research article cauliflower-like co /three-dimensional...

Research ArticleCauliflower-Like Co3O4Three-Dimensional GrapheneComposite for High Performance Supercapacitor Applications

Huili Liu12 Xinglong Gou2 Yi Wang1 Xuan Du1 Can Quan3 and Tao Qi1

1National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology Institute of Process EngineeringChinese Academy of Sciences Beijing 100190 China2College of Chemistry and Chemical Engineering China West Normal University 1 Shida Road Nanchong 637000 China3Division of Chemistry National Institute of Metrology Beijing 100013 China

Correspondence should be addressed to Yi Wang wangyiipeaccn and Can Quan superfluidcanhotmailcom

Received 5 December 2014 Accepted 11 February 2015

Academic Editor Jiayu Wan

Copyright copy 2015 Huili Liu et alThis is an open access article distributed under the Creative Commons Attribution License whichpermits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Cauliflower-like Co3O4three-dimensional (3D) graphene nanocomposite material was synthesized by a facile two-step synthesis

route (heat reduction of graphite oxide (GO) and hydrothermal synthesis of Co3O4) The phase composition morphology

and structure of the as-obtained products were characterized by scanning electron microscopy (SEM) transmission electronmicroscope (TEM) and X-ray diffraction (XRD) Electrochemical properties as supercapacitor electrode materials weresystematically investigated by cyclic voltammetry (CV) and constant current charge-discharge tests It was found that the Co

3O43D

graphene composite showed a maximum specific capacitance of 863 F gminus1 which was obtained by means of CVs at a scan rate of1mV sminus1 in 6M KOH aqueous solution Moreover the composite exhibited improved cycling stability after 1000 cycles The goodsupercapacitor performance is ascribed to the combination of 3D graphene and cauliflower-like Co

3O4 which leads to a strong

synergistic effect to remarkably boost the utilization ratio of Co3O4and graphene for high capacitance

1 Introduction

With the impending energy crisis the concerns about theenvironment are increasing The research on alternativeenergy conversion and storage systems which have highefficiency low cost and environmental benignity is urgentlycalled for Supercapacitors also known as electrochemicalcapacitors or ultracapacitors have been intensively investi-gated in the fields of energy storage and conversion [1 2]Compared with the conventional capacitors and secondarybatteries supercapacitors can provide high power densitystrong cycle stability and remarkable pulse charge-dischargeproperties [1ndash5] They are being employed in different appli-cations [1 6ndash8] such as mobile electronic devices back-up power supplies and hybrid electric vehicles The charge-storage mechanism in supercapacitor is usually electricaldouble-layer and Faradic capacitor [7 9] The electricaldouble-layer capacitor (EDLC) roots in charge separation at

the electrodesolution interface whereas the Faradic pseudo-capacitance arises from the reversible redox reactions occur-ring within the supercapacitor electrode material [10 11] Asthe core component of supercapacitors electrode materialplays an important role in determining the performance ofelectrochemical capacitors Recently tremendous attentionhas been focused on the development of new materials forsupercapacitor electrodes with higher specific capacitanceand better cyclic life whilst using environmentally benignmaterials in easily processable ways

Among the major materials studied for pseudo-capacitorelectrodes transition metal oxides such as RuO

2[12] InO

2

[13] NiO [14] MnO2[15] Co

3O4[16 17] and SnO

2[18]

have attracted much attention due to their high theoreticalcapacities and promising potential RuO

2is known as an

ideal electrode material for supercapacitors which showsoutstanding electrochemical properties In some reportsthe specific capacitance (SC) of RuO

2 which has a higher

Hindawi Publishing CorporationJournal of NanomaterialsVolume 2015 Article ID 874245 9 pageshttpdxdoiorg1011552015874245

2 Journal of Nanomaterials

conductivity than carbon materials and remarkable stabilityshows as high as 720ndash768 F gminus1 [19 20] However the highcost and toxic nature of ruthenium substantially limits itscommercial application [21] Hence the relatively inexpen-sive metal oxides have been studied widely Among themcobalt oxide is considered as a promising electrode materialfor supercapacitors due to its relatively low cost well-definedredox behavior ultrahigh theoretical SC (sim3560 F gminus1) andenvironmental friendliness [22 23] For instance it hasbeen reported that the CoO

119909xerogels thermally treated at

150∘C showed a good capacitive behavior with a SC value of291 F gminus1 [24] Tummala et al [7] have synthesized nanos-tructured Co

3O4by plasma spray technique and studied its

capacitance propertiesThe results showed a high SC value of162 F gminus1 in 6M KOH electrolyte

Graphene as a new carbon material has attracted muchattention since it was isolated from layer-by-layer graphite in2004 [25] It is a promising electrode candidate for EDLCsdue to its superb characteristics including the excellentmechanical strength and chemical stability high electricaland thermal conductivity and large surface area (a theoreticalvalue of 2630m2 gminus1) [25] But the serious agglomerationand restacking between individual graphene sheets result inlimited surface utilization percentage and a lower capacitancethan the anticipated value [26] In our recent work [2728] three-dimensional (3D) graphene (strictly speaking itshould be reduced graphene oxide (RGO) but it is stillcalled graphene here for convenience) was acquired by heatreduction of graphite oxide in vacuum which has a higherspecific surface area (487m2 gminus1) Different fromDongrsquoswork[29] it has a predominant mesoporosity which centers at4 nm and thesemesopores can provide a high specific surfacearea while allowing good mass transport in an electrodeprocess [30]

Recently many researchers have constructed a graphene-based metal oxide composite structure [17 31ndash34] It wasfound that these composites showed improved performancewith higher electron transport rate electrolyte contact areaand structural stability It can be attributed to the specialcomposite structure in which metal oxides are attached onthe surface or intercalated into the interlayer of large patchesof graphene [31] Yan et al [17] reported that graphenenanosheet (GNS)Co

3O4composite has a maximum specific

capacitance of 2432 F gminus1 at a scan rate of 10mV sminus1 in6M KOH aqueous solution However the composite of 3Dgraphene which was reported in our previous work [27] andCo3O4has never been investigated so far Herein a facile

hydrothermal strategy was proposed to deposit cauliflower-like nanostructured Co

3O4on 3D graphene nanosheets

Cyclic voltammetry (CV) and galvanostatic charge-dischargemeasurements were used to assess the electrochemical prop-erties of the Co

