Beyond the Pore Size Limitation of a Nanoporous Graphene Monolayer Membrane for Water Desalination Assisted by an External Electric Field

Abstract

One efficient strategy for addressing the global water shortage is advanced membrane separation, which depends on the precise pore size being close to the hydrated ion size and other surface properties like charge and polarity. However, it is very difficult to fabricate uniform pores with diameters of <1 nm on monolayer membranes. By applying an electric field (bias voltage) perpendicular to the direction of the pressure difference, herein we demonstrate for the first time that a monolayer nanoporous graphene membrane with pores much larger than hydrated ions exhibits high salt rejection and allows a high rate of water transport. This theoretical proposal goes beyond the pore size limitation and shows promise for the design of high-performance reverse osmosis membranes.

Exploring Water/Ion Transport through the Polarized Lamellar Channels

“…, where C f and C p the concentrations of ions in the feed and permeate reservoirs, respectively, when half of the water molecules have passed from the feed reservoir to the permeate reservoir. 30 Consistent with the previous studies, no ions can pass through the narrower channel with an interlayer height of 0.68 nm, 11,14 the salt rejection reaches 100% with different partial charges. For the channel with a height of 0.88 nm, the salt rejection slightly decreases for a partial charge q = ±0.4 e and then increases when the partial charge reaches 1.0 e. For h = 1.02 nm, salt rejection obviously decreases as the partial charge increases.…”

“…Meanwhile, in order to reduce the solid−liquid−gas contact interaction, the permeate (right) reservoir contained 1178 water molecules. 11,30 The dimension of the h-BN sheets is 4.34 × 8.0 nm 2 , and the interlayer height of the lamellar h-BN channel is considered as 0.68, 0.88, and 1.02 nm, which corresponds to an effective channel height (h eff ) of 0.34, 0.54, and 0.68 nm. 13,14 The channel height (h) is defined as the distance between the centers of mass of each sheet.…”

“…The constant pressure applied on each piston is given by ΔP = nf/A, (feed: 100 MPa, permeate: 0.1 MPa), where f is the constant external force, A is the membrane area, and n is the total number of C atoms in each piston (Figure S1a). 11,30 It should be noted that the external constant pressure difference applied here is higher than the realistic pressure for industry application (a few MPa); however, it is very common in the nonequilibrium MD simulation to reduce thermal noise and enhance signal/noise ratio within a nanosecond time scale. 5,32 Due to the water flux being positively relative to the applied pressure, our results can be extended to the low pressure.…”

“…The model of the simulation system is shown in Figure a, where a lamellar nanochannel consists of two h-BN sheets, 4141 water molecules, 79 Na + , and 79 Cl – ions that were filled in the feed (left) reservoir. Meanwhile, in order to reduce the solid–liquid–gas contact interaction, the permeate (right) reservoir contained 1178 water molecules. , The dimension of the h-BN sheets is 4.34 × 8.0 nm 2 , and the interlayer height of the lamellar h-BN channel is considered as 0.68, 0.88, and 1.02 nm, which corresponds to an effective channel height ( h eff ) of 0.34, 0.54, and 0.68 nm. , The channel height ( h ) is defined as the distance between the centers of mass of each sheet. The effective channel height ( h eff ), considering the atomic size, is estimated by subtracting the vdW size of atoms from the nominal channel height .…”

Atomistic Simulations of the Permeability and Dynamic Transportation Characteristics of Diamond Nanochannels

“…However, when channel height was larger than ~8 Å, became less dependent on channel height, though its magnitude was still as high as 5.8 GPa (for Å). It was expected that further increasing the channel height would further suppress , due to the reduced influence from the channel walls [ 64 , 65 ]. By varying the channel length from 30 to 60 Å (keeping a constant channel height of 10.2 Å), it was found that channel length exerted insignificant impact on water flux and ion retention rate ( Figure 6 c).…”