Liquid-Metal-Based Super-Stretchable and Structure-Designable

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Liquid Metal-Based Super-Stretchable and StructureDesignable Triboelectric Nanogenerator for Wearable Electronics Yanqin Yang, Na Sun, Zhen Wen, Ping Cheng, Hechuang Zheng, Huiyun Shao, Yujian Xia, Chen Chen, Huiwen Lan, Xinkai Xie, Changjie Zhou, Jun Zhong, Xuhui Sun, and Shuit-Tong Lee ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b00147 • Publication Date (Web): 08 Feb 2018 Downloaded from http://pubs.acs.org on February 8, 2018

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ACS Nano

Liquid Metal-Based Super-Stretchable and Structure-Designable Triboelectric Nanogenerator for Wearable Electronics

Yanqin Yang1,δ, Na Sun1,δ, Zhen Wen1, *, Ping Cheng1, Hechuang Zheng1, Huiyun Shao1, Yujian Xia1, Chen Chen1, Huiwen Lan1, Xinkai Xie1, Changjie Zhou1, Jun Zhong1, Xuhui Sun1, * and Shuit-Tong Lee1,*

1

Institute of Functional Nano and Soft Materials (FUNSOM), Jiangsu Key Laboratory for

Carbon-Based Functional Materials and Devices, and Joint International Research Laboratory of Carbon-Based Functional Materials and Devices, Soochow University, Suzhou 215123, China.

δ

Y. Yang and N. Sun contributed equally to this work.

*Corresponding Author: Z. Wen: [email protected]; X. Sun: [email protected]; S. T. Lee: [email protected]

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Abstract The rapid advancement of intelligent wearable electronics imposes the emergent requirement for power sources that are deformable, compliant and stretchable. Power sources with these characteristics are difficult and challenging to achieve. The use of liquid metals as electrodes may provide a viable strategy to produce such power sources. In this work, we propose a liquid metal-based triboelectric nanogenerator (LM-TENG) by employing Galinstan as the electrode and silicone rubber as the triboelectric and encapsulation layer. The small Young’s modulus of the liquid metal ensures the electrode to remain continuously conductive under deformations, stretching to a strain as large as ~300%. The surface oxide layer of Galinstan effectively prevents the liquid Galinstan electrode from further oxidization and permeation into silicone rubber, yielding outstanding device stability. Operating in the single-electrode mode at 3 Hz, the LM-TENG with an area of 6 × 3 cm2 produces an open-circuit voltage of 354.5 V, transferred short-circuit charge of 123.2 nC, short-circuit current of 15.6 µA, and average power density of 8.43 mW/m2, which represent outstanding performance values for TENGs. Further, the LM-TENG maintains stable performance under various deformations, such as stretching, folding and twisting. LM-TENGs in different forms, such as bulk-shaped, bracelet-like and textile-like, are all able to harvest mechanical energy from human walking, arm shaking or hand patting to sustainably drive wearable electronic devices.

Keywords:

liquid

metal,

triboelectric

nanogenerator,

structure-designable, wearable electronics


super-stretchable,

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The rapid development of wearable electronics is attracting intense attention because of their promising applications in many fields. One key challenge for practical applications of such wearable devices is the need of sustainable power sources applicable to biological environments.1, 2 To date, such electronic systems are largely made of hard and brittle materials such as silicon and metals, limiting their applications where mechanical deformation is minimal. For wearable applications, the power sources are generally required to be deformable and stretchable to adapt to common human motions, such as joint bending, telescopic and waist rotation.3-5 Extensive efforts have been devoted to develop stretchable and compliant energy storage and conversion systems for flexible electronic devices, such as the combination of organic solar cells and flexible supercapacitors/lithium-ion batteries.6-8 While such self-powering systems possess some degree of stretchability, however the operating of solar cells depends on cycling of day and night as well as weather conditions,9, 10 handicapping their applications as a continuous power source. Therefore, development of deformable, compliant and stretchable devices that can harness mechanical energy from human motions and directly power wearable electronics is highly desired but challenging. Triboelectric nanogenerator (TENG) is endowed with the natural advantages of lightweight, small size, simple structure, high integration and inexpensive materials.11-14 Further, as TENGs can effectively harvest energy from irregular and low-frequency human motions, they may provide a viable strategy to power wearable electronics.15-18 Large efforts have already been made for TENGs to be functionally stable and effective under different kinds of deformations, such as bending, twisting, and stretching. The common fabrication method for stretchable TENGs is to disperse or mix conductive particles, e.g., carbon nanotubes or silver nanowires, with


