A Highly Stretchable and Washable All-Yarn-Based Self-Charging

Sep 13, 2017 - Rapid advancements in stretchable and multifunctional wearable electronics impose a challenge on corresponding power devices that they ...
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A Highly Stretchable and Washable All-YarnBased Self-Charging Knitting Power Textile Composed of Fiber Triboelectric Nanogenerators and Supercapacitors Kai Dong,†,‡,⊥ Yi-Cheng Wang,†,⊥ Jianan Deng,†,⊥ Yejing Dai,† Steven L. Zhang,† Haiyang Zou,† Bohong Gu,‡ Baozhong Sun,*,‡ and Zhong Lin Wang*,†,§ †

School of Material Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States College of Textiles, Key Laboratory of High Performance Fibers and Products, Ministry of Education, Donghua University, Shanghai 201020, People’s Republic of China § Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences; National Center for Nanoscience and Technology (NCNST), Beijing 100083, People’s Republic of China ‡

S Supporting Information *

ABSTRACT: Rapid advancements in stretchable and multifunctional wearable electronics impose a challenge on corresponding power devices that they should have comparable portability and stretchability. Here, we report a highly stretchable and washable all-yarn-based self-charging knitting power textile that enables both biomechanical energy harvesting and simultaneously energy storing by hybridizing triboelectrical nanogenerator (TENG) and supercapacitor (SC) into one fabric. With the weft-knitting technique, the power textile is qualified with high elasticity, flexibility, and stretchability, which can adapt to complex mechanical deformations. The knitting TENG fabric is able to generate electric energy with a maximum instantaneous peak power density of ∼85 mW·m−2 and light up at least 124 light-emitting diodes. The all-solid-state symmetrical yarn SC exhibits lightweight, good capacitance, high flexibility, and excellent mechanical and long-term stability, which is suitable for wearable energy storage devices. The assembled knitting power textile is capable of sustainably driving wearable electronics (for example, a calculator or temperature−humidity meter) with energy converted from human motions. Our work provides more opportunities for stretchable multifunctional power sources and potential applications in wearable electronics. KEYWORDS: highly stretchable, all-yarn-based, self-charging, knitting textile, triboelectric nanogenerator, supercapacitor

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operation may be the most promising alternative to address such issues. Among various energy-harvesting devices, triboelectric nanogenerator (TENG) is most effective to convert almost all forms of mechanical energy in our living/working environment, including pressing, vibrations, airflow, rain drops, water wave, rotation, etc.,15−19 into electricity. Comparing with photovoltaic and thermoelectric energy harvesting approaches,20,21 mechanical energy harvesting is nearly independent of the weather and working environment. Moreover, owing to its advantage of lightweight, low-cost, high efficiency, and

ith the rapid development of multifunctional wearable consumer electronics, such as wearable displays,1,2 stretchable circuits,3,4 artificial electronic 5,6 skins, health monitoring devices,7,8 and motion tracking sensors,9,10 the requirements of flexible, lightweight, and, more importantly, sustainable power sources are receiving intensive attention. Nevertheless, in modern personal portable electronics, batteries cannot meet these energy requirements due to their rigid framework, limited lifetime, heavy package, and environmental pollution.11 Although significant progress has been made in improving the flexibility and maximum capacity of the lithium-ion batteries or supercapacitors (SCs),12,13 these energy storage devices still need to be charged frequently or even replaced/disposed.14 The employment of energy-harvesting technologies from ambient environment for sustainable © 2017 American Chemical Society

Received: July 27, 2017 Accepted: September 13, 2017 Published: September 13, 2017 9490

DOI: 10.1021/acsnano.7b05317 ACS Nano 2017, 11, 9490−9499

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TENG fabric with a flexible all-solid-state energy-storing yarn SC. The highly stretchable and shape-adaptive TENG fabric is knitted from a single energy-harvesting yarn that is fabricated by coating silicone rubber on the surface of three-ply twisted stainless steel/polyester fiber blended yarn. The all-solid-state symmetric yarn SC is fabricated by dip-coating carbon nanofiber (CNF) and poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) successively on a carbon fiber (CF) bundle. The as-prepared yarn SC is then knitted into the TENG fabric to form the self-charging power textile for wearable electronics. With the addition of a full-wave rectifier, our self-charging knitting power textile can continually drive a temperature−humidity meter or a calculator by hand tapping it. By taking advantage of the weft-knitting technique, the selfcharging power textile possesses high elasticity, flexibility, and stretchability. With all the components sealed by silicone rubber, our proposed self-charging power textile is also washable and can be integrated into regular fabrics. In addition, the knitting power textile is easy to shape and scale up as desired, suggesting its advantages for cost-effective and industrial-friendly manufacturability. Our work presents a soft and highly stretchable all-yarn-based self-charging power textile with knitting structure, which provides an overall design mentality for self-powering wearable electronics and presents extensive possibilities for sustainable energy systems.

