Polyvinylidene Fluoride Nanocomposite for Transparent

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Maghemite/Polyvinylidene Fluoride Nanocomposite for Transparent, Flexible Triboelectric Nanogenerator and Noncontact Magneto-Triboelectric Nanogenerator Bushara Fatma, Ritamay Bhunia, Shashikant Gupta, Amit Verma, Vivek Verma, and Ashish Garg ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.9b02953 • Publication Date (Web): 01 Aug 2019 Downloaded from pubs.acs.org on August 5, 2019

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ACS Sustainable Chemistry & Engineering

Maghemite/Polyvinylidene Fluoride Nanocomposite for Transparent, Flexible Triboelectric Nanogenerator and Noncontact Magneto-Triboelectric Nanogenerator Bushara Fatma†*, Ritamay Bhunia†, Shashikant Gupta†, Amit Verma‡, Vivek Verma†, Ashish Garg† †

Department of Material Science and Engineering, Faculty Building, Indian Institute of Technology Kanpur, Kalyanpur, Kanpur 208016, India. ‡ Department of Electrical Engineering, ACES Building, Indian Institute of Technology Kanpur, Kalyanpur, Kanpur 208016, India. *E-mail: [email protected] ABSTRACT: Propelled by development of IoT and other low power devices such as in health care or sensing applications, there is growing emphasis on development of energy harvesting devices based on piezoelectric and triboelectric harvesting. We demon54strate highly flexible and transparent triboelectric nanogenerator (TENG) prepared by incorporating maghemite (γ-𝐹𝑒2 𝑂3 ) fillers in polyvinylidene fluoride (PVDF) with polyethylene terephthalate (PET) as a triboelectric counterpart for potential application in powering wearable electronic devices. Addition of γ-𝐹𝑒2 𝑂3 fillers in PVDF matrix results in a power output with an average open circuit voltage of 250 V and short circuit current of 5 μA, which is substantially higher than that from only PVDF based TENG. With manually applied force, the light weight TENG device (area ~14.5 cm2 and weight ~1 g) can induce a maximum power output of 0.17 mW with a power density of 0.117 Wm-2. In addition, this device is extremely robust with excellent long-term stability for approximately 3000 seconds. We harvested biomechanical motion in the form of slow and fast foot movement by attaching this device to the sole of footwear. Moreover, the TENG device could continuously supply enough power to light up 108 light-emitting diodes (LEDs) connected in series, without the use of a capacitor and has potential applications in self-powered wearable and portable electronics obviating the use of batteries. Moreover, this device is shown to harvest energy from the rotary pump to charge a 1 F capacitor to the value of ~30 V in just 90 seconds. In addition, a thick magnetic γ𝐹𝑒2 𝑂3 /PVDF nanocomposite film was also successfully tested as a magneto-triboelectric nanogenerator (M-TENG) in noncontact mode showing potential for harvesting of stray magnetic field. KEYWORDS: PVDF composites, Triboelectric energy harvesting, Magnetic energy harvesting, TENG, Flexible energy harvesting.

