Triboelectric Nanogenerators for Blue EnergyHarvestingUsman Khan† and Sang-Woo Kim*,†,‡

†School of Advanced Materials Science and Engineering and ‡SKKU Advanced Institute of Nanotechnology, SungkyunkwanUniversity, 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 fullyexploited in order to make it available for the use of mankind. Blue energy harvesting is a challenging task as the kineticenergy 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 harvestmechanical 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 blueenergy harvesting. In this Perspective, we describe some of the recent progress and also address concerns related to durablepackaging of TENGs in consideration of harsh marine environments and power management for an efficient power transferand distribution for commercial applications.

Human society is heavily reliant on fossil fuels, such ascoal, oil, and natural gas, to meet its energy require-ments. However, the carbon emissions resulting from

the utilization of such fuels is causing problems such as climatechange, air pollution, and acid rain.1,2 Indeed, these seriousproblems in the long term may potentially put human civiliza-tion in jeopardy.2,3 Therefore, mankind has to exploit cleanand renewable energy resources such as sunlight, wind, oceanenergies, etc. for its sustainable development.4 Among therenewable energy resources, ocean energies are underexplored,1

even though water covers 70% of the face of the earth and hasan abundance of kinetic energy in the form of waves. Oceanenergy is also unique in that it is not strongly dependent onweather, time of day, or temperature.2 Therefore, blue energyfrom the oceans has the potential to contribute significantlyto 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 alternativeto fossil fuels, there is as yet insignificant development in thetechnology to exploit it fully.5

Electromagnetic Generators (EMGs) for Blue EnergyHarvesting. Blue energy in the form of water waves is amechanical kinetic energy.1 Its amplitude is random, andfrequency can be as low as <2 Hz.5 However, EMGs are well-known for converting mechanical kinetic energies from waterflow into electricity and, therefore, are candidates for harvesting

blue energy.6 Electromagnetic generators typically require anadditional turbine to convert water flow into rotational energy;the rotational energy is then utilized to move magnets aroundthe coils of an EMG in order for the electromagnetic induc-tion to occur.7 As a consequence, EMGs are complex, havehigh mass density, and have large volumes.1,7 Therefore, it ischallenging for EMGs to float on the ocean surface as they arebulky 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 thewave energies are present on the ocean’s surface.1 Electro-magnetic generators may also have serious limitations in makingnetworks of generators for energy harvesting over large areas dueto their bulkiness. Moreover, due to the high-quality materialsrequired for EMGs, the cost is also expected to be high.1,8

Published: July 13, 2016

In this issue of ACS Nano, Wen et al.note that although ocean energy ispotentially a great alternative to fossilfuels, there is as yet insignificantdevelopment in the technology toexploit it fully.

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© 2016 American Chemical Society 6429 DOI: 10.1021/acsnano.6b04213ACS Nano 2016, 10, 6429−6432

Another challenge with using EMGs for blue energy harvestingis that the irregular motion modes and low frequencies ofthe ocean waves comprise a major limitation.2,6 Indeed, EMGsare naturally suited for harvesting flow stream rather thanocean waves, which is also likely the reason that conventionalalternating current (ac) generators operate in frequencies from50 to 60 Hz.1,6 According to Faraday’s law of electromagneticinduction, the output voltage in an EMG is proportional to therate of change of flux through the coils. If ϕ is the magneticflux and N is the number of turns in the coils, then the outputvoltage of an EMG can be expressed as6

ϕ= −V Nt

ddEMG (1)

According to eq 1, the output voltage of EMGs is stronglydependent 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 theexpected 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 limita-tions in overcoming the losses during rectification and regulationfor powering electronic devices and/or charging batteries.6 Dueto the above-mentioned problems, generators for blue energyharvesting need to be lightweight, low cost, simple, easy to scaleup, and suitable for low-frequency kinetic energies.Triboelectric Nanogenerators (TENGs) for Blue Energy

Harvesting. Recently, TENGs have been introduced forharvesting mechanical energy present in the environment.9,10

Triboelectric nanogenerators are based on contact electrifica-tion between two materials and charge transfer between theirelectrodes due to electrostatic induction,9,10 and they typicallyutilize a polymer−metal pair as the friction layers.11,12 Therefore,they are low cost, lightweight, easy to fabricate, and offer anabundant choice of materials.4 Moreover, TENGs can haveenergy conversion efficiencies as high as 55%13 and can adaptwell to various mechanical energy types by different modesof 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 oceanwaves.5 Indeed, the output voltage of TENGs, VTENG, underopen-circuit conditions is given as6

=VQ

C x( )TENGsc

(2)

where Qsc is the short-circuit charge transfer amount and C(x)is the capacitance between the two electrodes at various dis-placements x. Therefore, for any TENG with displacementvarying from 0 to xmax, the peak value of the output voltageis independent of the frequency of the mechanical input.6

