Artificial leaf mimics photosynthesis to make fuel from water

Researchers have successfully developed an artificial leaf which mimics the process of photosynthesis to create fuel using water, carbon dioxide and sunlight.

The new system consists of three main components: two electrodes - one photoanode and one photocathode - and a membrane.

The photoanode uses sunlight to oxidise water molecules, generating protons and electrons as well as oxygen gas. The photocathode recombines the protons and electrons to form hydrogen gas.

A key part of the design is the plastic membrane, which keeps the oxygen and hydrogen gases separate. If the two gases are allowed to mix and are accidentally ignited, an explosion can occur; the membrane lets the hydrogen fuel be separately collected under pressure and safely pushed into a pipeline.

Semiconductors such as silicon or gallium arsenide absorb light efficiently and are therefore used in solar panels.

However, these materials also oxidise (or rust) on the surface when exposed to water, so cannot be used to directly generate fuel.

The new complete solar fuel generation system developed by Nate Lewis, professor at California Institute of Technology, and colleagues uses a 62.5-nanometre-thick titanium dioxide (TiO2) layer to effectively prevent corrosion and improve the stability of a gallium arsenide-based photoelectrode.

Another key advance is the use of active, inexpensive catalysts for fuel production. "The device reported here grew out of a multi-year, large-scale effort to define the design and materials components needed for an integrated solar fuels generator," said Harry Atwater, director of Joint Centre for Artificial Photosynthesis (JCAP) and Howard Hughes professor of Applied Physics and Materials Science at Caltech.

The photoanode requires a catalyst to drive the essential water-splitting reaction. The team discovered that it could create a cheap, active catalyst by adding a 2-nanometre-thick layer of nickel to the surface of the TiO2.

This catalyst is among the most active known catalysts for splitting water molecules into oxygen, protons, and electrons and is a key to the high efficiency displayed by the device.

The photoanode was grown onto a photocathode, which also contains a highly active, inexpensive, nickel-molybdenum catalyst, to create a fully integrated single material that serves as a complete solar-driven water-splitting system.

All of the components are stable under the same conditions and work together to produce a high-performance, fully integrated system.

The demonstration system is approximately one square centimetre in area, converts 10 per cent of the energy in sunlight into stored energy in the chemical fuel, and can operate for more than 40 hours continuously.

"This new system shatters all of the combined safety, performance, and stability records for artificial leaf technology by factors of 5 to 10 or more," Lewis said.