Potential role of motion for enhancing maximum output energy of triboelectric nanogenerator: APL Materials: Vol 5, No 7

Although triboelectric nanogenerator (TENG) has been explored as one of the possible candidates for the auxiliary power source of portable and wearable devices, the output energy of a TENG is still insufficient to charge the devices with daily motion. Moreover, the fundamental aspects of the maximum possible energy of a TENG related with human motion are not understood systematically. Here, we confirmed the possibility of charging commercialized portable and wearable devices such as smart phones and smart watches by utilizing the mechanical energy generated by human motion. We confirmed by theoretical extraction that the maximum possible energy is related with specific form factors of a TENG. Furthermore, we experimentally demonstrated the effect of human motion in an aspect of the kinetic energy and impulse using varying velocity and elasticity, and clarified how to improve the maximum possible energy of a TENG. This study gives insight into design of a TENG to obtain a large amount of energy in a limited space.

Energy harvesting, a technology that derives energy from external sources such as solar power, thermal energy, and mechanical energy, and converts it into electricity, has received great attentions from the perspective of clean and renewable alternative energy. In particular, with the miniaturization of portable and wearable devices, various energy harvesting systems have been explored as additional energy supplies to extend the conventional battery life. For portable and wearable devices, many studies have focused on harvesting the mechanical energy from motion, 1–3 17, R175 (2006). 1. S. P. Beeby, M. J. Tudor, and N. M. White, Meas. Sci. Technol., R175 (2006). https://doi.org/10.1088/0957-0233/17/12/r01 312, 242 (2006). 2. Z. L. Wang and J. Song, Science, 242 (2006). https://doi.org/10.1126/science.1124005 12, 2833 (2012). 3. Y. Yang, W. Guo, K. C. Pradel, G. Zhu, Y. Zhou, Y. Zhang, Y. Hu, L. Lin, and Z. L. Wang, Nano Lett., 2833 (2012). https://doi.org/10.1021/nl3003039 mechanical energy is ubiquitous and easily accessible independent of the environmental condition. One of the mechanical energy harvesters, the triboelectric nanogenerator (TENG), converts ambient energy into electrical energy using the coupling between triboelectrification and electrostatic induction. 4 1, 328 (2012). 4. F.-R. Fan, Z.-Q. Tian, and Z. Lin Wang, Nano Energy, 328 (2012). https://doi.org/10.1016/j.nanoen.2012.01.004 materials, 5,6 22, 2187 (2010). 5. D. Choi, M. Y. Choi, W. M. Choi, H. J. Shin, H. K. Park, J. K. Seo, J. Park, S. M. Yoon, S. Chae, Y. H. Lee, S. W. Kim, J. Y. Choi, S. Y. Lee, and J. M. Kim, Adv. Mater., 2187 (2010). https://doi.org/10.1002/adma.200903815 19, 39 (2016). 6. J. M. Wu, C. K. Chang, and Y. T. Chang, Nano Energy, 39 (2016). https://doi.org/10.1016/j.nanoen.2015.11.014 mechanical energy harvesters such as electromagnetic, 1 17, R175 (2006). 1. S. P. Beeby, M. J. Tudor, and N. M. White, Meas. Sci. Technol., R175 (2006). https://doi.org/10.1088/0957-0233/17/12/r01 2 312, 242 (2006). 2. Z. L. Wang and J. Song, Science, 242 (2006). https://doi.org/10.1126/science.1124005 3 12, 2833 (2012). 3. Y. Yang, W. Guo, K. C. Pradel, G. Zhu, Y. Zhou, Y. Zhang, Y. Hu, L. Lin, and Z. L. Wang, Nano Lett., 2833 (2012). https://doi.org/10.1021/nl3003039 energy supplies of commercial portable and wearable devices. Recently, the output energy of TENGs could operate various sensor units, 7–9 14, 102 (2015). 7. J. M. Wu, C. C. Lee, and Y. H. Lin, Nano Energy, 102 (2015). https://doi.org/10.1016/j.nanoen.2015.02.009 7, 9213 (2013). 8. Y. Yang, H. Zhang, Z. H. Lin, Y. S. Zhou, Q. Jing, Y. Su, J. Yang, J. Chen, C. Hu, and Z. L. Wang, ACS Nano, 9213 (2013). https://doi.org/10.1021/nn403838y 8, 6273 (2014). 9. J. Zhong, Y. Zhang, Q. Zhong, Q. Hu, B. Hu, Z. L. Wang, and J. Zhou, ACS Nano, 6273 (2014). https://doi.org/10.1021/nn501732z 10,11 8, 722 (2015). 10. C. Han, C. Zhang, W. Tang, X. Li, and Z. L. Wang, Nano Res., 722 (2015). https://doi.org/10.1007/s12274-014-0555-3 2, 688 (2013). 11. G. Zhu, P. Bai, J. Chen, and Z. Lin Wang, Nano Energy, 688 (2013). https://doi.org/10.1016/j.nanoen.2013.08.002 energy of TENGs is obtained under harsh conditions such as extremely high frequency for a long time. We should consider obtaining high output energy of a TENG in a user-friendly condition like human motion. harvesting, a technology that derivesfrom external sources such asthermalandand converts it into electricity, has received great attentions from the perspective of clean and renewable alternativeIn particular, with the miniaturization of portable and wearable devices, variousharvesting systems have been explored as additionalsupplies to extend the conventional battery life. For portable and wearable devices, many studies have focused on harvesting thefromsince theis ubiquitous and easily accessible independent of the environmental condition. One of theharvesters, the triboelectric nanogenerator (TENG), converts ambientinto electricalusing the coupling between triboelectrification and electrostatic induction.In terms of the industry, a TENG is advantageous in that it features cheap, lightweight, flexible, and biocompatiblein contrast to otherharvesters such as electromagnetic,piezoelectric,and pyroelectric generators.This advantage makes TENG one of the most likely candidates for additionalsupplies of commercial portable and wearable devices. Recently, the outputof TENGs could operate various sensor units,as well as it could charge portable and wearable devices.However, the outputof TENGs is obtained under harsh conditions such as extremely high frequency for a long time. We should consider obtaining high outputof a TENG in a user-friendly condition like human

