Comparison of the PVDF-based nanogenerator performance. The experimental conditions such as applied force, operation mode, area of nanogenerator, and load resistance were different in the literature.
This chapter deals with the development of ferroelectric polymer polyvinylidene fluoride (PVDF)-based nanogenerators. Due to its inherent flexibility, PVDF has been studied for application in nanogenerators. We first introduce PVDF and its copolymers, and briefly discuss their properties. Then, we discuss fabrication methods, including solution casting, spin coating, template-assisted method, electrospinning, thermal drawing, and dip coating. Using these methods, a wide variety of ferroelectric polymer structures can be fabricated. In addition to the performance enhancements provided by fabrication methods, the performance of PVDF-based nanogenerators has been improved by incorporating fillers that can alter the factors affecting the performance. Next, we review energy sources that can be exploited by PVDF-based nanogenerators to harvest electricity. The abundant energy sources in the environment include sound, wind flow, and thermal fluctuation. Finally, we discuss implantable PVDF-based nanogenerators. Another advantage of PVDF is its biocompatibility, which enables implantable nanogenerators. We believe that this chapter can also be helpful to researchers who study sensors and actuators as well as nanogenerators.
- ferroelectric material
All electronic devices need electrical energy to operate. Although fossil fuels have been the primary sources of that electrical energy to date, alternatives are emerging. This is particularly important given the ongoing proliferation of portable devices. For example, as “big data” has become increasingly important for monitoring structures, healthcare services, smart cites, and so on, there has been an explosion of sensors to collect that information [1, 2, 3]. These sensors can even be located in the human body, beneath the human skin, and inside personal wearable devices [4, 5, 6, 7]. In such cases, batteries cannot be easily charged or repeatedly replaced. To meet the energy requirements of remote or inaccessible application like these, one of the most promising alternatives to conventional batteries is a nanogenerator, which can convert mechanical energy or thermal fluctuations in the ambient environment into electricity.
Polyvinylidene fluoride (PVDF) and its copolymers, ferroelectric polymers, are ideal candidates for use in nanogenerators. Their unique properties include their high flexibility, lightness, chemical stability, and relatively simple manufacturing process [8, 9, 10, 11]. The limitation of PVDF-based nanogenerators has been their low power generation capacity. Hence, many research groups have studied to enhance the performance of nanogenerators, by utilizing various fabrication methods, and structures, by incorporating fillers (Table 1).
|Materials||Microstructures||Structures||Open-circuit voltage||Short-circuit current||Power||References|
|P(VDF-TrFE)||—||Flat film||7 V||58 nA||—|||
|P(VDF-TrFE)||—||Curved film||120 V||700 μA||3.9 mW·cm−2|||
|PVDF||—||Fabric||14 V @ 0.1 MPa||29.8 μA @ 0.1 MPa||5.1 μW·m−2|||
|PVDF/Ba(Ti0.9Zr0.1)O3||Nanocubes||Flat film||11.99 V @ 11 N||1.36 μA @ 11 N|||
|PVDF/SnO2||Nanosheets||Flat film||42 V||6.25 μA·cm−2||4900 W·m−3|||
|PVDF/ZnO||Nanoparticles||Flat film||24.5 V @ 28 N||1.7 μA @ 28 N|||
|PVDF/ZnO||Nanowires||Flat film||6.9 V||0.96 μA||6.624 μW|||
|PVDF/AlO-rGO||Nanoparticles||Flat film||36 V @ 31.19 kPa||0.8 μA @ 31.19 kPa||27.97 μW|||
|PVDF/BaTiO3||Nanoparticles||Flat film||10 Vpeak-peak @ 2 N||2.5 μApeak-peak @ 2 N||5.8 μW|||
|PVDF/BaTiO3||Nanowires||Flat film||14 V||4 μA||1.5 μW|||
|PVDF/NKNS-LT-BZ||Nanoparticles||Flat film||18 V @ 50 N||2.6 μA @ 50 N||—|||
|PVDF/NiO@SiO2||Nanoparticles||Flat film||53 V @ 0.3 MPa||0.3 μA·cm−2 @ 0.3 MPa||685 W·m−3|||
|PVDF||—||Electrospun membrane||48 V @ 8.3 kPa||6 μA @ 8.3 kPa||51 μW|||
|PVDF/ZnO||Nanorods||Electrospun membrane||85 V||2.2 μA||—|||
In this chapter, we discuss different fabrication techniques and developments of PVDF-based nanogenerators in detail. This book chapter is organized as follows. In Section 2, we introduce PVDF and its copolymers with their properties. In Section 3, we focus on fabrication methods used to prepare PVDF-based nanogenerators. In Section 4, we briefly cover conventional PVDF film-based nanogenerators. In Section 5, we review composite-based nanogenerators and how certain factors affect their performance of the nanogenerators. In Section 6, we introduce the energy sources that can be harvested by the nanogenerators. In Section 7, we review the biocompatibility of PVDF and related works.
