Synthesis of Graphene Hydrogel and Graphene Oxide/Polyaniline Composites for Asymmetric Supercapacitor

Abstract

As a kind of new carbon materials, graphene has excellent performances such as superior conductivity, high specific surface area (2675 m ² g ⁻¹ ), and high cycle life. But graphene sheets are easily restacked by van der Waals interactions during the processes of preparation, storage, and application of them, which can loss ultrahigh specific surface area. The restacking of graphene sheets could be prevented by constructing graphene hydrogel (GH) or graphene composite. In this work, GH was prepared by a one-step hydrothermal synthesis reaction and glucose is used as a reducing agent, while graphene oxide/polyaniline (GP) composites was prepared by coating polyaniline on GO through in situ chemical polymerization of aniline. Then an asymmetric supercapacitor (GH//GP) was further assembled, where GH and GP are the positive and negative electrodes, respectively. The electrochemical properties of GH, GP, and GH//GP are studied using voltammetry and galvanostatic charge/discharge. The consequences indicate that the energy density of GH//GP reaches 5.6 W h kg ⁻¹ at power density of 226.5 W kg ⁻¹ and 3.1 W h kg ⁻¹ at energy density of 8446.2 W kg ⁻¹ .

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IOP Conference Series: Materials Science and Engineering
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Synthesis of Graphene Hydrogel and Graphene Oxide/Polyaniline
Composites for Asymmetric Supercapacitor
To cite this article: Yangyang Shang et al 2019 IOP Conf. Ser.: Mater. Sci. Eng. 562 012105
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7th Annual International Conference on Materials Science and Engineering
IOP Conf. Series: Materials Science and Engineering 562 (2019) 012105
IOP Publishing
doi:10.1088/1757-899X/562/1/012105
1
Synthesis of Graphene Hydrogel and Graphene
Oxide/Polyaniline Composites for Asymmetric Supercapacitor
Yangyang Shang, Liao Xu, Linlin Cai and Xiaoqing Jiang*
Jiangsu Key Laboratory of New Power Batteries, School of Chemistry and Materials
Science, Nanjing Normal University, Nanjing 210023, China
Email: jiangxiaoqing@njnu.edu.cn
Abstract. As a kind of new carbon materials, graphene has excellent performances such as
superior conductivity, high specific surface area (2675 m2 g-1), and high cycle life. But
graphene sheets are easily restacked by van der Waals interactions during the processes of
preparation, storage, and application of them, which can loss ultrahigh specific surface area.
The restacking of graphene sheets could be prevented by constructing graphene hydrogel (GH)
or graphene composite. In this work, GH was prepared by a one-step hydrothermal synthesis
reaction and glucose is used as a reducing agent, while graphene oxide/polyaniline (GP)
composites was prepared by coating polyaniline on GO through in situ chemical
polymerization of aniline. Then an asymmetric supercapacitor (GH//GP) was further assembled,
where GH and GP are the positive and negative electrodes, respectively. The electrochemical
properties of GH, GP, and GH//GP are studied using voltammetry and galvanostatic
charge/discharge. The consequences indicate that the energy density of GH//GP reaches 5.6 W
h kg-1 at power density of 226.5 W kg-1 and 3.1 W h kg-1 at energy density of 8446.2 W kg-1.
1. Introduction
As novel power storage and energy storage device, supercapacitor with long cycle life, high charge
and discharge rates, and good operational safety, have recently attracted much attention [1]. The most
used materials for supercapacitors include carbon materials, metal oxides/hydroxides, and conductive
polymers. Among them, all-carbon materials such as graphene, active carbon, carbon nanotube, and
carbon fiber, have high power density and superior cyclic stability, due to rapid ion absorption and
desorption, which is called electric double layer capacitor (EDLC). However, this kind of rapid
process only occurs at the interface of the materials, so the capacitance of EDLC is very limited
(usually below 200 F g-1) [2]. In addition, metal oxides/hydroxides, such as RuO2, MnO2, and nickel
manganese double hydroxide, and conductive polymers, such as polyaniline, polypyrrole, and
polythiophene, usually exhibit higher capacitance than carbon materials, due to redox reactions, which
is called faradic pseudocapacitor (PC), but it also limits their charge and discharge efficiency at high
rates [3]. Therefore, composite materials such as carbon-metal oxide/hydroxide or carbon-conductive
polymer have become the research hotspot, in order to obtain high-performance electrodes with high
energy or power density.
In our work, a graphene/polyaniline composite (GP) was composited through in situ
polymerization of aniline to polyaniline (PANI) into the graphene oxide (GO). On the other hand, the
preparation of 3D graphene hydrogel (GH) by hydrothermal method using glucose as a reducing agent
is a simple method. In addition, an asymmetric supercapacitor was also assembled with GH and GP as
anode and cathode materials respectively, using a sandwich type construction. This design is
considered to be an effective and simple method to improve electrochemical performance, because the

