Tungsten Oxide@Polypyrrole Core–Shell ... - nanoctr.cn

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Tungsten Oxide@Polypyrrole Core–Shell Nanowire Arrays as Novel Negative Electrodes for Asymmetric Supercapacitors

Fengmei Wang , Xueying Zhan , Zhongzhou Cheng , Zhenxing Wang , Qisheng Wang , Kai Xu , Muhammad Safdar , and Jun He *

device typically consists of a positive electrode as the energy

source and a negative electrode as the power source. [ 4,5 ] Cur-

rently, signifi cant progress in the positive electrode materials

has been made in the fabrication of ASCs, such as amorphous

Ni(OH) 2 //active carbon (AC), [ 6 ] CoO@polypyrrole (PPy)//

AC, [ 7 ] H-TiO 2 @MnO 2 //H-TiO 2 @C, [ 8 ] and MnO 2 //carbon or

graphene etc. [ 9,10 ] Among these ASC devices, the negative

electrode usually acted by carbon-based materials due to

their good electrical conductivity and relatively large surface

area. However, low specifi c capacitance of carbon materials is

the major obstacle towards the high energy density of ASC. [ 4 ]

In order to overcome this shortcoming, a new class of nega-

tive electrode materials owning high capacitance in conjunc-

tion with excellent conductivity is essential for application of

ASC devices. In this regard, some new negative electrodes,

such as VN, [ 11 ] Co 9 S 8 [ 12 ] Fe 2 O 3 ,

[ 5,13 ] MoO 3−x , [ 14 ] and rGO@

Fe 3 O 4 [ 15 ] with a high energy density have been developed for

ASC. Among various pseudocapacitive materials, polypyr-

role (PPy) as a typical conjugated polymer holds great poten-

tial as a electrode for ASC because of good conductivity

(10–100 S/cm), ease of fabrication for large scale devices, low

environmental impact and suitable working window. [ 16–18 ]

However, to our best knowledge, almost all of the attention

is focused on application of PPy-based materials utilized as

Among active pseudocapacitive materials, polypyrrole (PPy) is a promising electrode material in electrochemical capacitors. PPy-based materials research has thus far focused on its electrochemical performance as a positive electrode rather than as a negative electrode for asymmetric supercapacitors (ASCs). Here high-performance electrochemical supercapacitors are designed with tungsten oxide@PPy (WO 3 @PPy) core–shell nanowire arrays and Co(OH) 2 nanowires grown on carbon fi bers. The WO 3 @PPy core–shell nanowire electrode exhibits a high capacitance (253 mF/cm 2 ) in negative potentials (–1.0–0.0 V). The ASCs packaged with CF-Co(OH) 2 as a positive electrode and CF-WO 3 @PPy as a negative electrode display a high volumetric capacitance up to 2.865 F/cm 3 based on volume of the device, an energy density of 1.02 mWh/cm 3 , and very good stability performance. These fi ndings promote the application of PPy-based nanostructures as advanced negative electrodes for ASCs.

Asymmetric Supercapacitors

DOI: 10.1002/smll.201402340

F. Wang, X. Zhan, Z. Cheng, Z. Wang, Q. Wang, K. Xu, M. Safdar, Prof. J. He National Center for Nanoscience and Technology (NCNST) No.11 ZhongGuanCun BeiYiTiao 100190 , Beijing , P.R. China E-mail: [email protected]

1. Introduction

The development and fabrication of high-energy-density and

long-lasting power sources has attracted attention from sci-

entifi c and technological researches. [ 1 ] Supercapacitors, with

the properties of high power density, long cyclic stability and

rapid charge/discharge rate, are considered to be potential

energy storage devices in future. [ 2 ] However, to meet the

increasing energy demands for next-generation electronic

devices, the energy density and operating voltage of super-

capacitors need to be further improved. Compared with

symmetric supercapacitors (SSCs), asymmetric supercapaci-

tors (ASCs) demonstrate better performance in much wider

potential windows and increased energy density. [ 3 ] An ASC

www.MaterialsViews.com

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http://doi.wiley.com/10.1002/smll.201402340

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full paperspositive electrode in ASCs. [ 7,19 ] There are few research works

on PPy acting as a negative electrode in ASCs. Therefore, it

is highly desirable to develop a PPy-based negative electrode

with high cycling performance and capacitance without sacri-

fi cing its excellent conductivity in electrolyte.

