An all-nanofiber-based ultralight stretchable triboelectric

May 10, 2018 - Here, we designed an all-nanofiber-based, ultralight, S-TENG that could be ... wearable devices, electronic skins and smart sensor netw...
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An all-nanofiber-based ultralight stretchable triboelectric nanogenerator for self-powered wearable electronics Shuyu Zhao, Jiaona Wang, Xinyu Du, Jing Wang, Ran Cao, Yingying Yin, Xiuling Zhang, Zuqing Yuan, Yi Xing, David Y. H. Pui, and Congju Li ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00439 • Publication Date (Web): 10 May 2018 Downloaded from http://pubs.acs.org on May 11, 2018

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An All-Nanofiber-Based Ultralight Stretchable Triboelectric Nanogenerator for Self-Powered Wearable Electronics Shuyu Zhaoa,b,c,1, Jiaona Wangb,1, Xinyu Du c,1, Jing Wangd, Ran Cao c, Yingying Yinc, Xiuling ,Zhang c, Zuqing Yuanc,*, Yi Xinga,*, David Y.H. Puie, Congju Lia,c*

a School of Energy and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, China.

b School of Materials Science & Engineering, Beijing Institute of Fashion Technology, Beijing 100029, China; Beijing Key Laboratory of Clothing Materials R &D and Assessment, Beijing 100029, China.

c Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P. R. China;School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, China.

d Institute of Environmental Engineering, ETH Zurich, 8093, Zurich, Switzerland; Advanced Analytical Technologies Laboratory, EMPA, Überlandstrase 129, 8600, Dübendorf, Switzerland

e Department of Mechanical Engineering, University of Minnesota, 111 Church Street SE, Minneapolis, MN 55455, USA

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KEYWORDS

stretchable electronics, nanofiber, wearable devices, triboelectric nanogenerators, tactile sensors

ABSTRACT

The flexible and stretchable electronics have been considered as next-generation electronics.

Stretchable

triboelectric

nanogenerator

(S-TENGs)

with

both

multifunction and comfort have become a hot field of research for wearable electronic devices recently. Here, we designed an all-nanofiber-based, ultralight, S-TENG that could be softly attached on skins for motion energy harvesting and self-powered biomechanical monitoring. The S-TENG consisted of only two nanofiber membranes: a

polyvinylidene

fluoride

nanofiber

membrane

(PVDFNM)

supported

by

thermoplastic polyurethane nanofiber membrane (TPUNM) was used as the frictional layer, and a multiwalled carbon nanotube (MWCNT) conductive material screen-printed on the TPUNM was used as the electrode layer. Due to the excellent stretchability of TPUNM, the S-TENG could generate electricity under various types of deformation, and regain its original performance after intense mechanical extension, even if it is partially cut or damaged. Owing to the great electronegativity of PVDFNM, the device generated a maximum voltage of 225 V and a current of 4.5

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µA with a electrode area of 6 × 1 cm2. The S-TENG has great potential applications in self-powered wearable devices, electronic skins and smart sensor networks.

The development of the deformable and stretchable devices has attracted great attention due to the commercial potential as the next-generation electronics1-4. Exceeding the limit of the traditional electronics, the applications based on the stretchable sensors4-6, wearable electronics2, 7-10, and implantable devices have been widely expanded11-15. However, a durable and sustainable power source to drive these electronic devices is still urgently needed. Although breakthrough has been made in increasing the capacity of batteries and energy efficiency of the electric systems, lithium ion batteries (LIBs) and other conventional sources, which provide limited power supply, greatly limit their applications and brings inconvenience16-17. For the electric biosensors, one of the reliable solution is to harvest the mechanical energy from the biomechanical motions and convert it to electricity for powering the wearable electronics, which can work continuously. Recently, TENGs, which harvest various types of mechanical energies and convert it to electricity based on the coupling effects of triboelectrification and electrostatic induction9, 18-23, had obtained special attention as a new sustainable power source. Compared with traditional power sources, TENGs have several advantages of low cost, simple manufacture process and wide selections of materials16, 24-26. Nowadays, various stretchable TENGs have been reported to harvest energy based on the soft surface, but most devices utilized the silicone rubber27-28 or kapton film29 which is 3 ACS Paragon Plus Environment

