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Wind-Driven Triboelectric Nanogenerators for Scavenging Biomechanical Energy Qiang Jiang, Bo Chen, and Ya Yang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00902 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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Wind-Driven Triboelectric Nanogenerators for Scavenging Biomechanical Energy Qiang Jiang†,‡⊥, Bo Chen†,‡⊥, and Ya Yang†,‡* †

CAS Center for Excellence in Nanoscience, 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. ⊥Jiang Qiang and Bo Chen contributed equally to this work. *To whom correspondence should be addressed: Email: [email protected]. ABSTRACT: Wind-driven triboelectric nanogenerator (TENG), considered as one of the most important tributaries of TENG's family, possesses high frequency signals and remarkable output power. Herein, a wind-driven TENG, employing silver nanowires (Ag NWs) and fluorinated ethylene propylene (FEP) as triboelectric materials, was designed with a purpose to act as a power unit to replace batteries in some wearable devices. Under the wind speed of 20 m/s, the as-fabricated TENG could generate an output voltage, current and power up to 150 V, 7.5 µA and 0.18 mW, respectively. Wind-driven TENGs were integrated into three types of self-powered devices (i.e. shoe, bracelet and mask) to play roles as energy source due to the high output power and high frequency signals. The wearable devices were utilized to monitor different motion states (e.g. walking, jogging and running) at various body positions. These prototypes of self-powered wearable devices, would offer new approaches to protecting our environment and improving the quality of human life. KEYWORDS: Triboelectric nanogenerator, silver nanowires, wind energy, biomechanical energy, self-powered, wearable device, monitoring human motion.

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1. INTRODUCTION Biomechanical energy, a resourceful and sustainable type of clean energy, is considered as a new generation of continuous power as well as an alternative powerful weapon to fight against energy crisis. Typically, human muscle can convert internal stored chemical energy into biomechanical energy with peak efficiencies of about 25%.1-2 Total available power for everyday activities of an adult can easily reach as high as more than 100 W. However, most of the energy flows away by the form of mechanical energy loss and heat loss, including the motions of lung breathing, walking/running, arm/leg swinging, eye blinking, heart beating, and blood flowing.1,

3-7

If the

"useless" energy of human beings and even animals' daily activities could be efficiently harvested and transformed into available power source, it would promisingly replace batteries as the power source of electronic devices. Meanwhile, with an increasing demand of wearable/portable devices, light-weight and renewable power units are expected to overcome the disadvantages of batteries (e.g. environmental pollution, large size, periodic replacement).8-10 Much effort has been devoted to harvesting biomechanical energy.11-13 Taking the advantages of high efficiency, small size and low cost, triboelectric nanogenerator (TENG) has drawn great public attentions and also has been employed as power supplement units in various portable devices, such as handheld printer,14 acceleration sensor,15 self-charging power unit,16-17 body motion sensor,18-19 and UV detector.20-21 Based on the mechanism of triboelectrification, most TENGs used in wearable devices harvest mechanical energy by utilizing contact-friction structure and contact-separation structure.22-24 These different working modes are designed corresponding to human motion, like arm swinging and heel strike from walking, resulting in a relative low signal frequency. In other words, these TENGs can only generate the same frequency of output signals as that of arm swinging or heel strike, which limits their energy density as well as output power. Thus, it is essential to find out a way to improve 2 ACS Paragon Plus Environment

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the frequency of the output signal of TENG. Wind-driven TENG, one of the most important tributaries of TENG’s family, presents high frequency signals and remarkable output power. Nevertheless, wind-driven TENG can hardly be utilized on human wearable electronics due to the relative low wind speed generated by human motion. Hence, it is significant to find a way to realize the combination of wind-driven TENG and wearable device. Apart from good antibacterial property, high electrical conductivity, and remarkable mechanical properties, silver nanowires (Ag NWs) are also the materials with inferior electron affinity and great durability, thus making themselves as promising triboelectric materials.25-29 In this paper, an Ag NWs-based wind-driven TENG has been successfully designed and fabricated for converting various human activities energy into available electricity via air flow introduced mechanical energy. Employing two Ag NWs-based electrodes (as positive triboelectric material) and a fluorinated ethylene propylene (FEP) film (as negative triboelectric material), the as-fabricated TENG could generate an output voltage, current and power up to 150 V, 7.5 µA and 0.18 mW, respectively, under the wind speed of 20 m/s. The TENG was imbedded into a shoe, a bracelet and a mask as an energy harvesting unit to scavenge heel strike energy, arm swing energy and lung breathing energy, respectively. The output signals were analyzed to reflect and monitor different motion states (i.e. walking, jogging, running). Besides, a capacitor (10 V, 4.7 µF) could be charged to 7 V in different times by the use of the same self-powered shoe at three different motion states. Here, for the first time, the biomechanical energy can be harvested by wind-driven TENG, thus providing a new design strategy, as well as a development direction of wearable device.