7th Annual International Conference on Materials Science and Engineering
IOP Conf. Series: Materials Science and Engineering 562 (2019) 012105
IOP Publishing
doi:10.1088/1757-899X/562/1/012105
2
voltage of the supercapacitor can be amplified by combining two different working potential electrode
materials. GH and GP composites have excellent electrochemical properties as electrode materials for
supercapacitors, which indicated that they have great potential in the field of energy storage.
2. Experimental
2.1. Reagents
Graphite powder (1200 mesh) was purchased from Aladdin. H2SO4 (98%), glucose, hydrochloric acid
(HCl), and potassium chloride (KCl) were from Sinopharm Chemical Reagent Co., Ltd. All of these
chemicals are analytical grade.
2.2. Sample Preparation
GO was first prepared through a modified Hummer’s method [4], then dispersed in water via
sonication for 30 min in order to prepare GO dispersion with a concentration of 2 mg mL-1. 30 mL of
this GO dispersion was mixed with 60 mg glucose by sonication for 10 min. Then, the mixture was
transferred into a Teflon-lined stainless steel reactor and hydrothermally treated at 120 ºC for 12 h.
Afterwards, the reactor was naturally cooled at room temperature and the cylinder GH was taken out
carefully with tweezers, being purified by dialysis in deionized water for 24 h. The dried GH was
obtained by freeze-drying for 12 h (20 Pa, -55 oC) in order to calculate the mass fraction of active
material (Wa) according to Eq. (1):
%100
t
d
a
×= W
W
W
(1)
Where Wt and Wd are the weight of electrode material. GP composite was synthesized using in situ
polymerization of aniline into PANI on GO within an acid condition. Generally, a uniform GO
dispersion was prepared with 24 mg GO and 180 mL distilled water through sonication for 10 min.
The purified aniline was dissolved in 8.2 mL 1 M HCl and added into the GO dispersion quickly (the
mass ration of aniline to GO is 4.1:1), and then the mixture was stirred for 30 min. Afterward,
ammonium persulfate (APS), with a mass ratio to aniline of 3.4:1, was dissolved in 8.2 mL distilled
water and added into the above reaction mixture rapidly. The obtained mixture was stirring vigorously
within an ice-water bath for 8 h. Finally, the GP composite was collected by filtration and repeatedly
washed by water until the filtrate was nearly neutral, followed by re-dispersed in water.
2.3. Characterization
Electrochemical measurements of experiment were performed on a CHI 660c electrochemical
workstation. The electrochemical properties of GH and GP were measured by cyclic voltammograms
(CV) and galvanostatic charge-discharge (GCD). In a three-electrode system, the working electrode
was the samples coated nickel foam, the reference electrode was Hg/HgO electrode, and the counter
electrode was a platinum ring. GH and GP were working electrodes fabricated process: a slice of GH
was cut and pressed onto a nickel foam sheet at 2 MPa for 30 s. The mass of active material was about
2 mg. CV and GCD were measured in the potential of -1.0 to 0 V at 10, 25, 50, 100, 150, 200, 250,
and 300 mV s-1 or 0.5, 1, 2, 3, 4, 6, and 10 A g-1. 5.0 μL of the GP solution was added dropwise onto a
glassy carbon electrode and dried at normal temperature. Then, the dried glassy carbon electrode,
Ag/AgCl, and Pt ring were the working electrode, reference electrode, and counter electrode,
respectively. And aqueous solution of 2.0 M H2SO4 was used as the electrolyte. The CV test and the
GCD curve test were performed between 0 and 0.7 V. In addition, two slices of GH and GP were used
as electrodes and a sandwich type construction (glass sheet/Pt foil/GH/separator/GP/FTO) was used to
fabricate an asymmetric supercapacitor with filter paper socked with 5.0 M KOH as the separator in
figure 1. CV and GCD curves of the GH//GP were measured in the potential of -1 to 0.7 V at different
scan rates or current densities.

