Natural Biopolymer-Based Triboelectric Nanogenerators via Fast

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Surfaces, Interfaces, and Applications

Natural Biopolymer-based Triboelectric Nanogenerators via Fast, Facile, Scalable Solution Blowing Seongpil An, Abhilash Sankaran, and Alexander L. Yarin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15597 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 6, 2018

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ACS Applied Materials & Interfaces

Natural Biopolymer-based Triboelectric Nanogenerators via Fast, Facile, Scalable Solution Blowing Seongpil An, Abhilash Sankaran, Alexander L. Yarin* Department of Mechanical and Industrial Engineering, University of Illinois at Chicago, 842 W. Taylor St., Chicago IL 60607-7022, USA

Abstract Here, we fabricated nanofiber (NF)-based triboelectric nanogenerators (TENGs) from natural biopolymers using the industrially scalable solution blowing. This technique eliminates severe restrictions on solutions to be used, and allows one to achieve biocompatible devices. Here solutions of soy protein and lignin were blown into continuous monolithic NFs of hundreds of nanometers in diameter. The technique we employed yields large-area NF mats within tens of minutes, and has never been employed to form TENGs. Furthermore, in contrast to electrospun and meltblown fiber mats, solution–blown NF mats are much fluffier/porous, which is beneficial for achieving higher voltages by means of triboelectricity. In particular, triboelectricity generated by our biopolymer-based TENGs revealed that they hold great promise as sustainable and environmentally-friendly selfpowered devices for biomedical applications with the highest efficiency in their class. Moreover, these are the first nano-textured plant-derived biopolymer-made TENGs.

Keywords: triboelectric nano-textured generator, environmentally-friendly, solution blowing, soy protein, lignin *

Corresponding author: [email protected]

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1. Introduction While concerns about energy crises and the corresponding demands for sustainable, renewable, and environmentally friendly materials grow, numerous miniaturized selfpowering generators have been hitherto developed,1 including triboelectric nanogenerators (TENGs),2-5 and piezoelectric nanogenerators (PENGs).6-9 Such generators can directly generate electric energy by harnessing ubiquitous sources of energy, for example, those accompanying vibrations and friction in nature and human motion.10 In particular, poly(methyl methacrylate) (PMMA)/polyimide (PI)-based TENG was introduced by using spin-coating and dry-etching methods,2 and from then on, TENGs aiming at the nextgeneration self-powered electronics became of great interest to academia and industry. This has accordingly resulted in a dramatic increase in development of various TENGs, which have employed numerous existing and emerging materials and fabrication methods (Figure S1). As a result, the output performance of the TENGs, for example, the output voltage has recently increased up to hundreds of volts.11-18 Employing new materials and highly sophisticated technologies not only causes the rise in costs, but also requires complicated multi-step fabrication processes that currently limit TENG’s potential uses in industrially feasible low-cost devices in the field of selfpowered electronics. With an increasing awareness of this issue, electrospun nanofiber (NF)based TENGs have recently attracted significant attentions because of their relative simplicity and low-cost, but also because electrospun triboelectric materials possess a superior surfaceto-volume ratio (S/V). The latter is considered as one of the most important key factors to enhance the triboelectric effect (Figure S1). Nevertheless, the NF-based TENGs still suffer from such issues as severe restrictions on required solutions for electrospinning and limited operating conditions. 2


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The present group demonstrated that solution blowing allows forming continuous monolithic NFs of hundreds of nanometers in diameter comprised of natural biopolymers.19-22 The present group scaled solution blowing to the industrial-scale, where a commerciallyavailable BIAX nosepiece normally used for meltblowing, which has 8 rows with 41 concentric annular nozzles was used.23 In contrast to a laboratory-scale solution blowing, an additional redistribution chamber and a slightly more viscoelastic polymer solution was required in the industrial-scale solution blowing to uniformly supply the solution in large quantities and prevent capillary instabilities (i.e. nozzle clogging, dripping and fly formation). Note also that for achieving a high-uniformity of solution-blown NF mats, it is also imperative to optimize such experimental parameters as the nozzle diameter, the solution flow rate, temperature and pressure. In the present work, using a laboratory-scale setup, largearea NF mats were formed within tens of minutes (Figures S2 and S3). Moreover, solution blowing removes severe restrictions on polymer solutions to be used, and in particular, allows for biopolymers,21 which are in focus here. In addition, porosity of NF mats formed by solution blowing is much higher than that of the electrospun NF mats.21 It should be emphasized that even though several groups previously introduced biopolymer-based filmlike TENGs (based on chitosan or lignin), but their performance was insufficient for applications.24-26 In contrast, the nano-textured TENGs fabricated from soy protein and lignin in the present study revealed performance attractive for biomedical applications. Note also that soy protein and lignin are not only natural biopolymers, but also easy-to-access, inexpensive, and abundant materials. For example, soy protein, which comprises about 80% of total mass of soy, is a residual product of soy biodiesel production.20 Lignin, which is also a waste byproduct from the pulp industry, etc., is known as the second-most abundant polymer among the natural polymeric materials.27 Such materials as soy protein and lignin 3



