Self-assembling porous network nanostructure 7-aminoindole decorated reduced graphene oxide for high-performance asymmetric supercapacitor

Article abstract of DOI:10.1016/j.jallcom.2020.155412

The design and preparation of novel green and efficient energy storage electrode materials is a crucial way to solve the problem of intermittent energy storage of renewable energy. In this paper, the composite electrode materials of rGO decorated by organic 7-aminoindole (7-Ai) molecule with the characteristics of fast reversible redox kinetics are synthesized successfully. The modified 7-AirGOs composites can not only produce extra pseudocapacitance, which greatly improve the specific capacitance, but also form a stable mesoporous nanostructure to facilitate the diffusion and transport of ions. By investigating the influence of 7-Ai molecule content on the electrochemical performance of the composite electrodes, the 7-AirGO1 electrode is chosen as the optimum electrode, which exhibits a high specific capacitance of 425.73 F g^?1 at 0.5 A g^?1 and an excellent rate capability with the current density increased to 20 A g^?1. Furthermore, the asymmetric supercapacitor assembled by the 7-AirGO1 and AC as positive and negative electrode respectively delivers an energy density of 14.60 W h kg^?1 and a power density up to 10,500 W kg^?1, as well as a cycling stability with a capacitance retention of 97.98% at a current density of 4 A g^?1 for over 20,000 cycles. These results indicate that rGO-aminoindole composites are of great potential in flexible and wearable micro energy storage devices.

Full text of DOI:10.1016/j.jallcom.2020.155412

Journal of Alloys and Compounds 835 (2020) 155412
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Self-assembling porous network nanostructure 7-aminoindole
decorated reduced graphene oxide for high-performance asymmetric
supercapacitor
,
Weiyang Zhang a, Chunyan Sun a, Yonggui Xu b, Hongwei Kang a *, Baocheng Yang a,
,
**
Huili Liu a, Zhikun Li a
a Henan Key Laboratory of Nanocomposite and Application, Zhengzhou City Key Laboratory of Supercapacitor, Institute of Nanostructured Functional
Materials, Huanghe Science and Technology College, Zhengzhou, 450006, China
b Henan Science and Technology Exchange Center with Foreign Countries, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The design and preparation of novel green and efficient energy storage electrode materials is a crucial
way to solve the problem of intermittent energy storage of renewable energy. In this paper, the com-
posite electrode materials of rGO decorated by organic 7-aminoindole (7-Ai) molecule with the char-
acteristics of fast reversible redox kinetics are synthesized successfully. The modified 7-AirGOs
composites can not only produce extra pseudocapacitance, which greatly improve the specific capaci-
tance, but also form a stable mesoporous nanostructure to facilitate the diffusion and transport of ions.
By investigating the influence of 7-Ai molecule content on the electrochemical performance of the
composite electrodes, the 7-AirGO1 electrode is chosen as the optimum electrode, which exhibits a high
specific capacitance of 425.73 F gꢀ1 at 0.5 A gꢀ1 and an excellent rate capability with the current density
increased to 20 A gꢀ1. Furthermore, the asymmetric supercapacitor assembled by the 7-AirGO1 and AC as
positive and negative electrode respectively delivers an energy density of 14.60 W h kgꢀ1 and a power
density up to 10,500 W kgꢀ1, as well as a cycling stability with a capacitance retention of 97.98% at a
current density of 4 A gꢀ1 for over 20,000 cycles. These results indicate that rGO-aminoindole composites
are of great potential in flexible and wearable micro energy storage devices.
Received 19 February 2020
Received in revised form
24 April 2020
Accepted 27 April 2020
Available online 1 May 2020
Keywords:
7-AirGO composites
Mesoporous nanostructure
Synergetic enhancement effect
Energy storage
Asymmetric supercapacitors
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
Among of them, introduction of pseudocapacitive materials into
carbon materials is currently one of the best ways to address this
In recent years, as high-power, long-life, green and sustainable
energy storage device, supercapacitors have attracted much
attention, and are promising candidates for the application of en-
ergy storage field [1e4]. In order to optimize these excellent char-
acteristics of supercapacitors and further improve the insufficiency
of its energy density, and obtain the state-of-the-art supercapacitor
devices, the scholars and engineers in this field have studied the
energy storage properties of various potential materials such as
carbon-based materials, metal oxides/hydroxides, nitrides and
organic polymers by using various methods and techniques [5e9].
low energy density problem for the application of the state-of-the-
art supercapacitors [10e12].
