Small-Sized, Lightweight, and Flexible Triboelectric Nanogenerator

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Small-Sized, Light-Weight and Flexible Triboelectric Nanogenerator Enhanced by PTFE/PDMS Nanocomposite Electret Zebin Li, Hua Yang Li, Youjun Fan, Lu Liu, Yang Hui Chen, Chi Zhang, and Guang Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04321 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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ACS Applied Materials & Interfaces

Small-Sized, Light-Weight and Flexible Triboelectric Nanogenerator Enhanced by PTFE/PDMS Nanocomposite Electret Ze Bin Li†,§,‖,⊥, Hua Yang Li ‡,⊥, Youjun Fan†,§,‖, Lu Liu†,§,‖, Yang Hui Chen†,§,‖, Chi Zhang*,†, §,

†CAS

Guang Zhu*,†,‡,§

Center for Excellence in Nanoscience, Beijing Key Laboratory of Micro-Nano Energy and

Sensor, Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China ‡New

Materials Institute, Department of Mechanical, Materials and Manufacturing Engineering, University of Nottingham Ningbo China, Ningbo 315100, China

§School

of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, China

‖Institute

of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

KEYWORDS: energy harvesting, triboelectric nanogenerator, corona charging, nanocomposite, flexible devices

ABSTRACT:

The rapid development of flexible and wearable electronics calls for a sustainable solution of power supply. In recent years, the energy-harvesting triboelectric nanogenerator (TENG) has

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attracted increasing intentions due to its sustainability, flexibility and versatility. However, achieving both high electric output and flexibility at the same time remains to be a challenge. In this work, we reported a corona charging enhanced flexible triboelectric nanogenerator (EFTENG) to harvest mechanical energy from human motions. The EF-TENG relied on the repeated contacts between a PTFE/PDMS nanocomposite electret and a nanofiber/AgNWs electrode on arrayed silicone pyramids. When the EF-TENG (3.5 × 3.5 cm2) was pressed, the open-circuit voltage (Voc), the short-circuit current (Isc) and the power density could reach 275 V, 9.5 μA and 802.31 mW/m2, respectively. The Voc of the EF-TENG was improved by 244% compared to the device of which the electret was not corona charged. Major factors that affected the electric output of the EF-TENG were discussed, including the height of the pyramids, the configuration of the pyramids array, and the properties of the electret nanocomposite. The EF-TENG only had an overall thickness of 1.3 mm and a weight of 1.7 g, making it especially suitable to be attached onto human body for harvesting mechanical energy from biomechanical motions.

INTRODUCTION With the advancement of flexible and wearable electronics, many emerging applications have attracted extensive attentions.1-10 However, due to their portability, the energy supply for this kind of devices has become a major concern. Energy harvesting technologies that convert energy from ambient environment have attracted increasing attentions in recent years as they can replenish the power for small electronics, which potentially enables prolonged and even sustainable operation. Especially recently, triboelectric nanogenerators (TENGs), as one of the promising techniques for harvesting mechanical energy, have shown great advantages such as high electric output, flexibility, and light weight.11-22 Many researchers have focused on improving the electric output


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of the TENGs by developing new materials or optimizing structural designs.23-31 For example, an arch-shaped TENG that had silicone micro-pyramids on its surface was reported, and its output voltage from a unit area reached 27.38 V/cm2.32 Besides, a frequency-multiplication TENG made of silicone and aluminum was developed, and an output voltage from a unit area of 29.06 V/cm2 was achieved.

