carbon coated textiles for flexible energy storage

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Carbon coated textiles for flexible energy storage†

Kristy Jost,ab Carlos R. Perez,b John K. McDonough,b Volker Presser,b Min Heon,b Genevieve Diona

and Yury Gogotsi*b

Received 19th August 2011, Accepted 26th September 2011

DOI: 10.1039/c1ee02421c

This paper describes a flexible and lightweight fabric supercapacitor electrode as a possible energy

source in smart garments. We examined the electrochemical behavior of porous carbon materials

impregnated into woven cotton and polyester fabrics using a traditional printmaking technique (screen

printing). The porous structure of such fabrics makes them attractive for supercapacitor applications

that need porous films for ion transfer between electrodes. We used cyclic voltammetry, galvanostatic

cycling and electrochemical impedance spectroscopy to study the capacitive behaviour of carbon

materials using nontoxic aqueous electrolytes including sodium sulfate and lithium sulfate. Electrodes

coated with activated carbon (YP17) and tested at �0.25 A$g�1 achieved a high gravimetric and areal

capacitance, an average of 85 F$g�1 on cotton lawn and polyester microfiber, both corresponding to

�0.43 F$cm�2.

1. Introduction

Smart textiles, also known as e-textiles or electronic textiles, feel

and function like fabrics but also have built-in functions, such as

sensing, data processing, actuation, storage (energy or data) and

communication.1–3 Examples of potential applications of this

technology include military garment devices, biomedical and

antimicrobial textiles, and personal electronics.1,4 The successful

commercialization of smart garments is hindered by the lack of

fully integrated energy storage, given that conventional batteries

and capacitors are too bulky and heavy to be considered wear-

aFashion, Product, Design and Merchandising Department, DrexelUniversity, 3141 Chestnut Street, Philadelphia, PA, 19104, USAbDepartment of Materials Science and Engineering & A.J. DrexelNanotechnology Institute, Drexel University, 3141 Chestnut Street,Philadelphia, PA, 19104, USA. E-mail: [email protected]; Fax: +1215 8951934; Tel: +1 215 895 6446

† Electronic supplementary information (ESI) available. See DOI:10.1039/c1ee02421c

Broader context

Electric energy storage is the bottleneck in implementation of rene

mobile user electronics. Further development of electronic textile

wearable, safe, reliable, and cost-efficient energy storage. Current ap

with carbon nanotubes, implementation of nanotube paper or elect

garments will greatly depend on the ability to use established textile

carbon/electrolyte combinations to obtain higher energy densities.

electrolytes is a more practical approach. In addition, textile-based

since they are flexible, pliable, do not kink, and recover their shape

based textile energy storage devices the potential to become succes

5060 | Energy Environ. Sci., 2011, 4, 5060–5067

able; this is why the integration of energy storage into garments

by building them upon pre-existing textile structures that are

widely used in the apparel industry is imparative. Super-

capacitors, which provide higher power than batteries and can

use nontoxic and non-flammable electrolytes, received much

attention in the past few years for energy storage.

Since electrodes are the active components that determine the

performance of supercapacitors, it is most important to fabricate

electrodes which are comparable to commercially available

supercapacitors5–7 in terms of device capacitance, mass and

thickness.7 Previous literature, however, focused primarily on

technologies which are either not direclty applicable for full

device integration (e.g., nonwoven8 or electrospun fiber9) or use

expensive carbon nanomaterials with low double layer capaci-

tance (e.g., carbon nanotubes8,10–12). In addition, the mass load-

ings of fabric supercapacitor electrodes employing CNTs are

usually very low (9.5 mg$cm�2 to 1.7 mg$cm�2 per 2-electrode

device)8,11,12 and the resulting device capacitance cannot compete

with conventional coin or pouch cell supercapacitors.

wable energy technologies, grid-scale energy management, and

s (specifically ‘‘smart garments’’) is held back by the lack of

proaches to electrode fabrication include coating natural fibers

rospun fibers. The success and wide-spread acceptance of smart

manufacturing technologies and integrate them with optimized

Thus, the use of nontoxic and inexpensive active materials and

energy storage has advantages over polymer films and paper,

. Their additional exceptional mechanical stability give fabric-

sfully integrated into various flexible electronic applications.

