Nano-Porous Graphene as Free-Standing Membranes
61
Fig. 6 Snapshot of typical bilayer simulation with effective parameters on the desalination
performance
[99]. They also found that the bilayer nano-porous graphene membrane is capable of
rejecting salt for sufficiently small nanopores (<4 Å) where salt rejection decreases
for greater pore alignment, larger layer spacing, or higher pressure. Pore offset and
layer separation considerably influences the membrane’s ability to reject salt. In the
limit of large H, it is expected that the two layers act as independent membranes. For
N-layer membranes, the permeability can be predicted using a resistance-in-series
model while increasing the number of layers results in greater salt rejection due to
a higher diffusion resistance for ions. Although nano-porous graphene membrane’s
permeability will drop linearly as the number of graphene layers increases, the
resulting permeability loss may still represent an acceptable trade-off [99].
Zhang et al. in [134] also investigated design parameters in multilayer nano-porous
graphene systems for water desalination. In addition to the offset O (12.6 Å) between
the nanopores and the interlayer spacing H (7–12 Å) between graphene layers, they
explored the number of pores and layers, considered more realistic pore diameters
(8–12 Å), optimized the parameters and examined salt rejection for NaCl, MgCl 2
and CaCl 2 solutions. Similar to [99], they found that salt rejection is influenced
substantially by the interlayer spacing distance. This was due to the large free energy
between ions and graphene sheets and the relatively large size of the hydration layer
around the ions. With the increase in the number of graphene sheets, the water flow
rate was slowed down. Alternatively, the salt rejection rate increased notably with
the number of graphene sheets in agreement with the report by Mooney et al. [135].
Water flux per area per time analyses revealed the dependence of the desalination
performance on the number of pores. With the fluctuation of salt rejection around
100% owing to the trapping phenomenon of ions in small interlayer spacing, water
flux increased with the increase in the pore numbers.
61
Fig. 6 Snapshot of typical bilayer simulation with effective parameters on the desalination
performance
[99]. They also found that the bilayer nano-porous graphene membrane is capable of
rejecting salt for sufficiently small nanopores (<4 Å) where salt rejection decreases
for greater pore alignment, larger layer spacing, or higher pressure. Pore offset and
layer separation considerably influences the membrane’s ability to reject salt. In the
limit of large H, it is expected that the two layers act as independent membranes. For
N-layer membranes, the permeability can be predicted using a resistance-in-series
model while increasing the number of layers results in greater salt rejection due to
a higher diffusion resistance for ions. Although nano-porous graphene membrane’s
permeability will drop linearly as the number of graphene layers increases, the
resulting permeability loss may still represent an acceptable trade-off [99].
Zhang et al. in [134] also investigated design parameters in multilayer nano-porous
graphene systems for water desalination. In addition to the offset O (12.6 Å) between
the nanopores and the interlayer spacing H (7–12 Å) between graphene layers, they
explored the number of pores and layers, considered more realistic pore diameters
(8–12 Å), optimized the parameters and examined salt rejection for NaCl, MgCl 2
and CaCl 2 solutions. Similar to [99], they found that salt rejection is influenced
substantially by the interlayer spacing distance. This was due to the large free energy
between ions and graphene sheets and the relatively large size of the hydration layer
around the ions. With the increase in the number of graphene sheets, the water flow
rate was slowed down. Alternatively, the salt rejection rate increased notably with
the number of graphene sheets in agreement with the report by Mooney et al. [135].
Water flux per area per time analyses revealed the dependence of the desalination
performance on the number of pores. With the fluctuation of salt rejection around
100% owing to the trapping phenomenon of ions in small interlayer spacing, water
flux increased with the increase in the pore numbers.