Surface slip on rotating graphene membrane
Membrane separation technology is dictated by the permeability-selectivity trade-off rule, because selectivity relies on membrane pore size being smaller than that of hydrated ions. We discovered a previously unknown mechanism that breaks the permeability-selectivity trade-off in using a rotating nanoporous graphene membrane with pores of 2 to 4 nanometers in diameter. The results show that the rotating membrane exhibits almost 100% salt rejection even when the pore size is larger than that of hydrated ions, and the surface slip at the liquid/graphene interface of rotating membrane enables concurrent ultra-selectivity and unprecedented high permeability. A novel concept of “temporal selectivity” is proposed to attribute the unconventional selectivity to the time difference between the ion’s penetration time through the pore and the bypass time required for ion’s sliding across the pore. The newly discovered temporal selectivity overcomes the limitation imposed by pore size and provokes a novel theory in designing high-performance membranes.
Fresh water is one of the most precious resources for the survival of human beings and other lives, but its availability is getting more and more strained because of population growth, climate change, and water pollution. Despite the abundance of seawater as an alternative water resource, seawater desalination is usually limited by relatively low productivity and high energy consumption. Membrane-based desalination technologies including the state-of-the-art reverse osmosis (RO) technology have been demonstrated to be more energy efficient than thermal desalination approaches ( 1 , 2 ). However, conventional polymeric RO membranes still suffer deficiencies such as low fouling resistance, poor selectivity, and low stability to resist chemical/heat-induced degradation. Therefore, it has been an ever-continuous endeavor to search and explore new materials for fabricating membranes with improved permeability, selectivity, chemical stability, and resistance to fouling simultaneously ( 3 ). Three types of novel membranes have demonstrated great promises in addressing water permeability and selectivity. They are ultrathin nanoporous membranes, e.g., porous graphene ( 4 – 11 ), membranes with artificial water channels such as carbon nanotubes (CNTs) ( 12 – 19 ), and layer-stacked membranes with two-dimensional water channels, including graphene oxide (GO) ( 20 – 24 ) and MoS2 ( 25 , 26 ).
Theoretically, a membrane with atomic thickness can lead to a water permeance two to three orders of magnitude higher than that of conventional membranes due to the inversely proportional relationship between water flux and membrane thickness ( 2 , 4 ). Thus, the atomic thickness of nanoporous graphene (~0.34 nm) may result in larger water permeability (6 to 66 liter/cm2 per day per MPa) than thin-film composite (TFC) membranes (~0.24 liter/cm2 per day per MPa) ( 27 ) while achieving a complete salt rejection (100%). It has been demonstrated by both the molecular dynamics (MD) simulations ( 4 ) and experiments ( 9 ). In addition, graphene materials exhibit fouling resistance, resistance to degradation ( 28 ), ultrahigh mechanical strength for preventing ripping, tunable small pore size for controllable permeability, chemical stability, and scalable synthesis method that may lead to cost-effective production. Because of these advantages, graphene-based membranes are identified as promising candidates for next-generation seawater desalination systems. However, the use of nanoporous graphene is still a work in progress for seawater desalination due to the difficulties in drilling defect-free subnanometer pores with uniform radius of <0.45 nm on a monolayer graphene ( 29 ). A primary reason for this is the requirement of uniform subnanoscale-sized pore distribution, which determines the selectivity of nanoporous graphene, posting a serious challenge in fabrication of the nanoporous membranes. This is the key factor that has been hindering the large-scale applications of porous graphene–based membrane technology.
CNTs are another class of materials that have great potential for RO desalination, which was first demonstrated by MD simulations ( 12 ). A single-walled (6,6) CNT with the diameter of 0.81 nm can reject salt completely and generate high water permeability of 14.33 liter/cm2 per day per MPa attributed to its smooth and nonpolar interior. However, the salt rejection decreases sharply to 58% when the CNT diameter increases to 1.1 nm. Subsequently, aligned CNT-based membranes were fabricated experimentally by chemical vapor deposition methods ( 14 , 15 ). The experimental results indicate that the water flux is three to five orders of magnitude higher than that predicted by continuum theory. Unfortunately, however, the fabricated CNT-based membranes with the CNT diameter ranging from 1.6 to 7 nm and above cannot reject the sub–2-nm particles, indicating that they are not suitable for seawater desalination at current stage unless the uniform single-walled (6,6) CNTs can be well dispersed and aligned in the membrane in the future.
