Improved triboelectric nanogenerator output ... - ACS Publications

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Applications of Polymer, Composite, and Coating Materials

Improved triboelectric nanogenerator output performance through polymer nanocomposites filled with core-shell structured particles Xinyu Du, Yuebo Liu, Jiaona Wang, Huidan Niu, Zuqing Yuan, Shuyu Zhao, Xiuling Zhang, Ran Cao, Yingying Yin, Nianwu Li, Chi Zhang, Yi Xing, Weihua Xu, and Congju Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05966 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 9, 2018

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ACS Applied Materials & Interfaces

Improved triboelectric nanogenerator output performance through polymer nanocomposites filled with core-shell structured particles Xinyu Dua,┴, Yuebo Liuc,┴, Jiaona Wangc,*, Huidan Niua, Zuqing Yuana, Shuyu Zhaob, Xiuling Zhanga, Ran Caoa, Yingying Yina, Nianwu Lia, Chi Zhanga, Yi Xingb, *, Weihua Xua, Congju Lia,b,* a

Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences,

Beijing 100083, P. R. China; School of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China. b

School of Energy and Environmental Engineering, University of Science and

Technology Beijing, Beijing 100083, China. c

School of Materials Science & Engineering, Beijing Institute of Fashion Technology,

Beijing 100029, China; Beijing Key Laboratory of Clothing Materials R&D and Assessment, Beijing 100029, China KEYWORDS: core-shell structure, nanocomposite, triboelectric nanogenerator, in situ polymerization, dielectric

ABSTRACT

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Core-shell structured BaTiO3-poly(tert-butyl acrylate) (PtBA) nanoparticles are successfully prepared by in situ atom transfer radical polymerization (ATRP) of tert-butyl acrylate (tBA) on BaTiO3 nanoparticle surface. The thickness of the PtBA shell layer could be controlled by adjusting the feed ratio of tBA to BaTiO3. The BaTiO3-PtBA nanoparticles are introduced into poly (vinylidene fluoride) (PVDF) matrix to form a BaTiO3-PtBA/PVDF nanocomposite. The nanocomposites keep the flexibility of the PVDF matrix with enhanced dielectric constant (~15@100 Hz) due to the high permittivity of inorganic particles and the ester functional groups in the PtBA. Furthermore, the BaTiO3-PtBA/PVDF nanocomposites demonstrate the inherent small dielectric loss of the PVDF matrix in the tested frequency range. The high electric field dielectric constant of the nanocomposite film was investigated by polarization hysteresis loops. The high electric field effective dielectric constant of the nanocomposite is 26.5 at 150 MV/m. The output current density of the nanocomposite-based triboelectric nanogenerator (TENG) is 2.1 µA/cm2, which is above 2.5 times higher than the corresponding pure PVDF-based TENG.

INTRODUCTION Recently, the triboelectric nanogenerator (TENG), a new type of nanogenerator based on the combination of triboelectrification and electrostatic induction,1 has been demonstrated and widely utilized in energy harvesting2,3 and self-powered sensing.4-9 TENGs have the advantages of low cost, lightweight, easy to fabricate, and vast 2 ACS Paragon Plus Environment

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material choice.10 According to the triboelectric series,11 the materials should be chosen that have very large ability differences of attracting and retaining electrons. To date, the most widely used negative triboelectric materials are polymers, such as polydimethylsiloxane (PDMS),12 polyimide (PI),13 fluorinated ethylene propylene (FEP),14,15 polytetrafluoroethylene (PTFE),16-19 and polyvinylidene difluoride (PVDF).20 Although the polymers have decent ability to attract and retain electrons, the output performances of TENGs are limited by their relative low dielectric constant. Polymer nanocomposites have drawn intensive interest due to its flexibility, easy processing, and tunable properties.21 Therefore, polymer nanocomposites have become promising candidates as triboelectric materials for breaking the output limitation of the TENGs. The common used method is filling the polymer matrix with inorganic powders to enhance the dielectric constant. However, the agglomeration and precipitation in the mixed polymer solution due to the high surface energy of nanoparticles, yielding poor mechanical property and high dielectric loss.22 To overcome the drawbacks of directly combination of polymer matrix and inorganic nanoparticles, the several surface treatments have been applied to reduce the surface energy and enhance the dispersity of nanoparticles, such as surface modification by silane couple agents,23 growth of a paraffin layer by thermal evaporation,24 and molecular polymerization from the nanoparticles surfaces via an in situ polymerization technique.25



