Graphene lets protons pass

In 2008 a landmark experiment by Paul McEuen and colleagues demonstrated that single-layer graphene is completely impermeable to air, argon, and helium. Since then, theory and experiment alike have suggested that pristine graphene should block the passage of all atoms and molecules. On the other hand, electrons can permeate atomically thin barriers with ease. Now Andre Geim (University of Manchester, UK), his students Sheng Hu and Marcelo Lozada-Hidalgo, and their collaborators have shown that protons, too, can pass through graphene monolayers. As illustrated in the figure, the researchers deposited a piece of crystalline graphene over a tiny hole they’d etched in a layer of silicon nitride; then they deposited Nafion (a polymeric solid that conducts protons but not electrons) and palladium hydride (an electrode material that converts electron currents to proton currents) on each side. The electrical conductivity per unit area of the hole was not only substantial, it varied little from device to device—evidence that protons were passing through the graphene itself, not through unnoticed holes or cracks. For comparison, the researchers tested two other two-dimensional crystalline materials: Molybdenum disulfide was impermeable to protons, but boron nitride let them through even more readily than graphene. The results position graphene and BN as promising new membrane materials for hydrogen fuel cells and hydrogen purification. The current favorite material for fuel-cell membranes is Nafion, but it’s hindered by poor performance above 80 °C or below 80% humidity; 2D materials could overcome both those limitations. (S. Hu et al., Nature 516, 227, 2014 .)