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GaP nanowires boost solar fuel cell efficiency

Eindhoven researchers increase hydrogen yield by a factor of ten

Above: Array of GaP nanowires made with an electron microscope

Researchers at the Eindhoven University of Technology (TU/e) and FOM Foundation have presented a promising prototype for a solar cell that generates hydrogen using nanowires made from GaP. The work is described in the journal Nature Communications.

Electricity produced by a solar cell can be used to set off chemical reactions. If this generates a fuel, then one speaks of solar fuels - a promising replacement for polluting fuels. One of the possibilities is to split liquid water using the electricity that is generated (electrolysis). As well as oxygen, this produces hydrogen gas that can be used as a clean fuel in the chemical industry or combusted in fuel cells - in cars for example - to drive engines.

To connect an existing silicon solar cell to a battery that splits the water is an efficient solution now but it is expensive. Many researchers are therefore looking for a semiconductor material that can both convert sunlight into an electrical charge and split the water, all in one; a kind of 'solar fuel cell'. Researchers at TU/e and FOM see GaP as their ideal candidate.

GaP has good electrical properties but the drawback that it cannot easily absorb light when it is a large flat surface as used in GaP solar cells. The researchers have overcome this problem by making a grid of very small GaP nanowires, measuring 500nm long and 90nm thick. This immediately boosted the yield of hydrogen by a factor of ten to 2.9 percent. A record for GaP cells, even though this is still some way off the fifteen percent achieved by silicon cells coupled to a battery.

According to research leader and TU/e professor Erik Bakkers, it's not simply about the yield "“ where there is still a lot of scope for improvement he points out: "For the nanowires we needed ten thousand less precious GaP material than in cells with a flat surface. That makes these kinds of cells potentially a great deal cheaper," he says. "In addition, GaP is also able to extract oxygen from the water - so you then actually have a fuel cell in which you can temporarily store your solar energy. In short, for a solar fuels future we cannot ignore GaP any longer."

'Efficient water reduction with gallium phosphide nanowires', by Anthony Standing et al, Nature Communications (17 July 2015) DOI: 10.1038/nscomms8824

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