Recent Structure Development of Poly(vinylidene fluoride)-Based Piezoelectric Nanogenerator for Self-Powered Sensor

As the internet of things (IoT) era approaches, various sensors, and wireless electronic devices such as smartphones, smart watches, and earphones are emerging. As the types and functions of electronics are diversified, the energy consumption of electronics increases, which causes battery charging and maintenance issues. The piezoelectric nanogenerator (PENG) received great attention as an alternative to solving the energy issues of future small electronics. In particular, polyvinylidene fluoride (PVDF) piezoelectric polymer-based PENGs are strong potential candidate with robust mechanical properties and a high piezoelectric coefficient. In this review, we summarize the recent significant advances of the development of PVDF-based PENGs for self-powered energy-harvesting systems. We discuss the piezoelectric properties of the various structures of PVDF-based PENGs such as thin film, microstructure, nanostructure, and nanocomposite.


Introduction
Due to the recent miniaturization of electronic devices and the addition of various functions, it is insufficient to supply power only with a battery. Therefore, energy harvesting technology that converts environmental energy into electrical energy is attracting attention, in particular, piezoelectric nano generators (PENGs), that convert mechanical energy into electrical energy, are in the spotlight. The piezoelectric effect, discovered by Jacques Curie and Pierre Curie in 1880, is the phenomenon of electricity generation when external pressure is applied to a piezoelectric material [1,2] that has a non-centrosymmetric crystal structure [3]. It is known that the piezoelectric effect appears when the polarization strength changes as the dipole moment changes due to external stress [4]. If electrodes are placed on both sides of the piezoelectric material, electrons flow through the external circuit to reduce the potential difference caused by the changed polarization. When the applied pressure is released, the polarization returns to its original state, and the electrons return to their original state through the external circuit. This process produces piezoelectric voltage and current from the PENG. Over the decades, various materials for high performance piezoelectric elements have been studied. Inorganic piezoelectric materials such as zinc oxide (ZnO) [5][6][7][8][9][10], lead zirconate titanate (PZT) [11][12][13][14], lead magnesium niobate-lead titanate (PMN-PT) [15][16][17], and barium titanate (BaTiO 3 ) [18][19][20], as well as organic materials such as polyvinylidene fluoride (PVDF), poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) [21][22][23], nylon [24], diisopropylammonium bromide [25], and biomolecular materials [26][27][28] have been developed. Compared to inorganic piezoelectric materials, organic piezoelectric material has various
In 2014, Pi et al. reported an poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) thinfilm-based PENG for a flexible nanogenerator [44]. A gold (Au) layer was deposited as a lower electrode on a silicon dioxide layer deposited on polyimide (PI) substrate. The P(VDF-TrFE) solution was spin coated on a lower electrode, and Au was deposited on a P(VDF-TrFE) thin film again as an upper electrode (Figure 2a). The manufactured P(VDF-TrFE)-based PENG was electrically poled by the application of an external electric field with a strength of 0.5-0.8 MV/cm. The output performance of the PENG was increased as the alignment of the polarization direction by the poling process due to exhibiting ferroelectric property as shown in the polarization-electric field (P-E) curve (Figure 2b). The output performance of the P(VDF-TrFE) thin-film-based PENG was measured with the repetition of a stretching and releasing process. The PENG produced an output voltage and current of 7 V and 58 nA, respectively, when a strain of 0.943% was applied with a frequency of 0.5 Hz. As
