Atomically-thin micas as proton conducting membranes

TL;DR

This study demonstrates that few-layer micas, after ion exchange with protons, serve as highly efficient proton-conducting membranes with superior conductivity over graphene and hBN, especially at high temperatures, due to their unique tubular channels.

Contribution

It reveals that thick 2D micas can be excellent proton conductors after ion exchange, challenging the notion that only monolayer materials are suitable for proton conduction.

Findings

Proton-exchanged micas exhibit areal conductivity >100 S/cm² at 500°C.

Ion-exchanged micas outperform graphene and hBN in proton conductivity.

Proton permeation is facilitated by 5 Å-wide tubular channels in mica structure.

Abstract

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.