2. PVDF and its copolymers
Semicrystalline PVDF has at least four crystalline modifications: α, β, γ, and δ . Generally, the α-phase is the most stable crystal phase of PVDF in ambient conditions. The conformation of α-PVDF, displayed in Figure 1a, is trans-gauche-trans-gauche (TGTG’). The α-phase is nonpolar because of its centrosymmetric symmetry and can be easily obtained from melt crystallization at atmospheric pressure. On the other hand, the β-phase is classified as a ferroelectric and exhibits the largest remnant polarization of ~ 13 μC/cm2 among the phases. Ferroelectricity in the β-PVDF is directly correlated to its macroscopic dipole moment. In the β-phase, the conformation is all trans (TTTT) as shown in Figure 1b. The transition from the PVDF α-phase to the β-phase can be induced by stretching the polymer. The γ-phase is also polar and its conformation is trans-trans-trans-gauche-trans-trans-trans-gauche (T3GT3G) as shown in Figure 1c. The γ-phase is also ferroelectric. However, the γ-phase is less frequently observed because it requires extreme temperature control and high pressures to develop. The δ-phase has the same configuration as the α-phase. The difference is that the δ-phase is a ferroelectric. Crystallization of the δ-phase can be achieved by electroforming from the bulk α-PVDF in a high electric field of about 170 MV/m. Recently, M. Li
Due to its electroactive properties, high β-phase content is desirable for applications. This can be achieved physically or chemically. The most common chemical derivatives of PVDF available are polyvinylidene fluoride-trifluoroethylene (P(VDF-TrFE)) and polyvinylidene fluoride-hexafluoropropene (P(VDF-HFP)). Despite its high cost, P(VDF-TrFE) is often preferred over PVDF. The main advantage of P(VDF-TrFE) is that the β-phase can easily develop without mechanical stretching or incorporating fillers. The mechanism behind the development of the β-phase is based on the introduction of additional fluorine atoms within a certain amount, which allows steric hindrance to occur. P(VDF-HFP) has received a lot of attention because of its extremely high electrostrictive response . As with PVDF, the β-phase in P(VDF-HFP) can be obtained by mechanical stretching . Another way to develop the β-phase in P(VDF-HFP) is casting from solution in dimethylformamide (DMF) .
Since the discovery of piezoelectricity in PVDF , the mechanism behind the piezoelectric response has been a subject of debate. Recently, L. Katsouras
In order to identify the phases of PVDF, analysis techniques based on electromagnetic radiations such as X-ray and infrared have been widely used. The crystalline structure in PVDF and its copolymers can be confirmed via X-ray diffraction (XRD). In general, the nonpolar α-phase and the polar β- and γ-phases of PVDF appear in XRD patterns (Figure 2a) . There are peaks at 18.4, 19.9, and 26.6° corresponding to (020), (110), and (021) reflections of the monoclinic α-phase, respectively. The peak at 20.6° is associated with the crystalline (200) and (110) of the orthorhombic β-phase. For the γ-phase, the dominant peaks appear at 18.5 and 20.2° corresponding to (020) and (110), respectively. In the case of P(VDF-TrFE), the peak corresponding to the (200) and (110) planes of the β-phase crystalline phase is represented at 19.7°. The position and width of peaks can change depending on experimental conditions and the ratio between VDF and TrFE. Therefore, the diffraction patterns can be differently observed in the literature.