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stretchable matrix materials PDMS or silicone rubber.19-23 However, the stretchability of those TENGs is greatly constrained by the sharp rise of resistance and irreversible damage of the flexible electrode during the motion process due to high Young’s modulus.24-26 Recently, shape-adaptable and stretchable TENGs employing ionic solutions of physiological saline as the electrode have been reported,27, 28 however the output performance of these TENGs was limited due to relatively lower conductivity of these ionic solutions. In this work, we report a super-stretchable and structure-designable liquid metal-based TENG (LM-TENG). The LM-TENG is made by using the silicone rubber layer as the triboelectric and encapsulation material and the liquid metal Galinstan as the stretchable electrode. A thin oxide layer naturally formed on the Galinstan electrode prevents further oxidization of Galinstan and its permeation into silicone rubber. The excellent fluidity and conductivity of the liquid metal enables the LM-TENG to be stretchable and functional to a strain as large as ~300%. Working in the single-electrode mode, the output performances of the LM-TENG under deformations such as stretching, folding and twisting were measured. In addition, three different forms of LM-TENGs, i.e. bulk-shaped, bracelet-like and textile-like devices, were fabricated and investigated. Lastly, the LM-TENG-based self-powered systems have been demonstrated to harvest mechanical energy from human walking, arm shaking or hand patting, and sustainably drive wearable electronic devices.

Results


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Figure 1. General characterization of the liquid metal based triboelectric nanogenerator (LM-TENG). (a) Photograph of the Galinstan liquid exposed in air (scale bar, 1 cm). (b) Ga 2p3/2 fitted XPS spectra of the natively oxidized Galinstan. (c) Schematic illustration of LM-TENG (inset shows detailed explanation of the protection of oxide layer). (d) Schematic illustration of the device structure in cross-sectional view. Contact angle of (e) an oxidized and (f) a non-oxidized Galinstan droplet on the silicone rubber surface. (g) Schematic illustration of the electricity generating mechanism. Photographs of the as-prepared LM-TENG at different deformed states, including (h) original, (i) folding, (j) twisting and (k) stretching state (scale bars, 2 cm). The general characterization of the components of the LM-TENG is depicted in Figure 1. Here, Galinstan, a eutectic alloy consisting of gallium, indium and tin, is selected as the liquid stretchable electrode for its low Young’s modulus, high conductivity and non-toxicity (Figure 1a).29-31 When Galinstan is exposed to air, a dense and thin oxidized layer instantly forms on the surface,32 for which the high-resolution X-ray photoelectron spectroscopy (XPS) shows two peaks at 1116.8 eV and 1118.9 eV corresponding to the Ga 2p3/2 signal of metallic gallium and Ga2O3,


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respectively (Figure 1b).33 The XPS spectra of tin and indium can be found in Figure S1, which shows only metallic tin and indium, respectively. The schematic structure illustrates that the LM-TENG mainly consists of three parts: the silicone rubber as the triboelectric and encapsulation layer, the natural gallium oxide layer as the protective layer, and the liquid metal as the stretchable electrode (Figure 1c and 1d). Note that the oxidized layer is dense enough to effectively prevent further Galinstan oxidization and its permeation into silicone rubber when stretched in a long-term period.34, 35 The wetting characteristic of a deformed Galinstan droplet on the silicone rubber surface further reveals the formation of an oxide layer in ambient (Figure 1e), which can be removed by etching in HCl.36 The non-oxidized Galinstan surface with higher surface tension becomes spherical with a contact angle of 143.2o (Figure 1f). Figure S2 shows the comparison of TENG composed of ionic solution and Galinstan as electrode. It is illustrated that weight of ionic solution-TENG dropped gradually while the LM-TENG kept constant. Due to the absence of proactive layer, the ionic solution is very easy to evaporate from the intermolecular space of silicone rubber. Thus, the oxide layer of Galinstan makes the stability and lifetime of LM-TENG both largely enhanced. Touching the LM-TENG enables the electricity generation due to coupling of contact triboelectrification and electrostatic induction (Figure 1g).37-42 The human body was applied as the ground for the LM-TENG mounted on the skin to form a single-electrode mode TENG (state I). When the skin touches the silicone rubber, negative charges would retain on silicone rubber and positive charges on the skin due to the order of electrification of these two materials (state II).43 When the skin moves away, positive charges would be induced on the liquid metal, driving free electrons flow from the liquid metal to the ground through the external circuit, forming an electrical signal (state III). When the skin and silicone rubber are separated far enough,


