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Knitted and screen printed carbon-fiber supercapacitors for applications in wearable electronics Published on 19 June 2013. Downloaded by Drexel University on 17/07/2013 16:11:50.

Cite this: DOI: 10.1039/c3ee40515j

Kristy Jost,ab Daniel Stenger,a Carlos R. Perez,a John K. McDonough,a Keryn Lian,c Yury Gogotsi*a and Genevieve Dion*b The field of energy textiles is growing but continues to face two main challenges: (1) flexible energy storage does not yet exist in a form that is directly comparable with everyday fabrics including their feel, drape and thickness, and (2) in order to produce an “energy textile” as part of a garment, it must be fabricated in a systematic manner allowing for multiple components of e-textiles to be integrated simultaneously. To help address these issues, we have developed textile supercapacitors based on knitted carbon fibers and activated carbon ink. We show capacitances as high as 0.51 F cm

2

per device

Received 13th February 2013 Accepted 19th June 2013

at 10 mV s

DOI: 10.1039/c3ee40515j

tested under the same conditions. We also demonstrate the performance of the device when bent at 90 , 135 , 180 and when stretched. This is the first report on knitting as a fabrication technique for

www.rsc.org/ees

integrated energy storage devices.

1

, which is directly comparable with those of standard activated carbon film electrodes

Broader context Novel electrical energy storage technologies for powering small portable electronics and computers are in high demand for these ubiquitous devices. Recharging electronic devices running on current battery technology may take an hour or more to complete. A combination of wearable energy storage and harvesting has been proposed to capture readily available energy from environmental sources such as sunlight, body heat, body movements, or ambient radio frequencies (e.g., Wi-Fi or Bluetooth). Self-powered smart garments can help reduce the user's carbon footprint and energy consumption, while providing new functionalities such as temperature regulation, monitoring of vital and real time information on health and the environment. At present, energy storage for smart garments remains a fundamental challenge. To be reliable, they will likely need comparable energy density to conventional batteries and supercapacitors, combined with excellent mechanical strength and exibility. Additionally, they will need to be made of non-toxic materials, and produced using scalable and well established textile manufacturing techniques capable of incorporating other types of components into the same garment/cloth. We address the above issues through the development of textile supercapacitors based on knitted carbon bers and activated carbon ink.

1

Introduction

Wearable electronics present exciting possibilities for interfacing computers/processors, sensors and other devices with the human body.1,2 Smart garments can be found commercially and have been described in the literature,1–4 but typically require bulky battery packs or have to be plugged into the wall, making energy storage one of the key challenges to the widespread usage of smart garments. Electric double layer capacitors (EDLCs) present a unique advantage for use in smart garment applications compared to batteries as they can be made entirely of non-toxic materials, do not ignite if punctured, and can be paired with textile-piezoelectrics,5 thermoelectrics,6 a

Department of Materials Science and Engineering & A.J. Drexel Nanotechnology Institute, Drexel University, 3141 Chestnut Street, Philadelphia, PA 19104, USA. E-mail: [email protected]; Fax: +1 215 895 1934; Tel: +1 215 895 6446

b

Shima Seiki Haute Technology Laboratory, ExCITe Center, Drexel University, 3401 Market Street, Philadelphia, PA 19104, USA. E-mail: [email protected]

c Department of Materials Science and Engineering, University of Toronto, Ontario, Canada. E-mail: [email protected]

This journal is ª The Royal Society of Chemistry 2013

or solar panels7,8 to efficiently harvest energy from body movements, body heat or sunlight, respectively (Fig. 1). EDLCs can be used to power wearable bio-sensors, antennas, and have the potential to be used in exible robotic prosthetics for rapid movements due to their high power and rapid discharge. Previous studies on textile energy storage devices (batteries9 and EDLCs10–13) report good gravimetric performance, but they are oen thin lms with low and impractical areal capacities. Chemical vapor deposition (CVD) and electrophoretic deposition (EPD) are coating techniques that result in very uniform lms, as evidenced in the literature,11 but they are limited to active material masses on the order of micrograms to a few milligrams. Previous studies14–16 have already pointed out that electrode lm thickness affects the electrochemical performance of EDLCs, meaning that even if many of the CVD/EPD coated textile devices were scaled up, they may have inconsistent gravimetric capacitances and resistances. In addition, Stoller and Ruoff16 point out that testing devices having less than 10 mg of active material can result in the overestimation of capacitance due to some pseudocapacitance from impurities

