synthesis and electrochemical characterization of reduced ... › article ›...

Journal of Electrochemical Science and TechnologyVol. 2, No. 1, 2011, 1-7DOI:10.5229/JECST.2011.2.1.001

− 1 −

Journal of Electrochemical Science and Technology

Synthesis and Electrochemical Characterization of Reduced

Graphene Oxide-Manganese Oxide Nanocomposites

Yu Ri Lee, Min-Sun Song, Kyung Min Lee, In Young Kim and Seong-Ju Hwang†

Center for Intelligent Nano-Bio Materials (CINBM), Department of Chemistry and Nano Sciences, Ewha Womans University,

Seoul 120-750, Korea

ABSTRACT:

Nanocomposites of reduced graphene oxide and manganese (II,III) oxide can be synthesized by the

freeze-drying process of the mixed colloidal suspension of graphene oxide and manganese oxide,

and the subsequent heat-treatment. The calcined reduced graphene oxide-manganese (II,III) oxide

nanocomposites are X-ray amorphous, suggesting the formation of homogeneous and disordered

mixture without any phase separation. The reduction of graphene oxide to reduced graphene oxide

upon the heat-treatment is evidenced by Fourier-transformed infrared spectroscopy. Field emission-

scanning electronic microscopy and energy dispersive spectrometry clearly demonstrate the formation

of porous structure by the house-of-cards type stacking of reduced graphene oxide nanosheets and

the homogeneous distribution of manganese ions in the nanocomposites. According to Mn K-edge

X-ray absorption spectroscopy, manganese ions in the calcined nanocomposites are stabilized in

octahedral symmetry with mixed Mn oxidation state of Mn(II)/Mn(III). The present reduced

graphene oxide-manganese oxide nanocomposites show characteristic pseudocapacitance behavior

superior to the pristine manganese oxide, suggesting their applicability as electrode material for

supercapacitors.

Keywords: Reduced graphene oxide, nanocomposite, manganese oxide, pseudocapacitance, porous

structure

Received December 11, 2010 : Accepted March 22, 2011

Introduction

Recently supercapacitors attract intense research

interest as auxiliary power source for hybrid electric

vehicles and plug-in electric vehicles.1) The electrochemical

activity of many transition metal oxides is investigated

to develop new efficient electrode materials with high

energy density, high power density, and excellent rate

characteristics.2-4) Among various transition metal oxides,

low price, rich abundance, low toxicity, and diverse

oxidation states of manganese element render its oxides

promising electrode materials for supercapacitors and

lithium secondary batteries.4-7) Since the formation of

electron double layer and the redox reactions of electrode

component near the surface are responsible for the charge

storage capability of supercapacitors,7-11) it is important

to increase the surface area of manganese oxide through

the formation of nanostructures. Thus, various types of nano-

structured manganese oxides such as 0D nanoparticles/

hollow spheres, 1D nanowires/nanorods/nanotubes, 3D

nanourchin, and mesoporous materials are synthesized and

many of them are applied as electrode for supercapa-

citors.5,11-14) To further improve the electrode performance

of nanostructured manganese oxides, it would be useful

to couple these nano-materials with conductive carbon

species.14-21) The resulting enhancement of electrical

conductivity can make better the electrode performance

of manganese oxide. As a new family of nanostructured

†Corresponding author. Tel.: +82-2-3277-4370

E-mail address: [email protected]

2 Yu Ri Lee, Min-Sun Song, Kyung Min Lee, In Young Kim and Seong-Ju Hwang

carbon, 2D graphene nanosheets receive particular

attention because of their high electrical conductivity and

high morphological anisotropy.22-25) Since the exfoliated

graphene nanosheets are lack of lattice energy stabilization

along the c-axis, they can easily hybrid with foreign

species. As homologues to graphene nanosheets, the

subnanometer-thick nanosheets of layered manganese

oxide can be synthesized by soft-chemical exfoliation of

the layered K0.45MnO2.26) This layered MnO2 nanosheet

can be transformed into different type of manganese

oxide nanocrystals upon the heat-treatment.27) Thus, the

nanocomposite of reduced graphene oxide and manganese

oxide would be synthesized by the sedimentation of

the mixed colloidal suspension of graphene oxide and

exfoliated manganate nanosheets and the following

heat-treatment at elevated temperature.

