Self-charging cell directly converts and stores energy

Researchers at the Georgia Institute of Technology (GATECH) 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.

They claim that, by eliminating the need to convert mechanical energy to electrical energy for charging a battery, the new hybrid generator storage cell utilises mechanical energy more efficiently than systems using separate generators and batteries.

Central to the system 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.

According to a statement, the lithium ions driven through the polarised membrane by the piezoelectric potential are directly stored as chemical energy using an electrochemical process.

By harnessing a compressive force, 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 walk.

‘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 GATECH. ‘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.’

How it works

According to GATECH, the power cell consists of a cathode made from lithium-cobalt oxide (LiCoO2) and an anode consisting of titanium dioxide (TiO2) nanotubes grown on top of 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 equilibriums 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 oxidises a small amount of LiCoO2 at the cathode to Li1-xCoO2 and reduces a small amount of TiO2 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.

Using a mechanical compressive force with a frequency of 2.3Hz, the researchers increased the voltage in the power cell from 327mV to 395mV in four minutes.

The device was then discharged back to its original voltage with a current of 1 milliamp for about two minutes. The researchers estimated the stored electric capacity of the power cell to be approximately 0.036 milliamp-hours.

The research was reported on 9 August 2012 in the journal Nano Letters.