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Fig. 1 Concept for integrated energy storage and other communication/energy harvesting components. This design intends to use textile-supercapacitors with antennas and piezoelectric materials.

that become more pronounced in very small devices. The majority of previously reported devices have very low masses per cm2, some with so small values that they were not reported by Bae et al.,11 El-Kady et al.,17 and Fu et al.18 These devices also fail to use entirely non-toxic, non-ammable, and inexpensive materials, nor scalable manufacturing techniques. Our approach to textile energy storage focuses on fabricating devices with mass, materials, and capacities comparable to their non-textile counterparts, and to do so with established industrial techniques (knitting and screen printing). The devices reported in the paper are primarily composed of carbon, and contain no metals, which makes them ideal for stealth applications. The most important metric to consider when designing energy storage for textile applications is capacitance per unit area (F cm 2), since the average human body only has 1.5 m2 of surface area. The thickness of textiles and our devices is also limited to the thickness of conventional textiles (usually 100 mm up to 1 mm thick). Areal capacitance depends upon two key factors: (1) mass loading of active material per area, and (2) the intrinsic capacitance of the selected active material. For example, Liu et al.19 report graphene painted onto cotton devices having 326 F g 1 tested in 6 M KOH. Even with such a high gravimetric capacitance, the authors' device only has about 1 mg cm 2 of active material, resulting in a device areal capacitance of 163 mF cm 2. Hu et al.,10 one of only a few papers that reports the use of a non-metallic current collector (carbon nanotubes acted as both electrode and current collectors), deposited only 0.8 mg cm 2 at 90 F g 1, resulting in 36 mF cm 2 per device. Therefore high gravimetric capacitance (F g 1) translates to high areal capacitance (F cm 2) only when also paired with high active mass loading per area (mg cm 2). However, as observed in industry, thicker carbon lms oen suffer from diffusion limitations of the electrolyte, resulting in lower gravimetric capacitances and slower charge–discharge rates. We will further elaborate on this point in the Results and Discussion section as it relates to our work.


Paper Our previous work reported woven cotton and polyester electrodes screen printed with activated carbon.14 In this work, we replaced the inactive cotton and polyester backbone with highly conductive carbon bers, and additionally eliminated the need for metal current collectors. We also use a solid electrolyte in place of a liquid electrolyte to prevent any possible leakage. In this work, we investigate the use of knitting combined with screen printing as fabrication techniques for wearable textile-supercapacitor electrodes. Knitting is a method of constructing a fabric by which a series of loops are interconnected with each other in any XYZ direction, while weaving limits a yarn to a single le/right or top/bottom (XY) direction. In knitting, many types of yarns can be integrated into a single sheet in any geometry, such as a single square of a new material embedded in the center of a full sheet of fabric, so-called intarsia knitting. Knitting is also not solely dependent on the contact of one yarn to another, since a sheet of knitted fabric can be made from a single continuous strand of yarn. For conductive materials, this structural continuity signicantly reduces resistance within the material. Knitting and screen printing are well established textile manufacturing techniques; using these techniques also allows us to tailor our textile supercapacitors for either high power, or high energy density applications by changing out the conductive/smart yarns18,20,21 and carbon printing inks. It is also possible to use knitting and screen printing to incorporate battery materials if desired, as well as batteries and supercapacitors alongside each other in the same piece of fabric. Screen printing is a coating technique where ink is passed through a screen with a shape masked off. The ink is slowly pushed through the screen with a squeegee, and the shape can be printed on the substrate below. In this study we report the fabrication of a textile EDLC that is made by screen printing an activated carbon paint onto a custom knitted carbon ber cloth that acts as the current collector and employs a solid, “no leak,” electrolyte (Fig. 3). Our electrodes have comparable active mass loading and capacitance per area (F cm 2) to their conventional lm counterparts when tested under the same conditions.