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Among these surface modification techniques, the atom transfer radical polymerization (ATRP) has attracted considerable attention in recent years owing to its outstanding controllability and diverse choices of monomers that can be polymerized. The key of surface modification by ATRP is to produce initiating sites for growing chains. The polymer chains are sturdily fixed on the surfaces of nanoparticles, forming a shell layer with a strong nanoparticle/polymer interface. Core-shell structured BaTiO3/poly(methyl methacrylate) (PMMA) nanoparticles were prepared by ATRP, the PMMA layer prevents nanoparticles aggregation, resulting in a stable dispersibility in organic solvents.26 Furthermore, ATRP has no limitation on the size of inorganic particles. Fukuda et al have used ATRP to prepare core-shell structured SiO2/PMMA with core diameter between 100 and 1500 nm.27 The ATRP technique brings not only improved dispersibility but also increased dielectric permittivity and ultralow dielectric loss.28 In this work, poly (tert-butyl acrylate) (PtBA) was in situ grown from the surface of the BaTiO3 nanoparticles to form core-shell structured BaTiO3-PtBA nanoparticles by ATRP technique. Polyvinylidene difluoride (PVDF) was chosen as polymer matrix to binding to

the

core-shell structured BaTiO3-PtBA

nanoparticles to form

BaTiO3-PtBA/PVDF nanocomposites. The PtBA shell layer not only can reduce the surface energy of the nanoparticles and avoid aggregation but also provide the net dipole moment by its ester groups. Therefore, the as-fabricated nanocomposites have high breakdown field, high dielectric constant and good mechanical properties so as to improve the output performance of TENGs. As the weight ratio increased to 30% 4 ACS Paragon Plus Environment

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(w/w), the dielectric constant increased from 8.5 to 15 at 100 Hz, which was attributed to the high dielectric constant of BaTiO3. The TENG fabricated with the BaTiO3-PtBA/PVDF nanocomposite generated a current density of 2.1 µA/cm2, above 2.5 times higher than the corresponding pure PVDF-based TENG. The current density can be further increased to 6.1 µA/cm2 by tuning the surface charge density with ion injection method.29 The BaTiO3-PtBA/PVDF nanocomposite film is highly flexible and can endure repeated bending and unbending treatments. The enhanced output performance was stable and reliable after the entire bending process. Furthermore, a higher polarization and energy density of the nanocomposite were also demonstrated. RESULTS AND DISCUSSION The preparation process and characterizations of the core-shell structured BaTiO3-PtBA nanoparticles are shown in Figure 1. The BaTiO3-PtBA nanoparticles were prepared via ATRP of tert-butyl acrylate (tBA) from the surface of BaTiO3 nanoparticles. Figure 1a schematically illustrates the preparation process of the BaTiO3-PtBA nanoparticles. The first step is hydroxylation of the BaTiO3 nanoparticles by aqueous solution of H2O2 for introduction of the -OH groups on their surface. The second step is linking the silane with the reactive functional groups of -OH by (3-aminopropyl)triethoxysilane (APS) treatment. The third step is reacting BaTiO3-APS with α-bromoisobutyryl bromide to obtain initiating sites for ATRP



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reaction. The last step is the polymerization reaction of tBA monomer from BaTiO3-APS-Br to form BaTiO3-PtBA nanoparticles. The FT-IR spectra (Figure 1b) show the characteristic absorption bands at different stages of the process. The as-received BaTiO3 show a broad absorption band in the range of 530 to 650 cm-1, which represents the vibration mode of a metal–dioxo bridge.30 After H2O2 modification, there is a deep absorption band at 3434 cm-1, which represents the stretching vibration mode of -OH, indicating that BaTiO3-OH nanoparticles were successfully obtained.31 The BaTiO3-APS-Br show an absorption band at 1622 cm-1, which represents the vibration mode of C=O of α-bromoisobutyryl bromide. The spectrum of BaTiO3-PtBA exhibits the characteristic PtBA absorption bands at 1150 (C-O-C groups) and 1726 cm-1 (C=O groups).31 Figure 1d-e displays the transmission electron microscopy (TEM) images of core-shell structured BaTiO3-PtBA. It can be seen that the thickness of the PtBA shell layer can be regulated by adjusting the feed ratio of tBA to BaTiO3 during ATRP. Thermogravimetric analysis (TGA) results (Figure 1c) are presented for quantitative analysis of the weight ratio of BaTiO3-PtBA. The weight loss of BaTiO3-PtBA increases from 15% to 28% as the feed ratio of tBA to brominated BaTiO3 increases from 1:4 to 1:10, which agree with observation of morphology by TEM. The weight percent of the core-shell structured BaTiO3-PtBA in the nanocomposite film was determined as 10, 20 and 30% (denoted as PVDF-W10, PVDF-W20 and PVDF-W30). The nanocomposite films were prepared by solution casting method and 6 ACS Paragon Plus Environment

