Highly flexible supercapacitors with manganese oxide nanosheet/carbon cloth electrode

Article abstract of DOI:10.1016/j.electacta.2011.05.090

We report a simple and cost-effective synthesis of hierarchically porous structure composed of Birnessite-type manganese dioxide (MnO₂) nanosheets on flexible carbon cloth (CC) via anodic electrodeposition technique. Petal-shaped MnO₂, having sheet thickness of a few nm and typical width of 100 nm, with a strong adhesion on CC is observed. This hierarchically porous MnO₂-CC hybrid structure dose exhibit not only excellent capacitance properties, such as up to 425 F g^-1 in specific capacitance, but also high crack resistance owing to its efficient release of bending stress, as observed by cyclic voltammetry and galvanostatic charge/discharge measurements under different curvature of bending configurations. Furthermore, flexible supercapacitors based on this kind of MnO₂ nanosheet/CC electrode showed significantly improved stability in capacitive performance over 3000 cycles under the bending test, which is highly promising for future applications in flexible energy storage device.

Full text of DOI:10.1016/j.electacta.2011.05.090

Electrochimica Acta 56 (2011) 7124–7130
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Highly flexible supercapacitors with manganese oxide nanosheet/carbon cloth
electrode
Ying-Chu Chena, Yu-Kuei Hsub, Yan-Gu Lina, Yu-Kai Linc,d, Ying-Ying Horngc, Li-Chyong Chena,
Kuei-Hsien Chena,c,∗
a Center for Condensed Matter Sciences, National Taiwan University, Taipei 106, Taiwan
b Department of Opto-Electronic Engineering, National Dong Hwa University, Hualien 97401, Taiwan
c Institute of Atomic and Molecular Science, Academia Sinica, Taipei 106, Taiwan
d Department of Physics, National Taiwan University, Taipei 106, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
We report a simple and cost-effective synthesis of hierarchically porous structure composed of Birnessite-
type manganese dioxide (MnO2) nanosheets on flexible carbon cloth (CC) via anodic electrodeposition
technique. Petal-shaped MnO2, having sheet thickness of a few nm and typical width of 100 nm, with
a strong adhesion on CC is observed. This hierarchically porous MnO2–CC hybrid structure dose exhibit
not only excellent capacitance properties, such as up to 425 F g−1 in specific capacitance, but also high
crack resistance owing to its efficient release of bending stress, as observed by cyclic voltammetry
and galvanostatic charge/discharge measurements under different curvature of bending configurations.
Furthermore, flexible supercapacitors based on this kind of MnO2 nanosheet/CC electrode showed sig-
nificantly improved stability in capacitive performance over 3000 cycles under the bending test, which
is highly promising for future applications in flexible energy storage device.
Received 28 January 2011
Received in revised form 21 May 2011
Accepted 21 May 2011
Available online 31 May 2011
Keywords:
Supercapacitors
MnO2
Nanosheet
Flexible
© 2011 Elsevier Ltd. All rights reserved.
Birnessite
1. Introduction
shown great promise due to their environmental friendly nature,
good electrochemical performance in neutral electrolyte, as well as
With the growing interest in recent years in development of
portable and flexible electronics, flexible and safe energy-storage
devices based on battery and/or electrochemical capacitors have
also attracted increasing attention to power such flexible devices
[1–3]. Electrochemical capacitors (also known as supercapacitors)
have, especially, shown great potential in recent years to meet the
short-term power needs [4–7]. Supercapacitors can be classified
into two broad categories, viz. electrical double-layer capacitor
and pseudocapacitor, based on their charge-storage mechanism
[8–12]. In theory, the capacitive ability of pseudocapacitors, whose
electron storage mechanism involves reversible Faradaic reac-
tion, is expected to be higher than that of the double-layer
capacitors, wherein the capacitance arises from electrostatic sep-
aration at the interface. Transition metal oxides and conducting
polymers with several reversible oxidation states/structures are
commonly regarded as potential active materials for pseudoca-
pacitors [13–17]. Among them, MnO2-based supercapacitors have
low cost of raw materials [18–20].
