Scientists are busy characterizing and finding applications for graphene, a young class of carbon materials, first discovered in 1859, with research ramping up in the 1950s. Simply put, graphene is a two-dimensional crystal best depicted as a honeycomb that is only one atom thick! Researchers at the Fluid Interface Reactions, Structures and Transport (FIRST) Center have discovered how protons selectively cross this material. This new understanding lays the foundation for innovative selective membranes for batteries, fuel cells, and other applications.
An atomic layer of graphene can be produced in large yard-sized sheets using chemical vapor deposition–a process in which a source of carbon is supplied in gaseous form over a heated piece of metal and graphene is formed by the subsequent chemical reactions. Graphene is of great importance because of its unusual electronic properties made possible by its unique geometry. It can conduct heat and electricity very efficiently. Moreover, it’s stretchable and impermeable. Flexible, mechanically stable graphene membranes can be made with tunable or adjustable nanosized pores. The use of single-layer graphene could potentially be better than the state-of-the-art polymer-based filtration membranes used for water desalination, fuel cells, and other applications.
But what can and cannot go through the single-layer of graphene before any modifications have been made? Can protons pass? As part of her FIRST-funded Ph.D. studies at Northwestern University, Jennifer Achtyl, her adviser Franz Geiger, and their collaborators asked these exact questions. And to produce a definitive answer, they used every sophisticated experimental and computational tool at their disposal.
They placed a well-characterized layer of graphene on a surface, immersed the composite in liquid, and cycled the pH, effectively increasing and decreasing the concentration of protons with each cycle. In theory, if graphene was impermeable to protons, these changes in pH would not affect the surface’s charge. But this was not the case. In fact, the charge response was nearly identical with and without the graphene “shield.”
“This is where the fun started,” said Achtyl. “We had this unexpected result–the graphene appeared to be permeable to protons–but now we needed to figure out why and how this was possible. Thankfully, because of the FIRST Center, we had the expertise needed to tackle these questions right at our fingertips. It was the comprehensive and cohesive team effort across four FIRST partner institutions that developed this initial finding into a truly awesome story.”
Achtyl and her team investigated pinhole defects that dot the surface of graphene sheets to see if the protons were leaking through. Using scanning electron microscopy followed by rigorous statistical calculations, they concluded that the number of larger gaps (that a single strand of human hair could fit through) known as pinholes was quite small and could not have accounted for that much proton transport.
In addition to the experimental work, the team performed complex calculations to quantify energy barriers for proton transport through the ideal graphene surface, as well as through various surface defect sites. Pathways that have too high of a calculated energy barrier are unlikely. They determined that a plausible pathway is through much smaller atomic-scale defect sites, where carbon atoms are missing. Calculating energy barriers for proton transfer through a defect site with one to four missing carbons indicate that a site with four vacancies would provide a favorable path (see figure). The existence of these rare and extremely small defects was confirmed by scanning transmission electron microscopy.
By combining sensitive surface-probing techniques with computer simulations the scientists provided a mechanistic answer as to how a proton may pass through single-layer graphene. They identified low-energy barriers for water-assisted proton transfer through one type of atomic defect, known as hydroxyl-terminated atomic defects, and high barriers for a different defect, known as an oxygen-terminated defect. Understanding the mechanisms at play is an important step in preparing zero-crossover proton-selective membranes. Further research will continue to lead to a deeper understanding of the physical and chemical phenomena associated with the 21st century superstar material–graphene.
This work was supported as part of the Fluid Interface Reactions, Structures and Transport (FIRST) Center, an Energy Frontier Research Center, funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences.
JL Achtyl, RR Unocic, L Xu, Y Cai, M Raju, W Zhang, RL Sacci, IV Vlassiouk, PF Fulvio, P Ganesh, DJ Wesolowski, S Dai, ACT van Duin, M Neurock, and FM Geiger. 2015. “Aqueous Proton Transfer across Single-Layer Graphene.” Nature Communications 6:6539. DOI: 10.1038/ncomms7539