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Electrochimica Acta 56 (2011) 5010–5015

Contents lists available at ScienceDirect

Electrochimica Acta

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reparation and electrochemical characterization of MnOOH nanowire–graphenexide

in Wang, Dian-Long Wang ∗

chool of Chemical Engineering & Technology, Harbin Institute of Technology, Harbin 150001, China

r t i c l e i n f o

rticle history:eceived 14 November 2010eceived in revised form 21 February 2011ccepted 18 March 2011vailable online 1 April 2011

eywords:

a b s t r a c t

MnOOH nanowire–graphene oxide composites are prepared by hydrothermal reaction in distilled wateror 5% ammonia aqueous solution at 130 ◦C with MnO2–graphene oxide composites which are synthe-sized by a redox reaction between KMnO4 and graphene oxide. Powder X-ray diffraction (XRD) analysesand energy dispersive X-ray analyses (EDAX) show MnO2 is deoxidized to MnOOH on graphene oxidethrough hydrothermal reaction without any extra reductants. The electrochemical capacitance of MnOOHnanowire–graphene oxide composites prepared in 5% ammonia aqueous solution is 76 F g−1 at current

−1

raphene oxidenOOH

upercapacitorydrothermal reactionanowire

density of 0.1 A g . Moreover, electrochemical impedance spectroscopy (EIS) suggests the electrochemi-cal resistance of MnOOH nanowire–graphene oxide composites is reduced when hydrothermal reaction isconducted in ammonia aqueous solution. The relationship between the electrochemical capacitance andthe structure of MnOOH nanowire–graphene oxide composites is characterized by cyclic voltammetry(CV) and field emission scanning electron microscopy (FESEM). The results indicate the electrochemicalperformance of MnOOH nanowire–graphene oxide composites strongly depends on their morphology.

. Introduction

Electrochemical supercapacitors are investigated in a wideange of energy storage applications and have become some ofost promising candidates for next generation power devices. The

apacitance of a supercapacitor consists of double-layer capaci-ance and pseudocapacitance [1–3]. Conventional electrochemicalapacitors are generated with state of the art electrode materialuch as high surface area carbon including activated carbon [4],arbon aerogel [5], and carbontube [6] which might not provideufficient energy and power densities or efficiencies for some elec-ric applications. Graphene which has a two dimensional carbonlane with one-atomic thickness is considered as a new carbonaterial for supercapacitors [7]. The chemical modified graphene

repared by thermal exfoliation of graphite oxide and chemicaleducing with hydrazine has many excellent performances, suchs good conductivity, large BET surface area, and good capacitiveroperties with specific capacitance of 135 F g−1 in 5.5 mol L−1 KOH

queous electrolyte [8].

Metal oxide modified graphene composites as candidate materi-ls for supercapacitors have attracted a great deal of attention when

∗ Corresponding author at: School of Chemical Engineering and Technology, POox 411, Harbin Institute of Technology, Harbin 150001, China.el.: +86 451 86413751; fax: +86 451 86413721.

E-mail address: dlwang@hit.edu.cn (D.-L. Wang).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.03.105

© 2011 Elsevier Ltd. All rights reserved.

ZnO–graphene [9], SnO2–graphene [10], Co3O4–graphene [11],Mn3O4–graphene [12] and MnO2–graphene [13] were recentlyexplored. Lu et al. [14] recorded ZnO–graphene and SnO2–grapheneachieved the excellent capacitive behavior with the specific capac-itances of 61.7 F g−1 and 41.7 F g−1. Yan et al. [15] studied thecapacitive behavior of MnO2–graphene and shown the excellentcapacitive performance of MnO2–graphene up to 310 F g−1 at scanrate of 2 mV s−1.

Manganese oxyhydroxide (MnOOH) is a stable polymorph oftrivalent hydroxide under ambient conditions, which is of consid-erable interest in many chemical processes including catalysis [16],model simulation [17], metal-air batteries [18], lithium ion batter-ies [19,20] and so on. And different nanostructures of MnOOH, suchas nanorods and nanobranches were fabricated via a hydrothermalprocess [21,22]. Recently, the redox transition of manganese diox-ide was found to involve the exchange of cation (M+) and electron,and crystalline MnOOH was shown to be the main electroactivespecies for the charge storage/delivery [23]. However, Hu et al. [24]studied the electrochemical performance of MnOOH nanowire sin-gle crystals and shown its poor capacitive performance. Therefore,the development of MnOOH nanowire–graphene oxide compositesis still significant to understand the charge storage mechanism ofmanganese dioxide in aqueous electrochemical capacitor. And theresearch of a simple, effective and mild pathway for the synthesis

of MnOOH–graphene oxide composites remains a big challenge.

