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Reduced graphene Oxide–MnO2 hollow sphere hybrid nanostructures ashigh-performance electrochemical capacitors†

Hao Chen,a Shuxue Zhou,a Min Chena and Limin Wu*ab

Received 30th July 2012, Accepted 10th October 2012

DOI: 10.1039/c2jm35054h

This paper presents the first successful fabrication of reduced graphene oxide (RGO)–MnO2 hollow

sphere (HS) hybrid electrode materials through a solution-based ultrasonic co-assembly method. The

porous structure of these MnO2 hollow spheres and the excellent dispersion of active materials give the

as-fabricated RGO–MnO2 HS hybrid electrodes excellent specific capacitance and energy density,

which can reach up to 578 F g�1 and 69.8 W h kg�1, respectively. These values are considerably larger

than those of most reported graphene–MnO2 based hybrid electrochemical capacitors. This solution-

processed method can also be used for the hybridization of graphene with other metal oxides in the

fabrication of high-performance electrochemical capacitors.

Introduction

Electrochemical capacitors (ECs) have attracted great interest

during the past decades because they charge and discharge

quickly, maintain a high power density (1–2 orders of magnitude

higher than that of batteries), superior cycle lifetime (2–3 orders

of magnitude better than that of batteries), and are highly reli-

able.1,2 Depending on the mechanism used to store charge and on

the active materials used, ECs are usually divided into three

types: electric double-layer capacitors, pseudocapacitors, and

hybrid electrochemical capacitors.3 Electric double-layer capac-

itors, which store charges electrostatically via reversible ion

absorption at the electrode/electrolyte interface, commonly use

carbon-based active materials with large surface areas.4 In

contrast, pseudocapacitors use fast and reversible redox reac-

tions at the surfaces of the electroactive materials that store the

charge. The large specific pseudocapacitance of faradaic elec-

trodes exceeds that of carbon-based materials using double-layer

charge storage. Typical active pseudocapacitive materials include

transition metal oxides, such as RuO2, MnO2, Fe3O4, and NiO,

and conducting redox polymers such as polyanilines, poly-

pyrroles, and polythiophenes.5–13 The third group of ECs

includes hybrid capacitors, which usually combine one battery-

type faradaic electrode (energy source) and a capacitive electrode

aDepartment of Materials Science, Fudan University, Shanghai 200433,China. E-mail: lmw@fudan.edu.cnbAdvanced Materials Laboratory, Fudan University, Shanghai 200433,China

† Electronic supplementary information (ESI) available: SEM images ofthe as-synthesized MnO2 hollow spheres and MnO2 nanoparticles;nitrogen (77 K) adsorption/desorption isotherms and BJH pore sizedistributions of as-synthesized MnO2 hollow spheres and MnO2

nanoparticles; FTIR spectra of graphene oxide and MnO2 HS powder;XRD patterns of the as-prepared MnO2 hollow spheres andnanoparticles. See DOI: 10.1039/c2jm35054h

This journal is ª The Royal Society of Chemistry 2012

(power source) in the same cell.14 This type of EC generally

shows profoundly enhanced capacitance and increased energy

or power density over electric double-layer capacitors and

pseudocapacitors.15–18

One of the most feasible ways to create high-capacitance

hybrid ECs with long cycle lives is to explore novel electrode

material systems with rationally designed morphology, size, and

combinations of materials. For example, incorporating abun-

dant-earth capacitive carbon materials with low-cost pseudoca-

pacitive metal oxides can offer both cost advantage and,

potentially, high performance. This allows the systems to benefit

from both electric double-layer capacitance and pseudocapaci-

tance.3 The combination of carbon-material composites (nano-

porous carbons, carbon nanotubes, graphene, etc.) with low-cost

metal oxides (MnO2, Fe3O4, NiO, etc.) into active electrode

materials has seen a great deal of progress.3,19–26 This is especially

true of graphene–metal oxide nanocomposite-based ECs, which

show a great deal of promise for large-scale energy storage

systems.27–30 However, most of the metal oxides in the graphene-

containing composites reported to date are in the form of

nanoparticles. Nanoparticles usually undergo severe aggregation

because of nonoptimal contact between the graphene sheets. This

prevents the active nanoparticles from producing an adequate

opposing force.26 High graphene loading also tends to promote

aggregation due to the attractive forces between layers during the

drying process.31,32 Because specific capacitance is related to the

effective specific surface area of electrode materials, these

aggregations of active materials will decrease surface areas and in

this way decrease the capacitance of electrode materials.

