When applied to semiconducting materials such as silicon, the nickel oxide film prevents rust buildup and 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," explained Nate Lewis, the George L. Argyros Professor and professor of chemistry at Caltech and a co-author of a study, published in the online issue of the journal the Proceedings of the National Academy of Sciences, that describes the film.
The development could help lead to safe, efficient artificial photosynthetic systems – also called solar-fuel generators or ‘artificial leaves’ – 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 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 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, 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.
Scientists have tried building the electrodes out of common semiconductors such as silicon or gallium arsenide – which absorb light and are also used in solar panels – but a major problem is that these materials develop an oxide layer (that is, rust) when exposed to water.
Lewis and other scientists have experimented with creating protective coatings for the electrodes, but 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," said 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."
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 far exceeded the performance of other similar films – including one that Lewis’s group created just last year. That film was more complicated – it consisted of two layers versus one and used as its main ingredient titanium dioxide (TiO2, also known as titania), a naturally occurring compound that is also used to make sunscreens, toothpastes, and white paint.
"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," explained Ke Sun, a postdoc in Lewis’s lab and the first author of the new study.
Lewis’s team developed a technique for creating the nickel oxide film that involves smashing atoms of argon into a pellet of nickel atoms at high speeds, in an
oxygen-rich environment. "The nickel fragments that sputter off of the pellet react with the oxygen atoms to produce an oxidized form of nickel that gets deposited
onto the semiconductor," said Lewis.
The team’s nickel oxide film works well in conjunction with the membrane that separates the photoanode from the photocathode and staggers the production of
hydrogen and oxygen 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," said Lewis. "With our film, you can build a safe device that will not explode, and that lasts and is efficient, all at once."
"Our team is also working on a photocathode," explained Lewis. "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 technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
The technical storage or access that is used exclusively for statistical purposes.The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.