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Nano Energy

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High-output acoustoelectric power generators frompoly(vinylidenefluoride-co-trifluoroethylene) electrospun nano-nonwovens

Chenhong Langa,b,1, Jian Fanga,1, Hao Shaoa, Hongxia Wanga, Guilong Yana, Xin Dingb,Tong Lina,⁎

a Institute for Frontier Materials, Deakin University, Geelong VIC 3216, Australiab College of Textiles, Donghua University, Shanghai 201620, PR China

A R T I C L E I N F O

Keywords:Acoustoelectric conversionNanofiberNonwovenP(VDF-TrFE)Power generatorNoise

A B S T R A C T

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 tothe potential in powering electronic devices. However, most of the small kinetic energy generators can onlygenerate electric power below 1 µW/cm3. As a result, an energy storage device is required to accumulate theenergy generated until it reaches a sufficient level. Herein, we report the exceptionally-high acoustoelectricconversion ability of randomly-orientated electrospun poly(vinylidenefluoride-co-trifluoroethylene) nanofibernonwoven webs. The optimized device under sound is able to generate peak voltage and current of 14.5 V and28.5 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 sufficientto light up tens of commercial LEDs, run electrochemical reactions and protect metals from corrosion. Such anovel acoustoelectric generator may offer an effective solution to recycling noise pollution into usable electricity.

1. Introduction

Small kinetic energies such as those from mechanical forces,vibrations and sound spread widely in surrounding environment andhuman body. Growing interest has been devoted to converting smallkinetic energy into electricity for powering miniature electronics [1–4].By integration of a small kinetic energy power generator (SKEG) intoelectronic devices or mini-robots, a self-powering ability is expected tobe rendered with the devices, so they can work self-sustainably withoutfrequent recharging, which will be extremely useful for applications inbiomedicine, healthcare, environment monitoring, industrial proces-sing and fundamental research.

Most of the studies on SKEGs developed so far are based on theconversion of mechanical forces. Despite new energy harvestingmaterials developed [5–10], effective conversion of sound energy intoelectricity for power generation purposes has been less reported. Intheory, sound carries sufficient energy to run microelectronic devices.Sound power (P) can be calculated by the equation:

P Apρc

θ= cos2

(1)

where A is the area of the surface, p is sound pressure, ρ is the massdensity, c is sound speed, and θ is angle between the directions of thesound propagation and the normal to the surface [11]. A sound thatpropagates in air through a surface of 1 m2 (θ=0°) carries a power of9.7 mW when the sound pressure level (SPL) at 100 dB. Sound at anoise 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 ofnoise into electricity could offer a way to power electronic device usingthis 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 electricoutputs generated. Acoustoelectric devices are developed based onpiezoelectricity [6] and triboelectricity [7]. However, only a few papersreport the acoustoelectric generators with a voltage output above 1 V(Tables S1 and S2). The power produced from those devices is very lowin density, and it has to be accumulated in an energy storage unit untilreaching a sufficient level.

Polyvinylidene fluoride (PVDF) and its copolymers such as poly(-vinylidenefluoride-co-trifluoroethylene) ((P(VDF-TrFE)) represent the

http://dx.doi.org/10.1016/j.nanoen.2017.03.038Received 23 November 2016; Received in revised form 3 March 2017; Accepted 21 March 2017

⁎ Corresponding author.

1 Both authors contributed equally to the manuscript.E-mail address: tong.lin@deakin.edu.au (T. Lin).

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Available online 22 March 20172211-2855/ © 2017 Elsevier Ltd. All rights reserved.

MARK

most commonly used piezoelectric polymers with the highest piezo-electric coefficient among piezoelectric polymers. Despite smallerpiezoelectric coefficient than inorganic piezoelectric materials, theyhave higher sensitivity than piezoelectric ceramics when used foracoustoelectric conversion, and thus can generate higher piezoelectricvoltage 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 reportedthat when PVDFs are electrospun into nanofibers, they already havepiezoelectricity. In particular for randomly-orientated nanofiber mats(also called nano-nonwovens in this article), they can have a voltageoutput as large as several voltages, though no extra stretching andpoling treatment are applied. More importantly, electrospun nanofiberwebs under compressive/decompressive deformation can generatemuch higher electric outputs than the piezoelectric dense film [10].