In the present study, homogeneous nanocomposites

of reduced graphene oxide-manganese (II,III) oxide are

synthesized by the freeze-drying process of the mixed

colloidal suspension of graphene oxide and manganese

oxide, and the following calcination process. The crystal

structure, crystal morphology, and chemical bonding nature

of the obtained materials are examined with diffraction,

microscopic, and spectroscopic tools. Their electrode

activity is investigated to probe the electrode functionality

of the obtained manganese oxide-based materials.

Experimental

Preparation

Layered potassium manganese oxide K0.45MnO2 and

its protonated derivative were prepared by the solid state

reaction with the stoichiometric mixture of K2CO3 and

MnO2 at 800oC, and the reaction of the obtained K0.45MnO2

powder with 1 M HCl aqueous solution at room temperature

for 4 days. The exfoliated nanosheets of layered manganese

oxide were obtained in the form of stable aqueous colloidal

suspension by the intercalation of tetrabutylammonium

(TBA) cations into the protonated manganese oxide.26)

The exfoliated nanosheets of reduced graphene oxide

were prepared by modified Hummer method;28-30) the

homogeneous suspension of graphite oxide (150 ml)

was reacted with 150 ml of distilled water, 150 ml of

hydrazine solution (35 wt% in water), and 1.05 ml of

aqueous ammonia solution (28 wt% in water). The

vigorous stirring of the resulting suspension in a water bath

(~85oC) for 1 h led to the formation of the colloidal

suspension of reduced graphene oxide. Both the aqueous

colloidal suspensions could be homogeneously mixed

each other. The mixing ratio of manganate:graphene oxide

for the formation of the nanocomposites was adjusted

as 2 : 1, 1 : 1, and 1 : 2 (The obtained nanocomposites

were denoted as MGO2 : 1, MGO1 : 1, and MGO1 : 2,

respectively). The powdery nanocomposites were

restored from these mixed colloidal suspensions via freeze-

drying process. For the reduction of graphene oxide in the

nanocomposites, the as-prepared MGO nanocomposites

(MGO2 : 1, MGO1 : 1, and MGO1 : 2) were heated at

300oC under N2 flow for 3 h (The calcined nanocom-

posites were denoted as RMGO2 : 1, RMGO1 : 1, and

RMGO1 : 2, respectively).

Characterization

Powder X-ray diffraction (XRD) analysis was carried

out to study the crystal structure of graphene oxide-

manganese oxide nanocomposites before and after the

heat-treatment. Their crystal morphology and chemical

composition were probed by field emission-scanning

electron microscopy (FE-SEM) and energy dispersive

spectrometry. The optical property of the nanocomposites

was examined by measuring UV-vis absorption

spectroscopy. The chemical bonding nature of the

nanocomposites was investigated with Fourier transformed

infrared (FT-IR) spectroscopy and Mn K-edge X-ray

absorption spectroscopy (XAS). The present XAS spectra

were measured with the extended X-ray absorption fine

structure (EXAFS) facility installed at the beam line 7C

at the Pohang Accelerator Laboratory (PAL) in Korea.

XAS data were measured in a transmission mode using

gas-ionization detectors. The energy of the Mn K-edge

XAS data was referenced to the spectrum of Mn metal foil.

The experimental spectra were analyzed by the standard

procedure reported previously.31)

Electrochemical Measurement

The electrochemical activity of the reduced graphene

oxide-manganese oxide nanocomposites was studied by

measuring cyclic voltammograms (CV) and galvanostatic

charge-discharge cycles of these materials. These electro-

chemical properties of the present materials were

characterized with conventional three-electrodes cell

and a potentiostat/galvanostat (WonA Tech). The working

electrode materials were prepared by mixing the nano-

composite, acetylene black, and polyvinylidenefluoride

(PVDF) in a mass ratio of 75 : 20 : 5. After stirring the

mixtures for 1 h in N-methylpyrrolidone, the resulting

slurry was pressed on a stainless steel substrate. The

stainless steel plate was used as substrate for composite

Journal of Electrochemical Science and Technology, Vol. 2, No. 1, 1-7 (2011) 3

electrode material and as a current collector. The resultingcomposite electrode was vacuum-dried at 80oC for 30 min.A platinum mesh and saturated calomel electrode (SCE)were used as a counter electrode and a reference electrode,respectively. 0.2 M aqueous solutions of alkali metalsulfates (Li2SO4, Na2SO4, and K2SO4) were employed aselectrolyte. CV data were collected in the potential rangefrom 0 to 1.0 V at a scan rate of 20 mVs−1. A constantcurrent density of 200 mAg−1 was applied for thegalvanostatic charge-discharge cycling.

