Every drop of water is stuffed with the greenest of fuels, hydrogen, but getting it out is a challenge. A new material raises the prospect of doing so using noise pollution – from major roads, for example.

A team at the University of Wisconsin-Madison made crystals of zinc oxide that, when immersed in water, absorb vibrations and develop areas of strong negative and positive charge. These charges rip apart nearby water molecules, releasing hydrogen and oxygen gas.

"This is like a free lunch," says lead researcher Huifang Xu. "You are getting energy from the environment just like solar cells capture energy from the sun."

Underwater operator
Xu and colleagues generate hydrogen using a new variation on piezoelectric crystals – materials that generate a voltage when strained and which are being investigated as a way to generate electricity from movement.

The new crystals, however, are designed to be submerged, so the charge they generate instead pulls apart water molecules to release hydrogen and oxygen gas, a mechanism Xu's team calls the piezoelectrochemical effect.

Xu and colleagues grew thin microfibers of highly flexible zinc oxide crystals that flex when subjected to vibration, for example due to sound waves. They showed that ultrasonic vibrations under water cause the fibres to bend between 5 and 10 degrees at each end, creating an electrical field with a high enough voltage to split water and release oxygen and hydrogen.

Growing fibres with different dimensions changes the type of vibration they absorb best. For instance, it should be possible to tune them to maximise energy production from the vibrations caused by water flowing past or any other sound, say Xu.

Efficiency issue
Xu says that lab tests suggested the material can convert 18 per cent of the energy it absorbs from vibration into energy locked up in hydrogen gas, which can be released by burning.

Conventional piezoelectric materials are not as efficient at converting vibrations into electricity, and typically achieve around a 10 percent conversion rate. Using the charge a material generates indirectly, to split water, instead of directly to drive current, accounts for the difference, says Xu. The new materials could be used to develop systems that generate hydrogen from the noise of anything from machinery to crashing waves, he adds.

"It's a good idea," says Jinhui Song of Georgia Tech University, Atlanta. Because there is no need to create a circuit, devices based on the new crystals could be simpler than those based on conventional dry piezoelectrics, he points out. "They can reduce the complexity of the device."

However, he's sceptical that the wet devices should necessarily be more efficient. In principle, says Song, the energy generated by a material should be the same however it is deployed.