Piezoelectric Energy Harvesting Nanofibers - Hierarchical Nanostructures for Energy Devices - page 168

in favor of b-phase PVDF while a high evaporation rate is in favor of a-phase
PVDF. 53 These are some examples from papers published recently on the
studies and possible enhancement of b-phase PVDF nanofibers. It is noted
that many of the aforementioned reports were conducted by different ex-
perimental setups such as different solvents, concentrations, electrical bias
and collectors. Therefore, it is possible that contradictory conclusions could
have been drawn on specific processing parameters or materials. It would be
desirable to have more uniform and optimized processing parameters with
good characterizations on the piezoelectric effects of electrospun nanofibers.

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7.6 Summary
Obviously, current and future stand-alone devices could all take advantage of
self-powering energy harvesters and piezoelectric nanofibers as discussed in
this chapter. In order to produce cost-effective piezoelectric nanofibers, the
electrospinning process is a good candidate as various materials and
nanoparticles have been successfully fabricated as nanofibers and/or
nanofibers with embedded nanoparticles. The conventional electrospinning
process has been modified for good alignment and position controllability
as well as the feasibility to embed nanoparticles. The nanofibers obtained,
such as PVDF and PZT for good piezoelectricity, are highly flexible and easy
to fabricate for possible integration in implantable and/or flexible devices, as
well as textile applications such as electric clothing. This provides tre-
mendous opportunities in various fields for fiber-based nanogenerators.
Several groups have demonstrated prototypes of nanofiber nanogenerators
as reviewed in this work. Beyond the current feasibility studies mainly from
academic institutes, further development of fiber based nanogenerators will
be necessary for practical applications. Here, several future prospects/dir-
ections are discussed.
High power nanogenerators: most of the current nanogenerators (including
nanofiber nanogenerators) are limited to low power generation in laboratory
environments. Some recent studies have advanced the records to higher
power outputs 73 as well as using real mechanical actuation sources such as
the human heartbeat 87 and mice 96 to drive active devices such as LEDs. 21
Nevertheless, the electrical power generated by current nanogenerators
made of nanostructures is often too small to have practical usage for com-
mercial hand-held systems such as an electrical watch, which typically
consumes electrical power in the range of a few mW. Therefore, if and when
the power generated by the current single nanofiber nanogenerator in the
recorded range of 10 11 Watt can be boosted up to the range of 10 6 Watt,
probably by multiple-nanofibers, many practical applications could attract
strong commercial interest in nanofiber nanogenerators.
Energy storage/regulation systems: similar to all electrical power generation
devices, suitable energy storage/regulation systems will be required for
nanofiber nanogenerators to store the generated energy and to release it at
the right

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time. For example, repetitive deformation of a piezoelectric