Underwater mechanical energy harvesters are of rising
interest
due to their potential for various applications, such as self-powered
ocean energy harvesters, monitoring devices, and wave sensors. Pressure-responsive
films and stretch-responsive fibers, which provide high electrical
power in electrolytes and have simple structures that do not require
packing systems, are promising as harvesters in the ocean environment.
One drawback of underwater mechanical energy harvesters is that they
are highly dependent on the direction of receiving external forces,
which is unfavorable in environments where the direction of the supplied
force is constantly changing. Here, we report spherical fleece, consisting
of wool fibers and single-walled carbon nanotubes (SWCNTs), which
exhibit repetitive electrical currents in all directions. No matter
which direction the fleece is deformed, it changes the surface area
available for ions to access SWCNTs electrochemically, causing a piezoionic
phenomenon. The current per input mechanical stress of the fabricated
SWCNT/wool energy harvester is up to 33.476 mA/MPa, which is the highest
among underwater mechanical energy harvesters reported to date. In
particular, it is suitable for low-frequency (<1 Hz) environments,
making it ideal for utilizing natural forces such as wind and waves
as harvesting sources. The operating mechanism in the nanoscale region
of the proposed fleece harvester has been theoretically elucidated
through all-atom molecular dynamics simulations.