triboelectric nanogenerators as new review energy ... · gress and potential applications as a new...

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CXXXX American Chemical Society

Triboelectric Nanogenerators as NewEnergy Technology for Self-PoweredSystems and as Active Mechanical andChemical SensorsZhong Lin Wang,,*

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245, United States and Beijing Institute of Nanoenergy andNanosystems, Chinese Academy of Sciences, Beijing, China

Themassive development of theworldelectronic technology follows a gen-eral trend of miniaturization, portabil-ity, and functionality. The development ofcomputers is a typical example of miniatur-ization, from the vacuum-tube-based huge-sized machine to solid-state MOSFET-basedmainframe computers and later laptop

computers. The tremendous increase ofpopularity of handheld cell phones is atypical example of portable/mobile electro-nics. The next few decades will be aboutbuilding functionality on existing electro-nics, which inevitably involves developing arange of sensors including but not limitedto navigation, motion, chemical, biological,

This Review is contributed by a recipient of the 2013 ACS Nano Lectureship Awards, presented at the InternationalConference on Nanoscience & Technology, China 2013 (ChinaNANO 2013) in September 2013. The ACS Nano Lectureshiphonors the contributions of scientists whose work has signifiantly impacted the fields of nanoscience and nanotechnology.

* Address correspondence [email protected].

Received for review September 3, 2013and accepted September 30, 2013.

Published online10.1021/nn404614z

ABSTRACT Triboelectrification is an effect that is known to each and every oneprobably since ancient Greek time, but it is usually taken as a negative effect and isavoided in many technologies. We have recently invented a triboelectric nanogenerator(TENG) that is used to convert mechanical energy into electricity by a conjunction oftriboelectrification and electrostatic induction. As for this power generation unit, in theinner circuit, a potential is created by the triboelectric effect due to the charge transferbetween two thin organic/inorganic films that exhibit opposite tribo-polarity; in theouter circuit, electrons are driven to flow between two electrodes attached on the backsides of the films in order to balance the potential. Since the most useful materials forTENG are organic, it is also named organic nanogenerator, which is the first using organicmaterials for harvestingmechanical energy. In this paper, we review the fundamentals ofthe TENG in the three basic operation modes: vertical contact-separation mode, in-planesliding mode, and single-electrode mode. Ever since the first report of the TENG in January 2012, the output power density of TENG has been improved 5 orders ofmagnitude within 12 months. The area power density reaches 313 W/m2, volume density reaches 490 kW/m3, and a conversion efficiency of60% has beendemonstrated. The TENG can be applied to harvest all kinds of mechanical energy that is available but wasted in our daily life, such as human motion, walking,vibration, mechanical triggering, rotating tire, wind, flowingwater, andmore. Alternatively, TENG can also be used as a self-powered sensor for actively detectingthe static and dynamic processes arising from mechanical agitation using the voltage and current output signals of the TENG, respectively, with potentialapplications for touch pad and smart skin technologies. To enhance the performance of the TENG, besides the vast choices of materials in the triboelectric series,from polymer to metal and to fabric, the morphologies of their surfaces can be modified by physical techniques with the creation of pyramid-, square-, orhemisphere-based micro- or nanopatterns, which are effective for enhancing the contact area and possibly the triboelectrification. The surfaces of the materialscan be functionalized chemically using various molecules, nanotubes, nanowires, or nanoparticles, in order to enhance the triboelectric effect. The contactmaterials can be composites, such as embedding nanoparticles in a polymer matrix, which may change not only the surface electrification but also thepermittivity of thematerials so that they can be effective for electrostatic induction. Therefore, there are numerousways to enhance the performance of the TENGfrom the materials point of view. This gives an excellent opportunity for chemists and materials scientists to do extensive study both in the basic science and inpractical applications. We anticipate that a better enhancement of the output power density will be achieved in the next few years. The TENG is possible not onlyfor self-powered portable electronics but also as a new energy technology with potential to contribute to the world energy in the near future.

KEYWORDS: triboelectrification . triboelectric nanogenerator . self-powered system . active sensor . organic nanogenerator

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and gas sensors. The near future development is aboutelectronics that are much smaller than the size of a cellphone, so that each person on average can have atleast dozens to hundreds of such small electronics.Such small-sized electronics operate at ultralow powerconsumption, making it possible to be powered by theenergy harvested from our living environment.1 It willbecome impractical if sensor networks have to bepowered entirely by batteries because of the hugenumber of devices, large scope of distribution, anddifficulty to track and recycle to minimize environmen-tal impact and possibly health hazards. Therefore,power sources are desperately needed for indepen-dent and continuous operations of such small electro-nics, which could be used widely for ultrasensitivechemical and bimolecular sensors, nanorobotics,microelectromechanical systems, remote and mobileenvironmental sensors, homeland security, and evenportable/wearable personal electronics.New technologies that can harvest energy from the

environment as sustainable self-sufficient micro/nano-power sources are newly emerging fields of nano-energy, which is about the applications of nano-materials and nanotechnology for harvesting energyfor powering micro/nanosystems. In the last 8 years,we have been developing nanogenerators (NGs) forbuilding self-powered systems and as active sensors.We havemainly utilized two physics effects for harvest-ing small-magnitude mechanical energies: piezoelec-tric effect and triboelectric effect. A basic introductionabout piezoelectric NG has been given in a recentbook2 and a few review articles.3,4 The objective of thispaper is to give a summary about the fundamentals ofthe triboelectric nanogenerator (TENG), which is adevice that converts mechanical energy into electricityusing the coupling effects between triboelectrifica-tion and electrostatic induction through the contact-separation or relative sliding between two materialsthat have opposite tribo-polarity and its updated pro-gress and potential applications as a new energytechnology and as self-powered active sensors.

Fundamentals of Triboelectrification. The triboelectriceffect is a contact-induced electrification in which amaterial becomes electrically charged after it is con-tacted with a different material through friction. Tribo-electric effect is a general cause of every day's electro-statics. The sign of the charges to be carried by amaterial depends on its relative polarity in comparisonto the material to which it will contact.

Triboelectric effect is probably one of a few effectsthat have been known for thousands of years. Althoughthis is one of the most frequently experienced effectsthat each and every one of us inevitably uses every day,the mechanism behind triboelectrification is still beingstudied possibly with debate.5,6 It is generally believedthat, after two different materials come into contact, achemical bond is formed between some parts of the

two surfaces, called adhesion, and charges move fromone material to the other to equalize their electroche-mical potential. The transferred charges can be elec-trons or may be ions/molecules. When separated,some of the bonded atoms have a tendency to keepextra electrons and some a tendency to give themaway, possibly producing triboelectric charges on sur-faces. The objective of our study is not to investigatethe mechanism of the triboelectrification, instead, useit for positive purposes.

Materials that usually have strong triboelectrifica-tion effect are likely less conductive or insulators, thus,they usually capture the transferred charges and retainthem for an extended period of time, building up theelectrostatic charges, which are usually considered tobe a negative effect in our daily life and technologydevelopments. We can use the following examples toillustrate the damages that can be caused by triboelec-trification. Aircraft flying will develop static chargesfrom air friction on the airframe, which will interferewith radio frequency communication. Electrostaticcharges are an important concern for safety, due tothe fact that it can cause explosion and ignite flam-mable vapors. Carts/cars that may carry volatile liquids,flammable gases, or explosive chemicals have to bedischarged properly to avoid fire. Some electronicdevices, most notably CMOS-integrated circuits andMOSFET transistors, can be accidentally destroyed byhigh-voltage static discharge that may be carried bygloves. Therefore, triboelectrification is mostly taken asa negative effect in our daily life, industrial manu-facturing, and transportation. Therefore, by surprise,although triboelectrification has been known for thou-sands of years, it has not been used for many positiveapplications. In this paper, we give a review on a new,innovative, and important application of triboelectriceffect for convertingmechanical energy into electricityand as self-powered active mechanical sensors.

Nanoscale Energy (Nanoenergy) and Macroscale Energy. Ingeneral, energy usually means the power required to

GLOSSARY: triboelectric effect - a contact-induced

electrification in which a material becomes electrically

charged after it is contacted with a different material

through friction;nanoenergy - the applications of nano-

materials and nanotechnology for harvesting energy for

powering micro/nanosystems; triboelectric nanogenera-

tor (TENG) - a device that convertsmechanical energy into

electricity using the coupling effects between triboelec-

trification and electrostatic induction through the contact-

separation or relative sliding between two materials that

have opposite tribo-polarity; self-powered active strain/

force sensors - a sensor that operates using the electric

signal generated by itself in responding to mechanical

triggering or agitation without applying an external

power source;PMMA - polymethyl methacrylate;PDMS -

polydimethylsiloxane;

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run a factory, a city, or a country. This is generallyreferred to as macroscale energy, which is measured inthe scale of gigawatt or megawatt. General character-istics of a technology for macroscale energy are thetotal power output, stability, conversion efficiency, andcost. In many cases, cost is the most important mea-sure, such as for solar cells (Figure 1).

