Atomically thin micas as proton-conducting membranes

Monolayers of graphene and hexagonal boron nitride (hBN) are highly permeable to thermal protons^1,2. 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 to 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 proton-exchanged 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 nanometre-scale channels, which could help close the materials gap in proton-conducting applications.