Design and synthesis of nanostructured hybrid mate-

rials play a vital role to fabricate electrodes. Based on our

previous result, WO 3 nanowires (NWs) on carbon fi bers

with high specifi c area and excellent conductivity may be

an outstanding scaffold to support electrochemically active

PPy. [ 20–22 ] Moreover, carbon fi bers can directly serve as a

lightweight and fl exible current collector to reduce the “dead

volume” of ASC device. Cobalt hydroxide (Co(OH) 2 ) could

be a positive electrode material because of its high capaci-

tance and simple synthesis method. [ 23–26 ] Herein, we report

an effi cient method of in-situ growing WO 3 @PPy nanowire

arrays on carbon fi bers as a negative electrode, assembled

with Co(OH) 2 nanowires on carbon fi bers as a positive elec-

trode, to fabricate a novel ASC device. As a consequence,

the resulting WO 3 @PPy nanowire arrays on carbon fi bers

exhibits high specifi c capacitance (253 mF/cm 2 ) at current

density of 0.67 mA/cm 2 in negative potential (–1.0–0.0 V).

Furthermore, the whole device reveals a high volumetric

capacitance of 2.865 F/cm 3 based on volume of the device, an

energy density of 1.02 mWh/cm 3 at scan rate of 20 mV/s and

good cycling performance.

2. Results and Discussion

The growth procedure of WO 3 @PPy nanowire arrays on

carbon fi bers is illustrated in Figure 1 a. The WO 3 nanowires

were grown on carbon fi bers by a catalyst-free method dem-

onstrated in our previous work. A thin layer of PPy shell was

coated on the surface of WO 3 nanowires by a simple electro-

deposition method. Figure 1 b and Figure 1 c shows scanning

electron microscopy (SEM) images of the WO 3 nanowires on

carbon fi bers, suggesting that the WO 3 nanowires uniformly

grow on whole carbon fi bers. After electrodeposition, the

WO 3 @PPy nanowire arrays are displayed in Figure 1 d. The

core–shell nanowire arrays were formed without any frac-

ture of WO 3 nanowires. Typical transmission electron micros-

copy (TEM) images of WO 3 @PPy core–shell nanowirea are

demonstrated in Figure 1 e. According to the low-resolution

TEM image, the PPy shell is homogeneous with a thickness

of ∼25 nm. As shown in Figure 1 f, the core of WO 3 is single

crystal with growth direction of [002] and the PPy shell is

amorphous. [ 20 ] For comparison, only PPy was deposited on

pure carbon fi bers with the same method. As displayed in

Figure S1, pure carbon fi bers were uniformly covered by PPy

fi lm, suggesting indispensability of WO 3 NWs for fabricating

low-dimensional core–shell nanostructure.

To confi rm the molecular identity of the polymer elec-

trodeposited on the WO 3 NWs, Raman spectroscopy


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Figure 1. (a) Schematic illustration of the growth procedure of WO 3 @PPy core–shell NWs arrays. (b,c) SEM images of the WO 3 NWs on the CFs with different magnifi cation. (d) SEM of WO 3 @PPy core–shell NWs on CFs. (e) TEM image of one WO 3 @PPy core–shell NWs and (f) HRTEM image of the core–shell nanowire.

751www.small-journal.com© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

was carried out and shown in Figure 2 . In particular,

the bands centered at 808 cm −1 , 706 cm −1 , 273 cm −1 and

133 cm −1 are attributed to vibrational modes of mono-

clinic WO 3 NWs. [ 20 ] Beyond these bands, several new bands

∼1585 cm −1 , ∼1404 cm −1 , ∼1049 cm −1 , ∼970 cm −1 observed

for v (C=C), v (C-N), β(C-H) and v (ring) of PPy polymer

(Figure 2 a). [ 27 ] The Raman spectra of pure CFs shown

in Figure S2 demonstrates that the small peaks around

∼1348 cm −1 and ∼1586 cm −1 can be attributed to the disor-

dered graphitic carbon from CFs. Additionally, Raman map-

ping based on core–shell NWs on one carbon fi ber provides

the spatial intensity maps of 1585–1049 cm −1 and 808 cm −1 for

PPy and WO 3 in Figure 2 b, demonstrating the uniformity of

our WO 3 @PPy core–shell nanowires. Thus, WO 3 NWs were

proved to be excellent scaffold for the growth of PPy.

