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Dual Friction Mode Textile-Based Tire Cord Triboelectric Nanogenerator

Wanchul Seung, Hong-Joon Yoon, Tae Yun Kim, Minki Kang, Jihye Kim, Han Kim, Seong Min Kim, and Sang-Woo Kim*

As vehicles become smarter, an alternative power solution will become increasingly important for future vehicle development. With this context, a triboelectric nanogenerator (TENG) is proposed which fully sits on tires and consists of textile-based tire materials. Both polydimethylsiloxane-coated silver textile, serving as an external tire tread material, and nylon woven textile, serving as an internal tire cord material, performing as opposing triboelec-tric materials, are well adaptable for rolling tires. It is demonstrated that tire material-based TENG performs at its maximum as it makes mutual contact with the road. The power generation property is characterized under different driving situations such as different tire rotation speeds and varying numbers of devices on the tires. The TENG demonstrates a maximum output voltage and a current of about 225 V and 42 µA, respectively, along with an output power of 0.5 mW at optimum load. The work offers the possibility to not only directly operate minute power-consuming electronics but also collect power and store it while driving a vehicle.

DOI: 10.1002/adfm.202002401

Dr. W. Seung, Dr. H.-J. Yoon, Dr. T. Y. Kim, M. Kang, Dr. J. Kim, H. Kim, Dr. S. M. Kim, Prof. S.-W. KimSchool of Advanced Materials Science and EngineeringSungkyunkwan University (SKKU)Suwon 16419, Republic of KoreaE-mail: kimsw1@skku.edu

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/adfm.202002401.

Mechanical energy harvesting (MEH) technologies have been widely explored for scavenging energy, not only for oper-ating sensors intermittently[11–13] but also for collecting power and charging relevant batteries or supercapacitors integrated in a variety of electronics/microsystems.[14–17] Recently, one of the MEH technologies, tri-boelectric nanogenerators (TENGs),[18–21] in which it is known that the displacement current is generated by the contact/sepa-ration of the materials, have been studied as an alternative to the aforementioned issues while harvesting an enormous amount of mechanical energy during the operation of the vehicle. A single-electrode mode TENG loaded on the rolling tires exhibits power generation.[22–24] However, the output performance is too low and the TENG seems to be a major hindrance to driving due to its bulk. The TENG with

an electric brush as an electrode has shown power generation without interfering with driving. However, in such a structure, inefficient energy harvesting is inevitable, and more impor-tantly, there is a lack of a design regarding the scale of a vehicle. The hexagonal structure-based TENG ensures continuous contact/separation while driving. However, the space between the tire tread and an inner liner is replaced by an elastic mate-rial, which is not as practical. Moreover, the output perfor-mance of TENG was characterized as being under 10  km  h−1 of velocity, 10 N of load, which limits its applicability. It would thus be highly desirable to design TENG in consideration of the expected load and the corresponding speed in application to a real vehicle, which matters substantially in terms of fuel efficiency.

In this work, we present a textile-based tire cord TENG (TC-TENG) that is adaptable to part of tires. Regarding the required features of being tire parts, we propose that the tire tread material, which consists of polydimethylsiloxane (PDMS)-coated silver (Ag) textile, makes contact with the road. The tire cord, another part working as a triboelectric material made of woven nylon textile, not only helps the treads stay flat but also leads to contact/separation with the tire tread for generation via the triboelectric effect.[25] By integrating the two tire parts of tread and cord, we demonstrate that dual friction is achieved: i) friction between the road and the tire tread and ii) friction between the tread and the tire cord. We found that TC-TENG can achieve maximum performance when it makes contact in

1. Introduction

Beyond serving as means of transportation, automobiles have recently gained the ability to perceive, learn, and navigate a nearly infinite range of driving scenarios, allowing for smarter and safer driving.[1,2] They use a power source to run their onboard sensors and conduct all of the analyses to make deci-sions, which demands substantial computing power. In order to address this problem, a number of approaches to supplying power from the surrounding environment while driving have been reported.[3,4] A powering vehicle, such as one using photo-voltaic cells,[5–7] may provide endless energy and free pollution, but there are critical barriers, such as the requirement of a large surface area for mounting, cost for a replacement, and limited energy storage capacity.[8–10] In this regard, an alternative approach using a design that is adaptable to driving situations, practicality, and energy capacity, is crucial.

