An Insight Into Cigarette Wrapper and Electroactive Polymer Based

Electroactive Polymer Based Efficient TENG as Biomechanical Energy Harvester for Smart Electronic Applications ... Publication Date (Web): August ...
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An Insight Into Cigarette Wrapper and Electroactive Polymer Based Efficient TENG as Biomechanical Energy Harvester for Smart Electronic Applications Sarbaranjan Paria, Ranadip Bera, Sumanta Kumar Karan, Anirban Maitra, Amit Kumar Das, Suman Kumar Si, Lopamudra Halder, Aswini Bera, and Bhanu Bhusan Khatua ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00951 • Publication Date (Web): 29 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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An Insight Into Cigarette Wrapper and Electroactive Polymer Based Efficient TENG as Biomechanical Energy Harvester for Smart Electronic Applications Sarbaranjan Paria, Ranadip Bera, Sumanta Kumar Karan, Anirban Maitra, Amit Kumar Das, Suman Kumar Si, Lopamudra Halder, Aswini Bera, Bhanu Bhusan Khatua* Materials Science Centre, Indian Institute of Technology Kharagpur, Kharagpur 721302, India

*Correspondence to Dr. B.B. Khatua (email: [email protected]) Materials Science Centre, Indian Institute of Technology, Kharagpur-721302, India Tel.:91-3222-283978

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Abstract: Here, we demonstrated an arc shaped flexible triboelectric nanogenerator (TENG) that efficiently harvests asymmetrical mechanical energy for powering several portable electronic devices. Here the fabrication of TENG is based on the integration of cigarette wrapper, spongelike polyvinylidene fluoride (PVDF) film and conducting carbon tape. Here, the spongelike PVDF film was prepared through non-solvent induced phase separation (NIPS) technique (by simple spraying PVDF solution on water surface in a Petri dish). Owing to low density of PVDF sponge and cigarette wrapper the final device with a volume of 6.6 cm3 shows a total mass of 1.259 g. The fabricated TENG exhibits open circuit voltage of ~342 V and short circuit current of ~8.1 µA on frequencies of ~3.4 Hz without electro-spinning and poling treatment under finger tapping which gives maximum area power density of ~0.37 mW/cm2 at a load resistance of 40 Mohm. The TENG shows excellent durability without any change in output performance for successive seven days (193200 cycles). Also, this device can directly lit up 136 commercial LEDs instantly without any rectification unit and senses different types of mechanical forces. With easy fabrication and cost effective technique, this work pave a new and smart way to fabricate high performance TENG for powering portable electronic appliances.

Keywords: TENG, NIPS, PVDF sponge, power density, durability

 

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1. Introduction Currently, researches on light weight, flexible, and wearable electronics have attracted huge attentions for their widespread applications together with but not inadequate to, wearable display, artificial electronic skin, and several sensing applications.1-4 The main aspect for these applications is the power source that is as flexible as the electronic sheet itself. Harvesting energy from the different types of ambient energies like solar, thermal, mechanical, wind could convince the independent and sustainable energy source for such systems without the use of conventional battery having limited life span.5-8 In recent times, triboelectric nanogenerators (TENGs) have drawn much interest due to their high performance, little maintenance, low asking price, easy fabrication, and durability.9-13 The human motion based TENG system has widespread applications in various self-powered electronic devices, which creates them as selfsufficient energy source without any external power sources, as human actions, e.g., stretching, pressing, and rubbing, are very common and easily available mechanical energy sources. As the triboelectrification takes place among many kinds of materials including polymers14-17 at the micro scale level18,19 thus it is worthy to design a TENG with low weight and high energy conversion efficiency.20,21 To address these later two issues, recently, paper based energy harvesting device attracts huge interests for green chemistry and green electronics due to its outstanding properties such as pliability, simple accessibility, disposability, retrievability and non-polluting nature22-26 The interesting feature of the paper is that the micro-fibrous, nano-fibrous, and nanocluster of the paper are interlocked mutually, making it suitable for friction layers which is very useful for flexible and wearable electronics. But, most of the studies involve some modifications of active materials like prior charge injection,27 plasma treatment of the friction layer.28 and chemical

 

