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An Effective Wind Energy Harvester by Paper-ash Mediated Rapid Synthesized ZnO Nano-particle Interfaced Electrospun PVDF Fiber Md. Mehebub Alam, Sujoy Kumar Ghosh, Ayesha Sultana, and Dipankar Mandal ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02441 • Publication Date (Web): 24 Nov 2017 Downloaded from http://pubs.acs.org on November 25, 2017

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ACS Sustainable Chemistry & Engineering

An Effective Wind Energy Harvester by Paper-ash Mediated Rapid Synthesized ZnO Nano-particle Interfaced Electrospun PVDF Fiber

Md. Mehebub Alam, Sujoy Kumar Ghosh, Ayesha Sultana and Dipankar Mandal*

Organic Nano-Piezoelectric Device Laboratory, Department of Physics Jadavpur University, Kolkata 700032, India *E-mail: [email protected], [email protected]

Keywords: paper-ash assisted ZnO nano-particle, PVDF nanofiber, electrospinning, βphase induction, wind energy harvesting

ACS Paragon Plus Environment


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Abstract In this paper, we report an innovative technique for rapid and simple synthesis of ZnO containing paper ash (ZPA) within 30 minutes. It is demonstrated that as synthesised ZPA can be an alternative of conventional semiconducting filler that is used to improve the electroactive β-phase content in the electrospun PVDF nanofiber (NF). Furthermore, the PVDF nanofiber based nanogenerator (NFNG) is fabricated which enables a high output voltage of 4.8 V under the exerted wind pressure of 145 Pa. In addition, the detection capability of minute wind pressure, even from human mouth blowing makes the NFNG as an active sensor that might be utilized for charging up the mobile phone during conversation. Thus, this strategy is expected to be a promising platform to harvest wind energy that is beneficial to design integrated self-powered wearable electronic devices.


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INTRODUCTION Mobile electronic devices and wireless sensors are gradually shrinking in size and the power requirements are shrinking as well due to tremendous technological development.1 However, powering these devices by conventional batteries becomes challenging as the number of devices is going to be huge and recharging or replacing the batteries become an ambitious task.2,3 In an alternative way, if the devices can be run through self-powered mode by using the energy that are available in the surrounding environments, then that would be the permanent solution for replacing the batteries. Therefore, harvesting energy from ambient mechanical vibrations turns a forthcoming strategy to design self-powered electronic devices. Thus, piezoelectric nanogenerator (NG) that can convert the mechanical vibration into electrical energy becomes the most possible power source as vibration is present at everywhere for forever.4-10 NG has been employed to harvest energy from various sources of mechanical vibration such as, body movement (handling, winding, pushing, stretching, bending, clapping, talking, breathing, eyelash trembling, etc.), air flow, friction, vibrations (acoustic and ultrasonic waves), and hydraulic forces (ocean waves, waterfall, or even body fluid and blood flow).11-12 Among various sources of ambient energy, wind energy harvesting becomes gradually interesting because of its abundance, ubiquity, steady supply, and large energy capacity.13-15 Lee et al. reported that piezoelectric NG can generate output power in a light wind and serve as an energy harvesting device under the waving motion of a flag.13 Sun et. al developed a NG to harvest energy from low-speed air flow and also demonstrated a possibility of harvesting energy from simulated respiration.14 Wind driven pyroelectric NG has been also recently reported by Xie et. al.15 Alongside experimental evolutions, theoretical and analytical efforts have also been undertaken to demonstrate the generation of electric potential in piezoelectric materials.16,17 Boxberg et. al. reports that piezoelectric property of



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compound semiconductor core-shell nanowires (NWs) might be utilized for the separation of photon-generated electron-hole pairs and, thereby, for photovoltaic applications.18 However, from the material point of view, PVDF is the best choice as an active material for NG fabrication since it is flexible, lightweight, cost-effective and easy to process in comparison to the inorganic piezoelectric materials and shows the maximum piezoelectric coefficient among the polymers.19-21 It exhibits different crystalline phases (i.e., α, β, γ, δ and ) among which the nonpolar α-phase is the most kinetically favoured phase.22 Thus, induction of electroactive β-phase that is most desirable phase for mechanical energy harvesting application, becomes a challenging task.22,