Results and Discussion

Powder XRD Analysis

The powder XRD patterns of the as-prepared grapheneoxide-manganese oxide nanocomposites and their calcinedderivatives are plotted in Fig. 1, compared with those ofthe reassembled nanosheets of graphene oxide andlayered manganese oxides.

The reassembled nanosheets of exfoliated layeredmanganate restored from the corresponding colloidalsuspension show well-developed (00l) reflections,reflecting the formation of TBA-intercalated layeredmanganate phase. Conversely, no distinct XRD peakcan be observed for the reassembled graphene oxidenanosheets, suggesting that the monolayer of graphene

oxide is too thin to form well-ordered intercalation structure.Instead, this material displays a very broad hump peakat 2θ = ~20-30o, indicating the disordered stacking ofexfoliated graphene oxide nanosheets.26) After thehybridization with graphene oxide, a series of (00l)XRD peaks newly appear at lower angle compared withthose of the reassembled layered manganese oxide,suggesting the lattice expansion of layered manganatealong the c-axis. As the content of graphene oxide in-creases, the intensity of newly observed Bragg reflectionsbecomes stronger. This finding suggests that the newXRD peaks originate from the interstratification structureof layered manganates monolayers and graphene oxidenanosheets with a high degree of hydration. The basalspacing calculated from the newly appeared XRD peaksis estimated as ~24.8-27.6 Å, which is much largerthan that of the reassembled manganese oxide (~16.7 Å).After the heat-treatment at 300oC, all of the (00l) peaks inthe nanocomposites are suppressed. Instead, a broadpeak at 2θ = ~20-30o is still observable, suggesting theformation of the disordered assembly of exfoliatednanosheets. No observation of distinct XRD peaks stronglysuggests that manganese oxide is homogeneously mixedwith reduced graphene oxide nanosheets without anyphase separation.

FE-SEM Analysis

The morphological change of the graphene oxide-manganese oxide nanocomposites before and after the heat-treatment is examined with FE-SEM analysis. The FE-SEMimages of the as-prepared nanocomposites and theircalcined derivatives are presented in Fig. 2. Wave-likesurface morphology formed by the stacking of exfoliatednanosheets is detected for all the present reassemblednanocomposites. After the heat-treatment at 300oC, allthe nanocomposites show house-of-cards type porousstacking structures of reduced graphene oxide sheets.Plenty of pores appear in the stacked structure of all thecalcined nanocomposites. The other nanocompositematerials composed of restacked layered MnO2 nanosheetsshow similar morphology in the FE-SEM images.11)

The N2 adsorption-desorption isotherm measurement forthese nanocomposite materials clearly demonstratedthe formation of porous structure with expanded surfacearea.11) Thus, the present FE-SEM results can be regardedas evidence for the formation of porous structure. Thereis no remarkable dependency of crystal morphology onthe mixing ratio of manganese oxide and graphene oxide,confirming homogeneous distribution of manganese oxide

Fig. 1. Powder XRD patterns of (a) reassembled graphene

oxide nanosheets, (b) reassembled manganese oxide nanosheets,

and the manganese oxide-graphene oxide nanocomposites of (c)

MGO 2 : 1, (d) MGO 1 : 1, (e) MGO 1 : 2, (f) RMGO 2 : 1,

(g) RMGO 1 : 1, and (h) RMGO 1 : 2.


and graphene oxide. The observed porous morphology of

the present nanocomposite is quite different from the non-

porous morphology of the pristine K0.45MnO2 material.26)

This result provides strong evidence for the usefulness of

reassembling with reduced graphene oxide nanosheets

in increasing the porosity of metal oxide.

The elemental distribution of manganese ions in the

nanocomposites is examined with EDS elemental mapping.

As can be seen clearly from Fig. 3, a uniform appearance

of Mn signal is obviously discernible in the nanocomposite

RMGO1 : 1, confirming the homogeneous distribution

of manganese oxide in the present nanocomposite.

