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To change a catalyst’s electrochemical properties, just ‘shuffle’

A standard deck of playing cards will feature the same individual cards, no matter how much you shuffle it. But the card on top – along with the arrangement of the other 51 cards – is likely to change each time you shuffle.

A research group led by Jin Suntivich, assistant professor and David Croll Sesquicentennial Faculty Fellow in the Department of Materials Science and Engineering (MSE), had an unexpected finding when it controllably “shuffled” the top layer of a well-known catalyst.

Manganese oxide (lanthanum-strontium-manganese oxide) catalyzes oxygen reduction reaction (ORR), a vital process for electrochemical energy technologies. And as Suntivich and his group found, the atomic arrangement of the four components makes a huge difference.

They found that placing the highly reactive strontium just below the surface of the material optimizes ORR catalysis, even more so than by putting it on the surface, where it can cause an unwanted side reaction that actually hinders catalysis.

“The bulk material is nominally identical, but by changing just the organization of these materials, you see that the chemical properties changed,” said John Eom, doctoral student in the Suntivich lab and first author of “Tailoring Manganese Oxide with Atomic Precision to Increase Surface Site Availability for Oxygen Reduction Catalysis,” which published Oct. 2 in Nature Communications.

Also contributing was Darrell Schlom, the Herbert Fisk Johnson Professor of Industrial Chemistry in MSE, and Ding-Yuan Kuo, doctoral student in MSE.

Oxygen reduction is a fundamental reaction related to energy conversion, but the mechanism of the ORR on metal surfaces is not well understood. The Suntivich lab focuses primarily on identifying design strategies based on optics and electronic structure engineering to discover new materials and devices for sustainable energy and environmental technologies.

Traditionally in the pursuit of new catalysts, researchers will vary the bulk composition or structure, but the placement of each component is often left to chance and doesn’t take into account how the location of the placement affects the material’s property.

By structuring the bulk material with thin films of each of the four components, using a process known as molecular beam epitaxy – a specialty of the Schlom lab – the team was able to precisely control the properties of catalyst while keeping its makeup the same.

“What we found is that if you take the same material, but just layer it differently, you get a catalyst with different properties,” said Suntivich, a member of the Kavli Institute at Cornell for Nanoscale Science, as is Schlom. “Think of a deck of cards: If you take a card from the top and put it on the bottom, you still have the same deck of cards, but how they’re dealt is different.

“That was what we found with this material,” he said, “but the question was, why?”

The research team first built the material without strontium, then placed the strontium gradually closer to the surface of the material. They found that by placing it just below the surface, ORR catalysis was superior compared to when it was placed at the surface.

“That’s when we realized that there must be another chemistry happening with the strontium at the surface,’” Suntivich said.

That other effect: Strontium at the surface can react with ambient air to create strontium oxide, which is detrimental to oxygen reduction catalysis.

The ability to tailor this catalyst at the atomic scale – “painting” thin films of each component in a precise way – could open up a whole new way of thinking about catalyst design, Suntivich said.

“Maybe catalysts that we once thought of as bad may behave differently if we can apply this approach,” he said. “This could provide new design variables, which gives you greater ability to tailor and fine-tune the material.”

Researchers from Stanford University, Drexel University and the Advanced Light Source at Berkeley National Laboratory in Berkeley, California also contributed.

Support for this work came from the U.S. Department of Energy Office of Basic Science. The researchers made use of the Cornell Center for Materials Research, which is supported by the National Science Foundation, and the Cornell NanoScale Science and Technology Facility.

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Jeff Tyson