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 INTRODUCTION Energy harvesters scavenge ambient energy in form of mechanical vibrations and motions, thermal energy, light energy and magnetic energy. Energy harvesting has gained substantial importance in recent times with the growth of wearable electronic devices. These devices can harness the energy in the form of portable flexible self-powered devices and avoid dependence on battery in some cases.1 These harvesters exploit phenomenon such as piezoelectricity,2 pyroelectricity,3 photoelectricity,4 and electromagnetic effect5 to harvest the energy present in the surroundings. While such stray energy forms in the ambient are typically low in magnitude and hence the devices produced are also limited by their low power output, these could be just sufficient for powering modern electronic devices which require power of the order of few mW or µW whilst also providing an advantage of smaller size. Conventionally, energy harvesters based on mechanical energy harvesting employ ceramic piezoelectrics such as Pb(Zr,Ti)O3 which are brittle and nonflexible, are opaque in nature and require high temperature processing.6 Another mechanism which can harness energy from mechanical energy is the triboelectric effect which is defined by generation of an electrical output when two materials with different positions in triboelectric series, say one positive and one negative, are brought in contact and removed periodically.7, 8 In 2012, Wang et al explored triboelectricity for scavenging ambient mechanical energy9 and since then, this technique has received substantial attention as against piezoelectric effect based energy generation devices, due to its high power output, durability, robustness, cost-effectiveness and ease of fabrication process of materials involved.10 Triboelectric generators have successfully harvested energy from biomechanical motion,11 mechanical vibration and rotation,12, 13 wind energy and blue energy.14, 15 The following approaches have been adopted to enhance the efficiency of triboelectric devices: (i) material exploration,16-21 (ii) surface and interface modification,22, 23 and (iii) adaptation of different design aspects in perspective of device architecture.24-27 Other approaches include TENG operation in different modes such as vertical contact separation mode,18 lateral sliding mode,25 single-electrode mode,28, 29 freestanding triboelectric-layer mode,30 and non-contact mode.31 Among these, material exploration offers a wide range of options in energy harvesting through triboelectric devices. In our surrounding, numerous materials such as silk, fur, metals, wool, human hair etc. manifest triboelectrification and hence the choices of materials for TENG are abundant.32 Among negative triboelectric materials, polyvinylidene fluoride (PVDF) is generally a well-researched material due to its flexibility, transparency, tunability with other polymers to design copolymers and cost effective processing in various forms such as films and fibers for a variety of applications which are not necessarily triboelectric.33 Triboelectric performance of PVDF and its copolymers can be enhanced by the formation of -phase which results in high surface charge density due to high spontaneous polarization.34 PVDF has lesser amount of -phase as compared to its copolymers but it is of lower cost. Therefore, it is necessary to develop various other strategies to enhance the output performance of cost effective PVDF based triboelectric generators. Incorporation of nanoparticles (NPs) in matrix can easily modify the intrinsic property of the PVDF composite film. In previous studies, conducting and dielectric nanoparticle incorporated composite films based TENG has shown increase in the power output.35, 36 However, less research has been done in this direction as the choice of NPs are widespread. Therefore, further study is needed to have deep understanding of charge accumulation and transfer while tuning the property of triboelectric layer with addition of NPs into PVDF. 1 ACS Paragon Plus Environment

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In this paper, we report the effect of magnetic γ-Fe2O3 NPs inclusion on the output performance of transparent TENG devices with PVDF matrix and PET as an opposite triboelectric layer. The results shows significantly improved power output, of the order of 0.117 Wm-2 from γ-Fe2O3/PVDF nanocomposites with substantial voltage output and excellent repeatability. Further, we demonstrate application potential of these devices in real applications such as in the form of power source for LED lights and capacitor charging as well as for harvesting high frequency vibrations from equipment such as a rotary pump. In addition, we also demonstrate the utility of thick γ-Fe2O3/PVDF nanocomposite as MTENG, used in noncontact mode using an external magnetic field. While there are few reports which have studied electromagnet assisted TENG devices and demonstrated energy harvesting capability,37-39 these devices do not actually consist of magnetic phase in the active energy harvesting layer and the device exploits the additional electric field created as a result of electromagnetic induction due to the presence of external magnet. In contrast, our work focuses on making a hybrid composite MTENG device using a nanocomposite made of magnetic γ-Fe2O3 particles dispersed in PVDF matrix which uses a simpler device structure and is completely based on triboelectric effect. The use of external magnet of magnetic field is only to move the composite layer of the device up and down and eventually causing triboelectric effect.  EXPERIMENTAL DETAILS

