Membrane Chemical Structure Research Articles

Fabrication and performance optimization of an advanced pervaporation desalination membrane: A study utilizing PVDF and hydrophilic active layer as composite

Pervaporation emerges as a promising technique for brine treatment. In this investigation, we employed a PVDF substrate layer and applied various coating solutions using a simple spray technique to produce a composite pervaporation membrane. The composite membrane's chemical structure, morphology, hydrophilicity, swelling property, surface roughness, and crosslinking conditions were thoroughly investigated. Notably, the PVDF/PEI/P(SS-MA) composite membrane exhibited outstanding desalination performance, demonstrating high salt rejection and flux of 308.7 ± 13.8 kg m−2 h−1 when subjected to a 3.5 wt% sodium chloride solution. Even at an elevated brine concentration of 20 wt%, the membrane maintained a commendable flux of 88.76 ± 5.4 kg m−2 h−1 at 72 °C. These findings underscore the efficacy of the developed composite membrane for brine desalination applications.

Plasma treatment of nafion membranes: Understanding the trade-off between surface modification and electrochemical degradation

Nafion is a polymer commonly used in electrochemical devices due to its excellent ionic conductivity. In applications such as proton-exchange membranes for fuel cells (PEMFCs) and ionomeric polymer metal composites (IPMCs), the interfacial properties are critical to device performance. Several studies use plasma treatment to modify the Nafion surface aiming for adhesion improvement through nanoroughness, however, most of them neglect the effects of this treatment on the membrane chemical structure and electrochemical performance. This work comprehensively investigates the Nafion properties, evaluating chemical, morphological, and electrochemical modifications trhough relevant characterization methods. Atomic force (AFM) and scanning electron microscopies (SEM) showed increased nanoroughness with plasma treatment time. Also, Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) analyses showed fluorine and sulfur abstraction, associated with side chain breakage. This effect was prejudicial to the water uptake and decreased the ionic conductivity, measured by Electrochemical Impedance Spectroscopy (EIS), from 140.8 to 1.0 μS cm−1 in the plasma-treated Nafion. Also, an applied electrochemical device was prepared and confirmed the ionic transport reduction using plasma-treated Nafion. This phenomenon is crucial for electrochemical devices, as fewer ionic channels remain on the treated membrane surface reducing water absorption, ionic conductivity, and charge density.

Elucidating mechanistic background of the origin and rates of peroxide formation in low temperature proton exchange fuel cells

Degradation of electrode-membrane assembly of the low-temperature hydrogen fuel cells represents one of the main obstacles in wider adoption of these clean and efficient electrochemical sources of electrical energy. Chemical degradation of proton exchange membrane is initiated by hydrogen peroxide formation, which forms in the fuel cell as a by-product to water in oxygen reduction reaction and decomposes to reactive radical species, damaging to the membrane chemical structure. Depending on the operating conditions of the fuel cell, the source of hydrogen peroxide can be either cathode, anode, or, as we argue in the paper, also the Pt particles in the membrane, which originate from the cathode catalyst dissolution, diffusion into the membrane and redeposition of Pt ions inside the membrane. In the paper we propose a mathematical model of intertwined physical processes in membrane and catalyst layer, aimed at unifying the description of hydrogen peroxide formation throughout entire membrane-electrode assembly at any fuel cell operating conditions. The model results, compared to experimental data, indicate that Pt particles inside the membrane can indeed be an important source of hydrogen peroxide in aged fuel cells. For a fresh fuel cell, numerical simulation using proposed model show that hydrogen peroxide can be formed at either cathode or anode, depending on the fuel cell operating condition, but with anode production being more prominent in standard fuel cell operating conditions.

