Flexible Single-Electrode Triboelectric Nanogenerator and Body

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A flexible single-electrode triboelectric nanogenerator and body moving sensor based on porous Na2CO3/PDMS film Chunmei Cui, Xingzhao Wang, Zhiran Yi, Bin Yang, Xiaolin Wang, Xiang Chen, Jing-Quan Liu, and Chunsheng Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17585 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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ACS Applied Materials & Interfaces

A flexible single-electrode triboelectric nanogenerator and body moving sensor based on porous Na2CO3/PDMS film Chunmei Cuia,b, , Xingzhao Wanga,b , Zhiran Yia,b, Bin Yanga,b,*, Xiaolin Wanga,b, Xiang Chena,b, Jingquan Liua,b and Chunsheng Yanga,b a

National Key Laboratory of Science and Technology on Micro/Nano Fabrication, Department of Micro/Nano electronics, Shanghai Jiao Tong University, Shanghai 200240, China b Key Laboratory for Thin Film and Micro fabrication of Ministry of Education, Shanghai Jiao Tong University, Shanghai 200240, China. *Corresponding author: Department of Micro/Nano electronics, Shanghai Jiao Tong University, Shanghai 200240, China. Email address: [email protected] (Bin Yang)

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Abstract Triboelectric nanogenerators (TENGs) converting mechanical energy into electrical energy have been widely concerned due to their huge potential applications for supplying power to electronic devices. Improving the performance of TENGs has become the research hot point due to their output limited current. In this paper, we propose a flexible single-electrode triboelectric nanogenerator based on porous Na2CO3/PDMS structure to enhance the triboelectric performance for nanogenerators. In order to compare their output performance, NaCl and sugar are normally used as sacrificial template for triboelectric nanogenerator. As an experimental result, the nanogenerator based on porous Na2CO3/PDMS structure obtains the open-circuit voltage of 125 V and maximum output current of 100 µA, which are higher than that generated by NaCl/PDMS and sugar/PDMS TENGs. And the generated electric energy of Na2CO3/PDMS TENG could instantaneously power 42 commercial LEDs without any energy storage devices. This developed porous Na2CO3/PDMS TENG could open a new application field for self-powered personal electronics, owing to its flexibility, simple manufacturing process, and the ability to harvest mechanical energy from human motions. Keywords: Triboelectric nanogenerators; Porous structures; Porosity; hydrophobicity; Self-powered sensors.


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1. Introduction With the rapid development of information technology and electronics industry, there are many low-power electronic devices emerging gradually. Meanwhile, energy harvesting technology has been extensively investigated because this technology is highly promising for self-powered electronic devices,1-13 which has a great significance for internet of things with long life-time. Therefore, in recent years most of researches mainly focused on micro energy scavenging coming from surroundings, especially mechanical energy such as body motion, machine vibration, wind, water wave and so on.14-22 Body motion is one of the most ubiquitous mechanical energies, which can be converted into electricity at anytime and anywhere.23-27 As a new type of energy harvester, triboelectric nanogenerators based on highly flexible, stretchable, transformable and mechanically durable materials may have a wide range of applications, especially for wearable or biomedical devices because of their high sustainability of all movements and body deformations.28-30 The basic working principle of the triboelectric nanogenerator is a combination of contact electrification and electrostatic induction effects.31-34 When one type of material is in contact with the other one, the electrons existed on the material surface are transferred due to the different abilities in capturing electrons of the two materials. The triboelectric charges generated on the dielectric surface can be retained for a long time, and thus serve as an electrostatic induction source for electricity generation process of TENGs.35,37 Different TENGs structures are targeted for different types of movements, resulting in various working modes.36 Traditional TENGs require two friction layers to contact as the function part of the devices, which limits their application.38 Thus the single electrode TENG has been widely utilized because it can be directly connected to any materials and placed anywhere for wearable application.39-42 But currently the output power of TENGs is too low to meet the power supplying requirement of some practical applications. Therefore, a surface or inner structure modification technique is an appropriate solution for improving output power because of the drastic increment of the effective contact area. Some



