Triboelectric Nanogenerators For Blue Energy Harvesting

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Triboelectric Nanogenerators for Blue Energy Harvesting Usman Khan† and Sang-Woo Kim*,†,‡ † School of Advanced Materials Science and Engineering and ‡SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, Republic of Korea ABSTRACT: Blue energy in the form of ocean waves offers an enormous energy resource. However, it has yet to be fully exploited in order to make it available for the use of mankind. Blue energy harvesting is a challenging task as the kinetic energy from ocean waves is irregular in amplitude and is at low frequencies. Though electromagnetic generators (EMGs) are well-known for harvesting mechanical kinetic energies, they have a crucial limitation for blue energy conversion. Indeed, the output voltage of EMGs can be impractically low at the low frequencies of ocean waves. In contrast, triboelectric nanogenerators (TENGs) are highly suitable for blue energy harvesting as they can effectively harvest mechanical energies from low frequencies (<1 Hz) to relatively high frequencies (∼kHz) and are also low-cost, lightweight, and easy to fabricate. Several important steps have been taken by Wang’s group to develop TENG technology for blue energy harvesting. In this Perspective, we describe some of the recent progress and also address concerns related to durable packaging of TENGs in consideration of harsh marine environments and power management for an efficient power transfer and distribution for commercial applications. H uman society is heavily reliant on fossil fuels, such as coal, oil, and natural gas, to meet its energy requirements. However, the carbon emissions resulting from the utilization of such fuels is causing problems such as climate change, air pollution, and acid rain.1,2 Indeed, these serious problems in the long term may potentially put human civilization in jeopardy.2,3 Therefore, mankind has to exploit clean and renewable energy resources such as sunlight, wind, ocean energies, etc. for its sustainable development.4 Among the renewable energy resources, ocean energies are underexplored,1 even though water covers 70% of the face of the earth and has an abundance of kinetic energy in the form of waves. Ocean energy is also unique in that it is not strongly dependent on weather, time of day, or temperature.2 Therefore, blue energy from the oceans has the potential to contribute significantly to the world energy requirements for powering our homes, businesses, and daily lives. In this issue of ACS Nano, Wen et al. note that although ocean energy is potentially a great alternative to fossil fuels, there is as yet insignificant development in the technology to exploit it fully.5 Electromagnetic Generators (EMGs) for Blue Energy Harvesting. Blue energy in the form of water waves is a mechanical kinetic energy.1 Its amplitude is random, and frequency can be as low as <2 Hz.5 However, EMGs are wellknown for converting mechanical kinetic energies from water flow into electricity and, therefore, are candidates for harvesting © 2016 American Chemical Society In this issue of ACS Nano, Wen et al. note that although ocean energy is potentially a great alternative to fossil fuels, there is as yet insignificant development in the technology to exploit it fully. blue energy.6 Electromagnetic generators typically require an additional turbine to convert water flow into rotational energy; the rotational energy is then utilized to move magnets around the coils of an EMG in order for the electromagnetic induction to occur.7 As a consequence, EMGs are complex, have high mass density, and have large volumes.1,7 Therefore, it is challenging for EMGs to float on the ocean surface as they are bulky and heavy and may require buoy platforms for floating.1 In order to harvest ocean wave energy most effectively, however, the generator would need to float on the surface as most of the wave energies are present on the ocean’s surface.1 Electromagnetic generators may also have serious limitations in making networks of generators for energy harvesting over large areas due to their bulkiness. Moreover, due to the high-quality materials required for EMGs, the cost is also expected to be high.1,8 Published: July 13, 2016 6429 DOI: 10.1021/acsnano.6b04213 ACS Nano 2016, 10, 6429−6432 Perspective www.acsnano.org ACS Nano Perspective Another challenge with using EMGs for blue energy harvesting is that the irregular motion modes and low frequencies of the ocean waves comprise a major limitation.2,6 Indeed, EMGs are naturally suited for harvesting flow stream rather than ocean waves, which is also likely the reason that conventional alternating current (ac) generators operate in frequencies from 50 to 60 Hz.1,6 According to Faraday’s law of electromagnetic induction, the output voltage in an EMG is proportional to the rate of change of flux through the coils. If ϕ is the magnetic flux and N is the number of turns in the coils, then the output voltage of an EMG can be expressed as6 VEMG = −N dϕ dt Generators for blue energy harvesting need to be lightweight, low cost, simple, easy to scale up, and suitable for low-frequency kinetic energies. reports on the subject. Zhu et al. demonstrated a liquid−solid electrification-enabled generator (LSEG) for harvesting water wave energy.7 It consists of a fluorinated ethylene propylene (FEP) friction layer with copper-based discrete electrodes