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If solar power is to become a primary source of electricity around the world, we’ll need cheap ways to store energy from the sun when it isn’t shining. A paper published in the journal Science this week reports a major step toward such a system. Researchers have developed a device that cheaply and efficiently converts the energy in sunlight into hydrogen, which can be used as a fuel and is easily stored.

Michael Graetzel, who directs the Laboratory of Photonics and Interfaces at the Ecole Polytechnique in Lausanne, Switzerland, along with colleagues in Korea and Singapore, built a device that uses electricity and catalyst materials to make hydrogen and oxygen from water. This new “water splitter,” as such devices are known, is highly efficient, uses cheap and abundant materials, and is easy to make.

Researchers have been pursuing solar-powered water splitting for decades, and while they’ve shown great performance in one or two parts of such a device, no one has built a complete system that’s practical.

The new device is remarkable because it meets three of the four criteria needed for a practical device: high efficiency, low cost, and the use of abundant materials (so it can be used at a large scale). The next step is to meet the fourth criterion—reliability. The device uses novel, relatively high-voltage solar cells to generate the needed electricity, along with inexpensive new catalyst materials based on nickel and iron for two electrodes—one produces hydrogen and the other makes oxygen.

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