Boosting the Efficient Energy Output of Electret Nanogenerators by

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Energy, Environmental, and Catalysis Applications

Boosting Efficient Energy Output of Electret Nanogenerator by Suppressing Air Breakdown under Ambient Conditions Zisheng Xu, Jiangjiang Duan, Wenbo Li, Nan Wu, Yuan Pan, Shizhe Lin, Jia Li, Fang Yuan, Shuwen Chen, Liang Huang, Bin Hu, and Jun Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19599 • Publication Date (Web): 03 Jan 2019 Downloaded from http://pubs.acs.org on January 3, 2019

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ACS Applied Materials & Interfaces

Boosting Efficient Energy Output of Electret Nanogenerator by Suppressing Air Breakdown under Ambient Conditions Zisheng Xua†, Jiangjiang Duana†, Wenbo Lia, Nan Wua, Yuan Pana, Shizhe Lina, Jia Lia, Fang Yuana, Shuwen Chena, Liang Huanga, Bin Hua, Jun Zhoua* a Wuhan

National Laboratory for Optoelectronics, Huazhong University of Science and Technology,

Wuhan 430074, China. Email: [email protected]; ‡These authors contributed equally for this work.

KEYWORDS： air breakdown, charge density, energy harvesting, electret nanogenerator, dielectrics ABSTRACT By virtue of simple fabrication, low cost and high conversion efficiency, nanogenerators are playing a key role in promoting the development of self-powered system and large-scale mechanical energy harvesting. Efforts have been ongoing for improving the output power of nanogenerators by maximizing their surface charge density via surface modification or structure optimization. Nevertheless, due to inevitable air breakdown during the operation process, enhancing charge density is not retainable, which is the most crucial limitation for output performance of nanogenerators. Here, a suppressing breakdown strategy is developed to remarkably enhance output charge density of the nanogenerator by embedding a dielectric film (PVDF) with high permittivity into air gap. Due to air breakdown suppression and strongly field-induced dielectric polarization effect, the output charge density of ~470 µC m-2 is obtained at ambient condition, which is ~4 times larger than the value of the conventional nanogenerator with air breakdown. In addition, the effects of different dielectrics materials and different thickness dielectrics are also studied for enhancing output charge density of the nanogenerator. These results provide a guide to design the state-of-the-art nanogenerator for efficient mechanical energy harvesting.

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Introduction To tackle with the threat of global warming and energy dilemma, more and more attentions have been given to researching renewable energy with reduced carbon footprinting.1 Apart from solar and wind power that are increasingly exploited, mechanical energy, ubiquitous in our everyday life, is a promisingly alternative for sustainable power sources. In the last decades, efforts have been implemented to develop various flexible harvesting energy technologies, including piezoelectric nanogenerators,2,3 triboelectric nanogenerators4–7 and electret nanogenerators8 have been dominantly studied, which can efficiently convert different types of mechanical energy into electricity and promote the development of self-powered wearable or portable electronics by harvesting mechanical energy from human body.9–13 It is well known that short-circuit current and open-circuit voltage of nanogenerators highly depend on their surface charge density, and corresponding output power density is quadratically related to transferred charge density.14 In the past years, numerous efforts have been devoted to pushing the charge density by means of surface modification, structural optimization, corona charging and so on.15–18 Note that injecting surplus charges by the corona discharging of air mode into the electrets is a more powerful strategy to invent nanogenerators (also called electret nanogenerators) with high charge density.8,19,20 For example, through the corona discharging mode, some electret materials, such as Fluorinated ethylene propylene (FEP), are capable of holding charge density over 1000 µC m-2.21 However, due to inevitable air breakdown during contact-separation process, most of enhanced surface charges are not retainable or usable for electricity generation.22 Air breakdown effect has been demonstrated to be the culprit of electrostatic discharging which leads to limited charge density retainable for nanogenerators. Recently, without the constraint of air breakdown, a triboelectric charge density of 1003 µC m-2 is attained in vacuum.23 In addition, a high charge density of 490 µC m-2 and 1020 µC m-2 can be obtained using other nanogenerator as charge pump to pump charges.24,25 Whereas, the maximum output charge density has

