Energy from motion is all around us — in the tides, in wind, when we walk, when we drive and even the vibrations from ambient sounds. A research team
at Georgia Tech has taken a step towards making that energy useful.
A group led by
materials science professor Zhong Lin Wang has built a power cell that
recharges when it's compressed or deformed. Current versions are small,
producing only a few hundred millivolts, but a larger one could supplement or
even a replace batteries in electronic devices.
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The power cell is made of a cathode and anode. The cathode
is lithium-cobalt-oxide (LiCoO2) and the anode is titanium dioxide (TiO2). The
anode is made up of tiny nanometer-sized tubes grown on a titanium film. The
anode and cathode are separated by a membrane made from polyvinylidene
fluoride film, also known as PVDF.
PVDF is a piezoelectric material, the kind that generates
a charge whenever it is put under a mechanical strain such as compression, stretching
or bending. The charge generated by the stress drives lithium ions from the
cathode to the anode. The lithium ions form lithium-titanium oxide, and store
the energy. Release the stress and the electric field disappears — but the
lithium ions stay in the anode.
Connecting an electrical circuit to the cathode and anode
causes the lithium ions to flow back to the cathode until the cycle is
Pressing on the cell more than two times per second produced up to 395
millivolts in four minutes. That's close to the frequency at which human steps
hit the ground during a walk.
The device was then discharged with a current of one
milliamp for about two minutes. Wang's team estimated the power cell held about
0.036 milliamp-hours. That's small — a typical lithium-ion battery holds four
orders of magnitude more. But it shows that this kind of technology works. The
big barrier to more efficiency is the metal casing of the cell, because it
doesn't transmit all the mechanical energy from pressing on it.
The research was supported by the Defense Advanced Research
Projects Agency, which has a real interest in powering mobile devices for the
military without the need for generators to charge them.
Credit: Gary Meek /