A flexible PI/MXene triboelectric nanogenerator for energy harvesting and motion monitoring in table tennis Open Access

Recently, advancements in flexible electronic technology, particularly wearable devices in the domain of intelligent sports, have been profoundly reshaping global society’s informatization and technological landscape.¹ These devices demonstrate remarkable potential not only in personal health monitoring and sports data acquisition but also in driving the evolution of intelligent sports ecosystems.² However, miniaturized portable devices, such as smartphones, smartwatches, and fitness wristbands, continue to face significant challenges in ensuring long-term comfortable wearability while meeting the ever-growing global demand for energy. Among these, the reliability and sustainability of their energy supply have emerged as critical bottlenecks hindering the widespread adoption and scalability of such technologies.^3,4 To overcome these challenges, self-powered wearable devices are increasingly replacing traditional battery-powered counterparts, becoming pivotal in advancing intelligent sports applications.^2,5 By eliminating the inconveniences associated with frequent charging and battery replacements, these devices significantly enhance user experience and practical utility. Conversely, conventional battery-powered devices are constrained in dynamic scenarios, such as intelligent sports, due to their limited flexibility and reliance on manual intervention for energy management. In this context, triboelectric nanogenerators (TENGs) have emerged as a groundbreaking energy harvesting technology, offering immense potential for addressing these limitations.^6–22 By coupling triboelectric and electrostatic induction effects, TENGs effectively capture mechanical energy—such as low-frequency human motion—from the environment, converting it into electrical energy to continuously power low-consumption electronic devices.²³ This capability positions TENGs as a transformative solution for intelligent sports and motion monitoring applications, where real-time, self-sufficient energy supply is critical.²⁴ Moreover, TENGs function not only as energy harvesters but also as active sensors, capable of autonomously generating electrical signals without requiring external power sources.⁶ This dual functionality enhances the independence and versatility of wearable devices while simultaneously enabling reliable, real-time data collection for intelligent sports monitoring systems. Their simple manufacturing process, exceptional adaptability, high output voltage, and superior energy conversion efficiency have positioned TENGs as one of the most promising technologies in the field of wearable electronics. Particularly within intelligent sports, TENGs offer innovative solutions for the development of lightweight, durable, and high-efficiency motion sensors.^25,26 The fundamental working principle of TENGs relies on charge separation when two materials with distinct electron affinities come into contact, underscoring the importance of selecting optimal triboelectric materials to achieve superior performance. The careful design and material selection directly influence the device’s output efficiency and application potential.

MXene materials, a class of two-dimensional transition metal carbides and nitrides, have emerged as highly promising candidates for enhancing the performance of TENGs.^27,28 Their exceptional electrical conductivity, high specific surface area, and tunable surface chemistry make them ideal for applications in TENG device fabrication. MXenes can serve as functional layers or additives in triboelectric materials to improve charge transfer efficiency and output performance.²⁹ When integrated into TENGs, MXenes enhance surface charge density due to their abundant surface functional groups, such as –OH and –O, which improve the dielectric properties of the triboelectric layer. Additionally, their excellent conductivity facilitates rapid electron transfer, reducing charge loss during operation. This leads to higher output voltages, currents, and energy conversion efficiencies compared to conventional triboelectric materials. MXene-based composites, formed by incorporating MXenes into polymer matrices, further improve the mechanical flexibility and durability of TENGs, enabling their application in wearable and flexible electronic devices. Moreover, MXenes’ structural versatility allows for tailoring material properties to meet specific device requirements, making them highly adaptable for diverse TENG applications. Electrospinning, recognized as a simple, efficient, and cost-effective technology, has been extensively employed in the fabrication of micro/nanofibers.³⁰ The resulting freestanding electrospun fibers exhibit a high specific surface area, remarkable porosity, and exceptional mechanical flexibility, making them an ideal substrate for wearable TENGs. These properties not only significantly enhance the contact area and frictional charge generation but also provide excellent mechanical stability, enabling superior performance in energy harvesting devices. Furthermore, the inherent multifunctionality and tunability of electrospun fibers allow their integration with various functional materials, such as MXene, to further optimize TENG performance.^31,32 Nevertheless, introducing MXene sheets into electrospinning processes presents a critical challenge: achieving a uniformly dispersed composite solution to prevent sheet aggregation. Uniform dispersion is essential for fabricating bead-free, smooth, and high-performance electrospun fibers. Despite MXene’s exceptional electrical conductivity and mechanical properties, studies investigating its role in enhancing the triboelectric and electrical performance of polyimide (PI)-based nanofibers remain limited. Importantly, fibers produced via electrospinning not only improve the energy conversion efficiency of TENGs but also allow for further optimization of device flexibility and overall performance through precise control over fiber structure, size, and material composition. These unique advantages position electrospinning as a crucial tool for the development of high-performance, flexible, and wearable TENGs, opening up new possibilities for their application across diverse fields.

