Research Article pubs.acs.org/journal/ascecg
Triboelectric Nanogenerators Based on Fluorinated Wasted Rubber Powder for Self-Powering Application Xiaohu Ren, Huiqing Fan,* Jiangwei Ma, Chao Wang, Yuwei Zhao, and Shenhui Lei State Key Laboratory of Solidification Processing, School of Materials Science and Engineering, Northwestern Polytechnical University, , No. 127 Youyixi Road, Beilin District, Xi’an 710072, China
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S Supporting Information *
ABSTRACT: We report a new high output triboelectric nanogenerator (TENG) based on the contact-separation mode using surface fluorinated wasted rubber powder (WRP) as a source material. The WRP-TENG’s configuration is designed by serving WRP as negatively charged friction electrode and water assisted oxidized (WAO) Al film as positively charged friction electrode. The open-circuit voltage (Voc) and short-circuit current density (Jsc) of the WRP based TENGs increase with decrease in the particle size of WRP. More importantly, after surface fluorination of WRP with modifier of trichloro (1H,1H,2H,2H-perfluorooctyl) silane (FOTS), the maximum Voc and Jsc of the WRP based TENG is further increased to 265 V and 75 mA/m2, respectively. The FWRP based TENG can drive 100 commercial LEDs directly. In addition, a self-powered hygrothermograph is designed, showing great potential application in daily life. The advantages such as simple fabrication process, low cost, and stability of this TENG make it a promising design for an energy harvesting device or for self-powered electronics. Furthermore, it presents a significant opportunity for the extension of waste utilization. KEYWORDS: Wasted rubber powder, Triboelectric nanogenerator, Surface fluorination, Energy harvesting, Solid waste reclamation
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INTRODUCTION Harvesting energy from sustainable, renewable, and eco-friendly resources has always been a hot issue and a viable route to alleviate the energy crisis. Among various renewable energy resources in the living environment of humans, the mechanical energy resource is a highly reliable and cost-effective energy resource that may originate from human motion such as vibration, rotation, and displacement, etc., and which is scarcely influenced by external conditions.1−4 In addition to the twocenturies-old technology of power generation through electromagnetic induction, novel mechanisms such as the piezoelectric5−7 and triboelectric8,9 effects have been under investigation to convert mechanical energy into electrical energy. Triboelectric nanogenerators (TENGs) can harvest energy with a coupling interaction of electrostatic induction and triboelectric effect, where electric power is generated from the charge transference between two materials with different triboelectric polarities when they are in contact and separate from each other.10,11 Recently, the TENGs are especially attractive owing to their high output, reliable robustness, environmental friendliness, and abundant friction energy sources.12,13 Since the first TENG was reported in 2012,14 many efforts have been made to optimize ambient energy harvesting styles, maximize the output power density, and realize the fabrication of self-powered devices.15 Generally, the most essential strategy for improving the performance of TENG is increasing surface charge density induced on the surfaces of triboelectric materials. Also, the charge © 2017 American Chemical Society
density induced is determined by the surface roughness degree and chemical component of triboelectric surfaces.16 Although almost all materials exhibit triboelectricity, maximum output can be obtained by finding the appropriate paired materials. From the triboelectric series,17 there is guidance that can qualitatively indicate the capability of a material to gain or lose electrons. However, there are more aspects that need to be studied such as choice of materials,18 surface structure configurations and patterning,19 surface functionalization,20 and so on. The used and wasted tires are tires that have completed their functional life and cannot be used on automobiles. According to the statistics, about 10 million tires are discarded worldwide every year.21 To date, they were usually buried with other industrial waste in landfill sites or added to dumps. Discarding waste tires rubber is a huge waste of resources and causes environmental problems because the rubber will not degrade.22 Therefore, how to deal with the large mass of wasted tires rubber has became a focus in the world. According to the triboelectric series, natural rubber contains negatively charged ions to absorb positive ions.17 Thus, fabricating TENGs utilizing wasted tires rubber as triboelectric materials is feasible and worth studying. Moreover, high friction properties, excellent fatigue properties, Received: November 14, 2016 Revised: January 1, 2017 Published: January 7, 2017 1957
DOI: 10.1021/acssuschemeng.6b02756 ACS Sustainable Chem. Eng. 2017, 5, 1957−1964
Research Article
ACS Sustainable Chemistry & Engineering
Figure 1. (a) FESEM image of waste rubber powder. The inset is the photograph of the WRP. (b) The topographic image of the surface of a single waste rubber particle.