2. EXPERIMENTAL SECTION Synthesis of Ag NWs. Ag nanowires were synthesized through a polyol synthesis method.30-31 Silver nitrate (AgNO3, 99.8%) and poly (vinyl pyrrolidone) (PVP, MW ≈ 1,300,000) were both 3 ACS Paragon Plus Environment

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purchased from Coolaber Science & Technology. Glycol (96%), sodium chloride, acetone (99.5%) and all other chemicals were obtained from Sinopharm Chemical Reagent Co., Ltd. In a typical synthesis process, the capping agent PVP (2.9 g) was dissolved in glycol (254 g, 4.09 mol) which acted as solvent as well as reductant to obtain a homogeneous solution A in a three-necked flask. AgNO3 (3 g, 0.018 mol) was sufficiently dissolved in glycol (9.9 g, 0.16 mol) to prepare solution B. Then, solution C was achieved by dissolving NaCl (0.146 g, 0.0025 mol) into glycol (10 mL, 0.18 mol). The three-necked flask with solution A and 480 µL solution C was heated in oil bath to 165 °C under mechanical stirring followed by injecting 1 mL solution B into the flask. After 1 min, the rest of solution B was slowly injected into the system at the speed of 0.5 cc/min utilizing a peristaltic pump. The reaction was stopped after 12 min and the resulting product was cooled down at room temperature. The resulting mixture was rinsed with acetone and distilled water for at least three times. Finally, the as-fabricated product, Ag NWs, was dissolved and stored in ethanol.

Fabrication of TENGs. According to our previous research, the fabricated TENG is based on double side-fixed structure wind-driven TENG. The as-prepared Ag nanowires were dissolved in water to form Ag slurry under continuous stirring. The well-mixed slurry was then pasted onto a photographic paper, and then dried at 80 °C in oven for 1 h. The Ag nanowires based electrodes were obtained by laminating the Ag nanowires slurry coating photographic paper to prepare uniform flat electrodes using a roll squeezer. The cut photographic paper coated with Ag nanowires was adhered on an acrylic bar (50 mm × 10 mm × 1 mm). The vibrating membrane (a FEP film with the thickness of 25 µm) was placed between two Ag electrodes and fixed by bolts at double sides, with the distance of air gap between the FEP film and top/bottom Ag electrode as 1 mm. The size of the as-fabricated TENG is 50 mm × 10 mm × 5 mm. And the shoe device was consisted of the TENG, and an aurilave, both of which were fixed onto the shoe insole. When heel touched the aurilave, 4 ACS Paragon Plus Environment

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airflow could be produced and drive the TENG. The bracelet device was mainly made up of the TENG and a flexible acrylic band which was used to fix the TENG on the wrist. The mask device was composed of the Ag nanowire-based TENG and a mask, where the TENG was placed on the gas outlet and was vertical to the airflow direction.

Characterization and Measurement. The morphology of Ag nanowires was characterized by using a field-emission scanning electron microscope (SEM, SU8020, Japan). The output voltage signals of the TENGs were measured on the mixed domain oscilloscope (Tektronix MDO3024) and 6514 System Electrometer (Keithley). Meanwhile, the output current signals of the TENGs were measured under the wind speed of 6, 8, 10, 15, and 20 m/s. The shoe device and bracelet device were applied to detect the output current/voltage signals of different movement states (walking, jogging and running). The mask device was used to detect the output current/voltage signals of different breathing frequencies. The output current signals of the devices were measured by the low-noise current preamplifier (Stanford Research SR570) and 2611B system source meter (Keithley), and the output voltage signals of the devices were measured on the mixed domain oscilloscope (Tektronix MDO3024) and 6514 system electrometer (Keithley).