23

Among different techniques (i.e.,

stretching, heat treatment, electrical poling, etc.), recently electrospining becomes the most fascinating technique as it is a one-step method for β-phase induction in PVDF fiber due to the in-situ mechanical stretching and electrical poling during fiber formation.24,

25

Thus,

recent literature survey shows that the electrospun PVDF nanofibers are extensively being utilizing in energy conversion,26,27 wearable power generation28,29 and various types of sensor30,31. However, most of the cases, PVDF electrospun fibers are still showed the coexistence of nonpolar α-phase with the induced polar β-phase and thus researchers focused on the doping of different types of inorganic nanofillers (i.e., MWCNTs, ZnO, MoS2, BaTiO3, Fe3O4) to improve the β-phase content in PVDF fiber.32-39 However, the synthesis of such inorganic fillers needs multi-steps fabrication procedure and thus time consuming. In this context, quick fabrication of the filler by a cost-effective approach is beneficial to improve the overall functionality of the PVDF based energy generation system. Zinc oxide (ZnO) is a well-known biocompatible nano-material and great attention is paid for its uses in photocatalytic devices, gas sensors, photovoltaics, piezoelectric energy harvesters and biomedical applications.40 Various techniques have been proposed for the synthesis of ZnO nanomaterials, such as, sol-gel, hydrothermal, precipitation, micro-emulsion,


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electrospinning, spray pyrolysis.40 But, in the most of the above mentioned methods involve complicated procedures, expensive materials, high reaction temperatures, long reaction time and toxic reagents.41 On the other hand combustion synthesis (CS) is an effective, ecofriendly, simple and rapid process and low-cost method for production of various industrially useful nanomaterial. 42 Meanwhile paper is a biodegradable material made of cellulose fibers and has various applications in our daily life. Paper is an attractive low-cost functional material that offers various sensing application. Paper is used for designing electronic devices, energy devices, strain sensors, gas sensors, and electroactive piezoelectric paper. 43 In this paper, we report the synthesis of nano-sized ZnO within 30 minutes by combustion method using paper as fuel. It is also demonstrated that the ZnO containing paper ash can effectively improve the piezoelectric β-phase content in the electrospun PVDF nano-fiber. In addition, nano-fiber based NFNG is found to be an excellent piezoelectric material, suitable for wind energy harvesting application. The NFNG also exhibits high output response under human mouth blowing. EXPERIMENTAL SECTION Materials: Zinc acetate (ZnAc2.2H2O) (Merck, India), Printing Paper, Poly(vinylidene fluoride) (PVDF) pellets (Mw ≈ 275 000, Sigma-Aldrich, USA), N,N-dimethylformamide (DMF) and acetone (Merck Chemical). Synthesis of ZnO containing Paper-ash (ZPA): ZnAc2.2H2O (1wt. %) was dissolved in deionized water and a clear solution was prepared under the constant stirring at room temperature. Printing paper were cut in to small pieces (dimension ~ 6×4 cm2) and immersed into the solution just for 10 min. The wet paper pieces were then collected carefully and dried by a blow dryer. Those pieces were then allowed to burn by a lighter. One edge of the paper was held by a tweezer and the other edge was put on the flame for burning. Then it was



