Cross-linked Membranes Research Articles

Construction of proton transport channels in cross-linked composite membrane with highly sulfonated SPEEK nanofibers

A series of novel nanofiber composite proton exchange membranes were fabricated by embedding highly sulfonated poly (ether ketone) (SPEEK) nanofibers in a cross-linked SPEEK matrix, showing both high proton conductivity and excellent stability. Composite membranes were prepared with salt-form SPEEK nanofiber mats filled with acid-form SPEEK solution. After thermal treatment at 180 °C in the presence of dimethyl sulfoxide, the cross-linking reaction occurred in the acid-form SPEEK matrix but not in nanofibers. The dimensional stability, mechanical strength, and oxidative stability of the composite membrane were significantly improved due to thermal cross-linking treatment. The swelling ratio of all the cross-linked membranes was below 30% in hot water, and the maximum tensile strength reached 68 MPa. The nanofibers formed non-cross-linked regions and retained rich sulfonic acid groups to construct fast, long-range proton transport channels. The proton conductivity of the nanofiber composite cross-linked membrane reached 200 mS/cm exceeding the pristine cross-linked membrane (135 mS/cm). The fuel cell performance showed a peak power density of 485 mW/cm2 at 80 °C under 100% relative humidity.

The Effects of Electrospinning Structure on the Ion Conductivity of PEO-Based Polymer Solid-State Electrolytes

To overcome the safety hazard of the liquid electrolytes used in traditional lithium batteries, solid electrolytes have drawn more attention because of their advantages such as non-volatility, easy processing, good mechanical properties, and stability. In this paper, sodium alginate (SA) nanofiber membranes were used as the backbone of PEO-based solid electrolytes. SA nanofiber membranes were prepared by electrospinning with assistance from PEO and cross-linked with calcium ions to construct a nanofiber network skeleton, which provided a guarantee for the stability of the subsequent electrolyte preparation process. The effects of spinning conditions and crosslinking time on the structure and performances of the nanofiber membranes were investigated. Meanwhile, the relationship between the skeleton of nanofiber membranes cross-linked with calcium ions and ion conductivity was investigated. The optimal parameters of the electrospinning process including concentration, voltage, distance, and SA content were discussed, and the fiber diameter and its distribution were analyzed. Furthermore, Fourier transform infrared (FTIR) spectrometer, thermal gravimetric analyzer analysis (TGA), X-ray diffraction (XRD), and energy dispersive spectrometer (EDS) maps were used to characterize the nanofiber membranes and electrolytes. The results showed that the thermal performance of cross-linked nanofiber membranes improved and the crystallinity of the PEO matrix decreased. The ion conductivity of the electrolytes was characterized by electrochemical impedance spectroscopy (EIS) testing, and the results showed that the assembled lithium symmetric battery had a good ion conductivity of 6.82 × 10−5 S/cm at 30 °C.

Phosphoric acid-doped cross-linked poly(phenylene oxide)-based membranes for high temperature proton exchange membrane fuel cells

In this study, a series of cross-linked poly (phenylene oxide) (PPO)-based membranes with and without rotatable ethylene oxide spacers are prepared. Both PPO-based membranes possess good thermal stability, excellent oxidative stability, and satisfactory dimensional stability in phosphoric acid (PA) owing to their cross-linked structure and PPO backbone. The cross-linked PPO-based membrane with a rotatable ethylene oxide spacer exhibits enhanced conductivity, higher tensile strength, and better oxidative stability compared with that without the rotatable spacer. Owing to a better trade-off effect, the cross-linked membrane with the rotatable ethylene spacer exhibits higher H2/air cell performance (maximum power density, 237 mW cm−2) than that without the rotatable spacer (maximum power density, 144 mW cm−2). The introduction of rotatable spacers is an effective method for enhancing the performance of high-temperature proton exchange membranes.