This journal is ª The Royal Society of Chemistry 2011

http://dx.doi.org/10.1039/c1ee02421c






http://pubs.rsc.org/en/journals/journal/EE

http://pubs.rsc.org/en/journals/journal/EE?issueid=EE004012

Fig. 1 Design concept of a porous textile supercapacitor integrated into

a smart garment, demonstrating porous carbon impregnation from the

weave, to the yarn, to the fibers.

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The current generation of energy storage systems often occupy

more space than the electronic devices that they power. There-

fore, areal capacitance is a key factor in smart garments since

there is a limited amount of surface to integrate electronics and

capacitive materials; therefore, achieving high mass loadings into

each square centimeter while maintaining the wearable quality of

the textile will most efficiently use the textile area to store energy.

Polymer,9 paper,11,13 and other nonwoven materials8,9 lack the

ability to be a continuous part of a pre-existing textile structure

since they are neither woven nor knitted, and the electrodes are

often made of expensive active materials such as CNTs,8,10–12 and

nanowires,14 employ toxic electrolytes,10 or use pseudocapacitive

materials15,16 not known to have cycle life as long as carbon

materials but have comparable areal capacitances.

The materials used in previous works on energy textiles were

primarily felt-like (nonwoven) fabrics that are not typically used

in apparel. Compared to most medium weight woven fabrics

(�200 mm), nonwoven textiles can be rather thick (up to 2 mm)

and may have lower strength depending on how tightly matted

(i.e., how dense) the fibers are within the textile.8,10 Nonwoven

fabrics are typically made of felted wool or matted synthetic

fibers (either melt spun or electrospun), both of which have

strength and permanent bonding between the fibers, while cotton

nonwovens used in other studies10 resemble a cotton ball

material.

Previous works on CNTs report films as thin as 100–500 nm 11

but it is important to note that the gravimetric and areal

capacitance usually decreases at higher carbon loading or

increased film thickness.17,18 Hu et al. reports this phenomenon

when they increase CNT film thickness from 500 nm to �14 mm

and gravimetric capacitance is reduced from 200 F$g�1 to

122 F$g�1. Stoller et al. also cited Hu et al. explaining that as the

device active mass of 72 mg$cm�2 was increased to 1.33 mg$cm�2,

the gravimetric capacitance dropped from 200 F$g�1 to 85 F$g�1

and further stating that using minute amounts of material can

overestimate the electrochemical performance.7 The carbon

electrode thicknesses of commercial supercapacitors can range

from 10 mm for high power density devices (often for micro-

electronics),19 to 100–300 mm for high energy density devices

often used for other applications.7

Using established methods (e.g., screen printing) and materials

(e.g., woven cotton or polyester) common in the fashion

industry, textile carbon electrodes can be manufactured on

a large scale without the expense of redesigning processes. Screen

printing and ink-jet printing14,20,21 of capacitive materials onto

fabrics are present methods used in manufacturing and have been

studied for applications in traditional thin film super-

capacitors.14,20 These techniques are suitable impregnation and

coating methods due to the precise control of carbon coated onto

the fabric.14 Ink-jet printing has the advantage of high precision

ink droplet spacing. Screen printing is material efficient, and can

coat large surface areas quickly, though may have a small vari-

ation in the carbon mass that is impregnated into the fabrics.

Dip-coating is another possible technique,8,10 but the amount of

carbon impregnated into the fabric depends greatly on the

hydrophilicity of the material, which can also vary from section

to section of the same fabric. This may result in non-uniform

coatings, limiting the types of fabrics that could be used as textile

electrodes.


The structure of fabrics, like 3D batteries, has hierarchical

porosity22 in the fiber, yarn, and woven or knitted structure and

can affect ion mobility (Fig. 1).23 In particular, woven and

knitted fabrics exhibit free space between individual fibers (2–

4 mm) and between yarns (10–30 mm) and it is the integration of

active carbon materials into this pore volume that will define

the resulting electrochemical performance of a textile

superacapcitor.

The objective of this study is to demonstrate supercapacitor

electrodes fabricated using a scalable and high mass loading

printing technique of commercially used porous carbon powders

onto commonly worn woven fabrics for use in wearable elec-

tronics and smart textiles. We understand this is a first, but very

important step for the application of textile supercapacitors, and

a fully integrated device will also require sealing of the active

surface to enclose and contain the electrolyte. However, this is

not part of the study.

2. Experimental

2.1 Carbon materials

CXV: activated carbon is derived from pinewood (Arkema,

France) and is a typical activated carbon used for preliminary

testing of fabric electrodes. Particle size: 10–32 mm; pore size:

broad distribution with a peak at �1 nm; surface area:

1300 m2$g�1.