In 2014, Pi et al. reported an poly(vinylidenefluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin-film-based PENG for a flexible nanogenerator [44]. A gold (Au) layer was deposited as a lower electrode on a silicon dioxide layer deposited on polyimide (PI) substrate. The P(VDF-TrFE) solution was spin coated on a lower electrode, and Au was deposited on a P(VDF-TrFE) thin film again as an upper electrode (Figure 2a). The manufactured P(VDF-TrFE)-based PENG was electrically poled by the application of an external electric field with a strength of 0.5-0.8 MV/cm. The output performance of the PENG was increased as the alignment of the polarization direction by the poling process due to exhibiting ferroelectric property as shown in the polarization-electric field (P-E) curve (Figure 2b). The output performance of the P(VDF-TrFE) thin-film-based PENG was measured with the repetition of a stretching and releasing process. The PENG produced an output voltage and current of 7 V and 58 nA, respectively, when a strain of 0.943% was applied with a frequency of 0.5 Hz. As the applied frequency and strain increased, the output voltage and current of the PENG also increased (Figure 2c). the applied frequency and strain increased, the output voltage and current of the PENG also increased ( Figure 2c). In 2013, Lee et al. reported a highly stretchable nanogenerator (HSNG) for mechanical and thermal energy harvesting based on P(VDF-TrFE) thin film [45]. The HSNG consists of three layers: a patterned polydimethylsiloxane-carbon nanotube (PDMS-CNT) which acts as the bottom electrode, P(VDF-TrFE) as the piezoelectric and pyroelectric polymer and graphene as the top electrodes ( Figure 2d). Since the HSNG is very elastic, flexible, and stretchable, it can be used as a power source for various flexible and stretchable electronics. In addition, the efficiency of pyroelectric power generation is increased by using a high thermal conductive graphene electrode. The measured piezoelectric output voltage was 0.7 V by 30% of the stretching-releasing cycle, and the measured pyroelectric output voltage was 0.4 V by the heating-cooling (ΔT = 30 °C) cycle ( Figure 2e). The measured output voltage was 1.1 V when the piezoelectric and pyroelectric effects were hybridized by a coupling phenomenon. In addition, the HSNG was mechanically stable up to the strain of 30%.
In 2016, Bhavanasi et al. reported a bilayer piezoelectric nanogenerator that added a graphene oxide (GO) layer to a P(VDF-TrFE) film to enhance Young's modulus and the piezoelectric properties ( Figure 2f) [46]. The PENG was fabricated by the drop casting of the P(VDF-TrFE) and GO solution on the indium tin oxide (ITO) electrode, and Au was deposited on the P(VDF-TrFE) layer as an upper In 2013, Lee et al. reported a highly stretchable nanogenerator (HSNG) for mechanical and thermal energy harvesting based on P(VDF-TrFE) thin film [45]. The HSNG consists of three layers: a patterned polydimethylsiloxane-carbon nanotube (PDMS-CNT) which acts as the bottom electrode, P(VDF-TrFE) as the piezoelectric and pyroelectric polymer and graphene as the top electrodes ( Figure 2d). Since the HSNG is very elastic, flexible, and stretchable, it can be used as a power source for various flexible and stretchable electronics. In addition, the efficiency of pyroelectric power generation is increased by using a high thermal conductive graphene electrode. The measured piezoelectric output voltage was 0.7 V by 30% of the stretching-releasing cycle, and the measured pyroelectric output voltage was 0.4 V by the heating-cooling (∆T = 30 • C) cycle ( Figure 2e). The measured output voltage was 1.1 V when the piezoelectric and pyroelectric effects were hybridized by a coupling phenomenon. In addition, the HSNG was mechanically stable up to the strain of 30%.
In 2016, Bhavanasi et al. reported a bilayer piezoelectric nanogenerator that added a graphene oxide (GO) layer to a P(VDF-TrFE) film to enhance Young's modulus and the piezoelectric properties ( Figure 2f) [46]. The PENG was fabricated by the drop casting of the P(VDF-TrFE) and GO solution on the indium tin oxide (ITO) electrode, and Au was deposited on the P(VDF-TrFE) layer as an upper electrode. The thickness of the GO and P(VDF-TrFE) layer was about 19 µm and 32 µm, respectively. The fabricated PENG is flexible enough for bending or rolling. The P(VDF-TrFE)-GO bilayer PENG produced the maximum output voltage and current of 4.26 V and 1.88 µA, respectively, whereas the P(VDF-TrFE) monolayer-based PENG only produced an output voltage and current of 2.1 V and 0.96 µA under the compression pressure of 0.32 MPa (Figure 2g), respectively. In the case of the P(VDF-TrFE) monolayer, the mechanical-electrical conversion efficiency of the PENG was determined by the direct influence of the P(VDF-TrFE) intrinsic permittivity and Young's modulus. However, in the case of the P(VDF-TrFE)-GO double layer, not only the effect of GO's dielectric constant and Young's modulus but also the electrostatic effect were combined to increase the mechanical-electrical conversion efficiency by two times the output voltage and 2.5 times the output power.
