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Bhadwal, N.; Ben Mrad, R.; Behdinan, K. Poly(Vinylidene Fluoride) and Copolymer-Based Piezoelectric Nanogenerators. Encyclopedia. Available online: https://encyclopedia.pub/entry/53652 (accessed on 21 May 2024).
Bhadwal N, Ben Mrad R, Behdinan K. Poly(Vinylidene Fluoride) and Copolymer-Based Piezoelectric Nanogenerators. Encyclopedia. Available at: https://encyclopedia.pub/entry/53652. Accessed May 21, 2024.
Bhadwal, Neelesh, Ridha Ben Mrad, Kamran Behdinan. "Poly(Vinylidene Fluoride) and Copolymer-Based Piezoelectric Nanogenerators" Encyclopedia, https://encyclopedia.pub/entry/53652 (accessed May 21, 2024).
Bhadwal, N., Ben Mrad, R., & Behdinan, K. (2024, January 10). Poly(Vinylidene Fluoride) and Copolymer-Based Piezoelectric Nanogenerators. In Encyclopedia. https://encyclopedia.pub/entry/53652
Bhadwal, Neelesh, et al. "Poly(Vinylidene Fluoride) and Copolymer-Based Piezoelectric Nanogenerators." Encyclopedia. Web. 10 January, 2024.
Poly(Vinylidene Fluoride) and Copolymer-Based Piezoelectric Nanogenerators
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This entry is adapted from the peer-reviewed paper 10.3390/nano13243170

The highest energy conversion efficiencies are typically shown by lead-containing piezoelectric materials, but the harmful environmental impacts of lead and its toxicity limit future use. At the bulk scale, lead-based piezoelectric materials have significantly higher piezoelectric properties when compared to lead-free piezoelectric materials. However, at the nanoscale, the piezoelectric properties of lead-free piezoelectric material can be significantly larger than the bulk scale. 

PVDF nanostructures piezoelectric nanogenerators piezoelectric energy harvesting piezoelectric thin films power output