In this work, to broad the synthesis methods ofMnOOH–graphene oxide composites (HMGO), a one-step

chimica Acta 56 (2011) 5010–5015 5011

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HMGO-A

HMGO-W

MGO

GO

MnO2

graphene MnOOHMn3O4

2

L. Wang, D.-L. Wang / Electro

ydrothermal procedure is developed. HMGO are synthesizedhrough hydrothermal reaction with MnO2–graphene oxide com-osites (MGO) which are synthesized by a redox reaction betweenMnO4 and graphene oxide. Furthermore, MnOOH nanowirean be directly formed on graphene oxide without any extraeductants. This method is easy and straightforward, thereforean be readily extended as a general procedure to the preparationf other metal oxyhydroxide–graphene nanocomposites. On thether hand, the electrochemical capacitance of HMGO is improved,hen MGO are treated in ammonia aqueous solution. Such a

esearch on the electrochemical performance of HMGO is reason-bly envisaged for understanding the charge storage mechanismf manganese dioxide in aqueous electrochemical capacitor. Andhe results show that the electrochemical specific capacitances ofanocomposites as prepared are in relation to their nanostructurerchitectures.

. Experimental

.1. Preparation of sample

Graphene oxide (GO) was prepared from natural graphite pow-ers according to modified Hummers method [25]. 6.5 g of graphiteowders was placed in 150 mL of cold (0 ◦C) concentrated H2SO4,nd then 19.5 g of KMnO4 was added gradually with stirring andooling, and the temperature of the solution was not allowed too up to 20 ◦C. The mixture was stirred for 40 min, and 460 mL ofistilled water was added slowly to an increase in temperature to8 ◦C. The temperature was held at 35 ± 3 ◦C 30 min. Finally, 1.4 Lf distilled water and 100 mL of 30% H2O2 solution were addedfter the reaction. The solution was held at room temperature for4 h and then the mixture was filtered and washed with 5% HClqueous solution until sulfate could not be detected with BaCl2.he graphite oxide obtained was dried at 50 ◦C for 72 h. Graphenexide was obtained after the treated graphite oxide being put intomuffle oven preheated to 1073 K for 30 s.

MnO2–graphene oxide composites were prepared by a redoxeaction between graphene oxide and potassium permanganate.5 mL of graphene oxide aqueous suspension (2.13 mg mL−1)as subjected to ultrasonic vibration for 1 h. And then 25 mL

f 0.1 mol L−1 KMnO4 aqueous solution was added into aboveraphene oxide suspension and stirred for 4 h. The black depositas collected by filtration and washed with distilled water, and

hen dried at 50 ◦CMnOOH nanowire–graphene oxide composites were prepared

y hydrothermal reaction in distilled water or 5% ammonia aque-us solution, respectively. 100 mg of MGO was added to 28 mLf distilled water or 5% ammonia aqueous solution in a Teflon-ined autoclave (45 mL). After stirring of 1 min with a stainless-steel

uddler, the autoclave was sealed, and heated at 130 ◦C for 24 h.he system was then allowed to cool to room temperature. Theesulting product was collected by filtration and washed with dis-illed water, and then dried at 40 ◦C. Herein, HMGO prepared byydrothermal reaction in distilled water and ammonia aqueousolution are designated as HMGO-W and HMGO-A, respectively.

.2. Characterization of sample

Powder X-ray diffraction (XRD) analyses were performed onigaku D/MAX-RC X-ray diffractometer with Cu Ka 1 (45 kV 50 mA,tep size = 0.02◦, 10◦ < 2� < 100◦) monochromated radiation in order

o identify the crystalline phase of the material. Morphologies ofhe products as prepared were observed on field emission scanninglectron microscopy (FESEM, Hitchi, S-4700 equipped with energyispersive X-ray analysis (EDAX)).

Fig. 1. XRD patterns of GO, MGO, HMGO-W and HMGO-A.

