High Permselectivity Research Articles

Serosa-Mimetic Nanoarchitecture Membranes for Highly Efficient Osmotic Energy Generation.

Osmotic energy stored between seawater and freshwater is a clean and renewable energy source. However, developing high-efficiency and durable permselective membranes for harvesting osmotic energy remains a longstanding bottleneck. Herein, we report that a nanocomposite membrane with a biological serosa-mimetic structure can achieve high-performance osmotic energy generation through the coupling of two-dimensional (2D) sulfonated covalent organic framework (COF) nanosheets and anion-grafted aramid nanofibers (ANFs). As verified by theoretical calculations and experimental investigations, the 2D COF nanosheets not only provide abundant one-dimensional (1D)/2D nanofluidic channels to synergistically benefit an ultrafast ion migration but also enable high cation permselectivity via the covalently tethered anions. The grafted ANFs increase the mechanical strength of the membrane and further improve the ion diffusion/rectification. When it was applied in an osmotic power generator, the biomimetic membrane delivered a power density of 9.6 W m-2, far surpassing the commercial benchmark of 5.0 W m-2. This work could boost the viability of osmotic energy conversion toward a sustainable future.

Ultrathin Sulfonated Mesoporous Silica Film with Well-Ordered Perpendicular Mesochannels: In-Situ Preparation, Characterization and Permselectivity

In this paper, a new type of ultrathin sulfonated mesoporous silica film (SMSF) with well-ordered perpendicular mesochannels was in-situ synthesized by Stöber approach and co-condensation method. The mesostructure of the synthesized SMSF was characterized. The results of small-angle XRD, high-resolution TEM, and cyclic voltammetry (CV) exhibited that, after loading of MPTMS, SMSF had an ordered mesostructure with a perpendicular orientation and open ends. SEM showed that SMSF could be entirely transferred and easily handled. FT-IR presented that sulfonic groups were successfully added to the surface of the nanochannels of silica film. Compared with mesoporous silica film (MSF) and commercial cation exchange membrane (CEM), SMSF had the highest permselectivity. The permselectivity of SMSF was not lined with the loading of the sulfonic groups (–SO3H). The highest permselectivity of SMSF to Na[Formula: see text] was 94% when the loading of MPTMS was 5.98% (wt.%). SMSF is a promising material in the application of salinity gradient energy harvest.

Unraveling the role of support membrane chemistry and pore properties on the formation of thin-film composite polyamide membranes

Nanoscale characteristics of the polyamide layer are key towards the high desalination performance of thin-film composite reverse osmosis (TFC-RO) membranes. Further advancements in the performance of TFC membranes necessitate a comprehensive understanding of the desired polyamide characteristics and its formation mechanisms. Empirical evidence has shown that the properties of the support layer is as equally important as the interfacial polymerization (IP) conditions in the fabrication of high permselectivity TFC membranes for desalination. Herein, we discuss the properties of polyamide layers formed using identical IP conditions over support membranes of different polymers and chemistries (polyethersulfone, polyetherimide and polysulfone) under fairly similar surface pore properties. The characteristics of the polyamide layers formed thereon displayed different physicochemical properties. It is postulated that the support membrane chemistry actually affects the IP reaction and polyamide formation by controlling the amine diffusion speed as well as the breadth of the IP reaction zone (i.e., the region between the interface and the furthest point in which the reaction occurs). Transmission electron microscopy analyses further revealed the nanoscale differences in the polyamide layer (heights ranging from 50 to 200 nm), including intrinsic thickness of basal layer (~10–35 nm) and leaf-like top layer (~20–85 nm), as well as the presence of nanovoids. Finally, we propose a conceptual model to underline the role of support membrane chemistry in the IP reaction, and consequently the formation mechanism of the nanoscale polyamide features. The mechanistic insights from this study are expected to provide more understanding towards a better control over the fabrication of polyamide layers for TFC membranes. The findings in this work are also expected to facilitate tailoring polyamide layers for specific osmotically driven processes.