The electrochemical studies of WO 3 @PPy core–shell

nanowires on carbon fi bers were conducted in a three-elec-

trode confi guration at potential region from –1.0 to 0.0 V. A

piece of core–shell NWs electrode with 0.75 cm 2 effective

area was dipped into 3 M NaOH electrolyte, with Pt wire

as the counter electrode and Ag/AgCl electrode as refer-

ence electrode, for a single electrode test. As displayed in

Figure 3 a, the WO 3 @PPy core–shell NWs electrode dem-

onstrates best capacitance compared with other electrodes

including pure carbon fi ber (CF), WO 3 NWs and PPy fi lm on

CFs electrodes (denoted as CF, CF-WO 3 NWs and CF-PPy

electrode). This result also reveals that the uniform PPy

shells on WO 3 NWs are the dominated capacitive materials.

The emergence of one pair of broad redox peaks at about

–0.7 V and –0.5 V arises from the Faradaic redox reaction

of PPy via the insertion/expulsion of Na + ions in alkaline

solution. [ 17,28 ] Figure S3 shows the typical galvanostatic

charge/discharge curves of CF-PPy and CF-WO 3 @PPy elec-

trodes at 1.33 mA/cm 2 . Obviously, the iR drop of CF-WO 3 @

PPy electrode ( iR = 0.0002 V) is substantially smaller than

the CF-PPy electrode ( iR = 0.1046 V), indicating superior

conductivity of CF-WO 3 @PPy core–shell electrode. And the

longer discharge time of the CF-WO 3 @PPy electrode again

confi rms its good electrochemical performance.

The galvanostatic charge/discharge curves of CF-WO 3 @

PPy electrode (Figure 3 b) at different current densities

(0.67–6.67 mA/cm 2 ) between –1.0 to 0.0 V display slight

nonlinearities due to the occurrence of Faradaic reactions

in the CF-WO 3 @PPy core–shell materials. The shape of the

discharge curves is representative of pseudo-capacitance,

which substantiates the CV curve result. Figure 3 c summa-

rizes the areal capacitance of the CF-WO 3 @PPy electrode

calculated from these discharge curves with the Equation S1.

The areal capacitances of the CF-WO 3 @PPy electrode were

calculated to be 0.253, 0.226, 0.178,0.160 and 0.152 F/cm 2 at

current densities of 0.67, 1.33, 4.00, 6.67 and 9.33 mA/cm 2 ,

respectively. The areal capacitance at 6.67 mA/cm 2 of this

electrode represents a 36.7% decrease compared with the

areal capacitance at a current density of 0.67 mA/cm 2 . This

result indicates the excellent capacitive behavior and high-

rate capacitance of the WO 3 @PPy core–shell NWs in a nega-

tive potential region. Furthermore, the cyclic voltammetry

curves at different scan rates are displayed in Figure 3 d. The

anodic and cathodic peaks are observed to be symmetric,

suggesting that the reversibility of the doping/dedoping

reaction on the as-synthesized WO 3 @PPy core–shell NWs

is excellent. The minimal change in shape of the CV curves

as the scan rate increases from 10 to 100 mV/s indicates the

superior electronic conductivity with small equivalent series

resistance. [ 6 ] The electrochemical impedance spectroscopy

(EIS) (Figure S4) also confi rms the smallest resistance of the

CF-WO 3 @PPy electrode with the equivalent series resistance

(ESR) of 4.98 Ω cm 2 . Therefore, such a good electrochemical

performance in negative potential region of the CF-WO 3 @

PPy electrode can be attributed to the increased surface area

and improved charge transport from the WO 3 @PPy core–

shell NWs. Signifi cantly, the excellent capacitive behavior

of CF-WO 3 @PPy electrode demonstrates that it will be a

promising candidate as a negative electrode in an asymmetric

supercapacitor (ASC).

