g–1/10,000
10 A g–1/10,000
10 A g–1/10,000
0.5 A g–1/10,000
2.5 A g–1/3000
0.5 A g–1/5000
1 A g–1/3000
1 M H2SO4
1 M H2SO4
1 M H2SO4
1 M H2SO4
6 M NaOH
1 M H2SO4
6 M NaOH
6 M NaOH
1 M H2SO4
1 M H2SO4
1 M H2SO4
1 M NaOH
91.9
91.9
88.9
120
97
9.4/2.5
this work
–
this work
PPBPa
–
this work
RGOa
–
this work
RGOb
188*
238.8
197
–
7
carbon/grapheneb
nitrogenꢀdoped graphenea
Nꢀcarbon nanorods/graphenea
graphene/PPy areogela
NH2ꢀRGO/PANIb
sulfonated graphene/PPyb
Graphene/carbon nanohornb
94
–
25
26
27
28
29
30
31
98
–
201
98.7
93
–
138
10 A g–1/2000
2 A g–1/1000
–
345
85
9.6/0.8*
360
1 A g–1/500
90
–
–
166
10 A g–1/1000
99
a. Cg measured in a three-electrode mode; b. Cg measured in a two-electrode mode.* Values estimated from the information given in references.
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Figure 7. Electrochemical performance of RGO/PPBP-2ǁRGO/PPBP-2 supercapacitor cell: (a) Galvanostatic charge/discharge curves; (b) cyclic voltammograms; (c) plot of Csingle as a
function of the current density; (d) cycling performance at 5 A g–1; inset is the GCD curves at the initial stage and after 10,000 cycles; (e) Nyquist plots and equivalent circuit; (f)
Ragone plots (1. N-RGO/PANI,48 2. RGO/PANI,32 3. RGO/PPy,32 4.MnO2 nanowire/Graphene,49 5. 3D N,B-codoped graphene.50
)
a typical capacitive behavior due to the simultaneous EDLC
and pseudocapacitance.25 Moreover, the CV retains the quasiꢀ
rectangular shape as the scan rate increases to 200 mV s–1,
indicating a good highꢀrate capability of the cell. The plot of
capacitance versus current density for a single electrode
represents EDLC and pseudocapacitance, respectively. The
fitted impedance data match well with the experimental data
(Figure S4 and Table S5, ESI).
The energy density and powder density of the cell are
calculated according to eqs 5 and 6. The Ragone plot of the
cell (Figure 7f) exhibits energy density values of 17.4, 16.5,
15.2, 13.6, 11.1, 9.4, 7.8 and 6.7 Wh kg–1 with corresponding
power density of 0.05, 0.1, 0.25, 0.5, 1, 2.5, 5, and 10 kW kg–1,
indicating that relatively high energy density can be achieved
without compromising the power density. Compared with the
advanced electrode materials in recent reports (Table 2 and
Figure 7f), our RGO/PPBP samples exhibit superior capacitive,
rate, and/or cycling performance to RGO or graphene,4ꢀ8,23,24
(
Csingle) exhibits Csingle values of 500.4, 474.8, 438.8, 392, 320,
272, 226, and 192 F g–1 at 0.1, 0.2, 0.5, 1, 2, 5, 10, and 20 A g–
1, respectively (Figure 7c). Compared with the threeꢀelectrode
system, the decrease in Csingle value measured by the twoꢀ
electrode mode is attributed to the mass/charge transfer
resistance caused by the separation film. The cycling stability
of the cell was tested at 5 A g–1, presenting a nearly 100%
Coulombic efficiency and a capacitance retention of 91.9%
after 10,000 cycles (Figure 7d). The high cycling stability of
the cell can be reflected by GCD curves after 10,000 cycles,
which are very similar to those of the initial cycles (Figure 7d,
inset). Moreover, the cycling stability is confirmed by the
Nyquist plots (Figure 7e), where the ohmic resistance of the
electrodes (Rs), derived from the electrolyte and the contact
between electrode and current collector, increases by only
0.12 and 0.14 ꢄ after 5,000th and 10,000th cycle, respectively.
Also, the chargeꢀtransfer resistance (Rct) and the Warburgꢀtype
PPBP,20
nitrogenꢀdoped
graphene,26
spineꢀlike
carbon/graphene,25 Nꢀdoped carbon nanorods/grahene,27
graphene/polypyrrole (PPy) nanotubes aerogel,28 NH2ꢀ
RGO/polyaniline (PANI),29,48 sulfonated graphene/PPy,30
nanoporous graphene/carbon nanohorn,31 RGO/conducting
polymers,32 MnO2 nanowire/graphene,49 and N,Bꢀcodoped
graphene.50 This result enables the tested cell to stay at the
high level for carbonꢀbased supercapacitors with aqueous
electrolyte (Figure 7f).
resistance (W) exhibit nearly no change after 10,000 cycles.
This result indicates the charge transportation has not been
blocked during the cycling process, evidencing high structural
stability of RGO/PPBPꢀ2 whose structural deformation arising
from PPBP component is alleviated by RGO due to the
synergistic effect between the two components. Using
Zsimpwin software, the impedance data are analyzed by
4. Conclusion
In summary, RGO/PPBP nanocomposites have been prepared
by grafting PPBP onto RGO through a microwaveꢀassisted
solvothermal approach followed by anneal treatment. Also,
the samples have been tested in the role of electrode materials
for supercapacitors. The scientific significance of this research
lies in: (1) grafting PPBP onto RGO not only suppresses
fitting to an equivalent circuit consisting of Rs, Rct, W, Cdl, and
Cps (Figure 7e, inset),47 where Cdl and Cps in the circuit
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14 D. Y. Yeom, W. Jeon, N. D. Kha Tu, S. Y. Yeo, S. S. Lee, B.
J. Sung, H. Chang, J. A. Lim and H. Kim, Sci. Rep., 2015,
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agglomeration and restacking of RGO but also tailor the
growth of PPBP on RGO, producing a developed 2D networkꢀ
structure beneficial for mass/charge transfer; (2) growth of
PPBP on RGO offers an enhanced degree of graphitization for
RGO/PPBP compared with that for individual RGO; (3) small
molecules (e.g. Br2, HBr, NH4Br, H2O, CO, CO2) released
during the polymerization of PBPs and formation of
RGO/PPBP serve as poreꢀmaking agents, allowing
RGO/PPBP to have abundant micropores coexisting with
mesopores; (4) integrated RGO/PPBP structure can be tuned
by varying the mass ratio of PPBP/RGO; (5) strong
combination between RGO and PPBP through microwave
irradiation in combination with anneal treatment ensures a
synergistic effect between RGO and PPBP; and (6) superior
capacitive, rate, and cycling performance as well as relatively
wide potential window with respect to RGO/PPBPꢀ2 result in
high energy density/power density couples for symmetric
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Acknowledgments
This research was financially supported by National Natural
Science Foundation of China under Grant No. 51272143 and
51472278.
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Journal of Materials Chemistry A
Page 12 of 12
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DOI: 10.1039/C5TA06751K
ARTICLE
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Graphical Abstract
An integrated structure has been designed by grafting polymer
of polybromopyrroles (PPBP) onto reduced graphene oxide
(RGO) to produce RGO/PPBP nanocomposites superior
electrochemical performance for supercapacitors.
12 | J. Name., 2012, 00, 1-3
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