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the negative charges on the surface of silicone rubber would be fully balanced by the induced positive charges on the liquid metal, therefore no signal would be observed (state IV). When the skin returns to the silicone rubber, the induced positive charges on the liquid metal would decrease, and free electrons would flow from the ground to the liquid metal, forming a reverse electrical signal (state V). By cycling the contact and separation process between the skin and LM-TENG, an alternating current would be generated. To obtain a quantitative understanding about the working mechanism, numerical calculations showing the induced potential fluctuation in successive motion steps were carried out by employing COMSOL software, as demonstrated in Figure S3. The simplest case, the LM-TENG can be designed to be bulk-shaped (Figure 1h), which allows the liquid metal to adapt to different shapes and retain continuous conductivity under different deformations.44 The photographs of the LM-TENG under different operating circumstances vividly demonstrate the foldable (Figure 1i), twistable (Figure 1j) and stretchable (Figure 1k) capability of the device, respectively. The LM-TENG can be stretched to as long as 300%, and the stretchability depends on the type, thickness and curing extent of the silicone rubber. When tapped by hand, the LM-TENG can easily light up dozens of commercial LEDs under various deformations, as demonstrated in Supporting Movie S1-4.


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Figure 2. Structure design and output performance of the bulk-shaped LM-TENG for harvesting human walking mechanical energy. (a) Schematic illustration of the fabrication process. (b) Electrical output under various motion frequencies ranging from 0.5 to 3 Hz, including open-circuit voltage (Voc), short-circuit current (Isc) and transferred charge (Qsc). (c) Electrical output at different stretchable strains at the motion frequency of 2 Hz. Photographs of the bulk-shaped LM-TENG (d) stuck to a shoe sole (inset shows the bottom view) and (e) driving several LEDs by harvesting human walking mechanical energy (scale bars, 5 cm). The structure design and output performance of the bulk-shaped LM-TENG are schematically displayed in Figure 2. A typical LM-TENG was fabricated via a simple scalable and low-cost dark engraving mould process (Figure 2a). Initially, the silicone rubber mixture was poured into an acrylic groove which was pre-assembled and patterned with Kapton tapes as mold for the desired shapes (state I and II). After solidifying naturally at room temperature, the cured silicone rubber with a groove in the middle was peeled off from the acrylic mold (state III and IV). With a conducting copper lead wire on one side, the as-prepared silicone rubber piece was glued to another one by liquid silicone rubber to form a sealed hollow cavity (state V). Finally, liquid metal was injected into the cavity by two syringe needles with one injecting


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fluid and another pumping remaining air (state VI). The basic structure of the bulk-shaped LM-TENG mainly consists of liquid metal electrode in the interior and silicone rubber triboelectric layers on the exterior together with a copper lead wire. A hog skin with an area of 3 × 6 cm2 was employed to substitute the human skin in the test. When the motion frequency varies from 0.5 to 3 Hz, the open-circuit voltage (Voc) and transferred short-circuit charge (Qsc) remain almost constant at ~354.5 V and ~123.2 nC, respectively. Meanwhile, the short-circuit current (Isc) increases from 2.4 to 18.6 µA with increasing frequency (Figure 2b). To investigate the output power of LM-TENG, external load resistors ranging from 0.1 MΩ to 2000 MΩ were connected in series with LM-TENG, as shown in Figure S4. The average power density increases with increasing contact frequency, reaching a maximum value of 8.43 mW/m2 at 3 Hz. It is observed that the optimum resistance decreases with the increasing frequencies. And this can be explained that the impedance of TENG decreases with the motion speed so that the optimum resistance would decrease accordingly.45 The electrical output of the bulk-shaped LM-TENG under different elongations was also measured. Here the elongation Lx is defined as Lx = (L - L0)/L × 100%

