Intrinsically Stretchable and Conductive Textile by a Scalable Process

Mar 27, 2017 - Key Laboratory of Opto-Electronics Technology, Ministry of Education ... Science and Technology, Faculty of Information Technology, Bei...
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Intrinsically Stretchable and Conductive Textile by a Scalable Process for Elastic Wearable Electronics Chunya Wang,†,‡ Mingchao Zhang,†,‡ Kailun Xia,†,‡ Xueqin Gong,§ Huimin Wang,†,‡ Zhe Yin,†,‡ Baolu Guan,§ and Yingying Zhang*,†,‡ †

Key Laboratory of Organic Optoelectronics and Molecular Engineering of the Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, P.R. China ‡ Center for Nano and Micro Mechanics (CNMM), Tsinghua University, Beijing 100084, P.R. China § Key Laboratory of Opto-Electronics Technology, Ministry of Education, College of Electronic Science and Technology, Faculty of Information Technology, Beijing University of Technology, Beijing 100022, P.R. China S Supporting Information *

ABSTRACT: The prosperous development of stretchable electronics poses a great demand on stretchable conductive materials that could maintain their electrical conductivity under tensile strain. Previously reported strategies to obtain stretchable conductors usually involve complex structure-fabricating processes or utilization of high-cost nanomaterials. It remains a great challenge to produce stretchable and conductive materials via a scalable and cost-effective process. Herein, a large-scalable pyrolysis strategy is developed for the fabrication of intrinsically stretchable and conductive textile in utilizing low-cost and mass-produced weft-knitted textiles as raw materials. Due to the intrinsic stretchability of the weft-knitted structure and the excellent mechanical and electrical properties of the as-obtained carbonized fibers, the obtained flexible and durable textile could sustain tensile strains up to 125% while keeping a stable electrical conductivity (as shown by a Modal-based textile), thus ensuring its applications in elastic electronics. For demonstration purposes, stretchable supercapacitors and wearable thermal-therapy devices that showed stable performance with the loading of tensile strains have been fabricated. Considering the simplicity and large scalability of the process, the low-cost and mass production of the raw materials, and the superior performances of the as-obtained elastic and conductive textile, this strategy would contribute to the development and industrial production of wearable electronics. KEYWORDS: carbonized Modal fabric, stretchable conductors, weft-knitted, wearable electronics, supercapacitors

1. INTRODUCTION Recent decades have witnessed the exciting development of flexible electronics such as flexible energy-related devices,1 smart sensors,2−4 and artificial electronic skins,5 putting forward great demand for stretchable conductors that could maintain high conductivity under various mechanical deformations.6 Flexible conductive materials that have intrinsic flexibility, such as conducting polymers,7 metallic nanomaterials,8 and carbon nanomaterials,9 have been intensively explored as electrical conductive components for stretchable conductors. Compared to conductive polymers10 and metallic nanomaterials,11 carbon nanomaterials possess the advantage of combining good electrical conductivity, chemical and thermal stability, and mechanical flexibility. Carbon nanomaterials have been assembled into stretchable conductors through various strategies, such as being embedded in or deposited on elastomers12,13 and being patterned in wavy or buckling structures.14,15 However, they still confront challenges such as the complexity in the structure-fabrication process and the inevitable increment in the resistance under tensile strain due to the disconnection of the conductive components.16 Recently, © XXXX American Chemical Society

carbon-based materials derived from natural biomaterials have attracted extensive research interest, due to their large-scale production capability at a relatively low-cost, environmental benignity, as well as satisfying electrical conductivity.17−19 Nevertheless, biomaterial-derived carbon materials usually lack stretchability, which imposes restrictions on their applications in stretchable electronics.20,21 On the other hand, textiles with knitted structure can be intrinsically stretched due to the fact that meandering loops in knitted textile can be extended in different directions.22 For example, Modal, a well-known semisynthetic cellulose fiber made by spinning reconstituted cellulose (primarily from wood pulp), has been widely used to manufacture knitted textiles because of its softness, biocompatibility, and large-scale production. At the same time, it is well-known that cellulose, which is a polysaccharide composed of a linear chain of β-(1− 4)-linked D-glucose units, could be easily converted into Received: March 1, 2017 Accepted: March 27, 2017 Published: March 27, 2017 A