In 2014, Chen et al. reported a microporous PVDF-based PENG [71]. Figure 3a shows a schematic illustration of the microporous PVDF-based PENG, which was composed of kapton substrate, bottom electrode, porous PVDF and upper electrode. The microporous film was fabricated by the solvent evaporation method. The porous surface increases the compressibility of PVDF, and the piezoelectric constant is proportional to the compressibility, Equations (1) and (2), and results in the increased output performance of the PENG: Actuators 2020, 9, x FOR PEER REVIEW 5 of 16 The output performance of the microporous PVDF film-based PENGs was measured by compression and tension at 1 Hz by bending. Bare PVDF film (0:5 = acetone:methyl ethyl ketone) produced an output voltage of 0.8 V and an output current of 160 nA. The microporous PVDF filmbased PENGs (3:2 = acetone:methyl ethyl ketone) produced an output voltage of 2 V and an output current of 300 nA under the same conditions due to the increased compressibility (Figure 3b). In addition, the porous PENG produced sufficient energy to charge a 1 μF capacitor up to 1.7 V for 360 cycles that showed two times the improved performance compared to the bare PVDF film PENG (0.82 V).
In 2015, Lee et al. reported a micropattern P(VDF-TrFE)-based PENG for a mechanical stimuli The output performance of the microporous PVDF film-based PENGs was measured by compression and tension at 1 Hz by bending. Bare PVDF film (0:5 = acetone:methyl ethyl ketone) produced an output voltage of 0.8 V and an output current of 160 nA. The microporous PVDF film-based PENGs (3:2 = acetone:methyl ethyl ketone) produced an output voltage of 2 V and an output current of 300 nA under the same conditions due to the increased compressibility (Figure 3b). In addition, the porous PENG produced sufficient energy to charge a 1 µF capacitor up to 1.7 V for 360 cycles that showed two times the improved performance compared to the bare PVDF film PENG (0.82 V).
In 2015, Lee et al. reported a micropattern P(VDF-TrFE)-based PENG for a mechanical stimuli sensor system [72]. Micropatterned P(VDF-TrFE) is fabricated using the Si mold made by simple photolithography and chemical etching process. The trigonal line-shaped and pyramid-shaped micropatterned PVDF was fabricated on the ITO-coated polyethylene naphthalate (PEN) substrate, and the Ag electrode was deposited on top of the patterned P(VDF-TrFE) (Figure 3c). Due to the patterned structure, the piezoelectric element can be highly deformed by vertical pressure, thereby improving the piezoelectric output performance. The output performance of the micropattern PENGs was measured by applying a vertical compressive force at the top of the PENG using a mechanical force stimulator. The piezoelectric output voltages and currents of a flat, trigonal line, and pyramid-patterned PENG were 0.4, 2.0, and 2.9 V, and 100, 500, and 900 nA, respectively, under the vertical compressive force of 1.5 N, as shown in Figure 3d. In addition, the output voltage and current were 4.4 V and 3.4 µA for the pyramid-shaped PENG at 16 N. The pyramidal patterned P(VDF-TrFE)-based PENGs exhibited a five times stronger output than the flat P(VDF-TrFE)-based PENGs. In practical application, the pyramidal pattern-based PENG can detect extremely low wind speeds of 0.5 m/s and can be used as self-powered active pressure sensors.
In 2015, Choi et al. reported vertically aligned P(VDF-TrFE) core-shell structures for PENG [73]. The P(VDF-TrFE) layers were coated onto the platinum/polyurethane acrylate (PUA) pillars and the Pt layer was subsequently deposited on the P(VDF-TrFE) layer (Figure 3e). This micro-sized pillar structure is also easier to deform than film by external mechanical force, thus improving the piezoelectric efficiency. In order to evaluate the piezoelectric properties of the core-shell structure P(VDF-TrFE), the reverse piezoelectric effect was measured by using voltage-displacement analysis. The displacement of the nanoindentation tip in contact with a 1 µm-thick micropillar was measured with a position accuracy of 0.01 nm. A voltage via of 5 V was applied between the tip and bottom electrodes. The P(VDF-TrFE) film showed a displacement of 0.2 nm, and the P(VDF-TrFE) core-shell micropillar showed a displacement of 0.37 nm. The average values of d 33 of the P(VDF-TrFE) film and the P(VDF-TrFE) core-shell micropillar were (40 ± 2) pm/V and (74 ± 2) pm/V, respectively (Figure 3f). The core-shell micropillar structure showed a 1.85-fold improvement in piezoelectric constant d 33 compared to the flat film.
