Nanochannel membranes for ion-selective electrodialysis: principles, materials, and environmental applications

Abstract

Ion-selective electrodialysis (SED) has emerged as a promising approach for water purification, resource recovery, and electrochemical processes. While conventional ion-exchange membranes (IEMs) enable efficient charge-based ion separation, their disordered polymer networks lack the structural precision needed to distinguish ions with similar valence or hydrated size. As separation demands become increasingly stringent, IEMs have evolved toward advanced ion-selective membranes that introduce nanoscale confinement and engineered interfacial chemistries. These developments have culminated in the emergence of nanochannel membranes, which feature geometrically defined sub-nanometer channels that promote surface-governed ion transport and enable ion–ion selectivity far beyond the capabilities of traditional IEMs. This review integrates fundamental principles of electrochemical ion transport with recent advances in nanochannel membrane design for SED. We first elucidate the key mechanisms governing ion selectivity, including dehydration-based partitioning at the channel entrance, intra-channel ion–pore interactions, and dimensionality-dependent transport in 1D, 2D, and 3D nanochannels. We then survey major material platforms used to construct nanochannel membranes, such as ultrathin polymeric layers, two-dimensional nanosheet laminates, crystalline porous frameworks, and ceramic nanochannels. Finally, we outline design principles for controlling channel dimensions, interfacial charge, and structural stability, and discuss remaining challenges in translating nanochannel-enabled SED into efficient, durable, and industrially relevant ion-separation technologies.

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Hanmin You

Hanmin You is an MS student in Chemical Engineering at Ajou University, South Korea, under the supervision of Prof. Jongkook Hwang. He received his BS degree in Chemical Engineering from Ajou University, South Korea. His current research focuses on membranes based on inorganic nanoporous materials for electrochemical ion-specific recovery.

Tae-Nam Kim

Tae-Nam Kim is a PhD student at the Ajou University, South Korea, under the supervision of Prof. Jongkook Hwang. He received his MSc degree in Environmental Engineering from the Korea Maritime and Ocean University, South Korea. His current research focuses on inorganic sol–gel processing in graphene oxide membranes for ion-specific recovery.

Jongkook Hwang

Jongkook Hwang is an associate Professor in the Department of Chemical Engineering at Ajou University, South Korea. He received his PhD from the Pohang University of Science and Technology (POSTECH), South Korea, under the supervision of Prof. Jinwoo Lee, followed by postdoctoral research with Prof. Markus Antonietti at the Max Planck Institute of Colloids and Interfaces, Germany, and Prof. Petra de Jongh at Utrecht University, Netherlands. His research focuses on inorganic nanoporous materials with well-defined architectures, emphasizing nanoconfinement and size-selective transport for alkali-ion batteries and electrochemical membrane systems for ion-specific recovery.

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Environmental significance

Ion-selective electrodialysis is a promising electrochemical method for recovering valuable ions in complex water sources. However, its broader application is hindered by the limited ion selectivity of conventional ion-exchange membranes. This review highlights the potential of nanochannel membranes with sub-nanometre confinement, charge modulation, and affinity-based interactions to address this challenge. By integrating transport theory with material design, this work offers guidance for developing next-generation membranes for sustainable lithium recovery and environmental separation processes.

1 Introduction

As industrialization accelerates, various metallic elements are becoming strategic resources that underpin current industries and technologies, including energy storage, catalysis, and electronics.^1–3 Representative examples include rare-earth elements, precious metals, and alkali-metal (Li⁺, Na⁺, and K⁺) and transition-metal (Ni²⁺, Co²⁺, and Mn²⁺) ions, which are critical in batteries and electronics.^4,5 A major challenge is the efficient and selective recovery of these ions from complex aqueous environments.^6–9

The main step in ion recovery is separation, in which aqueous solutions (natural brine, industrial wastewater, or leachates from solid waste) are processed to obtain a high-purity solution containing target ions. Regardless of the specific ion, the separation process must discriminate the target ion from other ions that exhibit physically or chemically similar properties. Traditional approaches, including chemical precipitation, solvent extraction, and crystallization, have long been employed; however, they have disadvantages such as high reagent consumption, generation of secondary waste streams, limited ion selectivity, and complex operation.^3,10 These limitations highlight the need for alternative separation strategies.

Membrane-based separation has attracted increasing attention in ion separation and high-value ion recovery due to its low specific energy demand and high separation purity (Fig. 1).^11–14 Pressure-driven membrane processes, most notably nanofiltration (NF) and reverse osmosis (RO), remain the most widely applied due to their technological maturity in desalination and high-purity water production. In these systems, hydraulic pressure drives water across the membrane while solutes are rejected to varying degrees depending on their size, charge, and specific interactions with the membrane matrix. Whereas RO membranes are essentially nonporous and allow only water passage, NF membranes feature nanoscale pores (1–5 nm) and statistically distributed fixed charges, characteristics that underpin their description as “nanochannel membranes”. Ion discrimination in NF stems largely from geometric size sieving: steric hindrance in sub-nanometer apertures limits the entry of bulky or highly hydrated ions, while Donnan exclusion further modulates ion permeation through charged pore walls (Fig. 2). Recent NF research has further advanced toward membranes with more geometrically defined nanochannels and tuneable interfacial chemistries, enabling enhanced ion selectivity through controlled surface charge, modulation of hydration energies, and incorporation of ion-specific functional groups.^15–22 However, NF intrinsically depends on high operating pressures, which impose limitations on energy efficiency and membrane durability. Treating dilute feeds requires disproportionately high energy input, and continuous operation at 5–6 MPa accelerates chemical degradation and mechanical fatigue.^7,23 Furthermore, because NF is fundamentally a water-flow-mediated process in which water, anions, and cations traverse shared transport pathways, its structural degrees of freedom for differentiating ions of identical charge are inherently limited.²⁴

Fig. 1 Annual number of publications on membrane-based ion selective separation technologies from 2005 to 2025.

Fig. 2 Schematic comparison of NF, ED, and SED highlighting distinct membrane architectures and characteristics.

Electrochemical membrane separation, driven by an applied electric field, has emerged as a versatile candidate for selective ion recovery.^25–28 Unlike pressure-driven systems, electrochemical processes operate under mild conditions and can be readily scaled in a modular manner. Moreover, the increasing availability of renewable electricity has further strengthened their relevance from an energy- and sustainability-governance perspective, as ion transport is driven directly by electrical energy. Among electrochemical methods, electrodialysis (ED) represents one of the most mature and widely implemented membrane-based platforms for desalination, wastewater treatment, and resource extraction.^4,29–34 ED offers several distinct advantages, including high ion recovery efficiency, low chemical consumption, and excellent scalability for continuous operation. A typical ED stack comprises alternating ion-exchange membranes (IEMs)—cation-exchange membranes (CEMs) and anion-exchange membranes (AEMs)—that separate diluate and concentrate compartments, enabling ions to migrate across the membranes under an applied electric field.^28,35

Conventional IEMs, such as sulfonated polystyrene-divinylbenzene (SPS-DVB) materials, consist of dense, non-porous polymer matrices that contain hydrated ionic clusters and free-volume pathways.^36–39 Ion transport proceeds through these disordered hydration networks and is governed by the distribution of fixed charges and local water content—resulting in counter-ion electromigration and co-ion exclusion via Donnan partitioning (Fig. 2). Because these pathways are not structurally defined channels and involve no bulk water flow, conventional IEMs function as ion-exchange media rather than “nanochannel membranes”. Although they efficiently separate cations from anions, their dense polymer networks and charge-based exclusion offer limited control over ions with similar valence or hydrated size.

To address this limitation, ED is now evolving toward selective electrodialysis (SED), where membranes incorporate nanoporous architectures, tuneable surface charge, and tailored chemical functionalities.^{25,33,34,40–44} By integrating NF-derived nanochannel engineering with electric-field-driven transport, SED shifts the role of the nanochannel from a predominantly convection-based geometric sieve, as in NF, to an electrostatic and dehydration-energy-regulated gating pathway. Through this nanochannel-mediated gating under an electric field, SED achieves confinement-, dehydration-, coordination-based discrimination among same-charge ions, surpassing the capabilities of conventional IEMs (Fig. 2).^45–47

Over the past decade, NF has progressed toward smaller pores and stronger surface-charge regulation, while traditional ED has increasingly adopted nanochannel design principles originally developed in NF—a convergence that has progressively blurred the structural and conceptual boundaries between the two technologies.^32,48,49 In some reports, the same membrane materials have been employed for both NF and SED, further contributing to conceptual ambiguity.^50–52 As a result, key factors governing SED performance—such as nanochannel transport mechanisms, the relative contributions of different selectivity-driving interactions under an electric field, the role of channel geometry and interfacial chemistry, and differences among material platforms—remain inconsistently defined. Despite numerous membrane-focused review articles,^{25,37,53–55} a comprehensive framework that integrates electrochemical driving forces with nanochannel-specific design concepts is still lacking.

To address this gap, this review aims to integrate the fundamental principles of electrochemical ion transport in SED with recent advances in nanochannel membrane design. Here, nanochannel membranes are defined as systems that are fundamentally based on geometrically defined sub-nanometer channels, with additional selectivity introduced through surface-governed transport arising from electric double-layer (EDL) overlap, ion dehydration, and coordination interactions. This definition primarily encompasses materials with structurally ordered nanochannels—such as aligned carbon nanotubes (CNTs), 2D laminates, metal–organic frameworks (MOFs)/covalent-organic frameworks (COFs), and crystalline ceramic nanochannels—but also includes selected polymeric membranes (e.g., ultrathin polyamide (PA) layers) that may not exhibit perfect geometric definition yet operate within the nanochannel regime under SED conditions.

This review first provides a systematic overview of ion-selective transport mechanisms in nanochannels, including (i) dehydration-based partitioning at the channel entrance, (ii) intra-channel ion–pore interactions governing migration, and (iii) dimensionality-dependent transport behaviour in 1D/2D/3D channels. Building on this mechanistic framework, we then examine representative material platforms for constructing nanochannel membranes and their corresponding SED performance. These include polyamide-based membranes (chapter 3), 2D nanosheet laminates (chapter 4), crystalline porous frameworks such as MOFs and COFs (chapter 5), and ceramics (chapter 6). Finally, we propose design criteria for controlling channel dimensions, interfacial chemistry, and structural stability, and discuss the remaining challenges that must be addressed to translate nanochannel-enabled SED concepts into practical, high-selectivity ion recovery technologies.

2 Mechanism of ion-selective transport

The general process of ion transport through membrane can be described by the extended Nernst–Planck equation, which incorporates the combined effects of diffusion, electromigration, and convection.^56–59
Here, J_i is the flux of ion i, D_i is the diffusion coefficient, C_i is the concentration, z_i is the ionic valence, ϕ is the electric potential, F is Faraday's constant, R is the universal gas constant, T is absolute temperature, and υ is the fluid velocity. In typical ED membranes, convection term is often negligible (C_iυ = 0), and ion transport is predominantly driven by diffusion and electric-field gradients, which can be expressed as following equation.
Building on this continuum description, the solution–diffusion framework has been widely used to elucidate the transport mechanism by using a single parameter (e.g., pressure gradient, concentration gradient).⁶⁰

However, ion transport through nanochannel membranes arises from a complex interplay of physical confinement, electrostatic modulation, and specific chemical affinity. Moreover, when the characteristic channel dimension approaches the Debye length or the hydrated ion diameter, continuum descriptions become inadequate. In such nanoconfined membranes, nanoscale confinement within well-defined pores gives rise to non-continuum transport, with emergent mechanisms that modulate permeability and selectivity far beyond bulk-scale predictions.^61–64

Where the solution–diffusion model falls short, transition-state theory provides a complementary, mechanistic account of ion transport under confinement.^65–67 Within the transition-state theory framework, ion transport proceeds a sequence of energy barriers associated with (i) partial dehydration at the pore entrance, (ii) ion migration through the nanochannel, and (iii) rehydration at the nanochannel exit (Fig. 3a).^55,63,66 Among these, the selectivity between ions primarily emerges in the first two stages, which are the focus of this section. In addition, the ion-transport behaviour in the nanochannel is strongly dictated by the confinement geometry. One-dimensional tubes, two-dimensional slits, and three-dimensional porous networks impose different levels of restriction and surface interactions, affecting the ionic mobility.

Fig. 3 (a) Schematic illustration of ion transport through nanochannels; (b–d) mechanism factors for ion selectivity and energy fluctuation in nanochannels, (b) steric effect; (c) charge density; and (d) coordination (adapted from ref. 63, © American Chemical Society, copyright 2024).

Collectively, ion transport through nanochannels arises from a complex set of coupled mechanism factors (e.g., steric effect (Fig. 3b), charge density (Fig. 3c), coordination (Fig. 3d)) that are highly sensitive to physicochemical confinement. These interdependencies highlight the strong correlation between the channel properties and ionic selectivity. This section presents a detailed discussion of these concepts.

