A Motion‐ and Sound‐Activated, 3D‐Printed, Chalcogenide‐Based Triboelectric Nanogenerator

Abstract

A multilayered triboelectric nanogenerator (MULTENG) that can be actuated by acoustic waves, vibration of a moving car, and tapping motion is built using a 3D-printing technique. The MULTENG can generate an open-circuit voltage of up to 396 V and a short-circuit current of up to 1.62 mA, and can power 38 LEDs. The layers of the triboelectric generator are made of polyetherimide nanopillars and chalcogenide core–shell nanofibers. Scavenging waste energy is an alternative prominent solution, which may play an important role regarding the world energy problems. Nowadays, many portable electronic devices are produced and they require very low power for functioning.1, 2 A compact mobile device design is assumed to be light weight with a long-lasting battery.3, 4 However, the end of the battery life is inevitable. Besides, a mobile device should be plugged into an energy-distribution source in order to charge its battery, which limits the mobility of the user. A portable energy-harvesting system is a considerable energy source that never depletes. There are many piezoelectric nanogenerators proposed for this purpose.5-9 Drawbacks of piezoelectric materials are the requirements of complex and high-cost manufacturing processes such as electric polarization using high electric field strengths to obtain high-efficiency piezoelectric materials, high brittleness, and toxic elemental composition.10 In order to overcome these drawbacks, very sophisticated processing conditions and techniques are required.5, 11 In the last few years, many researchers have reported that nanogenerators based on the triboelectric effect are promising energy sources; they can harvest mechanical energy from human motions, sound, hydraulic pressure, and wind using the nanostructured surfaces of the dielectric materials.12-21 The triboelectric effect has been known for a couple of centuries, but it has not been clearly understood yet. However, there are a few significant attempts based on charged atom transfer,22, 23 ion transfer,24 and the bonding of radicals25, 26 to explain the mystery behind the triboelectric effect. When micro- and nanostructured dielectric surfaces are in contact with each other or separated, there are positive and negative charges generated on coated electrodes depending on the triboelectric polarity of those dielectric materials. Therefore, a perfect design for a triboelectric nanogenerator (TENG) can be achieved by choosing the most distinct materials in terms of triboelectric polarity as well as diminishing of the feature sizes of those materials down to nanometer scale for obtaining maximum contact area.20 A list of the triboelectric materials in accordance with their polarity is presented in Table S1, Supporting Information. Working mechanisms of TENG devices are based on two mechanical motion modes, contact and sliding mode.20 Recent studies reveal that the sliding mode has a better voltage output.13 However, it requires a more complicated device design. The output voltage of TENGs is enhanced above 1 kV by using different material combinations and device geometries.12 In this study, we constructed core–shell nanostructures (polyethersulfone (PES) is in the core and As2Se3 is in the shell) for building a 3D TENG device (Figure 1a-6,a-7). Aluminum tape was used as a substrate and contact electrode for both polyetherimide (PEI) nanopillars and As2Se3 core–shell nanostructures (Figure 1a-1,a-5). Detailed information about the fabrication of the As2Se3 core–shell nanostructures and the PEI nanopillars is given in the Supporting Information (see Figure S1). The fluorination process used in the fabrication of the PEI nano­pillars and surface modification of the As2Se3 nanostructures is given in Figure S2 in the Supporting Information. Figure S3 in the Supporting Information illustrates the fabrication process of the nanopillars. Our device can be stimulated by both motion and acoustic waves at different frequencies. We combined fluorinated As2Se3 kilometer-long core–shell nanostructures, which are produced using an iterative fiber-drawing technique, with PEI nano­pillars, which are produced by a template-based method for the construction of the contact mode TENG in a multilayer fashion. Our device has an output of maximum 1.23 mW direct current (DC) and 0.51 W peak power and can power parallel connected 38 LEDs simultaneously. Our chalcogenide-based TENG has maximum 396 V and 1.6 mA peak-to-peak output voltage and current, respectively. In addition, a finite element model is developed to explain contact electrification between core–shell nanostructures and nanopillars using COMSOL Multiphysics. A perfect match between analytically calculated open-circuit voltage (OCV) and OCV measurement for a single layer generator was presented. To enhance the performance of triboelectric devices, there are two important parameters that play a major role in the selection of material combinations: the