Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 169

nanostructure is essential to generate energy and the generated energy could
be accumulated in an energy storage system such as a rechargeable battery
or supercapacitor for later release. For example, a ZnO-nanowire nanogen-
erator has been connected to a capacitor and macro scale electrical circuitry
for a system demonstration with discrete wires. 75 An integrated system that
combines an integrated nanowire nanogenerator, integrated circuitry and an
integrated battery/supercapacitor on a single chip could be an important
technology challenge for future developments.
Fundamentals on piezoelectricity: as stated and discussed in the earlier
section, the fundamentals of the piezoelectricity of nanofibers have to be
investigated systematically in all aspects to produce nanofibers with high
piezoelectricity. Specifically, there is still no persuasive evidence to clarify
the mechanism of the a-b phase transformation during the electrospin-
ning process and published literature often provides conflicting infor-
mation. Some groups have attributed this transformation to the whole
stretching process of the jet due to the comparatively high fraction of b-
phase observed in smaller-diameter fibers from lower-concentration solu-
tions, 43,49 while other groups relate the cause of b-phase formation to the
solvent evaporation rate 97 or the ion current, 98 rather than the jet elongation
experienced during the electrospinning process, because of a predominant
b-phase obtained in films consisting of small (charged) droplets. In
addition, some other works remain problematic with regard to identifying
the formation of crystalline phases during the electrospinning process,
especially the b- and g-phases. Hence, further systematic studies and
characterizations for the electrospun fibers are required. These include
optimizations in process parameters such as materials, solvents, concen-
trations, electrical bias, needle-to-electrode distance as well as electrospin-
ning methodologies. Furthermore, detailed analyses on the electrospun
nanofibers have to be fully characterized with tools such as XRD, FTIR,
PFM and SHG and Raman spectroscopy. As such, nanofiber nanogenerators
with high energy conversion eciency could be consistently built in the
future.
Wearable electronics: energy harvesters powered by human motion could
be interesting and important as devices for wearable electronics, such as
smart watches and displays. Directly charging and powering the wearable
electronics via human motion could be advantageous over battery-powered
systems, given that the form-factor and weight of wearable electronics are
limited. As such, piezoelectric fiber-based nanogenerators could provide a
potential breakthrough with the possibility of being integrated with fabrics
for these applications.

d n 3 r 4 n g | 2

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References
1. Z. L. Wang and J. Song, Science, 2006, 312, 242-246.
2. C. Sun, J. Shi, D. J. Bayerl and X. Wang, Energy Environ. Sci., 2011, 4,
4508-4512.