A facile direct spray-coating of Pebax® 1657: Towards large-scale thin-film composite membranes for efficient CO2/N2 separation

High-efficient CO2 capture using membrane separation technology is a promising approach towards environmental redemption in recent years. Thin-film composite (TFC) membrane with selective layer made of CO2-philic materials is the preferred membrane type for industrial-scale application, owing to its high permselectivity and good processability. Herein, we propose a facile direct spray-coating method to fabricate Pebax® 1657 thin-film on a porous polysulfone (PSF) substrate. Unlike most conventional multilayer TFC membranes, the unnecessary need for gutter layer in this work greatly simplifies the membrane fabrication process. Pebax® 1657 thin films were deposited through direct spraying of the polymer solution containing Pebax® 1657 on PSF substrate under ambient temperature. The as-prepared Pebax®1657 TFC membranes showed decent separation performances with CO2 permeance of >200 GPU and CO2/N2 selectivity of >50 for mixed gas (vol. (CO2)/vol. (N2) = 20/80) tests. The facile TFC membrane fabrication and good CO2 separation performances make the direct spray coated Pebax®1657 membranes a promising strategy for large-scale CO2 capture applications.

Dual-imprinted mixed matrix membranes for selective recognition and separation: A synergetic imprinting strategy based on complex initiation system

Molecularly imprinted membranes (MIMs) with sufficient and even-distributed recognition sites that can break the permeability-selectivity trade-off phenomenon are desirable in chemical field of selective separation. Herein graphene oxide (GO)/TiO2-loaded nanocomposite fibrous membranes were prepared by developing two kinds of tetracycline (TC)-imprinted systems in the same MIMs-based material. Thereinto, polydopamine-based and sol–gel-based imprinting processes were applied to the synthesis of GO/TiO2-loaded dual-imprinted mixed matrix membranes (GT-DIMs). The as-prepared GT-DIMs encompassed innovative GO/TiO2-based nanocomposite fibrous channels and two kinds of TC-imprinted systems, and critical comparisons regarding the fluxes, rebinding capacities and permselectivity were provided and studied. Importantly, dual-imprinted system of GT-DIMs could not only allow for largely enhanced rebinding result (70.63 mg/g) and fast adsorption equilibrium rate within 30 min, but also facilitate the high permselectivity of TC in complex separation systems and lab-simulated wastewater samples. The permselectivity factors were all around 5.0, which strongly demonstrated the efficiently selective recognition and separation performance of GT-DIMs. Overall, based on testing results of practical separation and scalability, excellent structural stability and separation continuity had been successfully obtained for selective separation applications of pollutants.

Thermal crosslinking membrane with enhanced CO2 separation performance derived from nitrile-containing phenolphthalein-based poly(arylene ether ketone)

Enhancing gas permeabilities of thermal crosslinking membranes while retaining adequate selectivity is of great significance for industrial applications. In this work, a novel and commercial polymer of nitrile-containing phenolphthalein-based poly (arylene ether ketone) (PEK-C(CN)) was introduced to prepare thermal crosslinking membranes via pretreatment in air and subsequent thermal crosslinking treatment in N2, and the high permselectivity for CO2/CH4 and CO2/N2 gas pairs was achieved. The ATR-FTIR and XPS results demonstrated that the nitrile groups might undergo thermal cyclotrimerization reaction with the formation of stable triazine rings across molecular chains during the pretreatment, which is conducive to the preparation of defect-free and highly permeable thermal crosslinking membranes. During thermal crosslinking treatment, the Cardo moieties decomposed and induced crosslinking with forming bulky biphenyl linkages between polymeric chains, accompanied with pyrolysis of some nitrile groups, which resulted in an increase in interchain distance and microvoid size. The CO2 permeability of the resulting thermal crosslinking membrane was increased to around 3 times without sacrificing CO2/CH4 and CO2/N2 selectivities due to the incorporation of nitrile groups, compared to the thermal crosslinking membrane derived from nitrile-free PEK-C. For equimolar binary mixture gases, the fully crosslinked membranes present the CO2 permeability of around 1300 Barrer with CO2/CH4 and CO2/N2 separation factors of 32.6 and 35.8, respectively, which offer a promising alternative in CO2 capture and separation.