are essentially, in search for novel methods of utilization, which for example, can make production of biodiesel sustainable due to development of new high-value products, like the eco-friendly biocompatible TENGs (eco-TENGs) aimed in the present work. Furthermore, the eco-TENGs formed from soy protein here revealed the output of the order of several volts, which is a sufficient voltage range for such medical electronic pacemakers, etc.28, 29 Accordingly, solution blowing of abundant biopolymers27, 30 (e.g., soy protein and lignin) is a cost-effective and non-toxic method that holds great promise for sustainable and environmentally friendly self-powered devices in particular, for biomedical applications.

2. Results and discussion 2.1 Eco-friendly biocompatible triboelectric nanogenerator and its operating principle Eco-friendly biocompatible triboelectric nanogenerator (eco-TENG) is illustrated in Figure 1a, where nanofibers (NFs) and a polyimide (PI) tape were located and adhered on separate copper (Cu) films. Then, they were also supported by a loop-shaped polyester (PET) tape with two Cu electrodes attached (cf. Experimental Section). The loop-shaped structure created by the flexible PET facilitated smooth reciprocation seesaw motion of eco-TENGs even on human joints in contact with clothes (for example, knees in walking, or elbows in bending).31-33 The attachment to the PET substrate caused no significant damage to either the NF mat or the PI tape. When dissimilar materials are contacting each other and then separating, positive versus negative net charges accumulate along the contact surfaces of both materials. This is known to be a result of several different interfacial properties of the contacting materials, 4


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including their chemical composition, surface roughness, and so on.34 Even though the underlying physical mechanism of this electrical interfacial phenomenon, which is known as contact electrification or static electricity, is still unclear,35 there is an ordered list called ‘triboelectric series’ (Table S1).34,

36

This list shows the relative charge polarity among

different insulators and organic polymers. In the present study, we used nylon 6, soy protein, and lignin NFs as positive charge materials, while the PI tape was used as the negative charge material. Note that soy protein and lignin are expected to have similar polarities with cellulose and wood, respectively, considering their chemical compositions and sources (Table S1). Figure 1b illustrates the operating principle of the eco-TENG based on the contact electrification. Note that Cu films on both parts were initially in the neutral state, and the asprepared NFs and the PI tape were also uncharged (cf. Ref. 36 for details on making samples neutral). As top and bottom parts of the device shown in Figure 1b (positive and negative triboelectric materials, respectively) are compressed toward each other, the opposite charges start to accumulate on each surface (the process from the first to the second steps in Figure 1b). Note that the exact mechanism of charge generation and transfer is still debated: it might be caused by electron-stealing via quantum tunneling, or by transfer of surface ions.37-40 Also, as listed in the triboelectric series in Table S1, different charges can be formed by the same material depending on its pair material. After being compressed, the entire contacting parts reach an electrically equilibrium state (the third step in Figure 1b, i.e. the charge magnitudes plateau). When the two surfaces are subsequently being released, each separating surface is tending independently to a new equilibrium state (the neutral state), which drives electrons from the bottom Cu film to flow to the top Cu film through the external circuit (the fourth step in Figure 1b). A subsequent new contact by compression with a high enough frequency 5



