Room-temperature synthesis of 3-dimentional Ag-graphene hybrid hydrogel with promising electrochemical properties

Materials Science and Engineering B 178 (2013) 769–774
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering B
journal homepage: www.elsevier.com/locate/mseb
Room-temperature synthesis of 3-dimentional Ag-graphene hybrid hydrogel
with promising electrochemical properties
Haocheng Quana,b
, Yuanlong Shaoa,b
, Chengyi Houa,b
, Qinghong Zhanga
,
Hongzhi Wanga,∗
, Yaogang Lib,∗∗
a
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, 201620, People’s Republic of
China
b
Engineering Research Center of Advanced Glasses Manufacturing Technology, College of Materials Science and Engineering, Donghua University, 201620, People’s Republic of China
a r t i c l e i n f o
Article history:
Received 24 October 2012
Received in revised form 26 January 2013
Accepted 11 March 2013
Available online 8 April 2013
Keywords:
Supercapacitor
Graphene
3-dimentional
Ag nanoparticles
Nanocomposites
Electrochemical properties
a b s t r a c t
In this article, we report a room-temperature synthesis of 3-dimentional (3D) Ag-graphene hybrid hydro-
gels and fabricate a symmetric supercapacitor with this hybrid material. The preparation of this 3D
Ag-graphene hybrid hydrogel is facile and its application in macroscopic devices is more convenient
than 2-dimentional (2D) graphene-based material. Our work may provide new insights into the room-
temperature synthesis of graphene-based materials. In this novel 3D graphene-based material, the unique
structure and combination with Ag nanoparticles made this material exhibit better electrochemical per-
formance compared with the pure graphene. Thus, the obtained Ag-graphene hybrid hydorgels could be
widely used in various energy storage devices.
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
As a promising candidate for alternative energy storage devices,
supercapacitors, also called electrochemical capacitors or ultraca-
pacitors [1,2], have generated great interest of researchers due to
their high rate capability, long cycle life, pulse power supply, simple
energy storage mechanisms, high dynamic of charge propagation
and low maintenance cost [2–5]. These excellent properties can
contribute to applications such as consumer electronics, memory
back-up systems and industrial power and energy management.
In general, supercapacitors can be divided into two categories on
the basis of the energy storage mechanism: electrical double layer
capacitors (EDLCs) and pseudo-capacitors [6]. Transition metal
oxides or hydroxides [7,8] and conducting polymers [9,10] are usu-
ally used as the electrodes for pseudo-capacitors, in which fast and
reversible faradic processes take place due to the electro-active
species and because of this, much higher pseudo-capacitance can
be achieved. However, pseudo-capacitors also reveal some draw-
backs. The lack of cycling stability is one of such drawbacks due
to the easily damaged structure of the materials during the redox
process, which limits the practical applications of such capacitors
∗ Corresponding author. Tel.: +86 21 67792881; fax: +86 21 67792855.
∗∗ Corresponding author. Tel.: +86 21 67792676; fax: +86 21 67792855.
E-mail addresses: wanghz@dhu.edu.cn (H. Wang), yaogang li@dhu.edu.cn (Y. Li).
[11]. Different from pseudo-capacitors, the capacitance of EDLCs
stems from the pure electrostatic charge accumulated at the elec-
trode/electrolyte interface. Carbon-based materials dominate the
electrodes of this kind of capacitors because of their excellent
properties on surface area, electrical conductivity and mechani-
cal strength [12], which perfectly compensates for the deficiencies
of pseudo-capacitors. Activated carbons (ACs) and carbon nano-
tubes (CNTs) are mostly used as electrodes of such capacitors
among carbon-based materials, owing to their excellent physical
and chemical properties. However, the energy density and rate
capability of ACs are limited, as well as the surface area and elec-
trical conductivity of CNTs are theoretically relatively low [12].
These deficiencies restricted their application in high-performance
EDLCs [13,14]. Graphene, with the structure of a flat monolayer of
carbon atoms tightly packed into a two-dimensional (2D) honey-
comb lattice [15], reveals a series of intriguing properties such as
outstanding mechanical strength, superb chemical and thermal sta-
bility, excellent electrical conductivity and high surface-to-volume
ratio [16–20]. These fascinating characteristics make graphene
become the most ideal material used as electrodes in EDLCs.
