Nanofibers with good designability, controllable morphologies, large specific areas, and unique physicochemical properties provide a promising platform for piezo-/tribo-electric nanogenerators (PTEGs) for various advanced applications. However, the further development of nanofiber-based PTEGs is limited by technical difficulties, ranging from materials to device designs. This thesis aims to design, prepare and study TENGs based on nanofibrous PVDF membrane structured from one-dimensional (1D) to three-dimensional (3D) for self-powered sensing and energy harvesting. The crystal phase, microstructure, antibacterial, triboelectricity and sensing properties of 1D PVDF nanofibers in electrospinning process were studied by organic-inorganic hybrid strategy. The 2D nanofibrous planar membrane with mechanical and electroactive enhancement was constructed by an effective solvent welding and thermal activation post-treatment process. Based on the previous monofunctional nanofibrous membrane, we attempt to construct the 3D multilayered fibrous membrane with thermal-moisture management function and dual-mode piezoresistive and triboelectric sensing. In addition, we have extensively discussed the wearable properties of nanofibrous PVDF membranes, such as breathability, mechanical properties, water resistance, antibacterial properties, biocompatibility, and thermal-moisture comfort. The specific content includes the following parts.

In chapter 1, we systematically reviewed the current developments in PTEGs based on electrospun nanofibers. The chapter begins with the mechanisms of PTEGs and the advantages of nanofibers and nanodevices, including high breathability, waterproofness, scalability, and thermal–moisture comfort. In terms of materials and structural design, novel electroactive nanofibers and structure assemblies based on 1D micro-/nano-structures, two-dimensional (2D) bionic structures, and 3D multilayered structures are discussed. Subsequently, nanofibrous PTEGs in applications such as energy harvesters, personalized medicine, personal protective equipment, and human-machine interactions are summarized. Based on the good processing properties and electroactivity of PVDF, we focus on the multiscale design of nanofiber structures, mechanical properties and electroactivity enhancement of nanofibrous PVDF membrane, and its applications in self-powered sensing, energy harvesting, and multifunctional wearable textiles are discussed in the following sections.

The 1D structure design can improve the surface electroactivity of nanofibers and improve the contact surface. In addition, the enhancement of nanofiber properties by traditional lead-based perovskites has raised concerns about biological toxicity. In chapter 2, we developed the biocompatible Cs₂InCl₅(H₂O)@PVDF-HFP nanofibers (CIC@HFP NFs) with 1D core-shell structure by one-step electrospinning for TENGs. By adopting the lead-free Cs₂InCl₅(H₂O) as an inducer, CIC@HFP NFs exhibited β-phase-enhanced and self-aligned nanocrystals within the uniaxial direction. The interface interaction was further investigated by the experimental measurements and molecular dynamics, which revealed that the hydrogen bonds between the Cs₂InCl₅(H₂O) and PVDF-HFP induced the automatically well-aligned dipoles and stabilized the β-phase in the CIC@HFP NFs. The TENG fabricated using CIC@HFP NFs and nylon 6,6 NFs exhibited significant improvement in output voltage (681 V), output current (53.1 µA) and peak power density (6.94 W m^-2), with the highest reported output performance among TENGs based on halide-perovskites. The energy harvesting and self-powered monitoring performance was further substantiated by human motions, showcasing its ability to charge capacitors and effectively operate electronics such as commercial LEDs, stopwatch, and calculator, demonstrating its promising application in biomechanical energy harvesting and self-powered sensing.

It is of great significance for the practical application of wearable devices to improve the electroactivity and wearability of nanofibers through effective 1D filler engineering. In chapter 3, a biocompatible and antibacterial all-textile structured TENG was designed for self-powered tactile sensing, constructed by 1D PVDF/MXene (P/M) nanofibers and 1D antibacterial Ag nanoparticles modified nylon 6,6 (Ag@nylon 6/6) nanofibers as the triboelectric negative and positive materials, respectively. The effect of MXenes in PVDF nanofibers for its mutual interaction, surface potential, breathability, tensile strength, biocompatibility, and triboelectric performance was evaluated thoroughly through experimental and theoretical investigations. As developed P/M nanofiber film was paired with tribopositive Ag@nylon 6/6 nanofibers to fabricate the TENG, exhibiting superior triboelectric output. The triboelectric harvesting properties were further demonstrated by capacitor charging and the operation of low power devices such as the clock, commercial LEDs. In addition, a self-powered tactile sensor based on the Ag@nylon 6,6 and P/M fibrous membranes realizes ultrasensitive pulse detection from different arteries, and the fabricated tactile sensor array shows a great potential in the application of smart wearable keyboard as well as high-resolution tactile mapping.

The poor strength and the instability of the nanofiber structures lead to the constraints on the durability and the lack of stability in the output performance. In chapter 4, 2D BaTiO₃@PVDF-HFP-p nanofiber membrane was prepared by electrospinning, in which BaTiO₃ nanoparticles enhanced the charge trapping ability of the nanofibers and provided nucleation sites for β-phase growth. Subsequent 2D planar post-treatment process crosslinked the nanofibers, which greatly improved the mechanical strength and stability of the nanofiber membranes. Thermal activation promoted the torsion and activation of the non-β-crystalline phase to the β-crystalline phase, which increased the surface charge density of the nanofibers. A flexible TENG based on BaTiO₃@PVDF-HFP-p nanofiber was prepared to harvest mechanical energy, which could achieve a maximum output of 482.5 V and 1.9 W m^-2, the voltage is more than 1.2 to 1.5 times higher compared to uncrosslinked nanofibers. The BaTiO₃@PVDF-HFP-p-based TENG maintains ultra-stable V_oc generation over 7000 cycles compared to the nanofibers without solvent and annealing treated. Finally, its practical application in harvesting ambient mechanical energy was demonstrated, which can harvest wind energy to power LEDs and small electronic devices such as temperature and humidity sensors.

Traditional physiological monitoring sensors usually have a single detection mode and lack of breathability, which can lead to skin allergies and even inflammation in long-term use, and are unable to realize the monitoring of multiple physiological functions. In chapter 5, we designed a bioinspired directional moisture-wicking electronic skin (DMWES) based on the 3D construction of heterogeneous fibrous membranes and the conductive MXene/CNTs electrospraying layer. Unidirectional moisture transfer was successfully realized by surface energy gradient and push-pull effect via the design of distinct hydrophobic-hydrophilic difference, which can spontaneously absorb sweat from the skin. The DMWES membrane showed excellent comprehensive pressure sensing performance, high sensitivity (maximum sensitivity of 548.09 kPa^-1), wide linear range, rapid response and recovery time. In addition, the single-electrode triboelectric nanogenerator (STENG) based on the DMWES can deliver a high areal power density of 21.6 µW m^-2 and good cycling stability in high pressure energy harvesting. Moreover, the superior pressure sensing and triboelectric performance enabled the DMWES for all-range healthcare sensing, including accurate pulse monitoring, voice recognition, and gait recognition.

In conclusion, we adopted various strategies in this project to try to optimally enhance the material properties of PVDF nanofibers from micro to macro and extensively explored the wearable properties. The crystalline phase, microstructure, triboelectricity, and sensing properties of 1D PVDF nanofibers were investigated using the organic-inorganic hybridization. The 2D post-treatment crosslinking enhancement process was used to strengthen the nanofibrous membrane. We also constructed 3D fibrous membranes with thermal-moisture management functions and bimodal sensing by multilayered design. It is believed that the research of nanofiber based PTEGs can breathe new life into a wider range of wearable textile products and applications in the future.