2


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been still limited to ~250 µC m-2 for a single nanogenerator in the air atmosphere due to the air breakdown effect.22,26 Herein, by embedding a dielectric film with high permittivity, Polyvinylidene Fluoride (PVDF), into the bottom of the air gap for nanogenerators, air breakdown effect is dramatically suppressed. Consequently, the charge density is boosted to 470 µC m-2 under ambient condition, which elevates the maximum output power density of the nanogenerator from 0.81 to 4.9 W m-2 at fairly low frequency of about 3 Hz and under 10 N force stimulated. We anticipate that this research provides guidance on design of state-of-the-art contact-separation nanogenerators. Results and discussion Electrets, such as FEP, Polytetrafluoroethylene (PTFE), can quasi-permanently preserve charges for a very long period of time. Typically, an electret nanogenerator is simply constructed by two electrodes and an electrified electret film, for example, a bottom Al electrode and a top FEP film attached with another Al electrode (Figure 1a). The FEP film was negatively charged by the corona treatment, and the two electrodes were positively charged due to the electrostatic induction. When compressing FEP film to the bottom electrode, the electric potential equilibrium between two electrodes will be broken, driving positive charges on the top electrode to the bottom electrode through the external load. Otherwise, the charge flow is reversed when releasing the nanogenerator (Figure S1). Ideally, the FEP film (~12.5 µm thickness) with surface charge density of 1000 µC m-2, as commonly observed in the electret film,27 will generate the electrostatic voltage of ~ 20 kV in surround environment during compressing process. This process can be evaluated by a finite element simulation shown in Figure S2. Factually, due to giant electrostatic electric field created by the electret film, air breakdown is likely to happen and results in immediate discharging for nanogenerators during the nanogenerator operation as illustrated in Figure 1a(ii). Therefore, even though an initial high surface potential distribution of ~1.01 kV can be obtained through corona discharging mode, the residual surface potential distribution significantly decreased to 3



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~0.36 kV due to air breakdown when the air gap distance narrowed down (Figure 1d and 1f). Indeed, such air breakdown effect is the major limitation on the output performance of nanogenerators. The Paschen’s law was widely employed to analyze the change of the critical breakdown voltage Vb described by a simple law28

Vb =

𝐵𝑝𝑑

(1)

1

ln (𝐴𝑝𝑑) ― ln [ln (1 + 𝛾 )] 𝑠𝑒

where p is the gas pressure. d is the gas distance between two boards. γse is the secondary electron emission coefficient. A is the saturation ionization in the gas at a particular electric field/pressure, and B is related to the excitation and ionization energies. The voltage between the electret film and the bottom electrode of the electret nanogenerator under short-circuit condition is given by29 𝜎𝑠𝑑1𝑑2

(2)

𝑉𝑔 = 𝜀0(𝜀2𝑑1 + 𝜀1𝑑2)

where Vg and d2 is the voltage between the bottom electrode and the FEP film. σs and d1 is the surface charge density and thickness of the FEP film. 𝜀0 is the vacuum permittivity (~8.85×10-12 F m-1). ε1 is the relative permittivity of FEP (~1.98), and 𝜀2 is the relative permittivity between the FEP film and the bottom electrode. As calculated from above equations, the Vb of air at 1 atm and Vg between the FEP film (1000 µC m-2) and electrode is plotted in Figure 1c. To avoid air breakdown, the Vg must be smaller than Vb. That means the gap distance between the FEP film and electrode is not less than ~ 20 µm based on curve intersections in Figure 1c. PVDF as a dielectric material has the high breakdown field (over 100 MV/m) and relative permittivity of ~10. When embedding a PVDF film into the bottom of the air gap, the minimum gap distance during compressing process is limited to the thickness of PVDF film. Herein, it is noting that 4


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PVDF use is totally different from the poled PVDF based nanogenerator,30,31 because the PVDF film is non-poled and is easy to prepared. Consequently, the operation process of nanogenerators is outside of the region of the Paschen’s law (Figure 1b). As shown in Figure 1e and 1f, the surface potential of the FEP film of the nanogenerator contains PVDF remained ~ 0.75 kV after several thousands of contactseparation cycles, indicating that the air breakdown was significantly suppressed. For the sight decline of the surface potential for the FEP film of the nanogenerator with PVDF during process, this may be attributed to discharging of contact electrification, and remarkably little is known about the mechanism underlying this phenomenon.32,33 Meanwhile, comparing with surface potential of PVDF before and several contact-separation process, there are only minor changes in surface potential (Figure S2b). Therefore, a minor charge transfer, between the FEP film and the PVDF film, can be ignored during the contact-separation process. Except from air breakdown effect, the effective transferred charge density of nanogenerators closely related to the gap distance between the electret film and electrode (Figure S1). For example, a conventional nanogenerator without PVDF obtains the maximum transferred charge density when the electret film closely contacting with electrode, namely, gap distance near to zero (Figure 2c). If the electret film has no full contact with the bottom electrode due to the coarse solid surfaces of two boards, it is hard to complete the effective charge transfer process.16 Unlike the conventional nanogenerator, the effect of embedding PVDF in the nanogenerator could not only dramatically suppress air breakdown, but also yield the maximum transferred charge density at a relative wide gap due to a big change in the strong electric field-induced polarization effect (Figure 2a). Herein, we proposed the PVDF and air as air/PVDF composites in series configuration (Figure S3a). Thus, when the volume of air is compressed during the operation process, the relative permittivity 𝜀2 of the air/polymer composites rises with the volume fraction f of the PVDF, which is related to the distance d2 (Figure S3b and Note 1). Simultaneously, As the distance d2 decreased, the potential between FEP film and the bottom electrode would increase, and therefore change dielectric polarization. Eventually, the dielectric tunability in the relative permittivity ε2 5