On the other hand, with the tremendous increase inthe number of portable electronics, developing en-ergy-storage-related technologies is vitally importantbecausemost are run by batteries. Although the powerconsumption of each is rather small, the total numberof devices is extremely huge. Over three billion peoplearound the world have cell phones. With the imple-mentation of sensor networks around the globe, agigantic number of sensors will be distributed world-wide; powering of such a horrendous network consist-ing of trillions of sensors would be impossible usingbatteries because one has to find the location, replacebatteries, and inspect the proper operation of batteriesfrom time to time. Energy harvesting from the envi-ronment in which the senor is employed is a possiblesolution. This is the field of nanoenergy, which is thepower for sustainable, maintenance-free, and self-powered operation of micro/nanosystems.7 The gen-eral characteristics for nanoenergy power sources areavailability, efficiency, and stability (Figure 1). In a casewhere a device is used under the light, the use of solarenergy would be a natural choice. In a case of a deviceused near an engine possibly in the dark, harvestingmechanical vibration energy would be the best choice.As for biological application, harvesting deformation

energy from muscle stretching would be a goodapproach. Althoughwemay have a super high efficientsolar cell, the condition under which the device willwork may have little light, the highly efficient solar cellis not the choice for this device. Therefore, the type ofenergy to be harvested depends on the workingenvironment of the device. This is what we mean bythe availability of the energy source for a particularapplication. The stability of the energy source is alsoimportant because it guarantees the long-term opera-tion of the device. Take solar cells as an example; it hasstrong dependence on the day or night, weather, oreven season. This is the reason that we have beendeveloping technologies for converting mechanicalenergy into electricity for self-powered sensors.

Traditional Triboelectric Generators. Traditional tribo-electric generator is amechanical device that producesstatic electricity or electricity at high voltage by contactcharging. The most popular ones are the Wimshurstmachine and Van de Graaff generator, which wereinvented in 1880 and 1929, respectively. Both ma-chines use the accumulated static charges generatedby triboelectrification; the tribo-charges are transferredfrom a rotating belt to a metal brush by the coronadischarging (e.g., the electric-field-induced arching ofair); once the accumulated charge density reaches acritical value, discharging over two opposite electrodesoccurs (Figure 2). It appears that the traditional tribo-electric generator is a high voltage source, and there isno current unless there is a discharging.

Triboelectric Nanogenerators. Although the triboelec-trification effect has been known for thousands of

Figure 1. Magnitude of power and its corresponding applications. Macroscale energy is for powering a city and even acountry; nanoscale energy is to power tiny small electronics. Both applications are measured by different characteristics.

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years, a fundamental understanding about it is ratherlimited. Research has been conducted to characterizethe triboelectrification process using various methodssuch as rolling sphere tool-collecting-induced chargesfrom rolling spheres on top of a dielectric disk8,9 andusing atomic force microscopy (AFM) to measure sur-face electrostatic force or potential on surfaces con-tacted by micropatterned materials.1012 However,

these methods either lack an accurate control of theelectrification process and/or cannot directly reveal thetriboelectric interface, thus hardly achieving a quantita-tive understanding about the in situ triboelectric process.

We have demonstrated an in situ method to quan-titatively characterize the triboelectrification at thenanoscale via a combination of contact-mode AFMand scanning Kelvin probe microscopy.13 As a benefitof the capability of controlled charge transferring andin situ measurement, AFM can be used to investigatethe triboelectric charge transfer at surfaces. As amodelsystem, a SiO2 thin film was rubbed for multiple cyclesat the same area with constant contact force. Thecorresponding scanning Kelvin probe microscopyimages after each friction cycle are shown in Figure 3a,and the extracted potential profiles are presentedin Figure 3b. Within eight cycles of friction, the magni-tude of the potential increased from 0.1 to 0.7 V at aslowing rate. As shown in Figure 3c, there is a clear trendfor the surface charge accumulation and saturationprocess. By quantitatively fitting the experimentaldata, surface charge density before the triboelectricprocess is 0 = (12( 3) C/m2, and saturation chargedensity after infinite numbers of cycles of friction is = (150( 8) C/m2. The electric field at the vicinityof the surface is 1.7 107 V/m, which can easilygenerate a high voltage. The contact-induced chargetransfer is the experimental base of our TENG.

Vertical Contact-Separation Mode-Based TENG:

Dielectric-to-Dielectric Case. The discovery of TENG

Figure 2. Wimshurst machine and Van de Graaff generator,developed a century ago, are the only two possible applica-tions of triboelectrification related to energy.

Figure 3. Triboelectric charge accumulation on the SiO2 surface with the increase of the number of repeated rubbings at thesame area. (a) Series of surface potential images taken in the same area from intact status to the one after the 8th rubbingcycles, and (b) their correspondingpotential profiles. (c) Derived surfacepotential as a functionof thenumber of friction cyclesand the simulation based on charge accumulation theory. Reproduced from ref 13. Copyright 2013 American Chemical Society.

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can be traced back to our development of piezoelectricnanogenerators,14 in which ZnO nanowires weregrown on polymer surfaces. The device was usuallyfully packaged without gap or void. However, in a casewhere a device was not fully packaged so that thebottom polymer substrate and the top packagingmaterial might have a relative sliding or contacting, avoltage of a few volts was generated. This phenomen-on was first considered as an artifact or surfaceelectrostatic charge andwas ignored. However, in early2012, we did a systematic study about this phenom-enon and found that it was a triboelectrification-drivenenergy conversion process.14,15 This was the birth ofthe TENG, which is distinctly different from the tradi-tional Van de Graaff generator in a way that electro-static induction is introduced for output power.

The operating principle of the TENG for the case ofdielectric-to-dielectric in contact mode can be describedby the coupling of contact charging and electrostaticinduction.16 Respectively, Figure 4a,b depicts electricoutput of open-circuit voltage and short-circuit cur-rent. In the original state, no charge is generated orinduced, with no electric potential difference (EPD)between the two electrodes (Figure 4aI). With an exter-nally applied force, the two polymers are brought intocontact with each other. Surface charge transfer thentakes place at the contact area due to triboelectrification.

According to the triboelectric series,17,18 which is a listof materials based on their tendency to gain or losecharges, electrons are injected from PMMA into Kap-ton, resulting in net negative charges at the Kaptonsurface and net positive charges at the PMMA surface,respectively. It is worth noting that the insulatingproperty of the polymers allows a long-time retentionof triboelectric charges for hours or even days. Sincethey are only confined on the surface, charges withopposite signs coincide at almost the same plane,generating practically no EPD between the two elec-trodes (Figure 4aII).

As the generator starts to be released, the Kaptonfilm tends to revert back to its original position due toits own resilience. Once the two polymers separate, anEPD is then established between the two electrodes(Figure 4aIII). If we define electric potential of thebottom electrode (UBE) to be zero, electric potentialof the top electrode (UTE) can be calculated by

UTE d0

0(1)

where is the triboelectric charge density, 0 is thevacuum permittivity, and d0 is the interlayer distance ata given state.

As the generator is being released, Voc increasesuntil reaching the maximum value when the Kapton

Figure 4. Sketch that illustrates the operating principle of the TENG for dielectric-to-dielectric in contact-separationmode. (a)Open-circuit condition. (b) Short-circuit condition. The polymer nanowires are not shown for the purpose of simplification.Reproduced from ref 16. Copyright 2012 American Chemical Society.

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film fully reverts to the original position (Figure 4aIVand Figure 4aV). Such a signal will remain constantprovided that the input impedance of the electrometeris infinitely large.

If pressing is immediately followed, the EPD startsdiminishing as the two polymer layers get closer toeach other. As a result, Voc drops from the maximumvalue to zero when a full contact is made againbetween the two polymers (Figure 4aV,VI).

If the two electrodes are shorted, any establishedEPD shown in eq 1 as the two polymers separate driveselectrons to flow from the top electrode (TE) to thebottom electrode (Figure 4bIII) in order to balancethe generated triboelectric potential, resulting in aninstantaneous positive current during the releasingprocess (Figure 4bIV). The net effect is that inducedcharges accumulate with positive sign on the topelectrode and negative sign on the bottom electrode(Figure 4bV). The induced charge density (0) when thegenerator is fully released is given by16

0 d0rkrp

d1rp d0rkrp d2rk (2)

where rk and rp are the relative permittivity of Kaptonand PMMA, respectively, and d1 and d2 are the thick-nesses of the Kapton film and the PMMA layer. Themaximum value of max

0is obtained by substituting d3

for d0 in the equation above.Once the generator is pressed again, reduction of

the interlayer distance would make the top electrodepossess a higher electric potential than the bottomelectrode. As a consequence, electrons are driven fromthe bottom electrode back to the top electrode, redu-cing the amount of induced charges (Figure 4bVI). Thisprocess corresponds to an instantaneous negativecurrent (Figure 4bV). When the two polymers are incontact again, all induced charges are neutralized(Figure 4bII).

