Comparison of self-standing and supported graphene oxide membranes prepared by simple filtration: Gas and vapor separation, pore structure and stability

A large series of self-standing and supported graphene oxide (GO) membranes were prepared via a facile synthetic approach involving the filtration of GO suspensions through polymeric and ceramic macroporous filters. Our overall aim was to develop a membrane that would be almost impermeable to helium and hydrogen, exhibiting in parallel very high water vapor and moderate alcohol vapor permeability, properties that constitute this type of membranes very promising for pervaporation and gas separation applications. Several of the derived self-standing membranes, especially those developed using aqueous GO suspensions of low concentration in GO, have met the above mentioned requirements. In particular, the development of highly efficient GO membranes using suspensions of low concentration (≤1.0 g/L in order to achieve individual GO flakes rather than GO stacks) and of high volume (>50 mL to avoid very thin membranes which in turn incorporate defects) is straightforward and independent of the filtration rate, while slow filtration rates lead to better results when employing higher GO concentrations (1.5 g/L), but never to a membrane with purely molecular sieving characteristics. Small-angle X-ray scattering (SAXS) measurements indicated better GO's sheet packing and, thus, smaller pore size/network available for gas diffusion in membranes with smoother surfaces. In addition, the in-plane distance between adjacent GO sheets (especially on the outermost layer of the membrane), and also the size of GO stacks, were found to have more impact on the performance of the membranes than the respective d-distance determined by X-ray diffraction (XRD). Overall, a self-standing membrane developed by using anodised alumina (AAO) filter exhibited exceptional stability coupled with an excellent water vapor flux and water/alcohol selectivity.