Module-Type Triboelectric Nanogenerators Capable of Harvesting Power from a Variety of Mechanical Energy Sources

micromachines

Article Module-Type Triboelectric Nanogenerators Capable of Harvesting Power from a Variety of Mechanical Energy Sources

Jaehee Shin 1 , Sungho Ji 1, Jiyoung Yoon 2 and Jinhyoung Park 1,*

1 School of Mechatronics Engineering, Korea University of Technology & Education, Cheonan 1600, Chungjeolro, Korea; [email protected] (J.S.); [email protected] (S.J.) 2 Safety System R&D Group, Korea Institute of Industrial Technology, 320 Techno sunhwan-ro, Yuga-myeon, Daegu 42994, Dalseong-gun, Korea; [email protected] * Correspondence: [email protected]; Tel.: +82-41-560-1124

Abstract: In this study, we propose a module-type triboelectric nanogenerator (TENG) capable of harvesting electricity from a variety of mechanical energy sources and generating power from diverse forms that ﬁt the modular structure of the generator. The potential energy and kinetic energy of water are used for the rotational motion of the generator module, and electricity is generated by the contact/separation generation mode between the two triboelectric surfaces inside the rotating TENG. Through the parametric design of the internal friction surface structure and mass ball, we optimized the output of the proposed structure. To magnify the power, experiments were conducted to optimize the electrical output of the series of the TENG units. Consequently, outputs of 250 V and ◦ 11 µA were obtained when the angle formed between the ﬂoor and the housing was set at 0 while nitrile was set as the positively charged material and the frequency was set at 7 Hz. The electrical

Citation: Shin, J.; Ji, S.; Yoon, J.; Park, signal generated by the module-type TENG can be used as a sensor to recognize the strength and J. Module-Type Triboelectric direction of various physical quantities, such as wind and earthquake vibrations. Nanogenerators Capable of Harvesting Power from a Variety of Keywords: energy harvesting; triboelectric nanogenerators; vibration energy Mechanical Energy Sources. Micromachines 2021, 12, 1043. https://doi.org/10.3390/ mi12091043 1. Introduction As the applications of Internet of Things sensors increase, much attention is given Academic Editor: to energy harvesting, which is a technology that enables self-generation [1]. Generators Nam-Trung Nguyen that adopt energy harvesting technology for using wasted energy include thermoelectric, piezoelectric, electromagnetic, and triboelectric nanogenerators (TENGs) [2–6]. The tribo- Received: 25 July 2021 electric nanogenerator operates by the movement of electrons from one object to another Accepted: 27 August 2021 Published: 29 August 2021 when two different objects are rubbed against each other. The polarity of the charges on the surface of the contacting triboelectric materials is determined by the triboelectric

Publisher’s Note: MDPI stays neutral series, and the surface charges differ according to the position of the external circuit, thus with regard to jurisdictional claims in inducing the flow of electrons. Among materials with different properties, negatively published maps and institutional affil- charged materials tend to gain electrons, whereas the positively charged materials tend iations. to lose electrons [7–9]. Triboelectricity can be used not only for power generation but also for sensors. The application of triboelectric generators as sensors is possible because the amount of electricity generated during friction depends on the degree of external friction [10–13]. The triboelectric nanogenerator-based sensors are self-generated and will be suitable for various applications, such as motion, vibration, pressure, and tactile sen- Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. sors. [14]. Additionally, the strength of ocean waves can be monitored by floating a circular This article is an open access article triboelectric generator device in the ocean. When multiple devices are combined, electric distributed under the terms and signals of a wide frequency range and high outputs can be achieved compared with when conditions of the Creative Commons a single device is used [15–19]. Finally, by using wind power, the direction of the wind can Attribution (CC BY) license (https:// be detected [20–23] and the amount of rainfall can be measured [24–28]. creativecommons.org/licenses/by/ It can be seen from these previous works that researchers have aimed to harvest 4.0/). kinetic energy effectively, but little research has been conducted on a module-type TENG

Micromachines 2021, 12, 1043. https://doi.org/10.3390/mi12091043 https://www.mdpi.com/journal/micromachines Micromachines 2021, 12, x FOR PEER REVIEW 2 of 11

It can be seen from these previous works that researchers have aimed to harvest kinetic energy effectively, but little research has been conducted on a module-type TENG that enables the harvesting of various types of kinetic energy rather than a single type. In this study, we propose a TENG in modular form, which consists of silicone rubber balls and wires embedded inside a cylindrical frame. In the module-type TENG, both the positively and negatively charged materials are placed in the cylindrical housing to form a single structure, unlike in the TENG of previous studies in which negatively and posi-

Micromachinestively charged2021, 12, 1043 materials are separated to form two structures. The proposed modular2 of 11 form enables versatility in the application of the TENG because it can be attached to various structures of objects that generate mechanical energy. To demonstrate the capability of the developed module-typethat enables theTENG harvesting in harves of variousting types various of kinetic types energy of rather kinetic than aenergy, single type. an In this study, we propose a TENG in modular form, which consists of silicone rubber output performance testballs was and wiresconducted embedded in inside vertical, a cylindrical pendulum, frame. Inand the rotational module-type motions. TENG, both the positively and negatively charged materials are placed in the cylindrical housing to 2. Materials and Methodsform a single structure, unlike in the TENG of previous studies in which negatively and positively charged materials are separated to form two structures. The proposed modular 2.1. Fabrication Processform of the enables Triboelectric versatility in Nanogenerator the application of the(TENG) TENG because Device it canand be Silicone attached to Rubber various structures of objects that generate mechanical energy. To demonstrate the capability of the Balls developed module-type TENG in harvesting various types of kinetic energy, an output Figure 1a illustratesperformance a schematic test was of conducted a TENG in vertical,in a dynamic pendulum, state. and rotational The housing motions. and mold that were used to create2. Materials the silicone and Methods rubber balls of the TENG were created by the fused deposition modeling (FDM)2.1. Fabrication method Process using of the Triboelectric a 3D printer Nanogenerator (DP203 (TENG) 3D WOX1, Device and Sindoh, Silicone Seoul, Rubber Balls Korea). Polylactic acid (PLA) was used as the printing filament. To determine the effect of Figure1a illustrates a schematic of a TENG in a dynamic state. The housing and mold the design and experimentalthat were usedvariables, to create the silicone housing rubber had balls an of outer the TENG diameter were created of 90 by themm fused and a height of 105 mm; andeposition inner modelingdiameter (FDM) of method80 mm using was a 3Dcovered printer (DP203 with 3D1 WOX1,mm of Sindoh, aluminum Seoul, Korea). Polylactic acid (PLA) was used as the printing ﬁlament. To determine the effect of tape and 1 mm of nitrile.the design The and angle experimental formed variables, by the the housing housing had with an outer the diameter floor was of 90 mmset andat a0°, 30°, 60°, and 90°. Thisheight size of 105was mm; optimize an inner diameterd to improve of 80 mm was the covered output with performance 1 mm of aluminum of tape the ◦ ◦ ◦ TENG. and 1 mm of nitrile. The angle formed by the housing with the ﬂoor was set at 0 , 30 , 60 , and 90◦. This size was optimized to improve the output performance of the TENG.