It is also well-known that several materials exhibit peculiar and
fascinating properties on a nano-scale, superior to their bulk coun-
terparts. Particularly, nanostructural MnO2 exhibits larger surface
area and shorter conduction lengths for electrons and cations;
in addition, such structures are amenable to release the bend-
ing stress, making it highly attractive for application as electrodes
for flexible energy-storage devices. To date, the two main meth-
ods used to produce nanosized MnO2 are hydrothermal approach
and electrochemical deposition [21–24]. Under different operat-
ing modes, the latter can be easily utilized to deposit a variety
of morphologies of MnO2 on a given substrate without requiring
any further process in electrode preparation [15,25]. Moreover,
a porous hierarchical nanostructure consisting of nanosized Mn
oxides by a simple two-step electrochemical deposition process
exhibited high specific capacitance (470 F g−1) and good cycle sta-
bility [26]. In addition, other approaches, such as self-limiting
reaction of permanganate with carbon and chemical reaction using
permanganate, have also been reported for synthesis of nanosized
MnO2 [27,28]. Typically, two-dimensional and rigid substrates,
such as the stainless steel and metal foil, are widely used as current
collectors [29,30]. Recently, the potential of composite materials of
carbon nanotubes and MnO2 via layer-by-layer method for flex-
ible supercapacitors have been demonstrated [31,32]. However,
∗
Corresponding author at: Center for Condensed Matter Sciences, National
Taiwan University, Taipei 106, Taiwan. Tel.: +886 2 2366 8232;
fax: +886 2 2362 0200.
E-mail addresses: chenlc@ntu.edu.tw (L.-C. Chen),
chenkh@pub.iams.sinica.edu.tw (K.-H. Chen).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.05.090

Y.-C. Chen et al. / Electrochimica Acta 56 (2011) 7124–7130
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mechanically flexible carbon fibers woven in form of cloth (car-
bon cloth, CC), which are commercially available materials as gas
diffusion layer in fuel cells [33,34], may be attractive alternative
for application as flexible electrodes due to their good electrical
conductivity, chemical stability, light weight, flexibility and high
porosity. In addition, CC also displays adequate electrochemical
properties, making them suitable for electrodeposition of MnO2
nanostructures. As presented here, our approach does not require
any binders or conductive agents; therefore, common problems
arising from their residues that lead to reduced electron storage
capacity and inconsistent cycle life-time in conventional MnO2
powder-based supercapacitors can thus be avoided [35].
In the present work, we utilized a simple and cost-effective
electroplating approach to directly deposit layered MnO2 nanos-
tructures on the surface of carbon cloth. The resulting hierarchical
electrode with strong adhesion between hybrid petal-shaped
MnO2 and flexible CC not only yields promising capacitive perfor-
mance due to fast penetration of the electrolyte, but also allows the
electrode to be bent without diminution of capacitance, via release
of the bending stress through the adjustment of the inter-particle
separation.
as electrolyte, using a conventional three-electrode system con-
sisting of the MnO2/CC electrode as the working electrode, square
platinum sheet as the auxiliary electrode, and an Ag/AgCl refer-
ence electrode in 3 M KCl solution. All potentials reported in this
article are with respect to that of Ag/AgCl (3 M KCl, 0.207 V vs. SHE)
reference electrode.
2.4. Assembly of symmetrical MnO2 /CC capacitors
Symmetrical capacitors, MnO2/cellulose/MnO2 (hereafter
referred to MnO2 capacitor), were assembled with cellulose film
as separator by sandwiching the MnO2/CC electrodes. Cellulose
film is the main constituent of paper and an inexpensive insulting
separator. Before assembly, the MnO2/CC electrodes were soaked
in an electrolyte of 0.1 M Na2SO4 for about 10 min. This step
allowed filling of the pores of polymer matrix by the aqueous
electrolyte.