surface properties and triboelectric polarity.20 Since the surface properties can be modified using various techniques, the triboelectric polarity is the most important inherent parameter that need to be considered. We analyzed the triboelectric response of many materials by measuring the OCVs and short circuit currents (SCCs) from 1 cm × 2 cm samples. The measurement setup and samples are illustrated in Figure S4a–c in the Supporting Information. Measurements were conducted under same conditions (3 g load and at 20 Hz tapping frequency) for all samples. Since the triboelectric effect is not well known and the triboelectric series is not perfectly and fully characterized, we combined different material sets which are promising for energy harvesting applications. The following materials were combined: carbon-loaded polyethylene (CPE), polycarbonate (PC), polyester (PEST), PEI, polydimethylsiloxane (PDMS), PES, poly(vinylidene flouride) PVDF, fluorinated polyetherimide (F–PEI), fluorinated arsenic selenide (F–As2Se3), and polytetrafluoroethylene (Teflon) (Table 1). Charge values and details of the measurement protocol are given in the Supporting Information. Teflon presents superior properties in terms of triboelectric properties. Unfortunately, it is not compatible with the iterative fiber-drawing process, which is an exciting fabrication technique11 that enables kilometer-long, parallel oriented, and polymer encapsulated nanostructures to be produced from a variety of materials, including As2Se327 and some fluorine-rich polymers such as PVDF.5 In the fabrication, there are at least two materials with compatible properties, a guard polymer jacket and a core material. Materials used in the iterative fiber-drawing process represent unique properties, e.g., low thermal expansion, low melting point (for core material), low glass-transition temperature, and no crystallinity. In this case, the core material is crystalline and the glass-transition temperature of the covering polymer preform should be higher than the melting point of the core material so that the core material can melt in the preform and the liquefied material takes the shape of the fiber (nanowire) during the process.5, 11 Unfortunately, Teflon is a highly crystalline polymer with a high melting temperature, which makes it incompatible with the iterative fiber-drawing process. Unlike Teflon, As2Se3 is perfectly compatible with the iterative fiber-drawing process because of its low glass-transition temperature, easy processibility, and amorphous structure.27 In addition, although arsenic is a toxic element, As2Se3 is an inert intermetallic material with no toxicity, that does not reflect any of the hazardous characteristics of arsenic and has been used in open cancer surgeries in fiber shape.28, 29 Although, the voltage peaks obtained from bare As2Se3 core–shell nanostructures were insignificant, the performance of the core–shell nanostructures was enhanced close to that of Teflon's after core–shell nanostructures were coated with fluorine monolayer. As a result of our experiments, we concluded that the materials with higher fluorine content tend to generate more triboelectric charges. The triboelectric effect is a surface contact and separation phenomenon, and increasing the surface area also increases the device performance. In addition to an increase in the total surface area by using nanoscale materials, using multilayers is also important to multiply the total converted energy. The working principle of a MULTENG device which consists of six alternating layers is illustrated in Figure 1b. We investigated the synchronism of the three units of MULTENG and voltage response at different frequencies (Figure S5, Supporting Information). To explain the working mechanism, we recorded video of a spare MULTENG during operation (tapping) using a high frame rate (HFR) camera (Video 1, Supporting Information). In addition, we designed a finite element simulation to confirm the HFR video results (Video 2, Supporting Information). Before examination of the working mechanism of the MULTENG, we should recall that the MULTENG has three units which consist of six layers making contact and separation in pairs (unit 1 consists of layer 1 and 2, unit 2 consists of layer 3 and 4, unit 3 consists of layer 5 and 6). The force for making contact between the layers of the MULTENG was applied through the top and the MULTENG was fixed from the bottom. The MULTENG started to squeeze when a triggering force was applied. In the early stage of its motion, the distances between the layers started to decrease in units 1 and 3 and slightly increase in unit 2. This is because the shape of the side connections of the three units was designed differently. The curvatures in convex connections of units 1 and 3 contracted and the curvature in concave connection of unit 2 expanded by applied force. Therefore, layers of units 1 and 3 approach each other whereas layers of unit 2 move apart from each other. According to simulation and HFR video recordings, the