wide choice of materials, TENG has been utilized for selfcharging power systems,22−25 active sensors,26−28 and sustainable energy sources.29,30 However, practically, TENGs usually have uncontrollable fluctuation/instability in their outputs, therefore, they cannot be directly used to drive most electronic devices that require a stable and continuous input power.23 The potential solution is to integrate an energy-harvesting TENG with an energy-storage battery/SC into a self-charging power unit, which provides more opportunities for sustainable and maintenance-free applications. Among most of the mechanical energy harvesting systems, converting or integrating most common fabrics into energyharvesting/energy-storing devices (i.e., TENG/SC) is ideal for wearable electronics, since it is low-cost, mechanically strong and flexible, and easy for industrial mass production. Moreover, the textile manufacturing technologies not only can provide a variety of fabrication methods for self-charging power textile but also can make them adapt to complex mechanical deformations. Owing to the extremely harsh operating circumstances of wearable devices, for example, stretching, bending, and twisting, a fully shape-adaptive, flexible, and stretchable self-charging power fabric is most desirable. As a consequence, several types of self-charging power textiles with woven structure have been recently reported for wearable electronics.22,23,31−33 Nevertheless, because of the nature of interior mutual interlacing yarns, which usually run straight horizontally and vertically, it is hard for woven fabric to achieve high stretchability, which may limit its practical applications. On the contrary, knitting fabric is comprised of consecutive rows of interconnected yarn loops that give a potential for being easily deformed. Due to the number of interdependent loops, the knitting structure shows outstanding elasticity. In addition, the space existing in each loop allows the knitting fabric to be stretched in all directions. In this work, we present a facile and scalable all-yarn-based self-charging knitting power textile that is capable of harvesting and simultaneously storing human motion energy for powering wearable electronics (Scheme 1). The self-charging knitting power textile is obtained by integrating an energy-harvesting

RESULTS AND DISCUSSION The integrated system of all-yarn-based knitting self-charging power textile is schematically illustrated in Scheme 1. The knitting power fabric can be a cloth worn on human body, an insole put in shoes, or a bracelet worn on wrists. The mechanical energy originating from human body movement is harvested by the knitting TENG fabric. Meanwhile, the generated alternating current electrical power can be stored in a SC after rectifying by a full-wave rectifier for sustainably driving wearable electronics. Before concurrent operations of our all-yarn-based self-charging knitting power textile, the characterizations of each functional device are carried out individually to evaluate its performance. As the fabrication procedures illustrated in Figure 1a, a commercial purchased three-ply twisted stainless steel/polyester fiber blended yarn (Figure 1b) is used as the conductive electrode, due to its good softness, tactile comfort, and excellent conductivity. Supersoft yet tough silicone rubber is chosen as the dielectric and encapsulated material of the TENG, owing to its good biocompatibility, superior mechanical properties, excellent flexibility/stretchability, and strong tendency to gain electrons.24,30,34,35 The energy-harvesting yarn is obtained by coating silicone rubber on the surface of conductive yarn. A homemade coating system (Figure S1) is designed to control the position of conductive yarn and keep the thickness of silicone rubber coating uniformly. The detailed fabrication method of the energy-harvesting yarn is described in the Experimental Section. According to the cross-section scanning electron microscopy (SEM) image of silicone rubber-coated energy-harvesting yarn (lower right in Figure 1b), the three-ply twisted stainless steel/polyester fiber blended yarn is approximately located in the center zone of the silicone rubber, which suggests the uniformity of the silicone rubber coating. Finally, with the weft-knitting technique, an asprepared energy-harvesting yarn is knitted into a TENG fabric, as illustrated in Scheme 1. The basic loop structure of the fabricated knitting TENG fabric consists of three parts: needle