2.1 Nanochannel entry: dehydration partitioning

The initial stage of ion permeation involves the transition from the bulk aqueous phase to the confined interior of the nanochannel. In aqueous environments, ions are stabilized by hydration shells and enter sub-nanometre pores, which typically require partial or complete dehydration, an energetically costly process governed by the hydration enthalpy of ions. The difference in hydration energies enables selective partitioning based on the thermodynamic cost. For example, multivalent ions such as Mg²⁺, with a small ionic radius and high charge density, exhibit strong dehydration energy (−1830 kJ mol⁻¹), whereas monovalent ions like Li⁺, Na⁺, or K⁺ are weakly hydrated and thus more readily undergo dehydration upon pore entry (Table 1).^68,69 Moreover, discrimination among monovalent cations can be realized, as their passage through sub-nanometre pores is modulated by distinct dehydration energies. In particular, Li⁺, with the highest hydration enthalpy among alkali ions, faces a greater energetic barrier for partial dehydration compared to Na⁺ or K⁺, allowing membranes to exploit this difference for enhanced ion selectivity.

Table 1 Physicochemical parameters of representative cations relevant to ion transport through membranes

Cations	Bare ion radius (Å)	Hydrated ion radius (Å)	ΔGhyd (kJ mol^−1)
H^+	—	2.82	−1050
Li^+	0.6	3.82	−475
Na^+	0.95	3.58	−430
K^+	1.33	3.31	−295
Rb^+	1.48	3.29	−275
Cs^+	1.69	3.29	−250
Ag^+	1.26	3.41	−430
Mg^2+	0.65	4.28	−1830
Ca^2+	0.99	4.12	−1505
Mn^2+	0.8	4.38	−1910
Co^2+	0.72	4.23	−1915
Ni^2+	0.7	4.04	−1980
Cu^2+	0.72	4.19	−2010
Zn^2+	0.74	4.3	−1955
Al^3+	0.5	4.75	−4525
Cr^3+	0.64	4.61	−4010
Fe^3+	0.6	4.57	−4265

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This selectivity can be modulated by the geometric constraints of the nanochannels. When the effective pore size lies between those of the hydrated and bare ionic radii, steric hindrance forces the ions to partially shed their hydration shells to enter the channels this results in coupling between steric exclusion and dehydration energetics, wherein only ions capable of tolerating the associated free energy penalty can enter the channels (Fig. 3b).

The surface chemistry at the channel interface also plays a critical role. Depending on the fixed surface charge of the pore walls, electrostatic interactions may further influence ion entry (Fig. 3c). For instance, negatively charged channel entrances can repel multivalent anions or strongly bind hydrated divalent cations such as Mg²⁺, introducing an additional electrostatic energy barrier. Conversely, monovalent cations such as Li⁺ or Na⁺—which experience weaker repulsion owing to their lower charge density—may more readily overcome this barrier.⁷

In addition to these structural effects, nanoscale confinement fundamentally alters the dielectric environment, which in turn affects the energetics of dehydration. Molecular dynamics (MD) simulations by Ritt et al. revealed that the dielectric constant of water confined in 2 nm channels is significantly lower than that of bulk water, decreasing the ability of water to stabilise an ionic charge.⁷⁰ This dielectric suppression originates from the restricted rotational degrees of freedom and the spatial alignment of water molecules near the solid interfaces. Epsztein et al. showed that interfacial water near pore walls formed layered structures with strongly oriented dipoles.⁷¹ These layers possess low polarizability, exhibiting dielectric constants as low as ε ≈ 2.1, substantially lower than ε_bulk ≈ 80. These confinement-induced polarisation effects reduce the effective solvation capability of water, thereby promoting dehydration upon channel ingress. Theoretical and experimental studies have demonstrated that the presence of an external electric field changes the hydration landscape. Under electric fields, particularly near the pore entrances, the orientation and dynamics of water dipoles can become more ordered, further limiting their ability to reorient and stabilise the ions. The electrostatic stiffening of the interfacial water layer enhances the free energy barrier for strongly hydrated ions and also modifies the energy landscape to preferentially accelerate ions with lower dehydration costs. Consequently, the field strength acts as an active tuning parameter for partition selectivity, enabling dynamic control over ion entry.

Overall, ion partitioning at the nanochannel interface is governed by an intricate balance between the hydration energy, pore geometry, interfacial dielectric effects, and applied electric field. These factors collectively contribute to the first, and often the most decisive, selectivity gate in nanochannel-based ion separation.

2.2 Intra-channel migration: ion–pore interactions

Once inside the nanochannel, ions traverse a confined environment where their mobility is affected by a subtle interplay between steric constraints, electrostatics, and specific ion–wall interactions. Thermodynamically, ion discrimination can be expressed by the relative binding free-energy difference. For example, K⁺ selectivity on Na⁺ with the Gibbs free energy difference can be explained by the following equation.⁷²

ΔΔG (K⁺ → Na⁺) = (G_pore (Na⁺) − G_bulk (Na⁺)) − (G_pore (K⁺) − G_bulk (K⁺)) = ΔG_pore (K⁺ → Na⁺) − ΔG_bulk (K⁺ → Na⁺)

When the free energy becomes larger than zero, it indicates that preferential stabilisation of K⁺ within the nanochannel. Whereas entry is largely governed by dehydration penalties, intra-channel migration proceeds over a position-dependent free-energy landscape set by the channel's architecture and chemistry and can involve intermittent partial de-/re-hydration at internal constrictions or coordination sites that act as ion diffusion barriers (Fig. 3a).

In charged membranes, fixed functional groups on surface contribute to Donnan exclusion, repelling co-ions and enriching counter-ions within the channel, whereas groups embedded inside the pore can invert these trends by strongly binding counter-ions and facilitating co-ion passage via transient ion pairing (Fig. 3d). This phenomenon is most effective under low-ionic-strength conditions, where the EDLs extend and overlap, leading to significant ion selectivity. In contrast, at high ionic strengths, charge shielding compresses the EDL, diminishing the Donnan potential and reducing the selectivity. Moreover, differences in the hydration number affect how closely ions approach the charged surface, further modulating the strength of electrostatic interactions.^55,73,74 To further illustrate how EDL overlap and nanoscale confinement result in intra-channel ion migration under ED condition, Razmjou et al. conducted MD simulation in lamellar vermiculite channels.⁷⁵ When the interlayer spacing was sub-nanometre (0.4–0.8 nm), Li⁺ followed a two-surface, wall-to-wall hopping pathway, while widened to ∼1.2 nm weakened coupling to the opposite EDL and yielded a single-surface mode. This width-controlled behaviour was attributed to charge regulation and spontaneous symmetry breaking between facing walls and was accompanied by field-assisted partial dehydration.

Beyond general electrostatics, specific chemical interactions between ions and functional moieties on the channel wall also play a decisive role. Groups such as –SO₃H and –COOH can form coordination complexes with target ions through Lewis acid–base pairing, hydrogen bonding, or ion–dipole interactions. These interactions can thermodynamically stabilise the partially dehydrated ions, effectively facilitating their transport across membranes. Consistent with this mechanism, DFT calculations by Zhang et al. show that an ionic imidazole group strongly anchors to a GO nanosheet (binding energy −60.62 kJ mol⁻¹), tightening the slit-like confinement and sharpening the exclusion of hydrated divalent ions, while the ionic sulfonic group exhibits appreciable affinity for water (binding energy to H₂O −31.34 kJ mol⁻¹), sustaining a hydrophilic corridor that promotes rapid water permeation and preferential migration of hydrated monovalent cations.⁷⁶ Notably, this coordination-driven selectivity can compensate for the dehydration penalty incurred at the pore entrance, particularly for divalent cations. However, excessively strong binding may result in kinetic trapping or reduced diffusivity, highlighting the trade-off between ion selectivity and permeability.⁷⁷

The influence of the channel architecture is also significant. Parameters such as pore length, tortuosity, surface roughness, and spatial periodicity of binding sites affect the transport dynamics.^17,78 In highly crystalline nanoporous materials, ion migration can occur via a site-to-site hopping mechanism rather than continuous diffusion.⁷⁹ In this regime, transport is controlled by the energy barriers between adjacent coordination sites, making it sensitive to ion size, charge density. Such mechanisms are particularly relevant under low-solvent or anhydrous conditions where traditional solvation-driven diffusion is suppressed.

In summary, intra-channel ion transport is a multifaceted balance between weak and strong interactions, structural confinement, and the dynamic field effect. By rationally tuning these parameters through surface functionalisation, pore design, and field modulation, it is possible to engineer ED membranes with precise discrimination of ions, even among species with similar sizes or charges.

2.3 Dimensional effects in ion transport

Ion transport through nanometre-scale channels is significantly affected by the geometry of the confined architecture. Structures such as narrow tubes (1D),⁶¹ layered slits (2D),⁸⁰ and porous networks (3D)⁸¹ impose distinct degrees of confinement and surface interactions, thereby modulating ion mobility. With recent advances in nanomaterial synthesis and structural control, these channels can now be fabricated with high precision, enabling systematic investigations of the effects of dimensionality on ionic behaviour. This section explores the ion transport across 1D, 2D, and 3D nanochannels (Fig. 4), emphasizing the representative materials, transport characteristics, and practical challenges associated with the integration of these structures into membrane systems.

Fig. 4 Comparative summary of nanochannel architectures and general pros and cons. Red arrows illustrate dominant ion pathways.

1D nanochannels, such as carbon nanotubes (CNTs)⁸² and boron nitride nanotubes (BNNTs),⁸³ provide highly confined pathways in which ions traverse straight, narrow conduits. This geometry restricts lateral motion and promotes unidirectional ion flow, directly influencing the transport dynamics. The ion mobility observed in these systems is attributed to the synergy between geometric confinement and surface smoothness. For instance, the inner surfaces of CNTs are atomically smooth, minimising frictional resistance for partially dehydrated ions. Furthermore, the linearity of 1D pathways reduces ion–ion collisions, in contrast to 2D or 3D channels, where ions may disperse or follow tortuous trajectories. This streamlined configuration reduces scattering and suppresses back-diffusion under strong driving forces. Studies have reported enhanced conductance and reduced resistance in 1D channels, particularly when the pore diameter closely matches the size of the hydrated ions.⁸⁴ Nevertheless, the performance of membranes with 1D channels is inherently defect-sensitive; vacancies, distortions, or chemical heterogeneities readily perturb the confined conduction pathway, increase resistance, and reduce selectivity. In addition, 1D channel systems inherently restrict the entry and exit points for mass transport, requiring vertical alignment of 1D materials to enable efficient ion permeation. However, owing to the laminar structure of macroscopic membranes, these 1D structures tend to lie flat along the membrane plane during fabrication, thereby impeding ion-selective transport through their axial pathways. These challenges highlight the gap between idealized nanoscale transport and practical, scalable implementation.

2D nanochannels—commonly formed between stacked nanosheets such as graphene oxide (GO),⁸⁵ molybdenum disulfide (MoS₂),⁸⁶ or MXene⁸⁷ materials—form slit-like pathways through which ions migrate between parallel atomic planes. These layers typically exhibit sub-nanometre to a few nanometre spacings, forming confined channels that selectively permit ions based on size, hydration energy, and charge. A defining structural feature of 2D nanochannels is their lamellar alignment that promotes lateral ion migration across the membrane plane. Critically, their transport behaviour can be systematically tuned by modifying three key geometric parameters: the interlayer spacing, membrane thickness, and nanosheet lateral size.^88,89 The optimisation of these variables enables control over the trade-off between selectivity and permeability, shortens the ion transit length, and reduces the internal resistance. In addition to tunability, 2D nanochannels offer advantages in membrane fabrication. Unlike 1D or 3D structures, which may suffer from limited membrane coverage and involve challenging assembly processes, 2D materials can be processed into large-area continuous films with uniform and controllable nanochannel architectures.⁸⁵ These features make 2D membranes promising candidates for water purification, energy storage, and lithium extraction applications.

3D nanochannels are interconnected networks typically found in porous crystalline materials.⁸¹ Unlike the linear or planar confinement observed in 1D and 2D systems, respectively, 3D channels facilitate isotropic ion transport through complex branching networks. These architectures often involve entry apertures that require partial ion dehydration upon access, followed by rehydration within interior cavities.^77,79,81 This sequential dehydration–rehydration process is a key determinant of ion selectivity and mobility. Despite the modest selectivity at a single constriction, successive dehydration–rehydration events along interconnected pores act as multiple energetic sieves, progressively amplifying subtle differences in the hydration energy and ion–channel interactions. From a transport perspective, 3D nanochannels offer an extensive internal surface area that can be chemically tailored to favour specific ion interactions. The modular design of framework materials enables the precise adjustment of pore size, charge distribution, and functional group placement, allowing for highly selective ion sieving. However, the tortuous pathways and frequent restructuring of the hydration shells generally slow ion migration compared to 1D or 2D systems, particularly for bulky or strongly hydrated ions.⁸¹ Nevertheless, 3D nanochannels are appealing because of their crystallinity, structural order, and exceptional mechanical and chemical stabilities. The tuneable architectures and rich chemistries of 3D membranes are favourable for applications in ion separation, electrochemical devices, and sensing. A critical challenge remains in integrating these materials into continuous membranes while preserving their intrinsic structures and avoiding transport bottlenecks at the interfaces or grain boundaries.

A wide range of materials has been explored to construct nanochannels, each with distinct structural and chemical characteristics (Fig. 5).^90–92 Recent research focused on modifying these materials to enhance their selectivity and transport performance and integrating them into membrane platforms. The following sections provide an in-depth overview of the representative material classes, highlight their unique features and modification strategies, and discuss recent advances in their application to ED-based separation technologies.