3. RESULTS AND DISCUSSION Due to the inferior electron affinity and uniform nanostructure, Ag NWs was employed as positive triboelectric material as well as the electrode material of the wind-driven TENG. As depicted in Figure 1a and 1b, the as-fabricated Ag NWs presented a mean diameter in the range of 50-80 nm and an average length of about 10 µm, indicating that uniform Ag NWs could be obtained through this polyol synthesis method. The Ag NWs based electrode showed a smooth and metal luster surface (Figure 1c) and excellent conductivity as well. Compared to the Ag NWs slurry, the Ag NWs were 5 ACS Paragon Plus Environment

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aligned more compactly on the photographic paper because of the oven drying process which removed the solvent and reduced the distance between each Ag NW (Figure 1d), thus increasing the conductivity of the electrode. A wind-driven TENG was designed and fabricated to scavenge human motion energy. Figure 1e and 1f display the schematic structure and the photograph of a typical as-fabricated TENG. A FEP film, acting as the negative triboelectric material as well as the vibrating membrane, was placed in the middle of two Ag NWs based electrode. To prepare a double side fixed TENG, spacers, bolts and screws were adopted to fortify at both ends of the TENG, resulting air gaps between FEP film and electrodes. When wind load was applied on the TENG, the FEP film would vibrate up and down contacting Ag NWs to generate positive and negative charges on the surfaces of electrode and FEP film, respectively. Figure S1 shows the schematic diagrams of the working principle for the TENG which is mainly based on the mechanism of triboelectrification and electrostatic induction.32-33 Overall, during the contact-separate activating states, there was no charge on electrodes and vibrating membrane at first. When wind went across the device, FEP film frequently contacted the upper and the lower electrodes frequently introducing the separation of positive and negative charges, which resulted in the positive and the negative charges accumulated on the surfaces of Ag NWs and FEP film. Next, TENG transferred into the working process, which was a circulation from state (i) to state (viii). In brief, when the FEP film moved down towards the lower electrode, electrons directionally flowed from the lower electrode via external circuit to the upper electrode, thus leading to a current signal in external circuit. Once the vibrating film contacted the lower electrode, it turned back towards the upper electrode. Hence, the TENG exported a current flow in the opposite direction. Three significant states are demonstrated in Figure 2a-2c, referring to the initial state, FEP film moving down and FEP film moving up, respectively. 6 ACS Paragon Plus Environment

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As illustrated in Figure 2d, the TENG can generate an open-circuit voltage up to 150 V under the wind speed of 20 m/s. Meanwhile, an output short-circuit current can reach as high as 8 µA in the same wind loading condition as presented in Figure 2e. The output power, which is regarded as the Joule heating of the load resistance, can be attained utilizing a formula of I2R, where I stands for the external current and R represents the external load resistance.34-35 In figure 2f, the curve increases rapidly first to a peak (0.18 mW) at the resistance of about 3 MΩ and then shows a down slope. The results indicate the impedance of the TENG is about 3 MΩ under the wind speed of 20 m/s. Since the devices were designed to monitor human motion, it is essential to investigate the relationship between wind speed and output signals. Aiming to demonstrate how wind speed affects the output signals of the TENG, the devices were driven under different wind speed air flow in rang of 6 m/s to 20 m/s. The output voltage and current under different wind speed were depicted in Figure 3a and 3b. The diagrams show that the values of output voltage as well as output current enhance with the increase of the wind speed. The results can be attributed to more wind mechanical energy is transferred into electric power via increasing the mechanical frictional force and the effective contact area of the TENG. Hence, a higher surface charge density has been achieved, while more charges transferred, resulting in a higher output current signal. The original voltage and current data for TENG under different wind speed are shown in Figure S2-S7. Each test was repeated at least three times and the statistical data was summarized in Figure 3c and 3d. According to the plots shown in Figure 3e, the output power of the TENG increases while the resistance decreases with the increase of the wind speed. The enhancement of the voltage and current could account for these results. The original power and resistance data under different wind speed are concluded in Figure S8.

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To illustrate the practicability of the TENG, three different kinds of self-powered devices (i.e. shoe, bracelet and mask) were designed and fabricated based on the TENG as the power unit to monitor human motion at different positions. First, the as-fabricated TENG was integrated in a shoe device that was used for detecting human motion state under feet, as shown in Figure 4a. The output voltage and current of the shoe device were displayed in Figure S9. On the basis of the results, the value of the output voltage, current as well as frequency were enhanced successively according to the sequence of walking, jogging and running, which would be ascribed to the mode of motion. Apparently, running is more vigorous comparing to the other two modes, that is to say higher heel striking frequency, higher wind speed and more mechanical energy can be attained when people are running. According to the experimental results above, a higher wind speed can generate larger output signals. In order to illustrate the practicability of the device, the output signals were rectified by a rectifier, thus realizing the transformation from alternative signals into direct current signals, as shown in Figure 4b, 4c and Figure S10. The electric power generated by heel striking could illuminate three light-emitting diodes (LEDs) (Figure 4d). The rectified output alternative current could also be utilized to charge capacitor. As demonstrated in Figure 4e, a capacitor (4.7 µF, 10 V) could be effectively charged by the shoe device when there is a strike on the device. Compared to jogging and walking modes, running mode offers less charging time, where the capacitor could be charged from 0 to 7 V in about 45 s. Hence, the shoe device can offer a new approach to collect biomechanical energy and storage the transferred energy as usable power source. Meanwhile, a self-powered bracelet device was developed for detecting human motion states on arms (Figure 5a). It is well known that the amplitude of a swing arm is shifty according to the human motion states, like walking or running, which will cause a fluctuation of the movement speed and frequency of the arms. Based on the value and frequency of the output signals, the output 8 ACS Paragon Plus Environment

