PEO/cysteine composite nanofiber-based triboelectric nanogenerators for harvesting tiny mechanical energy

Abstract

Triboelectric nanogenerators (TENGs) are promising devices for capturing mechanical energy. However, traditional polymer triboelectric materials result in a burden to the environment, and the natural/biodegradable tribo-materials have the disadvantage of poor output performance. For this purpose, we proposed a polyethylene oxide (PEO)/cysteine composite nanofiber film (PCF) which is prepared from biodegradable polymer PEO and natural cysteine. Thanks to the superior tribo-positive properties of PEO and cysteine, the electrical performance of a PCF-based TENG (PC-TENG) with 4 wt% cysteine is several times greater than that of a pure PEO nanofiber film. In addition, the PC-TENG exhibits better power density (6.6 W m⁻²), which is 3–110 times more than that in studies using related eco-friendly materials as the tribo-layer. Importantly, we designed a multi-layer funnel-shaped TENG (MF-TENG) constructed from 4 layers of PC-TENG, which can effectively harvest tiny mechanical energy to build self-powered electronic devices by integrating a power management circuit. This research offers an efficient approach for the practical application of natural and environmental-friendly material-based TENGs in energy harvesting and power supply in the Internet of Things.

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Introduction

With the continuous advancement of Internet of Things (IoTs) technology, its applications have expanded into numerous fields, driving connectivity, efficiency, and sustainability. Given the current state of IoTs' expansion, the energy consumption of sensors is an issue that urgently requires a solution.¹ At the same time, the traditional battery-based wearable electronic devices' limited battery capacity issues need to be figured out.² A triboelectric nanogenerator (TENG) was first proposed in 2012, and was founded on the principles of electrostatic induction and triboelectrification.³ TENGs can convert various forms of mechanical energy^4–6 in the environment into electricity with the prospect of powering thousands of sensors in IoTs.^7–12 A TENG's unique output can demonstrate excellent self-powered sensing characteristics,^13–16 meeting the requirements of future IoTs for self-driven sensor components. Meanwhile, TENGs exhibit high efficiency in converting mechanical energy into electrical energy,^17–20 particularly under low-frequency conditions. The TENGs' electrical performance has a strong relationship with the tribo-positive/negative material, which has an important influence on energy conversion efficiency.^21,22 To increase the efficiency of the mechanical/electric energy conversion, various triboelectric materials have been developed.^23–25 Although the study of various triboelectric materials based on traditional polymer materials has greatly promoted the progress of TENGs, they are difficult to degrade naturally. Therefore, among the increasingly serious environmental problems, the development of triboelectric materials with environmental protection, biodegradability and other advantages has gradually become an important research direction.^26–28 Biodegradable materials usually have the advantages of being widely sourced, environmentally friendly, and easy to process. However, compared with traditional polymers, low electric output, poor durability, and weak mechanical strength still hinder the large-scale utilization of natural and biodegradable materials in TENGs.

Biodegradable materials can currently be divided into natural materials and synthetic biodegradable polymers. Natural materials, such as amino acids,²⁹ proteins,³⁰ polysaccharides³¹ and other materials have unique functional groups that donate/accept electrons, showing good prospects for triboelectrification. Researchers have greatly improved the triboelectric properties, durability, extreme situation resistance of natural materials through physical processes,³² chemical modification³³ and genetic engineering.³⁴ In particular, amino acids contain amino and carboxyl functional groups, which have excellent electron donating ability and are very suitable for positive triboelectric materials. By combining natural materials with advanced processing technology, preparation methods and doping of high dielectric constant materials, amino acid-based materials can gradually become a reliable triboelectric layer. In addition, through adjusting the structure, molecular morphology and properties of synthetic biodegradable polymers, they can exhibit the same mechanical strength as traditional polymers and exhibit excellent biodegradable capability comparable to natural materials.^35–37 Many examples of synthetic biodegradable polymers being applied in the TENG field have been reported, and many results have been achieved. Excellent stability and biocompatibility have been demonstrated in both vivo and vitro self-powered devices.^38,39 According to a study by Zhong Lin Wang,⁴⁰ functional group density of the polymer side chain affects the triboelectric properties, indicating that polyethylene oxide (PEO) has the strongest tribo-positive capability in the synthetic biodegradable polymer series. Although self-powered systems with natural and synthetic biodegradable tribo-materials promote the progress of green TENGs, the study of high-performance eco-friendly triboelectric layers still needs to be deeply explored to achieve a sustainable relationship between human-beings and nature.

