on Carbon Nanotubes and Polyaniline Nanowire Arr

MUNICATION igh-Performance Two-Ply Yarn Supercapacitors Based Hon Carbon Nanotubes and Polyaniline Nanowire Arrays

Kai Wang ,Qinghai Meng ,Yajie Zhang ,Zhixiang Wei ,*and Menghe Miao * ntegration of electronic devices with textiles, widely known Ias wearable electronics or smart textiles, has led to the devel-[1–3opment of many interesting applications.

] As an example,

wearable electronics provide a platform for on-body sensing

to support people in various situations and activities, such as

monitoring of surrounding conditions and physiological sig-nals in sports, healthcare, rehabilitation, and high-risk environ-[4ments.

,

5

] All wearable electronic devices need wearable energy

[6storage devices to ensure high performance and safety.

,

7

]

Widely used energy storage devices include different kinds of

[8batteries and supercapacitors.

] Supercapacitors, also known

as electrochemical capacitors, can fulfi ll all the power require-ments by these wearable electronics thanks to their high power

[9density and reliability.

,

10

] Compared with batteries, supercapac-itors can cope with unstable energy input and output situations

because they can be charged and discharged to any potential

[9within their voltage window.

]

Generally speaking, energy storage devices are composed

of at least three layers, including two symmetric or asym-[11metric electrodes and a separator layer.

] Up till now, most

designs of fl exible energy storage devices are based on bulky

electrodes and separators and have not achieved the expected

[12–14]energy storage performance and fabrication simplicity.

In

addition, these fl exible energy storage devices are generally not

breathable, i.e., they do not allow sweat and air from human

body to pass through freely and thus cannot meet the wearer

comfort requirements for wearable electronics. To address

this issue, Wang, et al., reported a fi ber-like supercapacitor

that could be integrated with energy conversion devices such

[15as nano-generators.

,

16

] However, due to the presence of inor-ganic ZnO electrode materials, it is not fl exible enough for use

in wearable electronics. The capacitance of their device was also

relatively low because the fi ber substrate (Kevlar or poly(methyl

methacrylate) fi ber) makes no contribution to the capacitance

of the device.

In this communication, we present a simple design and

fabrication method for a high-performance, thread-like super-capacitor. The resultant supercapacitor, which is fi ner than a

conventional fi ne count cotton yarn, takes the form of a two-ply

composite yarn consisting of two carbon nanotube (CNT) sin-gles yarns that are infi ltrated with polyaniline nanowire arrays.

Since all the materials used are inherently fl exible, the resulting

supercapacitor is highly fl exible and can be easily woven or

knitted into conventional textile fabrics for uses in wearable

electronics.

A typical fabrication process of the thread-like supercapac-

itor is schematically shown in Figure 1 a. A pure CNT yarn was

used both as active material as well as current collector for the

thread-like supercapacitor. Conducting polyaniline (PANI) was

used as electrode material because of its high energy storage

capability and fl exibility. The pure CNT yarn was fi rst spun by

twisting a continuous CNT web drawn from a solid-state multi-walled carbon nanotube forest which was synthesized by the

[17,18]chemical vapor deposition (CVD) method.

The CNT yarn

possesses good mechanical properties, with tensile strength

[19normally in the range 500–800 MPa.

] As reported previously,

conducting polymer nanowire arrays are ideal electrode mate-rials for supercapacitors because of their large capacitance and

[20–24]their capability to charge and discharge at very high rate.

To maximize the capacitance of the thread-like supercapacitors,

conducting polyaniline nanowire arrays were deposited on the

CNT yarn by an in-situ polymerization process, resulting in a

composite yarn dubbed as CNT@PANI yarn (Step 1). In Step

2, a thin layer of polyvinyl alcohol (PVA)-H

2SO

4 gel electrolyte

was coated on the surface of the CNT@PANI yarn to form a

CNT@PANI@PVA yarn. Finally, two CNT@PANI@PVA yarns

were twisted together to form the two-ply yarn supercapacitor

(Step 3).

The morphologies of the initial pure CNT yarn and the

CNT@PANI composite yarn were examined using scanning

electron microscopy (SEM, Figure 1 b-e). Figure 1 b and c are the

SEM images of the as-spun pure CNT yarn at different magni-fi cations. The carbon nanotubes are highly aligned in the yarn.