Figure 1. Schematic and experimental conﬁguration of a triboelectric nanogenerator (TENG). (a) Schematic and movement of Figure 1. Schematic and experimental configuration of a triboelectric nanogenerator (TENG). (a) the TENG in a dynamic state, (b) material composition of the TENG, (c) photograph of the conﬁguration of the experimental Schematicequipment, and (d) plotmovement of the output of voltage, the TENG and (e) plotin a of dynamic the output currentstate, of(b the) material TENG. PLA: composition polylactic acid. of the TENG, (c) photograph of the configuration of the experimental equipment, (d) plot of the output voltage, and (e) plot of the output currentAs shown of inthe Figure TENG.1b, the PLA: interior polylactic housing of acid. the TENG was covered with an electrode and a positively charged material in that order. Aluminum tape was used as the electrode. The aluminum, nitrile, and paper were each used as positively charged materials to analyze As shown in Figuretheir effects1b, the on output.interior To createhousin theg ball of that the was TENG used as was the negatively covered charged with material,an electrode and a positivelya magneticcharged bar material was used to in stir th a 1:1at ratioorder. of part Aluminum A (D.S. NV10) tape and partwas B (curingused agent)as the for 5 min. A copper wool electrode of 0.9 g was mixed in the properly stirred silicone electrode. The aluminum, nitrile, and paper were each used as positively charged materials to analyze their effects on output. To create the ball that was used as the negatively charged material, a magnetic bar was used to stir a 1:1 ratio of part A (D.S. NV10) and part B (curing agent) for 5 min. A copper wool electrode of 0.9 g was mixed in the properly stirred silicone rubber; afterward, 17 g was poured into the 3D-printed mold created using

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rubber; afterward, 17 g was poured into the 3D-printed mold created using the FDM 3D printer and dried for approximately 6 h at room temperature (23 ◦C). Once created, the composite was inserted into the TENG housing, and a normal single wire (outer diameter of 1.3 mm) was connected to the top plate of the housing to create a pendulum motion, as illustrated in Figure1a.

2.2. TENG Device Assessment To measure the output performance of the constructed TENG device, we used an oscilloscope (TBS 2072, Tektronix, Oregon, U.S.A.), a voltage probe (P5100A, Tektronix), and a current preampliﬁer (DLPCA-200, Femto, Berlin, Germany), as shown in Figure1c . The maximum output was determined by applying resistance from 1 MΩ to 1 GΩ using a variable resistor. Additionally, we conducted an experiment in which we used a digital speed controller (subseries, SPG, Incheon, Korea) to create a frequency of 3–7 Hz for applying a pendulum motion to the TENG. The experiment also enabled us to determine the features that yielded an optimal dynamic behavior for the TENG device.

3. Results and Discussion 3.1. Preparation and Principle of the Potential Energy–TriboElectric NanoGenerator (P-TENG) A schematic of the TENG and its dynamic behavior are illustrated in Figure1, and detailed information is provided in the experimental section. The silicone rubber balls that were made using the mold were attached to the top plate of the housing using a normal single wire and located at approximately 70 mm to create a pendulum motion. As shown in Figure1b, the housing of the TENG consists of a PLA filament, aluminum tape, and nitrile. For the silicone rubber ball, copper wool was mixed into the silicone rubber interior to improve the output. As the copper wool was 400 µm thin, adding it to the interior of the silicone rubber yields a higher dielectric constant and capacitance than that of the normal silicone rubber. Hence, it is possible to obtain a higher output value because of the greater charge density that is transferred to the electrode [29,30]. Figure1d,e illustrate the output voltage and current of the TENG. To perform the output tests, the input frequency of the digital speed controller was fixed at 7 Hz, and the resulting output was a voltage of 210 V and a current of 9 µA. The output electricity generation can be described as the charge transfer that occurs between the silicone rubber ball and the Al electrode during physical contact. Figure2 illustrates the mechanism of current generation via charge transfer when the TENG operates in contact and separation modes. As shown in Figure2a, charge transfer does not occur in the completely separate initial state or the complete contact state, because electrical neutrality is maintained. However, while the silicone rubber ball and the positively charged electrical materials inside the housing are in the process of making contact or separation, charge transfer occurs to achieve electrical neutrality. During this time, the silicone rubber ball is charged as the negative pole, which has the quality of obtaining electrons easily, whereas the nitrile and aluminum are charged as the positive pole, which has the property of losing electrons easily. Owing to the opposite polarity of the two materials, charge transfer occurs via an external circuit to achieve electrical neutrality, and current is generated. Figure2b is a modeling image for performing electromagnetic field analysis using the finite element analysis (FEA) technique of COMSOL Multiphysics. To provide proof of the process described above, the results of an electrical field analysis using the COMSOL program are shown in Figure2c. The figure illustrates the changes in the electrical field according to the location of the silicone rubber ball. When the space (d) between the silicone rubber ball and the nitrile is 1 mm, a potential difference occurs in the contact part, and a voltage of approximately 200V is produced. This supports the claim that electron movement occurs when contact and separation occur between the silicone rubber ball and the interior of the housing. Micromachines 2021, 12, 1043 4 of 11 Micromachines 2021, 12, x FOR PEER REVIEW 4 of 11

Figure 2. Contact-Contact- and and separation-mode separation-mode mechanism mechanism an andd COMSOL COMSOL simulation simulation of the of theP-TENG P-TENG cyl- cylinderinder and and Dragon Dragon Skin Skin (D.S.) (D.S.) ball: ball: (a ()a schematic) schematic of of the the contact- contact- and and separation-mode separation-mode mechanism, ((b)) diagram showing showing modeling modeling for for the the simulation, simulation, and and (c) (gradientc) gradient image image depicting depicting COMSOL COMSOL simulation results. simulation results.

