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Abbreviation (ISO4): Prog Chem      Editor in chief: Jincai ZHAO

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Progress in Chemistry >

2025 , Vol. 37 >Issue 4: 564 - 574

DOI: https://doi.org/10.7536/PC240802

Review

Study and Applications of Two-Dimensional Nanochannel Ion Sieving Membranes

  • Jianyu Wang 1, 2, 3 ,
  • Shuai Wang , 2, 3, * ,
  • Chuanjie Fang 2, 3 ,
  • Baoku Zhu 2, 3, 4 ,
  • Liping Zhu , 2, 3, 4, *
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  • 1 School of Science，Kaili University，Kaili 556011
  • 2 Center of Health Care Materials，Shaoxing Research Institute，Zhejiang University，Shaoxing 312000，China
  • 3 Engineering Research Center of Membrane and Water Treatment Technology，Ministry of Education，Zhejiang University，Hangzhou 310030，China
  • 4 Department of Polymer Science and Engineering，Zhejiang University，Hangzhou 310030，China
*（Liping Zhu）；
（Shuai Wang）

Received date: 2024-08-19

  Revised date: 2025-01-08

  Online published: 2025-03-20

Supported by

Foundation Research Project of Kaili University(2025ZD007)

National Natural Science Foundation of China(U21A20302)

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Abstract

Two-dimensional nanochannel membrane is a new membrane composed of two-dimensional nanosheets with atomic layer thickness and stacked by self-assembly. Compared with traditional separation membranes，its ion separation behavior has many unique characteristics，and has important potential applications in seawater desalination，energy storage and conversion，rare element extraction and separation，and other fields. These materials have attracted great interest and wide attention from researchers. It has become an important development direction and research hotspot in the field of membrane separation science and technology in recent years. In this paper，the construction strategy，performance evaluation method and mass transfer mechanism of two-dimensional nanochannel membranes were systematically summarized from the perspective of two-dimensional nanochannel membranes used for accurate ion sieving. The latest research progress in the preparation and application of two-dimensional nanochannel membranes in recent years was reviewed，and the development trend was prospected. We hope this review can provide enlightenment for structure design and optimization，performance enhancement，large-scale preparation and engineering applications of two-dimensional nanochannel membranes in the future.

Contents

1 Introduction

2 Two-dimensional nanochannel ion sieving membrane and its construction methods

2.1 Two-dimensional nanochannel ion screening membrane

2.2 Construction method of 2D nanochannel ion sieving membrane

2.3 Characterization of structure and evaluation of properties of two-dimensional nanochannel ion sieving membranes

3 Mass transfer mechanism in two-dimensional nanochannels

3.1 Mass transfer mechanism of solvent in two-dimensional channels

3.2 Mass transfer mechanism of ions in two-dimensional channels

4 Application of two-dimensional nanochannel ion sieving membrane

4.1 Desalination of seawater

4.2 Energy conversion and storage

4.3 Extraction and separation of elements

5 Conclusion and outlook

Key words： two-dimensional materials; two-dimensional nanochannels; ion sieving; ion transport

Cite this article

Jianyu Wang , Shuai Wang , Chuanjie Fang , Baoku Zhu , Liping Zhu . Study and Applications of Two-Dimensional Nanochannel Ion Sieving Membranes[J]. Progress in Chemistry, 2025 , 37(4) : 564 -574 . DOI: 10.7536/PC240802

1 Introduction

Ion separation is a process and method for effectively separating two or more ions in a mixed state, serving as an important foundation in many scientific and applied fields such as seawater desalination, energy storage and conversion, extraction and separation of rare elements, and sensing.[1-3] Traditional ion separation methods include chemical precipitation, ion exchange, and adsorption, yet these approaches face limitations such as high cost, low efficiency, complex operation procedures, and the generation of secondary pollution.[4-7] In contrast, membrane separation technology, which relies critically on separation membranes, offers advantages including simple operation, energy efficiency, high separation capacity, and environmental friendliness, demonstrating notable technical and economic benefits along with broad application prospects.[8] Two-dimensional (2D) materials represent a new class of nanomaterials that have rapidly become a research hotspot across numerous disciplines including physics, chemistry, and materials science due to their ultrathin structure, high mechanical strength, and the ability to be manipulated at the atomic scale since the first discovery of graphene.[9] Unlike conventional polymeric membranes with disordered pores, 2D nanochannel membranes are highly ordered ultrathin layered structures formed by self-assembly of 2D nanosheets, offering an ideal platform for ion transport and selective sieving through their interconnected network of nanoscale 2D channels. Furthermore, the flexible tunability of channel size and internal chemical environments facilitates applications of 2D channel membranes in diverse fields. As a next-generation ion sieving membrane, precise design of 2D channel membranes holds promise for simultaneously enhancing both the transport efficiency and selectivity of specific ions, thereby overcoming the trade-off effect between ion transport rate and selectivity. However, limited by water-induced swelling of most 2D nanosheets and uncertainties in regulation techniques, achieving highly selective transport of ions or water molecules within 2D channels remains a limitation and challenge.[10] Therefore, precise structural design and performance modulation of 2D nanochannels are particularly crucial for improving their ion transport and selectivity.
In recent years, research on ion sieving membranes with two-dimensional nanochannels has flourished. Systematically reviewing and summarizing the reported findings is beneficial for understanding the key knowledge maps in this field, grasping the current research status, and identifying important scientific issues and core technical challenges that remain to be solved. To this end, this paper reviews the latest research advances in the preparation and application of two-dimensional nanochannel membranes, focusing on current studies regarding construction strategies, performance evaluation methods, and mass transfer mechanisms of two-dimensional nanochannel ion sieving membranes. The main challenges faced in applying such membranes to ion separation are discussed, and future development trends are also envisioned, aiming to provide references for optimizing membrane structures, enhancing performance, achieving large-scale fabrication, and promoting engineering applications of two-dimensional nanochannel membranes in the future.

