Nano Energy 76 (2020) 105066

Available online 6 July 20202211-2855/© 2020 Elsevier Ltd. All rights reserved.

Polarization-controlled PVDF-based hybrid nanogenerator for an effective vibrational energy harvesting from human foot

Dong Woo Lee a, Dong Geun Jeong a, Jong Hun Kim b,c, Hyun Soo Kim a,f, Gonzalo Murillo g, Gwan-Hyoung Lee b,c,d,e, Hyun-Cheol Song f, Jong Hoon Jung a,*

a Department of Physics, Inha University, Incheon, 22212, Republic of Korea b Department of Materials Science and Engineering, Seoul National University, Seoul, 08826, Republic of Korea c Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul, 08826, Republic of Korea d Institute of Engineering Research, Seoul National University, Seoul, 08826, Republic of Korea e Institute of Applied Physics, Seoul National University, Seoul, 08826, Republic of Korea f Center for Electronic Materials, Korea Institute of Science and Technology (KIST), Seoul, 02792, Republic of Korea g Department of Nano and Microsystems, Instituto de Microelectronica de Barcelona (IMB-CNM, CSIC), Bellaterra, 08193, Spain

A R T I C L E I N F O

Keywords: PVDF Hybrid nanogenerator Human foot Shoe insole Wireless pressure sensor network

A B S T R A C T

The effective conversion of vibrational energy from the motion of human body into electricity has been considered as one of the most promising technologies for charging portable electronic devices. Here, we report an electric polarization-controlled PVDF-based hybrid triboelectric-piezoelectric nanogenerator (TP-NG) as for an effective energy harvesting of various mechanical vibrations from human foot. The hybrid TP-NG simply consists of PVDF, Al, and acrylic, and the triboelectric NG component is vertically stacked on the piezoelectric NG component. We observed the strong electric-polarization-dependent electric power due to the modulated surface potential and negative piezoelectricity of PVDF. We also observed the in-phase power generation due to the vertical stacking of two flat NGs, irrespective of various loading rate, contact time, force, and frequency. Three hybrid TP-NGs were embedded at the forefoot, arch, and heel positions in a shoe insole. During normal walking, the shoe insole generated sufficient power to operate light-emitting diodes, which could be used in lightning at night. In addition, the insole operated a wireless pressure network, which could be used in monitoring and transmitting the pressure distribution on the foot to a cellular phone. This work provides an important step in the harvesting of random and irregular vibrational energy from the human foot, and in the realization of self- powered lightning for safety and self-powered wireless pressure monitoring system for diagnostic healthcare.

1. Introduction

The drastic increase of mobile devices in modern society has neces-sitated the development of lightweight and portable power sources [1, 2]. Traditional batteries require frequent replacement and can cause environmental pollution if not disposed of properly. Therefore, intensive research has been dedicated to scavenging wasted energy from ambient sources such as sun light, hot ground water, and wind [3,4]. Meanwhile, the human body acts as an energy reservoir with most of its thermal and mechanical energy being wasted [5–7]. Among them, the vibrational energy of footfall is reported to be the largest and recorded to several tens of watts [8,9]. Therefore, it would be a great challenge to effectively harvest the footfall energy and convert it into a portable and wearable

power source. A triboelectric nanogenerator (TENG) and piezoelectric nano-

generator (PENG) have been widely adapted to harvest the mechanical energy from human foot, because of their simplicity, cost-effectiveness, and high-efficiency [10–16]. Zhu et al. reported a flexible TENG-based power-generating shoe insole [10]. The TENG was composed of PTFE and Al with a zigzag-shaped multilayered structure. They developed a fully packaged self-lightning shoe which can power light-emitting di-odes (LEDs) during normal walking. Zhao et al. reported a piezoelectric harvester embedded in a shoe [11]. The device was composed of silicon rubber and multilayered PVDF films sandwiched by arch-shaped rib and groove. The piezoelectric harvester was located in the forefoot and heel places, and generated 0.12 mW. Rodrigues et al. reported a hybrid

* Corresponding author. E-mail address: jhjung@inha.ac.kr (J.H. Jung).

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https://doi.org/10.1016/j.nanoen.2020.105066 Received 7 April 2020; Received in revised form 26 May 2020; Accepted 4 June 2020

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triboelectric-electromagnetic-piezoelectric NG, composed of PTFE, nylon, NdFeB, magnet coil, and ZnO [12]. The hybrid NG generated a maximum power of 32 mW and turned on three green LEDs during walking. Although the above reports are innovative, any device should be comfort and have a strong resistance to sweat in order to adapt to a shoe. The device should also be simple and stable in order to harvest the random and irregular mechanical vibrational energies of the human foot. Furthermore, the device should be compact and efficient in order to generate sufficient power even in the small space of shoe.

