Conventional Polymeric Membranes Research Articles

Graphene Oxide and its Composites: Advanced Membranes for Selective Water Permeation.

Graphene oxide (GO) membranes have gained significant attention as a promising material for separation by selective permeation processes due to their advantageous structural and chemical properties, including high water permeability, chemical resistance, and mechanical strength. In this study, we explore the potential applications of GO membranes in pervaporation to separate liquid mixtures. The layered structure and hydrophilic nature of GO membrane facilitate rapid and selective water transport through angstrom-scale interlayer spacings, resulting in superior performance over conventional polymeric and inorganic membranes. The unique mass transport mechanisms - slip flow and molecular alignment - enable GO membranes to selectively permeate water over organic solvents. For chemical dehydration, GO membranes are the most potential candidates. Furthermore, advancements in composite GO membranes and cross-linking techniques that improve their stability and separation performance are discussed. This study highlights the advantages of GO membranes and their potential to replace or complement existing technologies, by emphasizing their role in advancing membrane-based separation and promoting environmental sustainability. Future research is expected to optimize the fabrication techniques for GO membranes and expand their application scope.

Material Aspects of Thin-Film Composite Membranes for CO2/N2 Separation: Metal–Organic Frameworks vs. Graphene Oxides vs. Ionic Liquids

Thin-film composite (TFC) membranes containing various fillers and additives present an effective alternative to conventional dense polymer membranes, which often suffer from low permeance (flux) and the permeability–selectivity tradeoff. Alongside the development and utilization of numerous new polymers over the past few decades, diverse additives such as metal–organic frameworks (MOFs), graphene oxides (GOs), and ionic liquids (ILs) have been integrated into the polymer matrix to enhance performance. However, achieving desirable interfacial compatibility between these additives and the host polymer matrix, particularly in TFC structures, remains a significant challenge. This review discusses recent advancements in TFC membranes for CO2/N2 separation, focusing on material structure, polymer–additive interaction, interface and separation properties. Specifically, we examine membranes operating under dry conditions to clearly assess the impact of additives on membrane properties and performance. Additionally, we provide a perspective on future research directions for designing high-performance membrane materials.

Alkaline-resistant covalent organic framework membranes for selective hydroxide ion separation

Alkali holds a pivotal role in the chemical industry, however, the substantial discharge of alkaline wastewater causes a severe waste of resources and environmental pollution. Conventional polymer membranes have ether bonded backbones and form randomly assembled nanochannels, suffering from a weak alkali resistance and a low hydroxide ion selectivity. Covalent organic frameworks (COFs), known for their robust framework structure, highly ordered channels, and tunable channel chemistry, have been considered as the competent candidate for alkaline-resistant membranes for selective hydroxide ion separation. Herein, we fabricate a β-ketoenamine COF (TpTAPB) membrane with a high alkaline resistance via the interfacial growth strategy. After post-sulfonation, the negatively charged channels of sulfonated TpTAPB membrane allows OH- with smaller size and lower charges to pass through while repulsing WO42- ions, achieving a selectivity of OH-/WO42- up to ∼ 60 and surpassing commercial and most reported polymeric membranes. This work expands the application scope of COF membranes in selective alkali separation.

Synergistic Construction of Sub-Nanometer Channel Membranes through MOF–Polymer Composites: Strategies and Nanofiltration Applications

The selective separation of small molecules at the sub-nanometer scale has broad application prospects in the field, such as energy, catalysis, and separation. Conventional polymeric membrane materials (e.g., nanofiltration membranes) for sub-nanometer scale separations face challenges, such as inhomogeneous channel sizes and unstable pore structures. Combining polymers with metal–organic frameworks (MOFs), which possess uniform and intrinsic pore structures, may overcome this limitation. This combination has resulted in three distinct types of membranes: MOF polycrystalline membranes, mixed-matrix membranes (MMMs), and thin-film nanocomposite (TFN) membranes. However, their effectiveness is hindered by the limited regulation of the surface properties and growth of MOFs and their poor interfacial compatibility. The main issues in preparing MOF polycrystalline membranes are the uncontrollable growth of MOFs and the poor adhesion between MOFs and the substrate. Here, polymers could serve as a simple and precise tool for regulating the growth and surface functionalities of MOFs while enhancing their adhesion to the substrate. For MOF mixed-matrix membranes, the primary challenge is the poor interfacial compatibility between polymers and MOFs. Strategies for the mutual modification of MOFs and polymers to enhance their interfacial compatibility are introduced. For TFN membranes, the challenges include the difficulty in controlling the growth of the polymer selective layer and the performance limitations caused by the “trade-off” effect. MOFs can modulate the formation process of the polymer selective layer and establish transport channels within the polymer matrix to overcome the “trade-off” effect limitations. This review focuses on the mechanisms of synergistic construction of polymer–MOF membranes and their structure–nanofiltration performance relationships, which have not been sufficiently addressed in the past.