Proton transfer efficiency enhanced over wide relative humidity: Etidronic acid-modified polyphosphazenes cross-linked polybenzimidazoles membranes

To enhance the proton conductivity and oxidative stability of the proton exchange membrane, an effective strategy to construct robust high-temperature PEMs (HT-PEMs) is proposed. The polyoxy(etidronic acid)phazenes cross-linked m-polybenzimidazole membranes (POHP-mPBI) are prepared by solution casting, thermal-crosslinking and acidification. Polyoxy(etidronic acid sodium)phazenes (Na–POHP) is cross-linkable proton conductors by partially phosphonic acid functionalized polyphosphazenes, which ensures proton transport while retaining the cross-linked groups. After covalent cross-linking between Na–POHP and mPBI, the cross-linked macromolecular structure stably locks the proton conductor in the polymer matrix, which ensures the construction of the proton transfer pathway at wide relative humidity (RH). The molecular dynamics simulation was conducted to verify the hydrogen bonds network with the POHP-mPBI membrane. The POHP-mPBI cross-linked membranes exhibit good oxidative stability, mechanical strength, proton conductivity and low fuel transmembrane permeability. At 180 °C and different RH (100% RH, 50% RH, 30% RH and 0 RH), the proton conductivity of POHP(50)-mPBI membrane is 0.150, 0.076, 0.055 and 0.048 S cm−1, respectively. After the water washing for 96 h, the durability of proton conductivity and weight loss shows a rare decline. The incorporation of proton conductors (POHP) into the PBI matrix by cross-linking is a perspective method to enhance the comprehensive performance of HT-PEM.

Strategies to overcome the limitations of cross-linked hydrophilic PVA membranes; carboxy methyl cellulose blending for epichlorohydrin-isopropanol-water pervaporation dehydration

Sodium carboxyl methyl cellulous (NaCMC)/Polyvinyl alcohol (PVA) coated Alumina hollow fiber (AHF) membrane was developed and applied to dehydration of ECH-IPA-water ternary feed mixture. CMC40 membrane, including 40 wt% of NaCMC showed the highest flux of 0.416 kg/m2h and a separation factor of 898 due to optimized packing density between PVA and the NaCMC polymer chain and relatively higher hydrophilic nature for PV separation to ECH-IPA-water ternary feed. After that, CMC40-coated AHF membrane was prepared according to the number of CMC40 coating and baking temperature. With an increase in baking temperature to 170 °C, the separation factor was significantly improved, while the flux decreased slightly. Moreover, NaCMC40-AHF showed robust properties with stable performance in the 50-h long-term PV test.

Swelling-Induced Cross-Linked Pyridine-Containing Membranes with High Stability and Conductivity for Vanadium Redox Flow Batteries

Swelling-Induced Cross-Linked Pyridine-Containing Membranes with High Stability and Conductivity for Vanadium Redox Flow Batteries

Phenyl Acrylate-Based Cross-Linked Anion Exchange Membranes for Non-aqueous Redox Flow Batteries

Phenyl Acrylate-Based Cross-Linked Anion Exchange Membranes for Non-aqueous Redox Flow Batteries

Polyamide⊥TpPa-PVDF membranes for aliphatic-aromatic solvent separations

To achieve net-zero emissions, the petrochemical industry requires energy-efficient processes for solvent recovery, and traditional distillation technologies may not be adequate. Membrane separations with 2D COFs (covalent organic frameworks) have shown potential due to their tunable pore sizes and modifiable properties. However, the large pores of the COFs have provided challenges in fabricating 2D COF membranes for solvent separation. Recent research has been focused on improving the selectivities and permeation fluxes for solvent mixtures, and to achieve long-term operation, the membranes must have high chemical and mechanical stabilities. To address the issue of large pores, our research group cross-linked the Tp (2,4,6-triformylphloroglucinol) and Pa (p-phenylenediamine) monomers of the COF with TMC (trimesoyl chloride), a monomer used traditionally for reverse osmosis interfacial polymerization, which resulted in COF(TpPa) with TMC cross-linking and small pore sizes for solvent separation. Polyamide⊥TpPa-PVDF membranes were fabricated for the first time with this method, and the TMC cross-linked COF membrane successfully rejected salts and separated aromatic/alkane solvents; a toluene/benzene mixture showed a higher separation factor than methanol/isopropanol. Use of the 2D materials for reverse osmosis at 20 bar enhanced the potential of the energy-efficient membranes for use in future petrochemical industrial applications.