Among of all carbon-based energy storage materials, graphene
is considered to be the most fascinating one for the next generation
of flexible and durable energy storage devices due to its excellent
electronic conductivity, thermal conductivity, mechanical proper-
ties, and super-large specific surface area [13e16]. However, due to
the lack of a large number of electroactive groups, the inherent
capacitance and energy density of graphene are very low. There-
fore, improving the energy storage performance of graphene by
modification, doping, recombination and other technologies have
become the focus of research [17e20]. Among them, the modifi-
cation of graphene by organic small molecules can improve the low
performance of the electrode, and have attracted people’s attention
in recent years [21e26]. Song et al. prepared a rGO-based com-
posite electrode using triethanolamine (TEA) as inter-layer spacer
* Corresponding author.
** Corresponding author.
E-mail addresses: hongweikang@infm.hhstu.edu.cn (H. Kang), zhikunli@infm.
hhstu.edu.cn (Z. Li).
https://doi.org/10.1016/j.jallcom.2020.155412
0925-8388/© 2020 Elsevier B.V. All rights reserved.

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W. Zhang et al. / Journal of Alloys and Compounds 835 (2020) 155412
via a controlled hydrothermal process, and the TEA/rGO electrode
exhibited a maximum energy density of 25.7 Wh kgꢀ1 in the
organic electrolyte and an outstanding cycling stability with 91.7%
capacitance retention after 10,000 cycling tests in 1 M H2SO4
electrolyte [27]. These composite electrodes combined the pseu-
docapacitance of small molecules with the excellent conductivity of
graphene, which ultimately improve the electrochemical
performance.
obtained sample was denoted as 7-AirGO1 (according to the mass
ratio of 7-Ai to GO is 1:1). A series of 7-AirGOs with different mass
ratios were synthesized through the same process, and denoted as
7-AirGO0.5 and 7-AirGO2 respectively.
2.3. Material characterizations
7-Ai was identified by a Bruker DPX-400 MHz NMR spectrom-
The graphene-based organic composite electrodes are of great
potential in the field of new green energy storage devices, based on
our previous work [28], we optimized the experimental scheme
and selected another electrolyte, and focused on the study of the
influence of different contents of 7-aminoindole modified rGO on
the electrochemical performance of the synthesized composite. In
this paper, the hierarchical mesoporous nanostructure 7-
aminoindole (7-Ai) modified rGO composites were successfully
synthesized by one-step hydrothermal method. Reduced graphene
oxide with porous nanostructure has abundant mesoporous
structure, which provides a broad space for the adhesion of 7-Ai
organic molecules. The synthetic 7-AirGO composites not only
have abundant electroactive sites, which can obtain a large number
of double-layer capacitance and pseudocapacitance, but also have
unique structural characteristics, which is in favor of the diffusion
of electrolyte ions, promoting the full contact between electrode
and electrolyte, and facilitating the rapid transmission of electrons
in the conductive network. Therefore, the results of these electro-
chemical performance tests showed that the optimal 7-AirGO1
eter (Billerica, MA, USA). XRD patterns were characterized on a
Bruker D8 Advance X-ray diffractometer with Cu K
a radiation
(l
¼ 1.5417 Å) at a step of 0.02ꢁ. Raman measurements were per-
formed on a Renishaw inVia Raman microscope employing a
633 nm laser excitation (25% laser power). The morphologies and
microstructures were characterized by field-emission scanning
electron microscopy (FESEM, Quanta 250 FEG) and transmission
electron microscope (TEM, JEOL JEM-2100). Thermogravimetric
analysis (TGA) data were obtained using a SDT Q600 (USA) in N2
from room temperature to 800 ꢁC at a heating rate of 2 ꢁC minꢀ1
.
Fourier transform infrared (FTIR) spectra were recorded on a
Thermo Scientific Nicolet iS5 with wavelength range from 400 to
4000 cmꢀ1. N2 adsorption/desorption isotherms were measured by
micromeritics ASAP 2020 analyzer. The samples were out-gassed at
120 ꢁC for 6 h before adsorption. The specific surface area (SBET
)
measurements were calculated by a Brunauer-Emmett-Teller (BET).
