  By systematically studying the proton selectivity of free-standing graphene
membranes in aqueous solutions we demonstrate that protons are transported by
passing through defects. We study the current-voltage characteristics of
single-layer graphene grown by chemical vapour deposition (CVD) when a
concentration gradient of HCl exists across it. Our measurements can
unambiguously determine that H+ ions are responsible for the selective part of
the ionic current. By comparing the observed reversal potentials with positive
and negative controls we demonstrate that the as-grown graphene is only weakly
selective for protons. We use atomic layer deposition to block most of the
defects in our CVD graphene. Our results show that a reduction in defect size
decreases the ionic current but increases proton selectivity.
