Core–shell yarn-structured triboelectric nanogenerator for harvesting both waterdrop and biomechanics energies

Wearable energy harvesters (WEHs) have garnered significant attention recently due to their promising capabilities in powering wearable devices. In this research, we present a core–shell yarn-structured triboelectric nanogenerator (CY-TENG) that operates in two modes: the single-electrode TENG (SE-TENG) and the droplet-based electricity generator. This design facilitates energy harvesting from both waterdrops and biomechanics. The CY-TENG is fabricated using fluorinated ethylene propylene ultrafine heat-shrink tubes combined with stainless-steel yarns, ensuring its flexibility, durability, and weavability. Such attributes underscore its potential as a dual-function WEH.

[3] Mechanical energy from sources like vibrations, water waves, raindrops, and human motions is emerging as a potential energy source.Successfully harnessing these mechanical energy sources not only can it address the capacity limitations of batteries, but also extend the operational lifespan.Various methods like piezoelectric nanogenerators, 4,5) conventional electromagnetic generators, [6][7][8] and magnetostrictive vibration energy harvesters, 9,10) yet, many lack efficiency or are unsuitable for wearable devices.However, triboelectric nanogenerators (TENGs), introduced by Wang's group in 2012, 11) present a promising solution. TENGsconvert mechanical to electrical energy using Maxwell's displacement current, principles of triboelectrification and electrostatic induction.12,13) TENGs have shown significant potential in wearable energy harvesters (WEHs) for harvesting human motions in recent years.[14][15][16][17][18][19] To further expand the range of application scenarios of WEHs based TENGs and harness a broader spectrum of environmental mechanical energy.
In this study, we introduce a core-shell yarn-structured TENG (CY-TENG) which could harvest energy from both waterdrops and biomechanics as a WEH.][22] The CY-TENG is fabricated by stainless-steel yarns and ultrafine fluorinated ethylene propylene (FEP) heat-shrink tubes, results show good flexibility and resilience, and can be woven into a mesh pattern.To evaluate the output performance, we conducted electricity generation tests (with an electrometer (Keithley 6517B) and an oscilloscope (OWON, VDS 6104) equipped with a high-impedance (10 MΩ) probe) based on contact separation with soft solid material (nitrile rubber) and waterdrops on the CY-TENG, verifying its efficient operation in both modes.Furthermore, by weaving multiple CY-TENGs into a mesh pattern, the output performance can be enhanced, highlighting its potential for large-scale integration in practical application.
The schematic diagram of CY-TENG is shown in Fig. 1(a).To fabricate the CY-TENG, first, a double-helix stainless-steel yarn as inner electrode [Nissa Chain Co., Ltd.(Japan)] with diameter of 200 μm was inserted into one of the FEP heat-shrink tube [Tokyo Garasu Kikai Co., Ltd.(Japan)] with an inner diameter of 0.55 mm, and was preheated in an constant-temperature oven (EYELA NDO-401, Tokyo Rikakikai Co., Ltd., Japan) at 200 °C for 20 min to obtain complete heat shrinkage, so that the inner electrode is wrapped around the heat-shrink tube.The wrapped stainless-steel yarn is spirally wrapped around another stainlesssteel yarn as outer electrode and inserted into one of the thicker heat-shrink tube with an inner diameter of 1.40 mm.Then the prepared sample was heated in the oven again at 200 °C for 20 min to form a cohesive whole.Ultimately, a CY-TENG is obtained.Figure S1 shows the fabrication flow in detail.SEM (FE-SEM, JSM-7001F, JEOL, Japan) images in Figs.1(b-i) and 1(b-ii) demonstrate the structure of CY-TENG.Since the CY-TENG is fabricated by stainless-steel yarns and ultrafine heat-shrink tubes, it shows flexible and weavable as illustrated in Fig. 1(c) and Video S1.In making the outer spirally wrapped electrode form an exposed structure, a notch with a width of about 1 mm is cut out using a stainless-steel knife to expose the partial electrode.The exposed electrode is then used as the surface electrode of CY-TENG in DEG mode, and the inner electrode acts as the bottom electrode.Figure 1(d) shows the inner and outer tube before and after shrink caused by heating.
][25] The primary reason is that waterdrops cannot effectively undergo contact separation, and they heavily rely on interfacial effects. 26,27)igure 2(a) schematically describes waterdrop contacting and detaching from the CY-TENG in SE-TENG mode.Throughout this interaction, the internal electric field between the waterdrop and CY-TENG remains unchanged.The electrostatically induced charge is predominantly related to the charge intrinsic to waterdrop or the charge post-triboelectrification. As the waterdrop detaches from the FEP, an electric signal emerges at the electrode due to electrostatic induction, originating from the excess negative charge on the FEP.This charge corresponds to the waterdrop's charge introduced above ground.However, the charge inherent to the waterdrop, or generated by triboelectrification, is typically minimal.Previous studies indicate that the charge basically kept around 0.1 nC level, 28) making this mode unsuitable for harvesting energy from waterdrops.Conversely, Fig. 2(b) shows an alternate power generation mechanism under DEG mode with a partially exposed electrode when in contact with a waterdrop.In this mode, the triboelectrification between waterdrop and FEP is consistent with SE-TENG mode.However, the output generation process diverges significantly.The reason is that the charges initially stored in the inner tube are fully "harnessed," and due to the electric double layer (EDL) formed at the interface between waterdrop and the outer tube, the charges are released instantaneously when the waterdrop contacts the surface exposed electrode. 