2

Experimental

2.1

Materials

Carbon staple ber yarn (knitted CF). Pharr Yarns, USA: staple bers, 99.9% carbon pyrolized from PAN (polyacrylonitrile), ber diameter: 6–7 mm, staple length: 3–5 cm, yarn thickness: 80 mm, 2-ply 1k tow. This material was knitted into a plain we knit fabric using a Shima Seiki (Japan) SSG122SV computerized knitting machine. The fabric has isolated regions of carbon ber squares (3 3 cm) within a larger sheet of wool (Fig. 2); fabric thickness: 400 mm. They were used as current collectors. 282 Plain weave carbon bers (woven CF). Applied Vehicle Technology, USA; 3k tow continuous monolament; 6–7 mm ber diameter, 1-ply yarn, 200 mm thick, 19.5 mg cm 2. They were used as current collectors.


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Fig. 2 Seamlessly knitted and woven carbon fiber electrodes. (a) 3D simulated model and (b) zoom-in of the shirt with a textile supercapacitor electrode, embedded as part of a long sleeve t-shirt. (c) Simulated knit structure rendered before fabrication. Developed on the SDS-One Design Software. (d) Carbon fiber current collector coming out of the knitting machine during fabrication. (e) Four current collectors knitted at once, demonstration of how lengths of electrodes can be knitted quickly and efficiently, (f) close-up of the CF electrode screen printed with activated carbon; outer green fabric made of wool. (g) Carbon fiber woven fabric before printing. (h) Carbon fiber woven fabric after printing. Only the overlapping section is coated in the electrolyte. (i) A shaped front bodice knitted as one piece with a sample electrode made as a part of the textile. Knitted on an SGG122SV Shima Seiki Machine.

YP17 activated carbon. YP17 activated carbon, Kuraray, Japan; SSA: 1500 m2 g 1, PSD: peak at 1 nm with 80% of pores below 1.2 nm, grain size: 2–3 mm. It was used as the active material in our EDLCs. Solid polymer electrolyte (SiWA). A solid polymer electrolyte (SiWA) was prepared as described by Gao and Lian:22 PVA (polyvinylalcohol), silicotungstic acid, orthophosphoric acid, and 85% water were mixed together to create a gelled solution (see ref. 22 for full details). The use of a solid electrolyte instead of liquid means the device will not leak onto the wearer. Porous membrane separator (PTFE). The membrane separator used is polytetrauoroethylene (PTFE) (Gore, USA, 50 nm pores, 23 mm thick). Two layers were used in each device. Matte medium (binder). Matte medium (binder) is an aqueous emulsion of polymethyl-methacrylate and polyethylene-glycol (PMMA + PEG) used conventionally to prime cotton canvas for acrylic painting.

2.2

Device fabrication and testing set-up

We tested two types of fabric structures, plain woven and we knitted carbon bers (Fig. 2 and 3). Woven CF electrode fabrication. Woven carbon ber sections were prepared by cutting the fabric into 6 2 cm strips (Fig. 2g and h) and nishing the edges with a small amount of binder to keep edges from fraying during handling. A 2 2 cm section was masked off for screen printing, and all devices have the bottom 2 2 cm section screen printed with 3 coats of carbon paint (10 minutes of air drying between coats), also see our previous work14 for the full printing procedure. Carbon ber This journal is ª The Royal Society of Chemistry 2013