Figure 2. (a) Schematic showing the structural design of the TENG based WRP. (b) SEM image of the surface of WRP/PDMS film. (c) The SEM image of the WAO aluminum foil. (d) A photograph of the TENG based on WRP. rubber particles sizes ranges from 50 to 200 μm. Polydimethylsiloxane (PDMS, Sylgard 184) and trichloro (1H,1H,2H,2H-perfluorooctyl) silane (FOTS, ≥97%) were purchased from Dow Corning and SigmaAldrich GmbH, respectively. Material Preparation. The waste rubber powders (WRPs) were washed by deionized water and acetone to remove the contaminations on their surface. The cleaned WRPs were sieved through the 40, 80, and 200 mesh screens to obtain the WRP with different particle size distributions, and are termed WRP-4, WRP-8, and WRP-20, respectively. To modify the WRPs with FOTS, the WRPs were first oxidized by aqueous hydrogen peroxide (H2O2) solution. The WRPs were placed in a glass reactor containing 10% H2O2 solution with continuous stirring at 80 °C for 3 h. Afterward, 1 g of WRP were put in a sealed vessel by dropping a few FOTS drops and then heating at 95 °C for 1 h. An Al film with micro/nanostructure acting as a positively charged friction electrode was prepared using a water assisted oxidation (WAO) process.23 Fabrication of the TENGs. To fabricate the WRP-TENGs, a piece of WAO Al film (4 cm × 4 cm) served as the matched friction and electrode material with a positively charged electrode, which was fixed on an acrylic glass substrate. To prepare the negatively charged electrode, a layer of PDMS was spin-coated onto another Al foil and precured 70 °C for 3 min. Then, the sieved WRPs were transferred onto the surface of the semicured PDMS film. After fully hardening, the WRP was tightly pasted on the Al foil through the PDMS film, and then the WRP/PDMS/Al was fixed on another acrylic glass to act as the negatively charged electrode. Subsequently, the two substrates were connected together through four springs, and a space of 5 mm was left. Finally, two Cu wires were connected to the two Al foils for subsequent
and chemical stability for rubber make it a valuable material for durably large-scale TENGs. Herein, we are the first to report triboelectric nanogenerator using wasted rubber powder (WRP) as negatively charged triboelectric material. The WRP with particle size range 50−200 μm is obtained by crumbing and grinding discarded rubber tires, having bump-like granules distributed over surface structures that are beneficial for triboelectric power generation due to a high surface-to-volume ratio. To improve the output performance of the TENG, the trichloro (1H,1H,2H,2H-perfluorooctyl) silane (FOTS) species were grafted on the surface of WRP using a facile dip-coating method. The TENG fabricated using surface fluorinated WRP (FWRP) exhibited a maximum open-circuit voltage of 265 V and short-circuit current density of 75 mA/m2 when the two electrodes are periodically put in contact and separated. It had about a 4 times higher output power density in comparison to that of the TENG with nonmodified WRP. Besides, the TENG based on FWRP demonstrated excellent stability over a 5000-cycle durability test. The TENGs based on FWRP introduced here are simple and cost-efficient, which can be potentially applied to the various energy harvesting and selfpowered devices.
