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SCIENCE & TECHNOLOGY

New Yale-designed semiconductor coating could open door to cheaper and more effi cient solar fuels

BY AISLINN KINSELLA CONTRIBUTING REPORTER

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 ef ciency — a measure of how much energy was produced by the device — was a record for solar-to-hydrogen conversion, and added that of Engineering and Applied Science, said he believes that future optimization the fi eld of solar-hydrogen production is will lead to much higher efficiency. currently dominated by materials such Hu agreed that the energy conversion as oxides and nitrides, as opposed to would increase with future research. He semi-conductive materials. While these said that their study had already looked materials are more stable than the semiat theoretical examples that could reach conductors used by Hu’s lab, they are not up to 10 percent in the near future, and ef cient enough to be practical. hoped that the process could eventually According to Yanagi, generating solar reach 20 percent ef ciency. fuels at a large scale will require using a

“If we even get to 10 percent, then material that is both ef cient and stable. the way that we produce solar fuel will Rather than trying to improve the ef completely change,” Hu said. “You see completely change,” Hu said. “You see ciency of a stable material, Hu’s group ciency of a stable material, Hu’s group

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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 ef ciency and stability.

Rito Yanagi GRD ’24, an author of the paper and graduate student at the School took the opposite approach and worked to improve the stability of an ef cient 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 signifi cant advance for the fi eld 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.

Contact AISLINN KINSELLA at aislinn.kinsella@yale.edu .

Mask mandates associated with people spending less time at home, Yale School of the Environment study finds

BY AMRE PROMAN STAFF REPORTER

The implementation of mask mandates around the country was correlated with people spending less time at home, a new study by a Yale School of the Environment team concluded.

Sparked by curiosity about the behavioral ef ects of various COVID-19 interventions, four researchers, including professor of natural resource economics Eli Fenichel and School of the Environment postdoctoral associate Youpei Yan, set out to understand the ef ect mask mandates had on how much time people spent at home. Using smartphone location data and careful analysis, they concluded that people leave their houses more when mask mandates are in place. This finding aligns with previous public health research into how people make trade-of s when it comes to their personal risk of infection.

“When you throw out a whole bunch of behavioral policies, they are not going to be additive,” Fenichel, the senior author of the study, said. “People will make trade-offs. And I think people need to be mindful about the trade-offs that they’re making.”

Back in April, Fenichel said, as COVID-19 cases and mask mandates were both on the rise, he noticed how many people were leaving their houses for non-essential trips with the mindset that wearing a mask would render their trip completely safe. At that time, and to this day, research has not shown that the cloth masks worn by average Americans completely eliminate the risk of getting COVID-19.

This led him to wonder whether there was any correlation between the implementation of mask mandates and how much time people spent away from their houses in the midst of the pandemic. Previous studies on similar phenomena — known as “risk compensation” — can be found in scientifi c literature, according to Fenichel.

“Probably one of the best examples is risk behaviors with HIV,” Albert Ko, chair of the Yale School of Public Health Epidemiology Department, said. “You have people who, you know, they’re on pre-exposure prophylaxis and they may not use condoms. There are people … entering on dangerous, potentially harmful risk behaviors.”

The central idea is that when certain preventative measures are implemented, some people may engage more often in risky behaviors. Understanding how these dynamics work in the context of COVID-19 is a new, yet vitally important, fi eld of study, according to the researchers.

As the article explains, the researchers used SafeGraph, a company which anonymizes and collects data on people’s whereabouts, and found that mask mandates are indeed correlated with Americans spending less time at home.

“This fi nding is not surprising and seems reasonable,” Paul Cleary, Anna M.R. Lauder professor of public health in the Department of Health Policy and Man-

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agement, wrote in an email to the News. “This is why so many commentators … emphasize that masking does not substitute for social distancing or … other preventive behaviors.”

The University’s own guidance, as laid out in the community compact, stipulates that all adherents must “wear face coverings in shared or public spaces,” but also compels all students, faculty and staf to practice social distancing and take other preventative measures to limit the spread of COVID-19.

According to Fenichel, public health guidance on masks has evolved since the study was in its early stages. Initially, he noted, there was not good communication on the effectiveness of masks. Ko also highlighted the ongoing discussion on the effectiveness of various types of masks. Fenichel pointed out that this is worrying, as people make trade-offs between the wide variety of policies being presented at the same time.

“The key message here is to be a little bit more mindful about the decisions you’re making to protect yourself during the pandemic,” Fenichel said. “I am not saying that masks are bad. What I’m saying is that I think what we did is we threw the kitchen sink from a policy perspective, at this, without being thoughtful about perhaps some of the interactions among those policies. And I think that was confusing for people.” The new question is whether another COVID-19 management strategy, vaccination, will cause a similar increase in risk compensation and, if so, what the resulting ef ects will be. Fenichel predicts that it will cause this type of behavior, citing comparable research conducted on vaccines for other pathogens. Yet Fenichel is slightly less worried about vaccines causing people to leave the house more, given how much protection vaccines provide against the virus as compared to wearing a mask. He points out that even in a setting where not the entire population is vaccinated, people will still be more protected in public than they might otherwise. Ko noted that the specifi c type of vaccine is also important when considering its ef ectiveness, given that not all are equally protective. “It depends on the vaccine,” Ko said. “Not all of the vaccines … may be … entirely transmission blocking, but they may reduce the risk of transmission. So, if people who are unvaccinated go out and aren’t using face masks and so forth, and, you know, but they could be infected and they can transmit [the virus].”

Nevertheless, as was reiterated by the University’s COVID-19 coordinator, Stephanie Spangler, in her most recent weekly update, wearing face coverings is a vital step in protecting the community. Fenichel highlighted that while it might be more complex on the policy level, wearing a mask is and should be an easy decision that all individuals make.

According to PR Newswire, the value of the worldwide reusable mask industry is expected to exceed $7 billion dollars by 2027.

Contact AMRE PROMAN at amre.proman@yale.edu .