Like an atomic-scale bucket brigade, molecular species residing at defects in graphene work together to shuttle protons through the ultrathin carbon film, according to a new study.
The investigation surprisingly shows that single layers of graphene, on their own, can selectively transmit protons in water. The finding deepens understanding of transport properties of the atomically thin carbon material and may lead to improved proton-selective membranes, a critical component of fuel cells.
This video shows the mechanism by which OH groups that ring a tiny graphene defect grab protons in water and pass them from one OH group to another, thereby mediating protons through graphene.
Credit: Matthew Neurock/University of Minnesota
In the ongoing push to explore graphene’s potential applications, several researchers have studied proton conduction through graphene. The results indicate that protons cannot pass through the material, unless researchers modify it with dopants, puncture it to form fine holes, or apply a voltage.
The new study, which was conducted by a multi-institution team led by Franz M. Geiger of Northwestern University, shows that those procedures are not required to coax protons through graphene (Nat. Commun. 2015, DOI: 10.1038/ncomms7539). Rather, a small number of atomic-scale defects that form naturally during graphene synthesis cause the material to rapidly transmit aqueous protons through the carbon network.
The team deposited a carefully characterized graphene film on a silica support and then added an aqueous solution. As they cycled the solution between low and high pH values, the team used a highly sensitive laser spectroscopy method to monitor protonation and deprotonation of silanol groups on the silica surface. When the solution pH was low, protons in solution moved through the graphene film to the silanol groups, and when the pH was high, protons traveled in the opposite direction. Through a combination of microscopy and other analyses, the group ruled out proton diffusion through pinhole defects and ensured that the film was not damaged by exposure to laser light and other probes.
The analysis, coupled with computations, shows that graphene exhibits rare defects—holes—surrounded by six carbon atoms that are either terminated with three oxygen atoms or six OH groups. The terminating oxygen atoms prevent proton transfer. But the hydroxyl groups work like an old-time bucket brigade grabbing protons from water and passing them quickly from one OH group to another, thereby transporting protons through the graphene membrane.
“The upshot is—for proton-separating membranes all you need is slightly imperfect single-layer graphene,” Geiger says.
Relative to earlier investigations of graphene proton transport, “this paper reports important technical and scientific advances,” says Mischa Bonn of the Max Planck Institute for Polymer Research, in Germany. The study, he says, provides insights into the proton conduction mechanism and describes a relay for protons that’s inaccessible to atoms and molecules.
Chemistry professor James T. (Casey) Hynes of the University of Colorado, Boulder, remarks that the study adds to the list of known proton relay chains, such as ones through proteins or at ice surfaces in the stratosphere. “This addition to the list, in an utterly hydrophobic environment, is a quite striking and pleasant surprise.”