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nanogenerators for harvesting human body motion energy have been reported and important progress has been achieved to enhance the output performance and converting efficiency. Zhang et al. fabricated pyramids structures on the surface of functional materials to increase output voltage.12, 43 Lin et al. designed nanorod array to promote the triboelectrification44,45 and Hou et al. fabricated patterned PDMS with convex dome-shaped bump on the surface2. However, these convex microstructures were easily worn during triboelectric process, resulting in the decreasing of output performance. In addition, the combination of photolithography and etching processes was absolutely essential to form microstructures on silicon substrate. Thus, these methods often suffered from high cost. Therefore, it is much desired to perform a simple, low cost approach to obtain high performance triboelectric nanogenerators with good stability. Recently,

some

porous

microstructures

are

formed

for

triboelectric

nanogenerators with high output due to large surface area-to-volume ratio. Fan et al.29 chose NaCl as the template to obtain a stretchable porous triboelectric nanogenerator, which could produce an open-circuit voltage of 60 V and a short-circuit current of 180 nA. Kim et al.46 fabricated a triboelectric PDMS sponge by a cube sugar template and concluded that as the pore size increases, the amounts of the induced triboelectric charges at the contact surfaces were decreased according to the triboelectric principle. The optimum open-circuit voltage and current density were 150 V and 0.14 mA cm−2. Although the above two methods have improved the output performance of TENGs, there are still several shortcomings. First of all, the thickness of the porous PDMS sponge prepared by the template is 5000 µm, which is too thick to meet the requirements of small size and light weight for wearable device. Secondly, the increase in thickness would greatly extend the cleaning time, which will destroy the characteristics of PDMS. Thirdly, the typical pore sizes formed by sugar and NaCl are in the range of several hundred micrometers, which cannot significantly improve the output performance of the nanogenerator. To address these issues of the NaCl or sugar based TENGs, Lee et al.47 utilized polystyrene (PS) microspheres to fabricate PDMS sponges with the range of pores from 0.5 µm to 10 µm. But the new problem also ACS Paragon Plus Environment

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comes with the fact that the PS spheres are insoluble in water and can only be dissolved by acetone. As a toxic substance, acetone not only pollutes the environment but also endangers human health. Moreover, the price of PS spheres is expensive, resulting in an increase in production costs. Therefore, a method of low cost, simple process, friendly environment and obtaining thin films with small size pores is urgently needed. In this paper, we demonstrate a new route to realize a high performance single-electrode TENG by taking advantage of ultrathin flexible porous film structure. We have not only used Na2CO3 as a sacrificial template for the first time to fabricate small porous structures, but also have performed comparative experiments based on NaCl and sugar pore structures. This proposed fabrication method for triboelectric nanogenerator with porous structures has some advantages as follows. In terms of the production cycle, the cleaning time is reduced as the thickness of the film is only 280 µm, which not only shortens the preparation time but also ensures that the properties of the PDMS were not changed. In the output performance, the diameter of the pores can be easily controlled at the range between 15 µm and 20 µm, which greatly increased the surface area-to-volume ratio so the output voltage is effectively increased. As far as biocompatibility is concerned, the production process of the device does not involve the use of any harmful substance, thus ensuring that the device can contact the human body directly. In addition to these advantages, this ultra-thin device offers great flexibility for its wearable applications. Meanwhile, the water contact angle on the pores surface increases, which is attributed to the increase in the hydrophobicity due to the upward force generated by the air inside the pores, lifting the water droplets.46,48 As a testing result, this developed Na2CO3/PDMS TENG generates the output voltage of 125 V and maximum output current of 100 µA when it interacts directly with human skin. This TENG is well suitable for body motions as self-powered moving sensor.

2. Experimental



The fabrication process of porous films is shown in Fig. 1a. The PDMS precursor mixture (1:10 ratio of curing agent and PDMS, Sylgard 184A, Dow Corning Company) was prepared and spin-coated on the clean and spotless glass wafer with the rotate speed of 500 r/s for 60 s. Then the wafer was cured for 60 min at 80℃. This PDMS layer with 100 µm thickness was intended as primary flexible substrate for the device. Since the samples of NaCl, sugar and Na2CO3 are normally agglomerated and the particles size is not uniform, we prepared NaCl, sugar and Na2CO3 micro particles by grinding with same time to obtain uniform size of the sacrificial templates. These materials with the same weight percentage doping of 33% were separately doped into another PDMS precursor mixture with sufficient dispersion in ultrasonic dispersion instrument. The mixtures were subsequently spin-coated on the PDMS primary membrane with the speed of 630 r/s for 60 s，while the pure PDMS was still spin-coated at a speed of 500 r/s for 60 s to ensure the thickness of four films prepared of 280 µm (PDMS primary layer with 100 µm and composite layer with 180 µm). After curing，three composite films and a pure PDMS film were obtained. In order to remove sacrificial templates, the composite membranes were peeled off from glass wafers and cleaned in the deionized water solution with the ultrasonic instrument (KQ-50B, Kunshan Ultrasonic Instruments, Co., Ltd.) for 24 h and the power of the instrument is 50 W. The cross-section scanning electron microscopy (SEM) images of pure film and composite membranes were shown in Fig. 1b.The interface between PDMS primary layer and porous layer was obviously observed, and there is no gap between the two layers after cleaning. The PDMS primary layers are smooth and flat, while the porous layer had different sizes pores for the different sacrificial templates. The pores diameter of NaCl/PDMS film was about 90 µm, and the pores with the diameter of about 15 µm were distributed in the layer of sugar/PDMS and Na2CO3/PDMS films. The enlarged views of the porous sugar/PDMS and Na2CO3/PDMS membranes were shown in inset b. It’s obviously found that several small pores were agglomerated to form a large pore in the porous sugar/PDMS film, ACS Paragon Plus Environment