Other types of nanomaterial membranes have been developed. For instance, the interlayer spacing of GO membrane can be controlled in the range of 0.64 to 0.98 nm to realize 97% rejection for NaCl ( 24 ). Fully hydrated MoS2 membranes with free spacing of 0.9 nm exhibit high water permeability up to 6 liter/cm2 per day per MPa and moderate-to-high ionic rejection of 90% ( 25 ). Recently, a large area graphene-nanomesh/CNT hybrid membrane with average pore size of 0.63 nm is fabricated experimentally to realize water permeability of 2.4 liter/cm2 per day per MPa with a stable salt rejection of 86% ( 30 ). The high salt rejection efficiency of 93 to 100% for a large pore size of 1.44 nm in diameter has been demonstrated for charged monolayer nanoporous graphene membranes, while the required pressure drop is reduced substantially ( 31 , 32 ). To break the limitation of porous graphene for molecular sieving, both the ion-gated and charged porous graphene membrane models have been proposed to tune nonselective larger pores of 0.52 to 0.6 nm to be selective for gas separation ( 33 , 34 ).
In summary, up to date, none of the novel membrane materials that have been explored in the literature is able to break the trade-off rule between permeability and selectivity. Although nanoporous monolayer graphene and CNT-based membranes are of high permeability because of their ultrathin thickness and fast water diffusion property, the fabrication of uniform subnanometer pores on a graphene sheet or the fabrication of uniformly dispersed small-diameter CNTs to form an assembled membrane presents great technical difficulties and challenges. The ultimate contradiction of selectively permeable membrane technology is that the size of nanopores should be smaller than the diameter of hydrated ions to exhibit enhanced ion selectivity while hoping for high permeability, which is a paradoxical issue or a trade-off that was thought as an insurmountable obstacle. In this study, we report a novel concept of desalination mechanism of temporal selectivity using slip-induced separation that breaks the permeability-selectivity trade-off without stringently relying on small, uniform pore sizes. The desalination is achieved by rotating porous monolayer graphene cylinder (GC) with large pores that are two to five times larger than the diameter of hydrated sodium ions (Fig. 1). The boundary slip at the water-GC interface significantly enhances the salt rejection. At the same time, the large nanopores results in an ultrahigh water flux under centrifuge-induced pressure. Therefore, this novel slip-induced separation bypasses the conventional limitation of pore size and breaks the trade-off between permeability and selectivity. Moreover, the required permeability and selectivity can be obtained by adjusting the porosity, the pore size, the thickness of membrane, and the rotating velocity of porous hydrophobic membranes. The findings reported in this work may open a new door of designing highly efficient RO desalination apparatus, triggering a boom of both theoretical and experimental researches on rotating/shearing membranes, which may further revolutionize the design of the next-generation desalination and water purification technologies.
Fig. 1 Schematic illustration of the large-scale MD desalination model.
( A ) A nanoporous rotating GC (red) of 36 pores with a diameter of 2 nm, seawater (sodium ions in blue, chlorine ions in yellow, and transparent water molecules in the left half) confined in GC, pure water out of GC (transparent), and two pored graphene sheets (blue) to confine the draw solution. The diameter of the pore on graphene sheets is identical to that of GC. The red arrow denotes the rotating velocity of GC, V GC. ( B ) Three-dimensional view of porous GC with 36 nanopores of 2 nm in diameter. ( C ) Dimension and relative position of each component. The feed solution is seawater confined within GC, and the draw solution is pure water confined by the outer surface of GC and two graphene sheets. The details of MD desalination model can be found in section S1. The movie of a representative graphene centrifuge with 36 pores rotating at angular velocity of 35 rad/ns is provided in movie S1.
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