3O43D graphene composite in KOH solu-

tions

2 Experimental

21 Materials Preparation In this study deionizedwater was used throughout Cobalt nitrate hexahydrate

(Co(NO3)2sdot6H2O) ammonia (37 NH

3sdotH2O) and potas-

sium hydroxide (KOH) were obtained fromXilong ChemicalEngineering Co Ltd Hydrochloric acid (37) and ethanolwere supplied by Chemical Plant of Beijing All the reagentsused in this experiment were of analytical grade and withoutfurther purification

Graphene oxide was prepared from nature graphite bya modified Hummersrsquo method 3D graphene material wasobtained by reducing graphite oxide powder in a glass bottleunder vacuum followed by heating to 250∘C from roomtemperature [27]

Co3O43D graphene composite was synthesized as fol-

lows First 15mg of 3D graphene was dispersed in 15mLethanol with the assistance of ultrasonication for 60minThen 05mmol Co(NO

3)2sdot6H2O and 34 120583L of 37 ammonia

were added to the 3D graphene suspension simultaneouslyThe as-obtained mixture was magnetically stirred for 20minAfter that the above suspension was loaded into a 25mL ofTeflon-lined stainless steel autoclave and heated at 170∘C for5 h The obtained black products were washed by centrifu-gation for several times using ethanol and deionized waterto remove excess ions and then dried in a vacuum oven at60∘C Afterwards the material was calcined at 350∘C for 3 hin air and then cooled to room temperature For comparisonCo3O4was also prepared by the same steps without the

addition of 3D graphene

22 Characterization Methods The crystallographic struc-tures of the samples were determined by X-ray diffractionsystem (XRD Xrsquo Pert-PRO MPD) equipped with Cu K

120572

(120582 = 15406 A) radiation The morphological analysis wasconducted using scanning electron microscope (SEM JEOLJSM-6700F) and transmission electron microscope (TEMJEOL JEM-2011 200 kV) The specific surface area and porevolume of the samples were determined by analyzing thestandard nitrogen adsorption isothermmeasured at 77 K Anelectrochemical workstation system (CHI760D ChenhuaShanghai) was employed to study the electrochemical prop-ertiesThe electrodeswere fabricated bymixing 80wt activematerials 15 wt acetylene black and 5wt polyvinylidenefluoride (PVDF) as binder The mixtures were well blendedusing alcohol coated the paste onto nickel foam currentcollectors (1 cm times 1 cm) were dried at 100∘C for 10 h andwere pressed under 20MPa for 3ndash5min Each electrode con-tains 2-3mg active materials A conventional three-electrodesystem was employed to investigate the capacitive behaviorsPlatinum foil and HgHgO (1M KOH aqueous solution)electrodes were used as the counter and reference electrodesrespectively

3 Results and Discussion

31 Structure and Morphology of the Samples The XRD pat-terns of 3D graphene Co

3O4 and the as-prepared composite

before and after calcination were shown in Figure 1 For3D graphene a broad peak was observed at 256∘ which isattributed to the C (002) plane [31 35] The main peaks inthe XRD pattern of pure Co

3O4(JCPDS number 42-1467)

are located at 190 313 368 448 594 and 653∘ which are

Journal of Nanomaterials 3

10 20 30 40 50 60 70 80 90

440511400

311

220

111 002

002

002

Inte

nsity

(au

)

(d)

(c)

(b)

(a)

2120579 (deg)

Figure 1 XRD patterns of (a) 3D graphene (b) Co3O4 (c) the

composite without calcination treatment and (d) the compositeafter calcination treatment

assigned to the crystal planes of (111) (220) (311) (400) (511)and (440) [36] FromFigure 1(c) it can be found that theXRDpattern of the as-prepared composite before calcination doesnot show any other peak except a broad hump at 2120579 of about256∘ which is attributed to the diffraction peak of C (002)in 3D graphene [35] However several peaks appear at 2120579 ofaround 19∘ 31∘ 37∘ 45∘ 59∘ and 65∘ obviously in the XRDpattern after calcinationThese diffraction peaks are assignedto the crystal planes of (111) (220) (311) (400) (511) and(440) which agree with the typical spinel Co

3O4[36] Thus

the chemical reactions in the synthetic process are supposedas follows

Co (NO3)

2+ 2NH

3sdotH2O 997888rarr Co (OH)

2+ 2NH

4

++ 2NO

3

minus

(1)

6Co(OH)2+O2997888rarr 2Co

3O4+ 6H2O (2)

The absence of diffraction peaks of Co(OH)2in Figure 1(c)

may result from the amorphous state of Co(OH)2 It is

inferred that the composite before calcination consists ofCo(OH)

2and graphene

Additionally from the XRD pattern of the compositeafter calcination the characteristic peak of 3D graphenefor C (002) is also found Based on these results the as-obtained calcinate material can be undoubtedly identifiedas Co

3O43D graphene composite Moreover it is notable

that the C (002) peaks in Figures 1(c) and 1(d) shift towardleft as compared to that in Figure 1(a) which indicates thatCo(OH)

2and Co

3O4may lead to the increase of the unit

cell volume of graphene because the shift of entire diffractionpeaks toward lower 2120579 usually suggests the increase of unitcell volume

The morphology of the as-prepared materials was inves-tigated by SEM as shown in Figure 2 The graphenematerial has a 3D structure composed of homogeneous

micrometer-sized flakes (Figure 2(a)) Moreover accordingto our previous work [27] the 3D graphene material hasmesoporosity centered at 4 nm demonstrating that it belongsto mesoporous materials Figure 2(b) shows the SEM imageof the composite before calcination (ie the precursor ofCo3O43D graphene composite) It can be seen that the

graphene skeleton maintains structural integrity except thatthe Co

3O4particles are distributed over the graphene surface

After the calcination treatment it seems that themorphologyand structure are similar to those of the precursor of thecomposite (Figure 2(c)) The high-magnification SEM imagein Figure 2(d) reveals that many small Co

3O4granules

agglomerated together to form some big circular particleswith nonuniform diameters from 50 to 250 nm Howeverfrom Figures 2(e) and 2(f) it is clearly seen that without theaddition of 3D graphene Co

3O4and its precursor display

cubic morphology and these particles have a size about 50ndash100 nm and were stacked irregularly It indicates that themorphology of Co

3O4in the composite material is different

from that of pure Co3O4 This may be because of the effects

of the electrostatic adsorption and other chemical bondsbetween the oxygen-containing functional groups on thesurface of 3D graphene and the precursor of Co

