A Flexible, Lightweight, and Wearable Triboelectric Nanogenerator for Energy Harvesting and Self- Powered Sensing

FULL PAPER Triboelectric Nanogenerators A Flexible, Lightweight, and Wearable Triboelectric Nanogenerator for Energy Harvesting and Self- Powered Sensing Fan Wu, Congju Li,* Yingying Yin, Ran Cao, Hui Li, Xiuling Zhang, Shuyu Zhao, Jiaona Wang, Bin Wang, Yi Xing, and Xinyu Du* Triboelectric nanogenerators (TENGs), which collect the energy neglected constantly in the daily life of humans, have been applied to various fields, such as energy harvesting, self-powered sensing, and environmental monitoring. With the growing demand for lightweight, flexible, and portable electric devices, TENGs have become a hot topic. In this work, a flexible, lightweight, and wearable TENG combining a nickel conductive mesh and a perfluorinated ethylene-propylene film via the ultrasonic-welding technique is presented. The TENG with an arch-bridge shape is in possession of plenty of excellent properties, including high output, resistance to destruction, and long-term stability, which not only owns a maximum power density of 36 mw m 2 at the optimal matching load impedance of 9 MΩ, but can also power a scientific calculator and 19 LEDs connected in series. Furthermore, the TENG could be used as a self-powered sensor to remotely control the state of electric fans or bulbs through a wireless sensing system. F. Wu, Prof. C. Li, Y. Yin, R. Cao, X. Zhang, Dr. X. Du Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences Beijing 100083, P. R. China E-mail: congjuli@126.com; duxinyu@binn.cas.cn F. Wu, Prof. C. Li, Y. Yin, R. Cao, X. Zhang, Dr. X. Du School of Nanoscience and Technology University of Chinese Academy of Sciences Beijing 100049, P. R. China Prof. C. Li, H. Li, S. Zhao, Prof. Y. Xing School of Energy and Environmental Engineering University of Science and Technology Beijing Beijing 100083, China Prof. C. Li, H. Li, S. Zhao, Prof. Y. Xing Beijing Key Laboratory of Resource-Oriented Treatment of Industrial Pollutants Beijing 100083, China J. Wang, Dr. B. Wang School of Materials Science and Engineering Beijing Institute of Fashion Technology Beijing 100029, China J. Wang, Dr. B. Wang Beijing Key Laboratory of Clothing Materials R&D and Assessment Beijing 100029, China The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admt.201800216. DOI: 10.1002/admt.201800216 1. Introduction As the increasing development of human civilization, the continuous breakthroughs of science and engineering have had impact on all sorts of aspects of people s life, including communication, medical treatment, and security. These fields cannot work smoothly without the collaboration of considerable electronics. Unfortunately, these conventional electronics always need an external power supply, such as a battery. The requirements of portability, small size, flexibility, environment friendly, and wearability of devices and systems are growing rapidly, especially for environmental monitoring, [1] biosensor, and mobile electronics. [2,3] For dealing with the emerging energy crisis, it is completely necessary to develop the devices and systems with low power consumption and sustainability. Additionally, there are many potential sources, such as waste heat, [4,5] friction energy, and vibration energy among the ambient environment of people s lives, [6 9] which could be collected for powering the electric micro/nanosystems. Since Wang s group developed a novel generator in nanoscale, called piezoelectric nanogenerator, which could covert mechanic energy into electricity with zinc oxide nanowires as functional material, [10] other functional materials such as GaN nanowires and ZnSnO 3 microbelts have also been used to work for piezoelectric nanogenerators. [11 13] These studies have provoked the interests in scavenging mechanic energy into electricity and have widened the methods of energy conversion. In 2012, the first triboelectric nanogenerator (TENG) was realized successfully. This TENG was formed by stacking Kapton film and polyester (PET) film on the basis of their different triboelectric property. On the top and bottom, the whole device was covered by two conductive layers to achieve the charge transfer. [6] Then, the fact is verified successively that formed by different friction materials and/or different structures the TENGs based on triboelectrification and electrostatic inducement have some fair different properties, [14,15] such as efficiently harvesting mechanic energy, [16,17] high signal outputs, [9] minimum abrasion, [18] and high removal efficiency for particulate matter. [19] Through the combination of polydimethylsiloxane (PDMS), 1800216 (1 of 7)