The X-ray photoelectron spectroscopy (XPS) analysis was per-
formed on a Thermo Scientific ESCALAB 250 Xi (Al target, Ka ra-
electrode exhibited a high capacitance of 425.73 F gꢀ1 at 0.5 A gꢀ1
,
diation, USA).
showing a distinct improvement in comparison to rGO electrode
and 7-Ai electrode. Furthermore, the assembled 7-AirGO1//AC
asymmetric supercapacitor device (ASD) displayed an energy
density of 14.60 W h kgꢀ1 (350 W kgꢀ1) and a maximum power
density of 10,500 W kgꢀ1 (10.50 W h kgꢀ1) at 15 A gꢀ1, as well as an
excellent electrochemical stability with approximately 97.98% of
the initial specific capacitance at 4 A gꢀ1 after 20,000 cycles. These
excellent electrochemical properties prove that this novel material
is of great potential in the application of novel supercapacitors.
2.4. Electrochemical measurements
All of the electrochemical performance tests were carried out on
a CHI 760E electrochemical workstation (Shanghai CH Instrument
Company, China). Cyclic voltammetry (CV), galvanostatic charge/
discharge and electrochemical impedance spectroscopy (EIS)
measurements were investigated in 2 M KOH aqueous electrolyte.
The working electrode was prepared by blending 7-AirGOs (80 wt
%), conductive carbon black (10 wt%) and polytetrafluoroethylene
(PTFE, 10 wt%), the mixture was produced a homogeneous phase by
adding a small amount of isopropyl alcohol. Then the slurry was
spread on a nickel foam sheet (0.9 ꢂ 0.9 cm2) and pressed together
followed by drying at 70 ꢁC under vacuum for overnight. The mass
of active materials coated on each work electrode was about 1.6 mg.
In a three-electrode system, platinum plate and Ag/AgCl elec-
trode were acted as counter electrode and reference electrode
respectively. Before the electrochemical performance test, the
working electrode was pre-soaked in electrolyte for about 6 h to
obtain a sufficient contact for working electrode and electrolyte.
Specific capacitance (C, F gꢀ1) of the working electrode was calcu-
lated from the galvanostatic discharge process based on the
following formula:
2. Experimental section
All of the chemical reagents were analytical grade and used
directly without further purification. Deionized water (DI water)
was used throughout the sample preparation.
2.1. Preparation of 7-aminoindole (7-Ai)
7-aminoindole (7-Ai) was synthesized according to the previous
literature [29]. A catalytic amount of palladium (10%) on activated
carbon was added to the 7-nitroindole ethanol solution, and reac-
ted at room temperature overnight with a hydrogen bag. Then, the
catalyst was removed by filtration over Celite, and the solvent was
evaporated to provide the product as a grey solid (96.5%). The 1H
NMR spectra and XRD pattern of 7-aminoindole sample were
shown in Fig. S1 and Fig. S2, respectively.
IDt
mDU
C ¼
(1)
2.2. Synthesis of 7-aminoindole/rGO composites (7-AirGOs)
Where C (F gꢀ1) is the specific capacitance, I (A) is the discharge
current, t (s) is the discharge time, U (V) is the voltage change
D
D
Ai-rGOs were synthesized in a simple hydrothermal process. GO
(85 mg) prepared according to our previous literature [30] was
dispersed in DI water and sonicated for 1 h to form a homogeneous
solution (1 mg mLꢀ1) at first. Then, 7-Ai (85 mg) was added to the
above suspension and stirred continuously overnight. The obtained
suspension was evenly allocated to two 50 mL Teflon-lined stainless
steel autoclave and reacted at 140 ꢁC for 15 h. The precipitate after
centrifugation with DI water was dried at 70 ꢁC for 12 h. The final
during the discharge process, m (g) is the mass of active material.