In this work, a fully biodegradable polyethylene oxide (PEO)/cysteine nanofiber film (PCF) with high tribo-positive properties is developed utilizing electrospinning technology, and a PCF-based TENG (PC-TENG) and a multi-layer funnel-shaped TENG (MF-TENG) are constructed, which not only maintained a high charge density in a limited space, but also ensured a high peak power density. The tribo-positive material of the PC-TENG has strong electron loss capability due to the synthetic biodegradable polymer PEO and amino functional group of cysteine. Through the optimization of cysteine doping parameters, it is found that the open circuit voltage, short circuit current, and transferred charge of the PCF containing 4 wt% cysteine is 325 V, 48 μA, and 168 nC, respectively, which is 1.47, 1.65, and 1.51 times better than those of a pure PEO electrospun mat, respectively. In addition, it benefits from excellent tribo-positive properties, and compared with previously reported TENGs with an eco-friendly natural material as the tribo-layer, the prepared PC-TENG's power density can reach 3–110 fold higher.^{10,20,26,28–30,33,35} In addition, combined with an MF-TENG and corresponding power management circuit, a self-powered sensor system was designed for harvesting tiny mechanical energy. Therefore, this research lays the foundation for the practical application of high performance, environmentally friendly self-powered devices for harvesting tiny mechanical energy for the IoTs.

Results and discussion

Structural design and fabrication of PEO/cysteine-TENGs

Natural materials have widely acted as tribo-layers in the field of TENG's energy harvesting and self-powered sensing. However, designing a high-performance sustainable power supply using eco-friendly materials remains a significant challenge for TENGs. Therefore, the exploration of green natural materials with high triboelectric properties is currently a top priority in research. As illustrated in Fig. 1a, cysteine, an amino acid, is found abundantly in various natural sources, such as feathers, hair, fur, and even plants. Its amino group exhibits excellent electron-donating capabilities, making it highly suitable as a triboelectric material, and is combined with a PEO polymer network, which has strong positive triboelectric characteristics. Meanwhile, FEP is employed as a tribo-negative layer. To optimize the space utilization and contact separation efficiency of TENGs, the integration of a multi-layer funnel-shaped TENG (MF-TENG) is proposed, which has the characteristics of generating multiple outputs, a stable structure and achieving better autonomous separation in limited volume. The as-fabricated MF-TENG with the 4 units' working principle (Fig. 1b(i)) is through repeated pressing and releasing actions, and the surfaces of PCF and FEP become oppositely charged. Related detailed working states are shown (Fig. 1b(ii)). Among them, a PEO/cysteine nanofiber mat (PCF) and FEP are used as the tribo-positive and negative materials, respectively. Meanwhile, electrodes are 4 pairs of copper foil. Underneath the PCF side, a foam pad is used as the elastic buffer layer to enhance the area of effective contact between the tribo-layers and improve MF-TENG's output performance. In addition, Kapton serves as both the upper and bottom packaging layers, shielding the electrodes and serving as a spring to ensure smooth contact-separation. The electron transfer schematic process is illustrated in Fig. S1,† which results in the generation of a corresponding voltage and current during the contact-separation process of tribo-layers. Thus, the MF-TENG is capable of harvesting tiny mechanical energy when driven by the corresponding vertical bodily movement.

Fig. 1 Structure design and working principle of the MF-TENG. (a) The wide source of cysteine, and molecule structures of cysteine, PEO, and FEP. (b) (i) MF-TENG's working diagram and (ii) detailed structure diagram in the working state.