In 2015, Chen et al. reported a microfiber PENG with vertically aligned P(VDF-TrFE) [74]. The P(VDF-TrFE) microfiber PENG was used to harvest the mechanical energy and operate the luminescent electronic devices. The vertically aligned P(VDF-TrFE) microfiber with 8 µm in diameter and 50 µm in height were fabricated by the electrohydrodynamically (EHD) pulling method. The P(VDF-TrFE) microfibers are vertically connected to both top and bottom electrodes (Figure 3g). The piezoelectric output performances of 10 mm x 10 mm of the PENG were measured using an electrochemical vibrator with a 25 N force and 1 Hz of frequency. The peak output voltage and current were 4 V and 2.6 µA, respectively (Figure 3h). The P(VDF-TrFE) microfiber PENG was able to charge the 47 µF capacitor to 5.1 V for 270 s and drive a four-bit liquid crystal display (LCD). In addition, it was able to control its output performance by controlling the diameter and shape of the P(VDF-TrFE). Since the air gap between the fibers makes the array structure porous, hence increasing the stress on the piezoelectric material. The finite element method (FEM) simulation performed to evaluate the effect of the diameter and shape of the P(VDF-TrFE) on the piezoelectric effect.
In 2014, Mao et al. reported a mesoporous PVDF for a scalable PENG and self-powered electronic system [91]. Mesoporous PVDF thin film was prepared by casting a mixture of ZnO nanoparticle and PVDF solution, and then removing the ZnO nanoparticle with HCl solution (Figure 4a). The HCl solution was chosen because the PVDF is a hydrophobic polymer with good chemical stability against corrosive solvent-containing acids. The ZnO nanoparticles create porosity in the PVDF films and help the PVDF to form a piezoelectric β-phase. The open-circuit voltage and short-circuit current were measured under a 40 Hz condition. The mesoporous PVDF-based PENG produced an 11.0 V average output voltage and 9.8 µA output current (Figure 4b). This output performance is much higher than that of the bare PVDF bulk film (5.0 V). The conversion efficiency of the mesoporous PVDF PENG improved as the ZnO mass fraction increased up to 50%, but decreased at a higher ratio due to the formation of the decreased β-phase (Figure 4c). The optimal porosity for achieving the maximum V oc output was identified to be -11.0 V. As a demonstration, parallelly connected four PVDF PENGs are attached on the back of the phone. When the oscillator was turned on, the PENG underneath was activated by the phone's weight, and the generated electricity can charge a 47 µF capacitor to 3.7 V.
In 2010, Chang et al. reported a PVDF nanofiber-based PENG for mechanical energy harvesting [80]. PVDF single nanofiber was prepared using the near field electrospinning method (Figure 4c). The crystal structure of the nanofiber was transformed into polar β-phase by strong electric fields (greater than 10 7 V/m) and stretching forces during the electrospinning process. The electrospun PVDF nanofibers have a diameter from 500 nm to 6.5 µm. The measured piezoelectric output voltage and current from the PVDF single nanofiber-based PENG were 5−30 mV and 0.5−3 nA, respectively, under an applied 0.085% of strain at 2 Hz (Figure 4d). The energy conversion efficiency of the PENG was calculated as the ratio between the generated electrical energy and the applied mechanical energy. The energy conversion efficiency of the nanofiber PENG was found to be as high as 21.8% with an average of 12.5%, which was much greater than conventional PVDF thin films PENG (0.5-2.6%) under the same conditions (Figure 4e) due to the higher crystallinity and sensitivity [84].
