Decoding ultrafast water transport in graphene oxide

Ultrafast water transport through the lamellar arrangement of flakes in graphene oxide is well established, but its origin has been debated in the literature. Here, we comprehensively examined water transport through graphene oxide membranes with sorption and permeation experiments across the entire RH interval, with varied membrane thicknesses and flake sizes. A series of experiments is necessitated since the diffusion process is influenced by the geometric tortuosity of pathways, degree of nano-confinement and associated surface interactions. The ratio of in-plane and out-of-plane Fickian diffusion coefficients, D_2D/D_out ∼ 10², which indicates that the water transport pathways have a tortuosity with an average step-size that is significantly smaller than the flake size. This is attributed to the presence of pinholes on the basal plane of graphene oxide. The thermodynamic correction to the Fickian D_2D is used to estimate the Maxwell-Stefan diffusion coefficient for in-plane transport of confined water, D_MS, which is a fundamental parameter representing molecular mobility. D_MS increases monotonically with the interlayer spacing of graphene oxide, which suggests a decrease in the frictional forces exerted by the confining walls on the diffusing molecules. Moreover, D_MS < 1.0 × 10^-9 m² s^-1 is lower than the self-diffusion coefficient of bulk water, and is typical of hydrophilic nanoporous systems. The large permeation rate is thus reconciled as a consequence of large sorption capacity, despite a modest D_MS. Finally, we also examined transport scenarios with single-sided condensation of water on the graphene oxide surface. A comparison of the permeation rates with the sorption-diffusion experiments is presented.