avoiding the fully neutral state to be reached, drives the electrons from the top Cu film to return to the bottom Cu film (the sixth step in Figure 1b). That is, how the operating cycle of the eco-TENGs is occurring via the red loop in Figure 1b. Note also that positive and negative output voltage V signals (and the corresponding electric current I signals) in the present study correspond to the compression (or the pressing) and releasing stages, respectively (Figure 1c). 2.2 Material characterizations Prior to the triboelectric performance of eco-TENGs, material characterization was conducted, with the results shown in Figure 2. Note that brush-casted nylon films and electrospun nylon NFs on Cu films (BC-NFLM and ES-NNF, respectively) were also fabricated and characterized for comparison with the solution-blown nylon 6 NFs (SB-NNF). Figures 2a–2d depict the photographs and scanning electron microscope (SEM) images of different nonwoven NF mats on Cu films: electrospun nylon 6 NFs (ES-NNF), SB-NNF, solution-blown soy protein NFs (SB-SNF), and solution-blown lignin NFs (SB-LNF), respectively. It should be emphasized that in contrast to the densely entangled electrospun NFs (Figure 2a), the solution-blown NFs revealed fluffier and rougher morphologies (Figures 2b–2d, cf. Figures S4a and S5a). In addition, the results of the wettability test confirmed that the surface of the solution-blown NFs was more heterogeneously textured than that of the electrospun NFs in terms of the Cassie-Baxter model (Figure S6).41, 42 The average diameters (Davg) of ES-NNF, SB-NNF, SB-SNF, and SB-LNF were 175 ± 49, 325 ± 110, 285 ± 136, 1139 ± 563nm, respectively (Figure S3 and Table 1). Figures 2e and 2f show the Fourier transform infrared (FTIR) spectra of all nylon 6based materials (BC-NFLM, ES-NNF, SB-NNF) and all solution-blown NF mats containing 6


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soy protein or lignin (SB-NNF, SB-SNF, SB-LNF), respectively. Figure 2e shows that all the nylon 6-based materials revealed two characteristic crystalline structures, the α- and γ-phases, of nylon 6, including N–H, CH2 asymmetrical stretch, CH2 symmetrical stretch, CH2 scissor, and CH2 twist, and the amide groups.43 Even though the peak intensities of the SB-NNF were slightly lower than those of the ES-NNF, both NF mats revealed shaper peaks than those corresponding to the BC-NFLM (Figure 2e). That was attributed to the unique characteristics of the NF fabrication processes, where the external forces, such as the electrostatic forces, or the aerodynamic drag forces were employed. That is, the electrostatic force associated with the applied high voltage in electrospinning, or the aerodynamic drag force imposed by the high-speed air jet in the solution blowing, cause the macromolecules in polymer jets to be significantly oriented along the fiber axis.30, 44-46 The

X-ray

diffraction

(XRD)

spectra

unequivocally

confirmed

that

the

macromolecular structures of both ES-NNF and the SB-NNF mats were highly oriented in comparison with that of the BC-NFLM film (Figure 2g). Two peaks at 20.1° and 24.1° corresponding to the (200) and mixed (002)/(202) planes of the α-phase, respectively, were revealed in the XRD spectrum of the BC-NFLM film (the green solid line in Figure 2g), which is characteristic of the non-parallel arrangement of molecular chains of the α phase.47, 48

In contrast, the NFs revealed the peak of the γ-phase at 21.3° corresponding to the (200)

plane, in which the molecular arrangement corresponds to that of the parallel chains (Figure 2g).47, 48 Broad peaks between 3700 and 2600 cm-1 were observed for both the SB-SNF and SB-LNF mats in the FTIR spectra (Figure 2f), which is attributed to many kinds of the O–H groups of different lengths and strengths in the biopolymers.49 Comparing to the SB-NNF, the 7



peaks assigned to the amide III and CH2 twist were enhanced and the peak for the CO stretch was newly exhibited for the SB-SNF and the SB-LNF. As shown in Figure 2h and Table 1, all the solution-blown NFs revealed decent mechanical properties, although the stiffness of both the SB-SNF and the SB-LNF biopolymer-containing mats were lower than those of the SB-NNF mat. However, it should be emphasized that this is essentially, beneficial, since such a softness is beneficial for flexibility that is one of the most important parameters related to the application as the wearable devices.1, 50 2.3 Performance and application Figures 3a–3e show the output voltage V signals of the ES-NNF- and the SB-NNFbased TENGs as functions of the frequency f. Note that the input force F (which means the compression force between the top and bottom surfaces of the TENG, cf. Figure 1) increased from 1.2 to 21.5 N as the frequency f increased from 2 to 10 Hz. That contributed to an increase of V because not only V is proportional to the compression velocity v,51 but also the effective friction area Aeff increased as both f and F increased.3 Here, the area Aeff is understood as the practically participating area generating the triboelectric effect, which is different from the projected contact surface area S of the sample. Note that the link of f and F was previously reported regarding the fiber-based TENGs.4, 13, 52-55 For the SB-NNF-based TENGs at all values of frequency f, the peak V values were an order of magnitude higher than those for the ES-NNF-based TENGs, which means that solution blowing forms much better TENGs than electrospinning. That was attributed to the different NF mat porosities, roughnesses, and morphologies corresponding to these processes (cf. Figures S4a and S5a, as well as Table S2). In general, solution-blown NF mats possess higher porosity and roughness with the fluffier morphology than those of electrospun NF 8