Fig. 5 Comparative summary of nanochannel characteristics and general pros and cons of representative membrane materials for SED (adapted from ref. 90 with permission from Elsevier, copyright 2024. Adapted from ref. 91 with permission from American Chemical Society, copyright 2007. Adapted from ref. 92, © 2025 Nature Portfolio. Published under the CC BY license).

3 Polyamide

PA-based membranes, used in pressure-driven NF and RO plants, are benchmark systems for desalination.⁹³ Their popularity is based on their high water/ion separation efficiency, scalable fabrication, and mechanical durability.^94–96 PA is synthesised via interfacial polymerisation (IP), a rapid cross-linking reaction between an amine monomer and trimesoyl chloride (TMC) that occurs at the interface of two immiscible solutions.⁹⁷ The aqueous phase contains diamine monomers, most commonly m-phenylenediamine (MPD) or piperazine (PIP), whereas the organic phase contains TMC. In a typical synthesis procedure, a porous support substrate is impregnated with an aqueous MPD solution, followed by exposure to the organic TMC phase to trigger IP. Upon contact, these monomers react instantly at the liquid–liquid interface, forming a dense and crumpled PA network with abundant nanochannels. The PA structure features interconnected nanoscale voids that form irregular but continuous 3D nanochannels as molecular transport pathways. This PA-selective layer is often fabricated as a thin film composite (TFC), where the ultrathin PA layer is deposited on a microporous membrane skeleton. In this configuration, sub-nanometre voids within the PA layer provide selective resistance against hydrated ions based on steric hindrance and partial dehydration, while enabling efficient water permeation.

Recently, PA membranes have attracted attention for their use in electrochemical ion recovery and selective ion separation from complex solutions.⁹⁸ Although the classical IP route is the current industrial standard, it often suffers from structural inhomogeneity, including depth-dependent variations in the cross-linking density and pore-size distribution.⁹⁹ These internal inconsistencies can generate local variations in the mass-transport resistance, thereby limiting the membrane performance. Accordingly, recent efforts have focused on tailoring the nanoarchitecture of PA membranes to optimize the 3D nanochannels. This section outlines the categories used for the development of PA membranes for ion-selective separation.

3.1 Fabrication of PA-based membrane for SED

3.1.1 Selection of monomers. To overcome the limitations of classical IP, researchers have explored the use of alternative monomers or novel reaction environments that modify the chemical nature and network topology of the PA layers. Kong et al. studied the role of the amine monomer chemistry in determining the Li⁺/Mg²⁺ separation performance of TFC membranes in electrodialysis (Fig. 6a).¹⁰⁰ PA membranes were fabricated by reacting TMC with three distinct amine monomers, MPD, PIP, and PEI (polyethyleneimine), on an amine-functionalised sulfonated poly(ether ketone) (SPEEK) substrate. This comparative study allowed the authors to isolate the effects of the monomer structure on ion selectivity and electrochemical performance. Among the candidates, the PEI-based membranes exhibited the highest Li⁺/Mg²⁺ selectivity, which was attributed to the branched architecture and higher amine density of PEI, which favour Li⁺ transport while effectively excluding divalent Mg²⁺. Additionally, the PEI membranes demonstrated superior current efficiency and a stable voltage response during long-term operation. These results demonstrate that monomer selection is important for controlling the PA network morphology and ion discrimination, especially for monovalent/divalent separations such as Li⁺/Mg²⁺. This study highlighted the importance of monomer-engineered IP for tuning the ion selectivity of PA membranes when used with chemically active substrates such as SPEEK.

Fig. 6 Strategies for tailoring PA membranes. (a) Schematic of PA synthesis on SPEEK supports using various amine monomers (PEI, PIP, MPD) (adapted from ref. 100 with permission from American Chemical Society, copyright 2025); (b) additive-assisted interfacial polymerization using sodium lauryl sulfate (SLS) to regulate monomer diffusion (adapted from ref. 102 with permission from John Wiley and Sons, copyright 2020); (c) TFNi strategy using GO nanosheets as an interlayer between substrate and PA to control monomer diffusion (adapted from ref. 107 with permission from Elsevier, copyright 2018).

A complementary strategy was demonstrated by Jiang et al., which involved the design of crown-ether-functionalised amine monomers capable of selectively coordinating Li⁺ ions.¹⁰¹ The IP reaction naturally embedded Li⁺-selective sites into the PA film by introducing an ion-specific recognition motif directly into the monomer backbone. This facilitated the selective transport of Li⁺, even in the presence of interfering multivalent ions such as Mg²⁺ or Ca²⁺, which are common in brine and battery leachate environments. The study reported a 2.5-fold increase in Li⁺/Mg²⁺ selectivity without compromising membrane integrity or mechanical stability.

3.1.2 Additive-assisted interfacial polymerization. Chemical additives are a powerful yet easily implementable tool for optimising IP without altering the primary monomeric structure. Additives can regulate factors such as the monomer diffusion rate, reaction zone stability, and polymer network compaction, all of which affect the resulting PA morphology. A notable example is the study by Sarkar et al., in which an anionic surfactant, sodium lauryl sulfate (SLS), was introduced into an aqueous amine solution (Fig. 6b).¹⁰² SLS addition reduced the interfacial tension and limited excessive cross-linking, allowing the formation of a more uniform and compact PA layer. This enhanced the rejection of divalent cations, which was attributed to better packing of the polymer chains and reduced nanochannel defect formation.

Shen et al. explored a salt-assisted IP strategy in which NaCl was added to both the aqueous and organic phases during the IP reaction.¹⁰³ The osmotic pressure gradient induced by salt addition increased the MPD diffusion rate during the IP process, resulting in a PA layer with a denser pore structure. This method was especially effective in reducing nonselective voids in the active layer and fine-tuning membrane porosity without modifying the monomer chemistry.

3.1.3 Filler-assisted interfacial polymerization. Recently, the addition of inorganic fillers has evolved beyond classical monomer-only strategies. These strategies are broadly classified into two categories: 1) thin-film nanocomposite (TFN) membranes, in which nanomaterials are directly incorporated within the TFC; and 2) TFN membranes with an interlayer (TFNi), in which a nanomaterial interlayer is deposited between the TFC and its substrate.^104,105 In TFN membranes, fillers such as GO, CNTs, and porous nanoparticles (e.g., zeolites and MOFs) are typically dispersed in either the aqueous or organic phase during IP.¹⁰⁴ These fillers alter the local polymerisation environment, introduce additional nanochannels for ion transport, and improve mechanical and chemical robustness. Sorribas et al. embedded various MOF nanoparticles, including hydrophilic MIL-101(Cr), into a PA layer during IP by dispersing them in an organic phase.¹⁰⁶ Among them, MIL-101(Cr), with its large pore size (∼3.4 nm) and high surface area (∼2300 m² g⁻¹), substantially increased solvent permeance (up to 11.1 L m⁻² h⁻¹ bar⁻¹ in THF) without compromising rejection. The MOF particles are homogeneously embedded within the PA matrix, providing permeation-facilitating pathways while preserving dense polymer packing. The hydrophilic MIL-101(Cr) helped retain the monomers near the interface, thereby promoting a more uniform and tightly cross-linked PA structure. Transmission electron microscopy and X-ray photoelectron spectroscopy analyses confirmed that the MOFs were well-integrated into the PA matrix, forming defect-free nanocomposite layers.

In contrast, TFNi membranes introduce a functional interlayer between the substrate and PA layer to regulate monomer diffusion and PA morphology. Lai et al. pre-deposited GO nanosheets on a support surface via vacuum filtration before IP, forming a GO-rich interlayer (Fig. 6c).¹⁰⁷ This hydrophilic interlayer serves as a monomer reservoir, selectively retaining PIP and facilitating its gradual release during contact with TMC. Consequently, the IP reaction occurs in a more controlled, diffusion-limited regime, producing a thinner, denser, and more cross-linked PA layer. At the optimal GO loading (0.02 g m⁻²), the membrane exhibited a 31.4% increase in water permeability and >95% salt rejection, without the structural defects observed at higher GO concentrations. Moreover, positron annihilation spectroscopy revealed a reduced free volume in the optimized PA layer, indicating tighter polymer packing owing to controlled monomer diffusion.

3.2 Surface coating

The surface modification of PA NF membranes with ammonium-rich polyelectrolytes can markedly improve ion selectivity. Foo et al. demonstrated that applying a surface coating on a PA membrane increased the density of positively charged NH₂⁺ groups in the active layer, sustaining a net positive surface charge over a wide pH range.¹⁰⁸ This modification amplified Donnan exclusion against multivalent cations without altering the intrinsic pore size distribution or compromising hydrophilicity. Although the coating slightly decreased water permeability (12–19%) owing to enhanced hydraulic resistance, it consistently enhanced monovalent/divalent separation. In salt-lake brine tests, the Li⁺/Mg²⁺ separation factor increased by up to 70% at pH 2, lowering residual Mg²⁺ in the permeate to 0.14%. In battery leachates, Li⁺ purity increased to ∼98% in a single NF stage, with the largest improvement observed at low pH, where NH₂⁺ group density was highest. At process scale, coated membranes reduced Mg²⁺ permeate concentration by one order of magnitude with only ∼14.7% higher specific energy consumption, demonstrating their operational viability for high-purity lithium recovery.

3.3 SED applications of PA membranes

PA membranes have recently attracted attention for use in Li recovery. Their sub-nanometre pore structures and tuneable surface charges are currently being investigated as functional elements for electrochemical separation under an applied electric field. A fundamental contribution to the understanding of PA membranes for Li recovery was provided by Wang and Lin (Fig. 7a). This study established a theoretical framework that compares the performance of monovalent-selective CEMs with that of PA-based NF membranes under ED conditions.⁵⁰ Using a unified solution–friction transport model, this study elucidated the effects of the pore size of the PA selective layer and surface charge density on Li⁺/Mg²⁺ separation. The PA-coated CEMs exhibited transport characteristics comparable to those of PA-based NF membranes, thereby positioning the PA-selective layer as a critical component for Li⁺/Mg²⁺ discrimination in ED systems. Thus, this foundational work demonstrates that PA chemistry, originally developed for desalination, can be rationally leveraged for selective lithium recovery.

Fig. 7 (a) Lithium recovery performance comparison of PA membranes under (left) ED and (right) NF conditions (adapted from ref. 50, © 2024 American Chemical Society. Published under the CC BY license); (b) mono-/di-valent cation transport characteristics of dendrimer-functionalised PA membranes (M-CEM_G₃), showing (left) permeation rate as a function of hydrated ion diameter, (middle) ion selectivity, and (right) composition of Li⁺/Mg²⁺ selectivity with literature values (adapted from ref. 109 with permission from American Chemical Society, copyright 2024).

Building on this conceptual basis, Wu et al. developed a dendrimer-assembled PA membrane, in which ionic polyamidoamine (PAMAM) dendrimers were integrated to form ångström-scale nanochannels within the PA layer (Fig. 7b).¹⁰⁹ The study aimed to mimic biological ion channels by combining exterior electrostatic exclusion with interior free-volume pathways. The incorporation of higher-generation dendrimers yielded thinner and smoother nanofilms, reduced steric hindrance, and formed well-defined voids accessible for transport. The resulting hybrid membranes exhibited remarkable ion selectivity (Li⁺/Mg²⁺ = 11) while maintaining a high monovalent flux. This study demonstrated that carefully designed nano-architectures can overcome the classical permeability–selectivity trade-off observed for conventional PA layers.

The SED performance of PA based membranes are summarised in Table 2. Despite their proven performance in SED applications, PA-based membranes still face critical durability challenges such as hydrolysis, scaling, and fouling. While these issues have been extensively studied in desalination processes, their impact under long-term SED operation remains underexplored.¹¹⁰

Table 2 SED performance of PA based membranes

Membrane	Fabrication method	Operating condition	Performance metrics	Ref.
PA-CEM	—	0.1 M LiCl, 0.1 MgCl2 (binary)	Li^+ flux 3.5 mol m^−2 h^−1	50
		100 A m^−2, coupon scale membrane	Li^+/Mg^2+ selectivity 25
		0.01 M LiCl, 0.1 MgCl2 (binary)	Li^+ flux 0.8 mol m^−2 h^−1	50
		100 A m^−2, coupon scale membrane	Li^+/Mg^2+ selectivity 6
M-CEM-G3	Dendrimer-assembled IP	0.1 M KCl, NaCl, LiCl, CaCl2, and MgCl2 (mixed solution), 20 mA cm^−2, membrane area 7.07 cm^−2	Li^+ flux 1.5 mol m^−2 h^−1	109
			Li^+/Mg^2+ selectivity 11
PA/SPEEK	IP using PEI as amine monomer	0.05 M LiCl, NaCl, KCl, MgCl2, CaCl2 (mixed solution) 0.04 mA cm^−2, membrane area 5 cm^−2	Li^+ flux 0.035 mol m^−2 h^−1	100
			Li^+/Mg^2+ selectivity 140
	IP using PIP as amine monomer		Li^+ flux 0.048 mol m^−2 h^−1	100
			Li^+/Mg^2+ selectivity 430
	IP using MPD as amine monomer		Li^+ flux 0.045 mol m^−2 h^−1	100
			Li^+/Mg^2+ selectivity 200

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4 2D nanosheet-based membranes

Since the emergence of 2D nanomaterials, laminar membranes composed of stacked 2D building blocks, such as graphene oxide (GO), MXenes, clays, and 2D MOFs, have attracted significant attention for membrane-based separation.^16,17,111 Their key structural advantage lies in the well-defined interlayer spacing between the nanosheets, which originates from the intrinsically low height-to-lateral length ratio (typically a few nanometres thick and several micrometres wide). These slit-like interlayer gaps, with dimensions ranging from sub-nanometres to a few nanometres, serve as ion-selective nanochannels.^112,113 In addition, the planar geometry of 2D materials allows scalable membrane fabrication via techniques such as vacuum filtration, blade coating, and spray deposition.¹¹⁴ However, the resulting lateral transport pathways are often highly tortuous, which significantly limits ion permeability.^115,116 Although shortening these pathways can enhance flux, it often disrupts the formation of high-quality laminated microstructures.¹¹⁷

Rational design of the interlayer architecture is essential to fully harness the potential of 2D lamellar membranes. This section reviews recent strategies for optimising the performance of laminated membranes, including interlayer texture engineering, surface charge modulation, and intercalation of functional additives.