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In principle, the energy generation of the TENG relied on the relative

displacement between two surfaces.34-36 As a result, it became challenging to obtain both high electric output density and flexibility at the same time. This tradeoff is one of the major issues that needs to be addressed. Therefore, it is highly desired to develop a high-performance TENG that is small-sized, light-weight, and still highly flexible. In this work, we reported an enhanced flexible triboelectric nanogenerator (EF-TENG) enabled by a corona-charged nanocomposite electret and a nanofiber/AgNWs electrode. A pair of contacted materials were the nanocomposite and the flexible nanofiber/AgNWs electrode that was conformally bonded on an array of silicone pyramids. The EF-TENG could be triggered by different mechanical stimuli. When being pressed, the open-circuit voltage (Voc), the output charge, and short-circuit current (Isc) were found to be 275 V, 90 nC, and 9.5 μA , respectively. This corresponded to an output voltage from a unit area of 30.56 V/cm2, which was superior to the stateof-the-art reports on flexible TENGs. Compared to the TENG that was not treated by the corona charging, the Voc of the EF-TENG could be enhanced by 244%. When being bent, the EF-TENG could produce a Voc of 85 V and an Isc of 140 nA with a curvature of 23.87 m-1. Major factors that affected the electric output of the EF-TENG were systematically investigated, including the height of the pyramids, the configuration of the pyramids array, and the properties of the electret nanocomposite. The EF-TENG only had an overall thickness of 1.3 mm and a weight of 1.7 g, making it especially suitable to be attached onto human body for harvesting mechanical energy


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from human motions. This work presented a novel design in materials for energy harvesting techniques, and showed huge potential in improving the electric output performance of TENG. RESULTS AND DISCUSSION The EF-TENG was composed of two separate parts, as drawn in the schematic diagram in Figure 1a. The upper part consisted of three layers, i.e., a conductive textile, a PTFE membrane and a PTFE/PDMS nanocomposite, which were stacked up in sequence. The lower layer was comprised of PDMS-based pyramid arrays that were in conformal bonding with a flexible nanofibers/AgNWs electrode. The cross-sectional scanning electron microscopy (SEM) image in Figure 1b clearly shows the layer-by-layer structure of the upper part. A magnified view of the nanocomposite layer in Figure 1c illustrates that the PTFE nanoparticles with an average diameter of 200 nm were uniformly distributed in the PDMS matrix. The top-down view of the lower part morphology is shown in Figure 1d. The nanofibers/AgNWs electrode was conformally attached to the pyramidstructured substrate because the electrode was ultra-soft and mechanically compliant, and a photograph of the electrode was shown in Figure S1 (Supporting Information). The surface morphology of the pyramid-structured substrate was barely affected by the coverage of the flexible electrode, as presented in Figure S2 (Supporting Information). The thermoplastic polyurethanes (TPU)-based nanofibers and the AgNWs were tangled together to form a homogeneous conductive network, which is clearly shown in the enlarged SEM image in Figure 1e.37 Figure 1f schematically shows the basic fabrication process of the EF-TENG. For the upper part, it was constructed by attaching the PTFE membrane onto the conductive textile and then blade-coating the PTFE/PDMS nanocomposite onto the PTFE membrane. Before the PDMS was complete cured, corona charging was conducted on the nanocomposite. For the lower part, the nanofibers/AgNWs electrode was fabricated by electrospinning TPU solution and electrospraying AgNWs dispersion


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simultaneously. Then the flexible electrode was spread onto a pyramid-structured substrate based on PDMS. Finally, the two separate parts were assembled into a complete EF-TENG with the edges taped together. The fabrication process was discussed in details in the Experimental Section. The photograph of a bent EF-TENG is exhibited in Figure 1g. The overall thickness was no more than 1.3 mm. The energy conversion principle of the EF-TENG relied on the coupling of contact electrification and electrostatic induction, which has been reported in literature.34 The EF-TENG reported in this work was capable of harvesting energy from different mechanical stimuli, including pressing (Figure 2a) and bending (Figure 2b). At the initial state, the upper part and the lower part were in contact at the tips of the pyramids. When a pressing force was loaded on the EF-TENG, the interface area between the pyramids and the PTFE/PDMS nanocomposite increased. Due to contact electrification, negative triboelectric charges were generated on the PTFE/PDMS nanocomposite, while positive ones formed on the nanofibers/AgNWs electrode. It was to be noted that the PTFE/PDMS nanocomposite was pre-charged with negative charges, resulting in an enhanced charge density. With further pressing, the pyramids became squeezed. Then the negative triboelectric/electret charges induced more positive induced charges on the nanofibers/AgNWs electrode. Accordingly, electrons flew from the nanofibers/AgNWs electrode to the conductive textile because of electrostatic induction. Subsequently, when the pressing force was released, the pyramids recovered to their initial state owing to its elastic resilience. Then the electrons flew back in the opposite direction. Therefore, an alternating current between the two electrodes was generated under the repeated pressing force. When the device was bent, as shown in Figure 2b, the process was similar to the case of pressing. As the bending curvature varied, the gap between the PTFE/PDMS nanocomposite and the pyramids changed, which drove electrons