Scheme 1. Systematic Configuration of the Highly Stretchable All-Yarn-Based Self-Charging Knitting Power Textile for Sustainably Powering Wearable Electronicsa

a

It integrated the energy-harvesting triboelectric nanogenerator (TENG) and the energy-storing supercapacitor (SC) into one fabric. 9491

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Figure 1. Schematic illustration and mechanical behaviors of the knitting TENG fabric. (a) Fabrication process of the knitting TENG fabric. (b) SEM images of the surface morphology of three-ply twisted stainless steel/polyester fiber blended yarn (upper), the cross section of this blended yarn (lower left), and the cross section of silicone rubber-coated energy-harvesting yarn (lower right) (scale bar: 1 mm, 500 μm, and 1 mm, respectively). The location of blended yarn is marked with yellow dotted line, and its three-strand yarns are separated with red dotted line. (c) Loop structure and type of the knitting fabric. (d) Knitting TENG fabric with top side of knit loops. (e) Photographs of the knitting TENG fabric under different tensile deformations, such as initial (0), elongated (1), widened (2), and distorted (3). (f) Schematic diagram of the corresponding deformations of a single knitting loop.

tapping frequency of 2 Hz, the increase of F (0.25−9.8 N) plays a significant role in improving the electrical output performances of energy-harvesting yarn (Figure S3d−f). It can be attributed to the corresponding increase of the contact area between silicone rubber and active layer (i.e., acrylic plate). All of the results (Figure S3) demonstrate that Ecoflex 0020 coating endows the energy-harvesting yarn with the best electrical output performances, compared to its counterparts. Technically, the harder silicone rubber will reduce the effective contact area when contacting. In addition, the higher viscosity of silicone rubber may lead to less separation distance during the contact−separation period. Since the better electrical output is achieved by larger contact area, along with great separation, the combination of moderate hardness and low viscosity of Ecoflex 0020 provides the maximum contact area and the largest contact-separation distance for the energyharvesting yarn. As a result, Ecoflex 0020 was chosen to further investigate the relationship between different coating thickness and electrical output of the energy-harvesting yarn. According to the diameter-dependent electrical outputs of these energyharvesting yarns in Figure S4, the electrical output performances of energy-harvesting yarns improve with an increase in the coating diameter of silicone rubber, which results in larger contact area between energy-harvesting yarns, and thus more charges are transferred. However, taking the practical knitting production and daily usage into consideration, a moderate diameter (3 mm) of energy-harvesting yarn with Ecoflex 0020 coating was chosen and adopted to fabricate the knitting TENG fabric. The working mechanism of the knitting TENG fabric is briefly demonstrated in Figure 2a, which is based on a conjunction of triboelectrification and electrostatic induction. To simplify the discussion, only a single energy-harvesting yarn is considered, since it is the basic unit of knitting TENG fabric.

loop (head), leg, and sinker loop. A stitch that is drawn through the previous loop from back to front is called a “knit” stitch, whereas a “purl” stitch is created when the stitch is drawn through the previous loop from front to back (Figure 1c). The knitted loops are arranged by suspension in the horizontal (course) or vertical (wale) direction, known as the weft-knitting method (Figure 1d). These meandering and suspended loops can be easily stretched in different directions that give the knitting fabrics much more elasticity than other types of textiles.36,37 Meanwhile, the space in each loop allows knitting fabrics to be elongated (1), widened (2), or distorted (3) by external or internal forces (Figure 1e), and the corresponding deformation states of a single knitting loop are also illustrated in Figure 1f. Before we knit the TENG fabric, two structural factors that affect the performance of energy-harvesting yarn, that is, the properties of silicone rubber and the diameter of energyharvesting yarn, have been comprehensively studied to optimize its electrical output performance. The experimental methods and design parameters for the electrical outputting measurement of energy-harvesting yarns are described in Figure S2. The effect of different silicone rubbers is first analyzed. Four kinds of silicone rubbers, that is, Ecoflex 0010, 0020, 0030, and 0050 (with Shore A Hardness of 00−10, 00−20, 00−30, and 00−50 and viscosity of 14,000, 3000, 3000, and 8000 cps, respectively), are used to fabricated energy-harvesting yarns with a fixed coating diameter of 6 mm, respectively. In addition, the relationships between the electrical outputs of these energyharvesting yarns and the tapping frequencies ( f, 1−5 Hz) at a fixed tapping force (F, maximum cyclic force) of 5 N are also investigated (Figure S3a−c). With an increase of f, the opencircuit voltage (VOC) and short-circuit transferred charge (QSC) experience no remarkable difference, while short-circuit current (ISC) increases gradually. On the other hand, with a fixed 9492