MnO2 is a very promising pseudocapacitive material due to its

low cost, environmental benignity, and high theoretical specific

capacitance, andmany graphene–MnO2 compositematerials have

been investigated in recent years.18,33–35Forexample, compositesof

graphene andMnO2 nanoparticles,36 nanowires,18,33 nanosheets,37

J. Mater. Chem., 2012, 22, 25207–25216 | 25207

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and flower-like,20 needle-like,38 honeycomb-like39 morphologies,

and the composites with textile,3 paper,40 and multilayer

morphologies,41 have all been used to enhance the capacitance of

hybrid electrodes. Activated carbon nanofibers, carbon nano-

tubes, and conducting polymers have also been incorporated into

graphene–MnO2 composites to improve electrical conductivity

and enhance specific capacitance.21,22,42 These interesting works

may motivate the further exploration and development of higher-

performance graphene–MnO2 hybrid ECs.

Hollow micro-/nano-structured materials with large surface

areas offer particular advantages to ECs due to their large

specific surface areas and easy transport of species.9,43–47

However, to date, little research involving the use of graphene-

hollow sphere hybrid structures in ECs has been performed. In

this study, we report the first successful fabrication of reduced

graphene oxide (RGO)–MnO2 hollow sphere (HS) hybrid elec-

trode materials using a solution-based ultrasonic co-assembly

process. This process differs from common methods of fabri-

cating ECs, such as mechanical blending and milling, in several

ways.21,33,37,39,48 First, graphene oxide (GO), MnO2 HS, and

acetylene black (AB, as electrical conductor) were dispersed

using an ultrasonic machine. They were then assembled into

hybrid structures by hydrogen bonding and electrostatic inter-

actions. After vacuum thermal treatment, GO was reduced to

RGO, forming RGO–MnO2 HS hybrid materials. Unlike

previous techniques and active materials, this process does not

suffer from the self-aggregation of RGO sheets.49,50 It facilitates

excellent dispersion of active materials and electrical conductors.

The as-fabricated active materials possess a porous structure for

electrolyte access, which enhances the specific capacitance and

energy density of the electrodes. The highest specific capacitance

and energy density of RGO–MnO2 HS hybrid material was

found to reach 578 F g�1 and 69.8 W h kg�1, respectively, at a

current density of 0.5 A g�1. These values are substantially larger

than those reported for graphene–MnO2-based hybrid

ECs.3,21,22,51,52 This suggests that the present RGO–MnO2 HS

hybrid electrode materials may have applications in high-

performance energy storage devices.

Experimental procedure

Materials

Concentrated sulfuric acid (98%, H2SO4), hydrochloric acid

(37%, HCl), and potassium permanganate (KMnO4) were

purchased from Shanghai Chemical Reagent Co., Ltd (China).

Sodium nitrate (NaNO3), potassium hydroxide (KOH), graphite

powder (8000 mesh), hydrogen peroxide (30%, H2O2), manga-

nese chloride (MnCl2$4H2O), and tetrahydrofuran (THF) were

purchased from Sinopharm Chemical Reagent Corp (China).

Acetylene black (F-900, AB) was purchased from Tianjin Ebory

Chemical Co., Ltd (China). All reagents were used as received.

Fabrication of RGO–MnO2 electrodes

GO was prepared from graphite powder using the method

described by Hummers and in our previous report.53,54 MnO2 HS

and nanoparticles (NP) were synthesized as previously repor-

ted.55 The as-synthesized MnO2 HS were found to have

diameters of about 300–600 nm and a shell thickness of about

25208 | J. Mater. Chem., 2012, 22, 25207–25216

40–100 nm as measured by SEM (Fig. S1a†). The typical solu-

tion-based ultrasonic co-assembly method can be described as

follows: a GO suspension in THF (0.405%, 5.0 g) was diluted in

THF (13 mL) and ultrasonically dispersed for 60 min. Then the

MnO2 HS aqueous suspension was added (0.046%, 0.5 g). This

suspension was ultrasonically dispersed for another 30 min, and

AB was slowly added (14.1 mg AB dispersed in 1.5 mL THF).