Transitioning from electrodialysis to reverse electrodialysis stack design for energy generation from high concentration salinity gradients

In this study, stack design for high concentration gradient reverse electrodialysis operating in recycle is addressed. High concentration gradients introduce complex transport phenomena, which are exacerbated when recycling feeds; a strategy employed to improve system level energy efficiency. This unique challenge indicates that membrane properties and spacer thickness requirements may differ considerably from reverse electrodialysis for lower concentration gradients (e.g. seawater/river water), drawing closer parallels to electrodialysis stack design. Consequently, commercially available electrodialysis and reverse electrodialysis stack design was first compared for power generation from high concentration gradients. Higher gross power densities were identified for the reverse electrodialysis stack, due to the use of thinner membranes characterised by a higher permselectivity, which improved current. However, energy efficiency of the electrodialysis stack was twice that recorded for the reverse electrodialysis stack at low current densities, which was attributed to: (i) an increased residence time provided by the larger intermembrane distance, and (ii) reduced exergy losses of the electrodialysis membranes, which provided comparatively lower water permeance. Further in-depth investigation into membrane properties and spacer thickness identified that membranes characterised by an intermediate water permeability and ohmic resistance provided the highest power density and energy efficiency (Neosepta ACS/CMS), while wider intermembrane distances up to 0.3 mm improved energy efficiency. This study confirms that reverse electrodialysis stacks for high concentration gradients in recycle therefore demand design more comparable to electrodialysis stacks to drive energy efficiency, but when selecting membrane properties, the trade-off with permselectivity must also be considered to ensure economic viability.

A photo-Fenton nanocomposite ultrafiltration membrane for enhanced dye removal with self-cleaning properties

Membrane fouling is an ongoing challenge in the membrane filtration process. Herein, a photocatalytic membrane comprising a reactive layer was fabricated by engineering partially reduced graphene oxide/Ag nanoparticles/MIL-88A (prGO/Ag/M88A, pGAM) photocatalysts on the PVDF substrate membranes. Benefiting from the high conductivity of prGO and the surface plasmon resonance (SPR) effect of Ag nanoparticles (Ag NPs), the photo-sensitivity of the prGO/Ag/M88A is significantly enhanced. Compared to the membrane in the dark condition, the pGAM membrane displayed an enhanced dye removal efficiency (∼99.7%) and significantly improved permeability (∼189 L·m−2·h−1 bar−1) towards dye contaminants based on the synergistic filtration/photo-Fenton processes. Significantly, the membrane retained high perm-selectivity after 10 cyclic runs (183 L·m−2·h−1 bar−1 of permeability and 98.1% of dye removal), and its nano-channel structure did not collapse under high pressure (0.1–0.4 MPa). The membrane also exhibits antifouling properties with a high water flux recovery of more than 90%. In addition, the pGAM membrane exhibited a high MB degradation efficiency (∼90%) when it is directly used as a photocatalyst in the photo-Fenton system. The mechanism of the self-cleaning is also proposed through quenching experiments. The results of this study demonstrate that this self-cleaning membrane has huge promise for membrane anti-fouling and wastewater remediation.

Silica Containing Composite Anion Exchange Membranes by Sol-Gel Synthesis: A Short Review.

This short review summarizes the literature on composite anion exchange membranes (AEM) containing an organo-silica network formed by sol–gel chemistry. The article covers AEM for diffusion dialysis (DD), for electrochemical energy technologies including fuel cells and redox flow batteries, and for electrodialysis. By applying a vast variety of organically modified silica compounds (ORMOSIL), many composite AEM reported in the last 15 years are based on poly (vinylalcohol) (PVA) or poly (2,6-dimethyl-1,4-phenylene oxide) (PPO) used as polymer matrix. The most stringent requirements are high permselectivity and water flux for DD membranes, while high ionic conductivity is essential for electrochemical applications. Furthermore, the alkaline stability of AEM for fuel cell applications remains a challenging problem that is not yet solved. Possible future topics of investigation on composite AEM containing an organo-silica network are also discussed.