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will result in dielectric polarization between the FEP film and the bottom electrode through changing the distance of air gap. Subsequently, the change in the polarization will drive more positive charges from the top electrode to the bottom electrode. Moreover, effect of the dielectric polarization and air breakdown on the transferred charge density for the electret nanogenerator was also proved, respectively (Figure S4). As shown in Figure 2b, the maximum dielectric polarization can be obtained when the FEP film approaches the PVDF film during compressing process. Otherwise, the polarization will decrease and drive charges in the bottom electrode back to the top electrode at releasing process (Figure S5). Based on the change in the dielectric polarization, the charge density 𝜎𝑖2 on the bottom electrode can be theoretically analyzed using a simple analytical model (see Supporting Note1 for details).

𝜎𝑖2 = 𝜀 𝜀 𝑑

𝑝 0 1

𝜎𝑠𝜀𝑝𝜀0𝑑1

(3)

+ 𝜀21(𝑑2 ― ℎ) + 𝜀𝑝𝜀1ℎ

where d1 is the thickness of the FEP film, and 𝜀2 and d2 is the relative permittivity of the space and the distance, respectively, between the FEP film and the bottom electrode. Through Equation 3, the transferred charge density ∆𝜎, the change in the charge density 𝜎𝑖2 of the bottom electrode, can be obtained (Note S1, Supporting Information). Therefore, as is shown in Figure 2c, compared with the conventional nanogenerator, the transferred charge density of nanogenerator with PVDF is more sensitive to gap distance, and will approach to the maximum at a relative wide gap about several micrometer. Therefore, the theoretical transferred charge density ∆𝜎 of the nanogenerator with PVDF is higher than that of conventional nanogenerator due to the its strong polarization effect. To validate theoretical calculation, we tested the performance of the nanogenerator with PVDF. When the nanogenerator was operated under the 10 N force with 3 Hz frequency simulating, the transferred charge density reached to about 470 µC m-2 (Figure 2d), which is 4 times than that of the conventional nanogenerator. Meanwhile, the charge density maintained around 470 µC m-2 when the nanogenerator was continuously operated for ~ 5000 cycle times (Figure 2e), indicating its robust stability for long time 6


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operation. It is worth noting that the transferred charge density of our nanogenerators is higher than that of previous works reported single nanogenerator (Figure 2f).17–18,26 Though a large output charge density of 1003 µC m-2 was reported under high vacuum (10-6 torr),34 this strategy is not viable for realistic energy harvesting application. Our design via integrating PVDF to improve output charge density was more feasible to the practical application. The effect of different dielectric materials on the transferred charge density was investigated. Herein, the

PTFE, FEP, polyethylene terephthalate (PET), polyimide (PI), and PVDF was selected as the

dielectric layer of the electret nanogenerator, respectively, and their Infrared spectrums is shown in Figure S6. The frequency-dependent permittivities are given in Figure 3a, which were tested by the LCR meter in the frequency range of 10-4000 Hz. The relatively permittivities of the PTFE, FEP, PET, polyimide PI and PVDF were 1.64, 2.01, 3.28, 3.79, and 9.79 at 100 Hz, respectively, and as the frequency increasing, the relatively permittivities of all above materials kept the relative stable values. Thus, based on the permittivities of all the above materials and air (𝜀0=1), the effective permittivities 𝜀2 of air/polymer composites was calculated according to the series model (Figure 3b). Through the Equation 3, the output charge densities with different dielectric materials (~50 µm thickness) were calculated with variation of the distance d2 (Figure 3c). Under the 10 N force with 3 Hz frequency simulating, the electrical performance of nanogenerators with different dielectric materials is shown in Figure S7a-c. Apparently, the output charge density of nanogenerators was positively correlated with the relatively permittivities of dielectric materials (Figure 3d). Compared with other dielectric materials, PVDF with the largest relative permittivity received the highest output performance. Hence, we introduced PVDF into nanogenerators in this work. The effect of thickness of PVDF films on the transferred charge density ∆𝜎 was also investigated. It is known that the permittivity rapidly decreases when the thickness of the dielectric material below the critical thickness due to the low crystallinity in the film (Figure S8a).35 Meanwhile, due to the effective 7