Fabrication and characterization of the PCF

Relative investigation indicates that PEO exhibits potential in the study of high-performance TENGs, since it is a synthetic biodegradable polymer material with good tribo-positive performance. To construct an environmentally friendly and high-performance TENG, further development for higher tribo-positive capability by doping with other biodegradable and natural additives is essential. Natural materials can be categorized into amino acids, proteins, polysaccharides, and other compound forms based on chemical foundations. For instance, outstanding performance is demonstrated by gelatin, silk fibroin, cellulose, lignin, and other biodegradable materials in terms of biocompatibility, degradability, and the triboelectric effect. However, there are few research studies about amino acid-based TENGs. Due to distinct –NH₂ functional groups, amino acids can be employed as tribo-positive materials for harvesting ambient mechanical energy. The results obtained from the density functional theory calculation of the DMol3 model using ‘Materials Studio’ simulation software are displayed in Fig. S2,† and the ground state electron density of functional groups in a molecule is obtained. First, the different coloured areas represent its ability to donate/accept electrons. In addition, according to the simulation results, the gap value between the HOMO and LUMO of PEO is 5.19 eV. In contrast, the gap of cysteine is 2.26 eV. The reduction of the HOMO–LUMO gap can lead to an increase in the tribo-positive ability of the material. Due to the unique electron donating functional group of cysteine and its lower HOMO–LUMO gap, the output of the PC-TENG can be improved by means of composite materials. Moreover, cysteine can also be highly soluble in water, which guarantees a smooth spinning process and the fiber's continuity. Additionally, the film's surface structure roughness and specific surface area can be efficiently increased by the electrospinning process, which also enhances the TENG's output performance. Fig. 2a provides a diagram of the PCF preparation process, and the Experimental section contains information on the related process. Scanning electron microscopy (SEM) surface morphology of a single PEO fiber and PCF is illustrated in Fig. 2b and c, where uniform fiber thickness is visible. Red dots in Fig. 2d are attributed to the existence of the S element on PCF. Thus, the surface structure and modification of PCF is accomplished successfully with an increase in the filler ratio. Besides, PCF is subjected to spectrum analysis using a Fourier transform infrared spectrometer (FTIR). As illustrated in Fig. 2e, the 1584 cm⁻¹ and 1612 cm⁻¹ absorption peaks are typical cysteine NH₃ symmetric bending signals related to cysteine's unique functional group. Besides, the peak at 2870 cm⁻¹ is assigned to PEO, which illustrates the substances' existence in PCF. By analyzing the XRD spectrum of PCF in Fig. 2f, it can be shown that the diffraction peaks at 2θ = 18.8° and 23.1° correspond to PEO which indicates that PEO is a high crystallinity polymer. At the same time, it can be found that cysteine does not affect the crystalline phase of PEO. As further demonstrated by X-ray photoelectron spectroscopy (XPS) spectrum results, the as-prepared PCF primarily consists of S, N, and C elements. The existence of amidogen and sulfur terminated groups in the polymer chain structure is suggested by the newly formed link in the amino group (–NH₂), sulfhydryl (–SH) and disulfide bond (–S–S–) owing to cysteine (Fig. 2g). The XPS S 2p, N 1s, and C 1s core–shell spectra of PCF are displayed in Fig. 2h–j, respectively. Furthermore, the O 1s core–shell spectra of the composite film are shown in Fig. S3,† and the peak at 533.68 eV is attributed to the ether group in the PEO polymer. Consequently, the aforementioned characterization clearly shows that cysteine and PEO have successfully developed excellent composite characteristics.

Fig. 2 Fabrication and characterization of the PEO/cysteine composite material. (a) The electrospun fabrication process. (b and c) SEM image of a single PEO and PEO/cysteine fiber, respectively. (d) Mapping of a single PEO/cysteine fiber. (e) FTIR, (f) XRD, and (g) XPS spectrum of cysteine powder, the PEO electrospun film, and the PEO/cysteine electrospun film. (h–j) XPS S 2p, N 1s and C 1s core–shell spectra of PCF.

