  Monolayers of graphene and hexagonal boron nitride (hBN) are highly permeable
to thermal protons. For thicker two-dimensional (2D) materials, proton
conductivity diminishes exponentially so that, for example, monolayer MoS2 that
is just three atoms thick is completely impermeable to protons. This seemed to
suggest that only one-atom-thick crystals could be used as proton conducting
membranes. Here we show that few-layer micas that are rather thick on the
atomic scale become excellent proton conductors if native cations are
ion-exchanged for protons. Their areal conductivity exceeds that of graphene
and hBN by one-two orders of magnitude. Importantly, ion-exchanged 2D micas
exhibit this high conductivity inside the infamous gap for proton-conducting
materials, which extends from 100 C to 500 C. Areal conductivity of
proton-exchanged monolayer micas can reach above 100 S cm-2 at 500 C, well
above the current requirements for the industry roadmap. We attribute the fast
proton permeation to 5 A-wide tubular channels that perforate micas' crystal
structure which, after ion exchange, contain only hydroxyl groups inside. Our
work indicates that there could be other 2D crystals with similar nm-scale
channels, which could help close the materials gap in proton-conducting
applications.
