Hydrogen fuel captures sun's energy for use at night

US researchers have developed a system that converts solar energy into hydrogen fuel, solving the problem of how to use the sun’s energy for power at night (when power is most used).

In one hour, the sun puts out enough energy to power every vehicle, factory and device on the planet for an entire year. Solar panels can harness that energy to generate electricity during the day. But the problem with the sun is that it goes down at night - and with it the ability to power our homes and cars. To give solar energy a shot at being a clean source for powering the planet, scientists had to figure out how to store it for night-time use.

Led by Tom Meyer, researchers at the Energy Frontier Research Center at the University of North Carolina (UNC) built a system that can store solar energy for later use.

“So-called ‘solar fuels’ like hydrogen offer a solution to how to store energy for night-time use by taking a cue from natural photosynthesis,” said 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 new system designed by Meyer and colleagues at UNC and with Greg Parsons’ group at North Carolina State University does exactly that. It is known as a dye-sensitised photoelectrosynthesis cell, or DSPEC, and 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 by-product, 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.”

Meyer’s design has two basic components: a molecule and a nanoparticle. The molecule, called a chromophore-catalyst assembly, absorbs sunlight and then kickstarts 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 both of 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 ultrathin layers, the 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.

With electrons flowing freely through the nanoparticle and the tether stabilised, 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. What’s more, the infrastructure to install these sunlight-to-fuel converters is based on existing technology. A next target is to use the same approach to reduce carbon dioxide, a greenhouse gas, to a carbon-based fuel such as formate or methanol.

“When you talk about powering a planet with energy stored in batteries, it’s just not practical,” said Meyer. “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.”