In this paper, we report a polarization-controlled PVDF-based hybrid triboelectric-piezoelectric nanogenerator (TP-NG) in order to easily adapt to a thin shoe insole and to effectively harvest the various me-chanical vibrational energies from the human foot. The TENG compo-nent is stacked on and electrically connected to the PENG component, so that there is little time delay between two NGs and it requires only limited separation distance for operation. Due to the modulated surface potential and negative piezoelectricity of PVDF, the hybrid TP-NG shows a strong polarization-dependent power output. The up- and down- polarization of PVDF for the triboelectric NG- and piezoelectric NG- component, respectively, generates the highest power of 127 μW while down-up polarization generates the lowest power of 52 μW. The encapsulated hybrid TP-NG in shoe insole sensitively generates different output for various human motions, and stable power even after exces-sive mechanical vibrations and washing several times. Using the vibra-tional energy from normal walking, the shoe insole generates sufficient power to operate LEDs and a wireless pressure sensor network. These results should imply that the polarization-controlled PVDF-based hybrid TP-NG should be quite useful for the mechanical energy harvesting from the human foot and could play a role of portable power source for various electronic devices.

2. Experimental section

2.1. Characterization of PVDF films

Commercially available β-phase PVDF films with a thickness of 28 μm (Precision Acoustics, UK) was used to fabricate the hybrid TP-NG. The up-/down-polarization and surface potential of PVDF films were characterized using a piezoresponse force microscopy (PFM) and Kelvin probe force microscopy (KPFM) (NX10, Park Systems), respectively. For the PFM measurement, we used a Pt-coated tip with an elastic constant of 0.2 Nm� 1. The loading force was set to 3 nN, and an AC signal of 5 V and 17 kHz was applied to the tip for a lock-in modulation technique. For the KPFM measurement, we used an Au-coated conductive tip with a mechanical driving frequency of 147 kHz and an AC modulation of 1 V at 17 kHz. The work function of the Au-coated tip was calibrated using an Au plate.

2.2. Fabrication and power measurement of the hybrid TP-NG

In order to fabricate the TENG, an Al electrode was deposited on the bottom side of a PVDF film. Another Al electrode was attached to an acrylic and periodically contacted to and separated from the top side of the PVDF. In order to fabricate the PENG, Al electrodes were deposited on both sides of the PVDF film. The TENG was stacked on top of the PENG, and the bottom electrode of the TENG was connected to the top electrode of the PENG. In order to prevent sweat from entering into TP- NG, the TP-NG was encapsulated by vinyl polymer.

The performance of the hybrid TP-NG was characterized using a custom mechanical system, in which a linear motor was used to peri-odically apply and release the compressive force to the device. The electrical outputs of the TP-NG were measured using a two-channel digital phosphor oscilloscope (DSOX2002A, KEYSIGHT), a program-mable electrometer (6517, Keithley), and a low-noise current pre- amplifier (SR570, Stanford Research Systems). A bridge rectification circuit was used to convert the AC signal to DC, and a capacitor and an

Li-battery were used to store the charge. In order to maximize the power output and optimize the energy storage, we used a power management circuit (LTC3330, Linear Technology) [17,18]. A wireless pressure sensor network (RA12P, Marveldex) was used to monitor and transmit the pressure distribution on the foot to a cellular phone. The pressure distribution was displayed in real-time on a cellular phone using an Arduino program.

2.3. Finite-element computer simulations

The voltage distribution of the PVDF-based PENG was simulated using a COMSOL Multiphysics 5.4 software. A tridimensional model was developed, composed of a sandwiched PVDF layer, Al electrodes, and acrylics. We assumed a piezoelectric coefficient of � 30 pC/N and the relative dielectric constant of 10 for PVDF [19], and applied compres-sive force of 40 N. Electrostatic physics and a stationary study were used to compute the electric field and potential generated by the piezoelectric charges for a fine tetrahedral mesh.

3. Results and discussion

3.1. Power generation mechanism of TP-NG

Fig. 1a shows a schematic illustration and a photograph of our hybrid TP-NG consisting of two PVDF films, three Al electrodes, and two acrylic supports. The upper TENG component is stacked on and electrically connected to the lower PENG component in the hybrid NG [20]. This simple layout allows the selection of the polarization directions of the PVDF with simultaneous power generation from both NGs without time-delay.