In 2014, Bhavanasi et al. reported P(VDF-TrFE) nanotube-based PENGs synthesized by nanoconfinement effect for mechanical energy harvesting [92]. PVDF nanotubes are prepared using an anodic aluminum oxide (AAO) template with a pore diameter of 200 nm. Due to the surface energy of the AAO wall, which is higher than that of the P(VDF-TrFE), the P(VDF-TrFE) is perfectly wetting on the AAO surface to form a uniformly shaped P(VDF-TrFE) nanotube. The fabricated P(VDF-TrFE) nanotube enhanced the ferroelectric phase (β-phase) formation and the reduced structural defects in the nanostructures formed from nanoconfinement in template pores, resulting in higher piezoelectric coefficients compared to bare films. The P(VDF-TrFE) nanotube PENG was prepared by depositing Au on both sides of the P(VDF-TrFE) as the upper and lower electrodes, following the encapsulation with poly(methyl methacrylate) (PMMA) to prevent gold penetration and device electric shortage (Figure 4g). The ferroelectric properties of the P(VDF-TrFE) nanotubes embedded in AAO were confirmed through the polarization-electric field (P-E) curve that shows strong ferroelectric behavior (Figure 4h  In 2010, Chang et al. reported a PVDF nanofiber-based PENG for mechanical energy harvesting [80]. PVDF single nanofiber was prepared using the near field electrospinning method (Figure 4c). The crystal structure of the nanofiber was transformed into polar β-phase by strong electric fields (greater than 10 7 V/m) and stretching forces during the electrospinning process. The electrospun PVDF nanofibers have a diameter from 500 nm to 6.5 μm. The measured piezoelectric output voltage and current from the PVDF single nanofiber-based PENG were 5−30 mV and 0.5−3 nA, respectively, under an applied 0.085% of strain at 2 Hz (Figure 4d). The energy conversion efficiency of the PENG was calculated as the ratio between the generated electrical energy and the applied mechanical energy. The energy conversion efficiency of the nanofiber PENG was found to be as high as 21.8% with an average of 12.5%, which was much greater than conventional PVDF thin films PENG (0.5-2.6%) under the same conditions (Figure 4e) due to the higher crystallinity and sensitivity [84].
In 2014, Bhavanasi et al. reported P(VDF-TrFE) nanotube-based PENGs synthesized by nanoconfinement effect for mechanical energy harvesting [92]. PVDF nanotubes are prepared using an anodic aluminum oxide (AAO) template with a pore diameter of 200 nm. Due to the surface energy of the AAO wall, which is higher than that of the P(VDF-TrFE), the P(VDF-TrFE) is perfectly wetting
Alluri et al. reported flexible BaTi (1-x) Zr x O 3 (BTZO)/PVDF nanocomposite film PENGs for a self-powered fluid velocity sensor [118]. They synthesized strong piezoelectric ceramic BTZO by the molten salt method and mixed it with PVDF to produce nanocomposite film. Then, the PENG was fabricated by coating Al electrodes on the top and bottom of the nanocomposite film (Figure 5a). The maximum open-circuit voltage and short-circuit current for the BTZO nanocube/PVDF-based PENG were 11.9 V and 1.36 µA, respectively, which are greater than the BaTiO 3 (BTO) nanocube/PVDF output of 7.99 V and 1.01 µA (Figure 5b). The improvement of the piezoelectric output performance of the PENG is due to the improved piezoelectric coefficient by mixing with BTZO nanoparticles. The output performance of BTZO/PVDF-film PENGs was measured with 11 N of mechanical load, applied at 3 Hz, 11 Hz and 21 Hz. As the frequency increased from 3 Hz to 21 Hz, the output voltages and current increased from 5 V and 0.71 µA to 12 V and 1.2 µA, respectively (Figure 5c). The fabricated PENG was used as an active sensor to the detect water velocities in the outlet pipe. The produced average maximum power was 0.2 nW at 31.43 m/s water velocity and increased up to 15.8 nW at 125.7 m/s (Figure 5d). In 2019 Ye et al. reported PENGs based on P(VDF-TrFE)/boron nitride nanotube (BNNT) nanocomposite micropillar for energy harvesting in space [119]. The P(VDF-TrFE)/BNNTs nanocomposite micropillar fabricated using the hot pressed method through the PDMS mold. Then, the PDMS is filled between the micropillars to act as a protective layer (Figure 5e). The improved output performance from the P(VDF-TrFE)/BNNT nanocomposite PENG was observed because BNNT has a higher piezoelectric coefficient and elastic modulus than P(VDF-TrFE). The piezoelectric output performance of the P (VDF-TrFE)/BNNT