Materials

The flexible plastic sheets were bought from Shanghai Rourou New Technology Co., Ltd. The conductive aluminum foil was purchased from Shanghai Lingguan New Materials Co., Ltd. PTFE tape was obtained from Taixing Chuangwei Composite Materials Co., Ltd. The polyester-paper cloth was purchased from Jinan Shengquan Group Co., Ltd. Flexible substrates, conductive electrodes, triboelectric layers, etc. are pasted with double-sided adhesive and compacted by mechanical presses.

Fabrication of PP-TENG

In this design, the preparation method of PP-TENG was very simple and suitable for large-scale production, as exhibited in Figure 1(a1–a4). Specifically, flexible plastic sheets were cut into rectangular substrates to serve as the load-bearing material for the entire PP-TENG device (Figure 1(a1)). The flexible substrate adapted to various deformation environments and restored its original state after deformation. Furthermore, conductive aluminum foil was pasted on the surface of the flexible substrate as the conductive electrode of PP-TENG to provide electrical energy output for external loads (Figure 1(a2)). Then, a layer of polyester-paper cloth was stuck on the surface of the other electrode as a positive triboelectric material (Figure 1(a3)). In addition, a layer of PTFE tape was pasted on the surface of the aluminum electrode as a negative triboelectric material (Figure 1(a4)). Finally, the flexible base was folded in half to form a Z-shaped structure, which achieved good response to mechanical motion due to its excellent elasticity (Figure 1(a5)).

Characterizations and Measurements

Figure 1(b) presented a picture of the polyester-paper cloth, and it could be observed that there were many patterns on the surface of the polyester-paper cloth. These fluffy textures contributed to the generation and accumulation of triboelectric charges. The physical image of PP-TENG was illustrated in Figure 1(c), and a coin was used as a size reference to evaluate the size of PP-TENG. In Figure 1(d), the image shows a physical prototype of the PP-TENG device. The PP-TENG is shown next to a coin, which provides a size reference for the device, indicating its compact dimensions. The PP-TENG is composed of layered materials, including polyester-paper cloth and PTFE tape, as described in the fabrication process. The wires attached to the device suggest it is ready for testing or energy harvesting applications. In Figure 1(e), a scanning electron microscopy (SEM) image of the polyester-paper cloth is displayed. The micrograph reveals the rough and fibrous structure of the material. This microstructure, with its intricate network of fibers, is advantageous for enhancing contact electrification, thereby contributing to the triboelectric charge generation capabilities of the PP-TENG. The scale bar indicates a size of 100 μm, providing a sense of the fine detail in the material’s surface. Moreover, the Keithley 6514 electrometer was used to test the I_sc and Q_sc of the PP-TENG device. The RIGOL DHO1102 digital oscilloscope was used to collect the V_oc generated by PP-TENG. The mechanical motor provided continuous mechanical power for PP-TENG operation.

2.1 The Working Mechanism of PP-TENG Device

The working mode of PP-TENG device follows a contact separation model, with no charge on the PTFE tape surface and polyester-paper cloth surface at the beginning (Figure 2(a1)). When the external force PP-TENG drives the PTFE tape to come into contact with the polyester paper cloth, electrons will migrate from the surface of the polyester paper cloth to the surface of the PTFE tape due to the strong electron affinity of the PTFE tape (Figure 2(a2)). After the external force is withdrawn, PP-TENG will drive the separation of PTFE tape and polyester-paper cloth surface due to the elastic effect of the flexible substrate, thereby generating induced current in the external circuit to balance the potential difference between PTFE tape and polyester-paper cloth surface (Figure 2(a3)). Until the separation gap between PTFE tape and polyester paper cloth reaches its maximum, the potential system of PP-TENG is in equilibrium and will not generate induced current (Figure 2(a4)). When PTFE tape and polyester-paper cloth approach again under external force, the current will flow from the back electrode of PTFE tape to the back electrode of polyester-paper cloth, thereby generating reverse current (Figure 2(a5)).

2.2 The Electrical Performance of PP-TENG Device

Figure 2(b) shows the electrical performance testing system of PP-TENG. The output current curves under different loads show that as the load increases, the output current decreases while the output voltage shows an opposite trend (Figure 2(c)). The intersection of the output current and output voltage is the maximum output power of PP-TENG. Figure 2(d) illustrates the variation curve of PP-TENG output power under different loads, and it can be observed that there is an optimal resistance (40 MΩ) that enables PP-TENG to reach its maximum power of 0.93 mW. Considering that the size of PP-TENG is 6 cm², the output power density of PP-TENG is 0.155 mW/cm². Moreover, at a working frequency of 6 Hz, the V_oc of PP-TENG can reach 466.64 V, according to results in Figure 2(e). The I_sc of PP-TENG can arrive at 48.73 μA (Figure 2(f)), which can provide sufficient electrical energy for low power electronic devices. The Q_sc of PP-TENG can get to 90 nC from the results in Figure 2(g). According to Table. 1, the output performance of PP-TENG is better than previous works.^43-46