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quickly removed from the flame to let it burn by spontaneous combustion with the liberation of large amount of gaseous products and becomes a burned solid. Finally, to get the ZnO containing paper-ash (ZPA) a pestle and mortar is employed for crushing and grinding the burned solid into powder. Preparation of NFs mat: PVDF (12wt.% (w/v)) was dissolved in a mixed solvent of DMF/acetone (6:4 v/v) under the constant stirring at room temperature. Therefore, a low content (0.5 wt. %) of ZPA was put into the solution and kept under constant stirring at room temperature. The prepared solution was filled into a 10 mL hypertonic syringe with a diameter of 0.8 mm needle and ZPA incorporated PVDF nanofibers (ZPA-PNFs) were fabricated by using commercial electrospinning equipment under a high voltage of 13kV with needle tip to collector distance (aluminium foil covered plate collector) of 12 cm. A syringe pump was applied to feed solutions to the needle at a rate of 1 mL/h. Pure PVDF nanofibers (PNFs) mat was also fabricated that serve as a reference specimen. Fabrication of NFNG: The as prepared ZPA-PNFs mat of thickness 150 µm was cut into a rectangular shape of dimension 9.5×8.7 cm2 and then two conducting fabrics (Ni−Cu−Ni plated polyester fabric) of area 8.2 ×7.2 cm2 are attached on the both side of the mat to prepare a sandwich-like nanofiber based nanogenerator (NFNG). Characterization: X-Ray Diffraction (XRD) pattern were carried out using a Bruker, D8 Advance diffractometer with the CuKα radiation (λ = 0.154 nm). The surface morphology was evaluated by Field Emission Scanning Electron Microscopy (FE-SEM, FEI, INSPECT F50). Fourier Transform Infrared Spectroscopy (FT-IR, Bruker, Tensor II) was executed with 16 numbers of scans and 4 cm-1 of spectral resolution. Optical properties were carried out by UV-visible (UV-vis) (Shimadzu, 3110PC) spectrometer. The ferroelectricity measurements were carried out by a FE test system (aixact TF Analyzer 2000). Thermogravimetric analyses


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(TGA) were performed in a thermo-gravimetric analyser (TGA/SDTA851e (Mettler Toledo AG) with a heating rate of 10 oC min-1 under the N2 atmosphere from room temperature to 1000 oC. Open-circuit output voltages were measured through a National Instrument (NI) Data acquisition (DAQ) system (NI, USB 6000) interfaced with a computer with a standalone program of LabVIEW.

RESULTS AND DISCUSSION Structural properties of ZPA The X-ray diffraction (XRD) pattern of the collected ZPA, after crushing the burned solid (shown in inset of Figure 1a) into powder shows the characteristic diffraction peaks of ZnO corresponding to typical wurtzite ZnO phase (JCPDS 36-1451) as shown in Figure 1a. Some weak peaks at 2θ~ 29.5, 39.5 and 43.3o are also observed which indicate the presence of small quantity of CaCO3 in the as-prepared ZPA powder. The average crystallite size of ZnO

is 26 nm, estimated by Debye-Scherrer’s equation, D = , where K is a constant (0.89), λ is the wavelength of the X-ray radiation, µ is the full-width (in radian) at half-maximum (FWHM) of the intense diffraction peak, is the angle of diffraction.The morphology of the synthesised ZPA powder is examined by FE-SEM image (Figure 1b). It revealed that ZPA powder is mainly composed of irregularly aggregated particles of 15-50 nm as shown in the histogram profile (inset of Figure 1b). In order to calculate the content of ZnO in ZPA, TGA measurements were carried out on both specimens, viz., only paper ash (PA) and ZnO containing paper ash (ZPA), from room temperature to 1000 °C in N2 atmosphere. It is found that PA and ZPA show 36.5% and 15 % weight loss respectively, at 1000 oC as shown in Figure 1c. Therefore the difference between the weight loss of PA and ZPA give rise the content of ZnO, which is 21.5% (wt./wt.). The UV-vis absorption spectrum (Figure 1d) shows well-defined exciton band at ~397 nm corresponding to the band gap value of 3.12 eV



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which is well matched with the typical UV-vis absorbance spectra of ZnO NPs.44, 45 However, it shows red shifting by ~24 nm relative to the bulk exciton absorption (373 nm). The reason of the shifting of absorption band could be due to the presence of defect energy levels in the synthesized ZnO nanostructures. In this method, defects in ZnO NPs might be originated by the surface defects in presence of paper-ash. Structural properties of NFs mat FE-SEM images of the fabricated PNFs and ZPA-PNFs mats along with the histogram profile are shown in Figure 2a and Figure2b, respectively.


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Figure 1. (a) XRD pattern of as synthesised ZPA in the region from 2θ=10-80o. Inset shows the digital image of burned solid that crushed into ZPA powder. (b) FE-SEM image of the as synthesised ZPA with a histogram profile of particle size in the inset. The TGA curves of PA and ZPA powders. (d) UV-vis absorption spectra of as synthesised ZPA in the wavelength region from 300 to 800 nm.