In this work, we used the polyimide/MXene (PI/MXene) spinning film and Cellulose Acetate/Polyamide 6 (CA/PA6) to fabricate a triboelectric nanogenerator (PC-TENG) for harvesting mechanical energy. The results demonstrate that 5% MXene is the optimal weight fraction for maximizing the performance outputs of the PC-TENG, highlighting the importance of balancing material properties for achieving peak energy harvesting performance. The PC-TENG achieves a peak power output of 1.16 mW at an optimal load resistance of ∼10 MΩ. Additionally, while the open-circuit voltage (V_OC) (∼105 V) and transfer charge (Q_SC) (∼63.14 nC) outputs remain stable across frequencies (2–6 Hz), the short-circuit current (I_SC) output increases from 14.91 μA at 2 Hz to 41.41 μA at 6 Hz. The PC-TENG demonstrates a strong linear correlation between applied force and output performance, with the V_OC increasing from 63.41 V at 10 N to 148.14 V at 50 N, the I_SC from 7.31 to 17.98 μA, and the Q_SC from 29.69 to 69.73 nC, showcasing its potential for precise force sensing. Additionally, we integrate PC-TENG into table tennis sports monitoring to demonstrate its practical application value. The remainder of the paper is organized as follows: Sec. II details the materials and methods used in the fabrication and characterization of the PC-TENG. Section III presents the experimental results, including the energy harvesting performance and force-sensing capabilities of the device. In addition, it discusses the implications of the findings and compares them with existing technologies. Finally, Sec. IV concludes with insights and prospects for future research.

The selection of materials for the PC-TENG was guided by their triboelectric properties, electrical conductivity, and compatibility with the fabrication process. Cellulose acetate (CA) was chosen as the primary material for the positive triboelectric layer due to its high triboelectric positivity, mechanical flexibility, and biocompatibility. Polyamide 6 (PA6) was added to enhance the mechanical strength and stability of the electrospun fibers. For the negative triboelectric layer, polyimide (PI) was selected for its excellent thermal stability, flexibility, and triboelectric negativity, while MXene was incorporated to enhance the electrical conductivity and charge transfer efficiency. To ensure optimal device performance, each material was evaluated based on its physical and chemical properties, as well as its compatibility with electrospinning. Table I summarizes the material properties, sources, and their specific roles in the PC-TENG.

Figure 1(a) demonstrates the fabrication process of CA/PA6 spinning films via electrospinning, detailing the critical steps from solution preparation to film formation. As shown in Fig. 1(a1), cellulose acetate (CA) and polyamide 6 (PA6) are dissolved in dimethylformamide (DMF) to prepare the polymer solution. In this experiment, 5 g of CA is used as the primary material, while PA6 is added at a mass of 1 g, maintaining a CA ratio of 5:1. DMF is utilized as the solvent, with a volume of 50 ml, to ensure complete dissolution and homogeneity. The chosen solvent volume corresponds to a polymer concentration of ∼10%, suitable for electrospinning. In Fig. 1(a2), the mixed solution is subjected to magnetic stirring to achieve a uniform dispersion of CA and PA6 within the solvent. This step ensures consistent polymer distribution, which is critical for fiber formation during electrospinning. In Fig. 1(a3), the homogeneous solution undergoes electrospinning. A high voltage is applied, facilitating the jetting of the solution and the formation of interconnected micro/nanofibers on the collector substrate. This process results in a controlled fiber morphology. Finally, Fig. 1(a4) illustrates the stripping of the spun fiber mat from the substrate, resulting in the formation of the CA/PA6 spinning film. The resulting film exhibits a well-structured fibrous network with optimized flexibility and mechanical integrity, suitable for advanced material applications. The precisely maintained material ratios and controlled fabrication steps ensure reproducibility and consistent film properties.

