Thin films of platinum on graphene make effective fuel cell catalyst

Films of platinum only two atoms thick supported by graphene could usher in fuel cell catalysts with unprecedented catalytic activity and longevity, according to a study by researchers at the Georgia Institute of Technology (Georgia Tech). The researchers report their findings in a paper in Advanced Functional Materials.

Platinum is one of the most commonly used catalysts for fuel cells because of how effectively it promotes the oxidation reduction reaction at the center of the technology. But the high cost of platinum has spurred research efforts to find ways to use smaller amounts of it while maintaining the same catalytic activity.

"There's always going to be an initial cost for producing a fuel cell with platinum catalysts, and it's important to keep that cost as low as possible," said Faisal Alamgir, an associate professor in Georgia Tech's School of Materials Science and Engineering. "But the real cost of a fuel cell system is calculated by how long that system lasts, and this is a question of durability.

"Recently, there's been a push to use catalytic systems without platinum, but the problem is that there hasn't been a system proposed so far that simultaneously matches the catalytic activity and the durability of platinum."

The Georgia Tech researchers tried a different strategy. In the paper, they describe creating several systems comprising atomically thin films of platinum supported by a layer of graphene, allowing them to maximize the total surface area of the platinum available for catalytic reactions while using a much smaller amount of the precious metal.

Most platinum-based catalytic systems use nanoparticles of the metal, which are chemically bonded to a support surface. But this means the surface atoms of the particles do most of the catalytic work, and the catalytic potential of the atoms beneath the surface is never utilized as fully as the surface atoms, if at all.

Additionally, the researchers showed that their new platinum films, which are at least two atoms thick, outperformed nanoparticle platinum in dissociation energy, a measure of the energy cost of dislodging a surface platinum atom. That measurement suggests the films could make potentially longer-lasting catalytic systems.

To prepare the atomically thin films, the researchers used a process called electrochemical atomic layer deposition to grow platinum monolayers on a layer of graphene, creating samples that had one, two or three atomic layers of atoms. The researchers then tested these samples for dissociation energy and compared the results to the energy of a single atom of platinum on graphene, as well as to the energy of common configurations of platinum nanoparticles used in catalysts.

"The fundamental question at the heart of this work was whether it was possible that a combination of metallic and covalent bonding can render the platinum atoms in a platinum-graphene combination more stable than their counterparts in bulk platinum used commonly in catalysts that are supported by metallic bonding," said Seung Soon Jang, an associate professor in the School of Materials Science and Engineering.

The researchers found that the bond between neighboring platinum atoms in the film essentially joins forces with the bond between the film and the graphene layer to provide reinforcement across the system. That was especially true in the platinum film that was two atoms thick.

"Typically, metallic films below a certain thickness are not stable because the bonds between them are not directional, and they tend to roll over each other and conglomerate to form a particle," Alamgir said. "But that's not true with graphene, which is stable in a two-dimensional form, even one atom thick, because it has very strong covalent directional bonds between its neighboring atoms. So this new catalytic system could leverage the directional bonding of the graphene to support an atomically-thin film of platinum."

Future research will involve further testing of how the films behave in a catalytic environment. The researchers found in earlier research on graphene-platinum films that the material behaves similarly in catalytic reactions regardless of which side – graphene or platinum – is the exposed active surface.

"In this configuration, the graphene is not acting as a separate entity from the platinum," Alamgir explained. "They're working together as one. So we believe that if you're exposing the graphene side, you get the same catalytic activity and you could further protect the platinum, potentially further enhancing durability."

This story is adapted from material from the Georgia Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.