DOI: 10.1021/acsami.7b02985 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. Fabrication and characterization of the intrinsically stretchable and conductive textile. (a) Schematic illustration showing the transformation of a Modal textile into a graphite-like carbon textile through a thermal treatment process. (b−e) Optical images of flexible commercial Modal textile (b), flexible individual CMT (c), and flexible CMT/Ecoflex composite (d, e). (f) SEM image of the front side of the CMT. (g) TEM image of the graphite-like carbon from the CMT (1050 °C), which was obtained through thermal treatment of Modal at 1050 °C. (h) Raman spectra of a pristine Modal textile and CMT (1050 °C). (i) High-resolution spectrum of the C 1s XPS peak of the CMT (1050 °C).

details). As shown in Figure 1a, each Modal fiber is composed of millions of cellulose molecules, the pyrolysis of which is controlled mainly by two predominant reactions (dehydration and depolymerization),23 resulting in a carbonized Modal textile (CMT) which is composed of highly conductive graphitic carbon fibers. The size of the textile shrank after thermal treatment because of the pyrolysis. For example, after thermal treatment under 1050 °C, the area of the textile shrank to about 33% of the original one. Importantly, the flexibility of the Modal textile was well-maintained after thermal treatment (Figure 1b,c), which is a critical precondition for the application of CMT as an intrinsically stretchable and conductive textile. In addition, the Ecoflex-encapsulated CMT also presented excellent flexibility (Figure 1d,e). As seen in Figure 1f, the typical scanning electron microscopy (SEM) image of the front side of a CMT shows that the visible portions of the loops are vertically arranged in a grid of “V” shapes, revealing the characteristic feature of the weft-knitted structure. On the back side of the CMT, the ends of the loops are visible, leading to a bumpy texture (see Figure S1). Figure 1g shows a typical transmission electron microscope (TEM) image of the CMT (1050 °C), which displays lattice fringes with an interlayer spacing of about 0.37 nm, belonging to the slightly expanded interplanar spacing of the (002) plane of hexagonal graphite.25 The slightly expanded interplanar spacing is related to the O-doping, which could be confirmed by the following X-ray photoelectron spectrum (XPS) characterization. The Raman spectrum of the CMT (1050 °C) (Figure 1h) presents a G-band at 1590 cm−1 (attributed to the vibrations of the sp2-bonded crystalline carbon) and a Dband at 1348 cm−1 (related to defects or heteroatom-doping). In addition, the Raman spectrum of the pristine Modal textile shows the presence of an intense peak located at ∼1096 cm−1,

electrically conductive graphite-like carbon materials through a simple thermal treatment, while the original fibrous or woven structures can be maintained.17,23,24 Inspired by the above, herein we ingeniously propose a large-scalable and costeffective pyrolysis strategy for the fabrication of intrinsically stretchable and conductive textile utilizing mass-produced knitted textile, such as Modal textile, as the raw material. The main advantage of this strategy is the combination of the unique and intrinsically stretchable, macroscale, weft-knitted structure and simple pyrolysis-induced, graphite-like carbon nanostructure through a facile and large-scalable process. The obtained textile can sustain tensile strain up to 125% while maintaining good electrical conductivity, thus ensuring its applications in wearable electronics. To the best of our knowledge, this is the first report on utilizing carbonized textile for stretchable conductors. The concept of taking advantage of the intrinsically stretchable, macroscale, weft-knitted structure and simple pyrolysis-induced, graphite-like carbon nanostructure for stretchable conductors is original and ingenious. As a proof of concept, we further fabricated stretchable supercapacitors and elastic thermal-therapy devices based on the stretchable and conductive textile, both of which showed superior performances, further demonstrating the great potentiality of the conductive textile for wearable electronics.