YP17: is a porous activated carbon (Kuraray Chemical,

Japan) derived from coconut shells and is a common material

used for supercapacitor electrodes. Particle size: 2–3 mm; pore

size: most of the pores (�80%) below 2 nm with a peak at�1 nm;

surface area: 1550 m2$g�1. (See supplementary information for

details†)

Carbon onions: also known as onion-like carbon (OLC) was

chosen as an additive to increase electronic conductivity of the

electrodes. OLC are mostly dense spherical carbon nanoparticles

consisting of concentric graphitic shells (multishelled fullerenes).

Energy Environ. Sci., 2011, 4, 5060–5067 | 5061


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They were produced by vacuum annealing of UD50 detonation

nanodiamond soot (NanoBlox, USA) at 1800 �C.19,24 Particle

size: 5–10 nm diameter; interparticle pores: 3–15 nm; surface

area: 500 m2$g�1.

2.3 Fabrics

An array of fabrics was selected to include both basic and

complex strutures made of fiber contents commonly used in the

textile and apparel industry. As described in Table 1,

A) Cotton lawn: a hydrophilic natural staple fiber, spun into

a yarn and woven as a plain weave fabric (yarn thickness: 50 mm;

textile thickness: 160 mm).

B) Polyester microfiber: a high wicking fiber, is made of fila-

ment fibers 10 mm in diameter, spun into a fine yarn and woven

into a twill fabric (yarn thickness: 50 mm; textile thickness:

200 mm).

C) Cotton twill: made of a natural staple fiber spun into 4-ply

yarns and woven into a twill fabric (yarn thickness: 100 mm;

textile thickness: 300 mm).

D) Double knit with silver: a polyester filament yarn knitted

into a double knit, with fine silver filaments knitted every

two rows on one side (yarn thickness: 60 mm; textile thickness:

1.5 mm).

E) Nylon neoprene: also a double knit made of filament yarns,

and a nylon monofilament pile looped between the top and

bottom layers, and is also called spacer fabric (yarn thickness:

50 mm; textile thickness: 2.6 mm).

2.4 Binder

Samples with 10 wt% polytetrafluoroethylene (PTFE) as a binder

were very brittle and resulted in carbon flaking off of the fabric

(section 3.1). The reason for this is that PTFE does not adhere to

fabrics because of the high electro-negativity of the outer shell of

fluorines. We chose to use Liquitex� matte medium, which is an

emulsion of polymethyl methacrylate (PMMA) and polyethylene

glycol (PEG) in water. It is used as a fine arts binder in acrylic

painting on canvas, binding the pigment particles together (in

our case the carbon particles) and permanently adhering the

pigments to a fabric canvas, often a basket-weave type of fabric

very similar to our plain weave cotton fabric.

2.5 Electrolyte

Tests were carried out in 1 M sodium sulfate and 2 M lithium

sulfate from Sigma Aldrich.25 These electrolytes were chosen

because they are nontoxic and inexpensive, making them safe for

use in smart garments as well as cost effective. Of course use of

Table 1 Dip coating textiles

Textile Weave, fiber content

A Cotton lawn Plain weave, 100% cottonB Polyester microfiber Twill weave, 100% polyesterC Cotton twill Twill weave, 100% cottonD Double knit with silver Double knit, 98% polyester, 2% siE Nylon neoprene Double knit with pile, 100% nylon


these or any other liquid of gel electrolyte will require sealing in

a polymer film. CXV samples were tested in 1M sodium chloride,

another nontoxic electrolyte.

2.6 Electrode preparation

For comparison with carbon coated textiles, YP17 conventional

electrodes were made with each electrode weighing 60 mg con-

taining 6 wt% PTFE binder (Section 4.5). 18 The electrodes were 2

� 2 cm2 in size, and �200 mm thick.

2.7 Carbon suspension

A carbon suspension comprised of 20 mgArkema CXV activated

carbon per ml ethanol, with 10 wt% PTFE was prepared for dip

coating fabrics as part of our preliminary testing and comparison

between dip coating and screen printing (Sections 3.1–3.3). The

fabric swatches soaked for 30 min while the suspension was

continually stirred, and then dried in air on a metal mesh rack

before being placed into a vacuum oven at 100 �C to degas for

16 h.

