Master of Engineering Science
Chemical and Biochemical Engineering
Boutilier, Michael S. H.
Membrane separation applications such as water desalination and carbon capture require high permeance and selectivity. For such processes, nanoporous graphene membranes promise 100-fold higher permeance at comparable selectivity to conventional polymer membranes, but remain under development. This thesis reports fluid permeance through both simulated and experimental graphene nanopores. Molecular dynamics simulations were performed to investigate liquid advection-diffusion through graphene nanopores and how the transport rates differ from continuum predictions. Furthermore, a technique for measuring the gas permeance of nanoscopic areas of graphene was developed. Here, a single layer of graphene seals a ~10 nm diameter hole in a multi-layer graphene nanoballoon over a pressurized cavity and atomic force microscopy is used to track its deflection over time. The results demonstrate helium/air selectivity through defects in five-layer graphene. We anticipate that this measurement technique can be adapted to determine the permeance of other atomically thin materials as well.
Summary for Lay Audience
Fluid separation strategies are necessary in industrial processes to meet demands including product output, standards of quality, emissions regulations, among other things. Techniques such as water desalination and carbon capture separate fluid components by forcing them across a porous membrane. A pressure gradient is used to control the direction of fluid flow and undesired materials are removed based on molecular size, spatial arrangement, or other interactions with the membrane. Membrane performance is measured by permeability and selectivity, which relate to how well a fluid can flow through the membrane and how well the membrane can remove unwanted components of the fluid, respectively. Typical membranes are unable to achieve both high permeability and high selectivity and may face problems with chemical reactivity and mechanical strength. Alternatively, single layer graphene provides low resistance to flow, supports selective nanopores, and is mechanically and chemically robust given its size. We analyzed the effects of pore size and applied pressure on various fluids across simulated and experimental single layer graphene nanopores. This thesis reports fluid permeation rates across 1-10 nm diameter pores at applied pressures up to 200 atm (simulated) and 1 atm (experimental). Simulations were performed to understand liquid flow through graphene nanopores, and how these flow rates differ from predictions for larger-scale pores. Furthermore, we discuss the development of a method for measuring gas flow rates through nanoscale areas of atomically thin materials. The results demonstrate helium/air selectivity through defects in five-layer graphene. We anticipate that this measurement technique can be adapted to determine the permeance of other atomically thin materials as well. However, significant development work is still required if graphene is to compete with current membrane technology.
Wong, Anika O. K., "Atomically Thin Nanoporous Graphene Membranes for Fluid Separation" (2021). Electronic Thesis and Dissertation Repository. 8135.