Nanocomposite synthesis and triboelectric device fabrication. Magnetic maghemite NPs (γ-Fe2O3) were synthesized from mild steel chips using cost effective oil reduction technique and the details of synthesis and characterization is reported elsewhere.40 PVDF (Sigma-Aldrich) was dissolved in dimethyl formamide (DMF) at 60°C for 15 h. Required percentage of γ-Fe2O3 was dispersed in DMF separately using probe sonication (VC 750 Ultrasonic Processor) for 30 min with 20 sec ON and 10 sec OFF cycles. Later, both the solutions were mixed together using bath sonicator for two hours to prepare 7.5% w/v PVDF with 0, 2.5, 5, 10 and 15 wt% γ-Fe2O3, with respect to PVDF, referred as PFx (where x =0, 2.5, 5, 10 and 15). The solution was spun onto ITO coated PET at 500 rpm and 1000 acceleration for one minute to prepare nanocomposite films. These films were further dried at 70°C for 4 h. Thick magnetic films are prepared using solution casting the solution on glass substrate and dried at 70°C for 15 h. Thickness of the composites are between 35-50 m. The PVDF based TENG devices were fabricated in sandwich pattern using two ITO coated PET sheet of area 1.5×1.5 inch2. Nanocomposites of γ-Fe2O3 and PVDF were spin coated onto one of the sheet acted as a negative triboelectric material with PET acting as a counter positive triboelectric layer as PET is relatively positive in the triboelectric series as compared to PVDF. Both the sheets are attached onto a curves Mylar sheet whose ends are stuck together using tape with PVDF and PET facing towards each other. Finally, the separation between them can be changed periodically with an external applied force and create potential difference (see Figure 1a,b).

Characterization. Microstructural characterization of the γ-Fe2O3-PVDF composite film was done using Field-emission scanning electron microscope (Nova NanoSEM 450) at 5 kV while crystal structure of the spin coated composite films was examined using grazing incidence X-ray diffraction (Panalytical MRD Pro X-ray Diffractometer) and Fourier transform infrared spectrometer (Agilent Cary 660). The morphology of the films was characterized using an Asylum Research MFP-3D atomic force microscope. Transmittance was measured in the range of 400 nm to 800 nm using Cary 7000 2 ACS Paragon Plus Environment


spectrophotometer (Agilent Technologies) and room temperature magnetic properties were measured using vibrating sample magnetometer (VSM).

Device Performance Test. The contact-separation motion of the triboelectric layers was done manually using fingertip force in a periodic manner to generate deformation in the device. The output voltage of the device in response to the external applied periodic external force was measured using a digital oscilloscope (Tektronics DPO4102B-L) with an internal impedance of 100 M while output current across load was measured using Keithley 6517B Electrometer.  RESULTS AND DISCUSSION The device structure and its photographic image is shown in Figure 1a,b with PVDF based nanocomposite and PET as two triboelectric layers and PET sheet as a support. As shown in Figure S1a,b, the weight and thickness of the device is 938 mg and 394 µm, respectively, suggesting that the complete device is thin, light weight and flexible enough to be used as a wearable and portable device. Figure 1c shows the plots of transmittance of PVDF and its composites. As we can see here that the percentage transmittance of the composite films reduce simultaneously with an increase in the nanoparticle incorporation. However, even with 15 wt% incorporation of γ-Fe2O3 NPs in PVDF, there is still 30% transmittance at 600 nm, which is close to the values reported in literature.41 There is almost 50% decrease in the transmittance of PF15 with respect to PF0.

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Figure 1. (a) TENG device structure (b) Device photographic image. (c) Percentage transmittance of composites in visible spectrum.

Surface morphological study of composites with different concentrations of NPs is done using FESEM images. Figure 2 shows FESEM images of spin coated nanocomposites films with different concentrations of γ-Fe2O3 NPs. It can be seen in Figures 2b-e, the nanocomposite films have randomly distributed γ-Fe2O3 NPs on the top surface up-to 5 wt% reinforcement. Further incorporation of NPs results in lower amount of visible NPs on the top surface (Figure 2d,e), due to agglomeration and settlement of particles on the lower surface towards ITO which is clearly evident in the FESEM image (Figure 2f) of the bottom surface of 15 wt% Fe2O3-PVDF nanocomposite.

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Figure 2. FESEM image of top surface of spin coated PVDF and its nanocomposites; (a) PF0, (b) PF2.5, (c) PF5, (d) PF10 and (e) PF15. (f) FESEM image of bottom surface of PF15. The scale bar is 30µm in all images.