Ultrathin Free-Standing Porous Aromatic Framework Membranes for Efficient Anion Transport.

Porous aromatic frameworks (PAFs) show promising potential in anionic conduction due to their high stability and customizable functionality. However, the insolubility of most PAFs presents a significant challenge in their processing into membranes and subsequent applications. In this study, continuous PAF membranes with adjustable thickness were successfully created using liquid-solid interfacial polymerization. The rigid backbone and the stable C-C coupling endow PAF membrane with superior chemical and dimensional stabilities over most conventional polymer membranes. Different quaternary ammonium functionalities were anchored to the backbone through flexible alkyl chains with tunable length. The optimal PAF membrane exhibited an OH- conductivity of 356.6 mS ⋅ cm-1 at 80 °C and 98 % relative humidity. Additionally, the PAF membrane exhibited outstanding alkaline stability, retaining 95 % of its OH- conductivity after 1000 hours in 1 M NaOH. To the best of our knowledge, this is the first application of PAF materials in anion exchange membranes, achieving the highest OH- conductivity and exceptional chemical/dimensional stability. This work provides the possibility for the potential of PAF materials in anionic conductive membranes.

Ultrathin Free‐Standing Porous Aromatic Framework Membranes for Efficient Anion Transport

AbstractPorous aromatic frameworks (PAFs) show promising potential in anionic conduction due to their high stability and customizable functionality. However, the insolubility of most PAFs presents a significant challenge in their processing into membranes and subsequent applications. In this study, continuous PAF membranes with adjustable thickness were successfully created using liquid‐solid interfacial polymerization. The rigid backbone and the stable C−C coupling endow PAF membrane with superior chemical and dimensional stabilities over most conventional polymer membranes. Different quaternary ammonium functionalities were anchored to the backbone through flexible alkyl chains with tunable length. The optimal PAF membrane exhibited an OH− conductivity of 356.6 mS ⋅ cm−1 at 80 °C and 98 % relative humidity. Additionally, the PAF membrane exhibited outstanding alkaline stability, retaining 95 % of its OH− conductivity after 1000 hours in 1 M NaOH. To the best of our knowledge, this is the first application of PAF materials in anion exchange membranes, achieving the highest OH− conductivity and exceptional chemical/dimensional stability. This work provides the possibility for the potential of PAF materials in anionic conductive membranes.

Tuning the assembly of MgNiO2 nanoparticles-infused polysulfone membranes for efficient gas separation: The selectivity-permeability conundrum

The persistent surge in global energy demand, coupled with escalating environmental apprehensions, has underscored the imperative for adept and sustainable gas separation technologies. Mixed matrix membranes (MMMs) have surfaced as a promising avenue to confront these multifaceted challenges, synergistically harnessing the strengths of conventional polymeric membranes and nanoparticle reinforcements. In this work, MMMs have been tailored for gas separation applications by incorporating MgNiO2 nanoparticles as an additive into the polysulfone (PSf) matrix as they provide additional space (in the form of nanoscale voids) for gas diffusion, facilitating faster gas transport, promoting selective permeation of specific gases, and enhancing membrane stability, in addition to their versatility, ease of preparation, cost-effectiveness and high activity. The crystalline nature of MgNiO2 nanoparticles, as characterized by finely tuned grain sizes, has been validated by powder X-ray diffraction (XRD) analysis. Gas permeation experiments on the developed MMMs encompassing a diverse range of pure gases and gas mixtures have been performed. Among the different MMMs investigated, PSM1 with a composition of 100 mg MgNiO2 nanoparticles has yielded substantial enhancements in permeability and selectivity. The PSM1 membrane exhibits remarkable permeance of 56.9 for H2, 16.3 for CH4, and 15 GPU for CO2, having ideal selectivity ratios of 3.5 and 3.8 for H2/CH4 and H2/CO2, respectively. Finally, this work provides new directions for improving the trade of relationship between the selectivity and permeability of gas mixtures under relevant process conditions.