Enhanced proton conductivity of viscose-based membranes via ionic modification and dyeing processes for fuel cell applications

Proton exchange membranes (PEMs), which are crucial fuel cell parts, play an important role in the field of energy science. However, the further development of conventional PEMs based on synthetic polymers is greatly limited by high energy consumption and difficult degradation. In this work, we reported the fabrication of a novel viscose-based PEM via cationic modification and dyeing treatment with the reactive dyes KE-7B1. High-efficiency proton transmission channels can be constructed due to the formation of the complex internal three-dimensional network of the as-prepared viscose-based PEM. H+ conductivity (σH+) and water uptake are intensively investigated by changing the cationic agents and KE-7B1, and the maximum σH+ reaches 44.19 mS/cm at 80 °C and 98% relative humidity (RH). Furthermore, the prepared membrane shows the lowest calculated activation energy value (12.25 kJ/mol), indicating that both Grotthuss and Vehicle mechanisms play an important role in ionic transport. The membrane chemical structure and micromorphology are analyzed and the proton transmission modes are explored in detail, supplemented with research on the hydrophilic/hydrophobic characteristics and crystallinity of the membranes. The application stability of the membranes is also evaluated analyzing the thermal, mechanical, and oxygen resistance properties, and the results show that all the prepared membranes can maintain good thermal stability within 200 °C. The maximum tensile strength reaches 42.12 MPa, and the mass losses of the membranes soaked in 30% (in mass, same below) H2O2 solution for 120 h can be restricted to 10%. Therefore, as a novel PEM, the obtained dye viscose-based membranes show great potential for application in fuel cells.

Effects of Reinforcement Type on the Structure and Properties of Perfluorosulphonic Acid Membranes for Polymer Electrolyte Membrane Fuel Cells

Fuel cell membrane durability remains one of the key challenges limiting the wide scale adoption of fuel cell technology. Membrane degradation in polymer electrolyte membrane (PEM) fuel cells limits the operational lifetime of the fuel cell and prevents the industrial targets from being met. A common approach to increasing the lifetime of membranes includes the addition of a reinforcement layer to increase the mechanical durability of the membrane while not adversely affecting the performance [1,2]. Although reinforced membranes are widely used in industry, there is a literature gap considering membrane structure and properties in relation to durability. This work contributes to characterizing the effects of reinforcement type on the membrane properties. In this study a selection of perfluorosulphonic acid (PFSA) ionomer membranes with expanded polytetrafluoroethylene (ePTFE) reinforcements were tested, these include two novel DMR100 membranes with different reinforcements and Nafion XL compared to a conventional, non-reinforced Nafion NRE-211 for reference. All of these membranes contain common PFSA ionomer and differ primarily in the type of reinforcement layer and membrane thickness. The study addresses comparison between the membrane chemical composition, water uptake, crystallinity, and mechanical strength. Methods of ex situ characterization include solid state nuclear magnetic resonance (SS_NMR), small angle X-ray scattering (SAXS), wide angle X-ray scattering (WAXS), Fourier transform infrared spectroscopy (FTIR), and dynamic mechanical analysis (DMA). Membrane chemical structure and properties are measured by SS-NMR and FTIR, whereas the water uptake, domain spacing, and crystallinity are assessed by SAXS/WAXS study of hydrated and dry membranes [3-5]. Tensile mechanical properties are measured under room temperature (23°C, 50% RH) and fuel cell conditions (80°C, 90% RH) by DMA [6]. Overall, this paper contributes both qualitative and quantitative understanding of the key structural properties of membranes with different reinforcement resulting in novel knowledge about membranes that can be leveraged for improved fuel cell durability.