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

In this study, we have proved that electrospun nano-nonwovensfrom P(VDF-TrFE) can be used to develop acoustoelectric generator.By modifying the nanofiber devices to allow maximum sound-nanofiberinteraction, we showed that a nanofiber acoustoelectric device cangenerate an electric output as high as 14.5 V and 28.5 μA with a volumeenergy 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 thatthe electric power generated in-situ from a small piece of our acousto-electric generator, without pre-storage in a capacitor, can be used todirectly light up 27 commercial LEDs, run electrochemical polymeriza-tion reactions and perform corrosion protection of metals. Such ahighly efficient acoustoelectric generator may offer an effective solutionto harvest noise pollution into usable electricity for self-poweredelectronics.

2. Experimental section

2.1. Materials

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

2.2. 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 roomtemperature for 2 h. The homogeneous P(VDF-TrFE) solution waselectrospun into nanofiber mat using a purpose-built needle electro-spinning setup [15]. During electrospinning, the flow rate of thepolymer solution, the applied voltage, and spinning distance were setat 0.5 ml/h, 23 kV, and 15 cm, respectively.

Fig. 1. (a) Schematic illustration of the energy harvester device; (b) digital photo of the device; (c) SEM image of the P(VDF-TrFE) nanofibers (scale bar, 2 µm); (d) schematicillustration of the acoustoelectric conversion testing; (e) voltage and current outputs of the device under sound; (f) Effect of sound wave frequency on the voltage output of the 8-holedevice (inset: magnified voltage outputs at the sound wave frequency range of 1000–2000 Hz); (g) peak voltage and current output changes with the external load resistance; (h) peakpower output change with the external load resistance; (i) effect of SPL on of 8-hole devices made of nanofiber web and commercial dense film (inset: log scale plot of voltage output atthe sound pressure levels lower than 95 dB). (8-hole device, hole diameter, 4.9 mm; web thickness, 20 µm; film area, 12 cm2; (a)–(e) and (g)–(i): sound pressure level, 115 dB andfrequency, 210 Hz).

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2.3. Device fabrication

A piece of electrospun P(VDF-TrFE) nano-nonwoven was sand-wiched between two gold-coated PET films (thickness 110 µm), withthe Au layer contacting with the nanofiber web. Holes were cut in themiddle area of each electrode. For comparison, a commercial piezo-electric P(VDF-TrFE) film and an electrospun PVDF nano-nonwovenwere also used as the active layer to fabricate devices with samestructure, dimension and electrode material.

2.4. Characterizations

The morphology of nanofibers was observed under a scanningelectron microscope (Zeiss Supra 55VP), and fiber diameter wasmeasured based on the SEM images using image processing software(ImageJ 1.45 s). XRD was measured on a diffractometer (PanalyticalXRD) with Cu-Ka radiation operated at 40 kV and 30 mA. The samplewas scanned in the 2θ range of 10–60°. FTIR spectra were recorded ona FTIR spectrophotometer (Bruker Optics) in ATR mode. Ramanspectra were recorded on a Raman spectrometer (Renishaw Invia)equipped with a 633 nm laser source.

2.5. Acoustoelectric conversion test

The energy conversion performance of the nanofibrous energyharvester was tested using a commercial PC speaker (Logitech, LS21)as the sound source. The sound frequency was controlled by acomputer and an Audio Tester software. A sound level meter(DIGITECH, QM-1589) was used to record the acoustic wave pressure.An electrochemistry working station (e-Corder 401) was used to recordelectric signals with a sampling frequency of 10,000 per second. Detailsof the testing platform have been reported in our previous publication[14].

2.6. FEM modelling

The vibration mode was calculated by a finite element method,using the Pressure acoustic and Solid mechanics interaction modulusin COMSOL Multiphysics 3.5. P(VDF-TrFE) nanofiber web, PET film,and sound-collecting cone were transformed into geometrical objects inCOMSOL, and their practical dimensions, locations, density, Young'smodulus, Shear modulus, Bulk modulus and Poisson's ratio were input.The two edge boundaries of film device and the domain boundary ofcone were set as fixed constraint. A monopole point source in the end ofcone was set as sound source. Due to the complexity of nanofibrousstructure, the (PVDF-TrFE) nanofiber web was set as nonlinearmaterial with the density of 450 kg/m3, Shear modulus of 7.2 MPaand Bulk modulus of 3.4 MPa. The PET film has the density of1380 kg/m3, Young's modulus of 3.6 GPa and Poisson's ratio of 0.35.