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EXPERIMENTAL SECTION
Materials. The reused waste rubber powders (WRP, ZN100-A1) were obtained from Sichuan Zhongneng Rubber Co., Ltd., and the 1958
DOI: 10.1021/acssuschemeng.6b02756 ACS Sustainable Chem. Eng. 2017, 5, 1957−1964
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1b; many granules of around 1 μm are randomly distributed throughout the surface, which increases the roughness of WRPs. In general, large roughness can raise the surface area of triboelectric materials and thus enable more charge carrying sites. As a result, the number of charge carriers can be increased by enhancing the surface roughness to improve performance of TENG.24 So, the rough surfaces of WRP make it potentially useful as a triboelectric material. The schematic diagram of the WRP-TENG is depicted in Figure 2a; it has a layered structure with two acrylic substrates for support. On the lower substrate, a layer of Al foil coated with a PDMS film was adhered as the electrode, and a layer of WRP was stuck on the PDMS as a negatively charged triboelectric material. An SEM image of the surface of WRP/PDMS film is displayed in Figure 2b, which indicates a uniform distribution of the rubber bumps with average size of around 1 μm on the film surface. On the upper substrate, a layer of WAO Al film designed as contact electrode was attached which plays dual roles of electrode and contact surface. Figure 2c shows an SEM image of the WAO Al film surface; the submicron pores with a diameter range from 100 to 300 nm were randomly distributed on the aluminum surface. The detailed fabrication process of TENG is presented in the Experimental Section, and the as-fabricated TENG is exhibited in Figure 2d. Figure 3 schematically illustrates the basic operating mechanism of the WRP-Al based TENG with contact-separation mode under the vertical compressive force. In general, the generation of electric charge of TENG is ascribed to the result of the cooperative effect between contact charging and electrostatic induction during the contact or friction process.25,26 While mechanical contact between two materials with different triboelectric polarities occurs, opposite charges generate at their surfaces, respectively. When they are separated, the flowing current generates through an external circuit due to electrostatic potential difference between the two materials. In the initial state as shown in Figure 3a, two electrodes are separated with a
electric measurement. Similarly, the FOTS modified waste rubber particles (FWRP) were attached on the PDMS film to fabricate the FWRP-TENG. Characterization. Surface morphologies of the waste rubber particles were investigated by field emission scanning electron microscopy (FE-SEM, FEI NANOSEM 450). Chemical compositions of WRP and FWRP were analyzed using X-ray photoelectron spectroscopy (XPS, Thermo Scientific, VG ESCALAB 220i-XL) with an Al Kα source (E = 1486.6 eV). The output voltage and current of the TENGs were measured using a source measurement unit (Keithley, 2410 SMU), and the data was recorded through a collecting program based on LabVIEW.
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RESULTS AND DISCUSSION The FE-SEM image of waste rubber particles is depicted in Figure 1a, revealing polydisperse amorphous particle shapes of
Figure 3. Working mechanism of WRP based TENG: (a) The two surfaces are separated in the original state. (b) The triboelectric charges are generated on the surfaces of WRP and Al foil when they are in contact with each other. (c) The electrons flow from top electrode to bottom electrode to keep electrical equilibrium in the releasing process. (d) No electron flows as the surfaces are completely separated and reaching an electrical equilibrium. (e) Pressing the surfaces into contact again, the electrons flow from top electrode to bottom electrode.
50−200 μm. The inset image of Figure 1a shows the photograph of WRP. The surface morphology of the WRP is shown in Figure
Figure 4. (a) Open-circuit voltage and (b) short-circuit current density of TENG based on WRP-20. (c) Output voltages and power density of the TENG based on WRP-20 under variable external load resistances. (d) Open-circuit voltage and short-circuit current of the TENG versus particle size of WRP. 1959
DOI: 10.1021/acssuschemeng.6b02756 ACS Sustainable Chem. Eng. 2017, 5, 1957−1964
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Figure 5. Stimulated results showing triboelectric potential differences of TENGs based on WRP with different particle sizes.