This mixture was ultrasonically dispersed for another 30 min and

then allowed to stand for 30 min, during which GO–MnO2 HS–

AB composites precipitated from the suspension. When most of

the solvent was removed by centrifugation, the GO–MnO2 HS–

AB composites were transferred to a flask and then dried at room

temperature under a vacuum for 12 h to remove residual solvent.

After that, the samples were heated at 150 �C under a vacuum for

1 h to reduce the GO into RGO, forming RGO–MnO2 HS

composites. The as-obtained composites were added to a poly-

tetrafluorene–ethylene solution as the binders to produce a

homogeneous paste and then pressed onto nickel foam current-

collectors to make RGO–MnO2 HS hybrid electrodes (here

called RGO–MnO2HS-U electrode, where U indicates the

ultrasonic co-assembly method). Using the same procedure,

RGO-U, MnO2HS-U, MnO2NP-U, and RGO–MnO2NP-U

electrodes could also be fabricated. The loading densities of

active materials were kept at about 9 mg cm�2 for all electrodes.

For the sake of comparison, composite electrodes were also

fabricated using the mechanical milling method as follows: a

mixture of RGO andMnO2 HS powder (53.3 wt% of MnO2 HS),

23 wt% of AB, 5 wt% of polytetrafluorene–ethylene (as a binder)

and a small amount of ethanol was prepared by milling,

producing a homogeneous paste. This paste was then pressed

onto nickel foam current-collectors to produce RGO–MnO2 HS

composite electrodes (here called RGO–MnO2HS-M electrodes,

where M indicates the simple milling method).

Characterization

The morphologies were observed by scanning electron micros-

copy (SEM, S-4800, Hitachi). The crystalline structure was

characterized by X-ray diffraction (XRD) patterns recorded in a

Rigaku D/max-kA diffractometer with Cu Ka radiation, and

Raman spectroscopy (Jobin Yvon LabRam-1B). The chemical

composition of the samples were investigated using Fourier

transform infrared spectroscopy (FTIR, Nicolet Nexus 470) and

X-ray photoelectron spectroscopy (XPS, PHI 5000C ESCA

System). The zeta-potentials were measured using Malvern

Zetasizer Nano ZS90 equipment. An automated adsorption

apparatus (Micromeritics, ASAP 2010) was used to analyze the

surface characteristics of the samples using gas physisorption at

77 K. The specific surface area and mesopore volume of each

sample were evaluated using the Brunauer–Emmett–Teller

(BET) and the Barrett–Joyner–Halenda (BJH) equations,

respectively.

Electrochemical measurement

The electrochemical properties of the as-obtained electrodes were

investigated under a three-electrode cell configuration at room

temperature. The electrodes were soaked in a 1 MKOH solution

and degassed in a vacuum for 5 h before the electrochemical test.

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Platinum foil and a saturated calomel electrode (SCE) were used

as the counter and reference electrodes, respectively. The cyclic

voltammetry (CV), galvanostatic charge–discharge, and elec-

trochemical impedance spectroscopy (EIS) measurements were

conducted on a CHI 660B electrochemical workstation

(Shanghai CH Instrument Company, China).

Results and discussion

Fabrication of RGO–MnO2 HS hybrid electrodes

Scheme 1 briefly describes the fabrication of RGO–MnO2 HS

hybrid materials through a solution-based ultrasonic co-

assembly process. The GO suspension was first exfoliated into

GO sheets by ultrasonic dispersion. Then MnO2 HS suspension

was added to the as-prepared GO suspension to formGO–MnO2

hybrids by hydrogen bonding between the carboxyl groups of the

GO sheets and the hydroxyl groups of MnO2 HS (ESI, Fig. S2†).