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modification 29, charge pumping.30 All these attempts have been focused on the development of the friction layers as triboelectric effect is a surface phenomenon. Nowadays, the electrospinning and poling techniques have been followed largely for advancing the output performance of the nanogenerator.31-33 In electrospinning technique the fibres are exhibiting more contact area and in poling technique the intrinsic and/ extrinsic dipoles get oriented along the applied electric field to make the friction surface more charged, leading to an increase in the performance of the nanogenerator. Recently, polyvinylidene fluoride (PVDF) and its co-polymer based TENGs have drawn huge attentions than the other polymers, due to the superior electron attracting properties and can be easily synthesized through a chemical method as it can be dissolved in frequent organic solvents.34 For instances, Li et al., fabricated a light weight TENG using the electro-spun PVDF membrane and nylon as friction layers.34 In another report, to enhance the output performance of the TENG, Huang et al., fabricated the device with graphene oxide doped electro-spun PVDF membrane35 Bai et al. have shown that the output performance of the TENG can be improved through the electrical poling treatment of the active friction layer and surface modification of the electrode.32 But, the existing micro/nano structure fabrication process and/or poling treatments involves intricate device designs, laborious and expensive which are naturally unreasonable for scalable fabrication of cheap TENGs and thus badly limits their practical implementation and market propagation to appropriate areas. Moreover, a lack of research with consider to TENGs constructed using simple fabrication process (like spraying) has emphasized the significance of this study as well. Thus, the objective of our work is to fabricate a high performance, light weight and durable TENG through a simple and cost effective method (i.e. without electro-spinning and/or electrical poling) for effectively harvesting mechanical energy and powering small portable electronic devices.

 

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In this effort, we have explored a novel and very simple designing of a light weight and flexible TENG for efficiently harvesting ambient mechanical energies available in the environment. Here, we choose cigarette wrapper as one of the friction layers due to its widespread availability as major wastage. Moreover, cigarette wrapper has micro cubic architectures on the conductive aluminium coating which excludes the additional electrode attachment on the active layer, as well as, eliminates any surface modification on the active part. On the other hand, PVDF was preferred as another friction layer because it simply attracts electrons from the other friction layer due to its superior electron affinity character. To reduce the cost of fabrication and mass of the device, we have prepared the spongelike PVDF film through nonsolvent induced phase separation (NIPS) technique,36 which excludes the expensive and tedious electrospinning technique. The total mass of the fabricated device with volume of 6.6 cm3 is about 1.259 g, which exhibits an open circuit voltage of ~342 V and short circuit current of ~8.1 µA under simple human finger tapping. The device shows excellent response to low force as well as high force. Also, this device can efficiently charge a capacitor of 22 μF capacitance up to ~5.1 V in ~243 sec which can be used for driving small electronics. Thus, this work highlights a novel and light weight TENG which shows potential application for sensing and harvesting energy from very small and large mechanical forces in our surroundings. 1.1. Schematic representation of the power generation from the TENG The electrical power generating mechanism of the TENG has been illustrated on the basis of coupling effect of contact electrification on the inner friction layer and induced electronic movement in the outer circuit, as represented in Figure 1.37 When the cigarette wrapper and PVDF come in contact with each other, positive charges develop on the cigarette wrapper whereas negative charges develop on the PVDF surface as PVDF has higher electronegativity

 

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than that of cigarette wrapper. When the cigarette wrapper and PVDF sponge are getting detouched from each other, the conducting carbon electrodes produce opposite charges to the triboelectric layer owing to the effect of electrostatic induction. Many theories and review articles consider that the electronic states lie within the huge band gaps of insulators and those states are mainly located on the surfaces.38-39As the insulators of various energy states at the surfaces are brought to contact, then the quasi work function inequality between the insulators govern the electron transfer process.38-40 That is, when the two insulators (here cigarette wrapper and PVDF sponge) come in contact, the electron may transfer from filled electronic state to unoccupied states of PVDF due to the difference in surface potential, leading to PVDF surface negatively charged.

Figure 1. Systematic diagrams of the power generation from the TENG structure under pressing and releasing conditions by human finger tapping, device structure (a) and steps involved in the power generation process (b).  

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Primarily, there is no charge on the friction surface, those are in arc shaped. When a vertical compression force is applied by human finger, the cigarette wrapper and the PVDF sponge come in contact and friction together, resulting in positively and negatively charged states, respectively. Once the compression force is removed, both the films will return to their original state due to flexibility of the device. At this moment the oppositely charged friction layers are getting quickly disconnectd by an air gap, building a dipole moment; an electrical potential difference developed between the electrodes.41 As a consequence, the electrons in the external circuit begin to flow for the bottom electrode (negatively charged PVDF sponge) to the top electrode (positively charged cigarette wrapper) till the accumulated charges attain an equilibrium state. Nevertheless, no electron will be ran between the sponge and wrapper due to their insulating nature. When the compressive force is reapplied, the dipole moment and potential difference start to decrease which offers the electron flow in the reverse direction. 2. Experimental 2.1. Materials and preparation of spongelike PVDF film PVDF was bought from Alfa-Aesar, India. N,N-dimethylformamide (DMF) was procured from Merck Chemicals, India. Conducting carbon tape was bought from Ted Pella, INC. Transparent polypropylene (PP) sheet for packaging was purchased from local market. Deionized (DI) water was collected from Merck Millipore, India. Cigarette wrapper and conducting Al foil were collected from thrown cigarette packet and market from India, respectively.

