  Graphene is impermeable to all gases and liquids, and even such a small atom
as hydrogen is not expected to penetrate through graphene's dense electronic
cloud within billions of years. Here we show that monolayers of graphene and
hexagonal boron nitride (hBN) are unexpectedly permeable to thermal protons,
hydrogen ions under ambient conditions. As a reference, no proton transport
could be detected for a monolayer of molybdenum disulfide, bilayer graphene or
multilayer hBN. At room temperature, monolayer hBN exhibits the highest proton
conductivity with a low activation energy of about 0.3 eV but graphene becomes
a better conductor at elevated temperatures such that its resistivity to proton
flow is estimated to fall below 10^-3 Ohm per cm2 above 250 C. The proton
barriers can be further reduced by decorating monolayers with catalytic
nanoparticles. These atomically thin proton conductors could be of interest for
many hydrogen-based technologies.