In recent years, more and more researchers have been focused
on graphene and graphene-based hybrid materials used as elec-
trodes for supercapacitors [21–24]. Combination with inorganic
nanoparticles [25,26], carbon nanotubes [27] or polymers [28],
graphene-based hybrids unfold an improved performance on a
variety of physical and chemical properties. Besides, a lot of
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http://dx.doi.org/10.1016/j.mseb.2013.03.004

770 H. Quan et al. / Materials Science and Engineering B 178 (2013) 769–774
researchers have demonstrated that two-dimensional structure
such as papers, thin films can be assembled by graphene-based
platelets [19,29–32]. In most cases, however, a large portion of
surface area of 2D graphene sheets becomes inaccessible dur-
ing fabrication of such graphene films, which is attributed to
the tendency of individual graphene sheet to irreversibly aggre-
gate and restack as a result of van der Waals interaction [33].
Decreased surface area obstructs the infiltration of electrolyte
and the transportation of ions, lowering their energy density and
limiting the potential applications of graphene materials in elec-
trodes of supercapacitors. Therefore, integration of 2D nanoscale
building blocks into 3-dimensional (3D) macroscopic structures is
attracting much attention since it is an essential step in explor-
ing the advanced properties of individual 2D sheets for practical
applications [33–36]. In addition, the methods of synthesis of
graphene and graphene-based materials have been developed since
the report of isolated graphene prepared by simple mechanical
cleavage of graphite crystals [37]. These methods include epitaxial
growth on metals or SiC [38,39], chemical exfoliation of graphite
oxide [32,36,40], liquid-phase ultrasonic exfoliation of graphite
powder [41], expanded graphite by oxidation or intercalation [42]
and chemical vapor deposition (CVD) growth on metal substrates
[43–47]. Nonetheless, most of these methods need relatively high
temperature condition, which costs a lot of energy and increases
the carbon emission. Literatures on synthesis of 2D graphene and
graphene-based materials at low temperature are finite [48–50]
and let alone the synthesis of 3D graphene materials.
In this paper, we attempt to prepare self-assembled Ag-
graphene hybrid hydrogels with 3D macroporous structure in a
room-temperature route. We also study the mechanisms of the
formation of the 3D graphene-based hybrid hydrogels and fab-
ricate them into electrodes of supercapacitor to investigate their
electrochemical performance.
2. Experimental
2.1. Preparation of graphene oxide
According to Hummer’s method [51], graphene oxide (GO) was
prepared using analytical grade reagents without further purifica-
tion. The process of synthesis can be divided into these steps: 6 g
natural flake graphite powder was added to 138 mL cold (0 ◦C) con-
centrated H2SO4. 18 g KMnO4 was gradually added to the mixture
of H2SO4 and graphite at 0 ◦C with stirring. After adding KMnO4,
the mixture was stirred at 35 ◦C for 2 h. 276 mL distilled water
was slowly added to the mixture and then the temperature was
maintained below 100 ◦C for 15 min. 840 mL H2O2 solution was
then added to the mixture. After that, the mixture was filtered and
washed with 1500 mL HCl aqueous solution to remove metal ions.
Finally, the product was thoroughly washed with distilled water.
2.2. Synthesis of Ag-graphene hybrid hydrogels
Graphene hydrogel (GH) was prepared by the following pro-
cedure with the reducing reagents of l-ascorbic and hydrazine
hydrate. In short, 0.24 g l-ascorbic acid was added to 20 mL
4 mg mL−1 GO aqueous dispersion loaded in a glass vial with 30 min
ultrasonication. Then, 0.25 mL hydrazine hydrate was added to the
mixture and dispersed by unltrasonication for 5 min. After ultra-
sonication, the mixture was placed at temperature of 25 ◦C for about
12 h without any disturbance to produce GH. The reduced graphene
oxide hydrogels were immersed in water to remove impurities such
as residual salts and acids and cleaned for further investigation.
Ag-graphene hybrid hydrogels were prepared through
the procedure as same as synthesis of graphene hydrogels.
0.24 g l-ascorbic acid was added to 20 mL 4 mg mL−1 GO aque-
ous dispersion with ultrasonication for 30 min. Then, 0.25 mL
hydrazine hydrate, 2 mL NH3·H2O and 170 mg AgNO3 were dis-
solved to the mixture by ultrasonication for 5 min. After being
placed at room-temperature for 12 h, the Ag-graphene hybrid
hydrogels were produced gradually.