sections were weighed before coating, and dried overnight in a vacuum oven heated to 80 C. Electrodes were re-weighed the next day to determine the mass of activated carbon on each electrode. Knitted CF electrode fabrication. Knitting is a fabric making technique where a continuous strand of yarn is looped over and over upon itself to form a sheet of fabric. Intarsia knitting is the insertion of other segments of yarn into the fabric to form “patterns” like polka dots, stripes, or argyle. This means we can knit capacitive materials within the textile while it is being manufactured, as well as any other conductive/smart yarn.3,4,23 In addition, these machines are like 3D printers for garments, and can construct fully nished garments (dress, sweater, pants, etc.) in a matter of minutes, allowing them to function as rapid prototyping machines. Knitted carbon ber electrodes were fabricated using a weknitting Shima Seiki (Japan) computerized knitting machine. The carbon staple ber yarn is embedded in a sheet of green wool (Fig. 2d–f), and continuous lengths of the CF current collectors (theoretically of innite size) can be knitted (Fig. 2e). CF electrodes had a 2 3 cm section screen printed with 3 coats of activated carbon ink (Fig. 2f), and the active material weight was determined using the procedure described in the Woven CF electrode fabrication section above. YP17 lm fabrication. As a reference, YP17 lm electrodes were prepared by a traditional rolling method.24 The lms contain 10% PTFE binder, and are 250 mm thick, 2 2 cm, and each electrode is precisely 60 mg, resulting in 15 mg cm 2 (Table 1). A set of stainless steel current collectors were also prepared. For full preparation details see our previous work.14


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Table 1

Paper

Mass loadings of YP17 activated carbon from screen printing carbon fiber textiles (mass values are an average of 4–6 samples)

Electrode

Structure

Fiber diameter (mm)

Thickness (mm)

Fabric mass per area (mg cm 2)

Active mass loading per area (mg cm 2)

YP17 lm Knitted CF Woven CF

Rolled lm Plain we Plain weave

— 6–7 6–7

250 400 200

— 30 20

15 12 6

Slurry/printing ink composition. The ink developed in our previous work14 that was used for screen printing onto the carbon ber current collector is comprised of 0.2 g of activated carbon per ml of water, with 5 wt% matte medium binder. The binder is rst quickly dispersed in water, as it is highly miscible, and then the mixture is slowly added to the activated carbon while stirring, to form a smooth paste, similar texture to toothpaste. Symmetric EDLC device fabrication. The polymer electrolyte22 precursor solution, is coated over the electrode to ensure full saturation through the electrode material and is dried in air. The electrodes are then assembled into a device, with screen-printed sections, overlapping with each other and uncoated sections of CF facing away from each other (Fig. 3). Once assembled, devices are placed between plates of PTFE and bound under pressure with binder clips. The devices are then placed in an oven at 90 C until completely dry. A drop of precursor solution was added between electrodes to wet the PTFE before being sandwiched together and dried again before testing to ensure good adhesion between electrodes. Devices without the PTFE separator were prepared, however bers from each electrode crossed the electrolyte membrane and resulted in shorted devices. Electrochemical testing. All experiments were carried out using a Biologic VMP3 potentiostat-galvanostat (BioLogic, USA). Devices were tested in a 2-electrode symmetric set-up and subjected to cyclic voltammetry (CV), galvanostatic cycling (GS), and electrochemical impedance spectroscopy (EIS) measurements,

following procedures fully described in previous studies.14 All devices were subjected to 100 pre-cycles at 20 mV s 1, and values of capacitance and ESR are taken from tests aer cycling.

