SHEN Chen-FeiZHENG Ming-BoXUE Lu-PingLI Nian-WuLü Hong-LingZHANG Song-TaoCAO Jie-Ming＊,(Nanomaterials Research Institute,College of Materials Science and Technology,Nanjing University of Aeronautics and Astronautics,Nanjing 006,China)(National Laboratory of Microstructures,School of Electronic Science and Engineering,Nanjing University,Nanjing 0093,China)

Graphene-RuO2Nanocomposites:Hydrothermal Synthesis and Electrochemical Capacitance Properties

SHEN Chen-Fei1ZHENG Ming-Bo2XUE Lu-Ping1LI Nian-Wu1Lü Hong-Ling1ZHANG Song-Tao1CAO Jie-Ming＊,1
(1Nanomaterials Research Institute,College of Materials Science and Technology,Nanjing University of Aeronautics and Astronautics,Nanjing 210016,China)(2National Laboratory of Microstructures,School of Electronic Science and Engineering,Nanjing University,Nanjing 210093,China)

Graphene-ruthenium oxide (G-RuO2)nanocomposite was prepared via a facile hydrothermal method.The sample was characterized by X-ray diffraction (XRD),scanning electron microscopy (SEM),transmission electron microscopy (TEM)and energy dispersive X-ray Spectroscopy(EDS).SEM result reveals homogeneous distribution of RuO2particles on the layers of graphene sheets.TEM images demonstrate that the average size of RuO2particles is around 3 nm.The electrochemical properties of the sample were examined by cyclic voltammetry(CV)and galvanostatic charge-discharge(GC).The specific capacitance value of the sample is about 219.7 F·g-1at the current density of 1 A·g-1in 1 mol·L-1H2SO4.

ruthenium oxide;graphene;nanocomposites;electrochemical capacitor;hydrothermal method

0 Introduction

Electrochemical capacitors(ESCs)have attracted attention as electricity storage devices due to their higher power capability and longer cycle life compared with conventional double-layer capacitors[1-2].With long cyclelife and high specic capacitance,metal oxide and carbonaceous materials have been viewed as promising electrode materials for ESCs.Metal oxides such as RuO2[3-7],MnO2[8-9],NiO[10-11]and Co3O4[12-13]have been investigated for their electrochemical behaviors.Among them,electrochemicalcapacitorsbased on RuO2electrode have been widely investigated due to their superior specfic capacitance,high electrochemical reversibility,and long cycle life[14-16].

In terms ofcarbonaceous materials,carbon nanotubes[17-18],carbon fibers[19],carbon aerogels[20]and activated carbons[21]have been studied for their electrochemical behaviors.Graphene,a new kind of carbonaceous material,has been reported with unique properties and thus has drew great interests[22-23].Vivekchand et al.[24]evaluated the capacitive behaviors of graphene and a capacitive performance of graphene up to 117 F·g-1in aqueous H2SO4solution was obtained.Stoller et al.[25]studied the electrochemical behaviors of graphene as the electrode of ESCs and a specfic capacitance of 135 F·g-1in 5.5 mol·L-1KOH aqueous electrolyte was achieved.Du et al.[26]used graphene as the electrode of ESCs to obtain a stable specfic capacitance of 150 F·g-1under specific current of 0.1 A·g-1for 500 cycles of charge/discharge.Recently,Du et al.[27]prepared functionalized graphene sheets(FGS)using graphite oxide(GO)as precursor via a low-temperature thermal exfoliation approach in air and the products exhibited good electrochemical behaviors.

To harness the good electrochemical properties of both metal oxide and graphene sheets,one possible route is to integrate these two kinds of materials into the electrodes of ESCs.The capacitive performance of the composites will be enhanced largely because most of the metal oxide can contribute pseudo-capacitance to the total capacitance apart from the double-layer capacitance from graphene sheets[28-29].Yan et al.[30]synthesized graphene-MnO2composites through the self-limiting deposition of nanoscale MnO2on the surface of graphene under microwave irradiation and studied the electrochemical behaviors of the products.Son et al.[31]fabricated NiO Resistive Random Access Memory (RRAM)nanocapacitor array on a graphene sheet.In this work,we synthesized graphene-RuO2nanocomposite with 15wt%RuO2loading via a facile hydrothermalmethod and studied electrochemical characteristics of the products in an aqueous electrolyte.

