Integrative square-grid triboelectric nanogenerator as a vibrational energy harvester and impulsive force sensor

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1 Nano Research Integrative square-grid triboelectric nanogenerator as a vibrational energy harvester and impulsive force sensor Chuan He 1,2, Weijun Zhu 3,4, Guang Qin Gu 1,2,5, Tao Jiang 1,2, Liang Xu 1,2, Bao Dong Chen 1,2, Chang Bao Han 1,2, Dichen Li 3,4, and Zhong Lin Wang 1,2,6 ( ) 1 Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing , China 2 CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology (NCNST), Beijing , China 3 State Key Laboratory of Manufacturing Systems Engineering, Xi'an Jiaotong University, Xi an , China 4 Collaborative Innovation Center of High-End Manufacturing Equipment, Xi'an Jiaotong University, Xi an , China 5 University of Chinese Academy of Sciences, Beijing , China 6 School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA , USA Received: 24 April 2017 Revised: 17 August 2017 Accepted: 25 August 2017 Tsinghua University Press and Springer-Verlag GmbH Germany 2017 KEYWORDS square grid, triboelectric nanogenerator, vibration, sensor ABSTRACT A square-grid triboelectric nanogenerator (SG-TENG) is demonstrated for harvesting vibrational energy and sensing impulsive forces. Each square of the three-dimensional (3D)-printed square grid is filled with an aluminum (Al) ball. The grid structure allows the SG-TENG to harvest vibrational energy over a broad bandwidth and operate at different vibrational angles. The most striking feature of the SG-TENG is its ability of being scaled and integrated. After connecting two SG-TENGs in parallel, the open-circuit voltage and short-circuit current are significantly increased over the full vibrational frequency range. Being integrated with a table tennis racket, the SG-TENG can harvest the vibrational energy from hitting a ping pong ball using the racket, where a direct hit by the racket generates an average output voltage of 10.9 ± 0.6 V and an average output current of 0.09 ± 0.02 μa. Moreover, the SG-TENG integrated into a focus mitt can be used in various combat sports, such as boxing and taekwondo, to monitor the frequency and magnitude of the punches or kicks from boxers and other practitioners. The collected data allow athletes to monitor their status and improve their performance skills. This work demonstrates the enormous potential of the SG-TENG in energy harvesting and sensing applications. 1 Introduction Aiming at harnessing small-scale ambient energy, various energy harvesters such as thermoelectric generators (TEGs) [1, 2], piezoelectric nanogenerators (PENGs) [3 5], and triboelectric nanogenerators Address correspondence to zlwang@gatech.edu

2 2 Nano Res. (TENGs) [6 14], have been developed as power sources for wearable electronics and sensor networks. Among them, TENG is of great interest for capturing lowfrequency mechanical energy owing to its low-cost fabrication and excellent robustness [15, 16]. Based on the coupling of the triboelectric effect and electrostatic induction, different TENG structures have been designed for harvesting mechanical energy of different forms, e.g., human motion [9, 10, 12], vibration [6 8], and water waves [11]. Among these, vibration is one of the most common mechanical motions ubiquitously available in our living environment. Spring-assisted TENGs were previously introduced to harvest vibrational energy, where the springs were used to move the triboelectric layers apart when the TENGs were subjected to external vibrations [6 8]. However, there are certain limitations to springassisted structures. First, the use of springs typically results in a relatively bulky volume of the TENGs. Secondly, an additional space is required for the vibrational part. Thirdly, considerable vibrational energy loss might occur when the springs become loose. To avoid these limitations, Wen et al. [11] reported a wavy TENG structure that allows self-restoration without using extra springs. In addition, package structures of TENGs based on the internal vibration of the oscillators have also been reported. The oscillators that have been used thus far are polymer-coated metal [17], polytetrafluoroethylene (PTFE) powders [18], or ferrofluid [19]. In this article, we demonstrate a square-grid TENG (SG-TENG) for harvesting vibrational energy and sensing impulsive forces. The demonstrated SG-TENG employs a package structure and uses a 3D-printed