Therefore, TENGs have no limitations for harvesting low-frequency mechanical energies such ocean waves. Moreover, thetypical output voltage of TENGs is quite high (∼100 V) and canovercome drops during rectification and regulation for chargingbatteries.6 In summary, because TENGs are lightweight, lowcost, easy to fabricate, and have the ability to harvest low-frequency kinetic energies effectively, they offer an effectivetechnology for blue energy harvesting.Triboelectric Nanogenerator Technology Development

for Blue Energy Harvesting. Due to the suitability of TENGsfor potentially rich blue energy, various important steps haverecently been made.1−7,16 Here, we outline two important

reports on the subject. Zhu et al. demonstrated a liquid−solidelectrification-enabled generator (LSEG) for harvesting waterwave energy.7 It consists of a fluorinated ethylene propylene(FEP) friction layer with copper-based discrete electrodes atits back (see Figure 1a). The FEP layer engages in contact

electrification with the water waves. Vertically aligned nanowiresare realized on the FEP (see Figure 1b) using plasma etching inorder to make the FEP film hydrophobic and also to enhancethe contact area and thus the output power. The LSEG isattached onto a substrate (Figure 1c). For the mechanism offunction, contact electrification with water waves results innegative triboelectric charges on the surface of the FEP layer.During submerging and surfacing of the LSEG due to travelingwater waves, current flows between the electrodes in order toscreen the triboelectric charges on the FEP surface, therebyproducing an electric power. The LSEG with a size of 6 cm ×3 cm has produced an average output power of 0.12 mW at awave velocity of 0.5 m/s.7 The LSEG structure is an all-in-onedesign type as it does not require additional components such asturbines for receiving the mechanical energy. Moreover, dueto the materials involved, the LSEG is low cost and can easilybe scaled up. Several LSEGs can be interconnected to form anetwork 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 forlarge-scale harvesting of ocean wave energy.1 The basic unit ofthe network consists of a box structure with walls composedof arch-shaped TENGs with a metal ball enclosed inside each,as shown in Figure 2a. The mechanical energy from waterwaves causes the metallic balls to collide with the arch-shapedTENGs, thereby producing power. The arch-shaped TENG isshown schematically in Figure 2c. It is composed of two metal-dielectric friction pairs. The top and bottom dielectric layersare made of polyethylene terephthalate (PTFE) with copper asthe back electrodes. Aluminum films with nanoporous surfaces

Generators for blue energy harvestingneed to be lightweight, low cost,simple, easy to scale up, and suitablefor low-frequency kinetic energies.

Figure 1. Structure of the liquid−solid electrification-enabledgenerator (LSEG). (a) Schematic description of the electrificationlayer with two electrodes on the back. (b) Scanning electron micro-scopy image of the polymer nanowires on the electrification layer.The scale bar is 1 μm. (c) Schematic description of a substrate-supported LSEG in water waves. The emerging and submerging ofthe LSEG in water waves produces electricity between the electrodes.Reprinted from ref 7. Copyright 2014 American Chemical Society.

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(see Figure 2f) supported on top and bottom of an acrylic sheetact as the complementary friction layers to the PTFE layers(see Figure 2c). A photograph of an as-fabricated arch-shapedTENG is shown in Figure 2d. In addition, PTFE nanowires(see Figure 2e) were realized on the PTFE surface usingreactive ion etching in order to enhance the charge density ofcontact electrification. For the working mechanism of theTENG, contact between the Al and PTFE layers results innegative triboelectric charges on the surface of the PTFE.When the friction layers separate, the current flows between theAl and the PTFE back electrodes due to electrostatic inductionin order to screen the triboelectric charges on the PTFEsurface. This current flow thereby produces power in anexternal circuit. The authors1 have also demonstrated a networkof four TENGs, which produce an open-circuit voltage of∼200 V, a short-circuit current of ∼320 μA, and a peak powerof ∼60 mW from the water waves. Remarkably, TENGs canconvert slow, random forces from all directions into electricpower, and they are extremely lightweight, low-cost, andcan float on the surface of water. According to the authors,1

once extended over an area of 1 km2 of the water’s surface, thenetwork of TENGs can produce an average power output of1.15 MW and, therefore, holds great potential for blue energyharvesting.

OUTLOOK AND FUTURE CHALLENGESBlue energy in the form of ocean waves offers a tremendousenergy resource and can significantly contribute to the energyrequirements of our daily life. Due to their low cost, light weight,

easy fabrication, and ability to harvest mechanical energyeven over low frequencies, TENGs offer an effective methodto harvest the energy from ocean waves. However, there areseveral critical challenges ahead for blue energy harvesting usingTENGs.

Since the water from ocean waves can short circuit the TENGs’electrodes, the TENGs have to be packaged in order to preventthis 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 heatand radiation, and, preferably, chemically inert.1,7

In order to harvest blue energy at large scales, a network ofTENGs is required. Therefore, the interconnection strategyamong TENGs is important. Since TENGs typically havevery high voltages (∼100 V) and low currents (from tens tohundreds of μA),10,17 several TENGs in the network should beconnected in parallel such that the total current of the networkis the sum of the individual currents of TENGs. The outputof TENGs is strongly dependent on the load resistance.10,17

Therefore, in order for maximum power transfer from the TENGnetwork, a power management module (PMM) is indispensable.

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-shapedTENG, 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.

TENGs can convert slow, random forcesfrom all directions into electric power,and they are extremely lightweight,low-cost, and can float on the surfaceof water.

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Since the kinetic energy of the ocean waves is irregular, the poweroutput of the TENG network will be irregular as well and socannot be directly available for commercial applications. There-fore, the power from the PMM has to be stored in the form ofbatteries and then made available for commercial applications.

AUTHOR INFORMATIONCorresponding Author*E-mail: kimsw1@skku.edu.NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTSWe acknowledge financial support from the Framework ofInternational Cooperation Program managed by NationalResearch Foundation of Korea (NRF-2015K2A2A7056357)and the National Research Council of Science & Technology(NST) grant by the Korea government (MSIP) (No. CRC-15-05-ETRI).

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