1. Introduction

With the increased push towards clean sustainable forms of energy, methods to convert ambient energy into useable electricity are being developed such as piezoelectric, thermoelectric, and triboelectric energy harvesters [1][2]. Due to the way dipoles are arranged in piezoelectric materials, they can transform mechanical energy (vibrations) into electrical energy and thus offer a way to harvest ambient mechanical energy. When a piezoelectric material is subjected to a force, the material deforms, causing the unit cells to deform, creating or enhancing the dipole moment in the unit cell due to the arrangement of atoms in the material. The material then has a net dipole moment, which induces charges on the electrodes of the material. This is the direct piezoelectric effect and can be used for energy harvesting. The choice between different methods of energy harvesting depends on the application. Piezoelectric energy harvesters offer the advantage of being durable and sensitive to mechanical vibrations. Lead-free piezoelectric materials are environmentally friendly when compared to chemical batteries and can supply clean renewable electrical energy [3]. A lead-free piezoelectric energy harvester could power biomedical implants, structural health-monitoring sensors, and electronics in space.
To enable the use of piezoelectric energy harvesters in more practical applications, their power density must be increased as current harvesters can only power ultra-low-power-consuming devices [4]. These harvesters should use materials with high piezoelectric properties in composite designs that improve the overall power output. The appropriateness of Poly(vinylidene fluoride) (PVDF) and Poly(vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) lead-free piezoelectric nanomaterials as the main piezoelectric material in piezoelectric energy harvesters is examined. The appropriateness is based on the nanomaterial’s piezoelectric properties and the power output of different PVDF/PVDF-TrFE composite structures.
The highest energy conversion efficiencies are typically shown by lead-containing piezoelectric materials. The harmful environmental impacts of lead and its toxicity will limit lead-based materials in future applications [3]. Several lead-free piezoelectric materials are being developed for use in energy harvesters such as zinc oxide (ZnO), aluminum nitride (AIN), barium titanate (BaTiO3), polyacrylonitrile (PAN), and polyvinylidene fluoride (PVDF) [3][5][6][7]. At the bulk scale, lead-free piezoelectric materials have significantly lower piezoelectric properties when compared to lead-based piezoelectric materials (can be an order of magnitude). However, at the nanoscale, the piezoelectric properties of lead-free piezoelectric materials can be significantly more substantial compared to the bulk level, primarily attributed to the heightened influence of surfaces, interfaces, decreased defects, stress concentrations, material disparities, unconfined volume expansion/contraction, elevated crystallinity, and precise management of crystal growth orientation [5][8][9][10][11]. Advances in the field of nanotechnology have paved the way for the creation of lead-free piezoelectric nanostructures undergoing further development to enhance their energy conversion efficiency [3][5][8][9][12]. Composites made from these piezoelectric nanostructures are called piezoelectric nanogenerators (PENGs) if used for energy harvesting. Piezoelectric nanogenerators (PENGs) exhibit greater flexibility compared to their larger counterparts and hold promise for electrical energy-harvesting applications.
PVDF is a piezoelectric polymer (chemical repeating formula (C2H2F2)n) discovered by Dr. Henji Kawai around 1970 in Japan [13][14][15][16][17]. PVDF is flexible [18], highly acid resistant [19], thermoplastic [20], biocompatible [20], nontoxic [20], and can be transparent under some conditions [8]. PVDF exhibits five phases, namely α, β, γ, δ, and ε, out of which the β, γ, and δ-phases show piezoelectric behavior [13][15][21][22]. The β-phase exhibits the most significant electric dipole moment, thus leading to the highest piezoelectric characteristics among all the phases [13][23]. Hence, the piezoelectric attributes of PVDF are contingent on the material’s crystallinity level and the relative composition of its various phases. The piezoelectric effect in PVDF can be limited due to its semicrystalline nature [24] and bulk PVDF is typically ~50% crystalline [25]. The α-phase has an alternating trans-gauche (TGTG) chain conformation, which leads to a mutual cancellation of dipole moments between the C–H and C–F bonds and hence the α-phase is non-polar (see Figure 1a) [13]. The β-phase has an all-trans (TTTT) conformation, where the H and F atoms are attached at both ends of the C–C chain such that the dipole moments of the two C–H and two C–F bonds add up perpendicular to the c-axis of the polymer, giving it piezoelectric properties (see Figure 1b) [13][15].
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Figure 1. Structures of PVDF and PVDF-TrFE. (a) Structure of PVDF ∝-phase; (b) structure of PVDF β-phase; (c) structure of PVDF-TrFE.
Poly (vinylidene fluoride-co-trifluoroethylene) (PVDF-TrFE) is a copolymer of PVDF and has been synthesized to achieve a higher amount of β-phase. The addition of TrFE (CF2–CFH) into VDF (CH2–CF2) promotes the immediate formation of the ferroelectric β-phase through crystallization [25][26][27][28]. In the PVDF all-trans state, the proximity of two large adjacent fluorine atoms disrupts the stability of the β-phase, leading to a preference for the α-phase with a TGTG conformation due to limited steric hindrance between the smaller hydrogen atoms and the neighboring fluorine atoms [24]. In PVDF-TrFE, the VDF and TrFE units are distributed without a specific pattern along the molecular chain and the introduction of a slightly larger fluorine atom, replacing a hydrogen atom, causes steric hindrance with neighboring G-bonds, leading to a preference for the trans bond over the gauche bond (see Figure 1c) [26]. When incorporating a minimum of 20% TrFE units, enough HH (CF2–CF2) and TT (CH2-CH2) defects are introduced, leading to significant steric hindrance within an α-phase crystal, which is not observed in a β-phase crystal, causing the copolymer chains to favor a polar all-trans conformation closely resembling the β-phase of PVDF [29]. The PVDF-TrFE copolymer with 20–50% TrFE content is more stable in the all-trans phases [24].
Table 1 lists the bulk piezoelectric properties of PVDF and PVDF-TrFE. The piezoelectric strain constant, d33, is the ratio of the strain produced to the applied electric field (m/V), or the ratio of electric charge generated per unit area to an applied stress (C/N) [30]. Εr is the relative dielectric constant and ε0 is the permittivity of free space. The piezoelectric voltage constant, g33, is the ratio of the strain produced to the applied electric displacement, or the ratio of the electric field generated per unit stress applied [31]. The dg product (d33g33, which is equivalent to d332/(εrε0)) is a performance metric utilized for assessing the energy-harvesting potential of a piezoelectric material [31][32]. 
Table 1. PVDF and PVDF-TrFE bulk-scale piezoelectric properties.
Material Values Crystallinity % β-Phase d33
(pC/N)		 εr d332/(εrε0)
or d33g33
(m2/N × 10−12)		 Reference
PVDF
(Bulk)		 Generally
Accepted Values		 ~50–60% [13] - −13–(−35) 8–15 17 [23][33][34][35][36]
Different
Industrial
Datasheets		 - - −30 8–10 12 [37]
- >80%
of total
crystallinity		 −23–(−28) 13.5 6.6 [38]
- - −33 - - [39]
PVDF-TrFE
(Bulk)		 Generally
Accepted Values		 - - −24–(−39) 5–20 34 [23][33][35][36][40]
Different
Industrial
Datasheets		 80–90% [41] - >25 7.5–8.5 9.5 [42]
- - −38 7.9 21 [43]