3. Results and discussion
3.1. Morphology
2. Experimental
The preparation procedure for MnO2 nanosheet/CC hybrid elec-
trodes is illustrated schematically in Fig. 1. First, nanostructured
MnO2 was anodically electroplated onto macroporous CC sub-
strates using MnSO4 as precursor solution. The electrode deposition
was carried out in Galvonostatic mode at 0.5 mA cm−2, correspond-
ing to the operational potential of 0.9 V vs. Ag/AgCl reference
electrode, for variable periods of time using 10 mm × 5 mm CC sub-
strate. Notably, the double layer current in this set-up was only
around few tens of ␮A. The application of higher current further
oxidizes the Mn2+ precursors into Mn oxidation state of 4. After
the electroplating process, nanostructured MnO2/CC hybrid struc-
ture was obtained in hydrated state, which offers an unobstructed
electron conduction path. These hierarchical nanocomposite elec-
trodes, which combine macroporous CC as reservoir and MnO2
nanostructures as active materials, facilitate the access to cations
from the electrolyte accompanying the reversible redox between
Mn3+ and Mn4+ oxidation states. Further, the evolution of morphol-
ogy corresponding to different deposition periods was examined by
FESEM, as presented in Fig. 2. The low magnification SEM images,
in Fig. 2a, clearly show the uniform distribution of nano-clusters of
MnO2 on carbon fiber surface. With increasing deposition time, the
density of nano-clusters gradually increased until the whole carbon
fiber surface was fully covered at a deposition time of 2000s. As the
deposition time was further increased to 2500s, the nano-clusters
of MnO2 merged together to form a highly fragile contiguous film
that easily cleaved due to the curvature of the electrode. How-
ever, the other samples with lower deposition times maintained the
original MnO2 nanostructure without apparent damage even after
bending the electrodes. Fig. 2b presents the corresponding high
magnification SEM images that clearly show each nano-cluster con-
sisting of irregular sheets to form the petal-shaped nanostructure of
MnO2. These nano-sized MnO2 clusters exhibit microporous archi-
tectures and strong adhesion on CC, giving high active surface area
and low interfacial resistance of the electrode. The BET surface areas
of the samples were also examined to be 25.5 m2 g−1 for bare CC
and 133.2 m2 g−1 for MnO2/CC composite electrode. The significant
increase in the surface area is due to the surface coverage of hierar-
chical nanoflakes MnO2, which is consistent with the observation
from the SEM images. With increase in deposition time, the isolated
clusters appeared to maintain their petal-shaped structure, while
the density of the clusters increased. The optimum uniformity and
coverage with petal-shaped MnO2 nano-cluster was obtained at the
deposition time of 2000s. The clusters appeared to consist of irreg-
2.1. Materials and chemicals
A
commercially available CC (E-TEK, USA), with specifi-
cations: B-1 Designation A (weave = plain, weight = 116 g m−2
thickness = 0.35 mm, and 0 wt wet proofing), was used as
,
%
the supporting material for MnO2. Other chemicals used for
MnO2 deposition, viz. H2SO4, MnSO4·5H2O and Na2SO4, were of
analytical-reagent grade from Aldrich.
2.2. Fabrication of MnO2/CC electrodes
The CC electrode was prepared by bonding a copper wire onto
the edge of approximately 1 × 0.5 cm rectangle of CC. The bond-
ing of Cu-wire was done by using silver paste, cured for 20 min
at 80 ◦C. The bonding pad was covered with epoxide in order
to allow exposure of only the CC surface to the electrochemical
deposition solutions. MnO2 was electrochemically deposited onto
this electrode in an electrolyte solution of 0.1 M H2SO4 + 0.1 M
MnSO4·5H2O under galvanostatic condition of 0.5 mA cm−2 for
durations of 500, 1000, 1500, 2000, and 2500s, and then rinsed
with distilled water. The current density was normalized by the
geometric area of CC. The amount of MnO2 deposited on the CC
electrode was determined from the difference in weight of the elec-
trode before and after electrodeposition, using a microbalance with
a measurement accuracy of 10 ␮g (Sartorius BP 211D, Germany).
The loading level of the active materials was found to be around
0.1–0.6 mg.
2.3. Measurement of the characteristics of MnO2/CC electrodes
The microstructure and morphology of MnO2 were investi-
gated by means of field-emission scanning electron microscopy
(FESEM, JEOL-6700), X-ray diffraction spectroscopy (Bruker D8
Advance diffractometer), and Raman spectroscopy (Jobin-Yvon
LabRAM HR800). The chemical states of the elements were
determined by electron spectroscopy (ESCA, Perkin-Elmer model
PHI 1600). Electrochemical measurements were conducted using
Solartron electrochemical test system (1470E) at ambient tem-
perature. In order to evaluate the electrochemical capacitance
performance of MnO2/CC, cyclic voltammetry (CV) and galvano-
static charge/discharge methods were used. All electrochemical
measurements were carried out in 0.1 M Na2SO4 aqueous solution

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Y.-C. Chen et al. / Electrochimica Acta 56 (2011) 7124–7130
Fig. 1. Schematic diagram for fabrication of MnO2/CC hybrid electrode.
ular sheets with thickness of a few nm and width of about 100 nm.