2.8 Slurry preparation

The carbon slurry constisted of 200 mg YP17 activated carbon

per ml water with 5 wt% Liquitex as the binder. Since the matte

medium is completely miscible in water, 5 ml of water was added

to the matte medium to disperse the binder. Binder and water

were added and mixed slowly into the carbon to create a paste-

like slurry. A second slurry was prepared with CXV and a third

with CXV and 10 wt% carbon onions, both containing 5 wt%

binder.

2.9 Cell assembly

Cells were assembled in a conventional symmetrical two elec-

trode setup (Fig. 2) as described in Ref. 26,27. The cell comprised

a porous PTFE separator (W.L. Gore, USA), stainless steel

sheets 50 mm thick that acted as current collectors, (dimensions:

2.5 � 2.5 cm with a 4 cm extended tab), and supporting PTFE

plates to apply evenly distributed pressure across the electrodes

while testing. The cells were put into a polypropylene bag filled

with the electrolyte and heat sealed into that bag over the tabs of

the current collectors to prevent evaporation and/or contami-

nation of the electrolyte.

2.10 Electrochemical performance testing

Cyclic voltammetry (CV), galvanostatic cycling, and electro-

chemical impedance spectroscopy (EIS) were performed on

Fabric mass(mg$cm�2)

Carbon loading(mg$cm�2)

Carbon:fabricratio (wt%)

6.8 1.2 17.213.3 2.2 16.219.8 0.7 3.7

lver 30.9 5.7 18.626.8 0.4 1.6



Fig. 2 Electrochemical capacitor cell assembly, a) front and back view

of the testing setup with fabric electrode on current collector and a PTFE

separator. b) Conventional double layer capacitor testing setup (side

view) with fabric electrodes. c) Simple circuit diagram of the device.

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a VMP3 potentiostat/galvanostat (Biologic, France). Cyclic

voltammetry data was collected at 1, 5, 10, 20, 50, and 100

mV$s�1, within a 1 V window ranging from �0.2 V to 0.8 V. For

each material system the stable voltage range was found to be

�0.2 to 0.8 V except for 1 M NaCl electrolyte, where it was

�0.2 to 0.6 V. Current was cycled galvanostatically at 2.5 and

5 mA$cm�2 within the same voltage window. Electrochemical

impedance spectroscopy (EIS) was performed between 10 mHz

and 200 kHz at the open circuit potential of the cell with a signal

peak to peak amplitude of 10 mV.

All experimental CV data was fitted using non-linear regres-

sion onto a series resistance-capacitance (RC) circuit model.6,28

The discharge capacitance was extracted from the integral of the

current during cyclic voltammetry. For galvanostatic data, the

equivalent series resitance (ESR) was calculated from the IR

drop at the begining of the discharge cycle and the capacitance

was also calculated from the slope of the discharge curve.6 For

impedance spectroscopy, the ESRwas taken to be the real part of

the impedance (Z0) at a frequency of 1 kHz.29

2.11 Gas sorption

Nitrogen gas sorption was performed at �196 �C using a Quad-

rasorb SI instrument (Quantachrome, USA) within a relative

pressure range between 0.01 and 1 with P0 being 760 mm Hg.

Pore size distribution and pore volumes were calculated with the

Quenched Solid Density Functional Theory kernel (QSDFT)

assuming a slit pore geometry.30 The Brunauer-Emmet-Teller

(BET) equation was used to determine the specific surface area

(SSA) between 0.05 and 0.3 P/P0.31

3. Carbon impregnation techniques

3.1 Dip coated fabrics

Cotton and polyester are the two most widely used natural and

synthetic fibers in the textile industry,23 and have also been studied

for supercapacitor applications in literature.8,10 Woven and


nonwoven fabrics by nature of their structure have little ability to

stretch unless the fiber contains an elastomer. Knit fabrics have

greater ease and stretch with or without an elastomer-based

yarn.23 Silver coated conductive yarnsmay be considered for their

improved conductivity, but knitted silver coated yarns were

reported to show increased resistivity after stretching and strain-

ing, due to silver interparticle breakage.32 Stretching of the fabric

carbon electrodes, rather than bending, may cause conductive

networks between carbon particles to break. Therefore, textiles

without elastomer were chosen for dip coating as extensive

stretching or deforming of a fabric is not a requirement in order to

be considered wearable, (e.g., button down woven shirt may not

have elastic fibers in the textile itself and can be quite taught).