Azo dye removal via surfactant-assisted polyvinylidene fluoride membrane

Background: Recently, concerns have been raised regarding the environmental and public health safety of azo dyes, the most widely used synthetic dyes. The membrane technique has been introduced as one of the efficient methods for dye removal treatments. Polyvinylidene fluoride (PVDF) membrane manipulated by surfactants was studied for removal of the azo dye, carmoisine. Methods: PVDF membrane was prepared via non-solvent-induced phase separation (NIPS) and used to remove the azo dye, carmoisine. Three nonionic surfactants including Tween 20, Tween 60, and Tween 80 were used individually as additives in casting solutions to improve PVDF membrane properties. Results: Fourier-transform infrared spectroscopy (FTIR) demonstrated the presence of functional groups of carbonyl (C=O) and hydroxyl (OH), assigned to Tween molecules, in the membrane chemical structure. All Tween species caused a decrease in the surface hydrophobicity of PVDF membranes illustrated by the reduced contact angles. Each Tween at a 2% concentration in the dope solution led to an increase in the pore-size of PVDF membranes, which was estimated by scanning electron microscopy (SEM). However, this impact was reversed at Tween concentrations of 4%. Membranes were assessed for dye removal efficiency and permeate flux in a cross-flow system. Permeability of PVDF membranes improved (~78%) with adding Tween 80 at a concentration of 2%. Tween 60 at a 2% concentration resulted in a ~45% increase in dye removal efficiency of PVDF membranes. And, atomic force microscopy (AFM) showed that Tween 60 increased membrane surface roughness. Conclusion: Surfactant-mediated changes in the surface properties of PVDF membrane improved dye removal efficacy.

Ameliorated Performance of Sulfonated Poly(Arylene Ether Sulfone) Block Copolymers with Increased Hydrophilic Oligomer Ratio in Proton-Exchange Membrane Fuel Cells Operating at 80% Relative Humidity

We designed and synthesized a series of sulfonated poly(arylene ether sulfone) (SPES) with different hydrophilic or hydrophobic oligomer ratios using poly-condensation strategy. Afterward, we fabricated the corresponding membranes via a solution-casting approach. We verified the SPES membrane chemical structure using nuclear magnetic resonance (1H NMR) and confirmed the resulting oligomer ratio. Field-emission scanning electron microscope (FE-SEM) and atomic force microscope (AFM) results revealed that we effectively attained phase separation of the SPES membrane along with an increased hydrophilic oligomer ratio. Thermal stability, glass transition temperature (Tg) and membrane elongation increased with the ratio of hydrophilic oligomers. SPES membranes with higher hydrophilic oligomer ratios exhibited superior water uptake, ion-exchange capacity, contact angle and water sorption, while retaining reasonable swelling degree. The proton conductivity results showed that SPES containing higher amounts of hydrophilic oligomers provided a 74.7 mS cm−1 proton conductivity at 90 °C, which is better than other SPES membranes, but slightly lower than that of Nafion-117 membrane. When integrating SPES membranes with proton-exchange membrane fuel cells (PEMFCs) at 60 °C and 80% relative humidity (RH), the PEMFC power density exhibited a similar increment-pattern like proton conductivity pattern.

Preparation and characterization of high conductivity comb polymer anion exchange membranes

A comb-shaped anion exchange membrane was successfully constructed by simultaneously introducing a hydrophobic aliphatic long-chain bromododecane (BA) and a hydrophilic short-chain bromoethanol (BE) as a quaternizing agent into the side chain of poly-4-vinyl-pyridine (P4VP). The aforementioned membrane chemical structure was investigated by FT-IR, and confirmed that the BA and BE were successfully grafted onto the P4VP backbone. A series of performance test results reveal that the water uptake (WU) and swelling ratio (SR) are effectively controlled with the BA content increased from 0% to 75.0%, which the value was corresponding to the water uptake decreases from 92.3% to 31.8%, and the swelling ratio decreases from 78.4% to 7.8%. Interestingly, the conductivity of synthesized membrane in this paper is much increased with substantially the same ion exchange capacity (IEC) (from 50.0 mS·cm−1 for BE100.0%-BA0%/P4VP to 75.0 mS·cm−1 for BE25.0%- BA75.0%/P4VP at 70 °C), which can be attributed to the microphase-separated structure. On the other hand, the membrane BE25.0%-BA75.0%/P4VP has the best overall performance, which with the conductivity of 75.0 mS·cm−1 at 70 °C, which the temperature closest to the alkaline fuel cell operating temperature. And it also had excellent tensile strength (16.0 MPa) and elongation at break (5.9%), indicating sufficient flexibility and processability, and it has excellent chemical stability immersed it into 3 mol·L−1 KOH for up to 240 h, which was the conductivity can still reach more than 70% of the initial value. And the aforementioned membrane also with the better thermal stability (the thermal decomposition temperatures above 200 °C). The single cell test result reveal that the membrane displayed good cell performance with a power density of 47.52 mW·cm−2 at 70 °C. In summary, the comprehensive performance of BE25.0%-BA75.0%/P4VP is expected to be applied to alkaline polymer fuel cells.