Paracetamol transport by ZnO nanoparticle-doped polymer-containing membrane

In this study, the transport performance of acetaminophen (paracetamol), which is most commonly prescribed and used for humans and animals and whose wastes are known to have toxic effects on the environment and some living organisms, was investigated using zinc oxide (ZnO)-reinforced polymer membranes. In this study, nanoparticle-containing polymer membranes were prepared from cellulose triacetate in dichloromethane and ZnO nanoparticles were synthesized to impart adsorption properties to the membrane in a single step, enabling adsorption and filtration to improve the removal of low molecular weight micropollutants that are poorly retained by conventional polymer membranes and by enabling re-release. Our membrane was prepared by phase inversion method by doping with cellulose triacetate (CTA) solution. Parameters such as carrier concentration, mixing rate, transport time, acceptor and feed phase concentrations were studied to determine the optimal conditions for the transport experiments of paracetamol (PARA). The presence of acid in the acceptor phase converted the hydrophilic part of paracetamol, which allowed the transport of PARA. The calculated flux values for different PARA concentrations ranged from (0.64) × 10−8 to (1.8) × 10−8 mol/(cm2s). Under optimal conditions, a transport efficiency of 84% was obtained for PARA with a CTA/ZnO polymer membrane. The obtained membranes can be used in wastewater treatment, recovery of pharmacological products from pharmaceutical industry waste, re-evaluation of hospital waste, etc.

A comprehensive study on transport behaviour and physicochemical characteristics of PU/based 3-phase mixed matrix membranes: Effect of [HNMP][HSO4] ionic liquid and ZnO nanoparticles

The gas separation industry has witnessed a significant focus on mixed matrix membranes (MMMs) due to their exceptional potential, surpassing the performance of conventional polymeric membranes. This study aimed to develop and evaluate MMMs using polyurethane (PU) as the polymer matrix and zinc oxide (ZnO) and ionic liquids (ILs) as fillers. PU/ZnO, PU/IL, and PU/ZnO/IL MMMs were prepared utilizing the dry-phase inversion method, and their efficacy in separating CO2, CH4, and N2 gases was investigated under various conditions. A comprehensive characterization protocol using FTIR, FESEM, TGA, AFM, and tensile testing was employed for their thorough characterization. The membranes’ transport properties were assessed at different pressures and filler concentrations. Results showed that IL incorporation increased gas permeability by enhancing solubility, diffusion, ionic interactions, swelling, and accessible volume. Incorporating ZnO and IL particles in the PU matrix simultaneously improved transport properties, particularly selectivity. Robeson’s diagram analysis shows that the PU/0.5%ZnO/2%IL MMM performed best. Furthermore, molecular simulation methods, including Monte Carlo and molecular dynamics simulations, assessed adsorption isotherms and physicochemical properties such as free volume, density, and X-ray diffraction. Simulation outcomes were highly reliable with experimental results, affirming the results’ efficiency and validation. Overall, this study demonstrated the efficiency of using ZnO and IL in the MMMs, highlighting their potential in gas separation applications.

Rational design of the PEBA2533 mixed matrix membrane containing metallo-supramolecular polymers for efficient CO2/H2 separation

Mixed-matrix membranes (MMMs) are an effective method for overcoming the trade-off limitations of conventional polymeric membranes and are attractive for CO2/H2 separation. However, major challenges remain in achieving high gas separation performance and long-term stability owing to undesirable polymer/filler interfaces. In this study, a novel metallo-supramolecular polymer (F127-Tpy-M) was designed and selected to engineer a polymer/filler interface. Benefiting from strong intermolecular interactions, F127-Tpy-M dispersed well in the PEBA2533 matrix with a defect-free interface. By synergistically combining the dipole–quadrupole effect of the ethylene oxide groups in F127 and gas–metal interactions, MMMs could significantly improve the gas separation performance. In comparison with the pristine membrane, the MMM with 60% F127-Tpy-M loading exhibited 175–194% and 215–259% increases in CO2/H2 selectivity and CO2 permeability, respectively, which far surpass the 2008 Robeson upper bound. The molecular dynamics simulations supported the experimental data, confirming that F127-Tpy-M is a promising candidate for designing next-generation high-performance MMMs.