It is found that both mats are made of randomly oriented fibers where ZPA-PNFs shows distinctly lower average fiber diameter (dav~195 nm) in comparison with that of the PNFs (dav~240 nm) mat due to increase in conductivity of PVDF solution after doping of ZPA. This decrease in the fiber diameter also promises the improvement of mechanical and piezoelectric properties of the ZPA-PNFs mat in comparison with the PNFs.36 The induction of piezoelectric β-phase in both the mats are confirmed from vibrational peaks at 841 and 1275 cm-1 in FTIR spectra (Figure 2c).23,25,46 The improved peaks intensity ensure the improvement in β-phase content in ZPA-PNFs compare to the PNFs. The relative proportional of β-phase (F (β)) is estimated from equation 1

β =

× 100 %

(1)

where Aα, Aβ are the absorbance intensity and Kα , Kβ are the absorption coefficient at 766 and 841 cm-1. 23 Interestingly, ZPA-PNFs mat shows higher content of (F (β) ~ 92 %) compare to that of PNFs mat (F (β) ~ 86 %). The F (β) content in ZPA-PNFs is similar with several previously reported results where different fillers are doped with PNFs as shown in table 1. Thus, the as synthesised ZPA may be the alternative of conventional semiconducting fillers to improve the piezoelectric β-phase content in PNFs for energy harvesting application. The XRD patterns also reveal that β-phase is induced in the NFs mats as shown in Figure 2d. It is also noticeable that corresponding peak intensity of β-phase at 2θ= 20.9o and 36.6o becomes



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intense in ZPA-PNFs compare to PNFs mat. 37,48 These observations are well matched with the FTIR results. The crystallinity ( ), was also calculated from XRD results (details of the calculations are shown in Text S1, supporting information). It is found that after incorporation of ZPA, ZPA-PNFs exhibits 12 % improvement in crystallinity compare to PNFs mat (Figure S1, supporting information).

Table 1: Comparison of F(β) in PVDF NFs when different fillers are used.

Filler

F (β)

References

ZnO

87 %

35

MoS2

95 %

36

BaTiO3

80 %

37

Fe3O4

95.1 %

39

CoFe2O4

90 %

46

CNTs

84 %

47

ZPA

92 %

This work

The simultaneous in-situ mechanical stretching and electrical poling at high electric field during the electrospining is responsible for the induction of β-phase in the NF mats. In addition, the improvement of β-phase in ZPA-PNFs is due to the doping of ZPA filler which has made an interfacial interaction with the macromolecular polymer chain of PVDF.46 Besides this, the increase in the conductivity of the electrospining solution also leads to decrease in fibers diameter which also improves the β-phase content.


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Figure 2. FE-SEM images of (a) PNFs and (b) ZPA-PNFs mat with corresponding fiber diameter distribution histogram profile in the insets. (c) FT-IR spectra of PNFs and ZPAPNFs mat in the wavenumber region from 1600 to 600 cm−1. (d) The XRD patterns of PNFs and ZPA-PNFs mat. (e) Typical P-E hysteresis loop and (f) corresponding S-E hysteresis loop of the mats under the electric field of ± 8 MV/m.

In order to investigate the effect of improved β-phase content in ZPA-PNFs, the macroscopic polarization responses have been measured. Figure2e illustrates the P-E loops of the NFs mats, measured at 10 Hz. It is found that under the electric field of 8MV/m the remnant polarization (Pr) of ZPA-PNFs is effectively increases in comparison to the PNFs. It is attributed to the fact that the β-phase content in ZPA-PNFs is larger than PNFs. The Pr also has a direct relation with the piezoelectric coefficient d33 as shown in equation 2,49 d33 = -S33Pr

(2)

where, S33 is the elastic compliance. Thus, ZPA-PNFs is more suitable for energy harvesting application in compare with PNFs mat. Furthermore, strain (S) developed in a piezoelectric materials under an electric field (reverse piezoelectric effect) is a quadric function of P as shown in equation 3,50,51 S = QP2

(3)

where Q is electrostriction constant ranging from -2.1– -2.5 m4/C2.52 Hence, butterfly shaped symmetrical S-E hysteresis loop corresponding to the P-E loop is found and shown in Figure 2d. It is found that much higher strain is developed at ZPA-PNFs mat at applied electric fields of 8MV/m. This is direct evidence that ZPA-PNFs mat is superior piezoelectric material compare to PNFs mat. It should also be mention that the strains are large compare to ceramics which is consistence with the previously reported results.53-55


