Output Power Density Enhancement of Triboelectric Nanogenerators

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Output Power Density Enhancement of Triboelectric Nanogenerators via Polarized Ferroelectric Polymers and Bulk MoS2 Composites Minje Kim, DaeHoon Park, Md. Mehebub Alam, Sol Lee, Pangun Park, and Junghyo Nah ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b00750 • Publication Date (Web): 15 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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Output Power Density Enhancement of Triboelectric Nanogenerators via Polarized Ferroelectric Polymers and Bulk MoS2 Composites

Minje Kim1, Daehoon Park1, Md. Mehebub Alam1, Sol Lee1, Pangun Park1, and Junghyo Nah1,* 1 Department

of Electrical Engineering, Chungnam National University, Daejeon 34134, Korea

*Corresponding author: [email protected]

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ABSTRACT Performance enhancement of triboelectric nanogenerators (TENGs) has been largely limited by the relatively low output current density. Thus, extensive research efforts have been made to increase the output current density. In this respect, this work presents a method to effectively increase output current density of TENGs by adopting polarized ferroelectric polymers and MoS2 composite. Specifically, by compositing bulk MoS2 flakes with both Nylon 11 and PVDF-TrFE, respectively, charge density of each triboelectric charging surface was significantly increased. In addition, proper polarization of both ferroelectric composite layers has also led to an additional increase in the charge density. Combination of them synergistically increases the surface charge density, generating huge output current and the power output density. By optimizing the fabrication process, the output voltage and current density up to ~145 V and ~350 A/cm2 were achieved, respectively. Consequently, the TENG exhibits a recordable output power density of ~ 50 mW/cm2, which is one of the highest output power densities reported to date. The method introduced in this work can greatly increase the output current density of TENGs, facilitating the development of high performance triboelectric energy harvesting devices.

Keywords: triboelectric generator, MoS2, ferroelectric composites, polarization, surface charge


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To date, great deal of efforts has been made to improve the output performance of triboelectric nanogenerators (TENGs). Indeed, various methods, such as dielectric permittivity modulation,1 , 2 surface pattering,3-5 chemical functionalization,6 , 7 and composite structure,8 , 9 have been adopted to enhance the output performance of TENGs. Through these efforts, their output power density has been significantly improved. However, further enhancement has been limited due to relatively low output current density compared to the output voltage. Since the current density of TENG depends only on the triboelectric surface charges of the friction surface, it is difficult to further increase the output current density. To address this problem, various methods, such as charge injection,10 , 11 embedded polymer layer,12-15 and grounded layer,16 have been widely investigated to effectively increase surface charge density of TENGs. Among these attempts, the enhancement of output current density of TENG has recently been demonstrated by using the composite layer consisting of monolayer MoS2 flakes and polyimide (PI), where the monolayer MoS2 flakes embedded in PI serve as electron acceptors.14 Thereby, obvious output current enhancement was achieved. However, it can be only effective for negative triboelectric friction surface and its output power is still not sufficient for practical application of TENGs. Therefore, further studies are necessary to develop more knobs that can be simultaneously applied to boost the output power density of TENGs. In this work, we report the performance of the TENG fabricated with ferroelectric Nylon 11/MoS2 and PVDF-TrFE/MoS2 composites. Both triboelectrically positive ferroelectric Nylon 11 and triboelectrically negative ferroelectric PVDF-TrFE layers were composited with bulk MoS2. Since both polymers have triboelectrically opposite characteristics, the frictional surface charge density can be maximized by adopting this contact pair. Furthermore, thanks to ferroelectric nature of both polymers, both friction surfaces can be further tuned to maximize surface charge density by utilizing dipolar polarization. To our best 3 ACS Paragon Plus Environment