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while micro pores were separated and uniformly dispersed in the porous Na2CO3/PDMS film. The reason for the above situation is that the surface of sugar always has a thin film containing water. The size of sugar grains will affect the moisture content of sugar, and the smaller the grain the higher the moisture content will be. Therefore, when the sugar was ground, the moisture content of the sugar will increase. So the ground sugar powder will be reunited and hardly completely dispersed in the PDMS. The Cr/Au (thickness of 30/300 nm) electrode of nanogenerator was subsequently sputtered by the magnetron sputtering system on the flat surface of PDMS primary layer. The electricity generation mechanism of the fabricated single electrode triboelectric nanogenerator with porous film is schematically shown in Fig. 1c. When the porous thin film contacts with skin, PDMS is more likely to capture electrons and injected from skin on the surface of porous thin film. In the original position shown in Fig. 1c(i), when a pressure force is applied on the device by hand, the porous thin film fully contacts with the surface of skin. The produced triboelectric charges with opposite polarities are fully balanced, so that no electron flows in the external circuit. Once the pressure force is removed, the porous thin film starts to separate from the skin and the interacting surfaces have opposite triboelectric charges. The negative charges on the surface of porous thin film induce positive charges on Au electrode. Thus free electrons flow from Au electrode to the ground, which leads to producing an output current signal, as shown in Fig. 1c(ii). When the porous thin film continues to completely separate from the skin, the device will reach a charge equilibrium state, no output signals can be observed in Fig. 1c(iii). As the porous thin film moves toward human skin, the electrons start to move back toward Au electrode, resulting in a reversed output current signal, as shown in Fig. 1c(iv). When the porous thin film and human skin overlap again, the electric potential will return to the original status. This output signal generation process is a full cycle for the single triboelectric nanogenerator.



Figure 1(a) Schematic diagram of composite film preparation. (b) Cross-sectional SEM images of (i)PDMS, (ii)NaCl/PDMS, (iii)Sugar/PDMS and (iv)Na2CO3/PDMS films, respectively. (c) The process of triboelectric effect generation when the nanogenerator attached with human skin.

3. Results and discussion 3.1 Characterization of four different TENGs The stress-strain experiments were tested using Zwick Roell machine (test standard: GB/T 228-2002) with the film dimension of 3 cm×1 cm, and the curves were shown in Fig. 2a. It can be found that pure PDMS film obtained the lowest elastic modulus with the value of 1.57 MPa. The elastic modulus of NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS films is 1.65 MPa, 1.66 MPa and 1.83 MPa, respectively. It could conclude that the porous structures do not obviously increase the elastic modulus of the membrane. Therefore, these films still have good flexibility and stretchability. The optical transmission spectrum (wavelength range from 200 nm to 1100 nm) of the samples is shown in Fig. 2(b). The maximum transmittance of pure PDMS, NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS films at the visible light range


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(380 nm~780 nm) is 91.39%, 53.49%, 9.86% and 21.63% ， respectively. Compared with pure PDMS film, the transparency of the porous films is obviously reduced. In order to keep triboelectric charges for long time with stable and even electrical output, at least one of the interacting friction layers is insulated. Moreover, the surface charges are significantly influenced by the ambient relative humidity, and generally degrade with the increasing of humidity in atmosphere. Especially, the electrical output performance will be greatly reduced when the relative humidity in air reaches above 80%. As a result, hydrophobic insulating materials are in urgent need. In this paper, the hydrophobicity of four types of films is evaluated by their water contact angles through optical contact angle measuring

instrument

(DSA30,KRUSS

GmbH,

Germany),

and

their

corresponding images are shown in Fig. 2c. The water contact angle on the porous surface of pure PDMS, NaCl/PDMS and sugar/PDMS are 102.4°, 104.8° and 107.5°, respectively. The contact angle of Na2CO3/PDMS film obviously increases to 114.4°, which is attributed to the improving in the hydrophobicity due to the upward force generated by the air inside the pores, lifting the water droplets.



