lable at ScienceDirect

Current Applied Physics 11 (2011) 462e466

Contents lists avai

Current Applied Physics

journal homepage: www.elsevier .com/locate/cap

Roles of nanosized Fe3O4 on supercapacitive properties of carbon nanotubes

Young-Ha Kim, Soo-Jin Park*

Dept. of Chemistry, Inha University, 253, Nam-gu, Incheon 402-751, Republic of Korea

a r t i c l e i n f o

Article history:Received 30 April 2010Received in revised form13 August 2010Accepted 20 August 2010Available online 27 August 2010

Keywords:Fe3O4

Multiwalled carbon nanotubesCoprecipitationSupercapacitors

* Corresponding author. Tel./fax: þ82 32 860 8438E-mail address: sjpark@inha.ac.kr (S.-J. Park).

1567-1739/$ e see front matter � 2010 Elsevier B.V.doi:10.1016/j.cap.2010.08.018

a b s t r a c t

We synthesized Fe3O4 nanoparticle-dispersed multiwalled carbon nanotubes (MWNTs) to evaluate theirpotential applicability to supercapacitor electrodes. Nanosized Fe3O4 was deposited by chemical copre-cipitation of Fe2þ and Fe3þ in the presence of MWNTs in alkaline solutions. Fe3O4 nanoparticles with anaverage particle size of 14 nm were prepared within several minutes. The structure and morphologicalcharacteristics of the Fe3O4/MWNT composites were studied by X-ray diffraction (XRD), X-ray photo-electron spectroscopy (XPS), and transmission electron microscopy (TEM). The electrochemical perfor-mance of a Fe3O4/MWNT composite electrode and a pure MWNT electrode was tested by cyclicvoltammetry (CV) and galvanostatic chargeedischarge in a sulfite electrolyte. The results showed thatthe Fe3O4/MWNT electrode had typical pseudo-capacitive behavior in a 1 M Na2SO3 solution anda significantly greater specific capacitance than that of the pure MWNT electrode. It could also retain85.1% of its initial capacitance over 1000 cycles.

� 2010 Elsevier B.V. All rights reserved.

1. Introduction

Electrochemical capacitors or supercapacitors have high powerdensity, long chargeedischarge cycle life, and high energy efficiency.A combined system of supercapacitors and rechargeable batteries orfuel cells is considered to be a potential power source for electricvehicles. In this system, supercapacitors provide the necessary highpower for acceleration and facilitate recuperation of brake energy[1e3]. According to the energy storage mechanism, electrochemicalcapacitors can be classified as carbon-based electrochemical doublelayer capacitors (EDLCs) [4e6] and conducting polymer [7e9] ormetal oxide based pseudo-capacitors [10e12]. In EDLCs, energystorage arises from the accumulation of electronic and ionic chargesat the interface between the electrode materials and the electrolytesolution. The capacitance of the latter is due to the reversible Faradaicreaction of electro-active species of the electrode, such as surfacefunctional groups and transition metal oxides. It is clear that theelectrode is the key element in the development of supercapacitors.

The development of advanced composite materials based onmetal oxide-carbon nanotubes for supercapacitor electrodes hasbeen studied recently in the hope that these materials will provideimproved capacitive behaviors due to their enhanced stability, highconductivity, and pseudo-capacitive property [13e16]. One of themost extensively studied metal oxides for use in supercapacitors ishydrous ruthenium oxide (RuO2), which has a pseudo-capacitance

.

All rights reserved.

of 720 F g�1 [17]. However, its prohibitive cost has motivated thesearch for alternative metal oxides. In this regard, magnetite(Fe3O4), which has a different valence state, has emerged asa promising supercapacitor material due to its low cost and envi-ronmentally benign nature [18,19]. Wu et al. [20] reported on thecapacitive characteristics of nanostructured Fe3O4 as an electrodematerial, noting that Fe3O4 nanocrystallites provide a pseudo-capacitance of 27 F/(g-Fe3O4) in 1 M Na2SO3 solution.