3O4

TEM images of Co3O4and Co

3O43D graphene com-

posite are shown in Figure 3 Figures 3(a) and 3(b) displaythe TEM images of pure Co

3O4with different magnification

It can be found that the pure Co3O4sample is composed

of a great deal of irregular cubic particles However Fig-ure 3(c) reveals that in the Co

3O43D graphene composite

many nearly spherical Co3O4particles are attached onto

3D graphene nanosheets which is similar to the results ofSEM Furthermore it can be seen that the Co

3O4cubic

particles with a size about 50ndash100 nanometers are completeand solid from the inset of Figure 3(a) However fromthe high magnification TEM image of Co

3O43D graphene

composite shown in Figure 3(d) it can be found that thebig spherical Co

3O4particles in the composite are com-

posed of numerous assembled nanoparticles with a size of10ndash25 nanometers and these small nanoparticles assembletogether to display cauliflower-like morphology This canlead to abundant pore structures which is beneficial to theelectrochemical utilization of metal oxides and the passing ofthe ions in the electrolyte Additionally the HRTEM imageof the composite is presented in the inset of Figure 3(d)The Co

3O4exhibits high crystallinity and the lattice spacing

of 026 nm is corresponding to the interspacing of the (311)planes [35]

Nitrogen isotherms of Co3O4and Co

3O43D graphene

composite are presented in Figure 4 It is interesting to seethat the composite adsorbs much more N

2than pure Co

3O4

It suggests that the structural characteristics change greatlyafter the addition of 3D graphene The BET surface areaincreases from 0111 to 697m2 gminus1 while the pore volumeincreases from 00016 to 017 cc gminus1 This is the normalsequence since 3D graphene possesses a large specific surfacearea and pore volume and the cauliflower-like morphologyof Co3O4in the composite further contributes to the surface

area and pore volume


1120583m

(a)

1120583m

(b)

1120583m

(c)

100nm

(d)

100nm

(e)

100nm

(f)

Figure 2 SEM images of (a) 3D graphene (b) the precursor of Co3O43D graphene composite Co

3O43D graphene composite with (c) low

and (d) high magnification and (e) the precursor of Co3O4and (f) Co

3O4

32 Electrochemical Properties For potential supercapacitorapplications the capacitor behaviors of the electrodes wereexamined

321 Cyclic Voltammetry (CV) Figure 5(a) displays the CVcurves of Co

3O43D graphene composite Co

3O4 and 3D

graphene at a scan rate of 10mV sminus1 in 1M KOH electrolytewithin the potential window ofminus02ndash05 V Evidently the areasurrounded by the CV curve is dramatically enhanced bythe introduction of Co

3O4nanoparticles onto 3D graphene

sheets as shown in Figure 5(a) This result indicates a largespecific capacitance associated with the composite electrodeThe CV curves are different from the ideal rectangular shapeof the typical electric double-layer capacitance Unlike 3D

graphene two pairs of redox reaction peaks are visible inthe CV curve of the composite which can be ascribed tothe redox process of Co

3O4 suggesting that the capacitance

mainly results from the pseudocapacitance of Co3O4 The

mechanism of electrochemical reactions of Co3O4in alkaline

electrolyte can be expressed as the following equations [36]

Co3O4+OHminus +H

2OlArrrArr 3CoOOH + eminus (3)

CoOOH +OHminus lArrrArr CoO2+H2O + eminus (4)

The SC of the electrode based on CV curves can becalculated as

119862 =

(int 119868119889119881)

]119898119881

(5)


200nm

200nm

(a)

100nm

(b)

200nm

(c)

100nm

026nm

5nm

(d)

Figure 3 TEM images of Co3O4with (a) low and (b) high magnification and Co

3O43D graphene composite with (c) low and (d) high

magnification Inset high resolution TEM (HRTEM) images

where 119862 is the SC of electroactive materials (F gminus1) 119868 isthe response current (A) 119881 is the potential (V) ] is thepotential scan rate (V sminus1) and119898 is the mass of the electrodematerials (g) According to (5) the SCs of the Co

3O43D

graphene composite Co3O4 and 3D graphene electrodes

are calculated to be about 457 134 and 97 Fg at a scanrate of 10mV sminus1 respectively If the SCs are representedby volumetric capacitance these values are 965 335 and103 Fcm3 respectively Clearly the SC of the composite isnot simple sum of Co

3O4and 3D graphene but signifi-

cantly larger than expected Moreover the SC value is muchhigher than that of a Co

3O4reduced graphene oxide scrolled

structure (1638 Fg) reported recently [31] The desirableperformance could be attributed to the synergistic effectbetween graphene sheets and Co

3O4nanoparticles in the

nanocomposite [37] This effect may be associated withthe fact that conductive graphene is hydrophobic whileCo3O4is hydrophilic but has poor conductivity It is very

likely that Co3O4increases the graphene hydrophilicity for

significantly increased utilization of the porous electrode and

the conductive graphene enhances the charge transfer rateof Co

3O4for higher pseudocapacitance Furthermore the

large specific surface area and pore volume resulting fromthe cauliflower-like morphology of Co

3O4in the composite

may also increase the utilization of Co3O4and thus further

enhance the pseudocapacitance These factors result in amuch larger SC value of the composite than the sum of that of3D graphene and Co

3O4 Additionally as compared to two-

dimensional (2D) graphenewhich has been reported [38] 3Dgraphene has amuch higher specific surface area (487m2 gminus1)and larger pore volume (104 cc gminus1) [27] The 3D graphenehas predominant mesoporosity which centers at 4 nm whichis in favor of improving the electrochemical accessibilityof electrolyte ions and providing preferable EDLCs [30]Both the pseudocapacitance derived from the cauliflower-likeCo3O4and the superior surface and pore properties of 3D

graphene contribute to the high capacitanceFigure 5(b) shows the variation of the calculated SC

values of Co3O43D graphene composite as a function of scan

rates in different concentration KOH electrolytes Obviously


00 02 04 06 08 10

0

20

40

60

80

100

120

Co3O43D graphene compositeCo3O4

N2

adso

rbed

vol

ume (

cm3g

)

Relative pressure (PP0)

Figure 4 N2adsorption-desorption isotherms of Co

3O4and

Co3O43D graphene composite

the SC values of the composite decrease gradually with theincrease of scan rate This may be because at high scanrates diffusion limits the movement of electrolyte ions dueto the time constraint and only the outer active surface isutilized for charge storage At lower scan rates however allthe active surface area can be utilized for charge storage andthe electrochemical utilization of Co

