Filling the holes in piezopolymers with a solid electrolyte: a new paradigm of poling-free dynamic electrets for energy harvesting

We report the design and fabrication of a novel poling-free dynamic polymeric piezoelectret generator that has outstanding properties in kinetic energy harvesting. The piezoelectret, i.e. a solid polyelectrolyte filled cellular piezopolymer, was fabricated through solution casting of a poly(vinylidene fluoride) (PVDF)-Nafion blend followed by high pressure crystallization. The size and morphology of the Nafion-filled PVDF cells, together with their crystalline forms and substructures, were controlled at high pressure with the variation of PVDF-Nafion composition. Without any treatment of electrical poling, the open-circuit voltage output density of the PVDF-Nafion generator developed at the optimized conditions, stimulated under its dynamic deformation, reached 14.6 V cm(-2), exceeding that of most of the state-of-the-art piezoelectric polymers reported in the open literature. This was attributed to a synergistic action of intrinsic polarity and orientation of molecular dipoles of PVDF cells, formed during pressure crystallization, with artificial macroscopic dipoles of Nafion fillers on a scale of several micrometers, generated by their inner ionic motions during the cell wall deformation. Moreover, the PVDF-Nafion generator showed good stability and durability, and no decay of electrical output was observed for more than 100 000 continuous working cycles. Evidently, the study presented here may lead to a new paradigm of piezoelectrets, enabling the facile fabrication of a class of unique electret-transducer materials and their follow-up applications in electromechanical energy conversion.