Figure 1(b) illustrates the preparation process of PI/MXene spinning films via electrospinning, detailing key steps from solution preparation to the final fiber film. As shown in Fig. 1(b1), PI and MXene are mixed in dimethyl sulfoxide (DMSO) to prepare a uniform spinning solution. In this experiment, PI is used as the primary material with a mass of 5 g, while MXene is added at a mass of 0.5 g (10% of the PI mass). DMSO serves as the solvent, with a volume of 50 ml, ensuring complete dissolution and effective dispersion of MXene nanosheets within the PI matrix. In Fig. 1(b2), the solution undergoes magnetic stirring to achieve a homogeneous dispersion, which is crucial for maintaining uniform fiber morphology during the electrospinning process. The stirring step ensures effective interaction between the MXene sheets and the polymer chains, promoting improved mechanical and electrical properties in the final composite material. As shown in Fig. 1(b3), the uniform solution is subjected to electrospinning under a high electric field. The applied voltage facilitates the formation of continuous PI/MXene composite fibers, which are collected on a grounded substrate. Finally, Fig. 1(b4) depicts the stripping of the spun fiber film from the substrate, resulting in a flexible and interconnected fibrous network. The presence of MXene contributes to enhanced conductivity and structural integrity, making the PI/MXene film suitable for applications in advanced electronic and energy devices. The carefully controlled material ratios and processing parameters ensure the reproducibility of high-performance films.

Figure 1(c) presents the structural schematic of the PC-TENG device. The device is composed of two complementary triboelectric layers: the CA/PA6 spinning film as the positive triboelectric layer and the PI/MXene spinning film as the negative triboelectric layer. Both layers are fabricated using the electrospinning method, ensuring a fibrous and porous structure that enhances the contact area and charge generation efficiency. The triboelectric layers are separated and supported by a Kapton substrate. The copper tape ensures effective charge collection and transmission during the triboelectric process. The flexible structure of the spinning films allows for excellent mechanical adaptability, which is crucial for energy harvesting under various mechanical deformations, such as pressing or bending. The schematic also illustrates the electrical connection, where the copper tape is linked to an external resistor (R), completing the circuit. This design enables the efficient conversion of mechanical energy into electrical energy, making the PC-TENG suitable for self-powered applications, such as wearable electronics and environmental sensing.

In Fig. 1(d), the CA/PA6 film (left) and PI/MXene film (right) are displayed side by side, demonstrating their distinct visual characteristics. The CA/PA6 film appears as a uniform, semi-transparent white membrane, indicative of its homogeneously distributed components. In contrast, the PI/MXene film exhibits a darker, grayish color due to the inclusion of MXene nanosheets, which impart conductive and triboelectric properties to the composite. Figure 1(e) presents the scanning electron microscopy (SEM) image of the PI/MXene film at high magnification. The micrograph reveals a well-interconnected fibrous network with a uniform distribution of nanofibers. The fibers are smooth, continuous, and exhibit an average diameter in the submicron range, indicative of a stable electrospinning process. The homogeneous dispersion of MXene nanosheets within the polymer matrix contributes to the structural integrity and enhances the functional properties of the film. The preparation and characterization processes were conducted using advanced laboratory equipment. A magnetic stirrer (Hei-PLATE Mix 20 L) was utilized to ensure vigorous and uniform stirring of the various solutions. The electrospinning system (ET-2535X) facilitated the fabrication of CA/PA6 and PI/MXene films, ensuring precise control over fiber morphology. A constant-temperature blast drying oven was employed to dry the materials thoroughly, preserving their structural integrity. The open-circuit voltage (V_OC), short-circuit current (I_SC), and transferred charge (Q_SC) were accurately measured using a Keithley 6514 electrometer, ensuring high sensitivity and reliability of the data. Additionally, a custom-designed adjustable mechanical motor was implemented to apply well-controlled actuation forces, enabling systematic investigation of the material performance under specific mechanical conditions.