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knowledge, the polarization of both friction surfaces on the performance of TENGs has not been investigated yet. Here, we have proposed the preferable polarization direction for maximized the output performance of Nylon 11 and PVDF-TrFE pair based TENGs. In addition to this, to further enhance surface charges in the friction layers, bulk MoS2 flakes are mixed with each polymer to form composite layers. By modulating dielectric permittivity, triboelectric surface charge density has been much improved for both surfaces. Thereby charge densities of both friction surfaces are promoted during the frictional contacts between two surfaces. Indeed, in this work, 500% higher current density and 220% higher output voltage were respectively obtained by employing the proposed methods, demonstrating more obvious output current density increase. Consequently, our results show that output power density up to 50 mW/cm2 has been achieved, which is one of the highest output power densities using a similar approach reported to date. By compositing with bulk MoS2 flakes and polarizing its friction surface, the TENG has demonstrated more than about 8-fold increase in the output power density by comparison to that of the TENG only with Nylon 11 and PVDF-TrFE.

Results and Discussion Figure 1a shows the TENG consisting of two polymer composites with opposite triboelectric charging surface. Here, MoS2 doped Nylon 11 and PVDF-TrFE composite layers work as positive and negative triboelectric charging surface, respectively. In Figure 1b, it should be mentioned that the polymers remain quite transparent even after incorporation of MoS2, exhibiting its potential utility for transparent TENG fabrication. The presence of MoS2 in the composites are also confirmed by XRD spectra in Figure 1c. There is a strong


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characteristic diffraction peak of MoS2 at 14.3° (002) in the composites and then all the other characteristic diffraction peaks of MoS2 and polymer matrixes are suppressed.17 , 18 Thus the enlarged view shows the other peaks corresponding to bulk MoS2 and the characteristic diffraction peaks of the polymer matrixes are shown in Figure S1. The XRD patterns of pure Nylon 11 and PVDF-TrFE exhibit typical ferroelectric characteristics peaks at 21.5° and 19.8°, respectively.19-22 Figure S2 shows the variation of peak-to-peak output performances with the increase of MoS2 content in the polymer matrixes. The Vpp increases with MoS2 content and get saturated beyond 0.5 wt %. Therefore, further experiments are carried out with Nylon 11 and PVDF-TrFE layers with MoS2 content of 0.5 wt.% only. The high voltage poling process was additionally performed to maximize the surface charge density. Since both polymers are ferroelectric materials, surface charge density can be further increased by the electrical poling process as shown in Figure 1d. Forward direction indicates that the polarized dipoles form positive charge center on the contact surface and the reverse direction did the opposite. To explicitly show the roles of MoS2-polymer composite formation and polarization, the surface potential of each substrate was measured under non-contact mode over an area of 25 m2 in Figure 1e. The results show that the surface potential of Nylon 11 increased from 400 mV to 733 mV by applying electrical poling and compositing with bulk MoS2 flakes. In the same way, the surface potential of PVDF-TrFE also increases in negative direction from -300 mV to -726 mV under the same treatment. Thus, poling and doping of MoS2 both are found to be independent ways of improving the surface potential of the ferroelectric polymer layers and combination of them can synergistically improve the surface potential more.20, 23-25 Similarly, dielectric properties of the polymer films are also enhanced as shown in Figure S3.26 , 27 It should be mentioned that dielectric constants exhibit similar trend of improvement with poling, doping or their combination treatments. 5 ACS Paragon Plus Environment

