Northwestern-led team finds slightly imperfect graphene can serve as a highly selective proton separation membrane
Researchers from Northwestern University, together with collaborators from Oak Ridge National Laboratory, the University of Virginia, the University of Minnesota, Pennsylvania State University and the University of Puerto Rico, have discovered that protons can transfer easily through graphene—conventionally thought to be unfit for proton transfer absent nanoscale holes or dopants—through rare, naturally occurring atomic defects.
In an open access paper published in the journal Nature Communications, the researchers reported that a slightly imperfect graphene membrane’s speed and selectivity are much better than that of conventional proton separation membranes, offering engineers a new and simpler mechanism for fuel cell design.
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.—Professor Franz M. Geiger, who led the research
A major challenge in fuel cell technology is efficiently separating protons from hydrogen. In the atomic world of an aqueous solution, protons are pretty big, and scientists have not believed that protons 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.)
When Geiger and his colleagues studied graphene exposed to water, they found that protons were indeed moving through the graphene. Using advanced laser techniques, imaging methods and computer simulations, they discovered that naturally occurring defects in the graphene—where a carbon atom is missing—trigger 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.
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.
Removing only 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.)
|Proton transfer through the hydroxyl-terminate graphene quad-vacancy, as obtained from an unbiased ReaxFF reactive force field molecular dynamics simulation at T=300K. Hydrogen atoms specifically relevant to the proton transfer event are colored blue. Credit: Murali Raju, Penn State.|
From the SHG signal jump rates and the time required for 2D proton diffusion, we estimate that the presence of as few as a handful of atomic defects in a 1 μm2 area sample of single-layer graphene is sufficient to allow for the apparent unimpeded protonation and deprotonation of the interfacial silanol groups within 10 s. Yet, we caution that given the limited accuracy with which the defect density can be determined in large (mm)-scale graphene, aqueous protons may transfer across single-layer graphene not only along the path discussed here but also along others as well. The identification of low barriers specifically for water-assisted transfer of protons through OH-terminated atomic defects in graphene, and high barriers for oxygen-terminated defects could be an important step towards the preparation of zero-crossover proton-selective membranes.—Achtyl et al.
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.—Franz Geiger
Jennifer L. Achtyl, Raymond R. Unocic, Lijun Xu, Yu Cai, Muralikrishna Raju, Weiwei Zhang, Robert L. Sacci, Ivan V. Vlassiouk, Pasquale F. Fulvio, Panchapakesan Ganesh, David J. Wesolowski, Sheng Dai, Adri C. T. van Duin, Matthew Neurock & Franz M. Geiger (2015) “Aqueous proton transfer across single-layer graphene” Nature Communications 6, Article number: 6539 doi: 10.1038/ncomms7539doi