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Figure 2. Working mechanism and output performances of the knitting TENG fabric at single-electrode mode. (a) Schematic illustrations of the charge distribution of a single energy-harvesting yarn under short-circuit conditions. (b) Numerical calculations of the potential distribution of a single energy-harvesting yarn under open-circuit conditions using COMSOL software. (c) Electrical outputs of the knitting TENG fabric (contact area: 40 × 40 mm2, and tapping force: 11 N), which include VOC, ISC, and QSC, at various motion frequencies (1−5 Hz). (d) Variation of the output current density and voltage of the knitting TENG fabric with the external loading resistance (tapping frequency, 3 Hz). (e) Dependence of the output peak power density on the external load (tapping frequency, 3 Hz).

When the inner blended conductive yarn is connected to the ground by a metal wire, the TENG fabric would work in the single-electrode mode.16,17,38 When an active object (such as glove, hand, or foot) contacts with the dielectric layer (silicone rubber), electrification occurs at their interface and generates the equivalent charges with opposite polarities at the two contact surfaces (Figure 2a, i). There is practically no electrical potential difference between the two surfaces, due to the two opposite charges coinciding at the same plane. Once the two surfaces are separating and moving away from each other, positive charges will be induced in the inner electrode by the negative charges on the dielectric layer, due to electrostatic induction effect. Potential difference between the inner electrode and the ground prompts electrons flowing, resulting in an electrical current (Figure 2a, ii). Until the active object is quite far away, a new electrical equilibrium achieves and the

electrons stop moving (Figure 2a, iii). As the object approaches the silicone rubber again, electrons flow inversely from the ground to inner electrode to make a charge balance (Figure 2a, iv). When the active object fully contacts with the silicone rubber-coated yarn, charge neutralization occurs again. Continuous contact−separation movements between the active object and the silicone rubber-coated yarn bring continuous alternating current outputs from the TENG fabric. To obtain a more quantitative understanding of the electricity generating process, the potential distribution of every component is simulated using COMSOL multiphysics software, as demonstrated in Figure 2b. To characterize the electrical output performances of the ultrastretchable knitting TENG fabric, we use a linear motor to provide periodic contact−separation movements between an acrylic plate and the TENG fabric, as schematically illustrated in 9493

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Figure 3. Schematic illustration of the yarn SC. (a) Fabrication process of the all-solid-state symmetric yarn SC composed of two PEDOT:PSS/CNF/CF electrodes in parallel. (b−d) SEM images of a pristine carbon fiber bundle (b), a CNF-coated yarn (c), and a PEDOT:PSS/CNF-coated yarn (d) (scale bar: 10 μm). The insets in (b−d) are their corresponding high-magnification SEM images (scale bar: 500 nm). (e) Photograph of the yarn SC under various mechanical deformations, such as folding, bending, and twisting.

During the test, the acrylic plate is always maintained to have the similar shape and size as the knitting TENG fabric. Compared with the initial state without strain (0% strain), about ∼15% decline of the electrical outputs of the knitting TENG is found after being stretched at 100% strain (Figure S9). These reductions in electrical outputs are attributed to the decrease of the surface area of the knitting TENG fabric and thus the contact area for electrification at stretched states. In addition, the durability and stability of our knitting TENG fabric is also tested over long-term motion cycles. After 50 thousand cycles of repeated contact−separation motion at a contact frequency of 4 Hz, the open-circuit voltage and shortcircuit current show no obvious degradation (Figure S10). The yarn SC is symmetrically assembled from two parallel arranged PEDOT:PSS/CNF/CF electrodes with a solid-state H3PO4/poly(vinyl alcohol) (PVA) gel as the electrolyte (Figure 3a). CNF and PEDOT:PSS are successively deposited on the surface of carbon fiber bundles by means of dip-casting processes and acted as active materials. The detailed fabrication processes can be found in the Experimental Section. Figure 3b shows the SEM images of a pristine carbon fiber bundle composed of thousands of carbon fibers. With CNF ink coating, the surface of carbon fibers is uniformly dispersed by tubular carbon nanofibers (Figure 3c). The connection between them form a three-dimensional nanonetwork structure (inset in Figure 3c and Figure S11a), which can lead to fast electrons/ protons transfer during charging/discharging process. After the PEDOT:PSS coating, a thin layer of PEDOT:PSS covers the surface of CNF/CFs as well as the gap between them (Figure 3d). According to the higher magnification SEM images (inset in Figure 3d, Figure S11b,c), the uniform PEDOT:PSS film can be observed on the CNF-coated carbon fibers. This conducting polymer reveals a dispersant effect over the CNFs at the interface level increasing the charge storage in the double layer. The synergic effect between CNF and PEDOT:PSS also improves the capacitive storage performance of the yarn SC.39 Regarding the potential application to serve as part of a