Dip coating was conducted in ambient air at room tempera-

ture. The dip coated textiles were soaked in a beaker of ethanol

carbon suspension (Section 2.7) with a magnetic stirrer mixing

the solution and textiles on low for 30 min. The textiles’ ability to

uptake the active carbon material can be found in Table 1.

Fabrics A, B and D held similar carbon to fabric mass ratios

(�17 wt%), while fabrics C and E held less than 4 wt%. Fabrics C

and E were two of the thickest fabrics, (C: 300 mm, E: 2600 mm)

and through the process of dip coating, carbons may not be

capable of penetrating and coating through such a thick and

dense material. Fabric D had a highly porous filament crimped

yarn,23 enabling carbon to penetrate through the fabric and into

the fiber bundles even at a thickness of 1.5 mm. Also, after

electrochemical testing, fabric D displayed erratic electro-

chemical reactions when tested in 1 M sodium chloride, most

likely due to the reaction between silver and sodium chloride.

Based on these results, cotton lawn and polyester microfibers

were selected for further experiments.

3.2 Screen printing

Dip coating does not impregnate a significant amount of carbon

into the fabric structure, and is highly dependant on the uptake

ability of the fibers, thus, no uniform coating on the textile was

achieved and the resulting coatings were not dense enough to

create conductive bonds similar to those in conventional thin film

supercapacitors. Therefore, 2� 2 cm2 swatches of fabric were cut

and aligned on a PTFE plate for screen printing. The screen

printing frame was a nylon mesh with filament spacing approx-

imately 0.3 mm apart. The screen was placed over the fabric with

the slurry directly applied to the swatches. The slurry was spread

to smoothly apply the carbon across the fabric and this process

was repeated 4 times to ensure full penetration of carbon through

the yarns and into the fiber bundles. All swatches were dried on

a small metal mesh drying screen and then degassed in a vacuum

oven for 16 h to remove any residual water prior to further

sample handling. The carbon mass was determined by weighing

the fabric electrodes before and after coating.

3.3 Adhesion of different carbons on polyester microfiber

To characterize carbon impregnation and adherence to fabrics,

different carbon materials were screen printed with a single coat

onto polyester microfiber and examined under a scanning elec-

tron microscope (SEM). Polyester microfibers are cylindrical

with a constant 10 mm diameter, while cotton fibers vary in size



Fig. 4 SEM images of weaves and their corresponding fibers a) polyester

microfiber twill weave before coating. b) Cotton lawn plain weave before

coating. c) Polyester fiber screen printed with YP17. d) Cotton fiber

screen printed with YP17.

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and length (16–30 mm). CXV activated carbon consists of

10–32 mm sized particles that are too large to adhere to the fibers

of polyester microfiber, which can explain why they flaked off.

YP17 particles (2–3 mm) coat and adhere to the fibers thoroughly

as seen in Fig. 3a. Carbon onions have excellent adhesion,

(Fig. 3b and 3d) and the coated fabric electrode has excellent

flexibility without carbon delamination (inset of Fig. 3d). Based

on the good particle adhesion and coating of YP17, a systematic

analysis of the material on both cotton lawn and polyester

microfiber fabrics was conducted. (Sections 4.1–4.3) Additional

testing using CXV and carbon onions was also conducted to

increase the electrical conductivity, where carbon onions were

used instead of carbon black as a conductive additive.24

4. Results and discussion

4.1 Carbon impregnation on polyester microfiber and cotton

lawn

The polyester microfiber fabric alone weighs 13.3 mg$cm�2 per

electrode, which is about twice the mass of the cotton lawn

(6.8 mg$cm�2). However, both fabrics regardless of mass or

carbon uptake ability were impregnated with the same amount of

carbon, on average �4.9 mg$cm�2. Fig. 4c–d show the thorough

coating and adhesion of the carbon to the polyester and cotton

fibers. Cotton lawn holds 81% of its weight carbon, while poly-

ester microfiber only holds 37% of its weight. It is important to

note that even this smaller value is about 20 times the amount of

single-wall carbon nanotubes (SWNT) dip coated into fabrics

reported in a recent study.8 The amount of carbon impregnated

into fabrics increases with the number of screen printed coatings,

which has also been observed from literature where each addi-

tional dip-coating increased the carbon mass by 5%. 8,10

4.2 Electrochemical performance of carbon coated textiles

Each data point in Fig. 5c represents the average of two exper-

iments, where the individual values never differed by more than

Fig. 3 SEM images of polyester microfiber impregnated with different

carbon materials via the screen printing process. a) YP17 thoroughly

coats the fibers b) 10% carbon onions were added to CXV. c) CXV coated

onto polyester microfiber, d) carbon onions coat very thoroughly around

polyester fibers. Inset: polyester electrode coated in carbon onions shows

high flexibility without carbon flaking or cracking.