2 2D Nanochannel Ion Sieving Membranes and Their Construction Methods

2.1 2D Nanochannel Ion Sieving Membranes

In 2004, Professors Geim and Novoselov at the University of Manchester in the UK discovered graphene by using adhesive tape to exfoliate highly oriented pyrolytic graphite. The discovery of graphene is a groundbreaking milestone, extending materials science research into the two-dimensional scale and providing new opportunities for technological advancement. Since then, graphene has become a research hotspot across various fields, significantly promoting the development of two-dimensional materials. Within merely 20 years, as many as thousands of two-dimensional materials have been developed and utilized, including elemental substances and their derivatives, III-V compounds, transition metal compounds, hydrotalcites, metal-organic frameworks, and covalent organic frameworks.
Two-dimensional nanochannel membranes are layered films formed by the self-assembly of two-dimensional nanosheets, creating highly ordered and interconnected two-dimensional channels. As shown in Figure 1, these membranes exhibit significant research importance and application value in ion sieving due to the flexible tunability of their nanoscale channels.
图1 二维通道的结构及传质过程（以氧化石墨烯GO为例）。（a）GO膜的断层结构SEM图；（b，c）GO膜的传质示意图[13]

Fig.1 Structure and mass transfer process of two-dimensional channel （take GO as an example）. （a） Cross-section structure diagram of GO membrane under SEM；（b，c） Mass transfer diagram of GO membrane[13]. Copyright 2012，American Chemical Society

2.2 Construction Methods of Ion Sieving Membranes with Two-Dimensional Nanochannels

The rich physical and chemical properties of nanosheets lead to complex and variable interlayer interactions, such as hydrogen bonding, electrostatic interactions, and π-π stacking. Therefore, diverse two-dimensional channel construction strategies can be employed to meet various requirements[14]. To date, several fabrication methods have been investigated, including vacuum-assisted filtration, spin coating, blade coating, and electrophoretic deposition.
(1) Vacuum-assisted filtration method
Vacuum-assisted method is the most commonly used approach for constructing two-dimensional channel membranes (Fig. 2a). Porous membranes that can be used as substrates include polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF), polycarbonate (PC), cellulose ester (MCE), and nylon[15]. Depending on different application purposes of the two-dimensional channel membranes, various pretreatments can be applied to the substrate prior to the vacuum filtration-assisted step. For example, after vacuum drying, vermiculite membranes can be easily peeled off from PVDF substrates to obtain self-supported two-dimensional channel films, which are used for evaluating ion sieving performance. To prepare ultrathin vermiculite membranes for pressure-driven solvent filtration and reduce the adhesion between the vermiculite membrane and the substrate, nylon substrates can be pretreated with polydopamine (PDA)[16]. Studies have shown that factors such as the concentration of nanosheet dispersion, deposition rate, and applied pressure during deposition can affect the uniformity and flatness of the resulting two-dimensional channel membranes[17].
图2 二维纳米通道膜的主要制备方法。（a）真空辅助过滤法；（b）旋涂法[22]；（c）刮涂法[20]；（d）电泳法[21]

Fig.2 Main preparation methods of two-dimensional nanochannel films. （a） Vacuum assisted filtration；（b） spin coating[22]，Cpyright 2020，Elsevier；（c） scraping coating[20]，Copyriht 2023，Wiley；（d） electrophoretic method[21]，Copyrigtht 2022，Elsevier

