Protons Pass Through The Notoriously Impermeable Material Graphene | December 8, 2014 Issue - Vol. 92 Issue 49 | Chemical & Engineering News
Volume 92 Issue 49 | p. 4 | News of The Week
Issue Date: December 8, 2014 | Web Date: December 4, 2014

Protons Pass Through The Notoriously Impermeable Material Graphene

Materials: Membranes made of this and other two-dimensional materials could improve hydrogen fuel cells, which require a proton-conducting barrier
Department: Science & Technology | Collection: Sustainability
News Channels: Materials SCENE, Nano SCENE
Keywords: Graphene, boron nitride, proton, conductivity
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CLOUD COVER
The thinner electron clouds between atoms in 2-D boron nitride and graphene membranes facilitate proton transport better than the denser clouds of monolayer MoS2.
Credit: Nature
Electron cloud structures of atomic lattices.
 
CLOUD COVER
The thinner electron clouds between atoms in 2-D boron nitride and graphene membranes facilitate proton transport better than the denser clouds of monolayer MoS2.
Credit: Nature

Graphene and atomically thin crystals of boron nitride conduct protons far better than predicted and could radically enhance the performance of hydrogen fuel cells, according to an international research team (Nature 2014, DOI: 10.1038/nature14015).

Over the past decade, scientists have found that forcing ions or atoms through even a single layer of graphene is virtually impossible without the help of a particle accelerator. Theorists had predicted that, at room temperature, hydrogen could take billions of years to cross a graphene monolayer. This meant graphene, and likely other two-dimensional crystals, would probably make for awful proton-conducting membranes, essential ingredients in certain hydrogen fuel cells.

But researchers led by Andre K. Geim of the University of Manchester, in England, have now experimentally demonstrated that pristine single layers of graphene and hexagonal boron nitride (hBN) conduct protons surprisingly well. At temperatures above 100 °C, the membranes achieve conductivities that make them attractive for use in fuel cells, Geim says, and unlike other proton-conducting materials, these membranes are impermeable to other ions and atoms.

In the context of hydrogen fuel cells, an ideal proton-exchange membrane separates a compartment containing a hydrogen fuel source from another chamber filled with an oxidant, passing only protons between the two. Current real-world membranes permit some mingling between fuel and oxidant, which saps device performance.

Graphene and hBN membranes could reduce this chemical crossover by orders of magnitude, says Geim, who won a share of the 2010 Nobel Prize in Physics for his pioneering work with graphene. “We just put another stake in the ground showing there is a blank spot in this huge area of research that hasn’t been touched before,” says Geim of his team’s new study.

Scaling up the production of such membranes for commercial fuel cells could be a challenge, but researchers are already making graphene sheets on the square-meter scale, says Rohit N. Karnik of Massachusetts Institute of Technology, a mechanical engineer who was not involved with the study. He adds that he is struck by how reproducible and conclusive the new results are. “I am really impressed with the quality of this work,” he says.

Geim and his team have yet to work out exactly how protons permeate the 2-D membranes, although they know that the density of the electron cloud between the membranes’ atoms plays a role.

At room temperature, hBN membranes conduct protons better than graphene does, the researchers found. Nitrogen atoms within an hBN network attract valence electrons more strongly than do the boron atoms within the network. The interstitial electron cloud within hBN becomes thin as a result. The symmetric arrangement of graphene’s carbon atoms creates a more uniform, denser electron cloud that is tougher for protons to penetrate.

The team also studied 2-D molybdenum disulfide membranes, which had the densest electron clouds according to the team’s simulations and the poorest proton conductivity according to their measurements.

Geim hopes theorists will help solve the remaining mysteries of proton transport, such as why graphene outperforms hBN at elevated temperatures. For now, he says, this study emphasizes the importance of experimental science. Without it, no one would know just how easily protons move through graphene, he adds.

According to Geim, “If you read too much theory before doing experiments, you might miss new phenomena.”

 
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Comments
A.Chandrasekaran (Wed Jan 14 14:35:28 EST 2015)
I fully agree with Prof. Geim's advise about not reading too much theory before doing experiments. I believe that theory is good for interpolation rather than extrapolation, especially farther from what is known and used for parameterization.

When coming to the exciting findings here, all considerations seem to be only about the protons going through a perfect monolayer, without defects. Considering defects may be a simpler approach than proton going through the six membered rings.

To me it looks that single atom defects/deficiencies can perfectly transport proton while blocking everything else, in terms of size. The ability of proton binding at such sites may determine the proton conducting ability, along with the extent of such defects.

Missed in the article is the interesting fact that graphene conducts electrons along the plane while now transporting protons across the plane!

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