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to flow between the two electrodes. It needed to be noticed that only when dynamic deformation took place could the electric current be generated. In the following sections, four major factors that might affect the electric output of the EFTENG were investigated, including the height of the pyramids, the density of the pyramid array, the corona charging duration, and the thickness of the PTFE/PDMS nanocomposite. For consistence, all of the EF-TENGs had a fixed size of 3.5 × 3.5 cm2. First, the height of the pyramids substantially affected the electric output of the EF-TENG. The Voc, the charge output and the Isc are shown in Figures 2c-e, respectively. The height of the pyramids varied, i.e. 500 μm, 1,000 μm and 1,500 μm, while the side length was fixed at 1,000 μm. It could be observed that the optimal height of the pyramids was 1,000 μm. This observation was also applicable to the case when the EF-TENG was bent (Figures S3a-c, Supporting Information). This was attributed to the following reasons. When the pyramid height was small, the pressing force could result in a complete contact between the pyramids and the upper part surface. As the height increased, the size of the pyramids became enlarged, corresponding to more contact area with the upper part surface. As a result, the electric output was enhanced. However, if the height became excessively high, the pressing force was not strong any more to bring the upper part into a full contact with the pyramids. Then the actual contact area was reduced. This was why an optimal pyramid height was found. Second, the density of the pyramid array also profoundly affected the electric output performance. As shown in Figures 2f-h, the 15 × 15 pyramid array was found to be superior to other configurations (e.g. 8 × 8, 10 × 10, 20 × 20) because it corresponded to the highest electric output. Similar observation was also found when the bending occurred (Figures S3d-f, Supporting Information). This was because the loose configurations, i.e. 8 × 8 and 10 × 10, resulted in a small


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effective contact area. In contrast, the compact matrix, i.e. 20 × 20, corresponded to a mechanically harder structure that could not be fully deformed. In this case, less contact area led to a decreased electric output. Therefore, an optimal configuration was identified, which was most beneficial to the electric output. Third, the properties of the PTFE/PDMS nanocomposite were also investigated. Here two parameters were discussed, i.e. the corona charging duration and the thickness of the nanocomposite. As shown in Figure S4, the corona charging duration had little effect on the electric output of the EF-TENG once it surpassed 5 minutes. This was because the electret charges became saturated in a short time during the corona charging, as demonstrated in Figure S5 (Supporting Information). In the first 3 minutes, the surface potential has reached the maximum value. Thus, the electric output barely changed after 5 minutes based on that. The surface potential of the PTFE/PDMS nanocomposite largely governed the electric output, as demonstrated in Figure S6. When compared with the TENG that was not treated by the corona charging, the Voc of the EFTENG that was treated by the corona charging for 10 minutes was enhanced by 244%, as shown in Figure S7. As for the thickness of the PTFE/PDMS nanocomposite, it did not profoundly affect the electric output, as shown in Figure S8 (Supporting Information). This observation was also reported previously.38, 39 Based on the above experimental results, the height of the pyramid was fixed at 1,000 μm, and the pyramid matrix configuration of 15 × 15 was utilized. The electric output of the EF-TENG with three different sizes were measured. First, when the devices were under repeated compressive force, the obtained Voc, the charge output and the Isc are presented in Figures 3a-c, respectively. It was observed that the electric output intuitively depended on the size of the EF-TENG. As the overall side length of the pyramid matrix increased at an increment of 1 cm, the electric output


