Kai Nip, Staff Photographer

A new Yale study tested a coating strategy for semiconductors that could improve the efficiency and lower the cost of solar fuel production.

Assistant professor of chemical and environmental engineering Shu Hu and his research group developed a new coating to protect semiconductors from corrosion. In order to produce solar fuels, semiconductors are illuminated, then within the semiconductor, specific materials allow for the splitting of water into hydrogen and oxygen. This produces energy in a process known as photocatalytic water splitting. These necessary materials, however, are prone to corrosion when illuminated and have to be replaced often.

Hu’s group developed a titanium dioxide coating that can protect and stabilize these semiconductors, a step closer to generating solar fuel at a large scale. The paper was published in the Proceedings of the National Academy of Sciences of the United States of America on Feb. 8.

“With this new coating, we not only improve the stability of the photocatalyst from a few hours to more than 150 hours, but it also improves the solar-hydrogen conversion efficiency above 1.7 percent,” postdoctoral associate and lead author of the study Tianshuo Zhao told the News.

Without the protective coating, semiconductors may only operate for a few hours before they become too corroded to use. Previous methods to protect the semiconductors interfered with the separation of charged particles within the semiconductor — a crucial part of the device’s functioning — but the new coating allows for this separation.

Zhao said that the 1.7 percent conversion efficiency — a measure of how much energy was produced by the device — was a record for solar-to-hydrogen conversion, and added that he believes that future optimization will lead to much higher efficiency. Hu agreed that the energy conversion would increase with future research. He said that their study had already looked at theoretical examples that could reach up to 10 percent in the near future, and hoped that the process could eventually reach 20 percent efficiency.

“If we even get to 10 percent, then the way that we produce solar fuel will completely change,” Hu said. “You see a pathway where the cost of fuels from sunlight is starting to be comparable to the gasoline price or natural gas price. That’s where the tipping point is.”

The importance of the study, Hu explained, was the combination of understanding the water-splitting process and applying a coating to improve the semiconductor efficiency and stability.

Rito Yanagi GRD ’24, an author of the paper and graduate student at the School of Engineering and Applied Science, said the field of solar-hydrogen production is currently dominated by materials such as oxides and nitrides, as opposed to semi-conductive materials. While these materials are more stable than the semiconductors used by Hu’s lab, they are not efficient enough to be practical.

According to Yanagi, generating solar fuels at a large scale will require using a material that is both efficient and stable. Rather than trying to improve the efficiency of a stable material, Hu’s group took the opposite approach and worked to improve the stability of an efficient one.

Yanagi noted the paper only focused on improving the hydrogen half-reaction of the water-splitting reaction, so future work will need to address the oxygen half-reaction.

“The other half-reaction is a little bit more challenging than this half-reaction,” he admitted. “But the basic strategy is the same.”

Jaehong Kim, senior professor and chair of chemical and environmental engineering at the School of Engineering and Applied Science, commented on the potential for hydrogen fuel to be produced cheaply with the use of photocatalysts.

“The cost reduction promised by using these photocatalysts is particularly noteworthy,” Kim wrote in an email to the News. “We now see a trajectory to achieve less than $2 per kilogram [of hydrogen], which is comparable to the gasoline price when you use [hydrogen] to run a fuel cell car.”

Professor of molecular biophysics and biochemistry and Director of the Yale Energy Sciences Institute Gary Brudvig told the News that the paper was a significant advance for the field of renewable energy production.

Brudvig’s group works on water-oxidation catalysts, which he believes could improve the efficiency of the reactions that Hu’s group focuses on. He said the two research groups are currently working together and discussing those possibilities.

Brudvig also identified some of the challenges posed by renewable energy sources. Because solar energy and wind energy cannot be produced all the time, it is necessary to store electricity in chemical bonds. Hu’s research is a step forward in improving the efficiency of solar energy, but more work is needed to address the difficulty of its storage.

“It would be ideal if the fuel could be stored for a long time, and it could be used when you need it,” Brudvig said. “The challenge is having systems that work efficiently and are scalable so that you can use them globally for solar energy storage.”

The Yale Energy and Sciences Institute is located on Yale’s West Campus.

Aislinn Kinsella | aislinn.kinsella@yale.edu

AISLINN KINSELLA