(2) Spin coating method
Spin coating is another important method for preparing two-dimensional channel membranes (Fig. 2b). Similar to vacuum-assisted filtration, spin coating also requires a substrate for support. However, spin coating demands a much smoother substrate surface to achieve a flatter membrane. During the spin coating process, shear forces reduce the capillary action between nanosheets, thereby removing water molecules. Thus, the flatness of the two-dimensional membrane on the substrate can be improved by adjusting the rotation speed of the substrate. Due to limitations in operational parameters, this method is not suitable for preparing large-sized two-dimensional membranes[18].
(3) doctor blading
Among the methods for preparing large-area two-dimensional channel membranes, the blade coating method is one of the most effective approaches (Fig. 2c). Under the shear force of the blade, two-dimensional nanosheets can align parallel to the substrate surface, forming a continuous and uniform film[19]. During the preparation of two-dimensional channel membranes via blade coating, the substrate imposes minimal constraints, offers better film-forming capability, and enables rapid fabrication. However, this method requires relatively high concentrations of the nanosheet suspension. Studies have shown that only when the concentration of the nanosheet dispersion exceeds 15 mg mL-1 can a relatively ideal uniform membrane be produced. In addition, this method is suitable for preparing membranes with thickness below the micrometer scale; if the membrane is too thick, a significant amount of time will be required during the subsequent drying process[20].
(4) Electrophoresis method
In addition to the aforementioned methods, large-area two-dimensional channel membranes can also be prepared through electrophoretic deposition. For example, Deng et al.[21] found that negatively charged Mexene nanosheets tend to migrate toward the anode under the influence of an electric field (Figure 2d). A large-area uniform two-dimensional channel membrane can be deposited on the surface of the anode material within a short time of less than 10 minutes[21]. Notably, because two-dimensional nanosheets with larger lateral dimensions are preferentially deposited during this stacking process, the resulting membrane exhibits a more ordered layered structure and superior desalination performance compared to vacuum-assisted filtration membranes. However, this method requires preparation on the surface of the anode material, which imposes certain limitations in practical applications.
Among the above methods, vacuum-assisted filtration is more suitable for preparing ultrathin two-dimensional nanochannel membranes due to its precise and controllable deposition of nanosheets, while spin coating is better suited for rapid preparation on smooth substrates; however, neither method can produce uniform large-area membranes. Electrophoretic deposition enables the preparation of membranes with controllable thickness and larger area within a short time, but it has certain limitations in processing conditions. Currently, for large-area fabrication, doctor blading is the most promising method, although issues such as thickness control and film uniformity still require further in-depth research and development.

2.3 Structural Characterization and Performance Evaluation Methods of Two-Dimensional Nanochannel Ion Sieving Membranes

2.3.1 Structural Characterization Methods of Two-Dimensional Nanochannel Membranes

The thickness of the monolayer two-dimensional nanosheets obtained by physical mechanical or chemical intercalation methods is approximately 1 nm, while the lateral dimensions range from hundreds of nanometers to tens of micrometers. The two-dimensional structure of these nanosheets can be observed and characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM), or transmission electron microscopy (TEM) (Fig. 3a, b).
图3 二维纳米片及二维通道膜的表征。MXene纳米片的（a）AFM图和（b）TEM图[25]；GO二维纳米通道膜的SEM（c）表面和（d）断层图[26]

Fig.3 Characterization of two-dimensional nanosheets and two-dimensional channel membranes. （a） AFM diagram and （b） TEM diagram of MXene nanosheets[25]；Copyright 2023，Wiley. SEM （c） surface and （d） tomography of GO 2D nanochannel membrane[26]. Copyright 2024，Elsevier

The 2D channel membranes composed of stacked two-dimensional nanosheets generally exhibit a wrinkled surface morphology as observed by SEM and AFM (Fig. 3c), while displaying a layered structure in the longitudinal direction (Fig. 3d). In these layered structures, the interlayer spacing between two adjacent nanosheets (denoted as d) is an important parameter for evaluating the ion sieving performance of the membrane. Due to the highly ordered stacking state of the nanosheets, the d value can be obtained through X-ray diffraction (XRD) measurements. When X-rays irradiate the sample surface, diffraction occurs due to differences in the atomic layer distances, with the diffraction angle and intensity closely related to the sample structure. The interlayer spacing can then be further calculated using the Bragg equation: 2d sinθ = nλ. Notably, the d value obtained in this case includes the thickness of a single nanosheet; therefore, the effective size of the 2D nanochannel should subtract the thickness of one individual nanosheet layer[23].
The stacking order of two-dimensional nanochannels can be determined by the full width at half maximum (FWHM) of the XRD peak. A smaller FWHM, indicating a sharper peak shape, suggests a higher stacking order of the nanosheets. Additionally, the stacking order of nanosheets can also be analyzed through Raman spectroscopy. Taking graphene-based materials as an example, their characteristic peaks are typically the D band, G band, and 2D band (G' band). Among them, the D band mainly appears near 1350 cm-1, representing defect scattering; the G band arises from the in-plane vibration of carbon atoms with sp2 hybridization and is centered around 1580 cm-1; while the 2D band near 2700 cm-1 is generally related to the electronic band structure. Therefore, the ratio ID/IG can be used to indicate the stacking order of the nanosheets, where a smaller value represents a higher stacking order[24].

2.2.2 Performance Evaluation Methods for Ion Sieving Membranes with Two-Dimensional Nanochannels

The performance evaluation methods for ion sieving membranes typically include forward osmosis and reverse osmosis.
In the testing of forward osmosis, the membrane is mounted at the junction of two osmotic chambers. Typically, a solution containing one or multiple metal ions (mixed salt solution) is added into one chamber, while an equal volume of deionized water is placed in the other chamber (Fig. 4a). Within a certain time frame, ions diffuse across the membrane driven by the osmotic pressure gradient. The transport efficiency of a specific ion and the selectivity between two ions can be evaluated by calculating the diffusion rate of individual ions and the ratio of their diffusion rates.
图4 二维纳米通道膜的性能评估方法。（a）正渗透装置示意图[27]；（b）反渗透死端过滤装置示意图；（c）反渗透错流装置示意图[23]

Fig.4 Performance evaluation method of two-dimensional nanochannel membrane. （a） Schematic diagram of the forward osmosis device[27]；Copyright 2024，Springer Nature. （b） Schematic diagram of reverse osmosis dead-end filter device. （c） Schematic diagram of reverse osmosis cross-flow device[23]. Copyright 2022，Elsevier