The diameter of the pure CNT yarn is approximately 20 μm(Figure 1 c). The pure CNT yarn is highly fl exible and can main-tain its structural integrity after being bent, folded, or knotted

[25hundreds of times.

] The ordered PANI nanowire arrays, a

high-performance pseudocapacitance electrode material, were

incorporated into the CNT yarn by in-situ dilute polymerization

to maximize the performance of the thread-like supercapac-[20itors.

,

22

] Figure 1 d and e show the morphology of the CNT@PANI composite yarn at different magnifi cations. The PANI

nanowires are uniform and aligned in the direction perpendic-ular to the CNT yarn surface. The PANI nanowires are about

50 nm in diameter and about 400 nm in length. The structures

of the pure CNT yarn and the PANI/CNT composite yarn were

further characterized by Raman spectroscopy (Figure S1 of the

Supporting Information (SI)). The D-band and G-band peaks

Dr. K. Wang, Q. H. Meng, Dr. Y. J. Zhang,

Prof. Z. X. WeiNational Center for Nanoscience and TechnologyBeijing 100190, P. R. ChinaE-mail: weizx@

Dr. M. H. Miao

CSIRO Materials Science and EngineeringP.O. Box 21, Belmont, Victoria 3216, AustraliaE-mail: @

DOI: 10.1002/adma.201204598

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Mater. 2013, 25, 1494–1498

ted together to form a two-ply yarn, the

fi nal thread-like supercapacitor, as show in

Figure 2 c. SEM images of the two-ply yarn

supercapacitor are provided in the SI (see

Figure S3). The two-ply yarn is a solid-state

supercapacitor that retains the characteris-tics of pure CNT yarn for conventional textile

processing, such as weaving and knitting.

Figure 2 d is an optical photograph of the as-prepared two-ply yarn supercapacitor. The

diameter of the two-ply yarn supercapacitor

is in the range 120 μm− 150 μm, fi ner than

fi ne count cotton yarns that are commonly

used in light weight shirting fabrics.

The as-prepared thread-like supercapacitor

can be woven or knitted into textiles alone or

together with other smart devices and used

in wearable electronics. A plain weave model

fabric was manually constructed from six two-ply yarn supercapacitors. An optical micro-photograph of the model fabric is shown in

Figure 2 e. The two-ply yarn supercapacitor

could also be co-woven with conventional tex-tile yarns using existing weaving technology.

Figure 2 f is an optical microphotograph of a

model fabric composed of four conventional

two-ply cotton yarns and four two-ply CNT@PANI@PVA yarn supercapacitors. Such a

blended weave construction provides mois-ture and air breathability, i.e., moisture and

air can pass through the fabric, and thus

offers comfort for wearers. The strength of

Figure 1. a) Schematics of the preparation procedures for the two-ply yarn supercapacitors;

conventional textile yarns is usually expressed

b,c) SEM images of pure CNT yarn; d,e) SEM images of CNT@PANI composite yarn. Ordered

in specifi c strength (called tenacity in cN/tex,

PANI nanowire arrays can be seen on the surface of CNT yarn in the CNT@PANI composite

where tex is the yarn linear density in terms

yarn.

of mg/m) to allow comparison between

yarns of differing thickness. The CNT yarn

[17[26

,

19

]

can be observed from the CNT yarn.

,

27

] After depositing PANI

used in our experiments has a tenacity of about 70 cN/tex.

Commercial cotton yarns typically have tenacity values below

nanowire arrays on the surface of the CNT yarn, the character-[28

] or less than 30% of the strength of our CNT yarn.

istic peaks of PANI appeared, and the D and G peaks for CNT

20 cN/tex,

All experiments suggested that the two-ply yarn supercapac-yarn were almost totally covered by the PANI peaks.

The uniformity of PVA coating on the yarn surface is critical

itors would perform very well in textile processes and that the

to the performance of the thread-like supercapacitors. If the

resultant co-woven fabric could stand the normal wear and tear

PVA gel does not cover the whole yarn surface, it will lead to

required for wearable electronics.