All hydrogels were freeze-dried for characterization and elec-
trochemical experiments.
2.3. Characterization
The morphology of the as-prepared products was determined at
5 kV with a JSM-6700F field emission scanning electron microscopy
(FESEM). The specific surface area was determined by the BET
method using nitrogen physisorption at 77 K on a Micrometritics
ASAP 2020 analyzer. Powder X-ray diffraction (XRD) spectroscopy
was carried out on a Rigaku D/max 2550 V X-ray diffractometer
using Cu Kairradiation (k = 1.5406 ˚A). The operating voltage and cur-
rent were kept at 40 kV and 300 mA, respectively. Raman spectra
were recorded on a Renishaw in plus laser Raman spectrometer
with exc = 785 nm.
The cyclic voltammetry (CV) and electrochemical impedance
spectroscopy were measured using a two-probe method. Cylin-
drical hydrogel samples were sandwiched between two platinum
foils and connected to a Zahner electrochemical workstation (Zen-
nium) for cyclic voltammetry and electrochemical impedance
spectroscopy. The mass specific capacitances were calculated
from CV curves according to the equations Cspec = 4 C/M and
C/M = IdV/ mV, where I is the current, is the voltage scan rate,
m is the total mass of both electrodes and V is the cell voltage. Spe-
cific energy (E) and specific power (P) were also calculated by the
equations E = CspecV2/2 and P = E/ t, respectively, where t is the
discharging time.
3. Results and discussion
3.1. Morphology analysis
3D Ag-graphene hybrid hydrogels were produced through
reduction of GO in aqueous solutions with present of Ag+. As
shown in the inset of Fig. 1(b), the macroscopic Ag-graphene hydro-
gel was self-assembled from initial solutions containing GO and
AgNO3. The strong reducing effect of hydrazine hydrate greatly
contributed to the gelation reaction without heating and in rela-
tively high speed. With the purpose of further characterization, the
as-prepared hydrogels were freeze-dried to aerogels.
FESEM of the freeze-dried graphene hydrogels and Ag-graphene
hydrogels are shown in Fig. 1. The well-defined interconnected 3D
porous network of the produced hydrogels is presented in Fig. 1(a)
and (b). Similar to the pure graphene sample, the pores sizes of
the network shown in the microstructure of the hybrid samples
are in the range of submicrometer to several micrometers and the
solid walls of these pores are composed by randomly cross-linked
and intertwisted graphene nanosheets. Also as shown in Fig. 1(c)
and (d), Ag nanoparticles were scattered on the graphene skele-
ton and can be observed on both outer and inner walls of the
porous. As the electrode material, the three-dimensional graphene
skeleton network with various pores is beneficial for electrode to
fully contact with the electrolyte and the Ag nanoparticles on the
network can facilitate the transportation of electrons because of
its excellent conductivity. Moreover, BET measurement suggests
that the specific surface area of Ag-graphene hybrid hydrogel is
620 m2 g−1 while the value of pure graphene hydrogel is only
19 m2 g−1. The contrast suggests that the decoration of Ag nanopar-
ticles can effectively prevent the restacking of graphene sheets and

H. Quan et al. / Materials Science and Engineering B 178 (2013) 769–774 771
Fig. 1. FESEM images of the graphene-based hydrogels. (a) Low-magnification FESEM image of graphene hydrogel. (b) Low-magnification FESEM image of Ag-graphene
hydrogel. Inset is a photograph of the obtained Ag-graphene hydrogels. (c and d) High-magnification FESEM images of Ag-graphene hydrogel.
increase electrode material’s surface area accessible to the elec-
trolyte.
3.2. XRD and Raman analysis
The XRD patterns of GO and Ag-GH are shown in Fig. 2(a).
