Fabrication of Porous Polymers for Triboelectric Generators using Microwave Irradiation and Electrospinning

Abstract

Recently, triboelectric generator (TEG) has attracted much attention as an energy harvester capable of sustainably harvesting renewable energy. TEG has many advantages in terms of high electrical performance, simple structure, and low-cost fabrication. Nevertheless, the improvement in electrical output is still a challenge. Polymers or composites with porous structure have a large surface area per unit volume and can generate more electric charges on the surface, which helps improve the performance of TEG and is well suited to flexible devices that are emerging as next-generation electronic devices. In addition, the lower the manufacturing cost of TEG, the better, and a simple manufacturing process is needed. The aim of this dissertation is to develop high-performance TEG based on porous polymers including micro/nanofibers. To fabricate the porous polymers, straightforward and novel fabrication methodologies that can be easily accomplished using electrospinning and microwave is introduced. Electrospinning is able to produce micro/nanofibers that is one of the porous structure, and the effect of various parameters on fiber structure was investigated. The relationship between a physical property of inks, and the onset condition for the ink to be ejected from the nozzle was analyzed. The fluid behavior near the nozzle tip is then investigated. Various fiber structures were confirmed to be successfully fabricated by controlling the ink property and fluid behavior. In addition, honeycomb-like nanofiber structure derived from self-assembling of nanofiber was observed under the specific physical property of the ink. Such a structure cannot be obtained through a typical electrospinning process; if the solvent is not completely evaporated before the ink is deposited on a substrate, and maintains sufficient viscosity, the nanofibers are self-assembled into a specific shape. The TEG based on nanofibers showed good electrical performance. A typical paper consisting of micro/nanofibers can be used for fabricating TEG instead of long-time electrospinning process that requires few hours, sometimes even tens of hours. Because the paper has poor triboelectric property in spite of fiber structure, a simple and rapid coating process was developed to enhance the property. A variety of low-concentration polymer solutions have been produced and when the solution is coated on paper, local polymer agglomeration is induced. This increased the surface roughness of microfibers in paper and ultimately led to enhanced electrical performance o TEG. In order to overcome the limitations of TEG using fibers which either requires a long-time fabrication or causes poor performance, a method of fabricating porous polymer composites using microwave irradiation was proposed. This allows rapid fabrication as well as higher electrical performance. For the porous structure, deionized water is mixed with a polymer. If the water and polymer mixture is irradiated by microwave, then water is firstly heated due to the oscillation of water molecules. At this time, the water evaporates and only the polymer remains. The area where water droplets were is replaced by air. If the Ni and BaTiO3 particles are embedded into the polymer with DI water, then the porous polymer composites are obtained. Composites and porous composites, as well as porous polymers, exhibited extremely enhanced electrical performance compared to pure solid polymer films. The methodologies suggested in this dissertation via electrospinning and microwave irradiation would be effective in the fabrication of porous polymers. In addition, the developed materials are expected to be practically used for the implementation of flexible and wearable energy harvesters and other electronic fields such as sensors and actuators.