3.2. ElectricalTo provide Performance proof of the of the process P-TENG described above, the results of an electrical field analysis Figureusing 3the shows COMSOL the process program of optimizing are shown the in electrical Figure 2c. output The characteristicsfigure illustrates when the thechanges TENG in operates the electrical in the field vertical according contact to separation the location mode. of the To obtainsilicone optimal rubber databall. forWhen the conditionsthe space (d) under between which thethe silicone TENG rubber device ball operates, and the experimentsnitrile is 1 mm, were a potential performed difference using differentoccurs in frequencies.the contact part, The and experiments a voltage of analyzed approximately the voltage 200V and is produced. current data This that supports were producedthe claim that as the electron inputfrequency movement of occurs a digital when speed contact controller and separation were changed occur from between 3 to 7 Hz.the Atsilicone a frequency rubber ofball 3 Hz, and a the voltage interior (V) of the 10 V housing. and a current (I) of 3 µA were obtained. It was found that the voltage and current both increased linearly with frequency. Based on this, it3.2. is Electrical observed Performance that the contact of the area P-TENG increases, and the output voltage and current increase becauseFigure of the 3 shows increase the in process the force of acting optimizing on the siliconethe electrical rubber output ball and characteristics the motion energy when asthe the TENG frequency operates increases. in the vertical At 4 Hz, contact there separation was a V of mode. 4 V and To anobtain I of 4.5optimalµA, and data at for5 Hzthe, thereconditions was aunder V of 150which V andthe TENG an I of device 7 µA. Atoperates, 7 Hz, theexperiments V and I were were 250 performed V and 11 usingµA, respectively.different frequencies. The experimental The experiments results show analyz thated the the output voltage values and rosecurrent sharply, data startingthat were at 5produced Hz. This as occurred the input because frequency as the of frequency a digital speed increased, controller the amplitude were changed of the from pendulum 3 to 7 motionHz. At a of frequency the silicone of 3 rubber Hz, a voltage ball increased, (V) of 10 and V and it was a current possible (I) forof 3 the μA top were and obtained. bottom surfacesIt was found of the that TENG the voltage housing and interior current to makeboth increased contact effectively. linearly with Owing frequency. to this effect,Based theon this, output it is at observed 7 Hz improved that the bycontact more area than increases, 200 V compared and the withoutput that voltage at 3 Hz, and and current the I increasedincrease because by more of thanthe increase 8 µA. This in the was force proven acting by on the the governing silicone rubber equation ball for and a contact-the mo- andtion separation-modeenergy as the frequency TENG, asincreases. shown below.At 4 Hz, there was a V of 4 V and an I of 4.5 μA, and at 5 Hz, there was a V of 150 V and an I of 7 μA. At 7 Hz, the V and I were 250 V and Q 11 μA, respectively. The experimental Eresults= show, that the output values rose sharply, (1) Sε ε starting at 5Hz. This occurred because as the 0frequencyr increased, the amplitude of the pendulum motion of the silicone rubberσ balld increased, σ andx(t) it was possible for the top and V = − + x(t) + , (2) bottom surfaces of the TENG housingε0 interiorεr to make εcontact0 effectively. Owing to this effect, the output at 7 Hz improved by more than 200 V compared with that at 3 Hz, and Sσ d v(t) the I increased by more than 8 μA.I This= − was provenεr by, the governing equation for a con- (3) 2 tact- and separation-mode TENG, as shownd below.+ x(t) εr 𝑄 where Q is the value of the transferred𝐸= charges between, the two electrodes, S is the dielectric(1) 𝑆𝜀𝜀 area size, ε0 is the vacuum permittivity, εr is the relative permittivity, σ is the friction charge surface density, x(t) is the distance between two contact surfaces, t is the time, and d is the

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Micromachines 2021, 12, x FOR PEER REVIEWeffective dielectric thickness. According to Equations (1)–(3), when the frequency increases,5 of 11 the pendulum motion amplitude of the silicone rubber ball increases, and the contact area with the surface of the housing interior increases, which increases the wattage and current. Figure4 shows the normal form of the silicone rubber ball compared with those that had dimple and bump patterns𝜎 newly𝑑 applied𝜎𝑥 to( them𝑡) in order to effectively increase their contact area. Figure4aV= shows − a comparison+𝑥(𝑡)+ of the four, shapes of the silicone rubber(2) 𝜀 𝜀 𝜀 balls. They are only-D.S. ball, D.S.+Cu wool ball, dimple ball, and bump ball, in that order. Figure4b shows a graph illustrating the analysis of the changes in the voltage and current 𝑑 𝑆𝜎 𝑣(𝑡) values according to the differences in the ball𝜀 shape. As shown in Equations (1)–(3), the I=− , electrical output performance of the TENG is signiﬁcantly affected by the contact area.(3) 𝑑 Figure4 shows the normally shaped silicone +𝑥( rubber𝑡) ball compared with silicone rubber 𝜀 balls that have newly applied dimple and bump-shaped patterns to effectively increase their contact area.

FigureFigure 3. 3. PlotsPlots depicting depicting changes changes in in output output according according to to changes in frequency: ( (a,ba,b)) voltage voltage and and currentcurrent occurring occurring from from 3 3 to to 7 7 Hz. Hz.