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costly and inconvenient to wear. Some works have reported a ultrathin stretchable TENG based electrospun polyurethane nanofibers for energy harvesting and gesture sensing, and the fibers were packaged with PDMS, which is not suitable for long time to contact with skin3. Thus, it is necessary to develop a ultralight TENG with high stretchability to form perfect suction with the skin and high power output to meet the demand of wearable electronics24, 27, 30-34. Here, we designed an all-nanofiber-based and ultralight S-TENG for energy harvesting and self-powered biomechanical monitoring. The S-TENG worked based on the single electrode mode and comprised two kinds of nanofiber membranes. To ensure the whole device stretchable and improve the electronegativity of the device, the fabrication of TENG was demonstrated by using PVDFNM supported by TPUNM as the frictional layer, MWCNT conductive material screen-printed onto TPUNM as the electrode layer through electrospinning and screen printing respectively35-36. Polyurethane is one ideal elastomer material with excellent tensile property, thus we consider Polyurethane as the substrate of frictional and electrode layers, in order to achieve the application in different deformation condition. The two layers of nanofiber membranes were pressed together at an appropriate temperature. The stretchable TENG can generate electricity by touching in spite of various deformation. But most of all, the output properties of the TENG is nearly not affected after cutting or twisting, indicating the device has the super-resilient property. At the same time, the nanofiber membrane is more comfortable to contact with the skin compared with other materials, such as silicone rubber or traditional casting film, which is one of the 4 ACS Paragon Plus Environment

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ideal materials for wearable electronics. Benefited from the above advantages, we designed the multifunctional nanofiber gloves based on the S-TENG to harvest biomechanical energy and sense the deformable motion for monitoring the fingers activities, such as patting, bending and holding, indicating its potential applications for smart manipulator or robot arm.

RESULTS AND DISCUSSION Figure 1a shows the schematic of the fabricated S-TENG, which consists of the frictional negative layer with PVDFNM on the TPUNM (PVDF/TPUNM), and an electrode layer with the MWCNT screen-printed on the TPUNM. The addition of PVDFNM can enhance the electronegativity for the triboelectrification, compared with the output performance of the pure TPUNM as the negative material. Thus we combine the two materials in order to achieve both high elasticity and electric performance. The working mechanism of the stretchable TENG is schematically illustrated in Figure S1 in the Supporting Information. Figure 1b shows the tensile properties of TPUNM electrospun on different substrates. The non-woven PET surface is rough, resulting in some holes on the surface of TPUNM when the PET substrate was removed, These holes severely affected its tensile properties. The structure of PP was porous, there is a big gap between the fibers. In the process of electrospinning, nanofiber membranes tended to fill the gap due to the local electrostatic attraction. Although TPUNM could be easily separated from the PP substrate, the fiber surface was not smooth or uniform, leading to the stress 5 ACS Paragon Plus Environment

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concentration when stretching the membranes. Compared with PP and PET, the PDMS surface was smooth and homogeneous. TPUNM was not damaged after the removal of the PDMS substrate. The SEM images of the corresponding substrates (PET, PP and PDMS) are illustrated in Figure S2 in the Supporting Information. The experimental results show that the strain of the TPUNM electrospun on the PDMS could reach over 400%, much larger than the results of on the PET and PP non-woven spinning substrates. Figure 1c and e are the SEM images of the PVDFNM and MWCNT on the TPUNM respectively. The all-fiber structure guarantees the flexibility and comfort of the TENG. Figure 1f shows the schematic fabrication process of the TENG. (i) electrospinning a certain thickness of TPUNM onto the PDMS substrate; (ii) electrospinning a thin PVDFNM onto TPUNM as the main friction layer and screen-printing MWCNT onto TPUNM as the electrode layer; (iii) the friction layer and electrode layer were hot-pressed to pack the whole device. Cross-sectional image of the device is shown as Fig S4, illustrating the device for each thickness of PVDFNM, TPUNM, and MWCNT.