energy of a TENG. The first approach is to increase the conversion efficiency of maximum possible energy to electrical output energy with load. Many researchers have reported that the conversion efficiencies can be achieved by more than 50% of the energy transferred to a TENG. 12,13 26, 6599 (2014). 12. Y. Xie, S. Wang, S. Niu, L. Lin, Q. Jing, J. Yang, Z. Wu, and Z. L. Wang, Adv. Mater., 6599 (2014). https://doi.org/10.1002/adma.201402428 7, 10987 (2016). 13. Y. Zi, J. Wang, S. Wang, S. Li, Z. Wen, H. Guo, and Z. L. Wang, Nat. Commun., 10987 (2016). https://doi.org/10.1038/ncomms10987 mechanical energy as possible to a TENG, i.e., to increase the maximum possible energy of a TENG. However, it is not clearly understood how the mechanical energy plays a role in increasing the maximum possible energy of a TENG yet. Further studies on the maximum possible energy of a TENG will make a significant contribution to the improvement of overall output energy. Generally, there are two main approaches to increase the outputof a TENG. The first approach is to increase the conversion efficiency of maximum possibleto electrical outputwith load. Many researchers have reported that the conversion efficiencies can be achieved by more than 50% of thetransferred to a TENG.The second approach is to transfer as muchas possible to a TENG, i.e., to increase the maximum possibleof a TENG. However, it is not clearly understood how theplays a role in increasing the maximum possibleof a TENG yet. Further studies on the maximum possibleof a TENG will make a significant contribution to the improvement of overall output

Herein, we focused on the role of motion in increasing the maximum possible energy of a TENG. We verified that the mechanical energy from human motion can cover the energy consumption of portable and wearable devices. We derived the relationship of the specific TENG form factor and the maximum possible energy of a TENG. In addition, we experimentally identified the effect of the motion on the maximum possible energy in terms of the kinetic energy and impulse. Understanding the relationship between the motion and the output energy allows us to improve the performance of a TENG when converting various motions to electrical energy.