whereIt 𝑄 is isobserved the value that of thethe D.S.+Cutransferred wool charges ball,dimple between ball, the and two bump electrodes, ball did S is not the show die- lectriclarge changes area size, in voltage𝜀 is the and vacuum current, permittivity, but the only-D.S. 𝜀 is the ball relative showed permittivity, markedly lower σ is the voltage fric- tionand charge current surface values. Thedensity, reason 𝑥(𝑡) for isthis the is distance because thebetween only-D.S. two ball contact was thesurfaces, only one 𝑡 is of the the time,four shapesand 𝑑 that is the operated effective in dielectric the single-electrode thickness. According mode because to Equations it did not contain(1–3), when Cu wool the frequencythat could increases, act as an electrode.the pendulum It is motion well known ampl thatitude the of the single-electrode silicone rubber mode ball increases, has lower andelectrical the contact output area performance with the surface than those of the of ho theusing contact interior and separation increases, modes.which increases Therefore, the it wattageis observed and thatcurrent. the only-D.S. Figure 4 ballshows obtained the normal markedly form lowerof the outputsilicone performance rubber ball compared compared withwith those that of that the had other dimple ball shapes. and bump Cu woolpattern wass newly added applied to achieve to them high in capacitance order to effec- and tivelyspeciﬁc increase inductive their capacity contact inarea. order Figure to receive 4a shows a high a comparison charge density of the from four the shapes electrode of the so siliconethat a high rubber output balls. value They could are only-D.S. be produced. ball, InD.S.+Cu the dimple wool model,ball, dimple a dimple ball, pattern and bump was ball,applied in that to the order. surface Figure to increase 4b shows the a contact graph area,illustrating but when the theanalysis model of was the used changes in practice, in the voltageit was found and current that the values generated according contact to area the wasdifferences slightly in reduced, the ball which shape. had As an shown adverse in Equationseffect on the (1–3), output the electrical value. It isoutput expected performa that thence contact of the TENG area will is significantly increase and affected the output by theperformance contact area. will Figure improve 4 shows if an the external normally force shaped makes silicone an extreme rubber change ball compared to the shape with of siliconethe ball. rubber However, balls within that have the frequencynewly app rangelied dimple of the and experiment, bump-shaped it is expected patterns that to effec- there tivelywill be increase no major their effect, contact because area. the outer shape of the silicone rubber ball was not greatly changed. In the case of the bump shape, wire shapes with a diameter of 0.6 mm and a height of 1 mm performed the role of improving the contact area when making contact

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Micromachines 2021, 12, x FOR PEER REVIEW with the surface of the housing interior. As shown in Figure4b, the highest6 outputof 11 voltage

and current were both obtained by the ball with bump shapes, which resulted in a V of 210 V and an I of 9 µA.

FigureFigure 4. Changes 4. Changes in the output in the values output according values according to the shape to ofthe the shape silicone of the rubber silicone ball. rubber (a) Pictorial ball. representation (a) Pictorial and photographsrepresentation of silicone and rubber photographs ball shapes: (i)of thesilicone modeled rubber shape ball and shapes: a 4× magnification (i) the modeled photograph shape and of the a silicone4× mag- rubber surfacenification of the ball photograph made only of of D.S., the (ii)silicone the modeled rubber shapesurface and of a the 4× ballmagnification made only photograph of D.S., (ii) of the the modeled silicone rubber surfaceshape of the and ball a made 4× magnification by mixing copper photograph wool into of the the silicone silicone rubber rubber ball surface interior, of (iii) the the ball modeled made by shape mixing and a 4× magnificationcopper photograph wool into ofthe the silicone silicone rubber surface ball interior of the ball, (iii) made the bymodeled addingdimple shape shapesand a to4× the magnification silicone rubber ball surfacephotograph to change the of contact the silicone area, and rubber (iv) the surface modeled of the shape ball and made a 4× bymagnification adding dimple photograph shapes ofto thethe silicone silicone rubber surfacerubber of the ballball made surface by addingto change bump-shaped the contact wires area, to theand surface (iv) the of modeled the silicon shape rubber and ball toa 4× change magnification the contact area. (b,c) Plotsphotograph showing theof voltagethe silicone and current rubbe accordingr surface toof differencesthe ball made in the by ball adding shape. bump-shaped wires to the surface of the silicon rubber ball to change the contact area. (b,c) Plots showing the voltage and current according to differencesFigure5 illustratesin the ball ashape. comparison and analysis of the changes in the output according to the size and quantity of dynamically behaving rubber balls in the TENG. First, to select It is observedthe that optimal the D.S.+Cu size for the wool silicone ball, rubber dimple balls, ball, the and mold bump was used ball to did prepare not show 10 pi, 20 pi, and large changes in voltage30 pi balls, and as current, shown in but Figure the 5only-D.S.a. Balls of ball sizes showed less than markedly 10 pi have lower an insufficient volt- mass age and current values.to facilitate The smoothreason operationfor this is of because contact/separation; the only-D.S. thus, ball 20 was pi and the 30 only pi balls one were also prepared based on the 10 pi ball. In addition, as the inner diameter of the housing is 80 mm, of the four shapes that operated in the single-electrode mode because it did not contain when a ball of 30 pi or larger is used, although the contact area increases and the output Cu wool that couldwill act increase, as an electrode. the contact/separation It is well known process that will the not single-electrode be performed smoothly. mode has Their masses lower electrical outputwere 0.73 perf g, 5.44ormance g, and than 15.8 g,those respectively, of the andcontact the masses and separation of the copper modes. wool within the Therefore, it is observedsilicone rubberthat the balls only-D.S. were 0.2 ball g, 0.4 obtained g, and 0.9 markedly g, respectively. lower As output shown perfor- in Figure5b,c, it mance comparedwas with found that thatof the the other voltage ball and shapes. current Cu values wool increased was added as the to sizes achieve of the high silicone rubber capacitance and specificballs increased. inductive This capacity can be explained in order byto Equationsreceive a (1)–(3).high charge As the density diameter from increased, the the electrode so thatfriction a high charge output surface value density could (σ be) value produced. increased In the along dimple with themodel, capacitance a dim- (C) value. ple pattern was appliedConsequently, to the the surface 30 pi siliconto increase rubber the ball contact created morearea, chargebut when transfers the model than those of the 10 pi ball, and it was able to produce a large output value. From the actual measurement was used in practice, it was found that the generated contact area was slightly reduced, results, the 10 pi ball had a V of approximately 30 V and an I of 2 µA, whereas the 30 pi ball which had an adversehad a Veffect of 200 on V the and output an I of 10value.µA. FigureIt is expected5d shows that a schematic the contact of silicone area will rubber balls increase and the outputconnected performance in quantities will of 1 impr to 4. Theove experimentalif an external conditions force makes were setan upextreme so that the balls change to the shapewere of fixed the ball. at 70 mmHowever, on the TENGwithin housing the frequency interior andrange a frequency of the experiment, of 7 Hz was applied, it is expected thatas there in the will experiments be no major with aeffect, single siliconebecause rubber the outer ball. Asshape the numberof the silicone of silicone rubber rubber ball was notballs greatly increased, changed. there was In nothe substantial case of the difference bump inshape, Vmax wireor Imax shapes. However, with by a observing diameter of 0.6 mm and a height of 1 mm performed the role of improving the contact area when making contact with the surface of the housing interior. As shown in Figure 4b, the highest output voltage and current were both obtained by the ball with bump shapes, which resulted in a V of 210 V and an I of 9 μA. Figure 5 illustrates a comparison and analysis of the changes in the output according to the size and quantity of dynamically behaving rubber balls in the TENG. First, to select the optimal size for the silicone rubber balls, the mold was used to prepare 10 pi, 20 pi, and 30 pi balls, as shown in Figure 5a. Balls of sizes less than 10 pi have an insufficient mass to facilitate smooth operation of contact/separation; thus, 20 pi and 30 pi balls were