Fabrication of the TENG is relatively simple, easy,low-cost, and sometimes no cleanroom equipment iseven needed. The TENG has a layered structure withtwo substrates, as schemed at the top left of Figure 5.19

Polymethyl methacrylate (PMMA) was selected as thematerial for substrates due to its decent strength, lightweight, easy processing, and low cost. On the lowerside, a layer of contact electrode is prepared. Thecontact electrode plays dual roles of electrode andcontact surface. It consists of a gold thin film and goldnanoparticles coated on the surface. In practice, suchnanoparticles can be replaced by any metallic nano-particles. On the other side, a thin film of gold islaminated between the substrate and a layer ofpolydimethylsiloxane (PDMS). This electrode is termedback electrode for later reference. The two substratesare connected by four springs installed at the corners,leaving a narrow spacing between the contact elec-trode and the PDMS.

A mechanical shaker was used to apply impulseimpact on the TENG. Here, the interaction force gen-erated between the gold and the PDMS is defined ascontacting force. Open-circuit voltage (Voc) and short-circuit current (Isc) were measured to characterize theTENG's electric performance.With a contacting force of10 N, the Voc and the Isc are presented in Figure 5a,b,respectively. The Voc switched between zero and aplateau value, respectively, corresponding to the con-tact position and the original position. The Isc exhibitsAC behavior, with an equal amount of electrons flow-ing in opposite directions within one cycle. The experi-mental data validate theworking principle described inFigure 4. It is observed that the current signal for theseparation process has a smaller magnitude but longerduration than that for the contact process (inset ofFigure 5b). It can be explained by faster contact result-ing from external impact compared to slower separa-tion caused by restoring force of the springs. Thepolarity of the measured electric signals can be re-versed upon switching the connection polarity be-tween the TENG and the measurement instrument.Furthermore, the AC output could be transferred topulse output in the same direction simply by a full-wave rectifying bridge (Figure 5c).

The NG's electric output is strongly related to thecontacting force, yielding higher output with largerforce. At a force as small as 10 N, the NG can stillproduce Isc ranging from 160 to 175 A (Figure 5d).When the force increases to 500 N, the electric outputreaches a saturated value, producing a peak Isc of 1.2mA. This result is due to increased contact area with alarger force. The two contacting surfaces are neitherabsolutely flat nor smooth. With a larger force, due toelastic property, thePDMScandeformandfillmorevacantspace, thus leading to larger contact area. As a result, theelectric output increases until all the vacant space is com-pletely filled by the PDMS, reaching a saturated limit.

Resistors were connected as external loads tofurther investigate the effective electric power of theTENG for driving electronics. As demonstrated inFigure 5e, the instantaneous current drops with in-creasing load resistance due to Ohmic loss, while thevoltage builds up. Consequently, the instantaneouspower output (W = Ipeak

2 R) reached the maximum at aload resistance of 1 M. At a contacting force of 500 N,a power output of 0.42 W was achieved (Figure 5f),corresponding to a power density of 109 W/m2 for onelayer of TENG. If themechanical shakerwas replaced byhuman footfalls, which can generate a contacting forcebetween 500 and 600 N, the maximum Isc could reachup to 2 mA, which can simultaneously light up 600LEDs (Figure 5g). It corresponded to an instantaneouscurrent of 1.1 mA at a load of 1 M, instantaneousoutput power of 1.2W, and power density of 313W/m2.The corresponding triboelectric surface charge densityof 594.2 C/m2 is demonstrated.

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Vertical Contact-Separation Mode-Based TENG:

Metal-to-Dielectric Case. The design of the TENG forthe metal-to-dielectric in contact-separation mode ispresented in Figure 6a.20 According to the triboelectricseries, electrons are injected from cellulose paper to

PTFE, resulting in net negative charges (Q) on the PTFEsurface. In a simplified model, the equivalent circuit ofthe TENG with an external load of R is illustrated inFigure 6bd, in which the device can be regarded as aflat-panel capacitor. If is the charge density of the

Figure 5. Top left: basic structure of the TENG for dielectric-to-dielectric in contact-separationmode. (a) Open-circuit voltage(Voc) at contacting force of 10 N. Inset: enlarged view of one cycle. Separation causes rising of the Voc to a plateau value, andcontact makes it fall back to zero. (b) Short-circuit current (Isc) at contacting force of 10 N. Inset: enlarged view of one cycle.Contact and separation correspond to a positive current pulse and a negative current pulse, respectively. (c) Rectified short-circuit current (Isc) by a full-wave bridge at contacting force of 10 N. Inset: enlarged view of one cycle, showing two currentpulses in the same direction. (d) Dependence of the Isc on the contacting force. Larger contacting force corresponds to highercurrent until saturation occurs. Inset: Isc at contacting force less than 50 N. The data points represent a peak value of currentpulses, while the line is the fitted result. (e) Dependence of the current output on external load resistance at a contacting forceof 10 N. Increased load resistance results in lower current output but higher voltage output. The points represent the peakvalue of electric signals, while the line is the fitted result. (f) Dependence of the power output on external load resistance at acontacting force of 500 N, indicating maximum power output when R = 1 M. The curve is a fitted line. (g) Photograph of asetup in which the TENG acts as a direct power source for 600 commercial green, red, and blue LED bulbs, when footstep fallson the NG, simultaneously lighting up the LEDs in real-time. The estimated output open-circuit voltage is1200 V. Note: allLEDs are connected serially. Reproduced from ref 20. Copyright 2013 American Chemical Society.

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PTFE surface, 1 is the charge density of the Cu surfacethat is contacted with PTFE, and 2 is charge density ofthe Ag upper surface (Figure 6b), we have21

1 1 d1

d2rp

(3)

2 1 (4)where d1 and rp are the thickness and permittivity ofPTFE, respectively, and chargeQ is stable for a relativelylong time on the PTFE surface; thus 1 is dictated by thegap distance d2. The variation of d2 will result in theredistribution of the charges between Cu and Agelectrodes through the load R that generates a currentthrough the load, so that mechanical energy is con-verted into electricity. The working mechanism of theTENG is similar to a variable-capacitance generator22

except that the charges are self-generated triboelectriccharges rather than an external power source. Oncethe TENG was pressed (Figure 6c), a reduction of theinterlayer distance of d2 would decrease 1 accordingto eq 3, which results in an instantaneous positivecurrent (Figure 6e). Upon the TENG being released(Figure 6d), the device would revert back to its originalarch shape due to resilience, the interlayer distance d2would increase, and the surface charge 1 increased aswell, resulting in an instantaneous negative current(Figure 6e).

Although the mechanism illustrated in Figure 6 isfor a flat surface, micro- or nanopatterns can be gen-erated on surfaces to enhance the contact areaand the effectiveness of the triboelectrification. TheTENG shown in Figure 7 is based on the contact

electrification between patterned PDMS as the topplate and patterned Al foil as the bottom plate(Figure 7a).18 According to the triboelectric series, thepurposely chosen PDMS and Al are almost at the twoends with very large differences in the ability to attractand retain electrons. The unique arch-shaped structureof the TENG from the naturally bent top plate, whichhelps to carry out the action of effective charge separa-tion and contact using the elasticity of the film, isachieved by the following innovative fabrication pro-cess (Figure 7b,c): The top part starts from a piece of flatKapton film (Figure 7b,i). A layer of 500 nm SiO2 film isdeposited using plasma-enhanced chemical vapordeposition (PECVD) at 250 C (Figure 7b,ii). Upon cool-ing to room temperature, the Kapton will shrink to amuch larger extent than the SiO2 film because of thelarge difference in thermal expansion coefficients, sothat thermal stress across the interface will make theplate bent naturally toward the SiO2 side. Then, theprefabricated PDMS film with pyramid patterns isglued to the inner surface through a thin PDMSbonding layer (Figure 7b,iii). Finally, the electrode isdeposited on top (Figure 7b,iv). As for the bottomplate, an aluminum foil (Figure 7c,i) is patterned witha typical photolithography process: defining the photo-resist to the array of square windows (Figure 7c,ii), depo-siting a layer of aluminum on top (Figure 7c,iii), andfinally lift-off, leaving the patterned Al cubes on thefoil (Figure 7c,iv). At last, the two as-fabricated platesof the same size are attached face-to-face and sealedat the two ends. The soft Al plate will be forced tobend outward under the contraction from the otherplate, so that a gap will form naturally between. Thepatterned surfaces of PDMS film (Figure 7d) and Al foil

Figure 6. Design, operating principle, and performance of a TENG in the metal-to-dielectric in contact-separation mode. (a)Schematic diagram and digital photography of an arch-structured flexible triboelectric nanogenerator. Equivalent circuit ofthe TENGwith an external load of Rwhen the device is at (b) origin, (c) pressing, and (d) releasing states and (e) correspondingcurrenttime curve, respectively. (f) Linear superposition tests of two TENGs (G1 and G2) connected in parallel with the samepolarity (G1 G2) and opposite polarity (G1 G2). Reproduced with permission from ref 21. Copyright 2013 Elsevier.

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(Figure 7e) are fabricated to enhance the triboelectriccharging.

Based on the contact-separation triboelectrificationprocess, multilayered and stacked TENG can be fabri-cated in order to enhance the total output power.23,24

Such designs extend TENG from a planar structure to avolume structure. In such a case, there are two para-meters for characterizing the output power: surfacepower density and volume power density. This typeof 3D-TENG has unique advantages compared tothe conventional solar cell and thermoelectric energy

generator. A solar cell can only receive light from itsnormal direction, in which any structure beneath acertain depth of the structure is ineffective for energyconversion. A thermal electric generator is limited bythe temperature drop between its two ends, and itsenergy harvesting efficiency may not be improved bydesigning a thermoelectric generator into a layered orstacked structure.

