Segmented wind energy harvester based on contact-electrification and as a self-powered flow rate sensor

Chemical Physics Letters 653 (2016) 96–100

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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett

Research paper

Segmented wind energy harvester based on contact-electrification and as a self-powered flow rate sensor Yuanjie Su ⇑, Guangzhong Xie, Fabiao Xie, Tao Xie, Qiuping Zhang, Hulin Zhang, Hongfei Du, Xiaosong Du, Yadong Jiang ⇑ State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Information, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, China

a r t i c l e

i n f o

Article history: Received 24 March 2016 Revised 22 April 2016 In final form 25 April 2016 Available online 26 April 2016 Keywords: Energy harvesting Contact electrification Wind energy Self-powered sensors

a b s t r a c t A single-electrode-based segmented triboelectric nanogenerator (S-TENG) was developed. By utilizing the wind-induced vibration of a fluorinated ethylene propylene (FEP) film between two copper electrodes, the S-TENG delivers an open-circuit voltage up to 36 V and a short-circuit current of 11.8 lA, which can simultaneously light up 20 LEDs and charge capacitors. Moreover, the S-TENG holds linearity between output current and flow rate, revealing its feasibility as a self-powered wind speed sensor. This work demonstrates potential applications of S-TENG in wind energy harvester, self-powered gas sensor, high altitude air navigation. Ó 2016 Elsevier B.V. All rights reserved.

1. Introduction Wind energy, as one of the most universal energy sources in nature, contains a gigantic reserve of renewable and green mechanical energy [1]. Making use of this sustainable energy carries immeasurable significance in large-scale electricity generation for public utilities and is considered as an alternative to fossil fuel. In general, the wind motions can be converted into electricity based on electromagnetic [2,3] and piezoelectric effect [4–10]. Given the significantly bulky volume and cost fabrication of electromagnetic generator and low efficiency of piezoelectric nanogenerator, a light weight, cost-effective, scalable, and simple structured harvester is desperately needed to scavenge a variety of wind motions. Recently, owing to the conjunction between triboelectrification and electrostatic induction, triboelectric nanogenerators (TENGs) have been demonstrated to harvest energy from a varieties of ambient mechanical motions, including human motion [11–14], vibration [15–18], rotating tire [19–21], sound wave [22,23], water wave and rain drops [24–26], which could be a new paradigm toward large scale energy. Through converting mechanical motion into electric signal, TENGs have been extensively utilized to successfully build up robust self-powered sensing systems with superior performance, including but not limited to tactile sensor [27], ⇑ Corresponding authors. E-mail addresses: [email protected] (Y. Su), [email protected] (Y. Jiang). http://dx.doi.org/10.1016/j.cplett.2016.04.080 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.

trajectory sensor [28,29], Mercury ion and ethanol sensors [30,31], and UV detector [32]. In fact, a majority of triboelectric effect based wind harvesters rely on single-electrode-mode [33– 35] and contact-separation mode [36–41]. Nevertheless, the inner counteraction dramatically limits output power and device performance of these two modes. Therefore, it is highly desired to develop a novel design of TENG that can effectively inhibit the inner counteraction. In this article, a newly designed thin film based segmented triboelectric nanogenerator (S-TENG) for scavenging wind energy was developed, which consists of copper (Cu) foils and a fluorinated ethylene propylene (FEP) thin film. Owing to the conjunction between triboelectrification and electrostatic induction, the prepared S-TENGs with a size of 10 cm  5 cm  2 cm can deliver an open-circuit voltage up to 36 V and a short-circuit current of 11.8 lA, corresponding to a maximum power output of 114.7 lW at external resistance of 300 MX, which is capable of powering tens of light emitting diodes (LEDs) instantaneously and efficiently charging capacitors. Furthermore, the prepared S-TENGs hold prominent linearity between output current and wind speed, unraveling the practicability as a self-powered sensor for detecting real-time wind speed. This work not only pushes forward a significant step toward the realization of large scale energy generation through triboelectrification but also provides a promising approach in self-powered air navigation, self-powered gas sensor, active wind vector sensor.

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Fig. 1. Single-electrode-based segmented triboelectric nanogenerator (S-TENG). (a) Schematic diagram of the fabricated S-TENG. (b) SEM image of the FEP surface with etched nanowire structure at the tilted view of 30°. (c) Working mechanism of the S-TENG for electricity generation process in a full cycle.

