The Preparation, Performance And Applicaton Of Electrospun PVDF Based Nanogenerators

Recent years, the growing popularity of portable smart electronics and wearable electronic systems are becoming more miniaturized and sophisticated, which brought a brand new challenge to the power system of these devices. The current technique is highly relied on batteries. However, because of a large development lag in the battery industry, this technology has its own limitations such as heavy weight, large volume, low power density, short battery life, environmental problems and also waste of resources, which cannot meet the requirement of the portable electronics development. In the near future, with the increasing development of micro/nano sensor networks, new energies and technologies are desperately needed for their application in in-situ health monitoring, infrastructure monitoring in remote areas, nanorobotics, homeland security and even portable/wearable personal electronics. Therefore, developing a self-sufficient power system and seeking for industry breakthrough, has becoming a very important issue in the development of future power technology industry.Nanogenerator that can convert mechanical energy into electricity to drive or charge the portable smart electronics at intervals would be a promising technology to solve the power problems of these devices. Electrospun nanofibers are lightweight, soft and have high porosity, which make them suited for wearable devices. However, nanofiber based nanogenerators are still lack of development. In order to enhance the output power of the nanofiber based nanogenerators and make a contribution to the power system of the wearable/portable electronics, we fabricated a series of nanogenerators which are composed of organic/inorganic hybrid materials, with high output power, good durability and stability based upon investigating the influencing factors of the performance the nanogenerators. This thesis is mainly conducted from the following aspects:1. PVDF/CNTs nanofiber-based piezoelectric nanogenerators are directly prepared via electrospinning without any post-poling treatment. The effect of the addition of CNTs on the fiber diameter, mechanical properties, β-phase composition, surface and volume conductivities, output voltage and output power are investigated. A full-wave rectifier bridge is used to convert the AC voltage outputs of the piezoelectric nanogenerator to DC signals and then to charge a capacitor, the commercial blue LED is successfully operated by the charged capacitor in about 3 minutes without any external electric power source. With the increasing concentration of CNTs, the average fiber diameter and distribution of PVDF/CNTs nanofibers are decreased. Appropriate amount of CNTs are helpful to the crystallization process and β phase formation which are confirmed by X-Ray diffraction（XRD） and Fourier transform infrared spectroscopy（FTIR）. Increased surface conductivity of the PVDF nanofiber mats, which plays an important role in the enhancement of output power, is found by the addition of an appropriate amount of CNTs. However, the over added CNTs would sharply increase the volume conductivity which led to the induced positive and negative charges flowing longitudinally, causing serious neutralizing or leakage effects, which in turn lowered the piezoelectric output voltage. The maximum generated piezo-voltage exhibited by PVDF nanofibers in the presence of 5 wt% CNTs is as high as 6 V, while the average capacitor charging power is 81.8 nW, increases of 200% and 44.8%, respectively, compared with bare PVDF nanofibers;2. A simple-to-fabricate, high-performance, wearable all-fiber triboelectric nanogenerator（TENG）-based insole composed of electrospun PVDF nanofibers sandwiched between a pair of conducting fabric electrodes that effectively harvests energy during human walking is reported. The effect of surface roughness of PVDF nanofibers and the morphology of the electrode on the performance of TENG-based insole are investigated through the following four groups of TENGs:（1） Al flat electrode with smooth PVDF nanofibers;（2） fabric electrode with smooth PVDF nanofibers;（3） Al flat electrode with secondary nanostructured PVDF nanofibers;（4） fabric electrode with secondary nanostructured PVDF nanofibers. The role of the piezoelectric effect in the electrospun PVDF nanofibers in this TENG-based insole is also systematically discussed. The maximum output voltage, instantaneous load power and output current from the insole reach 210 V, 2.11 mW and 45 μA, respectively, under the driving frequency of 1.8 Hz. This device is shown to be a reliable power source that can be used to light up 214 serially connected LED directly. The soft fiber-based electric power generator demonstrated is capable of meeting the requirements of wearable devices because of its good air permeability, high durability and user comfort;3. We developed a book-shaped triboelectric nanogenerator（TENG） that consists of graphene oxide（GO） modified electrospun PVDF composite nanofibers act as the electronegative material, and poly（3-hydroxybutyrate-co-3-hydroxyvalerate）（PHBV） nanofibers which have a large difference in the electronegativity, act as electropositive material to effectively harvest mechanical energy. The effect of GO concentrations and the possible mechanisms on the improving performance of the TENG are investigated. The diameter of PVDF/GO nanofibers decreased with the increasing GO dosages, which can improve the output voltage of the TENG to some extent, but are confirmed to be not the main origin of the improved performance. The dispersed GO in the PVDF nanofibers acts as charge trapping sites, which increased the interface for charge storage. Electrons attracted from PHBV nanofibers through the friction process were stored either in the interfaces or trapped in the amorphous GO dielectric which increased the surface potential and slowed the dissipation of surface charge on the PVDF/GO nanofibers, played a very important role in the improvement of the performance of TENG. The output performance of TENG improved with the increasing GO dosages. The maximum output peak-to-peak voltage and current in the presence of 0.7 wt% GO dosages are 340 V and 78 μA, respectively, which are 78.9%, 189% higher than those of a TENG with neat PVDF nanofibers. And the accumulated charges and energy in a 2.2 μF capacitor in 5 minutes are 431 μC and 42.3 mJ, respectively. These values give an average charging current of 1.44 μA and average power of 0.141 mW, which are increased by 148% and 513%, respectively;4. Based on the conclusions in the previous chapter, SiO₂ electrets are utilized to enhance the surface charge of PVDF nanofibers, and finally to improve the performance of PVDF/SiO₂-PHBV nanofiber based TENG. The effect of surface hydrophilicity and concentrations of SiO₂ nanoparticles on the output power of TENG are investigated in detail. A small electronic thermometer can be driven by a S-shape TENG which is fabricated by PVDF/SiO₂-PHBV nanofibers without any batteries. The output voltage of PVDF/SiO₂-PHBV nanofiber based TENG improves with the increase of SiO₂ dosages till the value is 0.6 wt%. However, it begin to decrease while the concentration of SiO₂ reaches 0.8 wt%, which is due to the hydrophilicity nature of SiO₂. The drawbacks can be avoided by using octanol modified SiO₂（mSiO₂） whose surface contact angel is 143°, thus confirmed as hydrophobic. The output power would not decrease any more even though the concentration of mSiO₂ is as high as 0.8 wt%. The peak to peak voltage and current of TENG fabricated with PVDF-0.8% mSiO₂-PHBV nanofibers are as high as 430 V and 85 μA, the average charging power in 5 minutes is 0.145 mW, increases of 126%,204% and 530%, respectively, compared with bare PVDF nanofibers.