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performance curves of the as-prepared device can reflect different motion states. Figure 5b-5d illustrate the output voltage as well as current curves when people are walking, running and swing arm in situ. For example, a higher output voltage and frequency was obtained during running compared to walking states, which can be used to distinguish these two motion states. What’s more, a mask like self-powered device can also be adopted to monitoring human motion states by collecting human breath-induced mechanical energy (Figure S11a). This device was designed to detect the variation of breath during exercise at head position. Obviously, it is easy to identify the difference of the output signal of human breath when people are in a good rest state (Figure S11b), walking (Figure S11c) and running (Figure S11d), respectively. The as-fabricated wind-driven TENG based three kinds of self-powered devices could monitor human motion state at different position on human’s body, which would extend the application of TENG in the field of portable devices.

4. CONCLUSION In summary, a wind-driven based self-powered TENG was designed and fabricated, by using Ag NWs and FEP as triboelectric materials. The as-fabricated TENG could generate an alternative voltage and current as high as 150 V and 7.5 µA under the wind speed of 20 m/s. The largest output power could reach up to 0.18 mW which can drive several LEDs. Increasing the loading wind speed, the output signals would be significantly enhanced, which can be utilized to detect the variation of the wind speed. For the first time, the combination of wind-driven TENG and wearable device were performed to widen the application field of TENG. Herein, the wind-driven TENG was integrated into a shoe, a bracelet and a mask as an energy harvesting unit to scavenge heel strike energy, arm swing energy and lung breathing energy. These different types of biomechanical energy were converted into electrical energy to power LEDs, charge capacitor and be utilized to monitor human 9 ACS Paragon Plus Environment

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motions. Each device could detect the motion state of a special position on human body, that is feet, arms and head, respectively. These various prototypes of self-powered wearable devices, would offer new approaches to protecting our environment and improving the quality of human's life.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. The schematic diagrams of the working principle for the TENG at different states of the working TENG.; Measured output current and voltage of the TENG under different wind speed ranging from 6 m/s to 20 m/s.; Dependence of the output current and the corresponding power for TENG on the external loading resistance under wind speed in the range of 6 m/s to 20 m/s.; Measured output voltage and current of the shoe device, and also rectified signals at different motion states.; Measured output voltage and current when people are in a good resting state, walking and running. (PDF)

AUTHOR INFORMATION Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Key R&D Program of China (Grant No. 2016YFA0202701), the National Natural Science Foundation of China (Grant No. 51472055, Grant 10 ACS Paragon Plus Environment

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No. 61404034), External Cooperation Program of BIC, Chinese Academy of Sciences (Grant No. 121411KYS820150028), the 2015 Annual Beijing Talents Fund (Grant No. 2015000021223ZK32), Qingdao National Laboratory for Marine Science and Technology (No. 2017ASKJ01), China Postdoctoral Science Foundation Grant (Grant No. 2018M631415), and the "thousands talents" program for the pioneer researcher and his innovation team, China.

