Carbon, 2020, January; 156:119-129

Hirunpinyopas, W, Iamprasertkun, P, Bissett, M, & Dryfe, R.

Graphene oxide (GO) and reduced graphene oxide (rGO) membranes have attracted significant attention as a potential technology for energy storage, gas separation, and water purification applications. However, these membranes have a significant drawback as they became swollen, hence unstable, after exposure to aqueous solutions. Here, we describe membranes produced from graphene prepared by liquid phase exfoliation, possessing a low oxygen content, unlike the GO/rGO systems typically used, and demonstrate their applicability for ion sieving in aqueous solutions. These low oxygen content graphene membranes formed from flakes of varying size were used to determine the effect of flake morphology on ion transport. Interestingly decreasing flake length and thickness leads to an increase in the number and tortuosity of nanochannels between the layers, resulting in a significant reduction of ion transport. The smaller flakes show an increased surface charge, due to the level of defects, which impedes chloride mobility allowing for both physical sieving and charge repulsion. Moreover, the graphene membranes reported here exhibit excellent Na+ rejection properties (∼97%) with water permeance ∼10 times higher than those reported for GO membranes, while demonstrating high stability in aqueous solutions with no observed swelling. These materials are therefore extremely promising for future applications in water purification.