Kapton, aluminum foil, and pyramid-feature structure, Wang et al. fabricated an arch-shaped TENG, which suggested that an arch-bridge shape is quite favorable for the improvement of the electric properties. [20] Recently, harvesting the ubiquitous energy from people s daily life has caught some eyes for addressing the limitation of the traditional power supplies. [9,21 24] A scale-like TENG was reported by Du et al. for its ability to harvest rotary mechanical energy and charge Li-ion batteries. [18] A network- integration TENG with an area of 10 cm 7 cm could accommodate various waves and generate electric power. [21] A single-electrode and a two-electrode TENG with the soft, flexible, and stretchable performances were manufactured by utilizing the hydrophilic hydrogel (polyacrylamide-sodium-alginate) and hydrophobic elastomer (PDMS) for scavenging the energies of biomechanics and human motion. [25] Lately, a washable, porous, and designable electronic textile has been developed by Li s group, which could not only work as a self-power gesture in the smart human machine interface, but also serve as a controlling system of household appliances through wireless sensing. [26] Nowadays the TENGs have been applied widely to sensors, [26 28] security monitoring, [14,29] gas detection, and energy harvesting. [1,2,30,31] Those applications and developments of the TENGs mentioned above were pleasant; however, all of them have been shadowed by the professional and complicated micromachining technologies, complex experiment procedures, and costly experimental installations, suffering from inflexibility, easiness of being damaged, and limited stability, which restrict their practical applications and mass production. Herein, we demonstrate an arch-structured, flexible, lightweight, and stable TENG using the nickel conductive mesh and the perfluorinated ethylene-propylene (FEP) film, which is called nickel-mesh and FEP triboelectric nanogenerator (NF-TENG). This NF-TENG has the outstanding ability to defend against corrosion and oxidation (shown in Figures S1 and S2, Supporting Information), low cost and flexibility of nickel mesh, and the favorable electronegativity, low cost, and flexibility of FEP film. The NF- TENG is fabricated by ultrasonic welding technology and works under a contact separation mode, which takes possession of the impressive performances of signal outputting, no matter the device is vertically pressed, folded, or even pressed under the extreme state of twisting at an angle of 180. Moreover, this device holds the resistance to breakage, and can accomplish wireless sensing to control the on/off state of the electric fans and light bulbs. 2. Results and Discussion The fabrication process of the NF-TENG is demonstrated in Figure 1a. As shown in the schematic diagram, the device is composed of two major parts: nickel conductive mesh and FEP film. More details are described in the Experiment Section. Figure 1c displays the photograph of the NF-TENG, which demonstrates the arch-shaped structure and lightweight of the device. The work mechanism of the NF-TENG is exhibited in Figure 1. a) An illustration of the NF-TENG. b) The working principle of the NF-TENG during a cycle of the contact and separation. c) An optical photograph of the NF-TENG. 1800216 (2 of 7)