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also prepared based on the 10 pi ball. In addition, as the inner diameter of the housing is 80 mm, when a ball of 30 pi or larger is used, although the contact area increases and the output will increase, the contact/separation process will not be performed smoothly. Their masses were 0.73 g, 5.44 g, and 15.8 g, respectively, and the masses of the copper wool within the silicone rubber balls were 0.2 g, 0.4 g, and 0.9 g, respectively. As shown in Figs. 5b and c, it was found that the voltage and current values increased as the sizes of the silicone rubber balls increased. This can be explained by Equations (1), (2), and (3). As the diameter increased, the friction charge surface density (σ) value increased along with the capacitance (C) value. Consequently, the 30 pi silicon rubber ball created more charge transfers than those of the 10 pi ball, and it was able to produce a large output value. From the actual measurement results, the 10 pi ball had a V of approximately 30 V and an I of 2 μA, whereas the 30 pi ball had a V of 200 V and an I of 10 μA. Figure 5d shows a schematic of silicone rubber balls connected in quantities of 1 to 4. The experimental conditions were set up so that the balls were fixed at 70 mm on the TENG housing interior and a frequency of 7 Hz was applied, as in the experiments with a single silicone rubber ball. As the num- Micromachines 2021, 12, 1043 ber of silicone rubber balls increased, there was no substantial difference in Vmax or7 ofImax 11. However, by observing the voltage and current waveforms, it can be seen that the density improved, and this is shown in Figure 5e. By observing the waveforms for a single silicone rubber ball, it can be seen that a single output was produced during the pendulum motion. the voltage and current waveforms, it can be seen that the density improved, and this is Similarly, it can be seen that in the case of two balls, two outputs were produced for a shown in Figure5e. By observing the waveforms for a single silicone rubber ball, it can single pendulum motion, while three and four balls produced three and four outputs, be seen that a single output was produced during the pendulum motion. Similarly, it can respectively. This helps to increase the Vrms and Irms values because the density in the be seen that in the case of two balls, two outputs were produced for a single pendulum waveform is increased. From the data values, one ball had a Vrms of 33.71 V and an Irms of motion, while three and four balls produced three and four outputs, respectively. This 0.904 μA, two balls had a Vrms of 39.954 V and an Irms of 0.948 μA, three balls had a Vrms of helps to increase the Vrms and Irms values because the density in the waveform is increased. 41,869 V and an Irms of 1.078 μA, and four balls had a Vrms of 45.825 V and an Irms of 1.27 From the data values, one ball had a Vrms of 33.71 V and an Irms of 0.904 µA, two balls had μA. Thus, it was found that the Vrms and Irms values increased as the number of balls in- a Vrms of 39.954 V and an Irms of 0.948 µA, three balls had a Vrms of 41,869 V and an Irms of creased. 1.078 µA, and four balls had a Vrms of 45.825 V and an Irms of 1.27 µA. Thus, it was found that the Vrms and Irms values increased as the number of balls increased.

FigureFigure 5.5. ((aa)) PictorialPictorial representationrepresentation of of various various sizes sizes of of silicone silicone rubber rubber balls; balls; (b ,c(b,c) plots) plots depicting depictingVmax Vmax, V,rms Vrms, I,max Imax,, and andI rmsIrms accordingaccording toto size;size; ((dd)) schematicschematic showingshowing aa comparisoncomparison ofof quantitiesquantities ofof siliconesilicone rubberrubber balls;balls; andand ((ee,f,f)) plotsplots ofof thethe voltagevoltage andand currentcurrent outputoutput waveform,waveform, andand VVmaxmax, ,VVrmsrms, I,maxImax, and, and IrmsI rmsaccordingaccording to quantity. to quantity.

3.3. Properties of LocationLocation and Material As shown in Figure6 6,, the the output output at at each each angle angle of of the the P-TENG P-TENG was was analyzed. analyzed. Experi- Exper- mentsimentswere were performed performed to to examine examine changes changes in in the the location location of of the the TENG TENG due due to changesto changes in thein the environment environment as as well well as as changes changes in in the the state state of of the the TENG TENG due due to to the the appliedapplied externalexternal force. As shown in Figure 6 6a,a, thethe experimentsexperiments werewereperformed performed on onframes frames built built with with angles angles of 0◦, 30◦, 60◦, and 90◦; the location of the TENG was ﬁxed at 70 mm inside the housing and the experiment was performed at a frequency of 7 Hz. In Figure6b,c, the highest values (200 V and 9 µA) were obtained at 0◦. Conversely, the lowest output values (40 V, 1.5 µA) were obtained at 90◦. The output values were the lowest at 90◦ because at 0◦ it was possible to make effective contact and separation on both sides of the housing during the pendulum motion, whereas at 90◦, a lower output value was obtained because of a smaller contact area in comparison with that for 0◦. Next, changes were made to the material of the positively charged materials inside the cylinder. A comparative analysis was performed on the effective positively charged materials following the triboelectric series, which are commonly encountered in daily life. Positively charged materials were created using Al, Al + nitrile, and Al + paper, as shown in Figure6d. The most basic way to improve the output performance of a TENG is to increase the surface potential difference. Depending on the choice of the positively charged materials used, the difference in surface potential can be increased. In Figure6e,f, when comparing the three materials, the output of Al + nitrile was similar to that of Al and Al + paper, but it was conﬁrmed that the RMS value was the largest, and thus Al + nitrile was selected as the positively charged material of P-TENG. Micromachines 2021, 12, x FOR PEER REVIEW 8 of 11

Micromachines 2021, 12, 1043 8 of 11 of 0°, 30°, 60°, and 90°; the location of the TENG was fixed at 70 mm inside the housing and the experiment was performed at a frequency of 7 Hz.