In the forward osmosis model, two variables are critically evaluated: the ion transport rate (Pi) and ion selectivity (Si/j), whose calculation formulas are as follows:
P i = C i × V A × Δ t
S i / j = ( C i / C j ) p ( C i ' / C j ' ) f
here, Ci and Ci' represent the initial concentration and the concentration on the permeate side of ion i, respectively; Cj and Cj' represent the initial concentration and the concentration on the permeate side of ion j, respectively; V represents the total volume on the permeate side, A represents the effective area of the membrane, and Δt represents the permeation time.
In addition, driven by the ionic osmotic pressure, water molecules diffuse in the direction opposite to that of ion transport, and the water flux of the membrane can be calculated based on the change in liquid level difference over a specific period of time[27].
Reverse osmosis is further divided into dead-end filtration (Fig. 4b) and cross-flow filtration (Fig. 4d). Both methods require an external pressure as the driving force to push the salt solution through the membrane. The difference lies in the direction of applied pressure: in dead-end filtration, the pressure is typically exerted perpendicular to the membrane surface, whereas in cross-flow filtration, the external pressure drives the salt solution to flow tangentially across the membrane surface[23].
Under the reverse osmosis model, water flux (Jw) and ion rejection rate (R) are important parameters for evaluating membrane performance, calculated using the following formulas:
J w = V Δ t × A × P
R = 1 - C p C f × 100 %
here, V is the total volume of the permeate liquid, P is the applied pressure, and Cp and Cf represent the ion concentrations on the permeate side and the feed side, respectively.
In addition, the membrane also needs to undergo long-term operation in these devices and operate under various specific environments to further evaluate its sieving stability, such as chemical stability, long-term operational stability, and so on.

3 Mass Transfer Mechanisms in Two-Dimensional Nanochannels

3.1 Mass Transfer Mechanism of Solvents in Two-Dimensional Channels

3.1.1 Spatial Confinement Effect

According to the different properties of two-dimensional nanosheets, the interlayer spacing of self-assembled two-dimensional channel films can be roughly divided into two categories: one is represented by graphene oxide (GO) and transition metal carbides/nitrides (MXene), where the nanosheets have functional groups on their surfaces. Due to the presence of these functional groups, the interlayer spacing is approximately 1 nm[13,28]; while the other type includes reduced graphene oxide (rGO), hydrotalcite (TMDs), and graphitic carbon nitride (C3N4), which are characterized by nanosheets with no or only trace amounts of surface functional groups. The interlayer spacing of these films matches the lattice spacing of the corresponding bulk crystals, being about 0.35 nm for rGO[29], approximately 0.6 nm for MoS2[30], and around 0.3 nm for C3N4[31].
Research indicates that the transport rate of water molecules within two-dimensional channel-confined spaces is remarkably high, approximately 1010 times faster than that of helium (He)[13]. There are two reasons for this novel phenomenon. First, hydrophilic groups at the edges of nanosheets can disrupt hydrogen bond coupling within polar clusters, facilitating the "capture" of water molecules into the two-dimensional channels, allowing frictionless slip flow in smooth regions and further enabling continuous and rapid transport of water molecules within the channels. Second, water molecules inside the confined channels are driven by ultra-high nanocapillary forces (>1000 bar)[32-33].
For two-dimensional materials without functional groups, the interlayer spacing is too narrow to allow water molecule transport, requiring functionalization treatment. For example, the channel width of a MoS2 self-assembled thin film is only 0.015 nm, preventing water molecules from entering the interlayer channels. Through functionalization of nanosheets, the channel width can be increased to 0.45–0.53 nm, achieving a maximum water flux of up to 43 L·m-2·h-1·bar-1[34].
For solvents with larger molecular sizes (such as organic solvents, ester solvents, etc.), functionalization or intercalation methods can also be used to expand the interlayer spacing, allowing solvent molecules to pass through quickly (Fig. 5).
图5 不同尺寸物质在通过二维通道时所受的作用力[35]

Fig.5 Forces on substances of different sizes passing through two-dimensional channels[35]. Copyright 2023，Wiley

3.1.2 Slippage of Water Molecules

Studies have shown that water molecules exhibit collective slip phenomena in the hydrophobic regions of two-dimensional channels. Boukhvalov et al.[32] discovered, through first-principles calculations of the transport mechanism of water molecules within GO channels, that water molecules tend to form ordered ice structures within two-dimensional channels. Such interlayer single-layer or double-layer ice structures are beneficial for enhancing the energy barrier. Subsequently, ordered ice structures within graphene nanochannels were directly observed via HRTEM characterization. Research also indicates that the ordered ice structures in these confined two-dimensional channels enable ultrafast water molecule transport through collective slip motions[36]. Similarly, the smooth surfaces between layers of MoS2 nanosheets provide favorable conditions for water molecule transport. When water molecules enter the MoS2 two-dimensional channels with a size of approximately 0.6 nm, they undergo a reorganization process from 3D to 2D. The strong hydrogen bonding between water molecules leads to the formation of highly ordered square network structures. These networks significantly enhance the transport rate of water molecules via collective slipping, which is about 4-5 times faster than that in GO two-dimensional channels[37].
Impurities on the channel walls can affect the slip rate of water molecules to varying degrees. For instance, the slip length of water molecules in a GO nanochannel exhibits an exponential decay with the hydration diameter of the inserted cations. Therefore, the slip of water molecules can be precisely manipulated by inserting different cations, thereby meeting the requirements of various scenarios[38].