motions of the three units were at different speeds and this was observed as unit 1 to make contact first while unit 3 was slightly apart and unit 2 was completely separated. Following unit 1, layers of unit 3 were brought in contact position by further pressing on top of the MULTENG. In the last stage, after layers of units 1 and 3 made contact, they transmit force to unit 2 and thus, layers of unit 2 came in contact position. When the applied force was released, the layers of units were separated vice versa with making the contact sequence. First, the layers of unit 2 separated, second, the layers of unit 3 separated, and finally, the layers of unit 1 separated. Considering the fact that a MULTENG should be durable against high cyclic motions and each layer should perform perfect contact and separation, we designed a MULTENG with 175 mm in length, 50 mm in width, and 49 mm in thickness as shown in Figure 1a. A detailed technical drawing of our device is given in Figure S6a–f, Supporting Information. Poly(lactic acid) (PLA) was used as 3D-printing material because it has low density, high flexibility, and low melting temperature. As shown in Figure S6a,d, Supporting Information, there are four 1 mm-thick PLA layers connected to each other with three half circular loops, which provide a good restoring mechanism and flexibility. There are thicker rectangular (75 mm in length and 35 mm in width) rigid regions in the middle of these layers which facilitate making a better contact between layers during the triboelectric cycle. In addition to the main body, six PLA thick layers (80 mm in length and 40 mm in width) were printed (Figure S6e, Supporting Information) as a holder for the Al electrodes, As2Se3 core–shell nanostructures, and PEI nanopillars. 27 mm long and 500 μm thick tails were extended on PLA holders which render the Cu tape contacts as durable under high cyclic motions. In the assembling process, Cu tapes (10 mm in width, 50 μm in thickness) were attached on PLA holder surface along the width of holders and extended through flexible tails, 50 μm thick double-sided Al conductive tapes (40 mm × 80 mm) were located on holders in contact with Cu tapes in all six holders. Three of the holders with Al electrodes were covered with PEI nanopillars (100 nm in diameter) (Figure S7a, Supporting Information). F–As2Se3core–shell nanostructures (180 nm in diameter) were dispersed on the last three of the holders with Al electrodes (Figure S7b, Supporting Information). Finally, PLA holders with nanostructures on top of them were attached to the main body. To our knowledge, this is the first study showing that a chalcogenide material was used for constructing a triboelectric device. In the fabrication process of nanopillar-decorated PEI films, the melt-infiltration method30, 31 was used (Figure S3, Supporting Information). Here, dissolved PEI behaves as an infiltration phase and the anodized aluminum oxide (AAO) membrane behaves as a template under the base of melt-infiltration. The AAO template was coated with 1% (v/v) 1H,1H,2H,2H-perfluorodecyl-trichlorosilane (FDTS) in n-heptane solution prior to use.30 Fluorination prevents polymer adherence into the membrane pores. In this process, the AAO membrane, which was grown on the aluminum sheet, was directly used as supported on its aluminum substrate. The template has 5 cm × 10 cm dimensions with a 100 nm pore diameter and a 2 μm pore depth. PEI was dissolved in dichloromethane (DCM) at a concentration of 0.3% (w/v). PEI solution was drop cast on the AAO template and left for solvent evaporation. After the evaporation step, PEI film was peeled off from the surface. Therefore, 4 cm × 8 cm PEI nanopillar films were obtained for the fabrication of MULTENG. F–As2Se3 core–shell nanostructures were produced using iterative thermal size reduction technique based on fiber drawing (Figure S1, Supporting Information). Initially, we produced a As2Se3 tube that was 10 cm in length and 10 mm in diameter using the sealed ampule method (Figure S8, Supporting Information). In this method, elements were inserted in a quartz tube and vacuum sealed at elevated temperatures to remove all impurities and oxygen as well as the moisture. The mixture in the tube was melted in a rocking oven at 750 °C. After overnight rocking of the samples, the molten material was quenched in tap water for increasing the glass formation by overcooling the material. Before quenching, molten As2Se3 was centrifuged for the formation of a 10 cm long hollow core structure with 10 mm outer diameter and 6 mm inner diameter. To absorb the impact during the tapping on the MULTENG, we filled inside of the tube with a PES rod which was mechanically extracted out from a PES preform. After three steps of iterative fiber drawing, we obtained 100 nm–1 μm thick core–shell nanostructures. Detailed SEM images of the nanowires are shown in Figure S9 in the Supporting Information. Finally, nanostructures were coated with self-assembled fluorine mono­layer. The fluorination process was