Bulk PVDF and PVDF-TrFE both have negative d33 coefficients, as opposed to most traditional piezoelectric ceramics, which have positive d33 coefficients [44][45][46][47]. Physically, this means that when an electric field is exerted in the direction of polarization of a traditional piezoelectric ceramic, it will expand, while PVDF and PVDF-TrFE contract [44].

To enable the use of piezoelectric energy harvesters in more practical applications, their power density must be increased as current harvesters can only power ultra-low-power-consuming devices. These harvesters should use materials with high piezoelectric properties in composite designs that improve the overall power output. The appropriateness of PVDF and PVDF-TrFE lead-free piezoelectric nanomaterials as the main piezoelectric material in piezoelectric energy harvesters is examined. The appropriateness is based on the nanomaterial’s piezoelectric properties and the power output of different PVDF/PVDF-TrFE composite structures.

2. Power Density-Improving Techniques

To increase the piezoelectric efficiency of PVDF, several methods and techniques have been implemented:
Stretching: Lovinger explained that mechanically stretching PVDF at a low temperature (~90 °C) causes the nonpolar α-phase spherulites to break and forces the molecular chains into their most extended conformation, which is the β-phase [13]. Although stretching induces the β-phase, the dipole vectors at this stage are randomly positioned within the plane perpendicular to the molecular chains [24]. Aligning the dipole vectors in the same direction is required to make the overall PVDF material demonstrate piezoelectric behavior, and this is usually accomplished by poling the material.
Poling: Subjecting PVDF to a strong electric field triggers the transition from the α-phase to the β-phase and aligns dipole moments along the applied electric field [13][48][49][50]. Gupta and Doughty speculated that the electrostatic force applied during the poling process can cause compression in the PVDF, leading to a relaxation of the C–C–C bond angle and an increase in the C–C separation, allowing the fluorine atoms to overcome steric hindrance, which permits the C–F bonds to rotate and facilitate the formation of the β-phase [50].
Quenching: Quenching PVDF at low temperatures can prompt the formation and arrangement of the self-aligned β-phase [51]. The O-H bonds in the water can form hydrogen bonds with the C–F groups of PVDF, leading to their orientation and the formation of the β-phase [51][52][53]. At lower quenching temperatures, crystallization occurs gradually, starting at the material’s surface and advancing inward through PVDF, which, in turn, results in the alignment of the β-phase during the stepwise crystallization process [51][52][53][54].
Annealing: Under the correct annealing process, thermal energy can promote the rearrangement of the polymer chains, inducing β-phase transformation and increasing the degree of crystallinity [55][56][57].
Press and Fold (Hot Pressing): High pressure and temperature can promote the α to β-phase transformation [58][59]. PVDF can be pressed and folded under high temperatures and during the process, spherulites are converted to small granular structures [58]. The β-phase has a smaller unit cell volume in comparison to the α-phase [13]. The pressure results in a closer packing of the atoms, resulting in the β-phase being preferred [58]. Temperatures between 100 and 165 °C are preferred as PVDF films prepared at temperatures below 80 °C showed obvious cracks [58].
Electrospinning: This technique combines both electric poling and stretching to increase the degree of crystallinity, β-phase, and dipole orientation in PVDF [60][61][62][63]. PVDF is typically dissolved in a solvent and then placed into a syringe with a metal tip. A grounded metal collector is placed in front of the syringe tip (typically 10–25 cm) and a large voltage is applied to the syringe tip, leading to a charged solution. When the electric charge force in the solution surpasses the surface tension force, a stream of PVDF dissolved in the solution is expelled from the nozzle [64]. During this process, the solvent starts evaporating, and the PVDF is poled and stretched due to the high electric field.
Copolymers and Terpolymers: As mentioned in Section 1, PVDF-TrFE is a copolymer that has an immediate formation of the ferroelectric β-phase through crystallization [25][26][27]. An example of a terpolymer is P(VDF-TrFE-CTFE), which is synthesized through the random addition of the third monomer chlorotrifluoroethylene (CTFE).
Addition of Fillers: The addition of fillers can also increase the power density of PVDF through the following mechanisms [8]:
β-phase increase—The filler particles, depending on their composition, may interact electrostatically with the fluorine atoms or hydrogen atoms in PVDF, leading to the F atoms in PVDF being aligned towards or away from the filler, inducing the formation of the β-phase. Thus, the filler can act as a nucleation agent, resulting in a higher crystallinity and β-phase [8].
Piezoelectric materials—If the filler itself is a piezoelectric material with higher piezoelectric properties than PVDF, the filler can contribute to increasing the power output of the nanocomposite.
Conductive materials—Conductive fillers have the capacity to establish electrical connections within the insulating piezoelectric material, helping induced piezoelectric charges flow between the inside of the PVDF and the electrode [8][65]. This can increase power density. However, too much conductive filler can electrically connect the top and bottom electrodes, causing charge neutralization and lower output [65].
Stress Concentrations—Hard fillers can act as stress concentrations when subject to external forces, leading to the development of higher piezoelectric potentials [8]. The local strain in the vicinity of each filler exhibits a significantly greater magnitude compared to the bulk strain observed in pure PVDF/PVDF-TrFE film, which induces higher potential [66].

