A mechanical energy transducer based on an ionic diode could enable a new way to harvest unused energy.

The diode captures low-frequency mechanical energy, such as the touch of a finger on a smart phone screen that then could be stored in a battery or used to power the phone or a tablet computer.

A cross-disciplinary team of materials scientists and electrical engineers at Pennsylvania State University announced their research results in the journal Advanced Energy Materials.

Touch-sensitive screen. Credit: Wang Lab/Penn State UniversityThe idea of harvesting unused energy is garnering much research attention (see here for an example). Devices that capture unused energy and convert it to electricity power many different small electronics. Most available devices operate on the piezoelectric effect, which is most efficient at higher vibration frequencies, over 10 vibrations per second.

The Penn State team addressed capturing lower-frequency motion.

"Our concept is to specifically design a way to turn low-frequency motion, such as human movement or ocean waves, into electricity," sats Qing Wang, professor of materials science and engineering, Penn State. "That's why we came up with this organic polymer p-n junction device."

The p-n junction device is the ionic diode, composed of two nanocomposite electrodes with oppositely charged mobile ions separated by a polycarbonate membrane. The electrodes are a polymeric matrix filled with carbon nanotubes and infused with ionic liquids.

When a mechanical force is applied, the ions diffuse across the membrane, creating a continuous direct current. At the same time, a built-in potential that opposes ion diffusion is established until equilibrium is reached.

The diode is flexible and light. Wang said the team hopes that the device can provide 40% of the power needed to operate a smart phone, putting less stress on the phone’s battery. Compared to piezoelectric devices, the ionic diode has equal or greater power density.