mechanical energy from human motion can cover the energy consumption of the devices. Figs. 1(a) 1(b) energy consumption of commercialized portable and wearable devices as well as the mechanical energy available from human motions for 1 min, respectively. The energy consumption of the devices for 1 min ranges from 3 μJ to over 60 J depending on the specifications of the device, such as display size, and the operation mode, such as video playback or stand-by mode. The mechanical energy from human motion for 1 min ranges from 60 mJ to 510 J, depending on the moving part of the body and velocity. It should be noted that, when assuming the mechanical energy is entirely converted into electrical energy, the mechanical energy generated by motion can sufficiently cover the energy consumption of the devices. For example, the energy generated by the daily motion of the arm can sufficiently cover the energy consumption of a smart watch and even the stand-by energy consumption of a smart phone. Thus, in order to fully utilize the mechanical energy from motion for the TENG, it is very important to increase the maximum possible energy of a TENG based on understanding the factors related to the motion. To utilize a TENG as an auxiliary power source for small devices, we need to ensure that thefrom humancan cover theof the devices. Figs.andshow theof commercialized portable and wearable devices as well as theavailable from humanfor 1 min, respectively. Theof the devices for 1 min ranges from 3J to over 60 J depending on the specifications of the device, such as display size, and the operation mode, such as video playback or stand-by mode. Thefrom humanfor 1 min ranges from 60 mJ to 510 J, depending on the moving part of the body and velocity. It should be noted that, when assuming theis entirely converted into electricalthegenerated bycan sufficiently cover theof the devices. For example, thegenerated by the dailyof the arm can sufficiently cover theof a smart watch and even the stand-byof a smart phone. Thus, in order to fully utilize thefromfor the TENG, it is very important to increase the maximum possibleof a TENG based on understanding the factors related to the

energy per one motion” (E MP ) of a TENG can be defined as follows: 15 6, 8376 (2015). 15. Y. Zi, S. Niu, J. Wang, Z. Wen, W. Tang, and Z. L. Wang, Nat. Commun., 8376 (2015). https://doi.org/10.1038/ncomms9376 E M P = 1 2 Q S C , m a x ( V O C , m a x + V m a x ′ ) , (1) where Q sc,max , V oc,max , and 𝑉 ′ max are the maximum short-circuit transferred charge, maximum open-circuit voltage, and maximum voltage at Q = Q sc,max , respectively. To be more specific, we derived the E MP of a TENG in the vertical contact-separation mode, which is one of the basic modes of a TENG, by substituting the theoretically defined Q sc,max , V oc,max , and 𝑉 ′ max 16 6, 3576 (2013). 16. S. Niu, S. Wang, L. Lin, Y. Liu, Y. S. Zhou, Y. Hu, and Z. L. Wang, Energy Environ. Sci., 3576 (2013). https://doi.org/10.1039/c3ee42571a E M P = 1 2 S σ 2 x m a x 2 ( 2 d 0 + x m a x ) ε 0 ( d 0 + x m a x ) 2 , (2) where d 0 denotes ∑ i d i / ε i , d the dielectric thickness, ε the dielectric constant, S the triboelectrification area, x max the maximum displacement, ε 0 the vacuum permittivity, and σ the surface charge density. According to recent research, the “maximum possibleper one motion” (E) of a TENG can be defined as follows:where, andare the maximum short-circuit transferred charge, maximum open-circuit voltage, and maximum voltage at, respectively. To be more specific, we derived the Eof a TENG in the vertical contact-separation mode, which is one of the basic modes of a TENG, by substituting the theoretically defined, and supplementary material , Note 2),wheredenotesthe dielectric thickness,the dielectric constant,the triboelectrification area, xthe maximum displacement,the vacuum permittivity, andthedensity.

