Confined orientation PVDF/MXene nanofibers for wearable piezoelectric nanogenerators

Abstract

The quest for high-performance wearable piezoelectric nanogenerators (PENGs) has intensified the focus on polyvinylidene fluoride (PVDF). The optimization of piezoelectric response in these nanofibers has traditionally been impeded by the difficulty in uniformly distributing the piezoelectrically active β-phase, leading to variable material characteristics and inconsistent device performance. Here, we propose a novel confined orientation structure of PVDF/MXene nanofibers, which significantly improves electromechanical performance without sacrificing flexibility. By incorporating MXene into the PVDF matrix, we successfully induce the formation of the β-phase, achieving a piezoelectric coefficient of 61.7 pC N⁻¹. This integration facilitates a synergistic enhancement of material and structure, leading to a high degree of orientation and confinement of MXene nanosheets within the fibers, which optimizes force transfer and enhances energy harvesting capabilities. Consequently, the nanofiber-based PENG demonstrates an exceptional response time of 14 ms and a pressure sensitivity of up to 19.29 mV kPa⁻¹. This work paves the way for advanced self-powered sensing technologies and mechano-electrodeposition applications, underscoring the pivotal role of PVDF-based nanofibers in the evolution of wearable PENGs.

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Introduction

The relentless pursuit of advanced wearable piezoelectric nanogenerators (PENGs) has led to a surge in interest for force–electric conversion materials.^1–4 Polyvinylidene fluoride (PVDF), with its exceptional piezoelectric properties, high mechanical strength, chemical stability, and biocompatibility, stands out as a leading candidate.^5–7 Electrospun PVDF nanofibers, in particular, have demonstrated enhanced piezoelectricity and adjustable morphology under electric field polarization and mechanical stretching.^8–12 However, further exploiting the piezoelectric performance potential of PVDF nanofibers remains a significant challenge.^13–15 The piezoelectricity of PVDF is fundamentally dependent on the β-phase, where aligned dipoles are perpendicular to the c-axis, and strategies to enhance this phase are crucial.^16–18 The β-phase, with its all-trans (TTTT) non-centrosymmetric structures, possesses the highest polar phase and contributes the most to the piezoelectric effect due to its enhanced dipole moment.^19–21 The transformation of the thermodynamically stable α-phase into the piezoelectrically active β-phase is not straightforward and often requires external stimuli such as mechanical stretching or electric field polarization.^22–25 And these methods may not uniformly distribute the β-phase, leading to inconsistent material properties and device performance.^26,27

To address this, researchers have introduced various fillers into PVDF to create composite materials aimed at boosting the performance through synergistic effects.^7,28–32 However, these strategies have often resulted in significant alterations to the mechanical properties of the resulting devices.³³ The challenge lies in the fact that while fillers can enhance the piezoelectric coefficient, they may also compromise the flexibility and processability of the nanofibers. As a result, this limits the effectiveness of PVDF nanofibers in cutting-edge applications such as wearable electronics and self-powered nanogenerators.^34–36 Moreover, the traditional approach of enhancing PVDF piezoelectricity through mechanical stretching or electric field polarization has drawbacks, as they fail to ensure a stable and uniform distribution of the β-phase.^37–41 This inconsistency in phase distribution and molecular orientation can lead to variability in device performance and limits the scalability of PVDF nanofibers for large-scale applications.

In this work, we propose a confined orientation structure of PVDF/MXene nanofibers for enhanced PENGs by molecular regulation and fiber design. The synergistic effect of material and structural enhancement is expected to markedly improve the performance of PVDF nanofibers. MXene, with its large specific surface area and rich functional groups, is integrated into the PVDF matrix. Confined within the nanofibers, MXene can induce the formation of the β-phase. This confinement, coupled with the oriented alignment of fibers, not only enhances the orientation of the crystalline region, but also optimizes the force transfer within the device structure, leading to a substantial enhancement in electromechanical performance. Meanwhile, the conductive nature of MXene improves the conductivity of the precursor solution, and the increased local electric field strength promotes electric field polarization and mechanical stretching during electrospinning. This results in a higher interaction density and a local anchoring effect of PVDF chains on the MXene nanosheets, which actively induces the optimal alignment of PVDF dipoles and the enhancement of spontaneous polarization. Consequently, this work enables the development of self-powered sensing technologies and mechano-electrodeposition applications, showcasing the active role of such PVDF-based nanofibers in advancing wearable piezoelectric nanogenerators.