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mats (cf. Figures 3g, 3i, S4a, and S5a). That means that the solution-blown NFs are more significantly microscopically separated from each other than the electrospun NFs ones, although NFs in both cases are fully entangled macroscopically. That is, when the same compressive force F is applied to the ES-NNF- and SB-NNF-based TENGs, the triboelectric effects associated with the actual contact are Aeff, and the friction forces between NFs, are more pronounced in the solution-blown NF mats, which are superior to those in the electrospun NF mats. Accordingly, a higher triboelectric voltage V is observed with the SBNNF. The BC-NFLM-based TENGs revealed a lower voltage V, even than the one observed with ES-NNF-based TENGs (Figure S7). The latter result shows that porous NF structures, especially those of the solution-blown NFs, are exceptionally useful for the TENG performance. On the other hand, it should be emphasized that additional secondary peaks between the pressing and releasing peaks were revealed as the frequency f increased over f = 8 Hz (Figures 3d and 3e). This phenomenon may be attributed to a stick-slip motions taking place not only between the NF mat and the PI film, but also between each individual pair of NFs.56 In spite of the fact that the compressive force F is perpendicular to the surfaces of the NF mat and the PI film (cf. Figure 1), the horizontal shearing forces (parallel to both surfaces) also inevitably arise while the value of F increases (cf. Figure S8). That is, when slip can happen, which essentially corresponds to the rolling motion of the individual NFs. Then, the dynamic friction coefficient which is smaller than the static friction coefficient, which, in turn, can cause the macroscopic slip between the two surfaces. Even though one cannot distinguish which effect (the macroscopic slip or the microscopic stick-slip motions, cf. Figure S8) is more dominant, it seems plausible that these sudden motions yielded the secondary V peaks at the higher frequency cases. This phenomenon is a unique characteristic of the NF-based 9



TENGs because this trend has not been observed with the BC-NFLM-based TENGs, as well as in other studies. Indeed, Figure S7 shows that even though minor multi-peaks were observed even in the low frequency cases, these were associated with the overall uneven surface of the BC-NFLM, rather than with the stick-slip motions. It should be emphasized that this phenomenon was also observed in the previous fiber-based TENG studies, albeit without any explanation of phenomenological or fundamental reasons.3, 57-59 The generation of such multi-peaks is strongly advantageous for TENGs in terms of the efficient energy harvesting because it allows a TENG to generate voltage multiple times during a single compression stage. Figure 3f shows the result of the durability tests for the ES-NNF-based and SB-NNFbased TENGs for about 100000 cycles (5 h) at f = 6 Hz. Surprisingly, the V signals of both TENGs increased as the cycle number increased although they revealed different slopes. The V signals of the ES-NNF-based TENG increased from 0.2 V to 3.2 V during 84000 cycles, whereas those of the SB-NNF-based TENG varied from 0.8 V to 3.2 V during 36000 cycles. The overall initial air gaps between the entangled NFs (or the overall porosity) decreased due to the repetitive compressions (Figures 3g–3j and Figure S10), which contributed to a larger actual contact area Aeff between two contacting NF mats and the corresponding increase in the value of V (cf. Figure S9).3 In the case of the high-porosity solution-blown NF mats, the initial compression decreases their porosity rapidly, and as a result, the actual contact area Aeff between two contacting NF mats and the value of V rapidly reach their ultimate values (cf. Figures 3f, S9, and S10). On the contrary, in the case of the low-porosity electrospun NF mats, the initial compression decreases their porosity much slower, and as a result, the actual contact area Aeff between two contacting NF mats and the value of V slowly reach the same saturation level as those for the solution-blown NF mats (cf. Figures 3f, S9, and S10). 10