3O4[39] As shown in

Figure 5(b) 6M electrolyte provided the largest SC valuesThe maximum SC value is 863 F gminus1 in 6M KOH electrolyteat a scan rate of 1mV sminus1 According to the equations of theelectrochemical reactions of Co

3O4in alkaline electrolyte

there is OHminus in the reactions Thus a high concentration ofhydroxyl ions is in favor of increasing the reaction rate Thisresults in the largest SC value in 6M KOH solution

322 Galvanostatic Charge-Discharge Measurement Fur-thermore galvanostatic charge-discharge measurement wasutilized to study the capacitor performance of the as-preparedsamples

Figure 6 depicts the charge-discharge behaviors ofCo3O4 3D graphene and the Co

3O43D graphene com-

posite electrodes between 0 and 05 V at a constant currentdensity of 5 A gminus1 in 1M KOH aqueous solution Fromthe charge-discharge curves of 3D graphene it is evidentlyseen that the voltage decreases linearly along the dischargetime which shows an ideal double-layer capacitive behav-ior However the discharge curve of Co

3O43D graphene

composite consists of a slow potential drop from 05 to02 V and a sudden potential decay from 02 to 0V Thelatter indicates short chargedischarge duration which isa pure double-layer capacitance behavior from the chargeseparation at the electrodeelectrolyte interface However theformer is a typical pseudocapacitance behavior caused byelectrochemical adsorptiondesorption or a redox reaction

at the electrodeelectrolyte interface [40] Usually Faradaicredox reaction is accompanied by the double-layer charge-discharge process and thus the combination of electricdouble-layer capacitance and Faradaic pseudocapacitance isresponsible for the longer chargedischarge duration [17]

From the discharge curves SC could also be calculatedusing the following equation

119862 =

(119868Δ119905)

(119898Δ119881)

(6)

where119862 is the SCof the activematerials (F gminus1) 119868Δ119905Δ119881 and119898 are constant discharge current (A) discharge time (s) thevoltage change after a full discharge (V) and the mass of theactive material (g) respectively [41]The SC values calculatedfrom the galvanostatic charge and discharge curves are 4046 and 337 F gminus1 for 3D graphene Co

3O4and the composite

respectively which are similar to the trend of those calculatedfrom CVs A larger specific capacitance value of 402 F gminus1for the composite is obtained at 05 A gminus1 The much largerSC value of the composite than the sum of that of grapheneand pure Co

3O4further proves that the combination of 3D

graphene and cauliflower-like Co3O4is very beneficial to

improving the supercapacitor performanceSubsequently the galvanostatic charge-discharge proper-

ties of the composite are investigated with different currentdensity in different concentration of KOH electrolytes TheSC values are calculated in accordance with (6) and listed inTable 1 It is found that the SC of the electrode based on theas-prepared composite reached amaximumvalue of 522 F gminus1in 6M KOH electrolyte at the current density of 05 A gminus1The results are similar to the results obtained from the CVsfurther indicating that a high concentration of hydroxyl ionsis in favor of increasing the reaction rate

To investigate the cycling performance of the Co3O43D

graphene composite the charge-discharge tests were per-formed at 2A gminus1 for 1000 cycles Figure 7 shows the results ofcycling stability measurement After 1000 cycles the specificcapacitance of the Co

3O43D graphene composite decreased

apparently An initial total capacitance of 75 was retainedafter the 1000th discharge cycle The decay of capacitancecan be ascribed to the increased impedance with increasingnumber of cycles and it is inferred that the increasedimpedance could be due to fatigue of the active materialduring cycling [7] As compared to some nanostructuredCo3O4electrodes reported recently [7] the retention rate

of capacitance of the Co3O43D graphene composite was

improved a little which may be attributed to the specialstructure of 3D graphene and cauliflower-like Co

3O4 During

cycling the three-dimensional nanoporous structure of 3Dgraphene as well as the cauliflower-like morphology ofCo3O4 can inhibit the volume change and detachment of

Co3O4particles more effectively and thus result in a better

cyclic life

4 Conclusions

In this work a Co3O43D graphene composite was synthe-

sized via a facile two-step synthesis route (heat reduction


00 01 02 03 04 05

000

001

002

Curr

ent (

A)

Potential (V)

3D graphene3O43D grapheneCo

Co3O4

minus003

minus002

minus001

minus02 minus01

(a)

0 20 40 60 80 100 120 140 160 180 200

0200

400

600

800

1000

1200

1M KOH2M KOH6M KOH

Spec

ific c

apac

itanc

e (F g

minus1)

Scan rate (mV sminus1)

(b)

Figure 5 (a) CV curves of 3D graphene Co3O4 and Co

3O43D graphene composite at a scan rate of 10mV sminus1 (b) the SC of Co

3O43D

graphene composite at various scan rates of 1 5 10 20 50 80 100 and 200mV sminus1 within the potential window of minus02ndash05 V

0 20 40 60 80 100 12000

01

02

03

04

05

Pote

ntia

l (V

)

Time (s)

3D graphene 3O43D grapheneCoCo3O4

Figure 6 Galvanostatic chargedischarge curves of 3D grapheneCo3O4 and Co

3O43D graphene composite at a current density of

5 A gminus1

of GO and hydrothermal synthesis of Co3O4) SEM and

TEM images showed that in the composite many smallnanoparticles with a size of 10ndash25 nm assemble togetherto display cauliflower-like morphology The supercapacitorperformance of the composite 3D graphene and Co

3O4

was investigated and compared by CV and constant currentcharge-discharge tests It was found that the composite

0 200 400 600 800 10000

100

200

300

400

500

Cycle number

Spec

ific c

apac

itanc

e (F g

minus1)

Figure 7 Cycling stability measurement result of Co3O43D gra-

phene composite within the potential window of 0ndash050V (versusHgHgO) at a current density of 2A gminus1

showed much larger specific capacitance than the sum ofthat of 3D graphene and Co

3O4 Moreover the composite

exhibited better cycling stability The good supercapacitorperformance is ascribed to the combination of 3D grapheneand cauliflower-like Co

3O4 which leads to a strong synergis-

tic effect to remarkably boost the utilization ratio of Co3O4

and grapheneThe cauliflower-likeCo3O43D graphene com-

posite is believed to be a promising electrode material forsupercapacitors


Table 1 SC values (F gminus1) of Co3O43D graphene composite atdifferent current density in different concentration electrolyteswithin the potential window of 0ndash05 V

