Energy question of the week: Can sunlight be used to split water directly into oxygen and hydrogen?

Many materials such as titanium disilicide and metal complexes containing ruthenium or cobalt oxide are suitable as catalysts. However, these materials are either too expensive or not stable enough for large-scale industrial applications. A Franco-American team of researchers has managed to stabilise the cobalt oxide catalyst with carbon-free ligands to form polyoxometallates. The yield was, however, too low to serve as the basis for viable hydrogen production. Chemists at the University of California in Berkeley were able to use a molybdenum-oxo-complex with a higher yield of 8500 hydrogen molecules per catalyst molecule. However, these catalysts are not yet at a suitable stage for engineering applications, because their efficiency rating started to decline after three days.

There are still a few hurdles along the road to artificial photosynthesis, which will need to be cleared over the next few years, but the efforts could reap rich dividends. This is because hydrogen will have a future as an energy source only if it can be obtained inexpensively and in a climate-neutral manner. In principle, this will be primarily through hydrolysis, which involves using electricity from renewable sources to split the water molecules. A current, cheaper, and therefore more widespread, method is the process of obtaining hydrogen from natural gas, a fossil fuel. Efficient artificial photosynthesis would have the potential to replace both of these conventional methods.

Using a different approach, one that does not involve using technology to copy the process of photosynthesis, DLR researchers at the Institute of Technical Thermodynamics are splitting water into its constituent elements. In the EU projects HYDROSOL I and II, a system installed at the Spanish Plataforma Solar de Almería makes it possible to split water molecules using concentrated sunlight at temperatures of 800–1200 degrees Celsius. This approach has the potential to economically produce hydrogen with the help of solar energy in the foreseeable future.

The DLR Energy question of the week in 'The future of energy' Year of Science