Dithiol cross-linked polynorbornene-based anion-exchange membranes with high hydroxide conductivity and alkaline stability

Vinylic-addition polynorbornenes possess high thermal stability, excellent mechanical integrity, and chemically inert backbones. Herein, we conveniently synthesize a series of vinylic-addition polynorbornene copolymers containing pendant alkenes from commercial vinyl-substituted norbornene (VNB) and alkyl-bromide-substituted norbornene (BrNB). Subsequently, the copolymers are photo-crosslinked with dithiol reagents through UV-initiated thiol-ene click reactions followed by quaternization to prepare the cross-linked anion-exchange membranes (AEMs). The extent of cross-linking and the hydrophobicity of the cross-linkers can be easily controlled, so that the absorption of excess water can be inhibited with enhanced membrane strength. The resultant AEMs have ultrahigh hydroxide conductivities (up to 199 mS/cm at 90 °C under humidified conditions) and excellent alkaline stability in 1 M KOH at 80 °C for 1200 h. The vinylic-addition AEMs are also successfully applied in alkaline anion-exchange membrane water electrolyzers with commercial noble-metal-free electrocatalysts.

Cross-Linked Polybenzimidazoles Containing Porous Organic Cages for High-Temperature Proton Conduction

We prepared a series of porous organic cages (POCs) with cross-linked polybenzimidazole membranes. Because of the introduction of the POC with numerous amine groups and ultramicroporous structures in the polymers, which enhanced the PA interactions and increased free volume, the POC-PBI membranes exhibited better proton conductivity and higher acid uptake. 20%-POC-OPBI showed the highest conductivity at 47.2 mS cm−1, followed by OPBI at 27.2 mS cm−1.

MAP4K4 regulates forces at cell-cell and cell-matrix adhesions to promote collective cell migration.

Collective cell migration is not only important for development and tissue homeostasis but can also promote cancer metastasis. To migrate collectively, cells need to coordinate cellular extensions and retractions, adhesion sites dynamics, and forces generation and transmission. Nevertheless, the regulatory mechanisms coordinating these processes remain elusive. Using A431 carcinoma cells, we identify the kinase MAP4K4 as a central regulator of collective migration. We show that MAP4K4 inactivation blocks the migration of clusters, whereas its overexpression decreases cluster cohesion. MAP4K4 regulates protrusion and retraction dynamics, remodels the actomyosin cytoskeleton, and controls the stability of both cell-cell and cell-substrate adhesion. MAP4K4 promotes focal adhesion disassembly through the phosphorylation of the actin and plasma membrane crosslinker moesin but disassembles adherens junctions through a moesin-independent mechanism. By analyzing traction and intercellular forces, we found that MAP4K4 loss of function leads to a tensional disequilibrium throughout the cell cluster, increasing the traction forces and the tension loading at the cell-cell adhesions. Together, our results indicate that MAP4K4 activity is a key regulator of biomechanical forces at adhesion sites, promoting collective migration.

Different Approaches for the Preparation of Composite Ionic Liquid-Based Membranes for Proton Exchange Membrane Fuel Cell Applications—Recent Advancements

The use of ionic liquid-based membranes as polymer electrolyte membranes for fuel cell applications increases significantly due to the major features of ionic liquids (i.e., high thermal stability and ion conductivity, non-volatility, and non-flammability). In general, there are three major methods to introduce ionic liquids into the polymer membrane, such as incorporating ionic liquid into a polymer solution, impregnating the polymer with ionic liquid, and cross-linking. The incorporation of ionic liquids into a polymer solution is the most common method, owing to easy operation of process and quick membrane formation. However, the prepared composite membranes suffer from a reduction in mechanical stability and ionic liquid leakage. While mechanical stability may be enhanced by the membrane’s impregnation with ionic liquid, ionic liquid leaching is still the main drawback of this method. The presence of covalent bonds between ionic liquids and polymer chains during the cross-linking reaction can decrease the ionic liquid release. Cross-linked membranes reveal more stable proton conductivity, although a decrease in ionic mobility can be noticed. In the present work, the main approaches for ionic liquid introduction into the polymer film are presented in detail, and the recently obtained results (2019–2023) are discussed in correlation with the composite membrane structure. In addition, some promising new methods (i.e., layer-by-layer self-assembly, vacuum-assisted flocculation, spin coating, and freeze drying) are described.

