Elsevier

Nano Energy

Volume 35, May 2017, Pages 146-153
Nano Energy

Full paper
High-output acoustoelectric power generators from poly(vinylidenefluoride-co-trifluoroethylene) electrospun nano-nonwovens

https://doi.org/10.1016/j.nanoen.2017.03.038Get rights and content

Highlights

  • Electrospun poly(vinylidenefluoride-co-trifluoroethylene) nonwovens show exceptional acoustoelectric conversion ability.

  • The device under sound can generate peak signal of 14.5 V and 28.5 μA with a volume power density output of 306.5 μW/cm3.

  • The electricity generated can directly light up LEDs, run electrochemical reactions and protect metals from corrosion.

  • The effect of hole number in electrodes on acoustoelectric conversion behavior was systematically examined.

Abstract

Conversion of mechanical forces, vibrations and sound which have energy in the range of milli- to tens of watts (also referred to as “small kinetic energies” in this paper) into electricity has received global attention owing to the potential in powering electronic devices. However, most of the small kinetic energy generators can only generate electric power below 1 µW/cm3. As a result, an energy storage device is required to accumulate the energy generated until it reaches a sufficient level. Herein, we report the exceptionally-high acoustoelectric conversion ability of randomly-orientated electrospun poly(vinylidenefluoride-co-trifluoroethylene) nanofiber nonwoven webs. The optimized device under sound is able to generate peak voltage and current of 14.5 V and 28.5 μA with a volume power density output of 306.5 μW/cm3 (5.9 mW/cm3 based on nanofiber web thickness). Without accumulation in any energy storage unit, the electricity generated by the nanofiber device is sufficient to light up tens of commercial LEDs, run electrochemical reactions and protect metals from corrosion. Such a novel acoustoelectric generator may offer an effective solution to recycling noise pollution into usable electricity.

Introduction

Small kinetic energies such as those from mechanical forces, vibrations and sound spread widely in surrounding environment and human body. Growing interest has been devoted to converting small kinetic energy into electricity for powering miniature electronics [1], [2], [3], [4]. By integration of a small kinetic energy power generator (SKEG) into electronic devices or mini-robots, a self-powering ability is expected to be rendered with the devices, so they can work self-sustainably without frequent recharging, which will be extremely useful for applications in biomedicine, healthcare, environment monitoring, industrial processing and fundamental research.

Most of the studies on SKEGs developed so far are based on the conversion of mechanical forces. Despite new energy harvesting materials developed [5], [6], [7], [8], [9], [10], effective conversion of sound energy into electricity for power generation purposes has been less reported. In theory, sound carries sufficient energy to run microelectronic devices. Sound power (P) can be calculated by the equation:P=Ap2ρccosθwhere A is the area of the surface, p is sound pressure, ρ is the mass density, c is sound speed, and θ is angle between the directions of the sound propagation and the normal to the surface [11]. A sound that propagates in air through a surface of 1 m2 (θ=0°) carries a power of 9.7 mW when the sound pressure level (SPL) at 100 dB. Sound at a noise level contains power (e.g. 0.3–9.7 mW when SPL 85–100 dB) equivalent to the power consumption level for class 2 Bluetooth (~2.5 mW) or running a RFID tag (~10 µW) [12], [13]. Conversion of noise into electricity could offer a way to power electronic device using this white pollution.

Conventionally, sound to electricity conversion, also referred to as “acoustoelectric conversion”, is used to develop acoustic sensors, microphones, speakers and music instruments due to small electric outputs generated. Acoustoelectric devices are developed based on piezoelectricity [6] and triboelectricity [7]. However, only a few papers report the acoustoelectric generators with a voltage output above 1 V (Tables S1 and S2). The power produced from those devices is very low in density, and it has to be accumulated in an energy storage unit until reaching a sufficient level.

Polyvinylidene fluoride (PVDF) and its copolymers such as poly(vinylidenefluoride-co-trifluoroethylene) ((P(VDF-TrFE)) represent the most commonly used piezoelectric polymers with the highest piezoelectric coefficient among piezoelectric polymers. Despite smaller piezoelectric coefficient than inorganic piezoelectric materials, they have higher sensitivity than piezoelectric ceramics when used for acoustoelectric conversion, and thus can generate higher piezoelectric voltage under the same sound pressure. They have greater flexibility, better response, and wider frequency response range (0.001–109 Hz) [9].

Recently, our and other research groups have separately reported that when PVDFs are electrospun into nanofibers, they already have piezoelectricity. In particular for randomly-orientated nanofiber mats (also called nano-nonwovens in this article), they can have a voltage output as large as several voltages, though no extra stretching and poling treatment are applied. More importantly, electrospun nanofiber webs under compressive/decompressive deformation can generate much higher electric outputs than the piezoelectric dense film [10].

More recently, we have uncovered that PVDF nanofiber webs produced by an electrospinning technique have strong acoustoelectric conversion ability [14]. They can detect sound with a sensitivity as high as 266 mV/Pa, which is over 5 times higher than the device made of piezoelectric dense PVDF film. However, nano-nonwovens as an acoustoelectric generator have not been reported.