In Fig. 2(a), the fundamental working mechanism of the PC-TENG is depicted, highlighting the interaction between the triboelectric layers and the copper electrodes under periodic pressing and releasing motions. The structure consists of two main triboelectric layers, PI/MXene and CA/PA6, sandwiched between copper electrodes. In the initial state [Fig. 2(a1)], both triboelectric layers are in full contact, and the system reaches an equilibrium in charge distribution. Upon releasing the external pressure [Fig. 2(a2)], the two layers begin to separate due to elastic recovery, resulting in a potential difference across the copper electrodes and driving electrons through the external circuit. In the fully separated state [Fig. 2(a3)], the potential difference reaches its peak, maximizing the electron flow. When the external pressure is reapplied [Fig. 2(a4)], the layers are brought back into contact, reversing the charge flow and completing the cycle. This repetitive motion underpins the energy harvesting process, with efficient charge transfer driven by the triboelectric and electrostatic induction effects. The current direction in the PC-TENG is governed by the charge transfer dynamics between the triboelectric layers and the electrodes. During the separation phase [Fig. 2(a2)], electrons flow through the external circuit to balance the potential difference created by charge separation. In contrast, during the contact phase [Fig. 2(a4)], electrons flow in the opposite direction as the potential difference diminishes and charges neutralize. While the voltage potential may appear similar at Figs. 2(a2) and 2(a4), the key difference lies in the state of motion of charges, resulting in opposite current directions.

Figure 2(b) examines the influence of the MXene weight fraction in the PI/MXene triboelectric layer on the voltage output of the PC-TENG. As the MXene content increases from 1% to 10%, the output voltage demonstrates a non-linear trend. At 1% MXene, the PC-TENG generates an output voltage of ∼55.18 V. This value increases to 79.36 V at 3% MXene and reaches a maximum of about 98.45 V at 5% MXene, representing the optimal weight fraction for achieving the highest voltage output. Beyond this point, the output voltage slightly declines, measuring ∼85.29 V at 7% and 65.53 V at 10%. The observed reduction at higher MXene concentrations could be attributed to a trade-off between enhanced dielectric properties and reduced mechanical flexibility, which may hinder triboelectric performance. This analysis highlights the critical role of material optimization in achieving peak device performance. Figure 2(c) presents the charge output of the PC-TENG as a function of the MXene weight fraction in the PI/MXene triboelectric layer. Similar to the voltage trend, the charge output increases with MXene content up to an optimal point and then declines. At 1% MXene, the charge output is ∼44.74 nC, increasing to 53.31 nC at 3% MXene. The maximum charge output of 61.82 nC is achieved at 5% MXene, reflecting the optimal composition for charge transfer efficiency. A reduction in charge output is observed at 7% and 10% MXene, where the values are around 52.45 and 48.31 nC, respectively. This behavior underscores the importance of balancing the MXene concentration to maximize charge transfer while maintaining the mechanical integrity of the triboelectric layer. Figure 2(d) illustrates the current output of the PC-TENG under varying MXene weight fractions. The current output follows a similar trend to the voltage and charge outputs. At 1% MXene, the current output is ∼13.75 μA, increasing to 15.44 μA at 3% MXene. The peak current output, ∼18.71 μA, is observed at 5% MXene, corresponding to the optimal weight fraction for achieving the highest current output. Beyond this concentration, the current output decreases slightly, with values of around 16.59 μA at 7% MXene and 13.52 μA at 10%. The reduction in current at higher MXene concentrations is consistent with the observed decline in voltage and charge outputs, likely due to diminished mechanical flexibility and excessive conductive filler loading. The combined analysis of Figs. 2(b)–2(d) indicates that 5% MXene represents the optimal weight fraction for maximizing the output voltage, charge, and current of the PC-TENG. These results provide critical insights into the design and optimization of triboelectric layers to achieve high-performance energy harvesting. The findings demonstrate the need for a careful balance between material properties, including electrical conductivity, dielectric enhancement, and mechanical flexibility, to ensure optimal device performance.

