Rotating-Sleeve Triboelectric–Electromagnetic Hybrid Nanogenerator

Aug 7, 2017 - Currently, a triboelectric nanogenerator (TENG) and an electromagnetic generator (EMG) have been hybridized to effectively scavenge mech...
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Rotating-Sleeve Triboelectric−Electromagnetic Hybrid Nanogenerator for High Efficiency of Harvesting Mechanical Energy Ran Cao,†,‡,§,⊥ Tao Zhou,†,‡,⊥ Bin Wang,†,‡ Yingying Yin,†,‡,§ Zuqing Yuan,†,‡,§ Congju Li,*,†,‡ and Zhong Lin Wang*,†,‡,∥ †

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing 100190, China § University of Chinese Academy of Sciences, Beijing 100049, China ∥ School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States ‡

S Supporting Information *

ABSTRACT: Currently, a triboelectric nanogenerator (TENG) and an electromagnetic generator (EMG) have been hybridized to effectively scavenge mechanical energy. However, one critical issue of the hybrid device is the limited output power due to the mismatched output impedance between the two generators. In this work, impedance matching between the TENG and EMG is achieved facilely through commercial transformers, and we put forward a highly integrated hybrid device. The rotatingsleeve triboelectric−electromagnetic hybrid nanogenerator (RSHG) is designed by simulating the structure of a common EMG, which ensures a high efficiency in transferring ambient mechanical energy into electric power. The RSHG presents an excellent performance with a short-circuit current of 1 mA and open-circuit voltage of 48 V at a rotation speed of 250 rpm. Systematic measurements demonstrate that the hybrid nanogenerator can deliver the largest output power of 13 mW at a loading resistance of 8 kΩ. Moreover, it is demonstrated that a wind-driven RSHG can light dozens of light-emitting diodes and power an electric watch. The distinctive structure and high output performance promise the practical application of this rotating-sleeve structured hybrid nanogenerator for large-scale energy conversion. KEYWORDS: triboelectric nanogenerator, electromagnetic generator, hybrid nanogenerator, impedance match, mechanical energy

B

The abundant motion-based mechanical energy makes the TENG useful for many applications. However, the characteristics of high output voltage but low output current of the TENG restrict its applications to some degree.20 To compensate for the imperfection of output intensity and increase the integration of the power source, researchers intend to hybridize various other generators with the TENG, such as a solar cell,21 a piezoelectric generator,22 a thermoelectric generator,23 or an electromagnetic generator (EMG)24 to solve the challenge mentioned above. Among them, the hybridization of a TENG and an EMG has been proved as an efficient way of harvesting mechanical energy.25 Nevertheless, the characteristics of the TENG and EMG are

oth the energy crisis and catastrophic global warming are becoming severe problems in the modern world, emphasizing the emergent needs of clean and renewable energy. In recent years, the triboelectric nanogenerator (TENG) has been created to harvest mechanical energy from various ambient mechanical motions, which provides a feasible way of converting daily mechanical energy into electric energy.1,2 Based on the coupling effect of triboelectrification and electrostatic induction,3 the TENG, with a considerable electric output, is capable of harvesting human-motion energy,4,5 wave energy,6,7 wind energy,8,9 and vibration energy.4,10 In the previous literature, a conversion efficiency of the TENG as high as 85% has been achieved.11 Up to now, the TENG has been widely applied in the area of energy harvesting,12,13 wearable electronics,14,15 biomedical devices,16−18 and sensor systems19 because it is sustainable, lightweight, environmentally friendly, and cost-efficient. © 2017 American Chemical Society

Received: May 25, 2017 Accepted: August 1, 2017 Published: August 7, 2017 8370

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Figure 1. Structure and fabrication of the RSHG. (a) Schematic illustration of the RSHG. (b) SEM image of the FEP film nanostructure (scale bar, 2 μm). (c) Schematic diagram illustrating the fabrication process of the RSHG.