2. RESULTS AND DISCUSSION Figure 1a illustrates the fabrication process of the stretchable and conductive textile that could be utilized as a stretchable conductor directly or after being encapsulated in elastic polymers such as Ecoflex. Pristine Modal textile with weftknitted structure was carbonized through a thermal treatment process under an inert atmosphere to produce the stretchable and conductive textile (see the Experimental Section for B

DOI: 10.1021/acsami.7b02985 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. Electrical, structural, and mech-electrical properties of the CMT-based, intrinsically stretchable conductors. (a) Electrical conductivity of CMT as a function of the pyrolysis temperature. (b) Raman spectra of CMT as a function of the pyrolysis temperature. (c, d) The relative change of the resistance of a bare CMT (1050 °C) (c) and a CMT (1050 °C)/Ecoflex composite (d) as a function of tensile strain along the course direction up to 75% and 125%, respectively. (e) Schematic illustration showing the weft-knitted structure of conductive yarns in CMT. (f, g) The variation in resistance of a bare CMT (1050 °C) (f) and a CMT (1050 °C)/Ecoflex composite (g) during 1000 stretching cycles at a strain of 50%.

(FWHM) of Raman bands and the intensity ratio of the Dband and G-band (ID/IG) (see Figure S3a,b) also provide information about the structure of carbonaceous materials,28 indicating the pyrolysis-temperature-induced increase in the structural order (see detailed discussion about Figure S3a,b in Supporting Information). In addition, the shift of the G peak from ∼1584 cm−1 (CMT treated at 600 °C) to ∼1590 cm−1 (CMT treated at 1050 °C) also suggests the increased structural order, which is in accordance with the previously reported trend that the G peak would increase while the samples transform from amorphous carbon to nanocrystalline graphite.29 The unique weft-knitted structure and the good electrical conductivity of the CMT make it a superior material as a flexible and stretchable conductor. As shown in Figure 2c,d, under uniaxial tensile strain along the course direction up to 70% and 125%, the resistances of the bare CMT and the CMT/ Ecoflex composite were almost constant with a variation of less than 3%, which is lower than the reported best results using CNT aerogels 30 and graphene foams 31 as stretchable conductors, indicating the superiority of CMT as a stretchable conductor. It is noteworthy that there exist three different regions for the resistance−strain behavior of both conductors (see Figure S4). In the first region, the resistance increased slightly with the increase of the applied strain and with the relative resistance change of less than 3%. In the second region, the resistance decreased with the increase of strain, and in the

which is the characteristic band of cellulose corresponding to the stretching mode of the CO ring and glycosidic linkage.26 XPS measurement was performed to further confirm the element composition (see Table S1) and the chemical state of the carbon element in the CMT. The C 1s XPS spectra of CMT (1050 °C) and pristine Modal textile are shown in Figure 1i and in Figure S2, respectively. The C 1s spectrum could be fitted by four peaks at the binding energy of 284.6 eV (belonging to CC/CC), 286.1 eV (belonging to CO C), 287.6 eV (belonging to CO), and 291.2 eV (belonging to ππ*).27 The appearance of the ππ* feature for the CMT (see Table S2) could be attributed to the development of a polycondensed carbon cluster, which results in the formation of a delocalized π electron system,27 and contributes to the good electrical conductivity of the CMT. As shown in Figure 2a, the electrical conductivity of the CMT increases rapidly with the pyrolysis temperature for the fact that a higher pyrolysis temperature brings about carbon with a higher degree of graphitization. Particularly, the electrical conductivity of the CMT treated at 1050 °C reaches 15.1 S/cm, which is much higher than that of traditional polymer composite conductors with carbon nanotubes (CNTs) or graphene as conductive fillers.12 Figure 2b shows Raman spectra of CMT carbonized at different temperatures, all of which exhibit the two first-order Raman bands of graphite carbon, namely, the G-band located at ∼1590 cm−1 and the Dband centered at ∼1348 cm−1, indicating the formation of sp2bonded crystalline carbon. The full width at half-maximum C

