Ion transport is crucial for biological systems and membrane-based technology. Atomic-thick two-dimensional materials, especially graphene oxide (GO), have emerged as ideal building blocks for developing synthetic membranes for ion transport. However, the exclusion of small ions in a pressured filtration process remains a challenge for GO membranes. Here we report manipulation of membrane surface charge to control ion transport through GO membranes. The highly charged GO membrane surface repels high-valent co-ions owing to its high interaction energy barrier while concomitantly restraining permeation of electrostatically attracted low-valent counter-ions based on balancing overall solution charge. The deliberately regulated surface-charged GO membranes demonstrate remarkable enhancement of ion rejection with intrinsically high water permeance that exceeds the performance limits of state-of-the-art nanofiltration membranes. This facile and scalable surface charge control approach opens opportunities in selective ion transport for the fields of water transport, biomimetic ion channels and biosensors, ion batteries and energy conversions.

2D materials' membranes with well-defined nanochannels are promising for precise molecular separation. Herein, the design and engineering of atomically thin 2D MXene flacks into nanofilms with a thickness of 20 nm for gas separation are reported. Well-stacked pristine MXene nanofilms are proven to show outstanding molecular sieving property for H 2 preferential transport. Chemical tuning of the MXene nanochannels is also rationally designed for selective permeating CO 2 . Borate and polyethylenimine (PEI) molecules are well interlocked into MXene layers, realizing the delicate regulation of stacking behaviors and interlayer spacing of MXene nanosheets. The MXene nanofilms with either H 2 -or CO 2 -selective transport channels exhibit excellent gas separation performance beyond the limits for state-of-the-art membranes. The mechanisms within these nanoconfined MXene layers are discussed, revealing the transformation from "diffusion-controlled" to "solution-controlled" channels after chemical tuning. This work of precisely tailoring the 2D nanostructure may inspire the exploring of nanofluidics in 2D confined space with applications in many other fields like catalysis and energy conversion processes.fabrication. Hence, MXene is considered to be a novel potential candidate for developing separative 2D-material membranes. However, there are a very few reports on MXene separation membranes. Gogotsi and co-workers first reported MXene membranes for rejection of trivalent cation in solution using nonpressure diffusion. [12] A high water permeance was obtained by applying MXene stacks in the pressure-filtration process, [13] but the membrane can only rejected matters with a size larger than 2.5 nm. Very recently, Wang and co-workers [14] reported the manufacturing of MXene membranes with highly ordered nanochannel structures for high-performance separation of H 2 /CO 2 , which opens the door of applying MXene membranes for molecular separation. It is considered that rationally regulating the nanostructure of 2D channels may, thereby, enlighten the exploring of MXene materials for sub-nanoscale separation with versatile functionalities.Herein, we report the design and engineering of MXene nanofilms with tunable transport channels for gas separation. Ultrathin pristine MXene nanofilms with a thickness down to 20 nm were fabricated by horizontally aligning the exfoliated MXene nanosheets on porous substrates. Molecular sieving channels within pristine MXene nanofilm can be formed to show highly selective H 2 permeation, as shown in Figure 1. Interestingly, these MXene laminates, as functionalized by borate and amine, exhibit different stacking behaviors and tunable interlayer spacing, allowing preferential CO 2 permeation. The resulting separation performance of either H 2 -or CO 2selective MXene nanofilms is beyond the performance limits for state-of-the-art membranes.

Two-dimensional (2D) material membranes have shown great potential for molecular separation.

Ion transport is crucial for biological systems and membrane-based technologies from both fundamental and practical aspects. Unlike biological ion channels, realizing efficient ion sieving by using membranes with artificial ion channels remains an extremely challenging task. Inspired by biological ion channels with proper steric containment of target ions within affinitive binding sites along the selective filter, herein we design a system of biomimic two-dimensional (2D) ionic transport channels based on a graphene oxide (GO) membrane, where the ionic imidazole group tunes the appropriate physical confinement of 2D ionic transport channels to mimic the confined cavity structures of the biological selectivity filter, and the ionic sulfonic group creates a favorable chemical environment of 2D ionic transport channels to mimic the affinitive binding sites of the biological selectivity filter. As a result, the as-fabricated ionic GO membrane demonstrates an exceptional K+ transport rate of ∼1.36 mol m–2 h–1 and competitive K+/Mg2+ selectivity of ∼9.11, outperforming state-of-the-art counterparts. Moreover, the semiquantitative studies of ion transport through 2D ionic transport channels suggest that efficient ion sieving with the ionic GO membrane is achieved by the high diffusion and partition coefficients of hydrated monovalent ions, as well as the large energy barrier and limited potential gradient of hydrated divalent ions encountered.

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Thin film composite (TFC) membranes have attracted great research interest for a wide range of separation processes owing to their potential to achieve excellent permeance. However, it still remains challenging to fully exploit the superiority of thin selective layers when mitigating the pore intrusion phenomenon. Herein, a facile and generic interface-decoration-layer strategy collaborating with molecular-scale organic-inorganic hybridization in the selective layer to obtain a high-performance ultrathin film composite (UTFC) membrane for CO 2 capture is reported. The interface-decoration layer of copper hydroxide nanofibers (CHNs) enables the formation of an ultrathin selective layer (≈100 nm), achieving a 2.5-fold increase in gas permeance. The organic part in the molecular-scale hybrid material contributes to facilitating CO 2 -selective adsorption while the inorganic part assists in maintaining robust membrane structure, thus remarkably improving the selectivity toward CO 2 . As a result, the as-prepared membrane shows a high CO 2 permeance of 2860 GPU, superior to state-of-the-art polymer membranes, with a CO 2 /N 2 selectivity of 28.2. The synergistic strategy proposed here can be extended to a wide range of polymers, holding great potential to produce high-efficiency ultrathin membranes for molecular separation.

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