5%, while the CVs were similarly repeatable. All data reported

(Tables 2 and 3) was averaged for all gravimetric and resistance

values.

Cotton lawn and polyester microfiber electrodes screen printed

with YP17 have similar gravimetric capacitances throughout all

three electrochemical techniques (Table 2), however, the capac-

itive bahavior of cotton lawn electrodes as seen in cyclic vol-

tammetry is less resistive and CVs are more rectangular than

polyester microfiber electrodes in both lithium and sodium

sulfate. Fig. 5a–b compare the capacitive behaviors of cotton

lawn and polyester microfiber tested in 1 M sodium sulfate. Both

voltammograms show capacitive behavior, yet both materials

become more resistive at faster scan rates exhibiting a fade in

Fig. 5 Capacitance versus scan rate a) Gravimetric capacitance vs.

voltage obtained from cyclic voltammetry of cotton lawn tested in 1 M

Na2SO4, at 10 and 100 mV$s�1 b) Cyclic voltammogram of polyester

microfiber tested in 1MNa2SO4 shows more resistive behavior compared

to cotton. c) Normalized capacitance versus scan rate d) Cyclic voltam-

mogram of a YP17 film tested in 1 M Na2SO4 under the same conditions

as the textiles electrodes.



Table 2 Gravimetric capacitance (F$g�1) determined by 3 methods

Electrode material Electrolyte CV (20 mV$s�1)Galvanostatic charge/discharge (20 mA) EIS (10 mHz)

Polyester microfiber (YP17) 2 M Li2SO4 90 83 74Polyester microfiber (YP17) 1 M Na2SO4 81 81 78Cotton lawn (YP17) 2 M Li2SO4 86 90 84Cotton lawn (YP17) 1 M Na2SO4 84 83 81YP17 film 1 M Na2SO4 60 55 50

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capacitance due to an increase in the resistance.28 The areal

capacitance of cotton lawn in sodium sulfate drops from

0.43 F$cm�2 at 1 mV$s�1 to 0.37 F$cm�2 at 100 mV$s�1 which is

a 14% drop in capacitance, (Fig. 5c). Polyester microfiber

swatches show a larger drop in capacitance at 100 mV$s�1, 23%

in sodium sulfate and 20% in lithium sulfate. All samples had

similar specific capacitances at low scan rates of 1, 5, and

10 mV$s�1 varying between 85–95 F$g�1. Cyclic voltammograms

and galvanostatic tests (see supplemental information†) also

show no visible electrochemical reactions occurring within our

voltage range, resulting in purely capacitive behavior.

Polyester microfiber and cotton lawn electrodes also exhibit

high specific capacitance from galvanostatic cycling (supple-

mental information†): polyester microfiber and cotton lawn

electrodes have average gravimetric capacitances of 85 F$g�1 at

�0.25 A$g�1. The average areal capacitance for both fabrics is

0.43 F$cm�2 at 5 mA$cm�2 due to their similar masses. These

capacitive values are dependent mainly on the YP17 active

material, however, it is the behavior as seen in cyclic voltammetry

(Fig. 5a–b) that changes depending on the carbon distribution

within the textiles (Fig. 6). As stated before, cyclic voltammo-

grams of cotton lawn swatches compared to polyester microfiber

based electrodes are more rectangular in shape indicating that

cotton electrodes are less resistive.

YP17 on cotton lawn was tested for 10,000 cycles at

100 mV$s�1 in 2 M lithium sulfate. The device had a decrease of

8% in capacitance after 10,000 cycles. This may be explained by

separation of carbon particles from the fabric during cycling.

While this is a more significant capacitance decrease than in

commercial supercapacitors, it is considerably less compared to

a study by Hung et al. in which woven activated carbon fibers34

showed a decay of �25% after 10,000 cycles.33

Cotton lawn electrodes tested in both sodium and lithium

sulfate exhibit similar ESR values from all three electrochemical

techniques, and polyester microfiber electrodes have a distinctly

higher resistance, especially when tested in lithium sulfate.

