A Scalable Nanogenerator Based on Self‐Poled Piezoelectric Polymer Nanowires with High Energy Conversion Efficiency

Abstract

nanowires, [ 7 ] PbZr x Ti 1-x O 3 nanowires [ 8 ] and nanoribbons [ 9,10 ] and poly(vinylidene fl uoride) (PVDF) nanofi bers [ 11 ] have all revealed promising energy harvesting performance. There has since been an ongoing concerted effort in developing this relatively new research fi eld, connecting nanotechnology with the fi eld of energy. [ 12 ] Piezoelectric nanowires are particularly attractive for energy harvesting due to their robust mechanical properties and high sensitivity to typically small ambient vibrations. [ 13 ] The implications of these properties, in fact, go beyond energy harvesting, as nanowirebased nanogenerators have recently been shown to function as bio mechanical sensors, [ 14 ] sensitive pressure sensors [ 15 ] and precision accelerometers. [ 16 ] The challenge lies in the large scale production of low-cost piezoelectric nanowires that can offer reproducible and reliable energy harvesting and/or sensor performance.The polymer PVDF [(CH2-CF2) n ] exhibits good piezoelectric and mechanical properties with excellent chemical stability and resilient weathering characteristics. [ 17,18 ] PVDF thin fi lms are thus commonly used as sensors and actuators. [ 19 ] However, the piezoelectric performance of PVDF is dependent on the nature of the crystalline phase present. Typically, PVDF occurs in the α, β and γ crystalline phases [20][21][22] and needs to be electrically poled (using an electric fi eld of the order of 100 MV m −1 ) and/ or mechanically stretched [ 20,22 ] to achieve the polar β-phase that shows the strongest piezoelectric behavior. [ 21 ] is a co-polymer that crystallizes more easily into the β-phase due to steric factors, [ 22 ] an advantage that we exploit in this work. Nanowires of PVDF and its copolymers have been previously incorporated into piezoelectric nanogenerators [ 11 ] but the relatively complex electrospinning fabrication process employed requires high voltages (5-50 kV) and specialized equipment. The associated high electric fi elds and stretching forces result in poled nanowires, however this fabrication process often suffers from poor control over nanowire size-distribution and alignment, and is yet to be conveniently and cost-effectively scaled up. [ 23 ] Here we report the growth of aligned P(VDF-TrFE) nanowires with a narrow size distribution using a simple, cost-effective and easily scalable template-wetting method, [ 24,25 ] where the template-induced space confi nement promotes high crystallinity and preferential orientation of the lamellar crystals in the polymer nanowires. [ 26,27 ] This results in the enhancement of piezoelectric properties, even without the need for electrical poling. A nanogenerator fabricated using template-grown, self-poled P(VDF-TrFE) nanowires is shown to have excellent electrical output when subjected to periodic vibrations. Using a circuit comprising a rectifi er to convert its AC output to DC, and a bank of capacitors to store the harvested energy, the nanogenerator is shown to be capable of lighting a commercial light emitting ...

Nanostructured polymer-based piezoelectric and triboelectric materials and devices for energy harvesting applications

“…Even then, it has been reported that electrically poled P(VDF-TrFE) nanowire arrays could have over 10 times higher piezoelectric coefficients than normal P(VDF-TrFE) thin films undergoing the same treatment [25]. More recently, nanowire synthesis approaches making use of a nanoconfinement growth effect have been found to be more likely to result in so-called 'selfpoled' β phase NWs [21,26], mostly observed, for example, in template-assisted growth whereby polymer NWs are grown within nanoporous templates by infiltration from solution or melt, and where the resulting NWs do not require external electrical poling in order to exhibit piezoelectric behaviour. Self-poling results from some degree of molecular alignment of the polymer chains along the length of the nanowire, which may arise as a result of the high degree of shear force experienced by the polymer solution during capillary infiltration.…”

“…In general, typical thin film fabrication often results in α phase PVDF (shown in figure 1(a)) which does not behave exhibit piezoelectricity due to net cancellation of dipoles along the chain [16]. Poling with either mechanical stretching, electric field or from controlled growth may force the polymer into a ferroelectric polar β phase which exhibits the strongest piezoelectricity [20][21][22][23][24]. Co-polymers of PVDF where some of the hydrogen atoms are replaced with fluorine atoms are more likely to crystallise in a polar phase due to steric factors.…”

“…In both cases, appropriate electrodes were applied on to the surfaces of the templates for charge collection from the piezoelectric ZnO NWs embedded within. The piezoelectric responses of these NGs were measured by using vibrational shakers similar to the one described in previous reports built by Whiter et al [21] ( figure 7(c)). The electrical output characteristics of the NGs, namely open-circuit voltage, short-circuit current and peak power output across impedance-matched loads are compared in figure 18, with typical V oc between 50 to 500 mV, I sc between 10 to 30 nA across a range of low-level vibrating frequencies.…”

“…Whiter et al reported a template-assisted approach for scalable growth of self-poled P(VDF-TrFE) nanowire arrays and their application in NGs [21]. To start the synthesis process, AAO template was prepared with a thin layer (~15 nm) of silver deposited on one side of the surface, which was required for easy and complete removal of the residual film formed after the complete infiltration of the polymer.…”

Roll type conducting polymer legs for rigid-flexible thermoelectric generator

“…20 In addition, the low output voltage could be accumulated and converted to a high voltage through the circuitry for a static energy harvester. 21,22 In summary, this is the first example of a rigid-flexible TEG based on the organic and inorganic hybrid TE legs with high output performance. It can generate more than 160 nW under a ∆T of 10 K and is flexible enough to apply it on the human wrist, taking advantage of the flexibility of PP-PEDOT thin film.…”