Figure 2 (a) Stress–strain curves of PDMS, NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS films. (b) The transmittance of PDMS, NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS films as the function of wavelength. (c)The water contact angle schematic of PDMS, NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS films.

3.2 Output performance of four different TENGs In order to further explore the output performance of fabricated nanogenerators, we pasted it to the human subject’s forearm and connected to the external electrical circuit. The open-circuit triboelectric output performance was shown in Fig. 3a, including pure PDMS, NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS triboelectric nanogenerators (TENGs). Obviously, Na2CO3/PDMS TENG had outstanding open-circuit voltage of 125 V, while pure PDMS, NaCl/PDMS, sugar/PDMS TENGs had the voltage of 25 V, 50 V, and 75V, respectively. And their large views of output signals with two cycles were shown in Fig. 3b. The


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output voltage of TENGs is related to the porosity, the size and shape of the pores. Firstly, the porosity obviously affected the electrical output of TENGs. The air with the lower dielectric constant (εair = 1.0) than PDMS (εPDMS = 3.0, measured in the air atmosphere) is mainly occupied in the pores47. Thus, when an external force is applied, the distance between the top and bottom electrodes on the film is significantly reduced, and the dielectric constant also rapidly increases under the mechanical force, which contributes to increasing the capacitance in the porous film. As a result, the large porosity effectively results in larger output voltage. The porosity (θ) was calculated by the following equation29, 49

ߠ = ቀ1 −

ெ

௏ఘ

ቁ × 100%

(1)

where M and V are the mass and the volume of a sample, respectively, and ρ is the density of a cavity-free control sample with the same volume. The porosity values of NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS films are calculated as 17.4%, 21.4% and 18.4%, respectively. Moreover, apart from the effect of the porosity on the performance of TENG, the size and shape of the pores also affect the output voltage. The decrease in pores diameter means an increase of the surface area-to-volume ratio. Therefore, the obvious increasing contact area will contribute to the static charges at the surface.47 The pores diameter of NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS films is about 90 µm, 15 µm and 15 µm. Although the pore sizes of sugar/PDMS film and Na2CO3/PDMS film are close, it is observed that the pores in the sugar/PDMS film are agglomerated, which will greatly reduce the area-to-volume ratio, as shown in Fig.1 b(iii). Therefore, the generated voltage of Na2CO3/PDMS TENG is obviously higher than that of NaCl/PDMS and sugar/PDMS TENGs.



Figure 3 (a) The open-circuit output performance of PDMS, NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS when the devices were slapped by hand. (b) Two cycle triboelectric performance of PDMS, NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS.

The close-circuit testing results were shown in Fig. 4(a-d). When the loading resistances varied from 10 kΩ to 150 MΩ, the output voltage was increasing and the current was decreasing. The maximum voltage and current value of pure PDMS, NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS TENG were 11.8 V and 0.8 µA, 40.5 V and 6.5 µA, 45.6 V and 36 µA, and 95.4 V and 71 µA, respectively. Further, their area power densities were calculated as 0.2 mW/m2, 12.0 mW/m2, 12.0 mW/m2 and 22.5 mW/m2, respectively. Apparently, the close-circuit output performance of Na2CO3/PDMS still expressed considerable results, while the performance of ACS Paragon Plus Environment

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NaCl/PDMS, sugar/PDMS and pure PDMS TENG presented reducing in turn. We found that there was a difference of the optimum matching resistance, which was the intersection point of the voltage and current curves for four devices. Compared with PDMS TENG, the resistance of NaCl/PDMS TENG (porosity value of 17.4%), sugar/PDMS TENG (porosity value of 21.4%), and Na2CO3/PDMS TENG (porosity value of 18.4%) had a decreasing trend due to the existence of pores in the film, and the high porosity could lead to the low optimum resistance. As we mentioned before, the pores distributed in the film could lift the droplets which would exist in inside the film. When an external force is applied, the pores would be compressed and the conductivity increases accordingly due to the increasing internal humidity. Most importantly, the droplets in the interior of the film would not affect the triboelectric effect because they are only related to surface effects. Therefore, the porosity of NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS films contributes to the low optimum internal resistance.