2.7. Application demonstrations

(1) LEDs: The electric output generated by the nanofiber deviceunder sound was rectified into DC power, which was connected directlywith commercial LEDs in series. (2) Polymerization reaction: Theelectric output after rectification was connected to a two-electrodeelectrochemical system. The positive electrode and negative electrodewere connected to a gold coated plastic film (working electrode) and aplatinum wire (counter electrode), respectively. The reaction wascarried out in propylene carbonate containing LiClO4 (0.1 mol/L)and EDOT (0.04 mol/L). (3) Corrosion protection: The electric outputafter rectification was connected a two electrode electrochemicalsystem. The cathode and anode were connected a steel sample (work-ing electrode) and a platinum wire (counter electrode), respectively.The corrosion reaction was carried out in 0.1 mmol/L aqueous NaClsolution.

3. Results and discussion

Fig. 1a schematically illustrates the structure of the acoustoelectricdevices 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-coatedAu on one side, and the Au layer was contacted with the nanofiberlayer. The Au layers function to collect electrical signals generated fromthe nanofibers. To maximize the sound-nanofiber interaction, throughholes were cut on each PET film. Fig. 1b is a digital photo to show theacoustoelectric device, which contains 8 through holes on bothelectrodes. The holes in the electrodes overlapped in location.

Fig. 1c shows the morphology of P(VDF-TrFE) nanofibers preparedby an needle-based electrospinning technique. The nanofibers had auniform morphology with a diameter of 240 ± 40 nm. The X-raydiffraction (XRD) pattern and Fourier transform infrared (FTIR)spectrum indicated that the nanofibers mainly contained β crystalphase (Fig. S1), which is critical for piezoelectric conversion.

The acoustoelectric conversion property of the nanofiber device wasmeasured by placing the device in front of a commercial speaker(Fig. 1d). A sound-collecting cone was set on the speaker to increasethe sound pressure. For the device containing eight through holes(diameter of each hole, 4.9 mm) on each electrodes and a nanofiberweb of working dimension 3×4 cm2 (thickness, 20 µm), under a sound(SPL 115 dB, frequency 210 Hz) it generated periodic voltage andcurrent outputs with peak values as high as 14.5 V and 28.5 μA (Fig. 1eand Fig. S2).

Fig. 1f shows the dependency of voltage outputs on soundfrequency for the nanofiber device. The large voltage outputs weregenerated mainly from low frequency sound. The inset in Fig. 1f showsthe voltage outputs at the sound wave frequencies above 1000 Hz. At105 dB, the device generated an obvious output at sound frequencybelow 500 Hz with the maximum output at 210 Hz. When the SPL wasincreased to 115 dB, the device showed higher outputs, and theresponding range was broadened to 1000 Hz (Fig. S3). Since the PCspeaker used for our experiment can’t reach 115 dB at the frequenciesbelow 200 Hz, the voltage output at lower frequency was not availableat this SPL. To get voltage output at lower frequency, the SPL wasdecreased to a lower level. Fig. 1f also includes the voltage outputs at105 dB and 95 dB, which shows a similar output profile except for lowvoltage values.

The energy parameters of the nanofiber device were estimatedaccording to the literature reported method [16]. The instantaneousmaximum power (P) calculated based on the maximum voltage (U) andthe maximum current (I) at the optimum load, i.e. P=UI, was141.3 µW. Based on the nanofiber and device dimensions, the instan-taneous maximum areal power density and instantaneous maximumvolume power density can be calculated, being 11.8 µW/cm2 and306.5 μW/cm3 (5.9 mW/cm3 based on nanofiber web thickness)respectively. The overall power of the device can be calculated by theequation: ∫P U t I t dt f= ( ) ( ) ×e Int t

ta( )

1

2(where fa is the frequency of the

sound wave). For the device output shown in Fig. 1e, the overall powerwas 25.8 µW. The device at this sound condition can generate electricenergy of 92.9 mW h.

Fig. 1g shows the effect of an external resistance on the electricoutputs of the nanofiber device. The output voltage increased withincreasing the external resistance until 2.5 MΩ. Further increasing theexternal resistance had little effect on the voltage output value. In contrast,the external resistance showed an opposite effect trend on the currentoutput. The output power of the device calculated based on the voltageand current outputs is shown in Fig. 1h. The device showed the maximumpower output of 141.3 µW at the external resistance of 470 kΩ. It isknown that power generators have the maximum power output when theexternal resistance is equivalent to the internal resistance. Therefore, theinternal resistance of out nanofiber device was 470 kΩ.