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dual friction mode. Based on this model, we not only experi-mentally study but also theoretically simulate how much power would be generated while driving a vehicle. The TC-TENG device, with dimensions of 5  ×  3 cm2, achieves ≈0.5  mW of output power through dual friction mode. We also demonstrate the working mechanism that explains the power generation in dual friction mode. This work may provide a new approach to efficiently harvesting mechanical energy by rolling tires with minimum load and space inside the tires.

2. Results and Discussion

In Figure 1a, we present a schematic description of the overall structure of tire and TC-TENG loaded inside the tire. Since tire balancing is one of the important design strategies for assuring the best performance from a vehicle, our device is located at the center of the tire.[26] TC-TENG, which is adaptable to tires, consists of two triboelectric layers serving as i) the tire tread and ii) the tire cord. The tire tread layer generally refers to the rubber on its circumference that makes contact with the road. Our tire tread, as one part of TC-TENG, is designed with PDMS-coated Ag textile (see details for fabrication in Figure S1, Supporting Information). The tire cord, a counter material of

TC-TENG, is made of stiff steel cord, which provides stiffness to the tread and protects the carcass by absorbing external shock. It is created by weaving nylon yarns into a plain structured tex-tile using a loom (see Figure  S2, Supporting Information, for description).[27] We note that both layers, i) tread and ii) cord, serve as part of TC-TENG where dual friction occurs as the tires rotate. When the vehicle is in motion, a rolling resistance, refer-ring to a force resisting the motion, occurs between the tire (particularly the tire tread) and the road (shown in Figure 2b), leading to a contact patch (shown in Figure 2c). This is the key to have a contact area corresponding to the weight, load of the vehicle, and roughness of the road surface. In order to experi-mentally characterize the output generation of TC-TENG, we prepare PDMS-coated Ag textile serving as the tire tread which is shown at a field emission-scanning electron microscope (FE-SEM) surface image in the inset of Figure 1d, woven nylon textile serving as the tire cord (in Figure 1e), and the TC-TENG device by integrating the prepared tire tread and thereby cord layers (in Figure 1f).

In order to elucidate the power generation via dual fric-tion as the tire rotates, we characterize the output voltage with devices in the absence/presence of the tire cord layer and cor-responding friction, as shown in Figure 2. Figure 2a shows that the TENG device only makes contact with the road (contact #1;

Figure 1. a) Schematic description of TC-TENG adaptable for structure of tires and a partial tire cross-section that shows how TC-TENG gets in contact with the road. b) Rolling resistance is caused by the deformation of the tire in the contact patch. c) This contact patch accounts for up to 95% of rolling resistance, which is inevitable but provides an aspect in which contact area is made in terms of triboelectrification. TC-TENG consists of d) tire tread designed with PDMS-coated Ag textile and e) a tire cord is prepared by weaving nylon yarns. f) Photo of TC-TENG showing its adapt-ability regarding its thin geometry.

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no tire cord layer), and further contact occurs in the pres-ence of the tire cord layer (contact #2), as shown in Figure 2b. All possible contacts (#1, 2) involving power generation are

described in Figure  2c. The output voltage was characterized by measuring the electric potential based on triboelectrification and electrostatic induction through contact/separation, where

Figure 2. Power generation via dual friction as tire rotates. Several types of TC-TENG models prepared a) in absence of tire cord layer, b) making mutual contact with both tire layers including tire tread and thereby cord layers, and c) under all possible contacts as well as resulting output voltage property of TC-TENG under d) a contact with the road, e) mutual contact opposite tread and cord materials, and f) dual friction.

Figure 3. Power generation mechanism of TC-TENG. a) The TC-TENG device remains electrostatically neutral in static state. As the tire rotates, the tire tread and cord materials are charged via triboelectrification, and following electrostatic induction, current flows in order to maintain the original equilibrium state. b–e) Schematic description showing the working principle of the TC-TENG with electron and current flow diagrams. f) Schematic of rolling tire with loaded TC-TENG.

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10  kgf of compressive force was applied using a mechanical force stimulator. We found that the device under dual friction (#1, 2) achieves the highest output voltage. Output voltages of ≈95, 416, and 472  V were respectively observed from #1, 2, and dual friction, as shown in Figure 1d–f. This can be mainly attributed to the significant difference in terms of triboelectric property between nylon and PDMS, which leads to the induc-tion of high electric potential. Note that TC-TENG generates power at such a minimum level of displacement, which indicates the applicability of TENG on tires.