composite PENG was measured at 2 Hz with an effective area of 1cm 2 under the compressive force of 40 N. The maximum output voltage and current were 40 Vpp and 640 nA, respectively. The output voltage of the bare P(VDF-TrFE) film, P(VDF-TrFE)/BNNT film, P(VDF-TrFE) micropillar, and the P(VDF-TrFE)/BNNT micropillar PENGs were 1 V, 2.6 V, 4.2 V, and 11 V, respectively ( Figure 5f). As for the introduced nanocomposite with microarrays structure, the output performance improved 11 times compared to the bare P(VDF-TrFE)-film PENG. The PENG can operate the RYG LED array after charging three capacitors. Moreover, due to the presence of isotope 10 B with a high neutron absorption rate, the ability of the BNNTs to shield neutron radiation was experimentally confirmed, showing the possibility of being In 2019 Ye et al. reported PENGs based on P(VDF-TrFE)/boron nitride nanotube (BNNT) nanocomposite micropillar for energy harvesting in space [119]. The P(VDF-TrFE)/BNNTs nanocomposite micropillar fabricated using the hot pressed method through the PDMS mold. Then, the PDMS is filled between the micropillars to act as a protective layer (Figure 5e). The improved output performance from the P(VDF-TrFE)/BNNT nanocomposite PENG was observed because BNNT has a higher piezoelectric coefficient and elastic modulus than P(VDF-TrFE). The piezoelectric output performance of the P (VDF-TrFE)/BNNT composite PENG was measured at 2 Hz with an effective area of 1cm 2 under the compressive force of 40 N. The maximum output voltage and current were 40 V pp and 640 nA, respectively. The output voltage of the bare P(VDF-TrFE) film, P(VDF-TrFE)/BNNT film, P(VDF-TrFE) micropillar, and the P(VDF-TrFE)/BNNT micropillar PENGs were 1 V, 2.6 V, 4.2 V, and Actuators 2020, 9, 57 9 of 15 11 V, respectively ( Figure 5f). As for the introduced nanocomposite with microarrays structure, the output performance improved 11 times compared to the bare P(VDF-TrFE)-film PENG. The PENG can operate the RYG LED array after charging three capacitors. Moreover, due to the presence of isotope 10 B with a high neutron absorption rate, the ability of the BNNTs to shield neutron radiation was experimentally confirmed, showing the possibility of being used as a PENG in space.
Kang et al. reported lead-free piezoelectric ceramic (Na 0.5 K 0.5 )NbO 3 (NKN) nanoparticles and P(VDF-TrFE) nanocomposite nanofiber-based PENGs [120]. The nanocomposite nanofiber was prepared by the electrospinning method after the mixing of P(VDF-TrFE) and NKN nanoparticles. The PENG was fabricated by the alignment of the P(VDF-TrFE)/NKN nanocomposite nanofibers in a direction parallel to the upper and lower ITO electrodes (Figure 5g). Due to the strong piezoelectric coefficient of a NKN nanoparticle, the output performance of the nanocomposite PENG was enhanced compared to the bare P(VDF-TrFE) nanofiber-based PENG. The output performance comparison was conducted for PENGs with different NKN concentrations (0 vol%, 5 vol% and 10 vol%). When the device bent, the measured output voltage and current from 0, 5, and 10 vol% nanocomposite PENGs were 0.45 V, 0.64 V, and 0.98 V, and 56 nA, 63 nA and 78 nA, respectively (Figure 5h).

Conclusions
The development of various future wireless electronics and sensors requires the development of the high-performance portable power generator. Recently, various studies are under way showing the possibility of supplying sufficient power from the PENGs. The developments of piezoelectric materials with high piezoelectric coefficients and robust physical properties, and an efficient PENG device structure for various applications have been conducted. In this review, recent studies on PENGs with various structures of PVDF and its applications are discussed. To improve the output performance of the PVDF PENGs, the design of the structure of the piezoelectric materials and devices are very critical for the various applications. The materials and devices preparation method and study of the output performance are summarized. To date, many PENG technologies including piezoelectric materials, material structure, and device structure were reported, contributing to the significant increase in the output power. However, further enhancement in the output power was still essential to the commercialization of the PENG. We believe this review can be useful and helpful in the design and development of PENGs that can provide sufficient energy for self-powered electronics.