Synthesis, characterization and antibacterial properties of reverse osmosis membranes from cellulose bromoacetate

The antibacterial alkyl bromide groups were introduced into cellulose diacetate (CDA) through the esterification between highly active bromoacetyl bromide and hydroxyl groups under mild conditions and the modified cellulose acetate reverse osmosis membrane (Br-CA-RO) was obtained subsequently after phase inversion process. The results of FTIR and 1H-NMR indicated the existence of alkyl bromide groups in the modified CDA powder. The optical density method was used to confirm its antibacterial ability. The chemical structure and morphology characterization demonstrated that the anchoring of bromoacetyl groups as antibacterial part brought out minimal alteration on membrane chemical structure and its asymmetric structure, but the regularity and density of membranes increased with the degree of esterification. The property testing results showed that the whole physical performance was enhanced with the thermal stability getting slightly worse. Both the flux and salt rejection of Br-CA-RO increased after modification and the latter was close to that of cellulose triacetate RO membrane. The dynamic contact antibacterial testing implied that Br-CA-RO showed much higher antibacterial ability than unmodified RO membranes, and its relative bactericidal rates against E. coli (gram negative) and S. aureus (gram positive) were higher than 73.0% and 93.5%, respectively. This method offers a new approach for antibacterial and functional modification of CA-RO membranes.

Towards a NMR-Based Method for Characterizing the Degradation of Nafion XL Membranes for PEMFC

PFSA-based reinforced membranes are used today as the benchmark material for the electrolyte in PEMFCs. Although greatly improved relatively to their unreinforced version, they still suffer from aging and degradation during fuel cell (FC) operation. In this study we first performed proton NMR to characterize the different water populations in the pristine Nafion XL reinforced membrane. Then we used proton and fluorine NMR, FTIR and sorption measurements in order to qualitatively observe the differences induced in the membrane's chemical structure and properties by long term FC operation. Proton NMR is seen to be an adapted tool to quickly measure a signature that is correlated to the degradation state while FTIR can serve as a local probe of the chemical structure.

Mechanical, thermal, and morphological properties of nanoporous reinforced polysulfone membranes

The effect of three different diphenols including 1,1′-thiobis(2-naphthol) (TBN), 2,2′-thiobis(4-methyl phenol) (TBMP) and curcumin (CUR) used in the preparation of sulfonated poly(ether sulfide sulfone) (SPESS) on the mechanical, thermal, and morphological properties of polysulfone (PSf) membranes was studied. In this regard, the morphological characteristics of the nanoporous membranes were explored by developed image analyzer program. Based on the obtained results, PSf membrane modified by SPESS-TBN copolymer showed the best nominal tensile strength in comparison to the other samples with the value of 206 MPa. The optimum real tensile strength with the value of 322 MPa was observed for SPESS-TBMP membrane. It was found that the addition of SPESS, improved the mechanical and thermal properties as well as the performance of membranes. Inspections of scanning electron microscopic images for evaluation of the void properties of membranes revealed that there were 42.3, 8.76, 41.45 and 15.92% of void contents within the structures of neat PSf membrane, SPESS-CUR, SPESS-TBMP and SPESS-TBN, respectively. Moreover, some conceptual relations between morphological and mechanical properties were presented. Finally, the effect of the membrane chemical structure on the mechanical, thermal, and morphological properties was discussed in this study.