The asymmetric supercapacitor device (ASD) was assembled by
using a relative size membrane between the 7-AirGOs electrodes
and AC electrode, and the insulation board was used as the shell for
packaging in 2 M KOH aqueous electrolyte. The specific capacitance
(C, F gꢀ1), energy density (E, W h kgꢀ1) and power densit y (P, kW
kgꢀ1) of the ASD were tested and calculated according to the
following equations:

W. Zhang et al. / Journal of Alloys and Compounds 835 (2020) 155412
3
graphitization and defect of the prepared samples were charac-
terized by XRD and Raman. As presented in Fig. 1a, a sharp peak at
IDt
C ¼
(2)
(3)
2q
¼ 10.2ꢁ is the characteristic peak of the (002) crystalline plane of
M
DU
GO. After hydrothermal reduction, the sharp characteristic peak
¼ 10.2ꢁ disappears and a broad diffraction peak
m
þ
m
ꢀ
Sc *
Sc *
þ
D
V
V
ꢀ
ꢀ
þ
located at 2
occurred at around 2
AirGO2 samples, corresponding to the typical drum peak of gra-
phene, which indicates that GO is reduced after hydrothermal re-
action [14,26]. It is worth noting that, after careful observation, the
q
¼
¼ 25.8ꢁ for rGO, 7-AirGO0.5, 7-AirGO1 and 7-
D
q
C*D
U2
2*3:6
E ¼
P ¼
(4)
diffraction peak of 7-AirGOs materials at around 2
q
¼ 25.8ꢁ shifts
slightly to a small angle than that of pure rGO material, which
means that the interplanar spacing of rGO becomes larger in 7-
AirGOs materials. What is more, the weak peak located at near
3600E
(5)
D
t
2q
¼ 43.1ꢁ indicates that a very thin rGO sheets are existed in 7-
Where I (A),
D
U (V),
D
t (s) and M (g) were the response current, the
AirGOs composites [14]. These results indicate that the introduc-
tion of 7-Ai molecule extends the interplanar spacing of rGO and
results in a thinner graphene lamellar structure. The Raman spectra
of GO, rGO, 7-AirGO0.5, 7-AirGO1 and 7-AirGO2 were exhibited in
Fig. 1b. The typical characteristic peaks located at 1334.12 cmꢀ1 and
1592.82 cmꢀ1 are corresponded to the D band and G band
respectively, which are represented the presence of structure de-
fects and disorder in graphitic structure derived from the A1g mode
of 3D graphitic-like lattice vibrations, and the E2g in-plane vibration
of sp2-bonded carbon atoms, respectively [31e33]. The intensity
range of potential, the discharge time and the total mass of both
electrodes, respectively. The m (g) is the mass of positive elec-
þ
trode, m- (g) is the mass of negative electrode, Sc (F gꢀ1) and Sc (F
þ
ꢀ
g
ꢀ1) is the specific capacitance of positive electrode and negative
electrode respectively calculated from a three-electrode system
tests.
3. Results and discussion
The crystal structure, phase composition, degree of
Fig. 1. (a) XRD patterns and (b) Raman spectra of GO, rGO, 7-AirGO0.5, 7-AirGO1 and 7-AirGO2; (c) FTIR spectra of rGO, 7-AirGO0.5, 7-AirGO1 and 7-AirGO2; (d) TGA and DTG
curves of 7-AirGO1 with a heating rate of 2 ꢁC minꢀ1 under N2 atmosphere.

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W. Zhang et al. / Journal of Alloys and Compounds 835 (2020) 155412
ratio of D band to G band (ID/IG) reflects the disorder degree of the
internal microstructures of carbon-based materials [34]. The
calculated ID/IG values of GO, rGO, 7-AirGO0.5, 7-AirGO1 and 7-
AirGO2 are 0.86, 1.25, 1.14, 1.17 and 1.21, respectively. The higher
ID/IG value in rGO, 7-AirGO0.5, 7-AirGO1 and 7-AirGO2 suggest that
more defects and more serious disorder are produced on rGO and
7-AirGOs materials after chemical hydrothermal reduction process.
Furthermore, a slight increase of the ID/IG value from 7-AirGO0.5 to
7-AirGO2 indicates more defects with the increase of reactant 7-Ai
content, which can be inferred that more 7-Ai molecules are grafted
onto the surface graphene sheets [27].
the increase of organic 7-Ai content, the rGO lamellar arrangement
is more orderly and denser. The high-magnification SEM image in
Fig. 2f further shows that 7-AirGO1 material has a very thin and
dispersed lamellar structure, and the porous structure with thin
pore wall that is very conducive to the transfer of electrolyte ions.
The microstructure of 7-AirGO1 composite was further character-
ized by TEM, as shown in Fig. 2g-h. The 7-AirGO1 material shows
the typical structural characteristics of graphene nanosheet, with
folded and highly transparent multilayered lamellar morphology
(Fig. 2g). The TEM image of 7-AirGO1 material with a higher
magnification shows that there are a lot of attachments evenly
distributed on the rGO nanosheets (Fig. 2h). Although single
organic 7-Ai molecule is not enough to be observed by SEM char-
acterization, while combined with the characteristics of the syn-
thetic material and the synthesis process, it is believed that these
attachments are formed by a small amount of organic 7-Ai mole-
cules gathered together due to the intermolecular force. The well-
defined rings shown in the selected-area electron diffraction
(SAED) are consistent with the XRD characterization results, indi-
cating a relatively good crystallinity for the nano-porous structure
of 7-AirGO1 composite.