3

Results and discussion

3.1

Screen printing/mass loadings

The carbon fabrics used in this study are 200 mm and 400 mm thick for woven and knitted CF, respectively. The thicknesses of these materials did not signicantly or appreciably increase aer carbon paint was screen-printed onto them. The woven devices, similar to cotton and polyester textiles in our previous work,14 had 6 mg cm 2 of electrode material (average of 6 electrodes). The knitted devices, being thicker understandably hold more material, had an average of 12 mg cm 2 (Table 1). Only devices that were reported by Bao and Li,25 and those reported in our previous work14 and in this paper have more than 5 mg electrodes, with knitted devices having 12 mg cm 2. Fig. 4 shows SEM micrographs of YP17 particles intermixed with carbon bers in both woven and knitted fabrics. Fig. 4b and d show close-ups of the individual carbon bers, each having diameters of 6–7 mm and similar structures (Table 1); we observe similar ber structures and similar networking and penetration of carbon particles (Fig. 4a, c, f and g) into the ber bundles. Fig. 4e shows the side view of the sandwiched woven device, being 250 mm in thickness. Knitted devices, though not shown, were found to be 1 mm thick. Devices were not electrochemically tested under pressure.

3.2

Fig. 3 Schematic of assembled devices comprised of a carbon fiber current collector (this geometry holds true for knitted or woven devices), a porous PTFE separator, SiWA polymer electrolyte, and the screen printed activated carbon ink. To the right is an assembled device demonstrating the flexibility as it is bent almost in half.


Electrochemical results

The knitted and woven CF devices were subjected to a series of testing experiments using cyclic voltammetry, galvanostatic cycling and electrochemical impedance spectroscopy, and values of capacitance and resistance are reported in Tables 2–4. Device capacities range from 2–5 F with 4–6 cm2 active areas. Woven CF electrodes have an average of 6 mg cm 2 ranging from 4–7 mg cm 2 across several sets of electrodes. Knitted devices have an average of 12 mg cm 2 ranging from 10–14 mg cm 2. Conventional supercapacitor lms also have lm masses within this range, and we compare a 60 mg per electrode YP17 device (4 cm2) directly with our own textile devices. All devices show similar capacitances across cyclic voltammetry, galvanostatic cycling and impedance spectroscopy. Galvanostatic testing is reported at 0.4 0.03 A g 1, and the differences in mass per area result in somewhat varying currents per area. We chose to use the same dataset for Tables 2


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Fig. 4 Comparison of screen printed carbon fibers from woven and knitted devices shows good integration of activated carbon particles and similar carbon fiber structures. (a and b) SEM micrograph of woven carbon fibers. (c and d) SEM micrograph of knitted carbon fibers. (e) SEM micrograph of the cross-section of a YP17 woven CF device, 350 mm thick. (f) Close-up of a few carbon fibers with a network of activated carbon formed between the fibers. (g) Multiple carbon fibers networked together by activated carbon particles.

Table 2 Gravimetric capacitance (F g 1) determined from cyclic voltammetry (CV), galvanostatic cycling (GS), and potentiostatic electrochemical impedance spectroscopy (PEIS) tested in SiWA; all with two layers of the PTFE porous membrane separator and using the same YP17 activated carbon. Capacitance is taken from the average of 2–3 samples

Electrode

CV (10 mV s 1) GS (0.4 0.03 A g 1) PEIS (10 mHz)

Film reference 75 Knitted CF 88 Woven CF 63

Table 3

52 76 63

Device capacitance per area (F cm

Electrode

66 77 42

2

) determined from CV, GS, and PEIS

CV (10 mV s 1) GS (0.4 0.03 A g 1) PEIS (10 mHz)

Film reference 0.66 Knitted CF 0.51 Woven CF 0.19

0.39 0.44 0.18

0.50 0.44 0.13

and 3 in order to keep consistency with the reported capacitance in all tables. Fig. 5a shows cyclic voltammograms of all devices with potential (V) vs. capacitance per area (F cm 2). The knitted device clearly has more capacitance (0.51 F cm 2) than the woven devices, attributed to the higher active material mass per area (Table 1). Galvanostatic cycling at 0.4 0.03 A g 1 conrms similar ESR values and capacitances to those taken from EIS


Table 4 Equivalent series resistance, ESR, (U cm2) determined from galvanostatic cycling (GS) and potentiostatic electrochemical impedance spectroscopy (PEIS)

Textile

GS (0.4 0.03 A g 1)