In this work, we present the first report on the synthesis andcapacitive behaviors of nanosized Fe3O4/multiwalled carbon nano-tube (MWNT) composites. Fe3O4 nanoparticlesweredeposited on tosurfaces of MWNTs by a simple chemical coprecipitation method.The electrochemical performance of the Fe3O4/MWNTcomposite asa supercapacitor electrode was investigated by cyclic voltammetry(CV) and galvanostatic charge/discharge testing. The results werecompared with those of a pure MWNT electrode and discussed.

2. Experimental procedure

Multiwalled carbon nanotube (MWNT) crude materials (purity:90%) produced via chemical vapor deposition (CVD) werepurchased from Nano Solution Co., Ltd (Korea). The diameter andlength of the MWNTs were 10e25 nm and 1e10 mm, respectively. Aferric chloride (FeCl3$6H2O), ferrous chloride (FeCl2$4H2O), andammonium hydroxide aqueous solution (NH4OH, 25 wt.%) wasobtained from Aldrich.

The commercial MWNTs were purified in a mixture of concen-trated sulfuric acid and nitric acid (1:3 v/v) at 80 �C with constant

20 30 40 50 60 70

(440)(511)

(422)(222) (400)

(311)

Fe3O

4/MWNTs

Inte

nsit

y

2 theta / degree

MWNTs

(220)

Fig. 1. Powder XRD patterns of MWNTs and Fe3O4/MWNT composites.

O 1s

C 1sa

Y.-H. Kim, S.-J. Park / Current Applied Physics 11 (2011) 462e466 463

stirring for 6 h with reflux. The solution was then diluted withdistilled water and rinsed several times until the pH value becameneutral. Finally, it was filtered and dried under vacuum at 60 �C forfurther use.

The Fe3O4/MWNT composites were prepared by suspending1.0 g of purified MWNTs in a 200 mL of distilled water containing1.0 g (4.33 mmol) FeCl2$4H2O and 2.7 g (8.66 mmol) FeCl3$6H2O at50 �C under a N2 atmosphere. After the solution was sonicated for10 min, 10 mL of 8 M NH4OH aqueous solution was added drop-wise to precipitate the iron oxides while the mixture solution wassonicated. To promote complete growth of the nanoparticle crys-tals, the reaction was carried out at 50 �C for 30 min underconstant mechanical stirring. The precipitate was isolated undera magnetic field and washed with copious amounts of doublydistilled water and 150 mL of absolute ethanol for three times.Subsequently, the composites were dried under vacuum at 100 �Cfor 12 h.

Fe3O4/MWNTs composites were characterized using X-raydiffraction (XRD, Rigaku D/MAX), X-ray photoelectron spectroscopy(XPS, Thermo, ESCALAB 250), and transmission electron micros-copy (TEM, A JEM 2100F).

Working electrodes the supercapacitors were prepared bymixing 90 wt.% of prepared samples with 10 wt.% of PVDF asa binder. A small amount of 1-methyl-2-pyrrolidone (NMP) wasthen added to the composites to obtain a more homogeneousmixture. The mixture was then cast onto a Ni-foam currentcollector about 1 cm2 in size. After vacuum drying overnight at100 �C, the electrodes were impregnated with the electrolyte(1 M Na2SO3) under vacuum to guarantee thorough wetting.

Electrochemical measurements were performed at roomtemperature using an electrochemical work station (Iviumstat,Ivium Technologies) with a conventional three-electrode cell.

200 400 600 800

Cou

nts

/s

Fe 2p

Binding Energy / eV

700 705 710 715 720 725 730

Cou

nts

/ s

Fe 2p1/2

Binding Energy / eV

Fe 2p3/2b

Fig. 2. (a) XPS wide scan and (b) Fe 2p spectra of Fe3O4/MWNT composites.