Lateral Sliding-Mode-Based TENG: Dielectric-on-

Dielectric Case. There are two basic friction processes:normal contact and lateral sliding. We demonstrated

Figure 7. Structure and fabrication process of the TENG for metal-to-dielectric in contact-separation mode. (a) Schematicdiagram showing the structural design of the arch-shaped TENG; the inset is the photograph of a typical arch-shaped TENGdevice. (b,c) Fabrication flowchart for (b) top plate and (c) bottom plate of the TENG. (d,e) Top view SEM images of (d) PDMSsurface with pyramid patterns and (e) Al surface with cubic patterns; insets are high-magnification images at a tilted angle.(c,d) TENGdriving a loadwith rectification, anddependenceof (c) the output voltage, current, and (d) instantaneous power onthe resistance of the load. All of the dots are measured values, and the solid lines are fitted curves. Reproduced from ref 19.Copyright 2012 American Chemical Society.

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here a TENG that is designed based on the in-planesliding between the two surfaces in lateral direction.25,26

With an intensive triboelectrification facilitated bysliding friction, a periodic change in the contact areabetween two surfaces leads to a lateral separation ofthe charge centers, which creates a voltage drop fordriving the flow of electrons in the external load. Thesliding-induced electricity generation mechanism isschematically depicted in Figure 8a. In the originalposition (Figure 8aI), the two polymeric surfaces fullyoverlap and intimately contact with each other. Be-cause of the large difference in the ability to attractelectrons, the triboelectrification will leave the nylonsurface with net positive charges and the PTFEwith netnegative charges with equal density. Since the tribo-charges on the insulators will only distribute in thesurface layer andwill not be leaked out for an extendedperiod of time, the separation between the positivelycharged surface and negatively charged surface isnegligible at this overlapping position, and thus therewill be little electric potential drop across the twoelectrodes. Once the top plate with the positivelycharged surface starts to slide outward (Figure 8aII),the in-plane charge separation is initiated due to thedecrease in contact surface area. The separatedcharges will generate an electric field pointing fromthe right to the left almost parallel to the plates,

inducing a higher potential at the top electrode. Thispotential difference will drive a current flow from thetop electrode to the bottom electrode in order togenerate an electric potential drop that cancels thetribo-charge-induced potential. Because the verticaldistance between the electrode layer and the tribo-charged polymeric surface is negligible compared tothe lateral charge separation distance, the amount ofthe transferred charges on the electrodes approxi-mately equals to the amount of the separated chargesat any sliding displacement. Thus, the current flow willcontinue with the continuation of the ongoing slidingprocess that keeps increasing the separated charges,until the top plate fully slides out of the bottom plateand the tribo-charged surfaces are entirely separated(Figure 8aIII). Themeasured current shouldbedeterminedby the rate at which the two plates are being slid apart.

Subsequently, when the top plate is reverted toslide backward (Figure 8aIV), the separated chargesbegin to contact again but with no annihilation due tothe insulator nature of the polymer materials. Theredundant transferred charges on the electrodes willflow back through the external load with the increaseof the contact area, in order to keep the electrostaticequilibrium. This will contribute to a current flow fromthe bottom electrode to the top electrode, along withthe second half cycle of sliding. Once the two plates

Figure 8. Working mechanism of the sliding mode TENG for dielectric-on-dielectric case. (a) Sketches that illustrate theelectricity generation process in a full cycle of the sliding motion. (bd) Finite element simulation of the potential differencebetween the two electrodes at consecutive sliding displacements: (b) 0 mm (the overlapping position); (c) 41 mm (slidinghalfway out); (d) 71 mm (fully sliding out). (e) Curves of the simulated potential difference V (red) and transferred chargedensity (blue) vs the sliding displacement from 11 to 91mm, in which the two plates fully slide out of each other at 71mm(marked by the purple dot line). Reproduced from ref 26. Copyright 2013 American Chemical Society.

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reach the overlapping position, the charged surfacesare fully in contact again. There will be no transferredcharges left on the electrode, and the device returns tothe state in Figure 8aI. In this entire cycle, the processesof sliding outward and inward are symmetric, so a pairof symmetric alternating current peaks should beexpected.

This in-plane charge-separation-induced potentialdifference and charge transfer can be verified throughnumerical simulation using COMSOL. The model con-structed here has the same structure and dimensions(71 mm 50 mm surface) with the real device, andthose two tribo-charged surfaces are assigned with acharge density of (70 C/m2. The device is in open-circuit condition, which means no electron transferbetween the two electrodes. As shown by the simula-tion results, when the two plates are in the fully alignedstacking position, which corresponds to the state inFigure 8aI, there is no potential difference generated(Figure 8b). When the top plate slides about halfwayout (with a displacement of 41 mm), there will be a2950 V potential difference between the two electro-des (Figure 8c), and this potential difference will in-crease to 1.03 105 Vwhen the top plate just slides outof contact with the bottom plate (with a displacementof 71 mm) (Figure 8d). In these simulation results ofFigure 8bd, the background planes show the poten-tial distribution in the free space of air surrounding theTENG, as a result of the in-plane charge separation.We have also simulated the voltage between thetwo electrodes at a series of displacements from11 to 91 mm. As shown in Figure 8e, the voltage keepsincreasing when the displacement gets larger, evenafter the plates slide out of each other. This is becausethe voltage is a path integral of the electric field alongthe displacement. On the other hand, the amounts oftransferred charges between the two electrodes underthese different displacements are also simulated byequating the potential of the electrodes at the short-circuit condition. As shown in Figure 8e, the amount oftransferred charges increases linearly with the displa-cement before the top plate slides out of the bottomplate (with a displacement smaller than 71 mm), butdifferent from the trend of the voltage, the amount oftransferred charges will saturate at the total amount oftribo-charges on one surface after the plates have fullyslid out of each other because there is no furthercharge separation here. So, the effective displacementregion for generating electricity is between 0 and 71mm,where the contact area of the two plates is changedduring the relative sliding of the two plates.

Lateral Sliding-Mode-Based TENG: Metal-on-Dielec-

tric Case with Linear Grating. In the sliding mode,introducing linear grating on the sliding surfaces isan extremely efficient means for energy harvesting.Linear grating with uniform period is fabricated onboth sliding surfaces. The rows of grating units have

the same size as intervals between, with all rows beingelectrically connected at both ends by two buses. Thegrating patterns on both sliding surfaces are identicalso that they can match well with each other whenaligned. Although the grating design reduces the totalcontact area by half, thus seemingly sacrificing half ofthe triboelectric charges, it increases the percentage ofthe mismatched area to 100% for a displacement ofonly a grating unit length rather than the entire lengthof the TENG so that it dramatically increases thetransport efficiency of the induced charges. Inducedfree electrons can be pumped back and forth betweenelectrodes multiple times due to the grating structure,providing multifolds of output charge compared to anongrating TENG. Every row of the grating units can beconsidered as a reduced-sized TENG, and it is in parallelconnection with all other rows through buses. Incontrast to a nongrating TENG that needs to be fullydisplaced in order to complete pumping of the in-duced charges for one time, the grating TENG onlyrequires a displacement of a unit length to completelytransport the induced charges, largely improving theenergy conversion efficiency. With further displace-ment of another length of the unit, back flow of theinduced charges can be realized. Therefore, for a one-way sliding process across the whole length of theTENG, the induced charges can be pumped for 2N 1times in total, where N is the number of grating units. Ifwe take into account that the contacting area de-creases as the two surfaces slide apart, the followingequation represents the total induced charges Q thatthe grating TENG can provide for a single sliding acrossthe entire length of the TENG:24

Q (2q0N)N=2 (5)where q0 is the induced charges generated from asingle grating unit for a displacement of the unitlength.

To demonstrate the capability of the new principleas a direct power source, a total of 80 commercial LEDbulbs were utilized as operating load (Figure 9a). Theywere divided into two groups, which were connectedto a TENG with reversed polarity in order to clearlydemonstrate the AC output without rectification(Figure 9b). Shown in Figure 9a, one substrate of theTENG was fixed on a breadboard where the LEDs wereinstalled, while the other one was attached to humanfingers. As the hand swept back and forth, the slidingwas realized. For a nongrating TENG, the output cur-rent it delivers to the load is displayed in Figure 9c. It isnoticed that faster sweeping generates higher currentpeaks as compared with those from slower sweeping.Every current peak was capable of simultaneouslylighting up one group of LEDs. Due to the AC output,the two LED groups were alternately lighted up, asindicated by ON and OFF states in Figure 9d.It is worth noticing that a single sliding process

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corresponds to only a single current peak, which isvisualized by snapshots in Figure 9d. In comparison, agrating TENG is able to power the load for multipletimes within a single sliding. For a TENG with 8 gratingunits, only a displacement of a unit lengthwas requiredto light up the LEDs. Therefore, with grating structure,not only can the TENG have substantial enhancementsof current and charges but also it can provide high-frequency AC output that enables continuous opera-tion of electric devices.

