Dielectric response and proton transport in water confined in graphene oxide

Graphene oxide (GO) membranes possess a hierarchical microstructure, with well-ordered crystalline lamellae combining to form a macroscopic membrane. Water can intercalate in GO either in the sub-nanometer interlayer spaces or in the gaps between the lamellae known as voids; distinguishing the contribution of these two has been challenging. Addressing this challenge, we systematically study various properties of GO membranes exposed to controlled humidity levels ranging from 0% to 90% RH. Thickness-dependent dynamic vapor sorption is used to quantify the water content under different humidity environments. Complementing the vapor sorption studies, the AC impedance response of the GO membrane is determined at different humidity values. Our findings suggest that (a) most water gets absorbed in interlayer spaces at low humidity (<25% RH), (b) the fraction of water in the void spaces increases with RH%, (c) the lower bound for the dielectric constant of confined water is estimated to be epsilon(water) > 17, and (d) the conductivity increases by 5 to 6 orders of magnitude over a narrow range of water content (13 wt% to 31 wt%). The rapid increase in conductivity over a narrow range of water content suggests a percolative process for the protons. The dielectric constant estimates suggest that confined water behaves distinctly differently in a hydrophilic environment than in a hydrophobic one.

Automated summary

Graphene oxide (GO) membranes with hierarchical microstructure have been studied under controlled humidity levels. Water content in the interlayer spaces and the voids between crystalline lamellae in GO membranes was quantified using dynamic vapor sorption and AC impedance response methods. The study reveals that water is primarily absorbed in the interlayer spaces at low humidity, while the fraction of water in the void spaces increases with humidity. The dielectric constant of confined water is estimated to be greater than 17, and the conductivity increases significantly within a narrow range of water content. The findings suggest a percolative process for protons and distinct behavior of confined water in hydrophilic and hydrophobic environments.