3.1.3 Hydrophilicity and Hydrophobicity

The hydrophilicity and hydrophobicity inside two-dimensional channels are also important factors affecting water molecule transport. As mentioned previously, in GO or other functionalized two-dimensional channels, hydrophilic groups located at the channel edges facilitate the "capture" of water molecules and their entry into the channel interior. Therefore, introducing appropriate hydrophilic groups within the channel can enhance the transport rate of water. However, excessive hydrophilic groups will lead to a "side-pinning effect," reducing the transport rate of water, as too many hydrophilic groups lock water molecules onto the channel walls.[39]

3.1.4 Influence of Other Solvent Properties

In addition to the aforementioned factors, the transport rate of solvents within two-dimensional channels is also closely related to the size, polarity, and viscosity of the solvent molecules. Generally, the permeation rate of solvent molecules is directly proportional to their own polarity, but inversely proportional to the viscosity of the solvent. Therefore, within two-dimensional channels, the transport rate of methanol is often higher than that of ethanol[10].

3.2 Mass Transport Mechanisms of Ions in Two-Dimensional Channels

3.2.1 Size Sieving Effect

In the ion sieving process of two-dimensional (2D) channel membranes, size effects dominate. Effective retention of specific ions can be achieved when the size of the 2D channels is less than or equal to the kinetic diameter of hydrated ions. Theoretically, 2D channels with a size-exclusion effect should possess uniform rigid structures[40]. However, it is important to note that due to the polar interactions between bare ions and water molecules, hydration shells are formed around the ions, resulting in what are known as "hydrated ions." For example, the diameter of a bare Na+ ion is 1.9 Å, but its hydrated ion diameter increases to 7.16 Å[41]. Therefore, 2D channels with sizes less than or equal to the hydrated diameter of Na+ are theoretically capable of effectively sieving Na+. However, in practice, when ions enter the 2D channels, they are also influenced by the entrance effect, causing deformation or partial dehydration of the hydrated ions, a phenomenon known as the ion dehydration effect. Furthermore, the dehydration effect is influenced by external factors such as osmotic pressure, applied pressure, and electric fields. Ion dehydration ability is closely related to the binding energy between the bare ion and water molecules. Consequently, when ions lack sufficient dehydration energy, the rejection effect of hydrated ions still plays a role[42-43].

3.2.2 Electrostatic Interaction

Due to the charged nature of ions, their transport through two-dimensional channels is also influenced by electrostatic interactions. Electrostatic interactions are divided into two types: internal electrostatic interactions within the channel and the driving effect of an external electric field[44]. An electrostatic interaction forms between the charged two-dimensional channel and the charged particle flow. According to classical mean-field electrostatic theory, when the channel length is smaller than the Debye length of the electrolyte (λD), the charges on the channel surface repel ions carrying the same charge and attract counterions with opposite charges. When a charged ionic solution comes into contact with a charged channel, a Donnan potential difference forms, establishing Donnan equilibrium. However, during pressure-driven filtration, the solution must maintain electroneutrality; thus, the Donnan potential repels ions in the solution carrying the same charge and simultaneously must also repel ions with opposite charges. It should be noted that when λD exceeds the dimensions of the two-dimensional channel and the solution flowing through the channel is dilute, the Donnan effect dominates. In contrast, in solutions with higher concentrations, where λD is comparable to the dimensions of the two-dimensional channel, the Donnan effect rapidly diminishes[45].
In addition, the number and distribution of ions can be influenced by different electric potentials, which affect ion transport within charged channels. According to the Poisson-Nernst-Planck model, in nanochannels smaller than 2 nm, the ion flux generated by the interfacial electric double layer (EDL) decreases as the channel entrance potential increases[46]. On the other hand, ion transport rates can be manipulated by applying an external electric field. Studies have shown that when the applied voltage on the graphene surface increases from 0 to -0.5 V, the instantaneous ion flux increases by sevenfold[47].
Although EDL can enhance ion sieving efficiency through electrostatic interactions, it usually compresses the effective transport space for solvents, which is detrimental to solvent transport.[48] Therefore, two-dimensional nanochannels with alternating positive and negative charges can be constructed to break the EDL effect, improve water flux, and cause charged particles to repeatedly collide along the tortuous path within the channels, thereby enhancing ion rejection rates.[49]