We show the power generation mechanism of the TC-TENG under a rolling resistance and the inner friction in the tire, thus achieving the dual friction mode. It is well known that periodic contact and separation between two materials with opposite triboelectric polarities drive electrons back and forth through an external circuit due to the coupling of the triboelectrification and the electrostatic effect.[28] The charge transfer characteristic between PDMS and the road strongly depends on their relative positions in the triboelectric series. In the present device configuration, the device initially remained neutral prior to rotation (as shown in Figure  3a). When the tire began to rotate, the PDMS and the road came into contact, and the tire cord also made contact with the PDMS simultaneously. The triboelectric charges with oppo-site signs were then generated depending on the position in the triboelectric series. Electrons are accepted into the PDMS

layer, denoted as the tire tread layer, from the road and nylon (a part of the tire cord layer), resulting in the road and nylon becoming positively charged and the PDMS becoming nega-tively charged (shown in Figure  3b). When the road and the tire are separated from each other, electrons flow from the electrode in the tire tread to the tire cord electrode in order to maintain the original state of electrostatic equilibrium (see Figure 3d). The equilibrium stage is schematically illustrated in Figure  3e and no electric signal is observed at this stage. When the road and the tire come into contact by the following rotation of tires, the electrons accumulated on the upper side flow back to the electrode in the tire tread side, and corre-sponding current can be observed in the opposite direction (as shown in Figure  3c). Therefore, continuous contact/separa-tion as the tire rotates allows for the generation of a complete cycle of an alternating current electric signal (Figure 3f). It is worth noting that the output generation would be proportional to the number of TC-TENG mounted on the tires.

TC-TENG can perform at its maximum as long as it is mounted on the tires at any location where dual friction occurs. Figure  4 shows the output power of the multiple TC-TENGs on tires, and thereby the output voltage and current as a function of the speed of rolling tires, denoted as revolu-tions per minute (RPM) varying from 0 to 1000 RPM. Figure 4a schematically describes the four multiple TC-TENGs that are mounted 90° apart. As the tire rotates, the generated output

Figure 4. Characterization study on power generation of TC-TENG. a) Schematic diagram of the four multiple TC-TENG loaded on the tires. b) Output voltage measured from four multiple TC-TENGs integrated into one performing under 1000 RPM. c) Output voltage and current measured from one integrated multiple TC-TENGs at different rotation speeds ranging from 30 to 1000 RPM. d) Output voltage and current from multiple TC-TENGs as a function of different external loads. e) Output power derived by varying external loads.

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performance of TC-TENGs is approximately four times greater than that of a single TC-TENG mounted on the tire, as shown in Figure  4b. Regarding its applicability, we characterized the output voltage and current of the multiple TC-TENGs as a function of the speed of the rolling tires, denoted as RPM, as shown in Figure 4c. Our experimental results demonstrate that the output voltage and current are proportional to the speed of rolling tires up to 1000 RPM. This can be understood by con-sidering that the flow of electrons along an external circuit is proportional to the speed, also referred to as frequency.

Note that contact/separation is triggered often, even at high RPM. This might be primarily attributable to the fact that i) TC-TENG consists of elastic materials that are apt to be recov-ered to their original shapes and ii) it is a woven structure-based device that is likely to have a relevant gap between each layer to make contact/separation while the tires are rolling. The highest output voltage and current measured values from the four multiple TC-TENGs at 1000 RPM were about 225 V and 41 µA, respectively. We characterized the output power at external load resistance (See Figure  3d), with the constant rotation of tires where four multiple TC-TENGs were mounted at 1000  RPM. The maximum power of the device is estimated to be 0.5 mW at 10  MΩ. In order to further study and estimate the power generation property of the TC-TENGs mounted on the real size vehicle, we calculated the output power values using COMSOL Multiphysics (see Figure  S3, Supporting Informa-tion). Through a general modelling approach, a contact patch area of 240 × 260 mm2 is formed by a pressure corresponding

to a typical car weight of 1200 kg. In this regard, the estimated output voltage and current values would be 1.1  kV and 1.84 mA, respectively, as calculated by considering the multiple TC-TENGs with 50  ×  30  mm2 that are mounted on the real vehicle (see Figure S4, Supporting Information).