PEIS (1 kHz)

Film reference Knitted CF Woven CF

3 14 50

2 14 56

(Table 4), which is equal to 14 U cm2. The woven devices show lower gravimetric capacitance; oen it is expected that devices with lower active masses result in higher gravimetric capacitance15 given that they are not restricted by the diffusion of the electrolyte. However, the woven devices suffer from high ESR (Table 4) due to the non-continuous yarn structure, which is a likely cause for a lower observed gravimetric capacitance. Cyclic voltammograms of the knit device are slanted upright with a small increase in current at 1 V. This behavior is likely due to the knitted carbon bers (Fig. 2f) having many bers standing upright which caused shorting in our initial devices that incorporated no separator (not reported in this paper). However, even with the separator some leakage current may still occur. Yarns in the woven devices are mostly at (Fig. 2g and h), only having to go over and under yarns perpendicular in the fabric, therefore we see more rectangular CVs. A small increase in current can be seen at 1 V for all the devices, indicating that


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Paper previously reported devices have not only lower resistance, but signicantly lower device capacitance. Our devices are very large (3 F) which contributes to the increase of the RC time constant.


3.3

Fig. 5 (a) Cyclic voltammograms scanned at 5 mV s 1 and normalized to capacitance per area; (b) galvanostatic curves taken at 0.4 0.03 A g 1; and (c) EIS plots measured from 200 kHz to 10 mHz and normalized per cm2. (d) Enlarged high frequency region of the knitted CF EIS curve. Resistance is taken at 1 kHz. All curves represent the best performing samples of each set.

the voltage window may be slightly too wide for this electrolyte, leading to electrolyte decomposition. EIS (Fig. 5c and d) also shows no obvious semi-circle at high frequencies, indicating that there is a good electrical contact between the current collector and active material. As conrmed by cyclic voltammetry and galvanostatic cycling, knit devices also exhibit the lowest ESR of 14 U cm2 (Table 4). Woven CF electrodes exhibit three times higher ESR than knitted CF electrodes. We attribute the lower resistance of the knitted CF to the textile structure, which is comprised of a single continuous strand of yarn. Data on this particular change in conductivity will be included in future studies. Additionally, a broad diffusion-controlled region is observed in Fig. 5d. For commercial aqueous or organic electrolyte based supercapacitors there is a short 45 angled region which is indicative of ion diffusion into the porous electrode material, transitioning to purely capacitive behavior of low frequencies. In our devices this region is extended, indicating slow ionic diffusion due to the use of a gel/ solid electrolyte. The YP17 lm devices show areal capacities as high as 0.66 F cm 2, ESR of 2 U cm2, and a 1 second time constant. Our knitted devices have 0.51 F cm 2 with ESR values of 14 U cm2 and a time constant of 7 seconds (Tables 3 and 4). The limiting factor for knitted devices is high ESR due to the lower conductivity of the carbon ber current collectors. Carbon bers are about ve orders of magnitude more resistive than 316 stainless steel (10 2 U cm vs. 7.5 10 7 U cm respectively26). Nonetheless, our knitted carbon ber device ESR (14 U cm2) is only one order of magnitude higher than a conventional YP17 device using a stainless steel current collector (2 U cm2). Clearly the conductivity of the current collector plays an important role in device ESR, but does not result in device ESR scaling linearly with current collector conductivity. Furthermore, many of the