3. Results and discussion

The Fe3O4/MWNT composites were synthesized by in situchemical coprecipitation of Fe2þ and Fe3þ in an alkaline solution inthe presence of MWNTs. The chemical reaction of the Fe3O4precipitation is as follows [21]:

2Fe3þ þ Fe2þ þ 8OH� / Fe3O4 þ 4H2O. (1)

The molar ratio of Fe2þ:Fe3þ was 1:2. The crystal phase andchemical composition of the synthesized samples were investi-gated by the X-ray powder diffractionmethod. Fig.1 shows the XRDpatterns of the Fe3O4/MWNT composites and pure MWNTs. Theresults indicate that the product was amixture of two phases: cubicFe3O4 (JCPDS file No. 75-0033) and MWNTs. Well-resolveddiffraction peaks reveal the good crystallinity of the Fe3O4 speci-mens. The diffraction peaks of MWNTs at 2q ¼ 26�and 43� corre-spond to the (002) and (100) reflection planes (inter-layeredspacing between adjacent graphite layers), respectively. No obviouspeaks from other phases were observed. Moreover, the diffractivepeaks of Fe3O4 were broadened, implying that the crystalline size ofthe Fe3O4 particles is quite small. The deposited Fe3O4 particleshave an average grain size of 14 nm, as calculated from theDebyeeScherrer equation, based on the line-broadening of themagnetite (311) reflection:

Lc ¼ Klb1=2cos q

; (2)

where Lc is the particle size (nm), K is the Scherrer constant (¼0.9),l is the X-ray wave length (CuKa ¼ 0.154 nm), q is the angle at the

peak maximum, and b1/2 is the width (radians) of the peak at halfthe height.

The Fe3O4/MWNTcomposites were also characterized using XPSanalysis. Fig. 2(a) shows the wide scan spectrum, where thephotoelectron lines at binding energies of about 285, 530, and

Y.-H. Kim, S.-J. Park / Current Applied Physics 11 (2011) 462e466464

711 eV are attributed to C1s, O1s, and Fe2p, respectively. The peaks at284.6 eV correspond with the sp2 hybridized carbon of theMWNTs.In Fig. 2(b), the peaks of Fe2p1/2 and Fe2p3/2, located at 711 and724.9 eV, further confirmed that the oxide in the sample was Fe3O4.

The detailed crystal structure of the Fe3O4/MWNT compositewas further examined by TEM and HRTEM analysis. Fig. 3(a) and (b)show a representative TEM image of the MWNT and the Fe3O4/MWNTs composite, respectively. This illustrates that the pureMWNTs present well-graphitized walls with a diameter of 20 nmand many additional Fe3O4 nanoparticles, with a size of about10e20 nm, were attached to the surface of the MWNTs and no freenanoparticles were found anywhere in the grid. The selected areadiffraction pattern of the Fe3O4 nanoparticle indicates that theFe3O4 nanoparticle was crystalline, and all diffraction rings could beindexed to the Fe3O4 nanoparticle with cubic symmetry [22]. Fig. 3(c) illustrates a corresponding HRTEM image, from which thedistance between adjacent lattice planes is measured to be about0.49 nm,which agrees well with the d-spacing of the (111) plane ofFe3O4 (0.484 nm). Furthermore, energy dispersive X-ray (EDX)analysis (Fig. 3(d)) confirms that only Fe, O and C are present in thecomposites, and the calculated atomic ratio of Fe to O is close to 3:4,which agrees well with the stoichiometric composition of Fe3O4.And the Cu signals are from the grid used for TEM examination.

Fig. 3. Microstructure of MWNT and Fe3O4/MWNT composites: (a) TEM image of normal Mcomposites; (c) and (d) show the HRTEM image and EDX spectrum of the Fe3O4/MWNT co

The CV method is considered a suitable tool for estimating thedifference between non-Faradaic and Faradaic reactions. Fig. 4(a)shows CV curves obtained in a three-electrode cell for the MWNTsand Fe3O4/MWNT composite electrodes at a voltage scan rate of5 mV/s in 1 M Na2SO3 electrolyte using a Pt plate as a counterelectrode and Ag/AgCl as a reference electrode in a potential rangesof �0.9e0.1 V. The MWNT electrodes presented a typical rectan-gular shape throughout the potential range, indicating an excellentcapacitive-like response. However, oxidative peaks were observedin the CV for the Fe3O4/MWNT composite electrodes. These peakswere attributed to redox reactions due to the deposited Fe3O4 onthe MWNT surfaces. For the Fe3O4/MWNT composite electrodes,the lack of symmetry of the curves is likely due to the combinedcontribution of the double layer capacitance from the MWNTs andpseudo-capacitances from Fe3O4 to the total capacitance. However,the hydrogen evolution became increasingly distinct at the upperrange of 0.2 V. This indicates that the potential range of Fe3O4/MWNT composites electrodes is limited to below 0.2 V. Thepseudo-capacitance reaction mechanisms of Fe3O4 in Na2SO3solution may result from the surface redox reaction of sulfur in theform of sulfate and sulfite anions, as well as the redox reactionsbetween Fe2þ and Fe3þ accompanied by intercalation of sulfite ionsto balance the extra charge with the iron oxide layers [20]:

WNT; (b) TEM image of Fe3O4/MWNT composites; the inset shows ED patterns of themposites, respectively.