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was also enhanced. In particular, the average Voc, the charge output, and the Isc of the EF-TENG with the size of 3 × 3 cm2 were measured to be 275 V, 90 nC, and 9.5 μA, respectively. The output voltage from a unit area was calculated to be 30.56 V/cm2, which was superior to the state-of-theart reports on flexible TENGs (Table 1). The same conclusion was drawn when the EF-TENG was repeated bent, as shown in Figures 3d-f. Excited by pressing force of different magnitude, the Voc of the EF-TENG was also different, as shown in Figure 3g. Similarly, when the device was bent, different bending curvatures also corresponded to different electric output, which is clearly shown in Figure 3h. When the device was bent at the maximum curvature of 23.87 m-1, the voltage was as high as 85 V. The average output power of the EF-TENGs was calculated as I2 R, where the I was the output current across an external load and the R was the load resistance. When an external load was applied, the amplitude of the output current decreased as the load resistance increased, as shown in Figure 3i. The maximum power reached 982.83 μW at the load resistance of 300 MΩ. The durability of the EF-TENG was also tested with a pressing force of 15 N at a frequency of 3 Hz. The result shown in Figure 3j reveals that after 8,000 cycles of compressing, the Voc hardly changed. Meanwhile, there was a slight increase of the voltage at the initial stage owing to the accumulation of triboelectric charges. 34 In addition, as a key part of the EF-TENG, the durability of the flexible electrode was also measured under the same compressing condition for more than 10,000 cycles. The result is presented in Figure S9 (Supporting Information). The initial resistance of the electrode was 2.5 kΩ. As the number of cycles increased, the resistance of the flexible electrode became larger. After being compressed for 10,000 cycles, the resistance of the electrode increased by 1,100%. However, considering that the impedance of the flexible electrode was substantially smaller than that of the EF-TENG, the influenced of the electrode resistance on the electric output became negligible.40


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Because of the excellent flexibility and its ability to generate considerable electric output in response to low-frequency mechanical stimuli, the EF-TENG was suitable to harvest biomechanical energy. An EF-TENG with dimensions of 3.5 × 3.5 cm2 was taped onto a shoe insole to demonstrate harvesting human motion mechanical energy, as shown in Figure 4a. Due to the small thinness (1.3 mm) and the light weight (1.7 g) of the EF-TENG, it had little influence on wearer’s normal gait. Two types of different motions, stomping (Figure 4b) and bending (Figure 4c), were adopted, and the generated electric output is shown in Figures 4d-f. It could be observed that the stomping motion could produce a higher electric output of approximately 150 V, 60 nC and 1.25 μA, whereas the electric output of the bending was lower—approximately 100 V, 35 nC and 250 nA, respectively. This was because the stomping brought about much more deformation of the EF-TENG than bending. During normal walking, a combination of pressing and bending was applied onto the EF-TENG sequentially and repeatedly. Therefore, it could continuously harvest the mechanical energy and convert it to electricity during walking. As shown in Movie S1 (Supporting Information), the EF-TENG could power several commercial LEDs during stomping and bending. Therefore, the EF-TENG showed a promise of harvesting mechanical energy from human motions. CONCLUSIONS In summary, a corona charging enhanced flexible triboelectric nanogenerator (EF-TENG) based on the PTFE/PDMS nanocomposite, the nanofibers/AgNWs electrode and the PDMS-based pyramid arrays is here developed to harvest human motion mechanical energy. Compared to the TENG without being treated by the corona charging, the Voc of the EF-TENG could be enhanced by 244%. This kind of EF-TENG could be excited by different mechanical stimuli, and when being loaded external mechanical forces, it could generate electricity owing to the contact electrification