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Similar values of the stiff thickness dstiff of the nylon 6-based NF mats (Table 1) and the corresponding similar values of porosities after the durability tests (Figure S10e) imply the same ultimate packing of fibers in these mats and their mutual contacts in compression. Even though electrospun and solution-blown NFs revealed similar ultimate voltage V after 100000 cycles, the latter are still superior compared to the former due to their industrial scalability and feasibility of the fast, scalable, and low-cost fabrication process. It should be emphasized that the ultimate value of Aeff and the corresponding value of Vmax depend on the compression stress proportional to the compression force F. Figure 4 illustrates the performances of the eco-TENGs (the SB-SNF- and the SBLNF-based TENGs). Similarly to the nylon 6-based TENGs, the eco-TENGs also revealed an increase in voltage V as the frequency f increased. The secondary peaks were also observed in both cases of f = 8 and 10 Hz. In particular, the remarkable secondary peaks were yielded by the SB-SNF-based TENGs, which allowed the crest factor of the SB-SNF-based TENG to be reduced from 4.57 (f = 2 Hz) to 4.45 (f = 10 Hz) despite the fact that V was significantly increased [where the crest factor is defined as the ratio of the maximum value of V, Vmax to the root-mean square (RMS) value of entire V signals].5 Since the crest factor significantly affects the energy harvesting efficiency, various methods have been proposed to reduce it.5 As shown in Figure 4k, the SB-NNF-based TENG revealed the highest Vmax, which was followed by the SB-SNF- and SB-LNF-based TENGs. Note that this tendency exactly agrees with the order of the triboelectric series (Table S1). To characterize the surface charge density σ during the compressing and releasing motions, a short-circuit current I signal of the SB-NNF-based TENG at f = 10 Hz was measured (Figure 4l). The values of charge Q at the compressing and releasing motions were 11



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calculated (red area in Figure 4l) and then each Q value divided by the projected contact surface area S of the TENG, i.e. the value of σ = Q S-1 was evaluated. The resulting values of σ corresponding to the compressing and releasing motions were σp = 1.23×10-9 C m-2 and σr = –1.25×10-9 C m-2. The almost identical magnitudes of these values imply that during TENG operation electrons associated with the compressing motion are repeatedly participating in the releasing motion (cf. Figure 1b). Niu et al.51 theoretically studied the contact-mode TENG, where the time-dependent voltage V at the short-circuit condition was described by the following equation: V (t ) = σ SR  exp ( − At − Bt 2 ) ( A + 2 Bt )

(

− 2 F × exp(− At − Bt 2 )( A + 2 Bt ) × D F

(

− A + 2A F

) (

2 + Bt × D F

2

)

(1)

)

2 + Bt  

x

D ( x ) = exp ( − x 2 ) ∫ exp( y 2 ) dy

(2)

0

where A = d0(RSε0)-1, B = v(2RSε0)-1, and F = A(2B)1/2. Also, σ is the surface charge density, S is the projected contact surface area of a TENG, R is the applied external resistance, d0 is the effective thickness defined as d0 = d1 εr1-1 + d2 εr2-1, where d1 and d2 are thicknesses of each material in contact, and εr1 and εr2 are the corresponding relative dielectric constants. In addition, ε0 is the vacuum permittivity, and v is the compression velocity. To compare the experimental results with the theoretical model, the positive V signal from the SB-NNF-based TENG at f = 10 Hz was analyzed in the case of the external load resistance of R = 100M Ω connected (Figure S11 and Table S3). Note that, since the theory does not include the humidity effect on the TENG performance, Eq. (1) was multiplied by the 12


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humidity factor of 0.6 because all the experiments in the present study were conducted at the relative humidity of 50%. This is related to the fact that the experiments of Ref.

17

revealed

that at humidity of 50%, the voltage value is about 0.6 of its maximum value corresponding to dry conditions. As shown in Figure S11, the sharp peaks and the Vmax value predicted using Eqs. (1) (with the multiplier 0.6) and (2) (the red solid line in Figure S11) agree fairly well with the experimental result (the black solid line in Figure S11). The feasibility of the eco-TENG for the future applications is demonstrated in Figure

5. First, Figures 5a and 5b depict the maximum V and I values (Vmax and Imax) of the SBSNF-based TENG at f = 10 Hz, and the corresponding power P values (P =VI=R V-2=I2R), while the external load resistance R varied from R = 100 kΩ to 100 MΩ. The value of Vmax increased, whereas the value of Imax decreased as the value of R increased. Accordingly, the maximum value of P was Pmax = 1.1 µW at R = 7 MΩ. Furthermore, using the full-wave bridge rectifier circuit allowed the negative V signals from the eco-TENG to be rectified to the positive V signals (Figures 5c and 5d). That facilitated that not only the electric energy storage in the energy storage device (i.e., capacitors or batteries, see Figure 5e), but also the operational electric device (producing LED light, see Figure 5f and Movie S1). It should be emphasized that the loop-shaped eco-TENG can be installed on various human joints (Figure

5g and Movie S2), which holds great promise as the wearable electronics for biomedical applications. In general, the performance of TENGs is enhanced as the values of the compression force F and the projected contact surface area S are increased [cf. Eq. (1)]. To evaluate the performance of the current TENGs with those of the previously reported fiber-based TENGs, the values of the efficiency η, defined as η =V F-1 S-1, were compared as shown in Figure 5h. 13



