Electrospun piezoelectric power generation

Piezoelectric polymers are attractive for many potential applications due to its availability as flexible thin film. Although their piezoelectric charge constants are much smaller than piezoelectric ceramic materials, their benefits lie in their flexibility and durability. These allow them to be used in areas which are not possible for the more fragile ceramic materials. Several piezoelectric polymers have already been routinely electrospun to form nanofibrous membrane. A study by Chang (2009) found that the average energy conversion efficiency for a single PVDF nanofiber was 12.5%, going as high as 21.8%, which is much higher than the energy conversion efficiency of PVDF thin film which average about 1.3%. This gives electrospun fibers the potential for use as power generator. Mandal et al (2011) have provided evidence of preferential orientation of CF₂ dipoles in poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) nanofiber by electrospinning. Current output from electrospun membrane can be further improved by stacking layers of fibers with the same polarization. Further increase in the electrical output from piezoelectric polymers may lies in fabricating composite fibers containing particles with greater piezoelectric characteristics. Yun et al (2016) loaded polyvinylidene fluoride (PVDF) with PZT (lead zirconate titanate, Pb(Zr_xTi _(1-x) )O₃) nanoparticles. The resultant electrospun PVDF/PZT membrane was found to be flexible and the optimum loading of PZT is 20 wt% for highest value of P_max (maximum polarization) at 2.64 µC/cm², 4 kV/mm. This is an increase of 27% of P_max over pure PVDF nanofiber. Bera et al (2017) loaded electrospun PVDF with gold nanoparticles to improve electrical generation under light. Their study showed improved output voltage generated by the PVDF/gold nanoparticles when blue light falls on it but no changes in output voltage when green light is shown. The effect is more apparent with higher loading of gold nanoparticles. However, it is not clear why gold nanoparticles increases output voltage of electrospun PVDF under blue light. Ji et al (2022) explored the use of nitrogen-doped-reduced-graphene-oxide (rGO) as a conductive material to a P(VDF-TrFE) polymer and BiScO₃-PbTiO₃ ceramic composite. Nitrogen doping removes defects in rGO which helps to promote electron transport. BiScO₃-PbTiO₃ nanoparticles and N-rGO were added to P(VDF-TrFE) solution prior to electrospinning. The resultant N-doped rGO/P(VDF-TrFE)/ BiScO₃-PbTiO₃ composite fibers were laid on interdigital electrodes to form a flexible piezoelectric energy harvester. With this setup the maximum power output was 6.3 µW at a voltage of 9.27 V and 0.68 µA and 0.63 mW/cm³ on the film. Application of mechanical force of 300 N at a rate of 1.5 Hz for 1000 cycles demonstrates the stability and durability of the flexible piezoelectric energy harvester. Li et al (2024) showed several advantages of adding Ag@BaTiO₃ (Ag@BTO) nanoparticles for electrospinning of PVDF. With the presence of Ag@BaTiO₃ (Ag@BTO) nanoparticles, the β-phase of the resultant electrospun Ag@BTO/PVDF fibers were greater compared to electrospun BTO/PVDF membrane and pure PVDF membrane at 83%, 80% and 75% respectively. BTO nanoparticles may function as nucleating agents for β-phase PVDF crystallization from hydrogen bonds formed between the hydroxyl group of BTO and fluorine atoms in PVDF. With Ag@BTO nanoparticles, the silver on the surface of the BTO nanoparticles may acquire induced charges by the electric field during electrospinning which in turn induce PVDF chains to crystallize on the surface of the silver nanoparticles hence increasing the β-phase in the resultant membrane. Higher β-phase of Ag@BTO/PVDF membrane is known to generate greater piezoelectric power. Further, the presence of silver atoms may also help to capture and transport electrons. The open circuit voltage of PVDF, BTO/PVDF and Ag@BTO/PVDF membranes were 3 V, 5.4 V and 6.2 V respectively with Ag@BTO/PVDF showing a significant increase in open circuit voltage over BTO/PVDF.

Having nanoparticles or other additives blended into nanofibers is not the only way to incorporate the additives. There is also a limit to the amount of additives that can be loaded into the solution to be electrospun before the electrospinning process is effected. Mirjalali et al (2023) used electrospinning and electrospraying to construct a multilayered mat with enhanced piezoelectricity. The material for electrospinning was Polyvinylidene Fluoride (PVDF) while zinc oxide nanoparticles were electrosprayed. Pristine electrospun PVDF nanofiber mat showed an output voltage of 0.24 V. A 5-layer PVDF+ZnO 50% showed a voltage of 0.91 V. This was greater than 50% ZnO blended into PVDF nanofibers which had an output voltage of 0.49 V. Further increase in ZnO concentration blended into PVDF nanofibers resulted in a decrease in the voltage. With the multilayered construct, further increase in the amount of ZnO particles is possible and a maximum voltage of 3V was recorded with 150% ZnO. The increase in voltage likely came from greater contribution of ZnO to the piezoelectric property of the mat. However, further increase in ZnO led to a decrease in the voltage which may be due to the greater thickness hindering harvesting of the excited charges. Mirjalali et al (2023) used the layered PVDF+ZnO mat to assemble a nanofiber-based device. By having the device in a shoe insole, jumping generated a pressure of 600 kPa and gave an output voltage of 2.7 V, sufficient to light 21 normal LEDs.