1 Experimental

1.1 Preparation of FGS

GO was prepared by Hummers method[32].To obtain FGS,certain amount of GO was thermally exfoliated at 300℃for 5 min under air atmosphere and denoted as FGS300[27].

1.2 Preparation of G-RuO2

All reagents were analytical grade and used without further purification.To prepare the sample with 15wt%RuO2loading of the composite,60 mg FGS300 was dissolved in 60 mL distilled water.After vigorous stirring,a stable suspension was obtained.1.7 mL of RuCl3(0.048 mol·L-1)was then dropped into the suspension with ultrasonication for 30 min.Then,the solution was transferred into a Teflon-lined autoclave with a capacity of 100 mL,and then the autoclave was sealed and maintained at 180℃for 6 h.After cooling to room temperature,the black product was washed with distilled water for several times,and dried under vacuum at 50℃for 48 h.

1.3 Characterization

The dimension and the morphology of the sample were observed by SEM (Gemini LEO1530)and TEM(JEOL JEM-2100).Composition of the samples was analyzed using TEM attached energy dispersive X-ray spectroscopy(EDS).XRD patterns were recorded by a BrukerD8-Advance diffractometerusing Cu Kα radiation (λ=0.15418 nm)with the scanning 2θ angles ranging from 10°to 80°,a graphitic monochrom at 40 kV and 40 mA.

1.4 Electrochemical tests

Cyclic voltammetry(CV)and galvanostatic chargedischarge (GC)were done in a three-electrode experimentalsetup using1 mol·L-1H2SO4as electrolyte.The prepared electrode,platinum foil,and saturated calomel electrode (SCE)were used as the working,counter,and reference electrodes in 1 mol·L-1H2SO4aqueous solution.The preparation of the working electrode for the three-electrode system was as follows:80wt%G-RuO2sample,15wt%acetylene black,and 5wt%polytetrafluoroethylene were well mixed,then the mixture was pressed onto a stainless steel grid under 10MPa.Each electrode contained 5.0 mg of G-RuO2(active material).The CV and GC measurements were carried out on CHI 440A electrochemical workstation at room temperature,the potential ranging from 0 V to 1.0 V (vs.SCE).The GC measurement was carried out in current density range of 0.5～5 A·g-1.

2 Results and discussion

Fig.1 shows the XRD patterns of FGS300 and GRuO2nanocomposite.The characteristic (002)peak of FGS300 is clearly observed on the XRD patterns of both FGS300 and G-RuO2[27].From a comparison of these two XRD patterns,no obvious diffraction peaks corresponding to RuO2are found in G-RuO2.It is supposed to be due to the small size of the RuO2particles.

Fig.1 XRD patterns of FGS300 and G-RuO2nanocomposite

Fig.2(a)shows the SEM image of FGS300.The wrinkle,a characteristic feature of graphene sheets,is observed.RuO2particles are observed decorated on the graphene sheets from Fig.2(b)and Fig.2(c).The uniform distribution of RuO2particles on graphene sheets guarantees the good electrochemical properties of G-RuO2[33].The HRTEM image(Fig.3(d))reveals the good crystalline nature of the nanoparticles.Besides,the size of the RuO2particles is observed to be around 3 nm,which explains the low intensity of diffraction peaks of RuO2in the XRD pattern of the composite.The selected area electron diffraction (SAED)pattern in Fig.2(e)shows a ring pattern,indicating that the obtained RuO2particles are polycrystalline,which is consistent with the HRTEM observation.Moreover,the first ring matches the (110)plane of RuO2.The other rings are very close to both structures of Ru and RuO2,which may be due to incomplete oxidation of RuCl3[34].The EDS spectrum (Fig.1(f))reveals the existence of Ru and O species,of which the Ru and O elements should be the main contribution of RuO2phase in the composite.