Existence of γ-Fe2O3 NPs in the PVDF matrix was further confirmed using XRD. As shown in Figure 3a, peaks at 2 value of 30.2, 35.4, 43.2, 57.2 and 62.7 correspond to (220), (311), (400), (511) and (440) crystal plane of γ-Fe2O3 NPs respectively.40 The two most intense peaks of γ-Fe2O3 NPs at 2 = 35.4and 30.2 overlap with the characteristic peaks of ITO.42 All these peaks are visible in each composite sample except in PF2.5 which is due to lower concentration of NPs in the composite. Moreover, these peaks gradually increase in intensity with increasing NPs incorporation. A broad peak at around 2 = 20° is a composite peak corresponding to (020), (100) and (110) planes of α-phase and (200) plane of β-phase.43 Deconvolution of this peak was done to observe the change in polar β-phase in PF0 and PF15. The peak intensity corresponding to β-phase does not increase with the incorporation of γFe2O3 NPs (Figure 3b) suggesting that the incorporation of γ-Fe2O3 NPs does not influence the extent of polar β-phase formation. Further confirmation was done by analyzing FTIR spectra of composites. In FTIR spectra (Figure 3c), peaks at 766 and 840 cm-1 correspond to α and β-phase of PVDF, respectively without any trace of γ-phase. These two peaks are used to evaluate the relative fraction of β-phase, F(β), for all the composites using the following equation;44 𝐹(𝛽) =

𝑋𝛽 𝐴𝛽 = 𝐾 𝑋𝛼 + 𝑋𝛽 ( 𝛽⁄𝐾 ) 𝐴𝛼 + 𝐴𝛽 𝛼

𝑒𝑞. (1)

Where, 𝑋𝛼 and 𝑋𝛽 are the mass fraction of α and β phase crystallites and 𝐾𝛼 (6.1×104 cm2/mol) and 𝐾𝛽 (7.7×104 cm2/mol) are the absorption coefficients at 766 and 840 cm-1respectively. 𝐴𝛼 and 𝐴𝛽 are the absorbance of vibration bands at 766 and 840 cm-1 respectively. Fractional amount of β-phase for all the composites is shown in the inset of Figure 3c. All the composites exhibit almost same amount of β-phase corroborating the XRD data.


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Figure 3. (a) Grazing incidence X-ray diffraction (GI-XRD), (b) Deconvolution of XRD peak of PF0 and PF15 around 20 and (c) FTIR pattern of spin coated pure PVDF and its composites with different incorporation of γFe2O3 NPs on ITO coated glass substrate. Inset of Figure 3c, shows the variation of β-phase fraction with varyingγFe2O3 NPs content.

Following structural characterization, PVDF and its composites were used as negative triboelectric layer to prepare TENG device for power generation in a vertical contact separation mode. The schematic for voltage and current generation during manually triggered vertical contact and separation mechanism is demonstrated in Figure 4. Initially, when both the surfaces are separated from each other, the device remains in an equilibrium position. Upon tapping the deformable upper surface of the device using a fingertip force, both the surfaces get in contact and rub against each other. This induces electrostatic charges of opposite polarity over the surfaces of both the polymers. Since, PVDF is relatively negative with respect to PET in the triboelectric series, the contact generates negative and positive charges on the respective surfaces. As long as these opposite charges lie in the same plane, the TENG device remains in electrical equilibrium with no output signal. When the external force is removed, the two active surfaces are separated leading to disruption of electrical equilibrium and hence, a potential difference is developed at the opposing triboelectric surfaces. In order to minimize energy, electron conduction takes place from lower ITO (PVDF side) surface to the upper ITO surface (PET side) and therefore, current flows in the outer circuit. When separation between two active layers is again decreased by exerting external force, charge equilibrium is perturbed again and causes free electrons to move in the opposite direction i.e. from 5 ACS Paragon Plus Environment

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PET side electrode to the PVDF side electrode. Therefore, for a complete periodic exerted force, AC voltage and current are generated by TENG. The periodic deformation cycles produce continuous signal from the device, which can further be used for various applications. Output behavior of TENG device made of pure PVDF, in response to vertically applied continuous periodic force is shown in Figure 4.