Chiral separation of β-cyclodextrin modified graphene oxide membranes with a complete enantioseparation performance

The separation of racemates remains a great challenge as they are enantiomers with similar chemical structures and physicochemical properties in achiral environments. Recently, graphene oxide (GO) based membranes modified with various chiral selectors had been proved to allow for both high permeation flux and enantioselectivity. Herein, mono(6-amino-6-deoxy)-beta-cyclodextrin (β-CD) was selected to serve as chiral selector in GO-based membranes for separating racemates, since it potentially forms specific inclusion complexes with guests. Results show that β-CD displays a much stronger enantioselective affinity toward l-enantiomer than d-enantiomer, so that such β-CD modified GO membrane (β-CD-GOM) exhibits a retarded transport mechanism, having exceptional enantioselectivities with the enantiomeric excess value of nearly 100%. Moreover, β-CD-GOMs are 1-2 orders of magnitude higher in flux than traditional enantioseparation membranes. These findings demonstrate that β-CD-GOMs could provide new opportunities for improving enantioseparation performances with both high permeability and high permselectivity and for the large-scale production of various enantiomers.

Preparation of monovalent cation perm-selective membranes by controlling surface hydration energy barrier

Cationic separating the with different valence states is greatly desirable but technically challenging. Achieving such a precise separation using membranes requires accurately distinguishing tiny differences of cations on physico-chemical properties. Here, inspired by the ion dehydration effect, a series of cation exchange membrane with adjustable surface hydrophobicity based on quaternized polypyrrole were prepared. We demonstrate that improving hydrophobicity of modified layer substantially hinder cations with higher hydration energy to enter the modified layer due to their stronger binding ability with water molecules. When applied in electro-driven cation separation, the optimal membrane exhibited an outstanding perm-selectivity between monovalent and divalent cations (PNaMg = 12.95, PLiMg = 1.28) than commercial monovalent cation perm-selective membrane (PNaMg = 2.46, PLiMg = 1.07). Overall, our results indicate that the hydrophobicity of the modified layer governs the cations dehydration behavior through the polymeric pores due to ion-pore wall interactions, providing the scientific basis for the construction of membranes with high monovalent cation perm-selectivity.

Functional UiO-66 Series Membranes with High Perm Selectivity of Monovalent and Bivalent Anions for Electrodialysis Applications

Metal–organic frameworks (MOFs) have a good separation potential due to their uniform pore size, easy functionalization, and adjustable structure. Therefore, we make use of this feature of MOFs for the separation of Cl–/SO₄²– systems with different charges and sizes. In this work, two kinds of UiO-66 membranes with different functional groups were synthesized and applied to the monovalent perm-selective separation of the Cl–/SO₄²– system in electrodialysis (ED). UiO-66-NH₂ membranes were used to investigate the influence of current density and initial solution concentration on perm selectivity. A layer of UiO-66-NH₂ was first grown on the anodic alumina oxide (AAO) disks. After that, short chains containing sulfonic acid groups were grafted onto the amino groups by the ring-opening reaction of 1,3-propanesulfonate. The structural stability of the MOF layer was confirmed by XRD analysis of the membrane surface after the ED test. Ion exchange capacity (IEC) and elemental mapping were used to confirm the comparison changes in the degree of sulfonation. It was found that UiO-66-NH₂ membranes exhibited a higher monovalent anion perm selectivity with a peak value of 36.23 (initial mixture solution of 0.05 M NaCl and 0.05 M Na₂SO₄) at lower current density (2.5 mA·cm–²). The perm selectivity of UiO-66-SO₃H membranes increased with running time under 24 h of sulfonation time. In particular, with low IEC value of 0.10–0.16, it still showed a good performance on perm selectivity. Compared with published polymer ion exchange membranes (perm selectivity < 5) for ED, it has higher perm selectivity, more convenient functionalization, and resistance to organic solvents.

