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The path to the hydrogen economy is getting visibly brighter--literally. Nano¿tubes that break apart water molecules to liberate hydrogen can now do so more efficiently and could soon use the optical spectrum of sunlight.
In dissociating water with sunlight, engineers have available three technologies: One is solar cells, which hold the record for water-splitting efficiency but are comparatively expensive. Another approach uses microorganisms, which are inexpensive but so far produce only minuscule amounts of hydrogen. The third option is photocatalysis, which relies on momentarily freed electrons in a semiconductor. Electrons that encounter water molecules replace the electrons in the bonds between hydrogen and oxygen. They thus break water apart and generate hydrogen gas. Photocatalysts are potentially less expensive than solar cells and produce more hydrogen than microorganisms.