4.1 Effects of fabrication methods on the microstructure of 2D-based membranes

Filtration and evaporation methods for assisting 2D nanomaterial stacking are among the most studied methods for fabricating 2D laminated membranes. By applying vacuum or pressure to a dispersion containing 2D nanomaterials, the nanosheets are deposited onto a substrate while the solvent is removed from the dispersion. This process is simple and applicable to various 2D materials, and the membrane thickness can be tuned by adjusting the dispersion concentration or volume. However, the electrostatic repulsion between nanosheets pushes them apart,¹¹⁸ resulting in defective membrane stacking. Tsou et al. investigated the effects of stacking methods on the microstructure of multi-layered GO membranes.¹¹⁹ They prepared three different GO membranes via pressure-, vacuum-, and evaporation-assisted self-assembly (PASA, VASA, and EASA, respectively) using the same GO stock solution. PASA, based on a downward pressure-driven filtration, formed the most compact and ordered GO laminates with the smallest and most uniform interlayer spacing (8.4 Å), while EASA, driven by upward solvent evaporation, resulted in the loosest and most disordered structure. These structural variations directly influenced membrane performance, with a GO-PASA/modified polyacrylonitrile membrane exhibiting the highest water/butanol separation performance. The authors stated that a highly ordered interlayer spacing is beneficial for maximising separation performance. Furthermore, Huang et al. demonstrated that shear forces applied to liquid-crystalline MXene (LCMM) dispersions induced highly aligned nanosheet stacking during membrane formation.¹²⁰ To induce shear forces, membranes were prepared by doctor-blade casting of a critical-concentration (∼15 mg mL⁻¹) LCMM dispersion. Although the resulting LCMM and non-LCMM membranes exhibited similar interlayer spacings (∼6 Å), their X-ray diffraction peak widths differed significantly; narrower peak widths reflect greater structural uniformity and reduced disorder in LCMM. The precise lamellar alignment in the membranes greatly enhanced Li⁺ permeability and Li⁺/M⁺ selectivity compared to conventionally cast membranes.

4.2 Functionalisation of 2D nanosheets

4.2.1 Modulating ion-selective interlayer spacing. The interlayer spacing significantly affects the ion-separation performance of laminated 2D membranes. In general, narrower interlayer spacings decrease permeability and increase selectivity for larger molecules. However, most laminated 2D membranes suffer from unwanted swelling of the interlayer spacing when exposed to aqueous environments, making it difficult to achieve high-precision separation. Typically, the interlayer spacing is tailored by the reduction or cross-linking of laminated membranes. For instance, Lu et al. conducted thermal annealing of MXene (Ti₃C₂T_x) stacked membranes, resulting in the reduction of the interlayer spacing of the membrane (Fig. 8a).¹²¹ The thermally treated membranes exhibited a 25% decrease in water permeability, with an NaCl rejection of 98.6%. In addition, compared with a bare MXene membrane, the thermally treated membranes showed a dramatic increase in interlayer stability in an aqueous environment.

Fig. 8 Strategies for nanochannel engineering in 2D-material-based membranes for ion-selective separations. (a) Interlayer spacing control via reduction (adapted from ref. 121 with permission from American Chemical Society, copyright 2019); (b) cross-linking by intercalating cross-linkers (adapted from ref. 122 with permission from American Chemical Society, copyright 2014); (c) surface functionalisation (adapted from ref. 126 with permission from Elsevier, copyright 2021); and (d) utilising intrinsic in-plane pores (adapted from ref. 130 with permission from Elsevier, copyright 2021).

Another approach for regulating the interlayer spacing is to bridge adjacent nanosheets via interlayer cross-linking. To date, various cross-linkers have been developed, including small molecules, inorganic sols, metal oxides, and polymers. The addition of cross-linkers significantly enhances the stability of the interlayer spacing against swelling and enables sub-nanometre control of the spacing. Pioneering research by Hung et al. added diamine monomers of different sizes to GO membranes to investigate the role of the cross-linker (ethylenediamine, butanediamine, or p-phenylenediamine (PPD)) size on the separation performance (Fig. 8b).¹²² The interlayer spacing was proportional to that of the cross-linker, with the largest interlayer spacing of 1.09 nm observed for the PPD/GO membrane. In contrast, the ethanol permeation rate of the PPD/GO membrane was the lowest of all membranes. This was attributed to the interlayer mass-transfer resistance imparted by bulky PPD with an aromatic ring structure.

In addition, intercalating active fillers that exhibit strong selectivity toward target ions has been studied. These intercalants act as spacers to stabilise the interlayer spacing, and as functional moieties to mediate selective coordination, ion–dipole interactions or hopping-based transport. The integration of functional polymers into MXene membranes offers an effective route for tailoring the ion transport. Lu et al. demonstrated that poly(sodium 4-styrene sulfonate) (PSS) chains intercalated into lamellar Ti₃C₂T_x MXene nanosheets enhanced Li⁺ selectivity due to the presence of sulfonate groups that served as hopping sites for partially dehydrated Li⁺.¹²³ The MXene@PSS composite membrane achieved a high Li⁺ permeation rate (0.08 mol m⁻² h⁻¹) while discriminating effectively against multivalent and monovalent cations (Li⁺/Mg²⁺ selectivity ∼28), highlighting the synergy of physical confinement and chemical affinity in transport pathways.

4.2.2 Surface functionalisation. Surface coating with polyelectrolytes is a simple and effective strategy for enhancing ion selectivity in 2D membranes. Meng et al. coated Ti₃C₂T_x MXene laminates with PEI, which converted the surface from negatively to positively charged.¹²⁴ This modification significantly improved MgCl₂ rejection (from ∼35% to ∼82%) while maintaining reasonable water permeance, demonstrating the combined benefits of electrostatic repulsion and size sieving. Similarly, Zhang et al. applied various cationic or anionic polyelectrolytes to GO laminates, enabling tuneable surface charge polarity.¹²⁵ This adjustment yielded the selective exclusion of AB₂ and A₂B⁻ type salts and markedly enhanced salt rejection while preserving the intrinsically fast water-transport channels of the GO membranes. Atomic layer deposition is another effective method to tailor the surface charge and improve separation efficiency without compromising permselectivity trade-offs. Kim et al. demonstrated that plasma-enhanced atomic layer deposition of Al₂O₃ (3–9 cycles, ∼1.44 nm coating) on GO membranes substantially improved water permeability (from 32.9 to 68.0 LMH per bar) and NaCl rejection (from 46.6% to 63.8%) while preserving the interlayer spacing (Fig. 8c).¹²⁶

4.2.3 Porous 2D nanosheets. 2D membranes can host in-plane defects or intrinsic nanopores in their nanosheets.¹²⁷ These features act as extra molecular diffusion pathways, providing transport shortcuts in addition to the conventional interlayer slit channels. Thus, the introduction of in-plane porosity or defects can mitigate the tortuous transport pathways typical of stacked 2D laminates, thereby boosting permeance without compromising selectivity. Broadly, this can be realised either by using intrinsically porous 2D materials, such as COFs or MOFs,¹²⁸ or by deliberately introducing defects into otherwise dense nanosheets.¹²⁹

For example, Sui et al. incorporated 2D COFs into reduced graphene oxide (rGO) laminates.¹³⁰ COFs act as nanospacers to relieve the restacking of rGO, as stabilisers to limit swelling, and as porous fillers to provide intrinsic in-plane pores (Fig. 8d). When intercalated into rGO stacks, the 2D COFs shortened transport pathways by ∼15% and introduced additional pore channels (∼1.4 nm) that facilitated solvent passage. The optimised COF@rGO hybrid membrane exhibited a 162% enhancement in methanol permeance while retaining high dye rejection. This work highlighted that porous 2D fillers with crystalline in-plane nanochannels can deliver shortcut diffusion routes, overcoming the typical permeability limitations of 2D-based membranes. An alternative approach is to directly construct membranes from 2D MOF nanosheets. Jian et al. exfoliated monolayer aluminium-porphyrin-based MOF nanosheets and assembled them into ultrathin water-stable laminar membranes.¹³¹ Unlike dense graphene sheets, these MOFs inherently contain uniform ångström-scale apertures (∼6.1 Å) aligned across the nanosheets that act as primary transport channels. The resulting membranes achieved a water flux up to 2.2 mol m⁻² h⁻¹ bar⁻¹ while maintaining nearly complete salt rejection, outperforming most reported 2D laminates in water/ion selectivity.

Introducing in-plane defects into 2D membranes can enhance mass transport. Kim et al. reported nanoporous rGO membranes with controlled pore generation induced by hot pressing.¹³² The resulting structure enabled higher permeability while retaining molecular sieving capability, demonstrating that defect-engineered rGO membranes are efficient platforms for ion and small-molecule separation.

4.3 SED applications of 2D nanosheet membranes

The unique structural features of 2D membranes make them promising candidates for ion–ion separation and recovery. By leveraging the subtle differences in hydrated ionic radii, especially the relatively small hydration shell of Li⁺, 2D nanochannels can precisely sieve Li⁺ from complex brine mixtures. Recent studies reported diverse strategies for overcoming the intrinsic limitations of pristine GO membranes, such as swelling-induced instability and poor ion–ion selectivity. Kim et al. recently demonstrated a heteroatom aluminosilicate (AS) sol-reinforced GO membrane (HARD–GO) in which the unique properties of the AS sol were exploited (Fig. 9a).⁹⁰ Because of their sub-nanometre size and tuneable surface charge, AS nanodots can be uniformly intercalated into GO lamellae, enabling precise ångström-scale tuning of the interlayer spacing. This design enabled the electrostatic regulation of ion transport and substantially enhanced the structural stability of the laminar framework against long-term aqueous exposure. The optimal HARD–GO membrane exhibited exceptional separation performance, with Li⁺/Fe³⁺ selectivity ratios up to 40 under continuous redox-ED, along with a Li⁺ permeability of 2.0 mol m⁻² h⁻¹.

Fig. 9 (a) Schematic of redox-ED cell incorporating 50AS/GO membranes for Li⁺/Fe³⁺ separation and Li⁺ permeability and Li⁺/Fe³⁺ selectivity (adapted from ref. 90 with permission from Elsevier, copyright 2024); (b) schematic of asymmetric interlayer spacing of a GO-based membrane and corresponding energy barrier of various cations through three distinct interlayer regions (adapted from ref. 134 with permission from Elsevier, copyright 2022); (c) schematic of ion entry and recognition within vermiculite-PTA layers and Li⁺ permeability and Li⁺/K⁺ selectivity (adapted from ref. 136 with permission from American Chemical Society, copyright 2025).

To address the limited ion selectivity of GO membranes towards Li⁺ and chemically similar monovalent cations (Na⁺ and K⁺), tannic acid (TA) was introduced into the interlayer spacing of GO laminates as an ion-specific binder.¹³³ TA molecules, which are rich in catechol and galloyl groups, form crown-ether-like coordination sites within the GO interlayers, selectively binding Li⁺ while maintaining membrane integrity. The resulting TA–GO membranes demonstrated affinity-driven ion trapping, enriching Li⁺ over Na⁺ and K⁺ with trapping factors of 1.18 and 1.49, respectively. This highlights the potential of functional-group engineering for introducing specific ion-recognition capabilities into otherwise non-discriminating GO channels.

Furthermore, an asymmetric GO architecture was developed to overcome the insufficient Li⁺ selectivity of conventional symmetric GO/rGO channels (Fig. 9b).¹³⁴ By inserting high-affinity guest molecules, such as 4,4 diamino-2,2-stilbenedisulfonic acid, local rGO domains were generated within the GO laminates, creating GO–rGO junctions with abrupt interlayer transitions. These junctions acted as “energy-surge baffles” that disrupted hydration shells while enhancing cation binding, thereby coupling steric exclusion with chemical affinity. The asymmetric design imparted additional structural stability and achieved Li⁺ selectivity factors up to 7.5, a nearly six-fold improvement over pristine GO membranes.