Rotation-Mode-Based TENG. The mechanism pre-sented in Figure 8 can work in either one directionalsliding between two plates or in rotation mode. Wedemonstrate a segmentally patterned disk-shapedTENG, in which a periodic overlapping and separationprocess of the two groups of sectors on the twoconcentric and closely contacted disks is achieved byrelative rotation.27 The basic structure of the disk TENGis composed of two disk-shaped components with foursectors each, as schematically illustrated in Figure10ac. The working principle of the disk TENG is basedon the triboelectrification and the relative-rotation-induced cyclic in-plane charge separation between Al

and Kapton, as shown in Figure 10d. In the relativerotation, the Al surface and Kapton surface slide onerelative to the other, so that the electrons will beinjected from the Al foil to the inner surface of theKapton film, leaving net positive charges on the Al foiland net negative charges on the Kapton film. Theelectricity generation process of the disk TENG canbe divided into four stages: In stage I, the two disks areat an overlapping position. Since the two chargedsurfaces are closely contacted with no polarization,there will be no potential difference between the twoelectrodes, thus no current flow in the external load.When the Al foil rotates in reference to the Kapton film,the corresponding two segments start to have apartially mismatched contact area (stage II), and thein-plane tribo-charges are thus separated in the direc-tion almost parallel to the sliding direction, which willinduce a higher potential on the Al layer than theKapton's electrode, thus the electrons in the electrodeattached to the Kapton filmwill be driven to flow to theAl foil through an external load (forming a current flowin the reverse direction), so that an opposite potentialis generated to balance the potential difference

Figure 9. Demonstration of the sliding mode TENG for metal-on-dielectric case with a fine grating structure. (a) Photographthat shows the experimental setup. Inset: four rows of LED bulbs that are being lighted up. (b) Electric circuit diagram,indicating the four rows of LEDs are divided into two groups with reversed connection polarities. (c) Short-circuit currentgenerated from a nongrating TENG as the relative displacement results from reciprocating sweeping of a human hand.(d) Enlarged view of the section highlighted in (c). Insets: snapshots that show the states of the two groups of LEDscorresponding to the current peaks. (e) Different stages of the sliding process as a TENGwith 8 grating units is being slid apart(left column) and corresponding states of the two groups of LEDs during the sliding process (right column). Reproduced fromref 25. Copyright 2013 American Chemical Society.

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created by the separated tribo-charges. In this process,the electrons keep flowing until the two disks are fully

mismatch in the contacting segmented areas (e.g., for45 rotation in this case), which is represented by stage

Figure 10. Rotating TENG in segmented disk configuration. (a) Schematic illustration showing the structure design of the diskTENG. The inset (bottom left) is an enlargedfigure showing the Kaptonnanorodarray createdon the surface area. (b) Top viewSEM imageof the Kapton nanorods showing its uniformity in a large range. The inset is a high-magnification SEM imageof theKapton nanorods in 30 tilted view. The scale bar is 500 nm. (c) Photograph showing the two parts of a real disk TENG. (d)Schematic illustrations showing the proposed working principle of the disk TENG with the electron flow diagram in fourconsecutive stageswithin a full cycle of electricity generation. Please note that only one pair of sectors (the cross-section areaentangled in Figure 1awas shownwith surface charges for clarity of illustration, and the surface charges on the interface areabetween Al foil and Kapton film are hidden and are not drawn for easy presentation. (e) relationship between the outputvoltage/current and the resistance of an external load. (f) Relationshipbetween the effective power density and the resistanceof the external load. The maximum power is received when the external resistance is 10 M. Reproduced from ref 27.Copyright 2013 American Chemical Society.

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III. At this moment, both the induced potential differ-ence and the amount of transferred charges betweenthe two electrodes reach themaximumvalues. In stageIV, as the top plate continues spinning, the Kaptonsurface begins to contact another adjacent sector of Alfoil, and the potential difference between two electro-des will drop with the decrease of the mismatch area.As a result, the electrons will flow back in the oppositedirection from the Al foil to the electrode attached tothe Kapton film. Thus, the entire processwill result in analternating current (AC) output. Such a charge transfercycle will start over from stage I when the two platesreach a complete overlapping again. Figure 10e showsthe resistance dependence of both output currentdensity and voltage, from 10 to 1 G. The outputcurrent density decreases with the increasing resis-tance, while the output voltage shows the reversetrend, but both the current and voltage tend tosaturate at both high and low ends of the resistance.The power density was also plotted as a function ofexternal resistance in Figure 10f. The output powerdensity first increases at a low resistance region andthen decreases at a higher resistance region. Themaxi-mum value of the power density of1 W/m2 occurs at10 M.

A schematic diagram of a cylindrical rotating TENGwith 6 strip units is shown in Figure 11a.28 The TENGhas a coreshell structure that is composed of a

column connected to a rotational motor and a hollowtube fixed on a holder. Here, acrylic was selected as thestructural material due to its light weight, good ma-chinability, and proper strength. On the inner surfaceof the fixed tube, metal strips made of copper areevenly distributed, with each having a central angle of30, creating equal-sized intervals between. Thesemetal strips, connected by a bus at one end, play dualroles as a sliding surface and as a common electrode.On the surface of the rotatable column, a collection ofseparated strips of foam tape were adhered as a bufferlayer with a central angle of 30 for each unit. Thebuffer layer is a critical design that ensures robustnessand tolerance on off-axis rotation, which will be dis-cussed in detail later. On top of the foam tape, a layer ofcopper and a layer of PTFE film are conformablyattached in sequence. The copper layer serves as backelectrode of the TENG. The outer surface of the PTFEfilm is modified by spreading a layer of PTFE nanopar-ticles to enhance energy conversion efficiency.

The working principle of a rotating TENG can bedescribed by the coupling of contact electrificationand electrostatic induction. The design of the cylind-rical rotating TENG is based on the relative slidingmotion of grated surfaces. Here, a pair of sliding unitsis selected to illustrate the process of electricity gen-eration, as schemed in Figure 11b. The foam tape andnanoparticles on the PTFE film are not presented for

Figure 11. Rotation TENG in cylindrical configuration. (a) Design and structure of the cylindrical rotation TENG. (b) Workingprinciple of the TENG. (c) Maximum Isc as a function of linear rotational velocity/rotation rate of 1.33 m/s (a rotation rate of1000 r/min). (d)MaximumVoc as a function of linear rotational velocity/rotation rate. (e) Equivalent direct current as a functionof linear rotational velocity/rotation rate. Reproduced from ref 28. Copyright 2013 American Chemical Society.

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simplification 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 film and themetal strips, chargetransfer takes place. Due to the difference on tribo-electric polarities, 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 flow in the external circuit (Figure 11bI). 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 slidingelectrode through the external circuit due to electro-static induction, neutralizing positive triboelectriccharges on the sliding electrode and leaving positivelyinduced charges behind on the back electrode(Figure 11bII). The flow 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 11bIII). As the relative rotation continues, thePTFE filmwill come into contact with an adjacentmetalstrip (Figure 11bIV). Thus, the induced electrons willflow back in an opposite direction until the fullyaligned position is restored (Figure 11bI). Therefore,in a cycle of electricity generation, AC electric output isgenerated. Since all of the sliding units are electricallyconnected in parallel, output current from differentunits are synchronized to constructively add up.

To characterize the electric output, Isc and Voc of arotating TENG with 6 strip units were measured at arotation rate of 1000 r/min, corresponding to anequivalent linear rotational velocity of 1.33 m/s. Anapproximately linear relationship between the linearrotational velocity and the current amplitude can bederived from the results shown in Figure 11c. The linearrelationship between the velocity and output currentcan be explained by the increasing frequency. The Vocstays stable at different rotation rate because thevoltage is only a function of percentage of mismatchas shown in Figure 11d. For output charge, a highercurrent frequency produces a larger amount of accu-mulative charges within the same period of time,resulting in a larger equivalent direct current. As shownin Figure 11e, the equivalent direct current increasesfrom 3.7 to 38.9 A when the linear rotational velocityincreases from 0.13 to 1.33 m/s.

