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C 2013 American Chemical Society

Cylindrical Rotating TriboelectricNanogeneratorPeng Bai,,,^ Guang Zhu,,^ Ying Liu, Jun Chen, Qingshen Jing, Weiqing Yang, Jusheng Ma,

Gong Zhang, and Zhong Lin Wang,,*

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States, Department of Mechanical Engineering,Tsinghua University, Beijing 100084, China, and Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, China.^These authors contributed equally.

As a type of contact electrication inwhich a material becomes electri-cally charged after contacting with

a dierent material, triboelectric eect isone of the most universal phenomena andis responsible for generating most every-day static electricity that usually leads toundesirable consequences, such as poten-tial hazard to delicate electronics and publicsafety.16 Though it has been known forcenturies, it is still an under explored eectwith the mechanism in debate.712 Recently,this eect was successfully used as an eec-tivemeans for harvestingmechanical energy,leading to the invention of the triboelectricnanogenerators (TENGs).13,14 Utilizing the tri-boelectric eect that couples with electro-static induction,15,16 the TENG oers a simple,reliable, cost-eective, and ecient methodto harvest mechanical energy for self-pow-ered applications without reliance on tradi-tional power supplies. Demonstrated applica-tions include micropatterning,17 poweringportable electronics,1820 and self-poweredsensors.21,22

Two basic operating principles of theTENG have been developed, that is, contact

mode and sliding mode. For the contactmode, it relies on repeated contact and se-paration between two materials that havedierent triboelectric polarities. Such an op-erating principle introduces a changing gapin the TENG, making it dicult for devicesealing and packaging and thus limiting theTENG's applications in harsh environment.More recently, sliding mode based on lateralelectrication hasbeen reported.23,24 Relativedisplacement between two contact surfacesof dierent triboelectric polarities creates un-compensated triboelectric charges that driveinduced free electrons to ow through theexternal circuit. The newly developed slidingmode greatly expands the TENGs' applicabil-ity to harvest energy from continuousmotionand makes it possible for a fully packageddevice.

Here, in this work, a coaxial cylindricalstructured rotating TENG is developed toharvest mechanical energy from rotationanalogous to an electromagnetic induc-tion-based generator. A TENG with a totalcontact area of 12 cm2 contains multiplestrip units that are connected in parallel.The instantaneous short-circuit current (Isc)

* Address correspondence tozlwang@gatech.edu.

Received for review May 16, 2013and accepted June 21, 2013.

Published online10.1021/nn402491y

ABSTRACT We demonstrate a cylindrical rotating triboelectric nanogenerator (TENG) based on sliding

electrication for harvesting mechanical energy from rotational motion. The rotating TENG is based on a

coreshell structure that is made of distinctly dierent triboelectric materials with alternative stripstructures on the surface. The charge transfer is strengthened with the formation of polymer nanoparticles

on surfaces. During coaxial rotation, a contact-induced electrication and the relative sliding between the

contact surfaces of the core and the shell result in an in-plane lateral polarization, which drives the ow

of electrons in the external load. A power density of 36.9 W/m2 (short-circuit current of 90 A and open-

circuit voltage of 410 V) has been achieved by a rotating TENG with 8 strip units at a linear rotational

velocity of 1.33 m/s (a rotation rate of 1000 r/min). The output can be further enhanced by integrating

more strip units and/or applying larger linear rotational velocity. This rotating TENG can be used as a direct power source to drive small electronics, such as

LED bulbs. This study proves the possibility to harvest mechanical energy by TENGs from rotational motion, demonstrating its potential for harvesting the

ow energy of air or water for applications such as self-powered environmental sensors and wildlife tracking devices.

KEYWORDS: triboelectric nanogenerators . rotation . energy harvesting

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and the open-circuit voltage (Voc) could reach 90 Aand 410 V, respectively, for a TENG with 8 strip units ata linear rotational velocity of 1.33 m/s, correspondingto an instantaneous maximum power density of36.9 W/m2 and an equivalent average direct currentof 45.6 A. Higher output power can be achieved byusing a higher density of strip units and/or applyinglarger linear rotational velocity. At a linear rotationalvelocity of 1.33 m/s, the TENG can be used as a directpower source for simultaneously powering 90 com-mercial light-emitting diode (LED) bulbs in real time.Since rotation is one of the most common forms ofmotion in ambient environment, a fully packagedcylindrical rotating TENG has potential applications inharsh environment, outdoors or even under water, toharvest energy from water ow.

