A missing link to artificial photosynthesis and 'solar fuels'

Because of its direct connection to solar energy, harnessing the power and beauty of photosynthesis has captivated scientists for decades. Now a group of Caltech scientists inspired by the photosynthetic process have developed a thin-nickel-oxide film coating that can replicate the natural process of photosynthesis to harness sunlight and split water into hydrogen fuel.

When applied to semiconducting materials such as silicon, the film prevents rust buildup and facilitates the solar-driven production of fuels such as methane or hydrogen.

According to George L. Argyros Professor and professor of chemistry at Caltech and a coauthor of a new study: "We have developed a new type of protective coating that enables a key process in the solar-driven production of fuels to be performed with record efficiency, stability, and effectiveness, and in a system that is intrinsically safe and does not produce explosive mixtures of hydrogen and oxygen."

The development could be used to develop safe and efficient solar-fuel generators that replicate the natural process of photosynthesis that plants use to convert sunlight, water, and carbon dioxide into oxygen and fuel in the form of carbohydrates, or sugars.

The artificial leaf that Lewis' team is developing is part of Caltech's Joint Center for Artificial Photosynthesis (JCAP). The leaf consists of three main components: a photoanode, a photocathode, and a membrane. The photoanode uses sunlight to oxidize water molecules to generate oxygen gas, protons, and electrons. 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.

In the past, scientists have tried building the electrodes out of common semiconductor materials such as those used in solar panels. These materials absorb light, but develop rust when exposed to water.

Lewis and other scientists have experimented with creating protective coatings for the electrodes, but didn't succeed for various reasons. Now, Lewis's group has reversed that trend.

"You want the coating to be many things: chemically compatible with the semiconductor it's trying to protect, impermeable to water, electrically conductive, highly transparent to incoming light, and highly catalytic for the reaction to make oxygen and fuels," says Lewis, who is also JCAP's scientific director. "Creating a protective layer that displayed any one of these attributes would be a significant leap forward, but what we've now discovered is a material that can do all of these things at once."

A study describing the new film appears the week of March 9 in the online issue of the journal the Proceedings of the National Academy of Sciences.

Source: Based on information from California Institute of Technology