By Jimmy Brancho
You know what they say: “You can’t store solar energy without cracking a few water molecules.”
Or, at least, many scientists around the world are working to make that so. As was discussed earlier on this blog, solar water splitting could enable a cleaner energy future by storing energy from the sun’s rays in a stable chemical fuel like hydrogen that can be used on-demand. Ideally, the only inputs needed would be water and sunlight, and the only waste product oxygen. However, the current state of technology is a long way off. Bart Bartlett, Charles McCrory, and Neil Dasgupta are among several faculty here at the University of Michigan that are working to make solar water splitting devices a reality. Each of them approaches the problem from a diverse angle.
A solar water splitting device needs to do two basic things. First, it needs a light absorbing material. By absorbing light, the device traps solar energy, making it available for storage in a molecule of fuel. Second, the device needs to be able to perform the water splitting reaction, generating hydrogen and oxygen gases. The material, or materials, that carry out the business of water splitting are called catalysts. Both functions need to be at their best for a water splitting device to work.
Metal oxides: Cheap, but promising
Bart Bartlett, associate professor of chemistry and associate director of the University of Michigan Energy Institute, has eight years of experience in water splitting research. (Full disclosure: he’s also the author’s Ph. D. advisor.) His group in inorganic chemistry studies applications for metal oxides, compounds formed when metals react with oxygen. Many metal oxides are cheap as dirt – some of them are dirt – and in many cases very abundant, making them attractive materials for large scale reactions. Initially, Bartlett wanted to leverage his inorganic chemistry knowledge to create metal oxides that strongly absorbed visible light, which makes up the most abundant part of the solar spectrum.
An associate professor of chemistry at the University of Michigan, Bart Bartlett investigates how complex metal oxides absorb light and perform reactions relevant to solar water splitting.
His group found some success, but their devices did not perform as well as anticipated. “I think what we learned, and what a lot of other people were learning around the same time, is that [light] absorption isn’t really the bottleneck,” Bartlett said. Instead, the catalytic part of the device – the part that actually rearranges bonds and splits water – is lagging.
Since that initial foray, Bartlett’s group has branched out to pursue other interesting questions related to photocatalytic water splitting. Instead of trying to make good light absorbers, his group now creates electrodes made from materials with different functions stacked on top of one another. This is in hopes of finding a system that works better than the sum of its parts. His group has had success coating metal oxides that are good light absorbers with other metal-based materials that make effective catalysts. The coating approach yields electrodes that last longer and waste less energy on side reactions, which means they’re one step closer to making it into a practical commercial device.
Honing in on long-lasting catalysts
Charles McCrory, a newly-minted assistant professor of chemistry, uses electricity to study a range of different catalyst materials. “What we look at is the catalysis side of a lot of reactions,” he says. “Some of them happen to be things related to solar fuels.”
McCrory came to the University of Michigan from the Joint Center for Artificial Photosynthesis (JCAP), a U.S. Department of Energy Innovation Hub dedicated to advancing solar energy storage. At JCAP, McCrory used a standard testing method to level the playing field for a variety of known catalysts. “In general,” he says, “when someone develops a catalyst, they like to report it using very specific conditions that make the catalyst look the best.” His job was to take those catalysts and test them all under identical conditions to compare how they performed.
He realized that it was fairly easy to say whether a catalyst was active or inactive for a particular reaction. What was more difficult, however, was determining whether a material was stable – whether it would remain active over a long period of use and re-use. Stability is a critical because devices that need to be replaced less often are cheaper in the long run.
McCrory has built his research program at Michigan around the stability question. Right now, if the activity of a particular catalyst doesn’t vary much over an arbitrary test period, the material is dubbed a stable one. “But that’s not been systematically applied,” says McCrory. “There’s no sort of systematic understanding of why some things are stable and some things aren’t.” Additionally, the test periods for stability used in most labs are very rarely longer than 100 hours, which is far, far shorter than the ideal lifetime for a device. Developing a broad and predictive understanding of catalyst stability is McCrory’s goal. This is a critical piece of designing water splitting devices that work for years on end.
Building better processes for water splitting at market scale
Water splitting research isn’t just limited to the chemistry department. Neil Dasgupta is a second-year assistant professor in the mechanical engineering department, “which is a bit unusual for water splitting,” he says.
Many semiconductors, like silicon, would be great for water splitting if they did not readily corrode and disintegrate during use. Dasgupta’s group is interested in figuring out how to take unstable semiconductors and chemically protect them so they can be used in a water splitting device. He studies a technique called atomic layer deposition, which uses gases to lay down a coating of a material one atomic layer at a time. Its ability to make conformal coatings at customizable thicknesses makes the technique very useful. “It’s used in […] processing at very large scale by companies like Intel and Applied Materials nowadays,” says Dasgupta. Atomic layer deposition is a critical step in Intel’s manufacturing of computer processors.
As a mechanical engineer who has studied chemistry, physics, and engineering, Dasgupta wants to hone in on the finest aspects of materials created by atomic layer deposition. He admits that plenty of researchers have used atomic layer deposition to protect unstable materials and show an increase in the stability. However, he thinks there are lots of unanswered questions about how the protective layers affect other details pertaining to the overall performance, such as how electricity moves through the device. “I think there’s a good opportunity for us to contribute as a group that studies the science of [atomic layer deposition], not just uses it as a tool,” says Dasgupta. Learning more about the details of how materials coated by atomic layer deposition behave is a frontier in water splitting research and could help make real technological strides towards commercialization.
A big part of that for Dasgupta, and the part which his engineering background plays directly into, is breaking out of laboratory-scale production. At lab scale, scientists make very small devices or amounts of substances to prove a concept without necessarily considering processing costs. “Can you bridge that gap between somebody making a little one square millimeter solar cell in a beaker and roll-to-roll processing at large scale sold for dirt cheap at Wal-Mart?” he asks. “That’s a pretty big leap. That’s a leap that often times, start-ups fail in.” Part of studying atomic layer deposition in Dasgupta’s group involves figuring out how to adapt lab syntheses to larger scale preparations so that someday, water splitting devices can be produced en masse.
Despite their differing approaches to solar energy storage via water splitting, each of the three researchers got into the field for a similar reason: to work on a pressing, technologically relevant problem while still propelling basic science forward. Says Bartlett, “What we’re learning has impact not only for this particular problem in fuel formation, but also, if you think about it, the questions we’re asking are fundamental. They’re not limited to fuels.”
“I think that’s critical in science,” he adds. “Yeah, you want to have your eye on the prize. But if all you do is swing for the fences and say whether it’s go or no go for accomplishing some major goal, there’s a lot left in between. And it’s the stuff that’s in between that’s really going to keep the field [of solar energy storage] moving forward.”
Read part one of the two part series on water splitting.
About the author
Jimmy is a 5th-year graduate student in the University of Michigan Department of Chemistry, exploring new chemical reactions to make photocatalysts for solar energy storage. He’s a southwestern Pennsylvania native and graduated from Duquesne Universtiy in Pittsburgh in 2011. Jimmy spends his off time at the roller hockey rink, playing involved board games, or annoying his cat. He also blogs chemistry and student issues at Tree Town Chemistry.
Read all posts by Jimmy here.