Figure 1b, which could be classified as a contact separation mode. [32,33] When the device stays in the original state without any external force, neither the nickel mesh nor the FEP film has any charges. Once an external force is enforced on the top of the device, the bottom surface of the mesh and the top surface of the FEP film can generate equally positive and negative charges when the two layers contact with each other, as shown in Figure 1b(i). As the distance between the nickel mesh and the FEP film increases, the positive charges spontaneously flow from the nickel mesh toward the mesh under the FEP film via the external circuit to balance the potential difference, which could generate an impulse current showing in Figure 1b(ii). Vice versa, there would be a reverse current, when the top layer and the bottom layer approach gradually to each other. With the offered periodic and stabilized force, the NF-TENG could generate periodic and stabilized current signals. To comprehend the behaviors of the NF-TENG, the electric properties of the NF-TENG in various situations have been explored, which are presented in Figure 2. As shown in Figure 2a c, a horizontal force was applied periodically to the device by a linear motor, and the NF-TENG achieved an opencircuit voltage and a short-circuit current up to 100 V and 3.5 µa, respectively. It should be noticed that the current signals are different from the current signals of the typical contact separation devices. There are multiple signals at each contact or separation process, which can be explained that the top layer and the bottom layer were not completely appressed during each contact or separation. When the NF-TENG was folded, as shown in Figure 2d f, it still generated an agreeable open-circuit voltage of 50 V and a short-circuit current of 3 µa. The device would generate electrical signals even though it was implemented pressing under the extreme state of twisting at an angle of 180, as shown in Figure 2g i. Compared with the results of folding and pressing under the state of twisting extremely, the advantages of pressing horizontally are rather dramatic, which could be attributed to the largest contacting area. [27,34] It is worth to highly praise that when the device was undermined with two breaches, it could still keep working just with a little reduction, which is revealed in Figure 2j l. The corresponding transferred charges in various situations could be obtained by the equation of Q= It where Q, I, and t are transferred charges, short-circuit current, and time, respectively (demonstrated in Figure S3, Supporting Information). This property implies that the device could maintain its completeness with pleasant signal outputs. The manual measurements of periodic contact separation in the mentioned manners are exhibited in Figure S4 (Supporting Information), which also indicates the outstanding performances of the NF-TENG. As shown in Figure S4a c (Supporting Information), when being pressed briskly by hand, the NF-TENG could produce rapid response signals. After being slightly folded by hand, the NF- TENG could generate impressive signals. The signals through twisting the device with a small angle by hand are remarkably higher than the results of pressing under the extreme state of twisting at an angle of 180 (see Figure S3g i, Supporting Information). As shown in Figure S3j l (Supporting Information), the signals of the device with two breaches by pressing manually are quite appreciable. Those favorable features allow that the NF-TENG holds the ability to supply power for the wearable devices or embedding in clothing to serve as a device for gathering body motion energy. The weight of the NF-TENG is displayed in Figure S5 (Supporting Information). The performances of the device under different loading resistance were also explored systematically. Figure 3a displays that as the load resistance increases, the short-circuit current of the NF-TENG presents a remarkable decrease. However, the power density builds up a tendency of first increase and then decrease. At the resistive load of 9 MΩ, the power density achieves the peak value of 36 mw m 2. The integration of electric properties of the device is discovered by increasing the number of arch units (see Figure 3b). There is a soaring in the voltage, when the number of units is double or triple. The increase of the output voltage value has a linear relationship with the increasing arch-unit number, which is attributed to the linear increase of the contact area. The long-term stability of the NF-TENG was also investigated for demonstrating the feasibility of the device. After running consecutively for around 3795 s at 1.7 Hz, the open-circuit voltage of the singlearch device had no distinct dropping as indicated in Figure 3c, which demonstrates the outstanding stability of the NF-TENG. Figure 3d is a representation that a capacitor with the capacity of 4.7 µf was charged by the NF-TENG through a rectifier. The voltage of the capacitor reached 10 V after being charged for 77 s. Figure 3e is the voltage curve of alternately charging the capacitor and powering the electronic calculator via the former capacitor, and Figure 3f is a photograph of a scientific calculator powered by the NF-TENG through the capacitor. Apart from the above, 19 green LEDs could be lighted up by the NF-TENG in the absence of capacitor or rectifier. Those results indicate that the NF-TENG would have extensive applications, such as charging the Li-ion batteries, [35] and MXene-based microsupercapacitors, [36] or powering electrospinning systems or electronic watches. [3,37] In addition, a wireless-sensing control system (WSCS), whose circuit diagram is indicated in Figure S6 (Supporting Information), was fabricated by connecting the NF-TENG with several electronic components, as represented in Figure 4. The WSCS was formed by three main segments: (1) the elements of the signal processing, (2) the emitter to emit the converted signals, and (3) the receiver to receive the triggering signals from the emitter and control the on/off state of home appliances such as bulbs and electric fans (see Figure 4a). The signal processing includes manually triggering the NF-TENG to form original signals, the conversion and amplification and transmission of the original signals by an instrumentation amplifier, and the conversion of the processed signals. Figure 4b demonstrates that the corresponding signals of the device in the process of the signals transmission. From top to down, the curves belong to the original signals by triggering manually, the conversed and amplified signals, and the triggering signals by the conversion of the latching relay, respectively. Figure 4c shows that the electronics were utilized in the process of the conversion and delivery of the NF-TENG signals, including the instrumentation amplifier, the latching relay, the emitter, and the receiver. From the left to right in Figure 4d, the first picture is the schematic of manually triggering the NF-TENG, which is followed by the optical pictures of controlling the on/off state of a bulb and an electric fan, and more details could be 1800216 (3 of 7)

Figure 2. The output performances of the NF-TENG. a) A photograph of the device being pressed horizontally by a linear motor, periodically. b,c) The open-circuit voltage and short-circuit current of the NF-TENG when being pressed, respectively. d) A photograph of the device being folded periodically by a linear motor. e,f) The open-circuit voltage and short-circuit current of the NF-TENG being folded, respectively. g) A photograph of the device being pressed periodically under the state of twisting at an angle of 180 by a linear motor. h,i) The open-circuit voltage and short-circuit current of the NF-TENG being pressed under the state of twisting at an angle of 180, respectively. j) A photograph of the NF-TENG with two breaches being pressed periodically by a linear motor. k,l) The open-circuit voltage and short-circuit current of the NF-TENG with two breaches being pressed periodically, respectively. 1800216 (4 of 7)