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25. Kang, H.; Kim, H.; Kim, S.; Shin, H. J.; Cheon, S.; Huh, J.-H.; Lee, D. Y.; Lee, S.; Kim, S.-W.; Cho, J. H. Mechanically Robust Silver Nanowires Network for Triboelectric Nanogenerators. Adv. Func. Mater. 2016, 26, 7717-7724. 26. Davoudi, Z. M.; Kandjani, A. E.; Bhatt, A. I.; Kyratzis, I. L.; O'Mullane, A. P.; Bansal, V. Hybrid Antibacterial Fabrics with Extremely High Aspect Ratio Ag/Agtcnq Nanowires. Adv. Func. Mater. 2014, 24, 1047-1053. 27. An, B. W.; Gwak, E.-J.; Kim, K.; Kim, Y.-C.; Jang, J.; Kim, J.-Y.; Park, J.-U. Stretchable, Transparent Electrodes as Wearable Heaters Using Nanotrough Networks of Metallic Glasses with Superior Mechanical Properties and Thermal Stability. Nano Lett. 2016, 16, 471-478. 28. Bellew, A. T.; Manning, H. G.; da Rocha, C. G.; Ferreira, M. S.; Boland, J. J. Resistance of Single Ag Nanowire Junctions and Their Role in the Conductivity of Nanowire Networks. ACS Nano. 2015, 9, 11422-11429. 29. Yoo, J. H.; Kim, Y.; Han, M. K.; Choi, S.; Song, K. Y.; Chung, K. C.; Kim, J. M.; Kwak, J. Silver Nanowire-Conducting Polymer-ITO Hybrids for Flexible and Transparent Conductive Electrodes with Excellent Durability. ACS Appl. Mater. Interfaces. 2015, 7, 15928-15934. 30. Lin, J. Y.; Hsueh, Y. L.; Huang, J. J. The Concentration Effect of Capping Agent for Synthesis of Silver Nanowire by Using the Polyol Method. J. Solid State Chem. 2014, 214, 2-6. 31. Li, B.; Ye, S.; Stewart, I. E.; Alvarez, S.; Wiley, B. J. Synthesis and Purification of Silver Nanowires To Make Conducting Films with a Transmittance of 99%. Nano Lett. 2015, 15, 6722-6726. 32. Jiang, Q.; Chen, B.; Zhang, K. W.; Yang, Y. Ag Nanoparticle-Based Triboelectric Nanogenerator To Scavenge Wind Energy for a Self-Charging Power Unit. ACS Appl. Mater. Interfaces. 2017, 9, 43716-43723. 14 ACS Paragon Plus Environment

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33. Zhao, K.; Wang, Z. L.; Yang, Y. Self-Powered Wireless Smart Sensor Node Enabled by an Ultrastable, Highly Efficient, and Superhydrophobic-Surface-Based Triboelectric Nanogenerator. ACS Nano. 2016, 10, 9044-9052. 34. Wang, S.; Mu, X.; Yang, Y.; Sun, C.; Gu, A. Y.; Wang, Z. L. Flow‐Driven Triboelectric Generator for Directly Powering a Wireless Sensor Node. Adv. Mater. 2015, 27, 240-248. 35. Wang, S.; Wang, Z. L.; Yang, Y. A One‐Structure‐Based Hybridized Nanogenerator for Scavenging Mechanical and Thermal Energies by Triboelectric–Piezoelectric–Pyroelectric Effects. Adv. Mater. 2016, 28, 2881-2887.

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FIGURE CAPTIONS

Figure 1. Materials and structure of the wind-driven TENG. (a,b) The SEM images of the Ag NWs at different magnification. (c) A photograph of an Ag NWs based electrode. (d) A SEM image of the surface of the as-fabricated electrode. (e,f) A schematic diagram (e) and a photograph (f) of the as-fabricated TENG.

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Figure 2. Mechanism and Output performance of the TENG (a-c) Different working states of the TENG including initial state (a), FEP film moving down (b) and FEP film moving up (c). (d,e) The output voltage (d) and current (e) of the TENG at the wind spend of 20 m/s. (f) Dependence of the output current and the corresponding power for TENG on the external loading resistance under the wind speed of 20 m/s.

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Figure 3. Output performance of the TENG at different wind speed. (a,b) Output voltage (a) and current (b) at wind speed ranging from 6 m/s to 20 m/s. (c,d) The statistic curve for output voltage (c) and current (d) at different wind speed. At least 20 successive peaks in the original voltage and current data had been selected and the values of the peaks were statistically calculated to attain each data and the standard deviation are also shown in figure (c) and (d). (e) Dependence of the output power and corresponding external loading resistance for TENG under different wind speed.

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Figure 4. Wind-driven TENG based shoe for detecting human heel striking. (a) A photograph of as-fabricated wind-driven TENG based shoe device. (b,c) The output voltage (b) and current (c) of the device during running. (d) The device generated electric energy to supply the working of LEDs. (e) The charging curves of a capacitor (10 V, 4.7 µF) which was charged by the self-powered shoe when people were walking, jogging and running.

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Figure 5. Wind-driven TENG based self-powered band for monitoring human motion. (a) Photographs of as-fabricated self-powered band. (b-d) Output signals for different motion states, walking (b), running (c) and swing arm in situ (d).

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TOC

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