Atomic clusters make light work of water splitting

Hydrogen could be an important part of our future energy supply – it can be stored, transported and burned as needed. However, most of the hydrogen available today is a by-product of natural gas production and thus contributes to global warming. To produce environmentally friendly ‘green hydrogen’, the best current strategy is to split water into hydrogen and oxygen using electricity that comes from renewable energy sources, such as photovoltaic cells.

It would be much easier, however, if sunlight could be used directly to split water. This is exactly what new catalysts are now making possible, in a process called ‘photocatalytic water splitting’, although such photocatalysts are not yet used industrially. At Vienna University of Technology (TU Wien) in Austria, important steps have now been taken in this direction, with scientists synthesizing a new combination of molecular and solid-state catalysts made from relatively inexpensive materials that can do the job. The scientists report their work in a paper in ACS Catalysis.

"Actually, to be able to split water with light you have to solve two tasks at the same time" says Alexey Cherevan from the Institute for Materials Chemistry at TU Wien. "We have to think about oxygen and about hydrogen. The oxygen atoms of the water must be transformed into O₂ molecules, and the remaining hydrogen ions – which are just protons – must be turned into H₂ molecules.”

Solutions have now been found for both tasks, by anchoring tiny inorganic clusters consisting of only a small number of atoms on a surface of light-absorbing support structures such as titanium oxide. The combination of the clusters and the carefully chosen semiconductor supports leads to the desired behavior.

The inorganic clusters responsible for oxidizing oxygen are made of cobalt, tungsten and oxygen atoms, while clusters of sulphur and molybdenum atoms are especially adept at producing hydrogen molecules. The researchers at TU Wien were the first to deposit these clusters on a surface of titanium oxide, where they can act as catalysts for water splitting.

"Titanium oxide is sensitive to light, that was already well known," says Cherevan. "The energy of the absorbed light leads to the creation of free-moving electrons and free-moving positive charges in the titanium oxide. These charges then allow the clusters of atoms sitting on this surface to facilitate the splitting of water into oxygen and hydrogen.

"Other research groups working on splitting water with light rely on nanoparticles that can take on very different shapes and surface properties. The sizes are hard to control, the atoms are not quite arranged in the same way. Therefore, in this case, it is not possible to explain exactly how the catalysis process takes place in detail."

At TU Wien, on the other hand, the exact structure of the clusters is determined with atomic precision, which allows Cherevan and his colleagues to gain full understanding of the catalytic cycle.

"This is the only way to get feedback on what the efficiency of the process really depends on," explains Cherevan. "We don't want to just rely on a trial-and-error approach and try different nanoparticles until we find the best one – we want to find out at the atomic level what the optimal catalyst really is."

Now the scientists have proven that the selected materials are indeed suitable for splitting water, the next step is to further tune their exact structure to achieve even higher efficiencies.

"The decisive advantage of our method over splitting water by electrolysis is its simplicity," Cherevan says.

Electric hydrogen production first needs a sustainable energy source – such as photovoltaic cells – as well as possibly an electric energy storage device and an electrolysis cell. This results in a relatively complex system consisting of a multitude of raw materials. For photocatalytic water splitting, on the other hand, all that is needed is a suitably coated surface that is covered by water and irradiated by sunlight.

In the long term, these findings could also be used to produce more complicated molecules for artificial photosynthesis. It might even be possible to use the energy of solar radiation to produce fuel-like hydrocarbons by utilizing carbon dioxide from the atmosphere and water.

This story is adapted from material from TU Wien, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.