 

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Figure 2. Schematic representation of the spongelike PVDF film formation and fabrication of the TENG. A clear solution of PVDF was obtained by dissolving 1 g PVDF in 20 mL DMF in a beaker and kept it under stirring on the hot plate at 60 oC for 2 h. Here, we have prepared the spongelike PVDF film through a NIPS method. 20 mL PVDF solution was taken in a syringe and sprayed over 20 mL of water in a Petri dish. Immediately after spraying the PVDF solution, a thin layer of spongelike PVDF film was formed due to the solvent exchange between water and DMF. After that, the sponge was taken out from the Petri dish and kept on a tissue paper to soak the residual solvent. Finally the sponge was kept in a hot air oven at 40 oC for a week to remove the solvent.  

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2.2. Fabrication of the TENG First, a transparent PP sheet was cut into 5  3 cm2 area and then the two edges were joined together with adhesive to form a transparent cylinder and pressed to form arc shaped device. Then, two conducting carbon tapes of same size were cut and attached with the two opposite sides of the arc shaped device. After that, two conducting copper wires were attached with the two carbon electrodes. For the final fabrication of the TENG, a cigarette wrapper and spongelike PVDF film with area of 4  2 cm2 were cut from the cigarette wrapper sheet and PVDF film. Then, the cigarette wrapper and spongelike PVDF film were attached on the carbon tapes previously attached with the two opposite concave surfaces of the device. Similarly we have fabricated another two devices where smooth aluminium electrodes were used instead of cigarette foil. Specifically the length and width of the arc shaped device were 5 and 3 cm, respectively, where the active area of the friction layer was  8 cm2. The schematic of the manufacture process of the TENG device is given in Figure 2 along with an actual photograph of the device and its weight. 2.3. Characterization Field emission scanning electron microscopy (FE-SEM, Carl Zeiss-SUPRA40) was used for the morphological analysis. Energy dispersive X-ray spectroscopy (EDX) assembled with FE-SEM was carried out for the elemental analysis of the spongelike PVDF film. For structural investigation of the spongelike PVDF film, X-ray diffraction (XRD) with X’Pert PRO diffractometer (PANalytical, Netherlands) with nickel-filtered CuKα (λ = 0.15404 nm) at a scanning rate of 0.25° min−1 and Fourier transform infrared spectroscopy (FTIR) (NEXUS-870, in attenuated total reflectance (ATR) mode) were carried out. Differential scanning calorimetry (DSC) analysis was conducted for the investigation of thermal and crystalline property of the

 

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PVDF sponge using TA instrument (Q20). The assessment of output voltage and short circuit current during finger tapping were performed by oscilloscope (ROHDE & SCHWARZ, RTM 2022, 200MHz, 5 GSa/s) and Keithley (model 4200-SCS) using four probe method, respectively. The output performances of the TENG under the various external load resistances ranging from

∼100 kΩ to ∼100 MΩ was measured by means of high impedance measurement instrument

(Keithley, 4200 SCS) with high sampling rate. The capacitors (capacitance of 1 to 22 µF) charging and discharging execution were carried out with a standard two electrode system CHI 760 D work station. 3. Results and discussion 3.1. Morphology The morphology of the spongelike PVDF film prepared with NIPS process was observed by the field emission scanning electron spectroscopy (FESEM), as shown in Figure 3. As can be seen (Figure 3 a, b, c), in the PVDF sponge, the fibres are entangled with one another to form uneven surface. This suggests that in case of any damage of the nanofibers in the PVDF sponge under cyclic pressure, the entangled-structured of PVDF sponge may preserve its capability to be charged since the other nanofibers would be charged instead of the smashed nanofibers, resulting in a type of fracture tolerant nature. Furthermore, the sprayed spongelike PVDF film has a specific surface area greater than that of cast PVDF film as surface casted film would be more smoother that the sprayed surface , leading to develop a large quantity of charge. The EDX spectrum of the spongelike PVDF film is shown in Figure 3d, which indicates the presence of  

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Figure 3. (a) FESEM image of the PVDF film surface and (b) enlarged view of the selected area in Figure 2a, (c) exaggerated view of the selected area in Figure 2b. (d) EDAX spectrum of the PVDF sponge, showing the presence of the elements C and F only. (e) FE-SEM image of the  

