click to enlarge Georgia Tech researcher Zhong Lin Wang holds the components of a new self-charging power cell that uses piezoelectric materials to directly convert mechanical energy to chemical energy. The chemical energy can be released as electricity. Photo: Gary Meek/Georgia Institute of Technology

Researchers have developed a self-charging power cell that directly converts
mechanical energy to chemical energy, storing the power until it is released as
electrical current. By eliminating the need to convert mechanical energy to
electrical energy for charging a battery, the new hybrid generator-storage cell
uses mechanical energy more efficiently than systems using separate generators
and batteries.

At the heart of the self-charging power cell is a piezoelectric membrane
that drives lithium ions from one side of the cell to the other when the
membrane is deformed by mechanical stress. The lithium ions driven through the
polarized membrane by the piezoelectric potential are directly stored as
chemical energy using an electrochemical process.

By harnessing a compressive force, such as a shoe heel hitting the pavement
from a person walking, the power cell generates enough current to power a small
calculator. A hybrid power cell the size of a conventional coin battery can
power small electronic devices—and could have military applications for
soldiers who might one day recharge battery-powered equipment as they walked.

“People are accustomed to considering electrical generation and storage as
two separate operations done in two separate units,” says Zhong Lin Wang, a
Regents professor in the School of Materials Science and Engineering at the Georgia
Institute of Technology. “We have put them together in a single hybrid unit to
create a self-charging power cell, demonstrating a new technique for charge
conversion and storage in one integrated unit.”

The research was reported in Nano Letters. The research was
supported by the Defense Advanced Research Projects Agency (DARPA), the U.S.
Air Force, the U.S. Department of Energy, the National Science Foundation, and
the Knowledge Innovation Program of the Chinese Academy of Sciences.

The power cell consists of a cathode made from lithium-cobalt oxide (LiCoO2)
and an anode consisting of titanium dioxide (TiO₂) nanotubes grown
atop a titanium film. The two electrodes are separated by a membrane made from
poly(vinylidene fluoride) (PVDF) film, which generates a piezoelectric charge
when placed under strain. When the power cell is mechanically compressed, the
PVDF film generates a piezoelectric potential that serves as a charge pump to
drive the lithium ions from the cathode side to the anode side. The energy is
then stored in the anode as lithium-titanium oxide.

Charging occurs in cycles with the compression of the power cell creating a
piezopotential that drives the migration of lithium ions until a point at which
the chemical equilibrium of the two electrodes are re-established and the
distribution of lithium ions can balance the piezoelectric fields in the PVDF
film. When the force applied to the power cell is released, the piezoelectric
field in the PVDF disappears, and the lithium ions are kept at the anode
through a chemical process.

The charging cycle is completed through an electrochemical process that
oxidizes a small amount of lithium-cobalt oxide at the cathode to Li1-xCoO2 and
reduces a small amount of titanium dioxide to LixTiO2 at the anode. Compressing
the power cell again repeats the cycle.

When an electrical load is connected between the anode and cathode,
electrons flow to the load, and the lithium ions within the cell flow back from
the anode side to the cathode side.

click to enlarge Components of a new self-charging piezoelectric power cell are shown in this image. The clear disc in the center is the piezoelectric film that serves as a charge pump for lithium ions. Image: Gary Meek/Georgia Institute of Technology

Using a mechanical compressive force with a frequency of 2.3 Hz, the
researchers increased the voltage in the power cell from 327 to 395 mV in just
four minutes. The device was then discharged back to its original voltage with
a current of one milliamp for about two minutes. The researchers estimated the
stored electric capacity of the power cell to be approximately 0.036
milliamp-hours.

So far, Wang and his research team—which included Xinyu Xue, Sihong Wang,
Wenxi Guo, and Yan Zhang—have built and tested more than 500 of the power
cells. Wang estimates that the generator-storage cell will be as much as five
times more efficient at converting mechanical energy to chemical energy for as
a two-cell generator-storage system.

Much of the mechanical energy applied to the cells is now consumed in
deforming the stainless steel case the researchers are using to house their
power cell. Wang believes the power storage could be boosted by using an
improved case.

“When we improve the packaging materials, we anticipate improving the
overall efficiency,” he says. “The amount of energy actually going into the
cell is relatively small at this stage because so much of it is consumed by the
shell.”

Beyond the efficiencies that come from directly converting mechanical energy
to chemical energy, the power cell could also reduce weight and space required
by separate generators and batteries. The mechanical energy could come from
walking, the tires of a vehicle hitting the pavement, or by harnessing ocean
waves or mechanical vibrations.

“One day we could have a power package ready to use that takes advantage of this
hybrid approach,” Wang says. “Almost anything that involves mechanical action
could provide the strain needed for charging. People walking could be generating
electricity as they move.”

Source: Georgia Institute of Technology