Voltage-free fabrication of high-performance porous polypropylene piezoelectrets via in-situ triboelectrification during constrained supercritical carbon dioxide foaming

Porous piezoelectric electrets are pivotal for advanced sensing and energy harvesting due to their outstanding piezoelectric coefficients and mechanical flexibility. However, conventional fabrication relies on high-voltage charging, which introduces manufacturing complexity and safety concerns. This work presents a groundbreaking voltage-free strategy for in-situ triboelectric charging of isotactic polypropylene (iPP) within a one-dimensional constrained space during supercritical carbon dioxide (scCO 2 ) foaming. This approach enables the direct generation of porous piezoelectrets in a single step, eliminating the need for post-foaming electrical polarization. We systematically investigated the effects of backing materials (e.g., polyimide, iron) and an electret masterbatch on the triboelectrification efficacy. The optimized porous iPP electrets achieved a remarkable piezoelectric coefficient (d 33 ) of ∼600 pC/N, significantly surpassing that of commercial corona-charged counterparts (∼400-450 pC/N). Moreover, these materials enhibit exceptional long-term charge stability, retaining >90% of their initial d 33 and surface potential after 90 days. A mechanistic model based on intermolecular electron transfer is proposed, wherein the scCO 2 environment reduces the interfacial energy barrier and amplifies the electron transfer driving force, thereby enabling efficient charge injection. Furthermore, the incorporation of the electret masterbatch is demonstrated to enhance crystallinity and promote the formation of dense α-crystals, which increases the charge trap density and consequently improves charge retention within the porous architecture. This novel approach establishes a green, safe and efficient pathway for manufacturing high-performance porous piezoelectrets with superior and stable electromechanical properties. (a) Schematic of the experimental setup for a one-dimension constrained foaming and tribo charging; (b) Foaming protocol for the iPP; (c) Schematic diagram illustrating the working mechanism of the tribo-charged PEG; (d) Circuit diagram of the reciprocating PEG system for capacitor charging and LED illumination.