The nitrogen adsorption/desorption isotherms of GO, rGO, and
7-AirGOs samples were shown in Fig. 3a. According to the IUPAC
classification, the isotherms displays a typical type-IV isotherm
with a hysteresis loop at P Pꢀ0 1 of around 0.4e0.6, indicating the
presence of a large number of mesoporous structures and a certain
amount of macroporous structures. In this hierarchical porosity
composed of mesopores connected with macropores nano-
structure, the macropores can be served as solution buffering
reservoir to minimize the diffusion distance to the mesopores,
facilitating mass transport, and also reducing the volume change
during the charge/discharge cycling, thereby ensuring a high
cycling performance [36e38]. However, for GO materials, the poor
porosity can be obtained from the isotherm, which is mainly due to
the serious agglomeration and stacking of GO nanosheets. Fig. 3b
shows the pore size distribution curves of the prepared samples. It
is found that, compared with the pure rGO, the 7-AirGOs com-
posites modified by organic 7-Ai molecules mainly reduce the
macropores and the larger mesopores in rGO, however, the porosity
of 7-AirGOs composites are greatly improved. In addition, the pore
size of the composite is mainly concentrated around 3e7 nm,
which can be observed from the insert in Fig. 3b. The BET surface
area (SBET) and textural properties of the GO, rGO, and 7-AirGOs
samples were further confirmed by the experimental data which
were summarized in Table S1. Compared with rGO materials, the
decrease of specific surface area and pore volume of 7-AirGOs
composites is mainly due to the existence of organic 7-Ai mole-
cule, whether intercalated in-between rGO layers or grafted on
outer rGO surfaces. And the intermolecular interaction between 7-
Ai and rGO, and the formation of amide bond reduce the layer
spacing as well as the pore size of rGO, which usually exists in
organic-based carbon (such as rGO/graphene, CNTs) composites
[31,39,40].
FTIR spectra of rGO, 7-AirGO0.5, 7-AirGO1 and 7-AirGO2 sam-
ples were shown in Fig. 1c. In the FTIR spectrum of rGO, a series of
characteristic weak peaks located at around 1015.8, 1560.1, 1712.3,
and 3400.0 cmꢀ1 can be attributed to CeO vibrations from the
alkoxy groups, C]C vibrations, C]O of carboxyl, and OeH
stretching vibration on the surface or edges of the rGO sheets,
respectively [35]. Upon functionalization, the more obvious ab-
sorption peaks at 1196.6, 1568.3, and 1714.4 cmꢀ1 are recognized as
the aromatic ring vibration of CeNeC originated from the surface
grafting of graphene with amines, NeH bending vibration of pri-
mary amine, C]O stretching vibration in amide bond [17]. And the
bulge peak located at ~3347.8 cmꢀ1 should be inferred as the eNH
stretching vibration (originated from the organic 7-Ai molecule and
the formed amide bond), eNH2 stretching vibration (in 7-Ai), and
the stretching vibration of eOH functional groups in a small
amount rGO that are not reduced in hydrothermal reduction re-
action. Compared with the FTIR spectrum of rGO, the appearance of
the peak (1196.6 cmꢀ1) and the unconspicuous peak (~1015.8 cmꢀ1
)
in the FTIR spectrum of 7-AirGOs, as well as the analysis of the
above results, confirm the presence of the 7-Ai molecules in the
rGO structure in 7-AirGOs composites [17,27].
The thermal stability of the 7-AirGO1 sample was evaluated by
thermogravimetric analysis (TGA), and carried out under nitrogen
atmosphere with a heating rate of 2 ꢁC minꢀ1. As illustrated in
Fig. 1d, the weight loss before 150 ꢁC is mainly due to the evapo-
ration of water in the composite material. While another weight
loss from 150 ꢁC to about 420 ꢁC is mainly attributed to the subli-
mation of organic 7-Ai material that intercalated in-between rGO
layers and the decomposition of a small amount of unstable oxygen
functionalities groups. The slight weight loss in the third stage
around 635 ꢁC should be attributed to the sublimation of 7-Ai that
grafted on the outer surface of rGO (i.e. 7-Ai molecules linked by
amide bond) and the decomposition of relatively stable functional
groups. In addition, combined with TGA curve analysis of rGO
(Fig. S3c), it is can be concluded that the content of organic 7-Ai
molecule in 7-AirGO1 composite is about 10.2 wt%. Similarly, we
inferred that the content of organic 7-Ai molecule in 7-AirGO0.5
and AirGO2 composites is about 9.0 wt% and 11.6 wt%, respectively
(Fig. S3 (a-b)). Simultaneously, comparing the TGA curves of 7-
AirGO0.5, 7-AirGO1 and 7-AirGO2 composites, the weight loss of
7-AirGO2 composite in the second stage (150 ꢁC to about 420 ꢁC) is
largest, which indicates that the organic 7-Ai molecules that mainly
exist in the form of intercalation in-between rGO layers.