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In case of poling, this trend of improvement is due to simply the preferable polarization direction as discussed below. To determine the preferable polarization direction for the TENG with Nylon 11 and PVDF-TrFE contact pair, different polarization pairs were examined. The schematic drawings in Figure 2a show the polarization pairs tested in this work. Additionally, Figure 2b shows the output voltages of the TENGs for each polarization pair. The raw output voltages are provided in Figure S4. The highest peak to peak output voltage of 270 V is achieved from the device with a contact pair of reverse polarized PVDF-TrFE and forward polarized Nylon 11 (Figure 2a-3), indicating that triboelectric surface charge density can be maximized with this contact pair. The effect of poling is also confirmed by P-E curve measurement of the friction layers. Remnant polarizations (Pr) is increased from 0.19 to 0.93 C/cm2 (Figure 2c) and 0.23 to 1.16 C/cm2 (Figure 2d) for Nylon 11 and PVDF-TrFE, respectively. There is about 5-fold improvement in Pr after poling both contact surfaces, demonstrating the ferroelectric nature of the friction layers.28-30 Consequently, the output voltage, current density, and charge density are increased from 120 to 220 V (Figure 2e), 130 to 280 A/cm2 (Figure 2f), and 4.2 to 7.8 nC/cm2 (Figure 2g), respectively, just by polarizing both ferroelectric polymer layers. After determining the preferable polarization direction, bulk MoS2 flakes were mixed with each polymer to further increase surface charge density. Figure 3a shows the scanning electron microscope (SEM) image of bulk MoS2 flakes, mixed with each ferroelectric polymer. Raman spectroscopy in the inset of Figure 3a also indicates bulk nature of MoS2 rather than mono layer of MoS2. In case of bulk MoS2, Raman shift value difference is obtained at 25 cm-1.31

, 32

Furthermore, the surface morphology of the polymer and

composites are shown in Figure S5 and it is found MoS2 flakes are well embedded in the polymer matrixes. In case of PVDF-TrFE/MoS2 composite, some MoS2 can be shown on surface also. Afterwards, the composite layers are polarized in the preferable polarization direction as demonstrated in Figure 2a-3. The poling process improves the Remnant 6 ACS Paragon Plus Environment

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polarization (Pr) from 0.51 to 1.56 C/cm2 (Figure 3b) and 0.58 to 1.8 C/cm2 (Figure 3c) for Nylon 11/MoS2 and PVDF-TrFE/MoS2 composite, respectively. It can also be clearly noticed that the area of hysteresis loop is expanded after compositing with bulk MoS2 flakes when compared with the results shown in Figure 2c, d.33-35 These results indicate that the poling effect can be further improved by compositing with MoS2 flakes. However, if the content of MoS2 flakes in the composite exceed 1 wt%, the P-E curve of the composite layer become that of a typical lossy dielectric, as shown in Figure S6. Figure 3d ~ f show the output characteristics of the TENG with the polarized composite friction layers. The Vpp of the TENG with MoS2-polymer composite layer shows more than 2-fold higher value by comparison to that of the TENG without it. After the poling process, the output voltage was further increased from 240 to 270 Vpp (Figure 3d). For current density, more noticeable enhancement was obtained. The peak-to-peak output current (Figure 3e) and charge density (Figure 3f) were increased from 130 A/cm2 and 4.2 nC/cm2 to 323 A/cm2 and 11.8 nC/cm2 by composite formation, which were further increased to 645 A/cm2 and 20.2 nC/cm2, respectively, after polarization of dipoles. Thus, the output current and charge density were increased by 5 times. Consequently, doping of MoS2 and poling of dipole synergistically improved the output performance of the TENG. We note that this output current density is one of the highest values reported to date (Table S1).36 , 37 Lastly, the output performance of the TENG was further investigated. Figure 4a exhibits the output voltage and current density as a function of load resistances. As the load resistance increases, the voltage increases proportionally and the current density decreases in inverse proportion. As a result, the maximum power density of ~50 mW/cm2 was obtained by connecting a 10 MΩ load resistance, which is significant the output performance compared with other TENGs using similar approaches (Table S1). The maximum output power density 7 ACS Paragon Plus Environment

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was ~ 8 times increased by the combination effect of doping bulk MoS2 flakes and polarizing the dipoles as shown in Figure 4b. An asterisk mark indicates the polarized samples. To examine stability of the TENG, output voltages were repeatedly measured for 12,000 cycles at an operation frequency of 6.5 Hz. Indeed, consistent output voltages were measured for the entire measurement cycles (Figure 4c). The raw output data can be found in Figure S7. Using the output of two different TENGs with contact area of 1 cm2, a capacitor of 0.47 F was charged by rectifying their outputs with a full-wave bridge rectifier (Figure 4d). The TENG fabricated with the polarized composite layers can rapidly charge the capacitor, reaching 6.6 V less than 10 s. The charging performance is expectedly higher than the TENG made of pure polymers pair without doping of MoS2 and poling treatment. Using the high performance TENG, 100 blue light emitting diodes (LEDs) can be instantaneously turned on by directly connecting to the output of TENG without any charge storages (Figure 4e).