Note that the other fiber-based studies, which did not provide values of F or S, were not included. Accordingly, one would expect a more complete information to be disclosed in future to make it possible to evaluate the efficiency of some designs reported in literature. Unfortunately, at present such information is lacking in a number of cases. Even though the studies in the blue domain in Figure 5h have revealed outstanding η values over η = 1000 V N-1m-1, it should be emphasized that they employed complicated processes with adding other materials, or additional post-treatments, such as the ion-gel materials,57 the silver nanowires with heat treatment,59 the freeze-drying process,55 and the negative charge-injection process.60 Thus, these methods require multi-step fabrication processes and expensive materials that significantly limit their industrial feasibility. Among the TENGs comprised of the asfabricated fibers (the red domain in Figure 5h),13, 53, 61 the present eco-TENGs formed from the solution-blown NFs made of natural biopolymers revealed higher η values than those in the other studies. That is, the eco-TENGs formed using the combination of the solution blowing and the natural biopolymers are expected to provide the industrially feasible, environmentally friendly, and cost-effective alternatives for the next-generation wearable electronics in biomedical applications.

3. Conclusion Nanofiber (NF)-based triboelectric nanogenerators (TENGs) can be formed using solution blowing of such abundant natural biopolymers as soy protein and lignin. Solution blowing is a fast and inexpensive process which has already been scaled-up to the industrial scale and large-area NF mats can be formed within tens of minutes. The solution-blown NF mats revealed the elastic behavior up to the strain of 20-40%. Accordingly, such NF mats can 14


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definitely be employed in stretchable devices. However, according to the operational principle of TENG, contacting and separating two different triboelectric materials is more important for charge accumulation at the contact surfaces rather than stretchability per se. The stretchability is a more useful feature for piezoelectric or flexoelectric nanogenerators, where voltage generation stems from mechanical stresses or a strain gradients.62-64 In the in-vivo electronics for biomedical applications, usage of some metals and synthetic polymers is common and inevitable for electronic and physical operation of the devices. The most important thing is that the exposure of metallic and synthetic polymeric parts should be minimized and they are non-toxic. The PET tape used in the present TENG is non-toxic and biocompatible and can be replaced by any other medical tapes. Moreover, a number of medical tapes are actually based on PET. In addition, the Cu film in the present study was covered by both PET tape and NF layer, resulting in its practical encapsulation, as it can be done in any other TENG based on our prototype. The elasticity of PET tape of our loop-shaped TENGs is definitely important because the releasing motion and durability are dependent on the elasticity of the PET tape. However, the NF layer microstructure, the material choice, and the compression force are more important factors for the output voltage of TENGs as revealed in Figures 3 and 4. There one can see that the porous structure of NFs enabled the TENG to demonstrate a higher performance than that of the flat film-shaped one. This was also true for the other types of TENGs studied in the literature.3, 13 In comparison to electrospun NFs mats, the solution-blown NFs mats possess a higher porosity, and, accordingly, generate higher triboelectric voltages. TENGs formed from solution-blown biopolymer NFs revealed the highest efficiency in their class, and we were 15



able to rectify the electric current generated by them and employ it in a meaningful prototype of wearable ‘self-powered’ electronics for biomedical applications.

4. Experimental section 4.1 Materials Nylon 6 pellets (1.084 g mL-1), low sulfonate lignin alkali powder (Mw = 10 kDa), and formic acid (≥ 95%) were obtained from Sigma-Aldrich (USA). Soy protein (Pro-Fam 781) was obtained from Archer Daniels Midland (USA). Polyimide (PI) adhesive tape (thickness = 100 µm) was purchased from ULINE (USA). Copper (Cu) film was purchased from a local market. Polyester (PET) adhesive tape (890MSR) was purchased from 3M (USA).