7th Annual International Conference on Materials Science and Engineering
IOP Conf. Series: Materials Science and Engineering 562 (2019) 012105
IOP Publishing
doi:10.1088/1757-899X/562/1/012105
3
Figure 1. Preparation and assembly process of GH, GP and GH // GP
The specific capacitance of GH is calculated by the discharge curve of CV (Cs-1, F g-1) in the
two-electrode system according to Eq. (2):
dVVI
VVmv
C
V
V
∫
−
=
2
1
)(
)( 4
12
1-s
(2)
where I is the current at a given potential V, V2-V1 is the potential window, v is the scan rate, and m
is the total mass of active material.
The specific capacitance of GH is calculated by the discharge curve of GCD (Cs-2, F g-1) in the
two-electrode system according to Eq. (3):
VmI
C∆
∆
=t4
2-s
(3)
where I is the discharge current,
△
t is the discharge time, m is the total mass of active material of
both electrodes, and
△
V is the potential window.
The specific capacitance of GH is calculated by the discharge curve of CV (Cs-1, F g-1) in a
three-electrode system according to Eq. (4):
∫
−
=
−
2
1)(
)( 1
12
1s
E
EdEEI
EEm
C
ν
(4)
The specific capacitance of GH is calculated by the discharge curve of GCD (Cs-2, F g-1) in a
three-electrode system according to Eq. (5):
Em
I
C∆
∆
=t
2-s
(5)
The energy density (E, W h kg-1) and power density (P, W kg-1) of a GH//GP asymmetric
supercapacitor can be estimated according to Eq. (6) and Eq. (7):
8
2
2
VC
E
s
∆
=
−
(6)
t
E
P∆
=
(7)

7th Annual International Conference on Materials Science and Engineering
IOP Conf. Series: Materials Science and Engineering 562 (2019) 012105
IOP Publishing
doi:10.1088/1757-899X/562/1/012105
4
3. Results and Discussion
3.1. The Morphology of GH and GP
The SEM photos of GH and GP are shown figure 2a and 2b. The SEM image of GH shows a 3D pore
structure in figure 2a. The GH structure can hinder restacking of graphene sheets and greatly enhance
the rapid diffusion and transport of ions, which increases the specific capacitance of the electrode
material. As can be seen from figure 2b, the PANI nanoparticles look like rods and combine well with
graphene sheets, indicating that the GP structure stability is better [5].
Figure 2. The SEM of GH (a) and GP (b).
3.2. The Electrochemical Test of GH, GP, and GH//GP
The CV and GCD curves of the GH are shown in figure 3a and 3c. The Cs-1 of the GH only decreases
slightly from 548.7 F g-1 at 10 mV s-1 to 500.0 F g-1 at 300 mV s-1, indicating again the superior
charge-discharge performance and outstanding specific capacitance. The Cs-2 of GH is 682.3 and 451.7
F g-1 at 0.5 and 6 A g-1. As can be seen from the figure, the GCD of GH is close to the isosceles
triangle, which implies the EDLC behavior of the electrode material. In 2 M H2SO4 solution with
three-electrode system, the CV and GCD curves of the GP were tested as shown in figure 4a and 4c.
The Cs-1 is 118.9 F g-1 at the scan rate of 10 mV s-1 and 33.4 F g-1 at the scan rate of 300 mV s-1. The
Cs-2 of GP is 122.3 and 81.7 F g-1 at the current density of 0.5 and 6 A g-1.
To further investigate the electrochemical performance of GH and GP in a two-electrode system, an
asymmetric supercapacitor device was constructed using GH and GP for the positive and negative,
which is designated as GH//GP. The CV and GCD curves of the GH//GP is shown in figure 5. The Cs-1
is 94.7 and 25.9 F g-1 at the scan rate of 10 and 300 mV s-1. The Cs-2 of the GH//GP is 82.3 and 51.4 F
g-1 at 0.5 and 6 A g-1. The energy and power density of GH // GP is shown in figure 6. The energy
density of GH // GP is 5.6 W h kg-1 at a power density of 226.5 W kg-1 and 3.1 W h kg-1 at 8446.2 W
kg-1, which is higher than reported in other literature [6].

