Cation-induced phase transformation in PVDF for efficient energy harvesting

Abstract

Piezoelectric poly (vinylidene fluoride) (PVDF) polymer and their nanocomposites have attracted significant interest for self-powered flexible electronics. Despite this, achieving PVDF with a distinct desirable phase remains a challenge. This study investigates the effects of cation-based halides doping (XCl₂ with X = Mg, Ca, Sr) on PVDF structure and properties with an electric field applied during heating. Molecular dynamics simulations and experimental investigations showed that ionic interactions with different cations induce distinct crystalline phases in PVDF. Mg²⁺ interactions primarily stabilize the α-phase, characterized by a higher gauche content, due to its smaller ionic size. In contrast, Ca²⁺, with a moderate ionic size, promotes the highly ordered β-phase by strongly aligning trans conformations. This effect is evident as the β-phase content increases from 67.36 % to 92.78 % upon incorporating 1.0 wt% CaCl₂. On the other hand, Sr²⁺, the largest ion among the three, induces the intermediate γ-phase, driven by stronger but more disruptive ionic interactions. We utilized PVDF-CaCl₂ films to fabricate a hybrid piezoelectric-triboelectric nanogenerator (PTNG). The CaCl₂ doping enhanced the surface charge density and polarization via increased β-phase content and electron affinity. The PVDF-CaCl₂/PA6 PTNG outperformed the control PVDF/PA6 nanogenerator, achieving a peak output voltage of ∼1553 V, a short-circuit current density of ∼253 mA/m², and a transferred charge density of ∼291.5 μC/m² at 1 Hz—over three times those of the control device. Furthermore, the PTNG device, coupled with an electronic switch, successfully demonstrated its functionality as a capacitive humidity sensor. These results highlight the importance of the size of cations in phase modulation of PVDF and demonstrate an efficient strategy for phase-specific PVDF applications in energy harvesting and sensor technologies.