Most previously designed devices for harnessing small motions have been based on the triboelectric effect or piezoelectrics. The researchers say 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 Professor Ju Li, lead researcher on the project. “Also, these devices have high electrical impedance and bending rigidity and can be quite expensive.”
By contrast, this system uses technology similar to that in lithium ion batteries; theoretically, it could be produced inexpensively at large scale. In addition, these 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. It 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.
When bent, the layered composite produces a pressure difference that squeezes lithium ions through the polymer. It also produces a counteracting voltage and an electrical current in the external circuit between the two electrodes, which can then be used directly to power other devices.
Because it requires only a small amount of bending to produce a voltage, such a device could have a tiny weight attached to one end to cause the metal to bend as a result of ordinary movements, when strapped to an arm or leg during everyday activities. Unlike batteries and solar cells, the output from the new system comes in the form of alternating current, with the flow moving first in one direction and then the other as the material bends first one way and then back.
The researchers say their device converts mechanical to electrical energy; meaning it is not limited by the second law of thermodynamics which sets an upper limit on the theoretically possible efficiency. “So in principle, the efficiency could be 100%,” said Prof Li.
In addition to harnessing daily motion to power wearable devices, the system might also be useful as an actuator with biomedical applications, or used for embedded stress sensors in settings such as roads, bridges, keyboards, or other structures.