gigantically different in terms of output impedance.26,27 Thus, the output of the hybrid device as a power source is greatly limited when a load is added. Previously, intricate power management circuits were adopted to hybridize the TENG and EMG in the same circuit to match their output impedance,28 which reduced the energy conversion efficiency due to the ohmic loss of the complex circuits. In this paper, we report a rotating-sleeve-based triboelectric− electromagnetic hybrid nanogenerator (RSHG) for converting mechanical energy into electric power with high energy conversion efficiency. The impedance match between the TENG and EMG is simply achieved through commercial transformers. The RSHG is based on the structure of a commercial EMG, typically adding the TENG in the device of the EMG without changing the original EMG machine dimensions. The highly integrated RSHG is speculated to have not only tremendous voltage but also high current at the same time, which guarantees the high efficiency of mechanical energy harvesting and converting. The characteristics of the TENG and EMG were systematically compared in this work, including short-circuit current (Isc), open-circuit voltage (Voc), output impedance, and their cooperation output performance. The output Voc and Isc of the EMG are proportional to the rotation speed of the rotator (no more than 400 rpm). However, the Voc of the TENG waves around 580 V even though the rotation speed varied from 100 to 400 rpm. With the utilization of transformers, the output impedance of the TENG and EMG appear at a similar level, at a load resistance of 8 and 7 kΩ, respectively. Moreover, the Isc of the TENG and EMG are adjusted to 1.2 and 1 mA, respectively; thus they can be connected in series directly in the same circuit and ensure a maximized output. Moreover, to estimate the practical energy conversion ability of the RSHG, wind energy is taken as a typical example to light dozens of commercial light-emitting diodes (LEDs) and power an electric watch. Given other compelling features of the RSHG including sufficient power sources, simplicity in fabrication, and low cost, it has great

significance in high-efficient harvesting and converting mechanical energy into electric power at large scale.

RESULTS AND DISCUSSION The device structure of the RSHG is schematically represented in Figure 1a, which mainly consists of two parts: a TENG and an EMG. The TENG contains a stator and a rotator. The support part of the stator is a double-layer tube that has been made by a 3D printer. Grating-structured copper (Cu) electrodes (10 mm × 100 mm for each electrode) with an electrode distance of 0.5 mm were fixed on the walls of the tubes as conductive electrodes. Furthermore, fluorinated ethylene propylene (FEP) film with a 50 μm thickness was deposited on the surface of the grating-structured Cu electrodes as one friction material. With respect to the rotator, Cu strips were anchored on the walls of the torus with a 10 mm spaced arrangement, which were regarded as the other friction materials. To further increase the frictional contact area,29 the surface of the FEP layer was modified via an inductively coupled plasma (ICP) etching process, as the scanning electron microscope (SEM) image shows in Figure 1b. The electret, FEP film, was polarized thereafter by injecting electrons toward its upper surface to increase the surface charge.30 Finally, the TENG can be accomplished by assembling the tubes and the torus coaxially. Relative movement between the FEP layer and Cu strips will lead to friction electrons flowing through the external circuit. As for the EMG, special strong magnets were anchored on the outer wall of the stator to provide a magnetic field, and the other portion is the coils intertwined in the grooves of the torus. Since the rotator is spinning while the magnets remain stationary, the coils will cut a magnetic induction line and induction electromotive force can be produced. A schematic illustration of the fabrication procedure of the RSHG is exhibited in Figure 1c. Typically, the hybrid nanogenerator is constructed by a stator and a rotator. In addition, a fan is added to represent one example of harvesting mechanical energy by means of wind drive. The detailed 8371

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Figure 2. Working mechanism of the RSHG. (a) Schematic working principle of the TENG. (b) Schematic working principle of the EMG.

Figure 3. Electrical output performance of the TENG and EMG when working simultaneously. (a) Short-circuit current of the TENG. (b) Open-circuit voltage of the TENG. (c) Dependence of the open-circuit voltage and short-circuit current of the TENG on different rotation speeds. (d) Short-circuit current of the EMG. (e) Open-circuit voltage of the EMG. (f) Dependence of the open-circuit voltage and shortcircuit current of the EMG on different rotation speeds.

fabrication process is described in the Methods section. In addition, photographs of the as-fabricated RSHG are shown in Figure S1 (Supporting Information).