Figure S5a,b. The chemical structure of the silicone rubber and acrylic plate is also presented in Figure S6a,b, respectively. The contact area is chosen as 40 × 40 mm2, tapping force is applied as ∼11 N, and the maximum movement distance per cycle is purposely set as 40 mm. Figure 2c exhibits the test results of VOC, ISC, and QSC for the knitting TENG fabric under various working frequencies (from 1 to 5 Hz). No obvious output changes can be observed in both the VOC (∼150 V) and QSC (∼52 nC) under different frequencies, while the peak value of the ISC increases with raising the operation frequency, from ∼0.55 μA at 1 Hz to ∼2.9 μA at 5 Hz. With the increasing of the varying external resistances, the maximum output current drops, while the maximum output voltage follows a reverse trend, which is due to the Ohm’s law (Figure 2d). The instantaneous power density is maximized at a load resistance of ∼100 MΩ, corresponding to a peak power density of ∼85 mW·m−2 (Figure 2e). The calculation method of output power density P can be calculated as P=

U2 R×A

(1)

where U is the output voltage across the external load, R is the load resistance, and A is the contacting area between the knitting TENG fabric and acrylic plate. To test the repeatability of above measured results, four more knitting TENG fabrics with small size (30 × 30 mm2) are fabricated (Figure S7a). Almost no obvious change is found by comparing their electrical outputs (Figure S7b−d), indicating that the measured results are reproducible in another similarly made device. As indicated above, our silicone rubber-based knitting TENG fabric is qualified with high stretchability, as its strain−stress curves (10% to 60% strain) shown in Figure S8. The viability of the energy harvesting of the knitting TENG fabric at stretched states is further evaluated. The knitting TENG fabric is uniaxially stretched for different strains (Figure S5c), and its corresponding electrical outputs (Figure S9) are recorded when having contact−separation motion relative to an acrylic plate. 9494

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Figure 4. Electrochemical characterization of the yarn SC. (a) CV curves of the SC under various scanning rates (1−100 mV/s). (b) GCD curves of the SC at different current loadings (5−100 μA). (c) CV curves of the SC at different bending states (scan rate: 50 mV/s); EIS of the SC is also shown in the inset. (d) Long-term stability of the SC for 6000 charging/discharging cycles. (e) Schematic illustrations of the integrated SC in series/parallel (left/right) connection. (f) CV and (g) GCD curves of the SC in series connection (1−4 SCs). (h) CV and (i) GCD curves of the SC in parallel connection (1−4 SCs).

electrochemical capacitive behavior, that is, the imaginary part of the impedance in the low-frequency region being perpendicular to the real part.41 Moreover, long-term durability of the yarn SC is also a critical parameter for a self-charging power textile. Although the capacitance had some subtle fluctuation during the cycling test, it was able to maintain as much as 78% after 6000 continual cycles at a current load of 25 μA (Figure 4d). The capacitance C can be calculated from the GCD curves as

wearable device, our yarn SC possesses a great advantage that can withstand complicated external mechanical deformations, such as folding, bending, and twisting (Figure 3e). This structural flexibility of our yarn SC makes it easy to be knitted into fabric, which has promising applications in wearable energy-storing device for portable electronics. In addition, it is worth noting that the improvement of capacitive storage performance of our yarn SC can be achieved by increasing the loading of CNF and PEDOT:PSS on the surface of carbon fibers.40 The electrochemical capacitance properties of our yarn SC are evaluated by cyclic voltammetry (CV), galvanostatic charging/discharging (GCD), and electrochemical impedance spectroscopy (EIS) techniques. Figure 4a shows the CV curves of the yarn SC recorded by a two-electrode system with a voltage window from 0 to 0.8 V. All the CV curves are close to rectangular at different scan rates from 1 to 100 mV s−1, which illustrates the great capacitive behavior of the yarn SC. The nearly symmetrical triangle-shaped GCD curves (Figure 4b) of the yarn SC with current loads of 5−100 μA are typical for SCs. There is no obvious ohm-drop (IR-Drop) phenomenon found even at a fast discharging rate (100 μA), further verifying the perfect capacitive behavior. As a wearable power textile, the mechanical stability of the device is particularly important. Figure 4c shows the CV curves of a yarn SC at different bending angles, and there is little capacitance drop observed under each bending angle. The inset of Figure 4c displays the EIS result, which indicates that the yarn SC has almost ideal

