Four years ago, we first heard about how Korean scientists had proposed using sound to charge mobile phones. They explained that it could be done via a piezoelectric effect, in which zinc oxide nanowires converted sound-caused vibrations into electricity. At the time, the researchers couldn't generate enough of a current to actually charge a phone. Now, however, scientists from Nokia and Queen Mary University of London (QMUL) have succeeded in doing so.

Like the Korean team, the Nokia/QMUL researchers utilized zinc oxide, in the form of a sheet of tiny nanorods. As is the case with other piezoelectric materials, zinc oxide produces an electrical current when subjected to mechanical stress. The nanorods will actually bend in response to sound waves, creating that stress in the process.

The scientists started by spraying a coating of liquid zinc oxide onto a plastic sheet. That sheet was then placed in a "mixture of chemicals" and heated to 90ºC (194ºF), which caused the zinc oxide to grow into an array of nanorods.

In order to harvest the voltage generated, the nanorod sheet was sandwiched between two electrical contact sheets. Whereas these contacts would typically be made from gold, the researchers developed a cost-cutting technique that allowed them to use ordinary aluminum foil instead.

The prototype energy-harvesting device is about the size of a Nokia Lumina 925, and is reportedly capable of generating up to five volts using "everyday background noise" such as traffic, music and voices. Five volts is enough to charge a mobile phone, and is significantly higher than the 50 millivolts managed in the previous effort.

"Being able to keep mobile devices working for longer, or do away with batteries completely by tapping into the stray energy that is all around us is an exciting concept," said QMUL's Dr. Joe Briscoe. "We hope that we have brought this technology closer to viability."

Source: Queen Mary University of London