Effects of Temperature and Catalyst Type on Chemical Degradation of Radiation Grafted Membranes in PEFCs

AbstractProton exchange membranes prepared by radiation grafting are a promising alternative material to perfluoroalkyl sulfonic acid (PFSA) for fuel cell application. The temperature effect on the chemical degradation of radiation grafted membranes is evaluated by testing styrene grafted and sulfonated membranes at elevated temperatures (90 to 110 °C) and open circuit voltage (OCV) conditions. The results show that the increased temperature can markedly enhance membrane degradation, as expected, which is similar to the case of PFSA membranes. Moreover, elevated operating temperature leads to brown discoloration of the tested membrane, which may be attributed to the formation of a large conjugated π‐electron system in the membrane chemical structure. The OCV tests in which the styrene grafted membranes were tested at 80 °C with different kinds of catalyst (Pt/C and Pt black, respectively) in the gas diffusion electrodes (GDE) show that the membrane combined with Pt black GDEs exhibited a significantly lower degradation rate than that combined with Pt/C GDEs. The influence of differences in the amount of contaminants and the possibility of H2O2 formation between these two types of catalyst are put forward to explain this phenomenon.

Influence of chitosan coating on the separation performance, morphology and anti-fouling properties of the polyamide nanofiltration membranes

Chitosan was coated into polyamide membranes by in situ reaction of amine and hydroxyl groups of chitosan and un-reacted of acylchloride of polyamide active layer. Effect of chitosan concentration and coagulation time on the membrane chemical structure, morphology, separation performances and anti-fouling properties were characterized by FT-IR, SEM, AFM, cross-flow membrane module device and cetyltrimethylammonium bromide experiment. Water flux of the polyamide membranes enhanced from 32.9 to 59.6L/m2h and anti-fouling results proved that excellent fouling resistance. Thus, chitosan as a secondary modifier has special potential to improve the performance of polyamide membranes.

The chlorination process of crosslinked aromatic polyamide reverse osmosis membrane: New insights from the study of self-made membrane

The development of chlorine-resistant membranes requires good understanding of membrane chlorination process. However, researches involving membrane chlorination processes are often influenced by experimental materials, especially the commercial crosslinked aromatic polyamide RO membranes, the preparation or modification technology and composition of which are not fully disclosed. In this work, the changes in membrane chemical structure and composition during the chlorination process were systematically studied by the use of self-made crosslinked aromatic polyamide RO membranes to avoid these influences. The effects of different chlorination conditions and pure water immersion time were analyzed. For the first time, the reversibility of the chlorination process was investigated. The ATR-FTIR and XPS results indicated that chlorination involves a multi-step process. The IR bands which characterize the amide groups (1540 and 1608cm−1) gradually disappear, followed by the appearance of the bands which characterize the chlorination of aromatic ring (780 and 811cm−1). The increase rates of the chlorine content are different at different stages. The changes in chemical structure and composition of self-made membrane were compared with those of two representative commercial membranes (RE2521 and BW30). Furthermore, the consequential effects of these changes on surface hydrophilicity, charge, permselectivity performance, and morphology of self-made membrane were discussed.

Interplay between the Structure and Relaxations in Selemion AMV Hydroxide Conducting Membranes for AEMFC Applications

Selemion AMV was studied to examine the relationship between the membrane chemical structure and its properties. The structure of AMV consists of two components: a functionalized polystyrene copolymer containing the ion-exchange moieties and PVC which is likely blended with copolymer. The PVC is primarily responsible for the mechanical properties of the membrane but seems to undergo degradation during the anion-exchange process that leads to a reduction of the storage modulus. The functionalized polystyrene copolymer with the ion-exchange groups is primarily responsible for the electrical properties of the membrane. The AMV and AMVOH membranes exhibited conductivities of 2 and 7 mS cm–1 at 25 °C, respectively. The membrane exhibited Arrhenius behavior in all conditions that suggests the dynamics of the membrane are not significantly involved in the mechanism of long-range conduction. The AMVOH membrane has two predominant pathways of charge exchange through the membrane: one through the bulk of the hydrop...