Conclusions We have demonstrated that the doping of ferroelectric polymers with MoS2 flakes and their polarization can greatly improve the performance of the TENGs. In addition to the opposing triboelectric properties of the two friction surfaces, these treatments significantly improve the output performance of TENG, producing a power density of ~50 mW/cm2, which is 8  higher value compared with that of the TENG without doping and polarization. Our work presented here provides useful knobs to greatly enhance the output power density of TENGs especially by boosting their output current density.

Experimental section 8 ACS Paragon Plus Environment

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Materials. The MoS2 flakes were purchased from US Research Nanomaterials, Inc. (diameter = 800 nm-1.2 m, 99.9%). Nylon 11 pellets were purchased from Sigma-Aldrich (particle size = 3 mm). PVDF-TrFE powder with a content of 30% (mol%) was purchased from Piezotech-Arkema Corp. All solvents, such as formic acid, dichloromethane, N,Ndimethylmethanamide (DMF) and acetone, were purchased from Alfa Aesar and SigmaAldrich. Fabrication of devices. Nylon 11 pellets (8 wt%) were stirred at 60°C for 1h with co-solvent of formic acid and dichloromethane (1:4 volume ratio). MoS2 flakes were added to Nylon 11 solution and stirred again at 60°C for 30 min to obtain uniform dispersion. The prepared Nylon 11/MoS2 solution was spin-coated on an ITO-coated PET film with a size of 1  1 cm2, which was subsequently cured at 40 °C for 30 min. For the negative friction layer, the mixture of PVDF-TrFE (13 wt%) and MoS2 flakes was dissolved in co-solvent of acetone and N,Ndimethylmethanamide (1:1 volume ratio). The prepared PVDF-TrFE/MoS2 solution was then coated on an ITO:PET film and cured under the same condition. It should be mentioned that both Nylon 1l and PVDF-TrFE composites are prepared by varying MoS2 concentration as 0.1, 0.2, 0.25, 0.5, 0.75, and 1wt% to determine optimal the MoS2 content in each composite layer. Finally, the composite layers were also polarized under high electric field (80 kV/cm) for 6h using a custom-built poling equipment. Characterization and Electrical Measurements. The surface potentials were measured by the non-contact mode Kelvin probe force microscopy (Park systems XE 7), where a Pt-coated cantilever (radius < 10nm) was used. The vibration of MoS2 molecule and crystallinity of layers were characterized by Raman spectroscopy (HORIBA HR-800 UV-Visible-NIR) using 514 nm laser and X-ray diffraction (Bruker AXS D8 DISCOVER), respectively. The polarization9 ACS Paragon Plus Environment

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electric field curves were measured by using a ferroelectric tester (Radiant Tech. RT66C) at room temperature. The triboelectric outputs were measured under the applied pressure of 0.15MPa using a oscilloscope (Teledyne wavesurfer 3022) with a 10 MΩ impendence probe. In addition, relative humidity was maintained at 27% by using a dehumidifier.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XRD patterns of pristine Nylon 11 and PVDF-TrFE, a comparison of peak-to-peak voltages with different concentrations of MoS2 in Nylon 11, a comparison of dielectric constant and loss obtained from Nylon 11 and PVDF-TrFE samples (pristine, poled, non-poled with MoS2, and poled with MoS2), Output voltages of the TENG with 4 different polarization pairs, SEMs of friction layers, P-E curve of PVDF-TrFE/MoS2 composite with 1 wt% MoS2 flakes in the composite, Cyclic output voltage measurement of TENG for durability test and a comparison of current density and power density of our TENG with other works reported previously.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] Author Contributions 10 ACS Paragon Plus Environment

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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. The authors declare no competing financial interest. Acknowledgments This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF-2017R1A4A1015744, NRF-2019R1A2C1010384).