The equivalent series resistance (ESR), calculated by

measuring the ohmic drop in galvanostatic charge/discharge

Table 3 Equivalent series resistance (ESR) determined by 3 methods and no

Electrode material Electrolyte CV (20

Polyester microfiber (YP17) 2 M Li2SO4

Polyester microfiber (YP17) 1 M Na2SO4

Cotton lawn (YP17) 2 M Li2SO4

Cotton lawn (YP17) 1 M Na2SO4

YP17 film 1 M Na2SO4


cycles,29 was found to be 3 U$cm2 for cotton lawn in both elec-

trolytes, and polyester microfiber electrodes had ESR values of

6 U$cm2 in sodium sulate and 12 U$cm2 in lithium sulfate

(Table 3).

Cotton lawn samples may have lower ESR than polyester

microfiber samples due to the improved carbon distribution

throughout the fabrics, as well as a smaller distance ions must

travel through a fabric in order to reach the active material. The

carbon network within the cotton lawn may have better conti-

nuity due to the highly porous curvilinear structure of cotton

lawn fibers compared to aligned synthetic fibers that are melt

spun (Fig. 6). The mass of the cotton lawn is also half the mass of

polyester microfiber, resulting in electrodes that contain more

carbon and less substrate material.

ESR values obtained from impedance spectroscopy (supple-

mental information†) have similar trends to values obtained

from galvanostatic cycling and cyclic voltammetry (Table 3).

Cotton lawn exhibited lower resistance than polyester microfiber

with values of 3–4 U$cm2 and 6–8 U$cm2, respectively. The time

constant is the reciprocal of the frequency at which the phase

angle of the device is �45�.24 A lower time constant corresponds

with the cell being able to charge and discharge faster. Cotton

electrodes tested in sodium sulfate had an average time constant

of 5.5 s and 6.3 s in lithium sulfate. Polyester microfiber elec-

trodes tested in sodium sulfate also achieved 6.2 s, while polyester

tested in lithium sulfate had a time constant of 22 s.

Polyester microfiber electrodes tested in lithium sulfate exhibit

the highest resistance of all samples in all three techniques (Table

3), with values as high as 18 U$cm2 when scanned at 20 mV$s�1.

Pasta et al. made an observation about lithium sulfate having

a slow H3O+ ion sorption that can achieve additional capaci-

tance.8 However, their reference was based on a study by Prosini

et al.35 that tested SWNTs in 2 M lithium sulfate, while disor-

dered activated carbons are different in structure, and thus may

not have the same ion sorption mechanism as nanotubes.

Comparing Li2SO4, Na2SO4, and K2SO4, the hydrated Na+ and

K+ ions are smaller than hydrated Li+. The smaller hydrated ions

can better access activated carbon pores resulting in higher ionic

rmalized by unit of electrode surface (U$cm2)

mV$s�1)Galvanostatic charge/discharge (20 mA) EIS (1 kHz)

18 8 811 6 65 3 45 3 38 2 3



Fig. 6 Model of carbon impregnation into yarns. a) Densely packed

cylindrical polyester fibers (10 mm diameter) do not allow for carbon

penetration into the fiber bundle. b) Cotton fibers’ organically shaped

structure (16–30 mm width) allows for improved impregnation of carbon

particles and ion transport. c) Structural formula of polyester. d) Struc-

tural formula of cotton.

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mobility and conductivity, thus, perform better than lithium

sulfate.25 The additional resistance of polyester electrodes tested

in lithium sulfate could be the result of the combination of larger

hydrated lithium ions, and the densely packed polyester fibers

(Fig. 6a), both resulting in higher device resistance.

Fig. 7 a) Cyclic voltammograms of CXV with and without 10wt%

carbon onions addition (scan rate: 10 mV$s�1 s). b) Rate handling ability

increases with the addition of carbon onions.

4.3 YP17 films compared to YP17 fabric electrodes

As a direct comparison of textile elctrodes to conventional films,

a YP17 film was prepared and tested in 1 M sodium sulfate. Each

elecrode had a mass of 60 mg active material and 6 wt% PTFE

binder. The YP17 carbon film had a specific capacitance of

60 F$g�1 and ESR of 8 U$cm2 from cyclic votammetry at

20 mV$s �1. The device has good capacitive behavior at low scan

rates, but the capacitance drops significantly when scanned up to

100 mV$s�1 (Fig. 5c–d). The bulk device also had a time constant

of 22 s, which is much higher than cotton lawn electrodes. The

increase in the specific capacitance and lower or similar ESR of

the textile electrodes (Tables 2 and 3) is likely due to the thinner

layer of carbon material across the fibers (Fig. 4c). The layers

may only be a few particles thick, meaning in combination with

the porous structure of textiles, that ions can easily access carbon

pores. The areal capacitance of the YP17 films is higher than the

textile electrodes having 0.9 F$cm2 and 0.43 F$cm2, respectively.