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Energy conversion efficiency of the devices were calculated as theratio of output electric power (Pe) and input acoustic power (Pa) (seeSupporting information for detailed calculation):

η PP

= e

a (2)

As listed in Table 1, the calculated energy conversion efficiency ofour device was as high as 60.3%, which is very promising for practicalenergy harvesting applications.

We also prepared similar devices using a commercial piezoelectricP(VDF-TrFE) film and compared them with our nanofiber devices.With the same device structure, working dimension and testingconditions, our nanofiber device generated higher voltage than thedense film counterpart in all the examined sound conditions (Fig. 1i,Fig. S4), the difference became much more obvious when the SPL wasabove 100 dB. When SPL was below 95 dB, the output voltage showedlittle difference (see the inset in Fig. 1i). The device made from thecommercial dense film had a low maximum output power (14.4 µW)with a very large internal resistance (2.5 MΩ, Fig. S5). In addition, wealso prepared the devices from PVDF nano-nonwoven. Table 1 alsolists the power generation features of the acoustoelectric devices madefrom commercial dense piezoelectric P(VDF-TrFE) film and electro-spun PVDF nano-nonwoven. Evidently, the P(VDF-TrFE) nano-non-woven device had the highest electric output and energy conversionefficiency.

To maximize the sound absorption, holes were drawn in theelectrodes. Changing the size and distribution of the hole in theelectrodes alters the area and location of nanofiber layer that exposeto the sound, hence the interaction between sound waves and nanofiberweb. Indeed, the through holes in the electrodes were found to play animportant role in deciding the device acoustoelectric conversion.Multiple holes were cut to allow nanofibers in different areas to exposeto sound source. Fig. 2a shows the effect of hole number on deviceoutputs. Here, the total area of the holes in each electrode wasmaintained the same (1.5 cm2) and the holes of equal size were cutin both electrodes. More holes in the electrode suggests smaller hole

Table 1Power generation features of P(VDF-TrFE) and PVDF.

P (VDF-TrFE) PVDFNano-nonwovencNano-nonwovena Commercial

filmb

8-hole 1-hole

Peak open-circuitvoltage (V)

14.5 7.8 4.6 5.3

Peak short-circuitcurrent (μA)

28.5 11.2 5.8 6.6

Optimum load R (kΩ) 470 2500 2500 3000Voltage on optimum

load (V)7.4 6.0 3.4 4.3

Current on optimumload (μA)

19.1 8.6 4.2 5.6

Instantaneous power(μW)

141.3 51.6 14.4 24.1

Areal power density(μW/cm2)

11.8 4.3 1.2 2.0

Volume power density(μW/cm3)

5900 2150 600 1000

Power (μW) Integral 25.8 12.5 5.2 7.6RMS 25.6 12.6 5.0 7.6

Energy (Wh) Integral 9.29×10−2 4.50×10−2 1.87×10−2 2.72×10−2

RMS 9.22×10−2 4.54×10−2 1.80×10−2 2.73×10−2

EnergyConversi-onEfficiency(%)

Integral 60.3 29.2 12.1 17.8RMS 59.8 29.4 11.7 17.8

a size, 3×4 cm2, thickness, 20 µm, nanofiber diameter, 240 ± 40 nm;b size, 3×4 cm2, thickness, 20 µm;c size, 3×4 cm2, thickness, 20 μm, nanofiber diameter, 310 ± 60 nm; calculated based

on the active layer.

Fig. 2. (a) Effect of hole number on voltage output (sound frequency, 210 Hz; SPL, 115 dB; web area, 12 cm2; web thickness, 20 µm); (b) relationship between voltage output and SPLfor the nanofiber devices with different numbers of holes (sound frequency, 210 Hz). FEM modelled vibration energy of (c) nanofiber web and (d) PET film in 1-hole and 8-hole devices(Sound frequency, 210 Hz) (insets: log scale plots of vibration energy at low SPLs), modelled vibration energy density of nanofiber webs in (e) 8-hole and (f) 1-hole device. (Soundfrequency, 210 Hz; sound pressure level, 115 dB).