3.2.3 Surface Effects of the Channel

As mentioned above, water molecules will exhibit collective slip in two-dimensional channels, thereby accelerating water transport. Ion transport is also affected by the water slip. For example, the migration rate of K+ in a graphene channel with a size of 6.8 Å reaches approximately 3×10-7 m2·V-1·s-1, a rate even higher than that of K+ in bulk solution. However, although the hydration size of Cl- is similar to that of K+, its transport rate is three times lower [50]. The transport rate of K+ is influenced by the rapid movement of water molecules; the polarization effect between water molecules and graphene causes the -OH groups to preferentially orient inward toward the channel walls. In contrast, in the hydration layer of Cl-, the -OH groups tend to orient outward, resulting in stronger interactions with the nanochannel walls and thus a lower transport rate of Cl- [51].
According to the inherent chemical properties of two-dimensional nanosheets, the prepared two-dimensional nanochannels can affect ion migration within the channels through different chemical bondings. For example, in GO two-dimensional channels, oxygen-containing functional groups in oxidized regions can act as Lewis coordination sites and bind with Lewis acid metal ions such as Cu2+, Cd2+, Mn2+. These heavy metal ions contain empty d-orbitals that can accept lone pair electrons provided by oxygen-containing groups, forming strong coordination complexes, thereby enabling them to enter the two-dimensional channels[52]. On the other hand, the unique aromatic ring π-electron orientation of graphene generates cation-π interactions with hydrated cations, pulling them from their hydration shells toward the nanosheet surface[53]. Additionally, studies have confirmed that similar to the recognition function of amino residues in cell membranes, carbonyl groups on GO nanosheets can specifically interact with K+, accelerating its transport within two-dimensional channels[5]. In MoS2 two-dimensional nanochannels, although the nanosheet surface lacks aromatic ring π-electron structures, S atoms serve as soft Lewis binding sites, fully exposed inside the two-dimensional channels, capable of undergoing Lewis acid binding with heavy metal ions (e.g., Hg2+). Therefore, MoS2 two-dimensional channel membranes outperform other two-dimensional channel membranes in regulating heavy metal ion transport[54].
It is worth noting that the electrostatic interactions on the channel surface should be considered a special type of surface effect.

3.2.4 Interaction Between Object Materials and Ions

Due to the large specific surface area and abundant binding sites of nanosheets, they can combine with guest materials of various dimensions, such as zero-dimensional materials (ions, small molecules, quantum dots, etc.), one-dimensional materials (nanotubes, nanowires, polymer molecules, etc.), and two-dimensional materials (nanosheets), to regulate the interlayer spacing and stability of the two-dimensional channels. Therefore, the properties of the interlayer guest materials will influence the ion transport and sieving characteristics of the two-dimensional channels. For example, Lv et al.[55] employed a molecular intercalation method to introduce crown ether molecules into the interlayers of GO. Based on the differences in affinity of crown ethers toward different cations, graphene oxide nanochannels intercalated with different crown ethers exhibited a "recognition" effect for specific ions: 12-crown-4 could recognize Li+, 15-crown-5 could recognize Na+, and 18-crown-6 could recognize K+. Thus, ion-selective channels can be designed according to specific requirements. By introducing ethylenediaminetetraacetic acid (EDTA) molecules, which contain a large number of negatively charged functional groups, into MXene two-dimensional channels, the channel size, chemical groups, and charge density can be precisely tuned. The resulting two-dimensional (2D) sub-nanometer biomimetic channels exhibit specific recognition capability for monovalent K+ while blocking divalent ions like Mg2+, showing excellent separation performance between monovalent and divalent cations[27].

4 Application of Two-dimensional Nanochannel Ion Sieving Membranes

4.1 Desalination of Seawater

Seawater desalination is one of the key solutions to global water scarcity. Due to their flexible and adjustable interlayer spacing, two-dimensional nanochannel membranes hold significant potential for application in seawater desalination. Since Na+ is the primary cation in seawater, with a hydrated diameter of 7.16 Å, achieving both high Na+ rejection and high water molecule flux through membrane technology has long been a scientific challenge. Therefore, designing an appropriate structure for two-dimensional channel membranes is crucial. Based on cation–π interactions, Chen et al.[56] developed a multi-stage seawater filtration platform by utilizing the characteristic of Na+ in high-concentration seawater to automatically regulate the channel size of GO two-dimensional membranes. After being filtered through the two-dimensional channel membrane five times and eleven times respectively, the Na+ concentration (~0.6 mol/L) in high-concentration seawater decreased to 0.123 mol/L (below the concentration of physiological saline) and 0.015 mol/L (below the concentration of freshwater). Compared with commercial seawater desalination membranes, this method consumes only 10% of the energy while increasing water flux tenfold, further advancing the practical application of two-dimensional channel membranes in seawater desalination. Chen et al.[57] prepared a multilayer two-dimensional graphdiyne membrane with submicron and nanopore structures by growing coupled multilayer graphdiyne (GDY) within porous copper hollow fibers. Further testing revealed that during vacuum membrane distillation using a 3.5 wt% NaCl solution, the membrane achieved nearly perfect NaCl rejection at 99.9% and an ultra-high water permeation rate of approximately 700 L·m-2·h-1. Moreover, through distillation and nanofiltration experiments conducted on highly saline seawater and real seawater, it was further confirmed that the membrane exhibits excellent structural stability. Experimental and theoretical studies have shown that compared to water molecule transport within the pores of nanosheets, interface transport between graphene's two-dimensional layers increases the water flux by several orders of magnitude (Fig. 6a).
图6 二维纳米通道膜的应用。（a）GO膜用于多级过滤海水淡化[56]；（b）MXene膜用于盐差能转化[67]；（c）GO膜用于核素筛分

