High performance composition-tailored PVDF triboelectric nanogenerator enabled by low temperature-induced phase transition

Polyvinylidene difluoride (PVDF) polymers are widely used in triboelectric nanogenerators (TENGs) due to their excellent dielectric and piezoelectric properties. To design a high-performance TENG, it is essential to investigate the individual contributions of these properties on contact electrification (CE). In this work, a range of PVDF, coand terpolymers are used to assess the distinct contributions of dielectric and piezoelectric properties on the TENG's output. As elucidated by a modified overlapped electron cloud model (OEC) supported with Kelvin Probe Force Microscopy (KPFM) measurements, the high dielectric constant in terpolymers hinders electron backflow after CE, while piezoelectric polarization in fl-phase PVDF shifts its electron energy levels before CE. Notably, in PVDF polymers, the dielectric and piezoelectric enhancements to TENGs come at the expense of one another. To verify this phenomenon, the terpolymers are cooled to alter their phases from relaxor to regular ferroelectrics, diminishing their dielectric constants while increasing their piezoelectric polarizations. Initially, the output is high (10.2 W/m2) at room temperature during the relaxor ferroelectric phase which is due to the high dielectric constant. Upon cooling, the output decreased to its lowest during the phase transition (4.3 W/m2 at 0 degrees C), as a result of the tradeoff between dielectric and piezoelectric enhancement. When the temperature was further tuned down to - 40 degrees C, the output reaches the maximum of 16 W/m2, which is attributed mainly to the ferroelectric phase. For the first time, the phase transition is observed and evidenced by electric field-induced dielectric and ferroelectric measurements, proving the piezoelectric polarization of PVDF terpolymers. These findings therefore provide guidelines on material selection and pave the way for potential low-temperature TENG applications.