According to XRD patterns in Fig. 2(a), the sharp peak at around
2Â = 10.8◦ corresponds to the (0 0 1) reflection of GO, which revealed
a well-ordered layered structure of GO. Moreover, the (0 0 1)
reflection peak of GO is replaced by the (0 0 2) reflection peak
(2Â = 25◦), which suggests that GO is reduced into GH. In addi-
tion, all diffraction peaks of Ag-graphene hybrids were also shown
in Fig. 2(a). Each of these sharp peaks at 2Â = 38.1◦, 2Â = 44.3◦,
2Â = 64.4◦, 2Â = 77.4◦, 2Â = 82◦ corresponds respectively to the
reflection peaks (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), which is con-
sistent with cubic spinel Ag (PDF no. 00-004-0783) very well. This
perfect correspondence indicates that purity of the Ag nanopar-
ticles scattered on 3D graphene-based hydrogels network is very
high.
The Raman spectrum reflects the reduced mechanism of GO
to reduced graphene sheets in graphene-based hybrid hydrogels.
The existence of the G band (E2g vibrational mode in plane) and
D band (A1g breathing mode) is proved by all spectra in Fig. 2(b).
The ratios of the intensity of the D band and G band (ID/IG)
for Ag-graphene hybrid and graphene hydrogel are 1.101 and
1.535, respectively, and both of them are higher than the value of
graphite oxide (1.087). These results manifest a trend of decrease
in the average size of the sp2domains upon the reduction of GO
[52,53].
3.3. Mechanism analysis
The diagrammatic sketches of the formation for Ag-graphene
hybrid hydrogels and the schematic illustration of the fabrication
process of a symmetric supercapacitor based on this hybrid are
demonstrated in Fig. 3. Room-temperature synthesis and decora-
tion by Ag nanoparticles can be accomplished due to co-reducing
effect of l-ascorbic acid and hydrazine hydrate. The formation
Fig. 2. (a) XRD patterns of GO and Ag-graphene hydrogels. (b) Raman spectra of GO, graphene and Ag-graphene.

Fig. 3. Schematic illustration of the formation of Ag-graphene composite hydrogels and the fabrication process of a symmetric supercapacitor.
of the 3D structure is attributed to the partial overlapping or
coalescing of graphene sheets via supermolecular interactions
such as ␲–␲ stacking and hydrogen bonding [54,55]. With the
reduction proceeding, the amount of ␲–␲ stacking cross-links
between graphene sheets is increased due to the growth of the
␲-conjugated structures of reduced GO sheets. This kind of 3D
structured graphene network and the residual oxygenated func-
tional groups [56–58] on it have the capability to entrap abundant
water and then from uniform aquatic gels with the help of l-
ascorbic acid. In addition, with the reducing effect of l-ascorbic
acid and hydrazine hydrate, Ag+ was reduced into Ag nanoparti-
cles, which were scattered well on the graphene skeleton. Such
decoration can facilitate electron transport in terms of the high
conductivity of Ag and graphene. Besides, electrons could transfer
stereoscopically in Ag-graphene hydrogels through 3D graphene
networks while in 2D structures they would be confined to a plane.
Moreover, this unique 3D structure of graphene hydrogels enables
the fabrication of self-supported electrodes of supercapacitor. With
the advantages of 3D Ag-graphene hydrogels, this kind of electrodes
is beneficial for improving the overall energy density and efficiency
of the supercapacitor.
3.4. Electrochemical properties analysis
To evaluate the performance of 3D Ag-graphene hydrogels as
electrochemical electrode, cyclic voltammetry (CV) measurements
were performed, using a two-electrode cell in 0.5 M Na2SO4
solution. The scan-rate-dependent CVs of pure graphene and
Ag-graphene hybrid, with a range of scan rates of 50–2000 mV s−1
and a potential window of −0.2–0.8 V, are illustrated by Fig. 4.
The CV curves of these two kinds of materials keep rectangular
in shape at a relatively fast scan rate, confirming the formation of
an efficient electrical double layers and fast charge propagations
within the electrodes. Even at a scan rate of 2000 mV s−1, the
CV curve remains quasi-rectangular with only a little variance.