21,22)n order to assess the output performances for waterdrops energy harvesting, under these two modes, a comparative experiment was conducted.As shown in Figs.2(c)-2(e), when the waterdrop contacts CY-TENG under SE-TENG mode, the output current, output voltage, and transfer charge are essentially remained around 50 nA, 380 mV, and 0.175 nC, respectively.In contrast, under DEG mode, CY-TENG exhibits an output current of 0.4 μA, an output voltage of 1 V, and a transfer charge of 0.3 nC, respectively as depicted in Figs.2(f)-2(h).Notably, the DEG mode generates a 8-fold increase in output current compared to the SE-TENG mode as shown in Fig. S2, even though there is only two-fold difference in transfer charge as mentioned above.This discrepancy stems from the varying electron flow mechanisms between the two modes which would discuss in the subsequent sections.As a result, for CY-TENG, the DEG compensates for the limitations of the SE-TENG mode in energy harvesting from waterdrops.
Meanwhile, the CY-TENG in SE-TENG mode is demonstrated to effectively address energy harvesting from biomechanicss. Figure 3(a) illustrates the work mechanism: a finger, acting as a tribo-positive material, contacts CY-TENG to generate electricity.Some other common materials, such as human skin, clothing, or even waterdrops, can induce triboelectrification with FEP.Due to the electronegativity of FEP relative to most materials, 29) it readily acquires tribonegative charge.As the finger recontacts the FEP, the resulting electrostatic field disturbance, caused by surface charge, compels electrons in the electrode to flow, producing voltage and current in the external circuit.However, in CY-TENG, some outer electrodes on the surface are exposed, that might influence the output.To investigate the impact, we stripped away the outer FEP tube, exposing the whole outer electrode, and then tested the output during contact separation (Unless otherwise specified, the contact-separation frequency is set at 1 Hz and a contact area of 35 mm 2 ).To simulate an elastic contact/separate happens between flexible materials (human skins or clothes etc.), a platform was designed as illustrated in Fig. S3 where a soft sponge substrate was used.The results are presented in Figs.electrodes exhibited an output current and voltage of approximately 40 nA and 6 V, respectively.Upon analysis, we consider that even though a portion of the outer electrodes is exposed, indicating the absence of corresponding parts of the outer tube, the inner tube continues to serve effectively as a triboelectric part and charge storage layer, thereby minimizing the impact.Without exposed outer electrodes, Figs.been substantiated in previous studies. 30)However, intriguingly, the effect of droplet releasing height on the output appears to diverge from those findings to date. Figure S5 shows the output current and output voltage generated by a droplet releasing from a height of 250 mm instead of 50 mm.Results show that the output current and voltage exhibit a consistent tendency to approach a value 0.4 μA and 1 V, which was not initially anticipated.This is attributed to the size of the surface onto which the waterdrop drops being smaller than waterdrop.On a planar and large surface, the expansion area of the droplet is proportional to its releasing height. 31,32)Conversely, when a droplet impacts a narrowdiameter tube, it hardly achieves expansive dispersion and might even result in splashing.This restricts the effective contact area, thus leading to diminished output.
To further verify the performance of CY-TENG in practical applications, two tests are conducted on a woven CY-TENG: a palm slapping test and a simulated rain test.Figure 5(a) shows the schematic diagram of CY-TENG harvesting energy from both waterdrops and biomechanics.The output performance after a long-term dropping time based on continuous waterdrops are presented in Fig. 5(b).With continuous contact between the waterdrops and CY-TENG, the output current not only decrease but even shows an upward trend.We suspect that the waterdrops here promotes the accumulation of charge on the FEP, leading to a positive feedback effect on the output.Figure 5(c) shows the output characteristic of CY-TENG by palm slapping.Notably, both the current and voltage surge to a proximate value of 10 μA and 100 V, respectively.Additional videos (Video S4) can be found in the Supplementary Information to show the power generation ability intuitively.To emulate rain-driven electricity generation, a commercial shower nozzle was utilized.The output performance obtained is shown in Fig. 5(d).It can be observed that the frequency of waveforms becomes intensive as the waterdrops come into contact at multiple locations, besides, also benefits from rapid triboelectric charge accumulation, the current and voltage can reach up to 2 μA and 3 V, respectively.That indicates the C'-TENG's capability to harness energy from multiple waterdrops simultaneously.
In conclusion, our introduced CY-TENG effectively harvests energy from both waterdrops and biomechanics operating in two modes of SE-TENG and DEG, simultaneously.Owing to the yarn structure of CY-TENG, it shows flexible and weavable.With a woven CY-TENG, palm slapping directly produces an output current and voltage of 10 μA and 100 V, respectively.Under simulated rainy conditions, we   recorded a consistent output current, with average peak values nearing 2 μA.Therefore, the proposed CY-TENG holds potential for the development of dual-function WEH.