Fig. 1b shows a schematic diagram of the power generation mecha-nism of the hybrid TP-NG during one cycle of vibration. When an external force is applied, triboelectric positive and negative charges are induced in Al and PVDF, respectively, after the contact [21]. At the almost same time, a piezoelectric potential is generated due to the deformation of PVDF. In order to balance the piezoelectric potential, the piezoelectric current flows along the external load from the bottom to the top Al electrode. When the external force is removed, the piezo-electric potential is disappeared due to the recovery of PVDF, which causes the piezoelectric current to flow reversely. At the almost same time, the triboelectric pairs of Al and PVDF separate from each other. In order to balance the triboelectric potential, the triboelectric current flows from top to bottom electrode. When an external force is applied again, the triboelectric current flows reversely. Thus, charge-flow occurs in contact and deformation during the approaching Al to PVDF (blue dashed box), and in recovery and separation during the receding Al from PVDF (red dashed box). Consistently, four current peaks are clearly discernible in the hybrid TP-NG when the force is intermittently applied and removed during one cycle of vibration (Fig. S1).

3.2. Power generation characteristics of each TENG and PENG

The PVDF film can be electrically polarized in two directions, i.e., up and down. Here, the up- (down-) polarization corresponds to the posi-tive (negative) and negative (positive) bound charges at the top and bottom Al electrode sides, respectively. Fig. 2a and c shows the electric polarization-dependent open-circuit voltage and short-circuit current of the PVDF-based TENG and PENG, respectively. In the TENG, the voltage and current outputs for up-polarization are significantly larger than those for down-polarization [22]. However, the voltage and current signs are the same regardless of the polarization direction. In order to investigate such behaviors, we show the surface potentials of PVDF and Al in Fig. 2b [23–25]. The surface potential of PVDF is clearly modulated depending on the polarization directions. In particular, the surface po-tential of the up-polarized PVDF is significantly smaller than that of Al, and that of the down-polarized PVDF is slightly larger than that of Al.

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The polarization-dependent output behaviors in a PVDF-based TENG can be understood as the induced two different charges during contact. As shown in Fig. S2, positive and negative piezoelectric charges are induced in the up- and down-polarized PVDF, respectively, when the PVDF is compressed during the contact. At the same time, positive and negative triboelectric charges are induced in Al and PVDF, respectively. In order to compensate the opposite piezoelectric charges, large and small amount of triboelectric charges are induced in up- and down-polarized PVDF, respectively. Therefore, a large and small current flow in the up- and down-polarized PVDF-based TENG, respectively, from the bottom to the top Al electrode, i.e. same directions.

In the PENG, the voltage and current signs for up-polarization are opposite to those for down-polarization. However, the voltage and current outputs are almost the same regardless of the polarization di-rection. In order to understand such behaviors, we consider the negative piezoelectricity of PVDF [26]. When the PVDF is pressed, the same amounts of positive and negative surface charges are generated in the up- and down-polarized PVDF surface, respectively. Therefore, current flows from the top to the bottom electrode in the up-polarized PVDF and reversely flows in the down-polarized PVDF. Opposite current-flow was supported by a finite-element COMSOL simulation for opposite voltage distribution in each polarization (Fig. 2d).

3.3. Polarization-dependent power generation in hybrid TP-NG

For the effective conversion of mechanical vibration into electricity in a hybrid TP-NG, in-phase power generation from each TENG and PENG is inevitable [27]. Fig. 3a and Fig. S3 show the voltage outputs from the TENG and PENG with respect to time for various loading rates and contact times. Compared to the peaks from the TENG, the peaks from the PENG are rather broad, probably due to the indirect application and removal of an external force in our hybrid TP-NG geometry. Nevertheless, the voltage peak positions from the TENG and PENG are almost coincided for a wide range of loading rate (800–1200 N/s) and contact time (0.1–0.6 s). In addition, the peak positions from the TENG are almost coincided with those of the PENG for wide range of forces (10–40 N) and frequencies (1.0–2.5 Hz) (Fig. 3b,c and Figs. S4a and b). Considering the random and irregular motions of the human body, these results should imply that a vertically stacked hybrid TP-NG should be

effective for scavenging vibrational energy from the foot. After confirming the in-phase power generation from each TENG and