The ferroelectric phase formation of PVDF and its copolymers have been also confirmed using Fourier transform infrared spectroscopy (FTIR). However, there exists a conflict on spectrum peaks corresponding the phases. Recently, X. Cai
where and are the absorbance at 840 and 763 cm−1, respectively, and and are the absorption coefficients at the respective wave numbers, whose values are 7.7 104 and 6.1 104 cm2 mol−1, respectively. If PVDF materials contain both phases, another method is needed to quantify individual β- and γ-phases. The quantification of β- and γ-phases can be demonstrated as follows:
where and are the height differences (absorbance differences) between the peak around at 1275 cm−1 and the nearest valley around at 1260 cm−1, and the peak around 1234 cm−1 and the nearest around at 1225 cm−1, respectively.
3. Fabrication techniques
PVDF and its copolymer-based nanogenerators consist of a layer of PVDF or its copolymers sandwiched between two electrodes, like a capacitor. As previously noted, PVDF and its copolymer-based nanogenerators have advantages over piezoelectric ceramics-based nanogenerators, because of their flexibility and high piezoelectric voltage coefficient (g33). One important consideration is the choice of a suitable technique for the fabrication of ferroelectric polymer-based nanogenerators. A great variety of methods to make ferroelectric polymer-based nanogenerators has been developed for a few decades: solution casting, spin coating, template-assisted method, electrospinning, thermal drawing, and dip coating, as described in Figure 3 [9, 12, 34, 35, 36, 37, 38]. The particular fabrication technique can affect the crystallinity and phase of ferroelectric polymers, influencing their electroactive and mechanical properties. This section covers conventional and recently developed fabrication techniques involving ferroelectric polymers in greater detail.
3.1 Solution casting
Usually, PVDF and its copolymer-based nanogenerators are prepared by solution casting because it is a simple and low-cost process and allows large-scale production. The casting method is also suitable for preparing composites of fillers and a polymer matrix because it is relatively insensitive to the viscosity and density of solution. As a result, solution casting has been used in most research on composite-based nanogenerators [16, 18, 22, 23, 39]. The thickness of the film can be controlled by the type of organic solvents employed and the concentration and amount of the ferroelectric polymer solution. Since evaporation speed can differ depending on the organic solvents used, the morphology of the ferroelectric polymer films can also be affected .
3.2 Spin coating
Spin coating is a universally used technique to fabricate uniform thin polymer films using the centrifugal forces induced by a spinning substrate. The thickness of the films can be controlled by a few parameters, such as solution concentration, viscosity, spin speed, and spin time. Generally, a low concentration polymer solution (<10%) is used for uniformity. In addition, the thickness of spin-coated ferroelectric polymer films can be controlled by the number of successive spin coatings.
Spin coating has been successfully used to prepare nanogenerators based on thin P(VDF-TrFE) films. For example, Z. Pi
3.3 Template-assisted method
Nanoconfinement involves confining dimensional geometries in a nanosized region. Typically, the template-assisted method from a melt or solution has been employed to fabricate one-dimensional nanostructures [8, 9]. In the template-assisted method, a polymer melt or solution is used to wet the surface of a porous template. It permeates into the pore of the template due to the high surface energy of the template walls. Finally, polymer nanotubes or nanowires form with a uniform size distribution. This technique has been reported to improve the performance of polymer-based nanogenerators. For example, V. Bhavanasi
The nanoconfinement via templates induces ferroelectric polymer nanowires to retain preferential orientation with the b-axis parallel to the long axis of the template. This orientation leads to a vertical direction of the polarization parallel to the channel axis. Y. Calahorra
When a template-assisted method is used, in-situ poling can be applied at the same time. This is known as the template-assisted electricity-grown method. X. Chen
Electrospinning is another common method used to fabricate β-phase ferroelectric fibers without further mechanical stretching or electrical poling processes . The electrospinning technique utilizes an electrical force to obtain fibers. The basic principle of conventional electrospinning is as follows: When a DC electric field is applied between the spinneret (metallic needle) and the collector (grounded conductor), a conical object called a Taylor cone forms at the tip of the needle. When the electric field rises above a threshold value, the resulting electrostatic force can overcome the surface tension and viscous force of the polymer solution. An electrified liquid jet ejected from the nozzle undergoes a stretching and whipping process and then splits into threads with diameters ranging from hundreds of micrometers to tens of nanometers.