We mainly explore the influence of the PVDFNM thickness and the electrode width relative to the same device area on the performance of the device. Different PVDFNM spinning time determines the thickness and further the electronegativity of the friction layer. We considered PVDFNM as the friction layer in order to improve the output compared with pure TPUNM, it would not be sure that TPUNM was completely covered by PVDFNM if the spinning time of PVDFNM, thus we explored the best spinning time of PVDFNM in order to balance the relationship between the 6 ACS Paragon Plus Environment

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thickness of PVDFNM and electric field. On the other hand, varying the electrode width changes the adhesive areas between the two layers, which affects the tensile properties of the packed device. As shown in Figure 2a, thin copper foil was regarded as the reference electrode to contact with the PVDF in the triboelectric process. The area of copper foil was 8×2.5 cm2 which was the same as the area of the whole S-TENG. As displayed in Figure 2b and 2c, with the increasing of the PVDFNM spinning time, the output of the device will increase. However, as indicated by the results of 30 min, the open-circuit voltage (Voc) and transfer charges (Q) of the TENG were a little less than the parameters of 60 min, but much spinning time was saved. In addition to PVDFNM spinning time, in hot-press process and under the condition of the same device area, varying electrode width affects the adhesive force between the friction layer and electrode layer. The purpose of hot pressing is to enhance the bonding of the TPU between the electrode layer and the friction layer as much as possible, so as to avoid the detachment phenomenon during the testing process which affected the output signal of the device. Thus we explored the influence of varying electrode width on the performance of the device. Figure 2d illustrates the test mode of varying electrode width on the output performance. In the course of the experiment, the signal was transferred through the screen-printed MWCNT network. The MWCNT length was 6 cm. Figure 2e showed that the output performance under the condition of varying electrode width which demonstrated that under the whole device area of 8×2.5 cm2, the Voc and Q of the S-TENG first increase with the electrode width and then decrease. the S-TENG obtains maximum Voc and Q when the 7 ACS Paragon Plus Environment

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electrode width is 1 cm. Because the binding force between MWCNT and TPUNM of friction substrate is not so good, there was an obvious spacer which lead to poor contact between friction and electrode layers when the electrode width was over 1cm regardless of the output of voltage and the transferred charge, Figure 2f indicates that the relationship between the strain and electrode width follows a negative correlation. It is noted that with the increase of the electrode width, obvious stratification phenomenon between the friction and electrode layer was observed during the process of testing, demonstrating that the electrode width affects the adhesive force between the friction layer and electrode layer and confirming previous assumptions.

Typical output performances of the fabricated device including Voc, short-circuit current (Isc), and Q were investigated, as shown in Figure 3a-c. The S-TENG can generated the Voc of approximate 218 V (Figure 3a), Isc of 4.5 µA (Figure 3b), and Q of 66 nC (Figure 3c) with a electrode area of 6×1 cm2. The negative peak came from the contact, and the positive peak indicated the output signal generated by separation. The reason that positive and negative peaks coexisted in pairs of peaks was a cavity between friction and electrode layers, which induced extra output signals because the two sides of the cavity would contact when separated. The transferred charge density can reach 44 µCm−2. Figure 3d shows the variations of the Isc and P of the fabricated S-TENG with different load resistance. The maximum output power of about 135 mW could be achieved when the load resistance is about 300 MΩ. Figure 3e presents the stability of the produced S-TENG. The Isc showed no significant degradation after

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7200 continuous cycles, indicating the durability of the device. Figure 3f illustrates the ultralight characteristic of the whole device. In order to prove that the device has great stretchable and deformable features, the device was tested for generating electricity in various deformation conditions. First, the device was stretched extremely along the long-axis direction, as shown in Figure 4a. And the corresponding response of the device at different tensile strains is shown in Figure 4b. Strain is defined as (L - Lo )/L o × 100% , where L is the elongated length and Lo is the original length of the device. The Voc displays no apparent change when stretched to 100% strain. Even after stretching to 200% strain, the device could recover to its original performance at the unstrained state. The resistance change of the electrode with the tensile strain is shown in Figure S3 (Supporting Information). Because the resistance of the device was very high, it had reached the MΩ level, the resistance change of the electrode has little effect on the output of the TENG. The superior shear strength of TPUNM enables the device to retain tensile properties even undergoing severe shear damages. The device was tested after being cut to small slits and stretched lengthwise. As shown in Figure 4c, the device is not completely cut off, the image of the part of the device had been cut is as shown in Figure S5, when the S-TENG cut partially and then was pulled by 50% ,the generated output had slight decrease compared with the output when it was pulled without cutting, however, the contact area was increasingly smaller when the S-TENG cut partially and pulled by over 50% , so the output of S-TENG was reduced significantly when cut and pulled by 100% and 125%. Due to the high shear strength of the TPUNM, the device can 9 ACS Paragon Plus Environment