Single-Electrode-Based TENG. The TENGs presentedin the last few sections must have two electrodes inorder to form a closed circuit for the electrons to flow.Such a configuration is possible for a working alonesystem, but it introduces difficulty in engineering de-sign for specific applications such as harvesting energyfrom a rotating tire. In this section, we introduce asingle-electrode-based TENG that ismore practical and

feasible design for some applications such as finger-tip-driven TENG.2932 To obtain a more quantitativeunderstanding about the TENG, we employed finiteelement simulation to calculate the electric potentialdistribution in the TENG and the charge transfer be-tween the ITO electrode and ground using COMSOL.The constructed model is based on a skin patch and aPDMS film on the ITO electrode with the dimensions of45 mm 65 mm and thickness of 1 mm for skin,0.5 mm for PDMS film, 1 mm for ITO electrode, asdepicted in Figure 12a. The ITO electrode was con-nected with the ground. The triboelectric charge den-sities on the skin and PDMS film were assumed to be10 and 10 C/m2, respectively. Figure 12b showsthe calculated results of the electric potential distribu-tion in the TENG under the different separation dis-tances of 0.5, 10, 30, and 60 mm. When the skin andPDMS fully contact with each other, the electric poten-tials on both the skin and PDMS approach zero. Theelectric potential difference was found to increase dra-matically with increasing separation distance. Whenthey are separated by 60 mm, the electric potential onthe skin surface reaches 1.2 104 V, while the electricpotential on the PDMS is still close to zero, which isassociatedwith contact with the ITO electrode/ground.As illustrated in Figure 12c, the amount of the totalcharges on the ITO electrode was plotted as a functionof separation distance between the skin and PDMS. Itincreases with increasing separation distance, reveal-ing that charge transfer decreases with increasingdistance. These results are consistent with the mea-sured change in charge quantity when the contactedskin was removed from the PDMS film, as shown inFigure 12d.

The working principle of the fabricated TENG isschematically shown in Figure 12e by the coupling ofcontact electrification and electrostatic induction. Inthe original position, the surfaces of skin and PDMSfully contact with each other, resulting in charge trans-fer between them. According to the triboelectric series,electrons were injected from the skin to the PDMSsince the PDMS is more triboelectrically negative thanskin, which is the contact electrification process. Theproduced triboelectric charges with opposite polaritiesare fully balanced/screened, leading to no electronflow in the external circuit. Once a relative separationbetween PDMS and skin occurs, these triboelectriccharges cannot be compensated. The negative chargeson the surface of the PDMS can induce positive chargeson the ITO electrode, driving free electrons to flow fromthe ITO electrode to ground. This electrostatic induc-tion process can give an output voltage/current signalif the distance separating the touching skin and thebottom PDMS is appreciably comparable to the size ofthe PDMS film. When negative triboelectric charges onthe PDMS are fully screened from the induced positivecharges on the ITO electrode by increasing the

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separation distance between the PDMS and skin, nooutput signals can be observed, as illustrated. More-over, when the skin was reverted to approach thePDMS, the induced positive charges on the ITO elec-trode decrease and the electrons will flow from groundto the ITO electrode until the skin and PDMS fullycontact with each other again, resulting in a reversedoutput voltage/current signal. This is a full cycle of theelectricity generation process for the TENG in contact-separation mode.

The mechanism of the single-electrode-based slid-ing TENG is schematically depicted in Figure 13a. In theoriginal position, the surfaces of PTFE and Al fullyoverlap and intimately contact with each other. Ac-cording to the triboelectric series, the contact betweenthe PTFE and Al will result in electrons being injectedfrom Al to PTFE since PTFE is much more triboelectri-cally negative than Al. The produced negative tribo-electric charges can be preserved on the PTFE surfacefor a long time due to the nature of the insulator. Oncethe top PTFE with the negatively charged surface startsto slide outward, it will result in the decrease of theinduced positive charges on the Al, thus the electronsflow from ground to Al, producing a positive currentsignal. When the top PTFE fully slides out of the bottomAl, the two triboelectric charged surfaces are entirelyseparated, and then an equilibrium state can be cre-ated with no output voltage/current. When the topPTFE plate is reverted to slide backward, the inducedpositive charges on the Al increase, driving the elec-trons to flow from Al to ground to produce a negativecurrent signal. Once the two plates completely reach

the overlapping position, the charged surfaces get intofull contact again and there will be no change of theinduced charges on the Al, thus no output current canbe observed. This is a full cycle of the single-electrode-based sliding TENG working process.

The electric potential distribution in the TENG andthe charge transfer between the Al and ground can beverified through numerical simulation using COMSOL.The proposed model is based on a PTFE patch and anAl plate with the same dimensions (5 cm 8 cm 0.1 cm), as shown in the inset of Figure 13b. Thetriboelectric charge density on the PTFE was assumedto be10 C/m2. The Al plate was connected with theground. Figure 13b depicts the calculated results of theelectric potential distribution in the TENG under fourdifferent sliding distances of 0, 16, 26, and 40 mm.When two plates are fully overlapped, the electricpotential on the PTFE surface approaches zero. Whenthe top PTFE plate slides out with a displacement of26 mm, the electric potential on the PTFE surface is upto6000V. It canbe clearly seen that the electric potentialdifference between the PTFE and the Al increasesdramatically with increasing sliding distance. As illu-strated in Figure 13c, the amount of the total chargeson the Al plate decreases linearly with increasingsliding distance, indicating that the transferred chargesbetween Al and the ground increase with the increas-ing sliding distance.

Usually, the effective output power of the TENGdepends on the match with the loading resistance.Figure 13d shows the resistance dependence of bothoutput voltage and the output current density with the

Figure 12. Single-electrode TENG in contact-separation mode. (a) Schematic diagram of the model for the calculation.(b) Finite element simulation of the potential distribution in the TENG for the different separation distances between the skinand PDMSfilm. (c) Total charges on the ITO electrode as a function of the separationdistance between the skin andPDMSfilm.The increase in total charges is a result of the electron flow from ITO to ground. (d) Measured charge quantity when theapplied pressure on the PDMS film was released. (e) Sketches that illustrate the electricity generation process in a full cycle.(I) Full contact between the human skin and PDMS film. (II) Two surfaces are separatedwith a small distance. (III) Two surfacesare separated with a large distance. (IV) Separated two surfaces are approaching. Reproduced with permission from ref 29.Copyright 2013 Wiley.

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resistance from 10 to 1 G. The output voltage ofthe device rises up with increasing the loading resis-tance, while the current density drops with the in-crease of the resistance. As shown in Figure 13e, theoutput power density was plotted as a function of theloading resistance. The instantaneous power densityremains close to 0 with the resistance below 1M andthen increases in the resistance region from1 to 100M.

The power density decreases under the larger loadingresistance (>100 M). The maximum value of theoutput power density reaches 350mW/m2 at a loadingresistance of 100 M.

Applications of TENG. TENG is a physical process ofconverting mechanical agitation to an electric signalthrough the triboelectrification (in inner circuit)and electrostatic induction processes (in outer circuit).

Figure 13. Single-electrode TENG in slidingmode. (a) Sketches that illustrate the electricity generation process in a full cycle.(b) Finite element simulation of the potential distribution in the TENG. The inset shows themodel for the calculation with theseparation distance of 0.2 mm between PTFE and Al. (c) Curve of the induced charge quantity on the Al electrode versus thesliding distance from 0 to 40 mm. (d) Dependence of the output voltage and the current density on the external loadingresistance. (e) Plot of the power density versus the loading resistance. Reproduced from ref 30. Copyright 2013 AmericanChemical Society.

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This basic process has been demonstrated for twomajor applications. The first application is energyharvesting with a particular advantage of harvestingmechanical energy. The other application is to serve asa self-powered active sensor because it does not needan external power source to drive. In this section, webriefly summarize the applications that have beendemonstrated in these two areas.

Harvesting Vibration Energy. Vibration is one of themost popular phenomena in our daily life, from walk-ing, voices, engine vibration, automobile, train, aircraft,wind, and many more. It exists almost everywhere andat all times. Harvesting vibration energy is of greatvalue especially for poweringmobile electronics. Basedon the fundamental principles introduced in section 4,various technologies have been demonstrated forharvesting vibration energy.

Cantilever-based technique is a classical approachfor harvestingmechanical energy, especially for MEMS.By designing the contact surface of a cantilever withthe top and bottom surfaces during vibration, TENGhas been demonstrated for harvesting ambient vibra-tion energy based on the contact-separation mode.With the assistance of nanowire arrays fabricated ontothe surfaces of beryllium copper alloys foils, the newlydesigned TENG produces an open-circuit voltage up to101 V, a short-circuit current of 55.7 A, with a corre-sponding peak power density of 252.3 mW/m2.33

To harvest the energy from a backpack, we demon-strated a rationally designed TENG with integratedrhombic gridding, which greatly improved the totalcurrent output owing to the structurally multiplied unitcells connected in parallel.34 With the hybridization ofboth the contact-separation mode and sliding electri-fication mode among nanowire arrays and nanoporesfabricated onto the surfaces of two contact plates, thenewly designed TENG produces an open-circuit vol-tage up to 428 V and a short-circuit current of 1.395mAwith a peak power density of 30.7 W/m2. Based on theTENG, a self-powered backpack was developed with avibration-to-electric energy conversion efficiency up to10.6%. The newly designed TENG can be a mobilepower source for field engineers, explorers, and disaster-relief workers.

With the use of four supporting springs, a harmonicresonator-based TENG has been fabricated based onthe resonance-induced contact separation betweenthe two triboelectric materials, which has been usedto harvest vibration energy from an automobile en-gine, a sofa, and a desk.35 It produces a uniform quasi-sinusoidal signal output, with an open-circuit voltageup to 287.4 V, a short-circuit current amplitude of 76.8A, and a peak power density 726.1 mW/m2. It caneffectively respond to input vibration frequency from 2to 200 Hzwith a considerably wideworking bandwidthof 13.4 Hz. The harmonic resonator-based TENG is verysensitive to small ambient vibrations such as an

operating automobile engine; it can also act as anactive vibration sensor for ambient vibration detection.This work not only presents a new principle in the fieldof vibration energy harvesting but also greatly expandsthe applicability of TENGs as power sources for self-sustained electronics.