Results and discussion

Material and structure design of the MXene/PVDF nanofiber membrane

Combat sports require precise movement and force application, where the PENG is an ideal candidate for advanced motion monitoring. In order to collect energy and monitor the pressure, the piezoelectric properties of the material and the piezoelectric response to both compression and bending deformations are effectively enhanced by the spatially confined orientation nanofiber structure in this work (Fig. 1a and S1†). The joints of the limbs are relatively prone to injury, so a PENG array was attached to a wrist strap to collect energy and monitor the stress experienced during the movement process (Fig. 2b). The functional nanofiber membrane was prepared through electrospinning. And by controlling the high-speed collector, the orientation of nanofibers could reach a high degree, which benefits the piezoelectric response in both compression and bending deformations. Although the high voltage and high ratio stretch during the electrospinning promote the formation of the polarization phase (β-phase), the piezoelectricity of pure PVDF is still limited. Thus, MXene nanosheets, with a large specific surface area and abundant functional groups (Fig. S2†), were introduced into the nanofibers. The physical and chemical structure endows MXene with a polarized surface to interact with the PVDF chains at the interface. The potential interactions, including van der Waals interaction and hydrogen bond interaction, enable the PVDF chains to form the all-trans (TTTT′) conformation via the interfacial polarization effect. Under the coupling effect of interfacial polarization and spatially confined structure, it is confirmed that a significant amount of β-phase would form in the PVDF (Fig. 1d), leading to a high piezoelectric response of the fabricated PENG.

Fig. 1 Conception and design of PENGs for combat sports monitoring. (a) Schematic illustration of monitoring compression and bending motion signals via PENGs. (b) Digital image and schematic illustration of the wearable PENG array worn at joints. (c) Schematic illustration of the piezoelectric nano-fiber design and structure design. (d) Schematic illustration of the oriented structure and spatially confined structure which enhance the piezoelectricity of the composite.

Fig. 2 Characterization of structure design. (a) SEM images of nanofibers prepared at various rotating speeds. The inset shows the statistical diameter distribution of the corresponding nanofibers. Scale bars are 1 μm. (b) FT images of nanofibers prepared at various rotating speeds derived from (a). (c) Relative angle distribution function of nanofibers prepared at various rotating speeds derived from (a). (d) Proportion of effectively oriented fibers. (e) 2D-SAXS patterns of nanofibers prepared at various rotating speeds. (f) HD-TEM image of the MXene/PVDF nanofiber. The inset shows the interplanar spacing of MXene nanosheets. Scale bar is 50 nm.

Characterization of the confined orientation structure

To obtain the orientation distribution structure, the orientation degree of the fibers can be improved in principle by increasing the rotation speed of the receiving collector. Thus, three kinds of rotating speed (100, 1500 2000 rpm and the corresponding samples are named S-100, S-1500, S-2000, respectively) were applied and studied for their effects on the nanofibers' structure. As presented in Fig. 2a and b, the scanning electron microscopy images (SEM) and corresponding Fourier transform (FT) images of the three samples clearly prove that increasing rotating speed improves the degree of orientation. The nanofiber possesses a uniform size and the nanofiber diameter slightly decreases with the increase of rotating speed. To provide a specific description of the orientation, the relative angle distribution (Fig. 2c) of the nanofibers was extracted from Fig. 2a. Benefiting from the high rotating speed, most nanofibers in S-2000 are oriented in the radial direction. And S-2000 possesses the highest degree of orientation among the three samples (Fig. 2d, effective orientation was defined within plus or minus ten degrees of the orientation direction). The orientation of fibers not only occurs locally, but also has structural characteristics of large-scale long-range directions, which can be affirmed by the results of two-dimensional small angle X-ray diffraction (2D-SAXS) (Fig. 2e). The dispersed ring in the middle gradually transforms into an ellipsoidal shape and forms scattering spots in S-2000, indicating the formation of a long-range oriented structure. In addition, the spatially confined structure was successfully constructed as revealed by the results of transmission electron microscopy (Fig. 2f). From the TEM image, it can be observed that the MXene nanosheets were completely confined with nanofibers and the structural characteristics of MXene nanosheets are distinct. This kind of harmonious coexistence and confined structure can promote the formation of the β-phase in PVDF, resulting in the enhancement of piezoelectricity.