Figure 3(a) presents the schematic architecture of the PC-TENG, highlighting its layered structure composed of triboelectric layers (CA/PA6 and PI/MXene) positioned between copper electrodes. The device is connected to an external load resistor (R) and voltage measurement system. This design ensures efficient charge generation and transfer during the periodic contact-separation process, leveraging the triboelectric and dielectric properties of the materials for energy harvesting. Figure 3(b) explores the relationship between external resistance and the output voltage and current of the PC-TENG. As the resistance increases from 0.1 to 1000 MΩ, the output voltage rises from ∼37 V to nearly 421 V, reflecting reduced current flow at higher resistances. Conversely, the current exhibits a decreasing trend, starting at around 37 μA at 0.1 MΩ and diminishing to less than 0.421 μA at 1000 MΩ. This trade-off between voltage and current highlights the critical role of resistance in modulating the device’s electrical performance. Figure 3(c) shows the output power as a function of load resistance. The power output increases with resistance initially, reaching a peak value of 1.16 mW at an optimal resistance of ∼10 MΩ. Beyond this point, the power declines due to the significant reduction in current. This observation underscores the importance of optimizing load resistance to maximize energy harvesting efficiency. Figure 3(d) examines the V_OC output of the PC-TENG at different frequencies ranging from 2 to 6 Hz. The results demonstrate that the output voltage remains largely unaffected by the frequency of the applied mechanical motion, with consistent peaks around 105 V. This frequency-independent behavior of the voltage is attributed to the intrinsic properties of the triboelectric layers, which enable stable charge transfer regardless of the operation speed. Figure 3(e) investigates the I_SC output under varying frequencies. Unlike voltage, the current exhibits a strong dependence on frequency, increasing progressively with the rise in frequency. At 2 Hz, the current output is ∼14.91 μA, increasing to around 41.41 μA at 6 Hz. This trend is driven by the higher rate of charge transfer associated with more frequent contact-separation cycles at elevated frequencies. Figure 3(f) depicts the Q_SC output of the PC-TENG under different frequencies. Similar to the voltage behavior, the transferred charge remains consistent across frequencies, with a stable peak around 63.14 nC. This frequency independence of charge output reflects the robust triboelectric performance of the materials used in the PC-TENG, ensuring consistent charge generation irrespective of the motion speed.

Figure 4(a) illustrates the PC-TENG connected to a rectification circuit for energy harvesting and storage. The structure integrates a bridge rectifier and a storage capacitor (C) to convert the alternating triboelectric signals into a unidirectional output voltage. This setup highlights the practical application potential of the PC-TENG in energy storage and management systems. Figure 4(b) shows the voltage output across different capacitors (1, 4.7, and 5.7 μF) as a function of time. Smaller capacitors (1 μF) charge rapidly, achieving nearly 60 V in less than 40 s. In contrast, larger capacitors (4.7 and 5.7 μF) require longer charging times but store more energy, reflecting the trade-off between charge speed and storage capacity. This performance demonstrates the PC-TENG’s compatibility with varying energy storage requirements. Figure 4(c) evaluates the long-term stability of the PC-TENG over 10 000 operational cycles. The voltage output remains consistent, confirming the device’s mechanical and electrical durability under repeated mechanical deformations. This stability underscores its potential for reliable performance in practical, long-term applications. Figure 4(d) examines the voltage output of the PC-TENG at different relative humidity (RH) levels (30%, 40%, 50%, 60%, and 70%). The results indicate a significant decrease in output voltage as humidity increases. At 30% RH, the device achieves a peak voltage of ∼119.12 V, whereas at 70% RH, the output drops to around 63.36 V. This decline is attributed to the increased surface conductivity under high humidity, which reduces charge retention and transfer efficiency. Figure 4(e) explores the current output under varying humidity levels. Similar to the voltage trend, the current decreases with increasing humidity. At 30% RH, the current output peaks at ∼16.89 μA, while at 70% RH, it declines to around 7.76 μA. This observation further supports the adverse impact of high humidity on triboelectric performance due to charge leakage and diminished surface charge density. Figure 4(f) presents the charge output at various humidity levels, revealing a trend consistent with the voltage and current outputs. At 30% RH, the device generates a charge of ∼71.59 nC, which decreases to about 33.16 nC at 70% RH. The stable performance under lower humidity conditions highlights the importance of environmental factors in optimizing triboelectric energy harvesting.