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cigarette foil (aluminium side). (f) X-ray diffraction pattern and (g) FT-IR spectra of the PVDF film. 3.2. Electroactive phase analysis of the spongelike PVDF film 3.2.1. XRD analysis Development of the crystalline polar beta () phase in the spongelike PVDF film was proved by XRD, DSC and FTIR analysis. As shown in Figure 3 f, several distinguished diffraction peaks of PVDF film are observed at 2θ of  17.6,  18.3,  19.9 and  26.5º, which are responsible for the reflection planes of (100), (021), (020), and (110) refer to the apolar alpha (α) phase.42,43 Nonetheless, a further strong peak is seen at 2θ of  20.2 in the PVDF sponge which corresponds to the sum of the diffraction in (200) and (110) planes.44 This peak confirms the pattern of polar β-phase in the PVDF sponge, in contrast to the formation of nonpolar α-phase in solvent casted PVDF thin film. The reduction of non-polar α-phase and formation of polar β-phase was clearly understood from the reduction in the intensity of the diffraction peaks associated with (100), (110), (020), and (021) planes correspond to apolar αphase. This consequence supported excellent interaction between water and air interface, as reported in previous literature.45 This significant switch of non-polar α-phase to polar β-phase is largely beneficial for the mechanical energy conversion through tribolectric phenomenon. 3.2.2. FTIR analysis FTIR gives detail interpretation about the particular phase development in the PVDF thin film and sponge as well. The FTIR spectra of the spongelike PVDF film and solvent casted thin film are shown in Figure 3 g. The characteristics absorption bands of apolar α, polar β and gamma (γ) phases of pure PVDF thin film and sponge are observed at 532, 763, 796, 854, 870, 1146, 1210, 1383 and 1423 cm1 for α- phase; 510, 841, 1276, 1286, 1431 cm1 for β-phase and

 

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812, 833, 838 and 1233 cm1 for γ-phase.46,47 As can be seen from the spectra, the intense peaks at 841 and 1276 cm1 clearly reveal the formation of β-phase over α- phase in case of spongelike film exclusively. 3.2.3. DSC studies The positive effect of the spreading of PVDF solution on the water surface to facilitate enhanced crystallization in the polar phase is additionally explained by the DSC analysis. For the DSC thermogram, as shown in Figure S1 in Supporting Information, unlike the PVDF film, the PVDF sponge exhibits a shoulder like structure around the key melting peak. It is widely accepted in the literature that β- phase crystallite melting appears in the area of 165–172 °C; αphase in the span of 172–175 °C with the γ-phase melting in between 175 and 180 °C.48-51 However, the melting temperature (T m ) of PVDF sponge is 165.5 °C, about 8 °C lower than the PVDF thin film. From the earlier reports, this decrease in T m is largely due to enhanced β-phase amount and development of porosity in the membranes.52,53 The values are in well harmony with those obtained from FTIR analysis, as discussed above. From earlier studies conducted by Gomes et al.,

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for PVDF, it has been well recognized that both the piezo and ferro-electric

nature of PVDF depend on the extent of crystalline β- phase and its saturation polarization increases with an augmentation in the ferroelectric β-phase content in the sample. Hence, the PVDF sponge with higher β-phase of PVDF should exhibits higher polarization and possesses better triboelectric charge density for gaining the electrons from the cigarette wrapper. 3.3. Output performance of the fabricated TENG The typical electrical output performance from TENG with an effective area of 4×2 cm2 was studied by periodically pressing and releasing condition by simple human finger tapping at controlled frequencies (~3.4 Hz) and amplitudes (~0.11 m). Upon pressing and releasing, the

 

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TENG develops alternating currents through the external circuit. The open circuit voltage and short circuit current generated from the spongelike film and casted film based TENG are shown in Figure 4 a-b, c-d, respectively. A voltage signal in positive direction was developed when the TENG was pressed to allow it into close contact with the surface of another friction layer, whereas a negative peak signal in a opposite direction was created on releasing the applied force. In order to study the effect of the spongelike film in TENG, two TENGs were combined with sponge type films by spraying and casting procedures; the smilar cigarette wrapper film was applied on the another sides. After that, the open-circuit voltages of the two TENGs were examined under the same condition, as given in Figure 4. The maximum V OC and I SC were obtained from the cigarette wrapper based TENG are ~342 V and ~8.1 µA, respectively, whereas thin film based TENG exhibits ~80 V and ~1.1 µA, respectively. The peak-voltage of the PVDF sponge based TENG is about ~4.3 times greater than that of the cast PVDF film based TENG. Also, the short circuit current generated from the spongelike film based TENG is about ~7.4 times higher than the casted film based TENG.

