Porous Graphene Membranes for Solute Separation …
149
on monolayer CVD graphene and the measurement results showed that the rejection
rates of KCl ions were 57 and 40% for two different membrane samples. However, the
rejection rates can be increased up to 84% by sealing the intrinsic defects on graphene
through interfacial polymerization. Li et al. [37] prepared the NPG membranes with
the carboxyl groups functionalized pores by using the irradiation with swift heavy
ions and investigated their ionic sieving performance; they found that the permeation
rate of ions had a great dependence on the temperature and H
+ concentration in the
solutions and that the external electric fields can enhance the permeation flux of
ions through the NPG membranes. A four-layer NPG membrane with nanopores
generated by using metal oxide nanoparticles at high temperature was fabricated by
Wei et al. [38] and they indicated that the water flux can be up to 4600 L · m
−2
· h
−1 for the pores of sizes 50 nm and of density 1.0 × 10
7 cm
−2 at the driving
pressure of 0.2 bar. In short, the macroscopic NPG sheets have been synthesized and
demonstrated to be competent to the ionic separations [2, 27, 39], which is a big
step toward the industrial applications of graphene-based membranes. Moreover, the
pore sizes of the NPG sheets can be precisely controlled and the high ion rejection
and water permeation flux can be achieved [40].
The group of Prof. Karnik at the Massachusetts Institute of Technology made a
significant contribution to the development of NPG water purification membranes
[41–46]. They fabricated large-scale atomically thick NPG membranes and demonstrated their potentials for purifying kinds of ions and solutes. Meanwhile, they
concluded that the single-layer graphene membranes can withstand ultrahigh applied
pressure by monitoring the failure of centimeter-scale NPG membranes on porous
supports. Although the graphene-based membranes work very well in laboratory,
many challenges remain to make the cutting-edge membranes practicable in industry.
The main challenges for the industrial-scale graphene-based membranes include the
fabrication of large-scale graphene-based membranes with high mechanical stability
and free of intricate defects and artificial damages, the generation of high-density
pores in the graphene sheets with precisely controlled sizes and compositions, and
other general challenges, such as supporting, blocking, fouling and concentration
polarization. Most recently, the group of Prof. Quan Yuan at Wuhan University and
the group of Prof. Xiangfeng Duan at the University of California Los Angeles cooperatively demonstrated a large-area hybrid membrane with graphene-nanomesh and
single-walled carbon nanotubes [47]. Their research indicated that such membranes
exhibited a high transport rate of water and a high rejection of ions owing to the highdensity selective nanopores in the graphene nanomesh. Meanwhile, these membranes
could well maintain the structural integrity and had a high mechanical strength under
the support of microscopic single-walled carbon nanotubes. Excitingly, the hybrid
membranes demonstrated in this work exhibited the advantages of both NPG and
graphene oxide membranes, namely high molecular and ionic permeance owing
to the permeable pores in the graphene nanomesh and they are easy to scale-up
with the help of macroscopic single-walled carbon nanotube networks. Additionally, these novel membranes overcame the key challenges faced currently for the
graphene-based membranes—fabrication of large-area graphene sheets with highdensity selectively permeable pores and high mechanical strength. This work further
149
on monolayer CVD graphene and the measurement results showed that the rejection
rates of KCl ions were 57 and 40% for two different membrane samples. However, the
rejection rates can be increased up to 84% by sealing the intrinsic defects on graphene
through interfacial polymerization. Li et al. [37] prepared the NPG membranes with
the carboxyl groups functionalized pores by using the irradiation with swift heavy
ions and investigated their ionic sieving performance; they found that the permeation
rate of ions had a great dependence on the temperature and H
+ concentration in the
solutions and that the external electric fields can enhance the permeation flux of
ions through the NPG membranes. A four-layer NPG membrane with nanopores
generated by using metal oxide nanoparticles at high temperature was fabricated by
Wei et al. [38] and they indicated that the water flux can be up to 4600 L · m
−2
· h
−1 for the pores of sizes 50 nm and of density 1.0 × 10
7 cm
−2 at the driving
pressure of 0.2 bar. In short, the macroscopic NPG sheets have been synthesized and
demonstrated to be competent to the ionic separations [2, 27, 39], which is a big
step toward the industrial applications of graphene-based membranes. Moreover, the
pore sizes of the NPG sheets can be precisely controlled and the high ion rejection
and water permeation flux can be achieved [40].
The group of Prof. Karnik at the Massachusetts Institute of Technology made a
significant contribution to the development of NPG water purification membranes
[41–46]. They fabricated large-scale atomically thick NPG membranes and demonstrated their potentials for purifying kinds of ions and solutes. Meanwhile, they
concluded that the single-layer graphene membranes can withstand ultrahigh applied
pressure by monitoring the failure of centimeter-scale NPG membranes on porous
supports. Although the graphene-based membranes work very well in laboratory,
many challenges remain to make the cutting-edge membranes practicable in industry.
The main challenges for the industrial-scale graphene-based membranes include the
fabrication of large-scale graphene-based membranes with high mechanical stability
and free of intricate defects and artificial damages, the generation of high-density
pores in the graphene sheets with precisely controlled sizes and compositions, and
other general challenges, such as supporting, blocking, fouling and concentration
polarization. Most recently, the group of Prof. Quan Yuan at Wuhan University and
the group of Prof. Xiangfeng Duan at the University of California Los Angeles cooperatively demonstrated a large-area hybrid membrane with graphene-nanomesh and
single-walled carbon nanotubes [47]. Their research indicated that such membranes
exhibited a high transport rate of water and a high rejection of ions owing to the highdensity selective nanopores in the graphene nanomesh. Meanwhile, these membranes
could well maintain the structural integrity and had a high mechanical strength under
the support of microscopic single-walled carbon nanotubes. Excitingly, the hybrid
membranes demonstrated in this work exhibited the advantages of both NPG and
graphene oxide membranes, namely high molecular and ionic permeance owing
to the permeable pores in the graphene nanomesh and they are easy to scale-up
with the help of macroscopic single-walled carbon nanotube networks. Additionally, these novel membranes overcame the key challenges faced currently for the
graphene-based membranes—fabrication of large-area graphene sheets with highdensity selectively permeable pores and high mechanical strength. This work further