Advances in Two-Dimensional Ion-Selective Membranes: Bridging Nanoscale Insights to Industrial-Scale Salinity Gradient Energy Harvesting

Salinity gradient energy, often referred to as the Gibbs free energy difference between saltwater and freshwater, is recognized as "blue energy" due to its inherent cleanliness, renewability, and continuous availability. Reverse electrodialysis (RED), relying on ion-selective membranes, stands as one of the most prevalent and promising methods for harnessing salinity gradient energy to generate electricity. Nevertheless, conventional RED membranes face challenges such as insufficient ion selectivity and transport rates and the difficulty of achieving the minimum commercial energy density threshold of 5 W/m2. In contrast, two-dimensional nanostructured materials, featuring nanoscale channels and abundant functional groups, offer a breakthrough by facilitating rapid ion transport and heightened selectivity. This comprehensive review delves into the mechanisms of osmotic power generation within a single nanopore and nanochannel, exploring optimal nanopore dimensions and nanochannel lengths. We subsequently examine the current landscape of power generation using two-dimensional nanostructured materials in laboratory-scale settings across various test areas. Furthermore, we address the notable decline in power density observed as test areas expand and propose essential criteria for the industrialization of two-dimensional ion-selective membranes. The review concludes with a forward-looking perspective, outlining future research directions, including scalable membrane fabrication, enhanced environmental adaptability, and integration into multiple industries. This review aims to bridge the gap between previous laboratory-scale investigations of two-dimensional ion-selective membranes in salinity gradient energy conversion and their potential large-scale industrial applications.

Micro and Nanoporous Membrane Platforms for Carbon Neutrality: Membrane Gas Separation Prospects

Recently, carbon neutrality has been promoted as a potentially practical solution to global CO2 emissions and increasing energy-consumption challenges. Many attempts have been made to remove CO2 from the environment to address climate change and rising sea levels owing to anthropogenic CO2 emissions. Herein, membrane technology is proposed as a suitable solution for carbon neutrality. This review aims to comprehensively evaluate the currently available scientific research on membranes for carbon capture, focusing on innovative microporous material membranes used for CO2 separation and considering their material, chemical, and physical characteristics and permeability factors. Membranes from such materials comprise metal-organic frameworks, zeolites, silica, porous organic frameworks, and microporous polymers. The critical obstacles related to membrane design, growth, and CO2 capture and usage processes are summarized to establish novel membranes and strategies and accelerate their scaleup.

Liquid-like and solid-like acetonitrile intercalated into graphite oxide as studied by the spin probe technique

The molecular mobility of acetonitrile intercalated into the inter-plane space of graphite oxide was studied using the spin probe technique. It was revealed that two types of intercalated substance - liquid-like and solid-like - are simultaneously present in between the oxidized graphene planes, and their ratio depends on temperature. The micro-viscosity of liquid-like intercalated acetonitrile was found to be higher than that of bulk acetonitrile and depends on the amount of intercalated liquid.

Sorption of Polar Sorbents into GO Powders and Membranes

The comparative study of sorption of polar substances acetonitrile and water into powders and membranes (>10 μm thick) of modified Hummers (HGO) and Brodie (BGO) graphite oxides was performed using isopiestic method (IM) and differential scanning calorimetry (DSC). Additional sorption data were obtained for pyridine and 1-octanol. Sorption measurements were accompanied by conventional XRD and XPS control. Electron paramagnetic resonance (EPR) was additionally used to characterize ordering of the membranes. The impact on sorption of synthetic procedure (Brodie or Hummers), method of making membranes, chemical nature of the sorbent, and method of sorption was systematically examined. It was demonstrated that variations in synthetic procedures within both Hummers and Brodie methods did not lead to changes in the sorption properties of the corresponding powders. Sorption of acetonitrile and pyridine was reduced by approximately half when switching from powders to membranes at ambient temperature. DSC measurements at a lower temperature gave equal sorption of acetonitrile into HGO powder and membranes. Water has demonstrated unique sorption properties. Equal sorption of water was measured for HGO membranes and powders at T = 298 K and at T = 273 K. It was demonstrated that lowering the orientational alignment of the membranes led to the increase of sorption. In practice this could allow one to tune sorption/swelling and transport properties of the GO membranes directly by adjusting their internal ordering without the use of any composite materials.