(1)

where L0 is the initial length and L is the length of the device after being stretched. It can be seen that the Isc, Voc and Qsc increase initially and then slightly decrease (Figure 2c). This change can be explained by coupling the effects of contacting surface area, thickness of dielectric layer and electrode resistance.27, 28 According to Poisson’s effect, when the device is stretched, the silicone rubber would become thinner which would increase the amount of induced charges in the liquid metal so that Isc would increase as well.20 Meanwhile, the area of the contacting surface would first increase and then decrease when the device is stretched so that the electrical


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output would change accordingly. The calculation process is illustrated in Supporting Note S1. We have also measured the resistance change of liquid metal electrode under elongation, as shown in Figure S5. When the device is stretched, it is obvious that the resistance of liquid metal slightly increases. However, compared with the natively large impedance of LM-TENG, this change is negligible for the output performance of LM-TENG.46 The electrical output of the bulk-shaped LM-TENG under other kinds of deformations such as twisting and folding are shown in Figure S6. After recovered from the deformed state, the device shows electrical outputs comparable to the initial values, indicating outstanding device durability and stability. Compared with previously reported single-electrode stretchable TENGs (Table S1), the present LM-TENG exhibits outstanding output performance in terms of Voc, Isc and Qsc as well as excellent stretchability. As a demonstration, the bulk-shaped LM-TENG was stuck to a shoe sole to harvest mechanical energy from human walking to light up several green LEDs in series (Figure 2d and 2e). The detailed energy harvesting and power supplying process can be found in Supporting Movie S5.


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Figure 3. Structure design and output performance of the bracelet-like LM-TEMG for harvesting mechanical energy from arm shaking. (a) Schematic illustration of the fabrication process. Photographs of the bracelet-like LM-TENG at (b) initial state and (c) enlarged state (scale bars, 1 cm). (d) Electrical output under different strain levels at the motion frequency of 2 Hz, including Voc, Isc and Qsc. Photographs of the bracelet-like LM-TENG (e) worn on the wrist and (f) driving several LEDs by harvesting mechanical energy from arm shaking (scale bars, 4 cm). The specific characteristics of liquid metal enable the fabrication of LM-TENGs into different shapes according to requirements, such as the bracelet-like form, as shown in Figure 3. A typical bracelet-like LM-TENG can be constructed by using the basic structure of a fiber-shaped device made of a hollow fiber-shape silicone rubber filled with liquid metal (Figure 3a). Initially, the silicone rubber mixture was poured over a twisted iron wire, on which release agent had been sprinkled uniformly (state I and state II). After solidifying naturally, a cured fiber-shaped silicone rubber with a


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hollow structure was obtained after peeled off from the wire (state III and IV). The two ends of the hollow fiber-shaped silicone rubber were then glued together to form a hollow bracelet, into which a conducting copper wire was stuck (state V). Finally, the liquid metal was injected into the hollow bracelet (state VI). The bracelet-like LM-TENG can be enlarged as much as 100% to adapt to different diameter (Figure 3b and 3c). Here the enlargement Dx is defined as Dx = (D - D0)/D × 100%

(2)

where D0 is the initial diameter and D is the diameter of the device after being enlarged. With increasing enlargement, the electrical outputs of the bracelet-like LM-TENG show the similar trend as the bulk-shaped device, with the Voc, Isc, and Qsc of 54 V, 1.3 µA, and 20 nC in the initial state, and maximum output of 64 V, 1.5 µA, and 21 nC in the 50% enlarged state, respectively. After recovery from the enlarged state, the LM-TENG shows electrical outputs comparable to the initial ones, indicating excellent device stability and durability (Figure 3d). When two bracelet-like LM-TENGs were connected in parallel and worn on the tester’s wrist to harvest mechanical energy from arm shaking motion (Figure 3e and 3f), the LM-TENGs could light up dozens of commercial LEDs, as displayed in Supporting Movie S6.