MP of a TENG is only related to the form factor of the TENG, such as triboelectrification area and maximum displacement, and the properties of materials, such as the surface charge density and the dielectric constant. It is ironic that the E MP of a TENG, which converts the mechanical energy of motion to electrical energy, is independent of motion factors such as velocity. Initially, motion such as velocity and/or frequency affects the surface charge density until the surface charge reaches its maximum. However, the amount of accumulated surface charge cannot exceed the maximum surface charge density of the material, and the fully accumulated surface charge does not decay well. 17 333, 308 (2011). 17. H. T. Baytekin, A. Z. Patashinski, M. Branicki, B. Baytekin, S. Soh, and B. A. Grzybowski, Science, 308 (2011). https://doi.org/10.1126/science.1201512 MP of a TENG is generally measured using the materials with maximum surface charge density, motion would hardly affect the surface charge density. It seems as if once the form factor of a TENG is determined, the maximum possible energy remains constant, even if the TENG is operated by various types of motion as shown in Fig. 1(b) motion triggers the movement of a TENG, and it can be explained by the concept of kinetic energy and impulse. 18 18. M. Thornton, Dynamics of a System of Particles, 4th ed. ( Harcourt Brace & Company , 1995). mechanical energy from human motion is applied to a TENG, one part of a TENG contacts another part with a kinetic energy corresponding to the velocity. The moving part experiences an impulse and stops or rebounds depending on the elasticity of the TENG. According to the derived Equation ( 2 ), the Eof a TENG is only related to theof the TENG, such as triboelectrification area and maximum displacement, and theofsuch as thedensity and the dielectric constant. It is ironic that the Eof a TENG, which converts theofto electricalis independent offactors such as velocity. Initially,such as velocity and/or frequency affects thedensity until thereaches its maximum. However, the amount of accumulatedcannot exceed the maximumdensity of theand the fully accumulateddoes not decay well.Since the Eof a TENG is generally measured using thewith maximumdensity,would hardly affect thedensity. It seems as if once theof a TENG is determined, the maximum possibleremains constant, even if the TENG is operated by various types ofas shown in Fig.. However, humantriggers the movement of a TENG, and it can be explained by the concept of kineticand impulse.When thefrom humanis applied to a TENG, one part of a TENG contacts another part with a kineticcorresponding to the velocity. The moving part experiences an impulse and stops or rebounds depending on theof the TENG.

motion on E MP by changing the velocity, which affects the kinetic energy, and the elasticity, which affects the impulse. Fig. 2(a) MP of a TENG in the vertical contact-separation mode, and Fig. 2(b) 2(a) μm-thick perfluoroalkoxy alkane film was utilized as the triboelectric layer. The triboelectrification area was determined as 20 mm × 20 mm based on the size of a smart watch and/or an activity band. Experiments were performed at velocity ranging from 50 to 200 mm/s based on the daily motion of a finger and forearm ( MP . 15,19 6, 8376 (2015). 15. Y. Zi, S. Niu, J. Wang, Z. Wen, W. Tang, and Z. L. Wang, Nat. Commun., 8376 (2015). https://doi.org/10.1038/ncomms9376 7, 7383 (2013). 19. G. Cheng, Z. H. Lin, L. Lin, Z. L. Du, and Z. L. Wang, ACS Nano, 7383 (2013). https://doi.org/10.1021/nn403151t surface charge density σ after contact electrification, the charged plates move away from x = 0 to x = x max without any connection (i.e., open-circuit condition) (Fig. 2(a) V oc,max as the capacitance of the TENG decreases (Fig. 2(b) max , the plates are instantaneously connected (i.e., short-circuit condition), and then charges in one plate move to the other plate to balance the potential difference in the plate (Fig. 2(a) Q sc,max (Fig. 2(b) 2(a) max with decreasing capacitance of the TENG (Fig. 2(b) 2(a) 2(b) 2(c) MP of the TENG corresponding to a smart watch in the vertical contact-separation mode. We examined the effect ofon Eby changing the velocity, which affects the kineticand thewhich affects the impulse. Fig.shows the process of measuring the Eof a TENG in the vertical contact-separation mode, and Fig.shows the output voltage (black) and charge (red) corresponding to each step (i–iv) in Fig.. In the experiment, a 50-m-thick perfluoroalkoxy alkane film was utilized as the triboelectric layer. The triboelectrification area was determined as 20 mm × 20 mm based on the size of a smart watch and/or an activity band. Experiments were performed at velocity ranging from 50 to 200 mm/s based on the dailyof a finger and forearm ( supplementary material , Note 1). The details of the TENG structure and the experimental procedure are presented in Note 3 of the supplementary material . We used the instantaneous discharging TENG to measure EBriefly, after the triboelectric layers of plates are charged withdensity σ after contact electrification, the charged plates move away from x = 0 to x = xwithout any connection (i.e., open-circuit condition) (Fig.i). In this case, the voltage increases toas the capacitance of the TENG decreases (Fig.i). When the distance between two plates becomes x, the plates are instantaneously connected (i.e., short-circuit condition), and then charges in one plate move to the other plate to balance the potential difference in the plate (Fig.ii). At this time, the voltage drops to 0, and the amount of transferred charge is(Fig.ii). When the plates approach each other, they are disconnected, and inductive charges remain on the plates (Fig.iii). The voltage decreases to −V′with decreasing capacitance of the TENG (Fig.iii). Finally, the distance between two plates becomes 0, and the plates are instantaneously connected again (Fig.iv). At this moment, the potential in the TENG disappears and the charges flow back (Fig.iv). A representative V-Q plot of the instantaneous discharging TENG is shown in Fig.. The enclosed area in the V-Q plot is the Eof the TENG corresponding to a smart watch in the vertical contact-separation mode.