Vermiculite is a naturally occurring 2D layered silicate composed of silica–oxygen tetrahedra and alumina–oxygen octahedra. Vermiculite layers are negatively charged owing to isomorphic Al³⁺ and Si⁴⁺ substitution. Compared with other 2D nanomaterials, vermiculite is naturally abundant, inexpensive (∼350 USD per ton), and exhibits excellent water/chemical stability in aqueous media.¹³⁵ Therefore, vermiculite is a practical platform for building nanofluidic channels for ion separation.

Dong et al. proposed a coordination-assisted ion-transport strategy by embedding phosphotungstic acid (PTA) ligands between layered vermiculite channels (Fig. 9c).¹³⁶ The strong Lewis acid sites provided by PTA created selective coordination environments that were coupled with Li⁺ dehydration, whereas the natural negative entry barrier of vermiculite further facilitated Li⁺ permeation. This hybrid design delivered a Li⁺ permeation rate of 2.96 mol m⁻² h⁻¹, nearly 16 and 9 times higher than those for K⁺ and Na⁺, respectively. A Li⁺/K⁺ separation factor of 6.07 under ED, while maintaining structural stability during extended operation. These results show that ligand-functionalised vermiculite nanochannels can overcome the long-standing limitations of Li⁺/Na⁺/K⁺ separation and move beyond the traditional permeability–selectivity trade-off.

The performance of 2D nanosheet-based membranes in SED applications is summarised in Table 3. These membranes leverage tuneable interlayer spacing and surface functionalities to achieve high ion selectivity, yet they remain susceptible to structural instability, swelling, and restacking during operation.

Table 3 SED performance of 2D nanosheet-based membranes

Membrane	Fabrication method	Operating condition	Performance metrics	Ref.
HARD–GO	Cross-linking GO interlayer by AS sol	0.01 M LiCl and FeCl3 (mixed solution), 1.2 V, membrane area 4 cm^2	Li^+ flux 2 mol m^−2 h^−1	90
			Li^+/Fe^3+ selectivity 42
TA–GO	Tannic acid incorporated into GO interlayer	0.5 M KCl, NaCl, LiCl, CaCl2, and MgCl2 (mixed solution), 1.0 V, 0.75 cm^2	Li^+ flux 0.61 mol m^−2 h^−1	133
			Li^+/Mg^2+ selectivity 3.4
Asymmetric-rGO/GO	Sulfonated guests drop-cast on half of the GO membrane surface	0.5 M KCl, NaCl, LiCl, CaCl2, and MgCl2 (mixed solution), 1.0 V, 1 cm^2	Li^+ flux 48 mol m^−2 h^−1	134
			Li^+/Mg^2+ selectivity 1.6
PTA/vermiculite	PTA incorporated into vermiculite interlayer	0.1 M LiCl and KCl (mixed solution), 0.8 V cm^−1	Li^+ flux 2.96 mol m^−2 h^−1	136
			Li^+/K^+ selectivity 6.1

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5 Crystalline porous frameworks

Crystalline porous frameworks, such as MOFs and COFs, have emerged as promising membrane materials owing to their unique pore structures and highly tuneable nanochannel chemistry.¹³⁷ MOFs comprise metal clusters and organic ligands,¹³⁸ whereas COFs comprise only light elements¹³⁹ (such as C, H, O, and N). These frameworks form rigid porous architectures through coordination or covalent bonding. Their unique pore environments have been extensively explored for the separation and adsorption of gases and heavy metals. In addition, their ångström-level nanochannels have recently attracted interest for challenging ion-separation processes, such as Li recovery.^140–142

The design flexibility of these frameworks allows systematic control over the pore size, geometry, and chemical functionality through the rational selection of linkers and nodes. In MOFs, open metal sites and functionalised organic linkers (e.g., –SO₃H and –COOH) serve as preferential binding sites for targeted cations.^143,144 In COFs, the use of π-conjugated linkers and chemically robust bonds (e.g., imine and keto-enamine linkages) results in highly ordered, 1D nanochannels that maintain structural integrity under an electric field.^145,146 Moreover, unlike conventional polymer membranes, the rigid skeletons of crystalline porous materials suppress swelling and distortion in aqueous environments, thereby offering stable ion-transport pathways.

Notably, the dimensionality of the nanochannels formed within these frameworks varies significantly for different architectures. Many MOFs, especially those with interconnected cage and window geometries, such as UiO-66, exhibit 3D ion-conduction networks. In addition, certain MOFs exhibit reduced dimensionality in their architectures.⁸¹ For instance, MIL-53 features 1D channels that enable the efficient transport of hydrated monovalent cations without significant dehydration, resulting in ionic conductivities comparable to those of bulk electrolytes.¹⁴⁷ In contrast, Al-tetrakis(4-carboxyphenyl) porphyrin, which exhibits a 1D axial channel network interlinked with 2D interlayer spaces, imposes partial dehydration and size-sieving effects, leading to selective transport behaviours for different ions. In 3D MOFs like UiO-66, ion conduction involves migration through a sequence of cages connected via narrow windows (∼6 Å), inducing multiple dehydration–rehydration steps. Similarly, COFs, which are often labelled as 2D materials owing to their layered stacking, can form 1D or 2D nanochannels depending on the stacking mode and linker planarity.^145,148 Eclipsed stacking creates straight 1D ion-conducting pores, whereas staggered or slipped arrangements result in tortuous 2D pathways with complex diffusion landscapes.

Despite their tuneable architectures, unmodified MOFs and COFs exhibit considerable limitations, such as poor processability and scalability for practical implementation. To overcome these challenges, researchers have focused on engineering the functionality of the framework and integrating it with other materials.^53,149 This section focuses on the synthetic modifications and integration strategies used to optimise the ion-transport characteristics of crystalline porous frameworks, culminating in a discussion of their recent applications in SED.

5.1 Synthetic modifications of frameworks

5.1.1 Post-synthetic modification of MOFs. Among the various types of MOFs, zirconium-based MOFs have garnered particular attention for ED applications owing to their exceptional chemical and thermal stabilities in aqueous and harsh environments.¹⁵⁰ UiO-66, constructed from 12-connected Zr₆O₄(OH)₄ nodes and 1,4-benzenedicarboxylate linkers, exemplifies a class of materials in which strong Zr–O bonds and a robust secondary building unit confer hydrolytic stability far exceeding that of most other MOFs.¹⁵¹ Such stability is a prerequisite for electrochemical separation in aqueous systems with an electric field as the driving force.

However, structural robustness alone does not guarantee selective ion transport. Achieving precise control over the ion mobility and selectivity necessitates further engineering of the internal pore environment. Therefore, post-synthetic modification (PSM) has emerged as a transformative strategy for endowing MOFs with tailored functionalities. PSM, as conceptualized and advanced by Cohen et al., enables the chemical elaboration of preformed MOF lattices through linker derivatisation, defect engineering, and node substitution without compromising the crystallinity or porosity.^152,153

A seminal demonstration of this concept is the asymmetric sub-nanochannel membrane based on UiO-66-(COOH)₂ reported by Lu et al.¹⁵⁴ This system integrates MOF crystals within polymeric nanochannels to create a spatially heterogeneous rectifying pathway that mimics the functionality of biological ion channels. The triangular windows (∼6 Å) in UiO-66-(COOH)₂ act as selective ion filters, whereas the carboxyl-rich pore environments introduce localised electrostatic fields and binding interactions. Through the pH-tuneable deprotonation of –COOH groups, the system achieved extraordinary K⁺/Mg²⁺ selectivity up to 4948 and high K⁺ permeation rates exceeding 10⁵ mol m⁻² h⁻¹. This study laid the groundwork for subsequent advancements in MOF-based ion-sieving membranes by highlighting the potential of PSM, even when confined within hybrid architectures, for synergistically modulating dehydration barriers, binding affinities, and ion mobilities.

Mo et al. further refined the nanochannel environment by a PSM method of grafting –OH and –OMe groups onto UiO-66 to modulate both the dehydration energy and ion–framework interactions (Fig. 10a).⁷⁷ The UiO-66-(OMe)₂ membrane exhibited an exceptional K⁺/Mg²⁺ selectivity of 1567.8, which was attributed to the enhanced activation-energy difference between the monovalent and divalent ions.

Fig. 10 (a) Post-synthetic grafting of –OH and –OMe groups onto UiO-66 (adapted from ref. 77, © 2025 Nature Portfolio. Published under the CC BY license); (b) functionalisation of TpPa COFs with –SO₃H, –PO₃H₂, and –COOH groups creating heterogeneous nanochannels (adapted from ref. 155, © 2022 Nature Portfolio. Published under the CC BY license); (c) dopamine-assisted pore engineering of TpPa–COOH COFs to reduce channel size and introduce catechol groups (adapted from ref. 156 with permission from John Wiley and Sons, copyright 2024); (d) randomly oriented COF domains forming sub-nanometre grain boundary constrictions (adapted from ref. 157, © 2022 Nature Portfolio. Published under the CC BY license).

Together, these studies highlight the potential of PSM for introducing specific functionalities, such as hydrophilic domains, localised charge densities, and steric constraints, into MOF nanochannels, offering powerful tools for fine-tuning ion selectivity and flux.

5.1.2 Synthetic modification and structural engineering of COF. The development of oriented and structurally engineered COFs has opened new avenues for enhancing ion-transport characteristics. Conventional COFs exhibit horizontal stacking owing to thermodynamic tendencies during growth. This in-plane alignment often leads to tortuous ion-diffusion paths that limit the transport rates. To overcome this, vertical alignment techniques, such as IP or the use of anisotropic templates, have been applied to fabricate COF membranes with nanochannels aligned perpendicular to the membrane surface. This orientation significantly improves through-plane conductivity, reduces tortuosity, and enhances ion flux under electric fields.

A compelling demonstration of PSM to enhance monovalent cation separation was reported by Wang et al.; acidic functional groups were introduced into COF nanochannels to construct membranes in spatially heterogeneous environments. By decorating 1,3,5-triformylphloroglucinol/p-phenylenediamine (TpPa)-based COFs with sulfonic (–SO₃H), phosphonic (–PO₃H₂), and carboxylic acid (–COOH) groups, 1D channels containing alternating acidic and acid-free domains were produced (Fig. 10b).¹⁵⁵ This design exhibited a “confined cascade separation” mechanism, wherein hydrated acid sites locally constricted channel diameters through water clustering, acting as enthalpic sieving stages, while unmodified segments preserved high flux. A phosphoric acid-functionalised TpPa–PO₃H₂ membrane exhibited an actual K⁺/Li⁺ selectivity of 4.2–4.7 and an ideal selectivity of ∼13.7, outperforming conventional membranes for monovalent cation mixtures. By systematically varying the group density, hydration energy, and number of stages, this study elucidated the modulation of confined hydration shells and multistage interactions to favour K⁺ over Li⁺. This work advanced the understanding of enthalpy-governed ion transport in COFs and established functional-group engineering as a powerful approach for creating size- and charge-discriminative channels within crystalline organic frameworks.

Wu et al. employed a dopamine-assisted pore-engineering strategy to tailor the transport properties of TpPa–COOH COF membranes for monovalent cation discrimination. Under acidic conditions, dopamine molecules undergo covalent esterification with the carboxylic acid moieties of TpPa–COOH, reducing the effective channel aperture from 1.25 to 0.71 nm (Fig. 10c).¹⁵⁶ This constriction establishes a pore-size cut-off between the hydrated diameters of K⁺ and Li⁺, enabling steric control over ion permeation. In addition to geometric tuning, the incorporated catechol groups introduced by dopamine modification modulated the local hydration structures and ion–channel wall interactions, further enhancing selectivity. As a result, the modified COF achieved an ideal K⁺/Li⁺ selectivity of 18.7 and an actual selectivity of 6.8 under continuous SED operation, maintaining structural and functional stability over prolonged use. This work demonstrated that post-synthetic covalent functionalisation can simultaneously tune the channel size and interfacial chemistry to achieve high-fidelity separation of monovalent cations within crystalline organic frameworks.

Interestingly, recent studies have challenged the conventional notion that perfect crystallographic alignment is beneficial. Bao et al. demonstrated that randomly oriented COF domains can induce sub-nanometre constrictions at grain boundaries, creating energetic and steric barriers that reject hydrated Li⁺ ions while allowing smaller or more mobile species, such as Na⁺ or K⁺, to permeate (Fig. 10d).¹⁵⁷ This counterintuitive result shows that the disruption of periodicity can sometimes enhance selectivity, particularly for size exclusion-driven separation. Moreover, the incorporation of sulfonic acid functional groups within the COF skeleton provides ion-specific interactions that further favour the transport of competing cations.

These advancements underscore the critical role of synthetic modifications, through precise post-synthetic functionalisation of MOFs or structural reconfiguration of COFs, in governing the ion selectivity, transport efficiency, and membrane stability. These strategies provide a robust foundation for integrating crystalline frameworks into next-generation ED systems, in which precise ion discrimination is required under challenging operational conditions.

5.2 Fabrication of porous framework-based membranes

Crystalline porous frameworks, such as MOFs and COFs, can be incorporated into membranes using two distinct approaches. First, in MOF channel membranes (MOFCs), the framework is grown as a continuous and independent separation layer without a polymer matrix. In this case, the intrinsic crystalline channels are directly used for ion or molecular transport. Second, the framework (or other porous filler) is dispersed within a polymer matrix to form mixed-matrix membranes (MMMs), where the polymer provides mechanical robustness and processability, and the embedded framework provides additional selective pathways. Porous frameworks offer precise molecular sieving owing to their uniform pore networks, but are often limited by poor mechanical stability and scalability. In contrast, MMMs are easier to fabricate and scale up, although their performance is strongly affected by the interfacial compatibility between the porous fillers and polymer matrix.