The electrochemical properties of the as-prepared two-ply

short-circuit when the two yarns are twisted together. On the

other hand, excessively thick PVA coating will cause problems

yarn supercapacitor were characterized by cyclic voltammetry

in subsequent weaving or knitting process. A home-made tool

(CV), galvanostatic charge–discharge and electrochemical

was used to control the coating thickness. The CNT@PANI

impedance spectroscopy (EIS). The two-ply yarn supercapacitor

yarn (or CNT yarn) was drawn through a hole with a diameter

is a symmetric two-electrode solid-state supercapacitor with a

of 200 μ m on which a drop of PVA gel was placed, as shown in

PVA gel coating as both electrolyte and separator and two sin-Figure 3 a presents the CV curves

Figure S2 of the SI. This ensured the CNT@PANI@PVA yarn

gles yarns as two electrodes.

to be controlled to their desired diameters. The PVA gel coating

of the two-ply yarn supercapacitors based on the pure CNT yarn

and the CNT@PANI composite yarn, respectively. The experi-decreased in thickness because the PVA gel shrank as it solidi-− 1 with a

fi ed in air. Figure 2 a shows an optical microscopy image of the

ments were conducted at a scanning rate of 20 mV s

CNT@PANI yarn and Figure 2 b after the PVA gel coating (i.e.,

potential range from 0 V to 0.8 V. Both of these supercapacitors

show the typical characteristics of capacitance materials. The

CNT@PANI@PVA composite yarn), both taken at transmis-pure CNT yarn supercapacitor shows a rectangular shaped CV

sion mode. The thickness of PVA coating was about 20 μm,

curve, which is typical of an electrochemical double layer super-giving an overall diameter of about 60 μ m for the CNT@[29] The CNT@PANI composite yarn supercapacitor

PANI@PVA yarn. Finally, two CNT@PANI@PVA yarns were

capacitor.

Adv. Mater. 2013, 25, 1494–1498© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, 1495COMMUNICATION

− 2 while the areal capacitance of 38 mF cm

the pure CNT yarn-based supercapacitor was

− 2only c.a. 2.3 mF cm , representing a 16-fold

improvement. It is noticed that this capaci-tance is two times of a very recently reported

capacitor comprising a carbon fi ber and pen

[32]

ink as electrode materials.

To our knowl-edge, the capacitance reported here is the

highest so far for thread-like supercapacitors.

The areal capacitance maintained a high level

− 2(12 mF cm

) even at the highest current

− 2density of 1 mA cm . The improvement in

capacitance can be attributed to the ordered

PANI nanowire arrays which are recognized

as a high-performance pseudocapacitance

[21–24]material.

To further differentiate the thread-like

supercapacitor based on the pure CNT yarn

and that based on the CNT@PANI yarn, we

measured the charge transport and ion diffu-sion of the yarn supercapacitors using elec-trochemical impedance spectroscopy (EIS),

from which their Nyquist plots were gener-ated, as shown in Figure 3 d. The x-intercept

of the Nyquist plots represents the equiva-lent series resistance (ESR) for two-electrode

supercapacitor and the charge transport

[33resistance.

] The slope of the Nyquist plots,

known as the Warburg resistance, is a result

of the frequency dependence of ion diffu-sion in the electrolyte to the electrode inter-[34]face.

As shown in Figure 3 d, both the ESR

and Warburg resistance of the pure CNT

Figure 2. Optical microphotographs. a) CNT@PANI yarn; b) CNT@PANI@PVA yarn; c) two

yarn supercapacitor are lower than the cor-CNT@PANI@PVA single yarns twisted together to form a thread-like, two-ply yarn superca-responding values of the CNT@PANI yarn

pacitor. d) Photograph of a longer supercapacitor. e) A model woven energy storage device

supercapacitor This is also consistent with

consisting of six two-ply yarn supercapacitors (refl ection mode); f) yarn supercapacitors are

the trend of the capacitance plots. As cur-co-woven with conventional cotton yarns to form a fl exible electronic fabric with self-suffi cient

rent density increased, the CNT@PANI yarn

power source.

supercapacitor showed a more rapid decrease

than the pure CNT yarn supercapacitor. A

demonstrates typical double redox peaks which are character-possible explanation is that the conductivity of the CNT yarn

istic of PANI materials. The fi rst peak can be ascribed to the

electrode decreases after coating the PANI polymer.

redox transition of PANI from the leucoemeraldine form to

As a textile material, the thread-like supercapacitors must

the emeraldine form, and the second peak corresponds to the

possess excellent fl exibility without scarifying their electro-[30]emeraldine-to-pernigraniline transformation.

chemical performance. The electrochemical properties of the

Gram capacitance does not provide a suitable basis for com-

CNT@PANI yarn supercapacitor under different bending states

[31]− 2

parison in this case. Instead, areal capacitance (F cm )is

were evaluated using an in-situ test. Figure 3 e shows the capac-calculated from the charge–discharge curve to evaluate the

itance retentions at different bending angles. The capacitance

charge-storage capacity of the two-ply yarn supercapacitors.

retentions were derived from their cyclic voltammetry curves at

− 1

The triangle-shaped galvanostatic charge–discharge curves in

a constant scanning rate of 20 mV s

. The capacitance of the

[9]Figure 3 b are typical for all supercapacitors.

