Scientists at Georgia Tech say that they have developed a new self-charging power cell technology that directly converts mechanical energy to chemical energy. Then, the power is stored until it is needed to generate electricity. This hybrid generator- storage cell utilizes mechanical energy more efficiently than systems using separate generators and batteries, they say.
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 pedestrian's shoe hitting the floor when walking, the hybrid power cell generates enough current to power small electronic devices, such as a calculator, or someday, such medtech applications as miniature implantable devices and handhelds.
“People are accustomed to considering electrical generation and storage as two separate operations done in two separate units,” said Zhong Lin Wang, a Regents professor in the School of Materials Science and Engineering at Georgia Tech. “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.” (See Figure 1)
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 the chemical equilibrium of the two electrodes are re-established and the distribution of lithium ions can balance the piezoelectric fields within the PVDF film. When the applied force is released, the piezoelectric field in the film disappears, and the lithium ions are kept at the anode by a chemical process.
The research was supported by the Defense Advanced Research Projects Agency, 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.