Coupling heat curing and surface modification for the fabrication of high permselectivity polyamide nanofiltration membranes

Surface modification is an efficient post-treatment method to optimize the properties of nanofiltration (NF) membranes. Here, we report a facile surface modification strategy coupling with heat curing for grafting monoethanolamine (MEA), a monomer containing both a primary amine and a primary alcohol group, onto a nascent polyamide NF membrane. With grafting 0.5 wt% MEA at 50 °C, the pure water permeability of the polyamide NF membranes was improved from 7.9 to 19.5 L m−2 h−1 bar−1 due to their enhanced physicochemical property, such as superior hydrophilicity, rough surface morphology, and enlarged membrane pores. Meanwhile, the rejection of Na2SO4 remained above 97.5%. More importantly, the optimal membrane modified with 0.5 wt% MEA exhibited a high Na2SO4 rejection of 99.1% and a negative NaCl rejection of −20.1% when treating a mixed salt solution containing 2 g/L Na2SO4 and 2 g/L NaCl. Our study provides a novel insight for the fabrication of high permselectivity NF membranes via surface modification.

Engineering block co-polymer anion exchange membrane domains for highly efficient electrode-decoupled redox flow batteries

The lifetime of the electrode-decoupled redox flow battery has been significantly improved (&lt;0.05% capacity fade per cycle) upon employing the block copolymer-based anion exchange membrane with high permselectivity.

Multilevel/hierarchical nanocomposite-imprinted regenerated cellulose membranes for high-efficiency separation: a selective recognition method with Au/PDA-loaded surface

Au/PDA-based multilevel nanocomposite imprinted membranes with high rebinding capacity and permselectivity performance were first proposed and developed for the selective recognition and separation of propranolol.

Comprehensive Review on High Hydrogen Permselectivity of Palladium Based Membranes: Part II

This article completes the presentation of various techniques reducing concentration polarisation in palladium based membranes for supplying ultra-high purity hydrogen to a polymer electrolyte fuel cell (PEFC), such as the implementation of baffles and the use of microchannel configuration. The present paper also reviews and reports the current methods for estimating hydrogen permeation flux under concentration polarisation influence, which will be a useful guide for academics and industrial practitioners.

Systematic Analysis Reveals Thermal Separations Are Not Necessarily Most Energy Intensive

Summary At the industrial level, separation of a variety of mixtures is predominantly carried out by thermally driven processes such as distillation. Nevertheless, the current literature seems to suggest that much is to be gained by replacing these processes with alternative non-thermal methods, which will potentially require a fraction of the energy used by distillation. This is primarily due to the widespread perception that thermal separation processes, particularly those that involve vaporization, are highly energy intensive. However, this belief is not well founded, because it is based on extrapolation from comparisons in the literature, many of which make ad-hoc assumptions and result in incorrect conclusions. Our analysis shows that this confusion arises because the processes utilize different types of energy: heat and electricity. Here, we present a consistent framework that enables a fair comparison between different processes that consume different forms of energies. Using this framework, we refute the general perception that thermal separation processes are inherently the most energy intensive and conclusively show through the examples of two challenging mixtures constituting components with low relative volatilities, that distillation processes, when run properly, can consume remarkably lower fuel than non-thermal membrane alternatives, which have often been touted as more energy efficient.