Recently, a three-dimensional triboelectric nano-generator (3D-TENG) has been designed based on ahybridization mode of conjunction the vertical con-tact-separation mode and the in-plane sliding mode.36

The innovative design facilitates harvesting randomvibration energy in multiple directions over a widebandwidth. An analytical model is established to in-vestigate the mechano-triboelectric transduction of3D-TENG, and the results agree well with experimentaldata. Compared with the state-of-the-art vibrationenergy harvesters, the 3D-TENG is able to harvestambient vibration in out-of-plane direction (z-axis)with extremely wide working bandwidth up to 75 Hzat a frequency of 63.5 Hz (f/f 1.18) and arbitraryin-plane (xy plane) directions with a bandwidth of14.4 Hz at a frequency of 38 Hz (f/f 0.38). Themaximum power densities of 1.35 and 1.45 W/m2 havebeen achieved under out-of-plane and in-plane excita-tions, respectively. The 3D-TENG is designed for har-vesting ambient vibration energy, especially at lowfrequencies, under a range of conditions in daily life,thus, opening the applications of TENG in environ-mental/infrastructure monitoring, charging portableelectronics, and Internet.

Harvesting Energy from Human Body Motion. Hu-manmotion has an abundant amount of energy, whichcan be useful for charging portable electronics andbiomedical applications. We have demonstrated apackaged power-generating insole with built-in flex-ible multilayered triboelectric nanogenerators thatenable harvesting mechanical pressure during normalwalking. The TENG used here relies on the contact-separation mode and is effective in responding to theperiodic compression of the insole. Using the insole asa direct power source, we develop a fully packagedself-lighting shoe that has broad applications for dis-play and entertainment purposes. Furthermore, a pro-totype of a wearable charging gadget is introduced tocharge portable consumer electronics, such as cellphones. This work presents a successful initial attemptin applying energy-harvesting technology for self-powered electronics in our daily life, which will havebroad impact on people's living style in the near future.

A TENG can be attached to the inner layer of a shirtfor harvesting energy from body motion. Under gen-eral walking, the maximum output of voltage andcurrent density are up to 17 V and 0.02 A/cm2,respectively. The TENG with a single layer size of2 cm 7 cm 0.08 cm sticking on clothes wasdemonstrated as a sustainable power source that notonly can directly light up 30 light-emitting diodes but

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also can charge a lithium ion battery by persistentlyclapping clothes. The electric energy stored in thelithium ion battery was used to power a biosensor fordetecting glucose. The detection of bioactive chemi-cals in our body using the energy harvested frombody motion is demonstrated. Moreover, due to thesensitivity and desirable stability to periodic vibra-tion, the TENGwas used tomeasure stride frequency,as well.

TENG as Self-Powered Active Strain/Force Sensors. ATENG automatically generates an output voltage andcurrent once it is mechanically triggered. The magni-tude or the output signal signifies the impact of themechanical deformation and its time-dependent be-havior. This basic principle of the TENG can be appliedas a self-powered pressure sensor.15,37,38 Figure 14aillustrates the voltage output signal of various types ofsensor devices to the applied pressure induced by adroplet of water (8mg,3.6 Pa of pressure). All types ofTENGs have a high sensitivity and fast response to theexternal force and show a sharp peak signal. Thesensitivities of the line-featured and cube-featuredsensors were about 5 and 10 times, respectively, largerthan that of the unstructured film-featured sensor. Thepyramid-featured device showed the highest sensitiv-ity in the four types of pressure sensors. Furthermore,we measured the response to the impact of a piece offeather (20 mg, 0.4 Pa in contact pressure), corre-sponding to a low-end detection limit of 13 mPa. InFigure 14b, the sensor shows two opposite voltagesignal curves indicating the feather loading (on) andunloading (off) process. In the real situation, when thefeather falls on the sensor, it will go through twoprocesses: initially touching the sensor and completelyfalling on the sensor. The sensor signal can delicatelyshow these details of the entire process. The existingresults show that our sensor can be applied tomeasurethe subtle pressure in real life.

In the case where we make a matrix array of TENGs,we can have a large-area and self-powered pressuremap applied on a surface.32 The response of the TENGarray with local pressure was measured through amultichannel measurement system. On the basis ofthis working principle, we demonstrated the tactileimaging capability of the TEAS matrix by loadingpressure through predesigned plastic architecturewith the calligraphy of the letters TENG. Beforeapplying the pressure, the voltage output from all ofthe pixels of the TEAS matrix was at the backgroundlevel, as displayed in Figure 15b. Figure 15cf showsthe two-dimensional contour plot of the peak value ofthe voltage responses that were measured when ex-ternal pressures were applied through each architec-ture. The highlighted color represents the area underpressing through each letter, as outlined by the whitedashed lines. These plots elaborate the spatial resolu-tion of the TEAS matrix for distinguishably mappingthe calligraphy of the applied pressure and its potentialapplications such as personal signature recognition. Inaddition, to gain a more intuitive understanding of theself-powered pressure mapping functionality of theTEAS matrix, each nine units of the same array devicewere connected in parallel to power up a seriallyconnected array of LEDs showing T, E, N, andG. This demonstration proves that the TEAS matrixcan work as an external power source and a sensorarray simultaneously for a truly stand-alone self-pow-ered system.

There are two types of output signals from theTENG: open-circuit voltage and short-circuit current.The open-circuit voltage is only dictated by the finalconfiguration of the TENG after applying a mechanicaltriggering, so that it is a measure of the magnitude ofthe deformation, which is attributed to the staticinformation to be provided by TENG. The outputcurrent depends on the rate at which the induced

Figure 14. TENG as self-powered pressure sensor. (A) Performance of the pressure sensor device induced by a droplet ofwater falling fromabout 5 cmhigh. The voltage responses of various types of sensor devicesweremeasured. (B) Performanceof the pressure sensor device inducedby a piece of feather. The top inset image illustrates the process that a double peakwasinduced by placement of the feather. The bottom inset image shows the photograph of the feather induced pressure sensor.Reproduced from ref 15. Copyright 2012 American Chemical Society.

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charge would flow, so that the current signal is moresensitive to the dynamic process of how the mechan-ical triggering is applied.

The active pressure sensor and the integratedsensor array based on the triboelectric effect presentedin this work have several advantages over conven-tional passive pressure sensors. First, the active sensorin this work is capable of both static pressure sensingusing the open-circuit voltage and dynamic pressuresensing using the short-circuit current, while conven-tional sensors are usually incapable of dynamic sensingto provide the loading rate information. Second, theprompt response of both static and dynamic sensingenables the revealing of details about the loadingpressure. Third, the detection limit of the TENG fordynamic sensing is as low as 2.1 Pa, owing to the highoutput of the TENG. Fourth, the active sensor arraypresented in this work has no power consumption andcould even be combined with its energy harvestingfunctionality for self-powered pressure mapping. Fu-ture works in this field involve the miniaturization ofthe pixel size to achieve higher spatial resolution, andthe integration of the TEAS matrix onto a fully flexiblesubstrate for shape-adaptive pressure imaging.

One of the materials for the TENG can be humanskin based on the single-electrode-based TENG.30 Thisallows a direct interface of fingertip and a transparentbottom electrode material as device for touch pad andsmart skin applications. As shown in Figure 16, theoutput voltage signals of 16 devices were recorded inreal-time as a mapping figure. By addressing andmonitoring the positive output voltage signals in 16channels of the tactile sensor system, the touchedinformation of a human finger including the positionand pressure can be attained by analysis of the mea-sured mapping figures. If larger pressure is applied onthe device, the larger output voltage signals can beobserved in themapping figures. Figure 16a presents aphotograph of the device when the sixth and 11thdevices in the matrix were simultaneously touched.Two, almost the same, pressures of about 4.9 kPa canbe confirmed by the analysis of the obtained sameoutput voltage signals in the corresponding mappingimage, as shown in Figure 16b. Figure 16c displays theresponse of the device to the localized pressure in-duced by two fingers when the matrix was rotated by90. The obtained mapping figure clearly reveals thatthe seventh and 10th devices were touched, where theproduced pressures of about 7.0 kPa are larger thanthose in Figure 16b. When the pressures were appliedalong the diagonal line of the matrix by using theside surface of a human hand, the distinctive changesin the output voltage signals can be observed, reveal-ing an increase of the pressure from 3.2 to 6.0 kPaalong the diagonal line, as illustrated in Figure 16d.When the pressures were applied on all 16 devices byusing the human hand, the recorded mapping figure(Figure 16e) shows that all the devices are functionaland the pressures on devices 18 are obviously largerthan those on devices 916. The real-time detection ofthe touched actions on the flexible devices is a desir-able feature for sensors embedded in robots or pros-thetic devices. Figure 16f shows a photograph of theflexible matrix attached to a transparent acrylic tube.When the pressures were applied on the tube surfaceby using the humanhand, the largest pressure of about7.3 kPa occurs at the marked area (the white dashedline in Figure 16g), which is consistent with the mea-sured mapping figure, as shown in Figure 16k. Such ademonstration can also be done by placing a trans-parent TENG array on the display panel of a cell phoneor underneath the keyboards during typing, clearlydemonstrating its potential as sensors and as potentialenergy harvester.