However, as discussed previously comparing the cotton lawn and

polyester microfiber electrodes with other polymer, paper, and

textile based electrodes, the pure YP17 films have a higher elec-

trode mass per area of 15 mg$cm�2. Yet the YP17 films do not

posses the same flexibility or mechanical strength of the textile

Table 4 Carbon onions as a conductive additive on polyester microfiber, va

Active Material (mV/s)Gravimetric

Capacitance (F$g�1)

CXV (activated carbon) 63CXV + 10 wt% carbon onions 65


electrodes, and improvements could be made to further increase

the textile mass loading.

4.4 Carbon onions as a conductive additive

YP17 coats fibers thoroughly and evenly when screen printed

onto both cotton and polyester (Fig. 4) and exhibits good

capacitive behavior as described in sections 4.1–4.3. However,

CXV activated carbon has larger particle sizes (10–32 mm), which

are too large to adhere to an individual fiber. Carbon onions are

highly conductive carbon nanoparticles24 and coat polyester

fibers better than any other carbon material (Fig. 3d). Therefore,

carbon onions have the potential to fill in the gaps between larger

activated carbon particles and act as conductive bridges between

large CXV particles to increase conductivity (Fig. 3b).

Two polyester devices were fabricated: one device has elec-

trodes screen printed with pure CXV slurry, while the other

device has electrodes screen printed with CXV slurry with 10 wt%

carbon onions. Electrodes with the addition of carbon onions are

more conductive (14 U$cm2 instead of 37 U$cm2) and have

a slightly higher specific capacitance (Table 4). Furthermore, the

rate handling ability (Fig. 7b) was significantly improved by

adding carbon onions 19,24 which is similar to the effect of onions

added to templated mesoporous carbon electrodes.36

5. Conclusions

Flexible and durable textile electrodes for supercapacitors were

fabricated using nontoxic electrolytes, common textiles used in

the apparel industry and inexpensive yet highly capacitve acti-

vated carbons. The electrodes achieved specific capacitances as

high as 90 F$g�1 with areal capacitances of 0.43 F$cm2 and

exceeded the gravimetric capacitances of YP17 films when tested

under the same conditions. These textile elctrodes are flexible

while conventional thin film electrodes are fragile and not ideal

for smart garment applications.

lues taken from cyclic voltammetry at 10 mV/s

DeviceESR (U)

NormalizedESR (U$cm2)

9 373.5 14



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The use of screen printing in combination with carbon mate-

rials (activated carbon and carbon onions) allows infiltration of

the porous structures of both cotton lawn and polyester micro-

fibers and acheives higher mass loading and areal capacitance

than reported previously in literature on polymer webs or CNT

textiles and paper.8–12

Carbon onions as a nanoscale conductive additive significantly

decrease ESR by acting as a conductive bridge filling the gaps

between micron sized activated carbon particles.

Cotton lawn and polyester microfiber based carbon electrodes

show similar gravimetric and areal capacitances. However,

because of the lower resistance of the electrode and mass of

cotton (about half the values of polyester) as well as capacitance

stability at higher scan rates, cotton lawn clearly is the better

candidate for real world textile supercapacitor applications.

Acknowledgements

Electron microscopy was carried out at the Drexel Centralized

Research Facilities (CRF). KJ was supported by the Fashionable

Technology Project funded by the Antoinette Westphal College

of Media Arts and Design (AW CoMAD) Synergy Grant 2010–

2011, the Research Co-op Office of the Steinbright Career and

Development Center (SCDC) at Drexel University, and NSF-

IGERT Fellowship under Grant No. DGE-0654313. CRP was

supported by the National Science Foundation ICC Project

under Grant No. CHE-0924570. YG, JKM, and VP were sup-

ported as part of the Fluid Interface Reactions, Structures and

Transport (FIRST) Center, an Energy Frontier Research Center

funded by the U.S. Department of Energy, Office of Science,

Office of Basic Energy Sciences under award no. ERKCC61. VP

acknowledges financial support by the Alexander von Humboldt

Foundation.

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