Atomically-thin micas as proton conducting membranes

Monolayers of graphene and hexagonal boron nitride (hBN) are highly permeable to thermal protons1,2. 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 protons1. 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 materials3, which extends from 100 ˚C to 500 ˚C. Areal conductivity of protonexchanged monolayer micas can reach above 100 S cm-2 at 500 ˚C, well above the current requirements for the industry roadmap4. We attribute the fast proton permeation to ~5 Å-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 crystals5 with similar nmscale channels, which could help close the materials gap in proton-conducting applications.