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contact area increasing as decreasing the thickness,36 the discharging of contact electrification is becoming pronounced (Figure S8b). These considerations suggest that the transferred charge density increases with the thickness decreasing firstly and reached a maximum around 10 µm, then begin to cut down shown in Figure 3e. As expected, when the thickness of PVDF increased from 9 µm to 50 µm, transferred charge density ∆𝜎 first increased and then decreased, and the maximum charge density ~470 µC m-2 was obtained at ~15 µm thickness of PVDF (Figure 3f and Figure S9). Moreover, the used film thickness is different in theoretical calculation and experimental measurement. This is because of contact discharging increasing as decreasing thickness and compression during operation process. Therefore, the experimental thickness of the PVDF film is slightly different from the theoretical thickness on the maximum transferred charge density. Owing to the absent air breakdown and strong electric field-induced dielectric polarization effect, the current amplitude of ~137 mA m-2 and the voltage amplitude of ~109 V for the nanogenerator with PVDF (~15 µm thickness) were much higher than those (~44 mA m-2 and ~32 V) of the device without PVDF (Figure 4a-b). Meanwhile, the output peak power density was enhanced from 0.81W m-2 with 10 MΩ to 4.6 W m-2 with 20 MΩ at fairly low frequency of 3 Hz (Figure 4c). Furthermore, the output of the nanogenerator with PVDF with a contact area of ~4 cm2 can be regulated to charge capacitors with 220 µF for powering the electronic watch (inset of Figure 4d). For demonstration, the voltage of the electronic watch was monitored by a voltmeter. When the electret nanogenerator was stimulated under 10 N force with 3 Hz for a short time of ~33 s, the voltage reached the starting voltage of the watch of ~1.8 V. Subsequently, due to the large output performance, the nanogenerator with PVDF operating at a low frequency of 3 Hz can not only drive the electronic watch sustainably, but also charge the capacitor simultaneously, indicating that the nanogenerator with PVDF generates more energy than the watch consumes (Figure 4d). However, when the conventional nanogenerator worked in air, the voltage of the capacitor charged can’t support the watch work .

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Conclusions In conclusion, surface charge density of nanogenerators was limited by the air breakdown under air atmosphere. In this work, we find a way to avoid air breakdown and push transferred charge density to 470 µC m-2 at ambient conditions, and better electrical performance is guaranteed. The way provides a new insight into the decisive factor on lasting development for the nanogenerator. It is predicted that the nanogenerator with PVDF will be useful in energy harvesting from ambient environment and enable their more promising usage in several important areas. Further work along this direction could involve further improvement of the out performance through increasing the surface charge density of the electret film and avoiding the contact discharging. Therefore, it will be significantly meaningful to fill the missing area and achieve a substantially improved charge density.

Experimental Section Preparation of the PVDF. PVDF was dissolved in dimethylformamide (DMF) at 70 oC under continuous stirring to obtain a solution with concentration of 0.2 g/mL. PVDF films with different thickness were deposited on the PET substrate by a spin coating method and then were dried for 24 h at 90 oC under vacuum. The XRD of PVDF is shown in Figure S9. Corona charging process. During charging, the FEP film was put 1cm below the corona needle tip, where the voltage was remained -5 kV for 5 minutes. Fabrication of the nanogenerator with PVDF. A conventional nanogenerator with PVDF was simply constructed by two pieces of the substrate PET coated by Al, PVDF and an electrified FEP film. The device was fabricated by laying a FEP film attached with an Al electrode on the top and a PVDF film with another Al electrode on the bottom of a PET substrate. The PVDF and FEP films were all of the same size of 1×1 cm, as is shown in Figure S10. 9