-0.8 -0.6 -0.4 -0.2 0.0-0.8

-0.6

-0.4

-0.2

0.0

0.2

0.4

0.6Fe

3O

4/MWNTs

Cur

rent

den

sity

/ A

g-1

Volts

MWNTs

-0.8 -0.6 -0.4 -0.2 0.0-3

-2

-1

0

1

2

Cur

rent

den

sity

/ A

g-1

Volts

5 mV s-1

10 mV s-1

20 mV s-1

a

b

Fig. 4. CVs of MWNTs and Fe3O4/MWNT composites electrodes in 1 M Na2SO3 solution.(a) CVs at a sweep rate of 5 mV s�1; (b) CVs of Fe3O4/MWNTs composites electrodes atdifferent sweep rates.

0 200 400 600 800 1000

-0.8

-0.6

-0.4

-0.2

0.0

Fe3O

4/MWNTs

Vol

ts

Time / seconds

MWNTs

0 100 200 300

-0.8

-0.6

-0.4

-0.2

0.0

Vol

ts

Time / seconds

1 A g-1

0.5 A g-1

0.2 A g-1

0 200 400 600 800 1000

-0.8

-0.6

-0.4

-0.2

0.0 1 A g-1

0.5 A g-1

0.2 A g-1

Vol

ts

Time / seconds

a

b

c

Fig. 5. Chronopotentiograms of (a) MWNTs and Fe3O4/MWNT composite electrodes at0.2 A g�1, (b) pure MWNT electrodes at different current densities, and (C) Fe3O4/MWNT composite electrodes at different current densities.

Table 1Specific gravimetric capacitances of the MWNTs and Fe3O4/MWNT electrodes atdifferent current density.

Samples Specific capacitance (F g�1)

0.2 A g�1 0.5 A g�1 1 A g�1

(1st cycle)1 A g�1

(1000th cycle)

MWNTs 58 53 51 e

Fe3O4/MWNTs 165 132 120 85.1

Y.-H. Kim, S.-J. Park / Current Applied Physics 11 (2011) 462e466 465

FeOþ SO2�3 5FeSO4 þ 2e�: (3)

2FeIIOþ SO2�3 5

�FeIIIO

�þSO2�

3

�FeOIIIO

�þþ2e�: (4)

CV curves under different sweep rates of the Fe3O4/MWNTcomposite electrode are shown in Fig. 4(b). It can be seen that theCV curves of the Fe3O4/MWNTcomposites retained a similar shape,except that the redox peak gradually decreased with increasingsweep rate. This indicates an irreversible process of charge transferin Fe3O4. This is understandable given that mass transfer is difficultin the framework of a crystalline oxide [23].

The galvanostatic chargeedischarge behaviors of MWNT andFe3O4/MWNT composite electrodes were investigated by chro-nopotentiometry from �0.9e0.1 V in 1 M Na2SO3 solution; theresults are shown in Fig. 5 and Table 1. It can be seen that thechargeedischarge curves of the MWNTs were very symmetricaland had a triangular shape (Fig. 5(b)). This implies that the MWNTelectrodes clearly had electrochemical double layer capacitivebehavior, which is in agreement with the result obtained from theCV in Fig. 4(a).

The Fe3O4/MWNT composite electrodes showed symmetric V-tresponse behavior (Fig. 5(c)). This indicates that the char-geedischarge process of the electrode is reversible. Compared withthe MWNT electrodes, a much larger capacitance was obtained dueto the presence of iron oxide. As shown in Fig. 5(c), a small IR drop

0 200 400 600 800 10000

20

40

60

80

100

120

140

Spec

ific

cap

acit

ance

/ F

g-1

Cycle number / n

Fig. 6. Cycle performance of Fe3O4/MWNT composite electrodes at 1 A g�1 for 1000cycles.