5.2.1 MOF channel membranes. The first MOFC, developed by Li et al., exhibited high selectivity for fluorine-ion transport. UiO-66-NH₂ was grown in situ with a polyethylene terephthalate (PET) support, combining the flexibility of a polymer with the high selectivity of a MOF.¹⁵⁸ Inspired by this research, Xu et al. made a significant advancement in Li recovery by fabricating an oriented UiO-67 membrane on a polyvinylpyrrolidone (PVP)-modified anodic aluminium oxide (AAO) substrate using a washing-assisted secondary growth strategy (Fig. 11a).¹⁵⁹ This approach enabled the formation of a well-intergrown UiO-67 layer with a preferential [022] crystal orientation, aligning the sub-nanometre channels across the membrane to facilitate directional ion transport. The resulting architecture exhibited minimal tortuosity and maximal exposure of triangular pore windows (∼8.0 Å), which imposed strong dehydration barriers on divalent ions while permitting efficient Li⁺ transport. As a result, the membrane exhibited an exceptional Li⁺/Mg²⁺ selectivity of 159.4 and Li⁺ permeability of 27.01 mol m⁻² h⁻¹—overcoming the typical trade-off between selectivity and flux. Mechanistic studies revealed that the highly aligned channels and structural rigidity and uniformity of the MOF framework created differential dehydration energy barriers between Li⁺ and Mg²⁺, enabling precise ion discrimination. This work marked a milestone in MOFC development by demonstrating the simultaneous enhancement of selectivity and permeability, and showcased the feasibility of pore orientation control as a design principle for advanced ion-separation membranes.

Fig. 11 (a) Oriented UiO-67 membrane on PVP-modified AAO fabricated by washing-assisted growth, exhibiting aligned [022] channels (adapted from ref. 159 with permission from John Wiley and Sons, copyright 2021); (b) PSS@HKUST-1 hybrid MMM formed by threading sulfonated polymer chains through MOF pores (adapted from ref. 161 with permission from John Wiley and Sons, copyright 2016); (c) COF-based MMM (NUS-2/SPEEK) containing sub-nanometre pores (adapted from ref. 162 with permission from Elsevier, copyright 2022); (d) oligoether-functionalised COF/PAN MMM demonstrating solvation-level control (adapted from ref. 163, © National Academy of Sciences. Published under the CC BY license).

5.2.2 Mixed-matrix membranes. In addition to their use in MOFC, crystalline porous frameworks are integrated in MMMs as an alternate strategy for ion separation. Although MMMs were initially developed for gas separation, they were subsequently adapted for ion separation by integrating MOFs into polymer matrices.¹⁶⁰ In 2016, Guo et al. developed a novel MMM by threading PSS chains through the nanopores of HKUST-1 to form a PSS@HKUST-1 hybrid structure via in situ confinement conversion (Fig. 11b).¹⁶¹ This architecture contained a 3D network of sulfonate-functionalised pathways within the MOF scaffold, which simultaneously enhanced the water stability and provided fixed-charge sites for cation exchange. The resulting membrane, PSS@HKUST-1-6.7, exhibited an impressive Li⁺ conductivity of 5.53 × 10⁻⁴ S cm⁻¹ at 25 °C, which was five orders of magnitude higher than that of pristine HKUST-1. The sulfonate network facilitated fast Li⁺ transport via a Grotthuss-like hopping mechanism, and its preferential coordination with Li⁺ over competing cations enabled exceptional selectivity. In binary ion-separation tests, the membrane achieved Li⁺/Na⁺, Li⁺/K⁺, and Li⁺/Mg²⁺ selectivities of 35, 67, and 1815, respectively, along with a Li⁺ flux of 6.75 mol m⁻² h⁻¹. These performance metrics highlight the synergistic advantage of polymer-MOF integration, where the structural regularity of MOFs is complemented by the ion-specific functionalisation of polymeric sulfonate domains to achieve both high selectivity and permeability.

Recent efforts have focused on the development of COF-based MMMs, which offer solutions to the limitations of conventional polymer membranes. Sun et al. presented one of the earliest demonstrations of this strategy by integrating a COF filler, NUS-2, into a SPEEK matrix for Na⁺/Mg²⁺ separation (Fig. 11c).¹⁶² The hybrid membrane leveraged size-exclusion effects via its sub-nanometre (∼0.8 nm) pores to inhibit Mg²⁺ while allowing monovalent Na⁺ transport. This resulted in a relative selectivity enhancement and sustained power density performance, even in brines with up to 30% Mg²⁺ content—conditions which significantly compromised commercial Neosepta CMX membranes. This study validated the structural compatibility and functional advantages of incorporating COFs into polymer matrices, emphasising the importance of geometrically optimised nanochannels for cation sieving.

Building on this foundation, Meng et al. advanced the MMM concept by shifting the focus from geometric control to solvation-level control (Fig. 11d).¹⁶³ They synthesized oligoether-functionalised COFs with tuneable ethylene oxide (EO) chain lengths and incorporated them into polyacrylonitrile (PAN) membranes. This design enabled the precise modulation of ion–solvent interactions within the nanochannels. The COF-EO₂/PAN membrane achieved an extraordinary Li⁺/Mg²⁺ selectivity of 1352 under electro-driven separation, owing to the fine-tuned balance between the partial dehydration of Li⁺ and favourable coordination within the oligomer-rich environment. Notably, longer EO chains facilitate a higher flux but reduce selectivity owing to decreased solvation energy differentials, highlighting the delicate interplay between coordination chemistry and ion partitioning.

In summary, these advances underscore the versatility of crystalline frameworks integrated into hybrid membrane architectures. The incorporation of MOFs and COFs with other materials, via direct channel alignment in MOFCs or synergistic interactions within MMMs, enables the tailoring of nanochannel environments at multiple levels. Such integration addresses the intrinsic limitations of standalone frameworks while enhancing transport selectivity, which is a critical requirement for scalable ED applications. As the field advances, further progress towards next-generation membranes (combining robustness, processability, and high-resolution ion discrimination) will likely be achieved by rational interfacial engineering and multifunctional filler design.

5.3 SED applications of crystalline framework membranes

Building on the structural and chemical tuneability of MOFs and COFs, recent studies have explored their integration into ED platforms to achieve high-precision ion separation. The sub-nanometre channels and charge-tailored surfaces of these frameworks offer critical advantages for SED, where ion–ion discrimination under electric fields is essential. Recent advances in the application of MOF- and COF-based membranes for SED are reviewed below.

Zhang et al. demonstrated the feasibility of using MOF membranes for SED by fabricating ultrathin ZIF-8 membranes on AAO supports via a GO-assisted interfacial growth strategy (Fig. 12a).¹⁶⁴ These membranes contained ångström-scale pore sizes (∼3.4 Å) and nanometre-scale cavities (∼11.6 Å), closely mimicking the selectivity filters and conduction pathways of biological ion channels. The ion selectivity order was reversed compared to bulk transport in conventional porous membranes, with observed selectivities of 4.6 (Li⁺/Rb⁺), 2.2 (Li⁺/K⁺), and 1.4 (Li⁺/Na⁺). MD simulations confirmed that the enhanced Li⁺ mobility resulted from size confinement-induced partial dehydration and weak water–framework interactions, which lowered the energy barrier for Li⁺ permeation. Moreover, the membrane sustained high ion transport rates at ultralow operating voltages (up to 10⁶ ions per s per channel at 200 mV), thereby establishing a benchmark for synthetic SED platforms with robust MOF architectures.

Fig. 12 (a) Ultrathin ZIF-8 membranes on AAO supports with ångström-scale pores mimicking biological channels (adapted from ref. 164, © 2025 American Association for the Advancement of Science. Published under the CC BY license); (b) MOF–PVC hybrid membranes fabricated by solution casting (adapted from ref. 165 with permission from Elsevier, copyright 2020); (c) urchin-like HKUST-1 arrays confined in PET pores via wettability-regulated growth (adapted from ref. 166 with permission from Johns Wiley and Sons, copyright 2023); (d) UiO-66-SO₃H/CTA MMM fabricated by solution casting (adapted from ref. 167. © American Chemical Society. Published under the CC BY license); (e) CC3 COF membranes on AAO produced by contra-diffusion growth (adapted from ref. 168 with permission from American Chemical Society, copyright 2022); (f) crown-POF/PDMS MMM with crown ether-embedded frameworks with dynamic Li⁺ binding sites (adapted from ref. 169 with permission from John Wiley and Sons, copyright 2023).

MOF–PVC hybrid membranes for Li⁺/Mg²⁺ separation from brines were fabricated by Zhang et al. by simply mixing MOF fillers into a PVC matrix (Fig. 12b).¹⁶⁵ The membrane was easily fabricated using the solution-casting method, and the representative HSO₃-UiO-66-0.6@PVC membrane exhibited an ion selectivity of 4.79. This performance was attributed to molecular sieving by the sub-nanometre pore structure and sulfonate functional groups of the MOF particles. This work demonstrated the feasibility of tuning membrane performance via polymer–filler compatibility and laid the groundwork for subsequent interface-engineered MMMs.

Wu et al. fabricated an urchin-like HKUST-1 array confined within the through-pores of a PET membrane (Fig. 12c).¹⁶⁶ Their approach involved a wettability-regulated confined interfacial reaction that enabled spatially controlled MOF nucleation inside micropores under ambient conditions. Unlike traditional polycrystalline coatings or dispersed fillers, the resulting MOFC architecture achieved continuous ionic pathways with sub-nanometre apertures formed by the HKUST-1 framework. Crucially, the sub-nanochannels in HKUST-1, with window sizes of ∼8–9 Å, enabled a pronounced size-sieving effect for ion separation. Under an applied electric field, the membrane exhibited an exceptional Li⁺/Zr⁴⁺ selectivity of 3930 ± 373, highlighting its ability to discriminate ions of similar hydrated size. Moreover, the MOFC membrane showed high ion-permeation fluxes, reaching 1.97 mol m⁻² h⁻¹ for Li⁺, underscoring the feasibility of achieving both high selectivity and throughput.

Eden et al. fabricated an MMM composed of a sulfonated metal–organic framework, UiO-66-SO₃H, embedded in a cellulose triacetate (CTA) matrix (Fig. 12d).¹⁶⁷ The membrane was prepared using a simple solution-casting method, achieving a high MOF loading of 60 wt%. This membrane exhibited electrochemical LiCl/NaCl selectivity of 1.21, with a Li⁺ flux of 3.64 × 10⁻¹¹ mol cm⁻² s⁻¹, outperforming the pristine CTA membrane with a selectivity of 0.90 and flux of 4.82 × 10⁻¹¹ mol cm⁻² s⁻¹. This enhanced selectivity is attributed to the fast diffusion of Li⁺ ions (diffusivity selectivity of 1.59), which overcomes the intrinsic solubility advantage of Na⁺ (solubility selectivity of 1.32). Electrochemical impedance spectroscopy confirmed a lower resistance for Li⁺ transport (1970 Ω) compared to Na⁺ (3760 Ω), supporting the preferential conduction pathway for Li⁺ through the sulfonic acid-functionalised MOF domains. This system represents a rare case in which Li⁺ selectivity over Na⁺ was achieved in an MMM under an electrochemical driving force, emphasizing the role of the tailored MOF chemistry and polymer–filler compatibility in promoting ion-specific transport. Xu et al. reported the pioneering application of COF membranes for SED by fabricating covalent cage 3 (CC3) membranes on AAO substrates using a contra-diffusion growth strategy (Fig. 12e).¹⁶⁸ The resulting membranes featured a hierarchical nanochannel architecture consisting of discrete internal cavities and external cage-aligned pathways connected through sub-nanometre-sized triangular windows (∼5.4 Å). The window cavity structure imposed a stringent size-exclusion mechanism, enabling the partial dehydration of cations during translocation. ED experiments demonstrated exceptional mono-/divalent ion discrimination, achieving ideal selectivities of 1031 (K⁺/Mg²⁺), 660 (Na⁺/Mg²⁺), and 284 (Li⁺/Mg²⁺) in single-ion systems with sustained selectivity (e.g., Li⁺/Mg²⁺ = 104), even in binary mixtures. Despite a moderate reduction in the monovalent ion flux under competitive multicomponent conditions, the CC3 membranes consistently outperformed commercial ion-exchange membranes (e.g., Selemion CSO) by nearly two orders of magnitude in selectivity while maintaining comparable fluxes.

Ruan et al. produced an MMM by embedding a mechanically interlocked crown ether porous organic framework (crown-POF) in a polydimethylsiloxane (PDMS) matrix for selective Li⁺ separation (Fig. 12f).¹⁶⁹ The crown-POF architecture contained 3D porous channels threaded with dibenzo-24-crown-8 rings, forming a dynamic host structure with penta-coordinated Li⁺ binding sites. This configuration enabled both high affinity and rapid shuttling of Li⁺ ions through the framework via a “mechanical flipping” motion, mimicking biological ion pumps. When 10 wt% crown-POF was integrated into PDMS, the resulting MMM exhibited an ideal Li⁺/Mg²⁺ selectivity of 14.4 and a permselectivity of 19.8 under ED operation, significantly outperforming conventional porous filler-based MMMs. Moreover, the Li⁺ conductivity of the membrane reached 4.03 × 10⁻⁸ S cm⁻¹—over five orders of magnitude higher than that of a PDMS-covered Nafion membrane without filler.