To simply clarify the operating principle of the RSHG, the working mechanisms of the TENG as well as the EMG are illustrated respectively in Figure 2. Figure 2a displays the 8372

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Figure 4. Output characteristics of the TENG and EMG under various load resistances. Dependence of the short-circuit current and output power on the load resistance of (a) the TENG and (b) the EMG. Dependence of the short-circuit current and output power on the load resistance of (c) the TENG and (d) the EMG after connecting with the transformer. The inset is the circuit diagram of the corresponding transformer of the TENG and EMG, respectively.

Another main part of the RSHG is the EMG, which operates based on electromagnetic induction. It is noteworthy that the special strong magnets we employed here are different from common ones. A lateral view of one magnet is exhibited in Figure 2b (I), where the blue part represents the N pole and the red represents the S pole. Accordingly, the magnet is magnetized in broadsides orientation. Figure 2b (II) displays the relative position of the coils and magnets. To further demonstrate the working mechanism of the EMG, the crosssectional view of the magnets and the coils are magnified and displayed in Figure 2b (III). The distribution of the magnetic induction lines reveals that the largest current is supposed to appear only when the coils are perpendicular to the magnets. Therefore, the coils were intertwined following a radial pattern to the torus center to obtain a maximum electromagnetic induction output. In a practical process, the magnetic flux crossing the coils changes with the movement of the torus, leading to the generation of inductive current. In particular, the adjacent two coils have reversed current flows, as the directions of the magnetic induction line through the coils are opposite. To sum up, all the working movement tracks of the rotator can be explained by the mechanisms presented in Figure 2 as the movements of the rotator are periodical. Before quantitatively studying the characteristics of the hybrid device, the rotation speed dependence of the output of both the TENG and EMG were analyzed respectively by utilizing a programmable electric motor to drive the rotator

electricity generation process of the TENG in one cycle. The friction charge generation progress based on the collaboration of the triboelectric effect and electrostatic induction31 can be classified into a freestanding-triboelectric-layer mode according to the four basic operation modes of the TENG.28 We define the initial state (stage I) and the final state (stage IV) as the states when the Cu strip on the rotator is aligned with the leftand right-hand electrode on the stator, respectively. During the rotation process, when the Cu strip is brought into contact with the FEP film, an equal quantity of positive charges and negative charges are generated on the surfaces of the Cu strip and FEP film, respectively, as the opposite tribo-polarity of the two friction materials (stage I).32 Negative charges will still be maintained in the surface of the FEP film even though the Cu strip slides forward due to the long-lasting property of the charges.17,33 As a result, the positive charges will transfer from the right-hand electrode to the left-hand electrode through the external circuit until the system reaches its equilibrium state (stage II). When the rotator continues to spin, the Cu strip stays above the right-hand electrode and a new charge balance is attained. Typically, equal amounts of the positive charges and negative charges exist on the surfaces of the Cu strip and FEP film (stage III), which is similar to stage I, where no current occurs in the circuit. The backward moving Cu strip leads to a reverse current flow, resulting from the opposite moving direction of positive charges (stage IV). 8373

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Figure 5. Electrical characteristic of the TENG, EMG, and RSHG. (a) Short-circuit current of the EMG, TENG, and RSHG when connected in parallel. (b) Open-circuit voltage of the EMG, TENG, and RSHG when connected serially. (c) Charging curves of the TENG, EMG, and RSHG. (d) Circuit diagram of the charging progress of the RSHG.

while keeping the stator stationary. As revealed in Figure 3a, the Isc created by contact and separation between the Cu strips and FEP film is as high as 150 μA at a rotation speed of 400 rpm. At the same time, the maximum Voc of the TENG reaches about 600 V (Figure 3b). By contrast, the output of the EMG is quite different from that of the TENG. Typically, the Isc of the EMG is 2 orders of magnitude higher than that of the TENG, while the Voc is much lower, as presented in Figure 3d and e, which are similar to previous work.34 It should be pointed out that the real-time signals of Isc and Voc of the EMG component are nonuniform because the device is unstable when rotating at a high speed (400 rpm). Although we use a 3D printer to fabricate the substrate and axle of the RSHG, it is hard to assemble the stator and the rotator completely coaxial because of the limited accuracy of the 3D printer. Figure 3c proves that the Isc of the TENG section continuously increases with the acceleration of the rotator, and the peak value (Isc = 150 μA) observed at the rotation rate of 400 rpm is 1 order of magnitude higher than that of the value obtained at the initial rotation rate of 100 rpm. Unlike the dependence of the Isc on the rotation speed, the Voc of the TENG remains stable around 580 V as the rotation speed increased from 100 rpm to 400 rpm, indicating that the Voc of the TENG is irrelevant to the rotation speed,35 which further proves the superiority of the TENG at low frequency. The Isc and the Voc of the TENG could be explained by using the equations of the capacitor model below:36