PENG component, we investigate the electric polarization-dependent open-circuit voltage and short-circuit current of hybrid TP-NGs in Fig. 3d and e, respectively. We want to emphasize that the hybrid TP-NG generates higher outputs than the single TENG as clearly shown in Fig. S5. For simplicity, we denote the polarization direction of PVDF in the hybrid TP-NG as the TENG first and the PENG second. For example, up-down corresponds to the up-polarization of the TENG and the down- polarization of the PENG. Clearly, the electric polarization of PVDF strongly affects the voltage and current of a TP-NG. The hybrid TP-NG with up-down polarization generates the largest outputs of 180 V and 5.3 μA, whereas that with down-up polarization does the smallest out-puts of 100 V and 1.8 μA. Such differences should be originated from the polarization-dependent surface potential and negative piezoelectricity of PVDF, as confirmed in Fig. 2. Note that, the hybrid TP-NG generates stable outputs for excessive mechanical vibrations and for wide range of external forces (Fig. S6).

The electric polarization of PVDF also affects the power generation of a hybrid TP-NG. Fig. 3f shows the external load-dependent electric power of a hybrid TP-NG. For all electric polarizations, the hybrid TP- NGs show the maximized power at 1 MΩ. The power is largest for the up-down polarization of 127 μW and smallest for the down-up polari-zation of 52 μW. Similarly, it takes the shortest time of 20 s for up-down polarized TP-NG and the longest time of 50 s for down-up polarization in order to charge a 1 μF capacitor up to 7 V (Fig. 3g).

3.4. Hybrid TP-NG embedded power generation shoe insole

The up-down polarized hybrid TP-NG was installed in a shoe insole in order to demonstrate the mechanical energy harvesting from the motion of the human body. Fig. 4a shows a schematic diagram and a digital photograph of a packaged, power-generating shoe insole. Three TP-NGs were connected in parallel and positioned at the forefoot, arch, and heel, where the pressure was highly concentrated [28]. A sponge spacer was inserted in-between the top Al electrode and the other PVDF/Al/PVDF/Al layers to act as a spring.

Fig. S7 and Fig. 4b show the rectified open-circuit voltage and short- circuit current of a power generating shoe insole, respectively, for

Fig. 1. (a) A schematic diagram and a digi-tal photograph, and (b) power generation mechanism of a hybrid triboelectric- piezoelectric nanogenerator (TP-NG). In (a) and (b), the up- and down-polarizations of PVDF are denoted by green and cyan, respectively. In (b), the rainbow color rep-resents the induced piezoelectric potential when the PVDF is deformed. The black and red arrows represent the triboelectric and piezoelectric currents, respectively. The blue and red dashed boxes include the processes for the Al approaching to and receding from the PVDF/Al/PVDF/Al layers, respectively.

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various human motions, including walking, jogging, running, and jumping. The pressure at the foot for a 70 kg adult is the same as 32 kPa. The output voltage and current are different according to different fre-quency of walking (1 Hz), jogging (1.6 Hz), running (2.3 Hz), and jumping (1.8 Hz) [29,30]. High-impact and high-frequency motions, such as running, generate relatively large currents, whereas low-impact and low-frequency motions, such as walking, generate relatively small currents.

The TP-NGs embedded shoe insole was encapsulated by a vinyl polymer in order to block sweats from foot (Fig. S8). As shown in Fig. 4c, the encapsulated TP-NG generated stable output even after washing 10 times. This result should imply that the encapsulation could be a simple and reliable method for a NG-embedded shoe insole for waterproof

without losing comfortability and light-weightiness.

3.5. Operation of light-emitting diodes and a wireless sensor network using vibrations on the foot

In Fig. 5a, we schematically illustrate the proposed application of our hybrid TP-NG-embedded shoe insole for self-powered lightning and self- powered pressure sensor network. During the various human motions, the hybrid TP-NG effectively converts the vibrational energy of the human foot into electricity. The harvested electricity can operate LEDs, which is useful to people for outdoor activity at night [31]. The har-vested electricity can also operate a wireless sensor network, which is useful to patients for pressure monitoring on the foot [32].

Fig. 2. Open-circuit voltage and short-circuit current of each (a) PVDF-based triboelectric nanogenerator (TENG) and (c) PVDF-piezoelectric nanogenerator (PENG) components in hybrid TP-NG. (b) Surface potentials of the up- and down-polarized PVDF, and Al electrode. In the inset of (b), the surface potential images are shown for each PVDF (area ¼ 360 � 360 nm2). (d) Finite-element COMSOL simulation of voltage distributions for up- and down-polarized PVDF-based PENGs.

