Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 166

d n 3 r 4 n g | 2

Figure 7.13 (a) Plots of measured energy conversion eciency of PVDF nanogen-

erators and thin films with different feature sizes. (b) Experimental
results of PVDF thin film and nanofiber charge density (generated
charges divided by electrode area) with respect to applied strain. The
charge density of the PVDF nanofiber increases nonlinearly when
the applied strain is larger than ca. 0.01%. The inset shows the
details under small strains. 22
Reprinted with permission from ref. 22. Copyright (2010) American
Chemical Society.

0.025 mV N 1 ). 14 In the study of single PVDF nanofibers, 45 PVDF nanofiber
nanogenerators have been tested that were 600 nm-6.5 mm in diameter and
100-600 mm in length. 22 The average conversion eciency was calculated as
12.5%, a value that is much greater than typical energy harvesters made from
reported piezoelectric PVDF thin films (0.5-4%) 87,88 and commercial PVDF
thin films (0.5-2.6%) tested under the same conditions as PVDF nanofibers
as illustrated in Figure 7.13(a). The eciency of the PVDF thin film was
estimated using the same method/experiment as the PVDF nanogenerators.
However, the d 31 mode was utilized in the PVDF thin film instead of the d 33
mode to accommodate experimental setup. The general trend in
Figure 7.13(a) indicates that nanogenerators with smaller diameters exhibit
higher energy conversion eciencies, even with variable piezoelectric
properties resulting from slight differences in processing conditions. Several
possible reasons have been proposed for the enhanced electromechanical
response, including a possible higher degree of crystallinity and chain
orientation, 89 a difference in elastic boundary conditions (thin film/bulk
samples have metal electrodes on both ends as constraints), the physical size
of the nanogenerators, which could promote size-dependent piezoelectricity
such as 'flexoelectricity', 90 and possible nonlinear piezoelectric responses
coming from extrinsic contributions known as 'domain wall motion'. 91 For
example, a much smaller domain wall motion barrier in PVDF nanofibers
(ca. 0.01%) as shown in Figure 7.13(b) was observed and it resulted in large
piezoelectric responses for strain higher than 0.01% for higher energy con-
version eciency.

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