4.2 Nanofibers The 15 wt% nylon 6 solution in formic acid was prepared by dissolving nylon 6 pellets and magnetically stirring for 24 h. A brush-casted nylon 6 film on the Cu film (BCNFLM) was fabricated by pouring 1 mL of the 15 wt% nylon 6 solution onto the Cu film of the area of 64 × 100 mm2, and uniformly brushing the solution. Then, the Cu film was left for a day until it was fully dried. Electrospun (used for comparison) and solution-blown nylon 6 nanofibers (NFs) on Cu films (ES-NNF and SB-NNF) were formed. First, the ES-NNF were fabricated by electrospinning the nylon 6 solution onto the Cu film,30, 65-67 with a syringe pump (NE-300, New Era Pump Systems, USA) equipped with an 18-gauge needle (Sigma-Aldrich, USA) and a DC power supply (PS/EH30P03.0, Glassman High Voltage, USA), and a house-made rotating drum being used to obtain large-area NF mats. The electrospinning flow rate was 30 16


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µL h-1, the applied high DC voltage was 15 kV, and the needle-to-collector distance was 7 cm. The total electrospinning time was 5 h. Next, the SB-NNF were prepared by blowing the nylon 6 solution onto the Cu film23, 68, 69

, with the syringe pump, a single house-made annular nozzle, and the rotating drum being

used. The annular nozzle consisted of an air nozzle and a 16-gauge needle, where the needle was located concentrically with the air nozzle in the form of a core-annulus structure. Then, the nylon 6 solution was supplied through the core needle with a fixed flow rate of 5 mL h-1 and the surrounding air was co-blowing through the annulus with a velocity of the order of 200 ms-1 sustained by a high pressure line of 2.8 bar. The total solution blowing time was around 80 min. Soy protein solution was prepared by first dissolving 1.5 g of soy protein in 13.2 g of formic acid and leaving it with magnetic stirring for 24 h. Then, 1.8 g of nylon 6 was added and the blend was left for 24 h with magnetic stirring. Lignin solution was also prepared by first dissolving 2.25 g of lignin powder in 14.25 g of formic acid and magnetically stirring until lignin was fully dissolved. Then, 2.25 g of nylon 6 was added and the blend was left with magnetic stirring for 24 h. These ratios were optimized according to our previous studies.19, 21, 23, 30, 69 To form solution-blown soy protein and lignin NFs on Cu films (SB-SNF and SBLNF), the same solution blowing method and experimental conditions, as with the solutionblown nylon 6 NFs (SB-NNF) were used except the supply pressure. In these cases, a higher pressure of 4.1 bar than that of the SB-NNF case (2.8 bar) was applied to the air annular nozzle.

4.3 Eco-friendly biocompatible triboelectric nanogenerator: 17



First, each NF sample (on the Cu film) was cut into rectangles of 25 × 35 mm2 (cf.

Figure 1a). Note that using the rotating drums in both electrospinning and solution blowing allowed us to make 15 triboelectric nanogenerators (TENGs) from one coating (Figure S2). Next, the PI tape (on Cu film) was also cut into rectangles of the same size (cf. Figure 1a). Finally, the NF mat and PI tape parts were attached onto PET tape with two Cu electrodes (Figure 1a). The total assembly time for this TENG was less than 5 min.

4.4 Characterization Morphologies and cross-sectional structures of NFs were characterized by a field emission scanning electrospun microscope (FE-SEM, JSM-6320F, JEOL, USA). Fiber size distributions were obtained by measuring 100 NFs in SEM images. Fourier transform infrared (FTIR) spectra of NF samples were obtained by a FTIR spectroscopy (Horiba LabRam Aramis IR2, Japan). A digital caliper (SPI, USA) was used to measure thicknesses of samples. The X-ray diffraction (XRD) analysis (SmartLab, Rigaku, Japan) was used to analyze the crystalline structures of the samples. Tensile tests of the NF samples were conducted by a universal testing machine (Model 5942, Instron, USA) with a 100 N load cell, where the size of the tested samples was 15 × 60 mm2 and the strain rate was 10 mm min-1. The output (short-circuit) voltage signals were measured by an oscilloscope (TBS2072, Tektronix, USA) with a passive probe (TPP0100, Tektronix, USA). A house-made tapping machine was used to induce reciprocation seesaw motion to pressure or release TENGs with different motion speeds (corresponding to different frequencies). Note that the contact surface area between the NF mat and the PI film in the tapping machine was 25 × 20 mm2 and the compression force imposed by the machine varied as the motion speed changed. The compression force was measured by a digital force gauge (M2-100, Mark-10, USA). The average of 5 measurements at each frequency was recorded. Three-dimensional (3D) images 18


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of NF mat surfaces were obtained by an optical profilometer (Nano Contour GT-K, Bruker). Photographs and a movie were obtained by a digital camera (D3200, Nikon, Japan).