In order to demonstrate the applicability of the multiple TC-TENGs as a power source to drive the wireless sensor system throughout the vehicle, we prepared a scale-car, as shown in Figure 5a, and characterized the electrical property of the device as it drove. We found that the charging property improves as the tire rotates at higher RPM, which is consistent with our results (see Figure 5b). Our results demonstrate that the device can charge faster at a higher RPM. The capacitor was charged to 1 V powered by TC-TENGs at 500 RPM, and to 4 V at 1000 RPM for 25  s. In order to provide practical insights into using TC-TENGs for vehicles, we designed an electrical circuit for a self-powered wireless sensor system, as shown in Figure  5c. A rectification circuit was used to convert the alternate cur-rent signal into a direct current signal. We directly ran six light-emitting diodes (LEDs) connected in series powered by the TC-TENG. The rectified output power generated from the TC-TENG was sufficient to simultaneously activate all six of the LEDs for the head and tail lights (see Figure  5d). Finally, a wireless sensor was operated using a commercial capacitor (1330 µF) that was only charged by the TC-TENG, without the need for an external power source. Figure  5e shows that the wireless sensor was activated when the corresponding switch was on.

Figure 5. a) Demonstration of TC-TENG-powered applications designed for mounting on a 1:8 scale car. b) Charging property of TC-TENG at 500 and 1000 RPM. c) Charging-management circuit system. d) Six LEDs are simultaneously lighting directly by the power output generated from the TC-TENG. e) Operation of a wireless sensor system using a commercial capacitor (1330 µF) charged only by the TC-TENG without the need to use an external charging source.

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3. Conclusion

In summary, the tire-adaptable mechanical energy harvesting system reported here represents a technology that generates power with the rolling of tires. We have demonstrated that the approach using a dual friction mode as a working mecha-nism can be effective in TC-TENGs, particularly for rolling tires concerned with i) friction between the tire tread and the road and ii) friction between the tire tread and the tire cord, which leads to maximum performance. The required features of a tire tread material include adaptability to different rolling tire speeds, triboelectric properties, and mechanical durabili-ties, which is enabled by coating Ag textile with PDMS. The tire cord, which is woven nylon textile capable of staying flat, gen-erated power by making contact with the opposite triboelectric tire tread material, PDMS. An experimental study demonstrated the possibilities, with a focus on measuring output voltage and current properties based on dual friction mode as well as with varying numbers of devices and loaded rotation speeds of tires. Simulation could approximately calculate an essential aspect of power generation property on a vehicle-scale, resulting in esti-mated output voltage and current up to 1.1  kV and 1.84  mA, respectively. Our results showed that TC-TENGs can not only directly light LEDs but also charge a capacitor to operate a wire-less speed sensor without requiring an external power source. These findings establish the engineering textiles for the class of mechanical energy harvesting technology that serves as func-tional tire materials. In this sense, the system introduced here provides a framework for future energy harvesting technology that is a suitable alternative power source for developing smart vehicles.

4. Experimental SectionFabrication of the TC-TENG: The TC-TENG consists of a PDMS,

Ag-coated textile, and nylon/copper (Cu) yarns. The tread/steel cord was fabricated using the following simple method: First, the commercially available Ag-coated textiles used to fabricate the TC-TENG were prepared (see Figure S1, Supporting Information). Next, a Kapton type film was attached on one side of the Ag textile. The Ag textile was then fully dipped in PDMS and the dipped Ag textile was vacuumed for 1 h to remove pores. After vacuuming, it was annealed at 60  °C for 1  h and then naturally cooled to room temperature. Then the nylon/Cu yarn was finally woven into the plain weaving structure for the tire cord component.

Characterization and Measurement: FE-SEM (Jeol Ltd., JSM-7500F at the MEMS Sensor Platform Center of SKKU) measurements were carried out to investigate the surface morphology of the PDMS-coated Ag textile. In order to measure the triboelectrification property of the device, a pushing generator (Labworks, Inc., Model No. ET-126-4) was used to create cyclic mechanical contact/separation. A Tektronix DPO 3052 Digital Phosphor Oscilloscope and 6514 system electrometer were used to measure the electrical signal. As for electrical measurement of the device while tires rotate, a brush-type electrode designed to get output signals without interrupting tires’ rotation was utilized.

Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsW.S. and H.-J.Y. contributed equally to this work. This work was financially supported by the Basic Science Research Program (2018R1A2A1A19021947) and the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (2013M3A6A5073177) through the National Research Foundation (NRF) of Korea Grant funded by the Ministry of Science and ICT in addition to the Korea Basic Science Institute (KBSI) National Research Facilities and Equipment Center (NFEC) grant funded by the Korea government (Ministry of Education) (No. 2019R1A6C1010031).

Conflict of InterestThe authors declare no conflict of interest.

Keywordsdual friction, energy harvesting, nanogenerators, textile tire cord, triboelectricity

Received: March 15, 2020Revised: May 30, 2020

Published online:

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