To fabricate ASCs with WO 3 @PPy core–shell nega-

tive electrodes, we selected Co(OH) 2 NWs grown on CFs

(denoted as CF-Co(OH) 2 electrode) as the positive electrode

because of its high capacitive performance and simple syn-

thesis method. In our experiment, Co(OH) 2 NWs on CFs

were fabricated by hydrothermal process. [ 26 ] The morphology

and detailed electrochemical characterization of the CF-

Co(OH) 2 electrode are presented in Figure 4 . As shown in

SEM images of our sample (Figure 4 a), uniform Co(OH) 2

NWs with the diameter of ∼100 nm and length of 3–5 µm

covered the whole CFs. TEM images (Figure 4 b) collected

from a typical Co(OH) 2 NW confi rm the growth direction

of the (100) plane and the lattice fringe spacing of ∼0.28 nm

(PDF#74–1057). In order to further estimate the various

chemical states of bonded elements, X-ray photoelectron


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Figure 2. (a) Raman spectra of the WO 3 @PPy core–shell NWs on CFs. (b) Raman mapping based on WO 3 @PPy core–shell NWs on one carbon fi ber.


full papers

spectroscopy was used to measure the binding energy. Typ-

ical spectra of the sample are displayed in Figure 4 c. Two

major peaks with binding energy at 797.3 eV and 781.2 eV

corresponding to Co 2p 1/2 and Co 2p 3/2 , respectively, yielding

a spin-energy separation of 15.9 eV characteristic of the

Co(OH) 2 phase. Moreover, the O 1s spectrum with a strong

peak at 531.2 eV is associated with bond hydroxide groups

(OH − ). [ 25,29 ]

Electrochemical measurements were conducted to eval-

uate the electrochemical performance in the same condi-

tion. Figure 4 d compares the CV curves at voltage region of

–0.2 to 0.4 V of the pure CF and CF- Co(OH) 2 electrodes

collected at a scan rate of 20 mV/s suggesting the superior

capacitance of Co(OH) 2 . The CV curves at different scan

rates in Figure S5a and EIS spectra in Figure S5b show the

ideal capacitive behavior, good rate-capability and good con-

ductivity with ESR of 3.83 Ω cm 2 . The galvanostatic charge/

discharge curves collected at different current densities of

this single electrode in Figure 4 e display the relatively sym-

metric shapes. This result indicates the ideal capacitive

characteristics and rapid charge/discharge properties of the

electrode. And its areal and specifi c capacitance calculated

from the discharge curves according to Equation S1 and S2

are shown in Figure 4 f. The areal capacitance of the Co(OH) 2

electrode achieved 746 mF/cm 2 (with the specifi c capacitance

of 800 F/g) at a current density of 0.67 mA/cm 2 , which is com-

parable to the reported results. [ 23,25 ]

An asymmetric supercapacitor (ASC) device was made

by using CF-Co(OH) 2 electrode as the cathode and the

CF-WO 3 @PPy electrode as the anode in 3 M NaOH solution

with one piece of cellulose paper as the separator (inset of

Figure 5 b and Experiment methods). This cell was encapsu-

lated by fl exible plastic fi lm sparing two pieces of pure CF

connected to the edges of the two electrodes (Figure S7a).

Figure S6a shows the CV curves of this liquid-state ASC at

different voltage windows. As expected, the stable electro-

chemical potential windows of this ASC can be extended

to 1.6 V. Typical CV curves at different scan rates from

20 to 500 mV/s between 0.0 and 1.6 V are presented in

Figure 5 a. The two strong redox peaks in each curve indicate

the pseudo-capacitive property of the supercapacitor due

to Faradaic redox reactions. Based on volume of the whole

device, Figure 5 b reveals the calculated volumetric capaci-

tance of the asymmetric cell at various scan rates. It is clear

that a volumetric capacitance of 2.865 F/cm 3 at a scan rate of

20 mV/s. To further evaluate the electrochemical properties

and estimate the stable potential windows of the as-fabricated

Co(OH) 2 -WO 3 @PPy based asymmetric capacitor, galvano-

static charging and discharging curves of this capacitor were

performed. The charge-discharge curves at various current


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Figure 3. (a) CV curves of different electrodes collected at 20 mV/s in 3 M NaOH. (b) Charge-discharge curves of CF-WO 3 @PPy core–shell NWs electrode at various current densities (ranging from 0.67 to 6.67 mA/cm 2 ). (c) Areal capacitance of the CF-WO 3 @PPy core–shell NWs electrode as a function of the current densities based on the charge/discharge curves. The effective area of the electrode is 0.75 cm 2 .