1MKOH 2MKOH 6MKOH05A gminus1 402 458 5221 A gminus1 380 431 4785A gminus1 337 367 39610A gminus1 296 338 363

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Acknowledgments

The authors are grateful for the financial support by OneHundred Talent Program of Chinese Academy of Sciences aswell as by the NSFC (51302264) of China

References

[1] P Simon and Y Gogotsi ldquoMaterials for electrochemical capaci-torsrdquo Nature Materials vol 7 no 11 pp 845ndash854 2008

[2] J Yan Z Fan W Sun et al ldquoAdvanced asymmetric super-capacitors based on Ni(OH)

2graphene and porous graphene

electrodes with high energy densityrdquo Advanced FunctionalMaterials vol 22 no 12 pp 2632ndash2641 2012

[3] L H Bao J F Zang and X D Li ldquoFlexible Zn2SnO4MnO

2

coreshell nanocable-carbon microfiber hybrid composites forhigh-performance supercapacitor electrodesrdquoNano Letters vol11 no 3 pp 1215ndash1220 2011

[4] L T Lam and R Louey ldquoDevelopment of ultra-battery forhybrid-electric vehicle applicationsrdquo Journal of Power Sourcesvol 158 no 2 pp 1140ndash1148 2006

[5] M Winter and R J Brodd ldquoWhat are batteries fuel cells andsupercapacitorsrdquo Chemical Reviews vol 104 pp 4245ndash42692004

[6] E Faggioli P Rena V Danel X Andrieu R Mallant and HKahlen ldquoSupercapacitors for the energymanagement of electricvehiclesrdquo Journal of Power Sources vol 84 no 2 pp 261ndash2691999

[7] R Tummala R K Guduru and P SMohanty ldquoNanostructuredCo3O4electrodes for supercapacitor applications from plasma

spray techniquerdquo Journal of Power Sources vol 209 pp 44ndash512012

[8] Y Hou Y Cheng T Hobson and J Liu ldquoDesign andsynthesis of hierarchical MnO

2nanospherescarbon nan-

otubesconducting polymer ternary composite for high perfor-mance electrochemical electrodesrdquo Nano Letters vol 10 no 7pp 2727ndash2733 2010

[9] B E Conway ldquoTransition from lsquosupercapacitorrsquo to lsquobatteryrsquobehavior in electrochemical energy storagerdquo Journal of theElectrochemical Society vol 138 no 6 pp 1539ndash1548 1991

[10] T Xue C-L Xu D-D Zhao X-H Li and H-L Li ldquoElectrode-position of mesoporous manganese dioxide supercapacitorelectrodes through self-assembled triblock copolymer tem-platesrdquo Journal of Power Sources vol 164 no 2 pp 953ndash9582007

[11] B E Conway V Birss and J Wojtowicz ldquoThe role andutilization of pseudocapacitance for energy storage by super-capacitorsrdquo Journal of Power Sources vol 66 no 1-2 pp 1ndash141997

[12] J P Zheng P J Cygan andT R Jow ldquoHydrous rutheniumoxideas an electrode material for electrochemical capacitorsrdquo Journalof the Electrochemical Society vol 142 no 8 pp 2699ndash27031995

[13] J Chang W Lee R S Mane B W Cho and S-H HanldquoMorphology-dependent electrochemical supercapacitor prop-erties of indium oxiderdquo Electrochemical and Solid-State Lettersvol 11 no 1 pp A9ndashA11 2008

[14] I Jung J Choi and Y Tak ldquoNickel oxalate nanostructures forsupercapacitorsrdquo Journal of Materials Chemistry vol 20 no 29pp 6164ndash6169 2010

[15] OGhodbane J-L Pascal and F Favier ldquoMicrostructural effectson charge-storage properties in MnO

2-based electrochemical

supercapacitorsrdquo ACS Applied Materials amp Interfaces vol 1 no5 pp 1130ndash1139 2009

[16] V R Shinde S B Mahadik T P Gujar and C D LokhandeldquoSupercapacitive cobalt oxide (Co

3O4) thin films by spray

pyrolysisrdquo Applied Surface Science vol 252 no 20 pp 7487ndash7492 2006

[17] J Yan T Wei W Qiao et al ldquoRapid microwave-assistedsynthesis of graphene nanosheetCo

3O4composite for super-

capacitorsrdquo Electrochimica Acta vol 55 no 23 pp 6973ndash69782010

[18] R K Selvan I Perelshtein N Perkas and A Gedanken ldquoSyn-thesis of hexagonal-shaped SnO

2nanocrystals and SnO

2C

nanocomposites for electrochemical redox supercapacitorsrdquoThe Journal of Physical Chemistry C vol 112 no 6 pp 1825ndash1830 2008

[19] J P Zheng and T R Jow ldquoHigh energy and high power densityelectrochemical capacitorsrdquo Journal of Power Sources vol 62no 2 pp 155ndash159 1996

[20] J P Zheng J Huang and T R Jow ldquoThe limitations ofenergy density for electrochemical capacitorsrdquo Journal of theElectrochemical Society vol 144 no 6 pp 2026ndash2031 1997

[21] G P Wang L Zhang and J J Zhang ldquoA review of electrodematerials for electrochemical supercapacitorsrdquo Chemical Soci-ety Reviews vol 41 no 2 pp 797ndash828 2012

[22] M-J Deng F-L Huang I-W Sun W-T Tsai and J-KChang ldquoAn entirely electrochemical preparation of a nano-structured cobalt oxide electrode with superior redox activityrdquoNanotechnology vol 20 no 17 Article ID 175602 2009

[23] Y Liu WW Zhao and X G Zhang ldquoSoft template synthesis ofmesoporous Co

3O4RuO

2sdotxH2O composites for electrochem-

ical capacitorsrdquo Electrochimica Acta vol 53 no 8 pp 3296ndash3304 2008

[24] C Lin J A Ritter and B N Popov ldquoCharacterization of sol-gel-derived cobalt oxide xerogels as electrochemical capacitorsrdquoJournal of the Electrochemical Society vol 145 no 12 pp 4097ndash4103 1998

[25] K S Novoselov A K Geim S V Morozov et al ldquoElectric fieldin atomically thin carbon filmsrdquo Science vol 306 no 5696 pp666ndash669 2004

