Graphene-based materials have attracted great interest to develop ultra-thin and high performance membranes due to their unique 2D structure, one-atomic thickness and high mechanical strength. While many studies have shown excellent performance of graphene-based membranes in fluid separations, scaling up of the membranes is still challenging. In addition, the transport mechanism in the membranes, especially in graphene oxide (GO) membranes is still not well understood.
In this study, graphene-based membranes were fabricated on hollow fibre substrates as a step forward to scaling up. High performance stainless steel-ceramic hollow fibres were developed while exploring suitable membrane substrates. GO and graphene membranes were fabricated on ceramic hollow fibres using two different approaches: solution-processing to produce GO membranes and chemical vapour deposition for graphene membranes. GO membranes were found unstable on hollow fibre substrates due to drying-related shrinkage. The stability problem was overcome by introducing a sacrificial layer during membrane synthesis. A post-treatment modification was carried out to improve the water permeation of GO membranes by creating extra microstructural defects in the membranes. Meanwhile, multi-layer graphene membranes were successfully synthesized on the nickel-plated ceramic hollow fibres. The graphene layer was exceptionally inert and could serve as an excellent protection layer against corrosive chemicals.
The water transport mechanisms in GO membranes in two different modes: pressure-driven filtration and pervaporation, were also studied. During filtration, GO membranes experienced a severe reduction in water permeation which was caused by the compaction of the loosely packed hierarchical microstructure. GO membranes showed a low steady state permeation flux because of the long transport path in the membranes. However, the water permeation was 1-2 orders of magnitude higher in pervaporation. The higher flux was due to the large capillary pressure in the interlayer space, which was induced by evaporation on the permeate side surface. In addition, an interface-related resistance was observed at the pore entrance of GO membranes when tested with water-ethanol mixture in pervaporation, which enabled the measurement of effective interfacial tension of miscible liquids using GO membranes.