In power generation using piezo-electric fibers, near field electrospinning is able to produce single fibers to determine its power generation ability or to construct highly oriented fibers which allow the control of current flow. Pan et al (2014, 2015a) had tested the power output from aligned nanofibrous membranes made from different piezoelectric materials. Through their test on single nanofibers from near field electrospinning, they found that the maximum power from poly(γ-benzyl α, l-glutamate) (PMLG) was 138 pW while polyvinylidene fluoride (PVDF) fiber was able to generate 266 pW. Interestingly, they found that having a fiber composite made from a blend of PVDF and PMLG was able to generate higher power output of 637.81 pW and maximum peak voltage of 0.08V. The improved performance has been attributed to better dipole orientation and higher dipole density [Pan et al 2015b].

Apart from the material, the structure of the fiber may also have an influence on power generation ability. Using electrospun hollow PVDF and solid PVDF, Pan et al (2015) found that hollow PVDF fibers were able to generate much higher maximum power output (856 pW) compared to solid PVDF fibers (348 pW). The higher power generation was attributed to greater stiffness of hollow PVDF. The relationship of induced electric current (I) and Young's modulus is given by,

I = dEAε

where
I is the induced electric current,
d is the piezoelectric coefficient,
A is cross-sectional area,
ε is the strain rate

Persano et al (2013) constructed highly aligned poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) nanofibers using a rotating drum. They were able to generate a maximum short-circuit current and voltage output of 40 nA and 1.5V respectively. Shafii (2014) electrospun randomly oriented PVDF fiber membrane and obtained a maximum power output of 2200 pW/cm² while Liu et al (2014) recorded a maximum power output of 577.6 pW/cm² from electrospun aligned PVDF fibers. Ji and Yun (2018) compared the power generation between electrospun aligned and randomly oriented BNT-ST (0.78Bi_0.5Na_0.5TiO₃-0.22SrTiO₃) ceramic/poly(vinylidene fluoride-trifluoroethylene) (PVDF-TrFE) copolymers. Aligned fibers were collected using a rotating collector. Electrospinning was found to encourage formation of β-phase PVDF-TrFE and does not affect the perovskite peaks of the piezoelectric BNT-ST ceramics. Using a rotating collector, the β-phase of PVDF-TrFE was not affected. However, the perovskite crystal peak intensity of BNT-ST was significantly increased. Therefore the resultant aligned nanofibers from a rotating collector would potentially perform much better than random nanofibers mat. Indeed, with a collector speed of 1500 rpm, the aligned nanofibers were able to give a P_max value of 1.62 µC/cm², which was 2.7 times (0.6 µC/cm²) of the randomly arrayed BNT-ST. Bu et al (2016) used near field electrospinning to generate a highly ordered array of aligned poly(vinylidene fluoride) fiber sandwiched between an elastomer frame [polydimethylsiloxane (PDMS)] and a Cu electrode on a polyethylene teraphthalate (PET) substrate. To enhance power output and prevention of second voltage poling, the nanofibers were isolated on the interdigitated electrodes. The power output was measured under different bend-release cycle. At frequency between 0.8 to 3.3 Hz, the highest output was measured at 3.3 Hz with average output current of the peak at 68 nA and output voltage of 128 mV. Further increase in frequency led to poorer output as the PDMS was unable to release fully and uniformly. Zaarour et al (2019) investigated the influence of fiber morphology and distribution on electrical generation. Comparing smooth, wrinkled and porous electrospun fibers, it was the wrinkled fibers that gave the greatest voltage and current output and the smooth fibers, the lowest. Zaarour et al (2019) attributed this result to greater friction in wrinkled surface fibers and porous fibers compared to smooth surface fibers. However, it is unclear how friction would have led to greater electrical generation. Wrinkled fibers showed the greatest β-phase and porous fibers showed the least. Therefore the wrinkled fibers have greater voltage and current generation than porous fibers. From these results, it can be deduced that greater roughness may contribute to higher voltage and current production. Between membranes made of aligned and randomly oriented electrospun fibers, the former consistently exhibited higher electrical output for the same fiber morphology. The highest voltage and current was obtained from wrinkled aligned electrospun fiber membrane at 2.8V and 3.9 µA respectively. While Zaarour et al (2019) once again attributed this to greater friction, an alternative explanation may be the greater packing density of aligned fibers compared to randomly oriented fibers. Aligned fibers will also ensure that all the fibers will be strained when the membrane was bent in the direction orthogonal to the fiber orientation and hence maximum piezoelectric production.

Representative pictures of samples fabricated by electrospinning of PVDF solutions with different morphologies. (a-c) Randomly oriented fibers, (a) wrinkled, (b) smooth, and (c) porous [Zaarour et al 2019].

