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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 this has to change for climate protection reasons. The best strategy so far to produce environmentally friendly “green hydrogen” is to split water into hydrogen and oxygen using electricity that comes from renewable energy sources, for example, photovoltaic cells.

However, it would be much easier 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.” The concept is not yet used industrially. At TU Wien, important steps have now been taken in this direction: on an atomic scale, scientists have realized a new combination of molecular and solid-state catalysts that can do the job while using relatively inexpensive materials.

Interaction of atoms

“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. Tiny inorganic clusters consisting of only a small number of atoms are anchored on a surface of light-absorbing support structures such as titanium oxide. The combination of clusters and carefully chosen semiconductor supports lead to the desired behavior.

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Structural models of two clusters that enable water splitting into O2 and H2 by means of light energy. Credit: Vienna University of Technology

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