Such a microstructure ensures optimum utilization of MnO2 within
the diffusion length of the cations. Therefore, this hierarchically
porous structure composed of microporous MnO2 and macrop-
orous CC appears to be a promising candidate for application as
electrode of a supercapacitor.
3.2. Structural characterization and composition analysis
Fig. 3a shows the XRD patterns of hybrid MnO2/CC elec-
trodes prepared at various deposition times. An XRD pattern for
CC substrate is also shown for comparison. It can be seen that
none of the XRD patterns of the hybrid electrodes with dif-
Fig. 2. FESEM micrographs of MnO2/CC hybrid electrodes, fabricated via anodic electrodeposition, for different deposition times (a) low magnification, (b) high magnification.

Y.-C. Chen et al. / Electrochimica Acta 56 (2011) 7124–7130
7127
Fig. 3. (a) XRD patterns of MnO2/CC hybrid electrodes, fabricated via anodic electrodeposition, for different deposition times. (b) Raman spectra of MnO2/CC hybrid electrodes
(deposition time of 2000s) and carbon cloth. (c) TEM image of MnO2 nanosheets; inset, the corresponding SAED pattern.
ferent deposition periods display any visible diffraction peaks
corresponding to MnO2; all of the strong diffraction peaks are
associated with graphite. This indicates a long-range disorder of
MnO2 in these composites. However, in order to explore the
local structure of MnO2 nanosheet/CC, Raman microscopy and
TEM, which is sensitive to the structural short-range order in
amorphous materials, was used. Fig. 3b shows the Raman spec-
trum of MnO2 nanosheet/CC hybrid electrode synthesized by
anodic electroplating for the deposition time of 2000s. A Raman
spectrum of CC is also shown for comparison. Three major fea-
tures corresponding to MnO2 can be identified at 500, 575 and
648 cm−1. The strong Raman band at 648 cm−1 can be attributed
to the symmetric stretching vibration (Mn–O) of the MnO6 groups
[36], while the second strong band located at 575 cm−1 is usu-
ally attributed to the (Mn-O) stretching vibration in the basal
plane of MnO6 sheet [36]. From these vibrational features, a
Birnessite-type MnO2 with layered structure can be concluded,
which also agrees well with the SEM observations. TEM micro-
graphs and corresponding selected area electron diffraction pattern
(SAED), shown in Fig. 3c, were taken from the MnO2/CC elec-
trodes. From TEM image, the layered structure composed of several
petal-shaped thin nanosheets. Continuous ring pattern of SAED
also confirmed their nanocrystalline nature. The d-spacings of
(2 0 0), (1 1 0), (1 1 1) and (2 0 1) measured from the SAED pat-
tern are consistent with Birnessite-type MnO2 (JCPDS 42-1317)
[21].
X-ray photo-electron spectroscopy (XPS) is a most widely used
technique for analyzing the chemical states of the surface ele-
ments. As shown in Fig. 4a, Mn 2p core level spectrum displays
two peaks at binding energies of 654.3 and 642.6 eV correspond-
ing to the spin-orbit doublet attributed to the Mn 2p1/2 and Mn
2p3/2. The spin-energy separation of 11.7 eV indicates the forma-
tion of MnO2 [9]. Moreover, the binding energies of Mn 2p3/2
electron for Mn3+ and Mn4+ states are reported to be 641.6 and
642.6 eV, respectively, which further confirms the composition
of the deposit as MnO2 [37]. Fig. 4b shows the O 1s spectrum
of the MnO2 nanosheet/CC electrode, which could be fitted into
three main constituent peaks corresponding to different oxygen-
containing species, viz. Mn–O–Mn bond at 529.7 eV, Mn–OH bond
at 531.8 eV, and H–O–H bond at 534.1 eV [9]. Based on the areas
under these peaks, the atomic ratio of Mn to O is estimated to be 1:
2.12. Notably, the 534.1 eV XPS peak corresponds to water molecule
(H–O–H), which confirms the MnO2 nanosheet to be in hydrated
form.