Effects of multi‐walled carbon nanotubes on structures and properties of heat‐resistant poly(m‐phenylene isophthalamide)‐based hollow fiber ultrafiltration membranes

AbstractThis study is the first attempt to fabricate a heat‐resistant hollow fiber ultrafiltration membrane using poly(m‐phenylene isophthalamide) (PMIA) polymer material, which has rarely been used as conventional polymeric membrane materials. Multi‐walled carbon nanotubes (MWCNTs) are incorporated as reinforcing agents. The PMIA polymer synthesized for this study exhibited an inherent viscosity of 1.6 dL/g, confirming its suitability for membrane fabrication, and successful synthesis was verified through Fourier‐transform infrared analysis. To enhance the dispersibility of MWCNTs during dope solution preparation and their interaction with the PMIA matrix, MWCNTs were acid‐treated and surface characteristics of the acid‐treated MWCNTs were confirmed through x‐ray photoelectron spectroscopy analysis. Scanning electron microscope analysis revealed that the introduction of MWCNTs resulted in thicker PMIA hollow fiber ultrafiltration membranes with symmetrical finger‐like and sponge‐like pore structures. Interestingly, unlike the typical polymer composite systems containing MWCNTs, which often show a dependence on MWCNT content, the mechanical properties of the PMIA/MWCNT membranes in this study seem to exhibit high variability and are not contingent on the MWCNT content. Additionally, filtration performance studies demonstrated that the introduction of MWCNTs substantially increased water permeability, particularly at 1.0 wt% MWCNT content, resulting in a remarkable 130% enhancement compared to pristine PMIA membrane. Simultaneously, even with a small quantity of MWCNTs, the rejection performance of the PMIA/MWCNT membrane witnessed significant improvement due to the reduction in average pore size, effectively overcoming the commonly observed trade‐off phenomenon. In summary, this study clearly showed the effects and changes on the structure and properties of a heat‐resistant PMIA‐based hollow fiber ultrafiltration membranes due to the introduction of MWCNTs.Highlights Heat‐resistant poly(m‐phenylene isophthalamide)‐based hollow fiber ultrafiltration membranes Introduction of multi‐walled carbon nanotubes (MWCNTs) into PMIA membrane as a reinforcing agent Unique structural changes of PMIA membrane originated from the MWCNTs Enhanced tensile strength of PMIA/MWCNT membranes Simultaneous improvement of filtration performance due to MWCNTs

Understanding the potentiometric response of cation-permselective hydrophilic nanopore-based electrodes in the low ion concentration range

Permselective membranes are at the core of important applications and devices, that include ion-selective electrodes, energy conversion, and separation. Here we report the potential response in the low ion concentration range of hydrophilic cation-permselective gold nanopore-based membranes obtained by surface modification with a sulfonate-bearing thiol derivative. Apart from conventional polymeric permselective membranes, we confirmed ‘equal’ potentiometric responses for the same valency cations, that include both inorganic and organic cations. A refined Nernst–Planck/Poisson (NPP) model to account for previously neglected mass transport effects in the low ion concentration domain was shown to accurately predict even the interesting zero current fluxes generated super-Nernstian responses for higher valency cations. For all investigated ions we found that their interaction with sulfonate-groups functionalized nanopores is dominantly electrostatic in its nature. Hence, the synthetic versatility of such permselective nanopores in conjunction with the possibility of a rational design of its properties based on the NPP model renders such nanopore-based membranes not only a convenient alternative to existent permselective membranes, but their unique properties show promises for novel ion-sensing and separation applications.

Rational design, synthesis, and characterization of facilitated transport membranes exhibiting enhanced permeability, selectivity and stability

Despite outperforming conventional polymer membranes in the short time frame, facilitated transport membranes (FTMs) are prone to photo/chemical aging, which causes a rapid decay of their performance. Moreover, when embedded in polymer materials to fabricate mixed matrix membranes, structural defects may form at the metal-polymer interface, causing a selectivity loss. To overcome these issues, a series of poly(amic acid)s (PAAs) have been synthesized using 4,4’-oxydiphathalic anhydride (ODPA) and Jeffamine monomers of varying molecular weight, and used to fabricate silver nanoparticles via the chelating reaction with silver ions, in attempts to simultaneously i) achieve defect-free mixed matrix membranes (MMMs), ii) target CO2 selective transport, and iii) enhance long-term stability. The occurrence of the chelating reaction between silver and the PAA was confirmed, and as a result, individual and un-aggregated PAA-coated silver nanoparticles were obtained and incorporated into a commercial polymer, Pebax 1657, to fabricate defect-free mixed matrix membranes. The effect of the PAA length and ether functional group concentration on the structure and performance of the resulting mixed matrix membranes was systematically investigated. Remarkably, inclusion of only 2 wt% nanoparticles in Pebax enhances the CO2 permeability by 50% and CO2 selectivity by 100% relative to neat polymer. A detailed analysis of the sorption and diffusion coefficients was performed to elucidate the molecular origin of the observed membrane performance. Finally, preliminary data show that the newly synthesized materials exhibit high chemical stability upon exposure to pure H2.

