Titanium dioxide key to solar water-splitter

‘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 lead researcher 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.’

Dubbed a dye-sensitised photoelectrosynthesis cell (DSPEC), the new system - designed by Meyer and colleagues at UNC and Greg Parson’s group at North Carolina State University - 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 oxygen by-product is released into the air.

‘But splitting water is extremely difficult to do,’ Meyer said in a statement. ‘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 is said to have two basic components: a molecule and a nanoparticle. The molecule, a chromophore-catalyst assembly, absorbs sunlight and kick starts the catalyst to strip electrons away from water. The nanoparticle, to which thousands of chromophore-catalyst assemblies are tethered, is part of a film of nanoparticles which shuttles the electrons away to make the hydrogen.

According to UNC-Chapel Hill, even with the best attempts, the system always crashed because either the chromophore-catalyst assembly broke 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 at NCSU to use a technique that coated the nanoparticle with a thin layer of titanium dioxide.

By using ultra-thin 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 ascertained 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 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.

The infrastructure to install these sunlight-to-fuel converters is in sight based on existing technology. A next target is to use the same approach to reduce carbon dioxide 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,’ 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.’