The triboelectric electricity is determined by many factors such as the material’s ability to gain or lose electrons, surface roughness, and affinity between materials. Based on this, we studied the electrical output performance of the combination of polyester-paper cloth and different triboelectric materials to find the optimal triboelectric pair combination. For example, the V_oc of TENGs with the triboelectric pair of polyester-paper cloth@PET, polyester-paper cloth@kapton, polyester-paper cloth@PVC, polyester-paper cloth@PDMS, and polyester-paper cloth@PTFE can arrive at 188.11 V, 241.38 V, 300.19 V, 349.49 V, and 419.25 V, from the results in Figure 3(a). The I_sc of TENGs with the triboelectric pair of polyester-paper cloth@PET, polyester-paper cloth@kapton, polyester-paper cloth@PVC, polyester-paper cloth@PDMS, and polyester-paper cloth@PTFE can arrive at 16.34 μA, 27.71 μA, 33.72 μA, 40.29 μA, and 46.36 μA. Hence, therefore, the combination of polyester-paper cloth and PTFE tape is due to the majority of triboelectric materials, reflecting the advantages of the combination. Due to the soft fiber structure of polyester-paper cloth, PP-TENG is endowed with pressure response function. The results in Figure 3(c) indicate that as the pressure increases from 1 N–8 N, the I_sc of PP-TENG also increases linearly from 21.29 μA–41.56 μA. The same trend of change also occurs in Q_sc of PP-TENG (Figure 3(d)), which is due to the fact that when polyester-paper cloth is subjected to pressure, the fibers in the polyester-paper cloth are compressed, resulting in an increase in the triboelectric region in PP-TENG, which in turn leads to an increase in the electrical output of PP-TENG. The triboelectric region is the key factor affecting the TENG output PP-TENG, and the expansion of the triboelectric region will provide higher output for PP-TENG. Figure 3(e) illustrates the V_oc of PP-TENG under the triboelectric region area of 1 cm², 2 cm², 3 cm², 4 cm², 5 cm², and 6 cm², with 204.66 V, 255.15 V, 260.21 V, 286.85 V, 298.18 V, and 333.21 V, respectively. When the triboelectric region area is 1 cm², 2 cm², 3 cm², 4 cm², 5 cm², and 6 cm², the I_sc of PP-TENG can reach 17.22 μA, 22.94 μA, 23.46 μA, 28.06 μA, 32.89 μA, and 38.12 μA, respectively, as illustrated in Figure 3(f). Hence, the increase of triboelectric region area is an effective means to improve PP-TENG.

The frequency of contact and separation between PTFE tape and polyester-paper cloth surface will affect the PP-TENG output. Figure 4(a) illustrates that the V_oc of PP-TENG with different frequencies from 2 Hz–6 Hz is maintained at around 410 V. However, the I_sc of PP-TENG can get to 23.16 μA, 31.26 μA, 36.99 μA, 46.31 μA, and 53.53 μA, when the working frequency is 2 Hz, 3 Hz, 4 Hz, 5 Hz, and 6 Hz. The contact and separation frequency between PTFE tape and polyester-paper cloth surfaces will affect the transfer rate of charges in the back electrodes of the two triboelectric materials. Therefore, the working frequency factor will mainly affect the I_sc of PP-TENG. Besides, the long-term stability of the PP-TENG electrical output were also developed. As illustrated in Figure 4(c), no significant decreases in V_oc of PP-TENG were observed even after 32000 cycles, demonstrating that PP-TENG is fit for mechanical energy needing a long lifetime with excellent cycling strength features.

After 5 hours of continuous operation, the micro surface of the polyester-paper cloth in PP-TENG still showed reliability and no damage occurred, as illustrated in inset of Figure 4(c). Moreover, the separation distance between PTFE tape and polyester-paper cloth surface will influence the PP-TENG output, because the separation distance influences the potential difference between two triboelectric material surfaces. The results that the V_oc of PP-TENG can reach 120.99 V when the maximum separation gap is 1 mm, as illustrated in Figure 4(d). When the maximum separation gap increases to 5 mm, the V_oc of PP-TENG will also increase to 389.61 V. The same influence also occurs in the I_sc of PP-TENG, as the separation gap on the surface of PTFE tape and polyester paper cloth increases from 1 mm–5 mm, the I_sc of PP-TENG will also increase to 32.29 μA, as illustrated in Figure 4(e). Due to the fact that the electrical output generated by PP-TENG is AC, PP-TENG cannot directly provide electrical energy to electronic devices. In this study, PP-TENG was rectified and charged to commercial capacitors of different capacities, including 1 μF, 2 μF, and 4.7 μF. Figure 5(f) illustrates the charging behavior of capacitors with different capacitors (1 μF, 2 μF, and 4.7 μF) when connected to the TENGs. The voltage across each capacitor increases over time, showcasing the TENG’s capability to store harvested energy. The capacitor with the smallest capacitance (1 μF) charges the fastest, reaching its maximum voltage in the shortest time. This is due to the lower energy requirement to achieve a given voltage in a smaller capacitance. Although the 4.7 μF capacitor takes the longest time to charge, it can store the most energy. The energy stored in a capacitor is given by W=CV²/2. Thus, the larger the capacitance, the greater the energy storage potential, even if the charging rate is slower. The voltage saturation levels differ among the capacitors. The 1 μF capacitor quickly reaches a higher voltage compared to the 2 μF capacitor and 4.7 μF capacitor. This indicates that for applications requiring higher voltage but lower energy storage, smaller capacitors may be preferred. Conversely, for applications demanding significant energy storage, larger capacitors, despite their slower charging rate, would be more suitable. The PP-TENG demonstrates effective charge storage capabilities, with the choice of capacitor size influencing the balance between charging speed and energy storage capacity. Also, 31 series connected light-emitting diodes (LEDs) are illuminated by PP-TENG device. This analysis underscores the versatility of PP-TENGs in various applications, depending on the specific energy and power requirements. Figure 4(g) shows the voltage output, Figure 4(h) depicts the current output, and Figure 4(i) presents the charge output, all measured over time at temperatures ranging from 20 °C–40 °C. The results demonstrate that the PP-TENG maintains a consistent and stable electrical output across a range of temperatures, underscoring its robust performance. This stability is essential for wearable applications, ensuring reliable energy harvesting from body movements under fluctuating thermal conditions. The device’s ability to produce consistent output even at elevated temperatures further highlights its potential for integration into wearable devices, offering a sustainable power source that can operate effectively in diverse environmental conditions.