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Characterization. The dielectric properties of the thick films were tested using Agilent E4980A LCR analyzer. The open-circuit voltage and transferred charge density were recorded in-situ by an electrometer (Keithley 6514B, USA). The output characteristics of samples were measured using a NI pci-6259 with a Standford Low-noise Current preamplifier (Model SR570). A resonator (JZK, Sinocera, China) controlled by a signal generator (YE 1311-D, Sinocera, China) was used to periodically trigger the nanogenerator attached on a force meter (Model ZPS-DPU-50N). The surface potential was tested using electrostatic voltmeter (Model 347, TRek). The XRD spectrum (X’Pert Pro, PANanalytical) was used to explored the structure of the PVDF. It should be noted that all the surface potential of the FEP film reached stable statement. Supporting Information. More detailed information about detailed device fabrication process and supporting figures. This material is available free of charge via the Internet at http://pubs.acs.org. Theoretical calculation of transferred charge density, a XRD pattern of PVDF, a photograph of the electret nanogenerator, the operational mechanism, infrared spectrums of four dielectric materials, the permittivity of PVDF with different thickness, the effect of dielectric polarization and air breakdown on the transferred charge density. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (61434001, 51672097), the National Program for Support of Top-notch Young Professionals, the program for HUST Academic Frontier Youth Team, the China Postdoctoral Science Foundation (2017M610468), the Fundamental Research Funds for the Central Universities (Huazhong University of Science and Technology: 2015MS004; 2018KFYXKJC025). The authors thank the facility support of the Center for Nanoscale Characterization & Devices, WNLO-HUST and the Analysis and Testing Center of Huazhong University of Science and Technology.

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(33) Baytekin, H.-T.; Patashinski, A.-Z.; Branicki, M.; Baytekin, B.; Soh, S.; Grzybowski, B.-A. The Mosaic of Surface Charge in Contact Electrification. Science 2011, 333, 308. (34) Wang, S.; Xie, Y.; Niu, S.; Lin, L.; Liu, C.; Zhou, Y.-S.; Wang, Z.-L. Maximum Surface Charge Density for Triboelectric Nanogenerators Achieved by Ionized‐air Injection: Methodology and Theoretical Understanding. Adv. Mater. 2014, 26, 6720. (35) Zhang, Q.-M.; Xu, H.; Fang, F.; Cheng, Z.-Y.; Xia, F.; You, H. Critical Thickness of Crystallization and Discontinuous Change in Ferroelectric Behavior with Thickness in Ferroelectric Polymer Thin Films. J. Appl. Phys. 2001, 89, 2613. (36) Chen, Q.; Shen, Y.; Zhang, S.; Zhang, Q.-M. Polymer-Based Dielectrics with High Energy Storage Density. Annu. Rev. Mater. Res. 2015, 45, 433.

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Figure 1. Air breakdown. a) air breakdown schematic of the conventional nanogenerator operation from (i) the static status to (ii) air breakdown status. b) Avoiding air breakdown schematic of the nanogenerator with PVDF from (i) initial status to (ii) equilibrium status. c) Air break voltage in 1 atm air and gap voltage of the nanogenerator with 1000 µC m-2 charge density. d-e) Surface potential images taken in same area (i) before compressing and (ii) after compressing for the conventional nanogenerator and the nanogenerator with PVDF, respectively. f) The surface potential as a function of the number of operational cycles. The black curve corresponds to the conventional nanogenerator and the red curve to the nanogenerator with PVDF.

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Figure 2. Enhanced output performance through suppressing air breakdown. a) the working mechanism of the nanogenerator with PVDF. b) The theoretical polarization as a function of the distance d2. c) Theoretical transferred charge density of the nanogenerator without and with PVDF. the black curve corresponds to the conventional nanogenerator with surface charge density of 400 µC m-2 and the red and blue curve to the nanogenerator with PVDF with 400 µC m-2 and 600 µC m-2. d) Comparison of transferred charge density of the nanogenerator without and with PVDF in experiment. e) Charge density stability measurement for the nanogenerator with PVDF. f) Comparison of the transferred charge density measured in this work with previously reported single nanogenerators with contact-separate mode.

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Figure 3. The output performances of different dielectric materials with different thickness. a) The relative permittivity of different dielectric materials versus frequency measured. b) Relative permittivity of the air/PVDF composite as function of d2. c) The theoretical transferred charge density and d) the experimental transferred charge density and peak current of the nanogenerator with different dielectric materials. e) The theoretical transferred charge density and f) the experimental transferred charge density and peak current of the nanogenerator with different thickness of the PVDF film.

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Figure 4. Output performance of the nanogenerator without and with PVDF. a) The current, b) open-circuit voltage, and c) the power density of the nanogenerator without and with PVDF. d) Charging curves of 220 µF capacitor when an electronic watch is driven by the nanogenerator without and with PVDF simultaneously. These insets are the charging management circuit system, and an electronic watch being lit by the nanogenerator with PVDF.

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Boosting the Efficient Energy Output of Electret Nanogenerators by

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