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To demonstrate such feasibility, we assembled hybrid TP-NGs, LEDs, and pressure sensors in a shoe insole, and assembled rectifiers, a battery, a switch, and power management circuit into a breadboard and wrapped in an ankle (Fig. 5b). The white LEDs embedded in shoe are brightly lightened at day- and night-time during normal walking (Fig. 5c and Movie S1). The 40 mAh Li-battery operates the wireless sensor network for 2 min after the charging for 50 min (Fig. 5d). The pressure is con-verted into resistance in a pressure sensor (Fig. S9). Fig. 5e shows the wirelessly received pressure distribution and corresponding motional images of the foot. For each normal walking step, the pressure on the foot distinctively changes in terms of the magnitude and distribution. Such pressure changes were immediately transmitted to a cellular phone

using an RF-transceiver (Movie S2). Increased number of TP-NGs in a shoe insole should generate higher power, which enables the operation of more pressure sensors for more accurate measurement of pressure distribution on foot.

Supplementary data related to this article can be found at https ://doi.org/10.1016/j.nanoen.2020.105066.

3.6. Merits of the hybrid TP-NG for the effective harvesting of footfall energy

There are several merits of a hybrid TP-NG for mechanical energy harvesting from the human foot in terms of the simple device structure,

Fig. 3. Voltage output behaviors of TENG (black lines) and PENG (red lines) with respect to time for (a) loading rate of 1200 N/s, (b) force of 20 N, and (c) frequency of 2.5 Hz. (d) Rectified open-circuit voltage and (e) short-circuit current of hybrid TP-NGs. (f) External load-dependent power output and (g) charging behavior of variously polarized hybrid TP-NGs.

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in-phase and large power generation, waterproof, and healthcare ap-plications. First, the hybrid TP-NG is simply composed of PVDF, Al, and acrylic, and has a vertically stacked structure. The minimal use of ma-terial species would be beneficial for cost reduction and simplified manufacturing. The vertically stacked structure would be highly sus-ceptible to mechanical impulses on the foot and easily placed in highly pressure-concentrated positions, such as the heel. Second, the power output is constructively added from each TENG and PENG, and is maximized for up-down polarization. The in-phase power generation of our hybrid TP-NG could be utilized for random and irregular mechanical vibrations, which is in sharp contrast to the arch-shaped hybrid NG [27]. The up-down polarized hybrid TP-NG fully utilizes the modulated sur-face potential and negative piezoelectricity of PVDF, which cannot be adapted in a single PVDF-based NG [25]. In addition, the TP-NG requires relatively short separation distance for operation, which could be applicable in a thin shoe insole. Third, the hybrid TP-NG-embedded shoe insole can generate stable power even in a humid condition of shoe due to sweats. Simple encapsulation of the TP-NG using vinyl polymer may

provide a chance to reuse the power generating insole after washing. Fourth, the hybrid TP-NG-embedded shoe insole generates sufficient power to operate a wireless pressure sensor network. The human foot can have a range of illnesses, such as plantar fasciitis, particularly for older people. During normal walking, the hybrid TP-NG can monitor and transmit the pressure distribution on the foot to a cellular phone. The real-time or stored pressure information should be quite useful to a hospital doctor in order to treat illnesses on the foot.

4. Conclusions

In summary, we fabricate a PVDF film-based hybrid TP-NG in order to effectively harvest the random and irregular vibrational energy from the human foot. The hybrid TP-NG simply consisted of PVDF polymer film, Al electrode, and acrylic supporter, and had a vertical stack structure. This simple layout is beneficial for reduced manufacturing, high-susceptibility to mechanical impulse on the foot, and easy instal-lation in a thin shoe insole. In addition, the layout provides in-phase

Fig. 4. (a) A schematic diagram and a digital photograph of hybrid TP-NGs embedded in a shoe insole. The TP-NGs were located at the forefoot, arch, and heel positions. The sponge spacer acts as a spring for contact and separation between the Al and the PVDF/Al/PVDF/Al layers. (b) Rectified short-circuit currents for various human motions of walking, jogging, running, and jumping. (c) Washing times dependent rectified open-circuit voltages of encapsulated TP-NG.

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power generation from each TENG and PENG during one cycle of vi-bration. The up-down polarized hybrid TP-NG generates the largest power of 127 μW due to the modulated surface potential and negative piezoelectricity of PVDF. Three hybrid TP-NGs were installed in the forefoot, arch, heel positions of the shoe insole, and encapsulated by a vinyl polymer. The hybrid TP-NG embedded shoe insole has a strong resistance to water and sensitively generates output powers from various human motions of different loading rates, contact times, forces, and frequencies. In addition, the shoe insole generates sufficient power to lighten LEDs and operate a wireless pressure sensor network. This work

demonstrates that the polarization-controlled hybrid TP-NG with a vertically stacked structure can be used in self-detection of human body motion for safety, self-powering LED for lightning at night, and self- monitoring of pressure distribution for healthcare.