Furthermore, we investigated the electrical output of the multi-layer folding structure PP-TENG with multiple PP-TENG units. The working size of each PP-TENG unit is 2 cm×3 cm. The results in Figure 5(a) indicates the multi-layer folding structure PP-TENG based on 1 unit, 2 units, 3 units, 4 units, and 5 units, with the I_sc of 10.48 μA, 13.48 μA, 15.21 μA, 19.85 μA, and 20.96 μA. The Q_sc of the multi-layer folding structure PP-TENG can reach 42 nC, 54 nC, 67 nC, 81 nC, and 85 nC, corresponding to 1 unit, 2 units, 3 units, 4 units, and 5 units, as presented in Figure 5(b). We further investigated the effect of ambient relative humidity on the PP-TENG output. Using the homemade humidity chamber, we measured the PP-TENG electrical output under different humidity conditions. As shown in Figure 5(c), when the ambient relative humidity increased from 20 %–60 %, the V_oc of the PP-TENG decreased from 437.93 V–172.57 V. This decrease is attributed to the fluffy structure of the polyester-paper cloth, which has an affinity for water vapor. The flexible PP-TENG device was constructed using PTFE tape, a flexible substrate, and polyester-paper cloth as the main components. Running is one of the most popular aerobic exercises worldwide, however, incorrect running posture and gait can lead to sports injuries. Traditional sports monitoring devices typically rely on external power sources, which can lead to issues such as short battery life and inconvenience in wearing. TENG sensors have the function of self powering by converting mechanical energy into electrical energy, greatly improving the portability and service life of the equipment. Its high sensitivity enables the TENG sensor to capture small mechanical changes during running, providing accurate gait analysis, monitoring foot pressure distribution, and collecting exercise energy. Applying TENG sensors to running not only enables real-time monitoring of runner’s exercise data, but also provides scientific exercise guidance and health management recommendations. Therefore, we applied PP-TENG to running and used the sensing signals generated by PP-TENG as analysis data for athletes’ running training, providing a reference basis for smart running. Figure 5(d1, d2) depict the experimental setup where the TENG is attached to the bottom of a runner’s socks, enabling real-time energy harvesting from various movements. The inset in Figure 5(d1) shows the picture of installation position about PP-TENG sensor (size: 3 cm×3 cm). Figure 5(e) demonstrates the voltage output generated during different activities: walking, running, and jumping. Walking produces lower voltage peaks compared to running and jumping, indicating a direct correlation between activity intensity and voltage generation. The increased mechanical force and frequency in running and jumping result in higher voltage outputs, highlighting the TENG’s sensitivity to different motion patterns. Figure 5(f–h) further analyze the current output at different step frequencies: 180, 240, and 480 steps per minute, respectively. The inset in Figure 5(f) illustrates the response time of PP-TENG sensor can reach 10 ms. The current output increases with step frequency, emphasizing the TENG’s capability to convert kinetic energy from rapid movements into electrical energy efficiently. This trend underscores the potential of TENGs in wearable electronics, where various human activities can be harnessed to generate sustainable power. The output current frequency of PP-TENG can reflect the athlete’s step movement frequency. Specifically, the number of current signals generated by PP-TENG is used to feedback the athlete’s steps, and the number of pulses within a certain period of time is calculated to reflect the athlete’s steps, which is independent of the peak output current of PP-TENG. This periodic current pulse indicates that PP-TENG can accurately capture changes in the pace, providing a stable output signal for real-time monitoring and analysis of the runner’s pace rate. This is of great significance for optimizing running training and monitoring the exercise status of runners.