Supercritical Fluid Foaming of Self-Polarized β-PVDF Piezoelectric Foam with Tailored Cells for Advanced Energy Harvesting

Porous poly(vinylidene fluoride) (PVDF) attracts considerable attention for mechanical energy harvesting and self-powered systems owing to its exceptional electromechanical coupling capabilities. Supercritical carbon dioxide foaming (SCF) that features solvent-free and green nature is one of the most preferable approaches to construct controllably porous polymers. Nonetheless, the electroactive β-phase of PVDF remains elusive during SCF due to its instability under an elevated temperature. Herein, we pioneer the use of ionic liquid (IL)-assisted SCF for the fabrication of self-polarized β-PVDF piezoelectric foam with tailored cells. IL incorporation not only facilitates CO₂ infusion but also catalyzes the formation and preservation of β-phase crystals throughout the SCF process, culminating in a PVDF foam with an exceptionally high β-phase content (98%). Furthermore, the constructed honeycomb-like pore can effectively absorb external forces and significantly amplify the compressive strain, substantially enhancing the piezoelectric output of the PVDF foam. Impressively, PVDF foam with circumferential pore delivers a maximum piezoelectric output of 19.1 V, setting a new benchmark for SCF fabricated PVDF-based foam PEH. This advanced piezoelectric PVDF foam demonstrates significant potential as a flexible sensor for real-time monitoring of human foot pressure, highlighting its utility in adaptive sensing applications.