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Figure 4. Structure design and output performance of the textile-like LM-TENG for harvesting mechanical energy from hand patting. (a) Schematic illustration of the fabrication process of a single fiber-shaped device. Photographs of the textile with knitting patterns of (b) 1×1, (c) 2×2 and (d) 3×3 nets (scale bar, 1 cm). (e) Voc and Isc of the three network textiles under different motion frequencies ranging from 0.5 to 2.5 Hz. (f) Voc and Isc of the 1×1 net textile-like LM-TENG under different strain levels at the motion frequency of 2 Hz. Photographs of the textile-like LM-TENG (g) fabricated on the clothes and (h) powering several LEDs by harvesting mechanical energy from hand patting (scale bars, 4 cm). To effectively accommodate the complex deformations induced by body motion, a fiber-based textile-like LM-TENG was also fabricated, as shown in Figure 4. The fabrication process of the fiber-shaped device is similar to that of the bracelet-like LM-TENG (Figure 4a). After obtaining the hollow fiber-shape silicone rubber (state I), the two ends of the device were then sealed (state II) and the liquid metal was injected into the hollow part (state III). By connecting several fiber-shaped LM-TENGs in parallel, three types of textile-like LM-TENG with knitting patterns of 1 × 1, 2 × 2 and 3 × 3 were constructed (Figure 4b, 4c and 4d). The output performances of the three kinds of textiles were measured under various motion frequencies ranging from


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0.5 to 2.5 Hz (Figure 4e). The area of hog skin was controlled to be the larger than the surface area of the LM-TENG, ensuring that the textile-like LM-TENG is in full contact with the hog skin. The total current output (ITsc) of LM-TENG increases with increasing number of single units under the same motion frequency due to increased contact area. For the same type of textile-like LM-TENG, the ITsc increases with increasing motion frequency. The total output voltage (VToc) remains almost unchanged in the different types under the different motion frequency due to their parallel connection. In addition, the photographs shown in Figure S6 reveal that the textile-like LM-TENG can be easily stretched. A relatively stable electrical output of 1 × 1 textile-like LM-TENG was measured (Figure 4f). As demonstrated in Supporting Movie S7, the textile-like LM-TENG can be fabricated into clothes and triggered by hand patting to light up several LEDs in series, providing an effective strategy to produce wearable power sources (Figure 4g and 4h).

Figure 5. Demonstration of the LM-TENG-based self-charging power system for powering wearable electronics. (a) Working circuit of the LM-TENG-based self-charging system, including LM-TENGs, rectifiers, commercial capacitors and wearable electronic devices. (b) Dependence of the charging voltage on different working frequencies of a bulk-shaped LM-TENG for charging a 33 µF commercial


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capacitor. (c) Charging curve of the capacitor connected to the power management and a wearable electronic device. The inset shows the rectified current of the LM-TENG. Photographs of the LM-TENG sustainably powering (d) a pedometer, (e) a minicalculator and (f) an electronic watch (scale bars, 1 cm) Since TENGs give pulse and alternating current that cannot be directly utilized by most conventional electronics, a management circuit is needed to enable TENGs for practical application, as demonstrated in Figure 5. The wearable self-charging power system was constructed with components including LM-TENG, bridge rectifier, commercial capacitor, and wearable electronic device (Figure 5a). The output of the LM-TENG is converted by the bridge rectifier from alternating to direct current, which then charges the capacitors. Although the rectifier and two controlling switches are not flexible or stretchable, it is possible to incorporate them into either a logo or a button due to their small size. A bulk-shaped LM-TENG was used to charge a 33 µF commercial capacitor with the simulated frequency ranging from 0.5 to 3 Hz. Figure 5b shows the charging speed increases with increasing working frequency. It takes the bulk-shaped LM-TENG 360 s to charge the commercial capacitor to 2 V at the frequency of 2.5 Hz, which is then able to drive wearable electronics such as an electronic watch for more than 50 s (Figure 5c). The wearable self-powered system was demonstrated to be capable of harvesting sufficient mechanical energy from human motion to power a pedometer (Figure 5d), a minicalculator (Figure 5e) or an electronic watch (Figure 5f), as displayed in Supporting Movie S8-10. Finally, the waterproof capability of the LM-TENG was investigated, by washing the device in a simulated sweat solution for over 2 hours, as shown in Supporting Movie S11. The electrical outputs of the LM-TENG before and after washing were almost the same, as shown in Figure S8, indicating that the LM-TENG would not be affected by human sweat or rainy weather. In addition, the long-term stability of LM-TENG was tested


