Electrochemical Opening of Impermeable Nanochannels in Laminar Graphene Membranes for Ultrafast Nanofiltration

Reduced graphene oxide (rGO) could be theoretically used to construct highly permeable laminar membranes with nearly frictionless nanochannels for water treatment. However, their pristine (sp2 C-C) regions usually restack into impermeable channels as a result of van der Waals interactions, resulting in a much low permeance. In this study, we demonstrate that the restacked regions could be electrochemically expanded to form ultrafast water transport nanochannels by providing a low positive potential (e.g., +1.00 V vs SCE) to the rGO membrane. Experimental investigations indicate that the structural expansion is attributed to the intercalation of water molecules into the restacked regions, driven by hydrogen bond interactions between water molecules and hydroxyl groups that are electrochemically produced on edges of rGO nanosheets. The structural expansion could be promoted by weakening the graphene-OH- interactions through intermittent application of the potential. As a result of more ultrafast water transport nanochannels available, the electrochemically treated rGO membranes could have a permeance 2 orders of magnitude higher than that of the pristine one and similar to 3 times higher than that of graphene oxide membranes. Because of their smaller average pore size, the rGO membranes also have a higher ionic/molecular rejection performance than graphene oxide membranes.

Automated summary

In this study, it was found that electrochemical treatment can expand the restacked regions of reduced graphene oxide (rGO) membranes, leading to the formation of ultrafast water transport nanochannels. The expansion is driven by hydrogen bond interactions between water molecules and electrochemically produced hydroxyl groups on the edges of rGO nanosheets. The treated rGO membranes exhibit a permeance 2 orders of magnitude higher than pristine rGO membranes and about 3 times higher than graphene oxide membranes. Additionally, the rGO membranes also show higher ionic/molecular rejection performance due to their smaller average pore size.