Figure 6. (a) Schematic representation of surface fluorinated modification of WRP by FOTS. (b) XPS survey spectrum of WRP before and after surface fluorination. Core level spectrum of C 1s in (c) WRP and (d) FWRP.
were connected as external loads. The effective power and maximum output voltage of the WRP-TENG made with WRP20 for a range of external load resistances at a cyclic triggering are shown in Figure 4c. The instantaneous output voltage across the load generally rises with an increase of the resistance, which becomes almost saturated to the open-circuit voltage as the resistors increase infinitely. Consequently, an instantaneous peak power of 68 mW/m2 (P = UI)28 was achieved across the 200 MΩ load resistance. Furthermore, to investigate the electrical performance of the WRP-TENG with different particle sizes of WRP, we fabricated a variety of WRP-TENGs with various size distributions of WRP (WRP-4, WRP-8, and WRP-20). Figure 4d shows the effects of particle sizes of WRP on the output performance of the WRPTENG. It is observed that both the open-circuit voltage and short-circuit current of these TENGs were enhanced as particle sizes of WRP decrease due to an increase of the interface roughness of the friction surfaces. Figure 4d indicates that the output voltage and current density of TENG with WRP-4 were decreased to 78 V and 3 mA/m2, respectively, when compared with that of the corresponding values for TENG with WRP-20. In addition, we investigated the effect of particle size distribution of WRP on the triboelectric potential of a TENG device through theoretical calculations using COMSOL multiphysics software.
distance; no charge transfer occurred, and thus no electric potential. When the WRP/PDMS film and Al electrode are instantaneously in contact with each other, the negative charges generate on the surface of WRP, and the positive charges generate on Al foil (Figure 3b).27 The negative charges on the surface of WRP induce positive charges on the top electrode due to the electrostatic induction effect during separation of the two surfaces in contact, and thus electrons on the top electrode move to the bottom electrode through an external circuit, resulting in the formation of a current flow (Figure 3c). Once a new equilibrium state is established, no current flows are generated (Figure 3d). Similarly, a reverse current can be detected through an external circuit during the pressing process (Figure 3e). Thus, during the periodic contact-separation process, the triboelectric charges on the WRP surface induce a periodic movement of the free electrons between the top and bottom electrodes to generate current flows in an external circuit. The electrical outputs of the WRP-TENG through the contact electrification between the WRP and Al electrode were measured during the periodic vertical pressing and releasing process. As shown in Figure 4a,b, the open-circuit voltage and short-circuit current density of the WRP-TENG made with WRP-20 are around 100 V and 5.5 mA/m2, respectively. To investigate the effective electric power of the WRP-TENG, different resistors 1960
DOI: 10.1021/acssuschemeng.6b02756 ACS Sustainable Chem. Eng. 2017, 5, 1957−1964
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ACS Sustainable Chemistry & Engineering
Figure 7. (a) Open-circuit voltage and (b) short-circuit current density of TENG based on FWRP. (c) Output voltages and power density of the TENG based on FWRP under variable external load resistances. (d) Instantaneous lighting of 100 commercial LEDs by the TENG driven with a hand. (e) Charging curves of the 10 μF capacitor by a rectifying bridge circuit from WRP-TENG and FWRP-TENG. (f) The test of stability and durability for the FWRP-TENG.
Table 1. Comparison of the Characteristics between the FWRP-TENG and Other TENGs with Contact-Separation Mode Previously Reported friction materials negatively
positively
Voc (V)
Isc (mA/m2)
power density (W/m2)
complexity
fabrication cost
ref
FWRP Kapton embossed PDMS PDMS graphite-PDMS PTFE PTFE PTFE PVDF fibers polyurethane
Al newspaper Al Au NPs Al Au rice-husk SiO2 Al Al
265 13 207 80 286 60 270 130 160 100
75 0.7 141 364 67 21 5.7 2.8 10.6 30
4.6 0.0025 5.42 11.5 3.7 1.75 0.84 0.12 1.6 1.6
simple simple complex complex simple complex simple complex complex complex
low low normal high normal high low normal normal normal
this work 4 34 40 41 42 43 44 45 46
According to theoretical studies,29 the open-circuit voltage generated is defined by the following equation: V=
σx(t ) ε0
between the electrodes at the instant time t. This is followed to calculate the triboelectric potential; a triboelectric charge density of tribocharged surfaces was assigned with 1 μC/m2. As shown in Figure 5, the triboelectric potential of the WRP/PDMS film obviously increased when the particle size of WRP was decreased, implying the generation of more charges on the surface of the WRP/PDMS film.
