Nano-Porous Graphene as Free-Standing Membranes
51
Water molecules would pass through the nanopores of the ideal membrane much
faster than across the polymeric membrane due to the different transport mechanisms,
active layer thickness and support layer thickness in addition to support layer porosity
(Fig. 4a). Therefore, the performance of the ideal membrane is already better in terms
of salt rejection and water permeation. After a few days or weeks (Fig. 4b), there
would be an accumulation of salt ions on the surface of both membranes. However,
there would also be an accumulation of salt ions in the pores of the support layer
of polymeric membranes due to the leakage (however low) in time. This would not
happen for the ideal membrane if its thickness and porosity are carefully designed.
A gentle cross-flow of water or other chemicals (Fig. 4c) would clean the flat surface
of the ideal membrane while the rough surface of the polymeric membrane would
not be cleaned entirely because of the surface roughness that traps ions. The support
layer with its complicated porous structure is not easily cleaned either, therefore
the polymeric membrane may be fouled and not useable anymore. That is if the
membrane is not damaged by chlorine during the cleaning process. In contrast, the
ideal membrane is completely clean and can be reused further in the RO process. It is
worthy to note that in polyamide membranes it is hard to synthesize films thinner than
100 nm because the film thickness is governed by the mass transfer of the diamine
to the organic phase during interfacial polymerization [94]. It is also uncertain that a
significantly thinner polyamide film would exhibit high enough salt rejection at any
rate [43].
With this introduction on the advantages and prospects of nano-porous atomically thin membranes over polymeric membranes for RO technology, it is inevitable
to explore nano-porous graphene as the mother of the 2D materials. Graphene-based
desalination membranes have been envisaged to serve as selective layers and operated as molecular sieves with the size-based exclusion of undesired solutes in two
forms: nano-porous graphene and graphene-based frameworks [15]. Graphene-based
framework membranes comprise a multi-layered stack of finite-sized graphene sheets
[95–97], formed as laminates through vacuum filtration or layer-by-layer deposition
of graphene oxide and differ in morphology and water transport mechanism from
nano-porous graphene [15] which are comprehensively explored in the literature and
are not the scope of this chapter.
3 Computer Simulations on Nano-Porous Graphene
Membranes; Water Desalination Perspective
In this section, we will review the recent computational findings on pristine and
functionalized nano-porous monolayer graphene membranes, inspect and summarize
simulation considerations and explore the latest results on the mechanical stability of
these membranes in a RO system. Further in this section, we will discuss simulation
achievements in the bilayer and multilayer nano-porous graphene and the raised
concerns in adding additional layers to permeability and selectivity. Finally, we will
51
Water molecules would pass through the nanopores of the ideal membrane much
faster than across the polymeric membrane due to the different transport mechanisms,
active layer thickness and support layer thickness in addition to support layer porosity
(Fig. 4a). Therefore, the performance of the ideal membrane is already better in terms
of salt rejection and water permeation. After a few days or weeks (Fig. 4b), there
would be an accumulation of salt ions on the surface of both membranes. However,
there would also be an accumulation of salt ions in the pores of the support layer
of polymeric membranes due to the leakage (however low) in time. This would not
happen for the ideal membrane if its thickness and porosity are carefully designed.
A gentle cross-flow of water or other chemicals (Fig. 4c) would clean the flat surface
of the ideal membrane while the rough surface of the polymeric membrane would
not be cleaned entirely because of the surface roughness that traps ions. The support
layer with its complicated porous structure is not easily cleaned either, therefore
the polymeric membrane may be fouled and not useable anymore. That is if the
membrane is not damaged by chlorine during the cleaning process. In contrast, the
ideal membrane is completely clean and can be reused further in the RO process. It is
worthy to note that in polyamide membranes it is hard to synthesize films thinner than
100 nm because the film thickness is governed by the mass transfer of the diamine
to the organic phase during interfacial polymerization [94]. It is also uncertain that a
significantly thinner polyamide film would exhibit high enough salt rejection at any
rate [43].
With this introduction on the advantages and prospects of nano-porous atomically thin membranes over polymeric membranes for RO technology, it is inevitable
to explore nano-porous graphene as the mother of the 2D materials. Graphene-based
desalination membranes have been envisaged to serve as selective layers and operated as molecular sieves with the size-based exclusion of undesired solutes in two
forms: nano-porous graphene and graphene-based frameworks [15]. Graphene-based
framework membranes comprise a multi-layered stack of finite-sized graphene sheets
[95–97], formed as laminates through vacuum filtration or layer-by-layer deposition
of graphene oxide and differ in morphology and water transport mechanism from
nano-porous graphene [15] which are comprehensively explored in the literature and
are not the scope of this chapter.
3 Computer Simulations on Nano-Porous Graphene
Membranes; Water Desalination Perspective
In this section, we will review the recent computational findings on pristine and
functionalized nano-porous monolayer graphene membranes, inspect and summarize
simulation considerations and explore the latest results on the mechanical stability of
these membranes in a RO system. Further in this section, we will discuss simulation
achievements in the bilayer and multilayer nano-porous graphene and the raised
concerns in adding additional layers to permeability and selectivity. Finally, we will