Characterization of the MXene/PVDF piezoelectric membrane

The membrane possesses excellent mechanical compliance and could endure the tensile force of objects weighing over 500 g with a film thickness of only 139 μm (Fig. 3a). Fig. 3b presents the spectra derived from X-ray diffraction (XRD) measurement, the characteristic peaks of MXene and PVDF β-phase are marked out. XRD results confirm that electrospinning promotes the formation of the β-phase, as the α-phase is thermodynamically stable under normal conditions. The promotion effect originates from the electric field and stretching effect during the electrospinning process. The electric field and the stretching effect favor a polarized phase and an extended molecular chain, where the β-phase is just the ticket. As the rotating speed increases, the characteristic peak of the β-phase turns out to be sharper, indicating the improvement of β-phase content. The above change can be attributed to the enhancement of the stretching effect as the rotating speed increases. The improvement of the β-phase is also validated by the results of Fourier transform infrared (FTIR) spectra, where the ratio of 840 cm⁻¹ (β-phase)/766 cm⁻¹ (α-phase) improved with the rotating speed. Combined with the results of differential scanning calorimetry (DSC), the β-phase in the crystal region forms the major part (Fig. S3†). From the above results, it can be inferred that the confined orientation structure improved the proportion of the β-phase, and the synergistic effect with the electric field is believed to be conducive to enhancing the piezoelectricity of the MXene/PVDF membrane.

Fig. 3 Characterization of the MXene/PVDF nanofiber membrane. (a) Digital images presenting mechanical compliance of the membrane. (b) XRD spectra of the membrane prepared at various rotating speeds. (c) FTIR spectra of the membrane prepared at various rotating speeds. (d) Schematic diagram illustrating the mechanism of membrane anisotropic mechanical properties. (e) Stress–strain curves of the membrane prepared at various rotating speeds. (f) Comparative mechanical properties of the membrane prepared at various rotating speeds. (g) Moisture permeability measurement of the membrane prepared at various rotating speeds. (h) Piezoelectric coefficient of the membrane prepared at various rotating speeds. (i) Comprehensive comparison of the membrane prepared at various rotating speeds.

The orientation structure will have a certain impact on the mechanical properties of the thin membrane. As illustrated in Fig. 3d, the aligned fibers would benefit tensile strength in the oriented direction as well as enhance elongation at break in the direction perpendicular to the orientation. This performance difference in directional selectivity derived from the orientation structure has been confirmed by mechanical experimental results (Fig. 3e). From the stress–strain curves, it can be validated that the mechanical performance changes of the membrane are as expected. The comparative mechanical performance of membranes is shown in Fig. 3f, where the influence of the orientation structure is clearly present. For the subsequent application of PENGs, it is imperative that the membrane employed possesses superior moisture permeability. From the results of moisture permeability measurement (Fig. 3g), three membranes prepared at various rotating speeds exhibited similar moisture permeability around 1700 g m⁻² d⁻¹. It is obvious that the membrane with the confined orientation structure possesses high moisture permeability and the potential for wearable devices. As a comparison, the structure effectively enhanced the piezoelectric coefficient of the membrane (Fig. 3h and S4†). The piezoelectric performance of the membrane was measured using a home-made test system, including a programmable electrometer, linear motor, and a force gauge. And the piezoelectric coefficient was calculated via the following formula.

The confined orientation structure significantly enhanced the piezoelectricity of the membrane, resulting in the piezoelectric coefficient reaching a relatively high value of 61.7 pC N⁻¹. The enhancement of piezoelectricity can be attributed to the high content of the β-phase and the synergistic effect of the electric field. In Fig. 3i, the comprehensive performances of the three types of membranes are compared. Obviously, S-2000 exhibited the best comprehensive qualities and qualified for subsequent applications.