In actual application of electrospun fiber piezoelectric generator, structural configuration may be used to increase the total power generated to the extent that the device may have practical usage. Instead of a single electrospun membrane layer, Lu et al (2016) stacked multiple layers of electrets films made of electrospun PVDF/PTFE nanofibers. The device comprised of the electrets films sandwiched between flexible copper electrodes films. In their setup, a three layers configuration gave the best output with an instantaneous output power of 45.6 µW. An important consideration for piezoelectric power generation is the durability of the device. Using electrospun P(VDF-TrFE) nanofibers only in a constructed nanofiber piezoelectric nanogenerators (PENG), Siddiqui et al (2017) showed that there was a sharp decline in output with Voc and Isc outputs of 1.5 V and 20 nA to 0.04 V and 1.21 nA respectively after 500 stretching cycles at 10% stretching strain. This may be due to low stretchability of the P(VDF-TrFE) nanofibers leading breakages. To improve stretchability of the electrospun piezoelectric membrane, electrospun elastomeric polyurethane containing barium titanate nanoparticles (BT-PU) was layered on electrospun P(VDF-TrFE) nanofibers. Electrospun BT-PU nanofibers alone exhibited only a small piezoelectric output. However, the hybrid stacked layers of P(VDF-TrFE) and BT-PU nanofibers gave peak values of 9.3 V and 189 nA for the 6-layered nanofibers PENG at 40% stretching strain. With 30% stretching strain, the device showed no significant drop in output up to 7000 stretching cycles. This shows that the layered construct was able to sustain large mechanical deformation without breakage.

A key requirement for practical utilization of piezoelectric nanogenerators (PENG) is that the membrane must remain intact under mechanical loading. To prevent damage to electrospun piezoelectric membrane when subjected to loading, Siddiqui et al (2017b) encapsulated electrospun nanocomposite of barium titanate nanoparticles (BT NPs) dispersed in poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) nanofibers in an elastomeric film using polydimethylsiloxane (PDMS). Without the elastomeric film protection, the voltage output steadily decreases from 12.46 V to 0.8 V after 500 tapping cycles with a force of 20 N. There was no output from the nanofiber piezoelectric nanogenerators (PENG) after 600 cycles. Cracks and compressed fibers were seen after 600 cycles and these would have caused the large drop in performance. With the PDMS coating, the voltage output was reduced to 3.4 V. However, the encapsulated PENG was able to maintain almost the same voltage output after 10 000 tapping cycles at the same force of 20 N. No cracked or compressed nanofibers were seen for this configuration and this could be attributed to better distribution of the applied strain on the nanofiber mat.

Schematic (a), photograph (b), and working principle (c) of the paper-based low-frequency eKEH [Lu et al. J. Phys.: Conf. Ser. 2016; 773: 012032. doi: 10.1088/1742-6596/773/1/012032. This work is licensed under a Creative Commons Attribution 3.0 Unported License].

For application of PENGs in clothes, one method is to have it in the form of yarn which can then be knitted to form fabrics. Borazan et al (2024) electrospun poly(vinylidene fluoride) (PVDF) nanofiber doped with zinc oxide (ZnO) nanoparticles onto a rotating, funnel-shaped collector. The deposited fibers were passed through guides and fed to a winding unit in the form of a yarn. The resultant yarn can be woven or knitted to form a self-standing piezoelectric nanogenerator (PENG) fabric. Electromechanical test showed that at 5% ZnO loading, the energy output from the fabric was doubled that from pure PVDF fabric, registering a peak power of 81 µW and power density of 30?µW/cm². This has been attributed to higher β-phase as a result of ZnO nanoparticles functioning as nucleating agents. However, higher addition of ZnO nanoparticles decreases the output probably due to nanoparticle agglomeration.

While a triboelectric nanogenerator uses friction to generate power, a piezoelectric nanogenerator uses mechanical stress on piezoelectric material to generate power. With the right assembly, it is possible to construct a power generator that utilizes both triboelectric and piezoelectric effects. Das et al (2024) constructed a tribo-enhanced piezoelectric nanogenerator (TPENG) comprised of AlFeO₃/PVDF nanofibers with two asymmetric electrodes (Al and ITO/PET). The assembly is in the form of a stator and a moving rotor. The PENG device was made of electrospun AlFeO₃/PVDF nanofibers sandwiched between an Al sheet base and an ITO/PET sheet and the three layers wrapped with Kapton tape which acts as a tribo-negative material. For the nanogenerator, the PENG assembly formed the stator. Nylon fabric which acts as the tribo-positive material to create friction was used to wrap the fin of the rotor. Hence the rotor rubbing the stator generates charges similar to a TENG and the periodic bending of the stator generates charges similar to a PENG. Each TPENG was able to produce an output of Voc and Isc at 52.3 V and 1.23 µA, respectively. With six TPENGs in a circular arrangement, the device was able to generate a Voc of 200 V and an Isc of 4.5 µA.

Schematics of TPENG, and the assembly of TPENG device with string-based pulley system [Das et al 2024].

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