Atomically thin micas as proton-conducting membranes.

Area of Science:

• Materials Science
• Solid-State Chemistry
• Nanotechnology

Background:

• Graphene and hexagonal boron nitride (hBN) monolayers are known proton conductors.
• Proton conductivity in thicker two-dimensional (2D) materials typically decreases significantly.
• This limitation suggested only atomic-layer-thin materials were suitable for proton-conducting membranes.

Purpose of the Study:

• To investigate the proton conductivity of few-layer micas after ion exchange.
• To explore the potential of thicker 2D materials as high-performance proton conductors.
• To address the materials gap in proton conductivity at elevated temperatures.

Main Methods:

• Ion exchange of native cations in few-layer micas with protons.
• Measurement of areal proton conductivity.
• Characterization of ion-exchanged mica structure and channel properties.

Main Results:

• Ion-exchanged few-layer micas demonstrate significantly higher areal proton conductivity than graphene and hBN (1-2 orders of magnitude greater).
• High proton conductivity is maintained within the 100°C to 500°C temperature range, bridging a known materials gap.
• Proton-exchanged monolayer micas achieve conductivities exceeding 100 S cm⁻² at 500°C, meeting industrial roadmap requirements.

Conclusions:

• Few-layer micas, ion-exchanged for protons, are excellent proton conductors, even when thicker than atomic layers.
• The high conductivity is attributed to ~5-Å-wide tubular channels containing hydroxyl groups after ion exchange.
• This discovery suggests other 2D materials with similar nanometer-scale channels could also be developed for advanced proton-conducting applications.