[26] Y Wang Z Q Shi Y Huang et al ldquoSupercapacitor devicesbased on graphenematerialsrdquoThe Journal of Physical ChemistryC vol 113 no 30 pp 13103ndash13107 2009

[27] Y Wang C Guan K Wang C X Guo and C M LildquoNitrogen hydrogen carbon dioxide and water vapor sorption


properties of three-dimensional graphenerdquo Journal of Chemicaland Engineering Data vol 56 no 3 pp 642ndash645 2011

[28] Y Wang C X Guo J Liu T Chen H Yang and C MLi ldquoCeO

2nanoparticlesgraphene nanocomposite-based high

performance supercapacitorrdquo Dalton Transactions vol 40 no24 pp 6388ndash6391 2011

[29] X C Dong J X Wang J Wang et al ldquoSupercapacitorelectrode based on three-dimensional graphenendashpolyanilinehybridrdquo Materials Chemistry and Physics vol 134 no 2-3 pp576ndash580 2012

[30] C X Guo F P Hu XW Lou and CM Li ldquoHigh-performancebiofuel cell made with hydrophilic ordered mesoporous carbonas electrode materialrdquo Journal of Power Sources vol 195 no 13pp 4090ndash4097 2010

[31] W Zhou J Liu T Chen et al ldquoFabrication of Co3O4-reduced

graphene oxide scrolls for high-performance supercapacitorelectrodesrdquo Physical Chemistry Chemical Physics vol 13 no 32pp 14462ndash14465 2011

[32] A Pendashteh M F Mousavi and M S Rahmanifar ldquoFab-rication of anchored copper oxide nanoparticles on grapheneoxide nanosheets via an electrostatic coprecipitation and itsapplication as supercapacitorrdquo Electrochimica Acta vol 88 pp347ndash357 2013

[33] S X Deng D Sun C H Wu et al ldquoSynthesis and electro-chemical properties of MnO

2nanorodsgraphene composites

for supercapacitor applicationsrdquo Electrochimica Acta vol 111pp 707ndash712 2013

[34] X Dai W Shi H Cai R Li and G Yang ldquoFacile preparationof the novel structured 120572-MnO

2graphene nanocomposites and

their electrochemical propertiesrdquo Solid State Sciences vol 27 pp17ndash23 2014

[35] X-C Dong H Xu X-W Wang et al ldquo3D graphene-cobalt oxide electrode for high-performance supercapacitorand enzymeless glucose detectionrdquo ACS Nano vol 6 no 4 pp3206ndash3213 2012

[36] X Wang S Liu H Wang F Tu D Fang and Y Li ldquoFacileand green synthesis of Co

3O4nanoplatesgraphene nanosheets

composite for supercapacitorrdquo Journal of Solid State Electro-chemistry vol 16 no 11 pp 3593ndash3602 2012

[37] G He J Li H Chen et al ldquoHydrothermal preparationof Co

3O4graphene nanocomposite for supercapacitor with

enhanced capacitive performancerdquoMaterials Letters vol 82 pp61ndash63 2012

[38] Y Wang J H Liu K Wang T Chen X Tan and C MLi ldquoHydrogen storage in Ni-B nanoalloy-doped 2D graphenerdquoInternational Journal of Hydrogen Energy vol 36 no 20 pp12950ndash12954 2011

[39] J Li H Xie Y Li J Liu and Z Li ldquoElectrochemical propertiesof graphene nanosheetspolyaniline nanofibers composites aselectrode for supercapacitorsrdquo Journal of Power Sources vol 196no 24 pp 10775ndash10781 2011

[40] S-J Bao C M Li C-X Guo and Y Qiao ldquoBiomolecule-assisted synthesis of cobalt sulfide nanowires for application insupercapacitorsrdquo Journal of Power Sources vol 180 no 1 pp676ndash681 2008

[41] R Z Ma J Liang B Q Wei B Zhang C L Xu and D H WuldquoStudy of electrochemical capacitors utilizing carbon nanotubeelectrodesrdquo Journal of Power Sources vol 84 no 1 pp 126ndash1291999

Submit your manuscripts athttpwwwhindawicom

ScientificaHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

CorrosionInternational Journal of

Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Polymer ScienceInternational Journal of



CeramicsJournal of


CompositesJournal of

NanoparticlesJournal of



International Journal of

Biomaterials


NanoscienceJournal of

TextilesHindawi Publishing Corporation httpwwwhindawicom Volume 2014

Journal of

NanotechnologyHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Journal of

CrystallographyJournal of


The Scientific World JournalHindawi Publishing Corporation httpwwwhindawicom Volume 2014


CoatingsJournal of

Advances in

Materials Science and EngineeringHindawi Publishing Corporationhttpwwwhindawicom Volume 2014

Smart Materials Research



MetallurgyJournal of


BioMed Research International

MaterialsJournal of


Nano

materials


Journal ofNanomaterials


conductivity than carbon materials and remarkable stabilityshows as high as 720ndash768 F gminus1 [19 20] However the highcost and toxic nature of ruthenium substantially limits itscommercial application [21] Hence the relatively inexpen-sive metal oxides have been studied widely Among themcobalt oxide is considered as a promising electrode materialfor supercapacitors due to its relatively low cost well-definedredox behavior ultrahigh theoretical SC (sim3560 F gminus1) andenvironmental friendliness [22 23] For instance it hasbeen reported that the CoO

119909xerogels thermally treated at

150∘C showed a good capacitive behavior with a SC value of291 F gminus1 [24] Tummala et al [7] have synthesized nanos-tructured Co

3O4by plasma spray technique and studied its

capacitance propertiesThe results showed a high SC value of162 F gminus1 in 6M KOH electrolyte

Graphene as a new carbon material has attracted muchattention since it was isolated from layer-by-layer graphite in2004 [25] It is a promising electrode candidate for EDLCsdue to its superb characteristics including the excellentmechanical strength and chemical stability high electricaland thermal conductivity and large surface area (a theoreticalvalue of 2630m2 gminus1) [25] But the serious agglomerationand restacking between individual graphene sheets result inlimited surface utilization percentage and a lower capacitancethan the anticipated value [26] In our recent work [2728] three-dimensional (3D) graphene (strictly speaking itshould be reduced graphene oxide (RGO) but it is stillcalled graphene here for convenience) was acquired by heatreduction of graphite oxide in vacuum which has a higherspecific surface area (487m2 gminus1) Different fromDongrsquoswork[29] it has a predominant mesoporosity which centers at4 nm and thesemesopores can provide a high specific surfacearea while allowing good mass transport in an electrodeprocess [30]