In 2011, D. Mandal
Unlike conventional electrospinning, near-field electrospinning using a short needle-to-collector distance enables superior location control, to produce orderly nanofiber patterns over large areas . Notably, C. Chang
where is the electromechanical coupling coefficient of the piezoelectric material. The electric energy was estimated by integrating the product of the output voltage and current of the PVDF nanogenerator when stretched. The elastic strain energy was estimated using
where is the Young’s modulus of the material, is the cross-sectional area, is the strain applied to the material, and is the length of the material. It was found that the energy conversion of the nanofiber-based nanogenerator (average 12.5%) was higher than experimental PVDF films (0.5–4%) and conventional PVDF thin films (0.5–2.6%).
3.5 Thermal drawing
Another interesting method to make ferroelectric polymer fibers is thermal drawing, as reported by S. Egusa
Due to flexibility of the thermal drawing, it has been successfully demonstrated for fabricating composite fibers such as BaTiO3-PVDF, Pb(Zr,Ti)O3 (PZT)-PVDF, and carbon nanotube (CNT)-PVDF . In that study, the piezoelectric performance of the fabricated fibers was systematically compared while bending and releasing. For a BaTiO3-PVDF fiber, the generated open-circuit voltage and short-circuit current were 1.4 V and 0.8 nA, respectively. For a PZT-PVDF fiber, the corresponding values were 6 V and 4 nA. A CNT-PVDF fiber generated 3 V and 1.2 nA. These results were attributed to the high piezoelectric coefficient (BaTiO3 and PZT) or the induced β-crystallization (CNT).
3.6 Dip coating
Dip coating is one of the most effective methods of depositing ferroelectric polymers on a 3D substrate. For example, D. Kim
4. Nanogenerators based on pure PVDF films
The typical type of nanogenerator is based on ferroelectric polymer films without any further processing or incorporation of fillers. Conventional PVDF film-based nanogenerators can be simply fabricated and easily stacked to improve performance. In addition, the flexible nanogenerators can be designed using various structures. For example, J. Zhao and Z. You fabricated a nanogenerator based on specially designed sandwich structure that was compatible with a shoe . The structure was composed of multilayered PVDF films and two wavy surfaces of a movable upper plate and a lower plate. The PVDF film layers were connected in parallel for high output current. Due to the structural design and stacked PVDF films, the nanogenerator provided an average output power of 1 mW while walking at a frequency of ~1 Hz. In another study, W.-S. Jung
5. Nanogenerators based on composites
In order to enhance the electrical and mechanical properties of ferroelectric materials, many research groups have employed the concept of composites, typically made of a polymer and a ceramic. Composite is a common approach in piezoelectric-based applications. Ultrasonic transducer is a typical example. The composites are used for impedance matching. Since piezoelectric ceramics like PZT, BaTiO3, and Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) have a relatively high acoustic impedance compared to water or human tissue, impedance matching is required. Such matching results in broad bandwidth and increased sensitivity.
Composites can also be utilized for nanogenerators based on PVDF and its copolymers to boost their performance. The outputs of nanogenerators depend on a variety of factors, which include the piezoelectric coefficient, elastic modulus, and dielectric constant. The output voltage generated by nanogenerators can be expressed as
where is the strain, is the elasticity modulus, and is the piezoelectric voltage constant, given by with being the piezoelectric coefficient,being the dielectric constant, and being the permittivity of free space . Hence, a higher piezoelectric coefficient and elastic modulus, and a lower dielectric constant result in higher nanogenerator performance. In this section, we will discuss the effect of fillers on composite-based nanogenerators. By varying the concentration, size, and morphology of fillers, the factors of composites can be controlled.