Isc =

dQ dt

(1)

Voc =

Q C

(2)

where Q is the frictional charge and C is the capacitance of the capacitor. The increasing Isc can be explained by the transferred charge quantity Q being constant while the time for each cycle is reduced as the rotation speed increased. As C is constant in this case as well, Voc of the TENG remains stable even though the rotation speed varied. Meanwhile, the same series of rotation speeds were employed toward the EMG to analyze its corresponding output. Compared with the TENG, the Isc and the Voc of the EMG follow an approximate linear relationship with the variation of the rotation speed (Figure 3f), correlating with Faraday’s law of induction.34 As far as we know, generators are supposed to operate with a matched external resistance to guarantee a maximum output power.37,38 Therefore, the output performances of the TENG and EMG were estimated to figure out their superior working conditions with various load resistances at a rotator speed of 250 rpm. Figure 4a reflects the resistance dependence of both the short-circuit current and output power of the TENG. The Isc remains at a stable level before the resistance increases to 70 kΩ and then decreases. The maximum power of the TENG reaches 12.7 mW at a load resistance of 9 MΩ. Various resistors were also adopted in the external circuit to explore the best 8374

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Figure 6. Application of the RSHG. (a) Demonstration of LED lights powered by the RSHG. (b) Demonstration of an electronic watch powered by the hybrid RSHG. (c) Extension applications of the RSHG as an energy converter.

TENG and EMG as the inner resistance of the EMG is constant. The efficiency of the RSHG can be defined as the ratio of the output electric energy of the hybrid device to the mechanical energy from the motor.13 The efficiency of the RSHG is calculated through formula 3 under the matched resistance of 8 kΩ at a rotation speed of 250 rpm, where F is the friction force and V is the line speed of the friction materials. The efficiency of the TENG and EMG is 33.3% and 3.1%, respectively. Thus, the total efficiency of the hybrid device is 36.4%, which proves the superiority of the TENG under such rotation speed.

operating conditions for the EMG. As illustrated in Figure 4b, the output power is maximized when the load resistance reaches 50 Ω and the largest power is 2.1 mW. The sizable difference of the output impedance between the TENG and EMG makes it difficult to synergistically use them in the same circuit directly. For the sake of obtaining the largest output power in the hybrid device, commercial transformers were adopted here to adjust the output impedance of the TENG and EMG to a similar level. A step-down transformer (TV600 GBE04, 12:1) adjusts the Isc and the Voc of the TENG from 120 μA and 580 V to 1.5 mA and 50 V, respectively. The detailed results of the output of the TENG after connecting with the transformer can be seen in Figure S2a,b (Supporting Information). Figure 4c exhibits the current and power curves of the TENG after connecting with a commercial transformer, and the dotted line highlights the maximum output power present at an external resistance of 8 kΩ. Similarly, a step-up transformer (model YD66X28, 1:10) is utilized with the EMG, and the Isc and Voc of the EMG turned out to be 1 mA and 7 V compared with the original state of 11.5 mA and 0.85 V. The instantaneously short-circuit current and output voltage of the EMG after connecting with the transformer are illustrated in Figure S2c,d in the Supporting Information. At the same time, the matched resistance of the EMG is modulated to about 7 kΩ (Figure 4d). Compared with previous work,34,39 the impedance matching of the TENG (8 kΩ) and EMG (7 kΩ) is achieved simply by utilizing transformers, without complex circuits, even not using a rectifier, which confirms the feasibility of the direct connection of the TENG and EMG in our work. It should be noted that the matched resistance of the TENG shows a decreasing tendency with increasing rotation speed.40 Compared with the matched resistance of 9 MΩ at the rotation speed of 250 rpm, the maximum output power of the TENG appears when the load is around 12 and 6 MΩ at rotation speeds of 150 and 350 rpm, respectively (Figure S3, Supporting Information). Corresponding to this, the transformers should be appropriately chosen to match the impedance between

