Solar energy has long been hailed as a clean alternative to fossil fuels, but it comes with a catch: The sun sets. In order to get around this, researchers have developed a system capable of converting the sun's energy into hydrogen fuel that can be stored for later.

"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. "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."

The system, known as a dye-sensitized photoelectrosynthesis cell (DSPEC), generates hydrogen fuel by splitting water into hydrogen and oxygen with help from the Sun. The hydrogen is then separated and stored while oxygen is simply released.

But while the process may sound straightforward enough, implementing is anything but, according to 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," he said. "How to design molecules capable of doing that is a really big challenge that we've begun to overcome."

Meyer's design breaks down into two basic parts - a molecule and a nanoparticle. The first is responsible for absorbing the sunlight and kickstarting the catalyst that separates the electrons while the second whisks the electrons away to make the fuel.

Time and time again, however, the system failed, either because the molecules kept escaping or the electrons were not transported quickly enough to produce hydrogen.

To solve this, Meyer teamed up with North Carolina State University to coat nanoparticles with a thin layer of a substance called titanium dioxide - atom by atom. Using ultra-thin layers caused the nanoparticles to carry electrons away much quicker. The system was complete when the researchers developed a protective coating that kept the molecules tied to the nanoparticles.

Going forward, the researchers plan on using the same approach to reduce carbon dioxide to a carbon-based fuel.

"When you talk about powering a planet with energy stored in batteries, it's just not practical," Meyer said. "It turns out that the most energy dense way to store energy is in the chemical bonds of molecules. And that's what we did - we found an answer through chemistry."