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Figure 4. (a) SEM and (b) TEM spectra of Co(OH) 2 NWs. (c) Core-level O 1s and Co 2p XPS spectra collected for Co(OH) 2 NWs. (d) CV curves of different electrodes collected at 20 mV/s in 3 M NaOH. (e) Charge-discharge curves of CF- Co(OH) 2 NWs electrode at various current densities (ranging from 0.67 to 6.67 mA/cm 2 ). (f) Areal capacitance and specifi c capacitance of the CF-Co(OH) 2 NWs electrode as a function of the current densities based on the charge/discharge curves. The effective area of the electrode is 0.75 cm 2 .

Figure 5. (a) CV curves of the Co(OH) 2 -WO 3 @PPy based liquid asymmetric capacitor at various scan rates. (b) Volumetric capacitance of the asymmetric capacitor as a function of scan rate based on the CV curves (the inset presents the schematic diagram of our device). (c) Cyclic performance of the asymmetric capacitor measured at a scan rate of 100 mV/s for 5000 cycles. (d) Power and energy density of the asymmetric supercapacitors.


full papersdensities (1.00–4.00 mA/cm 2 ) are shown in Figure S6b. These

curves display slight nonlinearities between 0.0 to 1.6 V, which

confi rms our CV results. Figure 5 c illustrates the cyclic per-

formance of Co(OH) 2 -WO 3 @PPy based asymmetric superca-

pacitor measured at a scan rate of 100 mV/s for 5000 cycles.

A total capacitance loss of only 15% is observed after the cell

underwent 5000 cycles, demonstrating the good long-term

stability of the asymmetric capacitor. The capacitance value

of our ASC gradually increases at the fi rst 2000 cycles, infer-

ring an activation process during the charge and discharge. It

is noteworthy that these results thus confi rm the high volu-

metric capacitance and remarkable rate capability of our

WO 3 @PPy core–shell NWs on CFs as a negative electrode

for high-performance electrochemical pseudocapacitors.

A Ragone plot showing the energy density as a function of

average power density of this ASC can be seen in Figure 5 d.

Taking the practical applications into account, we calculated

the energy density and power density with the total volume

for commercial SCs. The volumetric capacitance, energy den-

sity and energy density of our device were calculated using

the following Equations:

∫= Δ

1s

j( ) (F/cm )V3

a

CE

E dEE

Ec

(1)

= Δ

2 * 3600(Wh/cm )

23E C EV

(2)

= Δ

3600( / s)

(W/cm )3P EE

(3)

Where C V (F/cm 3 ) is the total volumetric capacitance of the

cell that can be achieved according to Equation ( 1) based on

CV curves, Δ E (V) is the cell voltage, j is the current density

(A/cm 3 ), s (V/s) is the scan rates, E (Wh/cm 3 ) is the energy

density and P (W/cm 3 ) is the power density. Our fabricated

ASC (the volume of the whole cell was about 0.06 cm 3 )

exhibits a highest energy density of 1.02 mWh/cm 3 , which

is about 3-fold higher than those in some latest papers that

employed an aqueous electrolyte, such as H-TiO 2 @MnO 2 //

H-TiO 2 @C-based ASCs (0.3 mWh/cm 3 ). [ 8 ] The maximum

power density is 0.12 W/cm 3 , which is comparable with some

graphene-based supercapacitors. [ 30 ] Fully charged two ASCs

also be used to power a light-emitting diode (LED), as shown

in Figure S7b and S7c. The success to fabricate these ASCs

suggests that the WO 3 @PPy core–shell NWs on CFs can be

used as novel negative electrodes in an ASC. The energy

storage performance of the device can be optimized through

improving the fabrication process.

3. Conclusion

We have fabricated, for the fi rst time, WO 3 @PPy core–shell

NWs on CFs demonstrating excellent electrochemical per-

formance in negative region. The excellent areal capacitance

of 0.253 F/cm 2 and the high-rate charge/discharge perfor-

mance reveal its potential as novel negative electrodes. An

asymmetric supercapacitor (ASC) device was packaged

with CF-Co(OH) 2 as cathode and CF-WO 3 @PPy as anode.