Figure 6. (a) Photographs showing the angles of the TENG cylinders; (b,c) plots of Vmax,Vrms,Imax, Figure 6. (a) Photographs showing the angles of the TENG cylinders; (b,c) plots of Vmax, Vrms, Imax, and Irms according to the cylinder angles; (d) photographs of the types of positively charged electrical and Irms according to the cylinder angles; (d) photographs of the types of positively charged electrical materials on the TENG interior surfaces; and (e,f) plots of Vmax, Vrms, Imax, and Irms values according materials on the TENG interior surfaces; and (e,f) plots of Vmax, Vrms, Imax, and Irms values according to the type ofof positivelypositively chargedcharged electricalelectrical materials.materials. 3.4. Application In Figures 6b,c, the highest values (200 V and 9 μA) were obtained at 0°. Conversely, As shown in Figure7, the output values found in the experiments were used to light the lowest output values (40 V, 1.5 μA) were obtained at 90°. The output values were the LEDs that can operate at low power. For experiments presented in Figure7a,d, the housing lowest at 90° because at 0° it was possible to make effective contact and separation on both applied with nitrile and the 30 pi bump ball were used at a frequency controlled at 7 Hz sides of the housing during the pendulum motion, whereas at 90°, a lower output value with a digital speed controller. For the experiment shown in Figure7e, the P-TENG was was obtained because of a smaller contact area in comparison with that for 0°. Next, fixed at the end of the cantilever structure, and pendulum motion was performed in the changes were made to the material of the positively charged materials inside the cylinder. left–right direction. To check the electrical power produced by the P-TENG, measurements A comparative analysis was performed on the effective positively charged materials fol- were taken at a variable resistance of 1 MΩ to 1 GΩ, as shown in Figure7a. As the lowing the triboelectric series, which are commonly encountered in daily life. Positively resistance increased, the voltage tended to increase and the current decreased. The results charged materials were created using Al, Al + nitrile, and Al + paper, as shown in Figure of calculating the output power values for each external resistance value showed that the 6d. The most basic way to improve the output performance of a TENG is to increase the maximum power values were 460 µW at 100 MΩ. As shown in Figure7b, a bridge rectifier wassurface used potential to assess difference. the charging Depending characteristics on the choice of the equipment,of the positively and thecharged AC signal materials was convertedused, the difference into a DC in signal. surface The potential capacitors can be used increased. in the charging In Figures tests 6e,f, had when the comparing following capacitances:the three materials, 0.82,1, the 1.5, output 2.2, 3.3, of 4.7,Al + and nitrile 10 nF.was Figure similar7c to shows that of a graphAl and of Al the + paper, charging but curveit was resultingconfirmed from that the the use RMS of a value variety was of the capacitors largest, under and thus the sameAl + nitrile input conditions.was selected The as timethe positively required tocharged charge material the 10 nF of capacitor P-TENG. to 14 V was 2.5 s. In Figure7c, a 10 nF capacitor and LEDs were connected to the rectifier circuit to conduct a charge/discharge test. The AC3.4. signalsApplication from the P-TENG was converted into DC signals with a rectifier circuit, and the 10 nFAs capacitor shown wasin Figure charged 7, the to lightoutput the values LEDs. found When in the the LEDs experiments were turned were on, used the to charge light storedLEDs that in the can capacitor operate at was low discharged. power. For experiments presented in Figures 7a,d, the housing appliedFigure7 withd is anitrile graph and of the 30 charge/discharge pi bump ball were curve. used To at verifya frequency the performance controlled at of 7 theHz P-TENGwith a digital as an speed actual controller. power source, For the a cantilever experiment and shown an LED in electricFigure 7e, circuit the wereP-TENG set up,was andfixed Figure at the7 eend shows of the the cantilever 30 LED lights.structure, The and proposed pendulum P-TENG motion has was a module-type performed in structurethe left–right for harvestingdirection. To various check kinetic the electr energies.ical power As a produced P-TENG generatesby the P-TENG, outputs measure- using a mechanismments were oftaken contact at a andvariable separation resistance modes, of 1 the MΩ experiment to 1 GΩ, as was shown conducted in Figure with 7a. a digitalAs the speedresistance controller increased, of a the vertical voltage motion tended structure to increase that allowsand the the current most decreased. effective operation The results of contact/separationof calculating the output of the power silicone values rubber for ball.each Aexternal frequency resistance of 7 Hz value was showed applied that for thethe experiment,maximum power and 30values LEDs were were 460 turned μW at on 100 with MΩ a. peakAs shown output in Figure of 250 V7b, and a bridge 11 µA. rectifier These detailswas used are to illustrated assess the in charging Figure7d. characterist The proposedics of P-TENG the equipment, is capable and ofharvesting the AC signal various was typesconverted of kinetic into energya DC signal. by being The attached capacitors to aus structureed in the that charging performs tests pendulum had the motionfollowing or rotational motion, in addition to the vertical motion. In Figure7e, the P-TENG was fixed at the end of the cantilever structure for pendulum motion, and the peak output of 130 V

Micromachines 2021, 12, x FOR PEER REVIEW 9 of 11

Micromachines 2021, 12, 1043 capacitances: 0.82, 1, 1.5, 2.2, 3.3, 4.7, and 10 nF. Figure 7c shows a graph of the charging9 of 11 curve resulting from the use of a variety of capacitors under the same input conditions. The time required to charge the 10 nF capacitor to 14 V was 2.5 s. In Figure 7c, a 10 nF capacitor and LEDs were connected to the rectifier circuit to conduct a charge/discharge test.was The obtained. AC signals In Figure from7 f,the the P-TENG P-TENG was was conv attachederted into to a DC rotating signals body with for a rectifier harvesting circuit,rotational and the kinetic 10 nF energy. capacitor The was peak charged output to of light 80 V the was LEDs. obtained When from the theLEDs experiment. were turned In this way, it was demonstrated that it is possible to obtain electrical output performance after on, the charge stored in the capacitor was discharged. attaching the proposed modular type P-TENG to various types of kinetic energy sources.

