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Sun's Energy Can Be Harnessed for Nighttime Use

15 January 2014

Researchers at the Energy Frontier Research Center at the University of North Carolina at Chapel Hill have built a chemical system that converts the sun's energy not into electricity but hydrogen fuel, storing it for later use after the sun goes down.

"So called 'solar fuels' like hydrogen offer a solution to how to store energy for nighttime use by taking a cue from natural photosynthesis," said Tom Meyer, Arey Distinguished Professor of Chemistry at UNC's College of Arts and Sciences.

It turns out the sun puts out enough energy in one hour to power every vehicle, factory and device on the planet for an entire year. Solar panels harness the energy to generate electricity during the day but need a storage mechanism for nighttime use.

The new system uses a chemical reaction to generate hydrogen fuel by splitting water into its component parts. After the split, hydrogen is sequestered and stored, while the byproduct, oxygen, is released into the air. Not an easy task.

"You need to take four electrons away from two water molecules, transfer them somewhere else and make hydrogen and, once you have done that, keep the hydrogen and oxygen separated," said Meyer.

Meyer investigated dye-sensitized photoelectrosynthesis cells (DSPECs) for such separation schemes and came up with a design using a molecule and a nanoparticle. The molecule—a chromophore-catalyst assembly—absorbs sunlight and then kick-starts the catalyst to rip electrons away from water. The nanoparticle, to which thousands of chromophore-catalyst assemblies are tethered, is part of a film of nanoparticles that shuttles the electrons away to make the hydrogen fuel.

In past attempts either the chromophore-catalyst assembly broke away from the nanoparticles or the electrons couldn't be shuttled away quickly enough to make hydrogen.

Meyer overcame that hurdle by coating the nanoparticle, atom by atom, with a thin layer of titanium dioxide. By using ultra-thin layers, his research team found that the nanoparticle could carry away electrons far more rapidly than before, with the freed electrons available to make hydrogen. The team also figured out how to build a protective coating that keeps the chromophore-catalyst assembly tethered firmly to the nanoparticle, ensuring that the assembly stayed on the surface.

Meyer's new system can turn the sun's energy into fuel while needing almost no external power to operate and releasing no greenhouse gases. According to Meyer, the infrastructure to install these sunlight-to-fuel converters is close to being based on existing technology.

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