Selective transport of water molecules through interlayer spaces in graphite

TL;DR

This study demonstrates electrochemical expansion of graphite interlayer spaces to significantly enhance water permeability while maintaining ion selectivity, advancing applications in filtration and energy storage.

Contribution

It introduces a novel electrochemical method to expand graphite layers, enabling high water permeability and ion selectivity, with detailed atomic-scale insights into transport mechanisms.

Findings

Water conductivity increased by several orders of magnitude.

Ion permeation remains weak and is ion-specific.

Spectroscopy reveals cation-π interactions influencing transport.

Abstract

Interlayer space in graphite is impermeable to ions and molecules, including protons. Its controlled expansion would find several applications in desalination, gas purification, high-density batteries, etc. In the past, metal intercalation has been used to modify graphitic interlayer spaces; however, resultant intercalation compounds are unstable in water. Here, we successfully expanded graphite interlayer spaces by intercalating aqueous KCl ions electrochemically. The water conductivity shows several orders of enhancement when compared to unintercalated graphite. Water evaporation experiments further confirm the high permeation rate. There is weak ion permeation through interlayer spaces, up to the highest chloride concentration of 1 M, an indication of sterically limited transport. In these very few transported ions, we observe hydration energy-dependent selectivity between salt ions. These strongly suggest a soft ball model of steric exclusion, which is rarely reported. Our spectroscopy studies provide clear evidence for cation-{\pi} interactions, though weak anion-{\pi} interactions were also detectable. These findings improve our understanding of molecular and ionic transport at the atomic scale.