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keep its properties even after extreme stretching to 150% strain. When the device restored the original length, it regained the primary performance. The resulting Voc is presented in Figure 4d. Furthermore, the S-TENG was tested under serious torsional deformation. The device was manipulated under the condition of both twisting and stretching, as shown in Figure 4e. The characteristics of the device was tested before and after distortion, which revealed that the Voc was significantly decreased shown in Figure 4f. This result was attributed to the smaller contact area, which could be recovered when mechanical force was released. Based on the above measurements and the comprehension of the working mechanism, we further proposed a gloved-like stretchable energy harvester and self-powered motion sensor, which could be utilized to monitor human motions. We fabricated functional gloves based on the all-nanofiber membrane. The image of the glove mark was shown in Figure S6, the glove mark was consisted of two independent devices, we connected the two independent devices with respective sensor channel in order to recognize the patting or bending. When people make different gestures, the output signals could be detected in the corresponding electrodes. Figure 5a left and Movie S1 in the Supporting Information illustrate simulation of the hand action of patting. Figure 5a right shows the output of different fingers in the clapping test. The difference of output signals from each finger is mainly induced by the different contact areas between each finger and friction layer. At the same time, because the frequency of each finger was slightly different in the response time had a certain error. Figure 5b left and Movie S2 in the Supporting 10 ACS Paragon Plus Environment

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Information illustrate simulation of the hand action of bending. Figure 5b right displays the output of different fingers in the bending test, the property was referring to the various bending levels, which was significantly lower than the property of clapping action. Because the flexural strength of the fingers was limited, the signal was not obvious, the different bending levels of the fingers led to the various signals. Figure 5c left and Movie S3 in the Supporting Information illustrate simulation of the hand action of holding. Figure 5c right shows the output of different fingers in the grabbing test. For example, in the process of holding the beaker, the fingers have both the action of clapping and bending, and the s-TENG demonstrated concurrent monitoring of the patting and bending signals. In addition to the application of finger motion sensing, the s-TENG can be directly attached to other parts of the body, for example, on the knee or elbow. Due to the tensile properties of the device, it can deform with the movement of the leg or arm and detect its action changes.

CONCLUSION In summary, an all-nanofiber-based, ultralight, super-stretchable triboelectric nanogenerator has been confirmed to generate electricity regardless of various extreme deformations and recover the performance when mechanical force was released. Instead of the reported silicone rubber and PDMS as the flexible substrate, the TPU nanofiber membrane with super-stretchability is more comfortable to contact with human skin. With the addition of PVDF nanofiber membrane, the device can generate a peak voltage of 218 V and transferred charge of 66 nC. As an electronic 11 ACS Paragon Plus Environment

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skin-like device, a self-powered and body-conformable e-skin system was realized to detect fingers signals. The device with simple structure was easy to manufacture, and the processes were cost-effective and suitable for industrial manufacture. These results are beneficial to open up a wide range of flexible electronics.

METHODS Preparation of PVDFNM and TPUNM. 10 wt% of PVDF dissolved in a mixed solution of dimethylformamide (DMF) and acetone with a volume ratio of 7:3, and 21 wt% of TPU dissolved in a mixed solution of DMF and dichloromethane (DCM) with a volume ratio of 5:1. Before electrospinning, the spinning solutions were stirred by a magnetic stirrer for 12 h until the solution became homogenous. The electrospinning process was operated at a voltage of 20 kV, the feed rate of spinning solutions were fixed at 0.6 mL h-1 by using a flow-metering pump. A layer of PDMS was pasted on the drum as the substrate of TPU nanofiber membrane, and electrospining PVDF onto TPU nanofiber membrane. Preparation of the whole device. (i): electrospinning a certain thickness of TPUNM onto the PDMS substrate; (ii): electrospinning a thin PVDFNM onto TPUNM used as main friction layer and screen-printing MWCNT onto TPUNM used as electrode layer, specifically, In the process of the second step, PDMS substrate never separates with nanofiber membrane; (iii) the friction layer and electrode layer were hot-pressed under 120℃ for 30 s to pack the whole device, here, the friction layer needs to

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separate with PDMS substrate, then attaching to the electrode layer by hot-press, finally, the electrode layer was s separated with PDMS substrate. Characterization. The Voc, Isc and Q were measured by an electrometer (Keithley, 6514). The S-TENG was operated by a linear motor (Linmot BF01-37). SEM images were carried out using a Nova Nano SEM 450 at a 5 kV beam voltage. The stress-strain curve was researched by a tensile machine (XLM-50N).