C=

I×t V

(2)

where I denotes the constant discharge current and V corresponds to the discharging potential drop within the discharge time t. By encapsulating a yarn SC in a flexible plastic tube, its capacitance endurances under continuous mechanical loadings are also tested. The capacitance of the yarn SC can maintain ∼85% and ∼70% after 6000 successive compression and bending cycle loadings, respectively (Figure S12a,b). Due to internal parallel arrangement of carbon fibers, the yarn SC cannot be stretched itself. However, the knitting structure endows the yarn SC with high stretchability. The capacitance stability of the yarn SC under tensile state is conducted by weaving the yarn SC into the TENG fabric (inset in Figure S12c). The capacitances of the yarn SC experience almost no change under various tensile strains (0%−100%, Figure S12c). At 100% tensile strain, the yarn SC can maintain ∼70% capacitance after 6000 stretching cycles (Figure S12d). 9495

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Figure 5. Applications of the energy-harvesting TENG fabric and the self-charging power textile. (a) An image of 124 blue LEDs lighted up by hand tapping the TENG fabric at initial state (above) and stretched state (below). (b) Transferred charge of the TENG fabric before and after washing in water. (c) LED warning sign on a shirt, that is, “FIB”, is lighted up by tapping a TENG bracelet wrapped on the wrist. (d) Yarn SCs are knitted into the normal knitting fabric at initial state (upper right) and stretched state (lower right). (e) Circuit diagram of the selfcharging power textile integrated by the TENG and SC. (f) Charging curves of two-series SCs by manually tapping the TENG fabric. The inset pictures are the temperature−humidity meter (left), calculator (upper right), and hand tapping fabric (lower right).

traditional wearable power sources based on solid materials like metal.32,42 When immersed in water and then naturally dried, our TENG fabric maintains its high output charge and can still power LEDs (Figure 5b and Movie S2), which clearly validates the speculation that our knitting TENG fabric is washable when it is stained. The contamination of silicon rubber will significantly reduce the electrical outputs of TENG fabric (Figure S14), due to the weakened ability of charge transfer. However, the electrical generating capability of TENG fabric can be recovered as long as the contaminations are washed away. Moreover, the electrical outputs of the knitting TENG fabric experience no decrease after multiple washing cycles (Figure S15a,b), which clearly demonstrates that this TENG fabric can be long-time used in our daily life. In practical wearable usages, our knitting TENG fabric can be fabricated into a large cloth as well as a small bracelet. As shown in Figure 5c, a knitted bracelet with the TENG fabric is worn on the wrist of an individual. A LED warning sign on the shirt such as the letters “FIB” can be lighted up during periodically tapping the bracelet (Movie S3). The combination of the energy-harvesting TENG fabric and the energy-storage yarn SC can be a potential strategy to further build up self-charging power textiles, which will make it possible to drive wearable electronics sustainably. The yarn SC can first be knitted into regular knitting fabric (for parallel connection) and later connected to TENG fabric (Figure 5d). In addition, they can also be directly knitted with TENG fabric (for series connection), as shown in Scheme 1. If the yarn SC is well encapsulated by a flexible plastic tube or silicone rubber, it can be washed multiple times. As shown in Figure S15c,d, the charging capabilities of the yarn SC are well maintained after washing 5 times, demonstrating that washing has no significant effect on the charge characteristics of the yarn SC. To demonstrate the practical application of our self-charging power textile, it is used for harvesting and simultaneously storing human hand tapping energy and then directly powering a calculator or temperature−humidity meter. The two-series yarn SC is charged by the rectified output of the TENG fabric

