Nanowires boost solar fuel cell efficiency

26th August 2015

Nat Bowers

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Researchers at Eindhoven University of Technology (TU/e) and FOM Foundation have presented a very promising prototype of a solar cell that produces fuel rather than electricity in the Nature Communications journal. The Gallium Phosphide (GaP) enables the solar cell to produce hydrogen gas, a clean fuel, from liquid water.

Processing the GaP in the form of very small nanowires is novel and helps to boost the yield by a factor of ten using ten thousand times less precious material.

The electricity produced by a solar cell can be used to set off chemical reactions. Alternatively it can generate solar fuel – a hugely 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.

Solar fuel cell

To connect an existing silicon solar cell to a battery that splits the water may be an efficient solution but it is still a very expensive one. Therefore, many researchers are targeting their search at a semiconductor material that is able to 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 their dream candidate in GaP, a compound of gallium and phosphide that also serves as the basis for specific coloured LEDs.

A tenfold boost

GaP has good electrical properties but the drawback is 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%, a record for GaP cells, even though this is still some way off the 15% achieved by silicon cells coupled to a battery.

Ten thousand times less material

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 explains: “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. 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 gallium phosphide any longer.”