η= =

E TENG + E EMG × 100% E in

∫ ITENG 2RTENG dt + ∫ IEMG 2REMG dt × 100% ∫ FV dt

(3)

The output performance of the RSHG was investigated by hybridizing the TENG with an EMG, and the total output is maximized by simply using commercial transformers. As shown in Figure 5a, the maximal Isc of the hybrid device is 2.2 mA, which is much higher than that of an independent EMG (1 mA) or TENG (1.2 mA). In other words, the Isc of the RSHG is equal to the sum of the current of the TENG and EMG when they are connected in parallel. As the Isc of the TENG and EMG are of the same order of magnitude, the features of the hybrid device when the TENG and EMG are connected serially were studied as well. According to Figure 5b, the peak output voltage of the hybrid nanogenerator is about 48 V, which is approximately equivalent to the sum of the Voc of the EMG (7 V) and of the TENG (40 V) when they are working simultaneously. To discuss the application of the hybrid nanogenerator in energy storage through a capacitor, a rectifier was employed here to transfer the signal to AC signs. The realtime-rectified current of the TENG and EMG are presented in Figure S4 (Supporting Information). A 470 μF capacitor was employed in the experiment to further compare the perform8375

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nanogenerator from 0 to 1 V within 0.8 s. The RSHG is capable of lighting dozens of LEDs as well as powering an electronic watch under blowing wind. Such a structure and widely distributed power sources make the RSHG a practicable device to convert mechanical energy into part of grid-tied electricity.

ance among the hybrid nanogenerator (TENG and EMG connected in series) and independent TENG and EMG after using a rectifier. The charging curves depicted in Figure 5c show that the EMG requires 13 s to charge the capacitor from 0 to 1 V. In contrast, the same capacitor can be charged to 1 V within 1 s by the TENG due to its inherent superiority of the high open-circuit voltage. The corresponding charging circuits of the TENG and EMG are provided in Figure S5a,b in the Supporting Information. The time demanded to charge the capacitor from 0 to 1 V reduces to 0.8 s for the hybrid device based on a self-designed charging circuit diagram, as illustrated in Figure 5d. It is true that the charging performance of the hybrid nanogenerator has a little enhancement compared with the independent TENG. Since the open-circuit voltage of the EMG is almost an order of magnitude lower than that of the TENG, the contribution of the EMG toward the total voltage of the hybrid device is negligible when the TENG and EMG are connected in series. It can be predicted that the charging performance of the hybrid nanogenerator could be improved if a better EMG was fabricated by twisting more turns of the coils or reducing the space between the magnets and coils. To demonstrate the practical applicability of the RSHG, here we take wind energy as a typical example to show its performance in converting ambient mechanical energy into electric power. As the photograph in Figure 6a shows, two rows of LEDs are lighted directly by the RSHG under blowing wind, and the related movie is provided in the Supporting Information. An electric watch is powered as well by the realtime output of the RSHG, which provides further evidence of the effectiveness of the hybrid device (Figure 6b). As depicted in Figure 6c, despite the wind energy, power sources of the RSHG can be expanded to widely distribute mechanical energy such as water waves, human-motion, and falling water. The output of the RSHG promises to be part of grid-tied electricity if appropriate technology is utilized to integrate this architectural design with widely used EMG equipment. After a power transmission system, the output electricity is capable of supplying home appliances, such as cell phones, scientific calculators, light bulbs, and computers even at low frequency. Moreover, the stability of the RSHG was investigated by estimating the Isc of the TENG since the TENG is the only abrasion part of the hybrid device. As displayed in Figure S6a (Supporting Information), the Isc of the TENG shows no trend of declining after 20 000 cycles. The surface of the FEP film has only some negligible scratches after the whole progress of the experiment (Figure S6b, Supporting Information).

