Five years in the making, Hu hopes it will be a breakthrough that could one day lead to making solar power storable at a large scale-something that would be a game-changer in the field, but has so far proven elusive.
"This is one of the very first all-in-one devices that does all the things that you need to split water into hydrogen and oxygen," said Hu, assistant professor of chemical & environmental engineering. "You just put water in, and sunlight will do the rest." It's known as the "artificial leaf," and as a handheld device, it works great. But Hu's real challenge is making it work on a much bigger scale. That's one of the projects that he set out for himself when he was hired at the ESI. It fits right in with the very ambitious aims of the Institute, just a few years into its existence.
Forming the ESI
The formation of the ESI was announced five years ago this September, with the vision of bringing together physicists, chemists, geologists, biologists, and engineers to develop solutions to the world's energy challenges. With four full-time faculty members and more hires planned over the next few years, the Institute now occupies two spaces on West Campus. The second, known as ESI 2, opened this year, bringing the total space to about 38,000 square feet.
The idea for the ESI emerged shortly after Yale purchased the West Campus property from Bayer Pharmaceuticals nine years ago. Several Yale researchers and officials convened to brainstorm how the space could be used. "The question was: If we had the opportunity to tackle the challenges of the 21st century, what should those topics be?" said Gary Brudvig, the Benjamin Silliman Professor of Chemistry and director of the ESI. "We had little breakout groups, and every group put energy at the top of the list as one of the grand challenges. We knew that this was something we needed to be pushing-we needed to have something on energy at the West Campus."
When Yale received a $25 million gift from Tom Steyer and Catherine Taylor to create the ESI, Yale's scientists and officials were able to fine-tune their vision for energy research. "We decided that we need to come up with a sustainable renewable energy basis to power our planet and focus on renewable energies - particularly, solar and storage of solar energy and wind energy," Brudvig said. "A big problem with solar and wind is that they're intermittent; you need to store them to use them on a large scale."
Using hydrogen as a sustainable source of energy has long intrigued researchers. One of the challenges, though, is finding an efficient way to split the molecules of water into oxygen and hydrogen. That's where Hu's artificial leaf comes in. It's a photoelectrochemical (PEC) device that mimics the photosynthesis of plants. Two electrodes split the water molecules to produce protons and electrons and oxygen.
When the second electrode recombines the electrons and protons, hydrogen gas is produced. Mixing hydrogen and oxygen can cause an explosion, so a polymer membrane is there to keep the two separate. Hu and his fellow researchers knew they were on to something when the technology could convert 10% of the sunlight's energy into stored energy. His lab is now working on one with 15% efficiency. The device is small, but it provides a big window into how we can mimic nature at the nanoscale, where photosynthesis takes place. And that could lead to designs that go beyond the conventional solar energy technologies to achieve high-efficiency and long-term stability at a low cost.
The Biggest Obstacle for Real-World Applications
Stability of materials in solar fuel devices has long been the biggest obstacle for turning handheld demonstrations into real-world applications. The oxidation process for the kind of device that Hu is working on requires a light-absorbing material. However, these materials corrode quickly when exposed to water. Finding a good protective layer has been tricky, since it needs to be conductive enough to allow charges to transport through and reach the device's catalyst, and thick enough to prevent the water to penetrate the coating.
Another big step toward that goal is Hu's rediscovery of a material often found in toothpaste and commercial sunblocks. Using what's known as the atomic layer deposition (ALD) technique, typically performed in the field of microelectronics, he and his fellow researchers created a protective coating of titanium dioxide over the device's semiconductor crystals. Acting as the catalyst that oxidizes the water are nanoscale dots of nickel oxide, an inexpensive and accessible material. The discovery has dramatically increased the materials' stability for up to thousands of hours, or about one year of sunlight cycles. "This is promising," Hu said. "Now we need to find ways to show the stability of tens of thousands of hours, which means they can work for 10 to 15 years."
Hu, who came to Yale from CalTech earlier this year, is the third faculty hire at ESI. He joined ESI's first affiliate, Judy Cha, the Melamed Professor of Mechanical Engineering & Materials Science, who came to Yale in 2013. In 2014, Yale hired ESI's second full-time faculty member, Hailiang Wang, assistant professor of chemistry. The fourth ESI faculty member, Owen Miller, assistant professor of applied physics arrived at Yale in July 2016. One branch of Cha's research is a class of materials known as chalcogenides, which include sulfides, tellurides and selenides. Like Hu, Cha is trying to find better ways to split water to produce hydrogen.
