Bidirectional Transport Phenomenon of Ions in Electric Fields Due to the Cluster Formation in Two-Dimensional Graphene Channels

In aqueous solution, cations and anions are known to be moving oppositely and unidirectionally in electric fields. In this work, we observe an abnormal bidirectional ion transport phenomenon in electric fields because of the formation of ion clusters in two-dimensional graphene channels. A series of molecular dynamic simulations reveal that the separated ions within two-dimensional graphene channels will rapidly assemble into individual clusters under the electric field, originating from the confinement-induced effective ionic interactions as well as the strongly compressed hydration shells of ions. Notably, owing to the competition between ionic interaction and electric field, the structure of ion clusters changes from a square-unit pattern to chain-like with the increase in electric field, and the cluster size displays a maximum behavior. As a result, the motion of cations and anions in these clusters will be strongly coupled to each other, leading to the bidirectional transport phenomenon. Furthermore, these ion clusters will disappear abruptly with the increase in graphene layer distance, and thus the ion transport becomes unidirectional as normal. Our findings shed light on the unusual ion dynamics in two-dimensional channels and provide a potential avenue for the design of novel nanofluidic devices.