Recently many researchers have constructed a graphene-based metal oxide composite structure [17 31ndash34] It wasfound that these composites showed improved performancewith higher electron transport rate electrolyte contact areaand structural stability It can be attributed to the specialcomposite structure in which metal oxides are attached onthe surface or intercalated into the interlayer of large patchesof graphene [31] Yan et al [17] reported that graphenenanosheet (GNS)Co

3O4composite has a maximum specific

capacitance of 2432 F gminus1 at a scan rate of 10mV sminus1 in6M KOH aqueous solution However the composite of 3Dgraphene which was reported in our previous work [27] andCo3O4has never been investigated so far Herein a facile

hydrothermal strategy was proposed to deposit cauliflower-like nanostructured Co

3O4on 3D graphene nanosheets

Cyclic voltammetry (CV) and galvanostatic charge-dischargemeasurements were used to assess the electrochemical prop-erties of the Co

3O43D graphene composite in KOH solu-

tions

2 Experimental

21 Materials Preparation In this study deionizedwater was used throughout Cobalt nitrate hexahydrate

(Co(NO3)2sdot6H2O) ammonia (37 NH

3sdotH2O) and potas-

sium hydroxide (KOH) were obtained fromXilong ChemicalEngineering Co Ltd Hydrochloric acid (37) and ethanolwere supplied by Chemical Plant of Beijing All the reagentsused in this experiment were of analytical grade and withoutfurther purification

Graphene oxide was prepared from nature graphite bya modified Hummersrsquo method 3D graphene material wasobtained by reducing graphite oxide powder in a glass bottleunder vacuum followed by heating to 250∘C from roomtemperature [27]

Co3O43D graphene composite was synthesized as fol-

lows First 15mg of 3D graphene was dispersed in 15mLethanol with the assistance of ultrasonication for 60minThen 05mmol Co(NO

3)2sdot6H2O and 34 120583L of 37 ammonia

were added to the 3D graphene suspension simultaneouslyThe as-obtained mixture was magnetically stirred for 20minAfter that the above suspension was loaded into a 25mL ofTeflon-lined stainless steel autoclave and heated at 170∘C for5 h The obtained black products were washed by centrifu-gation for several times using ethanol and deionized waterto remove excess ions and then dried in a vacuum oven at60∘C Afterwards the material was calcined at 350∘C for 3 hin air and then cooled to room temperature For comparisonCo3O4was also prepared by the same steps without the

addition of 3D graphene

22 Characterization Methods The crystallographic struc-tures of the samples were determined by X-ray diffractionsystem (XRD Xrsquo Pert-PRO MPD) equipped with Cu K

120572

(120582 = 15406 A) radiation The morphological analysis wasconducted using scanning electron microscope (SEM JEOLJSM-6700F) and transmission electron microscope (TEMJEOL JEM-2011 200 kV) The specific surface area and porevolume of the samples were determined by analyzing thestandard nitrogen adsorption isothermmeasured at 77 K Anelectrochemical workstation system (CHI760D ChenhuaShanghai) was employed to study the electrochemical prop-ertiesThe electrodeswere fabricated bymixing 80wt activematerials 15 wt acetylene black and 5wt polyvinylidenefluoride (PVDF) as binder The mixtures were well blendedusing alcohol coated the paste onto nickel foam currentcollectors (1 cm times 1 cm) were dried at 100∘C for 10 h andwere pressed under 20MPa for 3ndash5min Each electrode con-tains 2-3mg active materials A conventional three-electrodesystem was employed to investigate the capacitive behaviorsPlatinum foil and HgHgO (1M KOH aqueous solution)electrodes were used as the counter and reference electrodesrespectively

3 Results and Discussion

31 Structure and Morphology of the Samples The XRD pat-terns of 3D graphene Co

3O4 and the as-prepared composite

before and after calcination were shown in Figure 1 For3D graphene a broad peak was observed at 256∘ which isattributed to the C (002) plane [31 35] The main peaks inthe XRD pattern of pure Co

3O4(JCPDS number 42-1467)

are located at 190 313 368 448 594 and 653∘ which are


10 20 30 40 50 60 70 80 90

440511400

311

220

111 002

002

002

Inte

nsity

(au

)

(d)

(c)

(b)

(a)

2120579 (deg)




3O4[36] Thus


Co (NO3)

2+ 2NH


2+ 2NH

4

++ 2NO

3

minus

(1)


3O4+ 6H2O (2)




2and graphene




2and Co








3O4granules







3O4


3O43D graphene com-








3O4cubic


3O43D graphene







3O43D graphene


2than pure Co

3O4




1120583m

(a)

1120583m

(b)

1120583m

(c)

100nm

(d)

100nm

(e)

100nm

(f)




3O4




3O4 and 3D









Co3O4+OHminus +H




119862 =

(int 119868119889119881)

]119898119881

(5)


200nm

200nm

(a)

100nm

(b)

200nm

(c)

100nm

026nm

5nm

(d)





3O43D














3O4in the composite









00 02 04 06 08 10

0

20

40

60

80

100

120


N2

adso

rbed

vol

ume (

cm3g

)



3O4and



3O4[39] As shown in






3O43D graphene com-


3O43D graphene




119862 =

(119868Δ119905)

(119898Δ119881)

(6)














3O4 During



cyclic life

4 Conclusions




00 01 02 03 04 05

000

001

002

Curr

ent (

A)

Potential (V)


Co3O4

minus003

minus002

minus001

minus02 minus01

(a)

0 20 40 60 80 100 120 140 160 180 200

0200

400

600

800

1000

1200

1M KOH2M KOH6M KOH

Spec

ific c

apac

itanc

e (F g

minus1)


(b)



3O43D


0 20 40 60 80 100 12000

01

02

03

04

05

Pote

ntia

l (V

)

Time (s)




5 A gminus1



3O4


0 200 400 600 800 10000

100

200

300

400

500

Cycle number

Spec

ific c

apac

itanc

e (F g

minus1)















Acknowledgments


References






2



























2C







3O4RuO









































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




10 20 30 40 50 60 70 80 90

440511400

311

220

111 002

002

002

Inte

nsity

(au

)

(d)

(c)

(b)

(a)

2120579 (deg)




3O4[36] Thus


Co (NO3)