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Figure 1. (a) Schematic structure of the TENG with MoS2 composite layers. (b) The optical images of pure PVDF-TrFE layer & PVDF-TrFE/MoS2 composite layer (top) and pure Nylon 11 layer & Nylon 11/MoS2 composite layer with 0.5 wt% of MoS2 flakes (bottom). (c) XRD patterns of Nylon 11/MoS2 and PVDF-TrFE/MoS2 composite. (d) Schematic drawing of the poling process. (e) Surface potentials of Nylon 11 and PVDF-TrFE, and their composites with MoS2 before and after the poling process.


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Figure 2. (a) Schematic drawings of 4 different polarization pairs. (b) Comparison of the output peak-to-peak voltages for each poling pair. (c) P-E curves of non-poled Nylon 11 and poled Nylon 11. (d) P-E curves of non-poled PVDF-TrFE and poled PVDF-TrFE. The enhancement in (e) output voltage, (f) current density, and (g) charge density before (black colored line) and after (red colored line) polarization of the friction surfaces.


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Figure 3. (a) Scanning electron micrograph (SEM) of MoS2 flakes composited with ferroelectric polymers. (Inset) Raman spectroscopy of bulk MoS2 flakes. (b) P-E curves of nonpoled Nylon 11/MoS2 composite and poled Nylon 11/MoS2 composite. (c) P-E curves of nonpoled PVDF-TrFE/MoS2 composite and poled PVDF-TrFE/MoS2 composite. Comparison of (d) output voltage, (e) current density, and (f) charge density among the TENGs with Nylon: PVDF-TrFE pair, Nylon/MoS2: PVDF-TrFE/MoS2 composite pair, and polarized Nylon 11/MoS2: PVDF-TrFE/MoS2 composite pair.


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Figure 4. (a) Dependence of the output voltage and current density of the TENG on the externally connected load resistances. (b) Comparison of power density of the TENGs for externally connected load resistances. An asterisk indicates polarized frictional contact pair. The TENG fabricated with polarized ferroelectric polymers-MoS2 composite demonstrates greatly enhanced output power density in comparison to that fabricated just with ferroelectric polymers. (c) Output voltages of the TENGs measured for 12,000 cyclic frictional contacts at a frequency of 6.5 Hz. (d) A capacitor (0.47 F) charging behaviors by pristine TENG and the TENG with polarized ferroelectric composite contact pairs. (e) 100 blue LEDs lit by the rectified output of the TENG.