Declaration of competing interest

This work has not been published elsewhere and is not under consideration by other journals. All authors approved this publication and declare no competing financial interest.

Fig. 5. (a) Proposed application of hybrid TP-NG for lightning LEDs and wireless transmission of foot pressure to a cellular phone. (b) A circuit diagram and digital photographs of the hybrid TP-NGs, rectifier, Li-battery, switch, power management circuit (PMC), LEDs, and pressure sensor network. (c) A digital photograph of powered LEDs in the shoe during the day- and night-time. (d) Charging and discharging curve of a 40 mAh Li-battery. (e) Wirelessly received pressure distributions in a cellular phone for each step of normal walking.

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CRediT authorship contribution statement

Dong Woo Lee: Conceptualization, Methodology, Visualization, Data curation, Writing - original draft. Dong Geun Jeong: Methodol-ogy, Visualization, Data curation. Jong Hun Kim: Methodology, Vali-dation, Data curation. Hyun Soo Kim: Methodology, Validation, Data curation. Gonzalo Murillo: Methodology, Validation, Data curation. Gwan-Hyoung Lee: Supervision, Writing - original draft. Hyun-Cheol Song: Supervision, Writing - original draft. Jong Hoon Jung: Concep-tualization, Supervision, Writing - review & editing.

Acknowledgments

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2019R1F1A1058514 and 2016M3A7B4910940). G-H.L. also acknowledges the support from the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) (20173010013340). H.S.K. and H.-C.S. would like to acknowledge the support from the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CAP-17-04-KRISS). G. M. appreciates the financial support from La Caixa Foundation under the Junior Leader Retaining Fellowship (LCF/BQ/PR19/11700010) and EnSO project, accepted for funding within the Electronic Components and Systems For European Leadership Joint Undertaking in collabora-tion with the European Union’s H2020 Framework Programme (H2020/ 2014–2020) and National Authorities, under grant agreement no. 692482.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi. org/10.1016/j.nanoen.2020.105066.

References

[1] M.S. Dresselhaus, I.L. Thomas, Alternative energy technologies, Nature 414 (2001) 332–337.

[2] Z.L. Wang, Self-powered nanotech, Sci. Am. 298 (2008) 82–87. [3] D.S. Ginley, D. Cahen, Fundamentals of Materials for Energy and Environmental

Sustainability, Cambridge University Press, New York, 2011. [4] G. Boyle, Renewable Energy: Power for a Sustainable Future, Oxford University

Press, Oxford, 2012. [5] R.B. Barnes, Thermography of the human body, Science 140 (1963) 870–877. [6] M. Thielen, L. Sigrist, M. Magno, C. Hierold, L. Benini, Human body heat for

powering wearable devices: from thermal energy to application, Energy Convers. Manag. 131 (2017) 44–54.

[7] C. Dagdeviren, Z. Li, Z.L. Wang, Energy harvesting from the animal/human body for self-powered electronics, Annu. Rev. Biomed. Eng. 19 (2017) 85–108.

[8] T. Starner, Human-powered wearable computing, IBM Syst. J. (1996) 618–629. [9] L. Huang, S. Lin, Z. Xu, H. Zhou, J. Duan, B. Hu, J. Zhou, Fiber-based energy

conversion devices for human-body energy harvesting, Adv. Mater. (2019) 1902034.

[10] G. Zhu, P. Bai, J. Chen, Z.L. Wang, Power-generating shoe insole based on triboelectric nanogenerators for self-powered consumer electronics, Nano Energy 2 (2013) 688–692.

[11] J. Zhao, Z. You, A shoe-embedded piezoelectric energy harvester for wearable sensors, Sensors 14 (2014) 12497–12510.

[12] C. Rodrigues, A. Gomes, A. Ghosh, A. Pereira, J. Ventura, Power–generating footwear based on a triboelectric–electromagnetic–piezoelectric hybrid nanogenerator, Nano Energy 62 (2019) 660–666.

[13] H. Kalantaran, M. Sarrafzadeh, Pedometers without batteries an energy harvesting shoe, IEEE Sensor. J. 16 (2016) 8314–8321.