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and the variation of the surface potential. The electric output of the EF-TENG with dimensions of 3.5 × 3.5 cm2 could reach as high as 275 V, 90 nC and 9.5 μA at compressing mode, and 85 V, 30 nC and 140 nA at bending mode. Besides, its potential in harvesting human motion mechanical energy was also demonstrated in applications such as powering commercial LEDs when stomping and bending feet, and its light weight (1.7 g) resulted in little influence on people’s gait. Finally, the EF-TENG provided a new method for improving wearable electronics energy supply from the material design view, and showed great potential in powering portable and wearable electronics. EXPERIMENTAL SECTION Fabrication of the EF-TENG. The upper part: (1) The conductive textile was first attached on the PTFE membrane; (2) The PDMS base and the curing agent (Sylgard 184, Dow Corning) were mixed at a mass ratio of 10:1, then PTFE nanoparticles were added into the mixture at a mass ratio of 1:1; (3) The uncured PTFE/PDMS nanocomposite were blade-coated onto the PTFE membrane, after which corona charging was conducted by a voltage polarizer at a charging voltage of 8,000 V (ET2673A); (4) The nanocomposite after charging was then cured at 70 ℃ for 1 h in an oven. The lower part: (1) An acrylate-based mode was fabricated by stereolithography 3D printing; (2) The mode was coated with a layer of parylene to assist de-molding (PDS 2010); (3) Uncured PDMS elastomer (10:1 ratio between the base and the curing agent) was injected into the mode and degassed for 10 minutes, then cured at 70 ℃ for 1 h in an oven. The cured pyramids were then detached from the mode; (4) A thermoplastic polyurethanes TPU solution was prepared by dissolving the TPU (Sigma-Aldrich) in a solvent at a weight percentage of 15%, then stirred for 12 h at room temperature. The solvent was made of tetrahydrofuran (Aladdin) and dimethyl formamide (Aladdin) at a volume ratio 3:1; (5) The AgNWs dispersion in alcohol was diluted to 0.5 mg/mL; (6) The TPU solution and the AgNWs dispersion were loaded into two separate


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syringes that were attached to a syringe pump, respectively. The syringe with a capacity of 5 mL was used for the TPU solution, and the syringe with a capacity of 20 mL was used for the AgNWs dispersion; (7) At a positive voltage of 15 kV and a negative voltage of 2 kV, the electrospinning of the TPU solution and the electrospraying of the AgNWs dispersion were simultaneously conducted for 30 minutes at room temperature (Ucalery, ET-2535H). The distance between the receiver and the syringes was 14 cm, and the pumping rates of the TPU solution and the AgNWs dispersion were 0.05 mm/min and 1 mm/min, respectively. The rotation rate of the receiver was 100 r/min and the translation speed of the syringes was 100 mm/min. The translation distance of the syringes was 80 mm; (8) The fabricated flexible electrode was peeled off from the receiver and spread onto the silicone-based pyramids. Characterizations of the EF-TENG: The surface morphology of the flexible electrode, the PDMS pyramid substrate, the PTFE/PDMS nanocomposite and the cross section of the upper part were characterized by SEM (SU 8020). The photographs of the nanofiber electrode before and after being attached onto the PDMS pyramids were taken by a digital single lens reflex (Canon 5D mark3). The pressing force was loaded onto the EF-TENG by a commercial linear mechanical motor and measured by a dynamometer (MARK-10 M-5). The open-circuit voltage (Voc), the charge output and the short-circuit current (Isc) were measured by an electrometer (Keithley 6514). The surface potential on the PTFE/PDMS nanocomposite electret was measured by an electrostatic voltmeter (Marone model 279). The resistance of the flexible electrode was measured by a multimeter (Fluke 17B).

ASSOCIATED CONTENT


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Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Figure S1-S9, Movie S1

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C.Z.). *E-mail: [email protected] (G.Z.). ORCID Chi Zhang: 0000-0002-7511-805X Guang Zhu: 0000-0003-2350-0369 Author Contributions ⊥Z.B.L.

and H.Y.L. contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMETS


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This research was supported by the National Key R&D Project from the Ministry of Science and Technology, China (Grant Nos. 2016YFA0202701 and 2016YFA0202703), Natural Science Foundation of Zhejiang Province (Grant No. LR19F010001), National Science Foundation of China (Grant No. 51572030), Natural Science Foundation of Beijing Municipality (Grant No. 2162047).