Figure 7. Application: ( (aa)) Plots Plots depicting depicting the the influence inﬂuence of of load load resistance on the P-TENG, ( b) plot depicting the use of capacitors with different capacitance values (0.82,(0.82, 1, 1.5,1.5, 2.2,2.2, 3.3,3.3, 4.7,4.7, andand 1010 nF),nF), ((cc)) plotplot ofof thethe curvecurve ofof charge/dischargecharge/discharge when two LEDs were connected,connected, (d) imageimage ofof thethe “ASD”“ASD” patternpattern LEDLED byby thethe P-TENGP-TENG (photograph of the digital speed controller and electrical circuitcircuit conﬁguration),configuration), ((ee)) pendulumpendulum kinetickinetic energy,energy,and and ( f()f) rotational rotational kinetic kinetic energy. energy.

4. ConclusionsFigure 7d is a graph of the charge/discharge curve. To verify the performance of the P-TENGIn this as study,an actual we proposedpower source, and implemented a cantilever and a P-TENG, an LED which electric is acircuit pendulum-motion- were set up, andbased Figure TENG 7e thatshows effectively the 30 LED collects lights. the The potential proposed energy P-TENG of a ball. has Whilea module-type the negatively struc- turecharged for harvesting and positively various charged kinetic materials energies. are As separateda P-TENG and generates an external outputs device using performs a mech- anismthe contact/separation of contact and separation of the charged modes, materials the experiment in the conventional was conducted setup, in with the proposeda digital speedmodule-type controller P-TENG, of a vertical a silicone motion rubber structur ball (thee that negatively allows the charged most material)effective operation and a nitrile of contact/separationrubber (the positively of the charged silicone material) rubber areball. both A frequency placed inside of 7 theHz housing.was applied This for setup the experiment,enables contact/separation and 30 LEDs were without turned the needon with ofan a peak external output device. of 250 As noV and external 11 μA. device These is detailsrequired, are the illustrated P-TENG in can Figure be freely 7d. The attached propos toed various P-TENG places is capable to convert of harvesting mechanical various energy typessources of tokinetic electrical energy energy by being for power attached supply. to a Tostructure improve that the performs generation pendulum performance, motion a orcopper rotational wool motion, electrode in was addition inserted to the into ve thertical silicone motion. rubber In Figure ball, thereby 7e, the increasingP-TENG was the fixedcapacitance at the end and of achieving the cantilever a higher structure output. for Additionally,pendulum motion, the dynamic and thebehavior peak output of the of 130P-TENG V was was obtained. analyzed, In Figure and the 7f, designthe P-TENG variables was that attached affected to a the rotating output body were for identiﬁed. harvest- ingThe rotational shape, size, kinetic and quantityenergy. The of the peak silicone output rubber of 80 V ball was signiﬁcantly obtained from affected the experiment. the contact Inarea this with way, the it was interior demonstrated surface of that the it housing is possible during to obtain dynamic electrical behavior. output To performance effectively afterimprove attaching the contact the proposed area, four modular new silicone type P-TENG rubber ball to shapesvarious (only-D.S.types of kinetic ball, D.S.+Cu energy sources.wool ball, dimple ball, and bump ball) were proposed, and it was found that the bump ball provided the highest generation performance. Additionally, it was found that the 4.generation Conclusions performance increased as the size and quantity of balls increased, and this is believed to be due to the increase in friction surface charge density and capacitance. Finally, the P-TENG device was used as a power source to light 30 LEDs and power commercial electronic products, which proved that the kinetic energy created by the potential energy of the ball can be effectively harvested as electrical energy. Varying output values can be produced when different angles are assigned to the TENG, and its shape status changes for each angle. By using these output values, it is possible to create a monitoring sensor Micromachines 2021, 12, 1043 10 of 11

that can detect the functional status and deformation states without directly checking the status. Therefore, the results of this study may be used for wind volume and wind speed measurement sensors in the future. This is expected to have two effects because a TENG can be used in a variety of situations as a module type.

