Solar cell speeds hydrogen production

A solar cell that mimics photosynthesis has been used to make hydrogen directly from water. The prototype is inefficient, but the researchers who built it believe they can boost its efficiency, perhaps leading to a viable source of hydrogen to fuel cars and other vehicles.

The device, built by Thomas Mallouk of Pennsylvania State University and colleagues, works much like a solar cell called a Grätzel cell, using sunlight to knock electrons off dye molecules. But instead of being used to create a current, as in the Grätzel cell, the electrons are shuttled away from the dye and into a catalyst, where they split water molecules into oxygen and hydrogen ions in a reaction similar to one stage of natural photosynthesis.

Other dye-based approaches to splitting water haven't worked very well because the electrons often recombine with the dye before they can be used. Mallouk says that the problem was a matter of arranging the molecular circuitry to channel the electrons effectively, avoiding such “short circuits”.

His solution is to attach ruthenium-based dye molecules to a catalyst particle, clinging so closely that any electrons knocked out of the dye are directed into the catalyst. "The key thing is to get everything small and individually packaged," Mallouk told New Scientist.

In the new device, water is split a thousand times faster than in other dye-based cells.

Elegant and original
“The attractive feature of the work is that the dye is wrapped around the iridium oxide nanoparticles. This is a very elegant and original approach,” says Michael Grätzel of the Federal Institute of Technology in Lausanne, Switzerland, inventor of the Grätzel cell.

The approach may also be more promising than semiconductor approaches, which are incredibly complex, says James Durrant of Imperial College, London, UK.

Mallouk admits that so far the cell is very inefficient, as only around 1% of the light energy falling on it goes into splitting water, but he says that with some optimisation of the geometry and the molecules the efficiency could rise to 10% per cent. “Because we understand the relationship between intermolecular distances and electron transfer rates, we can in principle improve our system by changing the linking groups between molecules.”

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