2.3. Electrochemical performance of sample

Cyclic voltammetry and electrochemical impedance spec-troscopy of electrode as prepared were investigated under aconventional three-electrode cell with 1 mol L−1 Na2SO4 aque-ous solution as electrolyte at room temperature. The workingelectrodes were fabricated by mixing the powders as preparedwith 20 wt% acetylene black and 10 wt% polytetrafluorene ethy-lene (PTFE) binder. A small amount of distilled water was added tothe mixture to produce a homogeneous paste. The mixture waspressed onto nickel foam current collectors (1.0 cm × 1.0 cm) tomake electrodes. The mass of the active material was in the rangeof 3 mg. A platinum foil and a saturated calomel electrode (SCE)served as the counter and reference electrode respectively. Cyclicvoltammetry (CV) was collected between 0.05 and 0.85 V (vs. SCE).Electrochemical impedance spectroscopy (EIS) was recorded underthe following conditions: AC voltage amplitude 5 mV, frequencyranges 0.01 Hz–100 kHz. Cyclic voltammetry and electrochemicalimpedance spectroscopy were conducted on a CHI 604B electro-chemical workstation (Shanghai CH Instrument Company, China).For galvanostatic charge/discharge tests, the two-electrode config-uration was fabricated. The electrodes as prepared were separatedby a thin polypropylene film with 1 mol L−1 Na2SO4 aqueous solu-tion as electrolyte, which were sandwiched in a stainless steel cellwith a pressure of 10 MPa. Galvanostatic charge/discharge testswere measured by a Xin Wei electrochemical workstation.

3. Results and discussions

3.1. Characterization of sample

The XRD patterns of GO, MGO, HMGO-W and HMGO-A asprepared are shown in Fig. 1. The peak of GO at around 21.5◦ cor-responding to (0 0 2) crystal plane of carbon is weak, signifyingthat GO has a highly amorphous nature. From the XRD patternof MGO, the peaks of 36.6◦, 55.4◦ and 67.0◦ are consistent withthe standard values of MnO2 (JPSDS, Card No. 30-0820). In theXRD patterns of HMGO-W and HMGO-A, the peaks of 26.0◦, 33.8◦,35.5◦ and 37.1◦ match well with the standard values of MnOOH(JPSDS, Card No. 41-1379), as well as the peaks of 17.5◦, 29.0◦

and 60.1◦ match with the standard values of Mn3O4 (JPSDS, CardNo. 24-0734). This indicates that manganese oxides compoundsin HMGO-W and HMGO-A consist of MnOOH and few of Mn3O4.The results prove that MnO2 in MGO is deoxidized to MnOOH

5012 L. Wang, D.-L. Wang / Electrochimica Acta 56 (2011) 5010–5015

Fig. 2. FESEM micrographs of GO, MGO, HMGO-W and HMGO-A: (a) FESEM image of GO; (b) and (c) FESEM images of MGO at low magnification and high magnification; (d)and (e) FESEM images of HMGO-W at low magnification and high magnification; (f) and (g) FESEM images of HMGO-A at low magnification and high magnification.

L. Wang, D.-L. Wang / Electrochimica Acta 56 (2011) 5010–5015 5013

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ig. 3. EDAX spectra from the surface of HMGO-W and HMGO-A: (a) EDAX spectros

n HMGO through hydrothermal reaction without any extraeductants.

The FESEM images of GO, MGO, HMGO-W and HMGO-A arehown in Fig. 2(a) shows GO has a nanoporous structure, whichesults from thermal exfoliation. Fig. 2(b) shows that amorphousnO2 prepared through the redox reaction is coated on graphene

xide. The EDAX spectra of HMGO-W and HMGO-A show that thelements present include Mn, O and C on the surface of graphenexide in Fig. 3(a) and (b) when XRD patterns show MnOOH is

repared through hydrothermal reaction. This indicates MnOOHanowire is formed on the surface of graphene oxide. Fig. 2(c) andd) shows the morphology of manganese oxides on graphene oxides changed from amorphous structure to nanowire when XRD pat-

