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Materials Science and Engineering B 178 (2013) 769– 774

Contents lists available at SciVerse ScienceDirect

Materials Science and Engineering B

jou rn al hom ep age: www.elsev ier .com/ locate /mseb

oom-temperature synthesis of 3-dimentional Ag-graphene hybrid hydrogelith promising electrochemical properties

aocheng Quana,b, Yuanlong Shaoa,b, Chengyi Houa,b, Qinghong Zhanga,ongzhi Wanga,∗, Yaogang Lib,∗∗

State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Donghua University, 201620, People’s Republic ofhinaEngineering 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

rticle history:eceived 24 October 2012eceived in revised form 26 January 2013ccepted 11 March 2013vailable online 8 April 2013

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 3DAg-graphene hybrid hydrogel is facile and its application in macroscopic devices is more convenientthan 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 uniquestructure and combination with Ag nanoparticles made this material exhibit better electrochemical per-

eywords:upercapacitorraphene-dimentionalg nanoparticles

formance compared with the pure graphene. Thus, the obtained Ag-graphene hybrid hydorgels could bewidely used in various energy storage devices.

© 2013 Elsevier B.V. All rights reserved.

anocompositeslectrochemical properties

. Introduction

As a promising candidate for alternative energy storage devices,upercapacitors, also called electrochemical capacitors or ultraca-acitors [1,2], have generated great interest of researchers due toheir high rate capability, long cycle life, pulse power supply, simplenergy storage mechanisms, high dynamic of charge propagationnd low maintenance cost [2–5]. These excellent properties canontribute to applications such as consumer electronics, memoryack-up systems and industrial power and energy management.

In general, supercapacitors can be divided into two categories onhe basis of the energy storage mechanism: electrical double layerapacitors (EDLCs) and pseudo-capacitors [6]. Transition metalxides or hydroxides [7,8] and conducting polymers [9,10] are usu-lly used as the electrodes for pseudo-capacitors, in which fast andeversible faradic processes take place due to the electro-activepecies and because of this, much higher pseudo-capacitance cane achieved. However, pseudo-capacitors also reveal some draw-

acks. The lack of cycling stability is one of such drawbacks dueo the easily damaged structure of the materials during the redoxrocess, 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).

921-5107/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.mseb.2013.03.004

[11]. Different from pseudo-capacitors, the capacitance of EDLCsstems from the pure electrostatic charge accumulated at the elec-trode/electrolyte interface. Carbon-based materials dominate theelectrodes of this kind of capacitors because of their excellentproperties on surface area, electrical conductivity and mechani-cal strength [12], which perfectly compensates for the deficienciesof pseudo-capacitors. Activated carbons (ACs) and carbon nano-tubes (CNTs) are mostly used as electrodes of such capacitorsamong carbon-based materials, owing to their excellent physicaland chemical properties. However, the energy density and ratecapability 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-performanceEDLCs [13,14]. Graphene, with the structure of a flat monolayer ofcarbon atoms tightly packed into a two-dimensional (2D) honey-comb lattice [15], reveals a series of intriguing properties such asoutstanding mechanical strength, superb chemical and thermal sta-bility, excellent electrical conductivity and high surface-to-volumeratio [16–20]. These fascinating characteristics make graphenebecome the most ideal material used as electrodes in EDLCs.

In recent years, more and more researchers have been focusedon graphene and graphene-based hybrid materials used as elec-

trodes for supercapacitors [21–24]. Combination with inorganicnanoparticles [25,26], carbon nanotubes [27] or polymers [28],graphene-based hybrids unfold an improved performance on avariety of physical and chemical properties. Besides, a lot of

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70 H. Quan et al. / Materials Science

esearchers have demonstrated that two-dimensional structureuch as papers, thin films can be assembled by graphene-basedlatelets [19,29–32]. In most cases, however, a large portion ofurface area of 2D graphene sheets becomes inaccessible dur-ng fabrication of such graphene films, which is attributed tohe tendency of individual graphene sheet to irreversibly aggre-ate and restack as a result of van der Waals interaction [33].ecreased surface area obstructs the infiltration of electrolytend the transportation of ions, lowering their energy density andimiting the potential applications of graphene materials in elec-rodes of supercapacitors. Therefore, integration of 2D nanoscaleuilding blocks into 3-dimensional (3D) macroscopic structures isttracting much attention since it is an essential step in explor-ng the advanced properties of individual 2D sheets for practicalpplications [33–36]. In addition, the methods of synthesis ofraphene and graphene-based materials have been developed sincehe report of isolated graphene prepared by simple mechanicalleavage of graphite crystals [37]. These methods include epitaxialrowth on metals or SiC [38,39], chemical exfoliation of graphitexide [32,36,40], liquid-phase ultrasonic exfoliation of graphiteowder [41], expanded graphite by oxidation or intercalation [42]nd chemical vapor deposition (CVD) growth on metal substrates43–47]. Nonetheless, most of these methods need relatively highemperature condition, which costs a lot of energy and increaseshe carbon emission. Literatures on synthesis of 2D graphene andraphene-based materials at low temperature are finite [48–50]nd let alone the synthesis of 3D graphene materials.