2. Experimental methods 2.1. Fabrication of the S-TENG The S-TENG is composed of a FEP thin film and two Cu foils as electrode. A 3 mm thick acrylic sheet was cut into rectangle with dimensions of 10 cm  5 cm by precision laser cutter. A pair of tailored Cu films with a thickness of 35 lm was attached on the surface of two acrylic sheets to form the electrodes. The FEP film with a thickness of 50 lm was fixed to an acrylic supporting beam that had a separation of 1.0 mm away from the underlying electrode. Segmented electrode is realized by cutting the electrode into parallel units with similar size. The electrodes were grounded by lead wires for electric measurement. Nanowires on the surface of FEP were produced by using inductively coupled plasma (ICP) reactive ion etching. The FEP film with a thickness of 50 lm was clean with isopropyl alcohol and deionized water, then blown dry with nitrogen gas. In the etching process, Au particles were deposited by using DC sputter on the FEP surface as a mask. Subsequently, a mixed gas including Ar, O2, and CF4 was introduced in the ICP chamber, with corresponding flow rate of 15.0, 10.0, and 30.0 sccm, respectively. The FEP film was etched for 15 s to obtain nanowire

structure on the surface. One power source of 400 W was used to yield a large density of plasma, while another 100 W was used to accelerate the plasma ions. Fe bulk was fixed on the top of the acrylic sheet to add the total weight of the device. 2.2. Characterization and electrical measurement of the S-TENG The morphology and nanostructure of etched FEP film were characterized by Hitachi SU8010 field emission scanning electron microscopy (SEM) operated at 5 kV. The output performance of S-TENG was measured using Stanford Research Systems. Keithley 6514 system electrometer and SR570 low noise current amplifier were used to record the output voltage and current, respectively. 3. Results and discussion The segmented configuration of the S-TENG is schematically illustrated in Fig. 1a. The copper (Cu) foil plays dual roles of electrode and contact surface. As a triboelectric layer, one end of a fluorinated ethylene propylene (FEP) film is fixed to the supporting beam, leaving the other end free-standing. The air flow vibrates the triboelectrically charged FEP film and periodically changes

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Fig. 2. Finite-element simulation results of the S-TENG by COMSOL. (a) Model of the fabricated S-TENG for the calculation. (b) Finite-element simulation of the potential distribution in S-TENG at different position. (c) Calculated electrical potential difference between bottom electrode and ground as a function of the position of FEP film. (d) Calculated charge quantity on the bottom Cu electrode as a function of the position of FEP film.

Fig. 3. (a) Output current of the S-TENG with the nonsegmented ones as control. Insert: Numbered segments of the device. (b) Charge curves of S-TENG with the nonsegmented ones as control. Insert: Diagram of the rectified circuit.

the distance between Cu electrodes and FEP film, resulting in a measurable output current through the external load. This segmented structure is able to effectively inhibit the inner counteraction resulting from the diverse electric potential of the oscillating film. Each segment has a pair of electrodes and can be regarded as a reduced-sized TENG that operates independently. To increase the effective contacting area and triboelectric charge density in the

friction process, FEP film was etched though inductively coupled plasma (ICP) reactive ion etching to create nanowire structure on the surface as shown in Fig. 1b, in which polymer nanowires are uniformly distributed with an average length of 620 nm and diameters ranging from 90 nm to 130 nm. Fig. 1c elucidates the working principle of the S-TENG. In the initial state, the surfaces of FEP and Cu are in contact with each

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Fig. 4. (a) Dependence of the output voltage and current on the external loading resistance. (b) Plot of the output power versus the loading resistance. (c) Dependence of output current on the wind flow rate. Insert: Output current as a function of wind flow rate. (d) Photograph of 20 LED bulbs triggered by S-TENG at a flow rate of 17.9 m/s.

