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

Abstract

Energy harvesting from vibrations is demonstrated using a nanogenerator composed of piezoelectric polymer nanowires with high energy conversion efficiency. Nanowires of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) grown using a simple, scalable, and cost-effective template-wetting technique are shown to be successfully exploited in high-performance nanogenerators without the need for electrical poling. Nanogenerators based on piezoelectric materials convert ever-present mechanical vibrations into electrical power for energetically autonomous wireless and electronic devices. Nanowires of piezoelectric polymers are particularly attractive for harvesting mechanical energy in this way, as they are flexible, lightweight and sensitive to small vibrations. Previous studies have focused exclusively on nanowires grown by electrospinning, but this involves complex equipment, and high voltages of ≈10 kV that electrically pole the nanowires and thus render them piezoelectric. Here we demonstrate that nanowires of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) grown using a simple and cost-effective template-wetting technique, can be successfully exploited in nanogenerators without poling. A typical nanogenerator comprising ≈1010 highly crystalline, self-poled, aligned nanowires spanning ≈2 cm2 is shown to produce a peak output voltage of 3 V at 5.5 nA in response to low-level vibrations. The mechanical-to-electrical conversion efficiency of 11% exhibited by our template-grown nanowires is comparable with the best previously reported values. Our work therefore offers a scalable means of achieving high-performance nanogenerators for the next generation of self-powered electronics. Energy harvesting from ambient vibrations has generated significant interest1, 2 as it offers a fundamental energy solution for small-power applications including, but not limited to, ubiquitous wireless sensor nodes, portable and wearable electronics, biomedical implants and structural/environmental monitoring devices. Nanoscale piezoelectric harvesters, also known as nanogenerators, convert small-scale vibrations into electrical energy thus offering a means of superseding batteries that require constant replacing/recharging, and that do not scale easily with the size of the device. Nanogenerators were first demonstrated using ZnO nanowires3-5 and subsequent nanogenerators based on GaN nanowires,6 BaTiO3 nanowires,7 PbZrxTi1-xO3 nanowires8 and nanoribbons9, 10 and poly(vinylidene fluoride) (PVDF) nanofibers11 have all revealed promising energy harvesting performance. There has since been an ongoing concerted effort in developing this relatively new research field, connecting nanotechnology with the field 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 nanowire-based nanogenerators have recently been shown to function as bio mechanical sensors,14 sensitive pressure sensors15 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 films 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 phases20-22 and needs to be electrically poled (using an electric field of the order of 100 MV m−1) and/or mechanically stretched20, 22 to achieve the polar β-phase that shows the strongest piezoelectric behavior.21 P(VDF-TrFE) [(CH2-CF2)n-(CHF-CF2)m] is a co-polymer