High-Flux lamellar MoSe2 membranes for efficient dye/salt separation

Membrane-based technology is emerging as an efficient technique for wastewater treatment in recent years. Membranes made up of two-dimensional materials provide high selectivity and water flux compared to conventional polymeric membranes. Herein, we report the synthesis and use of MoSe2 membrane for dye and drug separation in wastewater, mainly from textile and pharmaceutical industries. The as-prepared MoSe2 membrane shows ∼ 100% rejection for organic dyes and ciprofloxacin drug with a water flux reaching up to ∼ 900 Lm-2h-1bar-1. Further, the MoSe2 membrane shows lower NaCl rejection of ∼ 1.9% for the dye/salt mixture. The interlayer spacing in the MoSe2 membrane allows the water molecules and ions from the salt to pass through freely but restricts the movement of large contaminants. The membrane is stable against the bovine albumin serum fouling with a flux recovery rate of 96%. It also shows good performance even in harsh environments (pH 3–10). To the best of our knowledge, the MoSe2 membranes were fabricated for the first time for wastewater treatment application. The dye/salt separation performance of the MoSe2 membrane is significantly better than several other membranes. This work highlights the promising potential for using two-dimensional materials for textile and pharmaceutical wastewater treatment.

Recovery of homogeneous photocatalysts by covalent organic framework membranes

Transition metal-based homogeneous photocatalysts offer a wealth of opportunities for organic synthesis. The most versatile ruthenium(II) and iridium(III) polypyridyl complexes, however, are among the rarest metal complexes. Moreover, immobilizing these precious catalysts for recycling is challenging as their opacity may obstruct light transmission. Recovery of homogeneous catalysts by conventional polymeric membranes is promising but limited, as the modulation of their pore structure and tolerance of polar organic solvents are challenging. Here, we report the effective recovery of homogeneous photocatalysts using covalent organic framework (COF) membranes. An array of COF membranes with tunable pore sizes and superior organic solvent resistance were prepared. Ruthenium and iridium photoredox catalysts were recycled for 10 cycles in various types of photochemical reactions, constantly achieving high catalytical performance, high recovery rates, and high permeance. We successfully recovered the photocatalysts at gram-scale. Furthermore, we demonstrated a cascade isolation of an iridium photocatalyst and purification of a small organic molecule product with COF membranes possessing different pore sizes. Our results indicate an intriguing potential to shift the paradigm of the pharmaceutical and fine chemical synthesis campaign.

Modification of polyacrylonitrile (PAN) membrane with anchored long and short anionic chains for highly effective anti-fouling performance in oil/water separation

Conventional polymeric membranes are usually prone to fouling by oily substances and therefore are greatly hindered in their applications for oil/water separation. In this study, we carried out a new strategy of modifying PAN membrane for highly effective anti-oil fouling performance. The modified membrane surface was anchored with negatively charged long and short chains, where the long chains could reject oil droplets at far away from the membrane surface while the short chains near the membrane surface could increase its hydrophilicity and thus enhance the permeate flux during the filtration of oil emulsion. The negatively charged long chains were constructed with sulfonate-terminated units and the short chains with carboxylate units. The prepared PAN membranes were tested to show both superhydrophilic and underwater superoleophobic surface properties, and displayed very high resistance to the adhesion of various types of oils. In the membrane separation experiments with 1000 mg‧L-1 surfactant-stabilized oil-in-water emulsion, the developed PAN membrane exhibited an initial water flux of 115.4 L·m−2·h−1, and less than 16% of flux decay but higher than 97% in flux recovery after 3 filtration cycles (45 min each). Furthermore, in a challenging test, the membrane also showed excellent recovery in its performance after being fouled by a heavy fuel oil but cleaned simply by the action of soaking it into water. Therefore, the current study appeared to provide a new and effective alternative in the modification of conventional PAN membrane to enhance or expand its anti-oil fouling performance, especially for oil recovery or oil removal in oily wastewater treatment.