Y.-H. Kim, S.-J. Park / Current Applied Physics 11 (2011) 462e466466

was observed, indicating a conductive characteristic of the ironoxide material, which decreased with a decrease in current density.The specific capacitance has been evaluated according to thefollowing equation and is shown in Table 1.

Csp ¼ IWðdV=dtÞ; (5)

where I is the applied current and W is the mass of each electrode.The specific capacitance values of MWNTs calculated from thedischarge curves using the above formula were 58, 53, and 51 F g�1,corresponding to discharge current densities of 0.2, 0.5, and 1 A g�1,respectively. For the Fe3O4/MWNT composite electrodes, thespecific capacitance was 165, 132, and 120 F g�1 at 0.2, 0.5, and1 A g�1, respectively. As the current density increased, the specificcapacitance decreased for both electrodes.

The chargeedischarge cycle stability of the Fe3O4/MWNTcomposite electrodes was also investigated by chro-nopotentiometry and the corresponding results with a currentdensity of 1 A g�1 for 1000 cycles are shown in Fig. 6. The depen-dence of the specific capacitance of the electrode on the cyclenumber could be divided into two ranges. The specific capacitanceof the composite decreased by about 2.4% from 1 to 400 cycles.From 400 to 1000 cycles, the decrease in the specific capacitancewas 12.7%. After, 1000 cycles, the specific capacitance was main-tained at 103 F g�1, which is 85.1% of the initial value.

4. Conclusions

We demonstrated a simple, effective, and reproducible copre-cipitation method to synthesize Fe3O4/MWNT composites. Nano-sized Fe3O4 particles with an average diameter of about 14 nmwereattached to the purified surfaces of MWNTs. The electrochemicalcharacteristics of the prepared Fe3O4/MWNT composite electrodesin 1MNa2SO3 solutionwere investigated to evaluate their potentialfor application in supercapacitors. From the CV results, Fe3O4showed a pseudo-capacitive behavior originating from redoxreactions with sulfite ions and iron oxide. Chargeedischargemeasurement of prepared electrodes at 0.2 A g�1 yielded a specificcapacitance of 58 and 165 F g�1 for pure MWNT and Fe3O4/MWNTcomposites, respectively. Furthermore, approximately 85.1% of thecapacitance of the Fe3O4/MWNT composites remained after 1000cycles. These results indicate that the Fe3O4/MWNT composites

have potential applicability as electrode materials ofsupercapacitors.

Acknowledgements

This work was supported by the IT Industrial Source TechnologyDevelopment Business of the Ministry of Knowledge Economy,Korea.

References

[1] B.E. Conway, Electrochemical Supercapacitors. Kluwer Academic/PlenumPublishers, New York, 1999.

[2] A. Burke, R&D considerations for the performance and application of elec-trochemical capacitors, Electrochim. Acta 53 (2007) 1083e1091.

[3] E. Raymundo-Piñero, F. Leroux, F. Béguin, A high-performance carbon forsupercapacitors obtained by carbonization of a seaweed biopolymer, Adv.Mater. 18 (2006) 1877e1882.

[4] J. Chmiola, G. Yushin, Y. Gogotsi, C. Portet, P. Simon, P.L. Taberna, Anomalousincrease in carbon capacitance at pore sizes less than 1 nanometer, Science313 (2006) 1760e1763.

[5] M.K. Seo, S.J. Park, Influence of air-oxidation on electric double layer capaci-tances of multi-walled carbon nanotube electrodes, Curr. Appl. Phys. 10 (2010)241e244.

[6] M.K. Seo, S.J. Park, Electrochemical characteristics of activated carbon nano-fiber electrodes for supercapacitors, Mat. Sci. Eng. B 164 (2009) 106e111.

[7] D.S. Dhawale, R.R. Salunkhe, V.S. Jamadade, D.P. Dubal, S.M. Pawar,C.D. Lokhande, Hydrophilic polyaniline nanofibrous architecture using elec-trosynthesis method for supercapacitor application, Curr. Appl. Phys. 10 (2010)904e909.