After that, AB was added to form the GO–MnO2 HS–AB hybrid

via electrostatic interactions between GO–MnO2 HS and AB

(zeta potentials of GO, MnO2 HS and AB are �54.8, �34.4, and

+18.2 mV, respectively). After most of the solvent had been

removed by centrifugation, the GO–MnO2 HS–AB hybrid was

transferred to a flask and dried at room temperature under a

vacuum for 12 h to remove residual solvent. It was then heat-

treated at 150 �C under a vacuum for another 1 h, during which

time GO was reduced to RGO,56 producing RGO–MnO2 HS

hybrid materials (RGO–MnO2 HS–AB).

Fig. 1a and b show typical SEM images of the as-obtained

RGO–MnO2 HS hybrid materials. The low-magnification image

indicates that the hybrid material possesses a multilayer-like

structure. Higher magnification shows that RGO sheets interact

with MnO2 HS to form porous hybrid structures: MnO2 HS and

AB nanoparticles are well dispersed throughout the hybrid

structure, and the self-aggregation of graphene sheets can be

obviously depressed. As shown in Fig. 1c, the C 1s, O 1s, andMn

2p peaks of samples in XPS scan spectra have binding energies of

about 286, 532, and 650 eV, respectively. The GO contains 72.3

and 27.7% of C and O elements, respectively, while RGO has

Scheme 1 Schematic of the fabrication of RGO–MnO2 HS hybrid ECs: (a)

addition of MnO2 HS suspension and interaction between GO sheets and M

interaction between GO–MnO2 HS and AB to form GO–MnO2 HS–AB hybri

temperature for 12 h, and vacuum heat treatment at 150 �C for 1 h to produ

This journal is ª The Royal Society of Chemistry 2012

81.3 and 18.7% of C and O elements, respectively. The decrease

in oxygen content shows that GO has been reduced to RGO by

vacuum thermal treatment. Fig. 1c also shows that the RGO–

MnO2 HS–AB hybrid consists of 66.8, 25.7, and 7.5% of C, O

and Mn, respectively. Compared with RGO, the increasing O

and Mn contents prove that the MnO2 exists in the hybrid. In

addition, besides the C 1s (286 eV) and O 1s (532.8 eV) signals

from graphene sheets, the O 1s peak observed at 530.1 eV is

assigned to the oxygen bonded with manganese (Mn–O) in

MnO2 HS (Fig. 1d). The Mn 2p XPS spectrum exhibits two

characteristic peaks at 643.0 and 654.5 eV, corresponding to the

Mn 2p3/2 and Mn 2p1/2 spin-orbit peaks of MnO2 HS (Fig. 1e),

further confirming the presence of MnO2 HS in the hybrid.18,57

The Raman spectra display a D band at 1332 cm�1, a G band at

1575 cm�1 for RGO and AB, and another band at 640 cm�1 for

the Mn–O stretching vibration of MnO2 HS (Fig. 1f). These

results further indicate that the hybrid structure is composed of

RGO and MnO2.33,58

Electrochemical properties

Fig. 2a–c presents the cyclic voltammetry (CV) curves of RGO-

U, MnO2HS-U, and RGO–MnO2HS-U electrodes within the

electrochemical window from �0.7 to 0.3 V. The nearly rectan-

gular CV curves of RGO-U, MnO2HS-U, and RGO–MnO2HS-

U indicate the strong supercapacitor nature of the fabricated

electrodes. The lack of symmetry of the CV curves of RGO-U

and RGO–MnO2HS-U can be attributed to the combined

double-layer and pseudocapacitive contributions to the total

capacitance.59,60 Because specific capacitance (Cs) is proportional

to the average area of a CV curve,61 the comparison of average

areas, as shown in Fig. 2d, demonstrates that the RGO–

MnO2HS-U electrode possesses an intermediate specific capaci-

tance between that of the RGO-U and MnO2HS-U electrodes

and much higher capacitance than either of these two types of

electrodes when the scan rate is decreased to 0.5 mV s�1 (Fig. 2e).

This can be attributed to the synergistic effect from the RGO–

MnO2 HS hybrid materials as follows: (i) the hybrid structure of

RGO–MnO2 HS can prevent the self-aggregation of RGO sheets

exfoliation of graphite oxide into GO sheets by ultrasonic dispersion, (b)

nO2 HS through hydrogen bonding, (c) addition of AB suspension and

d via electrostatic interactions, (d) centrifugation, vacuum drying at room

ce RGO–MnO2 HS hybrid materials.