(1)
Here, V is the difference of potential between the electrodes, ε the permittivity, σ the surface charge density, and x(t) the gap 1961
DOI: 10.1021/acssuschemeng.6b02756 ACS Sustainable Chem. Eng. 2017, 5, 1957−1964
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ACS Sustainable Chemistry & Engineering
Figure 8. Applications of the harvested electrical energy. (a) The circuit schematic of capacitor charging and discharging to drive a hygrothermograph. (b) Commercial hygrothermograph driven by a 100 μF capacitor that is charged by the FWRP-TENG.
performance. The output power from the TENG can be used to instantaneously light 100 commercial blue LEDs in series as shown in Figure 7d and Supporting Information Video 1. The energy generated from the TENG can be stored in a capacitor through a rectifying bridge circuit. To further illustrate the effect of surface modification on output performance, we utilized the TENG based on modified and unmodified WRP, respectively, to charge a capacitor of 10 μF, and the charging curves of the TENGs are shown in Figure 7e. The result shows that the conventional FWRP-TENG takes a much shorter time than WRP-TENG for charging the capacitor to 15 V, indicating the more efficient energy harvesting behavior of FWRP-TENG than WRP-TENG. Further, a test over 5000 cycles was carried out to determine the stability and durability of the FWRP-TENG, as shown in Figure 7f. The steady output voltage indicates exceptionally reliable and durable energy harvesting capability of FWRP-TENG. To evaluate improvement of this new design, a comparison with other reported TENGs with contact-separation mode is shown in Table 1. The evaluation criteria are opencircuit voltage (Voc), short-circuit current (Isc), power density, fabrication complexity, and cost. The FWRP-TENG shows the strong competitiveness for every evaluation criterion. Also, in fact, this method is simple and cost-efficient and can be promising for large-scale applications. Furthermore, it brings forward a new direction for recycling and reuse of the solid waste. In many instances, the ac pulses from the nanogenerators cannot be used to drive some electronic devices and systems directly due to their requirement for a constant dc current. So, the output power generated from the nanogenerator needs to be managed with a rectifier and energy storage unit such as capacitors, lithium ion batteries, etc. The electrical circuit of a 100 μF capacitor as energy storage unit to collect power form the TENG is shown in Figure 8a. The capacitor will be continuously charged by the TENG through a rectifying circuit when the switch is connected to point 1. As enough charges were stored, the capacitor can be disconnected from the TENG and power the electronic devices by switching 1 to 2. As shown in Figure 8b and Supporting Information Video 2, we demonstrated the powering of a hygrothermograph with charges stored in capacitor from the FWRP-TENG. As a feasible scenario, a self-powered electronic device by usual human motion could be realized with further optimization. In conclusion, the TENG based on the contactseparation mode using surface fluorinated wasted rubber powder as a source material shows outstanding performance. Because of the simple fabricating process, low cost, and stability, it is a promising design for an energy harvesting device or for selfpowered electronics. In addition, it presents a significant opportunity for the extension of waste utilization.