Fig.2 SEM images of FGS300 and G-RuO2nanocomposite(a,b),TEM and HRTEM images of G-RuO2nanocomposite(c,d),SAED pattern of G-RuO2composite(e),EDS spectrum of G-RuO2composite(f)

CV and GC were used to investigate electrochemical behaviors of G-RuO2in a threeelectrode system in 1 mol·L-1H2SO4electrolyte.Fig.3(a)shows the CV curves of G-RuO2at different scan rates(5～50 mV·s-1)in the potential range from 0 to 1.0V.Broad current peaks and almost mirror quasirectangular are observed in all the CV curves over the CV potential range.This indicates that the obtained GRuO2nanocomposite exhibits high redox reversibility and obvious pseudocapacitance character.[35]

Fig.3 (a)Cyclic voltammograms of G-RuO2obtainedat different scan rates.(b)Galvanostatic discharge curves of G-RuO2obtainedat different current densities and FGS300 at a current density of 1 A·g-1

Galvanostatic cycling of G-RuO2is performed at a current density of 0.5～5 A·g-1as shown in Fig.3(b).The specific capacitance of G-RuO2(Cs,composite)could be calculated from the slope of the charge-discharge curves,according to the equation:[36],where I is the current of charge-discharge,Δt is the time of discharge,m is the mass of active materials in the working electrode,and ΔV is 1.0 V.The calculated specfic capacitances of G-RuO2at different scan rates and the capacitance retention ofthe samples are listed in Table 1.The results demonstrate high capacitance retention of the sample.Besides,at the current density of 1 A·g-1,G-RuO2exhibits capacitance value of 219.7 F·g-1,which is much higher than that of FGS300(119.1 F·g-1).The specific capacitance of RuO2(Cs,Ru)could be calculated based on the equation:[33],where ωRuis the weight fraction of RuO2within the nanocomposite,Cs,FGS300is the specific capacitance of FGS300.At the current density of 1 A·g-1,Cs,Ruis calculated to be 789.8 F·g-1.

The distinguishing electrochemical behaviors of G-RuO2are due to the excellent electrochemical properties of FGS300 and the contribution of pseudocapacitance by RuO2.With nanoporous structure,the obtained FGS300 has fully accessible surface to electrolyte ion because both sides of a broad range of graphene sheets can be exposed to the electrolyte and contribute to capacitance[27].Furthermore,the residual functional groups on the surface of FGS may improve the hydrophilicity of electrode,which helps the RuO2particles to be loaded onto the surface of FGS300.The RuO2and the residual functional groups on graphene sheets all contribute to the pseudocapacitance and thus enhance the overall capacitance value of the composites.

Table 1 Specific capacitances of G-RuO2(Cs,composite)obtained in 1 mol·L-1H2SO4from GC method andcapacitance retention for G-RuO2

3 Conclusion

In summary,graphene-RuO2nanocomposite was prepared via a facile hydrothermal method.The SEM and TEM characterizations reveal homogeneous distribution of RuO2particles on graphene sheets.High capacitance value and capacitance retention of the composite are shown by capacitive behaviors of G-RuO2.

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石墨烯-氧化釕納米復(fù)合材料的水熱法合成及電化學(xué)電容性能

沈辰飛1鄭明波2薛露平1李念武1呂洪嶺1張松濤1曹潔明＊,1

(1南京航空航天大學(xué)材料科學(xué)與技術(shù)學(xué)院納米材料研究所，南京 210016)(2南京大學(xué)微結(jié)構(gòu)國家實(shí)驗(yàn)室電子科學(xué)與工程學(xué)院，南京 210093)

通過水熱法制備了石墨烯-氧化釕(G-RuO2)納米復(fù)合材料。對樣品進(jìn)行了X射線衍射(XRD)，掃描電子顯微鏡(SEM)，透射電子顯微鏡(TEM)和能量色散譜(EDS)表征。SEM結(jié)果表明氧化釕粒子均勻地分散在石墨烯層片上。TEM結(jié)果顯示氧化釕納米粒子的平均粒徑約為3 nm。對樣品進(jìn)行了循環(huán)伏安和充放電性能測試，結(jié)果表明在1 A·g-1的電流密度下，樣品在H2SO4(1 mol·L-1)溶液中具有219.7 F·g-1的比電容。

氧化釕；石墨烯；納米復(fù)合材料；電化學(xué)電容器；水熱

O613.71；O614.82+1

：A

：1001-4861(2011)03-0585-05

2010-08-03。收修改稿日期：2010-11-10。

江蘇省自然科學(xué)基金(No.BK2006195)資助項(xiàng)目。

＊通訊聯(lián)系人。 E-mail：jmcao@nuaa.edu.cn