 

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Figure 4. (a) and (b) The open circuit voltage generated from the spongelike film based TENG and solution casted thin film based TENG, respectively, under human finger tapping, (c) and (d) The short-circuit current from the spongelike film based TENG and solution casted thin film based TENG, respectively, under finger tapping. There is one probable interpretation for this outcome: well-entangled PVDF sponge prepared by spraying procedure has a superior surface area than that of the simple casted PVDF film, although the base material is same PVDF. The FESEM image of the solution casted PVDF film has been displayed in the Figure S2 (Supporting Information). As can be seen from the Figure S2 that there is no such kind of surface roughness as can be seen from the PVDF sponge. Several informations have displayed an identical behavior of rising output voltage with friction parts with extensive surface area.55,56 Also, the amount of tribo-charges on the contacting surface  

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induced by the triboelectric effect greatly depends on the contact surface area of the two friction layers and this generated triboelectric charge is very much responsible for enhancing the performance of the TENGs. We have calculated this developed charge on the friction surfaces under a complete pressing and releasing cycle by simply integrating the area under I SC vs. t curve for consecutive seven weeks, as shown in Figure S3 (Supporting Information). From the Figure S3 it is clear that the charge generated on the friction surfaces is about ~0.1 µC which is also accountable for the high output achievement of the TENG. Switch polarity tests was also executed to ensure that the measured output signals are originated from the TENG and not from the measurement instrument, as shown in Figure 5a. An opposite sign in the voltage peak was observed under reverse connection as in Figure 5b. The device exhibits an average peak-to-peak output voltage of ~342 V in both the cases, forward and reverse connections. Theoretically, the enhanced output performance can be interpreted as follows: The basic equation57 for the conductor-to-dielectric contact-mode TENG could be expressed by

V 

x(t ) Q d  (  x(t )) ……………………………(1) S 0  r 0

where ε 0 , ε r , Q and σ are the vacuum permittivity, relative permittivity of the PVDF, transferred charges within the two electrodes guided by the induced potential and tribo-charge density on the interior surface of the PVDF film. x(t), d, S and t are the interlayer distance, thickness of the PDMS film, area size of the dielectrics and time, respectively. For open circuit (OC) conditions, there will be no charge transfer process so the value of Q is 0. Thus, the open-circuit voltage can be resulting as

V  

x(t )

0

 ………………………………………......(2) 16 ACS Paragon Plus Environment

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The V OC is obtained at a maximum gap between the two tribo-electrification layers. As throughout the experiment the interlayer distance same so the V OC solely depends upon the surface charge density (σ) which is proportional to σ. σ is dependent on the contact surface area and surface roughness. As σ is increased significantly as compared to flat surface, so that the enhanced output of the TENG has been obtained. Again, the electric current generated by the PVDF sponge based TENG can be explained using the equation of charge-current relation:57

I SC 

dQ ……………………………………(3) dt

where I sc is the current developed from the contact between the nanostructured Al surface and PVDF sponge, Q is the total charge produced by friction electrification between the two contact surfaces, and t is the operating time. The built up charge can be estimated as:

Q    Acontact …………………………….(4)

 

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Figure 5. Output voltage generation performance of the TENG in forward connection (a) and

reverse connection (b). Open circuit voltage generated from the TENG fabricated with commercially available aluminium foil, (c) V OC from spongelike PVDF film and (d) V OC from PVDF thin film. Furthermore, the output voltage at any given time is low in case of releasing than that in the pressing condition, as returning of the TENG to its initial state takes place through structural elasticity rather than removal of the tapping force55,58 which results in the generation of the peaks of different heights. To check the effect of the micro-pattern of the cigarette wrapper on the output of the TENG we also carried out the experiment with commercially available Al foil as a friction layer. The result has been shown in Figure 5c and d. From the Figure 5c and d it is clear that the output performances are very lower than that of TENG fabricated with the cigarette  

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wrapper. This fact can be explained by the FESEM image of the commercially available Al foil, as shown in Figure S4. The microsturture on the cigarette foil, which is not seen for the commercial Al foil, is the responsible for the high output of the fabricated TENG. Table 1 Comparison of electric output of the designed TENG with the other PVDF and its

copolymer based TENG reported earlier

Current or current density

Power or power density

Nanogenerator

Poling/electrospinning/additi on of dielectric materials

Output voltage

PVDF/Nylon 34

Poling

75 V

6 μA

0.28 W/ kg

P(VDF-TrFE)/ PDMS-MWCNT59

Electrospinning

25 V

6.5 μA

1.98 mW/cm3

PVDFZnSnO 3 /PA660

Addition of dielectric materials 520 V

2.7 mA/m2

0.47 mW/m2

PVDF-TrFE/PI61

Not used

28.3 V

2.6 µA

64 mW/m2

P(VDFTrFE)/Graphene

Not used

11 V

0.6 μA

Not calculated

8.1 µA

0.37 mW/cm2

62

Present work

Not poled, not electrospinned and not added any dielectric 342 V materials

The output of our fabricated TENG has been compared with the earlier reported results, as shown in Table 1. To the best of our informations, the output power of the TENG (without poling and electrospinning technique) is much higher in comparison to numourous other TENG devices assesment under same condition. From the table it is evident that the fabricated TENG device in this work would be highly effective for harvesting electrical energy from the ambient  

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mechanical forces. Also, we have compared the output performances of the fabricated PVDF spongelike film based TENG with the TENG based on different materials, as shown in Table S1 (Supporting Information).

