Enhanced Permselectivity of Graphene-based Membranes through the Control of Nitrogen-bonding Configuration

Abstract

Studies of graphene-based membranes have focused on physicochemical properties that can supplant polymeric membrane for use in forward osmosis systems. However, recent research into graphene-based membranes has focused on mixtures of two or more different materials (e.g., graphene oxide and polymers) due to the need to reinforce underwater stability. Alternatives have included reduced forms including reduced graphene oxide to improve the stability and size-based selectivity, though have resulted in a narrow nanochannel that restricts water permeability. Herein, we propose the use of a nitrogen-doped graphene (NG) membrane to solve the trade-off between permeability and selectivity, investigating the bonding structure via hydrothermal reaction time. The NG membrane having a negatively polarized surface and narrow nanochannel exhibited significantly enhanced ion selectivity, based on the electrostatic interaction in the active layer facing the concentrated solution state. The pyridinic-N bonding structure improved the permeability and selectivity under a similar nanochannel size because of its hole defects, with the moderate energy barrier enabling water passage while blocking ions. Our experimental results confirm the possibility of fabricating novel graphene-based forward osmosis membranes by tailoring the nitrogen-bonding structure, which may significantly help develop a process for improving the scalability of membrane materials for water purification.