Deliquescent Salt Layer-Induced Asymmetric Porosity and Self-Polarized β-Phase in Spin-Coated PVDF Membranes

Achieving simultaneous control over the self-polarized β-phase and strain-amplifying microarchitectures in PVDF remains challenging. Here, we report a facile approach that introduces a deliquescent salt layer (e.g., CaCl2) during conventional spin-coating to address this issue. In this process, most deliquescent salt is trapped near the membrane bottom interface, while a minor fraction migrates toward the top surface. The resulting asymmetric salt distribution draws in ambient moisture, establishing a vertical water gradient that drives in situ nonsolvent-induced phase separation (NIPS), wherein cooperative interactions (hydrogen bonding, ion-dipole, and dipole-dipole) align PVDF chains into the self-polarized β-phase. After removing the residual salt from the membrane, an asymmetric porous architecture is formed. Under identical tapping conditions, this membrane produces a stable piezoelectric output of ∼3.69 V, outperforming KBr-based counterparts (∼1.65 V) and nondeliquescent salt systems. This low-cost, scalable strategy eliminates the need for expensive substrates or equipment, offering a practical route to high-performance flexible piezoelectric devices.