The study of PEO/cysteine-TENG electrical properties

A PEO/cysteine-based TENG (PC-TENG) utilized PCF as the tribo-positive layer, while the FEP acts as the tribo-negative material. Thorough triboelectric characterization studies have been conducted to study the composite material's triboelectric performance and mechanical energy harvesting ability. As illustrated by Fig. S4a–c,† electrical performances of the PC-TENG are studied based on the different mass fractions of cysteine in PCF, including the open-circuit voltage (V_OC), short-circuit current (I_SC), and transferred charge (Q_SC). Under 2 Hz and 10 N operating parameters are investigated by using a linear motor (Fig. 3a); as the cysteine mass fraction increases from 0 to 4 wt%, the PC-TENG's V_OC gradually increases from 218 to 321 V, I_SC gradually increases from 14.9 to 24.7 μA, and Q_SC gradually increases from 108 to 163 nC. The result of output varying with doping is shown in Fig. 3b. Surface transferred charge density of the PC-TENG reaches 18.11 nC cm⁻² which shows more potential in energy harvesting than previous biodegradable material-based TENGs (Fig. S5†).^18,20,26,28 Nevertheless, the addition of 5 wt% cysteine causes the V_OC, I_SC, and Q_SC to drop to 310 V, 21.7 μA, and 149 nC, respectively. The findings demonstrate that the triboelectric performance of composite nanofibers may be efficiently enhanced by doping with an appropriate amount of cysteine. In comparison to a pure PEO film, the voltage, current, and surface transferred charge output of the PC-TENG with 4 wt% cysteine PCF can be enhanced 1.47, 1.65, and 1.51 times, respectively. When filler concentration reaches 5 wt%, the reason for the PC-TENG's decreasing output performance is that cysteine crystals affect the formation of polymer networks. As illustrated in Fig. S6,† the diameter of a single fiber of PCF with 5 wt% cysteine is not uniform, which affects the triboelectric properties of the electrospun layer. Additionally, the TENG's electrical energy is used for powering external electronic equipment by utilizing the full-wave rectification circuit which is demonstrated in Fig. S7† to store it in energy storage capacitors or batteries. A PC-TENG with 4 wt% cysteine PCF is evaluated at various pressures and frequencies in order to thoroughly assess the electromechanical conversion ability of the device. Fig. 3c–e illustrate the PC-TENG's output performance at various loading vibration frequencies from 1 to 5 Hz. Under a given force of 10 N, it can be observed that the V_OC and Q_SC essentially remain constant at 313 V and 163 nC, whereas the I_SC exhibits an increasing trend as the frequency increases. PC-TENG's output performance under various applied forces is demonstrated in Fig. 3f–h. The force increases with an increasing trend in V_OC, I_SC, and Q_SC at a constant frequency of 2 Hz. With an increase in pressure from 2 N to 10 N, the V_OC, I_SC, and Q_SC increase to 310 V, 25.5 μA and 165 nC, respectively. This not only demonstrates that the prepared PC-TENG has a high triboelectric-generating output, but also shows that I_SC has a positive correlation with applied force frequency and a linear relationship with external force. Concurrently, a sequence of resistances from 10⁴ to 10⁹ Ω are connected with the PC-TENG with 4 wt% PCF under set conditions (10 N and 2 Hz) to determine the voltage and power density curve (Fig. 3i). This demonstrates that the voltage of load resistance initially stabilizes and then rapidly increases to the value of V_OC. Additionally, an illustration of the association between resistance and power density shows that, with an external resistance of 10 MΩ, the device with 4 wt% cysteine PCF can attain a power density of 6.60 W m⁻². However, environmentally friendly triboelectric material-based TENG systems have long struggled with power density. Thanks to PCF's superior positive triboelectric performance and its 4 wt% cysteine content, the power density of the related PC-TENG has improved significantly. As demonstrated in Fig. S8,† advanced biodegradable material-based TENGs consisted of an amino acid material (L-valine²⁹), two protein materials (keratin¹⁰ and fibroin^20,30), two polysaccharide materials (cellulose²⁶ and sodium alginate³³) and three biodegradable synthetic polymers (PLGA/PVA²⁸ and PBAT³⁵). From those related research studies, PC-TENG's power density is 3–110 times more than that in earlier biodegradable and eco-friendly tribo-material research. The conditions remain the same as in the previous test, based on PC-TENG's excellent energy harvesting capability, Fig. 3j illustrates the analysis and comparison of PC-TENG's charging capabilities for several commercial capacitors. The results show that the PC-TENG can charge a 0.47 μF commercial component to 11.45 V after 30 s, which exhibits potential for harvesting ambient energy. Furthermore, the TENG's life-span and operational reliability are crucial components that guarantee its implementation for self-powered devices. The stability of a PC-TENG with 4 wt% cysteine PCF over the long term is studied. Fig. 3k displays the V_OC results under a 10 000 s continuous impact at a 2 Hz and 10 N external force by using a linear motor. Inset data show that there is no discernible change in the V_OC value, which suggests its outstanding stability and durability for prolonged usage.