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for 50000 cycles at the working frequency of 1 Hz, as illustrated in Figure S9. It can be seen that the Qsc during the first 50 cycles is close to the last 50 cycles, which indicates excellent stability. A little lower Qsc in the first 50 cycle is due to the process of charge accumulation at the beginning.

Conclusions In summary, we have developed a super-stretchable and structure-designable LM-TENG power source for wearable electronics by employing Galinstan as the stretchable electrode and silicone rubber as the triboelectric and encapsulation layer. The LM-TENG exhibits super stretchability (~300%) owing to the flexibility of liquid metal and silicone rubber, and superior device stability due to prevention of electrode oxidation and permeation by the surface gallium oxide layer. Working in the single-electrode mode at 3 Hz, the LM-TENG with an area of 6 × 3 cm2 produces the Voc, Isc, Qscand average power density of 354.5 V, 15.6 µA, 123.2 nC and 8.43 mW/m2, respectively, which represent outstanding values achieved in TENGs. Furthermore, the LM-TENG maintains steady output performance under stretched, folded and twisted deformations. LM-TENGs in different structures, such as bulk-shape, textile-like, and bracelet-like, were all capable of harvesting mechanical energy from human walking, arm shaking, or hand patting motions. Integrated in a self-charging system, the LM-TENG could charge up capacitors to power several wearable electronics. This work demonstrates a promising application of liquid metal as a stretchable electrode in fabricating stretchable power sources. We anticipate further optimization would enable the LM-TENG-based self-charging system to drive high-power electronic devices, such as smart watches or portable bracelets.

Methods


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Materials. Galinstan was purchased from Geratherm Medical AG with the composition rate of Galinstan 68.5% gallium, 21.5% indium, and 10% tin. Silicone rubber (Ecoflex 00-30) was purchased from Smooth-On, Inc. and used as-received. Fabrication of the LM-TENG. The liquid silicone rubber was prepared by mixing the base and cure (1:1 volume ratio) in a beaker. After mixing uniformly, the mixture was poured into the prefabricated acrylic mold. After solidifying naturally at room temperature, the cured silicone rubber was peeled off from the acrylic mold. No release agent was needed here for peeling off the silicone rubber due to the smooth surface of the acrylic plates. Two silicone rubber pieces were sealed together forming a sealed cavity. A copper lead wire was attached on one side for electrical connection. Finally, the liquid metal (1 mL) was injected into the cavity using two syringe needles with one injecting fluid and another pumping residual air. Characterization. The chemical states and the composition of oxidized Galinstan were determined by XPS measurements (Kratos AXIS UltraDLD) with Al Kα radiation (1486 eV) as a probe. The binding energies in the XPS analyses were corrected for specimen charging by referring the C 1s peak to 285.5 eV. The contact angle measurement (Dataphysics TBU 90E) was employed to test the wetting characteristics between liquid metal and silicone rubber surface. 37 wt% HCl solution was used to remove the oxidized Galinstan. For stretchability test, hog skin was attached to another acrylic plate with the same length and width as the stretched device. The bulk-shaped LM-TENG was stretched by bounding the two ends with tapes, which controlled the elongation of the LM-TENG. For bracelet-like LM-TENG, the hog skin was controlled much larger than the stretched bracelet-like device, and a circular acrylic plate with different radius was used to control the enlargement. For the textile-like TENG, hog skin was controlled to be much larger than the stretched