square grid as the frame. Each square of the grid is filled with an aluminum (Al) ball as the oscillator. The grid structure allows the SG-TENG to operate at different vibrational angles. Owing to the special design and small size, the SG-TENG can easily be scaled and integrated into other structures. By connecting two SG-TENGs in parallel, both open-circuit voltage (V OC ) and short-circuit current (I SC ) are greatly increased over the entire vibrational frequency range. Furthermore, when integrated with a table tennis racket, the SG-TENG can harvest the vibrational energy of the racket from hitting ping pong balls during game play. Moreover, the SG-TENG integrated into a focus mitt can not only count the total number of punches but also track the force applied in every impact. The acquired data allow athletes to monitor their status and improve their performance during training. 2 Results and discussion The structural design of the SG-TENG is schematically illustrated in Figs. 1(a) and 1(b). As shown in Fig. 1(a), the SG-TENG has a sandwich structure consisting of four parts. In the middle, a square grid frame (83 mm 83 mm 2 mm) is fabricated using stereolithography (SL) and filled with Al balls. The Al balls act as both an oscillator and electropositive triboelectric layer. On each side of the square grid is a layer of PTFE film placed on top of an Al plate. Here, the PTFE film was chosen as the electronegative layer owing to its ability to attract electrons, whereas the Al plate acts as both an electrode and a protective layer. It can be seen in Fig. 1(b) that each square of the grid contains a single Al ball and the side length of the square and the diameter of the Al ball are 2 and 1 mm, respectively. This structure allows the vibration of the Al balls inside the SG-TENG when an external vibration is applied. Figures 1(c) and 1(d) show photographs of the side view of the SG-TENG and the front view of the square grid, respectively. The detailed fabrication process of the SG-TENG is presented in the Experimental section. Figure 2 illustrates the working principle of the SG-TENG. Considering that the SG-TENG has a grid structure, we only demonstrate one square unit for Figure 1 Structural design of the SG-TENG. (a) Schematic illustration of the device structure. (b) The square grid and Al balls inside. (c) Photograph of the side view of the SG-TENG. (d) Photograph of the front view of the 3D-printed square grid.

3 Nano Res. 3 Figure 2 The working principle of the SG-TENG. clarification. According to the triboelectric series, when the Al ball comes into contact with the PTFE films, the electrons will inject from the Al ball to the surfaces of the PTFE films through triboelectrification [11]. Thus, the total amount of positive triboelectric charges on the Al ball should be the same as the negative triboelectric charges on the PTFE films. Initially, as shown in Fig. 2(i), the Al ball is in contact with the PTFE film at the bottom, and thus the negative charges are attracted to the bottom electrode, leaving the same amount of positive charge on the top Al electrode. The Al ball then moves upward when the SG-TENG is subjected to an external vibration. As the Al ball approaches the top, the top electrode has a higher potential than the bottom electrode, and hence the electrons transfer from the bottom to the top in the external circuit (see Fig. 2(ii)). Once the Al ball reaches the top PTFE film, all electrons are transferred to the top electrode, as shown in Fig. 2(iii). When the Al ball takes the reverse course, a reverse transfer of the electrons occurs through the external circuit (see Fig. 2(iv)). Finally, the Al ball returns to the bottom of the PTEF film, and a full cycle is completed (see Fig. 2(i)). To evaluate the performance of the SG-TENG, an electrodynamic shaker (Labworks, Inc.) was used to produce sinusoidal vibrations with a fixed amplitude and tunable frequency. After attaching the SG-TENG to the shaker, the output dependence of the SG-TENG on the vibration frequency was measured at vibrational angles of 0, 45, and 90. The vibrational angle, α, is defined as the angle between the surface of the SG-TENG and normal to the ground. It should be noted that the surface of the SG-TENG is always perpendicular to the vibrational direction. As previously described, the sinusoidal vibration applied to the SG-TENG induces the vibration of the Al balls inside, and hence an alternating current (AC) is