Pebax-Based Composite Membranes with High Transport Properties Enhanced by ZIF-8 for CO2 Separation

A series of mixed matrix membranes containing poly (ether-block-amide) Pebax 1657 as matrix and polyethylene glycol (PEG) and Zeolitic Imidazolate Framework-8 (ZIF-8) as additives, were prepared and tested for CO₂ separation. The membranes were prepared by solvent evaporation method and were characterized by TGA, DSC, SEM, and gas permeation measurements. The effects of PEG and its molecular weight, and the percentage of ZIF-8 into Pebax matrix were investigated. The results showed that the addition of PEG to Pebax/ZIF-8 blends avoid the agglomeration of ZIF-8 particles. A synergic effect between PEG and ZIF was particularly observed for high ZIF-8 content, because the initial permeability of pristine Pebax was multiplied by three (from 54 to 161 Barrers) while keeping the CO₂ selectivity (α_CO2/N2 = 61, α_CO2/CH4 = 12 and α_CO2/O2 = 23). Finally, the mechanism of CO₂ transport is essentially governed by the solubility of CO₂ into the membranes. Therefore, this new Pebax/PEG/ZIF-8 system seems to be a promising approach to develop new selective membranes for CO₂ with high permeability.

Ultrathin Membranes for Separations: A New Era Driven by Advanced Nanotechnology

Ultrathin membranes are at the forefront of membrane research, offering great opportunities in revolutionizing separations with ultrafast transport. Driven by advanced nanomaterials and manufacturing technology, tremendous progresses are made over the last 15 years in the fabrications and applications of sub-50 nm membranes. Here, an overview of state-of-the-art ultrathin membranes is first introduced, followed by a summary of the fabrication techniques with an emphasis on how to realize such extremely low thickness. Then, different types of ultrathin membranes, categorized based on their structures, that is, network, laminar, or framework structures, are discussed with a focus on the interplays among structure, fabrication methods, and separation performances. Recent research and development trends are highlighted. Meanwhile, the performances and applications of current ultrathin membranes for representative separations (gas separation and liquid separation) are thoroughly analyzed and compared. Last, the challenges in material design, structure construction, and coordination are given, in order to fully realize the potential of ultrathin membranes and facilitate the translation from scientific achievements to industrial productions.

Ultralarge Free-Standing Imine-Based Covalent Organic Framework Membranes Fabricated via Compression

Demand continues for processing methods to shape covalent organic frameworks (COFs) into macroscopic objects that are needed for their practical applications. Herein, a simple compression method to prepare large-scale, free-standing homogeneous and porous imine-based COF-membranes with dimensions in the centimeter range and excellent mechanical properties is reported. This method entails the compression of imine-based COF-aerogels, which undergo a morphological change from an elastic to plastic material. The COF-membranes fabricated upon compression show good performances for the separation of gas mixtures of industrial interest, N₂ /CO₂ and CH₄ /CO₂ . It is believed that the new procedure paves the way to a broader range of COF-membranes.

Influence of the extent of hydrophobicity on water organization and dynamics on 2D graphene oxide surfaces

Graphene oxide (GO) nanomaterials are being extensively explored for a wide spectrum of applications, ranging from water desalination to fuel cell applications due to their tunable mechanical, thermal, and electrical...

Recent advances in polymeric nanostructured ion selective membranes for biomedical applications

Nano structured ion-selective membranes (ISMs) are very attractive materials for a wide range of sensing and ion separation applications. The present review focuses on the design principles of various ISMs; nanostructured and ionophore/ion acceptor doped ISMs, and their use in biomedical engineering. Applications of ISMs in the biomedical field have been well-known for more than half a century in potentiometric analysis of biological fluids and pharmaceutical products. However, the emergence of nanotechnology and sophisticated sensing methods assisted in miniaturising ion-selective electrodes to needle-like sensors that can be designed in the form of implantable or wearable devices (smartwatch, tattoo, sweatband, fabric patch) for health monitoring. This article provides a critical review of recent advances in miniaturization, sensing and construction of new devices over last decade (2011-2021). The designing of tunable ISM with biomimetic artificial ion channels offered intensive opportunities and innovative clinical analysis applications, including precise biosensing, controlled drug delivery and early disease diagnosis. This paper will also address the future perspective on potential applications and challenges in the widespread use of ISM for clinical use. Finally, this review details some recommendations and future directions to improve the accuracy and robustness of ISMs for biomedical applications.

Ultrafast H2-selective nanoporous multilayer graphene membrane prepared by confined thermal annealing

H₂ selective dense pores are generated in a graphene oxide (GO) layer by thermal-decomposition of oxygen-functional groups under high pressure. The nanoporous GO membrane shows H₂/CO₂ selectivity of 12.1 and H₂ permeability of 10360 Barrer.