Fig. 3 PC-TENG's electrical performance testing. (a) Schematic diagram of a linear motor. (b) V_OC, I_SC, and Q_SC of the PC-TENG with varying cysteine mass percentages. (c) V_OC, (d) I_SC, and (e) Q_SC of a PC-TENG with 4 wt% cysteine PCF at various impact frequencies. (f) V_OC, (g) I_SC and (h) Q_SC of a PC-TENG with 4 wt% cysteine PCF under various impact forces. (i) Voltage and power density under varied external load. (j) Commercial capacitors' charging curve. (k) 10k cycle stability test of the PC-TENG.

The study of MF-TENG electrical properties

To better harvest tiny mechanical energy, a multi-layer funnel-shaped TENG (MF-TENG) is developed to boost the device's harvesting efficiency in limited space. To assess the MF-TENG's electrical output thoroughly, measurements are made using a 4-layer MF-TENG containing 4 wt% cysteine PCF at different frequencies and applied forces. Fig. 4a–c illustrate the 5-layer MF-TENG's output performance at various loading vibration frequencies from 1 to 5 Hz. Due to the relatively small inherent capacitance of TENGs, precise measurement of the V_OC is unattainable for most measuring devices. Besides, multi-layer's high output can overload the electrometer. As a result, oscilloscope measurements of the V_OC of the MF-TENG are used as testing tools. As the frequency increases, the I_SC shows an increasing trend from 77 μA to 191 μA, while the V_OC and Q_SC essentially remain steady at the level of 833 V and 0.886 μC, respectively. Output performance of the 4-layer MF-TENG under various pressures is displayed in Fig. 4d–f, V_OC, I_SC, and Q_SC exhibit an increasing tendency at fixed 2 Hz as forces increase. The V_OC increases from 395 V to the level of 768 V, the I_SC increases from 66.4 μA to 105.7 μA, and the Q_SC increases from 583 nC to 856 nC as the external applied force ranges between 2 N and 10 N. The reason for this increase is that a foam pad as a buffer layer is added between the electrode and the packaging layer at the end of the high-performance tribo-material, which allows more effective contact area between the tribo-positive/negative materials. To better evaluate the actual effect of foam pads on the whole device, control groups without a buffer layer were manufactured.

Fig. 4 MF-TENG's electrical performance testing. (a) V_OC, (b) I_SC, and (c) Q_SC of the MF-TENG at various impact frequencies. (d) V_OC, (e) I_SC and (f) Q_SC of the MF-TENG under various impact forces. (g) Buffer layer's improvement to MF-TENG. (h) Voltage and power density under varied external load. (i) Commercial capacitors' voltage curve on charging by the MF-TENG.

As demonstrated in Fig. 4g, the MF-TENG with a buffer layer showed about 20% improvement in V_OC, I_SC, and Q_SC. The data demonstrate that the foam buffer layer can successfully increase TENG's output performance. To deeply verify the potential of MF-TENGs for energy harvesting, voltage curves and peak power density were obtained at 2 Hz and 10 N. The relevant output curves produced by adjusting the external load and measured using a programmed electrometer are displayed in Fig. 4h. When an external 50 MΩ is connected, peak power density of the MF-TENG reaches 3.65 W m⁻². Furthermore, as shown in Fig. 4i, a series of commercial capacitors are used to test the charging capability of the MF-TENG. With the linear motor set at 2 Hz and 10 N, the MF-TENG is used to charge capacitors. According to the comparison of the capacitor charging curve between the PC-TENG and MF-TENG, the MF-TENG can charge a 4.7 μF capacitor to 0.1 V in 3 s, while the result of the PC-TENG is 0.028 V. Furthermore, the durability test of the MF-TENG also shows that its output can still maintain the level of 95% within 10 000 s (Fig. S9†).