J. Mater. Chem., 2012, 22, 25207–25216 | 25209

Fig. 1 (a and b) SEM images of RGO–MnO2 HS hybrid materials. (c) XPS scans of GO, RGO, and RGO–MnO2 HS–AB hybrid. (d) O 1s and (e) Mn

2p XPS scans of RGO–MnO2 HS–AB hybrid. (f) Raman spectra of RGO, MnO2 HS, AB, and RGO–MnO2 HS–AB composite.

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through hydrogen bonding between the hydroxyl groups of

hydroxylated surfaces of RGO sheets and MnO2 HS; (ii) RGO

nanosheets with higher surface area can provide better conduc-

tive paths for fast electron transportation by excellent interfacial

contact between MnO2 HS and RGO sheets; (iii) the formed

porous hybrid strucutre promotes ion diffusion and electrolyte

access within the electrode materials, which facilitates RGO and

MnO2 HS to contribute their electric double-layer capacitance

(RGO) and redox-based pseudocapacitance (MnO2 HS). The

charge–discharge curves of the RGO–MnO2HS-U hybrid elec-

trode, as shown in Fig. 2f, are nearly symmetrical except for a

slight curvature, indicating pseudocapacitive contribution along

with the electric double-layer contribution.

According to Cs ¼ I � Dt/(DV � m) (where I (A) is the

discharge current, Dt (s) is the discharge time, DV (V) is the

potential change during the discharge, and m (g) is the mass of

the active material in the electrode), the Cs of RGO–MnO2 HS

hybrid electrode at current densities of 0.50, 0.75, 1.00, 1.25,

and 1.50 A g�1 can be calculated to be 578, 364, 241, 169, and

136 F g�1, respectively, from its galvanostatic discharge curves

(Fig. 3a). Using this method, we find that energy density, which

has a maximum of 69.8 W h kg�1, peaks at 0.50 A g�1,

according to the reported calculation method.3,21 The galvano-

static discharge curves of RGO-U, MnO2HS-U, and RGO–

25210 | J. Mater. Chem., 2012, 22, 25207–25216

MnO2HS-U electrodes show similar phenomena at current

densities of 1.00 and 0.50 A g�1 (Fig. 3b and c): RGO–

MnO2HS-U exhibits a longer discharge time than RGO-U or

MnO2HS-U at a low discharge current density, but an inter-

mediate discharge time was observed for RGO–MnO2HS-U at a

high discharge current density. To ascertain the origin of this

peculiarity of the RGO–MnO2HS-U capacitor, the Cs values of

RGO-U, MnO2HS-U, and RGO–MnO2HS-U electrodes were

compared at various current densities. The results showed the

Cs of RGO–MnO2HS-U to be larger than those of RGO-U and

MnO2HS-U when the current density was below 0.75 A g�1.

However, as the current density increased, the Cs of RGO–

MnO2HS-U dropped below that of MnO2HS-U. When the

current density exceeded 1.50 A g�1, it dropped below that of

RGO-U (Fig. 3d). This was mainly due to limited ion migration

into the pores of MnO2 HS as the current density increased.

This caused a rapid increase in resistance to charge transport.62

As a result, the effective redox reaction became more limited to

the outside surfaces of MnO2 HS.63 The double-layer capaci-

tance of RGO was less affected due to its larger surface area and

different mode of charge storage (electrostatic charge accumu-

lation). Therefore, at higher current densities, RGO is nearly

solely responsible for the capacitance of RGO–MnO2 HS

hybrid electrodes.

This journal is ª The Royal Society of Chemistry 2012

Fig. 2 CV curves of (a) RGO-U, (b) MnO2HS-U, and (c) RGO–MnO2HS-U at different scan rates (5–50 mV s�1). Comparisons of CV curves at a scan

rate of (d) 10 and (e) 0.5 mV s�1. (f) Galvanostatic charge–discharge curves of RGO–MnO2HS-U at a current density of 0.60 A g�1. (RGO–MnO2HS-U

contains 72 wt% of active materials (53.3 wt% of MnO2 HS in RGO–MnO2 HS), 23 wt% of AB, and 5 wt% of binder).