Fig.6 Application of two-dimensional nanochannel membranes. （a） GO membrane is used for multistage filtration of seawater desalination[56]；Copyright 2023，American Chemical Society. （b） MXene membrane for salt difference energy conversion[67]；Copyright 2023，Wiley. （c） GO membrane for the screening of nuclides. Copyright 2022，Springer Nature

Not only graphene two-dimensional channel membranes, but also COF two-dimensional membrane materials exhibit potential application value in seawater desalination. Wang et al.[58] designed a COF membrane structure with high-strength ordered two-dimensional channels through a strategy involving electrostatic interactions and π-π interactions between TaPa-SO3H nanosheets and TpTTPA nanosheets, achieving rapid seawater desalination. The TpTTPA nanoribbons enhance the stacking order of the COF two-dimensional nanosheets while providing strong rigid support for the two-dimensional channels. Under conditions of 50 °C, the target two-dimensional nanochannel membrane achieved a desalination rate of 99.91% during the filtration process of 3.5 wt% NaCl, with an extremely high water flux reaching 267 kg·m-2·h-1, which is 4 to 10 times higher than that of commonly used membranes.

4.2 Energy Conversion and Storage

The proton transport rate usually affects the energy conversion efficiency, so proton transport membranes are considered as the core components of fuel cells. In 2014, Professor Geim from the University of Manchester, UK[59] first discovered that protons can pass through monolayer graphene and h-boron nitride (h-BN), but not through monolayer MoS2. Further theoretical calculations revealed the underlying mechanism: both monolayer h-BN and graphene have single atomic layer structures with pores in their electron clouds that allow proton passage; whereas monolayer MoS2 contains a three-layer atomic structure with high electron cloud density and small pores, preventing proton penetration[60]. Moreover, the proton transport rates of monolayer two-dimensional material films are generally low. Therefore, multilayer self-assembled two-dimensional material films have become new research targets. For example, Professor Jiang Zhongyi's team at Tianjin University[61-62] prepared GO/polyphosphoric acid polymer and GO/montmorillonite/sulfonated polyvinyl alcohol composite two-dimensional channel membranes, significantly enhancing proton transport rates within the GO two-dimensional channels. The Chen group at Kean University, Australia[63] fabricated two-dimensional nanochannel membranes modified with hydrophilic groups such as amino, carboxyl, and hydroxyl groups by blending h-BN with urea; these functional groups provide protons and further promote proton transport.
The transport efficiency of lithium ions is also critical to lithium-ion batteries; therefore, their core component is the battery separator. Two-dimensional nanochannel membranes as battery separators exhibit excellent lithium ion transport characteristics. The research group of Professor Qiang Zhang from Tsinghua University[64] first utilized GO membranes as separators for lithium batteries and found that the lithium-sulfur batteries exhibited high stability and resistance to discharge characteristics. Further mechanistic studies revealed that the carboxyl functional groups on the surface of GO nanosheets can act as ion hopping transport sites, attracting ions while blocking negatively charged polysulfide ions from passing through the two-dimensional channels. Professor Yi Cui from Stanford University in the United States[65] first deposited two-dimensional black phosphorus nanosheets onto a Celgard separator and used it as a lithium battery separator, finding that the black phosphorus nanosheets could attract polysulfide molecules and form chemical bonding interactions, demonstrating better polysulfide molecule blocking capability and cycling stability.
Reverse electrodialysis is a clean energy technology that converts the salinity gradient in water bodies (such as at the interface of seawater and river water) into electrical energy through Nernst electrochemical driving, also known as "blue energy." In the process of reverse electrodialysis, ion-selective membranes are crucial as they enable anions and cations to move in opposite directions, thereby generating electric current in the external circuit. Due to the presence of functional groups with different charges on their surfaces, nanosheets with two-dimensional nanochannels are ideal media for reverse electrodialysis. Professor Wang Jin from Xi’an University of Architecture and Technology proposed a strategy for ion transport based on two-dimensional vermiculite (VMT) heterogeneous nanofluidic systems for efficient power generation via reverse dialysis. During transmembrane diffusion, cations are first separated and enriched within the micropores of the substrate, followed by secondary precise screening within the ultrathin VMT laminates with high ion flux, achieving a maximum power density of up to 33.76 W·m-2 under a salinity gradient of 50 times; in practical applications involving highly saline brine from natural salt lakes, a power density of 25.9 W·m-2 was achieved. A research team led by Professor Wang Haihui from Tsinghua University designed an asymmetric biomimetic MXene two-dimensional channel membrane structure. The positively charged amino-functionalized MXene two-dimensional channels combined with the originally negatively charged MXene exhibit excellent rectification performance, enabling selective transport of anions and cations. Under a salinity gradient of 500 times, the designed two-dimensional diode rectification channel achieved a power density as high as 17.8 W·m-2 (Figure 6b). Wang et al. developed a nanofluidic system based on two-dimensional copper tetra(4-carboxyphenyl)porphyrin (Cu-TCPP) MOF nanosheets aimed at enhancing ion sieving and stimulus-responsive transport characteristics within two-dimensional channels. This electrodialysis system achieved a power density of 16.64 W·m-2 in artificial seawater/rivewater reverse dialysis tests, while under photothermal conditions, the maximum power density reached 31.92 W·m-2, enabling efficient conversion of salinity gradient energy.