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Y.-C. Chen et al. / Electrochimica Acta 56 (2011) 7124–7130
Mn-O-H
Mn-O-H
(a)
(b)
O 1s
O 1s
Mn-O-Mn
Mn-O-Mn
H-O-H
H-O-H
540
535
530
525 540
535
530
525
Binding Energy/eV
Binding Energy/eV
Fig. 4. XPS spectra of (a) Mn 2p core level and (b) O 1s core level for MnO2/CC hybrid electrodes (deposition time of 2000s).
350
(a)
6
4
0.8
(b)
(c)
2500s
2000s
300
0.6
1500s
1000s
500s
250
200
150
100
50
0.4
0.2
0.0
2
0
0
500 1000 1500 2000 2500
t/s
-2
-4
-6
-8
CC
500s
1500s
2000s
2500s
1000s
0
0.0
0.2
0.4
0.6
0.8
1.0
0
500
1000
1500
2000
2500
Potential vs. (Ag/AgCl/3M KCl)/V
Time/s
500
400
300
200
100
0
(d)
0
100
200
300
400
500
Scan Rate/mV s-1
Fig. 5. (a) Cyclic voltammograms of MnO2/CC hybrid electrodes with different deposition times at a scan rate of 25 mV s−1, (b) charge–discharge curves of MnO2/CC hybrid
electrodes with different deposition times at a fixed current density of 0.13 mA cm−2, (c) variation in area-normalized capacitance of MnO2/CC electrodes as a function of
deposition time, and (d) dependence of the specific capacitance on the scan rate of CV from 10 mV s−1 to 1000 mV s−1 for MnO2/CC hybrid electrodes (deposition time of
2000s).

Y.-C. Chen et al. / Electrochimica Acta 56 (2011) 7124–7130
7129
Fig. 6. (a)Schematic of the flexible supercapacitor made of MnO2/CC, using a conventional two-electrode system. (b) MnO2 capacitor bended with diameters of curvatures
from 2.5 cm to 0 cm. (c) MnO2 capacitor returned the diameters from 0 cm to 2.5 cm at a scan rate of 10 mV s−1. (d) Cycle life of bent MnO2 capacitor; inset, FESEM micrograph
of MnO2/CC hybrid electrode after cycle life test.
ods at a fixed current density of 0.13 mA cm−2, within the potential
window of 0–0.8 V. The anodic charging segments were generally
symmetric to their corresponding cathodic discharging counter-
parts for all the samples, indicating a high reversibility of a typical
capacitive material, consistent with CV observations reported ear-
lier. The area-normalized capacitance (Ca) of an electrode in a given
electrolyte solution can be calculated using an equation as follows:
3.3. Capacitive properties
Electrochemical characteristics of the MnO2 nanosheet/CC elec-
trodes prepared for different deposition periods were evaluated by
using cyclic voltammetry (CV) and galvanostatic charge/discharge
methods. Fig. 5a presents the CV curves for various electrodes, mea-
sured in 0.1 M Na2SO4 electrolyte at 25 ◦C with a potential scan rate
of 25 mV s−1. As can be observed, all CV curves are essentially close
to rectangular shape and display symmetrical current-potential
characteristics of a capacitor, except at potentials near the two
extremes of the potential window. The mirror-image like char-
acteristics of the anodic and cathodic regimes suggest an ideal
pseudocapacitive behavior of the MnO2 nanosheet/CC electrodes,
whereas the areas enclosed under the CV curves correspond to
energy storage capability of the MnO2 nanosheet/CC electrodes.
More significantly, the voltammetric current response, and con-
sequently the capacitance of the coating, increased substantially
with increase in deposition time from 500 s up to 2000s. How-
ever, degradation in the voltammetric current accompanying with
a distorted CV curve was observed for the thicker MnO2 coating at
deposition time of 2500s, presumably due to the poor utilization
of electroactive species and reduced active surface area of MnO2 at
this thickness [38].
i ꢀt
Ca
=
A ꢀV
where i is the discharge current for the applied time duration ꢀt,
ꢀV is the potential window, and A is the electrode area. Fig. 5c
presents the area-normalized capacitance of MnO2 nanosheet/CC
electrodes as a function of deposition times. Note that the mass of
MnO2 deposited on the CC substrates was directly proportional to
the deposition time. For deposition time of up to 2000s, the capaci-
tance showed linear increase with deposited mass with an optimal
capacitance of 230 mF cm−2. The capacitance, however, decreased
for deposition times exceeding 2000s, which can be attributed to
aggregation of the MnO2 nanosheets. Thus, it appears that deposi-
tion time of 2000s is optimum for achieving maximum capacitance
and acceptable characteristics for application as supercapacitors.