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Fig. 4 depicts the impedance spectra of RGO-U, MnO2HS-U,

and RGO–MnO2HS-U electrodes recorded from 0.01 to 100

KHz with alternate current amplitude of 10 mV. All the

impedance plots are composed of a semicircle within the high-

frequency range and a nearly vertical line in the low-frequency

range. The intersection of the semicircle on the real axis repre-

sents the equivalent series resistance (Rs) of the electrode, and the

diameter of the semicircle corresponds to the charge-transfer

resistance (Rct) of the electrodes and electrolyte interface.20

RGO–MnO2HS-U has a visibly smaller Rct than RGO-U or

MnO2HS-U, indicating that the homogeneous dispersion of

MnO2 HS in hybrid structure broadens the distance between the

RGO sheets. This promotes ion diffusion within the electrode

materials, which facilitates efficient utilization of active mate-

rials. The nearly vertical line along the imaginary axis unveils an

ideally capacitive behavior of the as-fabricated RGO–MnO2HS-

U electrode.

We also used the galvanostatic charge–discharge measurement

to evaluate the durability of the as-fabricated electrodes. As

shown in Fig. 3e, the as-obtained RGO–MnO2HS-U retains

about 83% of its original capacitance after 1000 cycles. This is

comparable to existing reports of graphene-MnO2 composites

(typically 75–85% retention after 1000 cycles) and RGO-U

This journal is ª The Royal Society of Chemistry 2012

(z85% capacitance retention) but higher than the 75% capaci-

tance retention of MnO2HS-U.18,33,67 This indicates that our

hybrid materials also have very good cycling stability. The

difference in the cycling stabilities of MnO2HS-U and RGO–

MnO2HS-U may be attributable to their different double-layer

and pseudocapacitive contributions: double-layer processes only

involve the accumulation of charge or rearrangement, but

pseudocapacitive processes are related to chemical redox reac-

tions and the double-layer capacitors have a better electro-

chemical stability as compared with those of

pseudocapacitators.1,33

Furthermore, the increased specific capacitances caused by the

synergistic effect of the RGO–MnO2 HS hybrid structure with

53.3% MnO2 HS as an example can be calculated using the

following equation: increased Cs ¼ Cs(RGO–MnO2HS-U) �Cs(RGO-U) � 0.467 � Cs(MnO2HS-U) � 0.533. As shown in

Fig. 3f, with decreasing current density, increased Cs increases

from a negative to a positive value. This implies that low current

densities favor synergistic effects. At a current density of 0.5 A

g�1, increased Cs can reach up to 251 F g�1, which approaches

half of theCs of RGO–MnO2HS-U (578 F g�1 at 0.5 A g�1). This

means that RGO–MnO2 HS hybrid structures have an advan-

tage in energy storage at low current densities. Although the

J. Mater. Chem., 2012, 22, 25207–25216 | 25211

Fig. 3 (a) Galvanostatic discharge curves of RGO–MnO2HS-U at different current densities. Galvanostatic discharge curves at current densities of (b)

1.00 and (c) 0.50 A g�1. (d) Specific capacitances at various current densities. (e) Cycling performances of RGO-U,MnO2HS-U, and RGO–MnO2HS-U

at a current density of 0.60 A g�1. (f) The increased specific capacitance from the synergistic effect of RGO–MnO2 HS hybrid structure at different

current densities. (RGO–MnO2HS-U contains 72 wt% of active materials (53.3 wt% of MnO2 HS in RGO–MnO2 HS), 23 wt% of AB, and 5 wt% of

binder).

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as-fabricated RGO–MnO2HS-U electrodes had no visible

advantage at high current densities, the highest obtainable

specific capacitance (578 F g�1) and energy density (69.8 W h

Kg�1) were found to be significantly larger than those of most

graphene–MnO2-based hybrid ECs in previous reports (Table 1).

In this way, the ultra-high energy storage of the present electrode

material renders it suitable for applications in supplying

Fig. 4 (a) Nyquist plots of RGO-U, MnO2HS-U, and RGO–MnO2HS-U ele

RGO–MnO2HS-U and RGO–MnO2HS-M electrodes with insets showing th

25212 | J. Mater. Chem., 2012, 22, 25207–25216

electrical energy to small mobile devices, such as a substitute for

lithium ion batteries.