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Figure 3. Performance of a stretchable supercapacitor using the intrinsically stretchable and conductive textile as the electrodes. (a) Schematic illustration showing an all-solid stretchable supercapacitor using the CMT as the electrodes and H3PO4−PVA gel as the electrolyte and separator. (b) Photographs of the pristine supercapacitor and the supercapacitor under strain of 50%. (c) CV curves of the supercapacitor at different scan rates. (d) CV curves of the supercapacitor at a scan rate of 100 mV/s and under strains of 0%, 10%, 30%, and 50%. (e) CV curves of the supercapacitor with PANI-deposited CMT electrodes at different scan rates. (f) CV curves of the supercapacitor with PANI-deposited CMT electrodes at a scan rate of 100 mV/s and under strains of 0%, 10%, 30%, and 50%.

stretchability, and extraordinary electromechanical stability and durability, indicating its superiority for applications as stretchable conductive parts. Compared to the deteriorated conductivity under tensile strain of other reported stretchable conductors, the almost invariant conductivity and excellent stability and durability of the CMT-based, intrinsically stretchable conductor is superior and favorable. A detailed comparison of electrical properties of the CMT-based intrinsically stretchable conductor and other reported stretchable conductors is shown in Table S2, proving the superiority of the obtained intrinsically stretchable and conductive textile more clearly. Two kinds of flexible electronics, i.e., stretchable supercapacitors and wearable thermal-therapy devices, were fabricated on the basis of the CMT (1050 °C) to demonstrate the potential applications of the intrinsically stretchable and conductive textile. Both of them exhibited exciting performance, further showing the outstanding performance of the CMT for stretchable conductors. Flexible electrochemical energy storage devices have been booming as the promising power sources for the development of flexible and wearable electronics.1 Especially, flexible and even stretchable supercapacitors have attracted great attention because of their high power density, high charge−discharge rate, and long durability.33 Carbon materials have been reported as prominent materials for flexible supercapacitors because of their excellent properties such as high electrical conductivity, high surface area,

third region, the resistance increased again following the further increment of strain. The special phenomenon of the above electromechanical property of CMT could be interpreted by analyzing the deformation of the unique weft-knitted structure of the CMT under tensile strain. As shown in Figure 2e, in the single conductive course in the weft-knitted structure, conductive graphite-like carbon yarn formed into a series of loops which touched each other at adjacent parts to form contact points. The initial increased strain applied on the CMT would induce the decreased area of contact points, leading to the slight increase of the resistance in the first region. When further increased strain was loaded, the contact area and the force loaded on the contact points between loops of two adjacent courses increased, resulting in the decreased contact resistance (Rc) and the corresponding decreased overall resistance in the second region, which could be elucidated by Holm’s contact theory (see Supporting Information).32 For the third region, with the further increased strain, some of the carbon yarns in CMT would fracture, leading to an increment of resistance again. Beside the high electrical conductivity and the extraordinary stretchability, the CMT stretchable conductors also exhibit excellent electromechanical stability and durability (Figure 2f,g), which is of great significance for practical applications. From the above, it is concluded that the CMT showed concurrent merits of high electrical conductivity, outstanding D

DOI: 10.1021/acsami.7b02985 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Performance of a wearable heater based on the intrinsically stretchable and conductive textile. (a) Schematic illustration showing a stretchable CMT heater. (b) Evolution of the maximum temperature of a bare CMT under dc voltages from 1.0 to 3.5 V, where insets are temperature profiles captured by an IR camera at each applied voltage. (c) Evolution of the temperature of an Ecoflex-encapsulated CMT heater under stepwise strains of 0−70% at a constant dc voltage of 4.0 V, where insets are the corresponding temperature profiles taken at different tensile strain. (d) Photograph showing a wearable CMT heater affixed on a wrist. (e, f) IR images showing the temperature profiles of the CMT heater with the wrist under relax (e) and bending (f) conditions at a constant dc voltage of 5.0 V after about 120 s.