In comparison, the CVs of Ag-graphene hybrid exhibit a larger
rectangular area, indicating a more outstanding capacitive perfor-
mance than pure graphene. In addition, the specific capacitances of
Ag-graphene and pure graphen were calculated from the CV curves
at a scan rate of 5 mV s−1 on the basis of the total mass of active
materials on the two electrodes in the symmetric supercapacitors
[59,60]. The maximum specific capacitance for Ag-graphene is
147.1 F g−1, which is much higher than the value of 62.3 F g−1
for pure graphene. The specific energy and specific power of
Ag-graphene and graphene were also calculated from the CV
curves and it is worth to point out that at the scan rate of 5 mV s−1,
the specific energy of Ag-graphene is 20.4 Wh kg−1 and at the same
time the specific power value can maintain 368 W kg−1. In con-
trast, the specific energy of graphene is only 8.7 Wh kg−1 while the
value of specific power is only 43.5 W kg−1. Moreover, the cycling
performances of Ag-graphene and graphene were also tested by
successive charge-discharge cycles for 1000 times in the voltage
window from −0.2 to 0.8 and at a same scan rate of 50 mV s−1.
Fig. 5 shows the capacitance retention ratio of the symmetric
capacitor charged as a function of the cycle number. After 1000
cycles, although the specific capacitance of Ag-graphene retains
about 82.8% of the initial capacitance while that value of pure
graphene is 104.3%, the actual remanent specific capacitance of
Ag-graphene is 64.7 F g−1, which is still higher than the remanent
specific capacitance of pure graphene (41.3 F g−1). The superior
performances of Ag-graphene hybrid hydrogels clearly confirm the
success of such unique 3D structure in improvement of ion trans-
port and retaining a higher ion accessible surface area of energy
storage.
An electrochemical impedance spectroscopy (EIS) test was
conducted at a frequency range of 0.1 Hz to 100 kHz for further eval-
uation of electrochemical behaviors of the supercapacitor based on
Ag-graphene hydrogels, compared with the pure graphene based
supercapacitor. These examined capacitors all have same dimen-
sions. Fig. 6 shows the Nyquist plots of these two kinds of electrodes
Fig. 4. (a) Cyclic voltammograms of the supercapacitor based on graphene hydrogels. (b) Cyclic voltammograms of the supercapacitor based on Ag-graphene hydorgels.

H. Quan et al. / Materials Science and Engineering B 178 (2013) 769–774 773
Fig. 5. Cycle performance of the graphene (a) and Ag-graphene (b) symmetric supercapacitor with a voltage window of 1 V at scan rate of 50 mV s−1
in 0.5 M Na2SO4 aqueous
solution.
Fig. 6. Nyquistplots of graphene-based supercapacitor and Ag-graphene-based
supercacitor. Inset is a photograph of equivalent circuit.
and the measured impedance spectra were analyzed by fitting
method on the basis of equivalent circuit, which is given in the
inset of Fig. 6. In the high frequency region a semicircle arc has been
observed both in two lines and the charge transfer resistance can be
directly compared through the semicircle diameter. We calculated
that the electrode made of Ag-graphene had a charge transfer resis-
tance of 15.4 , which is much lower than pure graphene based
electrode (415 ), indicating the good conductivity of electrolyte
and very low internal resistance of the electrode. In the low fre-
quencies, the impedance plot turn into straight lines, and the shape
of Ag-graphene hybrid hydrogels are much more parallel to the
imaginary axis than the shape of pure graphene, which indicates
the attractive capacitive behavior of the device, representative of
the ion diffusion in the electrode structure [21,61]. In consider-
ation of this, Ag-graphene hydrogel is a more ideal candidate for
the electrode of supercapacitor than pure graphene.
4. Conclusion
In summary, 3D Ag-graphene hybrid hydrogels self-assembled
from graphene oxides were successfully prepared through
room-temperature synthesis. Then we fabricated a symmetric
supercapacitor based on this hybrid. An enhancement of capacitive
behavior was shown on such device compared to supercapacitor
based on pure graphene, owing to its unique 3D structure and the
high conductivity of Ag. This route of synthesis is environmen-
tal friendly and the produced Ag-graphene hydrogels would have
promising applications in various energy storage devices.
Acknowledgements
We gratefully acknowledge the financial support by Shanghai
Municipal Education Commission (No. 07SG37), Natural Science
Foundation of China (No. 51072034), Shanghai Leading Academic
Discipline Project (B603), the Cultivation Fund of the Key Scien-
tific and Technical Innovation Project (No. 708039), the Program
for Professor of Special Appointment (Eastern Scholar) at Shanghai
Institutions of Higher Learning, and the Program of Introducing
Talents of Discipline to Universities (No. 111-2-04).
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