Porous Graphene Membranes for Solute Separation …
157
different pressure, channel height and electric field intensity (Fig. 6b). The results
showed that the behaviors of salt ions and water molecules were diverse, namely, the
number of water molecules in the nanoscale channel was not affected by the driving
pressure, whereas the number of ions was sensitive to the pressure when the channel
thickness was small. The salt rejection decreased with the increase of the driving
pressure and channel height and it increased with the increase of the vertical electric
field. Lohrasebi et al. [3] also used multilayer graphene membranes as selective ion
transport membranes under the action of the applied external electric field. They
performed MD simulations and measured the ion selectivity, salt rejection rate and
water permeability. They used 3 box, 2 bilayer membrane with a rigid piston (for
applying pressure), plus 2 external electric fields in the opposite direction applied to
the porous graphene membranes. Four levels of external electric fields from 0.1 to
100 mV/Å were applied along with a pore radius of 4 Å and different width levels
between bilayer membranes. The pore size was designed according to the hydrated
ions so that no ion could pass through the membrane without applying the electric
field. The results indicated that the increase of the applied electric field improved ion
selectivity and the salt rejection rate was found to be more than 94% under 10 mV/Å.
4 Research Gaps
Graphene and its derivatives are naturally supposed to be a very promising candidate
for the separation membranes owing to their thinnest thickness so far, because the
membrane permeance, quantifying the permeation abilities of molecules and ions, is
generally accepted to be inversely proportional to the thickness of membranes. Such
high-efficiency membranes are very crucial for the energy-saving membrane separation technology, which is widely applied in the separation industries such as water
purification. Currently, two kinds of graphene-based membranes are proposed for the
separation technology; the first is NPG membrane while the second is graphene oxide
membrane. The mechanism of NPG membranes is basically the size-sieving effects
of the selective nanopores, while that of graphene oxide membranes is the selective
molecular and ionic transport through the interlayer channels among the laminated
graphene oxide sheets and the defects in the graphene oxide sheets. The molecular permeance of NPG membranes is very high comparing to the graphene oxide
membranes with a relatively thick thickness of layered structures. The fabrication
process of NPG membranes is very complex although the top-down and bottom-up
methods both seem to be effective; while that of graphene oxide membranes is relatively simple. In short, the advantage of NPG-based membranes is their ultra-high
molecular permeance and that of graphene oxide membranes is easy of fabrication.
People are trying to make the graphene-based membranes have the advantages of
both NPG and graphene oxide membranes, namely high molecular permeance and
ease of fabrication.
157
different pressure, channel height and electric field intensity (Fig. 6b). The results
showed that the behaviors of salt ions and water molecules were diverse, namely, the
number of water molecules in the nanoscale channel was not affected by the driving
pressure, whereas the number of ions was sensitive to the pressure when the channel
thickness was small. The salt rejection decreased with the increase of the driving
pressure and channel height and it increased with the increase of the vertical electric
field. Lohrasebi et al. [3] also used multilayer graphene membranes as selective ion
transport membranes under the action of the applied external electric field. They
performed MD simulations and measured the ion selectivity, salt rejection rate and
water permeability. They used 3 box, 2 bilayer membrane with a rigid piston (for
applying pressure), plus 2 external electric fields in the opposite direction applied to
the porous graphene membranes. Four levels of external electric fields from 0.1 to
100 mV/Å were applied along with a pore radius of 4 Å and different width levels
between bilayer membranes. The pore size was designed according to the hydrated
ions so that no ion could pass through the membrane without applying the electric
field. The results indicated that the increase of the applied electric field improved ion
selectivity and the salt rejection rate was found to be more than 94% under 10 mV/Å.
4 Research Gaps
Graphene and its derivatives are naturally supposed to be a very promising candidate
for the separation membranes owing to their thinnest thickness so far, because the
membrane permeance, quantifying the permeation abilities of molecules and ions, is
generally accepted to be inversely proportional to the thickness of membranes. Such
high-efficiency membranes are very crucial for the energy-saving membrane separation technology, which is widely applied in the separation industries such as water
purification. Currently, two kinds of graphene-based membranes are proposed for the
separation technology; the first is NPG membrane while the second is graphene oxide
membrane. The mechanism of NPG membranes is basically the size-sieving effects
of the selective nanopores, while that of graphene oxide membranes is the selective
molecular and ionic transport through the interlayer channels among the laminated
graphene oxide sheets and the defects in the graphene oxide sheets. The molecular permeance of NPG membranes is very high comparing to the graphene oxide
membranes with a relatively thick thickness of layered structures. The fabrication
process of NPG membranes is very complex although the top-down and bottom-up
methods both seem to be effective; while that of graphene oxide membranes is relatively simple. In short, the advantage of NPG-based membranes is their ultra-high
molecular permeance and that of graphene oxide membranes is easy of fabrication.
People are trying to make the graphene-based membranes have the advantages of
both NPG and graphene oxide membranes, namely high molecular permeance and
ease of fabrication.