good electrochemical stability, and flexibility.34 The high electrical conductivity, the porous structure, and the intrinsic stretchability of CMT ensure its applications in stretchable supercapacitors. We fabricated an all-solid, stretchable, symmetric supercapacitor using CMT as electrochemically active electrodes and phosphoric acid−poly(vinyl alcohol) (H3PO4−PVA) gel as the electrolyte and separator, which is shown in Figure 3a. The intrinsic stretchability of the CMT endows the corresponding supercapacitor with high stretchability. Figure 3b shows that a CMT-based supercapacitor could be easily stretched by 50% without obvious damage to the structure’s integrity. Cyclic voltammetry (CV) was used to characterize the electrochemical performance of the CMT-based supercapacitor. Figure 3c shows typical CV curves of the CMT-based all-solid stretchable symmetric supercapacitor measured by a twoelectrode system at different scan rates. The CV curves exhibit quasirectangular shapes within the selected potential window between 0 and 0.8 V even at a scan rate as large as 500 mV/s, showing the typical double-layer capacitor behavior. The CMTbased supercapacitor exhibited favorable rate capacitance with a maximum specific capacitance of 7.5 mF/cm2 (1.2 F/g) at the scan rate of 10 mV/s and the capacitance retention of 44% as the scan rate rose from 50 to 500 mV/s (see Figure S5), which is comparable to the reported stretchable supercapacitor based on buckled single-walled carbon nanotube macrofilms.35 It should be noted that the electrochemical performance of the supercapacitor could be optimized by using a higher carbonization temperature to decrease the internal resistance of the CMT and improving the specific surface area of the CMT by adding a pore-forming agent when it is being pyrolyzed or by depositing active materials with larger specific surface. The CV curves of the CMT-based supercapacitor with different strain loaded are shown in Figure 3d, which shows stable CV curves

with strain up to 50%, indicating that the CMT-based supercapacitor could be largely stretched with excellent, stable electrochemical performance. The capacitance of the intrinsically stretchable and conductive textile-based supercapacitor could be further improved by integrating electrochemical active materials. Polyaniline (PANI), which is a common conducting polymer and is widely utilized to improve the electrochemical capacitance of carbon materials because of its relatively large multiple redox-states-derived specific pseudocapacitance and low-cost,36,37 was utilized as an example to show the possibility of the performance improvement of the CMT-based supercapacitor. Flowerlike PANI was uniformly deposited on the surface of the CMT by eletropolymerization (see Figure S5), which is beneficial for the improvement of electrochemical performance. As shown in Figure 3e, the current density in the CV curves of the PANI-deposited CMT-based supercapacitor was much higher than that of pristine CMT, resulting in the greatly improved specific capacitance [246.3 mF/cm2 (41.6 F/ g) at the scan rate of 10 mV/s, 33 times that of the pristine CMT]. The rate capacitance of the PANI-deposited CMTbased supercapacitor became inferior (see Figure S6), which is consistent with reported results.38 In addition, as shown in Figure 3f, the PANI-deposited CMT-based supercapacitor also presented stable CV curves under different strains, indicating its superstretchable merit. Figure 4a illustrates the structure of a stretchable CMT heater that is composed of an Ecoflex-encapsulated CMT conductor. The Joule heating performance of the stretchable CMT heater was studied systematically. A bare CMT (n × m = 1.2 × 2.6 cm2; n and m indicate the length of CMT in the course and wale direction, respectively) was assessed under zero tensile strain at a stepwise direct current (dc) voltage from 1.0 to 3.5 V to evaluate its Joule heating characteristics. The E

DOI: 10.1021/acsami.7b02985 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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treatment. The electronic textile, which was able to sustain large tensile strains while keeping the electrical conductivity stable, could be used either directly or after being encapsulated in an elastomer. For a bare conductive textile, the relative variation of resistance is within 3% under strain up to 70%, while for a textile/Ecoflex composite, the variation is within 3% under strain up to 125%. The unique mech-electrical properties of the textile could be ascribed to the unique weft-knitted structure and the pyrolysis-induced formation of the graphite-like carbon fibers. For demonstration, we fabricated stretchable supercapacitors and heaters using the textile, which both showed stable performance with loading application of tensile strains. Particularly, the elastic heater could reach a temperature of about 150 °C under a voltage of only 3.5 V, and the heating performance would not deteriorate even under tensile strain up to 70%, which overmatches the strain-induced, obvious deterioration of the heating performance of other reported flexible heaters. Considering the simplicity and large scalability of the process, the low-cost and mass production of the raw materials, and the superior performances of the obtained material, we believe this strategy, which paves a new way for the large-scale and cost-effective fabrication of elastic electronic textiles, will contribute to the development and industrial production of wearable electronics.