Flexibility and stretching

The exibility of the devices is an important aspect to consider when designing supercapacitors for wearable applications (Fig. 3). To measure the performance while bent, we conducted cyclic voltammetry while the device was bent at 90 , 135 , and 180 . The device was mounted on a hinged wooden plank, with the middle of the device at the folding point (Fig. 6). Also aer observing possible electrolyte decomposition when cycling to 1 V (Fig. 5a), we chose to adjust our operational voltage window to 0.8 V. In this range we see no irregular increase in current at 0.8 V. Electrochemical testing was carried out at 2 mV s 1, aer initial testing of 200 charge–discharge cycles at scan rates from 2–20 mV s 1 and currents ranging from 0.1–0.5 A g 1. Bending tests were conducted one aer another, and the angle was changed manually. We nd good capacitance retention from all tests, as seen in Fig. 7. Aer the initial bending, the device retains 80% of its original capacitance. We attribute this initial capacitance loss to a loosening and delamination of carbon particles within the knitted CF as observed in Fig. 4f and g. It is also possible that an interfacial contact between the carbon particles and the electrolyte occurs. We also tested the devices when stretched 50% wider, and found that they also exhibited a small loss in capacitance which we attribute to the breaking of conductive networking between carbon particles when changing the dimensions of the device (Fig. 7a and b). We relaxed the fabric to a at state and remeasured the capacitance and again found a small decrease in capacitance. We believe the handling of the devices may be dislodging carbon particles with each movement. In order to determine whether or not continued degradation would occur from bending, we conducted four more experiments. Aer waiting 6 hours from the nal at test, we tested the device again at, then bent again at 180 , then at, then once more folded at 180 , and again at. The device regained some capacitance aer allowing the fabric to rest for 6 hours. It is possible that the fabric had not fully contracted back to its original dimensions during the at test immediately aer stretching, but had fully contracted within the 6 hours. Testing

Fig. 6

Bending set-up for electrochemical testing.


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Energy & Environmental Science Bao and Li.25 We also show the good mechanical and performance stability of the devices when stretched and bent to 180 . We chose a fabrication technique that results in high areal mass loading while retaining the high intrinsic capacitance of activated carbon. Knitted electrodes can also be quickly and easily scaled up for commercial manufacturing. The data reported in this paper represent a foundational set, as we chose to use an inexpensive and well-studied activated carbon while modifying the traditional supercapacitor design to a textile device. We fully expect that the use of conductive additives, and other more conductive carbon nanomaterials (e.g., graphene, carbon nanotubes, or onion-like carbons) can improve the rate performance, resistance, and specic capacitance, as evidenced in the literature.10,12,17,27

Acknowledgements

Fig. 7 (a) A series of cyclic voltammograms at 2 mV s 1 while under different bending conditions. (b) Capacitance retention under different bending conditions.

at 180 revealed almost no change in performance (Fig. 7b). However, it is in the recovery of the at state from the folded state that we see a decrease in device performance comparable to when stretched. We hypothesize that the folding compacts the carbon particles and bers together, and then these compacted particles do not “re-disperse” evenly into the fabric and further degradation of the carbon network and conductivity occurs. To be clear, no carbon material is aking off the electrodes as they are embedded into the gel electrolyte and carbon bers. However, particles may also be delaminating from the surface of the carbon bers (i.e., current collectors) decreasing the amount of material that is actually contributing to the overall capacitance.

4

Conclusions

This paper serves as the rst demonstration of knitted carbon ber electrodes, while achieving the highest reported capacitance per area for an all-carbon textile-supercapacitor (0.5 F cm 2), and having comparable mass and capacitance to conventional supercapacitors tested under the same conditions. The knitted devices reported in this paper have the highest reported active material mass loading for a textile supercapacitor, with the next best from recent literature having 5 mg cm 2 from the report by This journal is ª The Royal Society of Chemistry 2013

The authors thank Darin Tallman (Drexel University) for aid in the design of the exibility testing apparatus. K.J. would like to recognize the support of the Drexel-Penn NSF-IGERT Fellowship 2011–2012 (DGE-0654313) and the Department of Defense (DoD) through the National Defense Science & Engineering Graduate Fellowship (NDSEG) Program. D.S. would like to thank the Steinbright Career and Development Center, STAR (Students Tackling Advanced Research) Summer Program for support. C.R.P. was supported by the National Science Foundation ICC Project under Grant no. CHE-0924570. 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 supported J.K.M and helped to fund the electrochemical characterization of the activated carbon and textile supercapacitor devices.

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