FT-IR Spectroscopy

The effect of heat-treatment on the chemical bonding

nature of the graphene oxide component in the nanocom-

posites is examined with FT-IR spectroscopy. As plotted

in Fig. 4, the FT-IR spectra of the as-prepared nanocom-

posites demonstrate several IR bands related to the carbon-

oxygen bonds and sp3 carbon-hydrogen bonds in the wave-

number region of 900-1800 and 2900-3000 cm−1,32-34)

respectively. These spectral features indicate the presence

of graphene oxide and TBA+ ions, respectively. After the

heat-treatment, all these peaks are markedly depressed,

confirming the reduction of graphene oxide in the nano-

Figure 2. FE-SEM images of the graphene oxide-manganese oxide nanocomposites and their calcined derivatives: (a) MGO

2:1, (b) MGO 1:1, (c) MGO 1:2, (d) RMGO 2:1, (e) RMGO 1:1, and (f) RMGO 1:2.

Fig. 3. EDS mapping/FE-SEM image of the reduced graphene

oxide-manganese oxide nanocomposite RMGO 1 : 1.

Fig. 4. FT-IR spectra of the graphene oxide-manganese oxide

nanocomposites and their calcined derivatives: (a) MGO 2 : 1,

(b) MGO 1 : 1, (c) MGO 1 : 2, (d) RMGO 2 : 1, (e) RMGO 1 : 1,

and (f) RMGO 1 : 2.


composites into the reduced graphene oxide and the

removal of TBA+ cations.

Mn K-Edge XANES Analysis

The local structure and electronic configuration of man-

ganese ions in the as-prepared nanocomposite MGO

1 : 1 and its calcined derivative RMGO1 : 1 are investigated

with Mn K-edge X-ray absorption near-edge structure

(XANES) spectroscopy. Fig. 5 represents the Mn K-edge

XANES spectra of the nanocomposites, compared with

those of the pristine K0.45MnO2, the exfoliated MnO2nanosheets, MnO, Mn2O3, and β-MnO2. The as-prepared

nanocomposite shows nearly identical edge position to

those of exfoliated MnO2 nanosheets, which are higher

than that of the reference Mn(III)2O3 but slightly lower

than that of the β-Mn(IV)O2. This indicates that there

is no significant change in the Mn oxidation state upon

the nanocomposite formation with graphene oxide and

the as-prepared nanocomposite possesses the mixed

valent Mn(III)/Mn(IV) ions. After the heat-treatment, the

as-prepared manganese oxide-graphene oxide nano-

composite displays a remarkable red-shift of edge position

to the midway of the reference MnO and Mn2O3, indicating

the reduction of Mn oxidation state to Mn(II)/Mn(III).

All the present manganese compounds including both

the nanocomposites exhibit only a weak intensity for the

pre-edge peaks P and/or P’ corresponding to the dipole-

forbidden 1s→ 3d transition.35-37) Since this transition is

not allowed for the centrosymmetric octahedral symmetry,

the observed weak intensity of these features confirms

the stabilization of manganese ions in the octahedral

symmetry in the nanocomposites with graphene oxide

before and after calcination process. The main-edge peak

B related to the dipole-allowed 1s→ 4p transition reflects

sensitively the local atomic arrangement of manganese

oxide.36,37) A sharp and intense peak B is observable for

the layered manganese oxide consisting of edge-shared

MnO6 octahedra. Like the pristine K0.45MnO2, the as-

prepared nanocomposite shows a sharp peak B, indicating

the maintenance of layered manganate lattice after the

composite formation. After the heat-treatment, this peak

B becomes broad and weak, strongly suggesting a structural

modification of layered manganese oxide component into

non-layered manganese oxide phase.

Electrochemical Measurements

The electrode activity of the reduced graphene oxide-

manganese oxide nanocomposites is tested by measuring

the CV data of these materials with several electrolytes.

The CV data of the nanocomposite RMGO2 : 1 collected

with the aqueous solutions of 0.2 M alkali metal sulfate

are presented in the left panel of Fig. 6.

The rectangular-type CV curves are observable for the

present nanocomposite without any distinct redox peaks,

showing its pseudocapacitance behavior. Overall features

of the CV data are similar for the electrolytes of Li2SO4and K2SO4. A more severe distortion in the CV curve is

observed for the Na2SO4 electrolyte. In comparison with

the Li2SO4 and Na2SO4 solutions, a larger specific

Figure 5. Mn K-edge XANES spectra for the graphene

oxide-manganese oxide nanocomposite (a) MGO 1:1 and

its calcined derivative (b) RMGO 1:1, (c) MnO, (d) Mn2O3,

(e) b-MnO2, (f) the pristine K0.45MnO2, and (g) the

exfoliated MnO2 nanosheets.