The microstructure and morphology of the prepared samples
were characterized by SEM and TEM. As presented in Fig. 2a, the
prepared GO samples show multi-layer and yarn-like nanosheet
structure. However, due to the large number of functional groups
and the strong van der Waals force in GO materials, the agglom-
eration of obtained GO nanosheets is obviously. And after hydro-
thermal reduction, the degree of aggregation of graphene is
obviously reduced, and it presents a fluffy, porous, densely inter-
connected nanosheet structure (Fig. 2b). After functionalization of
rGO, we can found that all 7-AirGOs composites exhibit more
fragmented and dispersed lamellar structure (Fig. 2c-e). And with
The electrochemical performance of the prepared electrodes
was first measured in 2 M KOH electrolyte by means of a traditional
three electrode system. The GCD curves of 7-Ai, rGO, 7-AirGO0.5, 7-
AirGO1 and 7-AirGO2 electrodes at current density of 0.5 A gꢀ1
were described in Fig. 4a. The 7-AirGO1 electrode shows a superior
specific capacitance of 425.73 F gꢀ1, which is much higher than that
of 7-Ai (65.94 F gꢀ1), rGO (80.29 F gꢀ1), 7-AirGO0.5 (363.28 F gꢀ1) or
7-AirGO2 (322.60 F gꢀ1) electrodes. Furthermore, the GCD curves of
7-AirGOs electrodes present a nonlinear triangle, which indicate
that the high-capacitance exhibited by the composite electrodes is
composed of double-layer capacitance and pseudocapacitance. In
addition, the highest specific capacitance of electrode 7-AirGO1 in

W. Zhang et al. / Journal of Alloys and Compounds 835 (2020) 155412
5
Fig. 2. Low-magnification SEM images of (a) GO, (b) rGO, (c) 7-AirGO0.5, (d) 7-AirGO1, (e) 7-AirGO2 samples, (f) high-magnification SEM image of 2-AprGO1 sample, (geh) TEM and
(i) SAED images of 7-AirGO1 sample.
Fig. 3. (a) N2 adsorption-desorption isotherms, and (b) pore size distribution curves of the prepared GO, rGO, 7-AirGO0.5, 7-AirGO1 and 7-AirGO2 samples.
all 7-AirGOs electrodes indicates that rGO modified by an appro-
priate amount of 7-Ai molecule can effectively exert the advantage
of synergistic enhancement effect of rGO and the functional 7-Ai
molecules. The comparison of the electrochemical performance of
each electrode and the composition of the capacitance produced by
the composite electrode can be further proved by CV curves
(Fig. S5). Fig. 4b displays the GCD curves of 7-AirGO1 electrode at
different current densities from 0.5 to 20 A gꢀ1. The approximately
symmetrical triangle and the almost negligible voltage (IR) drop in
discharge curves demonstrate that the 7-AirGO1 electrode has the

6
W. Zhang et al. / Journal of Alloys and Compounds 835 (2020) 155412
Fig. 4. (a) GCD curves of 7-Ai, rGO, 7-AirGO0.5, 7-AirGO1 and 7-AirGO2 electrodes; (b) GCD curves of 7-AirGO1 electrode at different current densities; (c) rate capability and (d)
Nyquist plots of 7-Ai, rGO, 7-AirGO0.5, 7-AirGO1 and 7-AirGO2 electrodes.
characteristics of the relatively ideal capacitor behavior. The spe-
cific capacitance vs current densities of all prepared electrodes
were illustrated in Fig. 4c. It can be seen that with the increase of
current density, the specific capacitance of 7-AirGO1 electrode is
higher than that of other 7-AirGOs electrodes. And even when the
current density is increased to 20 A gꢀ1, the 7-AirGO1 electrode still
has a specific capacitance of 241.96 F gꢀ1, implying an excellent rate
capability. On the contrary, as the current density increase, the
capacitance of 7-Ai electrode decreases sharply, indicating a poor
rate capability, which is also the characteristic of pure organic
material electrode.
