A group of researchers at Caltech have designed a novel electrically conductive film that provides a promising means to develop devices that can harness sunlight to change water into hydrogen fuel.
The study has been published in the online issue of the journal Proceedings of the National Academy of Sciences.
We've discovered a material which is 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.
Prof. Nate Lewis
This nickel oxide film, when applied to a semiconducting substrate like silicon, prevents the formation of corrosion and promotes a key chemical process in the solar-mediated production of fuels such as hydrogen or methane.
Nate Lewis, coauthor of the study and the George L. Argyros Professor and professor of chemistry at Caltech, said:
"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."
This latest breakthrough could result in the development of safe and efficient artificial photosynthetic solutions, also known as artificial leaves or solar-fuel generators. These systems simulate the natural photosynthesis process, which is used by plants to change sunlight, carbon dioxide, and water into fuel and oxygen in the form of sugars or carbohydrates.
The researchers are developing the artificial leaf in part at the Joint Center for Artificial Photosynthesis (JCAP) of Caltech. This leaf includes three key components such as single membrane and two electrodes - photocathode and photoanode.
The photoanode is designed to harness sunlight and oxidize water molecules to produce electrons, protons, and oxygen gas, whereas the photocathode recombines the electrons and protons to create hydrogen gas.
The membrane is made of plastic and ensures that both gases are not mixed together so that any possible explosion is completely eliminated. It also allows the gas be collected under pressure so that the gas is safely pushed into a pipeline.
In the past, researchers had attempted to build the electrodes from standard semiconductors such as gallium arsenide or silicon which are capable of absorbing light and are also utilized in solar panels. However, these materials have a major drawback and tend to develop an oxide layer upon exposure to water.
Lewis and other researchers explored a number of options to develop protective coatings for the electrodes; however, all the earlier attempts did not work out for a number of reasons. Lweis commented:
"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. 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 research team has demonstrated that the new nickel oxide film can be used with a wide range of semiconductor materials, such as nickel oxide film and indium phosphide. When the nickel oxide film was applied to photoanodes, it surpassed the performance of other analogous films, including the one developed by Lewis's group.
That prior film included two layers instead of a single layer and was utilized as its core ingredient titanium dioxide, also called as titania. Titanium dioxide is a naturally occurring compound, which is also utilized to make white paint, toothpastes, and sunscreens.
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 postdoc in Lewis's lab and the first author of the new study.
Ultimately, the researchers developed a new technique for producing the nickel oxide film. In this method, atoms of argon are crashed into a pellet of nickel atoms at extreme speeds under oxygen-rich conditions
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, informed Lewis.
Most importantly, the nickel oxide film works suitably when combined with the membrane, which not only isolates the photoanode from the photocathode, but also staggers the production of oxygen and hydrogen 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. With our film, you can build a safe device that will not explode, and that lasts and is efficient, all at once, added Lewis.
However, more research is required to develop a commercial device that could change sunlight into fuel. Moreover, photocathode and other components of the system have to be perfected.
Our team is also working on a photocathode. 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, said Lewis.
In addition to Lewis and Sun, other authors of the study titled, Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films," include Caltech graduate students Michael Lichterman, Fadl Saadi, Xinghao Zhou, Stefan Omelchenko, and Noah Plymale; Bruce Brunschwig, the director of the Molecular Materials Research Center at Caltech; Kimberly Papadantonakis, a scientific research manager at Caltech; Jr-Hau He and Hsin-Ping Wang from King Abdullah University in Saudi Arabia; and William Hale from the University of Southampton.
The National Science Foundation, the Office of Science at the U.S. Department of Energy, the Gordon and Betty Moore Foundation, and the Beckman Institute funded the project.
References and further reading
One Step Closer to Artificial Photosynthesis and "Solar Fuels" - Caltech News
"Stable solar-driven oxidation of water by semiconducting photoanodes protected by transparent catalytic nickel oxide films" - Ke Sun et al., PNAS, 2015