Application of MF-TENGs

Due to the light weight, high space utilization and unique energy harvesting principle, combination of the prepared device with a high tribo-positive layer and the power-management (PM) circuit can provide energy for a large number of electronic devices in the IoTs system (Fig. 5a). As demonstrated in Fig. 5b–d, when the MF-TENG was placed on the sole of the foot, changes in V_OC, I_SC and Q_SC with gait can reach 402 V, 115 A and 1048 nC, respectively. It is considered that the prepared MF-TENG has advantages in harvesting tiny mechanical energy, and hand clapping is also a common electromechanical transformation method. Therefore, the electrical output produced by tapping the MF-TENG with the hand is tested. As shown in Fig. S10,† the relevant output can reach 315 V, 59 μA, and 742 nC, respectively. Due to the high inherent resistance characteristics of the TENG, a high voltage output with small current needs to be converted into high current results through the voltage-buck method. The PM circuit can achieve a lower voltage and high current. The integrated connection and a photograph of the human foot and MF-TENG, PM circuit, and electronic device is illustrated in Fig. 5e(i) and (ii), respectively. Besides, the as-demonstrated PM circuit can quickly charge the capacitor by using the gas discharge tube as the energy accumulation and release switch. As the voltage is reduced, the current increases due to the conservation of power (P = U × I), meaning that lowering the voltage results in higher output current which allows the acceleration of capacitor charging. In addition, as shown in Fig. 5f, after comparing the amount of transferred charge between the MF-TENG and MF-TENG with a PM circuit, the voltage buck value is 29 times higher than that of the MF-TENG without a PM circuit. In Fig. 5g and the inset figure, 10 μF capacitor's voltage curves on charging by a PC-TENG, MF-TENG, and MF-TENG with a PM circuit are shown, respectively. By placing the device on the sole of the foot, the 10 μF capacitor charge to 0.5 V takes 0.3 s, 1.0 s, and 25 s by gentle stepping, respectively. From the view of charging time, the charging efficiency of a MF-TENG with a PM circuit is 3.3 and 83.3 times that of a MF-TENG and PC-TENG, respectively. As demonstrated in Fig. S11,† tiny energy are collected by a MF-TENG with/without a PM circuit, according to E = C × U²/2, which also indicates the effectiveness of the integration of a PM circuit with a TENG and the performance of the MF-TENG in energy harvesting. Furthermore, a TENG-powered hygrothermograph sensor system was constructed. Benefiting from high triboelectric properties, the hygrothermograph is driven successfully and the working state related capacitor voltage is demonstrated in Fig. 5h. The inset figure is a photograph of the sensor in working mode. ‘Working’ is the result of the TENG converting mechanical energy into electrical energy to power the hygrothermograph, while ‘reset’ refers to the energy consumed by the device. In combination with Fig. 5i, it can be seen that when the system starts to self-powered supply, the hygrothermograph can be switched on. The capacitor voltage can always be stable at the rated voltage of the sensor, and the two states of ‘working’ and ‘reset’ change. These indicate that the MF-TENG has a strong ability to extract ambient mechanical energy and has great potential in future self-powered electronics in the IoTs system.

Fig. 5 Application of MF-TENGs. (a) Diagram of the as-fabricated MF-TENG harvesting energy from ambient and power electronics. (b) V_OC, (c) I_SC, and (d) Q_SC of the MF-TENG which are generated by stepping. (e) (i) Schematic of the PM circuit. (ii) Integrated system photograph of the foot, PM circuit and electronic device. (f) Transferred charge comparison between MF-TENGs with/without a PM circuit. (g) 10 μF commercial capacitor voltage curves on charging by a PC-TENG, MF-TENG, and MF-TENG with a PM circuit. (h) The voltage of the MF-TENG powered hygrothermograph sensor by stepping. (i) Voltage variation curve of the MF-TENG integrated PM circuit driving the hygrothermograph sensor.

