Scientists take step towards artificial leaves

Researchers in the US have taken their inspiration from nature to develop a special coating that they claim could pave the way for devices able to harness sunlight to split hydrogen fuel from water.

Developed by a team at Caltech’s Joint Center for Artificial Photosynthesis, the conductive nickel oxide film could, it is claimed, be applied to semiconducting materials to form so-called “artificial leaves” that replicate the natural process of photosynthesis used by plants.

The device  – which is the subject of a study in the Proceedings of the National Academy of Sciences – consists of three main components: two electrodes (a photoanode and a photocathode) and a membrane.

Conceptual drawing of an artificial phtosynthetic system
Conceptual drawing of an artificial phtosynthetic system

The photoanode uses sunlight to oxidise water molecules to generate oxygen gas, protons, and electrons, while the photocathode recombines the protons and electrons to form hydrogen gas. The membrane, which is typically made of plastic, keeps the two gases separate in order to eliminate any possibility of an explosion, and lets the gas be collected under pressure to safely push it into a pipeline.

According to Prof Nate Lewis, who has been heading up the research, the team experimented with a number of different light-absorbing semiconductor materials such as silicon or gallium arsenide (which are already used in solar panels) but found that they developed an oxide layer when exposed to water.

The group’s specially developed nickel oxide coating – created using a technique which involves smashing atoms of argon into a pellet of nickel atoms at high speed – offers a solution to this problem and, according to the team, enables fuels to be produced with efficiently and safely.

Crucially, the team’s nickel oxide film works well in conjunction with the membrane that separates the photoanode from the photocathode and staggers the production of hydrogen and oxygen gases.

‘Without a membrane, the photoanode and photocathode are close enough to each other to conduct electricity, and if you also have bubbles of highly reactive hydrogen and oxygen gases being produced in the same place at the same time, that is a recipe for disaster,’  Prof Lewis said in a statement. ‘With our film, you can build a safe device that will not explode, and that lasts and is efficient, all at once.’

Certain components of the system, such as the photocathode, will also need to be perfected before a commercial product converting sunlight into fuel can be developed, cautioned Prof Lewis.

‘Our team is also working on a photocathode,’ he said. ‘What we have to do is combine both of these elements together and show that the entire system works. That will not be easy, but we now have one of the missing key pieces that has eluded the field for the past half-century.’