Electrochemical impedance spectroscopy (EIS) test was used to
investigate the resistive properties and the ionic diffusion kinetics
features of the electrodes in electrolyte. The comparison of the
Nyquist plots of 7-Ai, rGO and 7-AirGOs electrodes in the frequency
range from 105 to 0.01 Hz were all presented in Fig. 4d. It can be
seen that these complex-plane impedance plots are composed of a
semicircle arc at the medium-high frequency region followed by an
approximate straight line at the low frequency region. The inter-
section of the semicircle arc with the real axis at high frequency
refers to the equivalent serial resistance (RESR), which reflects the
sum of ionic resistance of electrolyte, intrinsic resistance of elec-
trodes, and contact resistance at the electrolyte/electrode interface
[41e44]. The diameter of semicircle represents charge-transfer
resistance (Rct), by careful observation from the insert in Fig. 4d,
we can clearly obtained the order of Rct for 7-Ai, rGO, 7-AirGO0.5,
7-AirGO1 and 7-AirGO2 electrodes: Rct (rGO) ˂ Rct (7-AirGO2) ˂ Rct
(7-AirGO1) ˂ Rct (7-AirGO0.5) ˂˂ Rct (7-Ai), which shows that the
introduction of organic 7-Ai molecule slightly compromises the
charge transfer kinetic properties of rGO materials. The slope of the
straight line in the low frequency region represents the diffusion
resistance of electrolyte ions (Warburg resistance), and a vertical
line would be observed for an ideal capacitor. Therefore, the more
vertical line of the 7-AirGOs electrodes indicates a more ideal
capacitance behavior, especially for the case of 7-AirGO1 electrode
[27,43,45]. What is more, the slopy line of Nyquist plots of these
electrodes fabricated by composite materials also reveals that the
capacitance contributed by the 7-AirGOs electrodes not only in-
cludes the electric double layer capacitance, but also the
pseudocapacitance.
To further evaluate the practical applicability of electrochemical
energy storage performances of the 7-AirGO1 composite electrode,
an asymmetric supercapacitor device (ASD) was assembled based
on 7-AirGO1 electrode as the positive electrode and AC as the
negative electrode in 2 M KOH aqueous electrolyte. The mass ratio
of 7-AirGO1 electrode to AC electrode depended on the theory of
charge balance (equation (3)). The CV curves of 7-AirGO1//AC
asymmetric supercapacitor device (ASD) at different potential

W. Zhang et al. / Journal of Alloys and Compounds 835 (2020) 155412
7
Fig. 5. The electrochemical performances of 7-AirGO1//AC asymmetric supercapacitor device (ASD): (a) CV curves at different potential windows from 0-0.8 V to 0e1.5 V; (b) CV
curves at different scan rates; (c) GCD curves at various current densities; (d) Ragone plot, (e) cyclic stability and coulombic efficiency; (f) Nyquist plots before and after 20,000
cycles.

8
W. Zhang et al. / Journal of Alloys and Compounds 835 (2020) 155412
windows were shown in Fig. 5a. It can be seen that the voltage can
be extended to 1.4 V, so as to achieve better electrochemical
properties, while the polarization phenomenon in the reaction
process can be avoided as much as possible. Fig. 5b presented the
CV curves of 7-AirGO1//AC ASD at various scan rates from 10 to
200 mV sꢀ1. We can observe that the CV curves exhibit a quasi-
rectangular shape with less obvious wide redox peaks, even
though the scanning rate is as high as 200 mV sꢀ1, indicating that
the ASD has excellent capability. The GCD curves of the 7-AirGO1//
AC ASD recorded at different current densities display approximate
symmetrical distribution (Fig. 5c), implying an excellent electro-
chemical reversibility and capacitive behavior. The Ragone plots
obtained from the GCD curves are used to further evaluate the
energy-power properties of the ASD, as shown in Fig. 5d. It dem-
onstrates that the maximum mass energy density is 14.60 W h kgꢀ1
achieved at a power density of 350 W kgꢀ1, and even when the
power density is as high as 10,500 W kgꢀ1, the device still retains a
high energy density of 10.50 W h kgꢀ1, showing excellent energy
density and power density, which are both relatively higher than
the previously reported organic molecular based supercapacitors
[31,46e48].