Table 4 summarises the SED performance of crystalline porous framework membranes. Ion discrimination in these frameworks arises from coordination binding, size exclusion, and dehydration effects, yet their broader implementation is constrained by hydrolytic vulnerability and difficulties in scalable membrane fabrication.

Table 4 SED performance of crystalline porous framework membranes

Membrane	Fabrication method	Operating condition	Performance metrics	Ref.
ZIF-8/AAO	GO-assisted interfacial growth	0.1 M LiCl, KCl, NaCl membrane area ∼28.26 mm^2	Selectivity: 4.6 (Li^+/Rb^+), 2.2 (Li^+/K^+), 1.4 (Li^+/Na^+)	164
		I–V test −0.2–0.2 V	Flux: 10^6 ions per s per channel @200 mV
HSO3–UiO-66@PVC	Solution casting of MOF–PVC blend	1.0 M LiCl, MgCl2 membrane area 4.712 cm^2	Selectivity: 4.79 (Li^+/Mg^2+)	165
		I–V test −1–1 V
HKUST-1/PET	Wettability-regulated confined interfacial growth	500 ppm Li^+/Zr^4+ membrane area 0.8 mm^2	Selectivity: Li^+/Zr^4+ 3930 ± 373	166
		I–V test −1–1 V	Li^+ flux: 1.97 mol m^−2 h^−1
UiO-66-SO3H/CTA	Solution casting	0.1 M LiCl, NaCl, KCl, MgCl2 membrane area 9.62 cm^2	Selectivity: Li^+/Rb^+ 1.21	167
		I–V test −1–1 V	Li^+ flux: 3.64 × 10^−11 mol cm^−2 s^−1
CC3/AAO	Contra-diffusion growth	0.1 M KCl, NaCl, LiCl, MgCl2 membrane area 0.02 mm^2	Selectivity	168
		5 mA cm^−2	Single; 1031 (K^+/Mg^2+), 660 (Na^+/Mg^2+), 284 (Li^+/Mg^2+)
			Binary; 104 (Li^+/Mg^2+)
			Flux: ∼1.0 mol m^−2 h^−1 (K^+ Na^+, Li^+)
			0.002 mol m^−2 h^−1 (Mg^2+)
Crown-POF/PDMS	Embedding POF in PDMS	0.1 M LiCl, NaCl, KCl, MgCl2 membrane area 9.62 cm^2	Selectivity	169
		I–V test −1.5–1.5 V	Single; 14.4 (Li^+/Mg^2+)
			Perm-selectivity; 19.8 (Li^+/Mg^2+)
			Li^+ flux: 11.19 × 10^−9 mol cm^−2 s^−1

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6 Ceramics

Inorganic ceramic materials with intrinsic ionic conductivities, such as oxide-based Li_1+xAl_yGe_2−y(PO₄)₃ (LAGP),¹⁷⁰ Li_3−xLa_2/3−xTiO₃ (LLTO),¹⁷¹ sodium superionic conductor (NASICON)-type frameworks,¹⁷² and sulfide-based lithium superionic conductor (LISICON)-type frameworks,¹⁷³ have recently attracted attention as next-generation membrane materials for SED processes.⁷⁹ Originally developed for solid-state electrolytes (SSE) in alkali-ion batteries, these ceramics contain highly ordered crystalline channels that enable highly selective ion transport of alkali metal ions, particularly Li⁺ or Na⁺ ions.

Unlike fluid-filled nanochannel membranes, ceramics rely on vacancy-mediated or interstitial ion hopping mechanisms within dense solid-state lattices. These transport processes do not require solvated ion mobility, effectively bypassing the consecutive hydration shell barrier that limits aqueous-phase ion separation.^79,149,173 In ED systems, where an applied electric field drives directional ion migration, these unique transport kinetics enable high selectivity for co-ions of similar sizes and charges, which is challenging.

Ceramic membranes exhibit good long-term stability. Oxide-based SSE membranes contain densely packed crystalline frameworks, such as cubic or hexagonally close-packed lattices, where oxygen ions occupy octahedral and tetrahedral interstitial sites and metal ions are arranged at body-centred or face-centred positions. The highly ordered architectures impart excellent chemical and thermal properties, ensuring structural integrity under both hydrated and electrochemical conditions. These robust materials are suitable as ion-selective membranes for long-term ED operation.¹⁷⁴

Despite their potential, ceramic membranes continue to face persistent challenges, particularly owing to the lack of scalable fabrication methods and their limited stability in aqueous environments. Moreover, the use of ceramics as membrane materials for ion-selective applications remains relatively underdeveloped and requires further research. This section highlights the recent advances aimed at addressing these limitations by focusing on material innovations and representative milestones in their integration into SED systems.

6.1 Advances in ceramic membrane modification

The exploration of ceramic materials for SED began with a pioneering study by Hoshino, who reported the first dialysis system for lithium recovery using a lithium ionic superconductor (LIS) membrane in 2015 (Fig. 13a).¹⁷⁵ In this study, a dense ceramic membrane composed of Li-ion-conducting glass ceramics was fabricated, which exhibited exclusive Li⁺ permeability while effectively blocking competing cations such as Na⁺, K⁺, Mg²⁺, and Ca²⁺. Extending this concept, Li et al. established a continuous electrically driven membrane process using Li_0.33La_0.56TiO₃ (LLTO) ceramic membranes, marking a major advancement in ceramic-based SED (Fig. 13b).¹⁷⁶ LLTO contains a framework of interconnected TiO₆ octahedra forming sub-nanometre windows (∼1.07 Å), allowing selective Li⁺ transport while imposing substantial energy barriers for larger ions, such as Na⁺, K⁺, Mg²⁺, and Ca²⁺. Leveraging this intrinsic selectivity, dense, defect-free LLTO membranes were fabricated and integrated into a three-compartment electrochemical cell for seawater lithium extraction. Under an applied voltage of 3.25 V, lithium ions from the Red Sea (initial lithium concentration: 0.21 ppm) were continuously enriched across five cascaded stages, ultimately reaching a concentration of 9013 ppm, corresponding to an enrichment factor of ∼43 000.

Fig. 13 (a) First lithium recovery dialysis system using a lithium ionic superconductor (LIS) membrane (adapted from ref. 175, Elsevier. Published under the CC BY license); (b) continuous SED with LLTO ceramic membranes featuring sub-nanometre windows (adapted from ref. 176 © The Royal Society of Chemistry 2021. Published under the CC BY license); (c) LATP–polymer dual-channel hybrid membranes enabling complementary Li⁺ and anion transport (adapted from ref. 177 with permission from American Chemical Society, copyright 2023); (d) flexible LATP/PVDF-HFP composite membranes overcoming ceramic brittleness (adapted from ref. 178 with permission from Elsevier, copyright 2023).

A representative example of a ceramic–polymer hybrid membrane is a dual-channel ion-conductor system based on NASICON-type Li_1.5Al_0.5Ti_1.5(PO₄)₃ (LATP) (Fig. 13c), as reported by Ma et al.¹⁷⁷ To enable the simultaneous transport of Li⁺ and compensating anions in a concentration-driven process, a dual modification strategy was applied. Porous LATP pellets were fabricated via a sol–gel route, followed by sintering with PVA as a pore-forming agent. By tuning the PVA content, the porosity and grain connectivity were tailored, which in turn modulated the percolation of Li-ion channels within the ceramic matrix. The latent pore space within the LATP structure was then infiltrated with an anion-exchange polymer, poly(biphenyl N-methylpiperidine), and grafted with piperidinium/ethylene oxide side chains. This molecularly engineered polymer provided positively charged nanochannels that selectively conducted anions, forming a complementary path for maintaining charge neutrality. Via this dual-channel configuration, the membrane achieved remarkable Li⁺/Na⁺ selectivity (∼1389) and a Li⁺ flux of 21.6 mmol m⁻² h⁻¹ without the application of external voltage. This study illustrates a scientifically impactful direction for membrane design by integrating solid-state ion conductors with tailored polymer domains to decouple the cation and anion transport pathways.

In 2023, Shen et al. developed a flexible composite membrane by embedding LATP particles in a poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) polymer matrix to address the brittleness and mechanical limitations of ceramic electrolytes for lithium-selective ED (Fig. 13d).¹⁷⁸ The LATP/PVDF-HFP composite membrane (FLCM) prepared via solution casting exhibited high structural integrity and mechanical resilience under wet-state ED operation, while maintaining Li⁺ conductivity through the embedded inorganic phase. The optimised FLCM-2 (65 μm thickness) achieved a lithium transference number approaching 0.89 and exhibited negligible swelling or delamination. Importantly, FLCM-2 demonstrated exceptional selectivity, up to 24 845 for Li⁺/Na⁺, far surpassing those of commercial cation-exchange membranes. Even under an elevated voltage (2.0 V), FLCM-2 sustained high Li⁺ selectivity, with a Li⁺/Na⁺ selectivity of 7111. These results confirm that the synergistic integration of ion-conductive ceramics and flexible polymers is a viable route for scalable, high-selectivity lithium recovery from seawater.

In summary, the recent progress in ceramic membrane engineering has highlighted the adaptability of inorganic frameworks configured within composite and hybrid architectures. By modifying the original dense structures into dual-path and flexible composite systems, ceramics have demonstrated tuneable ion selectivity and mechanical resilience through interfacial modification and phase integration. These strategies overcome the brittleness and fabrication challenges inherent to ceramic SSEs. Future breakthroughs will likely be achieved by optimising the interphase compatibility, reducing the internal resistance, and designing hierarchically structured composites that balance mechanical durability with molecular-level ion discrimination.

6.2 SED applications of ceramic membranes

Although promising demonstrations of ceramic membranes in SED have emerged, a fundamental understanding of the ion-transport mechanisms within these systems under aqueous conditions remains limited. Most previous studies focused primarily on proof-of-concept enrichment and ion-selectivity metrics, but lack detailed explanations of key physicochemical phenomena, such as interfacial ion transfer, hydration–dehydration energetics, and the role of confined crystalline lattices in the presence of water. Nevertheless, by acknowledging the practical significance of these initial findings and addressing the critical research gaps, including long-term aqueous stability, interfacial impedance modulation, and integration within scalable membrane platforms, future studies can advance ceramics as a viable and robust class of ion-selective membranes for next-generation aqueous separation technologies. This section outlines recent studies on the development of ceramic membranes for SED.

As an early proof of concept, Jiang et al. demonstrated the utility of NASICON-type ceramics as lithium-selective ion-conducting membranes for aqueous ED (Fig. 14a).¹⁷⁹ By employing Li_1.5Al_0.5Ge_1.5(PO₄)₃ (LAGP) disks in a stacked ED configuration, the authors achieved efficient Li⁺ extraction from dilute LiOH solutions without co-ion migration or water crossover. The crystalline LAGP lattice, characterised by interconnected 3D channels and high Li⁺ mobility, enabled selective Li⁺ conduction via site-to-site hopping, whereas steric- and charge-based exclusion effectively suppressed the transport of other cations, including Na⁺ and Mg²⁺. High extraction efficiencies were observed, even at low feed concentrations, with >96.5% Li⁺ recovery from 0.01 M solutions. Furthermore, saturated LiOH was readily crystallised from the concentrate stream. This selectivity was attributed to the ability of the LAGP framework to accommodate dehydrated Li⁺ ions while presenting size- and charge-imposed barriers to competing species. Notably, the transmembrane resistance increased sharply at low concentrations, revealing the rate-limiting nature of Li⁺ ingress into the solid-state membrane. Nevertheless, the system operated without solvent transport or concentration polarisation, maintaining a stable performance under continuous flow. With an energy consumption of ∼4458 kWh per ton of LiOH—markedly lower than that of thermal evaporation—the study highlighted the potential of ceramic SSEs as high-fidelity ion-selective membranes for sustainable lithium refining from dilute or complex aqueous sources.

Fig. 14 (a) LAGP ceramic membranes used in stacked ED (adapted from ref. 179 with permission from Elsevier, copyright 2023); (b) flexible LLTO-based composite membranes with quaternary ammonium polymers (adapted from ref. 180 with permission from Elsevier, copyright 2024); (c) single-channel LAGP–PE membranes with bonded interfaces (adapted from ref. 181 with permission from John Wiley and Sons, copyright 2024); (d) performance comparison of SSE membranes with MOF, COF, polymer, and NF systems (adapted from ref. 92 © 2025 American Association for the Advancement of Science. Published under the CC BY license).