Effect of MnO2 HS contents

The effect of MnO2 HS content on the Cs of RGO–MnO2HS-U

was investigated by changing the MnO2 HS mass ratio in

ctrodes with insets showing the high-frequency parts. (b) Nyquist plots of

e high-frequency parts.

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Table 1 Comparison of specific capacitances of the reported graphene–MnO2 composite-based electrodes and the present work

Samples Cs (F g�1)Energy density(W h kg�1) Ref.

Graphene–MnO2 nanoparticles 324 (10 mV s�1) — 64Graphene–MnO2 nanoparticles 365 (5 mV s�1) 12.6 36Graphene Oxide–MnO2 nanowires 216 (0.15 A g�1) — 33Graphene–MnO2 nanowires 31 (0.5 A g�1) 30.4 18Reduced graphene oxide–MnO2 nanosheets 188 (0.25 A g�1) — 37Graphene-flower-like MnO2 328 (0.5 mA cm�2) 11.4 20Graphene-needle-like MnO2 260 (0.2 A g�1) — 38Graphene-flower-like MnO2 280 (0.2 A g�1) — 38Graphene-honeycomb-like MnO2 210 (0.5 A g�1) — 39Graphene–MnO2 nanostructured textiles 315 (2 mV s�1) 12.5 3Flexible graphene–MnO2 composite papers 256 (0.5 A g�1) — 40Graphene sheet–MnO2 sheet multilayers 263 (0.283 A g�1) — 41Graphene–MnO2//ACN (asymmetric) 114 (1 mV s�1) 51.1 21Graphene–MnO2-carbon nanotubes 193 (0.2 A g�1) — 42Graphene–MnO2-PEDOT:PSS 380 (0.1 mA cm�2) — 22Graphene nanosheets–MnO2 235 (20 mV s�1) 33.1 27Graphene–MnO2 film 400 (10 mV s�1) — 51Graphene–MnO2 nanowall 122 (10 mV s�1) — 65Graphene nanoplate–MnO2 309 (5 mV s�1) — 48Graphene–MnO2//graphene (asymmetric) — 10.0 66Hydrothermally reduced graphene–MnO2 212 (2 mV s�1) — 67Nanostructured graphene–MnO2 310 (2 mV s�1) — 52Reduced graphene oxide–MnO2 hollowsphere

578 (0.5 A g�1) 69.8 Present work

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RGO–MnO2 HS from 16.0 to 81.0 % (Fig. 5a). As the amount of

MnO2 HS increases, the Cs of RGO–MnO2HS-U first increases,

then gradually decreases. It is higher than that of either RGO-U

or MnO2 HS-U within a reasonable loading range. However,

excessive MnO2 HS loading was found to decrease the Cs of

RGO–MnO2HS-U because some of the MnO2 HS in the hybrid

materials would aggregate, as indicated in Fig. 5b.

Effect of fabriction methods

Traditionally, electrode materials are fabricated into EC elec-

trodes using mechanical blending or milling.21,33,37,39,48Herein, we

also fabricated RGO–MnO2HS-M electrodes by mechanical

milling of active materials and AB for the sake of comparison.

Fig. 6a and b show that the RGO–MnO2HS-M has considerably

lower capacitance than the RGO–MnO2HS-U due to their

different morphologies. In mechanical milling scenarios, AB and

MnO2 HS only cover the outside surfaces of close stacked RGO

multilayers, which suppresses the diffusion of ions across active

materials. And the hollow structure of MnO2 was broken into

Fig. 5 (a) Specific capacitance values of RGO–MnO2HS-U with various conc

of RGO–MnO2 HS with 81.0% MnO2 HS.

This journal is ª The Royal Society of Chemistry 2012

fragments during the milling process (Fig. 6c), which was found

to have a detrimental impact on the capacitance of the composite

electrodes. In contrast, the present solution-based ultrasonic

co-assembly process was associated with excellent dispersion and

porous structure (Fig. 1a and b).