Fig. 1 .
Fig. 1.(a) Schematic image showing the structure of CY-TENG which mainly contains FEP heat-shrink tube as triboelectric material and stainless-steel yarns as electrodes.(b) Cross-sectional and top-view SEM images of CY-TENG.(c) Photographs showing the flexibility and weave ability of CY-TENG.(d) SEM images show the inner and outer tube before and after shrink after heating.
3(d)-3(f) illustrate the output current, output voltage and transfer charge results, respectively, varying contactseparation frequencies of 1-5 Hz.However, it is noteworthy that due to the limited contact area, the output generated by one CY-TENG is still low.Here, multiple CY-TENGs were woven and then electrically tested at a contact area of 35 mm 2 .The weaving density is shown in Fig. S4.As shown in the Figs.3(g)-3(i), and it can be found that the output current, output voltage and transfer charge could reach up to 400 nA, 40 V and 15 nC, respectively, varying frequencies of 1-3 Hz, respectively.When CY-TENG works in DEG mode, Fig. 4(a) illustrates the states of a waterdrop as it contacts CY-TENG.Dynamic interactions are detailed in Video S2 and S3.A simplified circuit diagram is provided in Fig. 4(b) to show the behavior of CY-TENG in harvesting energy from waterdrops.When the waterdrop does not contact the exposed electrode (above the CY-TENG or spreading on the surface of FEP), the circuit is open with a fixed capacitance composed of the inner electrode and the outer FEP tube (C FEP/Inner electrode ).R L represents the external impedance.As the waterdrop continues to spread and contacts the exposed electrode, the circuit becomes open.Here, R W and C Water/FEP represent the impedance of waterdrop, and the EDL capacitance formed at the interface between waterdrop and FEP, respectively.Importantly, since the EDL capacitance is considerably larger than C FEP/Inner electrode , a rapid charge transfer ensues upon the waterdrop's contact with the exposed electrode.Within this analysis, the waterdrop serves three critical roles: as a tribo-positive material, a circuit switch, and a part to form EDL. That supports the CY-TENG to harvest energy from waterdrops efficiently.The current output characteristics of waterdrops based on differing droplet sizes and rates are investigated and depicted in Figs.4(c) and 4(d), respectively.The output voltage characteristics are shown in Figs.4(e) and 4(f), and basically correspond with the same trend as the current.Notably, while the peak current value across various dropping rates remains relatively constant, those waterdrops with larger size yield a higher output current.Such a relationship, underscoring the positive correlation between output and surface area, has

Fig. 2 .
Fig. 2. Schematic diagrams showing the working mechanisms of CY-TENG in (a) SE-TENG mode and (b) DEG mode, respectively.(c) Output current, (d) voltage, and (e) transfer charge generated by a waterdrop in SE-TENG mode.(f) Output current, (g) voltage, and (h) transfer charge generated by a waterdrop in DEG mode.

Fig. 3 . 4 ©
Fig. 3. (a) Schematic illustrations of the working mechanism for CY-TENG in SE-TENG mode contacting and separating with finger.(b) Output current and (c) voltage generated by CY-TENG with totally exposed outer electrode in SE-TENG mode.(d) Output current, (e) voltage, and (f) transfer charge generated by CY-TENG varying contact-separation frequencies of 1-5 Hz, respectively.(g) Output current, (h) voltage, and (i) transfer charge generated by woven CY-TENG varying contact-separation frequencies of 1-3 Hz.

Fig. 4 .Fig. 5 .
Fig. 4. (a) Photograph depicting the motion of a waterdrop upon contact with CY-TENG.(b) Simplified equivalent circuit diagram before and after waterdrops contacting outer exposed electrode.(c) Output current and (d) voltage generated by waterdrops across three different radius sizes (From left to right: 3.6, 4.1, and 5.0 mm).(e) Output current and (f) voltage generated by waterdrops across five different dropping rates.