Mild temperature-gas separation performance of graphene oxide membranes for extended period: micropore to meso- and macropore readjustments and the fate of membranes under the influence of dynamic graphene oxide changes

Abstract

Graphene oxide (GO) constitutes an ideal precursor for the creation of ultra-thin films of highly adaptable pore structure that could serve as microporous and mesoporous membranes in gas separation applications. The GO platform furnishes abundant opportunities for adjusting the gas diffusion pathways of membranes, either through the interlayer galleries and the defects of the basal plane or through the non-interlocked edges of GO stacks. Along with the pore structural adaptability of membranes, primary requirement for a sustainable gas separation process is their stable performance. However, the fate of GO membranes when integrated in a gas separation process at mild temperatures (<140 °C) and for extended periods of several months, was never studied up to date. Hence, the shortage of reports on the output of prolonged experimental campaigns raises significant doubts regarding the applicability of GO membranes. This work focused on this part of missing information, revealing that structural readjustments, which are mostly related with the loss of oxygenated functional groups during prolonged exposure at low to moderate temperatures, have a tremendous impact on the permeability properties. Before water removal all the membranes behaved as impermeable barriers but after their conditioning in high vacuum, they were transformed to effective molecular sieves, exhibiting high separation factors for small gas molecules. Structural changes started at 60 °C and escalated at 90 °C. These are reflected by a 2–3 orders of magnitude enhancement of the permeability factors and a change of the gas diffusion mechanism from microporous to Knudsen and eventually to viscous flow.