Figures 5(a)–5(c) present the voltage, current, and charge outputs of the PC-TENG under varying applied forces (10, 20, 30, 40, and 50 N). In Fig. 5(a), the output voltage increases linearly with the applied force, ranging from ∼63.41 V at 10 N to over 148.14 V at 50 N. This trend highlights the PC-TENG’s sensitivity to mechanical force, enabling precise force detection. Similarly, Fig. 5(b) shows an increase in current output, starting at about 7.31 μA for 10 N and reaching nearly 17.98 μA for 50 N. Figure 5(c) illustrates the charge output, which also increases with applied force, from 29.69 nC at 10 N to 69.73 nC at 50 N. These results demonstrate the strong correlation between applied force and output performance, showcasing the PC-TENG’s potential for force-sensing applications. Figure 5(d) displays a photograph of the PC-TENG sensor integrated into a table tennis paddle, where it is used to monitor batting actions. Figure 5(e) shows the real-time current signals generated during a table tennis training session. The data exhibit distinct peaks corresponding to each strike of the ball, providing detailed information on the frequency and intensity of the hits. This application highlights the PC-TENG’s capability to serve as a self-powered sensor for sports monitoring and training systems, offering precise and continuous feedback without requiring an external power source. Figures 5(f)–5(h) investigate the current output of the PC-TENG under different frequencies of mechanical input: high, medium, and low. Figure 5(f) shows a dense and consistent pattern of current peaks at high frequency, indicating rapid charge transfer due to frequent contact-separation cycles. In Fig. 5(g), the medium frequency results in fewer and slightly larger peaks, reflecting a moderate contact-separation rate. Figure 5(h) illustrates the low-frequency performance, where the current peaks are widely spaced and exhibit greater amplitude. These results emphasize the PC-TENG’s responsiveness to varying frequencies, making it suitable for dynamic sensing scenarios. The frequency thresholds for categorizing high, medium, and low frequencies were determined based on the mechanical inputs typically encountered in real-world applications. Low frequency (1–2 Hz) represents slow, deliberate motions; medium frequency (3–4 Hz) corresponds to moderate activity levels; and high frequency (5–6 Hz) simulates rapid, dynamic motions. These ranges ensure the PC-TENG’s performance is evaluated across a broad spectrum of practical operating conditions.

In summary, a polyimide/MXene (PI/MXene) spinning film and cellulose acetate/polyamide 6 (CA/PA6) were utilized to fabricate a triboelectric nanogenerator (PC-TENG) for efficient mechanical energy harvesting. The findings reveal that a 5% MXene weight fraction is optimal for maximizing the PC-TENG’s output performance, emphasizing the critical role of material optimization in achieving superior energy harvesting efficiency. The PC-TENG demonstrates a peak power output of 1.16 mW at an optimal load resistance of 10 MΩ. While the V_OC of 105 V and Q_SC of 63.14 nC remain stable across frequencies ranging from 2 to 6 Hz, the I_SC increases significantly from 14.91 to 41.41 μA within the same frequency range. Furthermore, the PC-TENG exhibits a strong linear relationship between applied force and output performance, with V_OC rising from 63.41 V at 10 N to 148.14 V at 50 N, I_SC from 7.31 to 17.98 μA, and Q_SC from 29.69 to 69.73 nC, demonstrating its potential for precise force sensing. The successful integration of the PC-TENG into a table tennis paddle highlights its applicability in real-time sports monitoring, while its frequency-responsive current output underscores its adaptability for dynamic sensing applications. These results showcase the PC-TENG’s potential as an efficient, self-powered device for energy harvesting and multifunctional sensing across diverse scenarios.

The authors have no conflicts to disclose.

Dazhong Xu: Conceptualization (lead); Data curation (lead); Formal analysis (lead); Funding acquisition (lead); Investigation (lead); Methodology (lead); Project administration (lead); Resources (lead); Software (lead); Supervision (lead); Validation (lead); Visualization (lead); Writing – original draft (lead); Writing – review & editing (lead). Xiaoxin Ma: Conceptualization (supporting); Data curation (supporting); Formal analysis (supporting); Funding acquisition (supporting); Investigation (supporting); Methodology (supporting); Project administration (supporting); Resources (supporting); Software (supporting); Supervision (supporting); Validation (supporting); Visualization (supporting); Writing – original draft (supporting); Writing – review & editing (supporting). Yong Ma: Conceptualization (equal); Data curation (equal); Formal analysis (equal); Funding acquisition (equal); Investigation (equal); Methodology (equal); Project administration (equal); Resources (equal); Software (equal); Supervision (equal); Validation (equal); Visualization (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.