Figure 6. (a,b) represent the performance of TENG under sewing machine and (c, d) represent

the tests for stability and durability of the TENG for different days. Again, we have carried out an experiment with the TENG to check its performance under a constant mechanical force exerted by a sewing machine. The V OC and I SC obtained from the TENG are shown in Figure 6 a, b. The maximum V OC and I SC developed from the TENG are ~46 V and ~1.8 μA, respectively. As the hitting probe area (diameter ~5.45 mm) under sewing machine is very less in comparison to finger’s area, the output voltage from the TENG reached the lower value than that under finger tapping.

 

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3.4. Durability of the TENG In the aspect of long term use, which would be required for the practical prospect, the durability of the fabricated TENG was performed, as demonstrated in Figure 6 c and d. Although we are concerned to develop the human motion based TENG but it is difficult to measure the performance of the nanogenerator for a long time under human motions i.e. finger tapping, walking, bending etc. For the simplicity we have carried out the test for durability of the TENG under a constant applying mechanical force given by a commercially available sewing machine. Typically, the TENG was attached to a plastic sheet through a wrapper, one end of which was tightly fixed to an x-y stage and the TENG was periodically compressed by a sewing machine. The voltage generated from TENG was around ~46 V on an average at a frequency of 4.6 Hz. We have carried out the test for consecutive seven weeks (100 min per day equivalent to 27600 cycles per day, total 193200 cycles) and did not observe any significant loss in output performance. This performance of the TENG suggests that the fabricated nanogenerator has excellent durability and can be used in long term practical purposes.

3.5. Sensitivity of the TENG under several mechanical forces Figure 7 a shows the output voltage generated from the TENG under single finger tapping. The minimum and maximum frequency given by a single finger tapping are ~ 2.8 and ~4.5 Hz, respectively. The Figure 6a clearly reveals the increasing trend of generated voltage of the TENG with enhancing the frequency. A promising cause of this occurrence is that the positively and negatively charge states on the rubbing layers experience less time to get neutralized upon high frequency of deformation, which leads to accumulation of more charges on the respective electrodes.61 This high sensitivity under different frequencies exhibited by the fabricated TENG signifies the possible implementation of the TENG for dynamical sensing.

 

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Figure 7. Measured V OC in response to four types of motion: (a) single finger tapping, (b) heel

pressing, (c) elbow pressing, and (d) mouth blowing. We also checked the output performance of the nanogenerator under heel pressing, elbow pressing and mouth blowing, as shown in Figure 7 b, c and d, respectively. These results unambiguously suggest that our fabricated device is highly capable of harvesting energy from tiny and high mechanical forces as well as senses them effectively. As the mechanical energy from an ambient environment largely varies and is variable, it is required to investigate the relationship between the output performances of PVDF sponge under various conditions. Thus, the output voltage generation performance of the TENG is subjective by the input state of the applied force, weight, and height of the falling object.

 

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Figure 8. The output voltage generation performance under different mass of the object (a: coin,

b: potato) under same falling height. (c) represents the performance characteristics of the TENG: Dependence of output from the TENG on external load (schematic of the circuit diagram is in inset). (d) Rectified open circuit voltage of the TENG, (e) enlarged view of selected area in Figure 7 (d). (f) Charging discharging behavior of a capacitor of 22 F capacitance. To examine the performance of the TENG with the height and force of the falling object we have carried out a series of experiments by coin and potato falling. The performances of the TENG under coin (two rupee, five rupee and ten rupee) and potato (of different weight) falling  

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are given in Figure 8 a and b, respectively. The dependability of the output of the TENG on the falling height and operating force are shown in Figure S5 and S6, respectively. The plot of output voltage versus time with various applied mass, as shown in Figure 8 a and b, indicated the linear connection among the outputs and masses, particularly in small mass range. Again, based on the rule of conservation of energy and momentum theorem, there is straight relationship between the height and useful force on device by hitting from the free descending object. Consequently, the TENG based on spongelike PVDF film can be applied as a height and force sensor. From these experiments it is obvious that we can harvest a significant amount of mechanical energy from small mechanical forces for powering small potable electronics. Also, this suggests the sensing behavior of the TENG towards small mechanical forces available in our surroundings. The corresponding video clips for sensitivity of the fabricated TENG under different types of applied force are shown in video S1 in Supporting Information.

