Energy harvesters based on fluorinated ethylene propylene unipolar ferroelectrets with negative charges

Energy harvesting devices can convert energy from various ambient sources (that are usually ignored) into electricity to power portable electronic devices and wireless sensor networks. Such devices have stimulated extensive interest in recent years. For the current study, we succeeded in designing and fabricating a new type of energy harvester using high performance unipolar ferroelectrets made from fluorinated ethylene propylene (FEP), consisting of a negatively charged and a noncharged wave-shaped FEP layer. Such materials are resistant to elevated temperatures owing to the thermally stable negative charges in this material. They also exhibit great stretchability due to the symmetric wave-shaped structure. By using a variety of test frequencies, seismic masses, external resistances, and wave-shaped FEP films with different surface potentials, we systematically investigated the energy harvesting performance of these devices. Typically, wave-shaped films with a material thickness of 12.5 μm and a maximum total thickness of the wavy structure (including the air thickness) of about 160 µm were employed. When charged to a surface potential of −500 V and operated with a seismic mass of 3 g fixed on the center of the band shaped unipolar ferroelectret sample, a power of 355 µW for an input acceleration of 1 g (g is the gravity of the Earth) was delivered to the optimal load resistance at the resonance frequency of 22 Hz. The relatively large power generated is due to the sizeable elasticity of the wave-shaped FEP film and the amplification of the force acting on the film in the specifically designed device.