Preparation and oxygen permeability of pure supramolecular polymer membranes synthesized from poly (substituted phenylacetylene)

We previously reported a new preparation method for supramolecular polymers having self-membrane forming ability from poly (substituted phenylacetylene)s using highly selective photocyclic aromatization (the SCAT reaction). However, since mechanical strengths of their membranes were not enough for oxygen permeation process, we could not estimate their oxygen permselectivities. In this study, we successfully obtained oxygen permselectivities from the experimental data for their pure supramolecular polymer membranes prepared on porous supports. This is the first example of gas permeability data for pure supramolecular polymer membranes. Surprisingly, the oxygen permselectivities were higher than those for their corresponding conventional polymer membranes. It may be because the supramolecular polymer membranes had denser and less defected structures than their corresponding conventional polymer membranes. It was thought that the supramolecularly-bonded main chains in these pure supramolecular polymers worked like the covalently-bonded main chains in the conventional polymers in the permeation.

Fast Magnesium Conducting Electrospun Solid Polymer Electrolyte

AbstractMagnesium Ion based Solid State Batteries (MIBs) are subject of intensive studies due to abundance of magnesium, its advantages in volumetric capacity, and the reduced dendrite growth. Here we report on a true solid polymer electrolyte system without liquid additives or plasticizers that reaches conductivities above 10−5 S cm−1 at room temperature and above 10−4 S cm−1 at 50 °C. An electrospun polymer electrolyte membrane fabricated from a polymer electrolyte featuring a composition of PEO : Mg(TFSI)2 36 : 1 [where PEO stands for poly(ethyleneoxide) and Mg(TFSI)2 for magnesium bis(trifluoromethanesulfonyl) imide] was identified as the best performing system. Magnesium transport was substantiated by different methods, and the electrochemical properties including solid electrolyte interface (SEI) formation were investigated. Electrospinning as a preparation method has been identified as a powerful tool to enhance the electrochemical properties beyond conventional polymer membrane fabrication techniques.

Surface Mineralization of the TiO2-SiO2/PES Composite Membrane with Outstanding Separation Property via Facile Vapor-Ventilated In Situ Chemical Deposition.

Conventional polymeric membranes are broadly employed in water treatment processes; however, most of them suffer from relatively low water permeance and severe membrane fouling phenomena owing to their relatively hydrophobic nature. In this work, a novel class of inorganic-organic composite membranes was developed through a newly developed vapor-ventilated in situ chemical deposition method, where the Ti and Si precursors were first hydrolyzed and then conferred into metal oxides to form a continuous TiO2-SiO2 modification layer. Owing to the distinct physicochemical properties, the Ti and Si precursors were leveraged as quasi-molecular regulators to tune the membrane surface chemistry and pore aperture (within the nanoscale) to benefit highly efficient water purification by underpinning the rapid transport of water molecules and featuring an excellent fouling-resistant and fouling-releasing property against typical pollutants. The as-developed TiO2-SiO2/PES composite membrane showed a high water permeance of 187.4 L·m-2·h-1·bar-1, together with a relatively small mean pore aperture of 4.2 nm, showing an outstanding permeating efficiency among state-of-the-art membranes with a similar separation accuracy. This study provides a paradigm shift in membrane materials that could open avenues for developing high-performance inorganic-organic composite membranes for complex wastewater treatment.

Essence of the Enhanced Osmotic Energy Conversion in a Covalent Organic Framework Monolayer.

Low membrane conductivity originated from a high membrane thickness has long been the "Achilles heel" of the conventional polymeric membrane, greatly hampering the improvement of the output power density in osmotic power generation. Herein, we demonstrate a molecularly-thin two-dimensional (2D) covalent organic framework (COF) monolayer membrane, featured with ultimate thickness, high pore density, and tight pore size distribution, which performs as a highly efficient osmotic power generator. Despite the large pore size up to 3.8 nm and relatively low surface charge density of 2.2 mC m-2, the monolayer COF membrane exhibits a high osmotic current density of 16.7 kA m-2 and an output power density of 102 W m-2 under 50 times the NaCl salinity gradient (0.5 M/0.01 M). This superior power density could be further improved to 170 W m-2 in the real seawater/river water gradient system. When the large pore size and low surface charge density are considered, this superior performance is not expected. Computational studies further reveal that the ultimate membrane permeability originated from the high membrane porosity, rather than ion selectivity, plays a dominant role in the production of high current density, especially under high salinity. This work provides an alternative strategy to realize improved output power density in ultrapermeable membranes.