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Figure 4. Mechanism of power generation in vertical contact separation mode of TENG device and the corresponding output voltage of TENG made of pure PVDF.

To obtain the real output of the device, the measurement was done in forward and reverse connection modes as shown in Figure S2a,b. Forward connection implies that the positive terminal of oscilloscope is connected to the PVDF-ITO electrode while ground is connected to the PET-ITO electrode. The terminals are interchanged accordingly for the reverse connection. For PVDF and PET, the average value of open circuit output voltage is 145 V at a frequency of 3.4 Hz whereas in reverse connection mode, polarity reversal of electrodes causes the inversion of output voltage. Moreover, the output behavior of TENG is similar to forward connection. Polarity dependent output signals specify the normal operating behavior of TENG device, confirming that the output electrical signals are caused only by mechanical deformation of the triboelectric layer. Figure S2b shows the zoom image of output signals, for a complete single cycle, in forward and reverse connection modes. This provides the insight into the consistency of the device output as the peaks overlap each other well. This spin coated composite based device structure has advantage over devices with films prepared by electro-spinning. As shown in literature, TENG based on web like nanofibrils of PVDF prepared using electo-spinning are less durable as compared to dense films prepared using spin coating.33



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Figure 5. (a) Open circuit voltage and (b) short circuit current of TENG with composites having different concentrations of γ-Fe2O3 NPs. (c) Variation of output current, power density with external loading resistance for pure PVDF and PVDF with 15 wt% γ-Fe2O3 NPs. (d) AFM image (20×20 𝜇𝑚2 ) of PF0 and PF15 composites with different surface roughness.

To study the effect of nanoparticle concentration, output performance of all the composite based devices is studied. The open circuit output voltage and short circuit current of TENG devices with varying concentration of γ-Fe2O3 NPs in PVDF are shown in Figure 5a,b. Increase in NPs concentration has no detrimental effect on the voltage and current output of the device. As the NPs content increases up to 15 wt%, the average short circuit output current increases from 1.8 to 5 μA while average value of open circuit output voltage also increases from 145 V to 260 V. Similarly charge density, as estimated from current vs time plots, increases from 15 to 40 nC/cm2 with 15 wt% addition of NPs while pressing whereas during release, change in charge density is from 10 to 35 nC/cm2 as shown in Figure S3. Load dependent study of TENG output, gives the complete evaluation of overall power generation capability. As presented 7 ACS Paragon Plus Environment

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in Figure 5c, there is a rapid decrease in output current for pure PVDF as well as 15 wt% γ-Fe2O3/PVDF based devices as the load resistance increases from 1 M to 30 M. Power output of both the devices increases with increase in the value of load resistance up to 20 M and then tend to decrease. For both the devices, the maximum power is obtained at 20 M. TENG device of 15 wt% γ-Fe2O3-PVDF generated a maximum power density of 0.117 Wm-2 at load resistance of 20 M which is higher than only PVDF based device. The power of the device is maximum, when internal impedance of the device is equivalent to the external impedance. Therefore, the internal impedance of PF0 as well as PF15 is close to 20 M. In case of PVDF nanocomposites, the increase in the output performance of TENG with NPs incorporation can be due to various factors such as increase in polar β-phase formation and surface roughness.33, 34 However, in our case β-phase content does not seem to be responsible for the enhancement in device output as there is no change in β-phase content on the NPs concentration (Figure 3b,c). Therefore, increase in the device output with NPs incorporation is more likely to be related to the surface roughness of nanocomposites. As seen in the AFM (Figure 5d) image of only PVDF and PVDF with 15 wt% γ-Fe2O3, with the incorporation of γ-Fe2O3 NPs there is more than two times increment in the surface roughness from 69 nm to 176 nm. With an increase of γ-Fe2O3 NPs in PVDF matrix, increase in the composite surface roughness is observed which is likely to be responsible for further increase in the open circuit voltage and short circuit current (Figure S4a-d). Increase in roughness may increase the contact area between triboelectric layers and thus increases the induced static charges on the surfaces (Figure S4e). Further increase in the nanoparticle concentration from 20 wt% to 35 wt% tends to decrease the output voltage and current of devices. This decrease in the output performance is attributed to the fact that at higher concentrations nanoparticles tend to agglomerate at the surface of film and hence the effective contact area between PVDF and PET reduces. Decrease in TENG’s output behavior at higher concentration of nanoparticles incorporation due to surface agglomeration is reported earlier as well.45 In summary, we can conclude here that γ-Fe2O3 NPs is incorporated in large amount to the value of 15 wt% inside PVDF matrix without affecting the performance of TENG device with less effect on its transparency. Moreover, the output performance of our TENG device is better than what was reported earlier with electrospun iron oxide/PVDF based TENG.45 A comparison of the electrical output with other different devices in literature is shown in Table S1. The output of this device is higher than most of the devices and comparable to some. Apart from the γ-Fe2O3 NPs incorporation, there are various other parameters that affect the output response of TENG such as tapping frequency etc.