METHODS Fabrication of the Stator. The double-layer tubes regarded as the main supporting part of the stator were made of polylactic acid (PLA) by 3D printing technology (Raise N2 Plus). The diameters of the inner tube and the outer tube are 84 and 96 mm, respectively, and the height of the tube is 130 mm. The outside wall of the inner tube and the inside wall of the outer tube were decorated with 12 and 19 pairs of grating electrodes (10 mm × 100 mm for each electrode), respectively. The special strong magnet anchored on the outer wall of the stator has a length of 100 mm, width of 5 mm, and thickness of 3 mm. The total number of magnets is 67 strips. Fabrication of the Rotator. A torus with grooves was fabricated by a 3D printer. The inner diameter and external diameter of the torus are 82 and 94 mm. Twenty-eight circular coils with a diameter of 0.2 mm were twined with 20 turns for each circle across the grooves of the torus. Twelve and 15 strips of copper sheets (10 mm × 100 mm for each sheet) were fixed on the inside wall and the outside wall of the torus with a distance of 10 mm. Assembly of the RSHG. A long axis with a snap ring groove was made by a 3D printer. The rotator and the stator were assembled coaxially through the axis and a bearing with lubricating balls. In addition, a fan was installed perpendicular to the axis to represent one example of harvesting mechanical energy by means of wind drive. Fabrication of the Nanostructure on the Surface of the FEP Film. First of all, a FEP film with a thickness of 50 μm was cleaned by acetone, ethyl alcohol, and deionized water in sequence. Magnetron sputtering (PVD75 Kurt J. Lesker) was used to create a Cu cover (10 nm) on the surface of the FEP film under DC sputtering. Afterward, 6 min was spent in the ICP (Sentech SI 500) chamber to manufacture nanostructures on the surface of the FEP film. O2, CF4, and Ar were brought into the chamber with air flow rates of 10, 30, and 60 sccm, separately, with a plasma source of 400 W and an ion speed up power of 100 W. Characterizations and Electrical Measurements. The scanning electron microscopy of nanostructures on the surface of the FEP was taken by a Hitachi SU8020. The short-circuit current and open-circuit voltage of the EMG and the short-circuit current were measured with a Keithley electrometer system (Keithley 6514). The open-circuit voltage of the TENG was measured by a mixed domain oscilloscope (Tektronix, MDO3024).

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.7b03683. Figure S1−S6 and photographs of the as-fabricated RSHG; the Isc and Voc of the TENG and EMG after using transformers; output characteristics of the TENG at different rotation speeds with loading resistances; rectified current of the TENG and EMG after using a transformer; circuit diagrams of the capacitor charging progress of the TENG and EMG; charging circuit of the TENG and EMG and the stability of the TENG (PDF) Movie S1: LED lights powered by the RSHG under blowing wind (AVI)

CONCLUSION In summary, we have successfully fabricated a rotating-sleeve triboelectric−electromagnetic hybrid nanogenerator to convert mechanical energy into electric energy with high efficiency. The RSHG is highly integrated by simulating the structure of commercial EMG equipment. Impedance matching between the TENG and EMG is achieved by simply using a commercial transformer without any intricate energy management circuits, which considerably reduces the power consumption. A systematic investigation of the output performance of the TENG, EMG, and RSHG was accomplished in this work. The RSHG showed an Isc of 1 mA and Voc of 48 V at a rotation speed of 250 rpm when the TENG and EMG are connected serially. The output powers of the TENG and EMG were 11.9 and 1.1 mW, respectively, on a matched load resistance of 8 kΩ. Moreover, a 470 μF capacitor can be charged by the hybrid

AUTHOR INFORMATION Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. 8376

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Zuqing Yuan: 0000-0003-3988-0618 Congju Li: 0000-0001-6030-7002 Zhong Lin Wang: 0000-0002-5530-0380 Author Contributions ⊥

R. Cao and T. Zhou contributed equally to this work.

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are thankful for support from the National Key R&D Project from the Minister of Science and Technology, China (2016YFA0202702), the Natural Science Foundation of China (NSFC Nos. 21274006 and 51503005), the Programs for Beijing Science and Technology Leading Talent (Grant No. Z16111000490000), and the “Thousands Talents” Program for Pioneer Researcher and His Innovation Team, China. REFERENCES (1) Fan, F.-R.; Tian, Z.-Q.; Lin Wang, Z. Flexible Triboelectric Generator. Nano Energy 2012, 1, 328−334. (2) Zi, Y.; Guo, H.; Wen, Z.; Yeh, M. H.; Hu, C.; Wang, Z. L. Harvesting Low-Frequency (