New system harnesses Sun's energy during day for use at night

Scientists have built a system that converts Sun's energy into hydrogen fuel during the day and stores it for later use, allowing people to power their devices even at night.
Solar energy has long been used as a clean alternative to fossil fuels such as coal and oil, but it could only be harnessed during the day when the Sun's rays are strongest.
Researchers led by Tom Meyer at the Energy Frontier Research Center at the University of North Carolina at Chapel Hill built the system that converts the Sun's energy not into electricity but hydrogen fuel and stores it for later use.

"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 Meyer.
"Our new findings may provide a last major piece of a puzzle for a new way to store the Sun's energy - it could be a tipping point for a solar energy future," he said.
The new system designed by Meyer and colleagues at UNC and with Greg Parsons' group at North Carolina State is known as a dye-sensitised photoelectrosynthesis cell, or DSPEC.

It generates hydrogen fuel by using the Sun's energy to split water into its component parts.

After the split, hydrogen is sequestered and stored, while the byproduct, oxygen, is released into the air.
"But splitting water is extremely difficult to do," said Meyer.
"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. How to design molecules capable of doing that is a really big challenge that we've begun to overcome," he said.
Meyer had been investigating DSPECs for years, his design has two basic components: a molecule and a nanoparticle.
The molecule, called 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.
However, even with the best of attempts, the system always crashed because either the chromophore-catalyst assembly kept breaking away from the nanoparticles or because the electrons couldn't be shuttled away quickly enough to make hydrogen.
To solve these problems, Meyer turned to the Parsons group to use a technique that coated the nanoparticle, atom by atom, with a thin layer of a material called titanium dioxide.
By using ultra-thin layers, researchers found that the nanoparticle could carry away electrons far more rapidly than before, with the freed electrons available to make hydrogen.

They 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.