Fuel for thought
Rice researchers detail mechanism of hydrogen storage on graphene

BY MIKE WILLIAMS
Rice News staff

Spill a glass of milk onto a table, and it’ll run all over the floor. But spill a theoretical glass of hydrogen gas onto the floor, and it’ll run up onto the table.

Huh?

That’s the mystery a team of Rice researchers tackled when they decided to look into the “spillover” effect that allows hydrogen to be stored on sheets of graphene, layers of carbon a single atom thick and the stuff buckyballs and carbon nanotubes are made of.

	A metal catalyst plays the central role in separating hydrogen atoms from each other and “spilling” them onto a graphene receptor, where they can be stored for future energy needs.

Their research details why graphene may be a viable carrier for hydrogen-based energy systems of the future, as small variations in temperature and pressure can effectively control the capture and release of hydrogen atoms.

Boris Yakobson, professor in mechanical engineering and materials science and of chemistry at Rice, and his team set out to define the mechanism by which this long-known effect takes place.

The paper by Yakobson, postdoctoral research associate Abhishek Singh and graduate student Morgana Ribas was published online in ACS Nano.

Spillover, they determined, takes place when a metallic catalyst (usually palladium, platinum or nickel) is introduced to the hydrogen/graphene soup, attracting hydrogen atoms to the point of overload. Unable to hold more, the catalyst splits the gaseous hydrogen and “spills” its component atoms onto the graphene. The hydrogen loosely binds with graphene under the right heat and pressure, which fortunately hovers around room temperature and one atmosphere.

That effect has long been known, if little understood. But the real mystery was why hydrogen made the leap to a more energetic state — pulling itself up onto the “table” — when it attached itself to the graphene substrate.

“Nobody knew the process by which this hydrogen was stored. Hydrogen prefers to stay as a gas, rather than going to the graphene, so it was a paradox,” said Singh, the paper’s primary author.

“It seems to be contradictory,” Yakobson said. “The hydrogen seems to go up in energy, which should not be happening.”

“The key,” Singh said, “is that the catalyst should be saturated enough that the chemical potential of hydrogen on the catalyst exceeds that in the hydrogenated graphene, essentially triggering the spillover.”

The hydrogen gas “must disassociate on the catalyst before it moves to the graphene substrate,” he said, “but once there the hydrogen interacts with itself again, which lowers the energy just enough to bind with the graphene.

“That’s the beauty of it,” Singh said. “It stays, but it’s not too strongly bound. If it stuck too well, it would form some kind of hydrocarbon, and you could not get the energy back. You’d have a new chemical compound. But graphene is flat, and it cannot be fully accommodating. So the energies are just balancing.”

Temperature and pressure are the other keys, Singh said, “and we can play with that. A little colder, and the hydrogen will stick to the graphene. A little hotter, and it will release.”

“It’s a good balance because it means that by tuning temperature and pressure in the gas phase, you can make the hydrogen flow one way to store it and the other way to use it as a fuel source,” Yakobson said. “It’s like a thermodynamic seesaw.”

The study was funded by the Office of Naval Research, the National Science Foundation and the Department of Energy’s Hydrogen Sorption Center of Excellence.