4.3 Extraction and Separation of Elements

The rapid development and widespread use of global electronic mobile devices and new energy vehicles have led to a sharp increase in demand for lithium-ion batteries. However, the Earth's lithium resources mainly come from lithium ores and salt lakes, with salt lake lithium accounting for about 70%[68]. Therefore, extracting lithium from salt lakes is an important approach to addressing lithium resource shortages. Membrane separation technology achieves salt lake lithium extraction by ion sieving, separating lithium ions from other ions. Two-dimensional membranes have shown significant advantages in this regard. Lv et al.[69] investigated the Li+ sieving ability of two-dimensional MOF channels functionalized with sulfonate groups and found that the abundant sulfonic acid groups inside the two-dimensional channels can provide hopping transport sites for Li+ conduction, thereby facilitating the separation of Li+ from impurity ions. Among them, the selectivity of Li+/Na+, Li+/K+, and Li+/Mg2+ reached as high as 11.0, 16.5, and 37.8, respectively. Lu et al.[25] introduced sodium polystyrene sulfonate molecular chains into MXene two-dimensional channels, creating favorable conditions for Li+ transport within the channels and significantly enhancing the Li+/Mg2+ sieving performance. Wang et al.[70] chemically bonded positively charged amino group-containing polyethyleneimine (PEI) onto the surface of GO nanosheets through bonding interactions between molecular chains and nanosheets, and studied the Li+/Mg2+ separation performance of such amino-functionalized GO two-dimensional nanochannels. Based on size sieving effects and electrostatic repulsion effects, the amino-functionalized two-dimensional channels exhibited high selectivity for Li+/Mg2+, improving by 13.5 times compared to the original channels, while maintaining a Li+ transport rate of 0.09 mol·m-2 ·h-1. Additionally, studies have found that the selectivity of Li+/Mg2+ is not only related to the size of the two-dimensional channels but also closely associated with the charge density inside the channels.
With the development of nuclear power, the pressure of treatment and disposal of hazardous radioactive wastewater is increasing. At the same time, uranium, as a strategic resource in nuclear energy, its extraction from radioactive wastewater is of great significance for sustainable resource development. Two-dimensional channel membranes used for radionuclide sieving have become a research hotspot in recent years. Wu et al. demonstrated that H+ under highly acidic conditions can cause expansion of GO nanochannels while regulating the interlayer spacing. Further adjustment of the oxidation degree of nanosheets during preparation enables dual regulation of interlayer spacing, precisely reducing it below the hydrated diameter of UO22+ ions, thereby enabling the extraction of UO22+ ions from radioactive wastewater and achieving recycling of uranium resources (see Fig. 6c)[71]. Inspired by cell ion channel proteins, Liang et al.[72] innovatively embedded biomembranes containing lanthanide ion-binding proteins with ultra-affinity into the interlayers of GO two-dimensional nanochannels, achieving precise recognition and separation of specific radioactive nuclide ions, with selectivity for Ce3+/Sc3+, La3+/Sc3+, and Yb3+/Sc3+ reaching up to 167, 103, and 69 respectively.

5 Conclusion and Prospect

Two-dimensional (2D) materials represent a class of emerging nanomaterials with numerous novel physical and chemical properties. These materials can be fabricated into ultrathin separation membranes containing interconnected nanopores through various technical strategies, demonstrating advantages in precise ion sieving that are difficult to achieve with traditional membrane materials, potentially overcoming the long-standing trade-off effect in the field of membrane separation science and technology. This review summarizes recent research advances in 2D-channeled membranes and their applications in ion sieving, focusing on construction strategies, structural and performance characterization methods, as well as solute and ion transport mechanisms within the 2D channels. It systematically introduces the current status of application studies for 2D-channeled membranes in fields such as seawater desalination, energy storage and conversion, and the extraction and separation of rare elements. To date, after more than a decade of development, 2D materials have been proven to be ideal candidates for high-performance ion separation platforms, offering significant research value and broad application prospects.
Although positive progress has been made in the study of two-dimensional channel membranes and their ion sieving capabilities, overall, this field is still in its early developmental stage and remains far from large-scale fabrication and practical engineering applications. Based on biomimetic strategies, introducing binding sites within channels that exhibit affinity toward specific ions can enable precise ion sieving and efficient transport. However, the synergistic effects among various mechanisms—such as size-exclusion effects, electrostatic interactions, and functional group effects—are complex, multivariate processes, and the underlying mechanisms are not yet fully understood and require further investigation. Recently emerging data-driven research paradigms based on artificial intelligence, machine learning, and theoretical simulations integrate physical and chemical theories, computational modeling, data science, and experimental approaches, offering great potential to significantly advance the rapid design and optimization of two-dimensional nanochannel membrane materials, structure-property relationships, and their assembled devices. Additionally, key fundamental scientific and technological challenges, such as precise structural control and scalable, green, and controllable fabrication methods for two-dimensional channel membranes, as well as the evolution mechanisms of membrane structures during application and their impact on ion separation performance, also demand sustained and systematic investigation. Two-dimensional nanochannel membranes and their ion separation applications present significant challenges but hold promising prospects. With continuous advancement in research and development efforts, it is believed that through the joint endeavors of researchers and engineering professionals, greater breakthroughs and developments will be achieved, ultimately leading to real-world engineering applications.

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