at its back (see Figure 1a). The FEP layer engages in contact (1) According to eq 1, the output voltage of EMGs is strongly dependent on the rate of change of the magnetic flux and, therefore, on the frequency of the input mechanical energy. Since the frequency of ocean waves can be low (<2 Hz),5 the expected low output voltages of EMGs (e.g., 2 V at 3 Hz in ref 6) are not practical. For instance, the low voltage may have limitations in overcoming the losses during rectification and regulation for powering electronic devices and/or charging batteries.6 Due to the above-mentioned problems, generators for blue energy harvesting need to be lightweight, low cost, simple, easy to scale up, and suitable for low-frequency kinetic energies. Triboelectric Nanogenerators (TENGs) for Blue Energy Harvesting. Recently, TENGs have been introduced for harvesting mechanical energy present in the environment.9,10 Triboelectric nanogenerators are based on contact electrification between two materials and charge transfer between their electrodes due to electrostatic induction,9,10 and they typically utilize a polymer−metal pair as the friction layers.11,12 Therefore, they are low cost, lightweight, easy to fabricate, and offer an abundant choice of materials.4 Moreover, TENGs can have energy conversion efficiencies as high as 55%13 and can adapt well to various mechanical energy types by different modes of operations such as contact-separation mode, sliding mode, single-electrode mode, and freestanding mode.14,15 Remarkably, TENGs can harvest energy over a broad frequency range, including vibration, human walking, body motions, and ocean waves.5 Indeed, the output voltage of TENGs, VTENG, under open-circuit conditions is given as6 VTENG = Figure 1. Structure of the liquid−solid electrification-enabled generator (LSEG). (a) Schematic description of the electrification layer with two electrodes on the back. (b) Scanning electron microscopy image of the polymer nanowires on the electrification layer. The scale bar is 1 μm. (c) Schematic description of a substratesupported LSEG in water waves. The emerging and submerging of the LSEG in water waves produces electricity between the electrodes. Reprinted from ref 7. Copyright 2014 American Chemical Society. electrification with the water waves. Vertically aligned nanowires are realized on the FEP (see Figure 1b) using plasma etching in order to make the FEP film hydrophobic and also to enhance the contact area and thus the output power. The LSEG is attached onto a substrate (Figure 1c). For the mechanism of function, contact electrification with water waves results in negative triboelectric charges on the surface of the FEP layer. During submerging and surfacing of the LSEG due to traveling water waves, current flows between the electrodes in order to screen the triboelectric charges on the FEP surface, thereby producing an electric power. The LSEG with a size of 6 cm × 3 cm has produced an average output power of 0.12 mW at a wave velocity of 0.5 m/s.7 The LSEG structure is an all-in-one design type as it does not require additional components such as turbines for receiving the mechanical energy. Moreover, due to the materials involved, the LSEG is low cost and can easily be scaled up. Several LSEGs can be interconnected to form a network for harvesting energy from water waves on a large scale. The original idea of using TENGs for blue energy was proposed by Wang. He suggested a network of TENGs for large-scale harvesting of ocean wave energy.1 The basic unit of the network consists of a box structure with walls composed of arch-shaped TENGs with a metal ball enclosed inside each, as shown in Figure 2a. The mechanical energy from water waves causes the metallic balls to collide with the arch-shaped TENGs, thereby producing power. The arch-shaped TENG is shown schematically in Figure 2c. It is composed of two metaldielectric friction pairs. The top and bottom dielectric layers are made of polyethylene terephthalate (PTFE) with copper as the back electrodes. Aluminum films with nanoporous surfaces Q sc C(x) (2) where Qsc is the short-circuit charge transfer amount and C(x) is the capacitance between the two electrodes at various displacements x. Therefore, for any TENG with displacement varying from 0 to xmax, the peak value of the output voltage is independent of the frequency of the mechanical input.6 Therefore, TENGs have no limitations for harvesting lowfrequency mechanical energies such ocean waves. Moreover, the typical output voltage of TENGs is quite high (∼100 V) and can overcome drops during rectification and regulation for charging batteries.6 In summary, because TENGs are lightweight, low cost, easy to fabricate, and have the ability to harvest lowfrequency kinetic energies effectively, they offer an effective technology for blue energy harvesting. Triboelectric Nanogenerator Technology Development for Blue Energy Harvesting. Due to the suitability of TENGs for potentially rich blue energy, various important steps have recently been made.1−7,16 Here, we outline two important 6430 DOI: 10.1021/acsnano.6b04213 ACS Nano 2016, 10, 6429−6432 ACS Nano Perspective Figure 2. Triboelectric nanogenerator (TENG) network for blue energy harvesting. (a) Photograph of a single unit of the TENG network. The scale bar is 5 cm. (b) Schematic description of a network of TENGs. (c) Schematic description and (d) photograph of an arch-shaped TENG, which forms