As important requirement for practical application of super-
capacitors, the long-term cycling stability of the ASD was evaluated
by performing galvanostatic charge-discharge at a current density
of 4 A gꢀ1. As shown in Fig. 5e, the ASD still displays excellent cycle
stability with 97.98% capacitance retention after 20,000 cycles.
What’s more, the ASD always maintains a relatively stable and very
high Coulomb efficiency over the 20,000 cycles. The EIS was carried
out to further examine the electrochemical performance of the ASD
before and after 20,000 cycles (Fig. 5f). A slight increase in the
diameter of the semicircle arc in the medium-high frequency re-
gions and the nearly vertical line in the low frequency regions in-
dicates the slight increase of Rct and Warburg resistance after
20,000 cycles, which means that the ASD still maintains good
charge transfer kinetics and electrolyte ion diffusion kinetics due to
the stable structure of the electrode material. The result further
proves the reason why the ASD has excellent ultra-long cycle
stability.
4. Conclusions
In summary, we demonstrated a promising composite electrode
based on the organic 7-Ai molecule decorated rGO nanosheets
successfully synthesized by a facile hydrothermal method. The
electrochemical performance test results of the composite elec-
trodes with different doping amount of 7-Ai show that the com-
posite electrode is of excellent electrochemical energy storage
performance rooted from the synergetic enhancement of the two
electrode materials. The 7-AirGO1 electrode exhibits optimal
electrochemical properties with
a
specific capacitance of
425.73 F gꢀ1 at a current density of 0.5 A gꢀ1 and an outstanding
rate capability. Furthermore, an asymmetric supercapacitor device
(ASD) of 7-AirGO1//AC assembled based on 7-AirGO1 electrode and
AC electrode displays a high energy density of 14.60 W h kgꢀ1
(350 W kgꢀ1) and a high power density of 10,500 W kgꢀ1
(10.50 W h kgꢀ1), as well as a super-long cycling stability with a
capacitance retention of 97.98% after 20,000 cycles at a large cur-
rent density of 4 A gꢀ1. These results show that the porous network
composite obtained by modifying graphene with redox functional
organic molecules can help to harvest the excellent electrochemical
energy storage performance of these novel electrode materials,
which is expected to be applied to the new generation of green and
efficient energy storage devices.
Declaration of competing interest
The authors declare no competing financial interest.
CRediT authorship contribution statement
Weiyang Zhang: Investigation, Writing - original draft. Chun-
yan Sun: Investigation, Writing - original draft, Validation. Yonggui
Xu: Data curation, Writing - review & editing. Hongwei Kang:
Conceptualization, Methodology, Writing - review & editing. Bao-
cheng Yang: Project administration. Huili Liu: Formal analysis.
Zhikun Li: Writing - review & editing.
These excellent electrochemical properties with high energy
and power density, super-long cycling stability and good Coulomb
efficiency for the synthesized composite electrodes were also
verified by 7-AirGO0.5//AC ASD and 7-AirGO2//AC ASD (Fig. S6).
The performance enhancement of 7-AirGOs composite electrodes is
mainly attributed to the following aspects: (1) in the charge-
discharge process, the introduction of the 7-Ai organic material
containing amino functional groups can formed electron donor/
acceptor with graphene, which can not only produce pseudoca-
pacitance generated by charge difference, but also promote the
electron transfer process between surface bound amino group and
rGO, thus improving the electrochemical performance of composite
electrode materials. In addition, the pyrrolic N, as an electro-
chemically active functional group that participates in hydrogen
bonding and easily disrupted, can also increase the capacitance,
thus improving the electrochemical properties of the composite
[49,50]; (2) the hydrophilicity of amino group is helpful to reduce
the contact resistance of electrode/electrolyte and enhances the
diffusion rate of electrolyte ions; (3) The interconnected and stable
porous network structure of 7-Ai molecules decorated rGO, as well
as the synergetic enhancement of 7-Ai and rGO, are conducive not
only to the rapid transport of ions/electrons, but also to the stability
of the nanostructure, thus improving the ultra-long cycling stability
of the composite electrode. As a result, the electrochemical energy
storage performance of the synthesized composite is significantly
enhanced, which is superior to the similar composite electrode
reported previously (Table S2).
Acknowledgements
The authors gratefully acknowledge the financial support from
the National Natural Science Foundation of China (51872110) and
Science -Technology Development Program of Henan Province
(202102210030).
Appendix A. Supplementary data
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jallcom.2020.155412.
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