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Figure 6. Variation in the voltage output of TENG in response to (a) different contact separation frequencies and (b) two and three finger tapping. (c) Durability of TENG for approximately 3000 seconds continuous operation.

Figure 6a,b show the output voltage of TENG device made of 15 wt% γ-Fe2O3/PVDF in response to different frequencies and finger tap. Figure 6a shows device output in response to two different frequencies of vertical contact-separation deformation i.e. 1.2 Hz and 3.4 Hz. The average output voltage increases from 175 V to 260 V as the contact-separation frequency increases from 1.2 Hz to 3.4 Hz. The corresponding change in current and charge density is shown in Figure S5a,c. The increment in the output is caused by charge build-up during continuous contact-separation cycle. The TENG output in response to two finger tap and three finger tap are shown in Figure 6b which shows that three finger tapping produces high voltage as compared to two finger tapping and the corresponding change in current and charge density is shown in Figure S5b,c. This is attributed to increase in the surface contact area of active material as triboelectric property is known to be a surface property. Therefore, this device prototype can easily sense the deformation frequency as well as difference between two fingers tap and three fingers tap. This also suggests that results would improve further when one uses more controlled and mechanized contact mechanisms. Finally, the mechanical endurance and long term working stability is tested to check the sustainability of the device. For this, the output response of 15 wt% γ-Fe2O3/PVDF nanocomposite 9 ACS Paragon Plus Environment

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is measured continuously for approximately 3000 seconds. Figure 6c shows the continuous output current of the device that remain consistent in response to external periodic force. With all the results, we can conclude that the performance of γ-Fe2O3/PVDF nanocomposite based TENG has stable and excellent performance for energy harvesting applications. 1F_PF15 22F_PF15

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Figure 7. Application of TENG (a) bridge rectifier circuit to convert AC into DC signal and (b) lighting up of 108 blue LEDs configured in series connection, in response to the periodic deformation in TENG made up of PF15. Charging voltage curve of (c) different capacitors for pure PVDF and nanocomposite with 15wt% γ-Fe2O3. Inset of Figure 7c shows the charging curve of 1 F capacitor.

To study the possible applications of TENG in powering devices, the output signal rectification is done using the circuit shown in Figure 7a.TENG can light up 108 LEDs arranged in series under manually driven periodic contact separation as shown in Figure 7b (see Video S1 and Video S2 in supporting information). All the LEDs are shown to light up continuously in response to finger tapping without any deterioration. Even a gentle low frequency tapping can light up all the LEDs (see Video S1 and Video S2 in supporting information). In practical situations, a constant DC source is used to power microelectronics devices and hence, rectified output of TENG device is stored either in a capacitor or a battery. Therefore, we have demonstrated capacitor charging ability of TENG device, as shown in Figure 7c. Here, a periodic contact separation deformation at 3.4 Hz is applied to only PVDF and 15 wt% γ-Fe2O3/PVDF composite based TENG devices and the generated rectified output is shown to continuously charge different capacitors. For 2 minute charging time, the composite TENG device is found to be capable of storing higher voltages as compared to pure PVDF for all the capacitors. In particular, the value of stored voltage in 1 μF capacitor increases from 22.1 V to 44.6 V in just 90 seconds. Therefore, incorporation of γ-Fe2O3 10 ACS Paragon Plus Environment


NPs increases the capability of TENG device to be used as sustainable source for powering microelectronic devices.

