Hydrogen is not only the most abundant element in the universe, it’s also one of the most promising green fuels. When it is used to power technologies like fuel cells, the only waste product is water, making it a carbon-free energy source.
However, most hydrogen on earth is locked up in compounds like water or hydrocarbons—aka fossil fuels—and the current industrial method for producing hydrogen uses steam to break apart natural gas molecules. It’s an extremely energy-intensive process that is not very sustainable.
Now, University of Wisconsin-Madison materials engineers have made “unlocking” that hydrogen more practical. Their advances could dramatically improve an alternative process for producing hydrogen using water and solar energy—potentially making future hydrogen production extremely inexpensive and sustainable.
In an April 2023 paper in the journal Nature Communications, the researchers describe their new synthesis technique for producing an ultrathin film that extends the lifespan of silicon photoelectrochemical electrodes. Those electrodes use the energy from sunlight to separate water into oxygen and hydrogen gas through a process called photoelectrochemical water splitting.
“This is one of the most environmentally friendly approaches to generating hydrogen fuel,” says Xudong Wang, a professor of materials science and engineering at UW-Madison. “There are many engineering challenges left for this technology, but extending the lifespan of the electrodes is one of the main challenges right now.”
The water-splitting process works much like a photovoltaic solar panel operates. A specialized silicon semiconductor (called a photoanode) is immersed in an ion-filled liquid, or electrolyte. Electrons from sunlight drive a reaction, causing the photoanode to split water molecules into oxygen and pure hydrogen gas—no fossil fuels required.
This promising system has one major flaw, however. The silicon photoanodes react—a lot—with the alkaline electrolyte solution. Without some protection, they fail after just a few hours of operation, at best. Several years ago, Wang and his students began developing a titanium oxide film to cover the photoanode which protects it while still allowing electrons to flow freely.
In their first efforts, the researchers found a disordered atomic structure was the most practical arrangement for the film. However, when they made the film using a technique called atomic layer deposition, they found that even among the disordered structure, small intermediate crystalline structures formed. These created “hot spots” in the film that conducted electrons, leading to pinholes and eventual failure.
So in subsequent trials, the researchers made the film super-thin, about 2.5 nanometers, which prevented the intermediate crystals from forming and boosted the lifespan of the semiconductor from 80 hours to 500 hours.
However, to be industrially useful, the film needs to last about 30,000 hours, meaning the titanium oxide protective layer needed to be much thicker and sturdier.
This time, the researchers produced a thicker, 15-nanometer layer of the titanium dioxide using the same atomic layer deposition technique—but by synthesizing it at a lower temperature, they largely prevented the intermediate crystal formation. The tradeoff was that more reactants were left behind in the film, including chlorine, which can cause it to degrade. So the team developed a new water-vapor-based treatment strategy, which removed most of the chlorine without inducing any more intermediate crystal formation.
The result was a film with many fewer defects, extending the semiconductor’s life to 600 hours and breaking the lab’s previous record. Wang says that he believes the film still has a lot of room for improvement and would like to investigate other strategies to further improve the longevity. “I think we are on the right track.” he says. “What we’re trying to do in our next project is understand how we can achieve better purity in a perfect random structure with a larger film thickness.”
The work was informed by simulations produced by co-authors Dane Morgan, a professor of materials science and engineering at UW-Madison, and postdoctoral researcher Jun Meng. “We were able to use quantum mechanical simulations to gain insights into the behavior of electrons in the titanium dioxide coatings,” says Meng.
“This project showcases how computational and experimental methods can work closely to help in modern materials development, particularly at the nanoscale,” says Morgan.
The work also utilized advanced electron microscopy of the structure and chemistry of the films, conducted by PhD student Mehrdad Abbasi and Jinwoo Hwang, an assistant professor of materials science and engineering at The Ohio State University. “Understanding the nanoscale structure of the film filled the gap in the scientific knowledge that has been preventing the advancement of this novel material,” says Hwang.
While Wang and his collaborators are currently focused on photoanodes for hydrogen production, this advanced film could be applied to many other chemical production and green-energy technologies. “This could be used on battery electrodes, surfaces of catalysts and in energy storage devices—anywhere that a functional material is interfacing with an electrolyte in a reactive environment,” says materials science and engineering graduate student Yutao Dong.
Dane Morgan is the Harvey D. Spangler Professor in materials science and engineering.
Other UW-Madison authors include Lazarus German, Corey Carlos, Jun Li, and Ziyi Zhang.
The authors acknowledge support the U.S. Department of Energy, Office of Science, Basic Energy Sciences, under Award # DE-SC0020283.
Featured image caption: PhD student Yutao Dong (left) and Professor Xudong Wang (right) are working to improve a thin film that could make hydrogen production via water splitting cheaper and more sustainable. Credit: Joel Hallberg.