Ion Permeation Through Thermally Tuned Microstructure in Graphene Oxide Membranes*

Graphene oxide (GO), an important material for sieving and filtration, has hierarchical microstructure; nanosheets of uniformly stacked GO with separations termed “nanochannels” form “lamellae”. In thick GO films, collections of lamellae are separated by voids. Current literature proposes ion permeation assuming GO as consisting only of lamellae. However, the contribution of voids cannot be ignored. In this work, electro-impedance spectroscopy was used to study GO films annealed between temperatures of 100 °C and 200 °C, aiming to discern contributions of nanochannels and voids. Responses were fit using equivalent circuits. Annealing increases microstructural disorder and thus increases number of voids and also decreases nanochannel width, as ascertained from X-ray diffraction. EIS detected two processes; decreasing charge transfer resistance R_ct (from 91 kΩm^-2 for as-prepared GO to 113 Ωm^-2 for GO annealed at 160 °C) and exponentially increasing permeation resistance R_P (from 112 Ωm^-2 for as-prepared GO to 1.5 kΩm^-2 for GO annealed at 160 °C). Increase in R_P despite increase in microstructural disorder suggests that nanochannels play a dominant role in ion permeation. Presence of strong dynamic for GO films annealed at 180 °C and 200 °C confirms this conclusion.