Imperfect graphene opens door to better fuel cells

March 17, 2015
Proton transfer channel across a quad-defect in graphene, as obtained from a ReaxFF molecular dynamics simulation. Credit: Murali Raju, Penn State

The honeycomb structure of pristine graphene is beautiful, but Northwestern University scientists, together with collaborators from five other institutions, have discovered that if the graphene naturally has a few tiny holes in it, you have a proton-selective membrane that could lead to improved fuel cells.

A major challenge in fuel cell technology is efficiently separating protons from hydrogen. In a study of single-layer graphene and water, the Northwestern researchers found that slightly imperfect graphene shuttles protons—and only protons—from one side of the graphene membrane to the other in mere seconds. The membrane's speed and selectivity are much better than that of conventional membranes, offering engineers a new and simpler mechanism for fuel cell design.

"Imagine an electric car that charges in the same time it takes to fill a car with gas," said chemist Franz M. Geiger, who led the research. "And better yet—imagine an electric car that uses hydrogen as fuel, not fossil fuels or ethanol, and not electricity from the power grid, to charge a battery. Our surprising discovery provides an electrochemical mechanism that could make these things possible one day."

Defective single-layer graphene, it turns out, produces a membrane that is the world's thinnest proton channel—only one atom thick.

"We found if you just dial the graphene back a little on perfection, you will get the membrane you want," said Geiger, a professor of chemistry in the Weinberg College of Arts and Sciences. "Everyone always strives to make really pristine graphene, but our data show if you want to get protons through, you need less perfect graphene."

Hydroxylated defect site that allows for facile proton transfer through the pristine single-layer graphene substrate. Credit: University of Minnesota

The study will be published March 17 by the journal Nature Communications.

Geiger's research team included collaborators from Northwestern, Oak Ridge National Laboratory, the University of Virginia, the University of Minnesota, Pennsylvania State University and the University of Puerto Rico.

In the atomic world of an aqueous solution, protons are pretty big, and scientists don't believe they can be driven through a single layer of chemically perfect graphene at room temperature. (Graphene is a form of elemental carbon composed of a single flat sheet of carbon atoms arranged in a repeating hexagonal, or honeycomb, lattice.)

The video will load shortly
Video simulation of the transport process

When Geiger and his colleagues studied graphene exposed to water, they found that protons were indeed moving through the graphene. Using cutting-edge laser techniques, imaging methods and computer simulations, they set out to learn how.

The researchers discovered that naturally occurring defects in the graphene—where a carbon atom is missing—triggers a chemical merry-go-round where protons from water on one side of the membrane are shuttled to the other side in a few seconds. Their advanced computer simulations showed this occurs via a classic "bucket-line" mechanism first proposed in 1806.

Hydroxylated defect site that allows for facile proton transfer through the pristine single-layer graphene substrate. Credit: Credit: University of Minnesota

The thinness of the atom-thick graphene makes it a quick trip for the protons, Geiger said. With conventional membranes, which are hundreds of nanometers thick, proton selection takes minutes—much too long to be practical.

Next, the research team asked the question: How many carbon atoms do we need to knock out of the graphene layer to get protons to move through? Just a handful in a square micron area of graphene, the researchers calculated.

The video will load shortly
Advanced theoretical simulations track the dynamics of proton transfer along a network of interconnected hydrogen bonds through the center of the defect in the graphene surface. Credit: Credit: University of Minnesota

Removing a few carbon atoms results in others being highly reactive, which starts the proton shuttling process. Only protons go through the tiny holes, making the membrane very selective. (Conventional membranes are not very selective.)

"Our results will not make a fuel cell tomorrow, but it provides a mechanism for engineers to design a proton separation membrane that is far less complicated than what people had thought before," Geiger said. "All you need is slightly imperfect single-layer graphene."

Explore further: Laying down a discerning membrane

More information: The paper is titled "Aqueous Proton Transfer Across Single-Layer Graphene."

Related Stories

Laying down a discerning membrane

October 4, 2013

One of the thinnest membranes ever made is also highly discriminating when it comes to the molecules going through it. Engineers at the University of South Carolina have constructed a graphene oxide membrane less than 2 nanometers ...

2-D materials' crystalline defects key to new properties

September 24, 2014

Understanding how atoms "glide" and "climb" on the surface of 2D crystals like tungsten disulphide may pave the way for researchers to develop materials with unusual or unique characteristics, according to an international ...

'Mind the gap' between atomically thin materials

November 23, 2014

In subway stations around London, the warning to "Mind the Gap" helps commuters keep from stepping into empty space as they leave the train. When it comes to engineering single-layer atomic structures, minding the gap will ...

Protons fuel graphene prospects

November 26, 2014

Graphene, impermeable to all gases and liquids, can easily allow protons to pass through it, University of Manchester researchers have found.

'Mind the gap' between atomically thin materials

December 24, 2014

When it comes to engineering single-layer atomic structures, "minding the gap" will help researchers create artificial electronic materials one atomic layer at a time, according to a team of materials scientists.  

Researchers pattern magnetic graphene

March 17, 2015

Graphene, an atomically thin sheet of carbon, has been intensively studied for the last decade to reveal exceptional mechanical, electrical, and optical properties. Recently, researchers have started to explore an even more ...

Recommended for you

Neuromorphic computing mimics important brain feature

August 18, 2016

(Phys.org)—When you hear a sound, only some of the neurons in the auditory cortex of your brain are activated. This is because every auditory neuron is tuned to a certain range of sound, so that each neuron is more sensitive ...

'Artificial atom' created in graphene

August 22, 2016

In a tiny quantum prison, electrons behave quite differently as compared to their counterparts in free space. They can only occupy discrete energy levels, much like the electrons in an atom - for this reason, such electron ...

Picoscale precision though ultrathin film piezoelectricity

August 10, 2016

Piezoelectricity (aka the piezoelectric effect) occurs within certain materials – crystals (notably quartz), some ceramics, bone, DNA, and a number of proteins – when the application of mechanical stress or vibration ...

2 comments

Adjust slider to filter visible comments by rank

Display comments: newest first

kafantaris
not rated yet Mar 18, 2015
Rebirth of the hydrogen fuel cell: All you need is slightly imperfect single-layer graphene.

"We found if you just dial the graphene back a little on perfection, you will get the membrane you want. Everyone always strives to make really pristine graphene, but our data show if you want to get protons through, you need less perfect graphene."
"Our results will not make a fuel cell tomorrow, but it provides a mechanism for engineers to design a proton separation membrane that is far less complicated than what people had thought before. All you need is slightly imperfect single-layer graphene."
"Imagine an electric car that charges in the same time it takes to fill a car with gas. And better yet-imagine an electric car that uses hydrogen as fuel, not fossil fuels or ethanol, and not electricity from the power grid, to charge a battery. Our surprising discovery provides an electrochemical mechanism that could make these things possible one day." -- Franz M. Geiger
PPihkala
not rated yet Mar 18, 2015
The next question will be what it will cost to make these leaky membranes of graphene.

Please sign in to add a comment. Registration is free, and takes less than a minute. Read more

Click here to reset your password.
Sign in to get notified via email when new comments are made.