Diamine vapor treatment of viscoelastic graphene oxide liquid crystal for gas barrier coating

A layered graphene oxide/ethylenediamine (GO/EDA) composite film was developed by exposing aqueous GO liquid crystal (GOLC) coating to EDA vapor and its effects on the gas barrier performance of GO film were systematically investigated. When a GO/EDA coating with a thickness of approximately 1 μm was applied to a neat polyethylene terephthalate (PET) film, the resultant film was highly impermeable to gas molecules, particularly reducing the gas permeance up to 99.6% for He and 98.5% for H₂ in comparison to the neat PET film. The gas barrier properties can be attributed to the long diffusion length through stacked GO nanosheets. The EDA can crosslink oxygen-containing groups of GO, enhancing the mechanical properties of the GO/EDA coating with hardness and elastic modulus values up to 1.14 and 28.7 GPa, respectively. By the synergistic effect of the viscoelastic properties of GOLC and the volatility of EDA, this coating method can be applied to complex geometries and EDA intercalation can be spontaneously achieved through the scaffold of the GOLC.

Mobility of liquids intercalated into the interplane space of graphite oxide as revealed by a combination of 19F NMR, 1H NMR and EPR spin probe methods

A combination of ¹⁹F and ¹H NMR with the Carr–Purcell–Meiboom–Gill (CPMG) pulse sequence was used to examine the mobility of liquids in the interplane space of graphite oxide (GO) for the first time.

Understanding Gas Transport Behavior through Few-Layer Graphene Oxide Membranes Controlled by Tortuosity and Interlayer Spacing

Here, we elucidate the gas transport behavior through few-layer graphene oxide membranes (FGOMs) that have a systematically controlled diffusion pathway, including tortuosity and channel width. The obtained unusual gas permeation order (especially, CH₄ > O₂ > N₂) of the FGOM provides strong evidence that gas molecules can indeed penetrate through the empty voids created by horizontally assembled GO, which allows selective gas transport features. These unique transport features of the FGOM originate from its continuously connected channel structure, which is an analogue of an ultrapermeable glassy polymer with extremely large free volumes in dense films. Furthermore, variation of the channel width in the range of 0.50-0.55 nm leads to notable changes in the gas permeance orders related to CH₄, indicating that there is a transition region for switching the gas transport mechanism between a molecular sieving character and the solution-diffusion model.

A GO-Induced Assembly Strategy To Repair MOF Nanosheet-Based Membrane for Efficient H2/CO2 Separation

Metal-organic framework (MOF) nanosheets have emerged as promising materials for the construction of molecular sieve membranes due to their nanoscale thickness and highly oriented pore arrays. However, the development of defect-free MOF nanosheet membranes from rigid MOF nanosheets with efficient permselectivity and robust mechanical strength still remains a big challenge. Herein, we develop a facile GO-induced assembly strategy to design continuous and uniform two-dimensional MOF composite membranes, in which the copper 1,4-benzene dicarboxylate (CuBDC) nanosheets present a layer-stacking arrangement. With the assistance of flexible GO, the voids between the junctions of rigid MOF nanosheets are well-repaired, and the interlayer bonding interactions are strengthened, resulting in the fabrication of a continuous nanosheet membrane with robust mechanical strength. The obtained MOF nanosheet-based membranes display remarkable H₂/CO₂ separation performance with a superior H₂ permeance of 9.6 × 10^-7 mol m^-2 s^-1 Pa^-1 and an ideal separation selectivity of 95.1 due to the well-defined size exclusion effect, showing great promise for H₂ separation and purification. The repair strategy described here could also provide the blueprint for the development of other nanosheet-based membranes.

The Preparation and Properties of Nanocomposite from Bio-Based Polyurethane and Graphene Oxide for Gas Separation

Petroleum depletion and climate change have inspired research on bio-based polymers and CO₂ capture. Tung-oil-based polyols were applied to partially replace polyether-type polyols from petroleum for sustainable polyurethane. Tung-oil-based polyurethane (TBPU), was prepared via a two-step polycondensation, that is, bulk prepolymerization and chain extension reaction. The graphene oxide (GO) was prepared via Hummer’s method. Then, TBPU was composited with the GO at different ratios to form a TBPU/GO hybrid film. The GO/TBPU films were characterized by fourier transform infrared spectroscopy (FTIR), differential scanning calorimeter (DSC), thermal gravimetric analysis (TGA) and scanning electron microscope (SEM), followed by the measurement of mechanical properties and gas permeability. The results showed that the addition of tung-oil-based polyols enhanced the glass transition temperature and thermal stability of TBPU. The mechanical properties of the hybrid film were significantly improved, and the tensile strength and elongation at break were twice as high as those of the bulk TBPU film. When the GO content was higher than 2.0%, a brittle fracture appeared in the cross section of hybrid film. The increase of GO content in hybrid films improved the selectivity of CO₂/N₂ separation. When the GO content was higher than 0.35%, the resulting GO agglomeration constrained the gas separation and permeation properties.

