Stone-Wales Defects Cause High Proton Permeability and Isotope Selectivity of Single-Layer Graphene.

While the isotope-dependent hydrogen permeability of graphene membranes at ambient condition has been demonstrated, the underlying mechanism has been controversially discussed during the past 5 years. The reported room-temperature proton-over-deuteron (H+ -over-D+ ) selectivity is 10, much higher than in any competing method. Yet, it has not been understood how protons can penetrate through graphene membranes-proposed hypotheses include atomic defects and local hydrogenation. However, neither can explain both the high permeability and high selectivity of the atomically thin membranes. Here, it is confirmed that ideal graphene is quasi-impermeable to protons, yet the most common defect in sp2 carbons, the topological Stone-Wales defect, has a calculated penetration barrier below 1 eV and H+ -over-D+ selectivity of  7 at room temperature and, thus, explains all experimental results on graphene membranes that are available to date. The competing explanation, local hydrogenation, which also reduces the penetration barrier, but shows significantly lower isotope selectivity, is challenged.