the walls of a single unit shown in panel a. Scanning electron microscope images of (e) PTFE nanowires and (f) nanopores on an aluminum electrode. Adapted from ref 1. Copyright 2015 American Chemical Society. easy fabrication, and ability to harvest mechanical energy even over low frequencies, TENGs offer an effective method to harvest the energy from ocean waves. However, there are several critical challenges ahead for blue energy harvesting using TENGs. (see Figure 2f) supported on top and bottom of an acrylic sheet act as the complementary friction layers to the PTFE layers (see Figure 2c). A photograph of an as-fabricated arch-shaped TENG is shown in Figure 2d. In addition, PTFE nanowires (see Figure 2e) were realized on the PTFE surface using reactive ion etching in order to enhance the charge density of contact electrification. For the working mechanism of the TENG, contact between the Al and PTFE layers results in negative triboelectric charges on the surface of the PTFE. When the friction layers separate, the current flows between the Al and the PTFE back electrodes due to electrostatic induction in order to screen the triboelectric charges on the PTFE surface. This current flow thereby produces power in an external circuit. The authors1 have also demonstrated a network of four TENGs, which produce an open-circuit voltage of ∼200 V, a short-circuit current of ∼320 μA, and a peak power of ∼60 mW from the water waves. Remarkably, TENGs can convert slow, random forces from all directions into electric power, and they are extremely lightweight, low-cost, and can float on the surface of water. According to the authors,1 once extended over an area of 1 km2 of the water’s surface, the network of TENGs can produce an average power output of 1.15 MW and, therefore, holds great potential for blue energy harvesting. TENGs can convert slow, random forces from all directions into electric power, and they are extremely lightweight, low-cost, and can float on the surface of water. Since the water from ocean waves can short circuit the TENGs’ electrodes, the TENGs have to be packaged in order to prevent this difficulty.4 In consideration of the harsh marine environment, the packaging has to have certain attributes in order to be durable: the packaging materials should be anticorrosive, resistant to heat and radiation, and, preferably, chemically inert.1,7 In order to harvest blue energy at large scales, a network of TENGs is required. Therefore, the interconnection strategy among TENGs is important. Since TENGs typically have very high voltages (∼100 V) and low currents (from tens to hundreds of μA),10,17 several TENGs in the network should be connected in parallel such that the total current of the network is the sum of the individual currents of TENGs. The output of TENGs is strongly dependent on the load resistance.10,17 Therefore, in order for maximum power transfer from the TENG network, a power management module (PMM) is indispensable. OUTLOOK AND FUTURE CHALLENGES Blue energy in the form of ocean waves offers a tremendous energy resource and can significantly contribute to the energy requirements of our daily life. Due to their low cost, light weight, 6431 DOI: 10.1021/acsnano.6b04213 ACS Nano 2016, 10, 6429−6432 ACS Nano Perspective and Electrostatic Induction at an Instantaneous Energy Conversion Efficiency of ∼55%. ACS Nano 2015, 9, 922−930. (14) Wang, Z. L. Triboelectric Nanogenerators as New Energy Technology and Self-Powered Sensors − Principles, Problems and Perspectives. Faraday Discuss. 2014, 176, 447−458. (15) Wang, Z. L. Triboelectric Nanogenerators as New Energy Technology for Self-Powered Systems and as Active Mechanical and Chemical Sensors. ACS Nano 2013, 7, 9533−9557. (16) Hu, Y.; Yang, J.; Jing, Q.; Niu, S.; Wu, W.; Wang, Z. L. Triboelectric Nanogenerator Built on Suspended 3D Spiral Structure as Vibration and Positioning Sensor and Wave Energy Harvester. ACS Nano 2013, 7, 10424−10432. (17) Zhu, G.; Peng, B.; Chen, J.; Jing, Q.; Lin Wang, Z. Triboelectric Nanogenerators as a New Energy Technology: From Fundamentals, Devices, to Applications. Nano Energy 2015, 14, 126−138. Since the kinetic energy of the ocean waves is irregular, the power output of the TENG network will be irregular as well and so cannot be directly available for commercial applications. Therefore, the power from the PMM has to be stored in the form of batteries and then made available for commercial applications. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS We acknowledge financial support from the Framework of International Cooperation Program managed by National Research Foundation of Korea (NRF-2015K2A2A7056357) and the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) (No. CRC-1505-ETRI). REFERENCES (1) Chen, J.; Yang, J.; Li, Z.; Fan, X.; Zi, Y.; Jing, Q.; Guo, H.; Wen, Z.; Pradel, K. C.; Niu, S.; Wang, Z. Networks of Triboelectric Nanogenerators for Harvesting Water Wave Energy: A Potential Approach toward Blue Energy. ACS Nano 2015, 9, 3324−3331. (2) Jiang, T.; Zhang, L. M.; Chen, X.; Han, C. B.; Tang, W.; Zhang, C.; Xu, L.; Wang, Z. L. Structural Optimization of Triboelectric Nanogenerator for Harvesting Water Wave Energy. ACS Nano 2015, 9, 12562−12572. (3) Wen, X.; Yang, W.; Jing, Q.; Wang, Z. L. Harvesting Broadband Kinetic Impact Energy from Mechanical Triggering/Vibration and Water Waves. ACS Nano 2014, 8, 7405−7412. 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