0.0 0.2 0.4 0.6 0.8 1.0-250-200-150-100

-500

50100150200250

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ific

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Potential vs. SCE/V

GO HMGO-A MGO HMGO-W

a

0.0 0.2 0.4 0.6 0.8 1.0-30

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ig. 4. Cyclic voltammetry curves of the products as prepared between 0.05 V and 0.85 VGO, HMGO-W and HMGO-A at scan rate of 5 mV s−1; (b)–(d) present cyclic voltamme

0 mV s−1), respectively.

from the surface of HMGO-W; (b) EDAX spectroscopy from the surface of HMGO-A.

terns show MnO2 is deoxidized to MnOOH through hydrothermalreaction. Fig. 2(e) presents MnOOH nanowire in HMGO-W showswire-like structure with 100 nm of diameter. Fig. 2(f) shows thatMnOOH nanowire in HMGO-A directly grows on graphene oxideand has exact contact with graphene oxide when ammonia aque-ous solution is used in hydrothermal reaction. Moreover, Fig. 2(g)shows the diameter of MnOOH nanowire is reduced to around50 nm.

3.2. Electrochemical testing

Fig. 4 shows cyclic voltammetry curves of the products as pre-pared in 1 mol L−1 Na2SO4 aqueous electrolyte at scan rates of 5,

0.0 0.2 0.4 0.6 0.8 1.0-250-200-150-100

-500

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5mV/s 10mV/s 20mV/s 30mv/s 40mv/s

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-100-80-60-40-20

020406080

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Potential vs. SCE/V

5mV/s 10mV/s 20mV/s 30mV/s 40mV/s

d

(vs. SCE) in 1 mol L−1 Na2SO4 aqueous electrolyte: (a) specific capacitances of GO,try curves of MGO, HMGO-W and HMGO-A at different scan rates (5, 10, 20, 30,

5014 L. Wang, D.-L. Wang / Electrochimica Acta 56 (2011) 5010–5015

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treatment of hydrothermal reaction. The excellent cycling stabilityof HMGO-W and HMGO-A could be ascribable to well-dispersedMnOOH nanowire on graphene oxide greatly reducing loss of the

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ig. 5. (a) Galvanostatic charge/discharge curves of the capacitor cells with electrharge/discharge cycling stability of capacitor cells with electrodes as prepared.

0, 20, 30 and 40 mV s−1. As seen in Fig. 4(a), the curves haveood rectangular shapes at scan rate of 5 mV s−1. This implies theood capacitive performance of the products as prepared. The spe-ific capacitances of GO, MGO, HMGO-W and HMGO-A are 56 F g−1,49 F g−1, 10 F g−1 and 68 F g−1 at scan rate of 5 mV s−1 respectively.he specific capacitances of products as prepared are calculatedrom the CV curves according to the following equation [26]:

rQ

�Vm

here Cp (F g−1) is the specific capacitance, m (g) is the mass of thective material, Q is the average charge during the charge/dischargerocess, �V is the potential window. Fig. 4(b)–(d) shows thatMGO-A has an excellent capacitance retention ratio over aide range of scan rate when the specific capacitances of MGO

nd HMGO-W degenerate very seriously. Under the scan raterom 5 mV s−1 to 40 mV s−1, the capacitance retention ratios at0 mV s−1 are 52%, 60% and 78% for MGO, HMGO-W and HMGO-, respectively. The remarkable rate capability of HMGO-A coulde attributed to the increased effective interfacial contact areaetween MnOOH nanowire and the electrolyte, as well as the

ncreased contact area between MnOOH nanowire and graphenexide, resulted from the small diameter of MnOOH nanowire andhe exact contact of MnOOH nanowire with conductive graphenexide, can promote both the electrochemical utilization of thenOOH and the electric conductivity of the electrode.The galvanostatic charge/discharge curves of capacitor cells

ith different electrodes at current density of 0.1 A g−1 (based onhe average mass of two electrodes) are depicted in Fig. 5. Thehapes of the curves are closely linear and charge/discharge curvesxhibit a typical triangle symmetrical distribution, which indicatell electrode materials have good electrochemical capacitive char-cteristics. The specific capacitances of the products as preparedre calculated from the discharge plot of E–t curves as followingquation [27]:

r = I�t

�V

1Cr

= 1Csm1

+ 1Csm2

here CT is the total series capacitance of two electrodes (F) in

he electrochemical capacitor cell, I is the current (A), �t is theischarging time (s), �V is the voltage difference of discharge (V), Cs

s the specific capacitance of a single electrode (F g−1), as well as m1nd m2 are the active masses of the two electrodes (g), respectively.

