Self-Poling Polyvinylidene Fluoride-Based Piezoelectric Energy Harvester Featuring Highly Oriented β-Phase Structured at Multiple Scales

Abstract

Self-poling poly(vinylidene fluoride) (PVDF) featuring uniformly aligned dipoles can convert mechanical vibrations into electrical energy without post-poling treatment, which is a topic of intense interest in self-powered flexible electronics. However, the current strategies to align PVDF rich in β-crystals have some limitations, such as complex processing, low productivity, and so on. Herein, a self-poling flexible PVDF generator structured with a highly oriented β-phase was prepared via a normal melt processing pathway, by combining flow-induced crystallization with the stabilizing effect of dopamine-functionalized carbon nanotubes (P-CNTs) on the oriented molecules. In this case, the intense flow field exerted by microinjection facilitated the gauche–trans transition and uniform orientation, while with the help of strong interaction between O–H groups of P-CNTs and F–C groups of PVDF, P-CNTs recruited and immobilized the neighboring short chains onto the surface, suppressing the molecular relaxation. As a result, rich electroactive β-phases with preferential alignment characteristics were cultivated in situ, resulting in intrinsic self-oriented dipoles. The extraordinary piezoelectric properties of the PVDF were fully utilized even without polarization treatment, and the open-circuit voltage density reached ∼11 V/cm2, with over 4-fold improvement as compared to that of the randomly oriented sample fabricated by conventional cast method. Finally, the potential application as a robust energy harvester was explored by converting irregular mechanical energy from finger movements into utilizable electric energy, lighting green light-emitting diodes. This multi-scale control from crystalline composition to morphology based on flow-induced crystallization is achieved by simple melt-processing and thus can be suitable for large-scale continuous fabrication of self-poling piezoelectric PVDF energy harvesting devices.