Electrical performance and applications of the nanogenerator

To evaluate the latent capacity of the high-performance membranes in applications, a sandwich structure was applied to fabricate the PENG. The explosive view of the PENG is shown in Fig. 4a and b presents the digital image of the PENG in reality. The response characteristics of the PENG to external stimulation are presented in Fig. 4c, where the PENG exhibits excellent response time (14 ms). It is reasonable to explore the impact of the confined orientation structure on the electrical performance, considering the material characterization results. By comparing the outputs of the three PENGs, it can be found that S-2000 has the highest current and voltage output (Fig. 4d and e). Such high output of the sample confirms the successful implementation of the strategy (Table S1†). In principle, the piezoelectric output should exhibit the same level and opposite direction when the connection is reversed. As expected, the output of the PENG completely conforms to the pattern (Fig. 4f). To quantify the perception of dynamic mechanical stimulation, the pressure sensitivity of the PENG is defined as the slope of the output voltage–pressure curve. The PENG possesses excellent capacity of electromechanical coupling (Fig. 4g), where the sensitivity is 19.29 mV kPa⁻¹ (<210 kPa) and 6.08 mV kPa⁻¹ (>210 kPa), respectively. In addition, the orientation nanofiber distribution endows the PENG with piezoelectric response capability with directional selectivity. To verify this conjecture, finite element simulations (FEA) were used to study the response of the oriented nanofibers under different external stimuli (Fig. S6†). Owing to the orientation structure, the nanofibers display the highest piezoelectric response and deformation when the bending direction is along the oriented direction. In the actual experimental measurements, the electrical outputs of the membranes showed experimental results consistent with simulation (Fig. S7†). This directional selectivity is expected to be used in developing devices to discriminate between different forms of external stimuli. In addition to the above performance characteristics, the PENG also has the ability to withstand long-term operation and fatigue (Fig. 4h).

Fig. 4 Electrical properties of the PENG based on the nanofiber membrane. (a) Explosive view of the fabricated PENG. (b) Digital image of the PENG. Arrows indicate the direction of fiber orientation. (c) Response time and recovery time of the PENG. (d) Voltage outputs and (e) current outputs of PENGs fabricated from membranes. (f) Measured outputs of the forward and reverse connections for the S-2000 sample. (g) Sensitivity of the PENG. (h) Durability test of the PENG.

Based on the above excellent electrical performance, the PENG was utilized in detecting the routine combat actions. Fig. 5a, b and c present the horizontal kick, downward kick, and upward kick and their corresponding piezoelectric signals, respectively. The signals of the three kinds of actions can be distinguished clearly. Although the PENG is capable of harvesting energy and discriminating different actions, there is still a need to monitor the distribution of stress. Given this requirement, a PENG array was designed and fabricated for subsequent applications. Boxing, the most typical combat action that applies vertical stress, was first applied to analyze and detect pressure distribution (Fig. 5d). The corresponding piezoelectric response and current outputs are displayed in Fig. 5e and f. However, in many combat situations, vertical stress and bending stress often coexist. Given that, the PENG array was employed to detect the regular gear shifting actions (Fig. 5g), where the PENG array simultaneously detects two forms of force (Fig. 5h). To provide a visual perception of the pressure distribution, the collected piezoelectric signals (Fig. S8†) were processed and displayed using a hotspot map (Fig. 5i), where the stress characteristics were fully present.

Fig. 5 Application of PENGs for energy collection and signal monitoring of various combat actions. Schematic diagram, digital images, and corresponding signals of (a) horizontal kick, (b) downward kick, and (c) upward kick. (d) Schematic illustration of the PENG array detecting boxing. (e) Corresponding voltage signals of the PENG array detecting boxing. (f) Current response of each unit in the PENG array. (g) Schematic diagram and digital image of the PENG array detecting the blocking of the upward kick with the forearm. (h) Schematic illustration of two types of corresponding piezoelectric response modes in the measurement (g). (i) The corresponding voltage outputs in the measurement (g).