3.6. Performance of the TENG under variable external load resistance Moreover, it is necessary to determine how the cigarette wrapper and PVDF based TENG can be made useful in real life. In order to determine this feasibility of the device for proper application, we have performed the power and accumulation tests under various load resistance, as the level of electrical power of the nanogenerator count on the load in the system. Thus the electric output of the TENG was measured by connecting the device with different electrical resistances, as shown in Figure 8 c. With increase in the resistance, the instantaneous maximum voltage increases and gets saturated to the open circuit voltage while the resistance is infinitely large. As a consequence, the instantaneous output power density (P = V2/R L *A) reached to ~0.37 mW/cm2 at the resistance of 40 Mohm, as shown in Figure 8 c. This result clearly reveals that the fabricated TENG is more effective when the load has the resistance in the order of tens of

 

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Mohm. The theoretical current (8.6 µA) can be gained from the maximum power value corresponding to load resistance from the formula (

). This theoretical value is

reasonably higher than the experimental value (8.1 µA) and this is because of the power expenditure by the internal resistance presence in the measurement assembly.

3.7. Capacitor charging-discharging behavior by TENG The process of rectification of the alternative output from TENG and the direct output stored in energy storage device like capacitor and batteries is demonstrated in Figure 8 d, e and f, through converting uneven mechanical energy, such as human movements into electricity. The corresponding circuit diagram has been shown in Figure S7 (Supporting Information). A four probe bridge rectifier was used for the rectification of the AC output of nanogenerator. From Figure 8 d and e, it is clear that on rectification of AC voltage generated from the TENG gives the DC output. With reference to Figure 4, it is obvious that the repeated pushing can make the build up voltage high due to the enlargement in the output density. As shown in Figure 8 f, the mount up voltage across a single capacitor of 22 F was measured under a constant human finger tapping. The maximum voltage reached in the capacitor is ~5.1 V in a time period of ~243 sec. The accumulated voltage across different capacitors of different capacitance charged by the TENG with the same finger tapping frequency is shown in Figure S8, by considering the threshold voltage of ~5.1V. The calculation of exerted pressure on the TENG is shown in Supporting Information and the approximated pressure under human finger tapping is about 26 kPa throughout the experiment. From this investigation, it can be concluded that the TENG has the ability to charge the capacitor of different capacitance within a very short time period and the stored charge in capacitor can be utilized for further purposes.

 

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3.8. Practical implementation of the TENG By organizing the properties of power production and mechanical motion monitoring, the TENG shows its prominent ability as an effective sensor for elegant home appliances. First of all,without using any rectification unit, it can easily power up number of LEDs (which can play as an alarm beacon) once a person applies force above the device, as shown in Figure 9. Under periodic tapping by human finger, the electric pulses and the open circuit peak voltage build up from the nanogenerator could immediately turn on all 136 LEDs (68 red and 68 blue) without using any rectification unit. In the form of practical application, the TENG could be positioned beneath the carpet in the entrance for turning on lights or start other electronic appliances. Secondly, the device can be connected to an electric circuit for powering the alarm system or for supplying electricity for household apparatus (Figure 9 b, c). To explore this practical implementation of the TENG, we have made an attempt to power up a mobile LCD screen, wrist watch and a digital hygrometer to symbolize household appliances. To validate the sensitivity of the device, the alternating electrical pulses were first rectified by a full wave bridge rectifier and stored in a capacitor for further utilizing the stored power; so that both the pressing and releasing processes would trigger the alert (the liquid crystal display (LCD) display), wrist watch and a digital hygrometer. The corresponding video clips for charging the capacitor and powering up the portable electronic devices are shown in video S2 in Supporting Information. Additionally, we have demonstrated the capability of our TENG to harvest electrical energy from a musical instrument (harmonium) when it is under playing condition. The fabricated TENG was placed inbetween the bellows and on periodic dragging and releasing of the bellows, it generated an output voltage of ~16.2 V (Figure S10, Supporting Information). Also, the TENG can power up several LEDs under harmonium playing, as shown in Figure S10, Supporting Information. The

 

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sensitivity of the TENG and powering of several LEDs under harmonium playing have been shown in video S1 and video S2, respectively. Thus, intelligent assembling of several musical instruments and TENGs (as the given design in Figure S10 in Supporting Information) may explore energy harvesting application where this highly robust TENG could be an alternative source for electrical energy from various musical instruments to power up various portable electronic devices.