motion and E MP , we measured E MP by changing the velocity (kinetic energy) and elasticity (impulse). The velocity increased by a factor of 4 from 50 mm/s to 200 mm/s, and consequently, the input kinetic energy increased by a factor of 16; however, E MP did not significantly change (Fig. 2(d) MP increased by 5.5% as the spring constant increased (black squares in Fig. 2(e) MP was increased by 6.9% at 200 mm/s (blue diamonds in Fig. 2(e) MP increase, we examined the change of Q sc,max , V oc,max , and V′ max depending on the spring constant, and all of them increased with the increase in the spring constant. The detailed statistical analyses are given in Note 4 of the surface charge density increased as the spring constant increased. We predict that the high impulse caused by the high elasticity could induce the additional surface charges. To clarify the relationship betweenand E, we measured Eby changing the velocity (kineticand(impulse). The velocity increased by a factor of 4 from 50 mm/s to 200 mm/s, and consequently, the input kineticincreased by a factor of 16; however, Edid not significantly change (Fig.). On the other hand, the average Eincreased by 5.5% as the spring constant increased (black squares in Fig.) at a velocity of 50 mm/s. When the velocity increases, the impulse effect is also increased; therefore, Ewas increased by 6.9% at 200 mm/s (blue diamonds in Fig.). To identify the cause of the Eincrease, we examined the change of, anddepending on the spring constant, and all of them increased with the increase in the spring constant. The detailed statistical analyses are given in Note 4 of the supplementary material . These results imply that thedensity increased as the spring constant increased. We predict that the high impulse caused by the highcould induce the additionalcharges.

MP provides important information to determine the output energy capability of a TENG for portable and wearable devices since the output energy generated by a TENG in actual use cannot exceed E MP . The maximum possible energy obtained from Eq. ( form factor and the enhanced maximum possible energy considering the kinetic energy and impulse are listed in Table energy. The structure of TENGs was designed in the vertical contact-separation mode using the entire space of the electronic device, and the movement of TENGs was established at 1 Hz for 1 min. The theoretical maximum possible energy is sufficient to cover the stand-by energy consumption of the smart watch and activity band. The enhanced maximum possible energy by the kinetic energy and impulse corresponding to the daily motion will further extend the stand-by time. Aside from the conversion efficiency, identifying the limitation of Eprovides important information to determine the outputcapability of a TENG for portable and wearable devices since the outputgenerated by a TENG in actual use cannot exceed E. The maximum possibleobtained from Eq. ( 2 ) based on the deviceand the enhanced maximum possibleconsidering the kineticand impulse are listed in Table I . And also we predict a charging possibility in various portable and wearable devices using maximum possibleThe structure of TENGs was designed in the vertical contact-separation mode using the entire space of the electronic device, and the movement of TENGs was established at 1 Hz for 1 min. The theoretical maximum possibleis sufficient to cover the stand-byof the smart watch and activity band. The enhanced maximum possibleby the kineticand impulse corresponding to the dailywill further extend the stand-by time.