Figure 4 The trend of output voltage and current external loads from 10 kΩ to 150 MΩ. (a)The output performance of PDMS TENG. (b) The output performance of NaCl/PDMS TENG. (c) The output performance of sugar/PDMS TENG. (d) The output performance of Na2CO3/PDMS TENG.



Considering that the surface condition of different human skin in the different ambient would change during friction, which maybe affects the experimental results, the aluminium foil is utilized as the friction layer to further verify the experimental results, as shown in Fig. 5a. Although the open-circuit voltages of the four nanogenerators are increasing, illustrated in Fig. 5b, the trend is consistent with Fig. 3a. The open-circuit voltage of Na2CO3/PDMS TENG remains the highest, reaching 240V, while pure PDMS, NaCl/PDMS, sugar/PDMS, TENGs have the voltage of 10 V, 75 V, and 80V, respectively. The results in Fig.3a and Fig.5b show that the output performance of Na2CO3/PDMS TENG is outstanding. Table 1 shows the output performance comparison of the triboelectric nanogenerators with different porous structures. It was clearly observed that the proposed triboelectric nanogenerator in this work has better performance due to its smaller porous and larger surface area-to-volume ratio compared to other nanogenerators. Moreover, this developed ultra-thin nanogenerator has very good flexibility for powering wearable devices.

Figure 5(a) The schematic diagram of aluminum foil as a friction layer. (b) The open-circuit output performance of PDMS, NaCl/PDMS, sugar/PDMS and Na2CO3/PDMS when the devices interact with aluminium foil.


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Table 1 The output performance comparison of triboelectric generators with porous structures. Reference

Area ( cm2)

Thickness (µm)

Sacrificial template

Friction material

Pores diameter (µm)

Output voltage (V)

[29]

2.5×2.5

5000

NaCl

PDMS, CNTs

200-400

60

[46]

5×5

4900

Sugar

PDMS, Au

300

150

Na2CO3

PDMS, skin PDMS, Al

15-20

This work

3×3

280

125 240

3.3 The application of fabricated TENG An energy storage unit such as a capacitor is needed to store the energy from the AC pulses generated by the TENG to provide a constant bias voltage or current to power electronic devices. To demonstrate this capability, a circuit integrating the TENG with a full-wave bridge rectifier and a capacitor is constructed shown in inset Fig. 6a. When the nanogenerator charges different capacitors, the higher value of the capacitor, the slower the voltage will rise. At beginning, the load capacitors were charging at a maximum speed, then the charging speed gradually decreased and finally they would saturate at different value. The charging curve is shown in Fig. 6a. Besides, 42 commercial LEDs connected in series could be instantaneously powered by the generated electric energy of Na2CO3/PDMS TENG without any energy storage process, shown in Fig. 6c.



Figure 6(a) Voltage-time relationship at different load capacitances. Inset (a) The circuit diagram design of the TENG charging characterization under periodic mechanical motion.(b) 42 LEDs are connected in series with the TENG. (c) 42 LEDs are lighted by TENG.

In order to evaluate the application of this developed TENG, we utilized them as self-powered wearable sensor to detect the motion signal. The Na2CO3/PDMS TENG was placed at the human wrist, leg, arm and ankle parts because of their large motion amplitude during the sports process. The Na2CO3/PDMS TENG attachment position of wrist was shown in Fig. 7a(i), and the triboelectric output voltage was shown in Fig. 7a(ii). It could be found that the voltage signal possessed a sharp and larger peak value upward, and lower peak value downward. Moreover, the device expressed stability of triboelectric output during a period cycle (12 times in 10 s) with the value of about 5 V, as shown in Fig. 7a(iii). While we attached the device on the leg, the testing position was shown in Fig. 7b(i), and the triboelectric output voltage was shown in Fig. 7b(ii). It’s observed that the voltage signal form of leg position is clearly different from one of wrist position, and the larger peak-peak voltage of 7 V is obtained. During a period cycle we achieved stable and uniform triboelectric signal as shown in Fig. 7b(iii). Identically, we also attached device to the arm and ankle positions, and the obtained corresponding triboelectric output voltages in 10 s were shown in Fig. 7c and Fig. 7d, respectively. The characteristic voltage forms were recorded, and their corresponding peak-peak voltage values of 2.0 V and 1.5 V were


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obtained. Thus, the fabricated Na2CO3/PDMS TENG could be applied as a flexible, wearable, and portable power device to harvest the mechanical energy from human body movements, and also used as a potential self-powered moving sensor for detecting the produced signal from different attachment human positions. Due to the unique voltage forms obtained from human different parts, the potential potable and low-cost moving sensor could be further studied in future.