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diameter. With increasing the hole number from 1 to 5, the voltageoutput increased from 7.9V to 12.2 V. However when the devicescontained 6–8 holes, the voltage outputs were almost unchanged. Theresults in Table 1 reveal that the energy conversion efficiency wasdoubled by increasing the hole number from 1 to 8, suggesting theimportance of device structure to the energy outputs.

The device with different numbers of holes showed slightly differentoutput responses to SPL (Fig. 2b). All devices had very small outputswhen the SPL was below 80 dB regardless of hole number. When theSPL was in the range of 80–107 dB, the devices with less holes hadhigher voltage outputs. However, when the SPL was above 107 dB, thedevices showed a reverse trend, in which the one with more holesgenerated larger voltage outputs. In the range of 107–115 dB, thevoltage output changed with SPL at a larger rate.

To understand the effect of holes on acoustoelectric response of thedevices, we analyzed the vibration characteristics of the nanofiberdevices under acoustic waves using a finite element method (COMSOLMultiphysics 3.5a). Fig. 2c shows the vibration energy of the nanofiberweb in the 8-hole and 1-hole devices. At a SPL above 105 dB, thenanofiber web in the 8-hole device had higher vibration energy thanthat in the 1-hole one. This can be explained by the strong soundabsorption ability of nanofibers. Changing the area and location wherethe nanofiber web directly receives sound could lead to change in thenanofiber vibration profile, because nanofibers in the hole vibrate morestrongly than those covered with the electrodes. Therefore, a multi-holestructure should include multiple vibration spots in the fibrous web,which facilitates to transport the vibration energy from the exposedareas to the entire device.

The hole showed a similar effect on the vibration energy of the PETfilms. The 8-hole film has much higher overall vibration energy thanthe 1-hole film (Fig. 2d). The inserts in Fig. 2c and d reveal moreobvious vibration energy difference of the PET film than the nanofiber

web at low sound pressure levels. These results were in good agreementwith our experiment results on device electric outputs (Fig. S7).

The 1-hole devices made of both nano-nonwoven and commercialfilm had lower electric outputs than their 8-hole counterparts. It hasalso been found that the hole size and nanofiber web thickness haveobvious effects on voltage output. When the hole diameter was smallerthan 6.4 mm, the voltage output was lower than 5.30 V. Larger holeincreased the voltage output until the diameter reached 13.8 mm.Further increasing the hole diameter resulted in decrease in the voltageoutput (Fig. S7(c)). When the thickness increased from 20 µm to40 µm, the output voltage keeps around 8 V. Further increasing thefiber web thickness led to decrease in voltage value (Fig. S7).

The vibration velocity for the nanofiber web increased withincreasing SPL (Fig. S8). At all examined SPLs, the nanofiber web inthe 8-hole device vibrated faster than that in the 1-hole device. Thevibration energy profile of the nanofiber web in the 1-hole and 8-holedevices is illustrated in Fig. 2e and f. The central part of the holeshowed higher vibration energy density than the periphery of the holes.The nanofiber web and the PET films in the devices showed noticeableincrease in vibration energy when the SPL was above 100 dB (Fig. S9).The 8-hole device exhibited higher average vibration velocity than the1-hole device (Fig. S8).

We also measured the vibration velocity at the central and the edgeparts of the entire device (Fig. S10). At a low SPL, the vibration velocityof nanofiber web in the central part was low until a SPL threshold,above which the vibration velocity increased very rapidly and thenstabilized at a higher SPL value. For the 8-hole device, the thresholdvalue was around 95 dB, while the 1-hole device had a much lowervalue, around 75 dB. The vibration velocity was stabilized at a similarlevel when the SPL above 105 dB and 85 dB for the 8- and the 1-holedevices, respectively. This was different to the modelling result sincethe calculation value showed no stabilized vibration velocity. It is

Fig. 3. Effect of average fiber diameter on (a) voltage and current output and (b) internal resistance (a and b: web thickness, 20 µm, the star marked data was obtained from P(VDF-TrFE) nanofibers containing beads). Effect of web thickness on (c) voltage and current output and (d) internal resistance of device. (fiber average diameter: 240 nm, 8-hole device; SPL,115 dB; sound frequency, 210 Hz).

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attributable to the simplified modelling conditions and the complexityof nanofiber vibration. In contrast, the vibration velocity in the deviceedge showed similar for both 8-hole and 1 hole devices.