This ASC exhibits stable performance between 0 and 1.6 V,

a high volumetric capacitance of 2.86 F/cm 3 (based on the

whole volume of the device), a superior energy density of

1.02 mWh/cm 3 and high-rate capability. Noticeably, the capac-

itance of our device in aqueous electrolyte achieves ∼90.5%

retention after 4000 cycles. These fi ndings open up the possi-

bility of uniform PPy based nanostructure for applications in

asymmetric supercapacitors as anodes with high voltage, high

energy and power densities to meet the diverse demands in

industry development.

4. Experimental Section

Synthesis of WO 3 @PPy Core–Shell NWs : The WO 3 NWs on carbon fi bers (CFs) were synthesized using catalyst free method demonstrated in our previous work. [ 20 ] The PPy shell was fabri-cated with the electrochemical polymerization method employing a CHI 660D electrochemical workstation. In this system, a piece of CF with effective area of 0.75 cm 2 acted as work electrode in the 60 mL solution containing 0.1 M pyrrole and 25 mM sodium dodecyl sulfate (SDS). An Ag/AgCl electrode and a Pt wire were utilized as reference electrode and counter electrode, respectively. Then, the deposition was performed in the potential windows from -0.4 V to 1.0 V by potentialdynamic method with 20 cycles at room temperature. After the deposition, the sample was dried at 60 °C. PPy fi lm was deposited on CFs under same conditions for comparison.

Synthesis of Co(OH) 2 NWs on CFs : The aligned Co(OH) 2 NW arrays on CFs were synthesized by a hydrothermal process. In brief, a seed layer was fi rst formed on the CFs by the dip-coating method using a 50 ml ethanol solution of 0.1 M cobalt chloride (CoCl 2 ) and 0.5 M urea (CO(NH 2 ) 2 ) followed by calcination at 450 °C for 4 h. Then, the seeded CFs were immersed in a 50 mL autoclave containing 40 ml aqueous solution of 0.1 M of CoCl 2 and 0.5 M CO(NH 2 ) 2 to grow the Co(OH) 2 NWs at 90 °C for 4 h. The loading mass of Co(OH) 2 NWs was 0.93 mg cm −2 by using an electronic bal-ance (BT 125D) with ±10 µg accuracy.

Characterization of Synthesized Samples : The structure of the as-prepared samples were examined with Hitachi S-4800 fi eld-emission scanning electron microscopy (SEM), transmission electron microscopy (TEM, JEM-2100F), and X-ray photoelectron spectroscopy (XPS, ESCALAB250Xi). Raman spectra of the samples were obtained using an InVo-RENISHAW system with an incident wavelength of 535 nm.

Electrochemical Characterization of Single Electrodes and Pseudocapacitors : The electrochemical measurements were con-ducted in a three-electrode electrochemical cell with a Pt counter electrode and an Ag/AgCl reference electrode in 3 M NaOH solu-tion. A piece of CF grown active materials with the effective area of 0.75 cm 2 was used as the working electrode. Cyclic voltammetry (CV) and galvanostatic charge and discharge (GCD) measurements were obtained using as electrochemical workstation in the scan range of –0.2 to 0.4 V and –1.0 to 0.0 V. The aqueous ASCs were assembled by a piece of CF-WO 3 @PPy and CF-Co(OH) 2 with a cel-lulose paper (NKK, TF40, 35 µm) separator in two-electrode simu-lation cells. 3 M NaOH was employed as the electrolyte. Then, this


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cell was encapsulated by fl exible plastic fi lm sparing two pieces of pure CF connected to the edges of the two electrodes. The thick-ness of the device was measured to be 0.8 mm, including the elec-trodes and the separator.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements

This work at National Center for Nanoscience and Technology was supported by 973 Program of the Ministry of Science and Tech-nology of China (No. 2012CB934103), the 100-Talents Program of the Chinese Academy of Sciences (No. Y1172911ZX), the National Natural Science Foundation of China (No. 21373065) and Beijing Natural Science Foundation (No. 2144059).

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Received: August 5, 2014 Revised: August 26, 2014 Published online: October 1, 2014


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