2+ 2NH


2+ 2NH

4

++ 2NO

3

minus

(1)


3O4+ 6H2O (2)




2and graphene




2and Co








3O4granules







3O4


3O43D graphene com-








3O4cubic


3O43D graphene







3O43D graphene


2than pure Co

3O4




1120583m

(a)

1120583m

(b)

1120583m

(c)

100nm

(d)

100nm

(e)

100nm

(f)




3O4




3O4 and 3D









Co3O4+OHminus +H




119862 =

(int 119868119889119881)

]119898119881

(5)


200nm

200nm

(a)

100nm

(b)

200nm

(c)

100nm

026nm

5nm

(d)





3O43D














3O4in the composite









00 02 04 06 08 10

0

20

40

60

80

100

120


N2

adso

rbed

vol

ume (

cm3g

)



3O4and



3O4[39] As shown in






3O43D graphene com-


3O43D graphene




119862 =

(119868Δ119905)

(119898Δ119881)

(6)














3O4 During



cyclic life

4 Conclusions




00 01 02 03 04 05

000

001

002

Curr

ent (

A)

Potential (V)


Co3O4

minus003

minus002

minus001

minus02 minus01

(a)

0 20 40 60 80 100 120 140 160 180 200

0200

400

600

800

1000

1200

1M KOH2M KOH6M KOH

Spec

ific c

apac

itanc

e (F g

minus1)


(b)



3O43D


0 20 40 60 80 100 12000

01

02

03

04

05

Pote

ntia

l (V

)

Time (s)




5 A gminus1



3O4


0 200 400 600 800 10000

100

200

300

400

500

Cycle number

Spec

ific c

apac

itanc

e (F g

minus1)















Acknowledgments


References






2



























2C







3O4RuO









































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




1120583m

(a)

1120583m

(b)

1120583m

(c)

100nm

(d)

100nm

(e)

100nm

(f)




3O4




3O4 and 3D









Co3O4+OHminus +H




119862 =

(int 119868119889119881)

]119898119881

(5)


200nm

200nm

(a)

100nm

(b)

200nm

(c)

100nm

026nm

5nm

(d)





3O43D














3O4in the composite









00 02 04 06 08 10

0

20

40

60

80

100

120


N2

adso

rbed

vol

ume (

cm3g

)



3O4and



3O4[39] As shown in






3O43D graphene com-


3O43D graphene




119862 =

(119868Δ119905)

(119898Δ119881)

(6)














3O4 During



cyclic life

4 Conclusions




00 01 02 03 04 05

000

001

002

Curr

ent (

A)

Potential (V)


Co3O4

minus003

minus002

minus001

minus02 minus01

(a)

0 20 40 60 80 100 120 140 160 180 200

0200

400

600

800

1000

1200

1M KOH2M KOH6M KOH

Spec

ific c

apac

itanc

e (F g

minus1)


(b)



3O43D


0 20 40 60 80 100 12000

01

02

03

04

05

Pote

ntia

l (V

)

Time (s)




5 A gminus1



3O4


0 200 400 600 800 10000

100

200

300

400

500

Cycle number

Spec

ific c

apac

itanc

e (F g

minus1)















Acknowledgments


References






2



























2C







3O4RuO









































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




200nm

200nm

(a)

100nm

(b)

200nm

(c)

100nm

026nm

5nm

(d)





3O43D














3O4in the composite









00 02 04 06 08 10

0

20

40

60

80

100

120


N2

adso

rbed

vol

ume (

cm3g

)



3O4and



3O4[39] As shown in






3O43D graphene com-


3O43D graphene




119862 =

(119868Δ119905)

(119898Δ119881)

(6)














3O4 During



cyclic life

4 Conclusions




00 01 02 03 04 05

000

001

002

Curr

ent (

A)

Potential (V)


Co3O4

minus003

minus002

minus001

minus02 minus01

(a)

0 20 40 60 80 100 120 140 160 180 200

0200

400

600

800

1000

1200

1M KOH2M KOH6M KOH

Spec

ific c

apac

itanc

e (F g

minus1)


(b)



3O43D


0 20 40 60 80 100 12000

01

02

03

04

05

Pote

ntia

l (V

)

Time (s)




5 A gminus1



3O4


0 200 400 600 800 10000

100

200

300

400

500

Cycle number

Spec

ific c

apac

itanc

e (F g

minus1)















Acknowledgments


References






2



























2C







3O4RuO









































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




00 02 04 06 08 10

0

20

40

60

80

100

120


N2

adso

rbed

vol

ume (

cm3g

)



3O4and



3O4[39] As shown in






3O43D graphene com-


3O43D graphene




119862 =

(119868Δ119905)

(119898Δ119881)

(6)














3O4 During



cyclic life

4 Conclusions




00 01 02 03 04 05

000

001

002

Curr

ent (

A)

Potential (V)


Co3O4

minus003

minus002

minus001

minus02 minus01

(a)

0 20 40 60 80 100 120 140 160 180 200

0200

400

600

800

1000

1200

1M KOH2M KOH6M KOH

Spec

ific c

apac

itanc

e (F g

minus1)


(b)



3O43D


0 20 40 60 80 100 12000

01

02

03

04

05

Pote

ntia

l (V

)

Time (s)




5 A gminus1



3O4


0 200 400 600 800 10000

100

200

300

400

500

Cycle number

Spec

ific c

apac

itanc

e (F g

minus1)















Acknowledgments


References






2



























2C







3O4RuO









































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials




00 01 02 03 04 05

000

001

002

Curr

ent (

A)

Potential (V)


Co3O4

minus003

minus002

minus001

minus02 minus01

(a)

0 20 40 60 80 100 120 140 160 180 200

0200

400

600

800

1000

1200

1M KOH2M KOH6M KOH

Spec

ific c

apac

itanc

e (F g

minus1)


(b)



3O43D


0 20 40 60 80 100 12000

01

02

03

04

05

Pote

ntia

l (V

)

Time (s)




5 A gminus1



3O4


0 200 400 600 800 10000

100

200

300

400

500

Cycle number

Spec

ific c

apac

itanc

e (F g

minus1)















Acknowledgments


References






2



























2C







3O4RuO









































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials








Acknowledgments


References






2



























2C







3O4RuO









































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials





































CeramicsJournal of







Biomaterials




Journal of


Journal of





CoatingsJournal of

Advances in








MaterialsJournal of


Nano

materials



research article cauliflower-like co /three-dimensional...

Documents