In this paper, we attempt to prepare self-assembled Ag-raphene hybrid hydrogels with 3D macroporous structure in aoom-temperature route. We also study the mechanisms of theormation of the 3D graphene-based hybrid hydrogels and fab-icate them into electrodes of supercapacitor to investigate theirlectrochemical performance.

. Experimental

.1. Preparation of graphene oxide

According to Hummer’s method [51], graphene oxide (GO) wasrepared using analytical grade reagents without further purifica-ion. The process of synthesis can be divided into these steps: 6 gatural flake graphite powder was added to 138 mL cold (0 ◦C) con-entrated H2SO4. 18 g KMnO4 was gradually added to the mixturef H2SO4 and graphite at 0 ◦C with stirring. After adding KMnO4,he mixture was stirred at 35 ◦C for 2 h. 276 mL distilled wateras slowly added to the mixture and then the temperature wasaintained below 100 ◦C for 15 min. 840 mL H2O2 solution was

hen added to the mixture. After that, the mixture was filtered andashed with 1500 mL HCl aqueous solution to remove metal ions.

inally, the product was thoroughly washed with distilled water.

.2. Synthesis of Ag-graphene hybrid hydrogels

Graphene hydrogel (GH) was prepared by the following pro-edure with the reducing reagents of l-ascorbic and hydrazineydrate. In short, 0.24 g l-ascorbic acid was added to 20 mL

mg mL−1 GO aqueous dispersion loaded in a glass vial with 30 minltrasonication. Then, 0.25 mL hydrazine hydrate was added to theixture and dispersed by unltrasonication for 5 min. After ultra-

onication, the mixture was placed at temperature of 25 ◦C for about2 h without any disturbance to produce GH. The reduced graphene

xide hydrogels were immersed in water to remove impurities suchs residual salts and acids and cleaned for further investigation.

Ag-graphene hybrid hydrogels were prepared throughhe procedure as same as synthesis of graphene hydrogels.

gineering B 178 (2013) 769– 774

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 mLhydrazine hydrate, 2 mL NH3·H2O and 170 mg AgNO3 were dis-solved to the mixture by ultrasonication for 5 min. After beingplaced at room-temperature for 12 h, the Ag-graphene hybridhydrogels 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 at5 kV with a JSM-6700F field emission scanning electron microscopy(FESEM). The specific surface area was determined by the BETmethod using nitrogen physisorption at 77 K on a MicrometriticsASAP 2020 analyzer. Powder X-ray diffraction (XRD) spectroscopywas carried out on a Rigaku D/max 2550 V X-ray diffractometerusing Cu Kairradiation (k = 1.5406 A). The operating voltage and cur-rent were kept at 40 kV and 300 mA, respectively. Raman spectrawere recorded on a Renishaw in plus laser Raman spectrometerwith �exc = 785 nm.

The cyclic voltammetry (CV) and electrochemical impedancespectroscopy were measured using a two-probe method. Cylin-drical hydrogel samples were sandwiched between two platinumfoils and connected to a Zahner electrochemical workstation (Zen-nium) for cyclic voltammetry and electrochemical impedancespectroscopy. The mass specific capacitances were calculatedfrom CV curves according to the equations Cspec = 4 C/M andC/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 theequations E = CspecV2/2 and P = E/�t, respectively, where �t is thedischarging time.

3. Results and discussion

3.1. Morphology analysis

3D Ag-graphene hybrid hydrogels were produced throughreduction of GO in aqueous solutions with present of Ag+. Asshown in the inset of Fig. 1(b), the macroscopic Ag-graphene hydro-gel was self-assembled from initial solutions containing GO andAgNO3. The strong reducing effect of hydrazine hydrate greatlycontributed to the gelation reaction without heating and in rela-tively high speed. With the purpose of further characterization, theas-prepared hydrogels were freeze-dried to aerogels.