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(29) Liao, M.; Huang, H.; Chuang, C. C. Performance Enhancement for the Triboelectric Energy Harvester by Using Interfacial Micro-Dome Array Structures. Appl. Phys. Lett. 2017, 110, 153901. (30) Fan, F.; Lin, L.; Zhu, G.; Wu, W.; Zhang, R.; Wang, Z. L. Transparent Triboelectric Nanogenerators and Self-Powered Pressure Sensors Based on Micropatterned Plastic Films. Nano lett. 2012, 12, 3109-3114. (31) Qian, J.; Wu, X.; Kim, D. S.; Lee, D. W. Seesaw-Structured Triboelectric Nanogenerator for Scavenging Electrical Energy from Rotational Motion of Mechanical Systems. Sensors and Actuators A: Physical 2017, 263, 600-609. (32) Wang, S.; Lin, L.; Wang, Z. L. Nanoscale Triboelectric-Effect-Enabled Energy Conversion for Sustainably Powering Portable Electronics. Nano lett. 2012, 12, 6339-6346. (33) Zhang, X.; Han, M.; Wang, R.; Zhu, F.; Li, Z.; Wang, W.; Zhang, H. X. FrequencyMultiplication High-Output Triboelectric Nanogenerator for Sustainably Powering Biomedical Microsystems. Nano lett. 2013, 13, 1168-1172. (34) Niu, S.; Wang, S.; Lin, L.; Liu, Y.; Zhou, Y. S.; Hu, Y.; Wang, Z. L. Theoretical Study of Contact-Mode Triboelectric Nanogenerators as an Effective Power Source. Energy & Environmental Science 2013, 6, 3576-3583. (35) Wang, S.; Xie, Y.; Niu, S.; Lin, L.; Wang, Z. L. Freestanding Triboelectric‐Layer‐Based Nanogenerators for Harvesting Energy from a Moving Object or Human Motion in Contact and Non‐Contact Modes. Adv. Mater. 2014, 26, 2818-2824.


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(36) Niu, S.; Liu, Y.; Wang, S.; Lin, L.; Zhou, Y. S.; Hu, Y.; Wang, Z. L. Theory of Sliding ‐ Mode Triboelectric Nanogenerators. Adv. Mater. 2013, 25, 6184-6193. (37) Fan, Y.; Li, X.; Kuang, S.; Zhang, L.; Chen, Y.; Liu, L.; Zhang, K.; Ma, S.; Liang, F.; Wu, T.; Wang, Z. L.; Zhu, G. Highly Robust, Transparent, and Breathable Epidermal Electrode. ACS Nano 2018, 12, 9326-9332. (38) Chen, G.; Lei, M.; Xiao, H.; Wu, L. Unique Charge Storage Characteristics of FEP/THV/FEP Sandwich Electret Membrane Polarized by Thermally Charging Technology. Chinese Physics Letters 2014, 31, 127702. (39) Zhong, J.; Zhong, Q.; Chen, G.; Hu, B.; Zhao, S.; Li, X.; Wu, N.; Li, W.; Yu, H.; Zhou, J. Surface Charge Self-Recovering Electret Film for Wearable Energy Conversion in a Harsh Environment. Energy & Environmental Science 2016, 9, 3085-3091. (40) Li, H.; Su, L.; Kuang, S.; Fan, Y.; Wu, Y.; Wang, Z. L.; Zhu, G. Multilayered Flexible Nanocomposite for Hybrid Nanogenerator Enabled by Conjunction of Piezoelectricity and Triboelectricity. Nano Research 2017, 10, 785-793. (41) Zhou, Q.; Park, J. G.; Kim, K. N.; Thokchom, A. K.; Bae, J.; Baik, J. M.; Kim, T. TransparentFlexible-Multimodal Triboelectric Nanogenerators for Mechanical Energy Harvesting and SelfPowered Sensor Applications. Nano Energy 2018, 48, 471-480.


