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Artificial leaf harnesses sunlight for efficient fuel production

29 August 2015

Researchers claim to have developed the first complete, efficient, safe, integrated solar-driven system for splitting water to create hydrogen fuels.

The stand-alone prototype includes two chambers separated by a semi-permeable membrane that allows collection of both gas products (photo: Lance Hayashida/Caltech)

The new solar fuel generation system, or artificial leaf, is described in the August 27 online issue of the journal, Energy and Environmental Science. The work was carried out by researchers at the Joint Centre for Artificial Photosynthesis (JCAP) at Caltech.

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 JCAP 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 enables the hydrogen fuel be collected separately under pressure and safely transferred via a pipeline.

Semiconductors such as silicon or gallium arsenide absorb light efficiently and are therefore used in solar panels. However, these materials also suffer surface oxidation when exposed to water, so cannot be used to generate fuel directly.

A major advance that allowed the integrated system to be developed was previous work in Nate Lewis's laboratory at Caltech, which showed that adding a nanometres-thick layer of titanium dioxide (TiO2) to the electrodes could prevent them from corroding while still allowing light and electrons to pass through.

The new complete solar fuel generation system developed by Lewis and colleagues uses such a 62.5-nanometre-thick 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 photoanode requires a catalyst to drive the essential water-splitting reaction. Rare and expensive metals such as platinum can serve as effective catalysts, but in its work the team discovered that it could create a much cheaper, 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 achieved 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.

A critical component that contributes to the efficiency and safety of the new system is the special plastic membrane that separates the gases and prevents the possibility of an explosion, while still allowing the ions to flow seamlessly to complete the electrical circuit in the cell. 

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 percent 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 five to ten or more," Lewis says. "Our work shows that it is indeed possible to produce fuels from sunlight safely and efficiently in an integrated system with inexpensive components. Of course, we still have work to do to extend the lifetime of the system and to develop methods for cost-effectively manufacturing full systems, both of which are in progress."

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