produced in the external circuit. Figures 3(a) 3(c) show the electrical output of the SG-TENG at α = 0, 45, and 90, respectively. The V OC and I SC of the SG-TENG over a vibration frequency at α = 0, 45, and 90 are plotted in Figs. 3(a)(i), 3(b)(i), and 3(c)(i), respectively. The vibrational frequency ranges from 10 to 180 Hz, which covers most of the ambient vibrations in our daily life [20]. At α = 0, 45, and 90, the bandwidths for the voltage are 92.6, 89.3, and 88.3 Hz, respectively, and for the current are 61.94, 84.6, and Hz, respectively; in addition, the measured peak-to-peak values of V OC are 3.76, 5.66, 5.28 V, respectively, whereas the amplitudes of I SC are 0.37, 0.41, and 0.39 μa, respectively. We can see that as α increases from 0 to 90, the electrical output also increases over the vibrational frequency, particularly within the low frequency range, which leads to an increase in the bandwidth. The reason for this can be ascribed to the fact that, at an angle of 90, the Al balls are at rest at the bottom of the SG-TENG, and thus more vibrational energy can be transferred to them than to the Al balls at an angle of 0, which are at rest at the side of the SG-TENG. Moreover, V OC and I SC of the SG-TENG in the time domain at α = 0, 45, and 90 are also depicted in Figs. 3(a)(ii) and 3(a)(iii), 3(b)(ii) and 3(b)(iii), and 3(c)(ii) and 3(c)(iii), respectively. The electrical signals at 20, 40, 60, and 80 Hz are chosen here for comparison. The insets in Figs. 3(a)(iii), 3(b)(iii), and 3(c)(iii) depict the corresponding transferred charges between two electrodes. As the frequency increases, the signal evolves from a pulsed output to an oscillatory output; in addition, the peak current also increases, whereas the total charge transferred during one cycle remains almost constant. At a frequency of 80 Hz, the total charge transferred at α = 0, 45, and 90 is 1.65, 1.68, and 1.35 nc, respectively. These results indicate that the SG-TENG is capable of harvesting vibrational energy over a broad bandwidth and at different vibrational angles. Nano Research

4 4 Nano Res. Figure 3 The VOC and ISC of the SG-TENG at different vibrational angles α. (a) α = 0, (b) α = 45, and (c) α = 90. The insets in (a)(iii), (b)(iii), and (c)(iii) show the corresponding transferred charges between two electrodes. One of the features of the SG-TENG is its scalability, and by connecting the SG-TENGs in parallel, the total output can be increased with an increased number of SG-TENGs. To prove this, we measured the frequency response of two parallel-connected SG-TENGs. A comparison of VOC and ISC for a single SG-TENG and two parallel-connected SG-TENGs is illustrated in Figs. 4(a)(i) and 4(a)(ii), respectively. The measurements were performed at a vibrational angle of 90. Clearly, VOC and ISC of the two parallel-connected SG-TENGs are greatly increased over the full vibrational frequency range. Compared to the single SG-TENG, the parallelconnection increases the contact area between the triboelectric layers, which produces more triboelectric charges, thus leading to an enhancement in the electrical output. At a frequency of 80 Hz, the peakto-peak value of VOC and the amplitude of ISC are 2.4 and 3.9 times greater than those of a single SG-TENG, respectively. Therefore, a higher electrical output is expected as the number of SG-TENGs increases. As mentioned before, owing to its small size, the SG-TENG can be easily integrated into any vibrational surfaces or structures for harvesting the vibrational energy. For example, as shown in Fig. 4(b), we demonstrated the ability of the SG-TENG for harvesting the vibrational energy of a table tennis racket. Because the SG-TENG is only 4 mm thick (see Fig. 1(c)), it can be integrated into the racket; in this study, we directly attached a SG-TENG to a racket for simplicity. Figure 4(b)(i) shows an image of a ping pong ball bouncing on the racket, with an image of the SG-TENG being attached to the racket shown in the inset of Fig. 4(b). The amounts of VOC and ISC generated by the SG-TENG are shown in Figs. 4(b)(ii) and 4(b)(iii), respectively, and enlarged views of the highlighted VOC and ISC are shown in the corresponding insets. As the ball bounces against the racket, a series of electrical signals is generated each time the ball hits it (Video S1 in the Electronic Supplementary Material (ESM)). The average output voltage of the SG-TENG is about 4.6 ±