3. Piezoelectric Nanogenerator Composite Structure and Power Output

3.1. (1-3 Composites) Vertically Aligned PVDF/PVDF-TrFE Nanowires and Nanotubes

PVDF/PVDF-TrFE 1-3 composites have vertically aligned PVDF/PVDF-TrFE nanotubes (NTs) or nanowires (NWs) which are uninterrupted in a single direction, while another material encapsulates the nanowires or nanotubes in all three directions as shown in Figure 2a,b. The second material may demonstrate piezoelectric activity but does not need to for example air.
Nanomaterials 13 03170 g002aNanomaterials 13 03170 g002b
Figure 2. (a) A 1-3 PVDF/PVDF-TrFE composite with vertically aligned PVDF/PVDF-TrFE nanowires (white) encapsulated in matrix (translucent blue-grey). (b) Cross-section of vertically aligned nanotubes grown by Template Assisted Method. (c) Nanoconfinement effect: Lamellae are aligned when in the nanopores but randomly aligned in the residual film. (d) Orientation of PVDF chains inside nanopores; the direction of polarization is along the length of the nanopore.
It is preferred to produce this composite in a way that the dipole moments of PVDF/PVDF-TrFE are oriented perpendicular to the substrate. The b-axis (axis in the direction of polarization [13][67]) of PVDF should be perpendicular to the substrate, while the c-axis (axis along the chain direction) should be parallel to the substrate.
Nanoconfinement:
Nanoconfinement is an important effect that is prominent in 1-3 nanocomposites of PVDF/PVDF-TrFE where the crystallization of PVDF/PVDF-TrFE in nanopores can lead to crystal alignment with the polar b-axis of the polymer running parallel to the length of the nanopore, resulting in large piezoelectric coefficients [68].
During crystallization, heterogeneous nucleation occurs in the residual film (PVDF/PVDF-TrFE film that has not infiltrated the template) as shown in Figure 2c [69][70]. Polymer lamellae in the residual film grow radially outward from the spherulites in all directions [70]. When the lamellae hit the nanopores of the template, only lamellae with a growth direction parallel to the length of the nanopore continue to grow while the lamellae with any other growth direction get blocked by the nanopore walls and cannot continue to grow [70]. PVDF/PVDF-TrFE recrystallized without a residual film has a random orientation of polymer chains in the nanopores and hence the residual film is essential for alignment [70]. In summary, if a residual film connects the nanostructures, the dominant growth direction of the crystals (polar b-axis) is aligned along the length of the nanopores as shown in Figure 2d [69].
Nanotubes (NTs)
In one study, a 1-3 composite with PVDF-TrFE NTs in an Anodized Alumina Membrane (AAM) was fabricated [70]. PVDF-TrFE was spin coated onto a porous anodized alumina template with one side closed (not porous) and the sample was heated to 250 °C. The polymer melt adhered to the template and entered the nanopores due to capillary forces, and this deposition process was repeated 15 times. A gold layer was deposited on the PVDF-TrFE and the bottom of the AAM template was exposed, after which the samples were poled under 800 V. There was space between the outer wall of the PVDF-TrFE nanotubes and AAM walls, which was important in not restricting the movement of nanotubes in the membrane.
Nanowires (NWs)
In one study, a 1-3 composite made of PVDF-TrFE nanopillars with a diameter <20 nm and aspect ratio up to 8.9 was created [71]. A silicon mold (NIL method) was used to create two 1-3 composite films. The films were flip-stacked on top of each other such that the PVDF-TrFE pillars were touching (the electrodes were facing opposite directions) and then poled. The maximum piezoelectric strain constant of a single PVDF-TrFE nanopillar was 210.4 pm/V, while the average was 72.7 pm/V as some nanopillars did not have contact in between them and only obtained partial polarization. The average strain constant of the developed PVDF-TrFE 1-3 composite structures was 5.19 times larger than that of the PVDF-TrFE flat thin film.
Impregnated and Non-Impregnated Microwires:
In another study, a 1-3 composite with PVDF-TrFE nanowires was fabricated using a NIL method and electrohydrodynamic (EHD) pulling [72]. PVDF-TrFE film was pressed against a PDMS mold for 30 min under a pressure of 4.8 MPa at 160 °C to form micropillars. Then, an ITO plate was placed above the pillars with an air clearance maintained with Kapton spacers and used as an upper electrode. A voltage was applied, which pulled the micropillars electrohydrodynamically (EHD) upwards, generating an array in contact with the upper electrode. The composite was annealed for 30 mins. The vertically aligned micropillars produced an output 5.4 times higher than the bulk-film-based generator, and the increase was attributed to good dipole alignment along the microwire length.