other, as depicted in Fig. 1c-I. Due to the large difference in triboelectric series [42], FEP attracts and retains electrons from the Cu foil, leaving net negative charges on FEP surface and equal amount of positive charges on the Cu film. Once the wind flow separates the FEP membrane from the bottom Cu foil, the electric potential change induce the electrons to move from top electrode to ground and drive electrons to flow from the ground to the bottom electrode (Fig. 1c-II). When the FEP film moves upward to reach the top electrode, the negative triboelectric charges on the FEP film will be fully screened by the induced positive charges as well as the contact-induced triboelectric positive charges on the top electrode, leading to a minimum induced charge density on the bottom electrode, as shown in Fig. 1c-III. Subsequently, as the FEP film moves back from the top Cu electrode to the bottom Cu electrode, the reverse electric potential variation induces an electron flowing from the bottom electrode to ground and an opposite trend on the top electrode, as revealed in Fig. 1c-IV. Once the FEP film comes in contact with the bottom Cu electrode, the negative triboelectric charges on the two surface of the FEP film are fully screened by the induced positive charges on the bottom electrode as indicated in Fig. 1c-V. As the FEP film was lifted up again by the air flow, the negatively charged FEP will force the electrons to move from top electrode to ground and induce electrons to flow from the ground to the bottom electrode (Fig. 1c-VI) until the FEP membrane reaches the top electrode to complete a whole cycle (Fig. 1c-I). Consequently, while FEP film vibrates as a result of wind flow, the S-TENG acts as an electron pump that drives electrons flowing back and forth between electrodes and ground, producing alternating current in the external circuit. To quantitatively study electricity generation mechanism of STENG, electric potential distribution and charge transfer process for the device are evaluated through numerical calculation using COMSOL. The quantity of triboelectric charges on the surface of

the FEP film was assumed to be 18.6 nC with a model sizes of 10 cm  5 cm  2 cm, as sketched in Fig. 2a. Fig. 2b exhibits the simulation results of the electric potential distribution in the STENG when the triboelectrically charged FEP film is at five different positions of 8, 4, 0, 4, and 8 mm, respectively. The electric potential difference is increased linearly when the vibrating film approaches the top or bottom Cu electrode, as shown in Fig. 2c. The calculated induced charge quantity on the bottom Cu electrode decreases monotonously with increasing distance (Fig. 2d), which is in agreement with the aforementioned working principle in Fig. 1c. To study the effect of segmented configuration on the device performance, a comparison measurement among each individual unit, the integrated segmented device and the nonsegmented one was carried out. According to Fig. 3a, the overall output current of the segmented S-TENG has obviously larger amplitude than the nonsegmented one, indicating that the divided electrode structure effectively diminishes the counteracting effect. Due to the restricted fluctuation of the FEP film by the supporting beam, the first unit generates much lower output current than the other units. To characterize the energy storage property of the S-TENG, the charging curve of a 30 lF capacitor has been investigated under a constant wind flow remained at 17.9 m/s. In order to rectify the alternating pulses into DC pulses, the S-TENG is connected to capacitor (or LEDs) by using two full-wave rectifying bridges, as shown in the insert of Fig. 3b. As plotted in Fig. 3b, the S-TENGs possess a steeper slope of charging curve compared to the nonsegmented one, which verifies the higher efficiency of the segmented configuration in converting the wind energy into the stored electrical energy. To further explore the load matching characteristic of the STENG, the output voltage and output current of the device were measured under different external resistance from 10 X to

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600 MX, as illustrated in Fig. 4a. The output voltage grows with increasing loading resistance, while the output current follows the opposite trend, rendering a maximal output power of 114.7 lW at a loading resistance of 300 MX (Fig. 4b). The relationship between electric output of S-TENG and wind flow rate was also tested as revealed in Fig. 4c, where the output current is proportional to the wind flow rate. This is because that the increasing wind speed not only enhances triboelectric charge density on FEP membrane but also boosts the rate of charge transfer between electrodes and ground. The apparent linearity presented in the insert of Fig. 4c demonstrates the feasibility of S-TENG as a selfpowered flow rate sensor. Furthermore, under a wind flow rate of 17.9 m/s, the prepared S-TENG can function as a sustainable power source to simultaneously light up 20 LEDs as exhibited in Fig. 4d. These demonstrations verify the practicability of S-TENG in powering small electronics. 4. Conclusions

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In summary, we presented a novel designed segmented TENG for harvesting wind energy. By converting the wind motion into electrical output, the S-TENG can be utilized as a sustainable power source to trigger 20 LEDs and charge capacitor with a maximized output power of 114.7 lW. The fabricated S-TENG possesses excellent current-wind speed linearity that can be applied as a selfpowered flow rate sensor. In addition, considering the small volume and light weight of the thin film based configuration, the generators can be easily scaled up by integrating an array of devices in both horizontal and vertical dimension to efficiently realize large scale power generation. This work also pushes forward a significant step toward the application of triboelectric sensor in wind energy harvesting, self-powered gas sensor, gas flow monitoring, high attitude navigation.

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Acknowledgments [33]

This work is partially supported by the Funds for Creative Research Groups of China (No. 61421002) and the National Natural Science Foundation of China (Grant No. 61571097). References

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