Solar-thermal membrane for dewatering aqueous organic-acid solutions

A thermally conductive porous membrane has been developed to directly absorb solar energy and conduct heat that can effectively evaporate liquid water at the interface between the membrane and the bulk feed solution. Black, porous, thermally conductive graphite foam support is employed as an effective photothermal-energy absorber and heat conductor that heats up an aqueous feed solution to produce vapor. Graphite nanoparticle-slurry coating is applied onto the foam surface to reduce the pore size and generate a microporous membrane. Then, application of a dense graphene oxide membrane coating as an active separation layer on the microporous nanoparticle surface of internal foam channels allows vapor permeation of volatile molecules. Acetic acid/water solution was studied as a model feed system to experimentally demonstrate effective separation. This model vapor permeation system demonstrates excellent acetic acid separation, with a separation factor of 8.3, under simulated 0.7 sun irradiation. The membrane hydrophobicity and nanostructure, including pore size and surface chemistry, played a significant role in enabling high permselectivity of water vapor over acetic acid molecules, as well as liquid solution. Pore size reduction from open pores to a nonporous dense layer in the membrane enables effective separation of water vapor from the organic vapor, while it reduces the permeation flux. Also, the hydrophilicity of the membrane surface shows ∼3 times higher permeation flux with higher permselectivity, compared with the hydrophobic membrane surface. This work introduces a process of directly using renewable energy instead of conventional heating to drive selective separation of water from organic acids.

An acid‐doped ice membrane for selective proton transport

Cation exchange membranes need to have high permselectivity for protons to make these membranes suitable for, for example, energy storage devices. Here, we present the proof of concept for a proton selective membrane made of hydrochloric acid-doped ice. The proton selectivity of this acid-doped ice membrane is the result of defects in the ice structure, caused by the acid. Ice membranes were made from different hydrochloric acid concentrations (0.1-2.0 M). The proton permselectivity of all ice membranes was above 99.7% when both Na+ and K+ were present. The resistivity decreased exponentially with the concentration of acid in the ice membrane, reaching a value of 12 Ω.cm. The ice membranes were tested in an electrochemical cell using the Fe/Fe2+ and Fe2+/Fe3+ redox couples, and a power density of 7 W/m2 and OCV of 0.87 V were measured. The resistance of the ice membrane increased with time as protons moved from the ice structure, as determined from the higher pH of the ice after melting. These expelled protons (and corresponding counter charged ions) were not replaced by other mobile cations, indicating a permanent loss in conductivity, but not selectivity. To apply the ice membrane as a selective separator for protons in energy storage devices in the future, the membrane thickness should be reduced and the protons should be retained inside the ice.

Novel thin-film composite membrane with ultrathin surface mineralization layer engineered by electrostatic attraction induced In-situ assembling process for high-performance nanofiltration

An ultrathin mineralization layer, comprising inorganic silica nanoparticles, was facilely in-situ engineered onto an organic polyamide (PA)-based membrane substrate via electrostatic attraction induced surface mineralization for nanofiltration. Based on transport behavior, the pre-infiltrated tetraethoxysilane monomers can act as spacers to break up the tightly structured PA-based matrix during the in-situ mineralization process, endowing the resultant thin-film composite (TFC) membrane with enlarged internal free volumes (voids) for the rapid transportation of water molecules. More importantly, the deposited mineralization layer further enriched the PA membrane with enhanced hydrophilicity and/or internal polarity with more accessible sites for water molecules to penetrate more easily into the membrane matrix. Consequently, the newly developed ultrafine silica nanoparticle-decorated PA-based (SiO2-d-PA) TFC membranes exhibited a high water permeance of 9.47 L m−2 h−1 bar−1 and a compelling rejection ratio of 95.2% toward Na2SO4, thereby generating an over two-fold increase in water permeability without sacrificing membrane selectivity. The surface mineralization layer endows the SiO2-d-PA TFC membrane with effective fouling-resistant/fouling-releasing property for comprehensive Ca2+-based suspensions containing bovine serum albumin, humic acid, and sodium alginate, whereby a full recovery of the permeating flux can be attained after three filtration-cleaning cycles. Simultaneously, the robust durability of the mineralization layer firmly anchored onto the PA-based membrane substrate can be obtained under harsh operating conditions (cross-flow, high-pressure, and continuous long-term filtration) owing to the electrostatic attraction incorporated with H-bond interaction. Overall, this study provides an insightful guideline for the development of novel inorganic/organic membranes with high permselectivity and low fouling propensity for high-performance nanofiltration.