Retarded Interfacial Formation of Polyamine-Based Nanofiltration Membranes with Pyromellitic Acid: Separation, Acid Stability, and Antifouling Performance

Crosslinked and additive-added polyamide thin-film composite (TFC) nanofiltration (NF) membranes formed by the conventional interfacial polymerization (IP) process have shown good performances. However, studies on polyamine TFC NF membrane prepared via an adjusted IP process with low-cost acid ligands as an interfacial additive for improved liquid-based separation are still lagging. In this regard, a polyamine-based TFC NF membrane synthesized by retarding the interfacial polymerization (IP) process with pyromellitic acid (PMA) (a polycarboxylate acid ligand) is herein reported. The PMA-adjusted membrane displayed a superb range of pure water fluxes from 9.8 to 20.4 L m–2 h–1 bar–1, with the best-performed membrane achieving up to 95.1% for Na2SO4 rejection. Thus, the water permeability of the PMA-TFC membrane is ∼2.4 times as high as the control TFC membrane. The best-performed PMA-TFC membrane showed good hydrophilicity, pressure resistance, and long-term filtration stability. In addition, the membrane structural stability was maintained with ∼3.9% declination in salt rejection after 30 days of immersion in 0.1 M HNO3 solution. Also, an impressive fouling resistance was achieved after three-cycle of bovine serum albumin fouling tests. This work, therefore, offers a more feasible way of designing polyamine-based NF membranes with good prospects in water reuse and nitric acid-generating wastewater treatment application.

Fabrication, Characterization and Permeation Studies of Ionically Cross-linked Chitosan/Kaolin Composite Membranes

This paper presents the successful preparation of porous membranes based on chitosan with enhanced mechanical, thermal and chemical properties applicable in water treatment field. Herein, chitosan/kaolin composite membranes with a cross-linking agent and a porogen were prepared using the solvent casting method. The characterization of the as-fabricated membranes indicated that the combined effect of kaolin as reinforcing agent, polyethylene glycol as pore former and citric acid as cross-linker in a chitosan matrix showed a significant influence on the membrane properties. The results indicated that the incorporation of a hydrophilic porogenic reagent into the collodion in addition to providing a porous morphology makes it possible to obtain a more hydrophilic membrane, and thus induces an increase in the pure water permeability. The cross-linked membranes exhibited an improved water resistance, better thermal and mechanical properties as compared to neat chitosan films. The cross-linked membranes had a mean pore size of 50 nm falling in the range of ultrafiltration. Their functional properties were determined in terms of pure water filtration and molecular weight cut-off tests.

Response Surface Optimization of Electro-Spun Polyvinyl Alcohol Nano-Fiber Membrane Process Parameters and its Characterization

An empirical exploration into the effects of time duration, voltage supply, concentration and flow rate on the membrane average fiber diameter and surface pore size distribution were done using response surface methodology (RSM) based on central compact design (CCD). The average fiber diameter and average surface pore diameter of the membrane (i.e. 110 nm and 130 nm, respectively) were obtained at optimum input parameters of 45 min, 10 wt. %, 12 kV and 1.0 mL/h for time duration, concentration, voltage supply, and flow rate, respectively. The optimization study shows that the predicted versus actual values of both membrane fiber diameter and surface pore diameter are at R2 = 0.96. In addition, the effect of glutaraldehyde on membrane crosslinking was also assessed for further studies. The results from FESEM images of the fabricated PVA nanofiber membranes using the optimized parameters revealed that the membranes showed smooth morphological structures without formation of beads. The thermo gravimetric analysis (TGA) results displayed an improvement in thermal stability after membrane crosslinking. From this study we have observed that the membrane average fiber diameter and surface pore diameter can be controlled by varying the electro-spinning parameters and can be utilized for wastewater treatment application.

Anion Receptor Enhanced Li Ion Transportation for High-Performance Lithium Metal Batteries.

High-potential lithium metal batteries (LMBs) are still facing many challenges, such as the growth of lithium (Li) dendrites and resultant safety hazards, low-rate capabilities, etc. To this end, electrolyte engineering is believed to be a feasible strategy and interests many researchers. In this work, a novel gel polymer electrolyte membrane, which is composed of polyethyleneimine (PEI)/poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) cross-linked membrane and electrolyte (PPCM GPE), is prepared successfully. Due to the fact that the amine groups on PEI molecular chains can provide the rich anion receptors and strongly pin the anions of electrolytes and thus confine the movement of anions, our designed PPCM GPE owns a high Li+ transference number (0.70) and finally contributes to the uniform Li+ deposition and inhibits the growth of Li dendrites. In addition, the cells with PPCM GPE as a separator behave the impressive electrochemical performances, i.e., a low overpotential and an ultralong and stable cycling performance in Li∥Li cells, a low overvoltage of about 34 mV after a stable cycling for 400 h even at a high current density of 5 mA/cm2, and, in Li∥LFP full batteries, a specific capacity of 78 mAh/g after 250 cycles at a 5 C rate. These excellent results suggest a potential application of our PPCM GPE in developing high-energy-density LMBs.