She thinks chalcogenides are a good bet. "Some of these materials have recently been found to be a reasonable catalyst to split water into hydrogen and oxygen," she said, "so people are looking at the chalcogenides to see if they can improve their catalytic performance, and perhaps replace some of the more expensive catalysts." To that end, her lab has studied molybdenum disulfide (MoS2), playing with its atomic structure to boost the hydrogen evolution reaction-that is, how well the material produces hydrogen by way of water electrolysis.
More recently, her lab has been making a semimetallic chemical compound known as tungsten telluride (WTe2). "So far, it appears that no one has studied the catalytic activity of tungsten telluride to see if it's better than the molybdenum disulfide," she said. Her hope is that it can be used for fuel cell type applications and anything that requires hydrogen production.
A Boon to Research
Cha, who splits her time between the main campus and West Campus, said the ESI space has been a boon to her research. The resources there allow her lab to take on experiments that she otherwise couldn't. She's particularly excited about the Materials Characterization Core, a new facility that houses state-of-the-art instruments for materials analysis. Capabilities include the investigation of crystal structure, surface topography, materials microstructure and chemical microanalysis. The instruments first arrived at the beginning of the year. Although primarily used by the ESI researchers, the facility is shared with researchers throughout Yale. "The type of experiments we can do expands because we have some capabilities that we didn't have until now," Cha said. "That's going to benefit us by accelerating the pace of our research, and being able to look at properties that we couldn't previously because we didn't have the right instruments. That could lead to answers to research questions that we couldn't address before."
Christopher Incarvito, Director of Research Operations & Technology at West Campus, said the purchase and installation of the instrumentation is a well-considered process. "Research instrumentation is ridiculously expensive, so we make sure that we put it in an environment where we get the return on investment. We want to make sure that it's getting used to its full capacity and that it's very accessible so that groups that might have a need for it know it's there and have access to the people who know how to use it."
Shu Hu's engineered semiconductor's stability is equivalent to one year of sunlight. But perhaps an even greater contribution to the research is a much lower-tech innovation. Approaching the lab spaces of the four full-time faculty members, Incarvito notes the lack of walls, and general flow of foot traffic. "The beauty of this space is that Judy's group is here, Hailiang's group is here, Shu's group is here, and Owen's group is here," he said. "You have four different departments represented in one open space, so you don't have to seek out collaborators who are focused on the same type of research. From a support perspective, you don't have to replicate all of those resources from each of the individual labs."
Promoting Collisions Among Researchers
For example, he said, Cha's group has thermal deposition equipment with a lot of experience making finely tuned materials. Now, the labs of her three fellow ESI faculty members can leverage that expertise. The famous layout at Bell Labs- designed to promote chance encounters, or "collisions," among researchers-served as inspiration for the ESI workspace, as well as the six other Institutes on West Campus. From the hallways to the offices and labs, everything is designed to foster collaborations and creativity and encourage people to meet and talk with each other. Just by leaving the lab area, Incarvito said, "you have to walk by three or four interesting things."
For her part, Cha said the emphasis on collaborations and chance encounters has been great for her work. "One thing that helps is that my colleagues are occupying the same space, because that can determine what kind of research you do together," she said. "Hailiang joined the ESI in my second year here, so now he and I work together, especially on the hydrogen evolution reactions and chalcogenides. My research direction has changed a little bit because we've started working together."
Wang's office is two doors down from Cha's, and their students and postdocs share the same space. "With the open design concept in the lab, I think there's a lot of fluidity," she said. "The students will initiate measuring something together and if the results are interesting, then they tell us. Now that Shu Hu has joined, I can see similar collaborations happening with him." In fact, Wang said, it's usually the postdocs and students who spark the collaborations. "They often have lunches or dinners together in the kitchen, they bring food and share it," he said. "They meet each other often and they have fantastic interactions between them."
That's how the research on the electrocatalysis of chalcogenides between Cha and Wang got started. "That has been going really well and we've published one paper in ACS Nanomaterials, and we have another paper in preparation now," Cha said. Cha said she's excited to work with ESI's fourth hire Owen Miller, a theoretician in applied physics, who came to the ESI from MIT. That adds one more discipline to the equation. "That'll be a nice balance," she said, "and it will truly be interdisciplinary in nature." Brudvig said the goal currently is to have 10 full-time faculty members, averaging one new hire per year. "They'll bring their different expertises together and collaborate and develop new initiatives that wouldn't have been possible from someone working only within their own department," he said. "They'll tackle problems that they wouldn't have been able to by themselves."
is director of news and outreach for the School of Engineering & Applied Science at Yale University, based in New Haven, CT. He can be reached at william.weir@yale. edu. This story originally appeared in the 2016-17 issue of Yale Engineering.