Figure 9. (a) Powering the commercially available red and green LEDs (136 pieces, 68 pieces

each), before connection (left), after connection (right) (b) Powering a mobile LCD screen by storing the charge in a capacitor, before connection with the capacitor (left), and after connection with the capacitor (right). (c) Powering a wristwatch by storing the charge in a capacitor, before connection with the capacitor left), and after connection with the capacitor (right). (d) Powering  

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a hygrometer by storing the charge in a capacitor, before connection with the capacitor (left), and after connection with the capacitor (right). (e) Overall schematics of the proposed fall detection system with the pressure sensing TENGs array on the floor. Furthermore we could utilize the fabricated TENG for fall detection systems through a intelligent design as shown in Figure 8d. With the swiftly increasing figure of older people in our societies, fall detection is becoming more important: Older adults may fall at home when they are alone and they may not be found in time for them to get help. In addition, a fall itself can cause severe injuries such as lacerations, fractures and hematomas etc. The 12 PVDF sponge based arch-shaped TENG unit cells generated analogous signals without any use of external power source when the TENGs are pressed and two friction surfaces come closer. Depending on the output voltage generated from the proposed array of TENGs, we could easily detect the fall from the bed. For the demonstration of the fall detection, we have carried out an experiment of applying pressure on the four TENGs (connected in series) by human being of different weights and the corresponding result has been provided in Figure S9. Owing to its little cost, durability, easy design, and installation of the TENGs, the proposed system can be directly applied at smart homes and hospitals to prevent additional injuries caused by falling. 4. Conclusion

In conclusion, designing of a novel and simple arch-shaped TENG based on PVDF sponge and cigarette wrapper is demonstrated that can be effectively used for harvesting variable mechanical energy. The fabricated device shows excellent output performance without any surface modification of the friction layers. By merely rectifying the output signal under the essential input signal pulse, it is viable to charge a capacitor in a short time span. The fabricated light weight TENG can be run by human hand or any various dynamic motion energy and blow

 

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energy, which extends their ubiquitous applicability. By the integrity of high durable and excellent output performance of the fabricated device, we can conclude that this approach for developing eco-friendly power sources to work out the electronic waste problem, concurrently enlarging the applications of paper and spongelike thin film based energy harvesting and sensing electronics. Supporting Information

DSC spectra of solvent casted PVDF film and PVDF sponge, transferred charges of the fabricated TENG during finger tapping, rectified open circuit voltage of the TENG, Open circuit voltage generated from the TENG fabricated with commercially available aluminium foil, (a) V OC from spongelike PVDF film and (b) V OC from PVDF thin film, output voltage from the TENG versus height of the falling coins, output voltage from the TENG versus force exerted by the falling objects, accumulated voltage across different capacitors of different capacitance, calculation of operating pressure and force under human finger tapping condition, and Video file 1: demonstration of sensitivity of TENG under various applying force conditions, Video file 2: demonstration of powering LCD screen, wrist watch and 136 red LEDs. Acknowledgements

The authors acknowledge the financial support provide by University Grant Commission (UGC), India and research facility provided by the Indian Institute of Technology, Kharagpur, India. The authors also acknowledge supports of Santimoy Khilari, Krishnedu Sarkar, Saikat Chakraborty and Abhishek Ghosh in measuring the output performance of the device. References

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(59) Wang, X.; Yang, B.; Liu, J.; Zhu, Y.; Yang, C.; He, Q. A Flexible TriboelectricPiezoelectric

Hybrid

Nanogenerator

Based

on

P(VDF-TrFE)

Nanofibers

and

PDMS/MWCNT for Wearable Devices. Sci.Rep. 2016, 6, 36409. (60) Soin, N.; Zhao, P.; Prashanthi, K.; Chen, J.; Ding, P.; Zhou, E.; Shah, T.; Ray, S. C.; Tsonos, C.; Thundat, T. High Performance Triboelectric Nanogenerators Based on PhaseInversion Piezoelectric Membranes of Poly (vinylidene fluoride)-Zinc Stannate (PVDFZnSnO 3) and Polyamide-6 (PA6). Nano Energy 2016, 30, 470-480. (61) Jang, S.; Kim, H.; Oh, J. H. Simple and Rapid Fabrication of Pencil-on-paper Triboelectric Nanogenerators with Enhanced Electrical Performance. Nanoscale 2017. (62) Cho, Y.; Park, J. B.; Kim, B.-S.; Lee, J.; Hong, W.-K.; Park, I.-K.; Jang, J. E.; Sohn, J. I.; Cha, S.; Kim, J. M. Enhanced Energy Harvesting Based on Surface Morphology Engineering of P (VDF-TrFE) film. Nano Energy 2015, 16, 524-532.

Table of Content (TOC)

 

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Triboelectric nanogenerator with excellent charging capability and durability has been fabricated using cigarette wrapper and spongelike PVDF film through a facile and cost effective method for energy harvesting and sensing applications.

 

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Graphical Abstract 48x28mm (300 x 300 DPI)

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