Electrospun polymers/nanoparticle composites for energy harvesting applications exploiting the triboelectric effect

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Καμηνιώτης, Μάριος

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Current wearable electronics technology and its connection to the Internet of Things (IoT) is increasingly based on processes which transform low-level mechanical energy or friction, which is ubiquitously present in any moving part of mechanical systems, including the biomechanical energy, into electricity. Given the particular energy harvesting mechanism involved, materials optimization and proper device assembly are required for obtained applications high potential. In contrast to high energy harvesting technologies, i.e. wind and solar, ultra-low power devices typically used as wearables, deal with much smaller energy scales. Such miniature devices can offer significant mass and size reduction, increased flexibility, providing energy autonomous systems which can lead to a batteryless transition. Devices that exploit the coupled effect of contact electrification and electrostatic induction, called triboelectric nanogenerators (TENGs), can convert mechanical energy into electricity. TENGs can be employed to harvest ambient mechanical energy (human motion, rain drops, wind etc.) to achieve self-powered sensing or power-packs for low-power devices. In this thesis we explore certain polymeric materials, either pristine or composites, that can be used as effective energy harvesting layers. Polymers are frequently selected as tribomaterials for TENGs as they are flexible and lightweight. As tribo-electrification is a surface phenomenon, increasing contact area, i.e. due to roughness or micro/nano-structuring, results in enhanced charge generation and hence better performance. Electrospinning is a suitable technique which can be used to prepare polymeric films of sub-micron fibrous structures, with enhanced roughness in comparison to bulky continuous polymer films prepared by other methods. Electrospinning also offers versatility in incorporating nanoparticles within the spun fibers. Therefore, a home -made electrospinning setup was built for the purpose of the current thesis. Using this set-up, a thorough optimization of electrospun PVDF fibrous films took place. Various parameters were optimized to obtain the best set resulting in homogeneous, bead-free fibers. In addition, composite fibers of PVDF with various loadings of ZnO and TiO2 nanoparticles were prepared and characterized. A special construction allowing the easy operation of the contact-separation triboelectric mode took place. Using this construction, a large number of TENG devices was fabricated and evaluated in regard to its electrical response in open and short-circuit conditions. The results show that certain loading levels of the ZnO or TiO2 nanoparticles can appreciably enhance the peak output power of the devices.



Electrospinning, Polymers, Triboelectric nanogenerators