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characterized by x-ray diffraction (Figure S1). After peeled off from the glass slice, an Al layer was sputtered on the film as back electrode. The nanocomposite film was subsequently adhered on an acrylic substrate, a layer of Al was prepared on the opposite substrate to form a contact-separation mode TENG, as shown in Figure 2a. For investigating the electric performance, the acrylic substrate with the nanocomposite film was fixed on a linear motor and the opposite substrate was fixed on an 3D stage. The separation distance and operation frequency were set to 10 mm and 2 Hz, respectively. The TENG with pure PVDF film generated an AC output of 0.8 µA/cm2. For the TENGs with nanocomposite films, the output current density was increased with increasing loading ratio of the BaTiO3-PtBA particles. The current density can be enhanced to1.1, 1.5, and 2.1 µA/cm2 for PVDF-W10, PVDF-W20, and PVDF-W30, respectively. The nanocomposite films with higher than 30% loading of nanoparticles had poor mechanical strength caused by the structural imperfections, thus unsuitable for TENGs. The instantaneous power densities were measured by connecting a resistance box to the PVDF-W30 based TENG, plotted in Figure 2c. The voltage builds up with increasing load resistance, while the current density drops due to the Ohmic loss. As a result, a maximum instantaneous power density of 224 mW/m2 is achieved at a load resistance of 5 MΩ. The capacitor charging characteristic of the PVDF-W30 TENG was investigated with a rectifier, as shown in Figure 2d. Under an operation frequency of 10 Hz, the capacitors of 2.2, 4.7, and 10 µF could be charged up to 3 V in 43, 55, and 127 s, respectively.



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The dielectric constant is a key feature of TENGs. The transferred charge density of the TENG is:32

σ =

⁄

(1)

where z is separation distance, σc is the surface charge density, d and ɛ are the thickness and dielectric constant of the nanocomposite films, respectively. This equation means that the transferred charge density is proportional to the dielectric constant of the dielectric film. Therefore, the output performance of the TENG could be enhanced by increasing the dielectric constant of the triboelectric materials. Frequency-dependent dielectric properties of the PVDF-based films were measured at room temperature, as depicted in Figure 3. The dielectric constant increases with increasing BaTiO3-PtBA content over the whole tested frequency range (Figure 3a). The dielectric constant at 100 Hz is 8.5 for pure PVDF and is enhanced to 15 for PVDF-W30. The dielectric properties of non-treated BaTiO3/PVDF (30% w/w) composite film are worse than that of the PVDF-W30, as shown in Figure S2. The worse dielectric properties can be attributed to the aggregation of the particles and poor dispersion of the BaTiO3 particles clusters in the solution. The aggregation of BaTiO3 particles can be observed in TEM image of the BaTiO3/PVDF film, as shown in Figure S3a. On the contrary, the core-shell structured BaTiO3-PtBA particles display a good dispersion and no aggregation can be observed (Figure S3b).

The

enhanced dielectric constant is the origin of the increased electrical output of TENG which is consistent with the above theoretical analysis. It should be noted that all the PVDF-based films show the characteristics of low dielectric loss over the whole 8 ACS Paragon Plus Environment

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tested frequency range. For frequencies above 500 Hz, the dielectric loss of the nanocomposite films even smaller than the pure PVDF film. These results demonstrate that ATRP could be a promising way to synthetize dielectric materials for high output performance TENGs. The PtBA shell in situ grown on the surface of the BaTiO3 particles, not only avoid the aggregation and phase separation but also keep the excellent mechanical properties of the polymer. For comparision, 30 wt% unmodified BaTiO3 particles were filled in PVDF solution to fabricate BaTiO3/PVDF nanocomposite film. The as-fabricated film is brittle and hard without any flexibility. A lot of defects and particle aggregations are observed in the SEM image (Figure S4a). On the other hand, the composite film fabricated by the modified core-shell BaTiO3-PtBA nanoparticles shows good flexibility and integrated surface micromorphology (Figure S4b). To demonstrate the durability and flexibility of the PVDF-based films, the films were bent 3000 times, as shown Figure 4a. Subsequently, the bent films were fabricated to TENGs. The output current densities of the pure PVDF and the PVDF-W30 TENGs have no significant change before or after bending, further proving that the core-shell structured nanoparticle could keep the excellent mechanical properties of the polymer matrix. The surface charge densities of the PVDF-based films were enhanced by ionized-air injection method.29 The surface charge density is determined by the dielectric constant which indicates the ability of the materials to retaining electrons. By ion injection, the output current density was enhanced for both pure PVDF and PVDF-W30 TENGs. After ion injection, the output current density of the PVDF-W30 9 ACS Paragon Plus Environment


