Gas Diffusion and Sorption in Graphene-based Membranes

Abstract

This thesis proposes the selective gas transport through graphene-based membranes, such as graphene, graphene oxide (GO) and reduced graphene oxide (RGO) in terms of diffusion and sorption. To use graphene-based materials as a membrane, two distinct research directions were considered: (1) creating pores in basal plane and (2) engineering gas diffusion channels in interlayer space. The gas transport through nano-channels in graphene-based membrane exhibits very intriguing separation properties. This dissertation will cover state-of-the-art of graphene, graphene oxide and reduced graphene oxide membranes, and also provide a material guideline on future research direction suitable for practical membrane applications. Chapter 2 describes the gas permeation through artificially stacked multi-layered, polycrystalline graphene on various polymer substrates. The grain boundaries in polycrystalline graphene are mechanically weak, so cleavages along with these grain boundaries happen during the transfer process. As expected, the gas permeability in single layer graphene/PTMSP film was not so significantly reduced. Also, no change in gas selectivity is an evidence that cleavage is still too large to separate molecules effectively. The more graphene sheets were transferred on PTMSP films by using the same procedure, anticipating that large defects or cleavages would be significantly covered with the random manner. As a result, the selective gas transport phenomena occurred through the graphene layers. Chapter 3 studies the gas transport properties of GO and RGO thick and thin membrane with different platelet sizes. In general, the layered materials have been considered as a gas barrier materials due to its long diffusion path length or high aspect ratio. However, the diffusion path length can be reduced by controlling platelet size and thickness. As expected, the gas permeability of GO and RGO gradually increased with decreasing platelet size due to the reduction of diffusion path length. The separation mechanism depends on size-sieving in both of thick and thin film. However, in thin composite (TFC) membrane, especially GO, the CO2 molecules diffuse abnormally through GO layers with highest permeance due to intercalated water in GO layers. In chapter 4, gas sorption properties of GO and RGO were investigated in their different forms, such as powders and films. Although the amount of N2 sorption was higher in that of RGO than GO in the powder form, the amount of CO2 adsorbed in GO powder was much higher than that of RGO powder due to the presence of accessible micropores and active sorption sites such as oxygen-containing functional groups with high CO2, leading to high CO2 sorption ability in GO powder. In the film form, CO2 molecules at low pressures are hard to access the interlayer space of GO and RGO due to highly interlocked lamellar structure. However, at high pressures above 20 bar, the amount of CO2 sorption dramatically increased even in RGO as well as GO, owing to swelling of interlayer spacing by intercalated and adsorbed CO2 molecules. In addition, adsorbed CO2 in GO still remained in GO layers, and leading to the change of GO layer structure disorder and interlayer space changing after desorption procedure at atmospheric pressure. Chapter 5 describes on the role of intercalated water molecules in GO layers including CO2 and N2 gas mixture transport. The water molecules can interact preferentially with oxygen-containing functional groups in GO and forming water cluster in GO layers in hydrated state. Most of polymeric membrane, the separation performances are degraded by the presence of water vapor in feed side. However, the CO2 permeance was significantly enhanced by intercalated water in GO layers in the humidified feed. However, N2 has relatively low solubility in water, and the diffusion could be hindered by the presence water clusters in hydrated GO layers. In addition, the structure stability was enhanced by intercalating amphiphilic bile salt which can act as a strong adhesive. By considering that the flue gas contains water molecules, water-enhanced CO2 separation in GO membrane will be a great advantage for post-combustion CO2 capture process