TENG as Self-Powered Active Chemical Sensors. Asfor TENG, maximizing the charge generation on oppo-site sides can be achieved by selecting the materialswith the largest difference in the ability to attractelectrons and changing the surface morphology. Insuch a case, the output of the TENG depends on thetype and concentration of molecules adsorbed on the

Figure 15. Self-powered pressure mapping using an arrayof TENG. (a) Schematic illustration of the TENG array. (b)Background signal with no pressure applied on the TENGmatrix. The inset is color scaling of voltage for all themeasurements in thisfigure. (cf) Two-dimensional voltagecontour plot from the multichannel measurement of theTENGmatrixwith an external pressure uniformly and locallyapplied onto the device through architectures with calli-graphy of T, E, N, and G. Reproduced from ref 37.Copyright 2013 American Chemical Society.

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surface of the triboelectric materials, which can beused for fabricating chemical and biochemicalsensors.39 In our case, the performance of the TENGdepends on the assembly of Au nanoparticles (NPs)onto themetal plate. These assembled Au NPs not onlyact as steady gaps between the twoplates at strain-freeconditions but also enable the function of enlargingthe contact area of the two plates, which will increasethe electrical output of the TENG. Through furthermodification of 3-mercaptopropionic acid (3-MPA)molecules on the assembled Au NPs, the high-outputnanogenerator can become a highly sensitive andselective nanosensor toward Hg2 ion detection be-cause of the different triboelectric polarity of Au NPsand Hg2 ions (Figure 17a). On the basis of this uniquestructure, the output voltage and current of the tribo-electric nanosensor (TENG) reached 105 V and 63 Awith an effective dimension of 1 cm 1 cm. Underoptimum conditions, this TENG is selective for thedetection of Hg2 ions, with a detection limit of 30nM and linear range from 100 nM to 5 M (Figure 17b).With its high sensitivity, selectivity, and simplicity, theTENG holds great potential for the determination ofHg2 ions in environmental samples. The TENG is a futuresensing system for unreachable and access-denied

extreme environments. As different ions, molecules,and materials have their unique triboelectric polarities,we expect that the TENG can become either an

Figure 16. TENG as self-powered touch pad sensor and smart skin. (a) Photograph of the matrix touched by two humanfingers. (be) Measured mapping figures under different pressures. (f) Photograph of the fabricated matrix attached to anacrylic tube. (g) Measured mapping figure when the device in (f) was touched by a human hand. Reproduced from ref 31.Copyright 2013 American Chemical Society.

Figure 17. TENG as self-powered chemical sensor for de-tecting Hg2 ions. Top: Fabrication of the TENG functiona-lized with Au nanoparticles at one side. Bottom: Sensitivityand selectivity of the as-developed TENG for the detectionof Hg2 ions. The concentration of all metal ions tested inthe selectivity experiment was 5 M. Reproduced withpermission from ref 39. Copyright 2013 Wiley.

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TABLE 1. Triboelectric Series for SomeCommonMaterials Following a Tendency To Easily Lose Electrons (Positive) andTo

Gain Electrons (Negative)6

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electrical turn-on or turn-off sensor when the analytesare selectively binding to the modified electrode sur-face. We believe this work will serve as the steppingstone for related TENG studies and inspire the devel-opment of TENG toward other metal ions and biomo-lecules such as DNA and proteins in the near future.

Water-Surface-Based TENG for Chemical Sensors.

Most recently, we have demonstrated a newly de-signed TENG based on the contact electrification be-tween a patterned PDMS pyramid array and water.40

This new prototype waterTENG provided an open-circuit voltage of 52 V and a short-circuit currentdensity of 2.45 mA m2 with a peak power density ofnearly 0.13 W m2. The dependence of the electricaloutputs on the contact frequency and motions ofwater wave has been systematically studied. Tap waterand deionized water with similar ion concentration toseawater were also evaluated and showed the poten-tial for harvesting water-related energy from the envi-ronment. Compared with traditional TENGs that aredesigned for the contact of solid materials, this studyopens the possibility of utilizing liquidmovements andextends its application scope in chemistry and chemi-cal sensors.

Theory of TENG. To help understand the basic outputof the TENG, analytical and numerical theories havebeen developed to calculate the output voltage, cur-rent, and charges.41 A theoretical model for the contact-separation mode TENG was first developed. Based onthe theoretical model, its real-time output characteristicsand the relationship between the optimum resistanceand TENG parameters were derived. The theory canserve as an important guidance for rational design ofthe TENG structure in specific applications.

A theoretical model for the sliding-mode TENG hasalso been developed. The finite element method wasutilized to characterize the distributions of electricpotential, electric field, and charges on the metalelectrodes of the TENG.42 Based on the FEM calcula-tion, the semianalytical results from the interpolationmethod and the analytical VQx relationship werebuilt to study the sliding-mode TENG. The analyticalVQx equation was validated through comparisonwith the semianalytical results. Furthermore, based onthe analytical VQx equation, dynamic output per-formance of sliding-mode TENG was calculated witharbitrary load resistance, and good agreement withexperimental data was reached. The theory presentedhere is a milestone work for in-depth understanding ofthe workingmechanism of the sliding-mode TENG andprovides a theoretical basis for further enhancement ofthe sliding-mode TENG for both energy scavengingand self-powered sensor applications.

Choice of Materials and Surface Structures. Almost allmaterials we know have triboelectrification effect,from metal, to polymer, to silk, and to wood, almosteverything. All of these materials can be candidates for

fabricating TENGs, so the material choices for TENGsare huge. However, the ability of amaterial to gain/loseelectrons depends on its polarity. John Carl Wilckepublished the first triboelectric series in 1757 on staticcharges.43,44 Table 1 gives such a series for someconventional materials. A material toward the bottomof the series, when touched to a material near the topof the series, will attain a more negative charge. Thefurther away two materials are from each other on theseries, the greater the charge transferred.

Beside the choice of the materials in the tribo-electric series, the morphologies of the surfaces canbe modified by physical techniques with the creationof pyramid-, square-, or hemisphere-based micro- ornanopatterns, which are effective for enhancing thecontact area and possibly the triboelectrification. How-ever, the created bumpy structure on the surface mayincrease the friction force, which may possibly reducethe energy conversion efficiency of the TENG. There-fore, an optimization has to be designed to maximizethe conversion efficiency.

The surfaces of the materials can be functionalizedchemically using various molecules, nanotubes, nano-wires, or nanoparticles, in order to enhance the tri-boelectrification effect. Surface functionalization canlargely change the surface potential. The introductionof nanostructures on the surfaces can change the localcontact characteristics, which may improve the triboe-lectrification. This will involve a large amount of studiesfor testing a range of materials and a range of availablenanostructures.

Besides these pure materials, the contact materialscan be made of composites, such as embedding nano-particles in a polymermatrix. This changes not only thesurface electrification but also the permittivity of thematerials so that they can be effective for electrostaticinduction.

Therefore, there are numerous ways to enhance theperformance of the TENG from the materials point ofview. This gives an excellent opportunity for chemistsand materials scientists to do extensive study both inbasic science and in practical applications. In contrast,materials systems for solar cell and thermal electrics,for example, are rather limited, and there are not verymany choices for high-performance devices.

SUMMARY AND PERSPECTIVES

As sparked by the first discovery of nanogeneratorsin 2006, research in nanoenergy has inspired world-wide interest. Nanoenergy is about the applications ofnanomaterials and nanotechnology for harvesting en-ergy for powering micro/nanosystems. The discoveryof the triboelectric nanogenerator (TENG) is a majormilestone in the field of converting mechanical energyinto electricity for building self-powered systems. Itoffers a completely new approach for harvesting me-chanical energy using organic and inorganic materials.

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Figure 18 gives a summary about our progress madefrom January 2012 to January 2013 in the performanceof the TENG, a 5 order of magnitude enhancement inoutput power density! The area power density reaches313 W/m2, and volume density reaches 490 kW/m3.The TENG can bemade intomultilayers so thatwe havea three-dimensional nanogenerator. Such high perfor-mance outplays many existing technologies in itscategory. We anticipate that muchmore enhancementof the output power density will be demonstrated inthe next few years. Such a huge output makes itpossible not only for self-powered portable electronicsbut also for harvesting energy from wind and oceanwave. Therefore, TENG is a new energy technologyfor the next century! We anticipate a worldwidestudy of TENG in the next few years, and soonsome industrial products and applications will beachieved. Since the mostly useful materials areorganic, TENG is an organic nanogenerator, whichis expected to be equally important as organic LEDsand organic solar cells.Furthermore, TENG has to be hybridized with other

technologies such as solar cell and thermal electricgenerators to simultaneously harvest multiple-typeenergies. TENG also has to be hybridi

triboelectric nanogenerators as new review energy ... · gress and potential applications as a new...

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