Study on the partially cross-linked poly(styrene-b-(ethylene-co-butylene)-b-styrene) anion exchange membrane remotely (spaced hexyl) grafted double cation separated by different alkyl groups

In order to improve the alkali stability and OH− conductivity of Poly (styrene-b-(ethylene-co-butylene)-b-styrene) (SEBS)-based anion exchange membranes (AEMs), double cations with different alkyl intervals are remotely grafted onto the SEBS skeleton with hexyl as a linker through reactions such as acylation and ketone reduction. Then, SEBS-0.8Cn-0.2C6 cross-linked membranes were prepared to study the effect of the length of the alkyl chain between the dications on the ion transport and other properties. The OH− conductivity of SEBS-0.8C4-0.2C6 cross-linked membrane can reach 85.27 mS cm−1 at 80 °C, and the peak power density can reach 225 mW cm−2 at a current density of 450 mA cm−2. As the dicationic spacer alkyl chains became longer, the swelling rate and water uptake of the membranes increased, resulting in significant improvements in mechanical properties and chemical stability. After soaking in 2 M NaOH solution at 80 °C for 1200 h, the conductivity of SEBS-0.8C6-0.2C6 decreased by only 5.76%. Optimizing the side chain structure of SEBS skeleton can effectively improve the comprehensive performance of AEM.

Rearrangement of GO nanosheets with inner and outer forces under high-speed spin for supercapacitor

The self-standing graphene membranes are considered as ideal electrode materials for supercapacitors. However, maintaining highly regularized and uniform graphene membranes with satisfied electrochemical performance is still a challenge. Herein, with the chelation of metal cation and the radial shear force introduced by high-speed spinning, the uniform interlayer channels and shrunken cracks between adjacent nanosheets can be achieved in the metal-intercalated graphene oxide (GO) membranes, thus realizing regularization both in normal and radial direction. With the promotion in electron transfer and electrolyte penetration, the iron cross-linked GO membrane with spin coating for 40 cycles exhibits a high specific capacitance (427 F g−1 at 1 A g−1) and rate capability (42.6% capacitance retention from 1 to 40 A g−1), as well as excellent cyclic capability (90.5% capacitance retention after 20,000 cycles). Particularly, a 21% increasement in capacitance can be achieved after high-speed spinning treatment. Moreover, the spin regularization strategy can be extended to GO membranes cross-linked by various multi-valence metal cations, the electrochemical performance of metal-cation cross-linked GO membrane electrodes after high-speed spinning treatment can also be improved obviously. Therefore, this paper provides a novel method to fabricate highly ordered GO membranes with promising electrochemical performance, which presents an immense potential application in membrane materials applied in energy storage, separation and catalysis.

Synthesis of a Cross-Linked Polymer Electrolyte Membrane with an Ultra-High Density of Sulfonic Acid Groups

Synthesis of a Cross-Linked Polymer Electrolyte Membrane with an Ultra-High Density of Sulfonic Acid Groups

Collagen Membrane as Water-Based Gel Electrolyte for Electrochromic Devices.

Bio-based polymers are attracting great interest due to their potential for several applications in place of conventional polymers. In the field of electrochemical devices, the electrolyte is a fundamental element that determines their performance, and polymers represent good candidates for developing solid-state and gel-based electrolytes toward the development of full-solid-state devices. In this context, the fabrication and characterization of uncrosslinked and physically cross-linked collagen membranes are reported to test their potential as a polymeric matrix for the development of a gel electrolyte. The evaluation of the membrane's stability in water and aqueous electrolyte and the mechanical characterization demonstrated that cross-linked samples showed a good compromise in terms of water absorption capability and resistance. The optical characteristics and the ionic conductivity of the cross-linked membrane, after overnight dipping in sulfuric acid solution, demonstrated the potential of the reported membrane as an electrolyte for electrochromic devices. As proof of concept, an electrochromic device was fabricated by sandwiching the membrane (after sulfuric acid dipping) between a glass/ITO/PEDOT:PSS substrate and a glass/ITO/SnO2 substrate. The results in terms of optical modulation and kinetic performance of such a device demonstrated that the reported cross-linked collagen membrane could represent a valid candidate as a water-based gel and bio-based electrolyte for full-solid-state electrochromic devices.