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TENG is about 6 µA/cm2 that is near 3 times than that of pure PVDF TENG due to its higher surface charge density. These results were higher than the values presented in previous reports, as summarized in Table S1. Because the excellent mechanical properties of the nanocomposite film, the PVDF-W30 TENG exhibits good stability and durability. The generated current of the PVDF-W30 TENG did not appear to change significantly through 21,000 cycles, as shown in Figure 4d. These results demonstrate the reliability and feasibility of the nanocomposite films as the TENG materials. As demonstrated in Figure 5a and b, a green light emitting diode (LED) can be lighted up by PVDF-based TENGs. The luminance of the PVDF-W30 TENG powered LED is higher than that of PVDF TENG powered one due to its higher output current density. In accordance with this result, the PVDF-W30 TENG could charge a 2.2 µF capacitor to 3 V in 45 s which is faster than PVDF TENG, as shown in Figure 5c. The brighter LED and faster charging time can be attributed to the high dielectric constant of the PVDF-W30 essentially. It is necessary to test the effective dielectric constant of the materials at high electric field considering the high output voltage of TENG.33 The effective dielectric constant was measured by polarization hysteresis loops, as shown in Figure 5d. For a linear dielectric material, such as polymers, the energy density (Ue) is proportional to the square of electric ﬁeld.34,35

=

（2）

where ɛ0 is the vacuum permittivity (ɛ0 = 8.85×10-12 F/m), k is the effective dielectric constant of the material at electric field of E. The energy density (Ue) of the material 10 ACS Paragon Plus Environment

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can be obtained by hysteresis loop (shaded area in Figure 5d). Therefore, the calculated effective dielectric constants at 150 MV/m are 26.5 and 20.4 for PVDF-W30 and PVDF films, respectively. This result means that the dielectric constant could be enhanced at both low and high electric field by inducing BaTiO3-PtBA nanoparticles into PVDF matrix. Furthermore, the nanocomposite film has better electrical energy density (2.6 J/cm3) than that of pure PVDF film (2.0 J/cm3). For the non-treated BaTiO3/PVDF composite films, the electric field are limited under 100 MV/m due to its poor mechanical properties, which causes a low electrical energy density (Figure S5).

CONCLUSION In summary, a PtBA shell layer was in situ grown on the surface of BaTiO3 nanoparticle. The core-shell structured nanoparticles were filled into PVDF matrix to form a BaTiO3-PtBA/PVDF nanocomposite film. The PtBA shell not only can reduce the surface energy of the nanoparticles and avoid aggregation but also provide the net dipole moment by its ester groups. As a result, a nanocomposite film with high dielectric constant (ɛ=15) was obtained. The TENG fabricated with the BaTiO3-PtBA/PVDF nanocomposite generated a current density of 2.1 µA/cm2, above 2.5 times higher than the corresponding pure PVDF-based TENG. The current density can be further increase to 6.1 µA/cm2 by tuning the surface charge density with ion injection method. Furthermore, the PtBA shell layer could keep the excellent mechanical properties of the polymer matrix. The nanocomposite film exhibits good 11 ACS Paragon Plus Environment


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flexibility and integrated surface micromorphology at a high loading ratio of 30%. The high electric field dielectric constant of the nanocomposite film was investigated by polarization hysteresis loops. A high effective dielectric constant of 26.5 at 150 MV/m was obtained from the nanocomposite film which is higher than of the pure PVDF film. The results of this work demonstrated that ATRP could be a promising way to fabricate nanocomposite films with high dielectric constant for improving the output performance of the TENGs. METHODS Materials: BaTiO3 (99.9%,

Improved triboelectric nanogenerator output ... - ACS Publications

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