TABLE I. Maximum possible energy of a TENG in the vertical contact-separation mode and the charging capability in various portable and wearable devices.

Maximum possible energy for 1 min (J)b Increased battery usage time (min) Devicea Energy consumption for 1 min.a (J) Based on form factor Based on form factor and motion Based on form factor Based on form factor and motion Tablet (Galaxy Tab S2) 12.0 7.61 8.13 0.63 0.68 Smart phone (Galaxy S7) 8.1 2.65 2.83 0.33 0.36 Smart watch (Gear S3) 0.47 0.99 1.06 2.13 2.28 Activity band (Gear Fit 2) 0.46 0.51 0.54 1.1 1.18

energy per motion with a load (E OL ) by varying the load resistance at various velocities and spring constants in the same manner as for E MP ( OL increased with the velocity as previously reported, 20 12, 6339 (2012). 20. S. Wang, L. Lin, and Z. L. Wang, Nano Lett., 6339 (2012). https://doi.org/10.1021/nl303573d MP did not change with the velocity. Meanwhile, the maximum E OL decreased with the spring constant even if E MP increased with the spring constant owing to additional surface charges (Fig. 3 elasticity, which resulted in the high E MP , might be an obstacle to obtaining E OL . Unlike in the measurement of E MP , the current is continuously measured in E OL ; therefore, E OL is also affected by the movement of a TENG before contact. The elasticity can affect not only the impulse but also the movement before contact and the repetitive motion; 21 31, 560 (2017). 21. T. Jiang, Y. Yao, L. Xu, L. Zhang, T. Xiao, and Z. L. Wang, Nano Energy, 560 (2017). https://doi.org/10.1016/j.nanoen.2016.12.004 elasticity control, and to optimize the elasticity of a TENG for each system and purpose. To apply a TENG in portable and wearable devices, it should be used in a steady state with a specific load, rather than at an instantaneous short-circuit condition. We investigated the change of outputperwith a load (E) by varying the load resistance at various velocities and spring constants in the same manner as for E supplementary material , Note 5). The maximum Eincreased with the velocity as previously reported,while Edid not change with the velocity. Meanwhile, the maximum Edecreased with the spring constant even if Eincreased with the spring constant owing to additionalcharges (Fig.). Presumably, the highwhich resulted in the high E, might be an obstacle to obtaining E. Unlike in the measurement of E, the current is continuously measured in E; therefore, Eis also affected by the movement of a TENG before contact. Thecan affect not only the impulse but also the movement before contact and the repetitivetherefore, it is necessary to consider other methods to increase the impulse besides thecontrol, and to optimize theof a TENG for each system and purpose.

In summary, we studied the role of motion in obtaining the maximum possible energy of a TENG. The mechanical energy from human motion for 1 min was found to range from 60 mJ to 510 J, depending on the moving part of the body and velocity. We confirmed that, if the mechanical energy is entirely converted into electrical energy, the energy generated by the daily motion of an arm can sufficiently cover the energy consumption of a smart watch (Gear series) and even the stand-by energy consumption of a smart phone (Galaxy S7). We understood that it is important to increase the maximum possible energy of a TENG for utilizing it in commercial devices. We confirmed by theoretical extraction that the maximum possible energy is related with specific form factors of a TENG. Furthermore, we demonstrated the effect of human motion in an aspect of the velocity (kinetic energy) and elasticity (impulse), to increase the maximum possible energy of a TENG. The velocity did not significantly change E MP , but the elasticity increased E MP by increasing Q sc,max , V oc,max , and V′ max . The optimization of output energy of a TENG in actual use remains a task for future work because a real system has many limitations such as impedance matching, frequency control, and the stability of the structure. Nevertheless, the results of this study give insight into the design of a TENG to obtain a large amount of energy in a limited space.

mechanical energy available from human motion, the TENG structure and the experimental procedure, the output characteristics of a TENG, the statistical analyses of E MP and E OL of the TENG as a function of the load resistance. See supplementary material for theavailable from humanthe TENG structure and the experimental procedure, the outputof a TENG, the statistical analyses of Eand Eof the TENG as a function of the load resistance.