The CNT@PANI

yarn supercapacitor changed very little even the bending angle

yarn-based supercapacitor exhibited longer charge–discharge

approached to 180 ° . The yarn supercapacitor had maintained its

time than the pure CNT yarn based supercapacitor. Figure 3 c

capacitance almost fully after having been bent for 150 cycles,

shows the dependence of areal capacitance on current density

as shown in Figure S4 of the SI.

calculated from the galvanostatic charge–discharge curves. The

The charge–discharge stability of the thread-like super-

areal capacitance of CNT@PANI yarn-based supercapacitor

capacitors was investigated using a cyclic galvanostatic

is much higher than that of the pure CNT yarn-based super-charge–discharge test. After 800 consecutive charge–discharge

− 2

capacitor. At the current density of 0.01 mA cm

, the CNT@cycles, both the pure CNT and the CNT@PANI composite

PANI yarn-based supercapacitor showed a capacitance of

yarn supercapacitors showed excellent capacitance retention

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Mater. 2013, 25, 1494–1498

UNICATIONFigure 3. Electrochemical properties of two-ply yarn supercapacitors: a) cyclic voltammograph; b) galvanostatic charge/discharge curves; c) areal

capacitance plots; d) impedance spectroscopy with a frequency loop from 50 KHz to 10 mHz using a perturbation amplitude of 5 mV at open-circuit

potential. e) Capacitance retention under different bending states (Inserted bottom-left image shows a connector used in electrochemical testing).

f) Cyclic charge–discharge stability.

(Figure 3 f). The pure CNT yarn supercapacitor did not show any

reduction in capacitance after the 800 charge–discharge cycles.

Conducting polymers often suffer from poor cyclic stability as

electrode materials for supercapacitors because of the cyclic

mechanical stress caused by material swelling and shrinking

[35]during the charge–discharge (doping-dedoping) process.

However, our CNT@PANI composite yarn supercapacitor

showed excellent cyclic stability. Its capacitance maintained

at 91% of its original value after 800 charge–discharge cycles.

This superior cyclic stability can be attributed fi rstly to the well-defi ned PANI nanowire arrays which can accommodate the

stress changes in the doping/dedoping process and thus less

[21–23]susceptible to fatigue caused by cyclic mechanical stress.

More importantly, the stability is attributable to the mechanical

reinforcement or toughening effect of the carbon nanotubes in

the CNT@PANI composite yarn supercapacitor.

Thread-like supercapacitors are constructed from two-ply

CNT and PANI composite yarns using a simple process. The

new supercapacitors can be used as energy storage devices and

woven into textile fabrics like conventional yarns. The CNT

yarn acts both as electrode and as high toughness substrate.

PANI nanowire arrays were in-situ polymerized on the surface

of the CNT yarn to improve the energy storage capacity of the

device. A layer of PVA gel was coated on the yarn surface to

form a separation layer containing electrolyte. Two such coated

yarns were twisted together to form a two-ply yarn supercapac-itor. The as-prepared CNT@PANI yarn supercapacitor has been

characterized by electrochemical experiments and has shown

1497Adv. Mater. 2013, 25, 1494–1498© 2013 WILEY-VCH Verlag GmbH & Co. KGaA,

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Supporting Information

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Acknowledgements

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Environ. Sci. 2012,5,8384 .This work was supported by National Natural Science Foundation of

[24 ]K. Wang ,J. Y. Huang ,Z. X. Wei ,J. Phys. Chem. C 2010,114,8062 .China (Grants 21125420, 20974029 and 91027031), The Ministry of

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Received: November 6, 2012

2008,8,3498 .

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The two-ply yarn supercapacitors have exhibited high fl exibility

and can be used as power source for fl exible electronic devices

for applications that require conventional fabric-like breath-ability and durability.

© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Mater. 2013, 25, 1494–1498