5.1 Piezoelectric coefficient
A piezoelectric coefficient is the most critical factor affecting the performance of nanogenerators. The piezoelectric coefficient (d33) of bulk PVDF and its copolymers is remarkably low compared to that of bulk ceramics such as PZT (700 pC/N) , BaTiO3 (350 pC/N) , and Pb(Zn1/3Nb2/3)O3-PbTiO3 (PZN-PT) (2500 pC/N) . As already mentioned in Section 2, the piezoelectric coefficient of PVDF and its copolymers is related to their crystallinity. In other words, the performance of nanogenerators can be improved by increasing the crystallinity of the ferroelectrics.
Recent studies have revealed that the crystallinity of ferroelectric polymers can change with fillers including organics, inorganics, and metals [16, 23, 39, 54]. The amount of β-phase in ferroelectric polymers can be enhanced by imbedding fillers into the ferroelectric polymer. For example, B. Dutta
In addition to considering the content of fillers, the geometric structures of fillers should be also considered for their effect on the extent of β-phase crystallization and their influence on piezoelectric performance in the composites. A recent example was demonstrated in Ag/PVDF composites, where the effect of mixing silver fillers with different morphologies into P(VDF-TrFE) was investigated . When Ag nanoparticles were added to P(VDF-TrFE), the crystalline transformation of P(VDF-TrFE) was not observed (Figure 4b). Similarly, work conducted by H. Paik
Interestingly, many research groups have used ferroelectric materials as fillers to enhance the performance of their nanogenerators. Even though ferroelectric fillers such as particles, nanowires, and nanorods have a poor piezoelectric coefficients, compared to that of bulk ceramics, they are still good candidates for piezoelectric nanogenerators. For example, PMN-PT nanofiber has a piezoelectric coefficient (d33) of 50 pC/N .
However, PVDF and its copolymers have a negative piezoelectric coefficient, as previously mentioned. If PVDF and its copolymers are used as matrices, the conflict of piezoelectric constants can limit its advantages and result in lower output. Previous reports indicated that PZT/P(VDF-TrFE) 0–3 composites with 50 volume percent ceramic did not exhibit a piezoelectric response because they had the opposite sign piezoelectric coefficient, thus canceled out . To circumvent this effect, different poling process can be adapted. For example, C. K. Jeong
5.2 Elastic modulus
It is also important to consider elastic modulus, the resistance to elastic deformation under load, according to the equation (Eq. (6)). A recent publication showed that Young’s modulus plays an important role in a piezoelectric nanogenerator . In that work, H. S. Kim
5.3 Dielectric constant
Dielectric constants are an important parameter that also can affect the performance of nanogenerators. In composites, the dielectric constant can change with the volume of fillers. In a BaTiO3-PVDF composite film, the dielectric constant increased from 8 to 31.8 at a frequency of 104 kHz when the volume fraction of fillers was increased from 0–30% . Furthermore, the size of the nanoparticles can modify the dielectric properties. When the BaTiO3 filler size was increased from 10.5 to 34.6 nm, the dielectric constant at a frequency of 104 kHz increased from 20.1 to 31. In another study, P. Kim
6. Energy sources
In addition to mechanical deformation, such as uniaxial compression and bending, energy sources such as air flow, sound, and thermal fluctuation are also available. A typical example of ambient natural sources is wind. Ferroelectric polymer-based nanogenerators can easily yield electricity from wind energy because of their considerable flexibility. For example, Li
Besides wind, sound is also an abundant energy source that can be found in the environment. PVDF films can be used as an active layer for harvesting sound waves. A variety of polymer-based nanogenerators have been developed to scavenge energy from sonic inputs. For example, S. Cha
More recently, nanoweb-type nanogenerators were developed to convert sound wave into electrical energy [62, 63]. The benefit of using nanowebs in nanogenerators compared to films is their high power generation because they are more flexible and easier to vibrate under the same acoustic waves.