[14] Z. Zhang, K. Du, X. Chen, C. Xue, K. Wang, An air–cushion triboelectric nanogenerator integrated with stretchable electrode for human–motion energy harvesting and monitoring, Nano Energy 13 (2018) 108–115.

[15] R. Zhang, M. Hummelgård, J. €Ortegrena, M. Olsena, H. Anderssonb, H. Olin, Interaction of the human body with triboelectric nanogenerators, Nano Energy 57 (2019) 279–292.

[16] G.Q. Gu, C.B. Han, J.J. Tian, C.X. Lu, C. He, T. Jiang, Z. Li, Z.L. Wang, Antibacterial composite film-based triboelectric nanogenerator for harvesting walking energy, ACS Appl. Mater. Interfaces 9 (2017) 11882–11888.

[17] H. Qin, G. Cheng, Y. Zi, G. Gu, B. Zhang, W. Shang, F. Yang, J. Yang, Z. Du, Z. L. Wang, High energy storage efficiency triboelectric nanogenerators with

unidirectional switches and passive power management circuits, Adv. Funct. Mater. 28 (2018) 1805216.

[18] H. Qin, G. Gu, W. Shang, H. Luo, W. Zhang, P. Cui, B. Zhang, J. Guo, G. Cheng, Z. Du, A universal and passive power management circuit with high efficiency for pulsed triboelectric nanogenerator, Nano Energy 68 (2020) 104372.

[19] https://www.acoustics.co.uk/pal/wp-content/uploads/2015/11/Propertie s-of-poled-PVDF.pdf.

[20] S. Chen, X. Tao, W. Zeng, B. Yang, S. Shang, Quantifying energy harvested from contact–mode hybrid nanogenerators with cascaded piezoelectric and triboelectric units, Adv. Energy Mater. 7 (2017) 1601569.

[21] A.F. Diaz, R.M. Felix-Navarro, A semi-quantitative tribo-electric series for polymeric materials: the influence of chemical structure and properties, J. Electrost. 62 (2004) 277–290.

[22] A. Sutka, K. Malnieks, A. Linarts, M. Timusk, V. Jurkans, I. Gornevs, J. Blums, A. Berzina, U. Joost, M. Knite, Inversely polarised ferroelectric polymer contact electrodes for triboelectric–like generators from identical materials, Energy Environ. Sci. 11 (2018) 1437–1443.

[23] W. Seung, H.-J. Yoon, T.Y. Kim, H. Ryu, J. Kim, J.H. Lee, J.-H. Lee, S. Kim, Y. K. Park, Y.J. Park, S.W. Kim, Boosting power–generating performance of triboelectric nanogenerators via artificial control of ferroelectric polarization and dielectric properties, Adv. Energy Mater. 7 (2017) 1600988.

[24] H.S. Kim, D.Y. Kim, J.E. Kim, J.H. Kim, D.S. Kong, Gonzalo Murillo, G.H. Lee, J. Y. Park, J.H. Jung, Ferroelectric-polymer-enabled contactless electric power generation in triboelectric nanogenerators, Adv. Funct. Mater. 29 (2019) 1905816.

[25] P. Bai, G. Zhu, Y.S. Zhou, S. Wang, J. Ma, G. Zhang, Z.L. Wang, Dipole–moment–induced effect on contact electrification for triboelectric nanogenerators, Nano Res. 7 (2014) 990–997.

[26] I. Katsouras, K. Asadi, M. Li, T.B. van Driel, K.S. Kjær, D. Zhao, T. Lenz, Y. Gu, P.W. M. Blom, D. Damjanovic, M.M. Nielsen, D.M. de Leeuw, The negative piezoelectric effect of the ferroelectric polymer poly(vinylidene fluoride), Nat. Mater. 15 (2016) 78–84.

[27] W.-S. Jung, M.-G. Kang, H.G. Moon, S.-H. Baek, S.-J. Yoon, Z.-L. Wang, S.-W. Kim, C.-Y. Kang, High output piezo/triboelectric hybrid generator, Sci. Rep. 5 (2015) 9309.

[28] P.J. Bennett, L.R. Duplock, Pressure distribution beneath the human foot, J. Am. Podiatr. Med. Assoc. 83 (1993) 674–678.

[29] S. Jung, J. Lee, T. Hyeon, M. Lee, D.-H. Kim, Fabric–based integrated energy devices for wearable activity monitors, Adv. Mater. 26 (2014) 6329–6334.

[30] Y. Guo, X.S. Zhang, Y. Wang, W. Gong, Q. Zhang, H. Wang, J. Brugger, All–fiber hybrid piezoelectric–enhanced triboelectric nanogenerator for wearable gesture monitoring, Nano Energy 48 (2018) 152–160.