5 5 Nano Res. Figure 4 (a) Comparison of (i) the VOC and (ii) ISC for a single SG-TENG and two parallel-connected SG-TENGs. (b) Demonstration of the SG-TENG as a vibrational energy harvester. (i) Photograph of a table tennis racket with an integrated SG-TENG. As the ball bounces on the racket, VOC and ISC generated by the SG-TENG are as shown in (ii) and (iii), respectively. The insets are the respectively enlarged views of one cycle of VOC and ISC. (iv) The electrical performance of the integrated SG-TENG during game play evaluated when hitting a series of ping pong balls using the racket. The corresponding VOC and ISC of the SG-TENG are shown in (v) and (vi), respectively. 0.9 V, whereas the average output current is 0.19 ± 0.02 μa. Furthermore, the electrical performance of the integrated SG-TENG during game play was also evaluated by hitting a series of ping pong balls using the racket (see Fig. 4(b)(iv)). The values of VOC and ISC of the SG-TENG during consecutive strokes are shown in Figs. 4(b)(v) and 4(b)(vi), respectively, where the signals generated in a single stroke are highlighted. A single stroke consists of two sequential processes: the swinging of the racket and a direct hit on the ball by the racket. As indicated in Figs. 4(b)(v) and 4(b)(vi), the entire process is delicately disclosed in the obtained signal. Because the swing process precedes the hitting process in a single stroke, the electrical signal produced is composed of two parts: signals generated by the swing process and signals generated by the hitting process that follows. During game play, the swing of the SG-TENG produces an average output voltage of 9 ± 1 V and an average output current of 0.05 ± 0.02 μa, whereas a direct hit by the racket generates an average output voltage of 10.9 ± 0.6 V and an average output current of 0.09 ± 0.02 μa (Video S2 in the ESM). This demonstration proves that an integrated SG-TENG can effectively harvest the vibrational energy. In addition, the SG-TENG is sensitive to different types of vibrations, i.e., swinging and hitting during play, and thus the SG-TENG also has a great potential in sensor applications. In addition to harvesting vibrational energy, the capability of the SG-TENG as an impulsive force sensor is demonstrated. Figure 5(a)(i) shows the dependence of ISC on the impulsive force applied on the SG-TENG. It is clear that the amplitude of ISC increases as the impulsive force increases. The relationship between Nano Research

6 6 Nano Res. Figure 5 Demonstration of the SG-TENG as an impulsive force sensor. (a) (i) The ISC of the SG-TENG under different impulsive forces. (ii) Amplitude of ISC as a function of impulsive force and the linear fit of the experiment data. (b) Photograph of a focus mitt with an integrated SG-TENG as an impulsive force sensor. The VOC and ISC of the focus mitt punched repeatedly by persons A and B. the amplitude of ISC and the impulsive force is plotted in Fig. 5(a)(ii). From Fig. 5(a)(ii), we can see that the amplitude of ISC is approximately linearly proportional to the impulsive force. This linearity is crucial for the SG-TENG as an impulsive force sensor. As is well known, the impulsive force is commonly encountered in various combat sports, such as boxing, taekwondo, and kickboxing. Taking boxing as an example, a focus mitt is a padded target that is generally used for training boxers and other combat athletes. For this study, we integrated a SG-TENG into the focus mitt, as shown in Fig. 5(b). When the focus mitt is punched repeatedly, a series of pulse signals is generated (Video S3 in the ESM). The electrical signals of the integrated SG-TENG punched by persons A and B are illustrated, where VOC and ISC generated by person A are shown in Figs. 5(b)(i) and 5(b)(ii), respectively, and VOC and ISC generated by person B are shown in Figs. 5(b)(iii) and 5(b)(iv). The insets in Figs. 5(b)(i) through 5(b)(iv) show enlarged views of the corresponding highlighted pulse signals. Because each punch produces a pulse signal, the SG-TENG can be utilized to count the total number of punches during training. In the meantime, as indicated in Fig. 5(a), the SG-TENG is also able to track the force of every punch. The frequency and magnitude of the punches collected by the SG-TENG can help athletes monitor their status and improve their performance during training. 3 Conclusions In conclusion, we demonstrated a square-grid TENG as a vibrational energy harvester and an impulsive force sensor. The design of the SG-TENG allows it to harvest the vibrational energy over a broad bandwidth at different vibrational angles (α = 0, 45, and 90 ). The SG-TENG can be easily scaled, and by connecting