Fig. 6. (Left) CV data of RMGO 2 : 1 collected with the electro-

lytes of 0.2 M Li2SO4 (dashed lines), 0.2 M Na2SO4 (dot-dashed

lines), and 0.2 M K2SO4 (solid lines) solutions. (Right) CV data

of (a) RMGO 2 : 1, (b) RMGO 1 : 1, and (c) RMGO 1 : 2 with

the electrolyte of 0.2 M K2SO4 solutions. A scan rate of 20 mVs−1

is applied.


capacitance can be obtained for the aqueous K2SO4 solution.

In general, two mechanisms are responsible for the specific

capacitance of metal oxide electrode; the surface adsorption/

desorption and the intercalation/deintercalation of alkali

metal ions.12) Due to the smaller hydration radius of

potassium cation, this ion is advantageous in terms of the

former mechanism. Conversely, in terms of the latter

mechanism, the small ionic size of lithium ions is helpful in

increasing the specific capacitance of manganese

oxide.12) That the specific capacitance of the present

nanocomposite is only slightly larger for the K2SO4electrolyte than for the Li2SO4 electrolyte suggests that

both the aforementioned mechanisms make contribution

to the capacitance behavior of the present nanocom-

posites. Based on this result, the CV data of all the present

nanocomposites are measured in the electrolyte of aqueous

0.2 M K2SO4 solution. As can be seen from the right

panel of Fig. 6, the nanocomposite of RMGO 2 : 1

shows a higher electrochemical activity than the other

nanocomposites.

Also the galvanostatic charge-discharge cycling is

carried out for the reduced graphene oxide-manganese

oxide nanocomposites. As illustrated in Fig. 7, the

pseudolinear time-dependences of potential confirm the

pseudocapacitance behaviour of the present nanocompos-

ites. From the present cycling data, the initial capacitance

is estimated as 80 Fg−1 for RMGO2 : 1, 77 Fg−1 for

RMGO1 : 1, and 25 Fg−1 for RMGO1 : 2, respectively.

There is no significant capacitance fading for initial

several cycles. The present results of electrochemical

measurement clearly demonstrate that the increase of

reduced graphene oxide leads to the decrease of the

specific capacitance of nanocomposite. This strongly

suggests that the high content of reduced graphene oxide

is not advantageous for improving the electrode perfor-

mance of the nanocomposite. Compared with these nano-

composites, the pristine K0.45MnO2 delivers smaller

capacitance of ~50 Fg−1,11) underscoring the benefit of

hybridization in improving the electrode performance.

The observed improvement of the specific capacitance

of manganese oxide is attributable to the formation of

porous structure during restacking of nanosheets.

Conclusion

In the present work, the porous nanocomposites

composed of reduced graphene oxide and manganese

(II,III) oxide are synthesized by mixing two precursor

colloidal suspensions and heating the as-prepared

nanocomposite. The calcined reduced graphene oxide-

manganese oxide nanocomposites possess a porous

morphology formed by the house-of-cards stacking of

reduced graphene oxide nanosheets. The heat-treatment

for the as-prepared nanocomposite induces the change of

layered manganese oxide to non-layered manganese oxide

with the reduced oxidation state of Mn(II)/Mn(III) and

the formation of reduced graphene oxide. The present

nanocomposites show pseudocapacitance behavior,

indicating the applicability of these materials as electrode

material of supercapacitor. Currently we are trying to apply

the present synthetic strategy to other 2D nanostructured

materials such as layered cobalt oxide, layered ruthenium

oxide, etc.

Acknowledgment

This work was supported by National Research

Foundation of Korea Grant funded by the Korean

Government(KRF-2008-313-C00442). The experiments

at PAL were supported in part by MOST and POSTECH.

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Fig. 7. Galvanostatic charge-discharge curves of several initial cycles at a constant current density 200 mAg−1 of (a) RMGO 2 : 1,

(b) RMGO 1 : 1, and (c) RMGO 1 : 2 collected with the electrolyte of 0.2 M K2SO4 solution.


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