Graphene, a strong, lightweight carbon honeycombed structure that’s only one atom thick, holds great promise for energy research and development. Recently scientists with the Fluid Interface Reactions, Structures, and Transport (FIRST) Energy Frontier Research Center (EFRC), led by the US Department of Energy’s Oak Ridge National Laboratory, revealed graphene can serve as a proton-selective permeable membrane, providing a new basis for streamlined and more efficient energy technologies such as improved fuel cells.
“Now you’re able to take a barrier that you can make very thin, like graphene, and change it so you build gates on a molecular scale,” says principal investigator Franz Geiger of Northwestern University, the senior author and a FIRST researcher.
With a tight lattice of carbon reminiscent of chicken wire, pristine graphene was believed to be impenetrable. Current studies, however, have shown that in aqueous solutions, graphene allows surprising numbers of protons to pass through its atomic structure.
The researchers’ first step was to create an atomically thin layer of graphene on fused silica, an effort led by ORNL’s Ivan Vlassiouk, an expert in the synthesis of two-dimensional materials including graphene using chemical vapor deposition techniques.
The scientists later isolated the paths of movement the protons followed. By creating a single-layer sliver of graphene on silica glass, separated from the glass by mere molecules of water, the scientists designed a trap for the hopping protons. Changes in the acidity of the aqueous solution on either side of the graphene layer revealed the covert gating mechanism in the material’s structure, which they were able to detect using a laser technique called second harmonic generation.
“The major advantage of second harmonic generation,” says Northwestern’s Jennifer Achtyl, lead author of the Nature Communication article, “is that it is highly sensitive to chemistry at the interface or, in this case, the nanometer-thick environment between the aqueous solution and the surface of the silica. This acute sensitivity and the fact that these experiments can be run nondestructively were critical to our ability to capture experimental evidence of the transfer of protons through graphene.”
Using computational methods to analyze the configurations of defects in the graphene, the FIRST researchers isolated proton-transfer occurrences at defect areas. In addition, the team demonstrated that even the smallest of molecules, hydrogen and helium, are unable to pass through the proton gates under normal conditions.
“Finally, when we were able to put all the pieces together, we made a conclusive statement that—even though there’s a high energetic barrier for proton transport through graphene—if you lower that energetic barrier, you can allow protons to pass right through,” says Unocic. “This opens a new pathway for the atomic-scale engineering of graphene.”
Key to energy’s future?
Although the scientists focused on the fundamental mechanics of graphene surfaces, the results of this study open the doors for further graphene development across the energy economy and beyond.
With fuel cells, to name but one area of promise, issues range from cumbersome size to fleeting efficiency. Isolating single ion-transfer mechanisms and structural gaps in graphene could facilitate improvements in the production, transportation and use of energy.
“We’ve looked at this problem from really as many sides as you can possibly look at it with today’s technology,” Geiger says. “It makes a very strong case for taking the effect that we’ve observed and the mechanism that we’ve found and doing something technologically relevant with it. There are so many people working with graphene that to show how aqueous protons actually transfer across graphene will make a big difference.”
Abstract – Aqueous proton transfer across single-layer graphene
Proton transfer across single-layer graphene proceeds with large computed energy barriers and is therefore thought to be unfavourable at room temperature unless nanoscale holes or dopants are introduced, or a potential bias is applied. Here we subject single-layer graphene supported on fused silica to cycles of high and low pH, and show that protons transfer reversibly from the aqueous phase through the graphene to the other side where they undergo acid–base chemistry with the silica hydroxyl groups. After ruling out diffusion through macroscopic pinholes, the protons are found to transfer through rare, naturally occurring atomic defects. Computer simulations reveal low energy barriers of 0.61–0.75 eV for aqueous proton transfer across hydroxyl-terminated atomic defects that participate in a Grotthuss-type relay, while pyrylium-like ether terminations shut down proton exchange. Unfavourable energy barriers to helium and hydrogen transfer indicate the process is selective for aqueous protons.
SOURCES – Oak Ridge National Lab, Nature Communications