Therefore, it can be seen that the fabricated yarn SC can withstand external forces in our daily life and meanwhile maintain its good energy-storage performance over a long period of time. In order to meet various energy requirements for practical applications, several yarn SCs are integrated into assemblies both in series and in parallel connection (Figure 4e). The CV and GCD curves of series-connected yarn SCs with different connected units (1−4) are shown in Figure 4f,g, respectively. The output voltage of the series-connected yarn SCs linearly increases with an increase in the number of connected FSC units, which provides a tunable operating voltage for driving various types of wearable electronics. In addition, our yarn SCs can also be connected in parallel configuration for enlarging its capacitance, and the CV and GCD curves of parallel-connected yarn SCs with different numbers of units (1−4) are shown in Figure 4h,i, respectively. All of these results show the tunable adaptability and reliable scalability of our yarn SC for practical applications. The abilities of accommodating to complex mechanical deformations and harvesting human body energy from diverse motion make the knitting TENG fabric become a desirable wearable power source. When manually tapped, the TENG fabric can sufficiency light up 124 light-emitting diodes (LEDs) in series before or even after being stretched (Figure 5a and Movie S1). For a better and more intuitive understanding this tapping process (Figure S13a), the tapping force of hand (Figure S13b) and the corresponding electrical outputs of the knitting TENG fabric are measured (Figure S13d−f). By wearing the TENG fabric on an elbow (Figure S13c), the electrical outputs of the knitting fabric under actual bending state are also presented (Figure S13g−i). Taking advantage of the knitting structure, the single yarn-based TENG fabric not only maintains excellent flexibility and stretchability but also keeps a high electrical output performance. Moreover, since the yarn-based electrode is well coated by silicone rubber, our TENG fabric is lightweight, waterproof, and anticorrosive, which can better suit the practical applications compared with 9496

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perfectly. Then, the upper and lower ends of the yarn were fixed by clamps. Third, silicone rubber was mixed with the two components in a 1:1 weight ratio, then blended, and degassed in vacuum for ∼5 min to thoroughly remove bubbles. Afterward, the as-prepared silicone rubber solution was syringed into the tube along the tube wall. After the tube was filled with silicone rubber, it was left for curing at room temperature (∼4 h). Finally, the tube was peeled off, and the energyharvesting yarn was obtained. Fabrication of the Yarn SC. The carbon fiber bundles were first ultrasonically cleaned in acetone, ethanol, and deionized water for 10 min, respectively. CNF was treated with HNO3/H2SO4, then collected, washed with copious of water, and dispersed in water as CNF ink. PEDOT:PSS was mixed with EG and Triton X-100 under sonication for 5 min. For the preparation of carbon fiber-based SC, CNF ink was first coated on the pretreated carbon fibers via drop casting and put in the 100 °C oven for 15 min. Then, PEDOT:PSS was drop coated on the carbon fibers, followed by annealing at 120 °C for 20 min. The procedures were repeated twice, respectively, to increase the loading of CNF and PEDOT:PSS. Two as-prepared carbon fiber-based electrodes were coated with PVA/H3PO4 (1 g of PVA and 1 g of H3PO4 in 10 mL of deionized water) gel electrolyte and assembled with separator. After the PVA/H3PO4 gel solidified at room temperature, the flexible yarn SC was obtained. Characterization and Measurement. A Hitachi SU8010 field emission SEM was used to measure the surface morphology of threeply twisted stainless steel/polyester fiber blended yarn and the carbon fiber electrodes as well as the cross section of the silicone rubbercoated energy-harvesting yarn. An electronic digital micrometer (L.S.Starrett, 733 Series) was adopted to measure the diameter. A step motor (LinMot E1100) was applied to mimic human motions, providing the periodic contact−separation movement for TENG. A compression dynamometer (Vernier LabQuest Mini) was used to measure the applied force. A programmable electrometer (Keithley, model 6514) was adopted to test the open-circuit voltage, short-circuit current, and transferred charge. The software platform is constructed based on LabView, which is capable of realizing real-time data acquisition. The mechanical tensile test was conducted by a universal materials tester (MTS, Model Insight 10). For the electrochemical performance (by using CV, GCD and EIS techniques) measurement of the SC, an electrochemical workstation (Princeton Applied Research, VersaSTAT3) was utilized.

