MIT's layered composite enables energy harvesting from small bending motions

January 11, 2016 // By Paul Buckley
Researchers at MIT have developed a new method of harnessing the energy of small motions based on electrochemical principles.  The discovery could be capable of harvesting energy from a broad range of natural motions and activities, including walking.

The method would be suitable for use in a number of biomedical, mechanical, or environmental monitoring devices.

The new system, based on the slight bending of a sandwich of metal and polymer sheets, is described in the journal Nature Communications, in a paper by MIT professor Ju Li, graduate students Sangtae Kim and Soon Ju Choi, and four others.

Most previously designed devices for harnessing small motions have been based on the triboelectric effect (essentially friction, like rubbing a balloon against a wool sweater) or piezoelectrics (crystals that produce a small voltage when bent or compressed). These work well for high-frequency sources of motion such as those produced by the vibrations of machinery. But for typical human-scale motions such as walking or exercising, such systems have limits.

“When you put in an impulse” to such traditional materials, “they respond very well, in microseconds. But this doesn’t match the timescale of most human activities,” said Li, who is the Battelle Energy Alliance Professor in Nuclear Science and Engineering and professor of materials science and engineering. “Also, these devices have high electrical impedance and bending rigidity and can be quite expensive,”  Li pointed out.
By contrast, the new system uses technology similar to that in lithium ion batteries, so it could likely be produced inexpensively at large scale. In addition, the devices would be inherently flexible, making them more compatible with wearable technology and less likely to break under mechanical stress.

While piezoelectric materials are based on a purely physical process, the new system is electrochemical, like a battery or a fuel cell and uses two thin sheets of lithium alloys as electrodes, separated by a layer of porous polymer soaked with liquid electrolyte that is efficient at transporting lithium ions between the metal plates. But unlike a rechargeable battery, which takes in electricity, stores it, and then releases it, this system takes in mechanical energy and puts out electricity.