In this study, we have proved that electrospun nano-nonwovens from P(VDF-TrFE) can be used to develop acoustoelectric generator. By modifying the nanofiber devices to allow maximum sound-nanofiber interaction, we showed that a nanofiber acoustoelectric device can generate an electric output as high as 14.5 V and 28.5 μA with a volume energy density of 306.5 μW/cm3 (5.9 mW/cm3 based on nanofiber web thickness), under a noise level of 115 dB. We further demonstrate that the electric power generated in-situ from a small piece of our acoustoelectric generator, without pre-storage in a capacitor, can be used to directly light up 27 commercial LEDs, run electrochemical polymerization reactions and perform corrosion protection of metals. Such a highly efficient acoustoelectric generator may offer an effective solution to harvest noise pollution into usable electricity for self-powered electronics.

Section snippets

Materials

P(VDF-TrFE) powder (55/45) (PIEZOTECH), N, N-dimethylformamide (DMF), acetone, lithium perchlorate (LiClO4), 3,4-ethylenedioxythiophene (EDOT), propylene carbonate and sodium chloride (NaCl) were purchased from Sigma-Aldrich and used as received.

Electrospinning

P(VDF-TrFE) solution was prepared by dissolving P(VDF-TrFE) powder in the solvent mixture of DMF and acetone (4/6, v/v) at room temperature for 2 h. The homogeneous P(VDF-TrFE) solution was electrospun into nanofiber mat using a purpose-built needle

Results and discussion

Fig. 1a schematically illustrates the structure of the acoustoelectric devices fabricated in this study. A piece of electrospun P(VDF-TrFE) nano-nonwoven was sandwiched between two PET film electrodes. Both the PET film electrodes contained a thin layer of sputter-coated Au on one side, and the Au layer was contacted with the nanofiber layer. The Au layers function to collect electrical signals generated from the nanofibers. To maximize the sound-nanofiber interaction, through holes were cut on

Conclusions

We have shown the incredible acoustoelectric conversion ability of randomly-orientated P(VDF-TrFE) nanofiber nonwovens, suitable for power supplying purposes. The devices generate higher electric outputs under a sound with pressure level above 100 dB. The optimized device can generate voltage and current outputs of 14.5 V and 28.5 μA with a volume power density output of 306.5 μW/cm3, which are much higher than these of commercial piezoelectric P(VDF-TrFE) films. The electricity generated from

Acknowledgements

Funding support from an ARC Future Fellow grant (ARC FT120100135) is acknowledged. C.H.L. acknowledges the scholarships from the Donghua University International Visiting Program for Excellent PhD Students and Deakin University to support her visit Deakin University. The authors thank Mr Hui Kong and Mr Jun Cheng for their assistance in corrosion protection demonstration, and Dr. Ben Allardyce for his technical assistance in vibration test.

Chenhong Lang received her B.S. degree in Textile Engineering from Donghua University in 2011. She is currently a Ph.D. candidate in Professor Xin Ding's group, Donghua University, and at the same time she is a visiting student at Prof. Tong Lin's group at Deakin University. Her current research focuses on piezoelectric nanofiber for acoustic applications.

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    Chenhong Lang received her B.S. degree in Textile Engineering from Donghua University in 2011. She is currently a Ph.D. candidate in Professor Xin Ding's group, Donghua University, and at the same time she is a visiting student at Prof. Tong Lin's group at Deakin University. Her current research focuses on piezoelectric nanofiber for acoustic applications.

    Jian Fang received his Ph.D. degree in Materials Engineering from Deakin University in 2009. He is currently a Research Fellow at Deakin University and his research focuses on electro-active fibrous materials for energy harvesting, biosensing and environment protection applications.

    Hao Shao received his Ph.D. degree from Deakin University in 2017. He won the 2016 Chinese Government Award for Outstanding Self-financed Students Abroad. He is currently a research assistant in Prof Tong Lin's group. His research focuses on electrospinning nanofibers and mechanical-to-electrical energy harvester devices.

    Hongxia Wang received her Ph.D. degree in Materials Engineering from Deakin University in 2010. After graduation, she joined Deakin University as an Alfred Deakin Postdoctoral Research Fellow, and has worked as ARC APD Fellow in 2011–2014 and Research Fellow since 2014. Her research interests include superhydrophobic/superoleophobic coatings, directional fluid transport fabrics, functional fabrics and nanofibers.

    Guilong Yan received his B.Sc. from Tianjin Polytechnic University. He is currently a Ph.D. candidate under the supervision of Prof. Tong Lin at Deakin University. His research mainly focuses on nanofibers, electrospinning and modelling.

    Xin Ding is currently a professor of Textile Engineering at the College of Textiles, Donghua University (formerly China Textile University), Shanghai. His research interests involve processing technology of textiles, for both fashion industry and nor-traditional areas such as civil and structural, aerospace, medical field and wearable technology. He is the recipient of the Governmental Prize for Outstanding Contributions in Education from the State Council of China and has won a number of national and international scholarships and prizes.

    Tong Lin received his Ph.D. degree in Physical Chemistry in 1998. He is an ARC Future Fellow, Professor and Personal Chair in Fibrous Materials at Deakin University, Australia. His research interests cover functional fibres, electrospinning and polymers.

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    Both authors contributed equally to the manuscript.

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