Ionic Liquid Selectively Facilitates CO2 Transport through Graphene Oxide Membrane

Membrane separation of CO₂ from H₂, N₂, or CH₄ has economic benefits. However, the trade-off between selectivity and permanence in membrane separation is challenging. Here, we prepared a high-performance CO₂-philic membrane by confining the [BMIM][BF₄] ionic liquid to the nanochannels in a laminated graphene oxide membrane. Nanoconfinement causes the [BMIM][BF₄] cations and anions to stratify. The layered anions facilitate CO₂ transportation with a permeance of 68.5 GPU. The CO₂/H₂, CO₂/CH₄, and CO₂/N₂ selectivities are 24, 234, and 382, respectively, which are up to 7 times higher than that of GO-based membranes and superior to the 2008 Robeson upper bound. Additionally, the resultant membrane has a high-temperature resistance, long-term durability, and high-pressure stability, indicating its great potential for CO₂ separation applications. Nanoconfining an ionic liquid into the two-dimensional nanochannels of a laminated membrane is a promising gas separation method and a nice system for investigating ionic liquid behavior in nanoconfined environments.

Performance of Nanocomposite Membranes Containing 0D to 2D Nanofillers for CO₂ Separation: A Review

Membrane technology has the potential to be an eco-friendly and energy-saving solution for the separation of CO₂ from different gaseous streams due to the lower cost and the superior manufacturing features. However, the performances of membranes made of conventional polymers are limited by the trade-off between the permeability and selectivity. Improving the membrane performance through the addition of nanofillers within the polymer matrix offers a promising strategy to achieve superior separation performance. This review aims at providing a complete overview of the recent advances in nanocomposite membranes for enhanced CO₂ separation. Nanofillers of various dimensions and properties are categorized and effects of nature and morphology of the 0D to 2D nanofillers in the corresponding nanocomposite membranes of different polymeric matrixes are discussed with regard to the CO₂ permeation properties. Moreover, a comprehensive summary of the performance data of various nanocomposite membranes is presented. Finally, the advantages and challenges of various nanocomposite membranes are discussed and the future research and development opportunities are proposed.

Permeability and Selectivity of PPO/Graphene Composites as Mixed Matrix Membranes for CO₂ Capture and Gas Separation

We fabricated novel composite (mixed matrix) membranes based on a permeable glassy polymer, Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO), and variable loadings of few-layer graphene, to test their potential in gas separation and CO₂ capture applications. The permeability, selectivity and diffusivity of different gases as a function of graphene loading, from 0.3 to 15 wt %, was measured at 35 and 65 °C. Samples with small loadings of graphene show a higher permeability and He/CO₂ selectivity than pure PPO, due to a favorable effect of the nanofillers on the polymer morphology. Higher amounts of graphene lower the permeability of the polymer, due to the prevailing effect of increased tortuosity of the gas molecules in the membrane. Graphene also allows dramatically reducing the increase of permeability with temperature, acting as a “stabilizer” for the polymer matrix. Such effect reduces the temperature-induced loss of size-selectivity for He/N₂ and CO₂/N₂, and enhances the temperature-induced increase of selectivity for He/CO₂. The study confirms that, as observed in the case of other graphene-based mixed matrix glassy membranes, the optimal concentration of graphene in the polymer is below 1 wt %. Below such threshold, the morphology of the nanoscopic filler added in solution affects positively the glassy chains packing, enhancing permeability and selectivity, and improving the selectivity of the membrane at increasing temperatures. These results suggest that small additions of graphene to polymers can enhance their permselectivity and stabilize their properties.