7 Nano Res. 7 two SG-TENGs in parallel, the values of both V OC and I SC are greatly increased over the full vibrational frequency range. At a frequency of 80 Hz, the peakto-peak value of V OC and the amplitude of I SC are 2.4 and 3.9 times as large as those of a single SG-TENG. By integrating the SG-TENG into a table tennis racket, the SG-TENG can harvest vibrational energy from hitting the ping pong balls using the racket. A direct hit by the racket generates an average output voltage of 10.9 ± 0.6 V and an average output current of 0.09 ± 0.02 μa. Furthermore, the SG-TENG can be used as the impulsive force sensor. We demonstrated the ability of applying the SG-TENG to boxing training to count the total number of punches and track the force of every punch. Owing to its lightness in weight and thinness, as well as its capability of being scaled and integrated, the SG-TENG has significant potential in energy harvesting and sensing applications. 4 Experimental section The SG-TENG consists of a square grid frame, Al balls, and two polytetrafluoroethylene (PTFE) films on Al plates. The square grid has a thickness of 2 mm and a side length of 83 mm. The square grid is fabricated using epoxy acrylate with a SL, which has a resolution of ±0.1 mm. The side length of the square is 2 mm. For each square, there is an Al ball with a diameter of 1 mm filled inside. On each side of the square grid is a layer of PTFE film placed on top of the Al plate. The Al plate has a thickness of 1 mm, whereas the thickness of the PTFE film is 80 μm. All of the electrical measurements of the SG-TENG were applied using a Keithley 6514 System Electrometer. A function generator (Stanford Research Systems DS345) and a linear power amplifier (Labworks PA-141) were used to produce the sinusoidal oscillations. A vibration shaker (Labworks ET-126B-4) was used to simulate mechanical vibration. The dynamic force applied was measured using a force gauge (HP-50) mounted on a linear motor. Acknowledgements Supports from the thousands talents program for the pioneer researcher and his innovation team, the National Key R&D Project from Minister of Science and Technology, China (No. 2016YFA ), National Natural Science Foundation of China (Nos , , , , and ), China Postdoctoral Science Foundation (No. 2015M581041), and Natural Science Foundation of Beijing, China (No ) are appreciated. Electronic Supplementary Material: Supplementary material (Videos S1 S3 demonstrate the electrical performance of the SG-TENG that being integrated into the table tennis racket and the focus mitt) is available in the online version of this article at References [1] Liu, W. S.; Jie, Q.; Kim, H. S.; Ren, Z. F. Current progress and future challenges in thermoelectric power generation: From materials to devices. Acta Mater. 2015, 87, [2] Bell, L. E. Cooling, heating, generating power, and recovering waste heat with thermoelectric systems. Science 2008, 321, [3] Wang, Z. L.; Song, J. H. Piezoelectric nanogenerators based on zinc oxide nanowire arrays. Science 2006, 312, [4] Hu, Y. F.; Xu, C.; Zhang, Y.; Lin, L.; Snyder, R. L.; Wang, Z. L. A nanogenerator for energy harvesting from a rotating tire and its application as a self-powered pressure/speed sensor. Adv. Mater. 2011, 23, [5] Zhang, Y.; Yan, X. Q.; Yang, Y.; Huang, Y. H.; Liao, Q. L.; Qi, J. J. Scanning probe study on the piezotronic effect in zno nanomaterials and nanodevices. Adv. Mater. 2012, 24, [6] Chen, J.; Zhu, G.; Yang, W. Q.; Jing, Q. S.; Bai, P.; Yang, Y.; Hou, T.-C.; Wang, Z. L. Harmonic-resonator-based triboelectric nanogenerator as a sustainable power source and a self-powered active vibration sensor. Adv. Mater. 2013, 25, [7] Yang, W. Q.; Chen, J.; Jing, Q. S.; Yang, J.; Wen, X. N.; Su, Y. J.; Zhu, G.; Bai, P.; Wang, Z. L. 3D stack integrated triboelectric nanogenerator for harvesting vibration energy. Adv. Funct. Mater. 2014, 24, [8] Wang, X. F.; Niu, S. M.; Yi, F.; Yin, Y. J.; Hao, C. L.; Dai, K. R.; Zhang, Y.; You, Z.; Wang, Z. L. Harvesting ambient vibration energy over a wide frequency range for self-powered electronics. ACS Nano 2017, 11, [9] Yang, Y.; Zhang, H. L.; Chen, J.; Jing, Q. S.; Zhou, Y. S.; Wen, X. N.; Wang, Z. L. Single-electrode-based sliding Nano Research

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