Guo et al. developed a flexible Li-selective composite membrane by embedding Li_0.33La_0.56TiO₃ (LLTO) into a semi-interpenetrating network of quaternary ammonium-functionalised polyepichlorohydrin (PECH-DABCO) and PVDF, resulting in an inorganic–organic composite (Fig. 14b).¹⁸⁰ The LLTO structure provided spatially confined lithium-ion pathways, whereas the positively charged quaternary ammonium groups induce Donnan exclusion, suppressing Na⁺, K⁺, Mg²⁺, and Ca²⁺ leakage. The authors revealed that loading 80 wt% of LLTO and a PECH-DABCO/PVDF mass ratio of 39% maximized Li⁺ selectivity, achieving a Li⁺/Na⁺ separation coefficient of 40.40 and a Li⁺ flux of 9–11 × 10⁻¹⁰ mol cm⁻² s⁻¹ in binary systems. In simulated high-Na/Li concentration from Yiliping brine, the membrane delivered Li⁺/Na⁺ separation coefficients 35.48–56.3 times higher than commercial CEMs—while maintaining high mechanical robustness (burst strength 0.91 MPa). Density functional theory and MD analyses identified an adsorption–dehydration–diffusion mechanism, with LLTO exhibiting strong Li⁺ adsorption energy (−0.47 eV), low dehydration energy (515 kJ mol⁻¹), and the lowest diffusion barrier (0.37 eV) among competing cations.

To further address interfacial leakage and scalability, Yang et al. proposed a novel “single-channel” design strategy, in which mono-layered LAGP particles were embedded within a polyethylene (PE) matrix, constructing continuous, exclusive pathways for lithium ions (Fig. 14c).¹⁸¹ Using triethoxyvinylsilane (TEVS) for chemical bonding at the LAGP–PE interface, the authors suppressed undesired interfacial leakage and enhanced mechanical integration. The resulting LAGP–PE membrane exhibited an exceptional Li⁺/Na⁺ separation factor exceeding 2.87 × 10⁷ and enabled direct lithium metal extraction from seawater with >98% coulombic efficiency at room temperature. This study represents a breakthrough in the development of ceramic-based Li⁺-selective membranes with practical scalability, high ion selectivity, and improved mechanical robustness.

Liu et al. addressed the longstanding scalability limitations of solid-state lithium-selective membranes by engineering a three-layer composite architecture (SLAM), comprising a porous ceramic support, dense LLTO selective layer, and PVDF/TiO₂ adsorption layer.¹⁷⁴ Large-area membranes (up to 25 × 100 cm²) were fabricated without fatal cracking. The LLTO layer provides Li⁺ conduction channels with dehydration size exclusion to suppress Na⁺, K⁺, Mg²⁺, and Ca²⁺ transport. The TiO₂-containing PVDF overlayer sealed the grain boundaries and enhanced the Li⁺ surface concentration via preferential adsorption, thereby increasing both the enrichment ratio and selectivity. Using an 8 × 8 cm² SLAM under 6 V in simulated seawater (0.21 ppm Li⁺, 1700 ppm Mg²⁺), the membrane enriched Li⁺ to 29.4 ppm while reducing Mg²⁺ to 2.7 ppm, achieving a Li⁺/Mg²⁺ selectivity of 87 000 and a 140-fold enrichment. At an industrial scale, a 25 × 100 cm² SLAM processed 1 ton per day of seawater, yielding 31.8 mg Li⁺ per day with a Li⁺/Mg²⁺ selectivity of 47 000 despite inevitable defect-related losses. The membrane retained >80 000 selectivity over five reuse cycles and performed effectively with brine and battery-leachate feeds. Furthermore, integration with a solar-powered pumping-ED system demonstrated 85.6% of the lab-scale Li⁺ recovery, highlighting the potential of this system for offshore energy-autonomous lithium extraction. Beyond aqueous demonstrations, Patel et al. demonstrated the potential of highly selective ion-conducting membranes for ED by solid-state diffusion (Fig. 14d).⁹² By incorporating a lithium-ion-conducting LiTi₂(PO₄)₃ framework, partially substituted with Ge and Al, into a batch ED configuration, the authors established that lithium ions could traverse the SSE lattice under an applied electric field, while Na⁺, Mg²⁺, and protons were effectively excluded. The observed selectivity was attributed to both steric and electrostatic constraints provided by the crystalline structure. Lithium ions, with smaller ionic diameters and monovalent charge, could hop through interconnected octahedral and tetrahedral lattice sites, whereas Na⁺ ions were blocked due to size exclusion at migration bottlenecks (∼1.8–2.0 Å), and Mg²⁺ ions faced high energy barriers due to their divalency and strongly bound hydration shells. Notably, even trace surface accumulation of sodium resulted in a measurable decline in the lithium flux, underscoring the interfacial exchange limitations and the need to prevent dopant-ion incorporation into the SSE lattice. Despite operating under anhydrous conditions, the SSE enabled stable lithium-ion migration without water co-transport, drastically lowering the energy cost relative to pressure-driven systems. This work marks a significant advancement in SSE ion separation, demonstrating near-perfect selectivity in real aqueous environments, and offering insights into the repurposing of ceramic materials as functional membranes for Li extraction from complex brines.

SED performance of ceramic membranes is summarised in Table 5. Overall, ceramic membranes provide a fundamentally distinct pathway for ion selectivity in ED, and their ion-hopping mechanism within solid crystalline matrices does not depend on aqueous-phase solvation dynamics, offering high selectivity against strongly hydrated or multivalent cations, such as Mg²⁺ and Ca²⁺. Despite the current limitations in scale-up and integration, the superior Li⁺ specificity under an applied voltage highlights the immense promise for sustainable lithium extraction from complex aqueous sources.

Table 5 SED performance of ceramic membranes

Membrane	Fabrication method	Operating condition	Performance metrics	Ref.
Li1.5Al0.5Ge1.5(PO4)3 (LAGP)	Conventional solid-state sintering reaction	Feed: 0.001–0.5 M LiOH	96.5% Li^+ recovery from 0.01 M solutions	179
		Receiving: 1–3 M LiOH	4458 kWh t^−1 LiOH
		Constant 4 V
		Membrane area 1.56 cm^2
LLTO/PECH-DABCO–PVDF	Embedding LLTO into semi-interpenetrating	0.05 M LiCl, NaCl, KCl, CaCl2, MgCl2	Selectivity	180
	PECH-DABCO/PVDF network	Simulated Yiliping brine	Binary; 40.40 (Li^+/Na^+), 71.77 (Li^+/Mg^2+)
		Constant 2 V	Li^+ flux: 8.33 × 10^−10 mol cm^−2 s^−1
		Membrane area 6.157 cm^2	Simulated brine; 35.48 (Li^+/Na^+), 2784.97 (Li^+/Mg^2+)
LAGP–PE	Mono-layered LAGP particles embedded in PE with TEVS chemical bonding	Seawater (Huludao, China)	Selectivity	181
		Li^+ 0.193 mg L^−1, Na^+ 9441 mg L^−1, K^+ 333 mg L^−1, Mg^2+ 1209 mg L^−1	Li^+/Na^+ > 2.87 × 10^7
		100–300 μA cm^−2	Li^+/K^+ > 1.01 × 10^6
		Membrane area 16 cm^−2	Li^+/Mg^2+ > 3.68 × 10^6
SLAM 100	Porous ceramic support + dense LLTO layer + PVDF/TiO2 overlayer	Simulated seawater	Selectivity: Li^+/Mg^2+ 47 000	174
		Li^+ 0.21 ppm, Na^+ 12 356 ppm, K^+ 746 ppm, Ca^2+ 483 ppm, Mg^2+ 1700 ppm
		Voltage 12.5 V
		Feed flux 150 L h^−1
LiTi2(PO4)3 (Ge,Al-doped)	SSE lattice in batch ED setup	Feed: 0.1 M LiCl, NaCl, MgCl2	No detectable Na^+ or Mg^2+ flux	92
		Constant 4 V	Conductivity: 0.05 mS cm^−1
		Membrane area 3.2 cm^2

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7 Conclusion and future perspectives

This review examined the mechanistic principles and material design strategies that underpin SED, focusing on the role of nanochannel membranes. By integrating nanoscale confinement with tuneable interfacial chemistry, these membranes enable simultaneous steric exclusion, electrostatic modulation, and dehydration effects that achieve the discrimination of ions of comparable sizes and charges. Across material platforms ranging from polymers and 2D nanosheets to crystalline porous frameworks and ceramics, recent advances demonstrate that ångström-level channel control and tailored surface functionality are critical for achieving high selectivity without compromising permeability. These findings establish a clear scientific foundation for advancing SED beyond the capabilities of conventional ion-exchange processes.

Although significant progress has been made in enhancing the ion selectivity and transport properties of nanochannel membranes, overcoming the inherent trade-off between permeability and selectivity remains a major obstacle. Fine-tuned confinement often improves the selectivity at the expense of ion flux. When the effective channel size approaches the (partially) dehydrated ion diameter and increasing the density of fixed charges and coordination sites raise the free-energy barriers for ions through stronger confinement, larger dehydration penalties, and stronger ion-pore binding. Conversely, efforts to increase the permeability can compromise the discrimination between the target and interfering ions. Achieving a membrane that overcomes this trade-off requires the simultaneous optimisation of structural parameters, such as channel length, interlayer spacing, tortuosity, and interfacial properties such as the surface charge density, hydrophilicity, and ion-specific binding affinity. Furthermore, standardised testing protocols and benchmarking against commercial IEMs are indispensable for reliable comparisons with existing IEMs and literature data, and for elucidating structure–performance relationships.

A critical, yet underexplored, area is the precise control of ion dehydration and rehydration dynamics within nanochannels. These mechanisms fundamentally govern selectivity for similarly sized ions. Selective dehydration mechanisms—enabled by ångström-scale confinement and tailored surface chemistry—have demonstrated promise in differentiating ions like Li⁺ from Mg²⁺ or Na⁺. However, a comprehensive understanding of these processes is currently lacking. Future studies should establish clear design guidelines by systematically decoupling these interconnected variables using model systems and in situ transport measurements.

Operational stability is another critical criterion for implementing membranes in SED. In practical separation environments, the membranes are exposed to a range of chemical, mechanical, and electrochemical stressors, such as pH changes, high current densities, and the use of biofouling or scaling agents. Membrane durability under such conditions must be validated through long-term cycling tests and accelerated aging studies. Materials such as thermally reduced GO, cross-linked COFs, and ceramic–polymer hybrids have shown promise in terms of their structural rigidity and chemical robustness. However, the systematic benchmarking of commercial ion-exchange membranes under realistic ED conditions remains largely unexplored.

From an industrial perspective, translating laboratory-scale prototypes into practical applications requires analyses of material cost, scalable fabrication, and process compatibility. Many high-performance nanochannel membranes rely on complex synthesis routes, expensive precursors, and throughput-limiting post-treatment processes. Thus, a shift towards scalable fabrication methods, such as continuous interfacial polymerisation, roll-to-roll assembly, and large-scale self-assembly, is crucial. These approaches must preserve nanochannel precision and functionality while enabling reproducible large-area membrane production. The economic feasibility of SED also depends on increasing the membrane lifetime and reducing the energy cost per unit separation. Therefore, membrane modules should be engineered for modular integration, rapid replacement, and maintenance-friendly operation.

Thus, polymer-based hybrid membranes represent the most realistic pathway among the available material options. Polymeric backbones provide mechanical strength, processability, and low cost, whereas nanofillers and crystalline domains provide molecular-level selectivity. Although membranes composed entirely of crystalline frameworks or nanosheets are theoretically achievable, their scalability, reproducibility, and long-term stability remain unclear. In contrast, hybrid architectures balance nanoscale precision with scalability, making them the most practical candidates for industrial-scale SED.

In conclusion, advances in nanochannel engineering have opened new opportunities for selective ion recovery. However, realising the full potential of SED requires both mechanistic precision and practical manufacturability. Hybrid membranes combining conventional polymers with functional nanochannel materials offer the most viable path toward scalable, durable, and cost-effective ion separation. By addressing the outstanding challenges of stability, scalability, and economic feasibility, SED is expected to become a cornerstone technology for sustainable water–energy–resource systems, contributing to both environmental resilience and a secure supply of critical elements.

Author contributions

H. You: conceptualisation, investigation, writing – original draft, reviewing and editing, T.-N. Kim: conceptualisation, writing – original draft, reviewing and editing, J. Hwang: funding acquisition, supervision, writing – reviewing and editing.

Conflicts of interest

There are no conflicts to declare.

Nomenclature

ED	Electrodialysis
SED	Selective electrodialysis
IEM	Ion-exchange membrane
CEM	Cation-exchange membrane
AEM	Anion-exchange membrane
NF	Nanofiltration
RO	Reverse osmosis
FO	Forward osmosis
MCDI	Membrane capacitive deionization
PA	Polyamide
PAMAM	Polyamidoamine
GO	Graphene oxide
MOF	Metal–organic framework
COF	Covalent–organic framework
SSE	Solid-state electrolyte
TFC	Thin film composite
MMM	Mixed-matrix membrane
EDL	Electrical double layer
MD	Molecular dynamics
DFT	Density functional theory
PSM	Postsynthetic modification
PSS	Polystyrene sulfonate
PET	Polyethylene terephthalate
PVDF	Polyvinylidene fluoride
CNT	Carbon nanotube
BNNT	Boron nitride nanotube

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Acknowledgements

This research was supported by Global-Learning & Academic research institution for Master's PhD students, and Postdocs (G-LAMP) Program of the National Research Foundation of Korea (NRF) grant funded by the Ministry of Education (No. RS-2023-00285390). This research was further supported by the NRF grant funded by the Korea government (MSIT) (2021R1C1C1009988).

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Footnote

† These authors contributed equally to this work.

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