[8] Q.F. Wu, K.X. He, H.Y. Mi, X.G. Zhang, Electrochemical capacitance of poly-pyrrole nanowire prepared by using cetyltrimethylammonium bromide(CTAB) as soft template, Mater. Chem. Phys. (2007) 367e371.

[9] H. Li, J. Wang, Q. Chu, Z. Wang, F. Zhang, S. Wang, Theoretical and experi-mental specific capacitance of polyaniline in sulfuric acid, J. Power Sources190 (2009) 578e586.

[10] V.D. Patake, S.M. Pawar, V.R. Shinde, T.P. Gujar, C.D. Lokhande, The growthmechanism and supercapacitor study of anodically deposited amorphousruthenium oxide films, Curr. Appl. Phys. 10 (2010) 99e103.

[11] M. Nakayama, T. Kanaya, R. Inoue, Anodic deposition of layered manganeseoxide into a colloidal crystal template for electrochemical supercapacitor,Electrochem. Commun. 9 (2007) 1154e1158.

[12] U.M. Patil, K.V. Gurav, V.J. Fulari, C.D. Lokhande, O.S. Joo, Characterization ofhoneycomb-like “b-Ni(OH)2” thin films synthesized by chemical bath depo-sition method and their supercapacitor application, J. Power Sources 188(2009) 338e342.

[13] G.X. Wang, B.L. Zhang, Z.L. Yu, M.Z. Qu, Manganese oxdie/MWNTs compositeelectrodes for supercapacitors, Solid State Ionics 176 (2005) 1169e1174.

[14] A. Leela Mohana Reddy, S. Ramaprabhu, Nanocrystalline metal oxidesdispersed multiwalled carbon nanotubes as supercapacitor electrodes, J. Phys.Chem. C 111 (2007) 7727e7734.

[15] K.W. Nam, C.W. Lee, X.Q. Yang, B.W. Cho, W.S. Yoon, K.B. Kim, Electro-deposited manganese oxides on three-dimensional carbon nanotubesubstrate: supercapacitive behaviour in aqueous and organic electrolytes,J. Power Sources 188 (2009) 323e331.

[16] Y.S. Lin, K.Y. Lee, K.Y. Chen, Y.S. Huang, Superior capacitive characteristics ofRuO2 nanorods grown on carbon nanotubes, Appl. Surf. Sci. 256 (2009)1042e1045.

[17] C.C. Hu, K.H. Chang, M.C. Lin, Y.T. Wu, Design and tailoring of the nanotubulararrayed architecture of hydrous RuO2 for next generation supercapacitors,Nano Lett. 6 (2006) 2690e2695.

[18] J. Chen, K. Huang, S. Liu, Hydrothermal preparation of octadecahedron Fe3O4thin film for use in an electrochemical supercapacitor, Electrochim. Acta 55(2009) 1e5.

[19] X. Du, C. Wang, M. Chen, Y. Jiao, J. Wang, Electrochemical performances ofnanoparticle Fe3O4/activated carbon supercapacitor using KOH electrolytesolution, J. Phys. Chem. C 113 (2009) 2643e2646.

[20] N.L. Wu, S.Y. Wang, C.Y. Han, D.S. Wu, L.R. Shiue, Electrochemical capacitor ofmagnetite in aqueous electrolytes, J. Power Sources 113 (2003) 173e178.

[21] K. Tao, H. Dou, K. Sun, Interfacial coprecipitation to prepare magnetitenanoparticles: concentration and temperature dependence, Colloids Surf.A: Physicochem. Eng. Aspects 320 (2008) 115e122.

[22] T. Yang, C. Shen, Z. Li, H. Zhang, C. Xiao, S. Chen, Z. Xu, D. Shi, J. Li, H. Gao,Highly ordered self-assembly with large area of Fe3O4 nanoparticles and themagnetic properties, J. Phys. Chem. B 109 (2005) 23233e23236.

[23] D. Yuan, J. Zeng, N. Kristina, Y. Wang, X. Wang, Bi2O3 deposited on highlyordered mesoporous carbon for supercapacitors, Electrochem. Commun. 11(2009) 313e317.