Enhanced energy density in piezoelectric PVDF-polymer nanocomposite via multiple mechanism synergetic action

Piezoelectric poly(vinylidene fluoride) (PVDF) polymer exhibits the flexible characteristic, however, a piezo-generator based on the PVDF polymer normally shows low power density because of its low piezoelectric coefficient. Here, we report a fundamental synergistic strategy to prepare PVDF polymer nanocomposite for enhancing electromechanical coupling performance and improving its energy density, including (i) the mechanically directional stress field (MDSF) fabrication of PVDF nanocomposite doped with polar Pb(Zr,Ti)O3 (PZT) ceramic nanoparticles via high-energy ball milling, (ii) introduction of micro-cavities holding polar nanoparticles, and (iii) floating electrode (Ag) films spaced uniformly inside the polymer nanocomposite. The investigations reveal that the MDSF fabrication can induce ordered, chain-shape nanocrystalline fibers with a high β-crystal phase, which makes 42% contribution to the piezoelectricity; while the micro-cavities and floating electrode films after poling exhibit a strong electret effect, which further make 18% and 27% contributions, respectively, to the piezoelectricity. This multiple-mechanism synergistic effect results in a much higher piezoelectric coefficient d33 increased from 4 pC N-1 for a pure PVDF (without bearing a uniaxial stretching) to 33 pC N-1 for the nanocomposite. It further dramatically increases to 228 pC N-1, a giant apparent piezoelectric charge coefficient d33*, for a 3D-printed 7-layer nanocomposite with 360 μm in thickness. An energy harvester (EH) based on the PVDF nanocomposite generates a record high peak power density of 13.2 mW cm-2 and the load power density of 5.46 mW cm-2 under a dynamic pressure of 255 kPa at the frequency of 5 Hz, which is near two orders of magnitude higher than that of a conventional PVDF polymer EHs. The fabricated flexible EH can even directly light up 46 LEDs in real-time or power a blind-man reading system when touching it. This work is of great significance in enlightening future flexible piezoelectronic designs.