Using 2-hole devices, we examined the effect of hole position onacoustoelectric response. With increasing the inter-hole distance in thedevices, the voltage output and web vibration energy decreased (Fig.S11). This suggests that multiple hole device with the holes close toeach other could generate higher electric outputs than that with theholes separate far apart.

Electrospinning allows producing nanofibers with different averagediameters. Using the solutions of different P(VDF-TrFE) concentra-tions in the range of 12–22%, we prepared P(VDF-TrFE) nanofiberswith different average fiber diameters (Fig. S12). Fig. 3a and b showsthe electrical outputs and internal resistance of the acoustoelectricdevices using different nanofiber webs. The one from uniform nanofi-bers with the smallest average diameter (240 ± 40 nm) generated thehighest outputs with the smallest internal resistance. The outputdecreased gradually with increasing the average fiber diameter. Thestar marks indicate the beaded nanofibers (Fig. 3a and b). The effect ofnanofiber web thickness on the electric outputs of the nanofiber deviceswas examined. With increasing the web thickness from 20 to 80 µm,both voltage and current outputs increased (Fig. 3c), and the webthickness showed a similar effect trend on the device internal resistance(Fig. 3d).

We further proved that the power generated by our nanofiberacoustoelectric device can be used directly to run electronics. Threeapplications, such as lighting up LEDs, carrying out electrochemicalpolymerization, and performing corrosion protection, were demon-strated as illustrated in Fig. 4. To convert the AC outputs into DC, abridge rectifier was integrated into the circuit. Without accumulating inany energy storage device such as capacitor, the DC pulse outputs(14.5 V) were applied directly to commercial LEDs. Under 115 dBsound (frequency, 210 Hz), a single piece of the nanofiber device

(working area 3×4 cm2) can light up 27 commercial LEDs simulta-neously (Movie S1). Running 9 LEDs in this way was adequate toillumination (Movie S2). For electrochemical polymerization, a two-electrode system was connected with the acoustoelectric device andrectifier. A gold coated plastic film and a platinum wire was used as theworking and the counter electrodes, respectively. The reaction wascarried out in propylene carbonate containing 0.1 mol/L LiClO4 and0.04 mol/L EDOT (3,4-ethylenedioxythiophene). After 6 h of acousticactivation (frequency 210 Hz, 115 dB), the immersed part on the goldsurface turned deep blue. The Raman spectra confirmed that the darkblue substance was polymerization product, poly(3,4-ethylenediox-ythiophene) (PEDOT, Fig. S13 and Movie S3). For comparison, wealso conducted the experiment without sound. As expected, no colorchange took place on the gold surface (Fig. S14 and Movie S4). Inanother experiment, corrosion protection of metals was performed.Our nanofiber device was used to charge steel for electrochemicalprotection. When a commercial dual phase steel stripe (Fe, 0.1 C,1.4Mn, 0.1Si, 0.9Al, 0.15Mo, wt%), which is often used in automotiveindustry, was immersed in an aqueous sodium chloride solution(0.1 mmol/L), it became corrosion in just 1 h, no matter whether thestripe was connected to our acoustoelectric device (without sound).However, when the steel stripe was connected to our acoustoelectricdevice under sound, the corrosion can be effectively suppressed(Fig. 4c, Figs. S15 and S16, and Movie S5 & S6).

Supplementary material related to this article can be found onlineat doi:10.1016/j.nanoen.2017.03.038.

Acoustoelectric conversion is conventionally used to develop acous-tic sensors, microphones, speakers and music instruments due to smallelectric outputs. It is mainly based on piezoelectric polymer films.Recently, efforts to develop acoustic energy harvesters using piezo-electric principle have been reported [17–21]. However, those piezo-electric acoustoelectric devices are typically low in power outputs,which are inadequate to directly drive an electronic device.

Fig. 4. (a) lighting up commercial LEDs; (b) running polymerization of EDOT; (c) performing metal corrosion protection. (No energy storage device was used in all threedemonstrations).

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Consequently, a storage unit (e.g. capacitor) has to be used toaccumulate the electric energy until the energy reaches the sufficientlevel.