s prepared between 0 and 0.9 V in 1 mol L−1 Na2SO4 aqueous electrolyte; (b) the

The specific capacitances of GO, MGO, HMGO-W and HMGO-A are 70 F g−1, 155 F g−1, 11 F g−1 and 76 F g−1 at current densityof 0.1 A g−1, respectively. Fig. 5(a) shows that the specific capac-itance of MGO is twice as much as that of graphene oxide whenamorphous MnO2 is prepared on graphene oxide. On the otherhand, the specific capacitance of HMGO-W decreases obviouslywhen amorphous MnO2 is transformed to MnOOH nanowire. How-ever, the specific capacitance of HMGO-A revives to 76 F g−1 whenHMGO-A are synthesized through hydrothermal reaction in 5%ammonia aqueous solution. Herein, this could be inferred that thespecific capacitance variability of HMGO-W is mainly attributedto the poor pseudocapacitive performance of MnOOH nanowirecompared to MnO2 [24]. And the electrochemical capacitive perfor-mance of HMGO-A is promoted, due to MnOOH nanowire directlygrowing on graphene oxide, as well as its small diameter and exactcontact with graphene oxide improving the electrochemical utiliza-tion and the electric conductivity of the electrode. Fig. 5(b) showsthe cycling stability of products as prepared in capacitor cell atcurrent density of 0.1 A g−1, which indicate that the capacitanceretention rates of MGO, HMGO-W and HMGO-A at the fiftieth cycleare 97%, 100% and 100%, respectively. The results suggest that thecycling stability of HMGO-W and HMGO-A are improved by the

Z'/ohm

Fig. 6. The Nyquist plots of the GO, MGO, HMGO-W and HMGO-A in 1 mol L−1

Na2SO4 aqueous electrolyte.

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ctive material caused by dissolution and detachment during theharge/discharge process.

To explain the relationship between the electrochemical per-ormance and the structure of the materials, the Nyquist plots ofll products are shown in Fig. 6. At very high frequencies, the inter-ept at real part (Z′) is a combinational resistance of ionic resistancef electrolyte, intrinsic resistance of substrate, and contact resis-ance at the active material/current collector interface (Re) [15].

major difference is the semicircle in the high frequency rangehich correspond to the charge transfer resistance (Rct) caused by

he Faradic reactions and the double-layer capacitance (CdL) on therain surface. The slope of the 45◦ portion of the curve is the Wur-urg resistance (Zw) which is a result of the frequency dependencef ionic diffusion/transport in the electrolyte and to the surfacef the electrode. CL is the limit capacitance. It can be seen thathe diameters of semicircles decrease in the order of HMGO-W,

GO, GO and HMGO-A. The results indicate that the charge trans-er resistance of HMGO-W increases when hydrothermal reactions conducted in distilled water, due to the poor pseudocapacitiveerformance of MnOOH nanowire increasing the electrochemicalolarization. However, the charge transfer resistance of HMGO-A

s reduced when hydrothermal reaction is conducted in ammoniaqueous solution, due to the small diameter of MnOOH nanowirend its exact contact with graphene oxide. The results suggest thathe electrochemical capacitive performance of MnOOH nanowirehich directly grows on graphene oxide is improved, due to its

mall diameter and exact contact with graphene oxide decreasingts charge transfer resistance.

. Conclusions

In summary, MnOOH nanowire can be directly formedn graphene oxide through hydrothermal reaction withnO2–graphene oxide composites and without any extra reduc-

ants. The specific capacitances of MGO, HMGO-W and HMGO-Are 155 F g−1, 11 F g−1 and 76 F g−1 at current density of 0.1 A g−1,

espectively. Cyclic voltammetry curves show that HMGO-A haven excellent capacitance retention ratio at scan rate from 5 mV s−1

o 40 mV s−1. Electrochemical impedance spectroscopy shows thatlectrochemical resistances of products decrease in the order of

[

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a Acta 56 (2011) 5010–5015 5015

HMGO-W, MGO, GO and HMGO-A. Moreover, it is found that theelectrochemical resistance of HMGO is reduced and the electro-chemical capacitance is improved when hydrothermal reaction isconducted in ammonia aqueous solution.

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