Figure 1. Structure design of the all-nanofiber-based S-TENG. (a) The structure diagram of the TENG. (b) The strain curve of the TPU nanofiber membrane at different substrate (c) SEM image of the PVDF nanofibers. (d) SEM image of the TPU nanofibers. (e) SEM image of the MWCNT. (f) The schematic diagram of fabrication processes of the TENG. 13 ACS Paragon Plus Environment

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Figure 2. The output properties of the TENG under different factor. (a) Photograph indicating that the test diagram of the influence of different PVDFNM spinning time onto the output performance. The Voc (b) and Q (c) of the TENG under different spinning time of PVDFNM. (d) Photograph indicating that the test diagram of the influence of different electrode width onto the output performance. The Voc (e) and Q (f) of the TENG under different electrode width.

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Figure 3. Basic property characterization of the device. (a) Voc, (b) Isc, and (c) Q of the TENG. (d) Relationship of the output current and peak power on the external load resistance. (e) Isc after continuously operation for 1800, 3600, and 7200 cycles. (f) Photograph showing that the quality of the whole device.

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Figure 4. Evaluation of the TENG under various deformations (a) Photograph indicating that the device can drive a load when it was extremely stretched in long-axial direction. (b) Voc at various strain levels in long-axial direction. (c) Photograph indicating that the device drove a load when it was extremely stretched after being cut. (d) Voc at various strain levels after being cut and stretched. (e)Photograph demonstrating that the device drove a load when it extremely stretched after being twisted. (f) Voc after being twisted and stretched.

Figure 5. Demonstration of the TENG for gesture sensing. (a) left: Photograph of simulating the hand action of touching, right: Output of different fingers in clapping test, left: Photograph of simulating the hand action of bending; (b) right: Output of different fingers in bending test; (c) left: Photograph of simulating the hand action of grabbing, right: Output of different fingers in grabbing test. 17 ACS Paragon Plus Environment

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ASSOCIATED CONTENT Supporting Information Figure S1: Schematic illustration of mechanism for generating electricity. Figure S2: SEM image of different spinning substrate. Figure S3: The resistance changes of the electrode with the tensile strain. Figure S4: Cross-sectional image of the device.

Figure S5: The image of the part of the device had been cut.

Figure S6: The image of the glove mark.

Movie S1: The simulation of the hand action of patting.

Movie S2: The simulation of the hand action of bending.

Movie S3: The simulation of the hand action of holding.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] *E-mail: [email protected] 1

These authors contributed equally to this work.

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ACKNOWLEDGMENT The authors are thankful for support from the Beijing Natural Science Foundation (Nos. 2182014), and the National Natural Science Foundation of China (NSFC Nos. 51503005, 21703010 and 21274006), National Key R&D Project from Minister of Science

and

Technology

(2016YFA0202702,2016YFA0202703,

and

2016YFA0202704), and the Programs for Beijing Science and Technology Leading Talent (Grant no. Z161100004916168), and the Beijing Hundred, Thousand and Ten Thousand Talent Project (110403000402), and the General Program of Science and Technology Development Project of Beijing Municipal Education Commission of China (SQKM201710012004), and Beijing Institute Of Fashion Technology Special fund translation for the construction of high-level teachers (BIFTQG201801), and the “Thousands Talents” Program for Pioneer Researcher and His Innovation Team, China.

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(23) Wang, S.; Wang, Z. L.; Yang, Y., A One-Structure-Based Hybridized Nanogenerator

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TOC Title An all-nanofiber-based ultralight stretchable triboelectric

nanogenerator for

self-powered wearable electronics 25 ACS Paragon Plus Environment

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Keyword

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Sensors

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