Potential role of motion for enhancing maximum output energy of triboelectric nanogenerator

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 since the 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 In terms of the industry, a TENG is advantageous in that it features cheap, lightweight, flexible, and biocompatible materials,^5,6 in contrast to other mechanical energy harvesters such as electromagnetic,¹ piezoelectric,² and pyroelectric generators.³ This advantage makes TENG one of the most likely candidates for additional energy supplies of commercial portable and wearable devices. Recently, the output energy of TENGs could operate various sensor units,^7–9 as well as it could charge portable and wearable devices.^10,11 However, the output 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.

Generally, there are two main approaches to increase the output 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 The second approach is to transfer as much 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.

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.

To utilize a TENG as an auxiliary power source for small devices, we need to ensure that the mechanical energy from human motion can cover the energy consumption of the devices. Figs. 1(a) and 1(b) show the 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.

According to recent research, the “maximum possible energy per one motion” (E_MP) of a TENG can be defined as follows:¹⁵ 

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 $𝑉′$_max¹⁶ (supplementary material, Note 2),

where d₀ denotes $∑ i d i / ε i$⁠, d the dielectric thickness, ε the dielectric constant, S the triboelectrification area, x_max the maximum displacement, ε₀ the vacuum permittivity, and σ the surface charge density.

According to the derived Equation (2), the E_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 Since the E_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). However, human motion triggers the movement of a TENG, and it can be explained by the concept of kinetic energy and impulse.¹⁸ When the 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.

We examined the effect of motion on E_MP by changing the velocity, which affects the kinetic energy, and the elasticity, which affects the impulse. Fig. 2(a) shows the process of measuring the E_MP of a TENG in the vertical contact-separation mode, and Fig. 2(b) shows the output voltage (black) and charge (red) corresponding to each step (i–iv) in Fig. 2(a). In the experiment, a 50-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 (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 E_MP.^15,19 Briefly, after the triboelectric layers of plates are charged with 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) i). In this case, the voltage increases to V_oc,max as the capacitance of the TENG decreases (Fig. 2(b) i). When the distance between two plates becomes x_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) ii). At this time, the voltage drops to 0, and the amount of transferred charge is Q_sc,max (Fig. 2(b) ii). When the plates approach each other, they are disconnected, and inductive charges remain on the plates (Fig. 2(a) iii). The voltage decreases to −V′_max with decreasing capacitance of the TENG (Fig. 2(b) iii). Finally, the distance between two plates becomes 0, and the plates are instantaneously connected again (Fig. 2(a) iv). At this moment, the potential in the TENG disappears and the charges flow back (Fig. 2(b) iv). A representative V-Q plot of the instantaneous discharging TENG is shown in Fig. 2(c). The enclosed area in the V-Q plot is the E_MP of the TENG corresponding to a smart watch in the vertical contact-separation mode.

To clarify the relationship between 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)). On the other hand, the average E_MP increased by 5.5% as the spring constant increased (black squares in Fig. 2(e)) at a velocity of 50 mm/s. When the velocity increases, the impulse effect is also increased; therefore, E_MP was increased by 6.9% at 200 mm/s (blue diamonds in Fig. 2(e)). To identify the cause of the 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 supplementary material. These results imply that 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.

Aside from the conversion efficiency, identifying the limitation of E_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. (2) based on the device form factor and the enhanced maximum possible energy considering the kinetic energy and impulse are listed in Table I. And also we predict a charging possibility in various portable and wearable devices using maximum possible 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.

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 output 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 (supplementary material, Note 5). The maximum E_OL increased with the velocity as previously reported,²⁰ while E_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). Presumably, the high 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 therefore, it is necessary to consider other methods to increase the impulse besides the elasticity control, and to optimize the elasticity of 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.

See supplementary material for the 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.