Inspired by a chemical process found in leaves, researchers at the California Institute of Technology have developed an electrically conductive film that could lead to devices that harness sunlight to split water (H2O), safely creating hydrogen fuel.
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.
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.”
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