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Figure 1. Structure and fabrication process of the EF-TENG. (a) Schematic diagram showing the structural design of the EF-TENG. (b) A cross-sectional SEM image of the upper part and (c) the zoom-in view of the PTFE/PDMS nanocomposite. (d) A SEM image of the PDMS pyramids with the nanofibers/AgNWs electrode and (e) zoom-in view of the nanofibers/AgNWs electrode. (f) Fabrication process of the EF-TENG. (g) A photograph of the EF-TENG.


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Figure 2. Energy-harvesting principle and electrical characterization of the EF-TENG with different heights of the pyramids and different densities of the pyramids. (a) Schematics of the working principle of the EF-TENG when being pressed and (b) when being bent. The corresponding (c) Voc, (d) the charge output and (e) the Isc of the EF-TENG with different heights of the pyramids (500 μm, 1,000 μm, 1,500 μm) when being pressed. The corresponding (f) Voc, the (g) charge output and (h) the Isc of the EF-TENG with different densities of the pyramids (8 × 8, 10 × 10, 15 × 15, 20 × 20) when being pressed. All EF-TENGs have the same dimensions of 3.5 × 3.5 cm2.


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Figure 3. Electrical characterization of the EF-TENG with different dimensions. The measured (a) Voc, (b) the charge output and (c) the Isc of the EF-TENG with different sizes (1 × 1 cm2, 2 × 2 cm2, 3 × 3 cm2) when the EF-TENG was pressed. (d) The Voc, (e) the charge output and (f) the Isc when the EF-TENG was bent. (g) The variation of the Voc when different pressing forces were


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loaded onto the EF-TENG. (h) The variation of the Voc when different bending curvatures were loaded onto the EF-TENG. The measured EF-TENG has the dimensions of 3.5 × 3.5 cm2, the height of the pyramids was 1,000 μm, and the configuration of the pyramids was 15 × 15. (i) Dependence of the peak power, the peak current, and the peak voltage on the load resistance of a single EF-TENG. (j) Stability and robustness measurement results, wherein the Voc was recorded for 8,000 cycles at a testing force of 15 N and a frequency of 3 Hz. Table 1: A summary of flexible TENGs with pyramid structure Tribo-layer Materials

Structure

Features

Size (area)

Output Voltage

Power Density

Output Current/Cu rrent Density

Output Voltage from a unit area

Ref

PDMS, ITO

archshaped

not mentioned

not mentioned

115V

not mention ed

30μA

/

[9]

PDMS, PET/ITO

archshaped

multilayer

1.5cm×2.5cm ×5

432V

not mention ed

12μA

23.04 V/cm2

[17]

PDMS mixed with BaTiO3 NPs and MWCNTs

archshaped

hybrid with piezoelect ric nanogener ator

2cm×4cm

22V

not mention ed

9μA/1.13 μA/cm2

2.75V/cm2

[24]

PDMS, PET/ITO

archshaped

transparen t

1.2cm×4.5cm

18V

not mention ed

0.13μA/cm2

3.33V/cm2

[30]

PDMS, Al

archshaped

Al surface has cubic structure

3cm×2.8cm

230V

128 mW/cm3

15.5μA/cm2

27.38 V/cm2

[32]

PDMS, Al

archshaped

double layers

2cm×4cm×2

465V

53.4 mW/cm3

13.4μA/cm2

29.06 V/cm2

[33]

PDMS, printed Agbased flexible transparent

sandwichshaped

transparen t, multimoda l

5cm×8cm

80.8V

3.2 mW/cm2

28.4μA/cm2

2.02V/cm2

[41]


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conducting electrode

Figure 4. Demonstration of the EF-TENG harvesting human biomechanical energy. (a) Photographs of an EF-TENG attached onto a shoe sole. Photographs of the shoe when (b) stomping and (c) bending was performed. The corresponding (d) Voc, (e) the charge output and (f) the Isc of the EF-TENG during the motions. The dimensions of of the EF-TENG was 3.5 × 3.5 cm2, the height of the pyramids was 1,000 μm, and the configuration of the pyramids was of 15 × 15.


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For Table of Contents Only


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Small-Sized, Lightweight, and Flexible Triboelectric Nanogenerator

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