FESEM of the freeze-dried graphene hydrogels and Ag-graphenehydrogels are shown in Fig. 1. The well-defined interconnected 3Dporous network of the produced hydrogels is presented in Fig. 1(a)and (b). Similar to the pure graphene sample, the pores sizes ofthe network shown in the microstructure of the hybrid samplesare in the range of submicrometer to several micrometers and thesolid walls of these pores are composed by randomly cross-linkedand 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 theporous. As the electrode material, the three-dimensional grapheneskeleton network with various pores is beneficial for electrode tofully contact with the electrolyte and the Ag nanoparticles on thenetwork can facilitate the transportation of electrons because ofits excellent conductivity. Moreover, BET measurement suggests

that the specific surface area of Ag-graphene hybrid hydrogel is620 m2 g−1 while the value of pure graphene hydrogel is only19 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

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ig. 1. FESEM images of the graphene-based hydrogels. (a) Low-magnification FEydrogel. Inset is a photograph of the obtained Ag-graphene hydrogels. (c and d) H

ncrease electrode material’s surface area accessible to the elec-rolyte.

.2. XRD and Raman analysis

The XRD patterns of GO and Ag-GH are shown in Fig. 2(a).ccording to XRD patterns in Fig. 2(a), the sharp peak at around� = 10.8◦ corresponds to the (0 0 1) reflection of GO, which revealed

well-ordered layered structure of GO. Moreover, the (0 0 1)eflection peak of GO is replaced by the (0 0 2) reflection peak2� = 25◦), which suggests that GO is reduced into GH. In addi-ion, all diffraction peaks of Ag-graphene hybrids were also shownn Fig. 2(a). Each of these sharp peaks at 2� = 38.1◦, 2� = 44.3◦,� = 64.4◦, 2� = 77.4◦, 2� = 82◦ corresponds respectively to theeflection peaks (1 1 1), (2 0 0), (2 2 0), (3 1 1), (2 2 2), which is con-

istent with cubic spinel Ag (PDF no. 00-004-0783) very well. Thiserfect correspondence indicates that purity of the Ag nanopar-icles scattered on 3D graphene-based hydrogels network is veryigh.

Fig. 2. (a) XRD patterns of GO and Ag-graphene hydrogels.

mage of graphene hydrogel. (b) Low-magnification FESEM image of Ag-grapheneagnification FESEM images of Ag-graphene hydrogel.

The Raman spectrum reflects the reduced mechanism of GOto reduced graphene sheets in graphene-based hybrid hydrogels.The existence of the G band (E2g vibrational mode in plane) andD 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 and1.535, respectively, and both of them are higher than the value ofgraphite oxide (1.087). These results manifest a trend of decreasein 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-graphenehybrid hydrogels and the schematic illustration of the fabrication

process of a symmetric supercapacitor based on this hybrid aredemonstrated in Fig. 3. Room-temperature synthesis and decora-tion by Ag nanoparticles can be accomplished due to co-reducingeffect of l-ascorbic acid and hydrazine hydrate. The formation

(b) Raman spectra of GO, graphene and Ag-graphene.

772 H. Quan et al. / Materials Science and Engineering B 178 (2013) 769– 774

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Fig. 3. Schematic illustration of the formation of Ag-graphene comp

f the 3D structure is attributed to the partial overlapping oroalescing of graphene sheets via supermolecular interactionsuch as �–� stacking and hydrogen bonding [54,55]. With theeduction proceeding, the amount of �–� stacking cross-linksetween graphene sheets is increased due to the growth of the-conjugated structures of reduced GO sheets. This kind of 3D

tructured graphene network and the residual oxygenated func-ional groups [56–58] on it have the capability to entrap abundantater and then from uniform aquatic gels with the help of l-

scorbic acid. In addition, with the reducing effect of l-ascorbiccid and hydrazine hydrate, Ag+ was reduced into Ag nanoparti-les, which were scattered well on the graphene skeleton. Suchecoration can facilitate electron transport in terms of the highonductivity of Ag and graphene. Besides, electrons could transfertereoscopically in Ag-graphene hydrogels through 3D grapheneetworks while in 2D structures they would be confined to a plane.oreover, this unique 3D structure of graphene hydrogels enables

he fabrication of self-supported electrodes of supercapacitor. Withhe advantages of 3D Ag-graphene hydrogels, this kind of electrodess beneficial for improving the overall energy density and efficiencyf the supercapacitor.