Pyroelectricity is a property of certain materials, which develop an electric field across the polar axis when there is a temperature change. According to the pyroelectric theory , the pyroelectric current is expressed as
where , , , andare the induced charge, the electrode surface of the material sample, the pyroelectric coefficient, and the temperature, respectively. Ferroelectric ceramics like PMN-PT and PZT have high pyroelectric coefficients above 550 μCm−2 K−1 compared to those of ferroelectric polymers (33μCm−2 K−1). As a result, a PZT-based nanogenerator delivered more current density than that of ferroelectric polymers. However, PVDF and its copolymers have also been employed to harvest thermal energy from temperature fluctuations, where they provide the benefits of light weight, low-cost, lead-free, flexible, and transparent properties [41, 65, 66]. A number of research groups have focused on improving the performance of such nanogenerators by changing their designed conditions. For example, D. Zabek
Lead (Pb) is considered one of the core materials in modern society because it is inexpensive and has high density and resistance to corrosion. However, it is also toxic and can harm multiple human body systems. Legislation has been adopted in many countries to prevent and reduce the use of Pb in many communities . Currently, many of the materials with high piezoelectric performance are lead-based materials, that is, PbTiO3, Pb(Zr,Ti)O3, PMN-PT, Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3 (PMN-PZT), and PIN-PMN-PT [70, 71, 72]. They are used in a wide variety of applications including sensors, actuators, and ultrasonic transducers. The EU’s Restriction of Hazardous Substances (RoHS) Directive of 2002 and its revision (RoHS 2) in 2011 designated certain piezoelectric devices as exemptions since lead-free piezoelectrics cannot completely replace all lead-based piezoelectrics at this time . Nevertheless, it is urgent to develop lead-free piezoelectrics that can perform as well as or better than PZT.
From the biocompatibility point of view, one of the most promising alternatives is a ferroelectric polymer. The unique advantage of PVDF and its copolymers lies in their good stability, similar to PTFE. They are chemically inert and resistant to sunlight [65, 73, 74]. There have been reports on the biocompatibility of PVDF-based nanogenerators. For example, Y. Yu
In this chapter, we summarized recent studies of ferroelectric polymer PVDF-based nanogenerators. PVDF and its copolymers are attractive materials for nanogenerator applications. The materials are flexible, transparent, chemically stable, easy to process, and biocompatible. Because of these advantages, PVDF-based nanogenerators can be placed anywhere, including bones, human skins, and wearable devices that usually have curved surfaces. In addition, they can harvest electricity from a variety of energy sources.
Considerable effort has been expended by numerous research groups to enhance the performance of PVDF-based nanogenerators using various fabrication methods, designing device structures, and incorporating fillers. However, studies on PVDF-based nanogenerators are fairly limited. The mechanisms leading to the enhanced performance after the incorporation of fillers should be comprehensively surveyed. Especially, in the case of ferroelectric fillers, a careful approach to the enhancement should be taken, since a piezoelectric effect also occurs with fillers. To optimize nanogenerator performance, circuits for nanogenerators should be developed. Their performance depends on the circuits and components, including load resistors, capacitors, and wiring. In order to realize the commercialization of PVDF-based nanogenerators, packaging of the devices is critical to prevent mechanical fatigue. The selection of substrates and electrode materials can also improve the mechanical properties of these devices. Therefore, the relationship between a ferroelectric polymer layer and other layers should be considered.
We hope that this chapter will help readers to better understand principles of PVDF-based nanogenerators. Furthermore, several new approaches that have been in this chapter can be adopted for other applications, such as sensors, actuators, and field-effect transistors.
This chapter is based on a research that has been conducted as part of the KAIST funded Global Singularity Research Program for 2019. Dr. Panpan Li was also supported by the Korea Research Fellowship Program funded by the National Research Foundation of Korea (no. 2017H1D3A1A01054478).