In order to compare the present results and those reported in the
literature, capacitance per mass of the coating (Cm) was also calcu-
lated. The optimum specific capacitance achieved in this study is as
To further examine the capacitive properties of hybrid MnO2
nanosheet/CC electrodes, Fig. 5b compares the charge-discharge
plots of the coatings corresponding to different deposition peri-

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Y.-C. Chen et al. / Electrochimica Acta 56 (2011) 7124–7130
high as 425 F g−1, which is higher than many of the data reported in
the literature [15,20], suggesting that the simple electrochemical
process proposed in this study for MnO2 deposition is promising
for fabrication of MnO2 nanosheet/CC composite electrodes with
superior pseudocapacitive performance. Moreover, only 37% of loss
in specific capacitance upon increasing the scan rate, as shown in
Fig. 5d, can be ascribed to the hierarchical nanocomposite elec-
trodes. The high gravimetric capacitance and enhanced stability
can be attributed to the direct-growth approach employed to fab-
ricate the MnO2 nanosheet/CC composite electrodes. This approach
can offer less resistive pathways for efficient electron transport
along with improved utilization of MnO2. Similar direct-growth
approach has been employed to RuO2 and PANI, the other two
popularly studied materials in the field of supercapacitors; superb
is also demonstrated. The present results demonstrate that the
petal-shape MnO2/CC is a promising active material for large-scale,
flexible and electrochemically stable supercapacitor.
Acknowledgements
This research was financially supported by the Ministry of Edu-
cation, Asian Office of Aerospace Research and Development under
AFOSR, National Science Council, National Taiwan University, and
Academia Sinica, Taiwan.
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In order to test the mechanical bendability of these electrodes
and its effect on the capacitance, the schematic diagram of the
flexible supercapacitor made of MnO2 nanosheet/CC with cellulose
film as separator by sandwiching the MnO2 electrodes and 0.1 M
Na2SO4 as the electrolyte is shown in Fig. 6a using a two-electrode
system. Comparison of MnO2 capacitors bended with diameters
of curvature from 2.5 cm to 0 cm (Fig. 6b) and MnO2 capacitors
returned the diameters from 0 cm to 2.5 cm (Fig. 6c) at a scan
rate of 10 mV s−1 showed similar capacitive behavior with capaci-
tance loss only of 0.05%, demonstrating a highly bendable MnO2
nanosheet/CC electrode with excellent mechanical stability. The
reason for such superior mechanical performance can be ascribed to
the efficient release of bending stress by nano-sized MnO2 through
inter-cluster gap adjustment. Furthermore, the specific discharge
capacitance of bent MnO2 capacitors as a function of the num-
ber of galvanostatic charge/discharge cycles is presented in Fig. 6d,
which shows that the specific discharge capacitance decreased by
about 6% during the first 1000 cycles, presumably related to the
equilibration of electrode potential. Once the system is stabilized,
the discharge capacitance displayed a constant magnitude through
the next 2000 cycles. On the other hand, the coulombic efficiency,
which is the ratio of charge capacitance to discharge capacitance,
remained constant at 98.5% through the entire 3000 cycles. After
galvanostatic charge/discharge cycles, the morphology of MnO2 on
CC, as shown in inset of Fig. 6d, still preserved without obvious
damages as compared with pristine sample. Thus, excellent cycle
life-time for MnO2 capacitors under bending is demonstrated in
this study, promising towards their potential application as flexible
supercapacitor.
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Hierarchically porous Birnessite-type MnO2 nanosheets on CC
substrates have been synthesized by anodic electrodeposition
method for application as flexible electrode. The layered MnO2
nanosheet/CC hybrid electrode with strong adhesion is shown
to consist of petal-shaped MnO2 in smaller units that form the
cluster, with the sheet thickness of few nm and ∼100 nm in
width. The amenability of these clusters consequently benefits
the release of bending stress. The optimal specific capacitance of
the MnO2 nanosheet/CC hybrid electrode is 425 F g−1, which is
higher than most of the literature reports. Moreover, such a high
capacitance performance can be maintained under highly bended
configuration, and excellent cycle life-time of over 3000 cycles
of galvanostatic charge/discharge process under the bending test