The enhancement of performance of TENGs has been effectively achieved by optimizing rational structural designs30−32 or roughing surface of friction materials.33−35 However, the performance of TENGs is also greatly influenced by the surface composition of the friction materials. In the case of materials, it has been proven that fluoride-rich materials are the most suitable for negative friction materials of TENG due to the strong electron affinity of the fluorine atom.36,37 Therefore, surface fluorination of friction materials can be seen as an effective approach to improve the performance of TENGs. Here, we have used FOTS as surface modifier to fluoridize the surface of WRP by grafting F-containing units to enhance the output capability of WRP-TENG. Figure 6a illustrates the schematic diagram of the fabrication process for fluorinated WRP. First, the WRPs were soaked in H2O2 solution for surface oxidation, by which part of C−C bonds were broken and a large number of active groups such as hydroxyl were generated on the surface of the WRP.38 Then, a stable chemical bond (Si−O) formed between WRP and the FOTS molecule as the hydroxyl groups (OH) on the surface of WRP reacted with head groups (Cl) of FOTS molecules; consequently, a layer of fluoride-rich molecules is coated on the surface of WRP. The surface composition of WRP before and after surface modification was analyzed by XPS. Figure 6b shows the full-scan XPS spectra of modified and unmodified WRP. As expected, the FOTS modified WRP sample shows the Si 2s and Si 2p peaks as well as an obviously strong F 1s peak, while there is no F 1s signal in the spectrum of the unmodified WRP, indicating fluoride on the surface of WRP via the chemical bond of FOTS. Figure 6c shows that the core level spectrum of C 1s for the WRP reveals only a peak at 284.6 eV corresponding to a C−C bond. For the WRP (Figure 6d), the C 1s core level spectrum can be deconvoluted into four components peaks at 284.5, 287.5, 290.4, and 292.9 eV corresponding to C−C, CH2−CH2, CH2−CF2, and CF2−CF2 bonds, respectively.39 Figure 7a,b shows that an open-circuit voltage and short-circuit current density of the TENG based on FWRP reach 265 V and 75 mA/m2, respectively. Figure 7c further exhibits the output voltage and power of the FWRP-TENG as a function of various loading resistance, and these are similar to the behavior shown in Figure 4c. The output power density under the load starts to dramatically increase above 1 MΩ and reaches the maximum of 4.6 W/m2 under a load resistance of 200 MΩ. In comparison, the fabricated TENG based on FWRP harvested more mechanical energy than WRP-TENG. The remarkable enhancement of output performance can be attributed to the observation that the fluorosilane grafted on WRP is more negative than the surface of the original WRP in the triboelectric series, which causes higher triboelectric potential difference between two electrodes of TENG and better output 1962
DOI: 10.1021/acssuschemeng.6b02756 ACS Sustainable Chem. Eng. 2017, 5, 1957−1964
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ACS Sustainable Chemistry & Engineering
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CONCLUSION A new high output TENG based on the contact-separation mode with surface fluorinated WRP as a negatively charged friction electrode was designed and fabricated. The open-circuit voltage and short-circuit current density of the WRP based TENGs increase with decrease of particle size of WRP. Moreover, after surface fluorination of WRP with FOTS modifier, the maximum open-circuit voltage and short-circuit current density of the WRP based TENG are further increased to 265 V and 75 mA/m2, respectively. The FWRP based TENG can drive 100 commercial LEDs directly without any energy storing device. In addition, a self-powered hygrothermograph is designed, showing great potential application in daily life. The advantages such as simple fabricating process, low cost, and stability of this TENG make it a promising design for sustainable energy harvesting device or selfpowered electronics. Furthermore, it presents a significant opportunity for the extension of waste utilization.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.6b02756. LED light-up demonstration (AVI) Self-powered hygrothermograph demonstration (AVI)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Fax: +86-29-8849 2642. Phone: +86-29-8849 4463. ORCID
Xiaohu Ren: 0000-0001-6199-0106 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation (51672220), the 111 Program (B08040) of MOE, the National Defense Science Foundation (32102060303), and the Fundamental Research Funds for the Central Universities (3102014JGY01004) of China.
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REFERENCES
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DOI: 10.1021/acssuschemeng.6b02756 ACS Sustainable Chem. Eng. 2017, 5, 1957−1964
Research Article
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DOI: 10.1021/acssuschemeng.6b02756 ACS Sustainable Chem. Eng. 2017, 5, 1957−1964
