The interaction between protons and graphene is attracting a large interest
due to recent experiments showing that these charged species permeate through
the 2D material following a low barrier (~ 0.8 eV) activated process. A
possible explanation involves the flipping of a chemisorbed proton (rotation of
the C-H+ bond from one to the other side of the carbon layer) and previous
studies have found so far that the energy barriers (around 3.5 eV) are too high
to explain the experimental findings. Contrarily to the previously adopted
model assuming an isolated proton, in this work we consider protonated graphene
at high local coverage and explore the role played by nearby chemisorbed
protons in the permeation process. By means of density functional theory
calculations exploiting large molecular prototypes for graphene it is found
that, when various protons are adsorbed on the same carbon hexagonal ring, the
permeation barrier can be reduced down to 1.0 eV. The related mechanism is
described in detail and could shed a new light on the interpretation of the
experimental observations for proton permeation through graphene.Comment: 16 pages, 5 figure