3.2. (3-1 Composites) Piezoelectric Vertically Aligned Nanorods Encapsulated in PVDF

A 3-1 PVDF nanocomposite has vertically aligned piezoelectric nanorods (the piezoelectric material is not PVDF/PVDF-TrFE) continuous in one direction, which are encapsulated by PVDF/PVDF-TrFE in all three directions (see Figure 3). The PVDF/PVDF-TrFE encapsulant can additively contribute to the piezoelectricity of the piezoelectric vertically aligned nanorods.
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Figure 3. A 3-1 composite with vertically aligned ZnO NWs (white pillars with hexagonal cross-section) encapsulated in PVDF (translucent purple grey).
Most research has been conducted on vertically aligned ZnO NRs encapsulated in PVDF to form a nanocomposite. It is preferred to fabricate this type of composite such that the c-axis of the ZnO crystals is normal to the substrate [4]. Vertically aligned ZnO nanorods are typically grown via the hydrothermal method on a substrate and then encapsulated in PVDF.
Anand and Bhatnagar encapsulated vertically aligned ZnO nanorods in PVDF [73]. PVDF dissolved in DMF was drop cast on top of vertically aligned ZnO nanorods to form the PENG. The PENG, without any prior poling, generated a power output that was 1800 times greater than that of PVDF alone. The vertically aligned ZnO nanorods played a crucial role in promoting the nucleation and formation of the β-phase within the PVDF, and the PVDF passivated the surface of ZnO nanowires. Choi et al. synthesized vertically aligned ZnO nanowires encapsulated in PVDF [74]. ZnO NWs were encapsulated in PVDF and the tops of the ZnO nanowires were completely covered in PVDF. The voltage output was 2.7 times that of pure PVDF. Experiments showed that the ZnO NWs may not have contributed significantly to surface charge generation, but the ZnO NWs increased the strain near PVDF, which was thought to be the reason for the power enhancement. In one study, an array of ZnO was encapsulated in PVDF [75].

3.3. (3-0 Composites) PVDF/PVDF-TrFE Composites with Non-Vertically Aligned Nanoparticles