Supporting Information Current publications related to triboelectric nanogenerators (TENGs). Photographs, fiber size distributions, cross-sectional optical microscope images of nanofiber (NF) mats. Water contact angles of electrospun and solution-blown nylon 6 NF mats. Output voltage signals of the TENGs comprised of brush-casted nylon 6 film. A schematic for the macroand microscopic movements of the two surfaces and of individual NFs. Sketches of the actual contact area changes. Porosity values for NF mats. Experimental and theoretical results for the output voltage signals. Triboelectric series of materials. Thickness and roughness values of electrospun and solution-blown nylon 6 NF mats. Movies for LED operation and clenching/unclenching motions with the TENG.

19



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Figure 1. (a) Schematic and structure of eco-TENGs. Insets are photographs of soy, wood (for lignin), their powders, and the resulting solution blown NFs on Cu films, and the ecoTENG (from left to right). (b) The operating principle for voltage generation by means of the eco-TENG. (c) Sketch of the corresponding short-circuit output voltage and the electric current signals.

25



Figure 2. Photographs and the corresponding SEM images of different NFs deposited on Cu films: (a) ES-NNF, (b), SB-NNF, (c) SB-SNF, and (d) SB-LNF. (e) FTIR spectra of the nylon 6-based materials: BC-NFLM, ES-NNF, and SB-NNF. (f) FTIR spectra of the solution-blown NF mats: SB-NNF, SB-SNF, and SB-LNF. (g) XRD spectra of all the materials. The characterization was conducted on the material from the red-encircled areas in the photographs of each sample (cf. Panels a–d). (h) Tensile test results of the solution-blown NF mats. Note that two trials of tensile test were conducted for each case.

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Figure 3. (a)–(e) Output voltage V signals of the ES-NNF- and SB-NNF-based TENGs for different input frequency f from 2 to 10 Hz (which also resulted in a change in the compression force F from 1.2 to 21.5 N). Note that, in each panel, the right-hand side red and blue graphs are, respectively, the zoomed-in V signals of the ES-NNF and the SB-NNF mats shown on the left-hand side. (f) Durability test (f = 6 Hz and F = 11.0 N) of the ES-NNF- and SB-NNF-based TENGs for around 100000 cycles (corresponded to around 5 h of non-stop operation). Optical profiler images of (g), (h) ES-NNF and (i), (j) SB-NNF (g), (i) before and (h), (j) after 100000 cycles. Inset scale bars in SEM images are 20 µm.

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Figure 4. Output voltage V signals of the eco-TENGs based on (a)–(e) SB-SNF and (f)–(j) SB-LNF with the input frequency f from 2 to 10 Hz (which also resulted in a change in the input compressive force F from 1.2 to 21.5 N). Note that, in each panel, the right-hand side graph is the zoomed-in V signal. (k) Maximum output voltage Vmax and the compressive input force F of all the eco-TENGs as functions of frequency f. (l) A short-circuit current I signal of the eco-TENG based on SB-NNF with f = 10 Hz (F = 21.5 N), which was obtained by connecting a resistor (Ω = 560 K) to the eco-TENG.

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Figure 5. (a) Maximum output voltage Vmax and the electric current Imax, and (b) the corresponding output power P as a function of the external load resistance. (c) A circuit for receiving a rectified output voltage signal, charging the capacitors, or operating an LED. (d) The rectified output voltage V signal. Performance of the eco-TENG based on SB-SNF: (e) The accumulated voltage V over time T with using two different capacitors. (f) Photographs of the LED operation. Note that the surrounding light is tuned off to clearly observe and highlight the LED light. (g) Output voltage V manually by clenching/unclenching motions and the corresponding photographs and illustrations (cf. Movie S2). (h) Comparative charts of the efficiency versus projected contact surface area for the NF-based TENGs. Our ecoTENGs are indicated by ‘star’ marks.

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Table 1. Average diameter Davg, Young’s modulus E, and stiff thickness dstiff values of different NF mats. The Young’s modulus E values were calculated based on the elastic-plastic Green equation, σxx = Y tanh(Eεxx/Y), where σxx is the tensile stress, E is Young’s modulus, εxx is the tensile strain, and Y is the yield stress.70 Note that the stiff thickness of the NF mats was measured by the digital caliper. ES-NNF

SB-NNF

SB-SNF

SB-LNF

Davg (nm)

175 ± 49

325 ± 110

285 ± 136

1139 ± 563

E (MPa)

-

10.68 ± 0.16

3.58 ± 1.32

6.27 ± 3.71

Y (MPa)

-

2.66 ± 0.07

0.82 ± 0.22

0.52 ± 0.16

dstiff (mm)

0.05

0.05

0.04

0.06

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Graphical Abstract

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Natural Biopolymer-Based Triboelectric Nanogenerators via Fast

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