Conclusions

In this work, in order to enhance efficiency of capturing tiny mechanical energy, we demonstrate a multi-layered funnel-shaped triboelectric nanogenerator (MF-TENG). As environmentally friendly and biodegradable materials, polyethylene oxide (PEO) and cysteine have advantages and potential in constructing ultra-high tribo-positive layers. The results of software simulation analysis, electrospinning technical processing, material technology characterization, and triboelectric performance testing show that a PEO/cysteine nanofiber film (PCF) with 4 wt% cysteine has higher energy collecting efficiency. Besides, benefiting from unique electron-donating functional groups in cysteine, the PCF-based TENG (PC-TENG) can improve the triboelectric property of the pristine PEO electrospun film by more than 65%. Furthermore, the peak power density of the PC-TENG reaches 6.6 W m⁻². From the perspective of power density, the energy harvesting capacity of the PC-TENG is 3–110 times that in previous studies which introduced environmentally friendly materials. Therefore, this study can provide significant guidance for subsequent research on natural material-based TENGs. Furthermore, the multi-layer funnel-shaped TENG (MF-TENG) has been developed on the basis of the PC-TENG, and combined with the power management circuit, it can provide a sustainable power supply for electronic devices. Therefore, our research offers a new energy harvesting strategy from the perspective of an environmental and sustainable power source, combining low-cost, environmentally friendly materials with high tribo-capability to overcome the short-comings. Furthermore, our research on flexible self-powered devices has broad application potential in future IoTs systems.

Experimental section

Preparation of polyethylene oxide/cysteine nanofibers

The polyethylene oxide (PEO)/cysteine nanofiber film (PCF) was fabricated by the electrospinning method. First, the target mass of cysteine powder and 2 g of PEO were pre-dispersed in a solution mixture composed of 10 g formic acid. The 1 to 5 wt% mixture from the first step was heated to 50 °C while stirring for 30 minutes in a magnetic mixer. After that, the evenly stirred solution was transferred to a 22 gauge syringe. Finally, 16 kV high voltage electricity was provided at the needle to polarize the solution, and a drum collector placed 12 cm away turned the load at 180 rounds per minute.

Fabrication method of PC-TENGs

The PC-TENG is used to collect mechanical energy in the vertical direction, and operates in contact-separation mode. The tribo-positive/negative material is the prepared PCF and commercial FEP films respectively. The tribo-positive/negative material's electrodes are copper foil which is pasted on its back. A wire is placed between the material and the electrode to transmit electrons.

Fabrication method of MF-TENGs

An MF-TENG is composed of four layers with different contact areas. Compared with PC-TENG, two improvements are the addition of Kapton as a partition between each layer of units, and Kapton as a protective and elastic layer on the outside. Furthermore, a foam pad was added between the PCF side electrode and Kapton as a buffer layer to increase the effective contact area. The structural parameters designed for the MF-TENG are shown in Note S1.†

Characterization and measurements

The crystal structure was analyzed and chemical analysis of the composite material was conducted by XRD (Japan Rigaku Smartlab SE), FTIR (American Thermo Fisher Scientific Nicolet iS20), and XPS (American Thermo ESCALAB 250Xi). The composite nanofiber's distribution and surface structure were tested by SEM (Germany ZEISS GeminiSEM 300). The PC-TENG and MF-TENG's electrical output performance was evaluated by using a programmable electrometer (Keithley 6514) and oscilloscope (InfiniiVision 2000 X) to test the open-circuit voltage, short-circuit current and transferred charge. The contact and separation motions were achieved by using a commercial linear motor (J-Best Technology RP-200).

Data availability

The data supporting this article have been included as part of the ESI.†

Author contributions

Yijun Hao: conceptualization, validation, investigation and writing – original draft. Chuguo Zhang: project administration and funding acquisition. Jiayi Yang and Xiaopeng Zhu: formal analysis and data curation. Keke Hong and Jiayu Su: resources and visualization. Wei Su and Hongke Zhang: supervision and project administration. Yong Qin and Xiuhan Li: conceptualization, supervision, project administration and funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Fundamental Research Funds for the Central Universities (2024YJS080 and 2020JBZD011), National Key Research and Development Program (2021YFB3203202 and 2021YFB3203200), Beijing Municipal Natural Science Foundation (4232074 and 4122058), National Natural Science Foundation of China (60706031 and 61574015), Talent Fund of Beijing Jiaotong University (2023XKRC034), China National Postdoctoral Program for Innovative Talents (BX20230037), and China Postdoctoral Science Foundation (2023M730205).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta06845a

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