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Figure 8. (a) Scavenging energy from rotary pump vibrations using TENG. (b) Voltage response, (c) current response and (d) capacitor charging ability of PF15 composite based TENG device in response to the rotary pump vibration of 25 Hz.

Finally, we also demonstrated the ability of this device to harvest energy from the rotary pump that will be otherwise wasted. For this purpose, the top and bottom sheets are joined using scotch tape at one edge of the device and the other end are free for contact and separation of layers as shown in Figure S6a. We fixed our device to the exhaust outlet of rotary pump (oil less vacuum pump, Rocker 400) using tape shown in Figure 8a. The air coming from exhaust as well as the rotary pump vibrations is causing contact and separation between layers and resulting in electrical power output. The maximum output voltage of 15wt% γ-Fe2O3-PVDF composite based TENG device in response to the rotary pump is ~ 50 V whereas maximum obtained value of output short circuit current of device is 4.7 μA, as shown in Figure 8b,c respectively. From the voltage response of the device, the vibration frequency is obtained to be equal to 25 Hz which is half of AC power frequency. The pump driven device is capable of efficiently charging a 1µF capacitor to the value of ~ 40 V in just 200 seconds as shown in Figure 8d (see Video S3 in supporting information). Therefore, this device is capable of successfully scavenging energy from


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vibrating equipment such as a rotary pump. As shown in Figure 8c, the current is continuously measured for approximately 30 minutes demonstrating the durability of TENG for long term continuous operation.

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Figure 9. (a) Voltage and (b) current output of flexible TENG in response to low and high frequency foot motion resembling to walking and running actions, when attached to footwear sole as self-powered wearable electronics for harvesting biomechanical motion. To check the ability of our flexible γ-𝐹𝑒2 𝑂3 /PVDF nanocomposite based TENG to be used as self-powered wearable electronics, it was attached to the sole of footwear for harvesting biomechanical motion in the form of low and high frequency foot motion resembling to walking and running. The device structure is shown in Figure S6b. The output of TENG in response to the foot strike while low and high frequency foot motion is shown in Figure 9a,b. As shown in Figure 9a, the peak output voltage for running actions (100 V) is higher than walking action (80 V). The higher output voltage while running is due to high frequency motion. Therefore, this TENG can be used as wearable generator to harvest energy from biomechanical motion as well as wearable sensor to monitor body movement as it can sense the difference between both motions. There is similar change in corresponding peak current in case of low frequency (2 A) and high frequency (3 A) foot motion as shown in Figure 9b.

Magneto-triboelectric nanogenerator (M-TENG). In order to take advantage of magnetic behavior of NPs, thick magnetic film of γ-𝐹𝑒2 𝑂3 /PVDF nanocomposite is prepared using solution casting method. The room temperature magnetic behavior (magnetization (M) vs. applied magnetic field (H)) of nanocomposites prepared using solution casting method is shown in Figure 10a. All nanocomposites exhibit a superparamagnetic behavior owing to the nanometer size of γ-Fe2O3 NPs. These NPs acts as nano magnetic domains throughout the matrix. With an increase in the NPs incorporation, the saturation magnetization (Ms) increases while there is hardly any change in the coercive field (Hc) of composites as shown in Figure 10b. With an increase of NPs incorporation from 5 wt% to 15 wt%, increase in Ms is linear from 1.94 to 6.25 emu/g and the coercive field increases from 238.8 to 278.3 Oe. These magnetic nanocomposites are used to prepare magnet responsive TENG. Due to strong magnetic response of nanocomposite, external magnetic field can be used to trigger the contact and separation of layers, instead of directly applied mechanical force. This M-TENG uses external magnetic field instead of mechanical force for the operation of M-TENG and therefore is operated in noncontact mode where the device is prevented from the direct contact to the external mechanical impact. Operating in non-contact mode can avoid damage to the electrode and triboelectric layers. Solution casting method is used to prepare 12 ACS Paragon Plus Environment