In simple terms, 3-0 PVDF/PVDF-TrFE nanocomposites are PVDF/PVDF-TrFE films with nanoparticles embedded in them. The nanoparticles are non-continuous in all directions and are encapsulated in PVDF/PVDF-TrFE, which is continuous in all three directions as shown in Figure 4.
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Figure 4. (a) A 3-0 composite with NPs (grey spheres) encapsulated in PVDF (translucent purple grey). (b) A 3-0 composite with NRs (grey rods) encapsulated in PVDF (translucent purple-grey).
To fabricate this kind of composite, PVDF/PVDF-TrFE is typically dissolved into a solvent and then nanoparticle fillers are added to the solution. The solution is subsequently applied to a substrate using either spin coating or drop coating, after which electrodes are positioned, resulting in the creation of a 3-0 composite.
Graphene and Derivatives
Since graphene is non-polar and PVDF is polar, the formation of homogenous composites is difficult; also, graphene is conducting, which can lead to significant dielectric loss [76][77][78]. The surface functionalization of graphene can improve dispersion. Graphene oxide (GO) has oxygen functional groups that improve dispersion by interacting with polymer chains in PVDF but it was observed that the addition of GO can lead to a deterioration in the electrical and mechanical properties of the polymer composite. Hence, Pusty et al. reduced GO to rGO to enhance dispersion in the PVDF [76]. They incorporated 1% by weight of reduced graphene oxide–silver (rGO–Ag) into the composite. The addition of Ag nanoparticles, which were easy to synthesize, was observed to improve the dielectric properties of PVDF. rGO–Ag enhanced the β and γ phases in PVDF due to the electrostatic interactions. The positively charged Ag ions were attracted to the –CF2– dipoles of PVDF, while they were repelled by the –CH2– dipoles. The PENG’s open circuit voltage and short circuit current increased by 180 and 35 times, respectively, compared to pure PVDF without poling.
ZnO
In one study, a PVDF film with nanopores was synthesized [66]. ZnO nanoparticles (50 wt%) were added to a solution of N, N-DMF, and PVDF. The solution was drop cast to form a film, which was annealed and then etched in HCl to remove the ZnO nanoparticles and form a porous PVDF film. The film was then poled. The PENG had an output current ~11 times higher compared to a pure PVDF-based PENG. ZnO was used to create porosity and to promote the formation of the β-phase through the dipolar interaction. The pores increased the stress in the PENG and boosted piezo potential.
Other Materials
Numerous studies have concentrated on increasing the β-phase in PVDF by incorporating nanoparticles (NPs). Karan et al. added 5 wt% vitamin B2 (VB2) powder to PVDF to create a completely organic-based biocompatible PENG [79]. VB2 contains hydroxyl, carbonyl, and amino groups that effectively stabilize the polar β-phase of PVDF through hydrogen bonding, leading to an increase in crystallinity and β-phase content. The PENG had an output voltage and current 26 times and 40 times higher, respectively, when compared to pure PVDF. Electrical poling resulted in only a 1.04 times increase in output voltage, suggesting that the PENG was self-poled. The device showed durability and stability in performance for 10 weeks.