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(21) Chen, S.; Yao, K.; Tay, F. E. H.; Chew, L. L. S. Comparative Investigation of the Structure and Properties of Ferroelectric Poly(vinylidene fluoride) and Poly(vinylidene fluoride-trifluoroethylene) Thin Films Crystallized on Substrates. J. Appl. Polym. Sci. 2010, 116, 3331–3337. (22) Feng, T.; Xie, D.; Zang, Y.; Wu, X.; Ren, T.; Pan, W. Temperature Control of P(VDF-TrFE) Copolymer Thin Films. Integr. Ferroelectr. 2013, 141, 187-194. (23) Cheon, S.; Kang, H.; Kim, H.; Son, Y.; Lee, J. Y.; Shin, H.-J.; Kim, S.-W.; Cho, J. H. HighPerformance Triboelectric Nanogenerators Based on Electrospun Polyvinylidene Fluoride-Silver Nanowire Composite Nanofibers. Adv. Funct. Mater. 2018, 28, 1703778. (24) Lee, K. Y.; Kim, S. K.; Lee, J.-H.; Seol, D.; Gupta, M. K.; Kim, Y.; Kim, S.-W. Controllable Charge Transfer by Ferroelectric Polarization Mediated Triboelectricity. Adv. Funct. Mater. 2016, 26, 3067-3073. (25) Seung, W.; Yoon, H.-J.; Kim, T. Y.; Ryu, H.; Kim, J.; Lee, J.-H.; Lee, J. H.; Kim, S.; Park, Y. K.; Park, Y. J.; Kim, S.-W. Boosting Power-Generating Performance of Triboelectric Nanogenerators via Artificial Control of Ferroelectric Polarization and Dielectric Properties. Adv. Energy Mater. 2017, 7, 1600988. (26) Meng, N.; Zhu, X.; Mao, R.; Reece, M. J.; Bilotti, E. Nanoscale Interfacial Electroactivity in PVDF/PVDF-TrFE Blended Films with Enhanced Dielectric and Ferroelectric Properties. J. Mater. Chem. C 2017, 5, 3296-3305. (27) Prashantha, K.; Lacrampe, M.-F.; Krawczak, P. Highly Dispersed Polyamide-11/Halloysite Nanocomposites: Thermal, Rheological, Optical, Dielectric, and Mechanical Properties. J. Appl. Polym. Sci. 2013, 130, 313-321. (28) Alkoy, E. M.; Berksoy-Yavuz, A. Electrical Properties and Impedance Spectroscopy of Pure and Copper-Oxide-Added Potassium Sodium Niobate Ceramics. IEEE Trans. Ultrason. Ferroelectr. Freq. Control. 2012, 59, 2121-2128. (29) Rojac, T.; Bencan, A.; Drazic, G.; Kosec, M.; Damjanovic, D. Piezoelectric Nonlinearity and Frequency Dispersion of the Direct Piezoelectric Response of BiFeO3 Ceramics. J. Appl. Phys. 2012, 112, 064114. (30) Zeng, J.; Zhao, K.; Shi, X.; Ruan, X.; Zheng, L.; Li, G. Large Strain Induced by the Alignment of Defect Dipoles in (Bi3+,Fe3+) Co-Doped Pb(Zr,Ti)O3 Ceramics. Scr. Mater. 2018, 142, 20-22. (31) Gołasa, K.; Grzeszczyk, M.; Korona, K. P.; Bożek, R.; Binder, J.; Szczytko, J.; Wysmołek, A.; Babiński, A. Optical Properties of Molybdenum Disulfide (MoS2). Acta Phys. Pol. A 2013, 124, 849851. (32) Yang, L.; Cui, X.; Zhang, J.; Wang, K.; Shen, M.; Zeng, S.; Dayeh, S. A.; Feng, L.; Xiang, B. Lattice Strain Effects on the Optical Properties of MoS2 Nanosheets. Sci. Rep. 2014, 4, 5649. (33) Gupta, S.; Tomar, M.; James, A. R.; Gupta, V. Study of A-Site and B-Site Doping on Multiferroic Properties of BFO Thin Films. Ferroelectrics 2013, 454, 41-46. (34) Gupta, S.; Tomar, M.; Gupta, V.; James, A. R.; Pal, M.; Guo, R.; Bhalla, A. Optimization of Excess Bi Doping to Enhance Ferroic Orders of Spin Casted BiFeO3 Thin Film. J. Appl. Phys. 2014, 115, 234105. (35) Lomenzo, P. D.; Zhao, P.; Takmeel, Q.; Moghaddam, S.; Nishida, T.; Nelson, M.; Fancher, C. M.; Grimley, E. D.; Sang, X.; LeBeau, J. M.; Jones, J. L. Ferroelectric Phenomena in Si-Doped HfO2 Thin Films with TiN and Ir Electrodes. J. Vac. Sci. Technol. B 2014, 32, 03D123. (36) Zhu, G.; Lin, Z. H.; Jing, Q.; Bai, P.; Pan, C.; Yang, Y.; Zhou, Y.; Wang, Z. L. Toward LargeScale Energy Harvesting by a Nanoparticle-Enhanced Triboelectric Nanogenerator. Nano Lett. 2013, 13, 847-853. (37) 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, 6720-6728.


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Output Power Density Enhancement of Triboelectric Nanogenerators

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