evolutions of the maximum temperature and the average temperature with time at various applied dc voltages are shown in Figure 4b and Figure S7, respectively. The corresponding temperature profiles captured by IR camera are shown in the insets of Figure 4b. It can be seen that the CMT heater can reach a stable temperature within ∼4 s at a low drive voltage (a dc voltage of 2.0 V could drive the heater up to a temperature around 60 °C), which is favorable for practical application of a wearable heater. To investigate the influence of tensile strain on the performance of the CMT heater, a stepwise tensile strain from 0% to 70% was applied to an Ecoflex-encapsulated CMT heater (n × m = 1.8 × 3.7 cm2, Ecoflex-encapsulated) under a dc voltage of 4.0 V. As shown in Figure 4c, when the applied tensile strain was increased from 0% to 70%, the temperature of the CMT-based stretchable heater remained stable with a relatively small variation within 120−140 °C, superior to the recently reported stretchable heaters, which showed obviously decreased temperature with increased strain.39 In detail, the first 10% tensile strain induced an inconspicuous decline in the Joule heating temperature, which could be attributed to the strain-induced, slightly increased resistance. Exceptionally, with application of tensile strain from 10% to 70%, the temperature was slightly increased, which could be attributed to the unique resistance variation behaviors of the CMT as shown in Figure 2d. To the best of our knowledge, this is the first report of wearable heaters that show a nearly stable Joule heating temperature under large tensile strain. In contrast, the previously reported flexible heaters based on metal and carbon nanostructures generally showed deterioration in temperature under strains or deformations (see detailed comparison in Table S4). It is noted that, compared to the bare CMT heater, the Ecoflex-encapsulated CMT heater needed longer time (more than 50 s) to reach a stable temperature, which could be attributed to the low thermal conductivity of Ecoflex. In addition, electrical heating stability and durability of a wearable heater are of significant importance for practical applications. Cyclic heating and cooling were conducted to attest to the electrical heating stability, which demonstrates the excellent stability of the CMT-based heater (see Figure S8a). Different cyclic loading of 70% tensile strain was applied to the CMT-based heater. The heater exhibited remarkably stable heating and cooling performance after different cycles of 70% strain (see Figure S8b). From the above assessment, the extremely low drive voltage, the fast electrical heating, the exceptional strain that induced no deterioration in heating temperature, and the extraordinary stability and durability of the CMT make it a potential material for wearable thermaltherapy devices. For demonstration, a CMT-based heater (n × m = 4.5 × 6.0 cm2, Ecoflex-encapsulated) was affixed onto a wrist of a volunteer to show its performance as a wearable heater (see Figure 4d), and its performance under bending deformation was examined. As shown in Figure 4e,f, compared to the results under the relax condition, under the bending condition, the CMT wearable heater showed a similar temperature profile, indicating the attractive performance of the intrinsically stretchable and conductive textile for wearable thermal-therapy devices.