repeated as performed in the template coating step. Then, the nanostructures were washed with n-heptane and dried at 100 °C in a vacuum oven. Voltage and current responses of the MULTENG in a moving car (2010 Model Ford Fiesta) were characterized during transportation in Bilkent University Campus with various speeds (15, 30, 40 km h−1). The MULTENG was positioned on the front panel of the car. An Al block 1.3 kg in weight, 60 mm in height, 25 mm in width, and 280 mm in length was located perpendicularly on top of the MULTENG as the centers of the device and the metal block coincide (Figure 4a). Because the Al block pushes on top of the device, the initial position of the MULTENG was set as full contact with no response and reaction, as shown in the fourth stage of the triboelectric cycle in Figure 1b. As the car moved forward at various speeds, the Al block on the top tilted the layers of the MULTENG backward and forward which caused partially separation and contact of layers. Therefore, small jumps cause quick and weak separations between the layers while the car moves in a routine way. So, the input mechanical energy here can be low and the output signal can be very complex because the response of the MULTENG to car vibration depends on many parameters, such as the roughness of the road, the car model, the skills of driver, and the geometry of the MULTENG. The highest instantaneous power and power density were obtained at a 40 km h−1 speed as 127 μW and 13.23 mW m−2, respectively, as seen in Figure 4b. The instantaneous power and power density were increased with increasing car speed. The OCV and SCC values were measured as the car passed over speed bumps (maximum 13 cm in height and 150 cm in width) at 15, 30, and 40 km h−1 speeds. The driver of the car slightly pushed the breaks just before passing through the speed bumps. This action caused the MULTENG to tilt backward and separate its layers. As the car moved further on the speed bumps, the vibration and separation also increased due to the greater inclination. The MULTENG was tilted forward while the car leaped over the top of the speed bump to access the flat asphalt. OCV and SCC graphs with respect to car speed while passing over a speed bump are shown in Figure 4c–h. The maximum peak-to-peak OCV was measured as 61.96 V at 40 km h−1 speed as shown in Figure 4g. The maximum peak-to-peak SCC was measured as 2.05 μA at 40 km h−1 speed, as shown in Figure 4h. Since the response of the MULTENG to car vibration was complicated because of the multiple contact and separation sequences, we pointed single, double, and triple contacts and separations in different units of the MULTENG during the measurements. Our observations about the motion of the MULTENG in car measurements are shown in Figure S15 and S16, Supporting Information. We observed two types of motions (linear and angular) in the MULTENG units. Linear motions caused a strong response in the OCV and SCC and angular motions caused weak responses. In conclusion, we have developed a multilayered triboelectric generator based on As2Se3 core–shell nanostructures and PEI nanopillars, which were produced by iterative size reduction and melt infiltration techniques, respectively. Triboelectric response of a new materials set was characterized and we deduced that increasing the fluorine content in the materials increases the triboelectric response. The MULTENG was activated using mechanical tapping, car vibration, and sound with different frequencies and forces. The mechanical tapping force was the most powerful input among these three conditions and we obtained the highest signals in the mechanical tapping condition, since it made a stronger contact between layers. The results are summarized in Table S3 in the Supporting Information. The maximum peak-to-peak OCV and SCC were obtained as 396 V at 7.5 Hz and 1.62 mA at 2.5 Hz. The energy conversion efficiency of the device was calculated as 5.3% at 1 Hz tapping frequency and it was decreased with increasing tapping frequency. The MULTENG can supply 1.23 mW constant DC power and directly illuminate 38 LEDs as well as charge a capacitor via an energy-harvesting circuit. The voltage and current signals were recorded in a car during transportation on campus road and leaping through speed bumps at various speeds. The signal intensities and output power density (13.23 mW m−2 at 40 km h−1 speed) were increased with increasing car speed. Measurements at sound frequencies between 10 and 100 Hz revealed that our device has a better response at 10 Hz with peak-to-peak 107.3 V OCV and peak-to-peak 0.49 μA SCC. The output power density (max. 5.47 mW of the MULTENG decreased with increasing sound frequency. The new material and processes proposed in this study such as the which enhance the triboelectric properties of materials, new studies to explain the mechanism of the triboelectric effect. of the MULTENG was for portable electronic devices such as and with an energy-harvesting circuit. 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