[31] P. Bai, G. Zhu, Z.-H. Lin, Q. Jing, J. Chen, G. Zhang, J. Ma, Z.L. Wang, Integrated multilayered triboelectric nanogenerator for harvesting biomechanical energy from human motions, ACS Nano 4 (2013) 3713–3719.

[32] K.Y. Lee, H.-J. Yoon, T. Jiang, X. Wen, W. Seung, S.-W. Kim, Z.L. Wang, Fully packaged self–powered triboelectric pressure sensor using hemispheres–array, Adv. Energy Mater. 6 (2016) 1502566.

Dong Woo Lee is a master’s course student at department of Physics, Inha University under the supervision of Prof. Jong-Hoon Jung. He received bachelor’s degree (2019) from Inha University. His current research interests are piezoelectric and triboelectric nanogenerators and their application to human- body mechanical energy.

Dong Geun Jeong is a bachelor’s course student at department of Physics, Inha University under the supervision of Prof. JongHoon Jung. His current research interests are mechanical energy harvesting using piezoelectric and triboelectric nanogenerators.

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Jong Hun Kim is a research professor in Department of Mate-rials Science and Engineering at Seoul National University. He earned his Ph.D. in Physics with scanning probe microscopy related study on local electrical properties of thin dielectric materials at Seoul National University. After graduation in 2009, he worked as a research engineer in Samsung electronics System LSI. After he moved to Korea Advanced Institute of Science and Technology as a postdoctoral fellow in 2012, he studied local tribological/electrical properties of either thin films or low-dimensional materials. After joining to Prof. Gwan- Hyoung Lee’s group in 2016, he has been carrying out surface studies to reveal local electrical, mechanical properties of two- dimensional materials.

Hyun Soo Kim is a Ph.D. candidate at department of Physics, Inha University under the supervision of Prof. JongHoon Jung. He is also a researcher at Center for Electronic Materials, KIST, under the supervision of Dr. Hyun-Cheol Song. His current research interest is a ferroelectric material based mechanical energy harvesting.

Gonzalo Murillo received his Electronic Engineering Degree (Bsc and MSc) from the Universidad de Granada (Spain) in 2007. He obtained his MSc in Micro and Nanoelectronics and his PhD from the Universitat Aut�onoma de Barcelona (UAB) in 2008 and 2011, respectively. He was a researcher at the Nanotech-DTU (Denmark), MINATEC (CEA-Leti, France), In-ternational Iberian Nanotechnology Laboratory (INL, Portugal) and Georgia Institute of Technology (Atlanta, GA, USA). Now, he is research fellow and junior leader at the Microelectronics Institute of Barcelona (IMB-CNM, CSIC). His current research field is the piezoelectric and triboelectric energy harvesting for self-powered flexible implantable and wearable devices and biological applications.

Gwan-Hyoung Lee is associate professor in Department of Materials Science and Engineering at Seoul National University. He received Ph.D. in Materials Science and Engineering from Seoul National University in 2006. During graduate school, he worked at University of Illinois at Urbana-Champaign as a visiting scholar in 2002. After graduation, he joined Samsung electronics as a senior engineer, developing LCD backlight unit and OLED devices. He moved to Columbia University as a postdoctoral research scientist in 2010. His research activities include the investigation of fundamental properties and syn-thesis of two-dimensional (2D) materials and van der Waals heterostructure devices for electrical and optical applications.

Hyun-Cheol Song is a senior research scientist in Korea Insti-tute of Science and Technology (KIST) after joined in 2006. He received the B.S. and M.S. degree in Materials Science and Engineering from Korea University, 2004 and 2006, respec-tively. In 2017, he received Ph.D. in the same field from Vir-ginia Tech. His research interests are now on, MEMS devices, nanostructured piezoelectric materials, and various energy harvesting devices including piezoelectric self-tuning har-vesters, triboelectric nanogenerators, and magneto- thermoelectric generators.

Jong Hoon Jung is a Professor in Department of Physics at Inha University. He received his Ph. D. from Seoul National Uni-versity in Department of Physics in 2000. After working as a postdoctoral researcher at Seoul National University and Spin superstructure ERATO project in Japan Science and Technol-ogy, he joined the Department of Physics, Inha University in 2004. His recent research interest is focused on piezoelectric/ pyroelectric/triboelectric nanogenerators in ferroelectric nano materials, and functional devices in flexible transition metal oxide films.

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