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Wind energy harvesting performance of the NFNG: The design for characterizing the wind energy harvesting capability of the ZPA-PNFs comprising NFNG is shown in Figure 3a. An air gun with adjustable flow speed was employed as a wind source and NFNG was attached to a rigid frame that was kept at 30 cm apart from the air gun. It is found that when the NFNG is subjected to a vertical wind pressure of wind velocity 10 m/s, an output voltage of 1.8 V is generated (Figure 3b). Basically, the NFNG experienced a compressive strain when it was subjected to a vertical wind pressure. As a result, the crystal structures of the PVDF NFs as well as ZnO were deformed that leads to piezo-electric output response through repeated flipping of stable to deformed state or vice versa.56 Thus, piezoelectric potential is developed between the top and bottom electrodes of the NFNG. A theoretical analysis has also been conducted using the Finite Element Method (FEM) in order to get a quantitative explanation on the piezopotential distribution inside the deformed ZPA-PNFs of the NFNG under wind pressure of wind velocity 10 m/s (material properties listed in table S1, supporting information). A networks structure was considered where eight NFs entangled with each other for the aforementioned theoretical analysis. The resulting deformation (i.e., displacement) distribution and the piezopotential distribution are shown in Figure 3c and 3d, respectively. It is found that, under wind pressure, maximum deformations (~34.8µm) of the ZPA-PNFs occur in the interaction region between lower and upper layer of the fibers. In the non-interactive regions, the deformations gradually diminish. It indicates that the network-structure of NFs enhance the resulting polarization in comparison to single fiber due to cooperative electromechanical mutual interaction among the adjacent fibers during applied pressure.57



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(a)

NFNG

air gun

(c)

(d)

Air Flow

(e)

NFNG

Figure 3. (a) Digital photograph shows that strain was given by an air gun and NFNG was attached to a rigid frame for piezoelectric voltage generation. (b) Output voltage under wind pressure. Simulation result of the (c) displacement and (d) piezopotential distribution in the


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network structure of NFs under wind pressure. (e) Driving of a blue LED by the NFNG under wind pressure. The NFNG is directly connected to a blue LED to lit it up. Inset shows a schematic of a circuit diagrams for LED lighting. (f) Performance of capacitor charging of NFNG under wind pressure. Inset shows a schematic of a circuit diagrams for capacitor charging. The AC output voltage generated from the NFNG is converted to DC voltage via the full wave rectifier and utilizes to charge up the capacitor.

Furthermore, this deformation yields a quantitative explanation on the piezopotential distribution inside the deformed NFs of the NFNG. The simulated piezopotential (maximum piezopotential 2.3V) is slightly higher than the experimentally measured piezopotential (~1.8 V).This deviation is probably due to the voltage drop resulting from internal leakage paths and charge loss in the conductor-dielectric-conductor (CDC) structure.49 It is also found that the output performance of the NFNG shows a proportional relation with the wind velocity (WV) which is in consistence with the previously reported results.14 The increase of wind velocity from 10 ms-1 to 15 ms-1results in improvement of output voltage from 1.8 to 4.8 V as shown Figure S2, supporting information. It should be mention that the output voltage is much higher than several reported wind energy harvesters as shown in table S2, supporting information. This is expected because of increasing wind speed results increase in the strain on the NFNG. Moreover, the output voltage is enough to power up a commercial blue LED continuously without any external power source and storage devices as shown in Figure 3e (the corresponding circuit diagram is shown in the inset). For better illustration, we have also shown the LED of being lit up at dark condition. It led us to assume that proper designing of the NG can explore its potential application to power up different portable electronic devices used in our everyday life. For example, relative airflow during the driving of a car usually exceeds 20 m/s, so energy can be harvested when the NFNG is attached onto a car. The



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applicability of the NFNG is also verified by testing the capacitor charging results that ensure the reliability and feasibility of the NFNG. For the experiments, the output voltage from the NFNG is first rectified through a typical bridge circuit and then a 1µF capacitor is connected across the rectified voltage to charge it up (the corresponding circuit diagram is shown in the inset of Figure 3f).The capacitor charging results are shown in Figure 3f. It is observed that the charging voltage curves are increased exponentially and finally reached to the steady state value of 0.6 V, 1.8 V and 4 V corresponding to the external pressure of 65, 93 and 145 Pa respectively generated from the wind velocity of 10 ms-1, 12 ms-1 and 15 ms-1, respectively. The Energy (W) and power (P) stored in the capacitor is calculated from equation 4 and 5 respectively, W=

!"

