Proton permeation through atomically thin crystals

It is well known that electrons can easily traverse through thin 2D crystals, including graphene, hexagonal boron nitride (hBN), molybdenum disulphide (MoS$_{2}$) etc.; at the same time pristine graphene and other 2D materials are impermeable to molecules and atoms including helium. Protons represent somewhat intermediate case, which, together with the fact that hydrogen technologies are extremely important nowadays, motivated us to study proton permeation through thin 2D crystals. Employing both liquid and solid proton conducting electrolytes we demonstrate that monolayers of graphene and hBN are permeable to protons at ambient conditions, while MoS$_{2}$, bilayer graphene and multilayer hBN show no proton conduction. Temperature dependence confirms the thermionic nature of the proton permeation with the activation energies of 0.3, 0.6 and 0.8 eV for monolayer hBN, monolayer graphene and bilayer hBN, respectively. Our findings suggest that atomically thin crystals can be promising for various hydrogen technologies, for instance, as proton exchange membranes for fuel cells.