by hand tapping, whose electrical circuit diagram is shown in Figure 5e. When the switch S1 is on and switch S2 is off in Figure 5e (first charging and then driving), the voltage of yarn SC increases in a nearly linear fashion (blue curve in Figure 5f). After about 260 s, a calculator is turned on and can work continuously (Movie S4). If both the switches S1 and S2 are on (charging and powering concurrently), a temperature− humidity meter can be driven when the voltage reaches its threshold voltage (red curve in Figure 5f). Due to the large current draw of the temperature−humidity meter during its initialization routine, the output voltage of yarn SC temporarily drops until the initialization routine completes. Once we continue to tap the TENG fabric, the V−t curve will raise again (Movie S5), which indicates that this electronic device can be powered and driven sustainably. All of the above shows that the as-proposed self-charging knitting power textile has great potential as a sustainable power source for wearable electronics.

CONCLUSION In summary, a highly stretchable all-yarn-based self-charging knitting power textile has been proposed and fabricated for harvesting and simultaneously storing human body motion energy. Due to the knitting loop structure, the fabricated selfcharging power textile possesses high elasticity, flexibility, and stretchability. The energy-harvesting TENG fabric with the maximum peak power density of ∼85 mW·m−2 is knitted with a single energy-harvesting yarn, which is obtained by coating silicone rubber on the surface of three-ply twisted stainless steel/polyester fiber blended yarn. By hand tapping, the TENG fabric can light up at least 124 LEDs even at a highly stretched state or after being washed. The energy-storing yarn SC is achieved by successively dip-coating CNF and PEDOT:PSS on the surface of carbon fiber electrodes. Besides good capacitance, high flexibility, tunable adaptability, and reliable scalability, the yarn SC exhibits excellent long-term and mechanical stability. With these excellent performances, the designed highly stretchable knitting self-charging power textile can be a practical alternative for modern popular wearable electronics.

ASSOCIATED CONTENT

S Supporting Information *

EXPERIMENTAL SECTION

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b05317. Figures S1−S15 (PDF) Movie S1: Lighting up 124 LEDs by tapping knitting TENG fabric before and after being stretched (AVI) Movie S2: Electrical output performance of knitting TENG fabric before and after being washed (AVI) Movie S3: Knitting TENG bracelet used for warning sign (AVI) Movie S4: Powering a calculator by tapping the knitting power textile (AVI) Movie S5: Powering a temperature−humidity meter by tapping the knitting power textile (AVI)

Materials. Commercial three-ply twisted stainless steel/polyester fiber blended yarn (20 and 80 wt %, respectively) was obtained from Sparkfun Electronics, Inc. Carbon fiber bundle (T700SC-12K) was provided by Toray, Inc. Ecoflex supersoft silicone (0010, 0020, 0030, and 0050) was manufactured by Smooth-On, Inc. Carbon nanofiber (CNF, Pyrograf III Carbon Nanofiber PR-24-HT-HHT) was purchased from Pyrograf Products, Inc. Poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS, 1% dispersion in H2O), Triton X-100, ethylene glycol (EG, anhydrous, 99.8%), poly(vinyl alcohol) (PVA), nitric acid (HNO3, 70%), and sulfuric acid (H2SO4, 95−98%) were obtained from Sigma-Aldrich. Phosphoric acid (H3PO4, 85%) was from Alfa Aesar. All materials or chemicals were used as received without any further purification unless mentioned elsewhere. All aqueous solutions were prepared using ultrapure water with resistivity ≥18.2 MΩ·cm. Fabrication of the TENG Fabric. The TENG fabric was fabricated from a single energy-harvesting yarn with the weft-knitting technique. The energy-harvesting yarn was obtained by coating silicone rubber on the surface of three-ply twisted stainless steel/ polyester fiber blended yarn. The specific fabrication processes were as follows: First, the blended yarn was inserted into a circular plastic tube, whose two ends were sealed by circular acrylic plates with epoxy resin glue. In the center of the circular acrylic plates, a hole was reserved for inserting the yarn. Second, the tube was placed along the gravitational direction that can ensure the yarn placed in the center of the tube

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected] *E-mail: [email protected]. ORCID

Zhong Lin Wang: 0000-0002-5530-0380 Author Contributions ⊥

9497

These authors contributed equally to this work. DOI: 10.1021/acsnano.7b05317 ACS Nano 2017, 11, 9490−9499

Article

ACS Nano Notes

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The authors declare no competing financial interest.

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DOI: 10.1021/acsnano.7b05317 ACS Nano 2017, 11, 9490−9499