Conclusion

In summary, we designed an MXene/PVDF piezoelectric membrane with a confined orientation structure for an enhanced PENG. The structural characterization results confirmed the successful construction of the confined orientation structure, with the film exhibiting a high degree of orientation and confinement of MXene nanosheets in the fibers. Combined with the results of material characterization, the confined orientation structure was shown to confer excellent piezoelectric properties to the membrane, as well as orientation selectivity in response to external stimuli. Through electrical performance measurement, the films demonstrated typical PENG characteristics and excellent sensing performance. By monitoring kick actions, it was validated that the PENG has an effective response to combat actions and the potential ability to distinguish between different actions. Finally, a PENG array was fabricated to explore the application in pressure distribution. It is confirmed that this work provides a worthwhile structural design idea and material design perspective.

Experimental section

Preparation of the MXene/PVDF nanofiber membrane and nanogenerator

Synthesis of MXene nanosheets. The Ti₃C₂T_x MXene was obtained through etching the Ti₃AlC₂ MAX powder by the LiF/HCl solution, which has been reported in our previous research. In particular, 2 g LiF (Sigma-Aldrich) was added into 40 mL 9 mol L⁻¹ hydrochloric acid solution (HCl, Sigma-Aldrich). Subsequently, 2 g MAX powder (400 mesh, 11 Technology Co., Ltd) was slowly added into the etching solution, and allowed to react for 24 hours in a 40 °C water bath. After 24 hours etching, the reaction product was washed with deionized water several times until the pH value of the solution was close to 7. Ultimately, the MXene nanosheet solution was obtained by ultrasonicating the suspension for 4 hours and extracting the upper clear liquid.

Preparation of the MXene/PVDF nanofiber membrane. 3 g PVDF (18 000 M_w, Archma) was added and dissolved in 5 mL N,N-dimethylacetamide (DMAc, Sigma-Aldrich) by stirring for 5 hours in a 60 °C water bath. After that, the MXene nanosheet DMAc solution obtained by solvent displacement was mixed with the PVDF solution. Subsequently, the MXene/PVDF solution was poured into a syringe for electrospinning with a 0.03 mL min⁻¹ spinning rate and 100/1500/2000 rpm rotating rate. More specifically, the applied high voltage was 16 kV and the distance between the needle tip and collecting roller was 13 cm. During the electrospinning, the temperature and humidity were 25 °C and 48%, respectively. Finally, the MXene/PVDF nanofiber membrane was collected on aluminum foil and annealed in an oven at 60 °C for 2 hours.

Fabrication of the PENG. The membranes were cut into a diameter of 16 mm square using a laser marking machine. The Ag electrode was coated on both sides of the functional membrane by magnetron sputtering in an argon atmosphere. Finally, the PENG was packaged tightly by using polyurethane (PU) medical tape.

Characterization

Material characterization. The morphologies and composition of the material were characterized by field emission scanning electron microscopy (FESEM, JSM 7800F) and transmission electron microscopy (TEM, JEM 2100F). The information of phase and crystal structure was characterized by X-ray diffraction (XRD, Empyrean), Fourier transform infrared spectroscopy (FTIR, Nicolet iS50), and differential scanning calorimetry (DSC 2500) with a temperature range of room temperature to 200 °C at a heating rate of 10 °C per minute. The mechanical properties of the membranes were characterized with a tensile tester machine (LDW-1, Songdu Instrument) at a tensile rate of 10 mm per minute. The long-range orientation degree of the membrane was characterized by SAXS (Xeuss 2.0). The FEA was performed through the COMSOL software.

Electrical performance measurement

The electrical performance of piezoelectric membranes and PENG was characterized using a programmable electrometer (Keithley 6517). The periodic pressure was supplied by a linear motor (LinMotH01-23 × 86/160) and was monitored using a force gauge (IMADA model ZPS-DPU-50N).

Data availability

The data are available from the corresponding author on reasonable request.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research is supported by the National Natural Science Foundation of China (No. 52303328), the Postdoctoral Innovation Talents Support Program (No. BX20220257), the Sichuan Science and Technology Program (No. 2023NSFSC0313), and the Sichuan Transportation Science and Technology Program (No. 2018-ZL-04). The authors are thankful for the help from the Analysis and Testing Center of Southwest Jiaotong University.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4ta08879d

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