Perfect proton selectivity in ion transport through two-dimensional crystals

TL;DR

This study demonstrates that defect-free monolayers of graphene and hexagonal boron nitride exhibit perfect proton selectivity, allowing only protons to permeate while blocking other ions, with implications for separation technologies.

Contribution

It provides experimental evidence that defect-free 2D crystals are perfectly selective for protons, advancing understanding of ion transport in these materials.

Findings

Protons can permeate defect-free graphene and hBN monolayers.

Counterions like chloride are completely blocked.

Proton selectivity is perfect with no detectable counterion flow.

Abstract

Defect-free monolayers of graphene and hexagonal boron nitride were previously shown to be surprisingly permeable to thermal protons, despite being completely impenetrable to all gases. It remains untested whether small ions can permeate through the two-dimensional crystals. Here we show that mechanically exfoliated graphene and hexagonal boron nitride exhibit perfect Nernst selectivity such that only protons can permeate through, with no detectable flow of counterions. In the experiments, we used suspended monolayers that had few if any atomic-scale defects, as shown by gas permeation tests, and placed them to separate reservoirs filled with hydrochloric acid solutions. Protons accounted for all the electrical current and chloride ions were blocked. This result corroborates the previous conclusion that thermal protons can pierce defect-free two-dimensional crystals. Besides importance for theoretical developments, our results are also of interest for research on various separation technologies based on two-dimensional materials.