RESULTS AND DISCUSSION

A schematic diagram of a rotating TENG with 6 stripunits is shown in Figure 1a. The TENG has a coreshellstructure that is composed of a column connected to arotational motor and a hollow tube xed on a holder.Here, acrylic was selected as the structural material dueto its light weight, good machinability, and properstrength. On the inner surface of the xed tube, metalstrips made of copper are evenly distributed with eachhaving a central angle of 30, creating equal-sizedintervals between. These metal strips, connected by abus at one end, play dual roles as a sliding surface andas a common electrode. On the surface of the rotatablecolumn, a collection of separated strips of foam tapewas adhered as a buer layerwith a central angle of 30for each unit. The buer layer is a critical design thatensures robustness and tolerance on o-axis rotation,which will be discussed in details later. On top of thefoam tape, a layer of copper and a layer of polytetra-uoroethylene (PTFE) lm are conformably attached insequence. The copper layer serves as the back elec-trode of the TENG. The outer surface of the PTFE lm ismodied by spreading a layer of PTFE nanoparticles forenhancing energy conversion eciency. PTFE nano-particles with an average diameter of 100 nm spreaduniformly on the surface of PTFE lm as shown inFigure 1b. As demonstrated in Figure 1a, the PTFE lmon the column and the metal strips on the tube werecongured to match for a full contact during rotation.These two components and the motor are set to becoaxial to minimize wobbling during rotation. Furtherdetails of the fabrication process will be discussed inthe Experimental Section.

The working principle of a rotating TENG can bedescribed by the coupling of contact electricationand electrostatic induction. The design of the cylind-rical rotating TENG is based on the relative slidingmotion of grated surfaces. Here, a pair of slidingunits is selected to illustrate the process of electricitygeneration, as schemed in Figure 2. The foam tape and

nanoparticles on the PTFE lm are not presented forsimplication and clear illustration. At the alignedposition, the two parts of the sliding pair completelymatch because of the same central angle. Upon con-tact between the PTFE lm and themetal strips, chargetransfer takes place. Due to the dierence on tribo-electric polarities,25 electrons are injected from metalinto the surface of PTFE. Since the two sliding surfacesare completely aligned, triboelectric charges with op-posite polarities are fully balanced out, making noelectron ow in the external circuit (Figure 2a). Oncea relative sliding occurs as a result of rotation, tribo-electric charges on the mismatched areas cannot becompensated. The negative ones on the PTFEwill drivefree electrons on the back electrode to the sliding

Figure 1. (a) Schematic of the rotating TENG with 6 stripunits. (b) SEM image of PTFE nanoparticles on the surface ofPTFE lm.

Figure 2. Working principle of the rotating TENG. Distribu-tions of charges at (a) fully aligned position, (b) two surfacesare sliding apart, (c) fully mismatched position, and (d) twosurfaces are sliding back together. (e) COMSOL simulationresult of the electric potential distribution when the twosurfaces are sliding apart (a constant value of triboelectriccharge density of 3.89 106 C/m2, and electric potential ofthe back electrode is set to be zero).

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electrode through the external circuit due to electro-static induction, neutralizing positive triboelectriccharges on the sliding electrode and leaving positiveinduced charges behind on the back electrode(Figure 2b). The ow of the induced electrons lastsuntil the mismatch between the two sliding surfacesreaches the maximum when the positive triboelectriccharges are fully screened by induced electrons(Figure 2c). As the relative rotation continues, the PTFElm will come into contact with an adjacent metal strip(Figure 2d). Thus, the induced electrons will ow backin an opposite direction until the fully aligned positionis restored (Figure 2a). Therefore, in a cycle of electricitygeneration process, AC electric output is generated.Since all of the sliding units are electrically connectedin parallel, output current from dierent units aresynchronized to constructively add up. As shown inFigure 2e, a simulation plot via COMSOL presentselectric potential dierence between the two electro-des in open-circuit condition when the pair of slidingparts are mismatched (a constant value of triboelectriccharge density of 3.89 106 C/m2, and electricpotential of the back electrode is set to be zero).

To characterize the electric output, Isc and Voc ofa rotating TENG with 6 strip units were measuredat a rotation rate of 1000 r/min, corresponding toan equivalent linear rotational velocity of 1.33 m/s.As shown in Figure 3a, Isc of 60 A was achieved. Anenlarged view in the inset exhibits continuous AC out-put current with a frequency of 100 Hz. The frequency