4

P=

!"

5

#

#$

where C is the capacitance of the capacitor, Vs is the saturation voltage, and t is the time required to reach the saturation voltage. Maximum energy and power stored in the capacitor is found to be 8 µJ and 0.47 µW, respectively. The storage performance of the capacitor at different wind velocity is summarized in Table S3, supporting information. This indicates that NFNG can be utilized as an alternative energy harvesting power source for tiny portable electronic devices.58


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(a)

NF NG

(c)

(d)

Figure 4. (a) Schematic demonstration of voltage generation under human mouth blowing. (b) Output voltage under human mouth blowing. Simulation result of the (c) piezopotential distribution and (d) displacement distribution in the network structure of NFs under human mouth blowing.

The flexible NFNG can also be used as an active sensor capable of detecting small amount of wind pressure from human mouth blowing. In order to demonstrate this possibility, NFNG was placed close (within 8 cm of distance) to the human mouth (Figure 4a), and driven by mouth blowing. Output voltage spike of 0.2 V was obtained when NFNG was deformed by the pressure of feeble mouth blowing (WV ~ 1 ms-1). In addition, the theoretical simulation on the piezopotential distribution (Figure 4c) shows that the resulting deformation (Figure 4d)



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distribution is confined in a small area. It is also found that output voltage linearly increases from 0.2 V to 1 V with the increase of mouth blowing wind flow from 1 ms-1 (corresponding to exerted pressure ~ 0.65 Pa) to 5 ms-1 (~16 Pa) as shown in Figure S3, supporting information. Therefore, the NFNG is demonstrated as an excellent candidate for day and night energy harvesting from respiration considering the fluctuating wind flow speed during normal breath (WV ~2 ms-1) which is generally increased 4-8 times during exercise.14 This capability is ideal for harvesting energy from environmental wind flow or respiration that make the NFNG suitable for various applications including charging up mobile phones during conversations. CONCLUSION In summary, ZnO containing paper ash (ZPA) is synthesised within 30 minutes through a simple but innovative synthesis technique. It indicates that ZPA may be the alternative of conventional semiconducting filler that can effectively improve the electroactive β-phase content of the electrospun PVDF nanofiber that make it superior for piezoelectric drive power generation. The as fabricated NFNG delivers high throughput of 4.8 V under 145 Pa of exerted pressure of wind flow. Upto 1.0 V is obtained when NFNG is deformed by human mouth blowing that shows its capability to serve as an active sensor. Thus it is expected that in near future it might be utilized for charging up the mobile phone during conversation. As a result, this strategy may provide a unique platform for wind energy harvesting that leads us to design self-powered wearable electronic devices.

ASSOCIATED CONTENT Supporting Information Additional information on material properties of PVDF fiber for finite element model simulation, comparison of NFNG with other reported data, output voltages of the NFNG


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under the wind pressure at different wind velocity, and as a function of human mouth blowing wind flow are presented. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected], [email protected] Tel: +91-8336-017243, Fax: +91-33-2413-8917 Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENT This work was supported by the Science and Engineering Research Board (SERB/1759/201415), Govt. of India. Md. Mehebub Alam is supported by the UGC-BSR fellowship (No. P1/RS/191/14). S.K.G acknowledges INSPIRE fellowship (IF130865). Ayesha Sultana is supported by Maulana Azad National Fellowship (F1-17.1/2015-16/MANF-2015-17-WES53885/(SA-III/Website)) from UGC.

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Graphical Abstract

An Effective Wind Energy Harvester is designed by Paper-ash Mediated Rapid Synthesized ZnO Nano-particle Interfaced Electrospun PVDF Fiber


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An Effective Wind Energy Harvester of Paper Ash ... - ACS Publications

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