.4. Electrochemical properties analysis

To evaluate the performance of 3D Ag-graphene hydrogels aslectrochemical electrode, cyclic voltammetry (CV) measurementsere performed, using a two-electrode cell in 0.5 M Na2SO4

olution. The scan-rate-dependent CVs of pure graphene andg-graphene hybrid, with a range of scan rates of 50–2000 mV s−1

nd a potential window of −0.2–0.8 V, are illustrated by Fig. 4.he CV curves of these two kinds of materials keep rectangular

n shape at a relatively fast scan rate, confirming the formation ofn efficient electrical double layers and fast charge propagationsithin the electrodes. Even at a scan rate of 2000 mV s−1, theV curve remains quasi-rectangular with only a little variance.

ig. 4. (a) Cyclic voltammograms of the supercapacitor based on graphene hydrogels. (b)

ydrogels and the fabrication process of a symmetric supercapacitor.

In comparison, the CVs of Ag-graphene hybrid exhibit a largerrectangular area, indicating a more outstanding capacitive perfor-mance than pure graphene. In addition, the specific capacitances ofAg-graphene and pure graphen were calculated from the CV curvesat a scan rate of 5 mV s−1 on the basis of the total mass of activematerials on the two electrodes in the symmetric supercapacitors[59,60]. The maximum specific capacitance for Ag-graphene is147.1 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 ofAg-graphene and graphene were also calculated from the CVcurves 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 sametime 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 thevalue of specific power is only 43.5 W kg−1. Moreover, the cyclingperformances of Ag-graphene and graphene were also tested bysuccessive charge-discharge cycles for 1000 times in the voltagewindow 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 symmetriccapacitor charged as a function of the cycle number. After 1000cycles, although the specific capacitance of Ag-graphene retainsabout 82.8% of the initial capacitance while that value of puregraphene is 104.3%, the actual remanent specific capacitance ofAg-graphene is 64.7 F g−1, which is still higher than the remanentspecific capacitance of pure graphene (41.3 F g−1). The superiorperformances of Ag-graphene hybrid hydrogels clearly confirm thesuccess of such unique 3D structure in improvement of ion trans-port and retaining a higher ion accessible surface area of energystorage.

An electrochemical impedance spectroscopy (EIS) test wasconducted at a frequency range of 0.1 Hz to 100 kHz for further eval-

uation of electrochemical behaviors of the supercapacitor based onAg-graphene hydrogels, compared with the pure graphene basedsupercapacitor. These examined capacitors all have same dimen-sions. Fig. 6 shows the Nyquist plots of these two kinds of electrodes

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 supercapacsolution.

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ig. 6. Nyquistplots of graphene-based supercapacitor and Ag-graphene-basedupercacitor. Inset is a photograph of equivalent circuit.

nd the measured impedance spectra were analyzed by fittingethod on the basis of equivalent circuit, which is given in the

nset of Fig. 6. In the high frequency region a semicircle arc has beenbserved both in two lines and the charge transfer resistance can beirectly compared through the semicircle diameter. We calculatedhat the electrode made of Ag-graphene had a charge transfer resis-ance of 15.4 �, which is much lower than pure graphene basedlectrode (415 �), indicating the good conductivity of electrolytend very low internal resistance of the electrode. In the low fre-uencies, the impedance plot turn into straight lines, and the shapef Ag-graphene hybrid hydrogels are much more parallel to themaginary axis than the shape of pure graphene, which indicateshe attractive capacitive behavior of the device, representative ofhe ion diffusion in the electrode structure [21,61]. In consider-tion of this, Ag-graphene hydrogel is a more ideal candidate forhe electrode of supercapacitor than pure graphene.

. Conclusion

In summary, 3D Ag-graphene hybrid hydrogels self-assembledrom graphene oxides were successfully prepared throughoom-temperature synthesis. Then we fabricated a symmetricupercapacitor based on this hybrid. An enhancement of capacitiveehavior was shown on such device compared to supercapacitor

ased on pure graphene, owing to its unique 3D structure and theigh conductivity of Ag. This route of synthesis is environmen-al friendly and the produced Ag-graphene hydrogels would haveromising applications in various energy storage devices.

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itor with a voltage window of 1 V at scan rate of 50 mV s−1 in 0.5 M Na2SO4 aqueous

Acknowledgements

We gratefully acknowledge the financial support by ShanghaiMunicipal Education Commission (No. 07SG37), Natural ScienceFoundation of China (No. 51072034), Shanghai Leading AcademicDiscipline Project (B603), the Cultivation Fund of the Key Scien-tific and Technical Innovation Project (No. 708039), the Programfor Professor of Special Appointment (Eastern Scholar) at ShanghaiInstitutions of Higher Learning, and the Program of IntroducingTalents of Discipline to Universities (No. 111-2-04).

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