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Journal of Electrochemical Science and TechnologyVol. 2, No. 1, 2011, 1-7DOI:10.5229/JECST.2011.2.1.001
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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]
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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
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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.
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4 Yu Ri Lee, Min-Sun Song, Kyung Min Lee, In Young Kim and Seong-Ju Hwang
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.
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Journal of Electrochemical Science and Technology, Vol. 2, No. 1, 1-7 (2011) 5
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.
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6 Yu Ri Lee, Min-Sun Song, Kyung Min Lee, In Young Kim and Seong-Ju Hwang
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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