Enhanced Gas Separation through Nanoconfined Ionic Liquid in Laminated MoS2 Membrane

Two-dimensional (2D) materials-based membranes show great potential for gas separation. Herein an ionic liquid, 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF₄]), was confined in the 2D channels of MoS₂-laminated membranes via an infiltration process. Compared with the corresponding bulk [BMIM][BF₄], nanoconfined [BMIM][BF₄] shows an obvious incremental increase in freezing point and a shift of vibration bands. The resulting MoS₂-supported ionic liquid membrane (MoS₂ SILM) exhibits excellent CO₂ separation performance with high CO₂ permeance (47.88 GPU) and superb selectivity for CO₂/N₂ (131.42), CO₂/CH₄ (43.52), and CO₂/H₂ (14.95), which is much better than that of neat [BMIM][BF₄] and [BMIM][BF₄]-based membranes. The outstanding performance of MoS₂ SILMs is attributed to the nanoconfined [BMIM][BF₄], which enables fast transport of CO₂. Long-term operation also reveals the durability and stability of the prepared MoS₂ SILMs. The method of confining ILs in the 2D nanochannels of 2D materials may pave a new way for CO₂ capture and separation.

Ultrathin graphene oxide-based hollow fiber membranes with brush-like CO2-philic agent for highly efficient CO2 capture

Among the current CO₂ capture technologies, membrane gas separation has many inherent advantages over other conventional techniques. However, fabricating gas separation membranes with both high CO₂ permeance and high CO₂/N₂ selectivity, especially under wet conditions, is a challenge. In this study, sub-20-nm thick, layered graphene oxide (GO)-based hollow fiber membranes with grafted, brush-like CO₂-philic agent alternating between GO layers are prepared by a facile coating process for highly efficient CO₂/N₂ separation under wet conditions. Piperazine, as an effective CO₂-philic agent, is introduced as a carrier-brush into the GO nanochannels with chemical bonding. The membrane exhibits excellent separation performance under simulated flue gas conditions with CO₂ permeance of 1,020 GPU and CO₂/N₂ selectivity as high as 680, demonstrating its potential for CO₂ capture from flue gas. We expect this GO-based membrane structure combined with the facile coating process to facilitate the development of ultrathin GO-based membranes for CO₂ capture.

Membrane separation technologies show promise for CO₂ capture, but typically suffer from a trade-off between permeance and selectivity. Here, the authors produce hollow fiber membranes coated with graphene oxide and a CO₂-philic agent that can efficiently separate CO₂ from flue gas under wet conditions.

Probe Into the Influence of Crosslinking on CO2 Permeation of Membranes

Crosslinking is an effective way to fabricate high-selective CO₂ separation membranes because of its unique crosslinking framework. Thus, it is essentially significant to study the influence of crosslinking degree on the permeation selectivities of CO₂. Herein, we report a successful and facile synthesis of a series of polyethylene oxide (PEO)-based diblock copolymers (BCP) incorporated with an unique UV-crosslinkable chalcone unit using Reversible Addition-Fragmentation Chain Transfer Polymerization (RAFT) process. The membranes of as-prepared BCPs show superior carbon dioxide (CO₂) separation properties as compared to nitrogen (N₂) after UV-crosslinking. Importantly, the influence of different proportions of crosslinked chalcone on CO₂ selectivities was systematically investigated, which revealed that CO₂ selectivities increased obviously with the enhancement of chalcone fractions within a certain limit. Further, the CO₂ selectivities of block copolymer with the best block proportion was studied by varying the crosslinking time which confirmed that the high crosslinking degree exhibited a better CO₂/N₂ (α_CO2/N2) selectivities. A possible mechanism model revealing that the crosslinking degree played a key role in the gas separation process was also proposed.

Synthesis of graphene oxide/polyimide mixed matrix membranes for desalination

Graphene oxide (GO) was incorporated into polyimide (PI) to fabricate GO/PI mixed matrix membranes (MMMs), which show a high water flux (36.1 kg m⁻² h⁻¹) and a high salt rejection (99.9%) for desalination of 3.5 wt% seawater at 90 °C.

Graphene-based membranes: status and prospects

Recently, graphene-based membranes have been extensively studied, represented by two distinct research directions: (i) creating pores in graphene basal plane and (ii) engineering nanochannels in graphene layers. Most simulation results predict that porous graphene membranes can be much more selective and permeable than current existing membranes, also evidenced by some experimental results for gas separation and desalination. In addition, graphene oxide has been widely investigated in layered membranes with two-dimensional nanochannels, showing very intriguing separation properties. This review will cover state-of-the-art of graphene-based membranes, and also provide a material guideline on future research directions suitable for practical membrane applications.

A highly proton-conducting, methanol-blocking Nafion composite membrane enabled by surface-coating crosslinked sulfonated graphene oxide

Coating an ultrathin graphene oxide film onto a Nafion membrane confers a distinct anti-tradeoff behavior: 93% decrease of methanol permeability while retaining the high proton conductivity.

Synthesis and applications of large-area single-layer graphene

The progresses in syntheses of large-area single-layer graphene and applications in membrane separation are summarized in this review.