is consistent with the one derived from the aforemen-tioned working principle. The Voc reached a maximumvalue of 373 V (Figure 3b), and the enlarged view in theinset of Figure 3b shows the oscillating behavior be-tween zero and the peak value at a frequency that isthe same as the output current. With a diode bridge (asinserted in Figure 3c), output current of dierent direc-tions can be added up constructively, leading to accu-mulative induced charges. As illustrated in Figure 3c,the accumulative charges can reach 24.5 C in 0.63 s,which corresponds to an eective direct current of38.9 A. To investigate the inuence of rotation rateon the output of the rotating TENG, Isc and Voc atdierent rotation rates were measured. At a rotationrate of 100 r/min (a linear rotational velocity of 0.13 m/s), the amplitude of the AC current generated by arotating TENG with 6 strip units is 9.0 A. When therotation rate increases to 1000 r/min (a linear rotationalvelocity of 1.33 m/s), the amplitude reaches 68.8 A.An approximately linear relationshipbetween the linearrotational velocity and the current amplitude can bederived from the results shown in Figure 3d. The linearrelationship between the velocity and output currentcan be explained by the increasing frequency. Becausethe linear rotational velocity has no inuence on thetransferred charge density under constant displace-ment, a higher linear rotational velocity results in afaster charge transfer rate.

I dqdt dq

dl 3v (1)

Figure 3. Electricity output of a rotating TENGwith 6 strip units. (a) Isc of a rotating TENG at a linear rotational velocity of 1.33m/s (a rotation rate of 1000 r/min). Inset: enlarged view of the current peaks. (b) Voc of a rotating TENG at a linear rotationalvelocity of 1.33 m/s (a rotation rate of 1000 r/min). Inset: enlarged view of the voltage peaks. (c) Accumulative inductivecharges generated by a rotating TENG. Inset: electric circuit diagram. (d) Maximum Isc as a function of linear rotationalvelocity/rotation rate. (e)MaximumVoc as a functionof linear rotational velocity/rotation rate. (f) Equivalent direct current as afunction of linear rotational velocity/rotation rate.

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where q is the quantity of the triboelectric charges onone sliding surface; t is the time needed for chargetransfer, which is the time needed for achieving themismatched displacement; l is the mismatched displa-cement between the two sliding surfaces; v is the linearrotational velocity. Thus, an increased Isc is expected.However, the Voc stays stable at dierent rotation ratebecause the voltage is only a function of percentageof mismatch, as shown in Figure 3e (SupportingInformation). For output charge, a higher current fre-quency produces a larger amount of accumulativecharges within the same period of time, resulting in alarger equivalent direct current. As shown in Figure 3f,the equivalent direct current increases from 3.7 to38.9 A when the linear rotational velocity increasesfrom 0.13 to 1.33 m/s.

It is to be noted that the density of sliding units in therotating TENG is a critical factor that directly deter-mines the electric output. As shown in Figure 4a, ata constant rotation rate of 1000 r/min, the output cur-rent is solely determined by the density of the slidingunits. Though the total amount of induced chargestransported between the two electrodes remains con-stant for a sliding distance of a unit, the frequency ofthe charge ow linearly depends on the number ofthe sliding units.23 An enhancement on Isc from 13.5 to89.9 A was achieved by increasing the number ofsliding units from one to eight. Besides, similar enhance-ment could also be realized for output charge from3.6 to 25 C in 0.55 s (Figure 4c), which correspondsto equivalent DC output from 7.0 to 45.6 A as insertedin Figure 4c. Because the length of strip units is muchbigger than the thickness of the PTFE lm, the densityof sliding units has little inuence on the amplitudeof Voc which stays stable around 360 V (Figure 4b).

23

However, the oscillation frequency of the voltagesignal increases from 16.7 to 133.3 Hz. For a TENGwith 8 sliding units, the instantaneous Isc and Voccould reach 90 A and 410 V, respectively, at a linearrotational velocity of 1.33 m/s (a rotation rate of1000 r/min), corresponding to an instantaneouspower density of 36.9 W/m2. It is expected that ahigher electric output can be achieved if more unitsare fabricated in the rotating TENG. Moreover, therobustness of the TENG was investigated by long timeoperation of the TENG at a rotation rate of 1000 r/min.As shown in Figure 5a, after 20 000 rounds of opera-tion, only a slight decline around 1.0% (from 68.81 to68.09 A) was observed for the amplitude of Isc from aTENG with 6 units while the Voc slightly dropped from374 to 371 V (0.7%). The minor decrease of theelectric output can be explained by the deformationof the PTFE nanoparticles on the contact surfaces, theincreased surface roughness of PTFE thin lm, and theaggravation of o-axis rotation, which could reducethe eective contact area and lead to a decreasedquantity of the triboelectric charges.