4. EXPERIMENTAL SECTION Fabrication of the Stretchable and Conductive Textile. Commercially available Modal textiles with weft-knitted structure were used as raw materials. The textiles were thermally treated under an argon- and hydrogen-mixed atmosphere in a tube furnace. The furnace was heated to the target temperature (600, 700, 800, 900, and 1050 °C) at a rate of 3 °C/min, kept for 200 min, and then cooled to room temperature. The CMT showed lower tensile strength and less stretchability after thermal treatment than the pristine Modal textile (see Figure S9). The CMT could be directly used as a stretchable conductor. It could also be encapsulated in Ecoflex (1:1 mixture of Ecoflex part A and part B, Ecoflex Supersoft 0050, Smooth-On, Inc.) for applications as composite stretchable conductors. In detail, the CMT was put on a solidified Ecoflex substrate, and then liquid Ecoflex was dropped on the surface for curing to fix and encapsulate the device. The thickness of the CMT/Ecoflex composite is about 1.1 mm, which could be observed from the SEM image of its cross section (see Figure S10). It should be noted that the electrical resistance of the CMT will slightly increase after being encapsulated in Ecoflex due to the penetration of the polymer into the fabric structure. Characterization of the Stretchable and Conductive Textile. The morphologies and microstructures of the CMT were characterized by a field emission SEM (FE-SEM, FEI Quanta 650) and a field emission TEM (FE-TEM, JEOL JEM2010F). Raman spectra were acquired with a Raman spectroscope (HORIBA HR800) with a laser excitation wavelength of 532 nm. XPS was performed using Al Kα radiation (Thermo Scientific Escalab 250Xi). The electrical conductivity of the CMT was measured by a four-probe method with a KDJ-1 analytical probe system (Guangzhou Kunde Technology Ltd., China) and a Keithley 2400 digital meter. Copper wires were connected to the CMT along the wale direction with silver paste for the measurement of the electromechanical performance. The loading of tensile strain along the course direction of the CMT was performed with a universal testing machine (SHIMADDZU AGS-X). The corresponding electrical signals were recorded in real time by a Keithley 2400 digital meter. Fabrication and Performance Characterization of the Stretchable All-Solid Supercapacitor. First, the H3PO4−PVA gel electrolyte was prepared as follows: 1 g of PVA powder was dissolved in 10 mL of deionized water and heated to 90 °C under vigorous stirring for 5 h to form a transparent gel. After the transparent gel cooled down to room temperature, 1 mL of H3PO4 (85 wt %) was

3. CONCLUSIONS In summary, we reported an intrinsically stretchable and highly conductive textile that could be prepared from low-cost and mass-produced weft-knitted textiles through a simple thermal F

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ACS Applied Materials & Interfaces blended into the gel, and the mixture was stirred for 3 h. Second, the PANI-deposited CMT was prepared by the electropolymerization method. In detail, a certain amount of aniline was mixed with hydrochloric acid (1 M) to form an electrolyte solution (0.5 M) for electropolymerization. The electropolymerization process was performed in a three-electrode system (the CMT as the working electrode, Pt electrode as the counter electrode, and a saturated calomel electrode as the reference electrode) for 5 min with a constant potential of 0.8 V. Then, two identical CMT or PANI-deposited CMT electrodes coated with H3PO4−PVA between them were stacked together and placed in the fume hood at room temperature to remove the excess water and to form the all-solid supercapacitor. The electrochemical measurement was performed in a two-electrode system with an RST 5000 electrochemical workstation (Zhengzhou Shiruisi Instrument Technology Co., Ltd., China). The tensile strain loaded on the supercapacitor was applied by a homemade tensile holder. Characterization of the Stretchable Heater. Ecoflex-encapsulated CMT was utilized as the stretchable heater. Copper wires were connected to the CMT along the wale direction with silver paste to make an electrical connection for the convenience of testing before being encapsulated by Ecoflex. The heater was held by a tensile holder for loading tensile strain. A Keithley 2400 digital meter was used to supply dc power for Joule heating. The images showing temperature distribution were captured by an infrared camera (SC5000, FLIR).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02985. Supporting figures, tables, and text including SEM, XPS, electrical data, and additional characterization information (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yingying Zhang: 0000-0002-8448-3059 Author Contributions

C.W. and M.Z. contributed equally to this work. Y.Z., C.W., and M.Z. conceived the project and designed the experiments. Y.Z. supervised the project. C.W., M.Z., K.X., X.G., H.W., Z.Y., and B.G. performed the experiments. C.W., M.Z., and Y.Z. analyzed the data and cowrote the paper. All authors discussed the results and commented on the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSF of China (51672153, 51422204, 51372132) and the National Key Basic Research and Development Program (2016YFA0200103, 2013CB228506).

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DOI: 10.1021/acsami.7b02985 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX