Triboelectric Nanogenerators Made of Porous Polyamide Nanofiber

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Triboelectric Nanogenerators Made of Porous Polyamide Nanofiber Mats and Polyimide Aerogel Film: Output Optimization and Performance in Circuits Hao-Yang Mi, Xin Jing, Mary Ann Babin Meador, Haiquan Guo, Lih-Sheng Turng, and Shaoqin Gong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b08098 • Publication Date (Web): 16 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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ACS Applied Materials & Interfaces

Triboelectric Nanogenerators Made of Porous Polyamide Nanofiber Mats and Polyimide Aerogel Film: Output Optimization and Performance in Circuits

Hao-Yang Mia,b,d, Xin Jinga,b,d, Mary Ann B. Meadore, Haiquan Guof Lih-Sheng Turngb,d*, and Shaoqin Gongb,c*

a

School of Packaging and Materials Engineering, Hunan University of Technology, Zhuzhou, 412007, China

b

Wisconsin Institute for Discovery, University of Wisconsin–Madison, Madison, WI 53715, USA

c

Department of Biomedical Engineering, University of Wisconsin–Madison, Madison, WI 53706, USA

d

Department of Mechanical Engineering, University of Wisconsin–Madison, Madison, WI 53706, USA

e

NASA Glenn Research Center, Cleveland, Ohio 44135, USA

f

Ohio Aerospace Institute, Brookpark, Ohio 44142

Corresponding Authors: S. Gong: [email protected]; L.S. Turng: [email protected]

Note: The authors declare no competing financial interest.

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Abstract: Triboelectric nanogenerators (TENGs) have been attracting a tremendous amount of attention since their discovery in 2012. Finding new means to enhance energy output is an ongoing pursuit. Herein, we introduce a new type of high performance TENG composed of highly porous polyamide (PA) nanofiber mats and polyimide (PI) aerogel films. We have demonstrated that the thickness of the porous triboelectric materials, which is attained by stacking multiple layers of triboelectric materials, has a profound effect on the triboelectric output performance of TENGs. The triboelectric output increased when PA increased from 1 layer to 6 layers. However, it decreased when PA was further increased to 12 layers. With an optimum material thickness, a TENG with only a 2 cm2 effective device size achieved a high output voltage of 115 V and a current of 9.5 µA under a small compressive pressure (30 kPa). A peak power density of 1.84 W/m2 was achieved on a 4.7 MΩ external load. The TENG was able to light 60 LEDs easily and quickly charge capacitors with different capacitance to 6 V, indicating an outstanding energy harvesting ability. In addition, the performance of multiple TENGs connected in different ways, as well as the performance of TENGs in resistive/inductive/capacitive (RLC) circuits, were investigated. These findings provide new insight into the working principles of TENGs in complex circuits.

Keywords: Triboelectric nanogenerator; polyamide; polyimide; highly porous; output optimization

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1. Introduction

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Sustainable and self-chargeable power sources are highly desirable for flexible, portable, and wearable electronics.1-2 Over the last decade, harvesting pervasive mechanical energy

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from ambient environment has been attracting a lot of attention due to the rising need for clean and alternative energy resources.3-6 Among all mechanical energy harvesting


technologies, triboelectric nanogenerators (TENGs) have the unique advantage of being able to harness ubiquitously chaotic mechanical energy in nature and human daily life.7-8 TENGs

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have been successfully used to harvest mechanical energy from human motion, machine vibration, wheel rotation, wind, and ocean waves.9-15

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The electrical energy generated by a TENG originates from the coupling effects of contact electrification and electrostatic induction between two layers of thin films that are

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typically within the range of 20 µm to 1 mm and have dissimilar tribopolarities.7,


16-20

.


Mechanical energy is converted to electric potential during the contact and separation of the two thin films. Apart from exploring new applications, improving the triboelectric output


performance of TENGs is another key research aspect for TENG.21-23 According to the

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working principles of TENGs, the selection of dissimilar materials and their microstructures are two important factors that significantly affect TENG performance. So far, the most commonly used tribopositive materials in TENGs are metals, metal oxides, polyamides (PA), and cellulose. propylene

24-27

(FEP),

. Commonly used tribonegative materials include fluorinated ethylene polyvinylidene

fluoride

(PVDF),

polydimethylsiloxane

(PDMS),

polytetrafluoroethylene (PTFE), polyvinyl chloride (PVC), and polyimide (PI).28-33 Compared to metals and metal oxides, polymers are much easier to process and can offer high flexibility; thus, they are highly desirable for flexible TENGs suitable for applications such as wearable electronics.

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In respect to the microstructure, it has been widely reported that structured surfaces possess superior charge generation capabilities as compared to smooth surfaces of the same material due to the enhanced contact and friction areas.34-36 By applying porous materials,

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such as nickel and polytetrafluoroethylene (PTFE), superior output performance was achieved.37-38 It was also found that a three-dimensional (3D) porous PDMS used as the negative material showed a significantly higher performance than solid PDMS.39-40 We

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recently demonstrated that an increase in the porosity of either the tribopositive or tribonegative material resulted in an increase in the energy output for various polymeric triboelectric materials.41 Hence, we believe that the 3D porous structures have profound

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effects on the triboelectric performance, yet these effects are not fully understood. Moreover, it is well-known that porous materials have a number of advantages, such as low cost, lightweight, high surface area, and high flexibility. Such materials have already been used in numerous fields such as in automobiles, airplanes, packaging, thermal insulation, sound barriers, air and water filtration, tissue engineering, supercapacitors, and batteries.42-46 The

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employment of porous materials in TENGs would not only enhance their energy harvesting performance, but also give the TENG all of the advantages mentioned above. To understand the influence of a 3D porous structure on TENGs, we investigated the thickness effect of highly porous triboelectric materials on the output performance. In this study, PA and PI were selected as the tribopositive and tribonegative materials, respectively, since they have distinctively dissimilar triboelectric properties according to the triboelectric series.16, 47 In addition, they are both highly flexible and durable polymeric materials that can

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be easily mass produced. PA nanofiber mats were fabricated by electrospinning, and porous

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PI aerogels were fabricated via supercritical fluid extraction. Various TENG devices were


fabricated by assembling different layers of PA nanofiber mats and PI aerogel films in order to investigate the effect of triboelectric material thickness. The optimum TENG with an effective device size of only 2 cm2 showed a high open circuit voltage of 115 V and a short 4



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circuit current of 9.5 µA under a small force (~ 6 N). A maximum power density of 1.84 W/m2 was achieved on a 4.7 MΩ resistor. The underlying mechanism for the enhanced triboelectric outputs using these 3D porous triboelectric materials was studied. Moreover, the


performance of multiple TENGs connected in series or in parallel, as well as the performance of TENGs in RLC circuits, was also investigated. Therefore, this study not only provides a high-performance polymer-based flexible TENG, but it also introduces a novel strategy for boosting the performance of porously polymer-based TENGs through optimization of the film thickness. In addition, we also provided some insights into the working principle of TENGs when integrated into complex circuits, which could guide the design of circuits or devices involving TENGs.

2. Experimental Methods

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2.1 Preparation of polyamide (PA) nanofiber mats PA nanofiber mats were prepared by electrospinning. Briefly, PA 6 (Mw = 12,000, Sigma–Aldrich) pellets were pre-dried and dissolved in formic acid to prepare a solution with a concentration of 25%. The freshly prepared solution was loaded into a plastic syringe that was connected to an 18-guage blunt needle and mounted on a digital pump. Electrospinning was carried out using a lab setup at a voltage of 20 kV, flow rate of 0.05 mL/min, and a working distance of 20 cm. The PA fibers were collected on aluminum foil in 1 h. 2.2 Preparation of polyimide (PI) aerogel film The

PI aerogel films were synthesized by crosslinking biphenyl-3,3’,4,4’-

tetracarboxylic dianydride (BPDA) with 50% 2,2’-dimethylbenzidine (DMBZ) and 50% 4,4’oxydianiline (ODA), followed by supercritical fluid extraction using CO2, according to previously reported procedures.48

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2.3 Fabrication of PA/PI triboelectric nanogenerators (TENGs) 5



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TENGs were assembled using PA nanofiber mats as the tribopositive material and PI aerogel films as the tribonegative material. As illustrated in Figure 1a, PA and PI were cut into rectangular pieces (2 cm × 1 cm) and attached to indium tin oxide (ITO)-coated flexible PET films that acted as current collectors using thin polypropylene tape. The two materials were separated using two PDMS spacers with a thickness of 2.3 mm. The generated charges were lead out using aluminum strands. The effect of the triboelectric material thickness was studied in order to optimize the performance of the TENG device. Various TENGs were fabricated using 1 to 12 layers of PA nanofiber mats and 1 or 2 layers of PI aerogels. 2.4 Characterization of materials and TENGs The microstructures of the PA nanofibers and PI aerogels were imaged using a scanning electron microscope (SEM, LEO GEMINI 1530, Zeiss) at an acceleration voltage of 3 kV. All samples were sputtered with a thin film of gold before imaging. The fiber diameter and pore size were measured from the SEM images using the Image Pro-Plus software. The triboelectric material thickness was measured using an Indi-X blue electronic indicator (Fowler). The porosity of the PI aerogel was calculated by P = (1 − ρb / ρ d ) ×100% , where ρb

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was the bulk density and ρd was the density of the solid material. The Brunauer–Emmett– Teller (BET) surface area of the PA nanofiber mats and the PI aerogel films were measured by N2 physisorption using a Gemini VII 2390 surface area analyzer (Micromeritics Instrument Corp.). All statistical measurements were performed at least in triplicate. To evaluate the triboelectric output performance of the TENG devices, they were periodically pressed by a shaker (LDS V201, Brüel & Kjær, Denmark) with controlled force (~6 N) and frequency (10 Hz). The open circuit voltage signal generated was recorded using an oscilloscope (DS1102E, Rigol, China) and the short circuit current signal was recorded using a potentiostat (versaSTAT-3, Princeton Applied Research, USA). the power density on the external load was calculated using PD = I 2 R / A , where I was the peak current on the 6


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external load, A was the effective device size of the TENG, and R was the resistance of


external load. The output current density was calculated using J = I / A . All tests were

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performed at least three times, and the most representative curves were reported.

3. Results and Discussion

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The TENG fabricated in this study (Figure 1a) used PA nanofiber mats as the tirbopositive material and PI aerogel film as the tribonegative material to generate triboelectric charges. These two materials were chosen because of the high tribopositivity of PA and the high tribonegativity of PI according to the triboelectric series, which should theoretically lead to a high triboelectric output.16, 47 Besides tribopolarity, surface morphology

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is a critical factor that determines a material’s ability to generate charge. Rough surfaces


generate more charges than flat surfaces due to their increased surface area. As shown in


Figure 1b and c, the prepared PA nanofiber mats showed a special fibrous structure that consisted of large fibers, with an average diameter of 151 nm, and small branched fibers, with an average diameter of 28 nm (Figure S1). This special branched fibrous structure contributed to the relatively high surface area of 10.8 m2/g. The PI aerogels, on the other hand, showed a nanoporous structure on the surface as shown in Figure 1d and e. Some cracks and large pores with sizes of around 500 nm can be seen from Figure 1d, and small pores with sizes ranging from 5 to 200 nm dominated the PI surface (Figure S2a). The porous structures of PA nanofiber mats and PI aerogels resulted in rough surfaces that are beneficial for electrostatic charge generation and electrification. Notably, the cross section of the PI aerogels showed a highly porous morphology with an average pore size of 686 nm (Figure S2b). The pore walls consisted of numerous nanofibrils, as shown in the inset image of Figure 1f. This special structure of the PI aerogel contributed to a high porosity of 92% and an extremely high surface area of 212 m2/g. The PI aerogel film was very flexible and could be easily rolled 7




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(Figure 1d). Previously, we found that 3D porous aerogels showed significantly higher triboelectric outputs than their dense counterparts, and the performance was higher for materials with high porosity and surface area, which proves that the porous structure facilitated charge generation in the triboelectric process.41 In this study, we investigated the effect


of triboelectric material thickness on the performance of TENGs composed of porous materials.

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Figure 1. (a) Schematic illustration of the TENG made of PA nanofiber mats and the PI aerogel film; (b, c) morphology of electrospun PA nanofibers at (b) low and (c) high magnification; (d, e) surface morphology of the PI aerogel at (d) low and (e) high magnification; (f) cross section morphology of the PI aerogel.

The representative triboelectric open circuit voltage (Voc) of different TENGs assembled with different layers of PA nanofiber mats and PI aerogels are shown in Figure 2, from which it can be seen that the material thickness had a significant effect on the output voltage. The thicknesses of PA nanofiber mats increased from 11 to 298 µm as the number of layers increased from 1 to 12 as shown in Table 1. The nonlinear increment of thickness was due to small gaps that formed when the thin PA nanofiber mats were manually stacked and fixed on 8


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the conductive PET substrate. The thickness of the PI aerogel film was 235 µm, and it was 526 µm when two layers of PI aerogel films were stacked together. Figure 2 shows that when paired with one layer of PI aerogel film, the output voltage increased from 59 V to 115 V, as the PA nanofiber mats increased from 1 layer to 6 layers (Figure 2a-d). As the PA layers increased further, the voltage started to decrease and the output signal became a bit noisy (Figure 2e-g). When 2 layers of PI aerogels were paired with different layers of PA nanofiber mats, the same trend held. A maximum voltage of 76 V was achieved with only 2 layers of PA nanofiber mats (Figure 2j). This was because the air gap decreased significantly when 2


layers of PI aerogels were used. The air gap became even smaller as the number of PA layers increased (Table S1), thereby leading to a lower electrostatic potential between the two current collectors as the increase of PA layers. As a result, a maximum output was obtained when only 2 layers of PA were paired with 2 layers of PI. The output voltage was only 33 V


when 12 layers of PA nanofiber mats were paired with 2 layers of PI aerogels. It was also noticed that the negative voltage signal greatly decreased, which was attributed to the


significant increase in PI thickness and the decrease of the air gap when two layers of PI aerogels were used.49

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Figure 2. Triboelectric open circuit voltage of the assembled TENG devices using (a-h) 1 layer of PI aerogel film and 1, 2, 4, 6, 8, 10, or 12 layers of PA nanofiber mats, respectively, and TENG devices using (i-p) 2 layers of PI aerogel films and 1, 2, 4, 6, 8, 10, 12 layers of PA nanofiber mats, respectively.

Figure 3 shows the short circuit current (Isc) results of different TENGs assembled with different layers of PA nanofiber mats and PI aerogels. It was found that the Isc showed the same trend as the Voc, and the Isc of the TENGs containing one layer of PI aerogel outperformed the corresponding TENGs containing two layers of PI aerogels. These results suggest that the thickness of the porous triboelectric materials had a significant effect on the output performance and that there is an optimum thickness that leads to a maximum output. The optimum combination for the TENG fabricated was 6 layers of PA nanofiber mats paired 10


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with one layer of PI aerogel, which provided a high Voc of 115V and a Isc of 9.5 µA. It should be noted that this high output performance was achieved with a small TENG device (2 cm2 effective device size) under a small compression force (~6 N). The same TENG showed a


higher output voltage with increased compression forces when tested with manual pressing. As shown in Figure S3, the voltage reached ~185 V at ~20 N, and further achieved ~220 V at a compression force of ~30 N. We also compared the performance of the optimum TENG fabricated (6PA/1PI) with a TENG made of relatively dense materials (casted solid PA film paired with a denser PI film with a porosity of 30%). From Figure S4, it was found that the denser TENG, which had about the same material thickness as the porous TENG, showed a lower voltage of 34.8 V and a lower current of 2.6 µA. This further demonstrates the advantages of a porous structure for improving TENG performance.

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Figure 3. Triboelectric short circuit current of the assembled TENG devices using (a-h) 1 layer of PI aerogel film and 1, 2, 4, 6, 8, 10, or 12 layers of PA nanofiber mats, respectively, and TENG devices using (i-p) 2 layers of PI aerogel films and 1, 2, 4, 6, 8, 10, 12 layers of PA nanofiber mats, respectively.

Generally, the thickness effect was explained by the change of capacitance of the triboelectric material as the thickness changes.50-51 This capacitive effect is discussed in the supporting information in detail for the PA/PI TENG fabricated in this study (Figure S5). However, we found that the capacitive effect is not enough to account for the dramatic increase of the output performance and also fails to explain the decrease in performance when the material thickness is too high. We believe that the improvement is highly related to the charge generation ability of the porous materials. In other words, the charges generated increased with the increase in material thickness, which means that the charges were generated not only on the outermost surface of the triboelectric material but also inside of the material to a certain depth from the surface. In order to clarify the thickness effects, we chose TENGs fabricated with 1 layer of PI aerogel film paired with 1, 6, and 12 layers of PA nanofiber mats as our model devices. Upon contact, equal amounts of charges were generated on both the fibrous PA and porous PI. Upon separation, the electrons would flow from the top current collector to the bottom current collector due to the electric potential difference created by the separated and oppositely charged PA and PI. This caused the current to flow in the opposite direction through the external circuit. Thus, the amount of charges generated on the triboelectric materials and the charges induced on the current collectors determined the output of the TENGs. As illustrated in Figure 4a–i, the charges generated on the triboelectric materials were low when the PA nanofiber mats had a small thickness. The current density (J) for the 1PA/1PI TENG was 27 mA/m2 (Figure 4a ii). With an optimum thickness, maximum charges could be generated on both PA and PI films, thereby resulting in a high J of 46 12



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mA/m2, (Figure 4b ii), for the 6PA/1PI TENG. However, when the thickness was further increased, the current density decreased to 29 mA/m2 for the 12PA/1PI TENG (Figure 4c ii). The performance of these TENGs in circuits with external loads was characterized by connecting them to resistors, with resistances ranging from 104 to 107 Ω, (Figure 4iii). It was found that the current density decreased as the load resistance increased, and the maximum power density was achieved on a 4.7 MΩ resistor. The maximum output power densities for 1PA/1PI, 6PA/1PI, and 12PA/1PI were 0.41 W/m2, 1.84 W/m2, and 1.06 W/m2, respectively. Therefore, the 6PA/1PI TENG exhibited the optimum output in circuits with external loads, which was 4.5 times of the 1PA/1PI TENG. With such a high power density, the TENG should be suitable for powering various small electronic devices. Figure 4d depicts the charge generation mechanism for the layered PA nanofiber mats. Positive charges are generated on PA nanofibers exposed on the surface in the first contact cycle with PI. Because PA is a dielectric material, charges cannot freely move across the material, while they can be transferred to other PA nanofibers that are initially neutral by contact. Upon compression, part of the charges are transferred from the surface to the interior PA nanofibers due to the squeezing of the pores. Therefore, additional charges can be generated on the surface in the next contact cycle with PI aerogel. Following this mechanism, charges gradually accumulate in the porous triboelectric materials, however, the amount of charges can be transferred keeps decreasing as the depth increases, thus an equilibrium state would be reached eventually. Figure S6 (a) illustrates the detailed charge transfer mechanism and Figure S6 (b) verifies the charge accumulation in our porous PA/PI-based TENG. This accounted for the increased voltage output as the number of PA layers increased from 1 layer to 6 layers. With a further increase in PA layers, although the maximum amount of charges could still be generated, the induction to the current collector was limited since the majority of the positive charges were located on the upper region of the layered PA nanofiber mats (Figure S6).24 Moreover, the air gap (da) decreased as the PA thickness (i.e., the number of 13




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PA layers) increased (Table S1). Thus, the air gap may not be large enough to induce high electric potential between the current collectors, as illustrated in Figure 4c i.

Figure 4. (i) Schematic illustration of the underlying reason for the effect of varying PA nanofiber mat thickness on TENG output performance, (ii) the current density on the current collectors, and (iii) the instantaneous current density and power density on external loads with resistances ranging from 10 kΩ to 10 MΩ of TENGs composed of (a) 1 layer, (b) 6 layers, and (c) 12 layers of PA nanofiber mats and 1 layer of PI aerogel film. (d) Diagrams showing the transfer of positive charges to the interior section of layered PA nanofiber mats.

Table 1. The thickness of the PA in TENGs assembled with different layers of PA nanofiber mats and 1 layer of PI aerogel film, and the figure of merit (FOM) for different TENG devices. The FOM for the TENGs composed of 1 layer of PA and PI was normalized to 1.

Thickness (µm) FOM

1PA

2PA

4PA

6PA

8PA

10PA

12PA

11.0

23.3

80.0

113.3

173.3

270.3

298.3

1

2.4

3.1

3.7

3.0

2.3

1.5

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The figure of merit (FOM) is normally used to evaluate the performance of a TENG device.52 In order to visually compare the differences, we used the method proposed by Snyder et al.

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to calculate the FOMs of TENGs composed of different layers of PA

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nanofiber mats and 1 layer of PI aerogel. As listed in Table 1, the FOM improved by 3.7 times when the thickness of the PA nanofiber mats increased from 1 layer to 6 layers. It decreased to 1.5 when the PA nanofiber mats reached 12 layers. Therefore, the TENG consisting of 6 layers of PA nanofiber mats and 1 layer of PI aerogel exhibited the best performance, so that it was used to demonstrate the energy harvesting abilities and stability of our novel, highly porous, polymer-based TENG. TENGs have the unique ability to convert ubiquitously chaotic mechanical energy to electrical energy.7 To demonstrate their energy harvesting abilities, a TENG was used to light

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up LEDs. A TENG with an effective device size of 2 cm2 was connected to 26 LEDs in series,


which spelled out “UW”, through a bridge rectifier, and was pressed periodically at 10 Hz. It can be seen from Figure 5 and Movie 1 that all LEDs were instantly lit by the TENG once it was subjected to an external force (~6 N), and they all emitted bright light, thus indicating a sufficient voltage supply. Moreover, the mini TENG was able to light a maximum of 60 green LEDs, as shown in Figure 5b and Movie 2, at the same condition, thus demonstrating an excellent output performance. Since TENGs are intended for harnessing energy from periodic motions, they must possess excellent long-term stability. In order to verify this, a TENG was stored in open air and tested over 8 weeks. During this period, it was pressed over 50,000 times. As shown in Figure 5c, the TENG maintained a very stable output over the 8 weeks, thus demonstrating the high durability and robustness of the porous PA/PI based TENG. Due to the nature of TENGs, the energy generated is alternating and periodic; thus, it is not applicable as a power source for most electronics as is. Therefore, harnessed energy needs to be stored first in a battery or a capacitor. We used a TENG to charge capacitors with different capacitances through a bridge rectifier for demonstration. As shown in Figure 5d, the TENG 15



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was able to charge capacitors with capacitances of 22 µF, 10 µF, and 6.8 µF to 6 V in 132 s, 197 s, and 324 s, respectively. The charged capacitors could be used to power various small electronics. For example, the capacitor charged by TENG can power a timer and light LEDs (Figure 5e, Movies 3 and 4). However, the energy harvesting rate of our TENG device was slower than the energy consumption rate. Thus, the capacitor needs to be charged repetitively. In practical application, the problem can be solved by using larger TENG device and longer charging time.

Figure 5. The energy harvesting and output performance of an optimum TENG consisting of 6 layers of PA nanofiber mats and 1 layer of PI aerogel with an effective device size of 2 cm2. (a) Twenty-six LEDs that spelled “UW” were instantly turned on through a bridge rectifier by a TENG under a compressive stress of 30 kPa. (b) The TENG could light a maximum of 60 green LEDs. (c) Output voltage stability test for the TENG over 8 weeks. (d) Charge capacitors with different capacitances (22 µF, 10 µF, and 6.8 µF) using the TENG through a bridge rectifier. (e) A timer and two LEDs powered by the energy stored in a capacitor (22 µF), which was charged by the TENG, through a bridge rectifier.

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For the ease of manufacture and scaling up, it is sometimes desirable to utilize or assemble multiple small TENGs rather than fabricate one with large size. Therefore, it is important to investigate the assembly mechanism of TENGs. Although TENGs generate instant alternating current in one press and release cycle, we can still define positive and negative terminals depending on the major direction of the current flow. It was found that the


voltage signal in the press cycle was greater than the output in the release cycle for our TENG (Figure S7). This was because charges on triboelectric materials were generated in the pressing cycle when two triboelectric materials contact, and the amount of charges generated is related to the applied force.

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Since an external force of ~6 N was applied during the

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press cycle while the force was removed during release, so that the pressing signal was stronger than the release signal. Thus, we can define the terminal attached to the tribonegative material (PI in this study) as the anode since positive charges were generated on the current collector it was attached. Similarly, the other terminal attached to tribopositive material (PA in this study) can be defined as the cathode. As plotted in Figure 6, we investigated the performance of two TENGs connected in three different ways. It was found that both the output voltage and current doubled when two TENGs were connected in parallel (Figure 6a). When connected in series, the output was identical to that of a single TENG since the negative charges on the cathode of the first TENG were neutralized by the positive charges on the anode of the second TENG (Figure 6b). There was no output signal when two TENGs were inversely connected as shown in Figure 6c. The underlying reason is easy to understand as illustrated in the schematic drawings. When the positively charged current collector is connected to a negatively charged current collector, the opposite charges neutralize one another immediately. Therefore, there was no output when two TENGs were inversely connected and the output was not enhanced when they were connected in series. When two TENGs were connected in parallel, the equivalent effective device size (4 cm2) was essentially doubled in comparison to the effective device size of a single TENG. The open 17




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circuit voltage and short circuit current reached 230 V and 18.6 µA, respectively, which was twice as much as a single TENG. This indicates that there is almost no energy loss when connecting our TENGs in parallel. Therefore, a parallel connection is an effective way to enhance the output of TENG devices. For example, multiple TENGs can be stacked and connected in parallel to boost the energy conversion efficiency from mechanical motion. For our PA nanofiber mats and PI aerogel film-based TENGs, theoretically, a high voltage of 1.2 kV and a current of 0.1 mA could be generated if the equivalent effective device size reached 20 cm2. This emphasizes the amazing energy generation capabilities of the TENGs fabricated in this study.

Figure 6. Performance of two TENG devices connected (a) in parallel, (b) in series, and (c) inversely. (i) Schematic illustration of equivalent circuit and charge transfer in two TENG devices. (ii) Open circuit voltage and (iii) short circuit current of two TENGs connected in different way.

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We also researched the performance of the TENG in RLC circuits, since TENGS may be integrated into more complicated circuits for small electronics. Their performance in resistive circuits was studied by placing them in parallel or series with two resistors of equal resistance. The voltage on the load was then measured (Figure 7a-d). When the resistance was relatively small (e.g., 100 kΩ), the voltage on parallel-connected load was 5 V, and it was 10 V on the series-connected load (Figure 7a, c). This was because the total resistance for the resistors connected in parallel was calculated to be 50 kΩ. The same trend held when the resistance was increased to 1 MΩ (Figure S8). However, when the load resistance increased to 10 MΩ, the voltage on the parallel-connected load was 90 V, and it was 70 V when the load was connected in series (Figure 7b, d). This was because the voltage on the 10 MΩ resistor was close to the open circuit voltage of the TENG, thus the output could not be further enhanced by increasing the external resistance, as shown in Figure S9. Therefore, it is important to measure the voltage output on different external loads for a specific TENG first in order to precisely control the voltage supply on specific components in a circuit. Since TENGs supply alternating current, it is also valuable to investigate their behavior in inductive and capacitive circuits. When a 1 MΩ load was connected in parallel to a 1mH inductor, no voltage was detected on the load, while the voltage was almost unchanged when the inductor was connected in series to the load (Figure 7e, f). This was due to the low inductive reactance of the inductor (0.06 Ω) under the TENG’s operating frequency (10 Hz). When the load was connected in parallel with a 1 µF capacitor, a very weak voltage signal (0.15 V) was detected on the load, thus indicating that charges were mostly stored in the capacitor (Figure 7g). However, when connected in series, the capacitor showed no effect on the voltage on the load (Figure 7h). In addition, we also investigated the behavior of a large load (10 MΩ), and the same trend held in both inductive and capacitive circuits (Figure S10). Therefore, we found that TENGs, as a power source, behave differently from normal AC and DC power sources. Thus, they may have favorable or unfavorable effects on different RLC circuits. The results of 19



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our study provide some general principles for designing RLC circuits in electronic devices that integrate TENG components.

Figure 7. Output performance of TENGs in RLC circuits. (a-d) A TENG connected to an external load and a resistor with different resistances: (a, b) parallel connection, (c, d) series connection; (e,f) TENG connected to an external load (1 MΩ) and an inductor (1 mH): (e) parallel connection, (f) series connection; (g, h) TENG connected to an external load (1 MΩ) and a capacitor (1 µF): (g) parallel connection, (h) series connection.

4. Conclusion

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High-performance TENGs composed of highly porous polyamide (PA) nanofiber mats and polyimide (PI) aerogel films were fabricated in this study. The significant effects of triboelectric material thickness on the triboelectric output performance were demonstrated for these highly porous polymer-based TENGs. An optimum material thickness exists for the TENGs to achieve maximum triboelectric output. The TENG composed of six layers of PA nanofiber mats and one layer of PI aerogel film achieved a high open-circuit voltage of 115 V and a short-circuit current of 9.5 µA under a small compressive pressure (30 kPa). 20


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Furthermore, a maximum power density of 1.84 W/m2 was achieved on a 4.7 MΩ resistor. A


TENG with only a 2 cm2 effective device size instantly lit 60 green LEDs and charged


capacitors with capacitances of 22 µF, 10 µF, and 6.8 µF to 6V within 350 s, thereby demonstrating its outstanding energy harvesting capabilities. The harvested energy can be used to power LEDs and small electronics. The underlying mechanism for the enhanced output with these 3D porous triboelectric materials has been proposed and discussed for the first time. In addition, we found that connecting two TENGs in parallel doubled the output voltage and current. The performance of TENGs in RLC circuits was also investigated. Our findings provide more insight into the working principles of TENGs when integrated into complex circuits, which can offer guidance on the design of circuits or devices involving TENGs.

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Supporting Information


Morphology and microstructure of the materials, additional triboelectric output signals and TENG performance in circuits, and discussion on the capacitive and charge accumulation mechanism. Movie 1 and 2: Powering LEDs using the PA/PI based TENG (AVI) Movie 3: Powering a digital timer using a capacitor charged by the PA/PI based TENG (AVI) Movie 4: Powering LEDs using a capacitor charged by the PA/PI based TENG (AVI)

Acknowledgements The authors would like to acknowledge the financial support of the National Natural Science Foundation of China (51603075; 21604026), the Kuo K. and Cindy F. Wang Professorship, the College of Engineering, Office of the Vice Chancellor for Research and

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Graduate Education, and the Wisconsin Institute for Discovery (WID) at the University of Wisconsin–Madison.

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Graphical abstract 68x41mm (300 x 300 DPI)


Triboelectric Nanogenerators Made of Porous Polyamide Nanofiber

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