Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 155

d n 3 r 4 n g | 2

Figure 7.6 (a) A schematic diagram of the near-field electrospinning process show-

ing possible dipole directions (black arrows) and electrical field direction
(red arrows). The PVDF polymer solution experiences mechanical
stretching and in situ electrical poling during the formation of nanofi-
bers due to the high electrostatic field toward the substrate. (b) SEM
microphoto showing an electrospun PVDF nanofiber on two contact
pads. (c) An optical image of a single nanofiber nanogenerator on top of
a plastic substrate with two electrical wires. (d) Testing results of a PVDF
nanofiber nanogenerator under a same mechanical strain but with
varying strain rate. As the strain rate increases, a higher output current
is produced while the total charges generated in both cases remain
approximately the same.
Reprinted with permission from ref. 22. Copyright (2010) American
Chemical Society.

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inherent high electric field with in situ mechanical stretching for the
possible alignment of dipoles along the longitudinal direction of the
nanofiber. 22 In the experimental demonstration, a single PVDF nanofiber
was electrospun across two adjacent electrodes placed 100-600 mm apart and
fixed to the contact electrode on either side, using conductive silver paste as
shown in Figure 7.6(b). It is noted that if the dipoles are aligned along
the longitudinal axis of the fiber, the d 33 mode could be responsible for the
main piezoelectric responses when the fiber is compressed or stretched in
the axial direction. Figure 7.6(c) shows an optical photo of the electrospun
PVDF nanofiber on top of a flexible plastic substrate with two contact
electrodes and electrical wires. Experimentally, a single PVDF nanofiber-
based nanogenerator was able to generate 0.5-3 nA of current and 5-30 mV
of voltage under repeated long term reliability tests without a noticeable
performance degradation. The study also validated that both voltage and
current generation are related to the strain rate. When keeping the
total strain at the same level, higher electrical responses were monitored