7th Annual International Conference on Materials Science and Engineering
IOP Conf. Series: Materials Science and Engineering 562 (2019) 012105
IOP Publishing
doi:10.1088/1757-899X/562/1/012105
5
Figure 3. CV (a) and GCD (c) curves of GH; Cs-1 (b) and Cs-2 (d) of GH.
Figure 4. CV (a) and GCD (c) curves of GP; Cs-1 (b) and Cs-2 (d) of GP.

7th Annual International Conference on Materials Science and Engineering
IOP Conf. Series: Materials Science and Engineering 562 (2019) 012105
IOP Publishing
doi:10.1088/1757-899X/562/1/012105
6
Figure 5. CV (a) and GCD (c) curves of GH//GP; Cs-1 (b) and Cs-2 (d) of GH//GP.
Figure 6. Energy and power densities of GH//GP.
4. Conclusions
In conclusion, The GH and GP composites were successfully prepared under simple and
environmentally friendly conditions. The assembly of the asymmetric supercapacitor was carried out,
and its electrochemical performance was tested by CV and GCD. The results show that the potential
window of the asymmetric capacitor reaches 1.3 V, which is larger than the electrode of the bipolar
material alone. Energy density of the asymmetrical supercapacitors arrives at 5.6 W h kg-1 at a power
density of 226.5 W kg-1 and remains 3.1 W h kg-1 at a power density of 8446.2 W kg-1.
5. Acknowledgments
This work is supported by the Jiangsu Collaborative Innovation Center of Biomedical Functional
Materials, Priority Academic Program Development of Jiangsu Higher Education Institutions, and
Huaian Bio-Medical Functional Materials and Analysis Technology Service Platform (HAP201612).

7th Annual International Conference on Materials Science and Engineering
IOP Conf. Series: Materials Science and Engineering 562 (2019) 012105
IOP Publishing
doi:10.1088/1757-899X/562/1/012105
7
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[2] Simon P and Gogotsi Y 2008 Materials for electrochemical capacitors NAT MATER 7 845-54
[3] Choi B G, Yang M H, Hong W H, Choi J W and Huh Y S 2012 3D macroporous graphene
frameworks for supercapacitors with high energy and power densities ACS nano 6 4020-28
[4] Xu Y X, Bai H, Lu G W, Li C and Shi G Q 2008 Flexible graphene films via the filtration of
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[5] Zhang Y L, Si L, Zhou B, Zhao B, Zhu Y Y, Zhu L H and Jiang X Q 2016 Synthesis of novel
graphene oxide/pristine graphene/polyaniline ternary composites and application to
supercapacitor CEJ 288 689-700
[6] Bai Y, Liu M M, Sun J and Gao L 2016 Fabrication of Ni-Co binary oxide/reduced graphene
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Ionics 22 535-44