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textile-like device, four sides were stretched together. Electrical measurement. A linear motor (Winnemotor, WMUC512075-06-X) was used to generate the contact and separation process controlling contact frequency. The hog skin was attached to an acrylic plate which was mounted onto the linear motor. The LM-TENG was attached to another acrylic plate perpendicular to the direction of the motor. A programmable electrometer (Keithley 6514) was adopted to test the output performance, including Voc, Isc, and Qsc. For electrical connection of the device, a copper foil is inserted into liquid metal to draw out electrical signal. In the quantitative test, the copper foil is connected with the one terminal of Keithley 6514, and the other terminal of Keithley 6514 is connected to the grounded terminal of the socket for grounding to form the testing circuit. In the demo experiments, human body can be treated as the ground. The software platform was constructed on the basis of LabVIEW, which is capable of realizing real-time data acquisition control and analysis. The typical output performances were investigated by applying a linear motor at different frequencies, wherein the maximum distance between the hog skin and silicone rubber surface was set at 5 cm. The triboelectric area between the silicone rubber and hog skin was controlled to be the same as that of the stretched electrode. The input mechanical energy was acquired by the impact between the two pads. A pressure sensor was applied to test the input force which was ~6 N. And the linear motor was controlled by the software to move back and forward which ensures the same distance and same frequency so that the input energy is the same. A photograph of the testing equipment has been added in Figure S10.

ASSOCIATED CONTENT Supporting Information.


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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano. Figure S1. High-resolution XPS spectra. Figure S2. Weight change of LM-TENG and NaCl-TENG remained in room temperature for several days. Figure S3. COMSOL numerical calculations on the induced potential difference in successive motion steps. Figure S4. Dependences of the average power density on the resistance of the external load. Figure S5. The resistance change of the liquid metal during elongation. Figure S6. Electrical output of the bulk-shaped LM-TENG under twisting and folding states. Figure S7. Photograph of the textile-like LM-TENG at original and stretched state. Figure S8. Electrical output before and after washing test. Figure S9. Short-circuit transferred charge of the LM-TENG cycling for 50000 cycles at the working frequency of 1 Hz. Figure S10. The testing equipment of quantitative measurement of the LM-TENG. Note S1. The contacting area of LM-TENG during elongation. Table S1. Comparison of the output performance of single-electrode mode stretchable TENG with different components. Movie S1. Hand-tapping a LM-TENG to generate electricity at original state. Movie S2. Hand-tapping a LM-TENG to generate electricity at stretched state. Movie S3. Hand-tapping a LM-TENG to generate electricity at folded state. Movie S4. Hand-tapping a LM-TENG to generate electricity at twisted state. Movie S5. Harvesting human walking energy by a bulk-shaped LM-TENG. Movie S6. Harvesting arm shaking energy by two bracelet-like LM-TENGs. Movie S7. Harvesting hand tapping energy by a textile-like LM-TENG. Movie S8. Powering a pedometer by a bulk-shaped LM-TENG based self-charging power system. Movie S9. Powering a minicalculator by two bracelet-like LM-TENGs based self-charging power system. Movie S10. Powering an electronic watch by a textile-like LM-TENG based self-charging power system. Movie S11. Waterproof performance test of the LM-TENG. ACS Paragon Plus Environment

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Note S1. Calculations of contacting area of LM-TENG during elongation.

AUTHOR INFORMATION Corresponding Author *E-mail: Zhen Wen ([email protected]); *E-mail: Xuhui Sun ([email protected]); *E-mail: Shuit-Tong Lee ([email protected]). ORCID Zhen Wen: 0000-0001-9780-6876 Xuhui Sun: 0000-0003-0002-1146 Shuit-Tong Lee: 0000-0003-1238-9802 Author Contributions Y. Yang and N. Sun contributed equally to this work. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes A patent has been filed based on the research results presented in this manuscript.

Acknowledgement The work was funded by Natural Science Foundation of China (NSFC) (Grant No. U1432249), the National Key R&D Program of China (Grant 2017YFA0205002), the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and China Postdoctoral Science Foundation. This is also a project supported by Collaborative Innovation Center of Suzhou Nano Science & Technology. Dr. Z. Wen

thanks

the

support

from

China

Postdoctoral

Science

Foundation

(2017M610346), Natural Science Foundation of Jiangsu Province of China (BK20170343) and Nantong Municipal Science and Technology Program.

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17. Zi, Y.; Guo, H.; Wen, Z.; Yeh, M.-H.; Hu, C.; Wang, Z. L. Harvesting Low-Frequency (

Liquid-Metal-Based Super-Stretchable and Structure-Designable

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