Figure 7 (a) (i)The diagram of the action when the device was fixed on the wrist.(ii) The cycle of voltage output in an operation.(iii) The voltage output of the device for ten seconds of movement.(b) (i)The diagram of the action when the device was fixed on the leg bend.(ii) The cycle of voltage output in an operation.(iii) The voltage output of the device for ten seconds of movement.(c)(i)The diagram of the action when the device was fixed on the arm.(ii) The cycle of voltage output in an operation.(iii) The voltage output of the device for ten seconds of movement.(d)(i)The diagram of the action when the device was fixed on the wrinkle.(ii) The cycle of voltage output in an operation.(iii) The voltage output of the device for ten seconds of movement.

4. Conclusions In this work, a porous Na2CO3/PDMS film based flexible wearable TENG was first proposed and used as a portable energy harvester and moving sensor. Considering the performance comparison, the pure PDMS, porous NaCl/PDMS, sugar/PDMS based TENGs were fabricated to evaluate the characterization and ACS Paragon Plus Environment


triboelectric performance. The experimental results show that the open-circuit voltage and maximum output current of the porous Na2CO3/PDMS TENG are 125 V and 100 µA, respectively. We demonstrated that the size and morphology of the pores could influence the effective contact area and hydrophilicity of the friction layers surface. Besides, we successfully utilized the Na2CO3/PDMS TENG to light 42 LEDs. Also we deployed this device as a moving wearable sensor on human different active positions including wrist, leg, arm and ankle, and achieved unique triboelectric output signals with different amplitudes. In summary, the low-cost, single friction layer mechanism, ultrathin and porous structure TENG will be a huge potential application for wearable and portable electronic devices.

Acknowledgements This research was supported in part by the 863 Program (2015AA043503) and Innovation Program of Shanghai Municipal Education Commission under Grant No. 14ZZ019 and the National Natural Science Foundation of China under Grant No. 61674104 and the Shanghai Committee of Science and Technology of China under Grant No. 14DZ1940305.

References (1) Dhakar, L.; Pitchappa, P.; Tay, F. E. H.; Lee, C. An intelligent skin based self-powered finger motion sensor integrated with triboelectric nanogenerator. Nano Energy. 2016,19,532-540. (2) Hou, T.-C.; Yang, Y.; Zhang, H.; Chen, J.; Chen, L.-J.; Lin Wang, Z. Triboelectric nanogenerator built inside shoe insole for harvesting walking energy. Nano Energy. 2013, 2, 856-862. (3) Jung, W.-S.; Lee, M.-J.; Kang, M.-G.; Moon, H. G.; Yoon, S.-J.; Baek, S.-H.; Kang, C.-Y. Powerful curved piezoelectric generator for wearable applications. Nano Energy. 2015, 13, 174-181. (4) Cheng, X.; Meng, B.; Zhang, X.; Han, M.; Su, Z.; Zhang, H. Wearable electrode-free triboelectric generator for harvesting biomechanical energy. Nano Energy. 2015, 12, 19-25. (5) Lee, M.; Chen, C. Y.; Wang, S.; Cha, S. N.; Park, Y. J.; Kim, J. M.; Chou, L. J.; Wang, Z. L. A hybrid piezoelectric structure for wearable nanogenerators. Adv. Mater. 2012, 24, 1759-1764. (6) Beeby, S. P.; Tudor, M. J.; White, N. M. Energy harvesting vibration sources for microsystems applications. Measure. Sci. Technol. 2006, 17, R175–R195. (7) Fan, F.-R.; Tian, Z.-Q.; Lin Wang, Z. Flexible triboelectric generator. Nano Energy 2012, 1, 328-334. (8) Fan, F. R.; Lin, L.; Zhu, G.; Wu, W.; Zhang, R.; Wang, Z. L. Transparent triboelectric nanogenerators and self-powered pressure sensors based on micropatterned plastic films. Nano Lett. 2012, 12, 3109-3114.


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Flexible Single-Electrode Triboelectric Nanogenerator and Body

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