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NOVEL piezoelectric crystals, which generate power when they come under mechanical stress, could be a useful way of generating hydrogen for use in hydrogen fuel cells, researchers say.

The crystals can split water into oxygen and hydrogen, using nothing more than small amounts of waste energy. The researchers have dubbed the process the piezoelectrochemical (PZEC) effect, and say it is almost twice as efficient as directly turning mechanical stress into electricity.

"This study provides a simple and cost-effective technology for direct water splitting that may generate hydrogen fuels by scavenging energy wastes such as noise or stray vibrations from the environment," says Huifang Xu, a geologist and specialist in crystals at University of Wisconsin-Madison, US, who led the research. “This new discovery may have potential implications in solving the challenging energy and environmental issues that we are facing today and in the future."

Xu’s team created nano-sized zinc oxide and barium titanate crystals and placed them underwater. At nanoscale the crystals lose some of their inherent brittleness, allowing the individual crystal fibres to flex, which in turn generates the piezoelectric effect. Xu found that when he target the crystals with ultrasonic vibrations, they split 5–10º each end, which creates enough voltage to split water into oxygen and hydrogen.

So far, the process was found to be 18% efficient – the hydrogen generated contained 18% of the energy present in the ultrasonic vibration. This is a considerable improvement on conventional piezoelectric materials, which convert mechanical stress into power with around 10% efficiency

Xu says: "Because we can tune the fibre and plate sizes, we can use even small amounts of [mechanical] noise — like a vibration or water flowing to bend the fibres and plates. With this kind of technology, we can scavenge energy waste and convert it into useful chemical energy."

Using piezoelectric energy to generate hydrogen means that its power can be stored and transported to where it is needed. While the amount of power generated this way is very small, the number of places the effect could be used is vast.

"We have limited areas to collect large energy differences, like a waterfall or a big dam," he says. "But we have lots of places with small energies. If we can harvest that energy, it would be tremendous."

The research was published in the 2 March issue of the Journal of Physical Chemistry Letters (doi 10.1021/jz100027t)