The acoustoelectric conversion demonstrated in this study repre-sents the highest in both electric output and energy density among thepiezoelectric acoustoelectric devices reported (Tables S1 and S2). Itsignificantly differs to the previous devices in that the electric energygenerated from the nanofiber device after a simple rectification can bedirectly used to run commercial electronic units without pre-accumu-lation in a capacitor. Such an incredible energy conversion ability isoriginated from the strong absorption of electrospun nanofibers tosound waves, the rational device structure facilitating effective sound-nanofiber interaction, and remarkable piezoelectric of P(VDF-TrFE).

The hearable sound for human is typically in a frequency ranging of20–20 000 Hz [11,22] and sound becomes a damaging noise when itsfrequency, intensity, or exposure time is beyond the human tolerablerange. The noise pollution primarily comes from low frequency sounds(20 Hz–200 Hz), which exist widely in road traffic, airports, industrialand many public places. The large voltage outputs under low frequencysound at a noise level make our acoustoelectric devices useful todevelop novel SKEGs for noise harvesting.

Here it should be pointed out that the electric energy harvested byour nanofiber devices can also be stored in capacitors or batteries. Inthis way, electrical energy from sound of various SPLs can beaccumulated for powering electronic devices. The nanofiber devicescan harvest sound energy not only from noise but also from normalsound.

4. Conclusions

We have shown the incredible acoustoelectric conversion ability ofrandomly-orientated P(VDF-TrFE) nanofiber nonwovens, suitable forpower supplying purposes. The devices generate higher electric outputsunder a sound with pressure level above 100 dB. The optimized devicecan generate voltage and current outputs of 14.5 V and 28.5 μA with avolume power density output of 306.5 μW/cm3, which are much higherthan these of commercial piezoelectric P(VDF-TrFE) films. The elec-tricity generated from the nanofiber device can be used to directly drivemicroelectronic devices and conduct electrochemical reactions, withoutusing any a storage unit. These incredible features make electrospunnano-nonwovens particularly useful for conversion of noise, a whitepollution, into usable electricity.

Acknowledgements

Funding support from an ARC Future Fellow grant (ARCFT120100135) is acknowledged. C.H.L. acknowledges the scholarshipsfrom the Donghua University International Visiting Program forExcellent PhD Students and Deakin University to support her visitDeakin University. The authors thank Mr Hui Kong and Mr Jun Chengfor their assistance in corrosion protection demonstration, and Dr. BenAllardyce for his technical assistance in vibration test.

Appendix A. Supplementary material

Supplementary data associated with this article can be found in theonline version at doi:10.1016/j.nanoen.2017.03.038.

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Chenhong Lang received her B.S. degree in TextileEngineering from Donghua University in 2011. She iscurrently a Ph.D. candidate in Professor Xin Ding's group,Donghua University, and at the same time she is a visitingstudent at Prof. Tong Lin's group at Deakin University. Hercurrent research focuses on piezoelectric nanofiber foracoustic applications.

Jian Fang received his Ph.D. degree in MaterialsEngineering from Deakin University in 2009. He is cur-rently a Research Fellow at Deakin University and hisresearch focuses on electro-active fibrous materials forenergy harvesting, biosensing and environment protectionapplications.

Hao Shao received his Ph.D. degree from DeakinUniversity in 2017. He won the 2016 ChineseGovernment Award for Outstanding Self-financedStudents Abroad. He is currently a research assistant inProf Tong Lin's group. His research focuses on electro-spinning nanofibers and mechanical-to-electrical energyharvester devices.

Hongxia Wang received her Ph.D. degree in MaterialsEngineering from Deakin University in 2010. After gradua-tion, she joined Deakin University as an Alfred DeakinPostdoctoral Research Fellow, and has worked as ARC APDFellow in 2011–2014 and Research Fellow since 2014. Herresearch interests include superhydrophobic/superoleo-phobic coatings, directional fluid transport fabrics, func-tional fabrics and nanofibers.

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Guilong Yan received his B.Sc. from Tianjin PolytechnicUniversity. He is currently a Ph.D. candidate under thesupervision of Prof. Tong Lin at Deakin University. Hisresearch mainly focuses on nanofibers, electrospinning andmodelling.

Xin Ding is currently a professor of Textile Engineering atthe College of Textiles, Donghua University (formerlyChina Textile University), Shanghai. His research interestsinvolve processing technology of textiles, for both fashionindustry and nor-traditional areas such as civil and struc-tural, aerospace, medical field and wearable technology. Heis the recipient of the Governmental Prize for OutstandingContributions in Education from the State Council of Chinaand has won a number of national and internationalscholarships and prizes.

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

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