One Step Closer to Artificial Leaves That Convert Water to Fuel

California, USA — When applied to semiconducting materials such as silicon, the nickel oxide film facilitates an important chemical process in the solar-driven production of fuels such as methane or hydrogen.

“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,” says Nate Lewis, a distinguished professor of chemistry at Caltech and coauthor of a new study that describes the film.

The development could lead to safe, efficient artificial photosynthetic systems – also called solar-fuel generators or “artificial leaves.”

Such a system would 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 in part at Caltech’s Joint Center for Artificial Photosynthesis (JCAP) consists of three main components: two electrodes – a photoanode and a photocathode – and a plastic membrane.

The photoanode uses sunlight to oxidize water molecules to generate oxygen gas, protons, and electrons, while the photocathode recombines the protons and electrons to form hydrogen gas. The membrane keeps the two gases separate to eliminate any possibility of an explosion, and lets the gas be collected under pressure to safely push it into a pipeline. All previous attempts have failed for various reasons.

“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,” he said.

The team has shown that its nickel oxide film is compatible with many different kinds of semiconductor materials, including silicon, indium phosphide, and cadmium telluride. When applied to photoanodes, the nickel oxide film exceeded the performance of other similar films – including one that Lewis’s group created just last year.

“After watching the photoanodes run at record performance without any noticeable degradation for 24 hours, and then 100 hours, and then 500 hours, I knew we had done what scientists had failed to do before,” said Ke Sun, a postdoctoral fellow in Lewis’s lab and the first author of the new study. 

The team’s nickel oxide film works well with the membrane that separates the photoanode from the photocathode and staggers the production of hydrogen and oxygen gases from water (H2O), which consists of the two 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,” Lewis explains. “With our film, you can build a safe device that will not explode, and that lasts and is efficient, all at once,” he said.

Lewis cautions that scientists are still far from developing a commercial product that can convert sunlight into fuel. Other components of the system, such as the photocathode, also need to be perfected. “Our team is also working on a photocathode,” Lewis says. “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.”

The study, “Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films,” was published the week of March 9 in the online issue of the journal “The Proceedings of the National Academy of Sciences.”

Additional authors on the paper include scientists from the University of Southampton and King Abdullah University in Saudi Arabia, and Bruce Brunschwig, the director of the Molecular Materials Research Center at Caltech.

Funding for this research was provided by the Office of Science at the U.S. Department of Energy, the National Science Foundation, the Beckman Institute, and the Gordon and Betty Moore Foundation.

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