  Proton transfer across single layer graphene is associated with large
computed energy barriers and is therefore thought to be unfavorable at room
temperature unless nanoscale holes or dopants are introduced, or a potential
bias is applied. Here, we subject single layer graphene supported on fused
silica to cycles of high and low pH and show that protons transfer reversibly
from the aqueous phase through the graphene to the other side where they
undergo acid-base chemistry with the silica hydroxyl groups. After ruling out
diffusion through macroscopic pinholes, the protons are found to transfer
through rare, naturally occurring atomic defects. Computer simulations reveal
low energy barriers of 0.68 to 0.75 eV for aqueous proton transfer across
hydroxyl-terminated atomic defects that participate in a Grotthuss-type relay,
while pyrylium-like ether terminations shut down proton exchange. Unfavorable
energy barriers to helium and hydrogen transfer indicate the transfer process
is selective for aqueous protons.