To demonstrate the ability of the rotating TENG asa direct power source for electronics, a total of 90commercial LED bulbs were utilized as external loads(Figure 5b). They were divided into two groups, whichwere connected to a rotating TENG with reversedpolarity. As illustrated in the circuit diagram inFigure 5c, 50 LED bulbs were connected in series,constituting the letter G on a circuit board shown inFigure 5b while the letter T consisted of another 40LED bulbs connected in series with reversed polarity.As shown in Figure 5d, at a rotation rate of 722 r/min,the 90 LED bulbs could be powered by the TENG in real

Figure 4. Electric output of a rotating TENG made of dier-ent strip units at a constant linear rotational velocity of1.33 m/s (a rotation rate of 1000 r/min). (a) Isc generated byrotatingTENGswithmultiple strip units. (b)Voc generated byrotating TENGs with multiple strip units. (c) Accumulativeinductive charges generated by rotating TENGs with multi-ple stripunits. Inset: equivalent direct current as a functionofnumber of strip units.

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time. Due to the high frequency around 70 Hz of theoutput current, the 90 LED bulbs with reversed pola-rities could be continuously lighted up (see video in theSupporting Information).

CONCLUSIONS

In summary, we demonstrated a design of cylindricalrotating TENG based on sliding electrication. Thisrotating TENG provides an eective way to harvestmechanical energy from rotational motion. A TENGwith 8 strip units on the surface could achieve a powerdensity of 36.9 W/m2 (Isc of 90 A and Voc of 410 V) ata linear rotational velocity of 1.33 m/s (rotation rate of1000 r/min). Higher output can be achieved if more

strip units are fabricated in the TENG and/or a higherlinear rotational velocity is applied. At a linear rota-tional velocity of 1.33 m/s, a rotating TENG can be usedas a direct power source which can drive 90 commer-cial LED bulbs in real time. This study demonstratesthe possibility to harvest mechanical energy by TENGsfrom rotational motion. It also can be used to designactive sensors of rotational motion. With this uniquedesign, fully packaged rotating TENGs can be furtherapplied in harsh environment, outdoors, or underwaterto harvest energy from air andwater ow. It is expectedto be an eective power source for applications suchas self-powered environmental sensors and wildlifetracking devices.

EXPERIMENTAL SECTIONFabrication of a Rotating TENG with Multiple Strip Units. An acrylic

column (diameter = 25.5 mm, length = 76 mm) and an acrylichollow tube (internal diameter = 31.5 mm, external diameter =33 mm, length = 38 mm) were used as the substrate. The size ofthe strip units was based on their quantity integrated in theTENG, and they were fabricated evenly with the same centralangle. As an example, a unit has a size of 8.2 mm 25.0 mm,6 of them are fabricated in the TENG. Stuck on the surface ofacrylic column, chemical-resistant polyethylene foam tape with

a thickness of 3.175 mm was used as the buffer layer. PTFE thinfilms with a thickness of 25 mwere prepared and depositedwith100 nm of copper by magnetron sputtering. They were adheredonto the top of foam tape with the uncoated side exposed. Then,a 0.5 g sample of commercial PTFE dispersion in which the PTFEnanoparticles were dissolved in isopropyl alcohol was sprayedon the surface of PTFE thin films and dried in air. Grid copperelectrodes (8.2 mm 25.0 mm) with a thickness of 100 nm weredeposited on a Kapton thin film (99 mm 25.0 mm 125 m)by magnetron sputtering. Subsequently, the Kapton thin film was

Figure 5. (a) Robustness of the rotating TENG. The electric output remains stable, although only a slight decline of themaximum Isc and Voc occurs. (b) Demonstration of the rotating TENG as a direct power source for driving 90 LED bulbs whenthe LED bulbs were o. (c) Electric circuit diagram, indicating that the two rows of LEDs are divided into two groups withreversed connection polarities, constituting the letters G and T, respectively. (d) Demonstration of the rotating TENG as adirect power source for driving 90 LED bulbs when the LED bulbs were on.

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adhered on the internal surface of the acrylic hollow tube atcorrespondingpositions. The strip unitswere connected inparallelby external wiring.

Conflict of Interest: The authors declare no competingnancial interest.

Acknowledgment. Research was supported by U.S. Depart-ment of Energy, Oce of Basic Energy Sciences (Award DE-FG02-07ER46394), NSF (0946418), and the Knowledge Innova-tion Program of the Chinese Academy of Science (Grant No.KJCX2-YW-M13). P.B. thanks the support from the ChineseScholars Council. Patents have been led based on the researchresults presented in this manuscript.

Supporting Information Available: (1) Analytical model forcalculating the maximum open-circuit voltage; (2) COMSOLsimulation result of electric potential distribution; (3) enlargedview of short-circuit current of rotating TENGs with multipleworking units; (4) supplementary video. This material is avail-able free of charge via the Internet at http://pubs.acs.org.

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