Effect of MnO2 nanostructures

We also used MnO2 nanoparticles to replace hollow MnO2

spheres in the fabrication of RGO–MnO2NP-U using a solution-

based ultrasonic co-assembly process. Fig. 7a and b show that

the addition of MnO2 nanoparticles can also increase the

capacitance of RGO, but the increased Cs is markedly smaller

than the increase caused by MnO2 HS (Fig. 7c and 3f). Also,

RGO–MnO2NP-U was found to show a smaller Cs than RGO–

MnO2HS-U at low current densities. The two had similar Cs

values at high current densities (Fig. 7d and e). Because the as-

prepared MnO2 nanoparticles also possessed the similar g phase

(lower degree of crystallinity) to MnO2 HS (Fig. S3†) and the

aggregated MnO2 particles were comparable to MnO2 HS in size

entrations of MnO2 HS at a current density of 0.50 A g�1. (b) SEM image

J. Mater. Chem., 2012, 22, 25207–25216 | 25213

Fig. 6 (a) CV curves of RGO–MnO2HS-U and RGO–MnO2HS-M electrodes at 10 mV s�1. (b) Specific capacitance values of RGO–MnO2HS-U and

RGO–MnO2HS-M electrodes at various current densities. (c) SEM image of RGO–MnO2HS-M.

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(Fig. S1a and S1b†), the specific surface area and the dispersion

of MnO2 nanomaterials may be responsible for the extent of the

increase in Cs. The nitrogen adsorption/desorption isotherms

and pore size distributions of MnO2 HS and MnO2 NP, which

are shown in Fig. S1c and S1d†, show that MnO2 HS has the

BET and Langmuir surface areas of 90 and 113 m2 g�1. These are

almost double those of the MnO2 NP, which are 49 and 63 m2

g�1, respectively. The pore volumes are 0.42 cm3 g�1 for MnO2

Fig. 7 (a) CV curves of RGO-U,MnO2NP-U, and RGO–MnO2NP-U electro

electrodes at various current densities. (c) The increased Cs from the synergisti

(d) CV curves of RGO–MnO2HS-U and RGO–MnO2NP-U electrodes at 5 m

various current densities. (f) Specific capacitance values of MnO2HS-U and M

25214 | J. Mater. Chem., 2012, 22, 25207–25216

HS and 0.26 cm3 g�1 for MnO2 NP. The larger surface area and

pore volume of MnO2 HS favor electrolyte access and ion

diffusion, allowing MnO2HS-U to exhibit higher specific

capacitance thanMnO2NP-U (Fig. 7f). Therefore, MnO2 HS can

provide a better capacitance contribution than MnO2 NP in

RGO–MnO2 hybrid structures. However, at high current

densities, there is little difference in specific capacitance between

RGO–MnO2HS-U and RGO–MnO2NP-U. This is because the

des at 10 mV s�1. (b)Cs of RGO-U,MnO2NP-U, and RGO–MnO2NP-U

c effect of RGO-MnO2 NP hybrid structure at different current densities.

V s�1. (e) Cs of RGO–MnO2HS-U and RGO–MnO2NP-U electrodes at

nO2NP-U electrodes at various current densities.

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pseudocapacitive contribution of the RGO–MnO2 hybrid

structure is only related to the chemical redox of the outside

surface of MnO2 nanostructures.

Conclusion

As shown here, we have successfully fabricated hybrid RGO–

MnO2 hollow sphere materials using a solution-based ultrasonic

co-assembly process. The present method can provide a better

dispersion of active materials and a richer pore-structure for

electrolyte access than previous techniques and active materials.

The as-fabricated RGO–MnO2 HS hybrid electrode exhibits

profoundly improved specific capacitance and energy density, as

much as 578 F g�1 and 69.8 W h Kg�1, respectively. These values

are far larger than those reported for graphene–MnO2 based

hybrid electrochemical capacitors. This method can also be used

for the hybridization of graphene with other metal oxides in the

fabrication of high-performance electrochemical capacitors.

Acknowledgements

Financial support was received from the National Natural

Science Foundation of China (Grant no.s 51133001 and

21074023), National ‘‘863’’ Foundation, the Shanghai Science

and Technology Foundation (10JC1401900), and the Science

and Technology Foundation of Ministry of Education of China

(IRT0911, 20110071130002).

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