Smart membrane technologies for precisely controlling the transport of water molecules are of intense interest for a variety of R&D areas, such as healthcare and water purification. In traditional polymeric membranes, control of water permeation is generally through structural modulation or surface physicochemical modification. Another long-sought-after approach is electrical control of water flow from complete blocking to ultrafast permeation. This has been experimentally demonstrated by these authors previously. However, the fundamentals for such control based on external electrical stimuli are debated with conflicted theories.
In the present work, coauthored by the graphene Nobel Laureates, µm-thick graphene oxide (GO) membranes are demonstrated to exhibit precise, reversible, and stable electrically controlled water permeation after initial electrical conditioning, thus showing promise for potential large-scale industrial applications. The initial electrical conditioning is confirmed to occur through formation of conductive filaments in the membranes via controllable electrical breakdown. High-resolution PeakForce TUNA images reveal that these filaments have diameters of <50 nm and membranes have a filament density of ~107 cm-2.
In addition to AFM, thermal, microscopic and spectroscopic techniques, together with molecular dynamics simulation, are employed in this work to address the debates on existing theories. The authors prove that the induced electric field around the current-carrying conductive filament dissociates the H2O molecules into H3O+ and OH- ions under applied bias. These ions form large hydrated clusters due to their strong interactions from the strong hydrogen bonds between the ions and the surrounding water, which reduces or impedes the water transport depending on the applied bias.