magnetic nanocomposite with drying time of 15 h. Thin film of aluminum is deposited onto nanocomposite film to serve as an electrode. PET serves as an opposite triboelectric layer having ITO as an electrode. The whole device is encapsulated inside acrylic petridish to avoid direct contact with magnet. PET side of device is fixed onto the petridish and gravitational force ensures separation between two triboelectric layers. The process of electricity generation is same as discussed in Figure 4a. Manually driven vertical movement of magnet causes contact and separation between layers is schematically shown in Figure 10c. The photographed image of TENG device in response to the vertical movement of magnet is shown in Figure 10d. When the magnet lies close to the device, magnetic nanocomposite is in contact to the upper surface having PET. Drawing the magnet away from the device causes nanocomposite layer to release away from PET due to gravitational force.

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Figure 10. (a) Magnetization vs applied magnetic field plot for nanocomposites. (b) Variation of saturation (e) (c) Schematic of M-TENG structure, with and magnetization and coercive field with γ-Fe2O3 NPs incorporation. without magnet. (d) Response of magnetic nanocomposite layer with and without magnet. (e) Current, (f) voltage output of magnet driven M-TENG made of PF15 nanocomposite. (g) Variation of output current and power in


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response to vertical movement of magnet with different resistances. Inset of Figure 10g, shows lighting up of 12 LEDs using M-TENG.

The output response of M-TENG is shown in Figure 10e,f. Variation of output current and power with resistance is shown in Figure 10g. The maximum output current and voltage of magnetically driven MTENG in noncontact mode is 0.8 μA and 90 V, respectively. The generated output power is sufficient to light up 12 blue LEDs connected in series (Inset of Figure 10g). The output can be further enhanced by increasing the magnetization of the nanocomposite. Therefore, this magnet driven device has substantial potential for improvement as the magnetic property of the composite can be tuned by the concentration or type of incorporated NPs.  CONCLUSION In conclusion, we propose a simple method to fabricate a transparent, flexible and stable TENG device using spin coated γ-Fe2O3/PVDF nanocomposites and PET as two triboelectric layers. The TENG device shows simple and low cost device preparation with high output performance. Using manual deformation with fingers only, we were able to obtain an average value of open circuit voltage and short circuit current as 260 V and 5 μA, respectively with power density of 0.117 Wm-2. Further, the effect of different forces and frequency on TENG were studied to check the biomechanical energy conversion ability and its future application as pressure sensor that are self-powered. The output of TENG is sufficient enough to directly power 108 LEDs with just manual tapping. Thus, the TENG can be used as a self-powered carbon free source to power street lights, portable electronics and wearable devices. This device is used to harvest energy from high frequency vibrations of rotary pump as well as biomechanical motion in the form of foot movement. The magnet responsive magneto-triboelectric nanogenerator can be operated in noncontact mode to avoid damage. This will increase the life of device as the device is not in direct contact to the impact. Therefore, this device has potential to detect and harvest stray magnetic field.  ASSOCIATED CONTENT The supporting information is available free of charge on the ACS Publication website. The supporting information consists of thickness and weight of device. AFM image of composites and relation between composite surface roughness to the output response of corresponding device. Device structure when used for harvesting energy from rotary pump and the comparison table. Three experimental videos.

 AUTHOR INFORMATION

Corresponding author *E-mail: [email protected]

Notes The authors declare no competing financial interest. 14 ACS Paragon Plus Environment


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 ACKNOWLEDGEMENTS We thank Dr. Nitesh Kumar (MSE department, IITK) for providing maghemite NPs used in the experiments and Vasav Guatam (Ph.D. student, Electrical Engineering Department, IITK). Authors also thank SERB, SUNRISE Project (EPSRC, UK) and DST (IMPRINT-II) for the financial support.

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Green, renewable and sustainable energy scavenging technique from PVDF/Fe2O3 triboelectric nanagenerator utilizing ambient biomechanical motions and varying magnetic field. 294x129mm (136 x 150 DPI)

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Polyvinylidene Fluoride Nanocomposite for Transparent

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