2, Department of Physics,, University of Ioannina, Ioannina, , Greece
Polymeric piezoelectric nanogenerators (PNGs) have emerged as a suitable candidates to harvest waste mechanical energy to power up portable electronic devices. Smart textiles, with piezoelectric functionalities integrated in the fabrics have been envisioned. In this regards, numerous piezoelectric (nano-) generators based on PVDF or P(VDF-TrFE) nanofibers have been reported.
However, the piezoelectric polymer (nano-)generators have typically shown low output energy densities; A the common issue hindering their application.
Introduction of porosity into the piezoelectric polymer has been proposed to increase the voltage output of PNGs. However, designing a process that allows introduction of pores in polymer fibers with typical diameter of the order 100 nm, would be a breakthrough in the field of PNGs. In this contribution, we discuss an elegant approach to tailor porosity in electrospun P(VDF-TrFE) nanofibers. The approach is based on the thermodynamics of polymer solutions, and solvent/non-solvent interactions with the polymer. We calculated the ternary phase diagram of P(VDF-TrFE)/ non-solvent (water)/solvent, and experimentally verified it. Based on the phase diagram, a conscious amount of water is intentionally added into the P(VDF-TrFE) solution to induce porosity in the fiber. PNGs based on the porous electrospun P(VDF-TrFE) nanofibers show systematic increases of the output voltage with porosity. The output power increased from 0.1 mW/cm3 for PNGs with zero porosity to 7 mW/cm3 for PNGs with 50% porosity. Dielectric spectroscopy of the nanofibers attributes the enhanced output to the reduced dielectric permittivity of the fibers and that the voltage generation in the porous fibers is of the same origin as in neat piezoelectric P(VDF-TrFE) films and is due to the relaxation of segments within the restricted amorphous phase.