Figure 3. The electric characterization and the applications of the NF-TENG. a) The dependence of the short-circuit current and the power density of the NF-TENG. b) The integration of the open-circuit voltage of the NF-TENG with the number of arches. c) The long-term stability of the NF-TENG. d) The rectified voltage of the capacitor charged by the NF-TENG. e) The voltage of the capacity that is charged by the NF-TENG and powers a scientific calculator. f) The optical photographs of the scientific calculator powered by that capacitor. g) An optical picture of 19 LEDs lighted by the NF-TENG without any capacitor or rectifier. found in Videos S7 and S8 (Supporting Information). It is firmly believed that the applications of the WSCS are not only limited to the given home appliances, but also include a series of other household appliances, such as doorbells, [38] televisions, and microwave ovens. [26] 3. Conclusion In summary, using a nickel conductive mesh and a FEP film, a flexible, lightweight, wearable, and cost-effective triboelectric nanogenerator (NF-TENG) with an arch-bridge shape is 1800216 (5 of 7)

Figure 4. The applications of the NF-TENG in a wireless-sensing control system. a) A scheme diagram of the WSCS involved in the NF-TENG. b) The curves of the changes of signals in the process of the signal transmission. From top to bottom, they belong to the original signals by triggering manually, the conversed and amplified signals, and the triggering signals by the conversion of the latching lay. c) Pictures of the electric elements involved in the WSCS. d) The illustrations of the applications of the WSCS in home appliances such as bulb and fan. reported. Based on the technique of ultrasonic welding, the device not only holds some impressive performances during all sorts of periodic contact and separation, but also achieves the integration of electric power when the number of arches increases. Furthermore, the NF-TENG could demonstrate resistance to damage and long-term stability. Therefore, this device has the potential for application in wearable devices or for embedding in clothing to serve as a device and gather body motion energies. Apart from these, the NF-TENG possesses a peak power density of 36 mw m 2 at a resistance load of 9 MΩ. Moreover, the device could charge capacitors, light up 19 LEDs in series, and power a scientific calculator. Above all, the NF- TENG could work as a self-powered sensor to wirelessly control home appliances such as bulbs and fans. Given that, it is believed that this device would have a promising prospect in smart home appliances and wearable portable electronics. 4. Experimental Section The Preparation of the NF-TENG: The NF-TENG was fabricated by the ultrasonic-welding technique. Before fabricating, the FEP film (50 µm) was attached to nickel mesh to form a complex layer. The conductive mesh of nickel (90 µm) was cut into the area of 5 10 cm 2, and the complex layer was kept 5 9 cm 2. In addition, some PET films with the areas of 5 1 cm 2 and 5 9 cm 2 were provided to protect the mesh and the FEP film against the damage of the violent energy of the ultrasonic-welding machine (HB-5015B) and the connection between the two layers. Then the two terminals of the nickel mesh with the size of 5 10 cm 2 were welded with the protective coating of PET (5 1 cm 2 ), causing the knurling of 5 1 cm 2, which is called part 1, and so the two terminals of the complex layer were, the latter is named part 2. Subsequently, the film of PET (5 9 cm 2 ) was placed between parts 1 and 2. After that, along the previous knurling parts 1 and 2 were welded together. At last, the device was finished by removing the part of PET out of the knurling and connecting external leads. All the materials were easy to be bought in the market. Electric Measurement: All the open-circuit voltage, short-circuit current, and power density were measured by Keithley 6514, and the device was driven by a linear motor (Linmot BF01-37) at a 10 m s 2 acceleration and a separation distance of 20 mm. The Achievement of the Wireless-Sensing Control: In the wirelesssensing control system, the wireless-sensing control depends on the conversion and amplification of the original signals and emitting. First, the element devices of the wireless-sensing control system were connected as the order of Figure 4b. Then the raw signals were emitted through triggering manually the NF-TENG and the original signals were conversed and amplificated by an AD623 instrumentation amplifier. When the conversed and amplificated signals were conveyed to the latching relay to convert into triggering signals, the latching relay transmitted the triggering signals to the emitter connecting the latching relay in series. Finally, the emitter emitted the triggering signals to the receiver. The receiver was also connected with the home appliances in series and it controlled the turning on/off of appliances. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. 1800216 (6 of 7)

Acknowledgements This work was supported by the Beijing Natural Science Foundation (Grant No. 2182014), the National Natural Science Foundation of China (NSFC Nos. 21703010, 51503005, and 21274006), the National Key R&D Project from the Minister of Science and Technology (Grant Nos. 2016YFA0202702, 2016YFA0202703, and 2016YFA0202704), the Programs for Beijing Science and Technology Leading Talent (Grant No. Z161100004916168), the General Program of Science and Technology Development Project of Beijing Municipal Education Commission of China (SQKM201710012004), Beijing Excellent Talent Training Subsidy Program-Youth Backbone (Grant No. 2017000020124G089), the Beijing Institute of Fashion Technology special fund translation for the construction of high-level teachers (BIFTQG201801 and BIFTQG201807), the Fundamental Research Funds for the Central Universities, and the Thousands Talents Program for Pioneer Researcher and His Innovation Team, China. 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