Author Contributions: Conceptualization, methodology, writing—original draft, writing—review and editing, J.S. and S.J.; simulation software, writing—review and editing, J.Y.; validation, resources, writing—review and editing, J.P. All authors have read and agreed to the published version of the manuscript. Funding: This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (2018R1D1A1B07040446), the Korea Institute of Indus- trial Technology (KITECH) under the “AI Platform for Vision Systems and Applications Project” (JA210002), and the Korea Institute for Advancement of Technology (KIAT) grant funded by the Korea Government (MOTIE) (P0008458). Acknowledgments: We would like to thank the Korea University of Technology and Education Common Equipment Center for the use of its scanning electron microscope. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Vu, R.J.M.; SchaijkaI, R.; Doms, I.; Hoof, C.V.; Mertens, R. Micropower energy harvesting. Solid-State Electron. 2009, 53, 684–693. 2. Je, I.; Sem, J.R.; Wang, J. On-Body Bioelectronics: Wearable Biofuel Cells for Bioenergy Harvesting and Self-Powered Biosensing. Adv. Funct. Mater. 2019, 30, 1906243. 3. Chi, S.; Wa, T.; Hi, Y.; Ma, K.; Ikoma, T. Consistent ocean wave energy harvesting using electroactive polymer (dielectric elastomer) artificial muscle generators. Appl. Energy 2013, 104, 497–502. 4. Wu, X.; Lee, D.-W. An electromagnetic energy harvesting device based on high efficiency windmill structure for wireless forest fire monitoring application. Sens. Actuators A Phys. 2014, 219, 73–79. [CrossRef] 5. No, A.; Kis, R.A.; Po, B.; Sa, U.; Li, W.; Cruz, R.; Priya, S. High Power Density Body Heat Energy Harvesting. ACS Appl. Mater. Interfaces 2019, 11, 40107–40113. 6. Fan, F.-R.; Tian, Z.-Q.; Lin Wang, Z.L. Flexible triboelectric generator. Nano Energy 2012, 1, 328–334. [CrossRef] 7. Kim, W.; Bhatia, D.; Jeong, S.; Choi, D. Mechanical energy conversion systems for triboelectric nanogenerators: Kinematic and vibrational designs. Nano Energy 2019, 56, 307–321. [CrossRef] 8. Wang, Z.L.; Chen, J.; Lin, L. Progress in triboelectric nanogenerators as a new energy technology and self-powered sensors. Energy Environ. Sci. 2015, 8, 2250–2282. [CrossRef] 9. Wu, C.; Wang, A.C.; Ding, W.; Guo, H.; Wang, Z.L. Triboelectric nanogenerator: A foundation of the energy for the new era. Adv. Energy Mater. 2019, 9, 1802906. [CrossRef] 10. Du, T.; Zuo, X.; Dong, F.; Li, S.; Mtui, A.E.; Zou, Y.; Zhang, P.; Zhao, J.; Zhang, Y.; Sun, P.; et al. A self-powered and highly accurate vibration sensor based on bouncing-ball triboelectric nanogenerator for intelligent ship machinery monitoring. Micromachines 2021, 12, 218. [CrossRef] 11. Pu, X.; Guo, H.; Tang, Q.; Chen, J.; Feng, L.; Liu, G.; Wang, X.; Xi, Y.; Hu, C.; Wang, Z.L. Rotation sensing and gesture control of a robot joint via triboelectric quantization sensor. Nano Energy 2018, 54, 453–460. [CrossRef] 12. Li, R.; Wei, X.; Xu, J.; Chen, J.; Li, B.; Wu, Z.; Wang, Z. Smart wearable sensors based on triboelectric nanogenerator for personal healthcare monitoring. Micromachines 2021, 12, 352. [CrossRef][PubMed] 13. Hwang, H.J.; Lee, Y.; Lee, C.; Nam, Y.; Park, J.; Choi, D.; Kim, D. Mesoporous highly-deformable composite polymer for a gapless triboelectric nanogenerator via a one-step metal oxidation process. Micromachines 2018, 9, 656. [CrossRef] 14. Gao, Q.; Cheng, T.; Wang, Z.L. Triboelectric mechanical sensors—progress and prospects. Extreme Mech. Lett. 2021, 42, 101100. [CrossRef] 15. Zhang, S.L.; Xu, M.; Zhang, C.; Wang, Y.-C.; Zou, H.; He, X.; Wang, Z.; Wang, Z.L. Rationally designed sea snake structure based triboelectric nanogenerators for effectively and efficiently harvesting ocean wave energy with minimized water screening effect. Nano Energy 2018, 48, 421–429. [CrossRef] 16. Xiao, T.X.; Liang, X.; Jiang, T.; Xu, L.; Shao, J.J.; Nie, J.H.; Bai, Y.; Zhong, W.; Wang, Z.L. Spherical triboelectric nanogenerators based on spring-assisted multilayered structure for efficient water wave energy harvesting. Adv. Funct. Mater. 2018, 28, 1802634. [CrossRef] 17. Lin, Z.; Zhang, B.; Guo, H.; Wu, Z.; Zou, H.; Yang, J.; Wang, Z.L. Super-robust and frequency-multiplied triboelectric nanogenerator for efficient harvesting water and wind energy. Nano Energy 2019, 64, 103908. [CrossRef] 18. Lee, K.; Lee, J.-W.; Kim, K.; Yoo, D.; Kim, D.S.; Hwang, W.; Song, I.; Sim, J. A spherical hybrid triboelectric nanogenerator for enhanced water wave energy harvesting. Micromachines 2018, 9, 598. [CrossRef] Micromachines 2021, 12, 1043 11 of 11

19. Wang, X.; Niu, S.; Yin, Y.; Yi, F.; You, Z.; Wang, Z.L. Triboelectric nanogenerator based on fully enclosed rolling spherical structure for harvesting low-frequency water wave energy. Adv. Energy Mater. 2015, 5, 1501467. [CrossRef] 20. Park, J.; Han, M.; Kim, G.; Jung, Y.; Cho, H. Tree-wrapped triboelectric generator for harvesting wind energy. J. Nanosci. Nanotechnol. 2020, 20, 239–244. [CrossRef][PubMed] 21. Bae, J.; Lee, J.; Kim, S.; Ha, J.; Lee, B.S.; Park, Y.; Choong, C.; Kim, J.B.; Wang, Z.L.; Kim, H.Y.; et al. Flutter-driven triboelectrification for harvesting wind energy. Nat. Commun. 2014, 5, 4929. [CrossRef] 22. Win Zaw, N.Y.; Roh, H.; Kim, I.; Goh, T.S.; Kim, D. Omnidirectional triboelectric nanogenerator operated by weak wind towards a self-powered anemoscope. Micromachines 2020, 11, 414. [CrossRef][PubMed] 23. Xia, Y.; Tian, Y.; Zhang, L.; Ma, Z.; Dai, H.; Meng, B.; Peng, Z. An optimized flutter-driven triboelectric nanogenerator with a low cut-in wind speed. Micromachines 2021, 12, 366. [CrossRef][PubMed] 24. Kwon, S.-H.; Park, J.; Kim, W.K.; Yang, Y.; Lee, E.; Han, C.J.; Park, S.Y.; Lee, J.; Kim, Y.S. An effective energy harvesting method from a natural water motion active transducer. Energy Environ. Sci. 2014, 7, 3279–3283. [CrossRef] 25. Chatterjee, S.; Saha, S.; Barman, S.R.; Khan, I.; Pao, Y.-P.; Lee, S.; Choi, D.; Lin, Z. Enhanced sensing performance of triboelectric nanosensors by solid–liquid contact electrification. Nano Energy 2020, 77, 105093. [CrossRef] 26. Kim, T.; Chung, J.; Kim, D.Y.; Moon, J.H.; Lee, S.; Cho, M.; Lee, S.H.; Lee, S. Design and optimization of rotating triboelectric nanogenerator by water electrification and inertia. Nano Energy 2016, 27, 340–351. [CrossRef] 27. Chung, S.-H.; Chung, J.; Kim, B.; Kim, S.; Lee, S. Screw pump-type water triboelectric nanogenerator for active water flow control. Adv. Eng. Mater. 2021, 23, 2000758. [CrossRef] 28. Kim, T.; Kim, D.Y.; Yun, J.; Kim, B.; Lee, S.H.; Kim, D.; Lee, S. Direct-current triboelectric nanogenerator via water electrification and phase control. Nano Energy 2018, 52, 95–104. [CrossRef] 29. Kim, J.; Cho, H.; Han, M.; Jung, Y.; Kwak, S.S.; Yoon, H.J.; Park, B.; Kim, H.; Kim, H.; Park, J.; et al. Ultrahigh power output from triboelectric nanogenerator based on serrated electrode via spark discharge. Adv. Energy Mater. 2020, 10, 2002312. [CrossRef] 30. Park, J.; Cho, H.; Lee, Y.-S. Enhancing the triboelectric nanogenerator output by micro plasma generation in a micro-cracked surface structure. Appl. Sci. 2021, 11, 4262. [CrossRef]