3.4. Electrospun Fibers

Electrospinning as a fabrication method of PVDF/PVDF-TrFE PENGs is considered to be a relatively simple low cost operation [80]. The piezoelectric properties of electrospun PVDF/PVDF-TrFE fibers are increased due to the mechanical stretching and poling of the fibers, and PENGs prepared by this method typically do not require a post-poling process as used in other fabrication methods.
The fabrication of this type of PENG typically involves the addition of PVDF/PVDF-TrFE into a polar solvent to form a solution. NPs may be added to the solution, which is then electrospun. The final structure is shown in Figure 5a,b. Several parameters that affect the quality and output of electrospun PENGs are humidity, temperature, applied electric field, solvent type, solution concentration, feed rate, collector type, distance between the syringe nozzle and collector, and syringe nozzle opening [80][81][82].
Nanomaterials 13 03170 g005aNanomaterials 13 03170 g005b
Figure 5. (a) ZnO NWs grown on the surface of a PVDF/PVDF-TrFE fiber. (b) NPs in a piezoelectric fiber of PVDF/PVDF-TrFE. (c) Only charges making direct contact with the electrode of an electrospun fiber are used. (d) Conductive NPs increase the horizontal flow and more charges can be used. (e) Addition of too many conductive NPs increases volume conductivity and can short the PENG.
Once the electrospun mat is created, typically electrodes are fixed on the top and bottom of the mat. The electrodes are only partially in contact with a restricted number of nanofibers on the surface layer of the PENG and as a result, the piezoelectric charges generated by nanofibers that are not in direct contact are not utilized, as shown in Figure 5c [65]. Increasing the surface conductivity of PVDF/PVDF-TrFE fibers with the aid of conductive nanoparticles facilitates the lateral movement of the generated piezoelectric charges between the fibers on the surface layer, as depicted in Figure 5d [65]. However, an excess of conductive nanoparticles in the electrospun PVDF/PVDF-TrFE PENG results in a more rapid increase in volume conductivity compared to surface conductivity, which causes the induced charges to flow longitudinally, leading to neutralization and leakage effects, ultimately reducing the piezoelectric output voltage, as illustrated in Figure 5e [65].
Increasing Dispersion of NPs
Several studies focused on improving the dispersion of NPs in electrospun PVDF-TrFE. Shi et al. created electrospun PVDF-TrFE with 10 wt% BaTiO3 nanowires (NWs) coated in PMMA via atom transfer radical polymerization to improve dispersion [83]. The NWs were well oriented along the length of the electrospun fiber. The maximum output power of the 10 wt% PMMA-coated BaTiO3/PVDF-TrFE PENG was 2.2 times greater than the maximum output power of the 10 wt% BaTiO3/PVDF-TrFE PENG (without PMMA coating), and it was also 7.6 times higher than the maximum output power of the PVDF-TrFE-based PENG. The higher output was due to the piezoelectricity of the BaTiO3 NWs, improved dispersion of the BaTiO3 NWs, and the high Young’s modulus of PMMA, which significantly enhanced the efficiency of stress transfer at the interface between the BaTiO3 nanowires (NWs) and the PVDF-TrFE matrix. The PMMA also reduced leakage current through the composite. The PENG was stable for 6000 cycles.
Carbon-based NPs
Yang et al. electrospun PVDF with (2 wt%) reduced graphene oxide nanoparticles in it [84]. GO nanoparticles were mixed in N, N-dimethylacetamide (DMF) and then PVDF was added and electrospun under a voltage of 1.4 kV/cm. The PENG was dried for 2 h at 80 °C after which it was heated for 1 h at 140 °C to reduce GO to rGO. The PENG had an open circuit voltage output ~11 times higher than pure PVDF and ~3.7 times higher than PVDF–GO PENG. The increased output was attributed to increased β-phase due to the graphene, the graphene acting as stress concentrations, and the conducting network being formed due to the graphene, which improved the transfer of induced charges generated by PVDF. Both graphene oxide (GO) and reduced graphene oxide (rGO) had a positive impact on the output of the PENG. However, rGO had a more pronounced effect in improving PENG performance because rGO is more conductive than GO because rGO has fewer functional groups attached to the carbon atoms, enhancing its conductivity. In one study, PVDF with a Ce3+ complex (Cerium (III)-N,N-dimethylformamide-bisulfate [Ce(DMF) (HSO4)3]) along with graphene NSs was electrospun into a PENG [85]. The PENG had an open circuit voltage ~2.5 times that of the PENG without graphene NSs. The enhanced output was attributed to the conductivity of the graphene NSs, the increase in crystallinity, and an increase in the β phase due to the interactions of the fillers with PVDF.
Other NPs
Tiwari et al. electrospun PVDF with two-dimensional nanoclay platelets, i.e., Cloisite 30B (bis-(hydroxyethyl) methyl tallow ammonium ion-exchanged montmorillonite [86]. The PENG had an output circuit voltage of 3.5 times higher than pure PVDF. The increase in performance was attributed to the increase in the β-phase due to the interaction between the nanoclay and PVDF matrix as well as the nanoclay acting as a nucleating agent. The nanoclay created a minute mesh-like structure, which, in turn, impeded the propagation of cracks, resulting in a more durable fiber when nanoclay was present. The tensile strength, Young’s modulus, and toughness of the PENG were 3.3, 3.4, and 3.5 times greater than that of pure PVDF, respectively. This enhancement in toughness and modulus was ascribed to changes in the material’s morphology, crystal structure, and the effective dispersion of the layered silicates within the polymer matrix.
ZnO
In one study, ZnO nanorods (27.3% by mass) were grown on top of electrospun PVDF [87]. The electrospun PVDF fibers were dip coated into a seed solution of ZnO and then ZnO was grown via hydrothermal synthesis. The PVDF–ZnO PENG produced an output voltage that was nearly three times higher compared to a pure PVDF PENG prepared using the same method. This boost in performance was attributed to the ZnO nanorods deflecting and sliding against each other during vibrations, causing greater deformation of both the ZnO nanorods and the PVDF fibers, ultimately increasing the power output. In another study, PVDF was electrospun, and ZnO nanorods were grown on top of the structure [88]. The open-circuit voltage experienced an approximately 2.3-fold increase in the presence of ZnO nanorods. Additionally, it was found that the mat possessed a level of breathability similar to cotton.

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